NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE...

9
NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther Cubo, MD, PhD, Saul-Indra Martinez-Horta, PhD, Frederic Sampedro Santalo, Asunci´ on Mart´ ınez Descalls, PhD, Sara Calvo, PhD, Cecilia Gil-Polo, MD, PhD, Ignacio Muñoz, MD, Katia Llano, Natividad Mariscal, Dolores Diaz, Aranzazu Gutierrez, Laura Aguado, and Mar´ ıa A. Ramos-Arroyo, MD, PhD, for the European HD Network Neurology ® 2019;92:e2101-e2108. doi:10.1212/WNL.0000000000007147 Correspondence Dr. Cubo mcubo@ saludcastillayleon.es Abstract Objective Because patients homozygous for Huntington disease (HD) receive the gain-of-function mutation in a double dose, one would expect a more toxic eect in homozygotes than in heterozygotes. Our aim was to investigate the phenotypic dierences between homozygotes with both alleles 36 CAG repeats and heterozygotes with 1 allele 36 CAG repeats. Methods This was an international, longitudinal, case-control study (European Huntingtons Disease Network Registry database). Baseline and longitudinal total functional capacity, motor, cog- nitive, and behavioral scores of the Unied Huntingtons Disease Rating Scale (UHDRS) were compared between homozygotes and heterozygotes. Four-year follow-up data were analyzed using longitudinal mixed-eects models. To estimate the association of age at onset with the length of the shorter and larger allele in homozygotes and heterozygotes, regression analysis was applied. Results Of 10,921 participants with HD (5,777 female [52.9%] and 5,138 male [47.0%]) with a mean age of 55.1 ± 14.1 years, 28 homozygotes (0.3%) and 10,893 (99.7%) heterozygotes were identied. After correcting for multiple comparisons, homozygotes and heterozygotes had similar age at onset and UHDRS scores and disease progression. In the multivariate linear regression analysis, the longer allele was the most contributing factor to decreased age at HD onset in the homozygotes (p < 0.0001) and heterozygotes (p < 0.0001). Conclusions CAG repeat expansion on both alleles of the HTT gene is infrequent. Age at onset, HD phenotype, and disease progression do not signicantly dier between homozygotes and heterozygotes, indicating similar eect on the mutant protein. Classification of evidence This study provides Class II evidence that age at onset, the motor phenotype and rate of motor decline, and symptoms and signs progression is similar in homozygotes compared to heterozygotes. MORE ONLINE Class of Evidence Criteria for rating therapeutic and diagnostic studies NPub.org/coe From the Neurology Department (E.C., C.G.-P., I.M., K.L., N.M., D.D., A.G., L.A.) and Research Unit (S.C.), Hospital Universitario Burgos; Movement Disorders Unit, Neurology Department (S.-I.M.-H., F.S.S.), Hospital de La Santa Creu I Sant Pau, Barcelona; Centro de Investigaci´ on en Red-Enfermedades Neurodegenerativas (CIBERNED) (S.-I.M.-H., F.S.S.), Madrid; Neurology Department (A.M.D.), Fundaci´ on Jim´ enez Diez, Madrid; and Genetic Department (M.A.R.-A.), Complejo Hospitalario de Navarra, Pamplona, Spain. Coinvestigators are listed at links.lww.com/WNL/A840. Go to Neurology.org/N for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article. Copyright © 2019 American Academy of Neurology e2101 Copyright © 2019 American Academy of Neurology. Unauthorized reproduction of this article is prohibited.

Transcript of NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE...

Page 1: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

NULL HYPOTHESIS CLASS OF EVIDENCE

Clinical manifestations of homozygote allelecarriers in Huntington diseaseEsther Cubo MD PhD Saul-Indra Martinez-Horta PhD Frederic Sampedro Santalo

Asuncion Martınez Descalls PhD Sara Calvo PhD Cecilia Gil-Polo MD PhD Ignacio Muntildeoz MD Katia Llano

Natividad Mariscal Dolores Diaz Aranzazu Gutierrez Laura Aguado and Marıa A Ramos-Arroyo MD PhD

for the European HD Network

Neurologyreg 201992e2101-e2108 doi101212WNL0000000000007147

Correspondence

Dr Cubo

mcubo

saludcastillayleones

AbstractObjectiveBecause patients homozygous for Huntington disease (HD) receive the gain-of-functionmutation in a double dose one would expect a more toxic effect in homozygotes than inheterozygotes Our aim was to investigate the phenotypic differences between homozygoteswith both alleles ge36 CAG repeats and heterozygotes with 1 allele ge36 CAG repeats

MethodsThis was an international longitudinal case-control study (European Huntingtonrsquos DiseaseNetwork Registry database) Baseline and longitudinal total functional capacity motor cog-nitive and behavioral scores of the Unified Huntingtonrsquos Disease Rating Scale (UHDRS) werecompared between homozygotes and heterozygotes Four-year follow-up data were analyzedusing longitudinal mixed-effects models To estimate the association of age at onset with thelength of the shorter and larger allele in homozygotes and heterozygotes regression analysiswas applied

ResultsOf 10921 participants with HD (5777 female [529] and 5138 male [470]) with a meanage of 551 plusmn 141 years 28 homozygotes (03) and 10893 (997) heterozygotes wereidentified After correcting for multiple comparisons homozygotes and heterozygotes hadsimilar age at onset and UHDRS scores and disease progression In the multivariate linearregression analysis the longer allele was the most contributing factor to decreased age at HDonset in the homozygotes (p lt 00001) and heterozygotes (p lt 00001)

ConclusionsCAG repeat expansion on both alleles of the HTT gene is infrequent Age at onset HDphenotype and disease progression do not significantly differ between homozygotes andheterozygotes indicating similar effect on the mutant protein

Classification of evidenceThis study provides Class II evidence that age at onset the motor phenotype and rate of motordecline and symptoms and signs progression is similar in homozygotes compared toheterozygotes

MORE ONLINE

Class of EvidenceCriteria for ratingtherapeutic and diagnosticstudies

NPuborgcoe

From the Neurology Department (EC CG-P IM KL NM DD AG LA) and Research Unit (SC) Hospital Universitario Burgos Movement Disorders Unit NeurologyDepartment (S-IM-H FSS) Hospital de La Santa Creu I Sant Pau Barcelona Centro de Investigacion en Red-Enfermedades Neurodegenerativas (CIBERNED) (S-IM-H FSS)Madrid Neurology Department (AMD) Fundacion Jimenez Diez Madrid and Genetic Department (MAR-A) Complejo Hospitalario de Navarra Pamplona Spain

Coinvestigators are listed at linkslwwcomWNLA840

Go to NeurologyorgN for full disclosures Funding information and disclosures deemed relevant by the authors if any are provided at the end of the article

Copyright copy 2019 American Academy of Neurology e2101

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

The presence of 36 or more CAG trinucleotide repeats in theHTT gene nearly ensures the development of Huntingtondisease (HD) as an autosomal dominantly transmitted disorderBecause homozygote patients (ie with 2 mutant alleles) for HDreceive the gain-of-functionmutation in a double dose onewouldexpect a more toxic effect in homozygotes than in the hetero-zygotes (ie with one mutant allele) similar to other poly CAGdiseases12 In this regard some publications in animalmodels andsmall HD human retrospective studies have reported a moreaggressive HD progression and brain atrophy in homozygotes23

In contrast other authors have reported an indistinguishablephenotype in HD homozygotes and heterozygotes4ndash8 Giventhese contradictory data and the sparse longitudinal informationwe aimed to further investigate the clinical differences betweenHD homozygotes (with both alleles ge36 CAG repeats) andheterozygotes (with one allele ge36CAG repeats) in terms of ageat onset phenotypic presentation and disease progression

MethodsDesignThis was an international retrospectivendashlongitudinal casendashcontrol study

Sample characteristics and ethicsClinical and sociodemographic data were obtained frompatients enrolled in the European Huntingtonrsquos DiseaseRegistry Database (European Huntingtonrsquos Disease Network[EHDN])9 For this observational study participants providedwritten informed consent following the International Confer-ence on HarmonizationndashGood Clinical Practice guidelines10

For participants who lacked capacity to consent study sitesfollowed country-specific guidelines for signing consent formsMinors agreed with both parents authorizing for them Ethicalapproval was collected from the local ethics committee for eachstudy site contributing to EHDN Registry9

Participants and clinical assessmentsData of individuals from the European Huntingtonrsquos DiseaseRegistry Database from July 1998 to December 2016 witha larger allele ge36 CAG repeats within the Huntingtin gene wereincluded in this study Data collection adhered to a standardprotocol including electronic case report forms and used iden-tical study protocols of assessment and sampling of biomaterials

Demographics number of years of education and body massindex (BMI calculated as weight in kilograms divided by

GlossaryBMI = body mass index CI = confidence interval EHDN = European Huntingtonrsquos Disease Network HD = Huntingtondisease SNP = single nucleotide polymorphism TFC = total functional capacity TMS = total motor scoreUHDRS = UnifiedHuntingtonrsquos Disease Rating Scale

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height in meters squared) were extracted from the EHDNregistry Motor and psychiatric signs were scored using theUnified Huntingtonrsquos Disease Rating Scale (UHDRS)11 Formotor and behavior UHDRS higher scores indicated worsemotor and higher psychiatric impairment For cognition weused the cognitive UHDRS composite score (UHDRS totalcorrect for letter fluency Symbol Digit Modalities Test andStroop subscores for word reading color identificationnaming and interference) with lower scores indicating worseperformance12 Disease stage was obtained from total func-tional capacity (TFC) scores with higher scores indicatingbetter functional status13 Patients were followed up ona yearly basis according to the EHDN Registry protocol912

Study site raters were annually trained evaluated and certifiedto lessen interrater and intrarater variability Data entry wasreviewed online and an on-site by monitors fluent in thelanguage of the study site912 TheHTT CAG genotyping wasperformed at each local genetic laboratory In addition freshblood samples were donated by patients and sent to thecentral laboratory in Milan Bio-Rep to be reanalyzed Clin-ically significant discrepancies defined as crossing theboundary at 35ndash36 or 39ndash40 CAG repeat lengths or mea-surement errors (plusmn1 for CAG repeat lengths le42 and plusmn3 forCAG repeat lengths ge43) were brought to the attention ofthe local site investigator and subsequently addressed

Data managementHDhomozygotes were defined as carriers of 2 alleles with ge36CAG repeats while HD heterozygotes were individuals withthe longer allele ge36 CAG repeats and the shorter allele lt36CAG repeats Demographics CAG repeat length and clinicalinformation including total motor score (TMS) cognitiveand behavior UHDRS scores and TFC information at base-line and after 4 years of follow-up were collected from theEHDN database With the motor UHDRS different domainsubscores were calculated chorea (sum of the chorea items[face buccolingual upper and lower extremities scores])dystonia (sum of the trunk upper and lower extremitiesscores) bradykinesia (sum of the finger taps pronatesupinate rigidity of each extremity and body bradykinesiascores) gait impairment (sum of gait tandem and retro-pulsion scores) and oculomotor performance (sum of ocularpursuit saccade initiation and saccade velocity) To deal withmissing values case-wise deletions were adopted

Statistical analysisAnalysis was done using IBM-SPSS 21 software (SPSS IncChicago IL) following the Reporting of ObservationalStudies in Epidemiology guidelines14 Normal distribution ofvariables was analyzed using the Kolmogorov-Smirnov testDescriptive analysis of the participantsrsquo characteristics wasperformed in terms of frequencies (percentage) meanmedian values with the corresponding SD or interquartilerange as appropriate and 95 confidence intervals (CI)

Clinical characteristics were analyzed in a cross-sectional andlongitudinal manner Baseline differences between HD

homozygotes and the total sample of HD heterozygous werefirst evaluated To balance differences in sample sizes a posthoc secondary analysis was carried out comparing homo-zygotes with a subset of heterozygotes paired by age andCAG larger allele (13) In addition because aging mightworsen the UHDRS scores especially bradykinesia and gaitscores independently of the genetic status15 a comparativeanalysis between homozygotes and heterozygotes was con-ducted including young and older participants All homozy-gote and heterozygote participants were classified as older(gt51 years old) or younger participants (le51 years old)based on the median age of homozygotes at registry entryDifferences were analyzed using the χ2 Phi and Cramer tests(categorical variables) and Mann-Whitney U tests (non-parametric for quantitative variables) A significance level of α= 0004 2-sided tests was applied after post hoc Bonferronimultiple comparison adjustments

To analyze the association between age length of the largerand shorter alleles BMI and UHDRS outcome measurescorrelations were calculated using Spearman (nonparametric)correlation coefficients To analyze the relationship betweenage at onset and the length of the larger and shorter alleles inthe homozygote and heterozygote groups a multivariate lin-ear regression analysis was conducted

For follow-up data linear mixed-effects models16 were con-structed to investigate the course of different outcomes overa 4-year period To account for the correlation between re-peated measurements on the same participant a random in-tercept and random time effect (slope) per participant wasused For all analyses an unstructured covariance for therandom intercepts and random slopes was used Differencesin the rate of progression (ie slope) between homozygotesand heterozygotes were compared

Data availability statementWe used the same methodology as that described in a pre-vious study15 The European HD Registry is a large pro-spective study observing the natural course clinicalspectrum and management of HD in 140 centers from 17European countries and 3 other countries912 More in-formation on the Registry can be found at euro-hdnethtmlregistry This study is registered with ClinicalTrialsgov numberNCT01590589

ResultsAs of December 2016 of 10921 participants with HD (5777female [529] and 5138 male [470]) with a mean age of551 plusmn 141 years 28 homozygotes (03) and 10893(997) heterozygotes were identified The median CAGrepeat lengths of the longer and shorter alleles were 45 (4247) and 38 (3740) respectively for the homozygote groupand 43 (4445) and 18 (1720) respectively for the hetero-zygote group (table 1)

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At baseline clinical characteristics of homozygotes and het-erozygotes were not significantly different although homo-zygotes showed a trend for lower age at HD onset and lowerBMI (table 1 and figure 1) Similar results were obtained inthe post hoc comparative analysis between homozygotes andthe age-larger allele-paired heterozygotes with a trend forgreater ocular disturbances in homozygotes (table 1)

After stratification of participants by age the median CAGrepeats of the longer allele in the younger group was 47 (4555)among homozygotes (n = 14) and 45 (4349) among hetero-zygotes (n = 4138) while in the older group it was 43 (4045)and 42 (4144) for homozygotes (n = 14) and heterozygotes (n= 6757) respectively No significant differences in BMI TMSCognitive Behavior-UHDRS andTFC scores were observed inthe young or old group except for a trend for lower age at HDonset in young homozygotes compared to young heterozygotes(270 [167365] vs 340 [280390] p = 004) and greater gaitUHDRS scores in old homozygotes compared to old hetero-zygotes (50 [4092] vs 40 [2270] p = 001)

The length of the longer allele was correlated with age at HDonset in both homozygotes (rs = minus086 p lt 00001) and

heterozygotes (rs = minus076 p lt 00001) while correlation withBMI was only seen in homozygotes (rs = minus050 p = 001) Thelength of the shorter allele did not have a significant effect oncognitive behavior ormotor (including bradykinesia chorea gaitand dystonia UHDRS) scores in either group Likewise thenumber of years of education did not show a significant correla-tion with age at HD onset among heterozygotes or homozygotes

In the multivariate linear regression analysis using the smallerand longer alleles as the independent variables the length ofthe longer allele contributed most to earlier age at HD onsetin the homozygote and heterozygote groups (table 2)According to these models in heterozygotes with 1-unitCAG repeat increase in the longer allele age at HD onsetdecreased 190 years 95 CI minus194 minus186 (p lt 00001)Likewise in homozygotes for 1-unit CAG repeat increase inthe longer allele age at HD onset decreased 172 years 95CI minus236 minus107 (p lt 00001) These models explained 448and 617 of the variability of age at HD onset in the het-erozygote and homozygote groups respectively

TMS cognitive behavior UHDRS BMI and TFC follow-updata are shown in boxplots (figure 2) Overall no significant

Table 1 Baseline characteristics of homozygotes and heterozygotes with Huntington disease (HD)

Homozygotes n = 28 Heterozygotes n = 10893 p Valuea Heterozygotesp n = 65 p Valueb

Age y 515 (367ndash717) 28 550 (450ndash650) 10886 031 520 (395ndash715) 65 076

Female 19 (679) 28 5758 (529) 10893 012 38 (585) 65 048

Shorter allele CAG repeats 38 (37ndash40) 28 18 (17ndash20) 10893 lt00001 18 (17ndash21) 65 lt00001

Longer allele CAG repeats 45 (42ndash47) 28 43 (41ndash46) 10893 003 45 (41ndash47) 65 037

Age at onset y 395 (237ndash530) 20 460 (370ndash560) 8442 004 445 (347ndash580) 46 013

HD duration y 125 (60ndash160) 20 100 (60ndash150) 8478 035 120 (90ndash162) 46 098

Education y 100 (90ndash130) 27 110 (90ndash140) 10391 012 120 (100ndash140) 65 001

BMI 221 (210ndash236) 25 235 (212ndash265) 10817 004 232 (209ndash267) 59 013

TFC 95 (45ndash127) 28 100 (60ndash130) 10717 050 110 (70ndash130) 65 015

TMS-UHDRS 350 (82ndash577) 28 270 (90ndash460) 10445 046 215 (40ndash450) 64 020

Chorea-UHDRS 65 (02ndash107) 28 50 (10ndash100) 10576 055 40 (00ndash90) 65 033

Bradykinesia-UHDRS 80 (32ndash167) 28 70 (20ndash120) 10535 044 60 (10ndash110) 65 028

Dystonia-UHDRS 10 (00ndash57) 28 00 (00ndash30) 10562 035 10 (00ndash20) 65 012

Gait-UHDRS 40 (00ndash67) 28 30 (00ndash50) 10542 023 15 (00ndash47) 65 015

Ocular-UHDRS 60 (15ndash165) 28 60 (12ndash110) 10569 031 40 (00ndash100) 65 006

Cognitive-UHDRS 2260 (1420ndash2690) 9 1740 (1220ndash2410) 4102 034 2100 (1270ndash3020) 35 071

Behavior-UHDRS 130 (40ndash270) 21 120 (50ndash210) 6753 081 80 (30ndash150) 47 051

Abbreviations BMI = body mass index Heterozygotesp = age-larger allele-paired heterozygotes TFC = total functional capacity TMS = total motor scoreUHDRS = Unified Huntingtonrsquos Disease Rating ScaleA significance level of α = 0004 2-sided tests was applied after post hoc Bonferroni multiple comparisons adjustments Values are expressed as median(interquartile range) n or n () Na p Value relates to the homozygotes vs heterozygotes comparisonb p Value relates to homozygotes vs age-larger allele-paired heterozygotes Baseline comparison values were compared using the Mann-Whitney U test

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differences were observed in homozygotes compared to het-erozygotes in terms of outcome variables over the course of 4years (table 3)

DiscussionIn this longitudinal analysis of the EHDN registry after cor-recting for multiple comparisons homozygote HD carriershad a similar age at onset phenotype and disease progressioncompared to heterozygotes These findings suggest thatoverall the shorter expanded allele does not have a significantand major influence in determining either the age at onset orthe phenotypic expression and progression of HD

In HD animal models and some CAG triplet diseases inhumans including spinal and bulbar muscular atrophy anddentatorubral-pallidoluysian atrophy homozygotes have con-sistently shown more aggressive neurodegeneration comparedto heterozygotes1718 In other polyQ diseases however theeffect of a second expanded allele remains unclear Singlereports and small series of homozygotes for spinocerebellarataxia type 6 and 3 suggest a gene dose effect on age at onsetand increased severity of the phenotype1920 For HD

assessment of double dose gene effect in the phenotype hasbeen extremely challenging given the rare occurrence ofhomozygotes While some publications in HD have reportedan indistinguishable phenotype in homozygotes andheterozygotes478 other studies have observed a nonchorei-form phenotype presentation more frequently in homo-zygotes with a significant increase in severity of progression ofmotor and psychiatric manifestations indicating that homo-zygotes may present with a wider spectrum of neurologicsymptoms other than chorea compared to heterozygotes221

Supporting findings included marked cerebellum atrophy inneuroimaging in 3 homozygotes and widespread brain atro-phy at autopsy of one homozygote patient with HD2 Ourstudy however cannot confirm this difference in phenotypeas clinical characteristics of homozygotes were similar toheterozygotes Even more we did not observe a differentclinical profile between younger and older homozygotes ex-cept for a trend for greater gait impairment in the old groupHomozygotes (especially young homozygotes) had lower ageat HD onset than heterozygotes but they also had highermedian CAG repeats of the longer allele which may partiallyaccount for an earlier initiation of symptoms Of notehomozygotes also showed a trend for lower BMI compared to

Table 2 Multivariate lineal regression analysis of clinical variables associated with age at Huntington disease onset

Homozygotes corrected R2 = 657 n = 20 n (95 confidenceinterval) p value

Heterozygotes corrected R2 = 487 n = 8442 Β (95 confidenceinterval) p value

Shorterallele

minus144 (minus365 to 076) 018 001 (minus004 to 007) 015

Longerallele

minus172 (minus236 to minus107) lt00001 minus190 (minus194 to minus186) lt00001

Figure 1 The relationship between expanded alleles CAG repeat length and age at onset of Huntington disease

The minimal adequate model for this heterozygote and homozygote dataset is shown as black (homozygotes) and red (heterozygotes) lines

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heterozygotes Explaining hypothesis includes that homo-zygotes might be at risk for a greater hypermetabolic state thatmay precede the occurrence of motor symptoms and con-tribute to weight loss2223

Heterozygote individuals with clinically diagnosed HD ex-perience a steady progressive decline in the cardinal featuresof the disease2425 In our study heterozygotes had a similarrate of decline compared to homozygotes in terms of BMITFC TMS behavior and cognitive UHDRS scores In con-trast Squitieri et al2 observed a faster rate of progression indisability measured by independence and the physical dis-ability scales in 8 homozygotes compared to 75 heterozygotesGiven the differentmethodology used in that study2 our resultsare not comparable Available clinimetric data show that in theoverall HD population items measured by the TMS-UHDRSthe only recommended rating scale to measure the severity ofmotor signs in HD that cover all motor domains in HD26 havevariable weights at different HD stages It seems that bradyki-nesia and dystonia are predominant in patients with greaterCAG repeats and younger onset of HD compared to lowerCAG repeats in the larger allele Instead chorea is predominantin earlier stages of manifest adult HD tends to plateau anddecreases later and parkinsonian features become progressivelymore severe and are more clinically significant in later stages ofthe disease27 Interestingly we found a similar motor pheno-type progression in homozygotes compared to heterozygotesin terms of choreiform and nonchoreiform manifestations overtime except for a trend for greater gait impairment in olderhomozygotes compared with heterozygotes in post hoc anal-ysis However the scarce and relatively short longitudinalclinical information on homozygotes limits the clinical rele-vance of these observations

Figure 2 Follow-up data

Follow-updata formotorUnifiedHuntingtonrsquosDisease Rating Scale (UHDRS)(A) cognitive UHDRS (B) behavior UHDRS (C) body mass index (D) and totalfunctional capacity (E) Number of participants participating in each visit isshown Homozygotes are represented in gray boxes and heterozygotes in redboxes Thedark line represents themedianof the outcome variable the bottomof the box indicates the 25th percentile and the top the 75th percentile

Table 3 Slope differences for the homozygotes vsheterozygotes

Slope difference (SE) p Value

TFC minus09 (06) 040

TMS-UHDRS 41 (42) 036

Chorea-UHDRS 02 (08) 078

Bradykinesia-UHDRS 10 (11) 037

Dystonia-UHDRS 05 (03) 044

Ocular-UHDRS 13 (10) 019

Cognitive-UHDRS 170 (264) 053

Behavior-UHDRS 15 (21) 047

BMI minus15 (12) 018

Abbreviations BMI = bodymass index TFC = total functional capacity TMS =total motor score UHDRS = Unified Huntingtonrsquos Disease Rating ScaleShown are parameter estimates from the linear mixed models The differ-ence between homozygotes and heterozygotes is expressed as a slope pValues relates to the difference in the homozygotes compared toheterozygotes

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In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

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Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

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Page 2: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

The presence of 36 or more CAG trinucleotide repeats in theHTT gene nearly ensures the development of Huntingtondisease (HD) as an autosomal dominantly transmitted disorderBecause homozygote patients (ie with 2 mutant alleles) for HDreceive the gain-of-functionmutation in a double dose onewouldexpect a more toxic effect in homozygotes than in the hetero-zygotes (ie with one mutant allele) similar to other poly CAGdiseases12 In this regard some publications in animalmodels andsmall HD human retrospective studies have reported a moreaggressive HD progression and brain atrophy in homozygotes23

In contrast other authors have reported an indistinguishablephenotype in HD homozygotes and heterozygotes4ndash8 Giventhese contradictory data and the sparse longitudinal informationwe aimed to further investigate the clinical differences betweenHD homozygotes (with both alleles ge36 CAG repeats) andheterozygotes (with one allele ge36CAG repeats) in terms of ageat onset phenotypic presentation and disease progression

MethodsDesignThis was an international retrospectivendashlongitudinal casendashcontrol study

Sample characteristics and ethicsClinical and sociodemographic data were obtained frompatients enrolled in the European Huntingtonrsquos DiseaseRegistry Database (European Huntingtonrsquos Disease Network[EHDN])9 For this observational study participants providedwritten informed consent following the International Confer-ence on HarmonizationndashGood Clinical Practice guidelines10

For participants who lacked capacity to consent study sitesfollowed country-specific guidelines for signing consent formsMinors agreed with both parents authorizing for them Ethicalapproval was collected from the local ethics committee for eachstudy site contributing to EHDN Registry9

Participants and clinical assessmentsData of individuals from the European Huntingtonrsquos DiseaseRegistry Database from July 1998 to December 2016 witha larger allele ge36 CAG repeats within the Huntingtin gene wereincluded in this study Data collection adhered to a standardprotocol including electronic case report forms and used iden-tical study protocols of assessment and sampling of biomaterials

Demographics number of years of education and body massindex (BMI calculated as weight in kilograms divided by

GlossaryBMI = body mass index CI = confidence interval EHDN = European Huntingtonrsquos Disease Network HD = Huntingtondisease SNP = single nucleotide polymorphism TFC = total functional capacity TMS = total motor scoreUHDRS = UnifiedHuntingtonrsquos Disease Rating Scale

e2102 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

height in meters squared) were extracted from the EHDNregistry Motor and psychiatric signs were scored using theUnified Huntingtonrsquos Disease Rating Scale (UHDRS)11 Formotor and behavior UHDRS higher scores indicated worsemotor and higher psychiatric impairment For cognition weused the cognitive UHDRS composite score (UHDRS totalcorrect for letter fluency Symbol Digit Modalities Test andStroop subscores for word reading color identificationnaming and interference) with lower scores indicating worseperformance12 Disease stage was obtained from total func-tional capacity (TFC) scores with higher scores indicatingbetter functional status13 Patients were followed up ona yearly basis according to the EHDN Registry protocol912

Study site raters were annually trained evaluated and certifiedto lessen interrater and intrarater variability Data entry wasreviewed online and an on-site by monitors fluent in thelanguage of the study site912 TheHTT CAG genotyping wasperformed at each local genetic laboratory In addition freshblood samples were donated by patients and sent to thecentral laboratory in Milan Bio-Rep to be reanalyzed Clin-ically significant discrepancies defined as crossing theboundary at 35ndash36 or 39ndash40 CAG repeat lengths or mea-surement errors (plusmn1 for CAG repeat lengths le42 and plusmn3 forCAG repeat lengths ge43) were brought to the attention ofthe local site investigator and subsequently addressed

Data managementHDhomozygotes were defined as carriers of 2 alleles with ge36CAG repeats while HD heterozygotes were individuals withthe longer allele ge36 CAG repeats and the shorter allele lt36CAG repeats Demographics CAG repeat length and clinicalinformation including total motor score (TMS) cognitiveand behavior UHDRS scores and TFC information at base-line and after 4 years of follow-up were collected from theEHDN database With the motor UHDRS different domainsubscores were calculated chorea (sum of the chorea items[face buccolingual upper and lower extremities scores])dystonia (sum of the trunk upper and lower extremitiesscores) bradykinesia (sum of the finger taps pronatesupinate rigidity of each extremity and body bradykinesiascores) gait impairment (sum of gait tandem and retro-pulsion scores) and oculomotor performance (sum of ocularpursuit saccade initiation and saccade velocity) To deal withmissing values case-wise deletions were adopted

Statistical analysisAnalysis was done using IBM-SPSS 21 software (SPSS IncChicago IL) following the Reporting of ObservationalStudies in Epidemiology guidelines14 Normal distribution ofvariables was analyzed using the Kolmogorov-Smirnov testDescriptive analysis of the participantsrsquo characteristics wasperformed in terms of frequencies (percentage) meanmedian values with the corresponding SD or interquartilerange as appropriate and 95 confidence intervals (CI)

Clinical characteristics were analyzed in a cross-sectional andlongitudinal manner Baseline differences between HD

homozygotes and the total sample of HD heterozygous werefirst evaluated To balance differences in sample sizes a posthoc secondary analysis was carried out comparing homo-zygotes with a subset of heterozygotes paired by age andCAG larger allele (13) In addition because aging mightworsen the UHDRS scores especially bradykinesia and gaitscores independently of the genetic status15 a comparativeanalysis between homozygotes and heterozygotes was con-ducted including young and older participants All homozy-gote and heterozygote participants were classified as older(gt51 years old) or younger participants (le51 years old)based on the median age of homozygotes at registry entryDifferences were analyzed using the χ2 Phi and Cramer tests(categorical variables) and Mann-Whitney U tests (non-parametric for quantitative variables) A significance level of α= 0004 2-sided tests was applied after post hoc Bonferronimultiple comparison adjustments

To analyze the association between age length of the largerand shorter alleles BMI and UHDRS outcome measurescorrelations were calculated using Spearman (nonparametric)correlation coefficients To analyze the relationship betweenage at onset and the length of the larger and shorter alleles inthe homozygote and heterozygote groups a multivariate lin-ear regression analysis was conducted

For follow-up data linear mixed-effects models16 were con-structed to investigate the course of different outcomes overa 4-year period To account for the correlation between re-peated measurements on the same participant a random in-tercept and random time effect (slope) per participant wasused For all analyses an unstructured covariance for therandom intercepts and random slopes was used Differencesin the rate of progression (ie slope) between homozygotesand heterozygotes were compared

Data availability statementWe used the same methodology as that described in a pre-vious study15 The European HD Registry is a large pro-spective study observing the natural course clinicalspectrum and management of HD in 140 centers from 17European countries and 3 other countries912 More in-formation on the Registry can be found at euro-hdnethtmlregistry This study is registered with ClinicalTrialsgov numberNCT01590589

ResultsAs of December 2016 of 10921 participants with HD (5777female [529] and 5138 male [470]) with a mean age of551 plusmn 141 years 28 homozygotes (03) and 10893(997) heterozygotes were identified The median CAGrepeat lengths of the longer and shorter alleles were 45 (4247) and 38 (3740) respectively for the homozygote groupand 43 (4445) and 18 (1720) respectively for the hetero-zygote group (table 1)

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Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

At baseline clinical characteristics of homozygotes and het-erozygotes were not significantly different although homo-zygotes showed a trend for lower age at HD onset and lowerBMI (table 1 and figure 1) Similar results were obtained inthe post hoc comparative analysis between homozygotes andthe age-larger allele-paired heterozygotes with a trend forgreater ocular disturbances in homozygotes (table 1)

After stratification of participants by age the median CAGrepeats of the longer allele in the younger group was 47 (4555)among homozygotes (n = 14) and 45 (4349) among hetero-zygotes (n = 4138) while in the older group it was 43 (4045)and 42 (4144) for homozygotes (n = 14) and heterozygotes (n= 6757) respectively No significant differences in BMI TMSCognitive Behavior-UHDRS andTFC scores were observed inthe young or old group except for a trend for lower age at HDonset in young homozygotes compared to young heterozygotes(270 [167365] vs 340 [280390] p = 004) and greater gaitUHDRS scores in old homozygotes compared to old hetero-zygotes (50 [4092] vs 40 [2270] p = 001)

The length of the longer allele was correlated with age at HDonset in both homozygotes (rs = minus086 p lt 00001) and

heterozygotes (rs = minus076 p lt 00001) while correlation withBMI was only seen in homozygotes (rs = minus050 p = 001) Thelength of the shorter allele did not have a significant effect oncognitive behavior ormotor (including bradykinesia chorea gaitand dystonia UHDRS) scores in either group Likewise thenumber of years of education did not show a significant correla-tion with age at HD onset among heterozygotes or homozygotes

In the multivariate linear regression analysis using the smallerand longer alleles as the independent variables the length ofthe longer allele contributed most to earlier age at HD onsetin the homozygote and heterozygote groups (table 2)According to these models in heterozygotes with 1-unitCAG repeat increase in the longer allele age at HD onsetdecreased 190 years 95 CI minus194 minus186 (p lt 00001)Likewise in homozygotes for 1-unit CAG repeat increase inthe longer allele age at HD onset decreased 172 years 95CI minus236 minus107 (p lt 00001) These models explained 448and 617 of the variability of age at HD onset in the het-erozygote and homozygote groups respectively

TMS cognitive behavior UHDRS BMI and TFC follow-updata are shown in boxplots (figure 2) Overall no significant

Table 1 Baseline characteristics of homozygotes and heterozygotes with Huntington disease (HD)

Homozygotes n = 28 Heterozygotes n = 10893 p Valuea Heterozygotesp n = 65 p Valueb

Age y 515 (367ndash717) 28 550 (450ndash650) 10886 031 520 (395ndash715) 65 076

Female 19 (679) 28 5758 (529) 10893 012 38 (585) 65 048

Shorter allele CAG repeats 38 (37ndash40) 28 18 (17ndash20) 10893 lt00001 18 (17ndash21) 65 lt00001

Longer allele CAG repeats 45 (42ndash47) 28 43 (41ndash46) 10893 003 45 (41ndash47) 65 037

Age at onset y 395 (237ndash530) 20 460 (370ndash560) 8442 004 445 (347ndash580) 46 013

HD duration y 125 (60ndash160) 20 100 (60ndash150) 8478 035 120 (90ndash162) 46 098

Education y 100 (90ndash130) 27 110 (90ndash140) 10391 012 120 (100ndash140) 65 001

BMI 221 (210ndash236) 25 235 (212ndash265) 10817 004 232 (209ndash267) 59 013

TFC 95 (45ndash127) 28 100 (60ndash130) 10717 050 110 (70ndash130) 65 015

TMS-UHDRS 350 (82ndash577) 28 270 (90ndash460) 10445 046 215 (40ndash450) 64 020

Chorea-UHDRS 65 (02ndash107) 28 50 (10ndash100) 10576 055 40 (00ndash90) 65 033

Bradykinesia-UHDRS 80 (32ndash167) 28 70 (20ndash120) 10535 044 60 (10ndash110) 65 028

Dystonia-UHDRS 10 (00ndash57) 28 00 (00ndash30) 10562 035 10 (00ndash20) 65 012

Gait-UHDRS 40 (00ndash67) 28 30 (00ndash50) 10542 023 15 (00ndash47) 65 015

Ocular-UHDRS 60 (15ndash165) 28 60 (12ndash110) 10569 031 40 (00ndash100) 65 006

Cognitive-UHDRS 2260 (1420ndash2690) 9 1740 (1220ndash2410) 4102 034 2100 (1270ndash3020) 35 071

Behavior-UHDRS 130 (40ndash270) 21 120 (50ndash210) 6753 081 80 (30ndash150) 47 051

Abbreviations BMI = body mass index Heterozygotesp = age-larger allele-paired heterozygotes TFC = total functional capacity TMS = total motor scoreUHDRS = Unified Huntingtonrsquos Disease Rating ScaleA significance level of α = 0004 2-sided tests was applied after post hoc Bonferroni multiple comparisons adjustments Values are expressed as median(interquartile range) n or n () Na p Value relates to the homozygotes vs heterozygotes comparisonb p Value relates to homozygotes vs age-larger allele-paired heterozygotes Baseline comparison values were compared using the Mann-Whitney U test

e2104 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

differences were observed in homozygotes compared to het-erozygotes in terms of outcome variables over the course of 4years (table 3)

DiscussionIn this longitudinal analysis of the EHDN registry after cor-recting for multiple comparisons homozygote HD carriershad a similar age at onset phenotype and disease progressioncompared to heterozygotes These findings suggest thatoverall the shorter expanded allele does not have a significantand major influence in determining either the age at onset orthe phenotypic expression and progression of HD

In HD animal models and some CAG triplet diseases inhumans including spinal and bulbar muscular atrophy anddentatorubral-pallidoluysian atrophy homozygotes have con-sistently shown more aggressive neurodegeneration comparedto heterozygotes1718 In other polyQ diseases however theeffect of a second expanded allele remains unclear Singlereports and small series of homozygotes for spinocerebellarataxia type 6 and 3 suggest a gene dose effect on age at onsetand increased severity of the phenotype1920 For HD

assessment of double dose gene effect in the phenotype hasbeen extremely challenging given the rare occurrence ofhomozygotes While some publications in HD have reportedan indistinguishable phenotype in homozygotes andheterozygotes478 other studies have observed a nonchorei-form phenotype presentation more frequently in homo-zygotes with a significant increase in severity of progression ofmotor and psychiatric manifestations indicating that homo-zygotes may present with a wider spectrum of neurologicsymptoms other than chorea compared to heterozygotes221

Supporting findings included marked cerebellum atrophy inneuroimaging in 3 homozygotes and widespread brain atro-phy at autopsy of one homozygote patient with HD2 Ourstudy however cannot confirm this difference in phenotypeas clinical characteristics of homozygotes were similar toheterozygotes Even more we did not observe a differentclinical profile between younger and older homozygotes ex-cept for a trend for greater gait impairment in the old groupHomozygotes (especially young homozygotes) had lower ageat HD onset than heterozygotes but they also had highermedian CAG repeats of the longer allele which may partiallyaccount for an earlier initiation of symptoms Of notehomozygotes also showed a trend for lower BMI compared to

Table 2 Multivariate lineal regression analysis of clinical variables associated with age at Huntington disease onset

Homozygotes corrected R2 = 657 n = 20 n (95 confidenceinterval) p value

Heterozygotes corrected R2 = 487 n = 8442 Β (95 confidenceinterval) p value

Shorterallele

minus144 (minus365 to 076) 018 001 (minus004 to 007) 015

Longerallele

minus172 (minus236 to minus107) lt00001 minus190 (minus194 to minus186) lt00001

Figure 1 The relationship between expanded alleles CAG repeat length and age at onset of Huntington disease

The minimal adequate model for this heterozygote and homozygote dataset is shown as black (homozygotes) and red (heterozygotes) lines

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2105

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

heterozygotes Explaining hypothesis includes that homo-zygotes might be at risk for a greater hypermetabolic state thatmay precede the occurrence of motor symptoms and con-tribute to weight loss2223

Heterozygote individuals with clinically diagnosed HD ex-perience a steady progressive decline in the cardinal featuresof the disease2425 In our study heterozygotes had a similarrate of decline compared to homozygotes in terms of BMITFC TMS behavior and cognitive UHDRS scores In con-trast Squitieri et al2 observed a faster rate of progression indisability measured by independence and the physical dis-ability scales in 8 homozygotes compared to 75 heterozygotesGiven the differentmethodology used in that study2 our resultsare not comparable Available clinimetric data show that in theoverall HD population items measured by the TMS-UHDRSthe only recommended rating scale to measure the severity ofmotor signs in HD that cover all motor domains in HD26 havevariable weights at different HD stages It seems that bradyki-nesia and dystonia are predominant in patients with greaterCAG repeats and younger onset of HD compared to lowerCAG repeats in the larger allele Instead chorea is predominantin earlier stages of manifest adult HD tends to plateau anddecreases later and parkinsonian features become progressivelymore severe and are more clinically significant in later stages ofthe disease27 Interestingly we found a similar motor pheno-type progression in homozygotes compared to heterozygotesin terms of choreiform and nonchoreiform manifestations overtime except for a trend for greater gait impairment in olderhomozygotes compared with heterozygotes in post hoc anal-ysis However the scarce and relatively short longitudinalclinical information on homozygotes limits the clinical rele-vance of these observations

Figure 2 Follow-up data

Follow-updata formotorUnifiedHuntingtonrsquosDisease Rating Scale (UHDRS)(A) cognitive UHDRS (B) behavior UHDRS (C) body mass index (D) and totalfunctional capacity (E) Number of participants participating in each visit isshown Homozygotes are represented in gray boxes and heterozygotes in redboxes Thedark line represents themedianof the outcome variable the bottomof the box indicates the 25th percentile and the top the 75th percentile

Table 3 Slope differences for the homozygotes vsheterozygotes

Slope difference (SE) p Value

TFC minus09 (06) 040

TMS-UHDRS 41 (42) 036

Chorea-UHDRS 02 (08) 078

Bradykinesia-UHDRS 10 (11) 037

Dystonia-UHDRS 05 (03) 044

Ocular-UHDRS 13 (10) 019

Cognitive-UHDRS 170 (264) 053

Behavior-UHDRS 15 (21) 047

BMI minus15 (12) 018

Abbreviations BMI = bodymass index TFC = total functional capacity TMS =total motor score UHDRS = Unified Huntingtonrsquos Disease Rating ScaleShown are parameter estimates from the linear mixed models The differ-ence between homozygotes and heterozygotes is expressed as a slope pValues relates to the difference in the homozygotes compared toheterozygotes

e2106 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2107

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

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httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology

Page 3: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

height in meters squared) were extracted from the EHDNregistry Motor and psychiatric signs were scored using theUnified Huntingtonrsquos Disease Rating Scale (UHDRS)11 Formotor and behavior UHDRS higher scores indicated worsemotor and higher psychiatric impairment For cognition weused the cognitive UHDRS composite score (UHDRS totalcorrect for letter fluency Symbol Digit Modalities Test andStroop subscores for word reading color identificationnaming and interference) with lower scores indicating worseperformance12 Disease stage was obtained from total func-tional capacity (TFC) scores with higher scores indicatingbetter functional status13 Patients were followed up ona yearly basis according to the EHDN Registry protocol912

Study site raters were annually trained evaluated and certifiedto lessen interrater and intrarater variability Data entry wasreviewed online and an on-site by monitors fluent in thelanguage of the study site912 TheHTT CAG genotyping wasperformed at each local genetic laboratory In addition freshblood samples were donated by patients and sent to thecentral laboratory in Milan Bio-Rep to be reanalyzed Clin-ically significant discrepancies defined as crossing theboundary at 35ndash36 or 39ndash40 CAG repeat lengths or mea-surement errors (plusmn1 for CAG repeat lengths le42 and plusmn3 forCAG repeat lengths ge43) were brought to the attention ofthe local site investigator and subsequently addressed

Data managementHDhomozygotes were defined as carriers of 2 alleles with ge36CAG repeats while HD heterozygotes were individuals withthe longer allele ge36 CAG repeats and the shorter allele lt36CAG repeats Demographics CAG repeat length and clinicalinformation including total motor score (TMS) cognitiveand behavior UHDRS scores and TFC information at base-line and after 4 years of follow-up were collected from theEHDN database With the motor UHDRS different domainsubscores were calculated chorea (sum of the chorea items[face buccolingual upper and lower extremities scores])dystonia (sum of the trunk upper and lower extremitiesscores) bradykinesia (sum of the finger taps pronatesupinate rigidity of each extremity and body bradykinesiascores) gait impairment (sum of gait tandem and retro-pulsion scores) and oculomotor performance (sum of ocularpursuit saccade initiation and saccade velocity) To deal withmissing values case-wise deletions were adopted

Statistical analysisAnalysis was done using IBM-SPSS 21 software (SPSS IncChicago IL) following the Reporting of ObservationalStudies in Epidemiology guidelines14 Normal distribution ofvariables was analyzed using the Kolmogorov-Smirnov testDescriptive analysis of the participantsrsquo characteristics wasperformed in terms of frequencies (percentage) meanmedian values with the corresponding SD or interquartilerange as appropriate and 95 confidence intervals (CI)

Clinical characteristics were analyzed in a cross-sectional andlongitudinal manner Baseline differences between HD

homozygotes and the total sample of HD heterozygous werefirst evaluated To balance differences in sample sizes a posthoc secondary analysis was carried out comparing homo-zygotes with a subset of heterozygotes paired by age andCAG larger allele (13) In addition because aging mightworsen the UHDRS scores especially bradykinesia and gaitscores independently of the genetic status15 a comparativeanalysis between homozygotes and heterozygotes was con-ducted including young and older participants All homozy-gote and heterozygote participants were classified as older(gt51 years old) or younger participants (le51 years old)based on the median age of homozygotes at registry entryDifferences were analyzed using the χ2 Phi and Cramer tests(categorical variables) and Mann-Whitney U tests (non-parametric for quantitative variables) A significance level of α= 0004 2-sided tests was applied after post hoc Bonferronimultiple comparison adjustments

To analyze the association between age length of the largerand shorter alleles BMI and UHDRS outcome measurescorrelations were calculated using Spearman (nonparametric)correlation coefficients To analyze the relationship betweenage at onset and the length of the larger and shorter alleles inthe homozygote and heterozygote groups a multivariate lin-ear regression analysis was conducted

For follow-up data linear mixed-effects models16 were con-structed to investigate the course of different outcomes overa 4-year period To account for the correlation between re-peated measurements on the same participant a random in-tercept and random time effect (slope) per participant wasused For all analyses an unstructured covariance for therandom intercepts and random slopes was used Differencesin the rate of progression (ie slope) between homozygotesand heterozygotes were compared

Data availability statementWe used the same methodology as that described in a pre-vious study15 The European HD Registry is a large pro-spective study observing the natural course clinicalspectrum and management of HD in 140 centers from 17European countries and 3 other countries912 More in-formation on the Registry can be found at euro-hdnethtmlregistry This study is registered with ClinicalTrialsgov numberNCT01590589

ResultsAs of December 2016 of 10921 participants with HD (5777female [529] and 5138 male [470]) with a mean age of551 plusmn 141 years 28 homozygotes (03) and 10893(997) heterozygotes were identified The median CAGrepeat lengths of the longer and shorter alleles were 45 (4247) and 38 (3740) respectively for the homozygote groupand 43 (4445) and 18 (1720) respectively for the hetero-zygote group (table 1)

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2103

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

At baseline clinical characteristics of homozygotes and het-erozygotes were not significantly different although homo-zygotes showed a trend for lower age at HD onset and lowerBMI (table 1 and figure 1) Similar results were obtained inthe post hoc comparative analysis between homozygotes andthe age-larger allele-paired heterozygotes with a trend forgreater ocular disturbances in homozygotes (table 1)

After stratification of participants by age the median CAGrepeats of the longer allele in the younger group was 47 (4555)among homozygotes (n = 14) and 45 (4349) among hetero-zygotes (n = 4138) while in the older group it was 43 (4045)and 42 (4144) for homozygotes (n = 14) and heterozygotes (n= 6757) respectively No significant differences in BMI TMSCognitive Behavior-UHDRS andTFC scores were observed inthe young or old group except for a trend for lower age at HDonset in young homozygotes compared to young heterozygotes(270 [167365] vs 340 [280390] p = 004) and greater gaitUHDRS scores in old homozygotes compared to old hetero-zygotes (50 [4092] vs 40 [2270] p = 001)

The length of the longer allele was correlated with age at HDonset in both homozygotes (rs = minus086 p lt 00001) and

heterozygotes (rs = minus076 p lt 00001) while correlation withBMI was only seen in homozygotes (rs = minus050 p = 001) Thelength of the shorter allele did not have a significant effect oncognitive behavior ormotor (including bradykinesia chorea gaitand dystonia UHDRS) scores in either group Likewise thenumber of years of education did not show a significant correla-tion with age at HD onset among heterozygotes or homozygotes

In the multivariate linear regression analysis using the smallerand longer alleles as the independent variables the length ofthe longer allele contributed most to earlier age at HD onsetin the homozygote and heterozygote groups (table 2)According to these models in heterozygotes with 1-unitCAG repeat increase in the longer allele age at HD onsetdecreased 190 years 95 CI minus194 minus186 (p lt 00001)Likewise in homozygotes for 1-unit CAG repeat increase inthe longer allele age at HD onset decreased 172 years 95CI minus236 minus107 (p lt 00001) These models explained 448and 617 of the variability of age at HD onset in the het-erozygote and homozygote groups respectively

TMS cognitive behavior UHDRS BMI and TFC follow-updata are shown in boxplots (figure 2) Overall no significant

Table 1 Baseline characteristics of homozygotes and heterozygotes with Huntington disease (HD)

Homozygotes n = 28 Heterozygotes n = 10893 p Valuea Heterozygotesp n = 65 p Valueb

Age y 515 (367ndash717) 28 550 (450ndash650) 10886 031 520 (395ndash715) 65 076

Female 19 (679) 28 5758 (529) 10893 012 38 (585) 65 048

Shorter allele CAG repeats 38 (37ndash40) 28 18 (17ndash20) 10893 lt00001 18 (17ndash21) 65 lt00001

Longer allele CAG repeats 45 (42ndash47) 28 43 (41ndash46) 10893 003 45 (41ndash47) 65 037

Age at onset y 395 (237ndash530) 20 460 (370ndash560) 8442 004 445 (347ndash580) 46 013

HD duration y 125 (60ndash160) 20 100 (60ndash150) 8478 035 120 (90ndash162) 46 098

Education y 100 (90ndash130) 27 110 (90ndash140) 10391 012 120 (100ndash140) 65 001

BMI 221 (210ndash236) 25 235 (212ndash265) 10817 004 232 (209ndash267) 59 013

TFC 95 (45ndash127) 28 100 (60ndash130) 10717 050 110 (70ndash130) 65 015

TMS-UHDRS 350 (82ndash577) 28 270 (90ndash460) 10445 046 215 (40ndash450) 64 020

Chorea-UHDRS 65 (02ndash107) 28 50 (10ndash100) 10576 055 40 (00ndash90) 65 033

Bradykinesia-UHDRS 80 (32ndash167) 28 70 (20ndash120) 10535 044 60 (10ndash110) 65 028

Dystonia-UHDRS 10 (00ndash57) 28 00 (00ndash30) 10562 035 10 (00ndash20) 65 012

Gait-UHDRS 40 (00ndash67) 28 30 (00ndash50) 10542 023 15 (00ndash47) 65 015

Ocular-UHDRS 60 (15ndash165) 28 60 (12ndash110) 10569 031 40 (00ndash100) 65 006

Cognitive-UHDRS 2260 (1420ndash2690) 9 1740 (1220ndash2410) 4102 034 2100 (1270ndash3020) 35 071

Behavior-UHDRS 130 (40ndash270) 21 120 (50ndash210) 6753 081 80 (30ndash150) 47 051

Abbreviations BMI = body mass index Heterozygotesp = age-larger allele-paired heterozygotes TFC = total functional capacity TMS = total motor scoreUHDRS = Unified Huntingtonrsquos Disease Rating ScaleA significance level of α = 0004 2-sided tests was applied after post hoc Bonferroni multiple comparisons adjustments Values are expressed as median(interquartile range) n or n () Na p Value relates to the homozygotes vs heterozygotes comparisonb p Value relates to homozygotes vs age-larger allele-paired heterozygotes Baseline comparison values were compared using the Mann-Whitney U test

e2104 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

differences were observed in homozygotes compared to het-erozygotes in terms of outcome variables over the course of 4years (table 3)

DiscussionIn this longitudinal analysis of the EHDN registry after cor-recting for multiple comparisons homozygote HD carriershad a similar age at onset phenotype and disease progressioncompared to heterozygotes These findings suggest thatoverall the shorter expanded allele does not have a significantand major influence in determining either the age at onset orthe phenotypic expression and progression of HD

In HD animal models and some CAG triplet diseases inhumans including spinal and bulbar muscular atrophy anddentatorubral-pallidoluysian atrophy homozygotes have con-sistently shown more aggressive neurodegeneration comparedto heterozygotes1718 In other polyQ diseases however theeffect of a second expanded allele remains unclear Singlereports and small series of homozygotes for spinocerebellarataxia type 6 and 3 suggest a gene dose effect on age at onsetand increased severity of the phenotype1920 For HD

assessment of double dose gene effect in the phenotype hasbeen extremely challenging given the rare occurrence ofhomozygotes While some publications in HD have reportedan indistinguishable phenotype in homozygotes andheterozygotes478 other studies have observed a nonchorei-form phenotype presentation more frequently in homo-zygotes with a significant increase in severity of progression ofmotor and psychiatric manifestations indicating that homo-zygotes may present with a wider spectrum of neurologicsymptoms other than chorea compared to heterozygotes221

Supporting findings included marked cerebellum atrophy inneuroimaging in 3 homozygotes and widespread brain atro-phy at autopsy of one homozygote patient with HD2 Ourstudy however cannot confirm this difference in phenotypeas clinical characteristics of homozygotes were similar toheterozygotes Even more we did not observe a differentclinical profile between younger and older homozygotes ex-cept for a trend for greater gait impairment in the old groupHomozygotes (especially young homozygotes) had lower ageat HD onset than heterozygotes but they also had highermedian CAG repeats of the longer allele which may partiallyaccount for an earlier initiation of symptoms Of notehomozygotes also showed a trend for lower BMI compared to

Table 2 Multivariate lineal regression analysis of clinical variables associated with age at Huntington disease onset

Homozygotes corrected R2 = 657 n = 20 n (95 confidenceinterval) p value

Heterozygotes corrected R2 = 487 n = 8442 Β (95 confidenceinterval) p value

Shorterallele

minus144 (minus365 to 076) 018 001 (minus004 to 007) 015

Longerallele

minus172 (minus236 to minus107) lt00001 minus190 (minus194 to minus186) lt00001

Figure 1 The relationship between expanded alleles CAG repeat length and age at onset of Huntington disease

The minimal adequate model for this heterozygote and homozygote dataset is shown as black (homozygotes) and red (heterozygotes) lines

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2105

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

heterozygotes Explaining hypothesis includes that homo-zygotes might be at risk for a greater hypermetabolic state thatmay precede the occurrence of motor symptoms and con-tribute to weight loss2223

Heterozygote individuals with clinically diagnosed HD ex-perience a steady progressive decline in the cardinal featuresof the disease2425 In our study heterozygotes had a similarrate of decline compared to homozygotes in terms of BMITFC TMS behavior and cognitive UHDRS scores In con-trast Squitieri et al2 observed a faster rate of progression indisability measured by independence and the physical dis-ability scales in 8 homozygotes compared to 75 heterozygotesGiven the differentmethodology used in that study2 our resultsare not comparable Available clinimetric data show that in theoverall HD population items measured by the TMS-UHDRSthe only recommended rating scale to measure the severity ofmotor signs in HD that cover all motor domains in HD26 havevariable weights at different HD stages It seems that bradyki-nesia and dystonia are predominant in patients with greaterCAG repeats and younger onset of HD compared to lowerCAG repeats in the larger allele Instead chorea is predominantin earlier stages of manifest adult HD tends to plateau anddecreases later and parkinsonian features become progressivelymore severe and are more clinically significant in later stages ofthe disease27 Interestingly we found a similar motor pheno-type progression in homozygotes compared to heterozygotesin terms of choreiform and nonchoreiform manifestations overtime except for a trend for greater gait impairment in olderhomozygotes compared with heterozygotes in post hoc anal-ysis However the scarce and relatively short longitudinalclinical information on homozygotes limits the clinical rele-vance of these observations

Figure 2 Follow-up data

Follow-updata formotorUnifiedHuntingtonrsquosDisease Rating Scale (UHDRS)(A) cognitive UHDRS (B) behavior UHDRS (C) body mass index (D) and totalfunctional capacity (E) Number of participants participating in each visit isshown Homozygotes are represented in gray boxes and heterozygotes in redboxes Thedark line represents themedianof the outcome variable the bottomof the box indicates the 25th percentile and the top the 75th percentile

Table 3 Slope differences for the homozygotes vsheterozygotes

Slope difference (SE) p Value

TFC minus09 (06) 040

TMS-UHDRS 41 (42) 036

Chorea-UHDRS 02 (08) 078

Bradykinesia-UHDRS 10 (11) 037

Dystonia-UHDRS 05 (03) 044

Ocular-UHDRS 13 (10) 019

Cognitive-UHDRS 170 (264) 053

Behavior-UHDRS 15 (21) 047

BMI minus15 (12) 018

Abbreviations BMI = bodymass index TFC = total functional capacity TMS =total motor score UHDRS = Unified Huntingtonrsquos Disease Rating ScaleShown are parameter estimates from the linear mixed models The differ-ence between homozygotes and heterozygotes is expressed as a slope pValues relates to the difference in the homozygotes compared toheterozygotes

e2106 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2107

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

Permissions amp Licensing

httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology

Page 4: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

At baseline clinical characteristics of homozygotes and het-erozygotes were not significantly different although homo-zygotes showed a trend for lower age at HD onset and lowerBMI (table 1 and figure 1) Similar results were obtained inthe post hoc comparative analysis between homozygotes andthe age-larger allele-paired heterozygotes with a trend forgreater ocular disturbances in homozygotes (table 1)

After stratification of participants by age the median CAGrepeats of the longer allele in the younger group was 47 (4555)among homozygotes (n = 14) and 45 (4349) among hetero-zygotes (n = 4138) while in the older group it was 43 (4045)and 42 (4144) for homozygotes (n = 14) and heterozygotes (n= 6757) respectively No significant differences in BMI TMSCognitive Behavior-UHDRS andTFC scores were observed inthe young or old group except for a trend for lower age at HDonset in young homozygotes compared to young heterozygotes(270 [167365] vs 340 [280390] p = 004) and greater gaitUHDRS scores in old homozygotes compared to old hetero-zygotes (50 [4092] vs 40 [2270] p = 001)

The length of the longer allele was correlated with age at HDonset in both homozygotes (rs = minus086 p lt 00001) and

heterozygotes (rs = minus076 p lt 00001) while correlation withBMI was only seen in homozygotes (rs = minus050 p = 001) Thelength of the shorter allele did not have a significant effect oncognitive behavior ormotor (including bradykinesia chorea gaitand dystonia UHDRS) scores in either group Likewise thenumber of years of education did not show a significant correla-tion with age at HD onset among heterozygotes or homozygotes

In the multivariate linear regression analysis using the smallerand longer alleles as the independent variables the length ofthe longer allele contributed most to earlier age at HD onsetin the homozygote and heterozygote groups (table 2)According to these models in heterozygotes with 1-unitCAG repeat increase in the longer allele age at HD onsetdecreased 190 years 95 CI minus194 minus186 (p lt 00001)Likewise in homozygotes for 1-unit CAG repeat increase inthe longer allele age at HD onset decreased 172 years 95CI minus236 minus107 (p lt 00001) These models explained 448and 617 of the variability of age at HD onset in the het-erozygote and homozygote groups respectively

TMS cognitive behavior UHDRS BMI and TFC follow-updata are shown in boxplots (figure 2) Overall no significant

Table 1 Baseline characteristics of homozygotes and heterozygotes with Huntington disease (HD)

Homozygotes n = 28 Heterozygotes n = 10893 p Valuea Heterozygotesp n = 65 p Valueb

Age y 515 (367ndash717) 28 550 (450ndash650) 10886 031 520 (395ndash715) 65 076

Female 19 (679) 28 5758 (529) 10893 012 38 (585) 65 048

Shorter allele CAG repeats 38 (37ndash40) 28 18 (17ndash20) 10893 lt00001 18 (17ndash21) 65 lt00001

Longer allele CAG repeats 45 (42ndash47) 28 43 (41ndash46) 10893 003 45 (41ndash47) 65 037

Age at onset y 395 (237ndash530) 20 460 (370ndash560) 8442 004 445 (347ndash580) 46 013

HD duration y 125 (60ndash160) 20 100 (60ndash150) 8478 035 120 (90ndash162) 46 098

Education y 100 (90ndash130) 27 110 (90ndash140) 10391 012 120 (100ndash140) 65 001

BMI 221 (210ndash236) 25 235 (212ndash265) 10817 004 232 (209ndash267) 59 013

TFC 95 (45ndash127) 28 100 (60ndash130) 10717 050 110 (70ndash130) 65 015

TMS-UHDRS 350 (82ndash577) 28 270 (90ndash460) 10445 046 215 (40ndash450) 64 020

Chorea-UHDRS 65 (02ndash107) 28 50 (10ndash100) 10576 055 40 (00ndash90) 65 033

Bradykinesia-UHDRS 80 (32ndash167) 28 70 (20ndash120) 10535 044 60 (10ndash110) 65 028

Dystonia-UHDRS 10 (00ndash57) 28 00 (00ndash30) 10562 035 10 (00ndash20) 65 012

Gait-UHDRS 40 (00ndash67) 28 30 (00ndash50) 10542 023 15 (00ndash47) 65 015

Ocular-UHDRS 60 (15ndash165) 28 60 (12ndash110) 10569 031 40 (00ndash100) 65 006

Cognitive-UHDRS 2260 (1420ndash2690) 9 1740 (1220ndash2410) 4102 034 2100 (1270ndash3020) 35 071

Behavior-UHDRS 130 (40ndash270) 21 120 (50ndash210) 6753 081 80 (30ndash150) 47 051

Abbreviations BMI = body mass index Heterozygotesp = age-larger allele-paired heterozygotes TFC = total functional capacity TMS = total motor scoreUHDRS = Unified Huntingtonrsquos Disease Rating ScaleA significance level of α = 0004 2-sided tests was applied after post hoc Bonferroni multiple comparisons adjustments Values are expressed as median(interquartile range) n or n () Na p Value relates to the homozygotes vs heterozygotes comparisonb p Value relates to homozygotes vs age-larger allele-paired heterozygotes Baseline comparison values were compared using the Mann-Whitney U test

e2104 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

differences were observed in homozygotes compared to het-erozygotes in terms of outcome variables over the course of 4years (table 3)

DiscussionIn this longitudinal analysis of the EHDN registry after cor-recting for multiple comparisons homozygote HD carriershad a similar age at onset phenotype and disease progressioncompared to heterozygotes These findings suggest thatoverall the shorter expanded allele does not have a significantand major influence in determining either the age at onset orthe phenotypic expression and progression of HD

In HD animal models and some CAG triplet diseases inhumans including spinal and bulbar muscular atrophy anddentatorubral-pallidoluysian atrophy homozygotes have con-sistently shown more aggressive neurodegeneration comparedto heterozygotes1718 In other polyQ diseases however theeffect of a second expanded allele remains unclear Singlereports and small series of homozygotes for spinocerebellarataxia type 6 and 3 suggest a gene dose effect on age at onsetand increased severity of the phenotype1920 For HD

assessment of double dose gene effect in the phenotype hasbeen extremely challenging given the rare occurrence ofhomozygotes While some publications in HD have reportedan indistinguishable phenotype in homozygotes andheterozygotes478 other studies have observed a nonchorei-form phenotype presentation more frequently in homo-zygotes with a significant increase in severity of progression ofmotor and psychiatric manifestations indicating that homo-zygotes may present with a wider spectrum of neurologicsymptoms other than chorea compared to heterozygotes221

Supporting findings included marked cerebellum atrophy inneuroimaging in 3 homozygotes and widespread brain atro-phy at autopsy of one homozygote patient with HD2 Ourstudy however cannot confirm this difference in phenotypeas clinical characteristics of homozygotes were similar toheterozygotes Even more we did not observe a differentclinical profile between younger and older homozygotes ex-cept for a trend for greater gait impairment in the old groupHomozygotes (especially young homozygotes) had lower ageat HD onset than heterozygotes but they also had highermedian CAG repeats of the longer allele which may partiallyaccount for an earlier initiation of symptoms Of notehomozygotes also showed a trend for lower BMI compared to

Table 2 Multivariate lineal regression analysis of clinical variables associated with age at Huntington disease onset

Homozygotes corrected R2 = 657 n = 20 n (95 confidenceinterval) p value

Heterozygotes corrected R2 = 487 n = 8442 Β (95 confidenceinterval) p value

Shorterallele

minus144 (minus365 to 076) 018 001 (minus004 to 007) 015

Longerallele

minus172 (minus236 to minus107) lt00001 minus190 (minus194 to minus186) lt00001

Figure 1 The relationship between expanded alleles CAG repeat length and age at onset of Huntington disease

The minimal adequate model for this heterozygote and homozygote dataset is shown as black (homozygotes) and red (heterozygotes) lines

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2105

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

heterozygotes Explaining hypothesis includes that homo-zygotes might be at risk for a greater hypermetabolic state thatmay precede the occurrence of motor symptoms and con-tribute to weight loss2223

Heterozygote individuals with clinically diagnosed HD ex-perience a steady progressive decline in the cardinal featuresof the disease2425 In our study heterozygotes had a similarrate of decline compared to homozygotes in terms of BMITFC TMS behavior and cognitive UHDRS scores In con-trast Squitieri et al2 observed a faster rate of progression indisability measured by independence and the physical dis-ability scales in 8 homozygotes compared to 75 heterozygotesGiven the differentmethodology used in that study2 our resultsare not comparable Available clinimetric data show that in theoverall HD population items measured by the TMS-UHDRSthe only recommended rating scale to measure the severity ofmotor signs in HD that cover all motor domains in HD26 havevariable weights at different HD stages It seems that bradyki-nesia and dystonia are predominant in patients with greaterCAG repeats and younger onset of HD compared to lowerCAG repeats in the larger allele Instead chorea is predominantin earlier stages of manifest adult HD tends to plateau anddecreases later and parkinsonian features become progressivelymore severe and are more clinically significant in later stages ofthe disease27 Interestingly we found a similar motor pheno-type progression in homozygotes compared to heterozygotesin terms of choreiform and nonchoreiform manifestations overtime except for a trend for greater gait impairment in olderhomozygotes compared with heterozygotes in post hoc anal-ysis However the scarce and relatively short longitudinalclinical information on homozygotes limits the clinical rele-vance of these observations

Figure 2 Follow-up data

Follow-updata formotorUnifiedHuntingtonrsquosDisease Rating Scale (UHDRS)(A) cognitive UHDRS (B) behavior UHDRS (C) body mass index (D) and totalfunctional capacity (E) Number of participants participating in each visit isshown Homozygotes are represented in gray boxes and heterozygotes in redboxes Thedark line represents themedianof the outcome variable the bottomof the box indicates the 25th percentile and the top the 75th percentile

Table 3 Slope differences for the homozygotes vsheterozygotes

Slope difference (SE) p Value

TFC minus09 (06) 040

TMS-UHDRS 41 (42) 036

Chorea-UHDRS 02 (08) 078

Bradykinesia-UHDRS 10 (11) 037

Dystonia-UHDRS 05 (03) 044

Ocular-UHDRS 13 (10) 019

Cognitive-UHDRS 170 (264) 053

Behavior-UHDRS 15 (21) 047

BMI minus15 (12) 018

Abbreviations BMI = bodymass index TFC = total functional capacity TMS =total motor score UHDRS = Unified Huntingtonrsquos Disease Rating ScaleShown are parameter estimates from the linear mixed models The differ-ence between homozygotes and heterozygotes is expressed as a slope pValues relates to the difference in the homozygotes compared toheterozygotes

e2106 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2107

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

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Page 5: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

differences were observed in homozygotes compared to het-erozygotes in terms of outcome variables over the course of 4years (table 3)

DiscussionIn this longitudinal analysis of the EHDN registry after cor-recting for multiple comparisons homozygote HD carriershad a similar age at onset phenotype and disease progressioncompared to heterozygotes These findings suggest thatoverall the shorter expanded allele does not have a significantand major influence in determining either the age at onset orthe phenotypic expression and progression of HD

In HD animal models and some CAG triplet diseases inhumans including spinal and bulbar muscular atrophy anddentatorubral-pallidoluysian atrophy homozygotes have con-sistently shown more aggressive neurodegeneration comparedto heterozygotes1718 In other polyQ diseases however theeffect of a second expanded allele remains unclear Singlereports and small series of homozygotes for spinocerebellarataxia type 6 and 3 suggest a gene dose effect on age at onsetand increased severity of the phenotype1920 For HD

assessment of double dose gene effect in the phenotype hasbeen extremely challenging given the rare occurrence ofhomozygotes While some publications in HD have reportedan indistinguishable phenotype in homozygotes andheterozygotes478 other studies have observed a nonchorei-form phenotype presentation more frequently in homo-zygotes with a significant increase in severity of progression ofmotor and psychiatric manifestations indicating that homo-zygotes may present with a wider spectrum of neurologicsymptoms other than chorea compared to heterozygotes221

Supporting findings included marked cerebellum atrophy inneuroimaging in 3 homozygotes and widespread brain atro-phy at autopsy of one homozygote patient with HD2 Ourstudy however cannot confirm this difference in phenotypeas clinical characteristics of homozygotes were similar toheterozygotes Even more we did not observe a differentclinical profile between younger and older homozygotes ex-cept for a trend for greater gait impairment in the old groupHomozygotes (especially young homozygotes) had lower ageat HD onset than heterozygotes but they also had highermedian CAG repeats of the longer allele which may partiallyaccount for an earlier initiation of symptoms Of notehomozygotes also showed a trend for lower BMI compared to

Table 2 Multivariate lineal regression analysis of clinical variables associated with age at Huntington disease onset

Homozygotes corrected R2 = 657 n = 20 n (95 confidenceinterval) p value

Heterozygotes corrected R2 = 487 n = 8442 Β (95 confidenceinterval) p value

Shorterallele

minus144 (minus365 to 076) 018 001 (minus004 to 007) 015

Longerallele

minus172 (minus236 to minus107) lt00001 minus190 (minus194 to minus186) lt00001

Figure 1 The relationship between expanded alleles CAG repeat length and age at onset of Huntington disease

The minimal adequate model for this heterozygote and homozygote dataset is shown as black (homozygotes) and red (heterozygotes) lines

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2105

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

heterozygotes Explaining hypothesis includes that homo-zygotes might be at risk for a greater hypermetabolic state thatmay precede the occurrence of motor symptoms and con-tribute to weight loss2223

Heterozygote individuals with clinically diagnosed HD ex-perience a steady progressive decline in the cardinal featuresof the disease2425 In our study heterozygotes had a similarrate of decline compared to homozygotes in terms of BMITFC TMS behavior and cognitive UHDRS scores In con-trast Squitieri et al2 observed a faster rate of progression indisability measured by independence and the physical dis-ability scales in 8 homozygotes compared to 75 heterozygotesGiven the differentmethodology used in that study2 our resultsare not comparable Available clinimetric data show that in theoverall HD population items measured by the TMS-UHDRSthe only recommended rating scale to measure the severity ofmotor signs in HD that cover all motor domains in HD26 havevariable weights at different HD stages It seems that bradyki-nesia and dystonia are predominant in patients with greaterCAG repeats and younger onset of HD compared to lowerCAG repeats in the larger allele Instead chorea is predominantin earlier stages of manifest adult HD tends to plateau anddecreases later and parkinsonian features become progressivelymore severe and are more clinically significant in later stages ofthe disease27 Interestingly we found a similar motor pheno-type progression in homozygotes compared to heterozygotesin terms of choreiform and nonchoreiform manifestations overtime except for a trend for greater gait impairment in olderhomozygotes compared with heterozygotes in post hoc anal-ysis However the scarce and relatively short longitudinalclinical information on homozygotes limits the clinical rele-vance of these observations

Figure 2 Follow-up data

Follow-updata formotorUnifiedHuntingtonrsquosDisease Rating Scale (UHDRS)(A) cognitive UHDRS (B) behavior UHDRS (C) body mass index (D) and totalfunctional capacity (E) Number of participants participating in each visit isshown Homozygotes are represented in gray boxes and heterozygotes in redboxes Thedark line represents themedianof the outcome variable the bottomof the box indicates the 25th percentile and the top the 75th percentile

Table 3 Slope differences for the homozygotes vsheterozygotes

Slope difference (SE) p Value

TFC minus09 (06) 040

TMS-UHDRS 41 (42) 036

Chorea-UHDRS 02 (08) 078

Bradykinesia-UHDRS 10 (11) 037

Dystonia-UHDRS 05 (03) 044

Ocular-UHDRS 13 (10) 019

Cognitive-UHDRS 170 (264) 053

Behavior-UHDRS 15 (21) 047

BMI minus15 (12) 018

Abbreviations BMI = bodymass index TFC = total functional capacity TMS =total motor score UHDRS = Unified Huntingtonrsquos Disease Rating ScaleShown are parameter estimates from the linear mixed models The differ-ence between homozygotes and heterozygotes is expressed as a slope pValues relates to the difference in the homozygotes compared toheterozygotes

e2106 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2107

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

Permissions amp Licensing

httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology

Page 6: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

heterozygotes Explaining hypothesis includes that homo-zygotes might be at risk for a greater hypermetabolic state thatmay precede the occurrence of motor symptoms and con-tribute to weight loss2223

Heterozygote individuals with clinically diagnosed HD ex-perience a steady progressive decline in the cardinal featuresof the disease2425 In our study heterozygotes had a similarrate of decline compared to homozygotes in terms of BMITFC TMS behavior and cognitive UHDRS scores In con-trast Squitieri et al2 observed a faster rate of progression indisability measured by independence and the physical dis-ability scales in 8 homozygotes compared to 75 heterozygotesGiven the differentmethodology used in that study2 our resultsare not comparable Available clinimetric data show that in theoverall HD population items measured by the TMS-UHDRSthe only recommended rating scale to measure the severity ofmotor signs in HD that cover all motor domains in HD26 havevariable weights at different HD stages It seems that bradyki-nesia and dystonia are predominant in patients with greaterCAG repeats and younger onset of HD compared to lowerCAG repeats in the larger allele Instead chorea is predominantin earlier stages of manifest adult HD tends to plateau anddecreases later and parkinsonian features become progressivelymore severe and are more clinically significant in later stages ofthe disease27 Interestingly we found a similar motor pheno-type progression in homozygotes compared to heterozygotesin terms of choreiform and nonchoreiform manifestations overtime except for a trend for greater gait impairment in olderhomozygotes compared with heterozygotes in post hoc anal-ysis However the scarce and relatively short longitudinalclinical information on homozygotes limits the clinical rele-vance of these observations

Figure 2 Follow-up data

Follow-updata formotorUnifiedHuntingtonrsquosDisease Rating Scale (UHDRS)(A) cognitive UHDRS (B) behavior UHDRS (C) body mass index (D) and totalfunctional capacity (E) Number of participants participating in each visit isshown Homozygotes are represented in gray boxes and heterozygotes in redboxes Thedark line represents themedianof the outcome variable the bottomof the box indicates the 25th percentile and the top the 75th percentile

Table 3 Slope differences for the homozygotes vsheterozygotes

Slope difference (SE) p Value

TFC minus09 (06) 040

TMS-UHDRS 41 (42) 036

Chorea-UHDRS 02 (08) 078

Bradykinesia-UHDRS 10 (11) 037

Dystonia-UHDRS 05 (03) 044

Ocular-UHDRS 13 (10) 019

Cognitive-UHDRS 170 (264) 053

Behavior-UHDRS 15 (21) 047

BMI minus15 (12) 018

Abbreviations BMI = bodymass index TFC = total functional capacity TMS =total motor score UHDRS = Unified Huntingtonrsquos Disease Rating ScaleShown are parameter estimates from the linear mixed models The differ-ence between homozygotes and heterozygotes is expressed as a slope pValues relates to the difference in the homozygotes compared toheterozygotes

e2106 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2107

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

Permissions amp Licensing

httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology

Page 7: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

In previous studies on HD homozygotes the most frequentvariable of disease severity was the difference in age at HDonset In agreement with other studies the length of the longerallele was the most contributing and consistent factor associ-ated with age at HD onset in both homozygotes andheterozygotes2457828 These results suggest that CAG repeatexpansion in HD determines age at onset in a fully dominantfashion2829 Age atHDonset in our homozygote participants wassimilar to that of the Lee et al28 study but lower than reported bySquitieri et al2 It should be mentioned however that this dif-ference might be due to the greater expansion in the longer allelein our homozygote sample compared to the Squitieri et al2

study In any case the small number of homozygotes in bothstudies228 does not allow us to draw definitive conclusions

The underlying mechanisms of the lack of a significant ex-pression of a double mutant gene dosage in HD are not well-understood On one hand it has been proposed that biallelicmutation in HD might be deleterious in patients due to lackof protective function of the wild-type huntingtin or mito-chondria impairment23031 On the other hand the level ofmutant huntingtin protein produced from a single allele ap-parently exceeds any minimum threshold required to triggerpathogenesis (at a rate determined primarily by its CAG re-peat length) and that hypothetically neither additional mu-tant protein nor the absence of any normal protein furtheralters the rate of pathogenesis leading to motor onset28 Inspite of that it is also important to consider the potentialmodifying effect of genetic factors on HD clinical coursebeside the CAG repeat length of the HTT gene In a recentgenome-wide disease progression association study singlenucleotide polymorphism (SNP) encoding an amino acidchange (Pro67Ala) inMSH332 was shown to have an effect ondisease progression Each copy of the minor allele at this SNPwas associated with a 04 units per year (95 CI 016ndash066)reduction in the rate of change of the TMS-UHDRS anda reduction of 012 units per year (95 CI 006ndash018) in therate of change of TFC in heterozygotes32 In this scenarioidentification and assessment of individual genetic and envi-ronmental factors should contribute to define the possibleeffect of the second expanded allele on the variance of HDphenotype and progression

The main limitations of this study are the small sample ofhomozygotes the limited clinical relevance of statisticallysignificant results obtained in post hoc analysis and the lack ofbiological data including measurement of mutant huntingtinprotein levels and other genetic modifiers neuroimaging andpostmortem pathologic results in the homozygotes support-ing our data Likewise we cannot rule out that homozygoteshave a significant distinct motor or cognitive phenotype thatcannot be adequately captured by the UHDRS or detecteddue to sample selectionattribution bias (loss of follow-up inparticipants with more severe cognitive or motor decline) orlimited statistical power In addition participants with HD inour study are very likely to be on medication a factor thatcould modify the phenotypic expression and outcome

measures Consequently the HD clinical manifestations andprogression in this study may not reflect that of the broaderHD population (homozygotes and heterozygotes) who mayhave less access to such care However despite the abovelimitations this study documents the clinical profile and dis-ease progression using the largest HD homozygote multi-center sample so far described Of note these observationaldata were obtained from a database without any prespecifiedhypotheses at the time of data collection which may precludesample selection bias

The results of this study extend previous reports examiningthe natural history of HD highlighting the importance of anobservational longitudinal disease registry Homozygosity inHD does not seem to modify significantly the age at onsetclinical phenotype or disease progression except for subtleclinical differences Environmental factors and compensatorygenetic factors might counteract the possible effect of themutation in a double dose This speculative hypothesis couldbe an area for future therapeutic investigation

Author contributionsE Cubo study concept design and writing the manuscriptMA Ramos-Arroyo interpretation critical revision of themanuscript S-I Martinez-Horta interpretation critical re-vision of the manuscript A Martinez-Descalls critical re-vision of the manuscript S Calvo F Sampedro Santalo datamanagement and analysis C Gil-Polo D Diaz A GutierrezI Muntildeoz K Llano NMariscal L Aguado interpretation andcritical review of the manuscript

AcknowledgmentThe authors thank the EHDN Registry Study Groupinvestigators for collecting the data all participating Registrypatients for their time and efforts and Margaret Kresse forediting the manuscript

Study fundingEuropean Huntington Disease Registry (data mining pro-ject 852)

DisclosureE Cubo has consulting fees for UCB Allergan and AbbVie SMartinez-Horta F Sampedro A Martinez-Descals S CalvoC Gil I Muntildeoz K Llano N Mariscal D Diaz A GutierrezL Aguado andM Ramos-Arroyo report no disclosures relevantto the manuscript Go to NeurologyorgN for full disclosures

Publication historyReceived by Neurology July 3 2018 Accepted in final form January 42019

References1 Gusella J MacDonald M No post-genetics era in human disease research Nat Rev

Genet 2002372ndash792 Squitieri F Gellera C Cannella M et al Homozygosity for CAG mutation in Hun-

tington disease is associated with a more severe clinical course Brain 2003126946ndash955

NeurologyorgN Neurology | Volume 92 Number 18 | April 30 2019 e2107

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

Permissions amp Licensing

httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology

Page 8: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

3 Southwell AL Smith-Dijak A Kay C et al An enhanced Q175 knock-in mouse modelof Huntington disease with higher mutant huntingtin levels and accelerated diseasephenotypes Hum Mol Genet 2016253654ndash3675

4 Wexler NS Young AB Tanzi RE et al Homozygotes for Huntingtonrsquos diseaseNature 1987326194ndash197

5 Myers RH Leavitt J Farrer LA et al Homozygote for Huntington disease Am J HumGenet 198945615ndash618

6 Laccone F Engel U Holinski-Feder E et al DNA analysis of Huntingtonrsquos diseasefive years of experience in Germany Austria and Switzerland Neurology 199953801ndash806

7 Durr A Hahn-Barma V Brice A Pecheux C Dode C Feingold J Homozygosity inHuntingtonrsquos disease J Med Genet 199936172ndash173

8 Kremer B Goldberg P Andrew SE et al A worldwide study of the Huntingtonrsquosdisease mutation the sensitivity and specificity of measuring CAG repeats N Engl JMed 19943301401ndash1406

9 Orth M European Huntingtonrsquos Disease Network Handley OJ et al ObservingHuntingtonrsquos disease the European Huntingtonrsquos disease Networkrsquos RegistryJ Neurol Neurosurg Psychiatry 2011821409ndash1412

10 ICH Available at ichorg Accessed March 1 201811 Huntington Study Group Unified Huntingtonrsquos Disease Rating Scale reliability and

consistency Mov Disord 199611136ndash14212 Orth M Handley OJ Schwenke C et al Observing Huntingtonrsquos disease the

European Huntingtonrsquos Disease Networkrsquos Registry PLoS Curr 20102RRN1184

13 Shoulson I Huntington disease functional capacities in patients treated with neu-roleptic and antidepressant drugs Neurology 1981311333ndash1335

14 Vandenbroucke JP von Elm E Altman DG et al Strengthening the reporting ofobservational studies in Epidemiology (STROBE) explanation and elaboration Int JSurg 2014121500ndash1524

15 Cubo E Ramos-Arroyo MA Martinez-Horta S et al Clinical manifestations of in-termediate allele carriers in Huntington disease Neurology 201687571ndash578

16 Verbeke G Molenberghs G Linear Mixed Models for Longitudinal Data New YorkSpringer 2000

17 Sato K Kashihara K Okada S et al Does homozygosity advance the onset ofdentatorubral-pallidoluysian atrophy Neurology 1995451934ndash1936

18 Graham RK Slow EJ Deng Y et al Levels of mutant huntingtin influence thephenotypic severity of Huntington disease in YAC128 mouse models Neurobiol Dis200621444ndash455

19 Matsumura R Futamura N Fujimoto Y et al Spinocerebellar ataxia type 6 molecularand clinical features of 35 Japanese patients including one homozygous for the CAGrepeat expansion Neurology 1997491238ndash1243

20 Soga K Ishikawa K Furuya T et al Gene dosage effect in spinocerebellar ataxiatype 6 homozygotes a clinical and neuropathological study J Neurol Sci 2017373321ndash328

21 Squitieri F Berardelli A Nargi E et al Atypical movement disorders in the early stagesof Huntingtonrsquos disease clinical and genetic analysis Clin Genet 20005850ndash56

22 Petersen A Bjorkqvist M Hypothalamic-endocrine aspects in Huntingtonrsquos diseaseEur J Neurosci 200624961ndash967

23 Aziz NA van der Burg JM Landwehrmeyer GB et al Weight loss in Huntingtondisease increases with higher CAG repeat number Neurology 2008711506ndash1513

24 Dorsey ER Beck CA Darwin K et al Natural history of Huntington disease JAMANeurol 2013701520ndash1530

25 Pagan F Torres-Yaghi Y Altshuler M The diagnosis and natural history of Hun-tington disease Handb Clin Neurol 201714463ndash67

26 Mestre TA Busse M Davis AM et al Rating scales and performance-based measuresfor assessment of functional ability in Huntingtonrsquos disease critique and recom-mendations Mov Disord Clin Pract 20185361ndash372

27 Folstein SE Jensen B Leigh RJ Folstein MF The measurement of abnormalmovement methods developed for Huntingtonrsquos disease Neurobehav Toxicol Ter-atol 19835605ndash609

28 Lee JM Ramos EM Lee JH et al CAG repeat expansion in Huntington diseasedetermines age at onset in a fully dominant fashion Neurology 201278690ndash695

29 Uhlmann WR Penaherrera MS Robinson WP Milunsky JM Nicholson JM AlbinRL Biallelic mutations in Huntington disease a new case with just one affectedparent review of the literature and terminology Am J Med Genet A 2015167A1152ndash1160

30 Scherzinger E Sittler A Schweiger K et al Self-assembly of polyglutamine-containinghuntingtin fragments into amyloid-like fibrils implications for Huntingtonrsquos diseasepathology Proc Natl Acad Sci USA 1999964604ndash4609

31 Squitieri F Cannella M Sgarbi G et al Severe ultrastructural mitochondrial changesin lymphoblasts homozygous for Huntington disease mutation Mech Ageing Dev2006127217ndash220

32 Hensman Moss DJ Pardinas AF Langbehn D et al Identification of genetic variantsassociated with Huntingtonrsquos disease progression a genome-wide association studyLancet Neurol 201716701ndash711

e2108 Neurology | Volume 92 Number 18 | April 30 2019 NeurologyorgN

Copyright copy 2019 American Academy of Neurology Unauthorized reproduction of this article is prohibited

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

Permissions amp Licensing

httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology

Page 9: NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations … · NULL HYPOTHESIS CLASS OF EVIDENCE Clinical manifestations of homozygote allele carriers in Huntington disease Esther

DOI 101212WNL0000000000007147201992e2101-e2108 Published Online before print March 13 2019Neurology

Esther Cubo Saul-Indra Martinez-Horta Frederic Sampedro Santalo et al Clinical manifestations of homozygote allele carriers in Huntington disease

This information is current as of March 13 2019

ServicesUpdated Information amp

httpnneurologyorgcontent9218e2101fullincluding high resolution figures can be found at

References httpnneurologyorgcontent9218e2101fullref-list-1

This article cites 30 articles 9 of which you can access for free at

Citations httpnneurologyorgcontent9218e2101fullotherarticles

This article has been cited by 3 HighWire-hosted articles

Subspecialty Collections

httpnneurologyorgcgicollectionhuntingtons_diseaseHuntingtons disease

httpnneurologyorgcgicollectioncohort_studiesCohort studies

httpnneurologyorgcgicollectionchoreaChorea

httpnneurologyorgcgicollectionall_movement_disordersAll Movement Disorders

httpnneurologyorgcgicollectionall_clinical_neurologyAll Clinical Neurology

httpnneurologyorgcgicollectionall_cbmrt_null_hypothesisAll CBMRTNull Hypothesisfollowing collection(s) This article along with others on similar topics appears in the

Permissions amp Licensing

httpwwwneurologyorgaboutabout_the_journalpermissionsits entirety can be found online atInformation about reproducing this article in parts (figurestables) or in

Reprints

httpnneurologyorgsubscribersadvertiseInformation about ordering reprints can be found online

rights reserved Print ISSN 0028-3878 Online ISSN 1526-632X1951 it is now a weekly with 48 issues per year Copyright copy 2019 American Academy of Neurology All

reg is the official journal of the American Academy of Neurology Published continuously sinceNeurology