Down Syndrome: A Complex Disease

Dr Una Fairbrother

Down Syndrome (DS)

Affects one in about 700-1000 newborns

Most significant genetic cause of mild to moderate mental retardation

Due to alterations in neural development.

Trisomy 21 or DS

Only autosomal aneuploidy not lethal in the fetal or early postnatal period

DS phenotypes show variable penetrance, affecting many different organs

Clinical features of neonates with DS

Flat facial profile, 90%

Hypotonia, 80% Poor moro reflex,

85% Hyperflexibility of

joints, 80% Excess skin on

back of neck, 80%

Slanted palpebral fissures, 80%

Pelvic dysplasia, 70% Anomalous ears, 60% Dysplasia of mid joint

of 5th finger, 60% Simian crease, 45%

Down Syndrome – Disease Phenotype Mental retardation, Congenital malformations of the heart and gastrointestinal tract, Duodenal stenosis or atresia, Imperforate anus, Hirschsprung disease, Leukemia, Hearing loss, Alzheimer disease and others

Human Chromosome 21 Many other disorders involving

Chr21 genes 165 hits on OMIM including: Familial amyotrophic lateral

sclerosis (SOD1 gene) Bethlem myopathy (COL6A1, A2

and A3) Acute myeloid leukemia (AML1)

Chromosome 21 Structure

Length of the short arm can vary greatly between individuals

First example of large genomic region that can expand or contract on a scale of many mega bases

Long arm: 225 genes originally identified (545 on chromosome 22)

How Does Trisomy Arise?

95% of DS arises from non-disjunction of homologous chromosomes during 1st meiotic division in oogenesis

Homogeneity means that phenotype severity cannot be predicted from the karyotype

Need to understand the pathology and identify candidate genes

Down Syndrome Research

Functions of most of the Chromosome 21 genes are largely unknown

As is their contribution, to the DS phenotype.

Trisomy and Disease How does an extra copy of all or part of

Chr21 result in the DS phenotype? Specific case of the more general

problem - how does chromosomal imbalance produce abnormalities in morphology and function?

No single mechanism explains the consequences

No simple solution to counteract its phenotypic impact.

Trisomy and Disease: Theories

Amplified developmental instability hypothesis:

DS phenotype due to non-specific disturbance of chr balance

Or… Gene dosage hypothesis:

DS due to cumulative effect of overexpression of specific chr 21 gene products

Corroborated experimentally in mouse models

Some genes undergo dosage compensation

Stage and tissue specific

Or Interaction effects

Through interaction of Chr21 genes products with:

each other the whole genome transcriptome Proteome Possibly through trans acting effects

 A proposed unifying model for the genetic mechanisms acting in autosomal trisomyFitzpatrick (2005) TIGS 21:249-253

Mouse Models useful to elucidate contribution of

specific genes to the DS phenotype. Strategies: assess single-gene by overexpressing

single or combinations of genes assess effects of overexpressing large

foreign DNA pieces, (YACs or BACs) in transgenic mice

mouse trisomies that carry all (usually lethal) or part of MuChr16, which has regions of conserved homology with HuChr21.

Down Syndrome Critical Regions (DSCRs)

Loci in the DS critical regions (DSCRs) may have major effect

Approach limited by high phenotypic variation among DS individuals

To date little evidence of association between any particular phenotypic trait and overexpression of a specific Chr21 gene.

Postgenomics Even knowing the molecular defect, it is

difficult to decipher the complex pathophysiology of the disease,

Developmental consequences of the trisomy

Impact on behaviour and cognitive function.

Facing a new era - postgenomics where the goal is to

identify protein function and physiological role of gene products.

Congenital Heart Defects and DS

40% of trisomy 21 individuals have CHDs

incidence in the euploid population, 0.8%

Frequency of types of CHD in Down Syndrome

Congenital Heart Defect (CHD) Frequency Atrioventricular septal defect (AVSD) Defect in fusion of endocardial cushions

43%

Ventricular septal defect (VSD) Failure of interventricular septum to close

32% (accounts for 35% of normal individuals)

Atrial spetal defect Incomplete fusion of endocardial cushions

10%

Tetralogy of Fallot, 4 defects including VSD

6%

Patient ductus arteriosus, closure usually occurs shortly after birth.

4%

Other 5%

CHD Schematic

Heart

VSD

Atrium

Ventricle

septa

ASD

AVSD

Endocardial cushion

Trisomy 21 and CHD If CHD phenotype is caused by

overexpression of a gene/s on chromosome 21 then

why do not all individuals with DS have a CHD?

May be related to specific alleles and their protein products, which function well in the diploid but not in the trisomic state.

CHD Critical Regions on Chr21

Fig.1: “Critical regions” of

chromosome 21

Represen-tative markers tested in our families to date: S=D21S

Genes in order cen gene tel (telomeric to marker at left)

Key genes and markers in region (>2000 total known)

"DOWN SYNDROME CRITICAL REGION"

TRISOMY IN Ts65Dn MOUSE (PARTIAL TRISOMY 16)

"CHD CRITICAL REGION" (Korenberg et al.)

REGION OF HETERO- TRISOMY IN TRISOMIC CHILDREN WITH VSD

Chromosome 21 cytogenetic band limits (not to scale)

Non-llinear scale: each Bar is approx 5Mb

kbp approx from pter

21p centromere S1951 12743 S215 21q11.1 S258 STCH,NRIP1 12825 S16,S120 S13,s46 21q11.2 13536 S1/S11 21q21.1 13657 S1905 17990 S8 NCAM2 19648 S111 GABPA 24101

APP 24246 S82 GAPDP, BACH1 21q21.2 27663

GRIK1 S300 21q21.3 28014 S226 SOD1,CTBP2, 30024 IFNAR2 D21S58 31594 IFNAR GART, SON3 31689 CRYZL1 31954 S235 ATP5O,DSCR1 32881 S65 RUNX1 33081 S17 34018 S167 KCNE1, TPRD 35153

DCRA,DYRK1A 35783 DCRB S259 36418 KIR4.2 S338 36660

S55 ERG 21q22.13 36745 ETS2 37169

S3 S343 37464 C21LRP(DSCR2)

37539

HMG14 37671 WRB 37741 SH3BGR S168 37813 PCP4 38228 DSCAM S23,S349 21q22.2 39207

S53 S64 21q22.3 39498 MX2 S1260,S356 39722 MX1 S19,S359 40810

S42 S49,S360,S1869

41261

HOXGT PKNOX1, S361 41386 CBS, CRYAA 41436 Cystatin B, HES1

S1979 42478

PFKL S400 42644 C21orf2 S1870, S154 42673

42917 S171 UBE2G2 43113

SMT3A, 43150 CD18 D21S112 43230 COL18A1 43800

COL6A1 44342 COL6A2 44467 S1575 44920

How Might haplotypic diversity Cause CHD in DS?

Variants interact indirectly/ directly with other loci modulating phenotypes

Functionality of gene drops below critical levels, endocardial cushions fail to fuse

Hypothesis: combinations of mutations embedded in particular haplotypes, in trisomic individuals, disturb the supramolecular structure of a vital protein and modulate the predisposition of an individual to a single or several types of CHD.

Do all CHD arise from the same mechanism?

4 CHDs in individuals+ DS, related but do not occur at same time in gestation

Counter-intuitive that a single, simple mechanism is responsible for all CHDs

Variants may interact indirectly/directly with other loci to modulate varying phenotypes

(compound?) heterozygosity for rare, functionally deficient alleles, may cause analogous CHD in non-syndromic, euploid individuals.

Heterotrisomy, a significant contributing factor to ventricular

septal defect associated with Down syndrome?

Baptista MJ, Fairbrother UL, Howard CM, Farrer MJ, Davies GE, Trikka D, Maratou K, Redington A, Greve G, Njolstad PR, Kessling AM. Hum Genet. 2000 107:476-82.

Heterotrisomy for a gene or genes in a narrowed region region is a contributing factor to the pathogenesis of VSD in trisomy 21

Either through the presence of three different specific alleles or through the presence of specific combinations of alleles.

Down syndrome congenital heart disease: a narrowed region and a

candidate gene. Barlow GM, Chen XN, Shi ZY, Lyons GE, Kurnit DM, Celle

L, Spinner NB, Zackai E, Pettenati MJ, Van Riper AJ, Vekemans

MJ, Mjaatvedt CH, Korenberg JR.Genet Med. 2001 3:91-101. These data suggest that the presence of three

copies of gene(s) from the region is sufficient for the production of subsets of DS-CHD.

Propose DSCAM as a candidate for VSD. Encodes a cell adhesion molecule, spans more

than 840 kb of the candidate region, and is expressed in the heart during cardiac development.

DSCR1 gene expression is dependent on NFATc1 during cardiac valve formation and

colocalizes with anomalous organ development in trisomy 16 mice.

Lange AW, Molkentin JD, Yutzey KE Dev Biol. 2004

15;266(2):346-60. Nuclear factor of activated T cells or DSCR1 regulatory protein in calcineurin/NFAT signal

transduction pathway. expressed during valvuloseptal development-

in the endocardium of the developing atrioventricular and semilunar valves, the muscular interventricular septum, and the ventricular myocardium.

Candidate for abnormal development of heart in DS

Cell type-specific over-expression of chromosome 21 genes in fibroblasts and fetal hearts with trisomy 21. Li CM, Guo M, Salas M, Schupf N, Silverman W, Zigman WB, Husain

S, Warburton D, Thaker H, Tycko B BMC Med Genet. 2006 15;7:24. Expression microarrays Expression differed across cell/tissue types. MX1 up in senescent +21 fibroblasts. (MX1 is an

interferon target) Interferon signaling a candidate pathway for

senescence GART up in fetal hearts with +21 Abnormal purine metabolism may have a role in

cardiac defects.

Complex disease non-mendelian- familial aggregation, but no clear

segregation. suggests causative agents and physiological

mechanisms; evolution, development, homeostatic processes.

generally more frequent than single-gene disorders. single-gene disorders and complex diseases have

multiple genetic, developmental, and environmental factors. In single-gene disorders, one gene has a pronounced effect in producing the phenotype.

Complex diseases is the major contributor to morbidity/mortality in developed countries.

Single-gene disorders are a smaller burden of disease and death than do complex diseases.