Inherited Ataxias

Molecular Pathways to Neurodegeneration

Professor Yasser Metwally

www.yassermetwally.com

CLASSIFICATION OF THE HEREDITARY CLASSIFICATION OF THE HEREDITARY ATAXIASATAXIAS

(HARDING, 1981)(HARDING, 1981)Congenital Cerebellar AtaxiasMetabolic Ataxias  Intermittent Metabolic Ataxias

  Progressive Metabolic Ataxias (AVED)Ataxias Associated With Defective DNA Repair (AT)Degenerative Ataxias

Early Onset Inherited AtaxiasFriedreich’s AtaxiaEOCA (ARSACS, AOA-1, SCAN-1)Cerebellar Ataxia with HypogonadismProgressive Myoclonic AtaxiaOther Early Onset Ataxias 

Late Onset Inherited AtaxiasADCA Type IADCA Type IIADCA Type IIIOther ADCA

Mitochondrial

Metabolic

Associated with Defective DNA Repair

Protein Folding & Degradation

Channelopathies

Toxic RNA

Others

Pathogenic Classification

Mitochondrial

Metabolic

Associated with Defective DNA Repair

Protein Folding & Degradation

Channelopathies

Toxic RNA

Others

Pathogenic Classification

Mitochondrial Ataxias

NUCLEAR GENES

• FRDA• MIRAS• IOSCA• ARCA2• SCA28

MITOCHONDRIAL GENES • MERRF• NARP• Kearns-Sayre syndrome

Mitochondrial Ataxias

NUCLEAR GENES

• Friedreich’s ataxia• MIRAS• IOSCA• ARCA2• SCA28

Mitochondrial AtaxiasÜber degenerative Atrophie der spinalen Hinterstränge; 1863

• Onset around puberty• Prominent ataxia and dysarthria • Inconstant nystagmus• Absent tendon reflexes• Late sensory loss and weakness• Scoliosis and pes cavus• Cardiac symptomsPrevalence 1:50,000

Carrier frequency 1:100

Ex 1 Ex 2 Ex 3 Ex 4 Ex 5a Ex 5b Ex 6

= Missense

= Truncation

GAA repeat

93%

7%

Homozygotes

Compound heterozygotes

FXN gene

Frequency Distribution of GAA

0300

600900

1200

0

10

20

30

40

Nu

mb

er

of

ch

rom

osom

es

GAA repeats

GAA1

GAA2

6 36Normal alleles

Expanded alleles60 1300

Frataxin

0

2

4

6

8

10

12

14

16

400 500 600 700 800 900 1000

GAA repeat number

Fra

taxi

n m

RN

A (

Arb

itra

ry U

nit

s)

0

5

10

15

20

25

30

35

A B C D E F G H Q R T

Fra

tax

in m

RN

A (

Arb

itra

ry u

nit

s)

Patients Controls

GAA expansion in the FXN gene causes frataxin mRNA reduction in FRDA patients

Frataxin reduction in FRDA patients

Iron Sulphur Centres

• Iron transport

• Iron storage

• Anti-oxidant

• Ox-Phos stimulation

• Fe-S centres biosynthesis

Frataxin deficiency

Oxidative stress

Oxidative stress

Increased Fe-S cluster byosynthesis

Reduction of Fe-Scluster proteins

Impaired energymetabolism

ROS productionFenton chemistry

Neurodegeneration

Fenton chemistry

Neurodegeneration

GAA1 Expansion is inversely correlated with onset age

r= -0.71r= -0.71

RR22 = 0.56 = 0.56

0

10

20

30

40

50

0 200 400 600 800 1000 1200

GAA1 repeats

Ag

e a

t on

set

Friedreich Ataxia Onset around puberty• Prominent ataxia and dysarthria • Inconstant nystagmus• Absent tendon reflexes• Late sensory loss and weakness• Scoliosis and pes cavus• Cardiac symptomsFRDA may show variant phenotype and the

molecular test should be considered in sporadic and autosomal recessive cases of ataxia, even with late onset and preserved tendon reflexes

DNA was extracted from EDTA-treated blood samples, and the (GAA)n repeat length in the first intron of the gene X25 was analyzed by PCR and separation on agarose gel. The size of alleles was estimated by leastsquare fit of fragment size to gel mobility.

NUCLEAR GENES

• FRDA• MIRAS• IOSCA• CoQ10 deficiency/ARCA2• SCA28

Mitochondrial Ataxias

Nuclear Mitochondrial Ataxias

Disease Acronym Gene Function

MITOCHONDRIAL RECESSIVE ATAXIA SYNDROME

MIRAS polymerase, dna, gamma; (POLG1)

Mitochondrial DNA replication

INFANTILE-ONSET SPINOCEREBELLAR ATAXIA

IOSCA

(mtDNA)-specific helicase (C10orf2)

Mitochondrial DNA replication

COENZYME Q10 DEFICIENCY

ARCA2

- mitochondrial parahydroxybenzoid-polyprenyltransferase (COQ2)

- decaprenyl diphosphate synthase subunit-1 gene (PDSS1)

- decaprenyl diphosphate synthase subunit-2 gene (PDSS2)

- aaarF-domain-containing kinase 3 (ADCK3/CABC1)

Coenzime Q Biosynthesis

Onset (years)Recessive ataxiaEpilepsyMental impairmentOphthalmoplegiaNeuropathyMyoclonusPyramidal signsOptic atrophyHearing loss HypogonadismHyperlactatemia

1-30++++++-----

MIRAS

1+++++--+++-

IOSCA

2-11+++-------+

ARCA2

Mitochondrial Ataxias

Cerebellar atrophy in ARCA 2

Cerebellar atrophy is present in MIRAS, IOSCA, and ARCA.It is absent or mild in FRDA.

NUCLEAR GENES

• FRDA• MIRAS • IOSCA• ARCA2• SCA28

Mitochondrial Ataxias

SCA28 – A Dominant Mitochondrial Ataxia

Genetics • mutations in the AFG3L2 (Afg3 like 2) gene (AAA family) • two m-AAA protease isoenzymes: a homo-oligomeric AFG3L2 complex and a hetero-oligomeric complex (paraplegin/AFG3L2) • dominant negative effect of AFG3L2 mutations • impaired quality system control of mitochondrial proteins

Phenotype• juvenile onset• slowly progressive ataxia• ophthalmoparesis

• cerebellar atrophy

Mitochondrial

Metabolic

Associated with Defective DNA Repair

Protein Folding & Degradation

Channelopathies

Toxic RNA

Others

Pathogenic Classification

INTERMITTENT ATAXIA

Deficiency of Urea CycleDisorders of Amino AcidDisorders of Pyruvate

PROGRESSIVE ATAXIA

Storage Diseases

Niemann-Pick type C KrabbeHexosaminidase A deficiency

KufsCholestanolosis (CTX)

Vitamin E Deficiency and Lipoprotein Disorders

AVED Abetalipoproteinemia Hypobetalipoproteinemia

Peroxysomal Diseases

Adrenomyeloneuropathy Refsum

Metabolic Ataxias

Metabolic Ataxias

• Rare disorders• Often autosomal recessive inheritance and early onset• Assay of enzymatic activities or measurement of serum or urine metabolite• Ataxia may be intermittent or progressive • Frequently associated to other neurological signs and multisystem involvement.

Mitochondrial Mitochondrial

MetabolicMetabolic

Associated with Defective DNA RepairAssociated with Defective DNA Repair

Protein Folding & DegradationProtein Folding & Degradation

ChannelopathiesChannelopathies

Toxic RNAToxic RNA

OthersOthers

Pathogenic Classification

Double Strand

• Ataxia Telangiectasia• AT like disorder

Single Strand

• AOA1• AOA2 (?)• SCAN1• Xeroderma Pigmentosum • Cockayne Syndrome

Disorders Associated with Defective DNA Repair

Neurological and non-neurological deficits

Absence of non-neurological deficits

Double Strand

• Ataxia Telangiectasia• AT like disorder

Associated with Defective DNA Repair

A-T is characterized by a triad of clinical manifestations:

a complex, progressive neurological syndrome

telangiectasias immunological deficiency

Onset (years)Recessive ataxiaOculomotor apraxiaNeuropathy Choreothetosis Mental impairment α-fetoprotein Cholesterol Albumin ImmunoglobulinsTelangiectasesMalignanciesEarly menopause

1-4++++-+--+++

-

AT

Single Strand

• AOA1• SCAN1• AOA2 (?)• Xeroderma Pigmentosum • Cockayne Syndrome

Disorders Associated with Defective DNA Repair

Ataxia oculomotor apraxia 1

AOA1 presents with early onset cerebellar ataxia, oculomotor apraxia, choreoathetosis, dystonia, axonal motor neuropathy, but no extra- neurological feature. Mild cognitive impairment may occur. In later stages, decreased serum albumin and increased total cholesterol may appear.

Aprataxin (APTX)

Imaging

[123I]FP-CIT

Diagnosis

We analyzed the PCR products by single-strand conformation polymorphism (SSCP) on PlusOne precast acrylamide gels (Pharmacia) with a Genephor electrophoresis device (Pharmacia) at two different running temperatures (5 °C and 20 °C) and then carried out silver staining. We sequenced the electrophoretic variants from both the forward and reverse strands after purification of the PCR products with the NucleoSpin Extract 2 in 1 kit (Macherey-Nagel GmbH). [Moreira et al. Nature Genetics 29, 189 - 193 (2001)]

Single Strand • AOA1 • SCAN1• AOA2 (?)• Xeroderma Pigmentosum • Cockayne Syndrome

Disorders Associated with Defective DNA Repair

121314 1615 17 1918 2021 232425 2622

TAGATG

123 4 5 6 7 89 10

Senataxin (SETX)

AOA2 presents with early onset cerebellar ataxia, oculomotor apraxia, choreoathetosis, dystonia, axonal motor neuropathy, but no extra- neurological feature. Early menopause may occur. Increased serum alphafetoprotein is a reliable peripheral marker.

Imaging

Back

Onset (years)Recessive ataxiaOculomotor apraxiaNeuropathy Choreothetosis Mental impairment α-fetoprotein Cholesterol Albumin ImmunoglobulinsTelangiectasesMalignanciesEarly menopause

1-4++++-+--+++

-

AT AOA12-30

++++

+/--++---

-

AOA210-22

++++-+++---

+

Double Strand

• Ataxia Telangiectasia• AT like disorder

Single Strand

• AOA1• AOA2 (?)• SCAN1• Xeroderma Pigmentosum • Cockayne Syndrome

Disorders Associated with Defective DNA Repair

Sporadic and autosomal recessive cases of ataxia with neuropathy negative for FRDA should be considered for screening of ataxia associated with defect of DNA repair (look for suggestive laboratory markers)