FA and Frataxin

Friedreich's Ataxia

Summary

Caused by mutations in the FRDA (Frataxin) gene on chromosome 9q13-q21.1. Inherited in a recessive fashion
Genetic Features

The FRDA gene codes for a protein called frataxin. A similar gene is found in yeast and has been used to study the function of frataxin.

The FRDA gene contains 7 exons (segments which are responsible for the final protein sequence): exons 1, 2, 3, 4, 5a, 5b and 6. The last exon is non-coding. It is involved in regulating the expression of the gene and does not lead to additions to the protein sequence. Exons 5a and 5b are alternatives and result in slightly different protein sequences. Most frataxin has the sequence involving exon 5a.

Frataxin protein is found in many tissues.

· Highest level of frataxin in non-neural tissues: Heart; Skeletal muscle; Liver; Kidney; Pancreas
· In the central nervous system: Spinal cord > Cerebellum > Cerebral cortex
· Localised to mitochondria
· ? Required for maintenance of mitochondrial genome

Function of frataxin

The frataxin homologue in yeast plays a role in iron metabolism
· Iron accumulates in mitochondria in yeast mutants

Yeast fratraxin
* Involved in iron homeostasis: Iron transport into mitochondria
* Respiratory function
* Disruption causes
* Multiple Fe-S-dependent enzyme deficiencies
* Aconitase
* Complexes I to III
* 10x increase in mitochondrial iron content where frataxin protein is deficient
· Mitochondrial intermediate peptidase promotes mitochondrial accumulation of iron in absence of frataxin

Possible disease mechanisms
· Sequestered iron not available for other iron proteins resulting in destabilization of these proteins
· Reduced activity of Fe-S cluster-containing subunits of mitochondrial complexes I, II, and III & aconitase
· Accumulation of reactive O2 species leading to oxidative stress
Changes in the FRCA gene in Friedreich ataxia
In FA, the FRDA gene is defective resulting in a reduced amount of frataxin protein. However, frataxin is not completely deficient in function.

The usual mutation (>90%) results in an excess of DNA triplet (GAA) repeats in the intron at the end of exon 1. GAA copy number

* Normal number is 6 to 34 with a bimodal distribution
* 6 to 12 repeEats (83%)
* 14 to 34 repeats (17%)
* Friedreich ataxia
* Range: 67 to 1,700
* Most commonly: 800 to 1,000 copies
* Intergenerational: Repeat length may expand or contract
* Long GAA repeats unstable during meiosis & mitosis
* Maternal transmission: increase or decrease in size with equal frequency
* Paternal transmission: more likely to decrease in size
* Expanded GAA repeats suppress FRDA (frataxin) gene expression
* Block transcriptional elongation
* Expanded GAA repeats bind to self: "Sticky" DNA
* Association of 2 purine-purine-pyrimidine (R-R-Y) triplexes at neutral pH
* Forms negatively supercoiled plasmids
* Forms folded, non-B DNA structure
* Residual FRDA (Frataxin)
* Some residual FRDA (Frataxin) probably expressed in all patients
* Amount: mRNA & FRDA protein inversely proportional to GAA expansion size
* No homozygous null mutations identified: ? Complete loss of FRDA (Frataxin) is lethal.
* Excess repeats or point mutations in 94% of Italian patients with FA phenotype.

Point mutations (6%)

* 17 different point mutations described
* Usually heterozygous with GAA repeat gene
* Occur in residues conserved with the homologous yeast protein
* Truncating mutations usually in exon 1
* Missense mutations sually in last 3 exons coding for FRDA (Frataxin) gene
* No homozygous point mutations identified yet
* 3 Point mutations common: Ile154Phe (Southern Italy); M1I; G130V
· Examples: GAA repeat + Missense mutation
* Exon 3 Stop
* Ile154Phe; Exon 4
* Most common point mutation
* Southern Italian families
* Similar disease severity to homozygous GAA repeat
* Intron 3: Disrupts acceptor splice site at end of intron
* G130V: Mild disease with slow progression; Same age of onset

Genotype-phenotype correlations

Clinical features correlate with lengths of mutant GAA alleles

* Correlate inversely with age of onset
* Correlation best with shorter allele or average length
* Milder phenotype & later onset with < 500 base pairs in shorter repeat
* Long shorter repeat: More cardiomyopathy; ê Reflexes in arms; Earlier disease onset
* Mean GAA allele length higher with: Diabetes; Cardiomyopathy; Scoliosis
* Pseudodominant inheritance may occur, due to manifesting carrier fathers: Onset > 40 years, or high carrier rate

Some associations with disease severity reported

* Intermediate number of GAA repeats (120-156)
* Spastic ataxia;
* Onset 38 to 45 yrs;
* Found in Acadians
* Missense or truncating mutations in carboxy half of mature frataxin protein
* Severe phenotype
* Missense mutations in amino-terminal half of frataxin protein
* Atypical & milder disease (early spastic gait)
* Compound heterozygotes
* Optic atrophy more common;
* ? 2° Lower residual FRDA levels frataxin protein

Epidemiology of Friedreich ataxia

· Incidence: 1 in 30,000 to 50,000
· Carrier frequency: 1 in 60 to 110
· Incidence low in: Asians; Africans
· Onset: Usually < 20 years with ataxia or scoliosis
· Allelic with a late-onset ataxia syndrome

Neurological features

* Cerebellar (100%): Ataxia (Limb & Trunk); Nystagmus; Dysarthria (95%)
* Corticospinal tract signs
* Extensor plantar responses (80%)
* Spasticity: Intermediate number of repeats
* Weakness (67% to 88%): Lower extremities
* Sensory loss (~80%): Especially vibration & joint position
* Tendon reflexes: Absent (75%) or reduced
* Chorea: Occasional patient may have chorea without ataxia * Electrophysiology
* Sensory nerve potentials: Absent or ê
* Motor: Normal or mildly reduced
* Spinal somatosensory evoked potentials: Absent

Variant neurological syndromes

* Late onset ataxia syndrome
* Onset: May be as late as 51 years
* Some late onset patients with normal reflexes
* Fewer skeletal deformities
* Slower progression
* Spastic ataxia: 2 molecular associations
* Intermediate number of GAA repeats (120-156)
* Onset 38 to 45 yrs
* Acadians
* Missense mutations in amino-terminal half of FRDA (Frataxin) in one allele
* Early onset with rapid progression
* Mutations
* One allele with missense mutation in carboxy half of FRDA (Frataxin): Exon 5a; R165P 2nd allele typical GAA expansion

Friedreich ataxia clinical syndrome

Early onset: Gait ataxia; 1st decade

Cerebellar
· Gait disorder
· Dysmetria: Upper limb; Mild
· Dysarthria: Absent or very mild

Upper motor neuron
· Weakness in lower extremities
· Upgoing toes

Tendon reflexes
· Arms normal
· Some retained knee jerks

Peripheral nerve involvement
· Slight to moderate
· Sensory potentials present

Systemic Features

* Skeletal
* Scoliosis (60% to 80%)
* Pes cavus (50% to 75%)
* Cardiac (50% to 75%)
* Hypertrophic cardiomyopathy
* Muscular subaortic stenosis
* Hypokinetic-dilated left ventricle
* Q waves; Poor prognosis
* Abnormal EKG (65%)
* Endocrine
* Diabetes mellitus (10%)
* Especially with FA onset < 10 years
* Sphincter disturbance (~25%) Prognosis

Course
* Slow progression – wheelchair in second decade
* Female: Time to wheelchair dependence shorter; Survival same
* Early onset: Time to wheelchair dependence shorter
* Death
* Mean: 38 years
* Range: 21 to 69 years
* Cause: Cardiomyopathy

Pathology

* Cellular: Dying back of distal axons
* Localization
* Spinal cord
* Spinocerebellar tracts
* Dorsal columns
* Pyramidal tracts
* Cerebellum
* Dentate nucleus
* Mild neuronal loss in cortex
* Medulla
* Cerebral cortex
* Mild neuronal loss

Biochemical

* Mitochondria
* Reduced level of aconitase
* Reduced level of mitochondrial complexes I, II and III
* Increased iron accumulation
* Decreased phospholipid levels in cerebellar & occipital cortex

Treatment

· ?Idebenone (5 mg/kg/day divided in 3 doses)

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