Francesca Cavalcanti

Friedreich's Ataxia: Autosomal Recessive Disease Caused by an Intronic GAA Triplet Repeat Expansion

Friedreichs ataxia (FRDA) is an autosomal recessive, degenerative disease that involves the central and peripheral nervous systems and the heart. A gene, X25, was identified in the critical region for the FRDA locus on chromosome 9q13. The gene encodes a 210-amino acid protein, frataxin, that has homologs in distant species such as Caenorhabditis elegans and yeast. A few FRDA patients were found to have point mutations in X25, but the majority were homozygous for an unstable GAA trinucleotide expansion in the first X25 intron.

Friedreich's ataxia : Point mutations and clinical presentation of compound heterozygotes

Friedreichs ataxia is the most common inherited ataxia. Ninety‐six percent of patients are homozygous for GAA trinucleotide repeat expansions in the first intron of the frataxin gene. The remaining cases are compound heterozygotes for a GAA expansion and a frataxin point mutation. We report here the identification of 10 novel frataxin point mutations, and the detection of a previously described mutation (G130V) in two additional families. Most truncating mutations were in exon 1. All missense mutations were in the last three exons coding for the mature frataxin protein. The clinical features of 25 patients with identified frataxin point mutations were compared with those of 196 patients homozygous for the GAA expansion. A similar phenotype resulted from truncating mutations and from missense mutations in the carboxy‐terminal half of mature frataxin, suggesting that they cause a comparable loss of function. In contrast, the only two missense mutations located in the amino‐terminal half of mature frataxin (D122Y and G130V) cause an atypical and milder clinical presentation (early‐onset spastic gait with slow disease progression, absence of dysarthria, retained or brisk tendon reflexes, and mild or no cerebellar ataxia), suggesting that they only partially affect frataxin function. The incidence of optic disk pallor was higher in compound heterozygotes than in expansion homozygotes, which might correlate with a very low residual level of normal frataxin produced from the expanded allele. Ann Neurol 1999;45:200–206

A New Locus for Autosomal Recessive Hereditary Spastic Paraplegia Maps to Chromosome 16q24.3

Hereditary spastic paraplegia is a genetically and phenotypically heterogeneous disorder. Both pure and complicated forms have been described, with autosomal dominant, autosomal recessive, and X-linked inheritance. Various loci (SPG1-SPG6) associated with this disorder have been mapped. Here, we report linkage analysis of a large consanguineous family affected with autosomal recessive spastic paraplegia with age at onset of 25-42 years. Linkage analysis of this family excluded all previously described spastic paraplegia loci. A genomewide linkage analysis showed evidence of linkage to chromosome 16q24.3, with markers D16S413 (maximum LOD score 3.37 at recombination fraction [theta] of .00) and D16S303 (maximum LOD score 3.74 at straight theta=.00). Multipoint analysis localized the disease gene in the most telomeric region, with a LOD score of 4.2. These data indicate the presence of a new locus linked to pure recessive spastic paraplegia, on chromosome 16q24.3, within a candidate region of 6 cM.

Determinants of onset age in Friedreich’s ataxia

We studied the factors that might influence onset age in Friedreich’s ataxia in 41 cases (20 male, 21 female) homozygous for GAA expansion on the first intron of X25 gene. Patients came from 18 multiplex families (13 couples, 5 triplets). Mean age (SD) was 18.1 (8.9) years and did not differ by gender. Onset age and the sizes of the smaller (GAA1) and the larger (GAA2) allele in each pair showed high intrafamily correlation. We found an inverse correlation between age at onset and GAA1 size, but not between age at onset and GAA2 size. Stepwise multiple regression of onset age on GAA1 size, sibling onset age, and GAA2 size showed that GAA1 accounts for 73% of onset age variance, and sibling onset age for an additional 13%. The study demonstrates that, in addition to GAA expansion size, other environmental or genetic familial factors influence disease expression.

Why do some Friedreich's ataxia patients retain tendon reflexes? A clinical, neurophysiological and molecular study.

Abstract Among 101 patients homozygous for GAA expansion within the X25 gene, 11 from 8 families had Friedreich’s ataxia with retained reflexes in the lower limbs (FARR). These patients had a lower occurrence of decreased vibration sense, pes cavus, and echocardiographic signs of left ventricular hypertrophy than the 90 FA patients with areflexia. The mean age at onset was significantly later (26.6 ± 11.4 vs. 14.2 ± 6.9 years), and the mean size of the smaller allele was significantly less (408 ± 252 vs. 719 ± 184 GAA triplets) in FARR patients. The neurophysiological findings were consistent with milder peripheral neuropathy and milder impairment of the somatosensory pathways in FARR patients.

Linkage disequilibrium analysis of Friedreich's ataxia in 140 Caucasian families: positioning of the disease locus and evaluation of allelic heterogeneity.

We investigated linkage disequilibrium between Friedreich’s ataxia (FRDA) and four tightly linked multi-allele markers in 140 families from France and Italy. These markers include three microsatellites (D9S111, D9S15 and D9S110) and one RFLP (D9S5). Their chromosomal order, D9S111-D9S15-D9S110-D9S5, had previously been established by physical mapping. Linkage disequilibrium was evaluated between each marker and FRDA and between markers. Extended haplotypes were obtained and their frequencies on FRDA and normal chromosomes were evaluated. We obtained evidence of strong allelic association of FRDA with D9S5 only. Analysis of linkage disequilibrium between markers revealed a significant decrease between D9S110 and D9S5, suggesting the presence of a recombination hot spot in the interval between these markers. Probably for this reason, no major FRDA-associated extended haplotype could be identified. Our data suggest the presence of a few common disease-causing mutations in the examined population, and indicate a putative localization for the FRDA gene. Transcribed sequences have been found in this candidate region.

Expanding the global prevalence of spinocerebellar ataxia type 42

Spinocerebellar ataxias (SCAs) are a heterogeneous group of neurodegenerative disorders that involve the degeneration of the cerebellum and brainstem.1 These genetic diseases are characterized by autosomal dominant inheritance with approximately 44 known subtypes. Recently, dominant mutations in the CACNA1G gene, encoding the voltage-gated calcium channel CaV3.1, have been linked to SCA42 in French2 and Japanese3,4 families. SCA42 prevalence elsewhere in the world has yet to be documented. Through a combination of whole-exome sequencing (WES) and linkage analysis, we have identified an SCA42 mutation in patients from 3 additional countries, expanding the worldwide prevalence of this disease.

Evidence of a genetic marker associated with early onset in Friedreich's ataxia.

We evaluated the association between age at onset of Friedreichs ataxia and alleles of two restriction fragment length polymorphisms (RFLP) at D9S15 and D9S5 in the 9gl3–9g21.1 region. We studied 65 Italian patients from 49 families. Age at onset was not normally distributed in our patients, suggesting allelic heterogeneity. Patients homozygous for allele 1 ofMspI RFLP detected by probe MCT112 at D9S15 (M1) had an earlier onset (mean 9.3, SD 3.4 years) than patients homozygous for allele 2 (M2; mean 12.1, SD 4.3). Heterozygotes had an onset age similar to that of the M2 homozygotes. These findings suggest that the M1 allele might be a marker of one allelic early-onset Friedreichs ataxia mutation.