GAA Expansion Size Research Articles

Clinical and genetic keys to cerebellar ataxia due to FGF14 GAA expansions

SCA27B caused by FGF14 intronic heterozygous GAA expansions with at least 250 repeats accounts for 10-60% of cases with unresolved cerebellar ataxia. We aimed to assess the size and frequency of FGF14 expanded alleles in individuals with cerebellar ataxia as compared with controls and to characterize genetic and clinical variability. We sized this repeat in 1876 individuals from France sampled for research purposes in this cross-sectional study: 845 index cases with cerebellar ataxia and 324 affected relatives, 475 controls, as well as 119 cases with spastic paraplegia, and 113 with familial essential tremor. A higher frequency of expanded allele carriers in index cases with ataxia was significant only above 300 GAA repeats (10.1%, n=85) compared with controls (1.1%, n=5) (p<0.0001) whereas GAA250-299 alleles were detected in 1.7% of both groups. Eight of 14 index cases with GAA250-299 repeats had other causal pathogenic variants (4/14) and/or discordance of co-segregation (5/14), arguing against GAA causality. We compared the clinical signs in 127 GAA≥300 carriers to cases with non-expanded GAA ataxia resulting in defining a key phenotype triad: onset after 45 years, downbeat nystagmus, episodic ataxic features including diplopia; and a frequent absence of dysarthria. All maternally transmitted alleles above 100 GAA were unstable with a median expansion of+18 repeats per generation (r2=0.44; p<0.0001). In comparison, paternally transmitted alleles above 100 GAA mostly decreased in size (-15 GAA (r2=0.63; p<0.0001)), resulting in the transmission bias observed in SCA27B pedigrees. SCA27B diagnosis must consider both the phenotype and GAA expansion size. In carriers of GAA250-299 repeats, the absence of documented familial transmission and a presentation deviating from the key SCA27B phenotype, should prompt the search for an alternative cause. Affected fathers have a reduced risk of having affected children, which has potential implications for genetic counseling. This work was supported by the Fondation pour la Recherche Médicale, grant number 13338 to JLM, the Association Connaître les Syndrome Cérébelleux - France (to GS) and by the European Union's Horizon 2020 research and innovation program under grant agreement No 779257 ("SOLVE-RD" to GS). DP holds a Fellowship award from the Canadian Institutes of Health Research (CIHR). SK received a grant (01GM1905C) from the Federal Ministry of Education and Research, Germany, through the TreatHSP network. This work was supported by the Australian Government National Health and Medical Research Council grants (GNT2001513 and MRFF2007677) to MB and PJL.

Nonneurological Involvement in Late-Onset Friedreich Ataxia (LOFA): Exploring the Phenotypes.

Friedreich's ataxia (FDRA) is the most common inherited ataxia worldwide, caused by homozygous GAA expansions in the FXN gene. Patients usually have early onset ataxia, areflexia, Babinski sign, scoliosis and pes cavus, but at least 25% of cases have atypical phenotypes. Disease begins after the age of 25 in occasional patients (late-onset Friedreich ataxia (LOFA)). Little is known about the frequency and clinical profile of LOFA patients. One hundred six patients with molecular confirmation of FDRA and followed in three Brazilian outpatient centers were enrolled. General demographics, GAA expansion size, age at onset, cardiac, endocrine, and skeletal manifestations were evaluated and compared between LOFA and classic FDRA (cFDRA) groups. We used Mann-Whitney and Fisher tests to compare means and proportions between groups; p values <0.05 were considered significant. LOFA accounted for 17% (18/106) and cFDRA for 83% (88/106) of the patients. There were 13 and 48 women in each group, respectively. LOFA patients were significantly older and had smaller GAA expansions. Clinically, LOFA group had a tendency toward lower frequency of diabetes/impaired glucose tolerance (5.8 vs. 17%, p = 0.29) and cardiomyopathy (16.6 vs. 28.4%, p = 0.38). Skeletal abnormalities were significantly less frequent in LOFA (scoliosis 22 vs. 61%, p = 0.003, and pes cavus 22 vs.75%, p < 0.001) as were spasticity and sustained reflexes, found in 22% of LOFA patients but in none of the cFDRA patients (p = 0.001). LOFA accounts for 17% of Brazilian FDRA patients evaluated herein. Clinically, orthopedic features and spasticity with retained reflexes are helpful tips to differentiate LOFA from cFDRA patients.

Frataxin gene point mutations in Italian Friedreich ataxia patients

Friedreich ataxia (FRDA) is associated with a GAA-trinucleotide-repeat expansion in the first intron of the FXN gene (9q13-21), which encodes a 210-amino-acid protein named frataxin. More than 95% of patients are homozygous for 90-1,300 repeat expansion on both alleles. The remaining patients have been shown to be compound heterozygous for a GAA expansion on one allele and a micromutation on the other. The reduction of both frataxin messenger RNA (mRNA) and protein was found to be proportional to the size of the smaller GAA repeat allele. We report a clinical and molecular study of 12 families in which classical FRDA patients were heterozygous for a GAA expansion on one allele. Sequence analysis of the FXN gene allowed the identification of the second disease-causing mutation in each heterozygous patient, which makes this the second largest series of FRDA compound heterozygotes reported thus far. We have identified seven mutations, four of which are novel. Five patients carried missense mutations, whereas eight patients carried null (frameshift or nonsense) mutations. Quantitation of frataxin levels in lymphoblastoid cell lines derived from six compound heterozygous patients showed a statistically significant correlation of residual protein levels with the age at onset (r = 0.82, p < 0.05) or the GAA expansion (r = -0.76, p < 0.1). In the group of patients heterozygous for a null allele, a strong (r = -0.94, p < 0.01) correlation was observed between the size of GAA expansion and the age at onset, thus lending support to the hypothesis that the residual function of frataxin in patients' cells derive exclusively from the expanded allele.

Friedreich's ataxia: cardiac evaluation of 25 patients with clinical diagnosis and literature review.

Cardiac evaluation (clinical, electrocardiographic and echocardiographic) of 25 Brazilian patients with clinical diagnosis of Friedreich's ataxia (FA) related to the frequency and the size of GAA repeats (unstable expansion of trinucleotide repeats that results in the disease). Clinical and cardiac study including electrocardiogram and echocardiogram of all patients and molecular analysis to detect the frequency and the size of GAA expansion, by polymerase chain reaction analysis. Homozygous GAA expansion was detected in 17 patients (68%) - all typical cases. In 8 (32%) cases (6 atypical and 2 typical), no GAA expansion was observed, therefore it was not considered Friedreich's ataxia. All patients with GAA expansion (100%) had electrocardiographic abnormalities, and only 25% of the cases without GAA expansion had some abnormality on this exam. However, only 6% of all patients revealed some signals/symptoms suggestive of cardiac involvement. A molecular analysis is essential to confirm the diagnosis of Friedreich's ataxia; however, an adequate cardiac evaluation, including an electrocardiogram, was extremely useful to better screening the patients which should perform these molecular analysis.

Friedreich's ataxia and frataxin: molecular genetics, evolution and pathogenesis (Review).

Friedreich's ataxia is an autosomal recessive neuro-degenerative disorder involving both central and peripheral nervous system. Patients also show a systemic clinical picture presenting heart disease and diabetes mellitus or glucose intolerance. The disease is caused by mutations in the FRDA gene mapped on chromosome 9q13. The product of the gene is frataxin, an 18 kDa soluble mitochondrial protein with 210 amino acids. Crystal structure suggests a new, not previously reported, protein fold. The most frequent mutation is the expansion of a GAA trinucleotide repeat located within the first intron of the gene, and represents 98% of the mutations. Point mutations are described in compound heterozygous subjects with one expanded allele. A two-step model of GAA normal alleles towards premutation alleles, which might generate further full expanded mutations in the population with Indo-European ancestry, has been postulated. Clinical phenotype is variable and an inverse correlation with the GAA expansion size has been observed. Analysis of the GAA triplet is a strong molecular tool for clinical diagnosis, genetic counselling and prenatal diagnosis. Friedreich's ataxia patho-genesis is not solved yet. Substantial data from organism models, such the S. cerevisae yeast and more recently conditioned knock-outs in mouse, and studies in heart biopsies and fibroblast cultures from patients suggest an important role of mitochondrial iron in the development of the disease. Iron is accumulated in the mitochondrial matrix of both the yeast frataxin deficient mutant and the patient fibroblasts. It has been postulated that iron-induced oxygen radical affects the oxidative phosphorylation in frataxin deficiency states favouring the disease pathology. A second hypothesis postulates a direct role of frataxin in the mitochondrial energy activation and oxidative phosphorylation. Iron chelator drugs and antioxidant drugs have been postulated for Friedreich's treatment. No results from clinical trials are available yet, but idebenone, a short-chain quinone, seems to reduce the size of hypertrophic cardiomyopathy and levels of oxidative stress molecules in patients.

Electrophysiological contribution to understanding the pathophysiology of Friedreich's ataxia

Friedreich's ataxia (FA) is the most frequent early onset recessive ataxia. Ataxia clinical progression is slow, but the majority of patients is wheelchair-bounded before 30 years of age. Some studies suggested that the peripheral sensory neuropathy was a dynamic process that could explain the clinical worsening. In 1983 an electrophysiological and histological study showed peripheral nerve axonal degeneration unrelated to disease duration or severity. A follow-up study confirmed that symptom progression is independent from peripheral neuropathy that is not progressive. A defective developmment of the largest neurons of the dorsal ganglion was supposed. When the unstable GAA expansion in the X25 gene was identified as the responsible mutation in the majority of FA patients, genotype-phenotype correlation studies were performed. A direct correlation between GAA expansion and involvement of peripheral nerve sensory fibers and central somatosensory pathway was shown. Disease duration correlates with pyramidal, auditory and visual pathway involvement, while the GAA expansion size has no effect.

Influence of GAA expansion size and disease duration on central nervous system impairment in Friedreich's ataxia: contribution to the understanding of the pathophysiology of the disease

Objective: To verify if GAA expansion size could account for the severity of the central nervous system involvement in Friedreich's ataxia (FA).Methods: Retrospective study of 52 FA patients (mean age 26.9±12.1 years; mean disease duration 10.6±7.6 years) homozygous for GAA expansion. Median nerve somatosensory evoked potentials (SSEPs) were available in 36 FA patients, upper limb motor evoked potentials (MEPs) to transcranial magnetic stimulation in 32, brainstem auditory evoked potentials (BAEPs) in 24, and visual evoked potentials (VEPs) in 34. N20, P100, MEP amplitude, SSEP and MEP central conduction time (CCT and CMCT), P100 latency and I-III and I-V interpeak latency, and a BAEP abnormality score were correlated with disease duration and GAA expansion size on the shorter (GAA1) and larger (GAA2) allele in each pair.Results: The GAA1 size inversely correlated with the N20 amplitude (r=−0.49; P<0.01). Disease duration directly correlated with CMCT (r=0.57; P<0.01) and BAEP score (r=0.61; P<0.01) and inversely with MEP (r=−0.40; P<0.05) and P100 amplitude (r=−0.39; P<0.05).Conclusions: Our data suggest that central somatosensory pathway involvement in FA is mainly determined by GAA1 expansion size. Vice versa, degeneration of pyramidal tracts, auditory and visual pathways seems to be a continuing process during the life of FA patients.

The Yeast Connection to Friedreich Ataxia

We thank John Ashkenas for his editorial assistance and David Koeller for data prior to publication. A.D. and D.P. are supported by grants DK53953-01 and GM57067-01, respectively, from the National Institutes of Health.

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.

Correlation between left ventricular hypertrophy and GAA trinucleotide repeat length in Friedreich's ataxia.

Friedreich's ataxia (FA), the most common inherited ataxia, is associated frequently with cardiac hypertrophy, and death is often cardiac related. Recently, the disease has been associated with a mutation that consists of an unstable expansion of GAA repeats in the first intron of the gene encoding frataxin on chromosome 9. We studied 44 consecutive patients with FA, determined the size of GAA expansions in the frataxin gene, and examined the relation between the genotype and cardiac phenotype assessed by M-mode and two-dimensional echocardiography. All the patients were homozygous for the mutation. The size of the GAA expansion on the smaller allele varied from 270 to 1200. We found a correlation between the size of GAA expansion and the left ventricular wall thickness (r = .51, P < .001) and the left ventricular mass index (r = .45, P = .002). Left ventricular hypertrophy was observed in 81% of patients with a number of GAA repeats above the median value of 770 compared with only 14% in the other group (P = .002). These data demonstrate that in FA, the severity of left ventricular hypertrophy is related to the number of GAA repeats. These results suggest that abnormalities of the gene encoding frataxin, a protein of unknown function highly expressed in the normal heart, may play an important role in the modulation of cardiac hypertrophy.