Progressive gaa expansions in dorsal root ganglia of Friedreich's ataxia patients

Abstract

Objective: Friedreich's ataxia patients are homozygous for expanded alleles of a GAA triplet-repeat sequence in the FXN gene. Patients develop progressive ataxia due to primary neurodegeneration involving the dorsal root ganglia (DRGs). The selective neurodegeneration is due to the sensitivity of DRGs to frataxin deficiency; however, the progressive nature of the disease remains unexplained. Our objective was to test whether the expanded GAA triplet-repeat sequence undergoes further expansion in DRGs as a possible mechanism underlying the progressive pathology seen in patients. Methods: Small-pool polymerase chain reaction analysis, a sensitive technique that allows the measurement of repeat length in individual FXN genes, was used to analyze somatic instability of the expanded GAA triplet-repeat sequence in multiple tissues obtained from six autopsies of Friedreich's ataxia patients. Results: DRGs showed a significantly greater frequency of large expansions ( p Ͻ 0.001) and a relative paucity of large contractions compared with all other tissues. There was a significant age-dependent increase in the frequency of large expansions in DRGs, which ranged from 0.5% at 17 years to 13.9% at 47 years (r ϭ 0.78; p ϭ 0.028). Interpretation: Progressive pathology involving the DRGs is likely due to age-dependent accumulation of large expansions of the GAA triplet-repeat sequence. Thus, somatic instability of the expanded GAA triplet-repeat sequence may contribute directly to disease pathogenesis and progression. Progressive repeat expansion in specific tissues is a common theme in the pathogenesis of triplet-repeat diseases.Ann Neurol 2007;61: [55][56][57][58][59][60] Friedreich's ataxia (FRDA) is characterized by progressive sensory ataxia with onset before 25 years of age, areflexia, loss of position and vibration senses, dysarthria, and extensor plantar responses. 1 Patients have primary degeneration of dorsal root ganglia (DRGs), associated with axonal degeneration of the posterior columns, spinocerebellar tracts, and corticospinal tracts, and large myelinated fibers in peripheral nerves. 2 In later stages, the cerebellum may be affected; however, other regions of the nervous system remain unaffected. Two-thirds of patients have cardiomyopathy. Although the rate of progression is variable, the mean age of loss of ambulation is 25 years, and patients often die prematurely. 1 FRDA is an autosomal recessive disease, and most patients are homozygous for expanded GAA tripletrepeat sequences (E alleles) in intron 1 of the FXN gene. 3 E alleles interfere with transcription in a lengthdependent manner, 4 resulting in deficiency of the mitochondrial protein frataxin. 5 E alleles contain 66 to 1,700 triplets, and the severity of disease and rate of progression correlate with repeat length. 6 -8 DRG neurons are hypersensitive to frataxin deficiency as seen in neuronal-specific, conditional, frataxin knock-out mice. 9 However, this mouse model did not accurately mimic the situation in FRDA, because the mice had normal levels th...

DNA Mismatch Repair Complex MutSβ Promotes GAA·TTC Repeat Expansion in Human Cells

“…We suggest that the low level of FXN expression in fibroblasts may have also contributed to the slower rate of expansion. Post-mitotic neurons in the DRG are characteristic sites of GAA⅐TTC repeat expansion (29) and neurodegeneration (51) in FRDA patients. These neurons are also particularly high in FXN gene expression (1), in contrast to the low level of transcription in native FRDA fibroblasts.…”

“…However, a somatic expansion bias exists within post-mitotic neurons of the spinal cord in FRDA patients (29) and mouse models (44,45). To investigate whether a lack of replication contributed to this bias, we performed parallel experiments with both dividing and non-dividing primary FRDA patient fibroblasts.…”

“…Significantly, a somatic expansion bias is evident in small pool PCR studies of FRDA disease relevant tissues such as the dorsal root ganglia (DRG) (29). While there are several excellent models available to study TNR contraction (30,31), the lack of higher eukaryotic cellular or animal models that exhibit robust TNR expansion has limited our understanding of this phenomenon.…”

Friedreich ataxia: The clinical picture

“…In this regard, FRDA resembles the other diseases associated with large expansions in non-coding regions of the respective genes, such as fragile X and myotonic dystrophy, and differs from diseases that result from CAG repeats in coding regions, such as the dominant ataxias and Huntington disease, in which further expansion typically occurs after paternal transmission. The GAA expansion is also unstable in somatic cells, including post-mitotic cells [26]. It is open to discussion whether the particular tendency of the GAA repeats to expand in some cell types, such as DRG sensory neurons, contributes to the selective vulnerability of these cells in FRDA [27].…”

DNA Methylation and Trinucleotide Repeat Expansion Diseases

“…Unaffected individuals have FXN alleles containing 5-32 GAA repeats, there is a premutation range of 33-65 GAA repeats, and affected individuals have alleles of 66-1700 GAA repeats. Both intergenerational and somatic instability of the GAA repeat are evident in FRDA, with expanded GAA repeats occurring prominently in disease-related CNS tissue (De Michele et al, 1998;De Biase et al, 2007a, 2007b. There is no disease-associated change in the DNA methylation status of the CpG island that spans the FXN 5-UTR and exon 1 regions.…”