Flowchart: Preparation: FXN


Text Box: FXN 



  Friedreich ataxia (FRDA)                                     


Text Box: DRG                       




 Neurogenetics. 2007 Nov;8(4):289-99. Epub 2007 Aug 17.Click here to read Links

Frataxin gene point mutations in Italian Friedreich ataxia patients.

Gellera C, Castellotti B, Mariotti C, Mineri R, Seveso V, Didonato S, Taroni F.

UO Biochimica e Genetica, Fondazione IRCCS Istituto Neurologico "Carlo Besta", via Celoria 11, 20133 Milan, Italy.

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.

PMID: 17703324 [PubMed - indexed for MEDLINE]


Ann Neurol. 2007 Jan;61(1):55-60.Click here to read Links

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

De Biase I, Rasmussen A, Endres D, Al-Mahdawi S, Monticelli A, Cocozza S, Pook M, Bidichandani SI.

Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA.

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 < 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 = 0.78; p = 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.

PMID: 17262846 [PubMed - indexed for MEDLINE]

1: Hum Genet. 2007 Jan;120(5):633-40. Epub 2006 Sep 21.Click here to read Links

The GAA triplet-repeat is unstable in the context of the human FXN locus and displays age-dependent expansions in cerebellum and DRG in a transgenic mouse model.

Clark RM, De Biase I, Malykhina AP, Al-Mahdawi S, Pook M, Bidichandani SI.

Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA.

Friedreich ataxia (FRDA) is caused by homozygosity for FXN alleles containing an expanded GAA triplet-repeat (GAA-TR) sequence. Patients have progressive neurodegeneration of the dorsal root ganglia (DRG) and in later stages the cerebellum may be involved. The expanded GAA-TR sequence is unstable in somatic cells in vivo, and although the mechanism of instability remains unknown, we hypothesized that age-dependent and tissue-specific somatic instability may be a determinant of the progressive pathology involving DRG and cerebellum. We show that transgenic mice containing the expanded GAA-TR sequence (190 or 82 triplets) in the context of the human FXN locus show tissue-specific and age-dependent somatic instability that is compatible with this hypothesis. Small pool PCR analysis, which allows quantitative analysis of repeat instability by assaying individual transgenes in vivo, showed age-dependent expansions specifically in the cerebellum and DRG. The (GAA)(190) allele showed some instability by 2 months, progressed at about 0.3-0.4 triplets per week, resulting in a significant number of expansions by 12 months. Repeat length was found to determine the age of onset of somatic instability, and the rate and magnitude of mutation. Given the low level of cerebellar instability seen by others in multiple transgenic mice with expanded CAG/CTG repeats, our data indicate that somatic instability of the GAA-TR sequence is likely mediated by unique tissue-specific factors. This mouse model will serve as a useful tool to delineate the mechanism(s) of disease-specific somatic instability in FRDA.

PMID: 17024371 [PubMed - indexed for MEDLINE]