Identification of three novel frameshift mutations in patients with Friedreich's ataxia.

EDITOR—Friedreich’s ataxia (FRDA, MIM 229300) is an autosomal recessive, progressive, neurodegenerative disorder. It is the most common of all hereditary ataxias, with an estimated prevalence of 1 in 50 000, and a carrier frequency calculated to be as high as 1 in 90 in white populations. Onset normally occurs between 8 and 15 years of age, presenting as ataxia of gait accompanied by dysarthria, areflexia, extensor plantar responses, and distal loss of position and vibration sense. The involvement of other systems is also apparent, and in particular most patients die prematurely from hypertrophic cardiomyopathy. The FRDA gene encodes a highly conserved protein, designated frataxin, which localises to the mitochondrial matrix and appears to be involved in the eZux of iron from mitochondria. Patients with FRDA have reduced levels of frataxin expression, which probably leads to mitochondrial iron accumulation and free radical damage of oxidative phosphorylation processes. The predominant mutation, accounting for approximately 98% of FRDA chromosomes, is expansion of a GAA trinucleotide repeat located within intron 1 of the FRDA gene. 9 Normal subjects have 6-36 GAA repeats at this position, whereas FRDA patients have expansions of 120-1700 repeats. The GAA expansion has been postulated to interfere with frataxin transcription by forming a triple helical based “sticky DNA” structure. 14 An estimated 96% of FRDA patients are homozygous for the GAA expansion and a correlation between severity of the disease phenotype and the size of the smaller of the two alleles has been identified. The remaining 4% of FRDA patients are compound heterozygotes, having a GAA expansion at one allele and either a known point mutation (2.5%) or an as yet unidentified mutation (1.5%) at the other allele. No instance of an FRDA patient carrying two frataxin point mutations has yet been described, and thus it is possible that complete frataxin loss may be lethal. To date there have been 20 diVerent FRDA point mutations reported (table 1), comprising single base pair deletions (two cases), insertions (one case), and substitutions (eight cases of transitions and nine of transversions). Ten of these point mutations are predicted to produce truncated or absent frataxin protein and, in general, these mutations are positioned within the 5' region of the FRDA gene. In contrast, 10 missense mutations have been identified primarily in the highly conserved regions of exons 3, 4, and 5a. We report here the identification of three novel FRDA mutations that all result in frameshifts of the frataxin coding sequence. This includes the detection of a mutation hot spot in exon 3, the first description of a mutation in exon 2, and the first identification of point mutations that involve more than a single base pair rearrangement. Investigation for mutation at the FRDA locus was undertaken in patients exhibiting early onset Friedreich’s ataxia from three unrelated families to confirm diagnosis. Patient 1 is a 7 year old male, who presented initially at the age of 31⁄2 years with poor balance and coordination. He now exhibits areflexia and impaired joint position sense. Right sural and digital sensory nerve action potentials are absent and median combined action potential is significantly diminished. ECG is normal. Patient 2 is a severely aVected 14 year old male who presented at the age of 3 years and was wheelchair bound by 10 years. An echocardiogram shows cardiomyopathy. Patient 3 is a 16 year old male who presented at the age of 3 years with a rapidly progressing ataxia. He became wheelchair bound at the age of 12 years and underwent vertebral fusion to correct his scoliosis at the age of 14 years. He now shows cardiac involvement. Genomic DNA was isolated from whole blood using standard procedures. Repeat expansions were detected by PCR amplification of 250 ng DNA using primers GAA-F and GAA-R together with the Expand long template PCR system (Boehringer Mannheim, Lewes, UK) under