Friedreich's ataxia is a mitochondri

Friedreich's ataxia (FRDA) is the most common of the early-onset inherited ataxias and accounts for approximately half of the cases of hereditary ataxia reported from Europe and the United States (1). FRDA is transmitted as an autosomal recessive trait, with a prevalence of about 2 per 100,000. Disease symptoms typically appear between the ages of 8 and 15 years, but some patients develop symptoms earlier in childhood. In rare instances, disease manifestations occur as late as the third or fourth decade (2, 3). Gait ataxia is the most common presenting symptom. Dysarthria, areflexia, pyramidal weakness of the legs, extensor plantar responses, distal loss of joint position, and vibration sense are not found in all patients at the time of presentation but are eventually universal. FRDA is usually defined as a neurological disorder, but most patients die from cardiomyopathy. The gene responsible for FRDA was identified by positional cloning and encodes a relatively small protein of 210 amino acids (4). The gene itself is small, encompassing only five exons. The most common mutation is an expansion of a GAA trinucleotide repeat within the first intron. Although normal individuals may have up to 27 GAA repeats, affected individuals have greater than 100 repeats, and repeat sizes of greater than 1,000 have been identified. Affected individuals synthesize a normal sized protein, termed Frataxin, but the expanded repeat decreases the amount of protein synthesized (5). During transcription, the expanded repeat induces a triple helical structure, which lowers the rate of transcription (6). The larger the repeat, the greater the effect on transcript and protein levels. Compound heterozygotes have been identified with a triplet expansion in one allele and either a nonsense mutation, missense point mutation, or initiation codon mutation in the other allele (7). To date, no patient has been identified with homozygosity for a null mutation, and it is predicted that a homozygous null would prove lethal in early embryonic life. The sequence of Frataxin yielded little insight into the function of the protein, but studies of the yeast homologue of Frataxin (YFH1) led to a hypothesis regarding the function of Frataxin and the pathophysiology of FRDA. YFH1 was found to encode a mitochondrial protein involved in iron metabolism (8, 9). Reduced levels of protein led to an increase in mitochondrial iron concentration, resulting in decreased mitochondrial respiration. The concept that FRDA is a mitochondrial disorder is strongly supported by the report of Lodi et al. (10) presented in this issue of the Proceedings. Deletion of the YFH1 gene in the budding yeast Saccharomyces cerevisiae resulted in a loss of mitochondrial respiration and the formation of petite strains (8, 11-13). Further studies demonstrated that the loss of respiration followed the accumulation of iron in mitochondria (9). These observations led to the hypothesis that the accumulated iron reacting with H202, a major byproduct of oxidative respiration, generated hydroxyl radicals that would damage lipids, proteins, and the mitochondrial genome. The defect in respiratory activity is associated with increased mitochondrial chromosomal mutations. Reduction in iron accumulation, either by limiting media iron or by deleting genes required for high-affinity iron