Expansion of GAA triplet repeats in the human genome: unique origin of the FRDA mutation at the center of an Alu.

Friedreich ataxia is caused by expansion of a GAA triplet repeat (GAA-TR) in the FRDA gene. Normal alleles contain <30 triplets, and disease-causing expansions (66-1700 triplets) arise via hyperexpansion of premutations (30-65 triplets). To gain insight into GAA-TR instability we analyzed all triplet repeats in the human genome. We identified 988 (GAA)(8+) repeats, 291 with >or=20 triplets, including 29 potential premutations (30-62 triplets). Most other triplet repeats were restricted to <20 triplets. We estimated the expected frequency of (GAA)(6+) repeats to be negligible, further indicating that GAA-TRs have undergone significant expansion. Eighty-nine percent of (GAA)(8+) sequences map within G/A islands, and 58% map within the poly(A) tails of Alu elements. Only two other (GAA)(8+) sequences shared the central Alu location seen at the FRDA locus. One showed allelic variation, including expansions analogous to short Friedreich ataxia mutations. Our data demonstrate that GAA-TRs have expanded throughout primate evolution with the generation of potential premutation alleles at multiple loci.