Somatic sequence variation at the Friedreich ataxia locus includes complete contraction of the expanded GAA triplet repeat, significant length variation in serially passaged lymphoblasts and enhanced mutagenesis in the flanking sequence.

TitleSomatic sequence variation at the Friedreich ataxia locus includes complete contraction of the expanded GAA triplet repeat, significant length variation in serially passaged lymphoblasts and enhanced mutagenesis in the flanking sequence.
Publication TypeJournal Article
Year of Publication1999
AuthorsBidichandani, SI, Purandare, SM, Taylor, EE, Gumin, G, Machkhas, H, Harati, Y, Gibbs, RA, Ashizawa, T, Patel, PI
JournalHum Mol Genet
Volume8
Issue13
Pagination2425-36
Date Published1999 Dec
ISSN0964-6906
KeywordsBase Sequence, Cells, Cultured, DNA Mutational Analysis, Friedreich Ataxia, Humans, Iron-Binding Proteins, Lymphocyte Activation, Lymphocytes, Molecular Sequence Data, Phosphotransferases (Alcohol Group Acceptor), Poly A, Trinucleotide Repeat Expansion
Abstract

The vast majority of Friedreich ataxia patients are homozygous for large GAA triplet repeat expansions in intron 1 of the X25 gene. Instability of the expanded GAA repeat was examined in 23 chromosomes bearing 97-1250 triplets in lymphoblastoid cell lines passaged 20-39 times. Southern analyses revealed 18 events of significant changes in length ranging from 69 to 633 triplets, wherein the de novo allele gradually replaced the original over 1-6 passages. Contractions and expansions occurred with equal frequency and magnitude. This behavior is unique in comparison with other large, non-coding triplet repeat expansions [(CGG)(n)and (CTG)(n)] which remain relatively stable under similar conditions. We also report a rare patient who, having inherited two expanded alleles, showed evidence of contracted GAA repeats ranging from nine to 29 triplets in DNA from two independent peripheral blood samples. The GAA triplet repeat is known to adopt a triplex structure, and triplexes in transcribed templates cause enhanced mutagenesis. The poly(A) tract and a 135 bp sequence, both situated immediately upstream of the GAA triplet repeat, were therefore examined for somatic mutations. The poly(A) tract showed enhanced instability when in cis with the GAA expansion. The 135 bp upstream sequence was found to harbor a 3-fold excess of point mutations in DNA derived from individuals homozygous for the GAA triplet repeat expansion compared with normal controls. These data are likely to have important mechanistic and clinical implications.

DOI10.1093/hmg/8.13.2425
Alternate JournalHum. Mol. Genet.
PubMed ID10556290