The repeat expansion detection method in the analysis of diseases with CAG/CTG repeat expansion: usefulness and limitations.

The repeat expansion detection (RED) method was described to detect expansions of trinucleotide repeats of unknown chromosomal location. We have improved the RED method by the use of 8-mer oligonucleotides and assessed its usefulness in 30 samples from patients with spinocerebellar ataxia type 1 (SCA1), Huntington's disease (HD), and Machado Joseph's disease (MJD), for which the number of CAG/CTG repeats was determined by sequencing. There was a good correlation between the number of repeats detected by sequencing and those identified by RED. However, in 17% of samples, the RED gave additional fragments for ligation products of different size than the CAG/CTG repeat expansion detected in the sample by sequencing. The same was observed in a group of control subjects (n = 78) without known clinical abnormalities in which products of more than 40 repeats were detected in 27% of them, indicating that CAG/CTG repeat expansions are common in the general population. Wether this corresponds to unidentified loci with expansions deserves further investigation.