Increased instability of human CTG repeat tracts on yeast artificial chromosomes during gametogenesis

Expansion of trinucleotide repeat tracts has been shown to be associated wi th numerous human diseases. The mechanism and timing of the expansion event s are poorly understood, however. We show that CTG repeats, associated with the human DMPK gene and implanted in two homologous yeast artificial chrom osomes (YACs), are very unstable. The instability is 6 to 10 times more pro nounced in meiosis than during mitotic division. The influence of meiosis o n instability is 4.4 times greater when the second YAC with a repeat tract is not present. Most of the changes we observed in trinucleotide repeat tra cts are large contractions of 21 to 50 repeats. The orientation of the inse rt with the repeats has no effect on the frequency and distribution of the contractions. In our experiments, expansions were found almost exclusively during gametogenesis. Genetic analysis of segregating markers among meiotic progeny excluded unequal crossover as the mechanism for instability. These unique patterns have novel implications for possible mechanisms of repeat instability.