Genetic instability of trinucleotide repeats in Saccharomyces cerevisiae

The genetic instability of trinucleotide repeats (TNR) is of interest both because of the central role they play in human diseases, such as Huntington's, and also due to their unique genetic features. Based on their behavior in affected human families, TNRs undergo mutations by a mechanism that is very different from other microsatellites. Biochemical and genetic experiments have led to a molecular model for instability in which TNR sequences form unusual hairpins during DNA replication. If these hairpins are not resolved, large expansions or contractions may occur. The purpose of studies presented in this dissertation was to examine the essential cis and trans-factors which cause instability of TNR. To this end, we have employed a qualitative and quantitative genetic assay to monitor expansion and contraction rates of TNR in yeast.;Herein we show that many of the unique genetic properties of unstable TNR observed in humans are also recapitulated in mitotically dividing yeast. In addition to the need for sequence specificity, we show that both CTG and CAG repeats are governed by a threshold for instability. This was the first demonstration of a TNR threshold in a model system. Further examination of interrupted TNR shows a novel role for mismatch repair in preventing expansions of interrupted repeats; however, contractions were not stabilized in the same mismatch repair-dependent manner. The examination of trans-factors which may influence TNR instability showed that the bypass polymerases eta and zeta, along with POL32 are not responsible for TNR contractions. In addition experiments examining simultaneous TNR mutations at different loci offer preliminary evidence that replication repair may contribute to the overall TNR instability observed in our model system. In conclusion yeast provide an excellent model organism to examine replication-mediated TNR instability, which we believe is governed by a complex mixture of essential cis and trans-factors.