Functional genetic tests of DNA mismatch repair protein activity in Saccharomyces cerevisiae

Hereditary non-polyposis colorectal cancer (HNPCC) is associated with mutations in four different genes encoding proteins involved in DNA mismatch repair (DMR). As many as 30% of the observed sequence variations in human DMR genes predict minor alterations in the encoded protein, such as amino acid (aa) replacements or small in-frame deletions/insertions. For such sequence variants, a functional genetic test will be required to discriminate mutations from polymorphisms. We have constructed a series of isogenic yeast strains in which individual genes involved in DMR are disrupted, and have standardized an assay which measures GT tract stability (Strand et al., 1993) to characterize these gene products. Disruptions of the yeast MSH2, MLH1, and PMS1 genes result in, respectively, a 290-, 450- and 390-fold increased tract instability over the wild type (wt) strain under optimized assay conditions. Expression of the wt MSH2 and PMS1 gene from plasmids results in complementation of the corresponding chromosomal gene disruption. Two different aa replacements which correspond to previously observed sequence variants of the human MSH2 gene, and implicated in HNPCC, were created in the conserved aa of the yeast MSH2 gene by site directed mutagenesis. Conversion of the Pro640 in the yeast protein to Leu resulted in a complete loss of protein function. In contrast, a yeast MSH2p protein in which the His658 is changed to Tyr retains full function in this in vivo assay. These results indicate that the Pro-->Leu and His-->Tyr variants observed in humans constitute, respectively, a mutation and a polymorphism. The system described here may be used for further structure/function analysis of yeast DMR proteins. Such studies may provide insight into the effects that specific sequence variations observed in human DMR proteins have on their function.