Mutator phenotypes of yeast strains heterozygous for mutations in the MSH2gene

Heterozygosity for germ-line mutations in the DNA mismatch repair gene MSH2 predisposes humans to cancer. Here we use a highly sensitive reporter to d escribe a spontaneous mutator phenotype in diploid T-east cells; containing a deletion of only one MSH2 allele, We also identify fire MSH2 missense mu tations that have dominant mutator effects in heterozygous cells when expre ssed at normal Levels from the natural MSH2 promoter. For example, a WO-fol d mutator effect Is observed in an MSH2/msh2 diploid strain in which Gly(69 3). which is invariant in MutS homologs and involved in ATP hydrolysis, is changed to alanine DNA binding data suggest that mismatch repair is suppres sed by binding of a mutant Msh2-Msh6 heterodimer to a mismatch with subsequ ent inability to dissociate from the mismatch in the presence of ATP. A dom inant mutator effect also is observed in yeast when Gly(693) is changed to serine, An early onset colorectal tumor is heterozygous for the analogous G ly --> Ser mutation in hMSH2, and a second hMSH2 mutation ws not found, sug gesting that this missense mutation may predispose to cancer,fa a dominant mutator effect. The mutator effects of the deletion mutant and the Gly --> Ala missense mutant in yeast MSH2 are enhanced by heterozygosity For a miss ense mutation in DNA polymerase delta that reduces its proofreading activit y but is not a mutator in the heterozygous state. The synergistic effects o f heterozygosity for mutations in two different genes that act in series to correct replication errors may be relevant to cancer predisposition.