Different mutator phenotypes in Mlh1- versus Pms2-deficient mice

Deficiencies in DNA mismatch repair (MMR) result in increased mutation rate s and cancer risk in both humans and mice. Mouse strains homozygous for kno ckouts of either the Pms2 or Mlh1 MMR gene develop cancer but exhibit very different tumor spectra; only Mlh1(-/-) animals develop intestinal tumors. We carried out a detailed study of the microsatellite mutation spectra in e ach knockout strain. Five mononucleotide repeat tracts at four different ch romosomal locations were studied by using single-molecule PCR or an in vivo forward mutation assay, Three dinucleotide repeat loci also were examined. Surprisingly, the mononucleotide repeat mutation frequency in Mlh1(-/-) mi ce was 2- to 3-fold higher than in Pms2(-/-) animals. The higher mutation f requency in Mlh1(-/-) mice may be a consequence of some residual DNA repair capacity in the Pms2(-/-) animals. Relevant to this idea, we observed that Pms2(-/-) mice exhibit almost normal levels of Mlh1p, whereas Mlh1(-/-) an imals lack both Mlh1p and Pms2p. Comparison between Mlh1(-/-) animals and M lh1(-/-) and Pms2(-/-) double knockout mice revealed little difference in m utator phenotype, suggesting that Mlh1 nullizygosity is sufficient to inact ivate MMR completely. The findings may provide a basis for understanding th e greater predisposition to intestinal cancer of Mlh1(-/-) mice. Small diff erences (2- to 3-fold) in mononucleotide repeat mutation rates may have dra matic effects on tumor development, requiring multiple genetic alterations in coding regions. Alternatively, this strain difference in tumor spectra a lso may be related to the consequences of the absence of Pms2p compared wit h the absence of both Pms2p and Mlh1p on as Set little understood cellular processes.