Different mutator phenotypes in Mlh1 - versus Pms2 -deficient mice

Abstract

Deficiencies in DNA mismatch repair (MMR) result in increased mutation rates and cancer risk in both humans and mice. Mouse strains homozygous for knockouts 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 each knockout strain. Five mononucleotide repeat tracts at four different chromosomal 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 ؊/؊ mice was 2-to 3-fold higher than in Pms2 ؊/؊ animals. The higher mutation frequency 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 ؊/؊ animals lack both Mlh1p and Pms2p. Comparison between Mlh1 ؊/؊ animals and Mlh1 ؊/؊ and Pms2 ؊/؊ double knockout mice revealed little difference in mutator phenotype, suggesting that Mlh1 nullizygosity is sufficient to inactivate MMR completely. The findings may provide a basis for understanding the greater predisposition to intestinal cancer of Mlh1 ؊/؊ mice. Small differences (2-to 3-fold) in mononucleotide repeat mutation rates may have dramatic effects on tumor development, requiring multiple genetic alterations in coding regions. Alternatively, this strain difference in tumor spectra also may be related to the consequences of the absence of Pms2p compared with the absence of both Pms2p and Mlh1p on as yet little understood cellular processes.

Elevated mutant frequencies and increased C : G→T : A transitions in Mlh1−/− versus Pms2−/− murine small intestinal epithelial cells

“…This study revealed a signi®cant di erence between the expansion and contraction mutations within these repeats, which resulted in elevated frameshift mutant frequencies in Mlh1 7/7 mice (Yao et al, 1999). With the lacI reporter system, however, we did not ®nd a similar di erence in expansions and contractions.…”

“…The discordant tumor spectrum of the Mlh1 7/7 and Pms2 7/7 mice has suggested a di erential role for these two proteins in tumor formation, likely as a result of di erences in genomic instability. In keeping with this, Yao et al (1999) recently demonstrated that mice de®cient in Mlh1 exhibited a greater degree of instability at repetitive sequences than did mice lacking Pms2.…”

“…Recently, a study of microsatellite instabilities and frameshift mutations in mono-and dinucleotide repeats was carried out in Mlh1 7/7 and Pms2 7/7 mice (Yao et al, 1999). This study revealed a signi®cant di erence between the expansion and contraction mutations within these repeats, which resulted in elevated frameshift mutant frequencies in Mlh1 7/7 mice (Yao et al, 1999).…”

“…Mutant frequencies and mutation spectra depend not only on the speci®c reporter genes employed, but also such variables as sequence context, DNA methylation, chromosomal location and transcriptional activity (Dogliotti, 1996). The lacI gene, for example, lacks long mononucleotide repeats such as those present within the supF reporter gene or microsatellite sequences used by Yao et al (1999). Indeed, as lacI gene mononucleotide repeats are generally 56 nucleotides in length, it is possible that di erences in the ratio of expansions-to-contractions in Mlh1 7/7 and Pms2 7/7 mice are not possible to observe with the lacI reporter system.…”

Models of Microsatellite Evolution

“…Another set of related experiments studies microsatellite mutations in vivo but in organisms whose mismatch repair system has been knocked out. For an example in mice see [55] and in yeast see [36]. There are many more microsatellite mutations in individuals with deficient mismatch repair systems, and this is informative for studying microsatellite models, but in addition to the rate the pattern of mutations also appears to be different in these individuals.…”