DNA mismatch repair haploinsufficiency leads to low-level microsatellite instability in normal mouse intestine

Tumors in Lynch syndrome (LS) patients display high levels of microsatellite instability (MSI), which results from complete loss of DNA mismatch repair (MMR). Why certain tissues, in particular the gastrointestinal tract, are especially prone to tumorigenesis in LS is not well understood. We hypothesized that MMR haploinsufficiency may compromise microsatellite stability in a tissue-specific manner before tumorigenesis. Using mouse genetics, we tested how levels of MLH1, a central MMR protein, contribute to microsatellite stability in vivo and whether elevated MSI is detectable prior to neoplastic growth. We assayed MSI by single-molecule PCR in normal jejunum and spleen of mice with different Mlh1 gene dosage ( Mlh1 +/+ , Mlh1 +/- , Mlh1 -/- ) at 4 and 12 months, and correlated our findings with Mlh1 mRNA and MLH1 protein expression levels. While spleen MLH1 levels of Mlh1 +/- mice were consistently reduced to, as expected, approximately 50% compared to Mlh1 +/+ mice, in jejunum, MLH1 levels varied between individual Mlh1 +/- mice and decreased with age (2-54% and 1-30% at 4- and 12-months, respectively), which was due to sporadic and progressive Mlh1 promoter methylation. The degree of MLH1 loss (prior to complete loss of the protein) correlated with MSI severity in Mlh1 +/- jejunum, while spleens remained microsatellite-stable. Our findings reveal MSI as a common pre-tumorigenic event that precedes (complete) loss of MMR function in Mlh1 +/- mice. A similar mechanism likely also operates in the GI epithelium of LS patients.