The Distinct Spectra of Tumor-associated Apc Mutations in Mismatch Repair-deficient Apc Mice Define the Roles of MSH3 and MSH6 in DNA Repair and Intestinal Tumorigenesis

In mammalian cells, mismatch recognition has been attributed to two partially redundant heterodimeric protein complexes of MutS homologues, MSH2-MSH3 and MSH2-MSH6. We have conducted a comparative analysis of Msh3 and Msh6 deficiency in mouse intestinal tumorigenesis by generating Apc mice deficient in Msh3, Msh6 or both. We have found that Apc mice defective in Msh6 show reduced survival and a 6–7-fold increase in intestinal tumor multiplicity. In contrast, Msh3-deficient Apc mice showed no difference in survival and intestinal tumor multiplicity as compared with Apc mice. However, when Msh3 deficiency is combined with Msh6 deficiency (Msh3 / Msh6 / Apc), the survival rate of the mice was further reduced compared to Msh6 / Apc mice because of a high multiplicity of intestinal tumors at a younger age. Almost 90% of the intestinal tumors from both Msh6 / Apc and Msh3 / Msh6 / Apc mice contained truncation mutations in the wild-type Apc allele. Apc mutations in Msh6 / Apc mice consisted predominantly of base substitutions (93%) creating stop codons, consistent with a major role for Msh6 in the repair of base-base mismatches. However, in Msh3 / Msh6 / Apc tumors, we observed a mixture of base substitutions (46%) and frameshifts (54%), indicating that in Msh6 / Apc mice frameshift mutations in the Apc gene were suppressed by Msh3. Interestingly, all except one of the Apc mutations detected in mismatch repair-deficient intestinal tumors were located upstream of the third 20-amino acid -catenin binding repeat and before all of the Ser-Ala-Met-Pro repeats, suggesting that there is selection for loss of multiple domains involved in -catenin regulation. Our analysis therefore has revealed distinct mutational spectra and clarified the roles of Msh3 and Msh6 in DNA repair and intestinal tumorigenesis.