The distinct spectra of tumor-associated Apc mutations in mismatch repair-deficient Apc(1638N) mice define the roles of MSH3 and MSH6 in DNA repair and intestinal tumorigenesis

In mammalian cells, mismatch recognition has been attributed to two partial ly redundant heterodimeric protein complexes of MutS homologues, MSH2-MSH3 and MSH2-MSH6. We have conducted a comparative analysis of Msh3 and Msh6 de ficiency in mouse intestinal tumorigenesis by generating Apc(1638N) mice de ficient in Msh3, Msh6 or both. We have found that Apc(1638N) mice defective in Msh6 show reduced survival and a 6-7-fold increase in intestinal tumor multiplicity. In contrast, Msh3-deficient Apc(1638N) mice showed no differe nce in survival and intestinal tumor multiplicity as compared with Apc(1638 N) mice. However, when Msh3 deficiency is combined with Msh6 deficiency (Ms h3(-/-)Msh6(-/-)Apc(1638N)), the survival rate of the mice was further redu ced compared to Msh6(-/-)Apc(1638N) mice because of a high multiplicity of intestinal tumors at a younger age. Almost 90% of the intestinal tumors fro m both Msh6(-/-)Apc(1638N) and Msh3(-/-)Msh6(-/-)Apc(1638N) mice contained truncation mutations in the wild-type Ape allele. Ape mutations in Msh6(-/- )Apc(1638N) Mice consisted predominantly of base substitutions (93%) creati ng stop codons, consistent with a major role for Msh6 in the repair of base -base mismatches. However, in Msh3(-/-)Msh6(-/-)Apc(1638N) tumors, we obser ved a mixture of base substitutions (46%) and frameshifts (54%), indicating that in Msh6(-/-)Apc(1639N) Mice frameshift mutations in the Ape gene were suppressed by Msh3. Interestingly, all except one of the Ape mutations det ected in mismatch repair-deficient intestinal tumors were located upstream of the third 20-amino acid beta -catenin binding repeat and before all of t he Ser-Ala-Met-Pro repeats, suggesting that there is selection for loss of multiple domains involved in beta -catenin regulation. Our analysis therefo re has revealed distinct mutational spectra and clarified the roles of Msh3 and Msh6 in DNA repair and intestinal tumorigenesis.