Tumor-associated Apc mutations in Mlh1(-/-) Apc(1638N) mice reveal a mutational signature of Mlh1 deficiency

Apc(1638N) mice, which are heterozygous for a germline mutation in Ape, typ ically de, clop three to five spontaneous intestinal tumors per animal, in most cases this is associated,vith allelic loss of wildtype Ape. We have pr eviously reported that the multiplicity of intestinal tumors is increased d ramatically by crossing Apc(1638N) with an Mlh1-deficient mouse strain that represents an animal model of hereditary non-polyposis colorectal cancer ( HNPCC). The increased tumor multiplicity in these mice was associated with somatic mutations in the Ape tumor suppressor gene, Here, we have examined the nature and distribution of 91 Ape mutations implicated in the de, devel opment of intestinal tumors in Mlh1 (-/-) Apc(1638N) animals. Protein trunc ation mutations were detected in a majority of tumor samples, indicating th at the prevailing mechanism of Ape mutation in tumors is altered from allel ic loss to intragenic mutation as a result of Mlh1 deficiency, The observed mutations were a mixture of base substitutions (27%) and frameshifts (73%) . Most frameshifts wire detected within dinucleotide repeats and there were prominent mutational hotspots within sequences of this sort at codons 927- 929, 1209-1211 and 1461-1464, The observed Ape mutations caused protein tru ncation upstream of the third 20 amino acid beta -catenin binding domain an d the first Axin-binding SAMP repeat, yielding Ape proteins that are predic ted to be deficient in destabilizing beta -catenin, Our results reveal a ch aracteristic mutational signature in APC that is attributable to Mlh1 defic iency. This demonstrates a direct effect of Mlh1 deficiency in the mutation of Apc in these tumors, and provides data that clarify the role of Mlh1 in mammalian DNA mismatch repair.