Induction of a mismatch repair deficient genotype by tailored chemical mutagenesis in experimental models of cancer.

Mismatch repair deficient (MMRd) tumors harbor thousands of somatic mutations enriched for insertion-deletion (indels) conferring high sensitivity to immunotherapy. We sought to reproduce this phenotype using mutagenic agents to engineer an MMRd genotype in immunoresistant cells. The combination of temozolomide (TMZ) and cisplatin led to a rapid accumulation of a high mutational load enriched for indels in murine cell lines resulting from the epigenetic loss of Msh2. Pretreated cells showed sensitivity to PD-1 blockade. Systemic treatment with TMZ, cisplatin, and anti-PD-1 bearing immunoresistant tumor cells led to increased survival, intratumoral T cell infiltration, and downregulation of Msh2 expression without affecting healthy tissues. In a clinical trial with 18 patients with refractory mismatch repair proficient colorectal cancer, no responses were seen, but MMRd signatures emerged in cell-free DNA. These findings show that recapitulating an MMRd genotype through chemical mutagenesis can generate an immunogenic phenotype.