Induction of mismatch repair deficiency by tailored chemical mutagenesis

Mismatch repair deficient (MMRd) tumors are immunogenic and highly responsive to checkpoint blockade. This immunogenicity is thought to be related to a mature MMRd genotype comprised of thousands of somatic mutations that are enriched for insertion/deletion mutations (indels) that are notably in microsatellite repeat sequences. To test this premise, we sought to reproduce this phenotype using mutagenic agents to engineer an MMRd genotype in cells that were not immunogenic. The combination of temozolomide (TMZ) with Cisplatin (CDDP), but not of either drugs alone, led to an accelerated accumulation a high somatic mutational load enriched for frameshift alterations and microsatellite instability in murine cell lines. This combination induced loss of MSH2/MSH6 complex through MSH2 promotor hypermethylation, and the pretreated cells showed delayed growth in syngeneic immunocompetent mice that was MHC class I dependent and sensitive to PD-1 blockade. Syngeneic tumors in immunocompetent mice treated systemically with TMZ, CDDP and PD-1 blockade led to increased survival and intratumoral T cell infiltration associated and downregulation of MSH2 expression, while healthy tissues remained MMR-proficient and did not accumulate excess mutations. We then tested this combination in 18 patients with treatment refractory mismatch repair proficient colorectal cancer. A MMRd signature emerged in mutations detected in cell-free DNA from these patients. These data demonstrate that specifically reproducing an MMRd genotype using tailored chemical mutagenesis can result in an immunogenic phenotype.