Abstract A49: Proof of principle that loss of mismatch repair protein reduces tumor burden in mouse model of gastric cancer

Abstract

Abstract Gastric cancer (GC) remains the third leading cause of cancer-related death worldwide. Unfortunately, only a subset of GC patients, characterized by tumors with high microsatellite instability (MSI), responds to immune checkpoint (ICI) therapy. In the context of GC, the mechanisms of how MSI improves ICI therapy responses remain poorly understood. Here we are undertaking a proof of principle study to demonstrate in novel GC mouse models that loss of mismatch repair protein MLH1 confers MSI phenotype and impairs tumor growth via altered anti-tumor immune responses. To study the functional and mechanistic effects of loss of MLH1 protein, we established MLH1-deficient (Kras-, Pi3kca-, Tp53-mutant) murine tumor organoids via CRISPR/Cas9 technology. These organoids were subcutaneously allografted into immunocompetent C57BL/6 mice to study tumor progression, immune surveillance, and responses to immunotherapy in vivo. MSI testing confirmed that MLH1-proficient parental organoids are MSI low, whereas MLH1-deficient organoids are MSI high. Low passage MLH1-deficient organoid tumors grew similarly to MLH1-proficient tumors. However, after culturing of organoids in vitro for a prolonged time prior to injection to allow accumulation of mutations, subcutaneous allograft MLH1-deficient tumors were considerably smaller compared to MLH1-proficient tumors. MLH1-deficient tumors showed a significantly higher number of CD8+ T cells. Additionally, MLH1-deficient tumors were similar in size to MLH1-proficient tumors when allografted into Rag1-/- mice. Interestingly, CD8- and CD4-depleting experiments showed an involvement of CD4+ T cells in the impairment of MLH1-deficient tumor growth. Treatment with anti-PD-1, but not anti-CTLA-4, reduced tumor mass further. We are currently investigating the underlying mechanism for the impaired growth of MLH1-deficient organoid allograft tumors via tumor mutational burden analysis and neoantigen testing. Taken together, we provide evidence that loss of MLH1 leads to high MSI in gastric tumors, reduces tumor growth after prolonged in vitro culture and increases response to anti-PD-1 therapy. Our findings encourage further studies to investigate the mechanisms of impaired tumor growth after MLH1 loss in GC and may provide insights leading to improve ICI therapy responses for GC patients. Citation Format: Anne Huber, Christine Dijkstra, Vicki Whitehall, Matthias Ernst, Moritz Eissmann. Proof of principle that loss of mismatch repair protein reduces tumor burden in mouse model of gastric cancer [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr A49.