HGG-32. UNCOVERING THERAPEUTIC VULNERABILITIES IN MISMATCH REPAIR-DEFICIENT GLIOMAS

INTRODUCTION: We have observed that approximately 26% of recurrent gliomas acquire hypermutation following treatment with temozolomide (TMZ). Intriguingly, 91% of these tumors harbor mutations in mismatch repair (MMR) genes. Strategies to target MMR-deficient gliomas thus stand to impact a large number of patients. METHODS: We ablated the MMR genes MSH2, MSH6, MLH1, and PMS2 using an all-in-one sgRNA-CRISPR/Cas9 expression vector to generate panels of isogenic MMR knockouts in patient-derived glioma cell lines. We have characterized the phenotype of these MMR-deficient glioma cells, and leveraged high-throughput drug screens to identify therapeutic vulnerabilities induced by loss of MMR. RESULTS: We demonstrate that sgRNA-CRISPR/Cas9 targeting of either MSH2 or MLH1 – the two obligatory components of the MutSα and MutLα complexes, respectively – also results in loss of protein expression of their respective binding partner MSH6 or PMS2. Moreover, we show that loss of each MMR component confers resistance to TMZ while maintaining sensitivity to the alkylating nitrosourea CCNU. Furthermore, we show that long-term TMZ treatment of MSH2 and MSH6 knockouts in an MGMT-methylated line induces hypermutation with enrichment of C > T mutations but not in MMR wild-type controls. Lastly, loss of MSH2 or MLH1 confers differential dependencies to small molecule inhibitors. CONCLUSIONS: CRISPR/Cas9 knockout of individual MMR pathway members allows us to systematically study the response of MMR-deficient cells to alkylating agents in an isogenic context. MMR deficiencies in glioma confer dependencies to small molecule treatment, which may inform future therapies for MMR-deficient tumors.