Development of Syngeneic Murine Glioma Models with Somatic Mismatch Repair Deficiency to Study Therapeutic Responses to Alkylating Agents and Immunotherapy

Glioblastoma (GBM) carries a dismal prognosis, with a median survival of less than 15 months. Temozolomide (TMZ), the standard frontline chemotherapeutic for GBM, is an alkylating agent that generates DNA O⁶-methylguanine (O⁶MeG) lesions. Without O⁶MeG-methyltransferase (MGMT), this lesion triggers the mismatch repair (MMR) pathway and leads to cytotoxicity via futile cycling. TMZ resistance frequently arises via the somatic acquisition of MMR deficiency (MMRd). Moreover, DNA-damaging agents have been shown capable of increasing tumor immunogenicity and improving response to immune checkpoint blockade (ICB), which has had limited success in glioma. The study of how alkylating chemotherapy such as TMZ impacts antitumor immunity in glioma has been hindered by a lack of immunocompetent models that incorporate relevant DNA repair genotypes. Here, we used CRISPR/Cas9 to generate models isogenic for knockout (KO) of Mlh1 in the syngeneic SB28 murine glioma cell line. MMR KO models readily formed intracranial tumors and exhibited in vitro and in vivo resistance to TMZ. In contrast, MMR KO cells maintained sensitivity to KL-50, a newly developed alkylating compound that exerts MGMT-dependent, MMR-independent cytotoxicity. Lastly, MMR KO tumors remained resistant to ICB, mirroring the lack of response seen in patients with somatic MMRd GBM. The development of syngeneic, immunologically cold glioma models with somatic loss of MMR will facilitate future studies on the immunomodulatory effects of alkylating agents in relevant DNA repair contexts, which will be vital for optimizing combinations with ICB. © 2025 Wiley Periodicals LLC. Basic Protocol 1: Validation of mismatch repair knockouts and in vitro sensitivity to alkylating agents Basic Protocol 2: Stereotaxic injection of isogenic SB28 cells in female C57BL/6J mice and in vivo treatment.