LAUSR

Diffuse midline gliomas (DMGs) are deadly brain tumors characterized by inactivating TP53 mutations and oncohistone H3.3K27M mutations. One potential strategy to improve outcomes for DMG patients is to inhibit Ataxia-telangiectasia mutated kinase (ATM), an orchestrator of the cellular response to the DNA double-strand breaks. Previous research showed that Atm inactivation strongly enhanced the efficacy of radiation therapy for primary mouse models of p53-inactivated DMG. However, it remained unclear whether Atm inactivation would also radiosensitize DMGs driven by both p53 loss and by H3.3K27M mutation. To induce primary DMGs driven by p53 deletion and H3.3K27M expression in a lineage-controlled manner, we leveraged the RCAS/TVA retrovirus system and a conditional loxP-Stop-loxP-H3f3a-K27M-Tag allele. We delivered retroviruses carrying (i) the oncogene platelet-derived growth factor B (PDGFB), (ii) Cre recombinase, and (iii) luciferase into the brainstems of pups with genotype Nestin-TVA, p53-FL/FL, loxP-Stop-loxP-H3f3a-K27M-Tag/+; Atm-FL/FL. In these mice, RCAS retroviruses specifically transduce TVA-expressing Nestin+ neural stem cells to drive primary brain tumors in which Cre recombinase induces H3.3K27M-Tag and inactivates both p53 and Atm. Mice were imaged bi-weekly through in vivo luciferase imaging until a tumor signal was detected. Mice then received 3 daily fractions of 10 Gy focal brain irradiation. Mice were monitored for survival and immunohistochemistry (IHC) was used to confirm appropriate expression or loss of different gene products. Preliminary analyses revealed similar time to tumor formation for these mice compared to littermate controls with intact ATM expression in their tumors (Atm-FL/+). Additionally, Kaplan-Meier survival analysis revealed a significant increase in post-radiation median overall survival for these mice compared to the controls with intact tumoral ATM expression. Overall, these data suggest that (i) the RCAS/TVA system can be combined with a unique conditional allele to generate primary DMGs bearing H3.3K27M in mice, (ii) Atm loss in the tumor cells does not appreciably affect tumor formation in these models, and (iii) Atm loss still significantly radiosensitizes p53-mutant tumors even in the presence of H3.3K27M. These results nominate ATM-directed therapies for further investigation as radiosensitizers in patients with TP53/H3.3K27M-mutant DMG. Citation Format: Sophie R. Wu, María E. Guerra Garcia, Harrison Liu, Nerissa T. Williams, Lixia Luo, Yan Ma, David G. Kirsch, Zachary J. Reitman. Combining the RCAS/TVA retrovirus system and a conditional oncohistone H3.3K27M allele to investigate radiosensitization strategies in primary mouse models of diffuse midline glioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2817.