DMG-34. Probing the impact of irradiation and ataxia-telangiectasia mutated (Atm) disruption in a primary immunocompetent mouse model of diffuse midline glioma

Diffuse midline glioma (DMG) is a universally fatal brain tumor affecting children and young adults. While radiation therapy (RT) is standard-of-care and results in transient improvement, its use is ultimately palliative. One approach to improve the efficacy of RT is to target ataxia-telangiectasia mutated (ATM), a kinase critical for orchestrating the cell’s response to RT. ATM inhibition (ATMi) may also serve to modulate innate immune signaling in the tumor microenvironment (TME) via the pro-inflammatory cGAS/STING pathway. Here we use an RCAS/t-va genetically engineered mouse model of DMG to investigate if ATMi + RT improves survival vs. RT alone and to probe changes in Sting expression in the DMG TME. DMGs were induced in neonate mice, and tumors were identified using bioluminescent imaging and/or neurological symptoms. Mice were then randomized 1:1 to receive RT alone or RT plus a brain-penetrant ATMi (AZD1390, dosed at 20 mg/kg 1 hour prior to RT). All mice received 3 daily fractions of 10 Gy focal brain RT. The survival data indicate a trend towards increased median overall survival when comparing ATMi + RT vs. RT alone (29.5 days vs. 10 days, p = 0.1 by log-rank test). To investigate whether Atm loss in neoplastic cells impacts Sting expression in the TME, we examined Xenium in situ spatial transcriptomic data from four tumor-bearing mouse brains (Atm+ vs. Atm- vs. Atm+/RT vs. Atm-/RT). Interestingly, we found that the tumor-myeloid compartment had lower Sting expression than resident microglial populations. The notable exception was the Atm- sample, where Sting expression in TME myeloid cells was elevated and comparable to levels in microglia. These results demonstrate that genetic deletion of Atm in neoplastic cells can mediate Sting expression in the broader TME. Future investigations into combining ATM-targeting therapies with immunotherapies may be warranted.