Enhancing the efficacy of radiation therapy for brainstem glioma by targeting ATM Project Summary/Abstract Brainstem gliomas are devastating pediatric brain tumors. Brainstem gliomas include “diffuse midline gliomas with H3K27M mutation” in the 2016 World Health Organization Classification of Tumors of the CNS, and includie tumors previously referred to as “diffuse intrinsic pontine gliomas” or DIPG. Brainstem gliomas are uniformly lethal to the patients. Radiation therapy is thought to be the only effective treatment for these tumors, providing temporary relief from symptoms and from tumor progression. However, brainstem gliomas inevitably progress after radiation therapy and result in death of the patient resulting in a median survival of less than one year. New strategies are needed to improve the efficacy of radiation therapy to improve patient survival. One promising investigational therapeutic strategy is to radiosensitize tumors by inactivating the serine/threonine kinase Ataxia Telangiactasia Mutated (ATM). ATM is the master sensor for DNA damage, and orchestrates the DNA damage response after cells are damaged by ionizing radiation or other DNA damaging agents. ATM inactivation dramatically radiosensitizes a genetically engineered mouse model of brainstem glioma. When ATM is inactivated in the tumor cells of our mouse model of brainstem glioma, radiation therapy is particularly effective and extends median overall survival of the mice by approximately threefold compared to mice bearing tumors with intact ATM. However, the specific cell populations that are radiosensitized by ATM inactivation, and the mechanisms by which ATM inactivation radiosensitizes brainstem gliomas, is unknown. A deeper understanding of the molecular mechansisms by which ATM inactivation can radiosensitize brainstem gliomas is needed to enable the rational design of combination therapies that combine ATM inhibition, radiation therapy, and other novel epigenetic and immunologic therapies to maximize survival of patients with brainstem gliomas. Here, I will test the hypothesis that ATM inactivation specifically radiosensitizes a population of progenitor-like tumor cells in our genetically engineered mouse model of brainstem glioma. In parallel with this work, I will dissect type I interferon signaling pathways that are contribute to radiosensitivity when ATM is inactivated. These experiments will map the tumor microenvironment of a mouse model of brainstem glioma at single cell resolution for the first time. They will also credential a genetically-engineered mouse model of brainstem glioma with an intact immune system for preclinical investigations of immunotherapeutic approaches. Additionally, the proposed work will provide me with critical expertise in genetically engineered mouse models and in immunologic investigations that will help me transition to a productive independent investigator.