Enhanced radiotherapy for pediatric glioma via synthetic lethal vulnerabilities

Project Summary Diffuse midline gliomas (DMG) are the most common malignant brain tumors of children. They are surgically inaccessible and refractive to chemotherapy. The prevalent oncogenic driver--an H3K27M mutation of histone H3--is undruggable. Radiotherapy is the standard of care for DMG, and the only treatment that prolongs patient

survival. However, H3K27M DMGs inevitably recur following radiotherapy and are uniformly fatal. Against this backdrop, we propose this R01 response to PAR-22-198 from the NCI entitled “Precision Approaches in Radiation Synthetic combinations (PAIRS).” Our broad goal is to improve survival of children with DMG.

The point of departure for our study plan is a recently completed genome-wide CRISPR screen aimed at identifying DMG-specific vulnerabilities. Our screen identified a conditional synthetic lethality at the junction of replication stress (RS) and the consequent DNA damage response (DDR). This vulnerability is druggable and

has potential to enhance the efficacy of radiotherapy in DMG synergistically--a strategy specifically proposed by PAIRS. Our testable hypothesis is that a sustainable balance between RS and DDR in DMGs can be tipped to therapeutic advantage by combining radiation and ATR antagonists (which increase RS) or by

radiation plus inhibitors of DNA polymerase Q (which decrease DDR). In collaboration with a corporate partner, we have identified clinical-stage, brain-penetrant antagonists of ATR and POLQ. For clinical trials we are an integral member of the international Pacific Pediatric Neuro- Oncology Consortium (PNOC). Drawing upon these resources, our aims are to test three specific predictions of

our hypothesis. Aim 1 (target specificity) tests the prediction that synergistic killing of DMG cells by the combination of radiation plus ATR or POLQ antagonist reflects “on target” responses to enhanced RS or suppressed DDR (induced by ATR or POLQ antagonist, respectively). Aim 2 (chromatin structure) tests the

prediction that the H3K27M driven altered chromatin propels dependency on ATR and POLQ. Aim 3 (therapeutic potential) tests the predictions that (a) the variability in DMG patient responses to radiotherapy reflects patient-specific differences in the pre-therapeutic burden of DDR and RS and (b) that markers of RS

and DDR will correlate with response to therapy in “avatar clinical trials” with patient-derived xenografts of DMGs. The study plan is enabled by an interactive collaboration among three investigators whose skill sets exceed the sum of their parts. Daphne Haas-Kogan, M.D. is a clinician-scientist with much practical experience in the

clinical care of children with DMG. As Chair of Radiation Oncology at Dana-Farber Cancer Institute, DHK interacts frequently with Dipanjan Chowdhury, Ph.D. - a DNA enzymologist with deep expertise in DNA replication and repair. In clinical studies proposed for Aim 3, DHK and DC will work closely with Sabine

Mueller, M.D. - an expert on the design and conduct of clinical trials and leader of the PNOC consortium.