Epigenetic therapies to augment radiation response in glioblastoma

Glioblastoma is the most prevalent and aggressive type of brain tumor, presenting limited treatment options that include surgery, chemotherapy, and radiotherapy. Radiotherapy aims to induce DNA damage in tumor cells to eradicate them. However, despite nearly all cancer patients receiving radiotherapy, recurrence occurs. Therefore, it is crucial to understand the molecular mechanisms underlying poor radiotherapy response and identify novel treatments. While genetic alterations have been extensively studied in glioblastoma, the role of epigenetic modifications in regulating radiotherapy response remains unresolved. In this study, we investigated the potential of epigenetic inhibitors as radiosensitizers in glioblastoma. A chemical screen was performed using a library of 152 small-molecule inhibitors targeting over 12 classes of epigenetic modifiers. Among them, Bromodomain-containing protein 9 (BRD9) inhibitors were identified as potent radiosensitizers. Combination treatment with ionizing radiation (IR) and BRD9 inhibitor, I-BRD9, decreased cell viability and colony formation ability of various glioblastoma cell lines. The IR treatment alone led to induction of DNA double-strand breaks (DSBs), which were repaired over time, as gauged by gH2AX staining. On the contrary, combination treatment with I-BRD9 led to delay in the clearance of gH2AX, indicating an attenuation of DNA repair. In parallel, long-term IR-exposed cell populations, as a model of IR-resistance, exhibited increased expression of BRD9 and low response to combination treatment. Complementary to chemical inhibition, CRISPR/Cas9-based ablation of BRD9 also sensitized glioblastoma cells to ionizing radiation. I-BRD9 and ionizing radiation treatment led to alterations in pathways related to cell cycle, DNA damage repair, Myc-, and E2F -targets, suggesting a link between BRD9, radiation- induced DNA damage repair, and cell cycle progression. Furthermore, cell cycle arrest at the G2/M phase was observed upon combination treatment. Taken together, our findings suggest a regulatory role for BRD9 in radiotherapy response, highlighting the potential of targeting BRD9 as a therapeutic approach for glioblastoma. This study provides new insights into the molecular mechanisms underlying the poor response to radiotherapy and identifies a potential avenue for improving treatment outcomes in glioblastoma patients.