BRD9 inhibition induces selective radiosensitivity in glioblastoma through MYC pathway modulation

Radiotherapy (RT) is a cornerstone of glioblastoma (GBM) treatment, yet therapeutic resistance remains nearly universal due to the rapid activation of stress-adaptive survival programs. Identifying molecular regulators that sustain these adaptive responses may reveal context-dependent vulnerabilities that can be therapeutically exploited. Here, we performed an epigenetic drug screen under low-dose irradiation to identify modifiers of radiotherapy response in glioblastoma. We identify BRD9 inhibition as a priming strategy that selectively enhances irradiation-induced lethality without inducing substantial cytotoxicity under baseline conditions. Mechanistically, BRD9 perturbation delays the resolution of irradiation-induced DNA damage, leading to increased apoptosis following irradiation. This effect is selective for malignant glioblastoma cell lines and patient-derived primary cells, while sparing non-malignant human astrocytes. Transcriptomic profiling reveals that BRD9 inhibition or genetic depletion produces a coordinated, MYC-centered suppression of translational programs, including ribosome biogenesis, rRNA processing, tRNA aminoacylation, and translational initiation. Ectopic MYC expression attenuates BRD9-dependent radiosensitization, functionally linking MYC suppression to the enhanced radiation response. Importantly, analysis of independent glioblastoma patient cohorts reveals a consistent positive association between BRD9 and MYC expression, alongside elevated BRD9 expression in recurrent compared with primary tumors. Together, these findings identify BRD9 as a regulator of MYC-associated translational programs and support its therapeutic targeting as a strategy to enhance radiotherapy efficacy in glioblastoma.