XRT-06. RAPID PTEFB-DEPENDENT TRANSCRIPTIONAL REORGANIZATION UNDERPINS THE GLIOMA ADAPTIVE RESPONSE TO RADIOTHERAPY

Dynamic regulation of gene expression is fundamental for cellular adaptation to exogenous stressors. PTEFb-mediated pause-release of RNA polymerase II (Pol II) is a conserved regulatory mechanism for synchronous transcriptional induction in response to heat shock, but this pro-survival role has not been examined in the applied context of cancer therapy. Using a combination of ChIP-, ATAC-, and RNA-seq in model systems of diffuse intrinsic pontine glioma and other pediatric high-grade gliomas, we observe a rapid genome-wide reorganization of active chromatin within hours of exposure to therapeutic ionizing radiation. This redistribution facilitates PTEFb-mediated pause-release of Pol II to drive nascent transcriptional induction of canonical DNA damage response programs. Concurrent inhibition of PTEFb imparts a transcription elongation defect, disrupting this chromatin reorganization and blunting transcriptional induction. Functionally, concurrent PTEFb inhibition abrogates key adaptive programs such as DNA repair and cell cycle regulation. This combination demonstrates a potent, synergistic therapeutic potential agnostic of glioma subtype, leading to a marked induction of tumor cell apoptosis and prolongation of xenograft survival. These studies reveal a central role for PTEFb underpinning the early adaptive response to radiotherapy, opening new avenues for combinatorial treatment in these lethal malignancies.