DMG-40. Targeting Pol II transcriptional elongation for the treatment of diffuse midline glioma

Abstract Diffuse midline glioma (DMG) K27-altered is one of the most malignant pediatric cancers. Many clinical trials involving different combinations of chemotherapy and radiation therapy have been unsuccessful, reinforcing the importance of identifying new therapeutic strategies based on molecular characteristics to improve treatment outcomes for DMG patients. We recently showed that targeting bromo- and extra-terminal (BET) domain protein 4 (BRD4) activity using small-molecule inhibitors results in delayed tumor progression and extended survival of animals bearing DMG patient-derived xenografts (PDX). However, tumors that initially respond to small-molecule inhibitors, such as those targeting BRD4 activity, eventually show resistance to monotherapy treatments. To find new therapeutic targets and discover novel combinatorial approaches to prevent or delay acquired resistance to monotherapy, we performed an unbiased genome-wide CRISPR/Cas9-based genetic screening of patient-derived DMG cells. We found that DMG cell maintenance depends on a set of downstream effectors including POLR2I, which encodes a subunit of RNA Polymerase II (Pol II) that is involved in transcriptional elongation. We subsequently observed that targeting POLR2I activity through short-hairpin RNA knockdown and small-molecule inhibitors block Pol II transcriptional elongation and inhibit the growth of DMG in vitro and in vivo. We hypothesized that dual inhibition of BRD4 and Pol II activity will further suppress gene transcription at levels of both transcriptional initiation and elongation and will either delay or prevent DMG from acquiring resistance to monotherapy. DMG cells treated with combined BRD4 + Pol II inhibitors showed time-dependent growth inhibition and increased apoptosis, outperforming each monotherapy. Mice bearing DMG PDX are now being treated with the combined therapy. Results and survival analysis will be reported at the meeting. This study has shown promising results demonstrating that transcriptional elongation is a potential strategy for treating DMG, and data from this research could lay the foundation for early clinical trials of this approach.