Targeting Transcriptional Elongation in Pediatric Glioma

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PROJECT SUMMARY/ABSTRACT - Diffuse intrinsic pontine glioma (DIPG) is one of the most devastating pediatric cancers. Numerous clinical trials in decades, involving different combinations of chemotherapeutic agents and radiation, have been ineffective in treating DIPG. The identification of efficacious therapeutic targets based on the molecular characteristic is of high importance for improving treatment outcomes for children with DIPG. The discovery of oncogenic histone gene mutations in DIPG has dramatically improved our understanding of disease pathogenesis, and stimulated the development of novel therapeutic approaches to target epigenetic modifiers. We have recently shown that targeted bromo- and extra-terminal (BET) domain protein 4 (BRD4) activity using JQ1 inhibitor results in a significant delay of tumor progression and prolonged survival of animals bearing DIPG patient-derived xenograft (PDX). Because of their promising anti-tumor activity, BRD4 inhibitors are being tested in a number of cancer patient clinical trials. However, tumors that initially respond to small molecule inhibitor therapies, such as those targeting BRD4 activity, eventually become resistant to monotherapy treatment, affirming the need for more effective therapeutic interventions. In order to identify new effective 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 screen of patient-derived DIPG cells. We identified nine “network modules” that are significantly enriched in CRISPR targets. One of these modules includes POLR2I, which encodes a subunit of RNA polymerase II (Pol II) that is involved in transcription elongation. We subsequently observed that targeting POLR2I activity through short-hairpin RNA knockdown and treatment of the small-molecule Pol II inhibitors block transcriptional elongation and inhibit the growth of DIPG in vitro and in vivo. Here, we will test the hypothesis that inhibition of Pol II transcriptional elongation in combination with BRD4 inhibition will further suppress gene transcription and will either delay or prevent DIPG from acquiring resistance to monotherapy. This dual inhibition approach will interfere with gene transcription at two levels: transcriptional initiation (BRD4) and elongation (Pol II). This project will also explore how these targeted therapies interact with radiation in treating DIPG, which is important due to the use of radiation in treating nearly all cases of DIPG in children. The successful completion of proposal study has significant impact on clinical practice and accumulating data from this research could therefore lay the foundation for early clinical trials of this approach, given the high unmet need and orphan disease status of DIPG.