Abstract BACKGROUND The highest-risk medulloblastomas are driven by recurrent Myc amplifications (Myc-MB) and experience poorer outcomes despite intensive multimodal therapy. The Myc transcription factor defines core regulatory circuitry for these tumors and acts to broadly amplify downstream pro-survival transcriptional programs. Therapeutic targeting of Myc directly has proven elusive, but inhibiting transcriptional cofactors may present an indirect means of drugging the oncogenic transcriptional circuitry sustaining Myc-MB. METHODS Independent CRISPR-Cas9 screens were pooled to identify conserved dependencies in Myc-MB. We performed chromatin conformation capture (Hi-C) from primary patient Myc-MB samples to map enhancer-promoter interactions. We then treated in vitro and xenograft models with the dual CDK9/7 inhibitor zotiraciclib to evaluate effect on Myc-driven programs and tumor growth. RESULTS Eight CRISPR-Cas9 screens performed across three independent labs identify CDK9 as a conserved dependency in Myc-MB. Myc-MB cell lines are sensitive to CDK9 inhibition at low nanomolar concentrations, and these synergize with CDK7 inhibition. We find that treatment with the dual CDK9/7 inhibitor zotiraciclib depletes Myc binding and transcriptional activity at enhancer elements within the Myc topologically associated domain, largely abrogating transcriptional output from the Myc promoter. This leads to a decrease in Myc binding genome-wide with a concordant downregulation of hallmark Myc-driven transcriptional programs. Clinically relevant CDK9 inhibitors show variable efficacy in vivo, but CNS-penetrant agents achieved a significant prolongation in xenograft survival. CONCLUSIONS CDK9/7 catalytic activity represents a druggable vulnerability underpinning Myc-driven transcriptional programs. The development of CNS-penetrant CDK9/7 inhibitors may open new avenues for rational therapy in these high-risk medulloblastomas.

MDB-88. DUAL TARGETING OF PROCESSIVE TRANSCRIPTION FOR MYC-DRIVEN CIRCUITRY IN MEDULLOBLASTOMA