MEDU-13. FUNCTIONAL CRISPR-CAS9 SCREEN IDENTIFIES DRUGGABLE DEPENDENCIES IN MYC-DRIVEN MEDULLOBLASTOMA

Myc-driven Medulloblastoma remains a major therapeutic challenge due to frequent metastasis and a poor 5-year survival rate. Myc overexpression results in transcriptional dysregulation, proliferation, and survival of malignant cells. However direct inhibition of Myc remains an obstacle to therapeutic interventions. To identify therapeutic targets in Myc amplified medulloblastoma we performed a CRISPR-Cas9 essentiality screen targeting 1140 genes annotated as the druggable genome. A strand guide library (4–10 Sg/gene) was transduced along with Cas9 into 3 different Myc amplified medulloblastoma cell lines in triplicate and DNA extracted and sequenced 18 days after transduction. Analysis of the resultant data revealed multiple gene networks as essential for Myc-medulloblastoma. The cyclin-dependent kinase, CDK7, was identified as a top candidate. CDK7 phosphorylates the c-terminal domain of RNA Pol II facilitating transcriptional initiation and elongation. We found that CDK7 expression is elevated in Myc-medulloblastoma patient samples and cell lines compared to other subgroups and normal pediatric cerebellum. Genetic depletion of CDK7 suppressed medulloblastoma cell clonogenicity in vitro. Recently a covalent inhibitor, THZ1, with high potency for CDK7 was identified and shown to efficiently reduce tumor growth in several pediatric cancers. To evaluate whether CDK7 is a therapeutic target in Myc-driven medulloblastoma we treated Myc-amplified medulloblastoma cells with THZ1. We observed a significant inhibition of clonogenic potential and induction of apoptosis in Myc-medulloblastoma cells in vitro. Using RNA-seq-based gene set enrichment analysis we show that THZ1 decreases c-Myc driven transcriptomic programs and enhanced inflammatory and apoptotic responses. Further studies established that chemical inhibition of CDK7 enhanced radiation sensitivity of Myc MB cells by potentiating DNA damage. Furthermore, CDK7 inhibition with THZ2 suppresses medulloblastoma tumor formation in vivo. Our studies are the first demonstration of CDK7 inhibition as a viable therapeutic strategy alone or in combination with radiation for Myc-amplified medulloblastoma.