MDB-26. CDK8 IS AN ONCOGENE IN GROUP 3 MEDULLOBLASTOMA THAT IMPACTS PROTEIN TRANSLATION BY REGULATING THE TRANSCRIPTIONAL ACTIVITY OF RIBOSOMAL GENES

Medulloblastoma (MB) is one of the most common malignant childhood brain tumors. Single-cell RNA-seq analysis of human medulloblastoma patients revealed that Group 3 MBs are dominated by undifferentiated progenitor-like cells with elevated expression of ribosomal genes, suggesting that overactive ribosome biogenesis is a hallmark of Group 3 MB. Using CRISPR-Cas9 screen, we found that Cyclin-dependent kinase 8 (CDK8) is an essential gene for MYC MB viability. We showed that CDK8 facilitates MYC-driven transcriptional programs and is necessary for sustaining expression of MYC. Furthermore, the loss of CDK8 resulted in a reduction in the phosphorylation of RNA polymerase II CTD and caused significant changes in genome transcription. Using cut&run, we found CDK8 directly regulated ribosomal gene transcription activity. CDK8 inhibition impaired ribosome biosynthesis suppressed protein translation and led to a striking reduction in phospho-Pol II binding at the promoter regions and gene bodies of SE-associated genes. The combination of an mTOR inhibitor with CDK8 inhibitors showed remarkable synergy in suppressing ribosome biogenesis, decreasing cell viability and attenuating tumor growth in vitro and in multiple models of MB. This proposal outlines the mechanistic consequences of CDK8 inhibition on ribosome biogenesis and provides pre-clinical validation of CDK8 inhibitor treatment as a novel therapeutic approach in MYC MB.