Abstract A082 Understanding the role of CDK8 in protein synthesis for treating MYC-driven medulloblastoma

Medulloblastoma (MB) is a prevalent malignant pediatric brain tumor with high mortality rates despite currentmultimodal therapies. MYC-driven Group 3 MB represents a highly aggressive subgroup with poor prognosis and limited treatment options. To systematically identify genes representing therapeutic vulnerabilities in MYC-driven MB, we performed a CRISPR-Cas9 screen with a specific focus on 1140 druggable genes across three MB cell lines. CDK8 emerged as a significantly selected gene crucial for MYC-driven MB growth, for which clinically relevant inhibitors are in early phase trials. CDK8 gene expression is high in group 3 MB (Colorado cohort) and specifically high in MYC driven MB subtypes and elevated expression of CDK8 is an adverse prognostic marker even in high-risk group 3 tumors. CDK8 is highly expressed in progenitor populations identified in our recent single-cell RNA-seq analysis and similarly expressed in cycling progenitors in murine models of MB. CDK8 depletion in MB cells using CRISPR-Cas9-induced knockout and lentivirus-mediated CDK8 shRNAs resulted in decreased i cell proliferation and MYC protein. Additionally, CDK8 depletion significantly decreased neurosphere growth in MB cells and supressed orthotopic xenograft growth in vivo. Evaluation of eight CDK8 selective inhibitors demonstrated a broad range of half-maximal inhibitory concentrations (IC50) across three G3-MB cell lines with RVU120 showing the maximum potency, RVu120 supressed colony growth in vitro in MB cells but not normal astrocytes. In vivo RVU120 potently supressed growth of orthotopic xenografts as evaluated by MRI and prolonged survival. RNA-seq analysis revelaed that CDK8 depletion leads to repression of mRNA translation and ribosome biogenesis. GSEA indicated that the loss of CDK8 resulted in strong negative enrichment in the transcriptomic profile associated with stemness gene sets and positive enrichment in differentiation gene sets. We performed a genome-wide analysis to map the occupancy of CDK8 and key histone markers using CUT&RUN in three G3-MB cell lines. We found that CDK8 functions as a transcriptional activator affecting the translation program. In vitro, inhibition of protein tranlation via mTOR supression was synergistic with CDK8 inhibition. We found that this synthetic lethality was relevant in vivo,where CDK8i and mTORi combined to supress xenograft growth and prolong survival. Taken together, these studies established the therapeutic efficacy of combination treatment with mTOR and CDK8 inhibitors in vivo and in vitro, opening an alternate path for biologically based therapeutic trials in MYC-driven MB. Citation Format: Dong Wang, Breunna Brunt, Bethany Veo, Milena Mazan, Tomasz Rzmyski, Sujatha Venkataraman, Nathan Dahl, Rajeev Vibhakar. Understanding the role of CDK8 in protein synthesis for treating MYC-driven medulloblastoma [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pediatric Cancer Research; 2024 Sep 5-8; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl):Abstract nr A082.