CSIG-19. UNRAVELING THE MECHANISMS OF THOC7 IN MEDULLOBLASTOMA PROGRESSION

Abstract Medulloblastoma (MB) is the most common malignant pediatric brain tumor, and although multimodal therapies have improved outcomes for standard-risk patients, children diagnosed with high-risk or recurrent MB, particularly those presenting with leptomeningeal dissemination (LMD), continue to experience poor prognoses, frequent relapses, and debilitating long-term treatment-related toxicities. To identify novel therapeutic vulnerabilities, we performed an unbiased genome-wide CRISPR knockout screen in MB cells alongside a meta-analysis of the Cancer Dependency Map (DepMap), which converged on THOC7 as a top-ranked dependency. THOC7, a highly conserved component of the TREX complex responsible for mRNA processing and nuclear export, was found to be significantly overexpressed in MB patient tumor samples compared to normal cerebellum, with elevated expression correlating with reduced overall survival, indicating potential clinical relevance. Functional depletion of THOC7 using stable shRNA-mediated knockdown in multiple MB cell lines resulted in a profound reduction in cell viability, proliferation, and anchorage-independent growth. Transwell migration assays revealed impaired invasive capacity, and apoptosis was significantly induced, as evidenced by Annexin V/PI staining and cleaved PARP accumulation. Mechanistically, THOC7 suppression led to the accumulation of DNA double-strand breaks, marked by persistent γH2AX foci and S/G2 cell cycle arrest, pointing to activation of DNA damage response pathways. In parallel, we observed reduced expression of stemness-associated transcription factors such as SOX2 and NANOG, suggesting that THOC7 supports MB maintenance by preserving tumor-initiating cell features. To further dissect the molecular consequences of THOC7 loss, we will employ RNA immunoprecipitation sequencing (RIP-seq) to identify transcripts directly bound and regulated by THOC7, potentially uncovering downstream oncogenic pathways dependent on its RNA-binding activity. Altogether, our findings define THOC7 as a multifaceted contributor to MB pathogenesis through regulation of genome integrity and stem-like identity, offering a promising therapeutic target for high-risk MB and informing future precision medicine strategies in pediatric neuro-oncology.