EPCO-35. IDENTIFYING NOVEL TUMORIGENIC MECHANISMS UNDERLYING GROUP 3/4 MEDULLOBLASTOMA USING HARMONIZED AND RHOMBIC LIP-ENRICHED CEREBELLAR ATLASES

Abstract Medulloblastoma (MB) is a malignant tumor of the cerebellum comprised of biologically distinct molecular subgroups. Among consensus subgroups, Group 3/4-MB tumors are the most molecularly and clinically heterogeneous. Using single-cell transcriptional atlases of cerebellar development, the origins of Group 3/4-MB have been linked to the early rhombic lip (RL), particularly mitotic unipolar brush cell (UBC) progenitors that emerge from this germinal zone. However, this lineage is underrepresented in existing fetal cerebellar atlases due to limited temporal coverage and sparse sampling, hampering deeper understanding of the developmental determinants underpinning Group 3/4-MB tumorigenesis. To overcome this challenge, we harmonized six published single-cell and single-nucleus RNA-seq datasets of the developing human cerebellum ranging from Carnegie Stage 18 to 25 post-conception weeks. Additionally, we generated a novel single-nucleus transcriptomic and epigenomic dataset using posterior cerebellar dissections designed to enrich RL populations. With ~500,000 cells collected from 26 developmental timepoints from different molecular modalities, the harmonized and RL-enriched fetal cerebellar datasets provide rich cellular composition, spatial localization, and gene regulatory network data for early cerebellar development. Comparative analysis with MB samples confirmed transcriptional similarity of Group 3/4-MB with progenitors from the RL-UBC lineage. Single-cell transcriptome comparisons between malignant MB cells and matched developmental origin uncovered novel, tumor-specific programs, such as prolactin signaling independent from MYC amplification, suggesting a role in tumorigenesis, warranting further experimental validation. This study delivers a multi-modal characterization of the developing human cerebellum, expanding on available timepoints and focusing on anatomical regions and cell types relevant to Group 3/4-MB developmental biology. These datasets, which we have made publicly accessible in a user-friendly data portal, provide a robust reference that will serve as a valuable resource for studying cerebellar development and MB pathogenesis.