MDB-29. MEDULLOBLASTOMA SUBGROUP-SPECIFIC CHROMATIN STATES REVEAL NOVEL THERAPEUTIC VULNERABILITIES

Medulloblastoma (MB) is a highly malignant childhood brain tumor comprising a collection of molecularly and clinically distinct entities. Mutations in chromatin-modifying genes distribute across subgroups and account for nearly half of all MBs. However, the molecular consequences of chromatin modifier alterations remain poorly understood. Herein, we characterize the chromatin landscape of fifty-two primary MBs representing each MB subgroup using chromatin immunoprecipitation followed by sequencing (ChIP-seq) for six histone marks and perform multi-layered integration with somatic genetics, genome-wide DNA methylation, and transcriptomic datasets extracted from the same tumors. Analysis of differential chromatin states across MB subgroups identified significant enrichment of a bivalent enhancer state (EnhBiv) in Group 4 MB. Group 4-specific EnhBivs mapped to promoters of key neurodevelopmental genes and transcription factors and was associated with increased DNA methylation and reduced expression. In accordance with higher EnhBivs coverage in Group 4 MB, we found that KDM2B, a H3K4me3 and H3K36me2 demethylase, is highly expressed in Group 4 MBs and patient-derived xenografts (PDXs). KDM2B knockout mediated by CRISPR targeting suppressed growth of both Group 3 MB cell lines in vitro and Group 4 MB PDXs in vivo but had no effect on control SHH-MB cells, suggesting that KDM2B is a novel selective dependency in MB. Knockout of KDM2B in MB cells induced profound upregulation of PRC2 targets, genes regulated by bivalent histone marks, as well as genes involved in neuronal differentiation. Further biochemical and molecular studies demonstrated that KDM2B interacts with PRC2 complex subunits and occupies gene promoters associated with Group 4-specific EnhBiv chromatin. In summary, our comprehensive characterization of the MB chromatin landscape in a large cohort of primary tumors provides novel insights into the epigenetic basis of MB and implicates KDM2B as a novel Group 4 MB dependency that should be prioritized as a therapeutic target in this subgroup.