MDB-09. IMPACT OF PRDM6 ON CHROMATIN ACCESSIBILITY, GENE EXPRESSION, AND MEDULLOBLASTOMA FORMATION

Abstract Group 4 medulloblastoma (MB) is the most common medulloblastoma subgroup and shows high incidence of metastasis and late-onset relapse cases. These tumors lack a unifying oncogenic driver and treatment targets, despite extensive genomic characterization. Group 4 MBs are characterized by recurrent genetic alterations in chromatin modifiers and amplification of stemness genes. A substantial fraction of Group 4 MBs are characterized by enhancer hijacking through tandem duplication of SNCAIP, resulting in high expression of PRDM6, a putative transcriptional repressor and histone methyltransferase. Some PRDM6-overexpressing medulloblastoma cases show additional mutations in chromatin regulators and high MYCN expression. We set out to elucidate the impact and oncogenic potential of sustained PRDM6 expression in early neural stem cell populations and patient-derived medulloblastoma cells. We find that PRDM6 expression in human iPSC-derived neuroepithelial stem cells (NESCs) results in tumor growth in mice at low penetrance. Moreover, we find that PRDM6 expression in MYCN-overexpressing NESCs does not further alter tumor aggressiveness or survival in vivo. Notably, PRDM6 overexpression in the patient-derived Group 3 MYC-amplified D283-Med cell line causes significantly increased aggressiveness of tumor growth and shorter survival in mice. At the cellular level, PRDM6 localizes to the nucleus, suggesting a role in gene expression regulation. Consistent with this notion, ATAC-seq and RNA-seq analysis of PRDM6-NESCs and PRDM6-D283-Med cells reveals major changes in chromatin accessibility and gene expression, specifically upregulation of integrin family members and gene family members of focal adhesion and extracellular matrix receptor interaction pathways. We conclude that PRDM6 promotes tumor growth in NESCs and may play a role in tumor maintenance through interactions with the extracellular matrix and tumor microenvironment.