MDB-09. Interrogating the role of histone methyltransferases or epigenetic regulators in MYC-driven medulloblastoma

Abstract Medulloblastoma (MB) is the most common malignant pediatric brain tumor of the cerebellum, with four distinct molecular subgroups, each carrying different clinical prognoses. Group 3 (G3) is the most aggressive subgroup and is associated with amplification of the MYC gene. MYC expression and function are controlled in part by epigenetic regulation, and interestingly, several genetic alterations in genes encoding epigenetic regulators are found in G3 medulloblastoma (G3MB). Surprisingly, many tumors have no mutations raising the question as to whether non-mutated or non-amplified epigenetic regulators may be drivers of G3MB tumorigenesis. To identify novel epigenetic drivers in G3MB, we performed an unbiased shRNA screen of 243 known chromatin regulators. The most intriguing candidate was SMYD3, a multidomain-containing protein with histone H4-lysine 5 (H4K5) methyltransferase activity. SMYD3 was not required for tumor initiation, but instead for tumor progression. Indeed, knockdown of Smyd3 suppressed G3MB proliferation in vitro and tumor progression in vivo in mouse G3MB. CRISPR-Cas9-mediated deletion of SMYD3 suppressed tumor progression of human G3MB patient-derived orthotopic xenograft (PDOX) as well as MYC, E2F, G2/M, and epithelial-to-mesenchymal transition’s target gene expression. To unravel the role of SMYD3 in G3MB tumorigenesis, we performed multi-omics analysis including CUT&RUN sequencing, ATAC-seq, RNA-seq and proteomics. Our results highlight the role of a potentially targetable new epigenetic regulator of tumorigenesis in G3MB for which inhibitors have been developed.