MEDB-45. Functional genomics identifies epigenetic regulators as novel therapeutic targets for sonic hedgehog medulloblastoma

Medulloblastoma (MB) is among the most common malignant childhood brain tumors that comprises a group of four molecularly distinct diseases. A significant proportion of these tumors is characterized by aberrant activation of the canonical sonic hedgehog (SHH) signaling pathway. Although small-molecule inhibitors targeting Smoothened (SMO) have proven a promising treatment approach for SHH-MB subgroup, primary or acquired resistance impedes its clinical efficacy. Therefore, novel targeted approaches are urgently needed to improve therapeutic strategies for this tumor entity. Here, we conducted a genome-wide CRISPR/Cas9 knockout screen in a murine and a human SHH-MB cell line, SMB21 and DAOY, respectively, in order to decipher tumor-specific genetic dependencies. Our data demonstrate that SMB21 cells highly depend on positive regulators of the SHH pathway, such as Smo and Gli1 for their survival, as opposed to DAOY cells, suggesting that the latter does not represent a faithful model of SHH-MB. Members of the epigenetic machinery such as Dnmt1 and Smarca5 scored strongly as SMB21-context specific essentialities. Pharmacologically, we show that DNMT1 inhibition is efficacious at clinically relevant concentrations against SMO inhibitor- sensitive, as well as resistant SHH-MB cell lines, indicating novel therapeutic avenues for SHH-MB. By performing RNA sequencing of SMB21 cells, we identified early and late changes in global gene expression induced by DNMT1 inhibition, including decreased expression of mediators of SHH signaling, such as Gli1 and Gli2. Of note, gene set enrichment analysis revealed that DNMT1 inhibition downregulates top gene sets associated with cell cycle progression, corroborating the screening results that Dnmt1 is essential for SMB21 proliferation. Further global DNA methylation profiling in SMB cells will help to define the molecular basis of sensitivity to DNMT1 inhibitors in SHH-MB. Summarizing, our data highlight the potential of inhibitors targeting epigenetic regulators in SMO inhibitor- sensitive and resistant MB for more efficacious treatment options.