Abstract
Our previous studies have shown that the epigenetic developmental regulator EZH2 is overexpressed in medulloblastoma (MB). EZH2 is known to repress developmental genes in embryonic stem cells to maintain their stem cell characteristics. To identify the role of EZH2 in medulloblastoma, we overexpressed EZH2 (EZH2Ox) in the C17.2 Myc immortalized cerebellar neural stem cell line. EZH2 overexpression in combination with Myc in the neural progenitor C17.2 cells generated orthotopic tumors that resembled clinical features of MYC-amplified group3 MB. To examine the mechanism of EZH2 mediated tumorigenesis we performed RNA-seq followed by Gene Set Enrichment Analysis (GSEA) of these EZH2 overexpressing neural stem cells. Transcriptomic analysis reveals that the gene sets associated with apoptosis were suppressed by EZH2 overexpression. Specifically, the p53 regulator and pro-apoptotic gene, HIPK2 was significantly suppressed by EZH2 overexpression. To examine the impact of EZH2 on apoptosis in Myc driven medulloblastoma cells we performed a CRISPR knock out of EZH2 in the D458 group 3 MB cell line. EZH2 deletion resulted in increased apoptosis leading to sensitization of cells to chemotherapy and radiation treatments. Further analysis of these CRISPR-edited EZH2 knockout cells showed a predominant upregulation in the HIPK2 expression compared to control D458 cells. While, overexpressing HIPK2 in D458 cells resulted in increased apoptosis after cisplatin or ionizing radiation, depleting HIPK2 in EZH2 CRISPR-KO cells rescued the proapoptotic phenotype. Treating mice with cisplatin resulted in significantly smaller orthotopic xenograft tumors of EZH2 knockout medulloblastoma cells compared to EZH2 sufficient medulloblastoma cells. Together, our results suggest that EZH2 contributes to treatment-resistance by inhibiting apoptosis via transcriptionally modulating HIPK2 expression in group 3 medulloblastoma. Therefore, targeting EZH2 will effectively sensitize tumors to standard therapies.