EZH2 targeting in medulloblastoma reduces tumor growth through epigenetic reactivation of the BAI1/p53 tumor suppressor pathway

Medulloblastoma is a malignant pediatric tumor for which new therapies are urgently needed. We demonstrate that treatment with EPZ-6438 (Tazemetostat), an enhancer of zeste homologue 2 (EZH2) inhibitor approved for clinical trials, blocks MB cell growth in vitro and in vivo, and prolongs survival in orthotopic xenograft models. We show that the therapeutic effect is dependent on epigenetic reactivation of adhesion G protein-coupled receptor B1 (ADGRB1), a tumor suppressor gene that controls p53 stability by blocking Mdm2. Histone 3 trimethylated on lysine 27 (H3K27me3), a marker of silent chromatin conformation is present at the ADGRB1 promoter, and inhibition of EZH2, the catalytic component of the Polycomb Repressive complex 2 (PRC2) that methylates H3K27, switches the gene into an active chromatin status and reactivates BAI1 expression. Mechanistically, targeting EZH2 promotes transition from H3K27me3 to H3K27ac at the promoter, recruits the C/EBPβ (CREB-binding protein) and CBP transcription factors and activates ADGRB1 transcription. Taken together, our results identified key molecular players that regulate ADGRB1 gene expression in MB, demonstrate that reactivation of BAI1 expression underlies EPZ-6438 anti-tumorigenic action, and provide pre-clinical proof-of-principle evidence for targeting EZH2 in patients with MB.