EXTH-10. COMBINATION OF EPIGENETIC ENZYME INHIBITORS, GSK-J4 AND BELINOSTAT, REVEALS HIGH EFFICACY IN IDH1 MUTANT GLIOMAS

Mutations in IDH1 and IDH2 genes are common in low grade gliomas and secondary GBM and are known to cause a distinct epigenetic landscape in these tumors. To interrogate the epigenetic vulnerabilities of IDH-mutant gliomas, we performed a chemical screen with inhibitors of chromatin modifiers and identified 5-azacytidine, Chaetocin, GSK-J4 and Belinostat as potent agents against primary IDH1-mutant cell lines. Testing the combinatorial efficacy of these agents, we demonstrated GSK-J4 and Belinostat combination as a very effective treatment for the IDH1-mutant glioma cells. Engineering established cell lines to ectopically express IDH1R132H, we showed that IDH1R132H cells adopted a different transcriptome with changes in stress-related pathways that were reversible with the mutant IDH1 inhibitor, GSK864. The combination of GSK-J4 and Belinostat was highly effective on IDH1R132H cells, but not on wt glioma cells or nonmalignant fibroblasts and astrocytes. The cell death induced by GSK-J4 and Belinostat combination involved the induction of cell cycle arrest and apoptosis. RNA sequencing analyses revealed activation of inflammatory and unfolded protein response pathways in IDH1-mutant cells upon treatment with GSK-J4 and Belinostat conferring increased stress to glioma cells. Specifically, GSK-J4 induced ATF4-mediated integrated stress response and Belinostat induced cell cycle arrest in primary IDH1-mutant glioma cells; which were accompanied by DDIT3/CHOP-dependent upregulation of apoptosis. Moreover, to dissect out the responsible target histone demethylase, we undertook genetic approach and demonstrated that CRISPR/Cas9 mediated ablation of both KDM6A and KDM6B genes phenocopied the effects of GSK-J4 in IDH1-mutant cells. Finally, GSK-J4 and Belinostat combination significantly decreased tumor growth and increased survival in an orthotopic model in mice. Together, these results suggest a potential combination epigenetic therapy against IDH1-mutant gliomas.