Abstract 3920: Differential effects of Jumonji family demethylase and EZH2 H3K27 methyltransferase inhibition on radiation sensitivity of H3K27M mutant DIPG

Abstract

Abstract Diffuse intrinsic pontine glioma (DIPG) is an aggressive pediatric brainstem tumor with a 5-year survival of <1% (1). Up to 80% of DIPG tumors contain a specific K27M mutation in one of two genes encoding histone H3 (H3K27M). 60-75% of H3K27M mutations occur in gene encoding histone variant H3.3, which has been linked to reduced overall survival and poor response to first line radiation therapy (1). Recent evidence indicates that H3K27M inhibits the enzymatic activity of Polycomb Repressive Complex 2 through interaction with the EZH2 histone methyl transferase (2). A specific inhibitor of Jumonji family histone demethylase GSK-J4 was recently reported to restore H3K27 tri-methylation patterns in human DIPG cells and improve survival of H3K27M mutant orthotopic xenograft brainstem tumor models (3). Furthermore, a genetic deletion of EZH2 had also been shown to improve survival of H3K27M mouse model of DIPG, suggesting that EZH2 may be a potential therapeutic target (4). Given these data, we investigated the effects of Jumonji demethylase inhibitor GSK-J4 and EZH2 inhibitor tazemetostat on radiosensitization of human H3K27M DIPG cells. We found that GSK-J4 inhibitor had a growth inhibitory effect on H3K27M mutant DIPG cells as a single agent (96% growth inhibition at 1.5 micro-molar concentration, P<0.0001). Strikingly, GSK-J4 inhibitor also displayed radiosensitizing effects when combined with ionizing radiation at sub-micromolar concentrations of the drug (34.4% growth inhibition at 0.37 micro-molar concentration when combined with 4 Gy XRT vs 23.3% with XRT alone, P<0.0001 for both; no significant growth inhibition as single agent). Contrary to previously reported results with EZH2 inhibition in H3K27M neural stem cell model of DIPG, EZH2 inhibitor tazemetostat had no effect on human H3K27M DIPG cells as a single agent or when combined with ionizing radiation. This result is, however, not unexpected, given the proposed inhibitory effect of H3K27M mutation on EZH2 activity in DIPG disease pathogenesis. In conclusion, Jumonji family methyltransferase inhibitor GSK-4J sensitizes H3K27M DIPG cells to ionizing radiation, while EZH2 methyltransferase inhibitor has no effect. This result appears to be consistent with the recently proposed epigenetic mechanism of DIPG pathogenesis (1, 2). 1. Johung TB, et al. Diffuse intrinsic pontine glioma: new pathophysiological insights and emerging therapeutic targets. Curr Neuropharmacol. 2017;15:88-97. 2. Lewis PW, et al. Inhibition of PRC2 activity by a gain-of-function H3 mutation found in pediatric glioblastoma. Science. 2013;340:857-61. 3. Hashizume R, et al. Pharmacologic inhibition of histone demethylation as a therapy for pediatric brainstem glioma. Nat Med. 2014;20:1394-6. 4. Mohammad F, et al. EZH2 is a potential therapeutic target for H3K27M-mutant pediatric gliomas. Nat Med. 2017;23:483-492. Citation Format: Anatoly Y. Nikolaev, John B. Fiveash, Eddy S. Yang. Differential effects of Jumonji family demethylase and EZH2 H3K27 methyltransferase inhibition on radiation sensitivity of H3K27M mutant DIPG [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3920.