P13.05.B EZH2 CONTRIBUTES TO THE MALIGNANT BEHAVIOR AND ITS PHARMACOLOGICAL DEGRADATION EFFECTIVELY INHIBITS CELL GROWTH IN GRADE 3 MENINGIOMA

Abstract BACKGROUND Anaplastic (CNS WHO grade 3) meningiomas are aggressive tumors of the meninges, for whom effective systemic treatment options are currently not available. In the past years, comprehensive molecular analyses of these tumors suggested that epigenetic dysregulation plays a fundamental role in their development and progression. These include the overactivation of the histone modifier Polycomb Repressive Complex 2 (PRC2), but the exact pathogenic relevance of its catalytic subunit, the histone methyltransferase Enhancer of Zeste Homolog 2 (EZH2) is still unclear. Therefore, we previously analyzed EZH2 expression in a cohort of CNS WHO grade 3 meningiomas, and found that higher EZH2 expression is related to poor overall and local progression-free survival. In addition, EZH2 expression positively correlates with mitotic and proliferative activity and is higher in tumors harboring homo- or hemizygous 9p21.3 deletion and H3 K27me3 loss. We also showed that the cofactor-competitive EZH2 inhibitor EPZ-6438 effectively inhibits cell growth in the IOMM-Lee and BEN-MEN-1 human meningioma cell lines in vitro, in a concentration-dependent manner in the 10-40 µM range. This effect is mediated by cell cycle block, as well as increased p21-, and decreased FOXM1 expression. In this study, we aimed to analyze the effects of another pharmacological approach, using the EZH2 selective degrader MS-1943. MATERIAL AND METHODS The effects of EPZ-6438 and MS-1943 were tested in vitro on the IOMM-Lee and BEN-MEN-1 human meningioma cell lines in parallel. Cell growth was characterized using the Alamar blue and sulforhodamine B tests. Cell cycle alterations and cell death were analyzed using propidium iodide staining and flow cytometry, while protein expression changes with WES Simple capillary immunoassay. RESULTS MS-1943 exerted a more potent inhibitory effect in both cell lines, in much lower concentration range (1.25-5 µM), then EPZ-6438. Similarly to EPZ-6438, these effects were more prominent in the IOMM-Lee cell line, in which MS-1943 also induced cell death (both necrotic and apoptotic), besides cell cycle block. Similarly to higher concentrations of EPZ-6438, MS-1943 increased p21-, and decreased FOXM1 expression, but also increased p53 expression at 5 µM concentration. CONCLUSION The EZH2 selective degrader MS-1943 exerts more potent effects in two different meningioma cell lines in vitro, then the histone methyltransferase inhibitor EPZ-6438. These findings suggest that non-conventional, histone methylation-independent functions of EZH2 play a fundamental role and maybe more promising targets of potential future therapeutic approaches in these tumors. SUPPORT National Research, Development and Innovation Office grant NKFIH-PD-146549.