Abstract 2717: Dual inhibition of PRC2 complex subunits EZH1/2 enhances DNA hypomethylation therapy in colorectal cancer

Abstract Aberrant DNA methylation patterning is an epigenetic hallmark of human cancers. DNA methyltransferase inhibitors (DNMTi) act by reversing these aberrant DNA methylation patterns and are clinically approved to treat hematologic cancers. However, there is limited evidence for clinical efficacy in solid tumors. Our prior work showed that compensatory transcriptional silencing through the Polycomb Repressive Complex 2 (PRC2) emerges following DNMTi to maintain epigenetic silencing of critical tumor suppressor gene networks. This provides mechanistic rationale for combining DNMTi with inhibitors of EZH2 (EZH2i), the most well studied lysine methyltransferase (KMT) subunit of PRC2 that catalyzes all three orders of lysine 27 methylation on histone H3 (H3K27me1/2/3). EZH1 is an interchangeable KMT subunit of the PRC2 complex whose role in cancer is less well studied. A small molecule screen of PRC2 inhibitors revealed that compounds targeting both EZH1 and EZH2 were more effective in combination with DNMTi than those that were selective for EZH2. Integrative proteomics and epigenomics analyses revealed a mechanism in which H3K27me1, supported by EZH1, blocks the activity of the lysine acetyltransferases p300/CBP towards the H3K27 substrate, which is critical for transcriptional activation downstream of DNMTi and EZH1/2i treatment. Collectively, these studies are defining the division of labor between the two KMT subunits of PRC2 in colon cancer cells, uncovering mechanisms of crosstalk between DNA methylation, histone methylation, and histone acetylation, and revealing therapeutic vulnerabilities that we propose are targetable with dual EZH1/2 inhibitors in combination regimens to enhance the clinical efficacy of DNMTi therapy. Citation Format: Ashley K. Wiseman, Ali A. Chomiak, Rochelle Tiedemann, Yanqing Liu, Scott Rothbart. Dual inhibition of PRC2 complex subunits EZH1/2 enhances DNA hypomethylation therapy in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2717.