Abstract
Germinal center B-cell-like diffuse large B-cell lymphoma (GCB-DLBCL) depends on the cooperative activity of the histone methyltransferases DOT1L and EZH2 to maintain its proproliferative GCB identity while repressing plasma cell (PC) differentiation. To explore the mechanisms underlying the codependency between DOT1L and EZH2 in GCB-DLBCL, we performed an EZH2 inhibitor (EZH2i)-anchored genome-wide CRISPR interference screen and identified multiple candidate genes encoding components of noncanonical Polycomb repressor complex 1 (ncPRC1), including USP7, KDM2B, RING1, and PCGF1. We identified USP7 (ubiquitin-specific protease 7) as a potential direct target of DOT1L, and its downregulation was associated with increased EZH2i sensitivity in multiple GCB-DLBCL cell lines. Furthermore, we observed that DOT1L influences the composition of chromatin-bound ncPRC1 and, in part, regulates the deposition of H2AK119 monoubiquitination (H2AK119ub1) at gene promoters co-occupied by H3K27 trimethylation (H3K27me3), here defined as PRC1/2 targets. These PRC1/2 targets were specifically enriched in PC signature genes, and their derepression was associated with DOT1L inhibition-mediated loss of H2AK119ub1. This study provides novel insights into the role of DOT1L and its functional codependency with EZH2 in maintaining GCB identity in DLBCL and supports a model in which concurrent reduction of H2AK119ub1 and H3K27me3 promotes differentiation toward an antiproliferative, PC-like state.