The EZH2 histone methyltransferase is highly expressed in germinal center (GC) B-cells and targeted by somatic mutations in B-cell lymphomas. Here we find that EZH2 deletion or pharmacologic inhibition suppresses GC formation and functions in mice. EZH2 represses proliferation checkpoint genes and helps establish bivalent chromatin domains at key regulatory loci to transiently suppress GC B-cell differentiation. Somatic mutations reinforce these physiological effects through enhanced silencing of EZH2 targets in B-cells, and in human B-cell lymphomas. Conditional expression of mutant EZH2 in mice induces GC hyperplasia and accelerated lymphomagenesis in cooperation with BCL2. GCB-type DLBCLs are mostly addicted to EZH2, regardless of mutation status, but not the more differentiated ABC-type DLBCLs, thus clarifying the therapeutic scope of EZH2 targeting.

The lysine-specific histone methyltransferase KMT2D has emerged as one of the most frequently mutated genes in follicular lymphoma (FL) and diffuse large B cell lymphoma (DLBCL). However, the biological consequences of KMT2D mutations on lymphoma development are not known. Here we show that KMT2D functions as a bona fide tumor suppressor and that its genetic ablation in B cells promotes lymphoma development in mice. KMT2D deficiency also delays germinal center (GC) involution, impedes B cell differentiation and class switch recombination (CSR). Integrative genomic analyses indicate that KMT2D affects H3K4 methylation and expression of a specific set of genes including those in the CD40, JAK-STAT, Toll-like receptor, and B cell receptor pathways. Notably, other KMT2D target genes include frequently mutated tumor suppressor genes such as TNFAIP3, SOCS3, and TNFRSF14. Therefore, KMT2D mutations may promote malignant outgrowth by perturbing the expression of tumor suppressor genes that control B cell activating pathways.

Somatic mutations in CREBBP occur frequently in B-cell lymphoma. Here, we show that loss of CREBBP facilitates development of germinal center derived lymphomas in mice. In both human and murine lymphomas CREBBP loss of function resulted in focal depletion of enhancer H3K27 acetylation and aberrant transcriptional silencing of genes that regulate B-cell signaling and immune responses including class II MHC. Mechanistically, CREBBP regulated enhancers are counter-regulated by the BCL6 transcriptional repressor in a complex with SMRT and HDAC3, which we find bind extensively to MHC class II loci. HDAC3 loss of function rescued repression of these enhancers and corresponding genes including MHC class II, and more profoundly suppress CREBPP mutant lymphomas in vitro and in vivo. Hence CREBBP loss of function contributes to lymphomagenesis by enabling unopposed suppression of enhancers by BCL6/SMRT/HDAC3 complexes, suggesting HDAC3 targeted therapy as a precision approach for CREBBP mutant lymphomas.

SUMMARY The BCL6 transcriptional repressor is required for development of germinal center (GC) B-cells and diffuse large B-cell lymphomas (DLBCL). Although BCL6 can recruit multiple corepressors, its transcriptional repression mechanism of action in normal and malignant B-cells is unknown. We find that in B-cells, BCL6 mostly functions through two independent mechanisms that are collectively essential to GC formation and DLBCL, both mediated through its N-terminal BTB domain. These are: i) formation of a unique ternary BCOR-SMRT complex at promoters with each corepressor binding to symmetrical sites on BCL6 homodimers, linked to specific epigenetic chromatin features, and ii) the “toggling” of active enhancers to a poised but not erased conformation through SMRT-dependent H3K27 de-acetylation, which is mediated by HDAC3 and opposed by p300 histone acetyltransferase. Dynamic toggling of enhancers provides a basis for B-cells to undergo rapid transcriptional and phenotypic changes in response to signaling or environmental cues.

Purpose The MURANO study demonstrated significant progression-free survival (PFS) benefit for fixed-duration venetoclax-rituximab compared with bendamustine-rituximab in relapsed/refractory chronic lymphocytic leukemia. With all patients off treatment, we report minimal residual disease (MRD) kinetics and updated outcomes. Methods Patients were randomly assigned to 2 years of venetoclax plus rituximab during the first six cycles, or six cycles of bendamustine-rituximab. Primary end point was PFS. Safety and peripheral blood (PB) MRD status—at cycle 4, 2 to 3 months after end of combination therapy (EOCT), and every 3 to 6 months thereafter—were secondary end points. Results Of 194 patients, 174 (90%) completed the venetoclax-rituximab phase and 130 (67%) completed 2 years of venetoclax. With a median follow-up of 36 months, PFS and overall survival remain superior to bendamustine-rituximab (hazard ratio, 0.16 [95% CI, 0.12 to 0.23]; and hazard ratio, 0.50 [95% CI, 0.30 to 0.85], respectively). Patients who received venetoclax-rituximab achieved a higher rate of PB undetectable MRD (uMRD; less than 10−4) at EOCT (62% v 13%) with superiority sustained through month 24 (end of therapy). Overall, uMRD status at EOCT predicted longer PFS. Among those with detectable MRD, low-level MRD (10−4 to less than 10−2) predicted improved PFS compared with high-level MRD (10−2 or greater). At a median of 9.9 months (range, 1.4 to 22.5 months) after completing fixed-duration venetoclax-rituximab, overall only 12% (16 of 130) of patients developed disease progression (11 high-level MRD, three low-level MRD). At the end of therapy, 70% and 98% of patients with uMRD remained in uMRD and without disease progression, respectively. Conclusion With all patients having finished treatment, continued benefit was observed for venetoclax-rituximab compared with bendamustine-rituximab. uMRD rates were durable and predicted longer PFS, which establishes the impact of PB MRD on the benefit of fixed-duration, venetoclax-containing treatment. Low conversion to detectable MRD and sustained PFS after completion of 2 years of venetoclax-rituximab demonstrate the feasibility of this regimen.