Mutations in the gene encoding the KMT2D (also called MLL2) methyltransferase are highly recurrent and occur early during tumorigenesis in diffuse large B cell lymphoma (DLBCL) and follicular lymphoma (FL). However, the functional consequences of KMT2D mutations and their role in lymphomagenesis are unknown. Here we show that FL/DLBCL-associated KMT2D mutations impair KMT2D enzymatic activity, leading to diminished global H3K4 methylation in germinal-center (GC) B-cells and DLBCL cells. Conditional deletion of Kmt2d early during B cell development, but not after initiation of the GC reaction, results in an increase in GC B-cells and enhances B cell proliferation in mice. In mice overexpressing BCL2, which develop GC-derived lymphomas resembling human tumors, genetic ablation of Kmt2d leads to a further increase in tumor incidence. These findings suggest that KMT2D acts as a tumor suppressor gene whose early loss facilitates lymphomagenesis by remodeling the epigenetic landscape of the cancer precursor cells. Eradication of KMT2D-deficient cells may represent a rational therapeutic approach for targeting early tumorigenic events.

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Inactivating mutations of the CREBBP acetyltransferase are highly frequent in diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL), the two most common germinal-center (GC) derived cancers. However, the role of CREBBP inactivation in lymphomagenesis remains unclear. Here we show that CREBBP regulates enhancer/super-enhancer networks with central roles in GC/post-GC cell fate decisions, including genes involved in signal transduction by the B-cell receptor and CD40 receptor, transcriptional control of GC and plasma cell development, and antigen presentation. Consistently, Crebbp-deficient B-cells exhibit enhanced response to mitogenic stimuli and perturbed plasma cell differentiation. While GC-specific loss of Crebbp was insufficient to initiate malignant transformation, compound Crebbp-haploinsufficient/BCL2-transgenic mice, mimicking the genetics of FL and DLBCL, develop clonal lymphomas recapitulating the features of the human diseases. These findings establish CREBBP as a haploinsufficient tumor suppressor gene in GC B-cells and provide insights into the mechanisms by which its loss contributes to lymphomagenesis.

The MEF2B gene encodes a transcriptional activator and is found mutated in ∼11% of diffuse large B cell lymphomas (DLBCLs) and ∼12% of follicular lymphomas. Here, we show that MEF2B directly activates the transcription of the proto-oncogene BCL6 in normal germinal-center B cells and is required for DLBCL proliferation. MEF2B mutations enhance MEF2B transcriptional activity either by disrupting its interaction with the co-repressor CABIN1, or by rendering it insensitive to phosphorylation- and sumoylation-mediated inhibitory signaling events. Consequently, Bcl-6 transcriptional activity is deregulated in DLBCL harboring MEF2B mutations. Thus, somatic mutations of MEF2B may contribute to lymphomagenesis by deregulating the expression of the BCL6 oncogene, and MEF2B may represent an alternative target to block Bcl-6 activity in DLBCLs.