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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.

Interleukin-17 (IL-17)-producing CD4M ore than 350 million people worldwide suffer from persistent infection with hepatitis B virus (HBV) and are at risk for developing liver cirrhosis and hepatocellular carcinoma. 1 HBV itself is noncytopathic, but immune-mediated liver damage often occurs in patients with both acute and chronic HBV infection. Such damage has conventionally been attributed to killing of infected hepatocytes by virus-specific cytotoxic CD8 ϩ T cells. [2][3][4] Increasing evidence, however, suggests that non-HBV-specific inflammatory infiltration into the liver is likely responsible for hepatic pathology in patients with chronic hepatitis B (CHB). 5,6 For example, in HBV infection activated HBV-specific CD8 ϩ T cells are often present at high levels in the livers of patients without evident liver inflammation; by contrast, nonantigen-specific lymphocytes were found to be massively infiltrated into the livers of patients with hepatic inflammation. 7 An HBV transgenic mouse model further reinforced the concept that liver inflammation initiated by virus-specific CD8 ϩ T cells is amplified by other lymphocytes. 4,8 Indeed, a large number of immune cells, including myeloid dendritic cells (mDCs), plasmacytoid dendritic cells, and FoxP3-positive regulatory T cells can