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.

No abstract

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.

The gene encoding the MEF2B transcription factor is mutated in germinal center (GC)-derived B cell lymphomas, but its role in GC development and lymphomagenesis is unknown. We demonstrate that Mef2b deletion reduces GC formation in mice and identify MEF2B transcriptional targets in GC, with roles in cell proliferation, apoptosis, GC confinement, and differentiation. The most common lymphoma-associated MEF2B mutant (MEF2B) is hypomorphic, yet escapes binding and negative regulation by components of the HUCA complex and class IIa HDACs. Mef2b expression in mice leads to GC enlargement and lymphoma development, a phenotype that becomes fully penetrant in combination with BCL2 de-regulation, an event associated with human MEF2B mutations. These results identify MEF2B as a critical GC regulator and a driver oncogene in lymphomagenesis.

Summary B lymphoblastic leukaemia (B‐ALL) cells are characterized by the expression of various B‐cell antigens. Expression of T/Natural Killer‐cell antigens, however, has rarely been reported in B‐ALL (TAg+ B‐ALL), and the significance of this aberrant antigen expression is unclear. We thus analysed the frequency of TAg+ B‐ALL at our institution and investigated its significance in the context of immunophenotypes, cytogenetic/molecular findings, and prognosis. We reviewed 134 consecutive cases of B‐ALL and found 18 cases (13·4%) of TAg+ B‐ALL. The most common aberrant T‐cell antigens expressed were CD2, CD5, and CD7 at equivalent rates (each in six cases), CD4 (two cases), and CD56 (three cases). Adverse cytogenetic abnormalities were seen in a significantly larger proportion of the TAg+ cases (72·2%) than the TAg− cases (32·2%; P = 0·003). Multivariate Cox‐regression analysis showed that the risk of relapse over time was higher in the TAg+ cases, independent of high risk status (based on age and white blood cell count) and the presence of adverse cytogenetic abnormalities (hazard ratio = 2·256, P = 0·065). These findings suggest that T‐cell antigen expression in B‐ALL may be an independent predictor of poor prognosis, and a useful marker to identify patients at increased risk for relapse and for harbouring adverse cytogenetic abnormalities.

AimsRefractory coeliac disease type II (RCDII), a rare complication of coeliac disease (CD) associated with high morbidity, requires identification of a clonal population of phenotypically aberrant intraepithelial lymphocytes (IELs) for diagnosis. However, data regarding the frequency and significance of clonal T cell receptor (TCR) gene rearrangements (TCR-GRs) in small bowel (SB) biopsies of patients without RCDII are limited.MethodsWe analysed results of TCR-GR analyses performed on SB biopsies at our institution over a 3-year period, which were obtained from eight active CD, 172 CD on gluten-free diet (GFD), 33 RCDI, and three RCDII patients and 14 patients without CD. TCR-GR patterns were divided into clonal, polyclonal and prominent clonal peaks (PCPs) and these patterns were correlated with clinical and pathological features.ResultsClonal TCR-GR products were detected in biopsies from 67% of patients with RCDII, 17% of patients with RCDI and 6% of patients with GFD. PCPs were observed in all disease phases (range 12%–33%). There was no significant difference in the TCR-GR patterns between the non-RCDII disease categories (p=0.39). A higher frequency of surface CD3(−) IELs was noted in cases with clonal TCR-GR, but the PCP pattern did not show associations with any clinical or pathological feature. Persistence of clonal or PCP pattern on repeat biopsy was seen for up to 2 years without evidence of RCDII.ConclusionsClonal TCR-GRs are not infrequent in cases lacking features of RCDII, while PCPs are frequent in all disease phases. TCR-GR results should be assessed in conjunction with immunophenotypic, histological and clinical findings for appropriate diagnosis and classification of RCD.

1983. The resistance of C57BL/6 mice to subcutaneous infection with Mycobacterium lepraemurium is dependent both on T-cells and other cells of the bone marrow origin.