Role of MEF2B mutations in lymphomagenesis

描述（由申请人提供）：本研究计划的总体目标是功能表征MEF2B， MEF2B是一种转录因子，最近被确定为生发中心（GC）反应的主要调节因子，其基因组位点在弥漫性大B细胞淋巴瘤（DLBCL）和滤泡性淋巴瘤（FL）中激活突变，这两种最常见的人类淋巴瘤。MEF2B基因编码一种转录激活因子，在约11%的弥漫性大b细胞淋巴瘤（DLBCL）和约12%的滤泡性淋巴瘤中发现突变。我们已经证明MEF2B直接激活正常生发中心（GC） B细胞中原癌基因BCL6的转录，并且是DLBCL增殖所必需的。MEF2B突变通过破坏其与协同抑制因子CABIN1的相互作用或使其对pka介导的磷酸化和sumoylation的抑制作用不敏感来增强MEF2B的转录活性。因此，在携带MEF2B突变的DLBCL中，BCL6的转录活性被解除调控。基于这些初步结果，本研究计划旨在研究MEF2B作为GC发展和淋巴瘤发生调节剂的作用。具体而言，将追求以下具体目标：1)通过分析MEF2B条件GC特异性失活小鼠的表型来确定MEF2B在正常GC发育中的作用；ii)通过ChIP-seq鉴定MEF2B靶基因；iii)通过质谱分析确定GC B细胞中调节MEF2B活性的蛋白质；2)在DLBCL和FL中鉴定的MEF2B突变的功能特征，包括影响转录共抑制因子结合的n端错义突变和通过磷酸化和/或sumo化影响其负调控的C端截断突变；3)通过分析GC B细胞中MEF2B突变等位基因条件激活的转基因小鼠，单独或与DLBCL和FL中共存的其他突变联合，确定MEF2B突变在体内的后果。

DESCRIPTION (provided by applicant): The overall goal of this research proposal is to functionally characterize MEF2B, a transcription factor that has been recently identified as a master regulator of the germinal center (GC) reaction and whose genomic locus is targeted by activating mutations in Diffuse Large B Cell Lymphoma (DLBCL) and Follicular Lymphoma (FL), the two most common forms of human lymphoma. The MEF2B gene encodes a transcriptional activator and is found mutated in ~11% of diffuse large B-cell lymphomas (DLBCL) and ~12% of follicular lymphomas. We have shown that MEF2B directly activates the transcription of the proto-oncogene BCL6 in normal germinal center (GC) 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 the inhibitory effects f PKA-mediated phosphorylation and sumoylation. Consequently, BCL6 transcriptional activity is deregulated in DLBCL harboring MEF2B mutations. Based on these initial results, this research proposal aims at investigating the role of MEF2B as modulator of GC development and lymphomagenesis. In particular, the following specific aims will be pursued: 1) determine the role of MEF2B in normal GC development by i) analyzing the phenotype of mice carrying conditional, GC-specific inactivation of MEF2B, ii) identifying the MEF2B target genes by ChIP-seq, and iii) the proteins that regulate MEF2B activity in GC B cells by mass spectrometry analysis; 2) functionally characterize the MEF2B mutations identified in DLBCL and FL, including both the N-terminus missense mutants that affect transcriptional co-repressor binding and the C- terminus truncating mutations that affect its negative regulation by phosphorylation and/or sumoylation; 3) determine the consequences of MEF2B mutations in vivo by analyzing transgenic mice with conditional activation of mutant MEF2B alleles in GC B cells, alone or in combination with other mutations co-existing in DLBCL and FL.