Role of MEF2B mutations in lymphomagenesis

描述(申请人提供)：这项研究提案的总体目标是从功能上表征MEF2B，这是一种转录因子，最近被确认为生发中心(GC)反应的主要调节因子，其基因组位置通过激活弥漫性大B细胞淋巴瘤(DLBCL)和滤泡性淋巴瘤(FL)这两种最常见的人类淋巴瘤的突变而成为靶点。MEF2B基因编码一种转录激活因子，在~11%的弥漫性大B细胞淋巴瘤(DLBCL)和~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，这是DLBCL增殖所必需的。MEF2B突变通过破坏MEF2B与共抑制子CABIN1的相互作用，或者使其对PKA介导的磷酸化和SUMO基化的抑制效应不敏感，从而增强MEF2B的转录活性。因此，在含有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发育和淋巴肿大的调节器的作用。具体而言，我们将追求以下具体目标：1)通过分析携带MEF2B条件性、GC特异性失活的小鼠的表型，通过CHIP-SEQ鉴定MEF2B靶基因，以及通过质谱学分析调节GC B细胞中MEF2B活性的蛋白质，来确定MEF2B在正常GC发育中的作用；2)从功能上表征在DLBCL和FL中发现的MEF2B突变，包括影响转录共抑制物结合的N端错义突变和通过磷酸化和/或总和作用影响其负调控的C端截断突变；3)通过分析转基因小鼠有条件地激活GC B细胞中突变的MEF2B等位基因，或与DLBCL和FL中共存的其他突变相结合，来确定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.