Role of MEF2B mutations in lymphomagenesis

描述（由申请方提供）：本研究提案的总体目标是对MEF 2 B进行功能表征，MEF 2 B是一种转录因子，最近被鉴定为生发中心（GC）反应的主调节因子，其基因组位点是弥漫性大B细胞淋巴瘤（DLBCL）和滤泡性淋巴瘤（FL）（两种最常见的人类淋巴瘤形式）中激活突变的靶点。MEF 2B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，MEF 2 B直接激活正常生发中心（GC）B细胞中原癌基因BCL 6的转录，并且是DLBCL增殖所需的。MEF 2B突变通过破坏其与辅阻遏物CABIN 1的相互作用或使其对PKA介导的磷酸化和类小泛素化的抑制作用不敏感来增强MEF 2B转录活性。因此，BCL 6转录活性在携带MEF 2B突变的DLBCL中失调。基于这些初步结果，本研究的目的是调查MEF 2B作为GC发展和淋巴瘤发生的调节剂的作用。1）通过i）分析携带MEF 2 B的条件性、GC特异性失活的小鼠的表型，ii）通过ChIP-seq鉴定MEF 2 B靶基因，和iii）通过质谱分析调节GC B细胞中MEF 2 B活性的蛋白质，确定MEF 2 B在正常GC发展中的作用; 2）在功能上表征DLBCL和FL中鉴定的MEF 2B突变，包括影响转录辅阻遏物结合的N-末端错义突变体和影响其通过磷酸化和/或类小泛素化的负调节的C-末端截短突变; 3）通过分析GC B细胞中突变MEF 2 B等位基因的条件性激活的转基因小鼠，单独或与DLBCL和FL中共存的其他突变组合，确定MEF 2 B突变的体内后果。

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.