Project 2: The cohesin complex as a tumor suppressor in myeloid leukemia

摘要-项目2（Melnick） 大多数B细胞淋巴瘤起源于生发中心（GC）B细胞，其在T细胞依赖性淋巴瘤形成后短暂形成。 抗原刺激GC B细胞经历大量增殖和基因组不稳定性作为副产品发生 免疫球蛋白体细胞超突变，这使他们处于恶性转化的危险之中。的 从静止幼稚B细胞到增殖和不稳定的GC B细胞的表型转变是大量的、快速的， 涉及数千个基因的差异表达。GC B细胞是易逝的，并迅速经历 抗原呈递后终末分化为浆细胞（或经历凋亡）。最常见的B- 细胞淋巴瘤（DLBCL和FL）本质上是GC B细胞持续异常存在并失败 进行终末分化。我们感兴趣的是表型的这些戏剧性变化是如何发生的， 以及这个过程是如何被破坏而导致淋巴瘤的。了解GC的机械基础 我们进行了全基因组染色体构象捕获（Hi-C，4C）沿着ChIP- 在B细胞发育过程中的不同时间点的组蛋白标记、粘附素和TF的seq。我们观察到 GC B细胞中染色体结构的大量变化，包括但不限于i）增加的启动子 连接性，ii）新增强子环的形成，iii）5'至3'基因成环，iv）离散边界的合并 划定基因邻域，形成更大的基因“城市”，导致 彼此正式分离的基因，和v）建立GC B细胞特异性基因座控制区 （LCR），其控制数百种GC B细胞基因增强子（Bunting et. 2016年）。引人注目的是，所有 这些结构变化与内聚体复合物的重新分布密切相关，值得注意的是， 在公共淋巴瘤中观察到复发性体细胞突变或粘附素卸载蛋白PDS 5 B缺失 基因组分析数据集。我们的初步研究表明，PDS 5 B调节参与退出GC的基因 反应和终末分化。在PDS 5 b敲除或点突变小鼠中的初步实验，点突变小鼠 破坏GC动力学和阻断GC出口。基于这些考虑，我们假设， 需要PDS 5 B来卸载GC特异性粘附素分布状态，使得GC B细胞的转录水平降低。 程序可以被消除并允许有利于浆细胞分化的不同配置。我们 预测从GC轻区中的GC T细胞接收的特异性信号直接诱导PDS 5 B依赖性的 粘着蛋白重新分布。我们认为PDS 5 B的遗传损伤导致基因组在结构上变得 卡在GC构型中，从而阻断终末分化所需的表观遗传重编程， 导致恶性转化。我们推测，尽管如此，粘连蛋白阻断可能是可逆的 并且可以被药物靶向，这些药物可以消除GC/淋巴瘤表观遗传编程。因此，该提案将确定 动态粘附素复合物重构在体液免疫应答中的作用及机制 和淋巴瘤发生，并开发新的粘附素治疗方法。

SUMMARY - PROJECT 2 (MELNICK) Most B-cell lymphomas arise from germinal center (GC) B-cells, which form transiently after T-cell dependent antigen stimulation. GC B-cells undergo massive proliferation and genomic instability occurring as a byproduct of immunoglobulin somatic hypermutation, which puts them in danger of malignant transformation. The phenotypic shift from quiescent naïve B-cells to proliferative and unstable GC B-cells is massive, rapid, and involves differential expression of thousands of genes. GC B-cells are evanescent, and quickly undergo terminal differentiation to plasma cells (or undergo apoptosis) after antigen presentation. The most common B- cell lymphomas (DLBCL and FL) in essence are GC B-cells that have continued to aberrantly persist and fail to undergo terminal differentiation. We are interested in how these dramatic changes in phenotypes occur, and how this process can be corrupted to cause lymphoma. To understand the mechanistic basis of the GC B-cell phenotype we performed genome-wide chromosomal conformation capture (Hi-C, 4C) along with ChIP- seq for histone marks, cohesin and TFs at different timepoints during B-cell development. We observed truly massive shifts in chromosomal architecture in GC B-cells including but not limited to i) increased promoter connectivity, ii) formation of novel enhancer loops, iii) 5' to 3' gene looping, iv) merging of discrete boundary delimited gene neighborhoods to form larger gene “cities resulting in de novo epigenetic coordination between genes formally isolated from one another, and v) establishment of GC B-cell specific locus control regions (LCRs) that control hundreds of GC B-cell gene enhancers (Bunting et. al. Immunity 2016). Strikingly, all of these architectural changes were tightly associated with cohesin complex redistribution and notably, we observed recurrent somatic mutation or deletion of the cohesin unloading protein PDS5B in public lymphoma genomic profiling datasets. Our pilot studies suggest that PDS5B regulates genes involved in exiting the GC reaction and terminal differentiation. Preliminary experiments in PDS5b knockout or point mutant mice, point to disruption of GC dynamics and blockade of GC exit. Based on these considerations we hypothesize that PDS5B is required to unload the GC specific cohesin distribution state so that the GC B-cell transcriptional program can be extinguished and allow for a different configuration that favors plasma cell differentiation. We predict that specific signals received from GC T-cells in the GC light zone directly induce PDS5B-dependent cohesin redistribution. We propose that genetic lesions of PDS5B cause the genome to become architecturally stuck in the GC configuration thus blocking epigenetic reprogramming required for terminal differentiation and leading to malignant transformation. We hypothesize that cohesin blockade may be nonetheless reversible and targetable by drugs that can erase GC/lymphoma epigenetic programming. This proposal will thus define the role and mechanism of action of dynamic cohesin complex remodeling in the humoral immune response and lymphomagenesis, and develop novel cohesin therapy approaches.