YY1-dependent chromatin structure stabilization of B lineage commitment

在B细胞谱系承诺期间，基因在转录受限和 前-PRO-B细胞向PRO-B细胞转变时转录允许的间隔导致激活 B世系计划和对替代世系计划的压制。而B血统承诺是 通常认为是由谱系特异的转录因子驱动的，我们做出了令人惊讶的发现 发现普遍存在的转录因子YY1的条件性敲除会导致B系的丧失 承诺，允许随后在体外和体内发育为T细胞谱系。拓荒者 EBF1等转录因子促进B系基因转录并抑制其表达 替代谱系基因启动B谱系承诺，但稳定的承诺需要改变 原B细胞期的染色质结构。因为YY1是控制谱系特异性基因的关键因素 调节长程染色质相互作用(LRCI)，我们假设在Pro-B细胞中YY1基因敲除 导致稳定维持B系特异性基因表达的染色质LRCI的丢失。一致 在此基础上，我们发现YY1基因敲除后B系转录产物减少。YY1也可以调解 Polycomb Group(PcG)抑制，我们发现YY1基因敲除导致表达增加 替代的谱系基因，表明YY1缺失废除了抑制染色质结构 以防止这些基因的表达。因此，我们假设在Pro-B细胞中YY1基因被敲除 导致稳定维持谱系特异性基因表达的染色质结构丢失，以及丢失 抑制染色质结构，以防止另一种谱系基因表达，从而导致 为失去的B血统承诺。为了测试这一点，我们将确定染色质折叠模式，核 野生型和YY1缺失型的关键基因定位、染色质可及性和表观遗传结构 以确定在B谱系承诺过程中受YY1调控的基因组结构。至 确定YY1的类似效应是否在T谱系中起作用，我们将确定YY1是否丢失 促进YY1缺失的DN3细胞的谱系可塑性。YY1在前B细胞中也是免疫球蛋白K基因所必需的 远端VK基因重排所需的收缩。有人认为YY1扮演着一种 结构在调节染色质结构中的作用，但目前尚不清楚这是否需要YY1转录 激活、PcG或自关联功能。我们将利用一个已建立的YY1突变体小组 在这些功能的折衷下并行评估，YY1调控的机制 B血统承诺所需的染色质结构，以及IgK基因座收缩和 JK-VK重排。由于YY1参与了多种组织类型的胚胎发生和发育， 确定YY1如何控制基因组结构以指定B血统承诺将提供一个新的 无处不在的因子在特定世系发育中的作用范例。

During B cell lineage commitment, a dynamic shift of genes between transcriptionally restricted and transcriptionally permissive compartments at the pre-pro-B to pro-B cell transition results in activation of the B lineage program and repression of alternative lineage programs. While B lineage commitment is generally believed to be driven by lineage-specific transcription factors, we have made the surprising discovery that conditional knock-out of the ubiquitous transcription factor YY1 results in loss of B lineage commitment, allowing subsequent development into the T cell lineage both in vitro and in vivo. Pioneer transcription factors such as Ebf1 promote transcription of B lineages genes and repress expression of alternative lineage genes to initiate B lineage commitment, but stable commitment requires changes in chromatin structures at the pro-B cell stage. As YY1 is a key factor controlling lineage-specific gene regulatory long-range chromatin interactions (LRCIs), we hypothesize that YY1 knock-out in pro-B cells results in loss of chromatin LRCIs that stably maintain B lineage-specific gene expression. Consistent with this, we found reduction of B lineage transcripts after YY1 knock-out. YY1 can also mediate Polycomb Group (PcG) repression, and we found that YY1 knock-out resulted in increased expression of alternative lineage genes, suggesting that YY1 loss abrogates repressive chromatin structures needed to prevent expression of these genes. Thus, we hypothesize that YY1 knock-out in pro-B cells results in lost chromatin structures that stably maintain lineage-specific gene expression, as well as loss of repressive chromatin structures needed to prevent alternative lineage gene expression, thus leading to lost B lineage commitment. To test this, we will determine chromatin folding patterns, nuclear localization of key genes, chromatin accessibility, and epigenetic structures in wild-type and YY1-null pro-B cells to define the genomic structures regulated by YY1 during B lineage commitment. To determine if analogous effects of YY1 are operative in the T lineage, we will determine if YY1 loss promotes lineage plasticity of YY1-null DN3 cells. YY1 is also necessary in pro-B cells for Igk locus contraction required for rearrangement of distal Vk genes. It has been suggested that YY1 plays a structural role in regulating chromatin structures, but it is unclear if this requires the YY1 transcriptional activation, PcG, or self-association functions. We will utilize an established panel of YY1 mutants that are compromised in these functions to assess in parallel, the mechanisms of YY1 regulation of chromatin structures needed for B lineage commitment, and those needed for Igk locus contraction and Jk-Vk rearrangement. As YY1 is involved in embryogenesis and development of multiple tissue types, determining how YY1 controls genomic structures to specify B lineage commitment will provide a new paradigm for the function of a ubiquitous factor in lineage-specific development.