Mechanisms of lineage plasticity revealed by YY1 deficiency.

造血发育是干细胞分化为多个谱系的有序过程。 虽然早期的祖先可以是多能性的，但特定于血统的祖先会达到这样一个阶段，即他们 变得完全忠于那个血统。例如，B和T细胞谱系区别于 在骨髓中产生的淋巴刺激的祖细胞，并专属于B细胞 当细胞从前-前-B细胞阶段过渡到前-B细胞阶段时，就会发生谱系。尽管有人承诺 从亲B细胞到B细胞谱系，我们有了一个令人惊讶的发现，即条件性敲除 Pro-B细胞中普遍存在的多功能转录因子YY1导致B血统丧失 承诺和随之而来的在体外和体内发展为T细胞谱系的能力。至 了解了这种令人惊讶的谱系可塑性的机制基础，我们开发了一个新的谱系 追踪小鼠品系将使我们能够确定YY1缺失的前B细胞是如何发育成T细胞的 (去分化为更原始的祖细胞，或转分化)，评估YY1- 去除前B细胞以发育成其他造血系，并确定YY1缺失的T细胞是否也 表现出谱系可塑性(目标1)。从机制上讲，谱系特异的转录因子与DNA和 在随后的大规模染色质结构改变之前调节基因表达 世系承诺。我们实验室和其他实验室的严格研究表明，尽管它 在普遍存在的表达模式中，YY1控制着一个谱系中的长程染色质相互作用(LRCI)- 特定的时尚。我们的发现支持DNA通过谱系特异性转录结合的假说 因子使YY1能够招募到不同的基因组座位，从而使YY1能够产生LRCI 稳定适合谱系的基因表达，并产生抑制性染色质标记 (H3K27me3)在谱系-不适当的基因。因此，我们将比较分子遗传表型 (基因表达模式、染色质可及性、表观遗传结构和染色质折叠) 与野生型相比，零PRO-B细胞发育成DN1、DN2a、DN2b、DN3、DP、CD4+和CD8+T细胞 类型T细胞，以及YY1条件性基因敲除T细胞(目标2)。我们假设在 YY1，T血统的缺失可以继续发展，但LRCI需要稳定地维持血统- 特定的基因表达和异染色质将无法抑制另一种谱系 充分发展，潜在地使持续的谱系可塑性。我们的实验可能揭示出一个共同的 控制谱系可塑性的机制，极大地扩大了YY1-NULL的潜在适用性 细胞分化成多个谱系。

Hematopoietic development is an ordered process in which stem cells give rise to multiple lineages. While early progenitors can be multipotent, lineage-specific progenitors reach a stage where they become exclusively committed to that lineage. For example, B and T cell lineages differentiate from lymphoid-primed progenitors produced in the bone marrow, and exclusive commitment to the B cell lineage occurs as cells transition from the pre-pro-B to the pro-B cell stage. Despite the commitment of pro-B cells to the B lineage, we have made the surprising discovery that conditional knock-out of the ubiquitous multi-functional transcription factor YY1 in pro-B cells, results in the loss of B lineage commitment and the consequent ability to develop into the T cell lineage both in vitro and in vivo. To understand the mechanistic basis for this surprising lineage plasticity, we have developed a new lineage tracing mouse line that will enable us to determine how YY1-null pro-B cells develop into T lineage cells (de-differentiation to more primitive progenitors, or trans-differentiation), assess the potential for YY1- null pro-B cells to develop into other hematopoietic lineages, and determine if YY1-null T cells also exhibit lineage plasticity (Aim 1). Mechanistically, lineage-specific transcription factors bind to DNA and regulate gene expression prior to subsequent large-scale alterations in chromatin structure needed for lineage commitment. Rigorous studies by our laboratory as well as others indicate that despite its ubiquitous expression pattern, YY1 controls long-range chromatin interactions (LRCIs) in a lineage- specific fashion. Our findings support the hypothesis that DNA binding by lineage-specific transcription factors enables YY1 recruitment to distinct genomic loci, thereby enabling YY1 to both generate LRCIs that stabilize lineage-appropriate gene expression, and to generate repressive chromatin marks (H3K27me3) at lineage-inappropriate genes. We will thus, compare the molecular genetic phenotype (gene expression patterns, chromatin accessibility, epigenetic structure, and chromatin folding) of YY1- null pro-B cells developed into DN1, DN2a, DN2b, DN3, DP, CD4+, and CD8+ T cells, compared to wild- type T lineage cells, as well as YY1 conditional knockout T lineage cells (Aim 2). We hypothesize that in the absence of YY1, T lineage development can proceed, but LRCIs needed to stably maintain lineage- specific gene expression, and heterochromatin needed for repression of alternative lineages will fail to fully develop, potentially enabling continuing lineage plasticity. Our experiments may reveal a common mechanism for controlling lineage plasticity, vastly expanding potential applicability of directing YY1-null cells into multiple lineages.