Mechanisms of lineage plasticity revealed by YY1 deficiency.

造血发育是一个有序的过程，其中干细胞产生多个谱系。 虽然早期祖细胞可以是多能的，但谱系特异性祖细胞达到了一个阶段， 完全忠于这个血统。例如，B和T细胞谱系从 在骨髓中产生的淋巴样引发的祖细胞，并且专门致力于B细胞 当细胞从前-原-B细胞阶段过渡到原-B细胞阶段时发生谱系。尽管承诺， 在将前B细胞敲除至B谱系的过程中，我们已经做出了令人惊讶的发现， 在前B细胞中普遍存在的多功能转录因子YY 1导致B谱系的丧失 在体外和体内，T细胞的定型和随后发育成T细胞谱系的能力。到 了解这种令人惊讶的谱系可塑性的机械基础，我们已经开发了一种新的谱系 追踪小鼠系，使我们能够确定YY 1-null pro-B细胞如何发育成T谱系细胞 （去分化为更原始的祖细胞，或转分化），评估YY 1- null pro-B细胞发育成其他造血谱系，并确定YY 1-null T细胞是否也 表现出谱系可塑性（目标1）。从机制上讲，谱系特异性转录因子与DNA结合， 在随后的大规模染色质结构改变之前调节基因表达， 血统承诺我们实验室和其他实验室的严格研究表明，尽管 普遍存在的表达模式，YY 1控制着谱系中的长距离染色质相互作用（LRCI）， 具体的时尚。我们的研究结果支持了这样的假设，即DNA通过谱系特异性转录结合 因子使YY 1能够募集到不同的基因组位点，从而使YY 1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 （H3 K27 me 3）在谱系不适当基因。因此，我们将比较分子遗传表型 (gene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，无效的pro-B细胞发育为DN 1、DN 2a、DN 2b、DN 3、DP、CD 4+和CD 8 + T细胞， 型T谱系细胞，以及YY 1条件性敲除T谱系细胞（Aim 2）。我们假设， YY 1缺失时，T谱系发育可以进行，但LRCI需要稳定维持谱系- 特异性基因表达和抑制替代谱系所需的异染色质将无法 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了一个共同的 控制谱系可塑性的机制，极大地扩展了指导YY 1-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.