Investigation into the function of RNA polymerase II promoter proximal pausing during terminal erythroid maturation

红系终末成熟涉及细胞核中基因表达的快速变化， 正在逐渐浓缩，为摘除眼球做准备。红系成熟的每个阶段都与 一个独特的基因表达谱，然而，与两者活跃相关的表观遗传标记的分布 在红系成熟的连续阶段，启动子和抑制的异染色质是相对静止的。 相反，我们的初步数据表明，组蛋白标记与活跃的转录相关 延伸率，如H3K36me3，在终末成熟过程中变化很大，表明是红细胞 优先在伸长水平上调节转录。RNA聚合酶II暂停(Pol II暂停)是一种 高度调控的转录调控机制，启动转录，但“暂停”30-60bp 转录起始点下游。停顿是基因表达的关键检查点，因为PolII不能 在不被pTEFb磷酸化的情况下转变为活跃的伸长。PTEFb可与组织结合 特定的转录因子，包括GATA1，以促进PolII在特定位点的暂停释放，或者它可以是 由HEXIM1隔离在7sk小核核糖核蛋白(SnRNP)复合体中，使其处于活性和 无法促进POL II的释放。我们的初步数据和发表的文献都表明，波尔二世暂停了 是红系成熟的关键调节因子，然而，Pol II暂停的调节机制在 人们对成熟的红血球知之甚少。支持Pol II在成熟过程中暂停发挥核心作用 红细胞，质谱分析表明，红系终末成熟与减少有关 在与主动转录延长相关的多个组蛋白标记的丰富中， 标记的变化提示Pol II停顿增加，而与异染色质增加无关。 CHIP-SEQ研究证实，H3K36me3丰度的减少与H3K36me3的丢失有关 &gt；1600个基因座的富集性。此外，POLII暂停的中心驱动因子HEXIM1在红系中高表达 与其他类型的细胞相比，其表达始终保持在RNA和蛋白质水平 终末期红系成熟。相反，pTEFb的表达下降，伸长水平也下降 最后，诱导HEXIM1促进红系终末成熟，并特别影响 成熟过程中失去H3K36me3浓缩的基因的表达。我们的初步数据加在一起 支持我们的核心假设，即波尔II停顿动态的转变越来越有利于“停顿” 状态是红系终末成熟的重要调节因子。在目标1中，我们将描述POL II的动态 在成熟的红细胞中暂停，我们将确定改变POL II暂停动力学的后果 关于红系终末成熟的多个方面。在目标2中，我们将确定通过哪些具体机制 在成熟的红系细胞中，POL II暂停在特定的位置被建立和释放。总而言之，这些研究将 有助于重新定义我们用来概念化正常和无序红细胞生成的范式。

Terminal erythroid maturation involves rapid changes in gene expression in the context of a nucleus that is progressively condensing in preparation for enucleation. Each stage of erythroid maturation is associated with a distinct gene expression profile, however the distribution of epigenetic marks associated with both active promoters and repressed heterochromatin is relatively static between successive stages of erythroid maturation. In contrast, our preliminary data demonstrates that histone marks associated with active transcriptional elongation, such as H3K36me3, change dramatically during terminal maturation, suggesting erythroblasts preferentially regulate transcription at the level of elongation. RNA Polymerase II pausing (Pol II pausing) is a highly regulated mechanism of transcriptional regulation whereby transcription is initiated, but “pauses” 30-60bp downstream of the transcription start site. Pausing is a critical checkpoint in gene expression, as pol II cannot transition into active elongation without being phosphorylated by pTEFb. pTEFb can associate with tissue specific transcription factors, including GATA1, to facilitate pol II pause release at specific loci, or it can be sequestered by Hexim1 in the 7sk small nuclear ribonucleoprotein (snRNP) complex rendering it in active and unable to facilitate Pol II release. Both our preliminary data and the published literature suggest that Pol II pausing is a critical regulator of erythroid maturation, however the mechanisms by which Pol II pausing is regulated in maturing erythroblasts are poorly understood. Supporting a central role for Pol II pausing in maturing erythroblasts, mass spectrometry demonstrates that terminal erythroid maturation is associated with a decrease in the abundance of multiple histone marks associated with active transcriptional elongation, coupled with changes in marks suggestive of increased Pol II pausing, without an associated increase in heterochromatin. ChIP-seq studies confirm that the decrease in abundance of H3K36me3 is correlated with loss of H3K36me3 enrichment at >1600 loci. In addition, Hexim1, a central driver of pol II pausing, is highly expressed in erythroid cells compared to other cell types and its expression is maintained at both the RNA and protein level throughout terminal erythroid maturation. In contrast, the expression of pTEFb declines, as does the level of elongation competent Pol II. Lastly, induction of hexim1 promotes terminal erythroid maturation, and specifically impacts the expression of genes that lose enrichment for H3K36me3 during maturation. Together our preliminary data support our central hypothesis that a shift in Pol II pause dynamics that increasingly favors the “paused” state is a critical regulator of terminal erythroid maturation. In aim1, we will delineate the dynamics of Pol II pausing in maturating erythroblasts and we will determine the consequences of altering Pol II pausing dynamics on multiple facets of terminal erythroid maturation. In aim 2, we will determine the specific mechanisms by which Pol II pausing is established and released at specific loci in maturing erythroid cells. Together, these studies will help to redefine the paradigm with which we conceptualize both normal and disordered erythropoiesis.