Heterochromatin on the human inactive X chromosome

描述（由申请人提供）：失活的 X 染色体 (Xi) 上的基因沉默是通过兼性异染色质的形成来实现的，该特征在随后的细胞分裂过程中非常稳定并忠实地保留在 Xi 上。我们发现人类 Xi 的异染色质被组织成占据确定的基因组间隔的非重叠类型的异染色质。根据特征染色质标记的存在，可以定义两种不同的异染色质类型：I、macroH2A 和 XIST RNA； II、HP1 和组蛋白 H3 在赖氨酸 9 处甲基化。这些表观遗传特征的严格空间排列与晚 S 期复制模式的变化以及 Xi 基因表达谱直接相关。 这些数据为测试 Xi 异染色质的表观遗传和表表型特征之间的相互关系提供了框架。 这里描述的实验有两个具体目标：（i）精确定义主要 Xi I 型区域的近端和远端边界； (ii) 研究每个区域的蛋白质成分在维持和/或限制 Xi 沿线的异染色质类型中的作用。 这些实验将在指定的时间间隔内生成详细的图谱，使我们能够确定表观遗传标记相对于彼此以及 Xi 表观型的精确组织，从而充当整个 Xi 的模型。通过选择性去除特定的异染色质标记，我们将能够监测对 Xi 所有特征的影响，从而使我们能够确定表观遗传成分在维持 Xi 异染色质中的功能意义。这些数据不仅将大大增进我们对 X 失活的理解，还将为基因表达的调控和特定染色质状态的遗传提供有价值的见解，这是所有人类发育和细胞分化的一个至关重要的方面。

DESCRIPTION (provided by applicant): Gene silencing at the inactive X-chromosome (Xi) is achieved through the formation of facultative heterochromatin, a feature that is remarkably stable and faithfully retained at the Xi throughout subsequent cell divisions. We have found that heterochromatin of the human Xi is organized into non-overlapping types of heterochromatin that occupy defined genomic intervals. Two distinct heterochromatin types can be defined based upon the presence of characteristic chromatin markers: I,macroH2A and XIST RNA; II, HP1 and histone H3 methylated at lysine-9. The strict spatial arrangement of these epigenetic features correlates directly with variation in the pattern of replication in late S-phase and with the Xi gene expression profile. These data provide the framework for testing interrelationships between the epigenetic and epiphenotypic features of Xi heterochromatin. The experiments described here have two specific aims: (i) To precisely define the proximal and distal boundaries of the major Xi Type-I territory; and (ii) To investigate the role of the protein components of each territory in maintaining and/or constraining the heterochromatin types along the Xi. These experiments will generate a detailed map across the specified interval, allowing us to determine the precise organization of epigenetic markers relative both to one another and to the Xi epiphenotypes, thus acting as a model for the entire Xi. By selectively removing specific heterochromatin markers, we will be able to monitor the effects on all features of the Xi, allowing us to determine the functional significance of the epigenetic components in maintaining Xi heterochromatin. Not only will these data substantially advance our understanding of X-inactivation, they will also provide valuable insight generally into the regulation of gene expression and the inheritance of defined chromatin states, a critically important aspect of all human development and cellular differentiation.