High-resolution Xist binding maps reveal two-step spreading during X-chromosome inactivation.
High-resolution Xist binding maps reveal two-step spreading during X-chromosome inactivation.
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Xist长非编码RNA（lncRNA）是X染色体失活（XCI）所必需的，XCI是哺乳动物补偿不相等数量的性染色体的过程。在XCI期间，Xist包覆未来失活的X（Xi）并将Polycomb抑制复合物2（PRC 2）招募到X失活中心（Xic）。Xist如何在150 Mb规模上传播沉默尚不清楚。在这里，我们使用CHART-seq生成了Xist在X染色体上结合的高分辨率地图。在经历XCI从头的雌性细胞中，Xist遵循两步机制，首先靶向基因丰富的岛屿，然后扩散到干预基因贫乏的区域。Xist是耗尽的基因逃脱XCI，但可能集中在附近的逃逸边界。Xist结合与PRC 2密度和H3赖氨酸27三甲基化（H3 K27 me 3）成线性比例，表明Xist和PRC 2的共迁移。有趣的是，当XCI后细胞中的Xi被急性剥离Xist时，Xist在基因丰富和基因贫乏的区域内以数小时而不是数天的时间尺度迅速恢复，这表明先前引发的Xi染色质状态。我们的结论是，Xist传播采取不同的阶段特定的形式：在最初的建立，Xist遵循两步机制，但在维护过程中，Xist迅速传播到基因丰富和贫困地区。
The Xist long noncoding RNA (lncRNA) is essential for X-chromosome inactivation (XCI), the process by which mammals compensate for unequal numbers of sex chromosomes. During XCI, Xist coats the future inactive X (Xi) and recruits Polycomb Repressive Complex 2 (PRC2) to the X-inactivation center (Xic). How Xist spreads silencing on a 150 Mb scale is unclear. Here we generate high-resolution maps of Xist binding on the X chromosome across a developmental time course using CHART-seq. In female cells undergoing XCI de novo, Xist follows a two-step mechanism, initially targeting gene-rich islands before spreading to intervening gene-poor domains. Xist is depleted from genes that escape XCI but may concentrate near escapee boundaries. Xist binding is linearly proportional to PRC2 density and H3 lysine 27 trimethylation (H3K27me3), suggesting co-migration of Xist and PRC2. Interestingly, when the Xi is acutely stripped off Xist in post-XCI cells, Xist recovers quickly within both gene-rich and -poor domains on a time-scale of hours instead of days, suggesting a previously primed Xi chromatin state. We conclude that Xist spreading takes distinct stage-specific forms: During initial establishment, Xist follows a two-step mechanism, but during maintenance, Xist spreads rapidly to both gene-rich and -poor regions.