At initiation of X chromosome inactivation (XCI), Xist is monoallelically upregulated from the future inactive X (Xi) chromosome, overcoming repression by its antisense transcript Tsix. Xist recruits various chromatin remodelers, amongst them SPEN, which are involved in silencing of X-linked genes in cis and establishment of the Xi. Here, we show that SPEN plays an important role in initiation of XCI. Spen null female mouse embryonic stem cells (ESCs) are defective in Xist upregulation upon differentiation. We find that Xist-mediated SPEN recruitment to the Xi chromosome happens very early in XCI, and that SPEN-mediated silencing of the Tsix promoter is required for Xist upregulation. Accordingly, failed Xist upregulation in Spen^−/− ESCs can be rescued by concomitant removal of Tsix. These findings indicate that SPEN is not only required for the establishment of the Xi, but is also crucial in initiation of the XCI process.

在 X 染色体失活 (XCI) 开始时，Xist从未来失活的 X (Xi) 染色体上单等位上调，克服其反义转录物Tsix的抑制。Xist招募各种染色质重塑因子，其中包括 SPEN，它们参与顺式X 连锁基因的沉默和 Xi 的建立。在这里，我们表明 SPEN 在 XCI 的启动中发挥着重要作用。Spen null 雌性小鼠胚胎干细胞 (ESC)在分化时Xist上调存在缺陷。我们发现，Xist介导的 SPEN 向 Xi 染色体的募集发生在 XCI 的早期，并且 SPEN 介导的Tsix启动子沉默是Xist上调所必需的。因此，Spen ^{−/− ESC 中失败的}Xist上调可以通过同时去除Tsix来挽救。这些发现表明 SPEN 不仅是 Xi 的建立所必需的，而且对于 XCI 进程的启动也至关重要。