<i>Xist</i> Exon 7 Contributes to the Stable Localization of Xist RNA on the Inactive X-Chromosome

To equalize X-linked gene dosage between the sexes in mammalian females, Xist RNA inactivates one of the two X-chromosomes. Here, we report the crucial function of Xist exon 7 in X-inactivation. Xist exon 7 is the second-largest exon with a well-conserved repeat E in eutherian mammals, but its role is often overlooked in X-inactivation. Although female ES cells with a targeted truncation of the Xist exon 7 showed no significant differences in their Xist expression levels and RNA stability from control cells expressing wild-type Xist, compromised localization of Xist RNA and incomplete silencing of X-linked genes on the inactive X-chromosome (Xi) were observed in the exon 7-truncated mutant cells. Furthermore, the interaction between the mutant Xist RNA and hnRNP U required for localization of Xist RNA to the Xi was impaired in the Xist exon 7 truncation mutant cells. Our results suggest that exon 7 of Xist RNA plays an important role for stable Xist RNA localization and silencing of the X-linked genes on the Xi, possibly acting through an interaction with hnRNP U.为平衡哺乳动物雌性个体两性间的X连锁基因剂量（X-linked gene dosage），Xist RNA（Xist核糖核酸）可介导两条X染色体中的一条发生X染色体失活（X-inactivation）。本研究揭示了Xist第7号外显子（Xist exon 7）在X染色体失活过程中的关键功能。Xist第7号外显子是真兽类哺乳动物（eutherian mammals）中的第二大外显子，其序列携带高度保守的重复序列E（repeat E），但该外显子在X染色体失活中的作用常被忽视。尽管携带Xist第7号外显子靶向截短的雌性胚胎干细胞（embryonic stem cells, ES cells），其Xist表达水平与RNA稳定性与表达野生型Xist（wild-type Xist）的对照细胞并无显著差异，但在该外显子截短的突变细胞中，研究者观察到Xist RNA的定位受损，且失活X染色体（Xi）上的X连锁基因沉默不完全。此外，在Xist第7号外显子截短的突变细胞中，突变型Xist RNA与介导Xist RNA向失活X染色体定位所需的异质性核糖核蛋白U（heterogeneous nuclear ribonucleoprotein U, hnRNP U）之间的相互作用受到损害。本研究结果表明，Xist RNA的第7号外显子对于Xist RNA的稳定定位以及失活X染色体上X连锁基因的沉默发挥着重要作用，其功能可能通过与异质性核糖核蛋白U的相互作用得以实现。