Shibata Shinwa, Lee Jeannie T
Howard Hughes Medical Institute, Department of Molecular Biology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
Hum Mol Genet. 2003 Jan 15;12(2):125-36. doi: 10.1093/hmg/ddg010.
In dosage compensation of female mammals, the accumulation of Xist RNA initiates silencing of one X-chromosome. Xist action is repressed by the antisense gene, Tsix, whose full-length RNA product is complementary to Xist RNA in mice. While previous work showed that Tsix transcription blocks the accumulation of Xist RNA, it is still unclear whether this repression requires the antisense RNA product or whether the antisense transcriptional movement is sufficient. A better understanding of potential mechanisms requires elucidation of Tsix RNA structure and determination of Tsix RNA copy number relative to that of Xist RNA. Previous work indicated that at least some of murine Tsix is spliced and that human TSIX truncates within the 3' end of XIST. Here, further characterization and quantitation of murine Tsix RNA reveal three new findings: first, in undifferentiated embryonic stem cells, Tsix RNA is present at 10-100-fold molar excess over Xist RNA. Second, only 30-60% of Tsix RNA is spliced at known exon-intron junctions. The nearly equal abundance of spliced and unspliced species leaves open possible roles for both isoforms. Finally, Tsix is spliced heterogeneously at the 5' end and most detectable splice variants exhibit only a 1.9 kb region of complementarity between sense and antisense RNAs. Implications for Tsix's possible mechanisms of action are discussed.
在雌性哺乳动物的剂量补偿过程中，Xist RNA 的积累启动了一条 X 染色体的沉默。Xist 的作用受到反义基因 Tsix 的抑制，在小鼠中，其全长 RNA 产物与 Xist RNA 互补。虽然先前的研究表明 Tsix 转录会阻止 Xist RNA 的积累，但仍不清楚这种抑制是否需要反义 RNA 产物，或者反义转录运动是否就足够了。要更好地理解潜在机制，需要阐明 Tsix RNA 的结构，并确定 Tsix RNA 相对于 Xist RNA 的拷贝数。先前的研究表明，至少部分小鼠 Tsix 是经过剪接的，而人类 TSIX 在 XIST 的 3' 端内发生截短。在这里，对小鼠 Tsix RNA 的进一步表征和定量揭示了三个新发现：第一，在未分化的胚胎干细胞中，Tsix RNA 的摩尔数比 Xist RNA 多出 10 - 100 倍。第二，只有 30 - 60% 的 Tsix RNA 在已知的外显子 - 内含子连接处进行了剪接。剪接和未剪接形式的丰度几乎相等，这使得两种异构体都可能发挥作用。最后，Tsix 在 5' 端存在异质性剪接，大多数可检测到的剪接变体在正义和反义 RNA 之间仅表现出 1.9 kb 的互补区域。文中讨论了 Tsix 可能的作用机制。
