Tsix, a gene antisense to Xist at the X-inactivation centre

Abstract

In mammals, dosage compensation is achieved by X inactivation and is regulated in cis by the X-inactivation centre (Xic) and Xist. The Xic controls X-chromosome counting, choice of X to inactivate and initiation of silencing. Xic action culminates in a change in Xist RNA property from a scarce, unstable RNA to highly expressed Xist RNA that coats the future inactive X. Deleting a 65-kb region downstream of Xist results in constitutive Xist expression and X inactivation, implying the presence of a cis-regulatory element. In this region, we now report the discovery of a gene antisense to Xist. Tsix is a 40-kb RNA originating 15 kb downstream of Xist and transcribed across the Xist locus. Tsix sequence is conserved at the human XIC. Tsix RNA has no conserved ORFs, is seen exclusively in the nucleus and is localized at Xic. Before the onset of X inactivation, Tsix is expressed from both X chromosomes. At the onset of X inactivation, Tsix expression becomes monoallelic, is associated with the future active X and persists until Xist is turned off. Tsix is not found on the inactive X once cells enter the X-inactivation pathway. Tsix has features suggesting a role in regulating the early steps of X inactivation, but not the silencing step.

A functional role for Tsix transcription in blocking Xist RNA accumulation but not in X-chromosome choice

“…Tsix initiates 15 kb downstream of the Xist 3Ј terminus and produces a noncoding transcript extending across the entire Xist locus. The expression pattern of Tsix is consistent with a role in regulating Xist (13). All active X chromosomes express Tsix before differentiation, a time when Xist expression is low.…”

“…The 3.7-kb region common to both deletions spans a CpG-rich domain and the putative promoter of Tsix, a gene antisense to Xist (13). Tsix initiates 15 kb downstream of the Xist 3Ј terminus and produces a noncoding transcript extending across the entire Xist locus.…”

PNA interference mapping demonstrates functional domains in the noncoding RNA Xist

“…The XIC has been shown to be a complex transcription unit consisting of at least two genes, Xist and Tsix, which are transcribed off the opposite strands of the same DNA duplex (3,(13)(14)(15)(16)(17)(18). Although the function of Xist is not known, deletion of the gene leads to failure of X-inactivation, and female knockout mice die near gastrulation (19,20).…”

Thioredoxin post-transcriptional regulation by H19 provides a new function to mRNA-like non-coding RNA

“…Some of these genes are included in an imprinted domain such as Xist, Tsix, IPW, KvDMR1 (Wevrick and Francke, 1997; Lee et al, 1999;Smilinich et al, 1999) and among them, some are transcribed as antisense mRNAs. Whereas the role of Xist/Tsix in chromosome X inactivation is well documented (Kelley and Kuroda, 2000), all are supposed to be implicated , are up-regulated in H19-transfected cells (H19 S14 3, H19 S14 4).…”