Academic journal article Genetics

A Boundary Element between Tsix and Xist Binds the Chromatin Insulator Ctcf and Contributes to Initiation of X-Chromosome Inactivation

Academic journal article Genetics

A Boundary Element between Tsix and Xist Binds the Chromatin Insulator Ctcf and Contributes to Initiation of X-Chromosome Inactivation

Article excerpt

ABSTRACT In mammals, X-chromosome inactivation (XCI) equalizes X-linked gene expression between XY males and XX females and is controlled by a specialized region known as the X-inactivation center (Xic). The Xic harbors two chromatin interaction domains, one centered around the noncoding Xist gene and the other around the antisense Tsix counterpart. Previous work demonstrated the existence of a chromatin transitional zone between the two domains. Here, we investigate the region and discover a conserved element, RS14, that presents a strong binding site for Ctcf protein. RS14 possesses an insulatory function suggestive of a boundary element and is crucial for cell differentiation and growth. Knocking out RS14 results in compromised Xist induction and aberrant XCI in female cells. These data demonstrate that a junction element between Tsix and Xist contributes to the initiation of XCI.

X-CHROMOSOME inactivation (XCI) equalizes sex chromosome- linked gene expression between male (XY) and female (XX) mammals (Lyon 1961). XCI is routinely studied in mouse embryonic stem (ES) cells, an ex vivo model that recapitulates the random form of XCI during cell differentiation (reviewed in Avner and Heard 2001; Lucchesi et al. 2005; Wutz and Gribnau 2007; Payer and Lee 2008). In ES cells and the early epiblast, all X chromosomes are active. As differentiation proceeds, each cell makes an autonomous decision regarding whether to inactivate one X. This decision is governed by a counting mechanism that determines X-chromosome number and inactivates all but one X in a diploid cell. At the same time, a choice mechanism randomly selects one of the Xs for inactivation.

Control of XCI is mediated by the X-inactivation center (Xic) (Brown et al. 1991b), an X-linked region that produces a number of noncoding RNAs (Figure 1A) (Simmler et al. 1996; Chureau et al. 2002). The X-inactive specific transcript (Xist) is responsible for initiating silencing along the X as the RNA accumulates and recruits silencing factors to the X in cis (Borsani et al. 1991; Brockdorffet al. 1991; Brown et al. 1991a; Clemson et al. 1996; Penny et al. 1996; Wutz et al. 2002). A major regulator of Xist is Tsix, which encodes another noncoding RNA, overlaps the Xist gene, and is transcribed from the opposite strand of DNA as Xist (Lee et al. 1999). Tsix expression suppresses Xist upregulation in cis and thereby designates the active X chromosome (Xa) (Lee and Lu 1999; Sado et al. 2001). Two other noncoding genes have been identified at the Xic. Xite lies upstream of Tsix and positively regulates the antisense RNA (Ogawa and Lee 2003; Stavropoulos et al. 2005). Jpx/Enox resides upstream of Xist (Chureau et al. 2002; Johnston et al. 2002), makes looping contacts with Xist (Tsai et al. 2008), and has recently been shown to activate Xist expression (Tian et al. 2010).

Genetic and physical interplay among Xite, Tsix, and Xist is central to the control of XCI. Its initiation is marked by homologous pairing of the Xs through the 59 ends of Xite and Tsix (Bacher et al. 2006; Xu et al. 2006, 2007) in an act that is necessary for counting and choice. Following pairing, Tsix expression is extinguished on the future inactive X chromosome (Xi) with the consequent activation of Xist and global silencing of the X in cis. On the future Xa, Tsix expression persists to block Xist induction and to ensure the continued active state of the X in cis. Several studies have shown that the coupling of Tsix and Xist chromatin states controls whether Xist will be transactivated. On the Xa, Tsix transcription through the Xist promoter results in recruitment of DNA methylation and repressive chromatin modifications to the Xist promoter. By contrast, loss of Tsix transcription on the Xi allows activating chromatin modifications to occur at the Xist promoter for transcriptional induction of Xist (Navarro et al. 2005; Sado et al. 2005; Sun et al. 2006).

A recent study has revealed a series of complex threedimensional interactions underlying the genetic interactions between the noncoding genes (Tsai et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.