XIST RNA and X-Silencing Mechanisms

The silencing of the inactive X chromosome is orchestrated by XIST, a long noncoding RNA transcribed from the X-inactivation centre. XIST is expressed only from the future inactive X, spreads along that chromosome in cis, and recruits the chromatin machinery that converts it into transcriptionally repressed heterochromatin. This topic covers the molecular cascade that establishes and maintains X-silencing.

Definition

XIST RNA is a long noncoding RNA expressed from the X-inactivation centre of the inactive X chromosome that coats that chromosome in cis and triggers the chromatin changes responsible for X-silencing.

Scope

The topic covers the X-inactivation centre and the XIST transcript, the cis-limited coating of the chromosome, the recruitment of repressive histone modifications and DNA methylation, the role of antisense and regulatory transcripts, and the distinction between initiation and maintenance phases. It is a mechanistic, reference-level account and does not provide clinical guidance.

Core questions

• What is XIST and why is it expressed only from the inactive X?
• How does XIST spread along and silence the chromosome from which it is transcribed?
• Which chromatin modifications convert the X into stable heterochromatin?
• How are the initiation and maintenance of silencing distinguished?

Key concepts

• X-inactivation centre (Xic)
• XIST long noncoding RNA
• Cis-limited chromosome coating
• Polycomb repressive complexes and H3K27 methylation
• Histone hypoacetylation and H4 changes
• DNA methylation of CpG islands
• Initiation versus maintenance of silencing

Key theories

Cis-acting noncoding-RNA silencing model

X-inactivation is initiated by XIST RNA that acts only on the chromosome that produces it, coating it and nucleating repressive chromatin so that silencing spreads from the X-inactivation centre across the chromosome.

Mechanisms

Silencing is controlled from the X-inactivation centre, where the XIST gene is expressed exclusively from the inactive X. The XIST transcript, a large nuclear-retained noncoding RNA, spreads in cis to coat the chromosome from which it is produced. This coating recruits chromatin-modifying complexes, including Polycomb complexes that deposit repressive histone marks such as trimethylation of histone H3 at lysine 27, together with histone hypoacetylation and, at gene promoters, DNA methylation. The combined marks establish transcriptionally silent heterochromatin during an initiation window early in development, after which the inactive state is locked in and maintained largely independently of continued XIST expression and is faithfully propagated through cell division. Targeted deletion experiments showed that XIST is required in cis for a chromosome to be inactivated.

Clinical relevance

Understanding the XIST-driven silencing pathway clarifies how a whole chromosome can be epigenetically silenced and why genes that escape this machinery remain biallelically expressed. The topic underpins interpretation of X-linked dosage effects and of research into reactivating silenced genes; it is descriptive and is not a basis for individual diagnosis or treatment.

History

The X-inactivation centre was mapped to a region of the X chromosome whose deletion prevents inactivation, and in 1991 Brown and colleagues identified XIST as a gene expressed only from the inactive X. The following year the human XIST transcript was characterised as a large, conserved, nuclear-localised noncoding RNA. In 1996 targeted deletion of Xist in mouse demonstrated that the transcript is required in cis for inactivation, cementing the noncoding-RNA model that later work extended with detailed chromatin and chromosome-architecture mechanisms.

Key figures

• Carolyn J. Brown
• Huntington F. Willard
• Neil Brockdorff
• Sohaila Rastan
• Edith Heard

Seminal works

• brown-1991
• brown-1992
• penny-1996

Frequently asked questions

What kind of molecule is XIST?

XIST is a long noncoding RNA: it is transcribed but not translated into protein, and it works as an RNA that physically coats the inactive X chromosome and recruits silencing factors.

Is XIST needed to keep the X chromosome silent forever?

XIST is essential to initiate inactivation, but once the inactive state is established it is maintained largely by stable chromatin marks and DNA methylation, so maintenance becomes less dependent on continued XIST expression.