Tsix transcription across the Xist gene alters chromatin conformation without affecting Xist transcription: implications for X-chromosome inactivation

Abstract

X-chromosome inactivation (XCI) is highly dynamic during early mouse embryogenesis and strictly depends on the Xist noncoding RNA. The regulation of Xist and its antisense partner Tsix remains however poorly understood. We provide here the first evidence of transcriptional control of Xist expression. We show that RNA polymerase II (RNAPolII) preinitiation complex recruitment and H3 Lys 4 (H3-K4) methylation at the Xist promoter form the basis of the Xist expression profiles that drives both imprinted and random XCI. In embryonic stem (ES) cells, which are derived from the inner cell mass where imprinted XCI is reversed and both Xs are active, we show that Xist is repressed at the level of preinitiation complex (PIC) recruitment. We further demonstrate that Tsix, although highly transcribed in ES cells, is not itself responsible for the transcriptional down-regulation of Xist. Rather, Tsix induces efficient H3-K4 methylation over the entire Xist/Tsix unit. We suggest that chromatin remodeling of the Xist locus induced by biallelic Tsix transcription renders both Xist loci epigenetically equivalent and equally competent for transcription. In this model, Tsix, by resetting the epigenetic state of the Xist/Tsix locus, mediates the transition from imprinted to random XCI.[Keywords: X-chromosome inactivation; histone methylation; antisense transcription; preinitiation complex of transcription; noncoding RNAs] Supplemental material is available at http://www.genesdev.org.

Eukaryotic regulatory RNAs: an answer to the ‘genome complexity’ conundrum

“…Furthermore, when Tsix RNA is prematurely truncated before it crosses into the Xist gene, Tsix no longer functions as a repressor of Xist, and XCI invariably occurs on the mutated X (Shibata and Lee 2004). It was also proposed earlier that the modulation of Xist chromatin structure might play a role in how Tsix regulates Xist (Navarro et al 2005;Sado et al 2005). Interestingly, a recent study has suggested that up-regulation of Xist RNA observed on the future inactive X is not due to the increased stability of the Xist transcript as suggested earlier but is regulated by Tsix Sheardown et al 1997;Sun et al 2006).…”

Tsixdefective in splicing is competent to establishXistsilencing

“…We recently showed that in the absence of Tsix, the Xist locus fails to establish the repressive chromatin configuration, and instead manifests the active modification in embryos, suggesting that Tsix mediates Xistsilencing through modification of the chromatin structure (Sado et al, 2005). Others also demonstrated Tsix-mediated chromatin effects using undifferentiated and differentiating embryonic stem (ES) cells as an in vitro model system (Navarro et al, 2005;Sun et al, 2006).…”

Inactivation du chromosome X et pluripotence

“…En effet, l'acquisition de la pluripotence s'accompagne d'une reprogrammation épigénétique à grande échelle qui pourrait prendre en charge la régulation de l'ICX, sans qu'aucun facteur spécifique n'y soit directement impliqué. [11], et que cette répression semble indépendante de la structure chromatinienne de ce gène [12]. Il y aurait donc des régulateurs spécifiques responsables de l'extinction de Xist dans les cellules ES.…”