The mammalian cell cycle is precisely controlled by cyclin-dependent kinases (CDKs) and related pathways such as the RB and p53 pathways. Recent research on long non-coding RNAs (lncRNAs) indicates that many lncRNAs are involved in the regulation of critical cell cycle regulators such as the cyclins, CDKs, CDK inhibitors, pRB, and p53. These lncRNAs act as epigenetic regulators, transcription factor regulators, post-transcription regulators, and protein scaffolds. These cell cycle-regulated lncRNAs mainly control cellular levels of cell cycle regulators via various mechanisms, and may provide diversity and reliability to the general cell cycle. Interestingly, several lncRNAs are induced by DNA damage and participate in cell cycle arrest or induction of apoptosis as DNA damage responses. Therefore, deregulations of these cell cycle regulatory lncRNAs may be involved in tumorigenesis, and they are novel candidate molecular targets for cancer therapy and diagnosis.

X-inactivation in female mammals is triggered by the association of non-coding Xist RNA in cis with the X chromosome. Although it has been suggested that the A-repeat located in the proximal part of the Xist RNA is required for chromosomal silencing in ES cells, its role in mouse has not yet been addressed. Here, we deleted the A-repeat in mouse and studied its effects on X-inactivation during embryogenesis. The deletion, when paternally transmitted, caused a failure of imprinted X-inactivation in the extraembryonic tissues, demonstrating the essential role of the A-repeat in X-inactivation in the mouse embryo. Unexpectedly, the failure of X-inactivation was caused by a lack of Xist RNA rather than by a defect in the silencing function of the mutated RNA, which we expected to be expressed from the mutated X. Interestingly, the normally silent paternal copy of Tsix, which is an antisense negative regulator of Xist, was ectopically activated in the preimplantation embryo. Furthermore, CpG sites in the promoter region of paternal Xist, which are essentially unmethylated in the extraembryonic tissues of the wild-type female embryo, acquire a significant level of methylation on the mutated paternal X. These findings demonstrate that the DNA sequence deleted on the mutated X, most probably the A-repeat, is essential as a genomic element for the appropriate transcriptional regulation of the Xist/Tsix loci and subsequent X-inactivation in the mouse embryo.

Expression of Xist, which triggers X inactivation, is negatively regulated in cis by an antisense gene, Tsix, transcribed along the entire Xist gene. We recently demonstrated that Tsix silences Xist through modification of the chromatin structure in the Xist promoter region. This finding prompted us to investigate the role of antisense transcription across the Xist promoter in Tsixmediated silencing. Here, we prematurely terminated Tsix transcription before the Xist promoter and addressed its effect on Xist silencing in mouse embryos. We found that although 93% of the region encoding Tsix was transcribed, truncation of Tsix abolished the antisense regulation of Xist. This resulted in a failure to establish the repressive chromatin configuration at the Xist promoter on the mutated X, including DNA methylation and repressive histone modifications, especially in extraembryonic tissues. These results suggest a crucial role for antisense transcription across the Xist promoter in Xist silencing.

Nucleus Cytoplasm TGF Smad3 P Smad4 ELIT-1 Positive feedback loop Snail, Vimentin, PAI-1 etc

In mammals, one of the two X chromosomes of female cells is inactivated for dosage compensation between the sexes. X chromosome inactivation is initiated in early embryos by the noncoding Xist RNA. Subsequent chromatin modifications on the inactive X chromosome (Xi) lead to a remarkable stability of gene repression in somatic cell lineages. In mice, reactivation of genes on the Xi accompanies the establishment of pluripotent cells of the female blastocyst and the development of primordial germ cells. Xi reactivation also occurs when pluripotency is established during the reprogramming of somatic cells to induced pluripotent stem cells. The mechanism of Xi reactivation has attracted increasing interest for studying changes in epigenetic patterns and for improving methods of cell reprogramming. Here, we review recent advances in the understanding of Xi reactivation during development and reprogramming and illustrate potential clinical applications.

The noncoding Tsix RNA is an antisense repressor of Xist and regulates X inactivation in mice. Tsix is essential for preventing the inactivation of the maternally inherited X chromosome in extraembryonic lineages where imprinted X-chromosome inactivation (XCI) occurs. Here we establish an inducible Tsix expression system for investigating Tsix function in development. We show that Tsix has a clear functional window in extraembryonic development. Within this window, Tsix can repress Xist, which is accompanied by DNA methylation of the Xist promoter. As a consequence of Xist repression, reactivation of the inactive X chromosome (Xi) is widely observed. In the parietal endoderm, Tsix represses Xist and causes reactivation of an Xi-linked GFP transgene throughout development, whereas Tsix progressively loses its Xist-repressing function from embryonic day 9.5 (E9.5) onward in trophoblast giant cells and spongiotrophoblast, suggesting that Tsix function depends on a lineage-specific environment. Our data also demonstrate that the maintenance of imprinted XCI requires Xist expression in specific extraembryonic tissues throughout development. This finding shows that reversible XCI is not exclusive to pluripotent cells, and that in some lineages cell differentiation is not accompanied by a stabilization of the Xi.

No abstract

The possible physiological significance of long non-coding RNAs (lncRNAs) has only recently been recognized. Technical innovations such as the super high-resolution tiling array and deep sequencing technology have indicated their importance. It has been proposed that lncRNAs such as HOTAIR are involved in the recruitment of chromatin modifiers to the target genes. The lncRNA ANRIL has been reported to be associated with a Polycomb complex, recruiting it to the target gene INK4 locus where it suppresses transcription via histone modification. Other lncRNAs such as Kcnq1ot1, AIR and Xist have also been found to recruit chromatin modifiers to their target loci. In this review, we discuss the function of lncRNAs such as HOTAIR, ANRIL, Kcnq1ot1, and Xist which recruit chromatin modifiers to target genes and discuss their involvement in cancer development and aggressiveness, and other cell fate determination.