Introduction:
Cells gain their identity by inheriting gene expression from their predecessors. X chromosome inactivation is a phenomenon that causes the silencing of one X chromosome due to the X inactivation center and contains Xist and various protein complexes. This occurs during embryogenesis. Replicated copies of both active and inactive X chromosomes are maintained through cell division. There are recent findings that include two types of inactivation: RNA Xist (ribonucleic acid X-inactive specific transcript) causing the silencing of the genes and changes in chromatin by physically coating it, anti-sense Tsix maintaining the active X chromosome, presence of Xist transgenes which affect the neighboring sequences, and the importance of the presence of two Xs one from each parent.
What Is X Chromosome Inactivation?
X chromosome inactivation is a type of epigenetic (changes in the physical characteristics of an individual without the alteration of DNA, deoxyribonucleic acid sequence) phenomenon that causes the inactivation of one of the X chromosomes in females. This process is carried out by the X inactivation center (Xic). Xic contains XIST and various protein complexes, causing the silencing of one X chromosome. X chromosome inactivation has two steps. The first step includes initiation, in which the inactive X chromosome goes through epigenetic transcriptional inactivation, and the second step includes the maintenance phase, where the replicated copies of inactive X chromosome are maintained inactive by multiple rounds of cell division. This phenomenon occurs during embryogenesis when the early female embryo cells undergo transcriptional silencing of genes in one X chromosome, the replicated copies of both active and inactive X chromosomes are maintained through mitosis, and the remaining cells maintain the imprinted X inactivation.
What Is Xist?
Xist (X-inactive specific transcript) is a long non-coding RNA and is involved in many steps of X inactivation, such as coating of inactive X chromosome, RNA polymerase exclusion, removal of active histone, and addition of repressive histone, DNA methylation, connecting the inactive X to the nuclear periphery, and chromatin packing.
What Are the Recent Advances in X Chromosome Inactivation?
The recent advances in X chromosome inactivation include:
X chromosome inactivation is of two types, which are:
Imprinted X Chromosome Inactivation:
This occurs in the embryo during the early pre-implantation development, where all the cells go through imprinted inactivation exclusively on the paternal X-chromosome.
Random X Chromosome Inactivation:
This occurs after the implantation of the embryo when the inner cell mass descends and randomly inactivates the paternal or maternal X chromosome.
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Inactivation of the X chromosome undergoes a series of events. One of the two Xs that are present in the female is first distinguished by the inactive X of the long non-coding RNA, called Xist. This causes the silencing of the genes, physically coats the expressed X chromosome, and causes changes in the chromatin. Whereas the active X chromosome is distinguished by Xist anti-sense long non-coding transcript Tsix, this Tsix prevents the inactivation of other X chromosomes. A defect or mutation of Xist causes inactive X to become active, and mutation in Tsix causes the inactivation of the active chromosomes.
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Xist RNA accumulates on the same chromosome, and polycomb group protein complexes are brought near the chromosome, and they alter the chromosome structure by transcription of the DNA into inert heterochromatin. This occurs during the initiation phase.
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Xist is also affected by ectopically expressed Xist transgenes. Multiple Xist transgenes can cause variable degrees of silencing in neighboring sequences, whereas single-copy Xist transgene does not cause any silencing.
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Xist-independent imprinted X-inactivation is a form of ancestral X-inactivation. For example, Marsupial mammals shared a common ancestor and showed imprinted X inactivation in all their cell types, along with a lack of Xist. Cis-acting elements mediated the inactivation of the X chromosome. In the case of random X inactivation, cellular machinery targets one of the X chromosomes to become inactive.
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X inactivation is the by-product of the sexual differentiation of a pair of homologous chromosomes (one from maternal and another from paternal) into X and Y chromosomes. The y chromosome declines to recombine with the X, and therefore, inactivates Xist. Therefore, Xist alone does not trigger imprinted X-inactivation, and X ist originates after the loss of the Y chromosome.
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Polycomb group genes encode Polycomb group protein complexes (EED) during the activation. If female embryos lack these genes, it can result in an imprinted X-inactivation defect. These defects affect the maintenance phases of X-inactivation.
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In the case of random X-inactivation, the Tsix functions complementary to the Xist, and in the case of imprinted X-inactivation, the active maternal X expresses the Tsix, and its absence can cause X-inactivation. Tsix can act independently and cause maternal epigenotype in the ovary cell.
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X inactivation depends on the number of cells, too. If the X-chromosome number exceeds the other, then only inactivation occurs. Therefore, the two X chromosomes sense each other by physically pairing before inactivation. This can occur only in the case of the XX chromosome, and XY chromosomes are immune to this inactivation.
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X-linked Rnf12 gene codes a protein that can sense the number of x chromosomes and can trigger the expression of Xist during random X-inactivation. This also explains why inactivation occurs only in females and not in males. Rnf12 is an X-linked gene. Therefore, two X chromosomes produce proteins twice as much as produced by the XY chromosome. However, according to studies, RNF12 is not sufficient to cause inactivation.
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Random X-inactivation can occur even in the absence of RNF12. But in the case of imprinted X-inactivation, RNF12 is required. The function of RNF12 is derived from oocytes and has two types: mutant and wild type. Mutant RNF12 is destroyed due to an imprinted X-inactivation defect. RNF12 protein derived from oocytes is necessary for proper imprinted X-inactivation.
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In the case of parthenogenetic (asexual reproduction, the embryo develops from the unfertilized egg), two Xs from the maternal side cannot undergo inactivation. Therefore, the X chromosome of the maternal side during early embryo is immune to inactivation, and RNF12 selectively targets the X chromosome of the paternal side, leaving the Xist to express the maternal side X chromosome.
Conclusion:
Therefore, the discovery of long non-coding RNA Xist, in the case of X inactivation, has led to understanding many steps that occur during inactivation by using newer technologies such as mutagenesis in experimental animals, techniques to detect expression of X-linked gene, high-resolution microscopy, and genome sequencing efforts.
