Xist and the order of silencing
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XIST
Dosage compensation
Gene dosage
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Dosage compensation
Skewed X-inactivation
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X-chromosome inactivation (XCI) results in the transcriptional silencing of one X-chromosome in females to attain gene dosage parity between XX female and XY male mammals. Mammals appear to have developed rather diverse strategies to initiate XCI in early development. In placental mammals XCI depends on the regulatory noncoding RNA X-inactive specific transcript (Xist), which is absent in marsupials and monotremes. Surprisingly, even placental mammals show differences in the initiation of XCI in terms of Xist regulation and the timing to acquire dosage compensation. Despite this, all placental mammals achieve chromosome-wide gene silencing at some point in development, and this is maintained by epigenetic marks such as chromatin modifications and DNA methylation. In this review, we will summarise recent findings concerning the events that occur downstream of Xist RNA coating of the inactive X-chromosome (Xi) to ensure its heterochromatinization and the maintenance of the inactive state in the mouse and highlight similarities and differences between mammals.
XIST
Dosage compensation
Skewed X-inactivation
Genomic Imprinting
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XIST
Dosage compensation
Skewed X-inactivation
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XIST
Dosage compensation
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XIST
Dosage compensation
Skewed X-inactivation
Gene dosage
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X chromosome inactivation achieves dosage equivalence for most X-linked genes between the two X chromosomes in females and the single X chromosome in males. In this article the evidence for random inactivation of an X chromosome is reviewed, along with the exceptions that result in nonrandom inactivation. Another exception to X chromosome inactivation is the presence of genes that escape inactivation and are expressed from both the active and inactive X chromosomes. The phenotypic consequences of such expression from the inactive X chromosome are discussed. The major players in the process of inactivation are presented. Initiation of inactivation requires the functional RNA, XIST, and the subsequent stable inactivation of the X chromosome relies upon the recruitment of many other factors, the majority of which are generally associated with heterochromatin.
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Dosage compensation
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Gene dosage
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Abstract Dosage compensation is the process by which the amount of X‐linked gene products between individuals with one and two X chromosomes is equalized. In mammals, dosage compensation is achieved by the transcriptional silencing of one X chromosome in female cells. Inactivation is attained by the establishment of several sequential epigenetic modifications in the future inactive X triggered by expression of the Xist gene in cis, and including different histone modifications and methylation of CpG islands. These transformations occur during early embryonic development and require that the cell counts the X chromosomes and chooses one to be active per diploid genome, inactivating the others. Here, we will review what is currently known about the early events of X‐chromosome inactivation, and discuss the mechanisms by which a cell counts and chooses X chromosomes.
Dosage compensation
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Skewed X-inactivation
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X-chromosome inactivation (XCI) compensates for differences in X-chromosome number between male and female mammals. XCI is orchestrated by Xist RNA, whose expression in early development leads to transcriptional silencing of one X chromosome in the female. Knockout studies have established a requirement for Xist with inviability of female embryos that inherit an Xist deletion from the father. Here, we report that female mice lacking Xist RNA can, surprisingly, develop and survive to term. Xist -null females are born at lower frequency and are smaller at birth, but organogenesis is mostly normal. Transcriptomic analysis indicates significant overexpression of hundreds of X-linked genes across multiple tissues. Therefore, Xist -null mice can develop to term in spite of a deficiency of dosage compensation. However, the degree of X-autosomal dosage imbalance was less than anticipated (1.14-fold to 1.36-fold). Thus, partial dosage compensation can be achieved without Xist , supporting the idea of inherent genome balance. Nevertheless, to date, none of the mutant mice has survived beyond weaning stage. Sudden death is associated with failure of postnatal organ maturation. Our data suggest Xist-independent mechanisms of dosage compensation and demonstrate that small deviations from X-autosomal balance can have profound effects on overall fitness.
XIST
Dosage compensation
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XIST
Dosage compensation
Genomic Imprinting
Skewed X-inactivation
MEG3
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XIST
Dosage compensation
Chromosome conformation capture
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