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Theoretically, X-inactivation should eliminate the differences in gene dosage between affected individuals and individuals with a typical chromosome complement. In affected individuals, however, X-inactivation is incomplete and the dosage of these non-silenced genes will differ as they escape X-inactivation, similar to an autosomal aneuploidy.
Nonrandom X-inactivation leads to skewed X-inactivation. Nonrandom X-inactivation can be caused by chance or directed by genes. If the initial pool of cells in which X-inactivation occurs is small, chance can cause skewing to occur in some individuals by causing a bigger proportion of the initial cell pool to inactivate one X chromosome.
Not all random X-inactivation is entirely random. Some alleles, generally mutations in the X-inactivation center on the X-chromosome have been demonstrated to confer a bias towards inactivation for the chromosome on which they sit. [1] Truly random X-inactivation may also appear to be non-random if one X-chromosome carries a deleterious mutation.
Xist (X-inactive specific transcript) is a non-coding RNA transcribed from the X chromosome of the placental mammals that acts as a major effector of the X-inactivation process. [5] It is a component of the Xic – X-chromosome inactivation centre [6] – along with two other RNA genes (Jpx and Ftx) and two protein genes (Tsx and Cnbp2). [7]
X chromosome reactivation (XCR) is the process by which the inactive X chromosome (the Xi) is re-activated in the cells of eutherian female mammals. Therian female mammalian cells have two X chromosomes, while males have only one, requiring X-chromosome inactivation (XCI) for sex-chromosome dosage compensation .
XCI is usually divided in two phases, the establishment phase when gene silencing is reversible, and maintenance phase when gene silencing becomes irreversible. [2] During the establishment phase of X Chromosome Inactivation (XCI), Xist RNA, the master regulator of this process, is monoallelically upregulated [3] and it spreads in cis along the future inactive X (Xi), relocates to the nuclear ...
The SHOX gene in the PAR1 region is the gene most commonly associated with and well understood with regards to disorders in humans, [17] but all pseudoautosomal genes escape X-inactivation and are therefore candidates for having gene dosage effects in sex chromosome aneuploidy conditions (45,X, 47,XXX, 47,XXY, 47,XYY, etc.).
Mammalian X-chromosome inactivation is initiated from the X inactivation centre or Xic, usually found near the centromere. [6] The center contains twelve genes, seven of which code for proteins, five for untranslated RNAs, of which only two are known to play an active role in the X inactivation process, Xist and Tsix. [6]