Tsix

Tsix is a

Xist is only expressed from the future inactive X chromosome in females and is able to "coat" the chromosome from which it was produced. Many copies of Xist RNA bind the future inactivated X chromosome. Tsix prevents the accumulation of Xist on the future active female X chromosome to maintain the active euchromatin state of the chosen chromosome.^[3][4]

Function in mammals

In the extra-embryonic lineage in mice and some other mammals, all female individuals have two X chromosomes. However, during embryonic development, an X chromosome is deactivated, while the other X chromosome is left untouched, in a process called

Tsix and Xist regulate X chromosome protein production in female mice to prevent early embryonic mortality.[6] X inactivation allows for equal dosage of X-linked genes for both males and females by inactivating the extra X chromosome in the females.^[7] Mutation of the maternal Tsix gene can cause over accumulation of Xist on both X chromosomes, silencing both X chromosomes in females and the single X chromosome in male. This can cause early mortality. However, if the paternal Tsix allele is active, it can rescue female embryos from the over-accumulation of Xist.^[8]

Mutations

When one allele of Tsix in mice is null, the inactivation is skewed toward the mutant X chromosome. This is due to an accumulation of Xist that is not countered by Tsix, and causes the mutant chromosome to be inactivated. When both alleles of Tsix are null (

In development, X chromosome inactivation is a part of cellular differentiation. This is accomplished by normal Xist function. To confer pluripotency in an embryonic stem cell, factors inhibit Xist transcription. These factors also upregulate transcription of Tsix, which serves to inhibit Xist further. This cell is then able to remain pluripotent as X inactivation is not accomplished.^[10]

The marker

• Cellular differentiation
• Xist
• X-inactivation
• Barr body

References