An essential role for the DXPas34 tandem repeat and Tsix transcription in the counting process of X chromosome inactivation.

A counting process senses the X chromosome/autosome ratio and ensures that X chromosome inactivation (XCI) initiates in the early female (XX) embryo and in differentiating female ES cells but not in their male (XY) counterparts. Counting depends on the X inactivation center (Xic), which contains the Xist gene encoding a nuclear RNA, which coats the inactive X chromosome and induces gene silencing. A 37-kb sequence lying 3' to the Xist gene is known to prevent initiation of XCI in male differentiating ES cells. This region contains the major and minor promoters of the Tsix gene, which runs antisense to Xist, and the DXPas34 tandem repeat lying close to the Tsix major promoter. We have addressed the role of these elements in counting by using male ES cells. Targeted deletion of DXPas34 impaired recruitment of RNA-polymerase II and TFIIB at the Tsix major promoter, resulting in low levels of Tsix expression in ES cells and moderate ectopic initiation of XCI upon differentiation. A deletion extending 3' to Xist and including the Tsix major promoter resulted in almost complete impairment of Tsix transcription and in efficient ectopic XCI upon differentiation of male ES cells. Internal deletions within the Tsix gene did not affect significantly the level of antisense transcription within Xist and had only minor effects upon differentiation. Our results identify a function for DXPas34 in murine XCI and demonstrate the critical role of Tsix transcription in preventing XCI in differentiating male ES cells and in normal functioning of the counting pathway.