Human XIST yeast artificial chromosome transgenes show partial X inactivation center function in mouse embryonic stem cells

Initiation of X chromosome inactivation requires the presence, in cis, of t he X inactivation center (XIC), The Xist gene, which lies within the XIC re gion in both human and mouse and has the unique property of being expressed only from the inactive X chromosome in female somatic cells, is known to b e essential for X inactivation based on targeted deletions in the mouse. Al though our understanding of the developmental regulation and function of th e mouse Xist gene has progressed rapidly, less is known about its human hom olog. To address this and to assess the cross-species conservation of X ina ctivation, a 480-kb yeast artificial chromosome containing the human XIST g ene was introduced into mouse embryonic stem (ES) cells. The human XIST tra nscript was expressed and could coat the mouse autosome from which it was t ranscribed, indicating that the factors required for cis association are co nserved in mouse ES cells, Cis inactivation as a result of human XIST expre ssion was found in only a proportion of differentiated cells, suggesting th at the events downstream of XIST RNA coating that culminate in stable inact ivation may require species-specific factors. Human XIST RNA appears to coa t mouse autosomes in ES cells before in vitro differentiation, in contrast to the behavior of the mouse Xist gene in undifferentiated ES cells, where an unstable transcript and no chromosome coating are found. This may not on ly reflect important species differences in Xist regulation but also provid es evidence that factors implicated in Xist RNA chromosome coating may alre ady be present in undifferentiated ES cells.