Unite de Genetique Moleculaire Murine; Institut Pasteur
Initiation of X chromosome inactivation requires the presence in cis of the X-inactivation centre (XIC). The Xist gene, which lies within the XIC region in both man and mouse and has the unique property of being expressed only from the inactive X chromosome in female somatic cells, is essential for X inactivation based on targeted deletions in the mouse. Although our understanding of the developmental regulation and function of the mouse Xist gene is progressing rapidly, less is known about its human homolog. To address this and to assess the cross-species conservation of X inactivation, a 480 kb YAC containing the human XIST gene was introduced into mouse embryonic stem (ES) cells. The human XIST transcript was expressed and could coat the mouse autosome from which it was transcribed, indicating that the factors required for cis-association are conserved in mouse ES cells. Cis-inactivation as a result of human XIST expression was found only in a proportion of differentiated cells, suggesting that the events downstream of XIST RNA coating which culminate in stable inactivation may require species-specific factors. Surprisingly, human XIST RNA coats mouse autosomes in ES cells prior to 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 suggests that factors implicated in Xist RNA chromosome coating may already be present in undifferentiated ES cells. It may also reflect important species differences in Xist regulation during early development.