INDUCTION OF XIST EXPRESSION FROM THE HUMAN ACTIVE X-CHROMOSOME IN MOUSE HUMAN SOMATIC-CELL HYBRIDS BY DNA DEMETHYLATION/

X chromosome inactivation occurs early in mammalian development to tra nscriptionally silence one of the pair of X chromosomes in females. Th e XIST RNA, a large untranslated RNA that is expressed solely from the inactive X chromosome, is implicated in the process of inactivation. As previous studies have shown that the XIST gene is methylated on the active X chromosome, we have treated a mouse/human somatic cell hybri d retaining an active human X chromosome with demethylating agents to determine whether expression of the human XIST gene could be induced. Stable expression of XIST was observed after several rounds of demethy lation and stability of XIST expression correlated with the loss of me thylation at the three sites analysed. We conclude that methylation is sufficient to inhibit expression of the XIST gene in somatic cell hyb rids. No loss of expression was detected for eight other X-linked gene s from the active X chromosome that was expressing XIST, suggesting th at additional developmental or species-specific factors are required f or the inactivation process.