DNA hypomethylation can activate Xist expression and silence X-linked genes.

Xist and other X-linked gene expression was examined by fluorescence in situ hybridization in cells of wild type and DNA methyltranferase (Dnmt) mutant embryos and embryonic stem (ES) cells to determine whether demethylation-induced Xist expression leads to inappropriate X chromosome inactivation. In undifferentiated ES cells low-level Xist expression was detected from the single active X chromosome (Xa) in male cells and on both Xa's in female cells. Upon differentiation Xist expression was detected only in female cells, in which Xist RNA colocalized with the entire inactive X chromosome (Xi). Differentiated Dnmt mutant ES cells or cells of mutant postgastrulation embryos showed aberrant patterns of Xist expression: Xist transcripts colocalized with the single X chromosome in male cells and with both X chromosomes in female cells. X-linked gene expression was not detected from chromosomes coated with Xist RNA. These results suggest that ectopic Xist expression, induced by DNA hypomethylation, may lead to the inactivation of X-linked genes. We conclude that Xist-mediated X chromosome inactivation can occur in the absence of DNA methylation, arguing that DNA methylation may be required to repress Xist expression for the maintenance of a transcriptionally active Xa. In differentiated Dnmt mutant ES cells the activation of Xist expression correlated with a dramatic increase in apoptotic bodies, suggesting that Xist-mediated X chromosome inactivation may result in cell death and contribute to the embryonic lethality of the Dnmt mutation.