Activation of the Inactive X Chromosome Induced by Cell Fusion between a Murine EC and Female Somatic Cell Accompanies Reproducible Changes in the Methylation Pattern of theXistGene

Abstract

Mouse embryonal carcinoma (EC) cell lines are divided into two classes with or without the capability of reactivating the inactive X chromosome from a fusion partner of female lymphocyte. The 5′ region ofXistwas partially methylated in reactivating-competent EC cells but was fully methylated in reactivating-incompetent EC cells having a single X chromosome. Partial or heterogeneous methylation implies methylation of each CpG site in about half of the cell independently of methylation status of neighboring CpG sites. Fusion of the reactivating-competent EC cells with female lymphocytes induced not onlyde novomethylation in the 5′ region ofXistallele on the hitherto inactivated X chromosome, but also demethylation of the same region ofXiston the other X chromosome from the female somatic cell. In contrast, no such changes occurred in hybrid cells involving reactivating-incompetent EC cells. Thus, partial methylation of the 5′ region ofXistmost probably maintained by low maintenance and highde novomethylation efficiency is correlated with reactivation potential of the EC cell. It is possible that this unique methylation pattern is implicated in random X inactivation in EC-hybrid cellsin vitroand in epiblast cellsin vivo.