Control of Xist expression for imprinted and random X chromosome inactivation in mice

Applying RNA fluorescence in situ hybridization to parthenogenetic embryos with two maternally derived X (X-M) chromosomes and embryos with X chromoso me aneuploidy such as X(P)0 (X-P, paternally derived X chromosome), (XXXP)- X-M-X-M and (XXY)-X-M-Y-M. We studied the control of Xist/Tsix expression f or silencing the entire X chromosome in mice. The data show that the patern ally derived Xist allele is highly expressed in every cell of the embryo fr om the 4-cell stage onward, irrespective of the number of X chromosomes in a diploid cell. The high level of Xist transcription is maintained in non-e piblast cells culminating in X-P-inactivation, whereas in X(P)0 embryos it is terminated by the blastocyst stage, probably as a result of counting the number of X chromosomes in a cell occurring at the morula/ blastocyst stag e. Xist is also down-regulated in epiblast cells of (XXP)-X-M and (XXXP)-X- M-X-M embryos to make X-inactivation random. In epiblast cells, Xist seems to be up-regulated after counting and random choice of the future inactive X chromosome(s). Although the maternal Xist allele is never activated in fe rtilized embryos before implantation, some parthenogenetic embryos show Xis t up-regulation in a proportion of cells. These and other data reported ear lier suggest that imprinted X-inactivation in non-epiblast tissues of roden ts had been derived from the random X-inactivation system.