Human XIST yeast artificial chromosome transgenes show partial X inactivation center function in mouse embryonic stem cells

X-chromosome inactivation in XX androgenetic mouse embryos surviving implantation

Using genetic and cytogenetic markers, we assessed early development and X-chromosome inactivation (X-inactivation) in XX mouse androgenones produced by pronuclear transfer. Contrary to the current view, XX androgenones are capable of surviving to embryonic day 7.5, achieving basically random X-inactivation in all tissues including those derived from the trophectoderm and primitive endoderm that are characterized by paternal X-activation in fertilized embryos. This finding supports the hypothesis that in fertilized female embryos, the maternal X chromosome remains active until the blastocyst stage because of a rigid imprint that prevents inactivation, whereas the paternal X chromosome is preferentially inactivated in extra-embryonic tissues owing to lack of such imprint. In spite of random X-inactivation in XX androgenones, FISH analyses revealed expression of stable Xist RNA from every X chromosome in XX and XY androgenonetic embryos from the four-cell to morula stage. Although the occurrence of inappropriate X-inactivation was further suggested by the finding that Xist continues ectopic expression in a proportion of cells from XX and XY androgenones at the blastocyst and the early egg cylinder stage, a replication banding study failed to provide positive evidence for inappropriate X-inactivation at E6. 5.

The role of chromosomal RNAs in marking the X for dosage compensation

Both flies and mammals remodel the architecture of the X chromosome to achieve dosage compensation. A novel class of noncoding RNAs that paint entire chromosomes are centrally involved in this process. The genes encoding these unusual RNAs are themselves located on the X, and are key sites that target the X for dosage compensation.

Maternally inherited X chromosome is not inactivated in mouse blastocysts due to parental imprinting

Mouse embryos having an additional maternally inherited X chromosome (X(M)) invariably die before midgestation with the deficient extraembryonic ectoderm of the polar trophectoderm lineage, whereas postnatal mice having an additional paternally inherited X chromosome (X(P)) survive beyond parturition. A cytogenetic study led us to hypothesize that abnormal development of such embryos disomic for X(M) (DsX(M)) is attributable to two doses of active X(M) chromosome in extraembryonic tissues. To test the validity of this hypothesis, we examined the initial X chromosome inactivation pattern in embryos at the blastocyst stage by means of replication banding method as well as RNA FISH detecting Xist transcripts. X(P) was the only asynchronously replicating X chromosome, if any, in X(M)X(M)X(P) blastocysts, and no such allocyclic X chromosome was ever detected in X(M)X(M)Y blastocysts. In agreement with these findings, only one Xist paint signal was detected in 79% of X(M)X(M)X(P) cells, whereas no such signal was found in X(M)X(M)Y embryos. Thus, the present study supports the hypothesis that two X chromosomes remaining active in the extraembryonic cell lineages due to the maternal imprinting explain the underdevelopment of extraembryonic structures and hence early postimplantation death of DsX(M) embryos.

A shift from reversible to irreversible X inactivation is triggered during ES cell differentiation

Xist is required for X inactivation. To study the initiation of X inactivation, we have generated a full-length mouse Xist cDNA transgene and an inducible expression system facilitating controlled Xist expression in ES cells and differentiated cultures. In ES cells, transgenic Xist RNA was stable and caused long-range transcriptional repression in cis. Repression was reversible and dependent on continued Xist expression in ES cells and early ES cell differentiation. By 72 hr of differentiation, inactivation became irreversible and independent of Xist. Upon differentiation, autosomal transgenes did not effect counting, but transgenic Xist RNA induced late replication and histone H4 hypoacetylation. Xist had to be activated within 48 hr of differentiation to effect silencing, suggesting that reversible repression by Xist is a required initiation step that might occur during normal X inactivation in female cells.