A new Xist allele driven by a constitutively active promoter is dominated by Xist locus environment and exhibits the parent-of-origin effects
The dosage difference of X-linked genes between the sexes in mammals is compensated for by genetically inactivating one or the other X chromosomes in XX females. A noncoding RNA transcribed from the Xist gene at the onset of X-inactivation coats the X chromosome in cis and induces chromosome-wide heterochromatinization. Here, we report a new Xist allele (Xist CAG ) driven by a CAG promoter, which is known to be constitutively active in many types of cells. The paternal transmission of Xist CAG
... ission of Xist CAG resulted in the preferential inactivation of the targeted paternal X (Xp) not only in the extraembryonic, but also embryonic lineage, whereas maternal transmission ended with embryonic lethality at the early postimplantation stage with a phenotype that resembled mutant embryos carrying a maternal deficiency in Tsix, an antisense negative regulator of Xist, in both sexes. Interestingly, we found that the upregulation of Xist CAG in preimplantation embryos temporarily differed depending on its parental origin: its expression started at the 4-to 8-cell stages when paternally inherited, and Xist CAG was upregulated at the blastocyst stage when maternally inherited. This may indicate that the Xist locus on Xp is permissive to transcription, but not on the maternal X (Xm). We extrapolated from these findings that the maternal Xist allele may manifest a chromatin structure inaccessible by transcription factors relative to the paternal allele. This may underlay the mechanism for the maternal repression of Xist at the early cleavage stage when Tsix expression has not yet occurred on Xm.