IMPRINTING AND X-CHROMOSOME COUNTING MECHANISMS DETERMINE XIST EXPRESSION IN EARLY MOUSE DEVELOPMENT

In mice, X inactivation is preceded by in cis Xist expression. Initial ly, normal female embryos express the paternal Xist allele exclusively , preceding imprinted X inactivation in the trophectoderm Later expres sion of Xist alleles is random, preceding random X inactivation in the epiblast lineage. In this study using uniparental embryos, we demonst rate that Xist expression is initially dictated solely by parental imp rinting, causing expression of all paternal alleles. Maternal alleles remain repressed, irrespective of X chromosome number. At the compacti ng morula stage, this parental imprint is erased, and the mechanism co unting the X chromosomes imposes appropriate Xist expression with resp ect to chromosome number. Our results also suggest that Xist expressio n may itself be regulated by a novel imprinted maternally expressed ge ne.