Primary non-random X inactivation associated with disruption of Xist promoter regulation

In this report we demonstrate primary non-random X chromosome inactivation following targeted mutagenesis of a region immediately upstream of Xist pro moter P-1. In heterozygous animals there is a preferential inactivation of the targeted X chromosome in 80-90% of cells, The phenotype correlates with inappropriate activation of Xist in a proportion of the mutant XY embryoni c stem cells. Strand-specific analysis revealed increased sense transcripti on initiating upstream of Xist promoter P-1. There was, however, no discern ible effect on transcription from the antisense Tsix gene, We demonstrate t hat the in vitro and in vivo phenotypes are specifically attributable to th e presence of a PGKneo cassette at the targeted locus. These findings are d iscussed in the context of understanding mechanisms of Xist gene regulation in X inactivation.