Title:
X-inactivation initiated by Xist RNA defective in silencing in the mouse embryo

Takashi Sado
Associate Professor
Division of Epigenomics, Department of Molecular Genetics, Institute of Bioregulation, Kyushu University

Abstract:
A big breakthrough in the field of X-inactivation was brought about in the early 90’s by the discovery of noncoding XIST/Xist RNA as a transcript exclusively expressed from the inactive X chromosome. Although most of the studies on X-inactivation until then had been mainly conducted by classic cytogenetic approaches, cloning of the Xist sequence accelerated the studies at the molecular level. Extensive efforts have been made since then to understand the role for Xist in X-inactivation and the regulatory mechanism by which the Xist gene is monoallelically upregulated at the onset of X-inactivation. We now know that Xist is essential for X-inactivation to occur in cis and its expression is positively and negatively regulated by some additional noncoding RNAs occurring in the X inactivation center. Xist RNA has also been anticipated to play a role in recruiting proteins involved in heterochromatinization onto the X chromosome. The detailed mechanism by which Xist RNA induces chromosome-wide silencing is, however, still largely unknown. This is partly attributable to the fact that there has been no mutant that expresses Xist RNA defective in silencing. If such mutants were available, they would provide opportunities to dissect how the inactive state of the X chromosome is established following the accumulation of Xist RNA. Accordingly, we decided to generate mice that express Xist RNA lacking the evolutionally conserved repeat present in the 5′ region, which has been suggested to be essential for the silencing function of the RNA. I will talk about an unexpected finding brought about by a simple deletion of the conserved repeat as well as more recent analyses of X-inactivation initiated by the mutant Xist RNA in the mouse embryo.