The mechanism of homologous interaction at the X-inactivation center

Random X-chromosome inactivation (XCI) is a dosage compensation process by which females transcriptionally silence one of their two X chromosomes to achieve the equal X-linked gene expression level with XY males. The X-inactivation center (Xic) is the X-linked sequence necessary and sufficient for counting, choice and cis -spreading of silencing. Xist, Tsix and Xite are three well-studied non-coding RNA genes within the Xic that function in cis. On the future active X (Xa), Xite activates Tsix, which in turn represses Xist. On the future inactive X (Xi), Xite and Tsix are turned off, which allows Xist accumulation. Following Xist -mediated gene silencing, the Xi is maintained in a stable heterochromatic state that is inherited in all subsequent cell divisions. Only one X chromosome remains active while the other becomes inactive. Therefore, the two Xs must be coordinated in trans to ensure mutually exclusive silencing. I hypothesized that interchromosomal pairing mediates this communication. To examine the potential spatial association of the two Xs, fluorescence in situ hybridization (FISH) and chromosome conformation capture (3C) were used in mouse embryonic stem (ES) cells at di