The inactive X chromosome adopts a unique three-dimensional conformation that is dependent on Xist RNA.. Mus musculus

3D-topology of DNA in the cell nucleus provides a level of transcription regulation beyond the sequence of the linear DNA. To study the relationship between transcriptional activity and spatial environment of a gene, we have used allele-specific 4C-technology to produce high-resolution topology maps of the active and inactive X-chromosomes in female cells. We found that loci on the active X form multiple long-range interactions, with spatial segregation of active and inactive chromatin. On the inactive X, silenced loci lack preferred interactions, suggesting a unique random organization inside the inactive territory. However, escapees, among which is Xist, are engaged in long-range contacts with each other, enabling identification of novel escapees. Deletion of Xist results in partial re-folding of the inactive X into a conformation resembling the active X, without affecting gene silencing or DNA methylation. Our data point to a role for Xist RNA in shaping the conformation of the inactive X-chromosome independently of transcription. Overall design: Five or six 4C viewpoints were applied on the mouse female wild type active X-chromosome, the wild type inactive X-chromosome, the conditional Xist X-chromosome and the Xist knock out X-chromosome