Deletion of Xist upstream sequences alters TAD interactions and leads to defects in Xist coating and expression

Abstract

The topological organization of the genome plays an important role in regulating gene expression. However, the connection between the two remains poorly understood. X-chromosome inactivation is a unique model system to explore the interlink between topologically associated domains (TADs) and gene expression. TADs are largely lost upon X-inactivation, and the inactive-X gets bipartitely reorganized into two large mega domains. However, the X-inactivation center (XIC) harbors two TADs - at the locus of long non-coding RNA Xist (Xist-TAD) and Tsix (Tsix-TAD). Xist is the master regulator of X-inactivation, which coat the inactive-X and facilitates heterochromatinization. Here, we deleted Xist upstream sequences ([~]6 kb) near the Xist TADs boundary in extraembryonic endoderm stem cells (XEN), which undergo imprinted X-inactivation. This deletion led to the major rearrangement of TADs and affected the expression of genes located within Xist and Tsix TAD, specially the expression of Xist was upregulated, suggesting TADs are essential for proper transcriptional regulation. On the other hand, Xist-upstream deletion on the inactive-X resulted in dispersal of Xist coating and loss of enrichment of repressive chromatin marks on the inactive-X but no effect on X-linked gene silencing. However, we found that autosomal genes were dysregulated in Xist-upstream deleted cells, probably because of misregulation of genes located in Xist and Tsix-TAD, specially Xist. We conclude that Xist upstream sequences are necessary for proper organization of the TADs at the XIC, maintenance of Xist coating/expression and autosomal gene expression.