Inversion of a topological domain leads to restricted changes in its gene expression and affects inter-domain communication

The interplay between the topological organization of the genome and the regulation of gene expression remains unclear. Depletion of molecular factors underlying genome topology, such as CTCF and cohesin, leads to modest alterations in gene expression, while genomic rearrangements involving boundaries of topologically associating domains (TADs) disrupt normal gene expression and can lead to pathological phenotypes. Here we inverted an almost entire TAD (245kb out of 300kb) within the X-inactivation centre (Xic), leaving its boundaries intact. This led to a significant rearrangement of topological contacts within the TAD, mostly in accordance to the orientation of underlying CTCF binding sites but suggesting heterogeneity in the "contact" potential of different CTCF sites. The inversion also led to increased contact insulation with the neighbouring TAD. Expression of most genes within the inverted TAD remained unaffected in mouse embryonic stem cells and during differentiation. Interestingly, expression in the neighbouring TAD of the noncoding transcript Xist, which controls X-chromosome inactivation, was ectopically upregulated. The same inversion in mouse embryos led to a bias in Xist expression, but X-inactivation choice ratios did not significantly deviate from wild type. Smaller deletions and inversions of specific clusters of CTCF sites within the TAD led to similar results: rearrangement of contacts, limited changes in local gene expression but significant changes in Xist expression. Our study suggests that the wiring of regulatory interactions within a TAD can influence the expression of genes in neighbouring TADs, highlighting the existence of mechanisms for inter-TAD communication.