Impact of Genome Architecture Upon the Functional Activation and Repression of Hox Regulatory Landscapes

Background The spatial organization of the mammalian genome relies upon the formation of chromatin domains of various scales. At the level of gene regulation in cis , collections of enhancer sequences define large regulatory landscapes that usually match with the presence of topologically associating domains (TADs). These domains are largely determined by bound CTCF molecules and often contain ranges of enhancers displaying similar or related tissue specificity, suggesting that in some cases such domains may act as coherent regulatory units, with a global on or off state. Results By using the HoxD gene cluster as a paradigm, we investigated the effect of large genomic rearrangements affecting the two TADs flanking this locus, including their fusion into a single chromatin domain. We show that, within a single hybrid TAD, the activation of both proximal and distal limb enhancers initially positioned in either TADs globally occurred as when both TADs are intact. We also show that the timely implementation of distal limb enhancers depends on whether or not target genes had previously responded to proximal enhancers, due to the presence or absence of H3K27me3 marks. Conclusions From this work, we conclude that antagonistic limb proximal and distal enhancers can exert their specificities when positioned into the same TAD and in the absence of their genuine target genes. We also conclude that removing these target genes reduced the coverage of a regulatory landscape by chromatin marks associated with silencing and thus prolonged its activity in time. Since Polycomb group proteins are mainly recruited at the Hox gene cluster, our results suggest that Polycomb Repressive Complex 2 (PRC2) can extend its coverage to far- cis regulatory sequences as long as confined to the neighboring TAD structure.