A regulatory 'landscape effect' over the HoxD cluster.

Faithful expression of Hox genes in both time and space is essential for proper patterning of the primary body axis. Transgenic approaches in vertebrates have suggested that this collinear activation process is regulated in a largely gene cluster-autonomous manner. In contrast, more recently co-opted expression specificities, required in other embryonic structures, depend upon long-range enhancer sequences acting from outside the gene clusters. This regulatory dichotomy was recently questioned, since gene activation along the trunk seems to be partially regulated by signals located outside of the cluster. We investigated these alternative regulatory strategies by engineering a large inversion that precisely separates the murine HoxD complex from its centromeric neighborhood. Mutant animals displayed posterior transformations along with subtle deregulations of Hoxd genes, indicating an impact of the centromeric landscape on the fine-tuning of Hoxd gene expression. Proximal limbs were also affected, suggesting that this 'landscape effect' is generic and impacts upon regulatory mechanisms of various qualities and evolutionary origins.