Polarized regulatory landscape and Wnt responsiveness underlie Hox activation in embryos.

Sequential 3'-to-5' activation of the Hox gene clusters in early embryos is a most fascinating issue in developmental biology. Neither the trigger nor the regulatory elements involved in the transcriptional initiation of the 3'-most Hox genes have been unraveled in any organism. We demonstrate that a series of enhancers, some of which are Wnt-dependent, is located within a HoxA 3' subtopologically associated domain (subTAD). This subTAD forms the structural basis for multiple layers of 3'-polarized features, including DNA accessibility and enhancer activation. Deletion of the cassette of Wnt-dependent enhancers proves its crucial role in initial transcription of HoxA at the 3' side of the cluster.