Control ofHoxdgene transcription in the mammary bud by hijacking a preexisting regulatory landscape

Abstract

VertebrateHoxgenes encode transcription factors operating during the development of multiple organs and structures. However, the evolutionary mechanism underlying this remarkable pleiotropy remains to be fully understood. Here, we show thatHoxd8andHoxd9, two genes of theHoxDcomplex, are transcribed during mammary bud (MB) development. However, unlike in other developmental contexts, their coexpression does not rely on the same regulatory mechanism.Hoxd8is regulated by the combined activity of closely located sequences and the most distant telomeric gene desert. On the other hand,Hoxd9is controlled by an enhancer-rich region that is also located within the telomeric gene desert but has no impact onHoxd8transcription, thus constituting an exception to the global regulatory logic systematically observed at this locus. The latter DNA region is also involved inHoxdgene regulation in other contexts and strongly interacts withHoxd9in all tissues analyzed thus far, indicating that its regulatory activity was already operational before the appearance of mammary glands. Within this DNA region and neighboring a strong limb enhancer, we identified a short sequence conserved in therian mammals and capable of enhancer activity in the MBs. We propose thatHoxdgene regulation in embryonic MBs evolved by hijacking a preexisting regulatory landscape that was already at work before the emergence of mammals in structures such as the limbs or the intestinal tract.