Control of Hoxd gene transcription in the mammary bud by hijacking a pre-existing regulatory landscape

In this work we have analyzed the transcriptomic profiles of E13.5 mouse embryonic mammary buds. We show that Hoxd8 and Hoxd9, two gene members of the HoxD cluster, are transcribed during mammary bud development. Yet, unlike in other developmental contexts, their co-expression does not rely upon the same regulatory mechanism. Hoxd8 is regulated by the combined activity of closely located sequences and the most distant telomeric gene desert. On the other hand, Hoxd9 is controlled by an enhancer sequence also located within the telomeric gene desert, but which has no impact on Hoxd8 transcription, thus constituting an exception to the global regulations systematically observed at this locus. The latter DNA region is also involved in Hoxd gene regulation in other contexts and strongly interacts with Hoxd9 in all tissues analyzed so far as well as in other vertebrate species, indicating that its regulatory activity was already operational before the appearance of mammary glands. Within this DNA region and neighboring the CS39 limb enhancer, we further identified a short sequence conserved in therian mammals and capable of enhancer activity in the mammary buds. We propose that Hoxd gene regulation in embryonic mammary buds evolved by hijacking a preexisting regulatory landscape, which was already at work before the emergence of mammals in structures such like the limbs or the intestinal tract. Overall design: RNA-seq analysis of e13.5 mammary buds and adjacent embryonic skin

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Schep R, Necsulea A, Rodríguez-Carballo E, Guerreiro I, Andrey G, Nguyen Huynh TH, Marcet V, Zákány J, Duboule D, Beccari L