A Complex Regulatory Landscape Involved In The Development Of External Genitals

ABSTRACTIn vertebrates, developmental genes are often controlled by large regulatory landscapes matching the dimensions of topologically associating domains (TADs). In various ontogenic contexts, the associated constitutive chromatin backbone is modified by fine-tuned specific variations in enhancer-enhancer and enhancer-promoter interaction profiles. In this work, we take one of the TADs flanking the HoxD gene cluster as a paradigm to address the question of how these complex regulatory architectures are formed and how they are de-constructed once their function has been achieved. We suggest that this TAD can be considered as a coherent functional unit in itself, with several regulatory sequences acting together to elicit a transcriptional response. With one notable exception, the deletion of each of these sequences in isolation did not produce any substantial modification in the global transcriptional outcome of the system, a result at odds with a conventional view of long-range enhancer function. Likewise, both the deletion and inversion of a supposedly critical CTCF site located in a region rich in such sequences did not affect transcription of the target gene. In the latter case, however, slight modifications were observed in interaction profiles in vivo in agreement with the loop extrusion model, despite no apparent functional consequences. We discuss these unexpected results by considering both conventional explanations and an alternative possibility whereby a rather unspecific accumulation of particular factors within the TAD backbone may have a global impact upon transcription.