SPECIFIC ECTODERMAL ENHANCERS CONTROL THE EXPRESSION OFHoxcGENES IN DEVELOPING MAMMALIAN INTEGUMENTS

VertebrateHoxgenes are key players in the establishment of structures during the development of the main body axis. Subsequently, they play important roles either in organizing secondary axial structures such as the appendages, or during homeostasis in postnatal stages and adulthood. Here we set up to analyze their elusive function in the ectodermal compartment, using the mouse limb bud as a model. We report that theHoxCgene cluster was globally co-opted to be transcribed in the distal limb ectoderm, where it is activated following the rule of temporal colinearity. These ectodermal cells subsequently produce various keratinized organs such as nails or claws. Accordingly, deletion of theHoxCcluster led to mice lacking nails (anonychia) and also hairs (alopecia), a condition stronger than the previously reported loss of function ofHoxc13, which is the causative gene of the ectodermal dysplasia 9 (ECTD9) in human patients. We further identified two ectodermal, mammalian-specific enhancers located upstream of theHoxCgene cluster, which act synergistically to regulateHoxcgene expression in the hair and nail ectodermal organs. Deletion of these regulatory elements alone or in combination revealed a strong quantitative component in the regulation ofHoxcgenes in the ectoderm, suggesting that these two enhancers may have evolved along with mammals to provide the level of HOXC proteins necessary for the full development of hairs and nails.Significance StatementIn this study, we report a unique and necessary function for theHoxCgene cluster in the development of some ectodermal organs, as illustrated both by the hair and nail phenotype displayed by mice lacking theHoxc13function and by the congenital anonychia (absence of nails) in fullHoxCcluster mutants. We show thatHoxcgenes are activated in a colinear manner in the embryonic limb ectoderm and are subsequently transcribed in developing nails and hairs. We identify two mammalian-specific enhancers located upstream of theHoxCcluster with and exclusive ectodermal specificity. Individual or combined enhancer deletions suggest that they act in combination to raise the transcription level of severalHoxcgenes during hairs and nails development.