1443 Skin regional specificity and the regulation of HoxC gene cluster

Skin exhibits remarkable regional specificity. However, the epigenetic mechanism underlying this process remains elusive. In this study, we used the avian scalp skin as a model and leveraged the finding that a tandem 195-bp duplication in the intron of HoxC10 causes the chicken Crest phenotype. Crested chickens (e.g., Polish chickens) exhibit elongated feathers in the scalp region and the comb is minute. Along the anterior-posterior midline dermis of the control White Leghorn chicken, we showed a co-linear expression of HoxC members (using RNA seq,in situ hybridization), which correlated with region-specific chromatin accessibility (using ATAC seq). Configurations of topologically associating domains (TADs) over the HoxC gene cluster in the scalp, dorsal and tail dermis show reduced HoxC gene interactions with outside regulatory elements particularly in the scalp region (using Micro-C). In contrast, crested Polish scalp dermis exhibits a TAD configuration that is similar to the configuration found in dorsal dermis. HoxC8 over-expression in the scalp led to new feather bud formation at the expense of the comb domain, representing a homeotic transformation. Further analyses revealed six transcription factors as major potential upstream modulators of the HoxC cluster in the chicken scalp region, and the tandemly duplicated 195 bp in Polish chickens may facilitate the generation of ectopic chromatin loops to interact with distant putative enhancers. Our results provide a new clue to dissect how the HoxC gene cluster is regulated in different skin regions, setting up a platform to study the epigenetic control of skin regional specificity, and how the scalp region can exhibit so many diverse skin appendage phenotypes in birds as well as in mammals.