Dissection of a Complex Enhancer Element: Maintenance of Keratinocyte Specificity but Loss of Differentiation Specificity

This study investigated the mechanisms of keratinocyte-specific and differentiation-specific gene expression in the skin, focusing on the keratin 5 (K5) promoter and enhancer activity. Researchers identified five keratinocyte-specific open chromatin regions within a 6 kb upstream regulatory sequence. Among these, DNase I-hypersensitive site 4 (HSs 4) was notable for independently driving keratinocyte-specific gene activity in culture and transgenic mice, despite not being essential for K5 enhancer activity. The study highlighted the 125-bp 3' half of HSs 4, known as 4.2, as the smallest strong enhancer with keratinocyte-specific activity in vivo, active in a subset of K5-expressing cell progeny in sebaceous glands. The 4.1 half of HSs 4 suppressed sebocyte-specific expression and induced expression in inner root sheath cells. The findings suggested that keratinocyte gene expression is regulated by multiple modules containing AP-2 and Sp1/Sp3 binding sites, with unique sites for specificity, indicating that transcriptional repressors and activators play roles in determining specificity.