Congenital Recessive Ichthyosis : Studies of Gene Expressions Related to Keratinocyte Differentiation and Skin Barrier Repair

Autosomal recessive congenital ichthyosis (ARCI) is a rare monogenetic disorder characterized by a defective skin barrier, hyperkeratosis, and dry, scaly skin. It affects keratinocyte differentiation and is caused by mutations in any of at least 12 genes believed to control the formation of ω-O-acylceramide and the corneocyte lipid envelope (CLE): ABCA12, ALOXE3, ALOX12B, CERS3, CYP4F22, ELOVL4, LIPN, NIPAL4, PNPLA1, SDR9C7, SLC27A4, and TGM1. Studies of keratinocyte differentiation and gene expression in ARCI may help us better understand the pathobiology of skin barrier formation. One way to verify that ARCI-related gene products are operating in a chain of events essential for lipid barrier formation is to use immunofluorescence and in situ proximity ligation assays to demonstrate the proteins’ colocalizations in the epidermis. In paper I, a new method for the objective quantitative image analysis of protein expression and colocalization in different epidermal layers of skin sections was developed using free, open-source software, CellProfiler. Using this method and microarray analyses of skin biopsies from ARCI patients with TGM1 mutations (n = 5) compared with those of healthy controls (n = 4), many ARCI-related genes were found to be upregulated in patient epidermis (paper II). Because many other genes involved in keratinocyte differentiation and immune/inflammatory response, including PPARD, were also induced in the patients’ microarrays, the effects of a ligand-dependent transcription factor, PPARδ, encoded by PPARD, were studied on ARCI-related gene expression in cultured keratinocytes, usually showing the pronounced upregulation by PPARδ agonists (paper III). Furthermore, using previous array data obtained from cultured differentiated keratinocytes and from skin biopsies of patients with TGM1 mutations, nine novel candidate markers of differentiation were identified, and the upregulation was verified by qPCR of mRNA from cultured keratinocytes and skin biopsies. These transcripts were also induced by PPARδ agonists in cultured proliferating keratinocytes (paper IV). To conclude, the upregulation of other ARCI-related genes in patients with TGM1 mutations might reflect a feedback mechanism in ω-O-acylceramide biosynthesis, which, however, is unable to restore the patients’ skin barrier. In theory, substitution therapy with ω-O-acylceramide and recombinant TGm-1 may be required. Because PPARδ activation appears involved in upregulating ARCI-related genes and nine novel differentiation marker genes, all potentially important for barrier repair, this approach could become a treatment option for several types of ichthyosis and wound healing