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SVEP1 is a recently identified multi-domain cell adhesion protein, homologous to the mouse polydom protein, which has been shown to mediate cell-cell adhesion in an integrin dependent-manner in osteogenic cells. In the present study, we characterized SVEP1 function in the epidermis. SVEP1 was found by qRT-PCR to be ubiquitously expressed in human tissues, including the skin. Confocal microscopy revealed that SVEP1 is normally mostly expressed in the cytoplasm of basal and suprabasal epidermal cells. Down-regulation of SVEP1 expression in primary keratinocytes resulted in decreased expression of major epidermal differentiation markers. Similarly, SVEP1 down-regulation was associated with disturbed differentiation and marked epidermal acanthosis in three-dimensional skin equivalents. In contrast, the dispase assay failed to demonstrate significant differences in adhesion between keratinocytes expressing normal vs. low levels of SVEP1. Homozygous Svep1 knockout mice were embryonic lethal. Thus, to assess the importance of SVEP1 for normal skin homeostasis in vivo, we down regulated SVEP1 in zebra fish embryos with a Svep1-specific splice morpholino. Scanning electron microscopy revealed a rugged epidermis with perturbed microridge formation in the center of the keratinocytes of morphant larvae. Transmission electron microscopy analysis demonstrated abnormal epidermal cell-cell adhesion with disadhesion between cells in Svep1-deficient morphant larvae compared to controls. In summary, our results indicate that SVEP1 plays a critical role during epidermal differentiation.

Congenital erythroderma is a rare and often life-threatening condition, which has been shown to result from mutations in several genes encoding important components of the epidermal differentiation program. Using whole exome sequencing, we identified in a child with congenital exfoliative erythroderma, hypotrichosis, severe nail dystrophy and failure to thrive, two heterozygous mutations in ABCA12 (c.2956C>T, p.R986W; c.5778+2T>C, p. G1900Mfs*16), a gene known to be associated with two forms of ichthyosis, autosomal recessive congenital ichthyosis, and harlequin ichthyosis. Because the patient displayed an atypical phenotype, including severe hair and nail manifestations, we scrutinized the exome sequencing data for additional potentially deleterious genetic variations in genes of relevance to the cornification process. Two mutations were identified in CAPN12, encoding a member of the calpain proteases: a paternal missense mutation (c.1511C>A; p.P504Q) and a maternal deletion due to activation of a cryptic splice site in exon 9 of the gene (c.1090_1129del; p.Val364Lysfs*11). The calpain 12 protein was found to be expressed in both the epidermis and hair follicle of normal skin, but its expression was dramatically reduced in the patient's skin. The downregulation of capn12 expression in zebrafish was associated with abnormal epidermal morphogenesis. Small interfering RNA knockdown of CAPN12 in three-dimensional human skin models was associated with acanthosis, disorganized epidermal architecture, and downregulation of several differentiation markers, including filaggrin. Accordingly, filaggrin expression was almost absent in the patient skin. Using ex vivo live imaging, small interfering RNA knockdown of calpain 12 in skin from K14-H2B GFP mice led to significant hair follicle catagen transformation compared with controls. In summary, our results indicate that calpain 12 plays an essential role during epidermal ontogenesis and normal hair follicle cycling and that its absence may aggravate the clinical manifestations of ABCA12 mutations.

The present data suggest that ADG is genetically homogeneous and result from perturbed expression of epidermal differentiation-associated genes.