HMCN1 variants aggravate epidermolysis bullosa simplex phenotype.

Epidermolysis bullosa simplex (EBS) refers to a heterogeneous group of inherited skin disorders characterized by blister formation within the basal cell layer. The disease is characterized by marked variations in phenotype severity, suggesting co-inheritance of genetic modifiers. We identified three deleterious variants in HMCN1 that co-segregated with a more severe phenotype in a group of 20 individuals with EBS caused by mutations in KRT14, encoding keratin 14 (K14). HMCN1 codes for hemicentin-1. Protein modeling, molecular dynamics simulations, and functional experiments showed that all three HMCN1 variants disrupt protein stability. Hemicentin-1 was found to be expressed in human skin above the BMZ. Using yeast-2-hybrid, co-immunoprecipitation, and proximity ligation assays, we found that hemicentin-1 binds K14. Three-dimensional skin equivalents grown from hemicentin-1-deficient cells were found to spontaneously develop subepidermal blisters, and HMCN1 downregulation was found to reduce keratin intermediate filament formation. In conclusion, hemicentin-1 binds K14 and contributes to BMZ stability, which explains the fact that deleterious HMCN1 variants co-segregate with a more severe phenotype in KRT14-associated EBS.