Structural and functional consequences of loricrin mutations in human loricrin keratoderma (Vohwinkel syndrome with ichthyosis)

Although loricrin is the predominant protein of the cornified envelope (CE) in keratinocytes, loss or gain of loricrin function in mouse models produces only modest skin phenotypes. In contrast, insertional mutations resulting in a frameshift in the C-terminal domain of loricrin produce the characteristic ichthyosis of loricrin keratoderma in mouse and man. To ascertain the basis for the loricrin keratoderma phenotype, we assessed epidermal structure and stratum corneum (SC) function in a previously genotyped human loricrin keratoderma kindred. Our studies revealed abnormal corneocyte fragility and basal permeability barrier function, but accelerated repair kinetics. Despite fragility, increased water loss occurred predominantly via extracellular domains, which correlated with disorganized lamellar bilayers that were linked spatially to discontinuities of the CE. Accelerated barrier recovery was explicable by amplified lamellar body secretion, while partial normalization of the CE in the outer SC correlated with persistence of abundant calcium in the extracellular spaces (positioned to activate transglutaminase-1). These results show that the barrier abnormality in loricrin keratoderma is linked to a defective CE scaffold, resulting in increased extracellular permeability, as shown previously for another "scaffold disorder", lamellar ichthyosis. But in contrast to lamellar ichthyosis, the CE scaffold partially normalizes in the outer SC in loricrin keratoderma.