Loricrin and human skin diseases: molecular basis of loricrin keratodermas

The cornified cell envelope (CE) is a tough structure formed beneath the plasma membrane of terminally differentiated keratinocytes. Recent progress in understanding the molecular organization of the CE has disclosed the complex, yet orderly structure that functions as a protective barrier against the environment. We have recently demonstrated that two inherited skin diseases, Vohwinkel's syndrome (VS) and progressive symmetric erythrokeratoderma (PSEK) may result from mutations in the gene encoding loricrin, a major constituent of the CE. In adult human epidermis, loricrin is diffusely distributed within the superficial granular cells. In the cornified cells, loricrin is associated with CEs. In some patients with VS and PSEK skin, however, granular cells contain many intranuclear granules which are labeled with an amino-terminal loricrin antibody. CEs are thinner than normal and sparsely labeled with the loricrin antibody. Parakeratotic cornified cells contain loricrin-positive granules. Sequencing of the loricrin gene has disclosed heterozygous mutations; insertion of one nucleotide (730insG, 709insC) that shifts the reading frame in these patients. Consequently the carboxyl-terminus are replaced by highly charged missense sequences that override the endogeneous termination codon extending the protein with an additional 22 amino acids. Elucidation of the molecular biology of "loricrin keratodermas" adds to our understanding of the complexity and biological significance of the CE.