Loricrin keratoderma: a novel disease entity characterized by nuclear accumulation of mutant loricrin

Loricrin is the major protein of the cornified cell envelope, a structure that replaces the plasma membrane during keratinocyte terminal differentiation. Recently, unique heterozygous, insertion mutations in the loricrin gene have been found to underlie certain congenital skin abnormalities, the phenotypes of which vary considerably. Clinically, these patients can be diagnosed as suffering from an ichthyotic variant of Vohwinkel's syndrome (VS), progressive symmetric erythrokeratoderma, or congenital ichthyosiform erythroderma born as a collodion baby. Common clinical features include hyperkeratosis of the palms and soles with digital constriction. Histologic characteristics include parakeratotic hyperkeratosis with hypergranulosis and nuclear accumulation of mutant loricrin. The unique mutations in the glycine-rich domain of the mutant loricrin form arginine-rich nuclear localization sequences (NLSs) that disrupt differentiation of keratinocytes. This group of unique genodermatoses caused by distinct loricrin mutations is collectively termed as loricrin keratoderma (LK).