Loricrin is a major constituent of the epidermal cornified cell envelope (CE). We have recently identified heterozygous loricrin gene mutations in two dominantly inherited skin diseases, the ichthyotic variant of Vohwinkel syndrome (IVS) and progressive symmetric erythrokeratoderma with palmoplantar keratoderma (PSEK-PPK), collectively termed loricrin keratoderma (LK). In order to see whether the mutant loricrin molecules predicted by DNA sequencing are expressed in vivo and to define their pathological effects, we raised antibodies against synthetic peptides corresponding to the mutated sequences of loricrin. Immunoblotting of horny cell extracts from LK patients showed specific bands for mutant loricrin. Immunohistochemistry of LK skin biopsies showed positive immunoreactivity to the mutant loricrin antibodies in the nuclei of differentiated epidermal keratinocytes. The immunostaining was localized to the nucleoli of the middle epidermal layer. As keratinocyte differentiation progressed the immunoreactivity moved gradually into the nucleoplasm leaving nucleoli mostly non-immunoreactive. No substantial staining was observed along the CE. The present study confirmed that mutant loricrin was expressed in the LK skin. Mutant loricrin, as a dominant negative disrupter, is not likely to affect CE crosslinking directly, but seems to interfere with nuclear/nucleolar functions of differentiating keratinocytes. In addition, detection of the mutant loricrin in scraped horny layer could provide a simple non-invasive screening test for LK.
