Defective cathepsin Z affects EGFR expression and causes autosomal dominant palmoplantar keratoderma

Abstract Background Abnormal function of the epidermal growth factor receptor (EGFR) has been recently shown to underlie various disorders of cornification. Objectives Here we aimed at delineating the genetic basis of a novel dominant form of palmoplantar keratoderma (PPK). Methods We used whole exome and direct sequencing, RT-qPCR, protein modelling, confocal immunofluorescence microscopy, immunoblotting, three-dimensional skin equivalents and an enzyme activity assays. Results Whole exome sequencing revealed heterozygous variants (c.274T > C and c.305C > T) in CTSZ encoding cathepsin Z in four individuals with focal PPK, who belong to three unrelated families. Bioinformatics and protein modeling predicted the variants to be pathogenic. Previous studies suggested that EGFR expression may be subject to cathepsin regulation. Immunofluorescence staining revealed reduced cathepsin Z expression in the upper epidermal layers and concomitant increased epidermal EGFR expression in patients harboring CTSZ variants. Accordingly, human keratinocytes transfected with constructs expressing PPK-causing variants in CTSZ displayed reduced cathepsin Z enzymatic activity as well as increased EGFR expression. In line with the role played by EGFR in the regulation of keratinocyte proliferation, human keratinocytes transfected with the PPK-causing variants, showed significantly increased proliferation which was abolished upon exposure to erlotinib, an EGFR inhibitor. Similarly, downregulation of CTSZ resulted in increased EGFR expression and increased proliferation in human keratinocytes, suggestive of a loss-of-function effect of the pathogenic variants. Finally, 3-dimensional organotypic skin equivalents grown from CTSZ-downregulated cells showed increased epidermal thickness and EGFR expression as seen in patient skin; here too, erlotinib was found to rescue the abnormal phenotype. Conclusions Taken collectively, these observations attribute to cathepsin Z a hitherto unrecognized function in epidermal differentiation.