Loss-of-function variants in KLF4 underlie autosomal dominant palmoplantar keratoderma

HMCN1 variants aggravate epidermolysis bullosa simplex phenotype

Epidermolysis bullosa simplex (EBS) refers to a heterogeneous group of inherited skin disorders characterized by blister formation within the basal cell layer. The disease is characterized by marked variations in phenotype severity, suggesting co-inheritance of genetic modifiers. We identified three deleterious variants in HMCN1 that co-segregated with a more severe phenotype in a group of 20 individuals with EBS caused by mutations in KRT14, encoding keratin 14 (K14). HMCN1 codes for hemicentin-1. Protein modeling, molecular dynamics simulations, and functional experiments showed that all three HMCN1 variants disrupt protein stability. Hemicentin-1 was found to be expressed in human skin above the BMZ. Using yeast-2-hybrid, co-immunoprecipitation, and proximity ligation assays, we found that hemicentin-1 binds K14. Three-dimensional skin equivalents grown from hemicentin-1-deficient cells were found to spontaneously develop subepidermal blisters, and HMCN1 downregulation was found to reduce keratin intermediate filament formation. In conclusion, hemicentin-1 binds K14 and contributes to BMZ stability, which explains the fact that deleterious HMCN1 variants co-segregate with a more severe phenotype in KRT14-associated EBS.

A loss-of-function variant in KLF4 affecting zinc finger motifs causes progressive symmetric erythrokeratodermia

We identified a novel de novo variant (c.1234C > A, p.His412Asn) in KLF4, which is located within the first zinc finger motifs of KLF4, in a patient with progressive symmetric erythrokeratodermia. By dual-luciferase reporter assay, quantitative reverse transcriptase-polymerase chain reaction and immunofluorescence, we demonstrated that the KLF4 variant is a loss-of-function mutation, and the expression of SLURP1 and DSG1, both of which are transcriptionally regulated by KLF4, was downregulated.

Proteolytic and Antiproteolytic Activity in the Skin: Gluing the Pieces Together

Epidermal differentiation is ultimately aimed at the formation of a functional barrier capable of protecting the organism from the environment while preventing loss of biologically vital elements. Epidermal differentiation entails a delicately regulated process of cell-cell junction formation and dissolution to enable upward cell migration and desquamation. Over the past two decades, the deciphering of the genetic basis of a number of inherited conditions has delineated the pivotal role played in this process by a series of proteases and protease inhibitors, including serpins, cathepsins, and cystatins, suggesting novel avenues for therapeutic intervention in both rare and common disorders of cornification.

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

BACKGROUND

The abnormal function of epidermal growth factor receptor (EGFR) has recently been shown to underlie various disorders of cornification.

OBJECTIVES

To delineate the genetic basis of a novel dominant form of palmoplantar keratoderma (PPK).

METHODS

Whole-exome (WES) and direct sequencing, quantitative real-time polymerase chain reaction, protein modelling, confocal immunofluorescence microscopy, immunoblotting, three-dimensional skin equivalents and an enzyme activity assay were used to delineate the genetic basis of a novel dominant form of PPK.

RESULTS

WES revealed heterozygous variants (c.274T > C and c.305C > T) in CTSZ (encoding cathepsin Z) in four individuals (belonging to three unrelated families) with focal PPK. Bioinformatics and protein modelling predicted the variants to be pathogenic. Previous studies have suggested that EGFR expression may be subject to cathepsin regulation. Immunofluorescence revealed reduced cathepsin Z expression in the upper epidermal layers and concomitant increased epidermal EGFR expression in patients harbouring 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 that 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, three-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.

Autosomal recessive congenital ichthyosis caused by a pathogenic missense variant in CLDN1

Autosomal recessive congenital ichthyosis (ARCI) refers to a large and genetically heterogenous group of non-syndromic disorders of cornification featuring diffuse scaling. Ichthyosis, leukocyte vacuoles, alopecia, and sclerosing cholangitis (ILVASC) syndrome is a rare autosomal recessive syndromic form of ichthyosis. The disease usually results from premature termination codon-causing pathogenic variants in CLDN1 encoding CLAUDIN-1 (CLDN1). We used whole exome sequencing (WES), Sanger sequencing, 3D protein modeling, Western blotting, and immunofluorescence confocal microscopy to delineate the genetic basis of ichthyosis in two siblings with ichthyosis but no other ectodermal abnormalities. One of the two siblings underwent liver transplantation in early childhood due to biliary atresia. Both patients were found to carry a homozygous missense pathogenic variant, c.242G>A (p.Arg81His), in CLDN1. The variant resulted in decreased CLDN1 expression in patient skin. 3D protein modeling predicted that p.Arg81His induces deleterious conformational changes. Accordingly, HaCaT cells transfected with a construct expressing the mutant CLDN1 cDNA featured decreased levels and mislocation of CLDN1 as compared with cells expressing the wildtype cDNA. In conclusion, we describe the first pathogenic missense variant in CLDN1 shown to result in ARCI.