Epidermal growth factor receptor inhibition leads to cellular phenotype correction of DSP-mutated keratinocytes.

Desmoplakin (DSP) is a desmosomal component expressed in skin and heart, essential for desmosome stability and intermediate filament connection. Pathogenic variants in the DSP gene encoding DSP, lead to heterogeneous skin, adnexa and heart-related phenotypes, including skin fragility, woolly hair (WH), palmoplantar keratoderma (PPK) and arrhythmogenic/dilated cardiomyopathy (ACM/DCM). The ambiguity of computer-based prediction analysis of pathogenicity and effect of DSP variants, indicates a necessity for functional analysis. Here, we report a heterozygous DSP variant that was not previously described, NM_004415.4:c.3337C>T (NM_004415.4(NP_004406.2):p.(Arg1113*)) in a patient with PPK, WH and ACM. RNA and protein analysis revealed ~50% reduction of DSP mRNA and protein expression. Patient's keratinocytes showed fragile cell-cell connections and perinuclear retracted intermediate filaments. Epidermal growth factor receptor (EGFR) is a transmembrane protein expressed in the basal epidermal layer involved in proliferation and differentiation, processes that are disrupted in the development of PPK, and in the regulation of the desmosome. In skin of the abovementioned patient, evident EGFR upregulation was observed. EGFR inhibition in patient's keratinocytes strongly increased DSP expression at the plasma membrane, improved intermediate filament connection with the membrane edges and reduced the cell-cell fragility. This cell phenotypic recovery was due to a translocation of DSP to the plasma membrane together with an increased number of desmosomes. These results indicate a therapeutic potential of EGFR inhibitors for disorders caused by DSP haploinsufficiency.

Functional investigation of two simultaneous or separately segregating DSP variants within a single family supports the theory of a dose-dependent disease severity.

Desmoplakin (DP) is an important component of desmosomes, essential in cell-cell connecting structures in stress-bearing tissues. Over the years, many hundreds of pathogenic variants in DSP have been associated with different cutaneous and cardiac phenotypes or a combination, known as a cardiocutaneous syndrome. Of less than 5% of the reported DSP variants, the effect on the protein has been investigated. Here, we describe and have performed RNA, protein and tissue analysis in a large family where DSPc.273+5G>A/c.6687delA segregated with palmoplantar keratoderma (PPK), woolly hair and lethal cardiomyopathy, while DSPWT/c.6687delA segregated with PPK and milder cardiomyopathy. hiPSC-derived cardiomyocytes and primary keratinocytes from carriers were obtained for analysis. Unlike the previously reported nonsense variants in the last exon of DSP that bypassed the nonsense-mediated mRNA surveillance system leading to protein truncation, variant c.6687delA was shown to cause the loss of protein expression. Patients carrying both variants and having a considerably more severe phenotype were shown to have 70% DP protein reduction, while patients carrying only c.6687delA had 50% protein reduction and a milder phenotype. The analysis of RNA from patient cells did not show any splicing effect of the c.273+5G>A variant. However, a minigene splicing assay clearly showed alternative spliced transcripts originating from this variant. This study shows the importance of RNA and protein analyses to pinpoint the exact effect of DSP variants instead of solely relying on predictions. In addition, the particular pattern of inheritance, with simultaneous or separately segregating DSP variants within the same family, strongly supports the theory of a dose-dependent disease severity.

Keratinocyte Binding Assay Identifies Anti-Desmosomal Pemphigus Antibodies Where Other Tests Are Negative.

The serological diagnosis of pemphigus relies on the detection of IgG autoantibodies directed against the epithelial cell surface by indirect immunofluorescence (IIF) on monkey esophagus and against desmoglein 1 (Dsg1) and Dsg3 by ELISA. Although being highly sensitive and specific tools, discrepancies can occur. It is not uncommon that sera testing positive by ELISA give a negative result by IIF and vice versa. This brings diagnostic challenges wherein pemphigus has to be ascertained or ruled out, especially when no biopsy is available. We utilized the ability of anti-Dsg3 and anti-Dsg1 IgG to bind in specific desmosomal patterns to living cells to investigate these discrepancies between IIF and ELISA. Living cultured primary normal human keratinocytes were grown under differentiating conditions to induce adequate expression of Dsg1 and Dsg3, incubated with patient serum for 1 h, and then stained to visualize bound IgG. We investigated two different groups; sera from patients with a positive direct immunofluorescence (DIF) and inconsistent serological findings (n = 43) and sera with positive ELISA or IIF but with negative DIF (n = 60). As positive controls we used 50 sera from patients who fulfilled all diagnostics criteria, and 10 sera from normal human subjects served as negative controls. In the DIF positive group, IgG from 39 of the 43 sera bound to the cells in a desmosomal pattern while in the DIF negative group none of the 60 sera bound to the cells. This shows that for pemphigus patients, ELISA and IIF can be negative while anti-desmosomal antibodies are present and vice versa that ELISA and IIF can be positive in non-pemphigus cases. In absence of a biopsy for DIF, such findings may lead to misdiagnosis.