Identification of clinically useful predictive genetic variants in pachyonychia congenita

BACKGROUND

Pachyonychia congenita (PC) refers to a group of autosomal dominant disorders caused by mutations in five keratin genes (KRT16,KRT6A,KRT17,KRT6B or KRT6C). Current disease classification is based on the gene harbouring disease-causing variants.

AIMS

We harnessed the International Pachyonychia Congenita Research Registry (IPCRR) containing both clinical and molecular data on patients with PC worldwide, to identify genetic variants predicting disease severity.

METHODS

We ascertained 815 individuals harbouring keratin mutations registered in the IPCRR. We looked for statistically significant associations between genetic variants and clinical manifestations in a subgroup of patients carrying mutations found in at least 10% of the cohort. Data were analysed using χ² and Kruskal-Wallis tests.

RESULTS

We identified five mutations occurring in at least 10% of the patients registered in the IPCRR. The KRT16 p.L132P mutation was significantly associated with younger age of onset, presence of palmar keratoderma oral leucokeratosis and a higher number of involved nails. By contrast, the KRT16 p.N125S and p.R127C mutations resulted in a milder phenotype featuring a decreased number of involved nails and older age of onset. Patients carrying the p.N125S mutation were less likely to develop palmar keratoderma while p.R127C was associated with an older age of palmoplantar keratoderma onset. Moreover, the KRT17 p.L99P mutation resulted in an increased number of involved fingernails and patients demonstrating 20-nail dystrophy, while the opposite findings were observed with KRT17 p.N92S mutation.

CONCLUSIONS

We have identified novel and clinically useful genetic predictive variants in the largest cohort of patients with PC described to date.

Evidence for cutaneous dysbiosis in dystrophic epidermolysis bullosa

BACKGROUND

The human microbiome project addresses the relationship between bacterial flora and the human host, in both healthy and diseased conditions. The skin is an ecosystem with multiple niches, each featuring unique physiological conditions and thus hosting different bacterial populations. The skin microbiome has been implicated in the pathogenesis of many dermatoses. Given the role of dysbiosis in the pathogenesis of inflammation, which is prominent in dystrophic epidermolysis bullosa (DEB), we undertook a study on the skin microbiome.

AIM

To characterize the skin microbiome in a series of patients with DEB.

METHODS

This was a case-control study of eight patients with DEB and nine control cases enrolled between June 2017 and November 2018. The skin of patients with DEB was sampled at three different sites: untreated wound, perilesional skin and normal-appearing (uninvolved) skin. Normal skin on the forearm was sampled from age-matched healthy controls (HCs). We used a dedicated DNA extraction protocol to isolate microbial DNA, which was then analysed using next-generation microbial 16S rRNA sequencing. Data were analysed using a series of advanced bioinformatics tools.

RESULTS

The wounds, perilesional and uninvolved skin of patients with DEB demonstrated reduced bacterial diversity compared with HCs, with the flora in DEB wounds being the least diverse. We found an increased prevalence of staphylococci species in the lesional and perilesional skin of patients with DEB, compared with their uninvolved, intact skin. Similarly, the uninvolved skin of patients with DEB displayed increased staphylococcal content and significantly different microbiome diversities (other than staphylococci) compared with HC skin.

CONCLUSIONS

These findings suggest the existence of a unique DEB-associated skin microbiome signature, which could be targeted by specific pathogen-directed therapies. Moreover, altering the skin microbiome with increasing colonization of bacteria associated with nonchronic wounds may potentially facilitate wound healing in patients with DEB.

Molecular epidemiology of pachyonychia congenita in the Israeli population

BACKGROUND

Pachyonychia congenita (PC) is a rare autosomal dominant disorder featuring palmoplantar keratoderma, nail dystrophy, oral leucokeratosis, pilosebaceous cysts and natal teeth. PC results from dominant mutations in one of five genes (KRT6A, KRT6B, KRT6C, KRT16, KRT17) encoding keratin proteins.

AIM

To delineate the clinical and genetic features of PC in a series of Israeli patients.

METHODS

We used direct sequencing of genomic DNA, and also used cDNA sequencing where applicable.

RESULTS

We collected clinical information and molecular data in a cohort of Israeli families diagnosed with PC (n = 16). Most of the patients were Ashkenazi Jews and had a family history of PC. The most common clinical findings were painful focal plantar keratoderma (94%) accompanied by nail dystrophy (81%), pilosebaceous cysts (31%) and prenatal/natal teeth (13%). In contrast to the high prevalence of KRT6A mutations in other populations, we found that KRT16 mutations were the most common type among Israeli patients with PC (56%). Most (77%) of the Israeli patients with PC with KRT16 mutation carried the same variant (c.380G>A; p.R127H) and shared the same haplotype around the KRT16 locus, suggestive of a founder effect.

CONCLUSION

The data gleaned from this study emphasizes the importance of population-specific tailored diagnostic strategies.

ST18 affects cell-cell adhesion in pemphigus vulgaris in a tumour necrosis factor-α-dependent fashion

BACKGROUND

Pemphigus vulgaris (PV) is a life-threatening mucocutaneous autoimmune blistering disease. We previously showed that genetic variants within the ST18 gene promoter area confer a sixfold increase in the propensity to develop PV. ST18, a transcription factor, was found to be overexpressed in the epidermis of patients with PV. In addition, it was found to promote autoantibody-mediated abnormal epidermal cell-cell adhesion and secretion of proinflammatory mediators by keratinocytes.

OBJECTIVES

To delineate the mechanism through which ST18 contributes to destabilization of cell-cell adhesion.

METHODS

We used quantitative reverse-transcriptase polymerase chain reaction, immunofluorescence microscopy, a luciferase reporter system, site-directed mutagenesis, chromatin immunoprecipitation (ChIP) and the dispase dissociation assay.

RESULTS

The ChIP and luciferase reporter assays showed that ST18 directly binds and activates the TNF promoter. Accordingly, increased ST18 expression contributes to PV pathogenesis by destabilizing cell-cell adhesion in a tumour necrosis factor (TNF)-α-dependent fashion. In addition, dual immunofluorescence staining showed increased expression of both ST18 and TNF-α in the skin of patients with PV carrying an ST18-associated PV risk variant, which was found to be associated with a more extensive PV phenotype.

CONCLUSIONS

Our findings suggest a role for TNF-α in mediating the deleterious effect of increased ST18 expression in PV skin.

Palmoplantar keratoderma caused by a missense variant in CTSB encoding cathepsin B

BACKGROUND

Palmoplantar keratoderma (PPK) refers to a large group of disorders characterized by extensive genetic and phenotypic heterogeneity. PPK diagnosis therefore increasingly relies upon genetic analysis.

AIM

To delineate the genetic defect underlying a case of diffuse erythematous PPK associated with peeling of the skin.

METHODS

Whole exome and direct sequencing, real-time quantitative PCR, protein modelling and a cathepsin B enzymatic assay were used.

RESULTS

The patient studied had severe diffuse erythematous PPK transgrediens. Pedigree analysis suggested an autosomal dominant mode of inheritance. Whole exome sequencing revealed a heterozygous missense mutation in the CTSB gene, encoding the cysteine protease cathepsin B. Genomic duplications in a noncoding region, which regulates the expression of CTSB, were recently found to cause erythrokeratolysis hiemalis, a rare autosomal dominant disorder of cornification. This mutation affects a highly conserved residue, and is predicted to be pathogenic. Protein modelling indicated that the mutation is likely to lead to increased endopeptidase cathepsin B activity. Accordingly, the CTSB variant was found to result in increased cathepsin B proteolytic activity.

CONCLUSION

In summary, we report the identification of the first gain-of-function missense mutation in CTSB, which was found to be associated in one individual with a dominant form of diffuse PPK.

A novel homozygous deletion in EXPH5 causes a skin fragility phenotype

Epidermolysis bullosa simplex (EBS) is the most common form of EB. Eight different genes have been implicated in the pathogenesis of different types of EBS, but a substantial portion of the cases cannot be attributed to mutations in known genes. Recently, recessive mutations in the gene EXPH5 (encoding exophilin-5, also known as Slac2-b) were identified in patients affected with a mild form of EBS. We used immunohistochemistry, Sanger sequencing and PCR-restriction fragment length polymorphism analysis to identify the cause of mild congenital skin fragility in a 3-year-old girl. No mutations were detected in KRT5 or KRT14, but we identified a novel homozygous deletion in EXPH5, which was found to cosegregate with the disease phenotype in the family. Our results further expand the spectrum of mutations in EXPH5. Appraisal of the present case against previously reported patients indicate that EXPH5 mutations result in a distinctive skin fragility phenotype, with minimal blistering compared with other forms of basal EBS.

Pyoderma gangrenosum, acne and ulcerative colitis in a patient with a novel mutation in the PSTPIP1 gene

BACKGROUND

Pyogenic sterile arthritis, pyoderma gangrenosum and acne (PAPA) syndrome is a rare hereditary, autosomal dominant, auto-inflammatory disease caused by mutations in the PSTPIP1 gene, which encodes proline-serine-threonine phosphatase interacting protein 1. The fact that PSTPIP1 is involved in immune regulation provides a rationale for treatment of this rare disease with interleukin (IL)-1 signalling blocking agents.

AIM

We investigated a 33-year-old man with a long-standing history of ulcerative colitis, severe acne and recurrent skin ulcerations, and a 3-year history of a recalcitrant pustular rash.

METHODS

We used direct sequencing to search for mutations in the PSTPIP1 gene.

RESULTS

Examination of biopsies obtained from pustules and skin ulcers revealed folliculitis and ulceration with a diffuse neutrophilic dermal infiltrate, consistent with a diagnosis of pyoderma gangrenosum. Because of the known association of acne and pyoderma gangrenosum in PAPA syndrome, we determined the entire coding sequence of the PSTPIP1 gene, and identified a hitherto unreported heterozygous mutation predicted to alter a highly conserved residue (p.G403R) and to be damaging to the protein function. Based on this finding, we initiated treatment with a human IL-1 receptor antagonist, anakinra, which led to a dramatic improvement in the patient's condition.

CONCLUSIONS

We describe a novel mutation in PSTPIP1 resulting in pyoderma gangrenosum, acne and ulcerative colitis. This novel constellation of clinical manifestations, which we term 'PAC syndrome', suggests the need to regroup all PSTPIP1-associated phenotypes under one aetiological group.