Identification of somatic and germline mosaicism for a keratin 5 mutation in epidermolysis bullosa simplex in a family of which the proband was previously regarded as a sporadic case

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

A KRT6A mutation p.Ile462Asn in a Chinese family with pachyonychia congenita, and identification of maternal mosaicism: a case report

Background

Pachyonychia congenita (PC, OMIM #167200, #167210, #615726, #615728, and #615735) is a rare autosomal dominant disorder caused by keratin gene mutations in KRT6A,KRT6B,KRT6C,KRT16 or KRT17. It is characterized with nail dystrophy and palmoplantar keratoderma (PPK). The most prominent manifestation is plantar pain. This is a further unusual case of parental mosaicism in PC. Although very rare, germ cell mosaicism should be considered when providing genetic counselling for unaffected parents of a child with PC.

Case presentation

We report the case of a 5-year-old boy with thickening nails and oral leukokeratosis at birth. He began to develop palmoplantar keratoderma at 2 years old and his sister has similar clinical manifestation characterized with nail discoloration and thickening. A previously reported heterozygous mutation, p.Ile462Asn, was identified in KRT6A in the proband and his affected sister. SNaPshot sequencing revealed mosaicism at a level of 2.5% and 4.7% in DNA from blood and hair bulbs from the unaffected mother. HiSeq deep sequencing demonstrated low-grade mosaicism in the patient’s younger sister and parents.

Conclusion

These findings indicate the ability of WES and SNaPshot sequencing to detect low-frequency mosaic mutations. Although very rare, germinal mosaicism should be considered when genetic counseling is given to families with presumed spontaneous cases of PC.

Severe Generalized Epidermolysis Bullosa Simplex in Two Hong Kong Children due to De Novo Variants in KRT14 and KRT5

We report two Hong Kong children with severe generalized epidermolysis bullosa simplex (EBS), the most severe form of EBS, without a family history of EBS. EBS is a rare genodermatosis usually inherited in an autosomal dominant fashion although rare autosomal recessive cases have been reported. Genetic studies in these patients showed that the first case was due to a novel de novo heterozygous variant, c.377T>G (NM_000526.5 (c.377T>G, p.Leu126Arg)) in the KRT14 gene and the second case was due to a rare de novo heterozygous variant c.527A>G (NM_000424.4, c.527A>G, p.Asn176Ser) in the KRT5 gene. To our knowledge, the c.377T>G variant in the KRT14 gene has not been previously reported, and the c.527A>G variant in the KRT5 gene is a rare cause of severe generalized EBS. In severe generalized EBS, infants exhibit severe symptoms at the onset; however, they tend to improve with time. A precise genetic diagnosis in these two cases aided in counseling the families concerning the prognosis in their affected children and the recurrence risk for future pregnancies.

KIT-related piebaldism in a Chinese girl

Piebaldism is a rare, autosomal dominant and congenital pigmentary disorder characterized by stable depigmentation of the skin and white forelock. Mutations in KIT or SNAI2 genes result in piebaldism. Most individuals with piebaldism have a family history of the disorder. Herein, we report a 5-month-old Chinese girl with severe piebaldism but no family history thereof. She has white forelock and large patches of depigmentation in the jaw, central anterior trunk, perineum and extremities. We performed whole-exome and Sanger sequencing and identified a de novo KIT mutation (NM_000222.2: c.2657G>A, p.Gly886Val) in exon 18 of KIT in the proband. Currently, this mutation is located in the most extreme C-terminal of the tyrosine kinase domain 2 of the KIT gene amongst all reported mutations and causes a severe clinical phenotype. We further reviewed literature on piebaldism and summarized 79 KIT gene mutations that lead to this disease. Our study may expand knowledge on the genotype-phenotype correlation in piebaldism and serve as a reference for genetic counseling and prenatal diagnosis of affected families.

A novel mutation of keratin 5 in epidermolysis bullosa simplex with migratory circinate erythema

Epidermolysis bullosa simplex migratory circinate erythema (EBS-Migr) is an uncommon subtype of EBS. We report a case of EBS-MIGR with a novel heterozygous pathogenic mutation in exon 9 (frameshift deletion c.1650delC) and likely benign heterozygous mutation in exon 2 (missense c.591C > A) of keratin 5. This novel pathogenic mutation in KRT5 expands the molecular spectrum of this rare subtype of EBS.

[Current approaches to the morphologic diagnosis of different types of congenital epidermolysis bullosa]

Congenital epidermolysis bullosa (CEB) is an extensive group of hereditary skin diseases, the differential diagnosis of which is a challenge due to the rarity of this pathology and the diversity of its clinical manifestations. The determination of the type of CEB makes it possible to estimate its prognosis and to facilitate a prenatal diagnosis.

AIM

to optimize the morphological diagnosis of different types of CEB.

MATERIAL AND METHODS

28 skin biopsies from 14 patients with different types of CEB were investigated. The investigators performed routine histological examination of skin fragments taken from a bullous area and immunofluorescence antigen mapping using the indirect immunofluorescence test (IIFT) with antibodies against structural proteins of the dermal-epidermal junction (laminin α3, β3, and γ2 chains, keratins 5 and 14, types VII and XVII collagen, α6 and β4 integrin subunits, desmoplakin, plectin, kindlin-1, and plakophillin) of the apparently unaffected skin. The intact skin of healthy individuals, which had been obtained during cosmetic operations, was used as controls in IIFT.

RESULTS

Immunofluorescence antigen mapping could determine the type of CEB in all cases and in 86% of cases identify the protein, the impaired production of which was responsible for the development of the disease.

CONCLUSION

Immunofluorescence antigen mapping is an integral part of the comprehensive morphological diagnosis of CEB, acting as an intermediate between the morphological verification of CEB diagnosis and the targeted search for mutations by a molecular genetic method.

Mutations in KRT5 and KRT14 cause epidermolysis bullosa simplex in 75% of the patients

BACKGROUND

Epidermolysis bullosa simplex (EBS) is a mechanobullous genodermatosis that may be caused by mutations in the genes KRT5 and KRT14 encoding the basal epidermal keratins 5 (K5) and 14 (K14). Three main clinical subtypes of EBS exist, differing in onset, distribution and severity of skin blistering. Previous reports of KRT5 and KRT14 mutations suggest a correlation between the location of the mutation and the severity of the associated EBS phenotype.

OBJECTIVES

The prevalence of KRT5/KRT14 mutations and the genotype-phenotype correlation in the largest tissue-confirmed EBS population is investigated.

METHODS

KRT5 and KRT14 genomic DNA and cDNA sequences of 76 clinically well-defined unrelated EBS probands were amplified and then subjected to direct sequencing and product length analysis. Immunofluorescence microscopy on patients' skin biopsies with antibodies against K5 and K14 was performed to study protein expression.

RESULTS

In 57 of 76 (75%) probands 41 different KRT5 and KRT14 mutations were identified, of which 12 were novel. Mutations affecting the highly conserved helix boundary motifs of the rod domains of K5 and K14, and the K14 helix initiation motif in particular, were associated with the severest, EBS Dowling-Meara, phenotype. In 21 EBS probands (37%) the mutation was de novo. In 19 probands (25%) KRT5 or KRT14 mutations were excluded.

CONCLUSIONS

The phenotype-genotype correlation observed in this large EBS population underscores the importance of helix boundary motifs for keratin assembly. Only three-quarters of biopsy-confirmed EBS probands have KRT5 or KRT14 mutations, indicating genetic heterogeneity in EBS. Alternative gene candidates are discussed.

Mutation mechanisms

A mutation is an event that produces heritable changes in the DNA. There are many different types of mutations, including point mutations (changes that imply loss, duplication, or alterations of small DNA segments, often involving a single or a few nucleotides) and major DNA changes (loss, duplication, or rearrangements of entire genes or of gene segments). This article reviews how different types of mutation may result in defective gene expression.

Keratin function in skin epithelia: a broadening palette with surprising shades

Keratins make up the largest subgroup of intermediate filament (IF) proteins and form a dynamic network of 10-12 nm filaments, built from type I/type II heterodimers, in the cytoplasm of epithelial cells. A major function of keratin IFs is to protect epithelial cells from mechanical and non-mechanical stresses that cause cell rupture and death. Interference with this role is the root cause of a large number of inherited epithelial fragility conditions. Additional functions, non-mechanical in nature, are manifested in a way that depends on the specific keratin and on the epithelial context. The recent discovery of unusual mutations affecting keratin proteins has uncovered a novel dimension of their mechanical support function, and has synergized with mouse genetics to reveal a role in skin pigmentation. Other studies extended the role of keratin proteins in regulating the response to pro-apoptotic signals, and revealed their ability to modulate protein synthesis and cell size in epithelial cells challenged to grow.