A Heterozygous Frameshift Mutation in the V1 Domain of Keratin 5 in a Family with Dowling–Degos Disease

Epidermal keratin 5 expression and distribution is under dermal influence

Human skin melanin pigmentation is regulated by systemic and local factors. According to the type of melanin produced by melanocytes, the transfer and degradation of melanosomes differ, thus accounting for most variations between ethnicities. We made the surprising observation that in a drastically changed environment, white and black phenotypes are reversible since Caucasian skin grafted onto nude mice can become black with all black phenotypic characteristics. Black xenografts differed essentially from other grafts by the levels of epidermal FGF-2 and keratin 5. In vitro analysis confirmed that FGF-2 directly regulates keratin 5. Interestingly, this phenomenon may be involved in human pathology. Keratin 5 mutations in Dowling-Degos Disease (DDD) have already been associated with the pheomelanosome-eumelanosome transition. In a DDD patient, keratin 5 was expressed in the basal and spinous layers, as observed in black xenografts. Furthermore, in a common age-related hyperpigmentation disorder like senile lentigo (SL), keratin 5 distribution is also altered. In conclusion, modulation of keratin 5 expression and distribution either due to mutations or factors may account for the development of pigmentary disorders.

Updated review of genetic reticulate pigmentary disorders

Reticulate pigmentary disorders are a group of disorders characterized by hyper- and/or hypopigmented macules with varying sizes and amounts of pigment. Some of the disorders are heritable, such as Dowling-Degos disease, dyschromatosis universalis hereditaria, dyschromatosis symmetrica hereditaria, reticulate acropigmentation of Kitamura and X-linked reticulate pigmentary disorder. Although each condition possesses unique phenotypic characteristics and the prognosis for each is somewhat different, there is a large degree of overlap between the disorders and therefore they are difficult to differentiate in the clinical setting. This updated review provides a clinical and molecular delineation of these genetic reticulate pigmentary disorders and aims to establish a concise diagnostic strategy to allow clinical dermatologists to make an accurate diagnosis, as well as to provide useful information for clinical and genetic counselling.

Recognizing syndromic hidradenitis suppurativa: a review of the literature

Hidradenitis suppurativa (HS) is an inflammatory skin disease causing painful inflammation and suppuration. It may occur in rare syndromes: follicular occlusion, Bazex-Dupré-Christol, Down's, KID, PAPASH, PASS, PASH, and SAPHO syndromes, as well as Dowling-Degos disease. An overview of syndromic HS may inform the search for aetiological factors in HS. PubMed, Ovid and Web of Science were systematically searched using '(hidradenitis OR acne invers*) AND (syndrome OR KID OR PASS OR PAPA OR PASH OR SAPHO OR bazex-dupre OR 'dowling degos' OR triad OR tetrad)' and Cochrane Library using 'hidradenitis OR acne invers*'. A total of 82 articles were included in the final review. We summarize 134 cases collected from the 82 included articles. The syndromes are discussed, focusing on etiopathogenesis, clinical presentation and treatment. This study is based on case reports; therefore, conclusions may be subject to the selection bias. These syndromes are rare; however, it is important to recognize them, as treating them may require a different approach. Three subtypes of syndromic hidradenitis are suggested: syndromes with known genetic abnormalities, syndromes characterized by follicular plugging or structural defects, and syndromes with possible autoinflammatory pathogenesis. There was no universally effective treatment for syndromic HS, and treatment was individualized.

A novel KRT5 mutation associated with generalized severe epidermolysis bullosa simplex in a 2-year-old Chinese boy

Mutations in keratin 5 (KRT5) or KRT14 genes are responsible for the most severe form of epidermolysis bullosa simplex (EBS), which is EBS generalized severe (EBS-gen sev). To date, only four pathogenic mutations (p.Arg165Ser and p.Lys199Asn in KRT5; p.Arg125Cys and p.Arg125His in KRT14) have been reported to be responsible for EBS-gen sev in the Chinese population. In the present study, a 2-year-old Chinese boy was clinically suspected to suffer from EBS, and thus Sanger sequencing was performed in the extracted genomic DNA samples from the patient, his parents and 100 healthy controls. A novel de novo heterozygous missense mutation c.503A>G (p.Glu168Gly) located at the N-terminal end segment of the 1A domain in KRT5 was identified by molecular analysis. In silico analysis tools were used to predict the pathogenicity of the novel missense mutation. A diagnosis of EBS-gen sev was thus confirmed according to the clinical presentations and molecular results.

Heterozygous frameshift mutation in keratin 5 in a family with Galli-Galli disease

BACKGROUND

Reticulate pigmentary disorders include the rare autosomal dominant Galli-Galli disease (GGD) and Dowling-Degos disease (DDD). Clinical diagnosis between some of the subtypes can be difficult due to a degree of overlap between clinical features, therefore analysis at the molecular level may be necessary to confirm the diagnosis.

OBJECTIVES

To identify the underlying genetic defect in a 48-year-old Asian-American woman with a clinical diagnosis of GGD.

METHODS

Histological analysis was performed on a skin biopsy using haematoxylin-eosin staining. KRT5 (the gene encoding keratin 5) was amplified from genomic DNA and directly sequenced.

RESULTS

The patient had a history of pruritus and hyperpigmented erythematous macules and thin papules along the flexor surfaces of her arms, her upper back and neck, axillae and inframammary areas. Hypopigmented macules were seen among the hyperpigmentation. A heterozygous 1-bp insertion mutation in KRT5 (c.38dupG; p.Ser14GlnfsTer3) was identified in the proband. This mutation occurs within the head domain of the keratin 5 protein leading to a frameshift and premature stop codon.

CONCLUSIONS

From the histological findings and mutation analysis the individual was identified as having GGD due to haploinsufficiency of keratin 5.

The First Report of KRT5 Mutation Underlying Acantholytic Dowling-Degos Disease with Mottled Hypopigmentation in an Indian Family

Galli Galli disease (GGD) is the name given to a rare form of acantholytic Dowling-Degos disease. (DDD), the latter itself being a rare condition. We believe we are describing for the first time in Indian dermatologic literature a case of GGD in a family where 25 persons have DDD and have been able to document a KRT5 mutation in four members of the family. Whereas reticulate pigmentation is a hallmark of DDD there are rare reports of mottled pigmentation with multiple asymptomatic hypopigmented macules scattered diffusely along with the pigmentation. All the cases described here show a mottled pigmentation comprising hypo and hyperpigmented asymptomatic macules. After the clinical diagnosis was made by one of the authors (SV) in India, the German authors repeated histological examination and successfully demonstrated a heterozygous nonsense mutation, c.C10T (p.Gln4X), in exon 1 of the KRT5 gene, from various centers in Munich, Bonn, Dusseldorf and Friedrichschafen in Germany.

Impact of human pathogenic micro-insertions and micro-deletions on post-transcriptional regulation

Small insertions/deletions (INDELs) of ≤21 bp comprise 18% of all recorded mutations causing human inherited disease and are evident in 24% of documented Mendelian diseases. INDELs affect gene function in multiple ways: for example, by introducing premature stop codons that either lead to the production of truncated proteins or affect transcriptional efficiency. However, the means by which they impact post-transcriptional regulation, including alternative splicing, have not been fully evaluated. In this study, we collate disease-causing INDELs from the Human Gene Mutation Database (HGMD) and neutral INDELs from the 1000 Genomes Project. The potential of these two types of INDELs to affect binding-site affinity of RNA-binding proteins (RBPs) was then evaluated. We identified several sequence features that can distinguish disease-causing INDELs from neutral INDELs. Moreover, we built a machine-learning predictor called PinPor (predicting pathogenic small insertions and deletions affecting post-transcriptional regulation, http://watson.compbio.iupui.edu/pinpor/) to ascertain which newly observed INDELs are likely to be pathogenic. Our results show that disease-causing INDELs are more likely to ablate RBP-binding sites and tend to affect more RBP-binding sites than neutral INDELs. Additionally, disease-causing INDELs give rise to greater deviations in binding affinity than neutral INDELs. We also demonstrated that disease-causing INDELs may be distinguished from neutral INDELs by several sequence features, such as their proximity to splice sites and their potential effects on RNA secondary structure. This predictor showed satisfactory performance in identifying numerous pathogenic INDELs, with a Matthews correlation coefficient (MCC) value of 0.51 and an accuracy of 0.75.

Mutations in POGLUT1, encoding protein O-glucosyltransferase 1, cause autosomal-dominant Dowling-Degos disease

Dowling-Degos disease (DDD) is an autosomal-dominant genodermatosis characterized by progressive and disfiguring reticulate hyperpigmentation. We previously identified loss-of-function mutations in KRT5 but were only able to detect pathogenic mutations in fewer than half of our subjects. To identify additional causes of DDD, we performed exome sequencing in five unrelated affected individuals without mutations in KRT5. Data analysis identified three heterozygous mutations from these individuals, all within the same gene. These mutations, namely c.11G>A (p.Trp4*), c.652C>T (p.Arg218*), and c.798-2A>C, are within POGLUT1, which encodes protein O-glucosyltransferase 1. Further screening of unexplained cases for POGLUT1 identified six additional mutations, as well as two of the above described mutations. Immunohistochemistry of skin biopsies of affected individuals with POGLUT1 mutations showed significantly weaker POGLUT1 staining in comparison to healthy controls with strong localization of POGLUT1 in the upper parts of the epidermis. Immunoblot analysis revealed that translation of either wild-type (WT) POGLUT1 or of the protein carrying the p.Arg279Trp substitution led to the expected size of about 50 kDa, whereas the c.652C>T (p.Arg218*) mutation led to translation of a truncated protein of about 30 kDa. Immunofluorescence analysis identified a colocalization of the WT protein with the endoplasmic reticulum and a notable aggregating pattern for the truncated protein. Recently, mutations in POFUT1, which encodes protein O-fucosyltransferase 1, were also reported to be responsible for DDD. Interestingly, both POGLUT1 and POFUT1 are essential regulators of Notch activity. Our results furthermore emphasize the important role of the Notch pathway in pigmentation and keratinocyte morphology.

Grover's disease following multiple bilateral Blaschko lines: a rare clinical presentation with genetic and electron microscopic analyses

Grover's disease is an acquired dermatosis of unknown cause histopathologically characterized by the presence of acantholysis. We report an 83-year-old Japanese man who showed multiple pruritic papular lesions distributed bilaterally along Blaschko lines, necessitating the exclusion of segmental Darier's disease. No mutations in ATP2A2, ATP2C1 or keratin 5 genes were found both in the lesional skin and in peripheral leukocytes, suggesting that putative pathogenesis of Grover's disease is distinct from those of other acantholytic dermatoses. Electron microscopy revealed poorly developed tonofibrils in the basal cells, and the structure of desmosomes appeared normal, with an increase in the number of desmosomes in the spinous layer, indicating compensation of defective desmosomal function. Impairment of desmosomal plaque proteins linking tonofilaments to desmosomal cadherins may thus account for acantholysis. The unusual bilateral mosaic arrangement in our patient may offer valuable clues to the genetic basis of Grover's disease.

Progress towards genetic and pharmacological therapies for keratin genodermatoses: current perspective and future promise

Hereditary keratin disorders of the skin and its appendages comprise a large group of clinically heterogeneous disfiguring blistering and ichthyotic diseases, primarily characterized by the loss of tissue integrity, blistering and hyperkeratosis in severely affected tissues. Pathogenic mutations in keratins cause these afflictions. Typically, these mutations in concert with characteristic features have formed the basis for improved disease diagnosis, prognosis and most recently therapy development. Examples include epidermolysis bullosa simplex, keratinopathic ichthyosis, pachyonychia congenita and several other tissue-specific hereditary keratinopathies. Understanding the molecular and genetic events underlying skin dysfunction has initiated alternative treatment approaches that may provide novel therapeutic opportunities for affected patients. Animal and in vitro disease modelling studies have shed more light on molecular pathogenesis, further defining the role of keratins in disease processes and promoting the translational development of new gene and pharmacological therapeutic strategies. Given that the molecular basis for these monogenic disorders is well established, gene therapy and drug discovery targeting pharmacological compounds with the ability to reinforce the compromised cytoskeleton may lead to promising new therapeutic strategies for treating hereditary keratinopathies. In this review, we will summarize and discuss recent advances in the preclinical and clinical modelling and development of gene, natural product, pharmacological and protein-based therapies for these disorders, highlighting the feasibility of new approaches for translational clinical therapy.

Grover disease: a reappraisal of histopathological diagnostic criteria in 120 cases

Grover disease (GD) is a rather common papular pruritic dermatosis that can be transient, persistent, or asymptomatic. The microscopic diagnosis of clinically suspected lesions can be challenging because GD can adopt different patterns, and involved areas are generally admitted to be mostly focal. The histopathologic hallmark of the disease is acantholysis, frequently combined with dyskeratosis, which confers the lesions an appearance similar to Darier disease, Hailey-Hailey disease, or pemphigus. Eczematous features can be observed as well. In this study of 120 consecutive cases of GD, we have found a sex and age incidence similar to what has been previously described, with no obvious seasonal influence, but careful evaluation of their microscopic features suggests that the histopathological diagnostic criteria of GD should be expanded. Specifically, in addition to the commonly described GD findings, we have detected cases with porokeratosis-like oblique columns of parakeratosis, lesions showing a nevoid or lentiginous silhouette, intraepidermal vesicular lesions, lichenoid changes with basal vacuolization and dyskeratosis, and dysmaturative foci with keratinocyte atypia. Moreover, quite often the dermal infiltrate was composed not only of lymphocytes intermingled with eosinophils, but also of neutrophils. In many cases, the capillary vessels showed hints of vascular damage including endothelial tumefaction due to cytoplasmatic edema and erythrocyte extravasation. Finally, because involved areas were larger than 2 mm in more than 50% of our cases, we should assume that GD lesions are not always as small as commonly claimed. Awareness of the patterns newly described herein may be important to avoid underdiagnosis of GD and may contribute to understand the pathogenesis of this acantholytic disease.

Galli-galli disease: a rare acantholytic variant of dowling-degos disease

Galli-Galli disease is a rare acantholytic variant of Dowling-Degos disease, with few cases reported in the literature. We describe a case of Galli-Galli disease and review the literature.

Keratin gene mutations in disorders of human skin and its appendages

Keratins, the major structural protein of all epithelia are a diverse group of cytoskeletal scaffolding proteins that form intermediate filament networks, providing structural support to keratinocytes that maintain the integrity of the skin. Expression of keratin genes is usually regulated by differentiation of the epidermal cells within the stratifying squamous epithelium. Amongst the 54 known functional keratin genes in humans, about 22 different genes including, the cornea, hair and hair follicle-specific keratins have been implicated in a wide range of hereditary diseases. The exact phenotype of each disease usually reflects the spatial expression level and the types of mutated keratin genes, the location of the mutations and their consequences at sub-cellular levels as well as other epigenetic and/or environmental factors. The identification of specific pathogenic mutations in keratin disorders formed the basis of our understanding that led to re-classification, improved diagnosis with prognostic implications, prenatal testing and genetic counseling in severe keratin genodermatoses. Molecular defects in cutaneous keratin genes encoding for keratin intermediate filaments (KIFs) causes keratinocytes and tissue-specific fragility, accounting for a large number of genetic disorders in human skin and its appendages. These diseases are characterized by keratinocytes fragility (cytolysis), intra-epidermal blistering, hyperkeratosis, and keratin filament aggregation in severely affected tissues. Examples include epidermolysis bullosa simplex (EBS; K5, K14), keratinopathic ichthyosis (KPI; K1, K2, K10) i.e. epidermolytic ichthyosis (EI; K1, K10) and ichthyosis bullosa of Siemens (IBS; K2), pachyonychia congenita (PC; K6a, K6b, K16, K17), epidermolytic palmo-plantar keratoderma (EPPK; K9, (K1)), monilethrix (K81, K83, K86), ectodermal dysplasia (ED; K85) and steatocystoma multiplex. These keratins also have been identified to have roles in apoptosis, cell proliferation, wound healing, tissue polarity and remodeling. This review summarizes and discusses the clinical, ultrastructural, molecular genetics and biochemical characteristics of a broad spectrum of keratin-related genodermatoses, with special clinical emphasis on EBS, EI and PC. We also highlight current and emerging model tools for prognostic future therapies. Hopefully, disease modeling and in-depth understanding of the molecular pathogenesis of the diseases may lead to the development of novel therapies for several hereditary cutaneous diseases.

Galli-Galli disease: A case report with review of the literature

Galli-Galli disease is a rare variant of the genodermatosis Dowling-Degos disease with the histologic finding of acantholysis. We present the case of a patient who presented with reticulated pigmentary changes in the flexures as well as a pruritic papular eruption and histologic features consistent with Galli-Galli disease. A literature search revealed 3 previous case reports of Galli-Galli disease. These case reports were reviewed and summarized, and the clinical and histologic presentations were compared with those of our patient. In addition, another case report of patients with similar clinical manifestations was found and is presented herein. The differential diagnosis of Galli-Galli disease includes classic Dowling-Degos disease, transient acantholytic dermatosis, Darier's disease, and epidermolysis bullosa with mottled pigmentation. Galli-Galli disease is best considered a subtype of Dowling-Degos disease with clinical and histologic features similar to transient acantholytic dermatosis.

Diseases of epidermal keratins and their linker proteins

Epidermal keratins, a diverse group of structural proteins, form intermediate filament networks responsible for the structural integrity of keratinocytes. The networks extend from the nucleus of the epidermal cells to the plasma membrane where the keratins attach to linker proteins which are part of desmosomal and hemidesmosomal attachment complexes. The expression of specific keratin genes is regulated by differentiation of the epidermal cells within the stratifying squamous epithelium. Progress in molecular characterization of the epidermal keratins and their linker proteins has formed the basis to identify mutations which are associated with distinct cutaneous manifestations in patients with genodermatoses. The precise phenotype of each disease apparently reflects the spatial level of expression of the mutated genes, as well as the types and positions of the mutations and their consequences at mRNA and protein levels. Identification of specific mutations in keratinization disorders has provided the basis for improved diagnosis and subclassification with prognostic implications and has formed the platform for prenatal testing and preimplantation genetic diagnosis. Finally, precise knowledge of the mutations is a prerequisite for development of gene therapy approaches to counteract, and potentially cure, these often devastating and currently intractable diseases.

Identification of a keratin-associated protein with a putative role in vesicle transport

Protection of skin against UV light requires a coordinated interaction between melanocytes and keratinocytes. Melanosomes are lysosome-related organelles that originate in melanocytes and are transferred into keratinocytes where they form a supranuclear cap. The mechanism responsible for melanosome transfer into keratinocytes and their intracellular distribution is poorly understood. Recently, we reported for the first time that loss-of-function mutations in the keratin K5 gene affect melanosome distribution in keratinocytes and results in a reticulate hyperpigmentation disorder, called Dowling-Degos disease. Here, we characterise the distribution and behaviour of individual K5 and K14 domains following transient and stable transfection into cells. We report that the K5 head domain is considerably more stable than the K14 head. Moreover, the distribution of the K5 head domain is altered following depolymerisation of microtubules. Following co-immunoprecipitation, we verified a specific interaction between the head domain of K5 with Hsc70, a chaperone also involved in vesicle uncoating. We hypothesise that this interaction is involved in melanosome formation or transport in keratinocytes. Alternatively, it may have a general function in the regulation of keratin assembly.

Structural and regulatory functions of keratins

The diversity of epithelial functions is reflected by the expression of distinct keratin pairs that are responsible to protect epithelial cells against mechanical stress and to act as signaling platforms. The keratin cytoskeleton integrates these functions by forming a supracellular scaffold that connects at desmosomal cell-cell adhesions. Multiple human diseases and murine knockouts in which the integrity of this system is destroyed testify to its importance as a mechanical stabilizer in certain epithelia. Yet, surprisingly little is known about the precise mechanisms responsible for assembly and disease pathology. In addition to these structural aspects of keratin function, experimental evidence accumulating in recent years has led to a much more complex view of the keratin cytoskeleton. Distinct keratins emerge as highly dynamic scaffolds in different settings and contribute to cell size determination, translation control, proliferation, cell type-specific organelle transport, malignant transformation and various stress responses. All of these properties are controlled by highly complex patterns of phosphorylation and molecular associations.