Mutation in the gene for connexin 30.3 in a family with erythrokeratodermia variabilis

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

Partial Loss of Function ABCA12 Mutations Generate Reduced Deposition of Glucosyl-Ceramide, Leading to Patchy Ichthyosis and Erythrodermia Resembling Erythrokeratodermia Variabilis et Progressiva (EKVP)

Ichthyoses are genetically determined cornification disorders of the epidermis characterized by the presence of different degrees of scaling, hyperkeratosis, and erythroderma often associated with palmoplantar keratoderma. Different classifications of these diseases have been proposed, often based upon the involved genes and/or the clinical presentation. The clinical features of these diseases present some overlap of phenotypes among distinct genetic entities, depending mainly on the penetrance of mutations. In this study, using a clinical, genetic, and molecular approach, we analyzed a family with two affected members who had clinical and histological features resembling erythrokeratodermia variabilis (EKV) or a type of erythrodermic hyperkeratosis with palmoplantar keratoderma. Despite of the clinical presentation, we demonstrated that the affected patients were genetically double heterozygous for two different mutations in the ABCA12 gene, known to be responsible for harlequin ichthyosis. To explain the mild phenotype of our patients, we performed a molecular characterization of the skin. In the upper layers of the epidermis, the results showed a patchy presence of the glucosyl-ceramides (GlcCer), which is the lipid transported by ABCA12, fundamental in contributing to skin impermeability. Indeed, the two mutations detected do not completely abolish ABCA12 activity, indicating that the mild phenotype is due to a partial loss of function of the enzyme, thus giving rise to an intermediate phenotype resembling EKVP, due to a partial depletion of GlcCer deposition.

GJB4 variants linked to skin disease exhibit a trafficking deficiency en route to gap junction formation that can be restored by co-expression of select connexins

Epidermal keratinocytes are enriched with at least nine connexins that are key regulators of epidermal homeostasis. The role of Cx30.3 in keratinocytes and epidermal health became evident when fourteen autosomal dominant mutations in the Cx30.3-encoding GJB4 gene were linked to a rare and incurable skin disorder called erythrokeratodermia variabilis et progressiva (EKVP). While these variants are linked to EKVP, they remain largely uncharacterized hindering therapeutic options. In this study, we characterize the expression and functional status of three EKVP-linked Cx30.3 mutants (G12D, T85P, and F189Y) in tissue-relevant and differentiation-competent rat epidermal keratinocytes. We found that GFP-tagged Cx30.3 mutants were non-functional likely due to their impaired trafficking and primary entrapment within the endoplasmic reticulum (ER). However, all mutants failed to increase BiP/GRP78 levels suggesting they were not inducing an unfolded protein response. FLAG-tagged Cx30.3 mutants were also trafficking impaired yet occasionally exhibited some capacity to assemble into gap junctions. The pathological impact of these mutants may extend beyond their trafficking deficiencies as keratinocytes expressing FLAG-tagged Cx30.3 mutants exhibited increased propidium iodide uptake in the absence of divalent cations. Attempts to rescue the delivery of trafficking impaired GFP-tagged Cx30.3 mutants into gap junctions by chemical chaperone treatment were ineffective. However, co-expression of wild type Cx30.3 greatly enhanced the assembly of Cx30.3 mutants into gap junctions, although endogenous levels of Cx30.3 do not appear to prevent the skin pathology found in patients harboring these autosomal dominant mutations. In addition, a spectrum of connexin isoforms (Cx26, Cx30, and Cx43) exhibited the differential ability to trans-dominantly rescue the assembly of GFP-tagged Cx30.3 mutants into gap junctions suggesting a broad range of connexins found in keratinocytes may favourably interact with Cx30.3 mutants. We conclude that selective upregulation of compatible wild type connexins in keratinocytes may have potential therapeutic value in rescuing epidermal defects invoked by Cx30.3 EKVP-linked mutants.

Ichthyosis

The ichthyoses are a large, heterogeneous group of skin cornification disorders. They can be inherited or acquired, and result in defective keratinocyte differentiation and abnormal epidermal barrier formation. The resultant skin barrier dysfunction leads to increased transepidermal water loss and inflammation. Disordered cornification is clinically characterized by skin scaling with various degrees of thickening, desquamation (peeling) and erythema (redness). Regardless of the type of ichthyosis, many patients suffer from itching, recurrent infections, sweating impairment (hypohidrosis) with heat intolerance, and diverse ocular, hearing and nutritional complications that should be monitored periodically. The characteristic clinical features are considered to be a homeostatic attempt to repair the skin barrier, but heterogeneous clinical presentation and imperfect phenotype-genotype correlation hinder diagnosis. An accurate molecular diagnosis is, however, crucial for predicting prognosis and providing appropriate genetic counselling. Most ichthyoses severely affect patient quality of life and, in severe forms, may cause considerable disability and even death. So far, treatment provides only symptomatic relief. It is lifelong, expensive, time-consuming, and often provides disappointing results. A better understanding of the molecular mechanisms that underlie these conditions is essential for designing pathogenesis-driven and patient-tailored innovative therapeutic solutions.

Connexinplexity: the spatial and temporal expression of connexin genes during vertebrate organogenesis

Animal development requires coordinated communication between cells. The Connexin family of proteins is a major contributor to intercellular communication in vertebrates by forming gap junction channels that facilitate the movement of ions, small molecules, and metabolites between cells. Additionally, individual hemichannels can provide a conduit to the extracellular space for paracrine and autocrine signaling. Connexin-mediated communication is widely used in epithelial, neural, and vascular development and homeostasis, and most tissues likely use this form of communication. In fact, Connexin disruptions are of major clinical significance contributing to disorders developing from all major germ layers. Despite the fact that Connexins serve as an essential mode of cellular communication, the temporal and cell-type-specific expression patterns of connexin genes remain unknown in vertebrates. A major challenge is the large and complex connexin gene family. To overcome this barrier, we determined the expression of all connexins in zebrafish using single-cell RNA-sequencing of entire animals across several stages of organogenesis. Our analysis of expression patterns has revealed that few connexins are broadly expressed, but rather, most are expressed in tissue- or cell-type-specific patterns. Additionally, most tissues possess a unique combinatorial signature of connexin expression with dynamic temporal changes across the organism, tissue, and cell. Our analysis has identified new patterns for well-known connexins and assigned spatial and temporal expression to genes with no-existing information. We provide a field guide relating zebrafish and human connexin genes as a critical step toward understanding how Connexins contribute to cellular communication and development throughout vertebrate organogenesis.

Genome-wide characterization of gap junction (connexins and pannexins) genes in turbot (Scophthalmus maximus L.): evolution and immune response following Vibrio anguillarum infection

Gap junction (GJ), a special intercellular junction between different cell types, directly connects the cytoplasm of adjacent cells, allows various molecules, ions and electrical impulses to pass through the intercellular regulatory gate, and plays vital roles in response to bacterial infection. Up to date, the information about the GJ in turbot (Scophthalmus maximus L.) is still limited. In current study, 43 gap junction genes were identified in turbot, phylogeny analysis suggested that gap junctions from turbot and other species were clustered into six groups, GJA, GJB, GJC, GJD, GJE and PANX, and turbot GJs together with respective GJs from Japanese flounder, half-smooth tongue sole and large yellow croaker, sharing same ancestors. In addition, these 43 GJ genes distributed in different chromosomes unevenly. According to gene structure and domain analysis, these genes (in GJA-GJE group) were highly conserved in that most of them contain the transmembrane area, connexin domain (CNX) and cysteine-rich domain (connexin CCC), while PANXs contain Pfam Innexin. Although only one tandem duplication was identified in turbot gap junction gene, 235 pairs of segmental duplications were identified in the turbot genome. To further investigate their evolutionary relationships, Ka/Ks was calculated, and results showed that most ratios were lower than 1, indicating they had undergone negative selection. Finally, expression analysis showed that gap junction genes were widely distributed in turbot tissues and significantly regulated after Vibrio anguillarum infection. Taken together, our research could provide valuable information for further exploration of the function of gap junction genes in teleost.

Interrogation of Carboxy-Terminus Localized GJA1 Variants Associated with Erythrokeratodermia Variabilis et Progressiva

Although inherited GJA1 (encoding Cx43) gene mutations most often lead to oculodentodigital dysplasia and related disorders, four variants have been linked to erythrokeratodermia variabilis et progressiva (EKVP), a skin disorder characterized by erythematous and hyperkeratotic lesions. While two autosomal-dominant EKVP-linked GJA1 mutations have been shown to lead to augmented hemichannels, the consequence(s) of keratinocytes harboring a de novo P283L variant alone or in combination with a de novo T290N variant remain unknown. Interestingly, these variants reside within or adjacent to a carboxy terminus polypeptide motif that has been shown to be important in regulating the internalization and degradation of Cx43. Cx43-rich rat epidermal keratinocytes (REKs) or Cx43-ablated REKs engineered to express fluorescent protein-tagged P283L and/or T290N variants formed prototypical gap junctions at cell-cell interfaces similar to wildtype Cx43. Dye coupling and dye uptake studies further revealed that each variant or a combination of both variants formed functional gap junction channels, with no evidence of augmented hemichannel function or induction of cell death. Tracking the fate of EKVP-associated variants in the presence of the protein secretion blocker brefeldin A, or an inhibitor of protein synthesis cycloheximide, revealed that P283L or the combination of P283L and T290N variants either significantly extended Cx43 residency on the cell surface of keratinocytes or delayed its degradation. However, caution is needed in concluding that this modest change in the Cx43 life cycle is sufficient to cause EKVP, or whether an additional underlying mechanism or another unidentified gene mutation is contributing to the pathogenesis found in patients. This question will be resolved if further patients are identified where whole exome sequencing reveals a Cx43 P283L variant alone or, in combination with a T290N variant, co-segregates with EKVP across several family generations.

Erythrokeratoderma variabilis (EKV) - First Nepalese case documenting GJB3 mutation

Erythrokeratoderma Variabilis (EKV) is a rare genodermatosis, characterized by variable erythematous and hyperkeratotic skin lesions. It is most often transmitted in autosomal dominant manner (AD). Casual mutations were found in the GJB3 and GJB4 genes encoding connexins 31 and 30.3, respectively. We report a 7-year-old girl with multiple dusky red and brown skin lesions on face, buttock, both arms and legs. This diagnosis was made on the basis of clinical and histological findings and further genetic analysis detected a G > C transition at position 125 of the coding sequence, which replaces arginine with a proline at residue 42 of the protein (R42P). Here, we report a first case of Nepalese patient with EKV resulting from the GJB3 mutation.

Erythrokeratodermia variabilis et progressiva due to a novel mutation in GJB4

Erythrokeratodermia variabilis et progressiva (EKVP) is a rare genodermatosis of clinical and genetic heterogeneity, characterized by the manifestations of localized or disseminated persistent hyperkeratotic plagues and stationary to migratory transient erythematous patches. The majority of EKVP cases display an autosomal dominant mode of inheritance with incomplete penetrance, although recessive transmission has also been described. Mutations associated with EKVP have been primarily detected in connexin (Cx) genes. We herein reported a Chinese sporadic case of late-onset EKVP with a novel heterozygous missense mutation c.109G>A (p.V37M) in GJB4 (Cx30.3) gene, which resulted in a significant reduction of GJB4 expression in the epidermis of the patient. In accordance, while wild-type GJB4 localized at the cell membrane of HeLa cells forming intercellular junctions and intracellular puncta, V37M mutant variant was diffusely expressed within HeLa cells at a considerably lower level. Our findings reveal an essential role of GJB4 in the pathogenesis of EKVP and provides insights into the therapeutic potential of the disease.

Expanding the clinical phenotype associated with NIPAL4 mutation: Study of a Tunisian consanguineous family with erythrokeratodermia variabilis-Like Autosomal Recessive Congenital Ichthyosis

Erythrokeratodermia variabilis (EKV) is a rare disorder of cornification usually associated with dominant mutations in the GJB3 and GJB4 genes encoding connexins (Cx)31 and 30.3. Genetic heterogeneity of EKV has already been suggested. We investigated at the clinical and genetic level a consanguineous Tunisian family with 2 sisters presenting an autosomal recessive form of EKV to better characterize this disease. Mutational analysis initially screened the connexin genes and Whole-exome sequencing (WES) was performed to identify the molecular aetiology of the particular EKV phenotype in the proband. Migratory shaped erythematous areas are the initial presenting sign followed by relatively stable hyperkeratotic plaques are the two predominates characteristics in both patients. However, remarkable variability of morphological and dominating features of the disease were observed between patients. In particular, the younger sister (proband) exhibited ichthyosiform-like appearance suggesting Autosomal Recessive Congenital Ichthyosis (ARCI) condition. No causative mutations were detected in the GJB3 and GJB4 genes. WES results revealed a novel missense homozygous mutation in NIPAL4 gene (c.835C>G, p.Pro279Ala) in both patients. This variant is predicted to be likely pathogenic. In addition, in silico analysis of the mutated 3D domain structure predicted that this variant would result in NIPA4 protein destabilization and Mg²⁺ transport perturbation, pointing out the potential role of NIPAL4 gene in the development and maintenance of the barrier function of the epidermis. Taken togheter, these results expand the clinical phenotype associated with NIPAL4 mutation and reinforce our hypothesis of NIPAL4 as the main candidate gene for the EKV-like ARCI phenotype.

Gap Junction Channelopathies and Calmodulinopathies. Do Disease-Causing Calmodulin Mutants Affect Direct Cell-Cell Communication?

The cloning of connexins cDNA opened the way to the field of gap junction channelopathies. Thus far, at least 35 genetic diseases, resulting from mutations of 11 different connexin genes, are known to cause numerous structural and functional defects in the central and peripheral nervous system as well as in the heart, skin, eyes, teeth, ears, bone, hair, nails and lymphatic system. While all of these diseases are due to connexin mutations, minimal attention has been paid to the potential diseases of cell–cell communication caused by mutations of Cx-associated molecules. An important Cx accessory protein is calmodulin (CaM), which is the major regulator of gap junction channel gating and a molecule relevant to gap junction formation. Recently, diseases caused by CaM mutations (calmodulinopathies) have been identified, but thus far calmodulinopathy studies have not considered the potential effect of CaM mutations on gap junction function. The major goal of this review is to raise awareness on the likely role of CaM mutations in defects of gap junction mediated cell communication. Our studies have demonstrated that certain CaM mutants affect gap junction channel gating or expression, so it would not be surprising to learn that CaM mutations known to cause diseases also affect cell communication mediated by gap junction channels.

Hearing Impairment with Monoallelic GJB2 Variants: A GJB2 Cause or Non-GJB2 Cause?

Recessive variants in GJB2 are the most common genetic cause of sensorineural hearing impairment. However, in many patients, only one variant in the GJB2 coding region is identified using conventional sequencing strategy (eg, Sanger sequencing), resulting in nonconfirmative diagnosis. Conceivably, there might be other unidentified pathogenic variants in the noncoding region of GJB2 or other deafness-causing genes in these patients. To address this, a next-generation sequencing-based diagnostic panel targeting the entire GJB2 gene and the coding regions of 158 other known deafness-causing genes was designed and applied to 95 patients with nonsyndromic sensorineural hearing impairment (including 81 Han Taiwanese and 14 Mongolian patients) in whom only a single GJB2 variant had been detected using conventional Sanger sequencing. The panel confirmed the genetic diagnosis in 24 patients (25.3%). Twenty-two of them had causative variants in several deafness-causing genes other than GJB2, including MYO15A, MYO7A, TECTA, POU4F3, KCNQ4, SLC26A4, OTOF, MT-RNR1, MITF, WFS1, and USH2A. The other two patients had causative variants in GJB2, including a Taiwanese patient with a mosaic maternal uniparental disomy c.235delC variant (approximately 69% mosaicism) and a Mongolian patient with compound heterozygous c.35dupG and c.35delG variants, which occurred at the same site. This study demonstrates the utility of next-generation sequencing in clarifying the genetic diagnosis of hearing-impaired patients with nonconfirmative GJB2 genotypes on conventional genetic examinations.

A DSG1 Frameshift Variant in a Rottweiler Dog with Footpad Hyperkeratosis

A single male Rottweiler dog with severe footpad hyperkeratosis starting at an age of eight weeks was investigated. The hyperkeratosis was initially restricted to the footpads. The footpad lesions caused severe discomfort to the dog and had to be trimmed under anesthesia every 8–10 weeks. Histologically, the epidermis showed papillated villous projections of dense keratin in the stratum corneum. Starting at eight months of age, the patient additionally developed signs consistent with atopic dermatitis and recurrent bacterial skin and ear infections. Crusted hyperkeratotic plaques developed at sites of infection. We sequenced the genome of the affected dog and compared the data to 655 control genomes. A search for variants in 32 candidate genes associated with human palmoplantar keratoderma (PPK) revealed a single private protein-changing variant in the affected dog. This was located in the DSG1 gene encoding desmoglein 1. Heterozygous monoallelic DSG1 variants have been reported in human patients with striate palmoplantar keratoderma I (SPPK1), while biallelic DSG1 loss of function variants in humans lead to a more pronounced condition termed severe dermatitis, multiple allergies, and metabolic wasting (SAM) syndrome. The identified canine variant, DSG1:c.2541_2545delGGGCT, leads to a frameshift and truncates about 20% of the coding sequence. The affected dog was homozygous for the mutant allele. The comparative data on desmoglein 1 function in humans suggest that the identified DSG1 variant may have caused the footpad hyperkeratosis and predisposition for allergies and skin infections in the affected dog.

Identification of a CDH12 potential candidate genetic variant for an autosomal dominant form of transgrediens and progrediens palmoplantar keratoderma in a Tunisian family

Molecular diagnosis of rare inherited palmoplantar keratoderma (PPK) is still challenging. We investigated at the clinical and genetic level a consanguineous Tunisian family presenting an autosomal dominant atypical form of transgrediens and progrediens PPK to better characterize this ultrarare disease and to identify its molecular etiology. Whole-exome sequencing (WES), filtering strategies, and bioinformatics analysis have been achieved. Clinical investigation and follow up over 13 years of this Tunisian family with three siblings formerly diagnosed as an autosomal recessive form of Mal de Melela-like conducted us to reconsider its initial phenotype. Indeed, the three patients presented clinical features that overlap both Mal de Meleda and progressive symmetric erythrokeratoderma (PSEK). The mode of inheritance was also reconsidered, since the mother, initially classified as unaffected, exhibited a similar expression of the disease. WES analysis showed the absence of potentially functional rare variants in known PPKs or PSEK-related genes. Results revealed a novel heterozygous nonsynonymous variant in cadherin-12 gene (CDH12, NM_004061, c.1655C > A, p.Thr552Asn) in all affected family members. This variant is absent in dbSNP and in 50 in-house control exomes. In addition, in silico analysis of the mutated 3D domain structure predicted that this variant would result in cadherin-12 protein destabilization and thermal instability. Functional annotation and biological network construction data provide further supporting evidence for the potential role of CDH12 in the maintenance of skin integrity. Taken together, these results suggest that CDH12 gene is a potential candidate gene for an atypical presentation of an autosomal dominant form of transgrediens and progrediens PPK.

The Role of Desmoglein 1 in Gap Junction Turnover Revealed through the Study of SAM Syndrome

An effective epidermal barrier requires structural and functional integration of adherens junctions, tight junctions, gap junctions (GJ), and desmosomes. Desmosomes govern epidermal integrity while GJs facilitate small molecule transfer across cell membranes. Some patients with severe dermatitis, multiple allergies, and metabolic wasting (SAM) syndrome, caused by biallelic desmoglein 1 (DSG1) mutations, exhibit skin lesions reminiscent of erythrokeratodermia variabilis, caused by mutations in connexin (Cx) genes. We, therefore, examined whether SAM syndrome-causing DSG1 mutations interfere with Cx expression and GJ function. Lesional skin biopsies from SAM syndrome patients (n = 7) revealed decreased Dsg1 and Cx43 plasma membrane localization compared with control and nonlesional skin. Cultured keratinocytes and organotypic skin equivalents depleted of Dsg1 exhibited reduced Cx43 expression, rescued upon re-introduction of wild-type Dsg1, but not Dsg1 constructs modeling SAM syndrome-causing mutations. Ectopic Dsg1 expression increased cell-cell dye transfer, which Cx43 silencing inhibited, suggesting that Dsg1 promotes GJ function through Cx43. As GJA1 gene expression was not decreased upon Dsg1 loss, we hypothesized that Cx43 reduction was due to enhanced protein degradation. Supporting this, PKC-dependent Cx43 S368 phosphorylation, which signals Cx43 turnover, increased after Dsg1 depletion, while lysosomal inhibition restored Cx43 levels. These data reveal a role for Dsg1 in regulating epidermal Cx43 turnover.

Two de novo GJA1 mutation in two sporadic patients with erythrokeratodermia variabilis et progressiva

Background

Erythrokeratodermia variabilis et progressiva (EKVP, OMIM 133200) is a rare hereditary disorder characterized by varies from transient, fast moving erythema to persistent brown hyperkeratotic plaques. Recently, mutations in the genes gap junction alpha 1 gene (GJA1), GJB3, and GJB4 have been reported to cause EKVP. Here, we report the identification of two de novo missense mutations in the GJA1 gene in two unrelated individuals with EKVP.

Methods

The patients and his family members were subjected to mutation detection in the candidate gene GJA1, GJB3, and GJB4 by Sanger sequencing. The expression of connexin (Cx) 43 was detected by immunohistochemistry and immunofluorescence (IF) studies in the lesions.

Results

A 12‐year‐old boy presented with multiple hyperkeratotic plaques on the face, neck, elbows, wrists, limbs, knees, inguinal region, hands, and feet. A 7‐year‐old girl presented with symmetrical erythematous, plaques on the hands, feet, wrists, and ankles. A novel heterozygous missense mutation c.848C > T (p.P283L) in exon 2 of the GJA1 gene was identified in both patients. A novel heterozygous missense mutation c.869C > A (p.T290N) in exon 2 of the GJA1 gene was also identified in the boy. These mutations were not found in the unaffected family members and 100 normal controls. In the patients’ lesions, Cx43 protein was located to the cytomembrane and cytoplasm in the stratum corneum, and granular layer. Compound heterozygous mutations in the boy showed a more severe clinical phenotype and cytoplasmic mislocalization.

Conclusions

The novel mutations c.848C > T (p.P283L) and c.869C > A(p.T290N) arose de novo and were considered as the cause of two Chinese EKVP. GJA1 P283L and T290N mutations lead to Cx43 protein cytoplasmic mislocalization. Our finding expands the mutant spectrum of GJA1 gene and adds new understanding of the genotype‐phenotype correlation.

Mutations in PERP Cause Dominant and Recessive Keratoderma

Investigation of genetic determinants of Mendelian skin disorders has substantially advanced understanding of epidermal biology. Here we show that mutations in PERP, encoding a crucial component of desmosomes, cause both dominant and recessive human keratoderma. Heterozygosity for a C-terminal truncation, which produces a protein that appears to be unstably incorporated into desmosomes, causes Olmsted syndrome with severe periorificial and palmoplantar keratoderma in multiple unrelated kindreds. Homozygosity for an N-terminal truncation ablates expression and causes widespread erythrokeratoderma, with expansion of epidermal differentiation markers. Both exhibit epidermal hyperproliferation, immature desmosomes lacking a dense midline observed via electron microscopy, and impaired intercellular adhesion upon mechanical stress. Localization of other desmosomal components appears normal, which is in contrast to other conditions caused by mutations in genes encoding desmosomal proteins. These discoveries highlight the essential role of PERP in human desmosomes and epidermal homeostasis and further expand the heterogeneous spectrum of inherited keratinization disorders.

Mutations in KDSR Cause Recessive Progressive Symmetric Erythrokeratoderma

The discovery of new genetic determinants of inherited skin disorders has been instrumental to the understanding of epidermal function, differentiation, and renewal. Here, we show that mutations in KDSR (3-ketodihydrosphingosine reductase), encoding an enzyme in the ceramide synthesis pathway, lead to a previously undescribed recessive Mendelian disorder in the progressive symmetric erythrokeratoderma spectrum. This disorder is characterized by severe lesions of thick scaly skin on the face and genitals and thickened, red, and scaly skin on the hands and feet. Although exome sequencing revealed several of the KDSR mutations, we employed genome sequencing to discover a pathogenic 346 kb inversion in multiple probands, and cDNA sequencing and a splicing assay established that two mutations, including a recurrent silent third base change, cause exon skipping. Immunohistochemistry and yeast complementation studies demonstrated that the mutations cause defects in KDSR function. Systemic isotretinoin therapy has achieved nearly complete resolution in the two probands in whom it has been applied, consistent with the effects of retinoic acid on alternative pathways for ceramide generation.

Human diseases associated with connexin mutations

Gap junctions and hemichannels comprised of connexins impact many cellular processes. Significant advances in our understanding of the functional role of these channels have been made by the identification of a host of genetic diseases caused by connexin mutations. Prominent features of connexin disorders are the inability of other connexins expressed in the same cell type to compensate for the mutated one, and the ability of connexin mutants to dominantly influence the activity of other wild-type connexins. Functional studies have begun to identify some of the underlying mechanisms whereby connexin channel mutation contributes to the disease state. Detailed mechanistic understanding of these functional differences will help to facilitate new pathophysiology driven therapies for the diverse array of connexin genetic disorders. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Recent advances in understanding ichthyosis pathogenesis

The ichthyoses, also known as disorders of keratinization (DOK), encompass a heterogeneous group of skin diseases linked by the common finding of abnormal barrier function, which initiates a default compensatory pathway of hyperproliferation, resulting in the characteristic clinical manifestation of localized and/or generalized scaling. Additional cutaneous findings frequently seen in ichthyoses include generalized xerosis, erythroderma, palmoplantar keratoderma, hypohydrosis, and recurrent infections. In 2009, the Ichthyosis Consensus Conference established a classification consensus for DOK based on pathophysiology, clinical manifestations, and mode of inheritance. This nomenclature system divides DOK into two main groups: nonsyndromic forms, with clinical findings limited to the skin, and syndromic forms, with involvement of additional organ systems. Advances in next-generation sequencing technology have allowed for more rapid and cost-effective genetic analysis, leading to the identification of novel, rare mutations that cause DOK, many of which represent phenotypic expansion. This review focuses on new findings in syndromic and nonsyndromic ichthyoses, with emphasis on novel genetic discoveries that provide insight into disease pathogenesis.

Connexin channels in congenital skin disorders

Gap junctions and hemichannels comprised of connexins influence epidermal proliferation and differentiation. Significant advances in our understanding of the functional role of connexins in the skin have been made by studying the diseases caused by connexin mutations. Eleven clinically defined cutaneous disorders with an overlapping spectrum of phenotypes are caused by mutations in five different connexin genes, highlighting that disease presentation must be deciphered with an understanding of how connexin functions are affected. Increasing evidence suggests that the skin diseases produced by connexin mutations result from dominant gains of function. In palmoplantar keratoderma with deafness, the connexin 26 mutations transdominantly alter the function of wild-type connexin 43 and create leaky heteromeric hemichannels. In keratitis-ichthyosis-deafness syndrome, different connexin 26 mutations can either form dominant hemichannels with altered calcium regulation or increased calcium permeability, leading to clinical subtypes of this syndrome. It is only with detailed understanding of these subtle functional differences that we can hope to create successful pathophysiology driven therapies for the connexin skin disorders.

The Renal Connexome and Possible Roles of Connexins in Kidney Diseases

Connexins are membrane-spanning proteins that allow for the formation of cell-to-cell channels and cell-to-extracellular space hemichannels. Many connexin subtypes are expressed in kidney cells. Some mutations in connexin genes have been linked to various human pathologies, including cardiovascular, neurodegenerative, lung, and skin diseases, but the exact role of connexins in kidney disease remains unclear. Some hypotheses about a connection between genetic mutations, endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in kidney pathology have been explored. The potential relationship of kidney disease to abnormal production of connexin proteins, mutations in their genes together with ER stress, or the UPR is still a matter of debate. In this scenario, it is tantalizing to speculate about a possible role of connexins in the setting of kidney pathologies that are thought to be caused by a deregulated podocyte protein expression, the so-called podocytopathies. In this article, we give examples of the roles of connexins in kidney (patho)physiology and propose avenues for further research concerning connexins, ER stress, and UPR in podocytopathies that may ultimately help refine drug treatment.

Erythrokeratodermia variabilis et progressiva allelic to oculo-dento-digital dysplasia

Erythrokeratodermia variabilis et progressiva (EKVP) is a genodermatosis with clinical and genetic heterogeneity, most often transmitted in an autosomal dominant manner, caused by mutations in GJB3 and GJB4 genes encoding connexins (Cx)31 and 30.3, respectively. In this issue, Boyden et al. (2015) report for the first time de novo dominant mutations in GJA1 encoding the ubiquitous Cx43 in patients with EKVP. These results expand the genetic heterogeneity of EKVP and the human disease phenotypes associated with GJA1 mutations. They disclose that EKVP is allelic to oculo-dento-digital dysplasia, a rare syndrome previously known to be caused by dominant GJA1 mutations.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.

Dominant De Novo Mutations in GJA1 Cause Erythrokeratodermia Variabilis et Progressiva, without Features of Oculodentodigital Dysplasia

Genetic investigation of inherited skin disorders has informed understanding of skin self-renewal, differentiation, and barrier function. Erythrokeratodermia variabilis et progressiva (EKVP) is a rare, inherited skin disease characterized by transient figurate patches of erythema, localized or generalized scaling, and frequent palmoplantar keratoderma. By employing exome sequencing, we show that de novo missense mutations in GJA1 (gap junction protein alpha 1) cause EKVP. The severe, progressive skin disease in EKVP subjects with GJA1 mutations is distinct from limited cutaneous findings rarely found in the systemic disorder oculodentodigital dysplasia, also caused by dominant GJA1 mutations. GJA1 encodes connexin 43 (Cx43), the most widely expressed gap junction protein. We show that the GJA1 mutations in EKVP subjects lead to disruption of Cx43 membrane localization, and aggregation within the Golgi. These findings reveal a critical role for Cx43 in epidermal homeostasis, and provide evidence of organ-specific pathobiology resulting from different mutations within GJA1.

No exonic mutations at GJB2, GJB3, GJB4, GJB6, ARS (Component B), and LOR genes responsible for a Chinese patient affected by progressive symmetric erythrokeratodermia with pseudoainhum

OBJECTIVE

Progressive symmetric erythrokeratodermia (PSEK) is characterized by symmetric and growing erythematous hyperkeratotic patches over the body shortly after birth, particularly trunk and limbs, the buttocks, and the face, sometimes together with palmoplantar keratoderma (PPK). The GJB2, GJB3, GJB4, GJB6, ARS (Component B), and LOR gene mutation might contribute to PSEK manifestation. This study aimed to identify sequence alteration of these genes in a Chinese PSEK patient with pseudoainhum.

METHODS

Genomic DNA was purified from the patient's peripheral blood. Mutation analysis of target genes was performed by direct sequencing using ABI 3730 sequencer

RESULTS

No exonic mutations was identified in the aforementioned genes.

CONCLUSIONS

The result underlines the genetic heterogeneity of PSEK and other related erythrokeratodermas.

Gap junctions and blood-tissue barriers

Gap junction is a cell-cell communication junction type found in virtually all mammalian epithelia and endothelia and provides the necessary "signals" to coordinate physiological events to maintain the homeostasis of an epithelium and/or endothelium under normal physiological condition and following changes in the cellular environment (e.g., stimuli from stress, growth, development, inflammation, infection). Recent studies have illustrated the significance of this junction type in the maintenance of different blood-tissue barriers, most notably the blood-brain barrier and blood-testis barrier, which are dynamic ultrastructures, undergoing restructuring in response to stimuli from the environment. In this chapter, we highlight and summarize the latest findings in the field regarding how changes at the gap junction, such as the result of a knock-out, knock-down, knock-in, or gap junction inhibition and/or its activation via the use of inhibitors and/or activators, would affect the integrity or permeability of the blood-tissue barriers. These findings illustrate that much research is needed to delineate the role of gap junction in the blood-tissue barriers, most notably its likely physiological role in mediating or regulating the transport of therapeutic drugs across the blood-tissue barriers.

[Erythrokeratodermia variabilis]

BACKGROUND

Erythrokeratodermia variabilis (EKV) is a rare genodermatosis associated with keratinisation disorders. Mutations are found in genes encoding connexin 31 and 30.3 mapped to chromosome 1 p34-35. We report two cases of EKV, one of which presented dramatic improvement with oral retinoids.

PATIENTS AND METHODS

A 15-month-old boy was referred to us with reddish-brown hyperkeratotic and well-demarcated plaques on the extremities, axillary space and face. The lesions started when he was 6months of age. Cutaneous histopathology showed acanthosis and papillomatosis associated with orthokeratotic hyperkeratosis. Anatomoclinical comparison confirmed the diagnosis of EKV. A second child aged 10years was referred to us with fixed, well-demarcated hyperkeratotic plaques associated with transient red patches. The lesions began when she was 1month old. Anatomoclinical comparison confirmed the diagnosis of EKV and the patient showed dramatic improvement after 2weeks on acitretin.

DISCUSSION

EKV is characterized by the association of fixed well-demarcated plaques and transient erythematous patches. Although cutaneous histopathology is not specific, a typical physical examination and a compatible cutaneous histopathology can aid the diagnosis. Oral retinoids are often very rapidly effective.

Molecular diagnosis of genodermatoses

The progress of molecular genetics helps clinicians to prove or exclude a suspected diagnosis for a vast and yet increasing number of genodermatoses. This leads to precise genetic counselling, prenatal diagnosis and preimplantation genetic haplotyping for many inherited skin conditions. It is also helpful in such occasions as phenocopy, late onset and incomplete penetrance, uniparental disomy, mitochondrial inheritance and pigmentary mosaicism. Molecular methods of two genodermatoses are explained in detail, i.e. genodermatoses with skin fragility and neurofibromatosis type 1.

GJB2 Gene Mutations in Syndromic Skin Diseases with Sensorineural Hearing Loss

The GJB2 gene is located on chromosome 13q12 and it encodes the connexin 26, a transmembrane protein involved in cell-cell attachment of almost all tissues. GJB2 mutations cause autosomal recessive (DFNB1) and sometimes dominant (DFNA3) non-syndromic sensorineural hearing loss. Moreover, it has been demonstrated that connexins are involved in regulation of growth and differentiation of epidermis and, in fact, GJB2 mutations have also been identified in syndromic disorders with hearing loss associated with various skin disease phenotypes. GJB2 mutations associated with skin disease are, in general, transmitted with a dominant inheritance pattern. Nonsyndromic deafness is caused prevalently by a loss-of-function, while literature evidences suggest for syndromic deafness a mechanism based on gain-of-function. The spectrum of skin manifestations associated with some mutations seems to have a very high phenotypic variability. Why some mutations can lead to widely varying cutaneous manifestations is poorly understood and in particular, the reason why the skin disease-deafness phenotypes differ from each other thus remains unclear. This review provides an overview of recent findings concerning pathogenesis of syndromic deafness imputable to GJB2 mutations with an emphasis on relevant clinical genotype-phenotype correlations. After describing connexin 26 fundamental characteristics, the most relevant and recent information about its known mutations involved in the syndromic forms causing hearing loss and skin problems are summarized. The possible effects of the mutations on channel expression and function are discussed.

The role of connexins in ear and skin physiology - functional insights from disease-associated mutations

Defects in several different connexins have been associated with several different diseases. The most common of these is deafness, where a few mutations in connexin (Cx) 26 have been found to contribute to over 50% of the incidence of non-syndromic deafness in different human populations. Other mutations in Cx26 or Cx30 have also been associated with various skin phenotypes linked to deafness (palmoplanta keratoderma, Bart-Pumphrey syndrome, Vohwinkel syndrome, keratitis-ichthyosis-deafness syndrome, etc.). The large array of disease mutants offers unique opportunities to gain insights into the underlying function of gap junction proteins and their channels in the normal and pathogenic physiologies of the cochlea and epidermis. This review focuses on those mutants where the impact on channel function has been assessed, and correlated with the disease phenotype, or organ function in knock-out mouse models. These approaches have provided evidence supporting a role of gap junctions and hemichannels in K(+) removal and recycling in the ear, as well as possible roles for nutrient passage, in the cochlea. In contrast, increases in hemichannel opening leading to increased cell death, were associated with several keratitis-ichthyosis-deafness syndrome skin disease/hearing mutants. In addition to providing clues for therapeutic strategies, these findings allow us to better understand the specific functions of connexin channels that are important for normal tissue function. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

А case of erythrokeratodermia variabilis (Mendes da Costa type) in a three-year-old child

The authors describe a case of erythrokeratodermia variabilis (Mendes da Costa type) in a three-year-old child. They present data on the clinical picture and histological examination. Major stages of the etiology and pathogenesis of the diseases are described.

Key functions for gap junctions in skin and hearing

Cx (connexin) proteins are components of gap junctions which are aqueous pores that allow intercellular exchange of ions and small molecules. Mutations in Cx genes are linked to a range of human disorders. In the present review we discuss mutations in β-Cx genes encoding Cx26, Cx30, Cx30.3 and Cx31 which lead to skin disease and deafness. Functional studies with Cx proteins have given insights into disease-associated mechanisms and non-gap junctional roles for Cx proteins.

The role of gap junctions in the brain in health and disease

Gap junctions connect the cytosolic compartments of adjacent cells for direct electrotonic and metabolic cell-to-cell communication. Gap junctions between glial cells or neurons are ubiquitously expressed in the brain and play a role in brain development including cell differentiation, cell migration and survival, tissue homeostasis, as well as in human diseases including hearing loss, skin disease, neuropathies, epilepsy, brain trauma, and cardiovascular disease. Furthermore, gap junctions are involved in the synchronization and rhythmic oscillation of hippocampal and neocotical neuronal ensembles which might be important for memory formation and consolidation. In this review the accumulated evidence from mouse mutant and pharmacological studies using gap junction blockers is summarized and the progress made in dissecting the physiological, pathophysiological and behavioral roles of gap junction mediated intercellular communication in the brain is discussed.

Structure of the gap junction channel and its implications for its biological functions

Gap junctions consist of arrays of intercellular channels composed of integral membrane proteins called connexin in vertebrates. Gap junction channels regulate the passage of ions and biological molecules between adjacent cells and, therefore, are critically important in many biological activities, including development, differentiation, neural activity, and immune response. Mutations in connexin genes are associated with several human diseases, such as neurodegenerative disease, skin disease, deafness, and developmental abnormalities. The activity of gap junction channels is regulated by the membrane voltage, intracellular microenvironment, interaction with other proteins, and phosphorylation. Each connexin channel has its own property for conductance and molecular permeability. A number of studies have tried to reveal the molecular architecture of the channel pore that should confer the connexin-specific permeability/selectivity properties and molecular basis for the gating and regulation. In this review, we give an overview of structural studies and describe the structural and functional relationship of gap junction channels.

Do cell junction protein mutations cause an airway phenotype in mice or humans?

Cell junction proteins connect epithelial cells to each other and to the basement membrane. Genetic mutations of these proteins can cause alterations in some epithelia leading to varied phenotypes such as deafness, renal disease, skin disorders, and cancer. This review examines if genetic mutations in these proteins affect the function of lung airway epithelia. We review cell junction proteins with examples of disease mutation phenotypes in humans and in mouse knockout models. We also review which of these genes are expressed in airway epithelium by microarray expression profiling and immunocytochemistry. Last, we present a comprehensive literature review to find the lung phenotype when cell junction and adhesion genes are mutated or subject to targeted deletion. We found that in murine models, targeted deletion of cell junction and adhesion genes rarely result in a lung phenotype. Moreover, mutations in these genes in humans have no obvious lung phenotype. Our research suggests that simply because a cell junction or adhesion protein is expressed in an organ does not imply that it will exhibit a drastic phenotype when mutated. One explanation is that because a functioning lung is critical to survival, redundancy in the system is expected. Therefore mutations in a single gene might be compensated by a related function of a similar gene product. Further studies in human and animal models will help us understand the overlap in the function of cell junction gene products. Finally, it is possible that the human lung phenotype is subtle and has not yet been described.

Mutations in connexin genes and disease

BACKGROUND

Connexins are a family of transmembrane proteins that are widely expressed in the human body. Connexins play an important role in cell-cell communication and homeostasis in various tissues by forming gap junction channels, which enable a direct passage of ions or metabolites from one cell to another. Twenty-one different connexins are expressed in humans, each having distinct expression patterns and regulation properties. Knowledge on this family of proteins can be gained by making an inventory of mutations and associated diseases in human.

DESIGN

PubMed and other relevant databases were searched. In addition, key review articles were screened for relevant original publications. Sections of representative organs were photographed and annotated.

RESULTS

The crucial role of connexins is highlighted by the discovery of mutations in connexin genes which cause a variety of disorders such as myelin-related diseases, skin disorders, hearing loss, congenital cataract, or more complex syndromes such as the oculodendrodigital dysplasia. This review systematically addresses current knowledge on mutations in connexin genes and disease, focusing on the correlation between genetic defects, cellular phenotypes and clinical manifestations.

CONCLUSIONS

The review of diseases caused by mutations in connexin genes highlights the essential nature of connexin function and intercellular communication in tissue homeostasis.

Revised nomenclature and classification of inherited ichthyoses: results of the First Ichthyosis Consensus Conference in Sorèze 2009

BACKGROUND

Inherited ichthyoses belong to a large, clinically and etiologically heterogeneous group of mendelian disorders of cornification, typically involving the entire integument. Over the recent years, much progress has been made defining their molecular causes. However, there is no internationally accepted classification and terminology.

OBJECTIVE

We sought to establish a consensus for the nomenclature and classification of inherited ichthyoses.

METHODS

The classification project started at the First World Conference on Ichthyosis in 2007. A large international network of expert clinicians, skin pathologists, and geneticists entertained an interactive dialogue over 2 years, eventually leading to the First Ichthyosis Consensus Conference held in Sorèze, France, on January 23 and 24, 2009, where subcommittees on different issues proposed terminology that was debated until consensus was reached.

RESULTS

It was agreed that currently the nosology should remain clinically based. "Syndromic" versus "nonsyndromic" forms provide a useful major subdivision. Several clinical terms and controversial disease names have been redefined: eg, the group caused by keratin mutations is referred to by the umbrella term, "keratinopathic ichthyosis"-under which are included epidermolytic ichthyosis, superficial epidermolytic ichthyosis, and ichthyosis Curth-Macklin. "Autosomal recessive congenital ichthyosis" is proposed as an umbrella term for the harlequin ichthyosis, lamellar ichthyosis, and the congenital ichthyosiform erythroderma group.

LIMITATIONS

As more becomes known about these diseases in the future, modifications will be needed.

CONCLUSION

We have achieved an international consensus for the classification of inherited ichthyosis that should be useful for all clinicians and can serve as reference point for future research.