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

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.

Rare mosaic variant of GJA1 in a patient with a neurodevelopmental disorder

GJA1 is the causative gene for oculodentodigital dysplasia (ODDD). A novel de novo GJA1 variant, NM 000165:c263C > T [p.P88L], was identified in a mosaic state in a patient with short stature, seizures, delayed myelination, mild hearing loss, and tooth enamel hypoplasia. Although the patient exhibited severe neurodevelopmental delay, other clinical features of ODDD, including limb anomalies, were mild. This may be due to differences in the mosaic ratios in different organs.

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.

Increased Hemichannel Activity Displayed by a Connexin43 Mutation Causing a Familial Connexinopathy Exhibiting Hypotrichosis with Follicular Keratosis and Hyperostosis

Mutations in the GJA1 gene that encodes connexin43 (Cx43) cause several rare genetic disorders, including diseases affecting the epidermis. Here, we examined the in vitro functional consequences of a Cx43 mutation, Cx43-G38E, linked to a novel human phenotype of hypotrichosis, follicular keratosis and hyperostosis. We found that Cx43-G38E was efficiently translated in Xenopus oocytes and localized to gap junction plaques in transfected HeLa cells. Cx43-G38E formed functional gap junction channels with the same efficiency as wild-type Cx43 in Xenopus oocytes, although voltage gating of the gap junction channels was altered. Notably, Cx43-G38E significantly increased membrane current flow through the formation of active hemichannels when compared to wild-type Cx43. These data demonstrate the association of increased hemichannel activity to a connexin mutation linked to a skeletal-cutaneous phenotype, suggesting that augmented hemichannel activity could play a role in skin and skeletal disorders caused by human Cx43 mutations.

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.

StarD7 deficiency hinders cell motility through p-ERK1/2/Cx43 reduction

StarD7 belongs to START protein family involved in lipid traffic, metabolism, and signaling events. Its precursor, StarD7.I which is important for mitochondrial homeostasis, is processed to the StarD7.II isoform that lacks the mitochondrial targeting sequence and is mainly released to the cytosol. StarD7 knockdown interferes with cell migration by an unknown mechanism. Here, we demonstrate that StarD7 silencing decreased connexin 43 (Cx43), integrin β1, and p-ERK1/2 expression in the non-tumoral migratory HTR-8/SVneo cells. StarD7-deficient cells exhibited Golgi disruption and reduced competence to reorient the microtubule-organizing center. The migratory capacity of StarD7-silenced cells was reestablished when Cx43 level was resettled, while p-ERK1/2 expression remained low. Importantly, ectopic expression of the StarD7.II isoform not only restored cell migration but also ERK1/2, Cx43, and integrin β1 expression. Thus, StarD7 is implicated in cell migration through an ERK1/2/Cx43 dependent mechanism but independent of the StarD7.I function in the mitochondria.

Towards a Better Understanding of Genotype-Phenotype Correlations and Therapeutic Targets for Cardiocutaneous Genes: The Importance of Functional Studies above Prediction

Genetic variants in gene-encoding proteins involved in cell−cell connecting structures, such as desmosomes and gap junctions, may cause a skin and/or cardiac phenotype, of which the combination is called cardiocutaneous syndrome. The cardiac phenotype is characterized by cardiomyopathy and/or arrhythmias, while the skin particularly displays phenotypes such as keratoderma, hair abnormalities and skin fragility. The reported variants associated with cardiocutaneous syndrome, in genes DSP, JUP, DSC2, KLHL24, GJA1, are classified by interpretation guidelines from the American College of Medical Genetics and Genomics. The genotype−phenotype correlation, however, remains poorly understood. By providing an overview of variants that are assessed for a functional protein pathology, we show that this number (n = 115) is low compared to the number of variants that are assessed by in silico algorithms (>5000). As expected, there is a mismatch between the prediction of variant pathogenicity and the prediction of the functional effect compared to the real functional evidence. Aiding to improve genotype−phenotype correlations, we separate variants into ‘protein reducing’ or ‘altered protein’ variants and provide general conclusions about the skin and heart phenotype involved. We conclude by stipulating that adequate prognoses can only be given, and targeted therapies can only be designed, upon full knowledge of the protein pathology through functional investigation.

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.

The Genomic and Phenotypic Landscape of Ichthyosis: An Analysis of 1000 Kindreds

Importance

Ichthyoses are clinically and genetically heterogeneous disorders characterized by scaly skin. Despite decades of investigation identifying pathogenic variants in more than 50 genes, clear genotype-phenotype associations have been difficult to establish.

Objective

To expand the genotypic and phenotypic spectra of ichthyosis and delineate genotype-phenotype associations.

Design, Setting, and Participants

This cohort study recruited an international group of individuals with ichthyosis and describes characteristic and distinguishing features of common genotypes, including genotype-phenotype associations, during a 10-year period from June 2011 to July 2021. Participants of all ages, races, and ethnicities were included and were enrolled worldwide from referral centers and patient advocacy groups. A questionnaire to assess clinical manifestations was completed by those with a genetic diagnosis.

Main Outcomes and Measures

Genetic analysis of saliva or blood DNA, a phenotyping questionnaire, and standardized clinical photographs. Descriptive statistics, such as frequency counts, were used to describe the cases in the cohort. Fisher exact tests identified significant genotype-phenotype associations.

Results

Results were reported for 1000 unrelated individuals enrolled from around the world (mean [SD] age, 50.0 [34.0] years; 524 [52.4%] were female, 427 [42.7%] were male, and 49 [4.9%] were not classified); 75% were from the US, 12% from Latin America, 4% from Canada, 3% from Europe, 3% from Asia, 2% from Africa, 1% from the Middle East, and 1% from Australia and New Zealand. A total of 266 novel disease-associated variants in 32 genes were identified among 869 kindreds. Of these, 241 (91%) pathogenic variants were found through multiplex amplicon sequencing and 25 (9%) through exome sequencing. Among the 869 participants with a genetic diagnosis, 304 participants (35%) completed the phenotyping questionnaire. Analysis of clinical manifestations in these 304 individuals revealed that pruritus, hypohydrosis, skin pain, eye problems, skin odor, and skin infections were the most prevalent self-reported features. Genotype-phenotype association analysis revealed that the presence of a collodion membrane at birth (odds ratio [OR], 6.7; 95% CI, 3.0-16.7; P < .001), skin odor (OR, 2.8; 95% CI, 1.1-6.8; P = .02), hearing problems (OR, 2.9; 95% CI, 1.6-5.5; P < .001), eye problems (OR, 3.0; 95% CI, 1.5-6.0; P < .001), and alopecia (OR, 4.6; 95% CI, 2.4-9.0; P < .001) were significantly associated with TGM1 variants compared with other ichthyosis genotypes studied. Skin pain (OR, 6.8; 95% CI, 1.6-61.2; P = .002), odor (OR, 5.7; 95% CI, 2.0-19.7; P < .001), and infections (OR, 3.1; 95% CI, 1.4-7.7; P = .03) were significantly associated with KRT10 pathogenic variants compared with disease-associated variants in other genes that cause ichthyosis. Pathogenic variants were identified in 869 (86.9%) participants. Most of the remaining individuals had unique phenotypes, enabling further genetic discovery.

Conclusions and Relevance

This cohort study expands the genotypic and phenotypic spectrum of ichthyosis, establishing associations between clinical manifestations and genotypes. Collectively, the findings may help improve clinical assessment, assist with developing customized management plans, and improve clinical course prognostication.

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.

Heterozygous GJA1 variants with ocular phenotype: Missense in domain but truncation out of domain

Purpose

Oculodentodigital dysplasia (ODDD) is a group disorder caused by GJA1 variants, of which glaucoma leading to blindness is a frequent complication of the ocular phenotype. In this study, the correlation of the GJA1 genotype with the ocular phenotype was analyzed systematically.

Methods

GJA1 variants were collected from in-house whole-exome sequencing data of 5,307 individuals. Potentially pathogenic variants (PPVs) were defined based on prediction of multiple in silico tools, related phenotypes, and previously established evidence. The characteristics of GJA1 PPVs were evaluated based on our data, gnomAD, and HGMD.

Results

In total, 21 rare variants in GJA1 were detected in 32 subjects from the study cohort. Four of the 21 variants were classified as PPVs, including two frameshift, one missense, and one in-frame deletion. The four PPVs were detected in four probands with microcornea or high hyperopia; two developed glaucoma. A systematic review of GJA1 variants in literature suggested that most heterozygous missense PPVs are located inside the connexin domain. All truncations downstream of the connexin domain are associated with autosomal dominant disease, while most truncations within the domain are associated with autosomal recessive ODDD. Ocular signs were present in 80.0% (116/145) of patients with GJA1 PPVs. Of the 116 patients, glaucoma was observed in 26.7% (31/116), among whom 77.4% (24/31) of cases occurred in patients ≥10 years old.

Conclusions

Eye abnormalities are the most common signs associated with GJA1 PPVs, and they carry a high risk of developing glaucoma. The identification of GJA1 PPVs needs further attention and clarification.

Cutaneous and hepatic vascular lesions due to a recurrent somatic GJA4 mutation reveal a pathway for vascular malformation

The term “cavernous hemangioma” has been used to describe vascular anomalies with histology featuring dilated vascular spaces, vessel walls consisting mainly of fibrous stromal bands lined by a layer of flattened endothelial cells, and an irregular outer rim of interrupted smooth muscle cells. Hepatic hemangiomas (HHs) and cutaneous venous malformations (VMs) share this histologic pattern, and we examined lesions in both tissues to identify genetic drivers. Paired whole-exome sequencing (WES) of lesional tissue and normal liver in HH subjects revealed a recurrent GJA4 c.121G>T (p.Gly41Cys) somatic mutation in four of five unrelated individuals, and targeted sequencing in paired tissue from 9 additional HH individuals identified the same mutation in 8. In cutaneous lesions, paired targeted sequencing in 5 VMs and normal epidermis found the same GJA4 c.121G>T (p.Gly41Cys) somatic mutation in three. GJA4 encodes gap junction protein alpha 4, also called connexin 37 (Cx37), and the p.Gly41Cys mutation falls within the first transmembrane domain at a residue highly conserved among vertebrates. We interrogated the impact of the Cx37 mutant via lentiviral transduction of primary human endothelial cells. We found that the mutant induced changes in cell morphology and activated serum/glucocorticoid-regulated kinase 1 (SGK1), a serine/threonine kinase known to regulate cell proliferation and apoptosis, via non-canonical activation. Treatment with spironolactone, an inhibitor of angiogenesis, suppressed mutant SGK1 activation and reversed changes in cell morphology. These findings identify a recurrent somatic GJA4 c.121G>T mutation as a driver of hepatic and cutaneous VMs, revealing a new pathway for vascular anomalies, with spironolactone a potential pathogenesis-based therapy.

Harnessing the therapeutic potential of antibodies targeting connexin hemichannels

BACKGROUND

Connexin hemichannels have been implicated in pathology-promoting conditions, including inflammation, numerous widespread human diseases, including cancer and diabetes, and several rare diseases linked to pathological point mutations.

METHODS

We analysed the literature focusing on antibodies capable of modulating hemichannel function, highlighting generation methods, applications to basic biomedical research and translational potential.

RESULTS

Anti-hemichannel antibodies generated over the past 3 decades targeted mostly connexin 43, with a focus on cancer treatment. A slow transition from relatively unselective polyclonal antibodies to more selective monoclonal antibodies resulted in few products with interesting characteristics that are under evaluation for clinical trials. Selection of antibodies from combinatorial phage-display libraries, has permitted to engineer a monoclonal antibody that binds to and blocks pathological hemichannels formed by connexin 26, 30 and 32.

CONCLUSIONS

All known antibodies that modulate connexin hemichannels target the two small extracellular loops of the connexin proteins. The extracellular region of different connexins is highly conserved, and few residues of each connexins are exposed. The search for new antibodies may develop an unprecedented potential for therapeutic applications, as it may benefit tremendously from novel whole-cell screening platforms that permit in situ selection of antibodies against membrane proteins in native state. The demonstrated efficacy of mAbs in reaching and modulating hemichannels in vivo, together with their relative specificity for connexins overlapping epitopes, should hopefully stimulate an interest for widening the scope of anti-hemichannel antibodies. There is no shortage of currently incurable diseases for which therapeutic intervention may benefit from anti-hemichannel antibodies capable of modulating hemichannel function selectively and specifically.

Comparative Analysis of Cx31 and Cx43 in Differentiation-Competent Rodent Keratinocytes

When considering connexin expression and regulation, the epidermis of the skin is one of the most complex tissues found in mammals even though it largely contains a single cell type, the keratinocyte. In the rodent epidermis, up to 9 connexin family members have been detected at the mRNA level. Many of these connexins are temporally and spatially regulated in coordination with keratinocyte progenitor cell differentiation and migration from the stratum basale to form the stratum spinosum and stratum granulosum layers before finally forming the stratum corneum. Cx43 is the principal connexin found in basal keratinocytes and to a lesser degree found in keratinocytes that have begun to differentiate where Cx26, Cx30 and Cx31 become prevalent. Here we show that the CRISPR-Cas9 ablation of Cx43 reduces overall gap junction coupling in monolayer cultures of rat epidermal keratinocytes (REKs) and dysregulates the differentiation of REKs when grown in organotypic cultures. Natively found in differentiated keratinocytes, Cx31 readily assembles into gap junctions when expressed in REKs where it can extensively co-assemble into the same gap junctions with co-expressed Cx30. Time-lapse imaging indicated that many Cx31 gap junctions are mobile within the plasma membrane undergoing both fusion and fission events. Finally, the persistence of pre-existing Cx31 gap junctions in the presence of the protein trafficking blocker, brefeldin A, is longer than that found for Cx43 gap junctions indicating that it has a distinctly different life expectancy in REKs. Collectively, this study highlights the importance of Cx43 in rodent keratinocyte differentiation and suggests that Cx31 acquires life-cycle properties that are distinct from Cx43.

Fulminant myocarditis following recurrent generalized erythrokeratoderma in a child with a heterozygous GJA1 variant

Pathogenic germline variants in the gap junction protein alpha 1 (GJA1) gene have been identified in several congenital disorders affecting cutaneous, skeletal, and cardiac tissues. Here, we describe a 12-year-old patient with a GJA1 c.113G>A, p.(Gly38Glu) variant, who presented with fulminant myocarditis following recurrent generalized erythrokeratoderma. His mother and younger sister had the same clinical manifestations with the same GJA1 variant, but did not have cardiac dysfunction. GJA1 variants have been reported in patients with congenital cardiac malformations, while acute myocarditis in GJA1-related disorders has not been reported so far.

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.

Connexin 43 Mutations Lead to Increased Hemichannel Functionality in Skin Disease

Gap junctional channels are specialized components of the cellular membrane that allow the intercellular passage of small metabolites, ions, and second messengers to maintain homeostasis. They are comprised of members of the connexin gene family that encode a wide array of proteins that are expressed in nearly every tissue type. Cx43 is perceived to be the most broadly expressed connexin in humans, with several genetic skin diseases being linked to Cx43 mutations specifically. These mutations, in large, produce a gain of functional hemichannels that contribute to the phenotypes of Erythrokeratoderma Variabilis et Progressiva (EKVP), Palmoplantar Keratodemra Congenital Alopecia-1 (PPKCA1), and others that produce large conductance and increased permselectivity in otherwise quiescent structures. Gaining functional hemichannels can have adverse effects in the skin, inducing apoptosis via Ca²⁺ overload or increased ATP permeability. Here, we review the link between Cx43 and skin disease. We aim to provide insight into the mechanisms regulating the normal and pathophysiological gating of these essential proteins, as well as address current therapeutic strategies. We also demonstrate that transient transfection of neuro-2a (N2a) cells with mutant Cx43 cDNA resulted in increased hemichannel activity compared to wild-type Cx43 and untransfected cells, which is consistent with other studies in the current literature.

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.

Connexin43 mutations linked to skin disease have augmented hemichannel activity

Mutations in the gene (GJA1) encoding connexin43 (Cx43) are responsible for several rare genetic disorders, including non-syndromic skin-limited diseases. Here we used two different functional expression systems to characterize three Cx43 mutations linked to palmoplantar keratoderma and congenital alopecia-1, erythrokeratodermia variabilis et progressiva, or inflammatory linear verrucous epidermal nevus. In HeLa cells and Xenopus oocytes, we show that Cx43-G8V, Cx43-A44V and Cx43-E227D all formed functional gap junction channels with the same efficiency as wild-type Cx43, with normal voltage gating and a unitary conductance of ~110 pS. In HeLa cells, all three mutations also localized to regions of cell-cell contact and displayed a punctate staining pattern. In addition, we show that Cx43-G8V, Cx43-A44V and Cx43-E227D significantly increase membrane current flow through formation of active hemichannels, a novel activity that was not displayed by wild-type Cx43. The increased membrane current was inhibited by either 2 mM calcium, or 5 µM gadolinium, mediated by hemichannels with a unitary conductance of ~250 pS, and was not due to elevated mutant protein expression. The three Cx43 mutations all showed the same gain of function activity, suggesting that augmented hemichannel activity could play a role in skin-limited diseases caused by human Cx43 mutations.

Gain-of-Function Mutations in TRPM4 Activation Gate Cause Progressive Symmetric Erythrokeratodermia

Transient receptor potential (TRP) channels respond to various chemical and physical stimuli by mediating cation influx. The skin expresses abundant TRP channels of different subtypes, which play an essential role in the maintenance of skin functionality. Here, we report cases of mutations in TRPM4, which encodes TRPM4, a Ca²⁺-activated monovalent cation channel, as a cause of an autosomal dominant form of progressive symmetric erythrokeratodermia. In three separate families with progressive symmetric erythrokeratodermia, we identified two missense mutations (c.3099C>G and c.3119T>C) that produce p.Ile1033Met and p.Ile1040Thr, both of which are located in the S6 transmembrane domain of the TRPM4 protein. The substitutions are expected to directly affect activation gating of TRPM4 according to the cryo-EM structures. Electrophysiological studies of the mutants showed substantial hyperactivity, as evidenced by pronounced baseline activity, enhanced sensitivity to intracellular Ca²⁺, and an elevated resting membrane potential. In vitro studies showed enhanced proliferation in keratinocytes overexpressing either of the mutants. We also detected an up-regulation of markers for proliferation and differentiation of keratinocytes in the affected skin tissues. Our study identified TRPM4 as an important player in the pathogenesis of skin TRP channelopathies and a potential target for treatment of skin hyperkeratotic disorders.

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.

Connexin 43: Key roles in the skin

Gap junctions are tightly packed intercellular channels that serve a common purpose of allowing the intercellular exchange of small metabolites, second messengers and electrical signals. Connexins (Cxs) are gap junction proteins. Currently, 20 and 21 members of Cxs have been characterized in mice and humans, respectively. Connexin 43 (Cx43) is the most ubiquitously expressed type of Cx in the skin. It is produced by various different types of skin cell, such as keratinocytes, fibroblasts, endothelial and basal cells, melanocytes and dermal papilla cells. At present, more evidence indicates that Cx43 has an important role in skin repair and skin tumor development, as well as in skin cell invasion and metastasis. In this review, current knowledge regarding the regulation and function of Cx43 is summarized and the therapeutic potential of regulating Cx43 activity is discussed.

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.

The Molecular Revolution in Cutaneous Biology: Era of Next-Generation Sequencing

Like any true conceptual revolution, next-generation sequencing (NGS) has not only radically changed research and clinical practice, it has also modified scientific culture. With the possibility to investigate DNA contents of any organism and in any context, including in somatic disorders or in tissues carrying complex microbial populations, it initially seemed as if the genetic underpinning of any biological phenomenon could now be deciphered in an almost streamlined fashion. However, over the past recent years, we have once again come to understand that there is no such a thing as great opportunities without great challenges. The steadily expanding use of NGS and related applications is now facing biologists and physicians with novel technological obstacles, analytical hurdles and increasingly pressing ethical questions.

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.

Connexin26 Mutations Causing Palmoplantar Keratoderma and Deafness Interact with Connexin43, Modifying Gap Junction and Hemichannel Properties

Mutations in GJB2 (Cx26) cause either deafness, or deafness associated with skin diseases. That different disorders can be caused by distinct mutations within the same gene suggests that unique channel activities are influenced by each class of mutation. We have examined the functional characteristics of two human mutations, Cx26-H73R and Cx26-S183F, causing palmoplantar keratoderma (PPK) and deafness. Both failed to form gap junction channels or hemichannels when expressed alone. Co-expression of the mutants with wild-type Cx43 showed a trans-dominant inhibition of Cx43 gap junction channels, without reductions in Cx43 protein synthesis. In addition, the presence of mutant Cx26 shifted Cx43 channel gating and kinetics towards a more Cx26-like behavior. Co-immunoprecipitation showed Cx43 being pulled down more efficiently with mutant Cx26, than wild-type, confirming the enhanced formation of heteromeric connexons. Finally, the formation of heteromeric connexons resulted in significantly increased Cx43 hemichannel activity in the presence of Cx26 mutants. These findings suggest a common mechanism whereby Cx26 mutations causing PPK and deafness trans-dominantly influence multiple functions of wild-type Cx43. They also implicate a role for aberrant hemichannel activity in the pathogenesis of PPK, and further highlight an emerging role for Cx43 in genetic skin diseases.

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.

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.

Connexin hemichannels influence genetically determined inflammatory and hyperproliferative skin diseases

Connexin mutations underlie numerous human genetic diseases. Several connexin genes have been linked to skin diseases, and mechanistic studies have indicated that a gain of abnormal channel function may be responsible for pathology. The topical accessibility of the epidermal connexins, the existence of several mouse models of human skin disease, and the ongoing identification of pharmacological inhibitors targeting connexins provide an opportunity to test new therapeutic approaches.