Bigenic connexin mutations in a patient with hidrotic ectodermal dysplasia

Gap junctions are formed by a polygenic family of more than 20 different connexin proteins. They mediate intercellular communication via the direct exchange of ions, metabolites and secondary messengers, thus controlling and coordinating cellular activities. Mutations in five gap junction genes, including GJB2 (Cx26), GJB3 (Cx31), GJB4 (Cx30.3), GJB6 (Cx30) and GJA1 (Cx43) are known to cause inherited hearing loss and/or disorders of the skin and its appendages, often giving rise to overlapping phenotypes. In this study we present a patient with hidrotic ectodermal dysplasia, who had abortive features of oculo-dento-digital dysplasia, extensive hyperkeratosis of the skin. The patient harbored a novel sporadic mutation (V41L) in GJA1 (Cx43) as well as a heterozygous coding variant (R127H) of GJB2 (Cx26). Our findings suggest that GJA1 mutations can produce variable clinical phenotypes on the background of sequence variants in other connexins.     

Genetic heterogeneity of KID syndrome: identification of a Cx30 gene (GJB6) mutation in a patient with KID syndrome and congenital atrichia

Connexins are integral membrane proteins forming aqueous gap junction channels that allow the diffusional exchange of ions and small metabolites between cells, thus coordinating metabolic activities in multicellular tissues. Dominant mutations in the Cx26 gene GJB2 have been shown to cause keratitis-ichthyosis-deafness (KID) syndrome, palmoplantar keratoderma associated with hearing loss, and Vohwinkel syndrome. Missense mutations in the closely related Cx30 gene GJB6 underlie Clouston syndrome (autosomal dominant hidrotic ectodermal dysplasia). We report a 6-y-old boy with phenotypic characteristics of KID syndrome as well as atrichia. In contrast to other KID syndrome patients, molecular analysis of the connexin gene GJB2 did not disclose a pathogenic mutation, although the patient was homozygous for a common polymorphism (V27I) in the coding sequence of Cx26. Nevertheless, screening of GJB6 revealed a heterozygous missense mutation (V37E) predicted to alter sequence and charge of the first transmembrane helix of Cx30, which was previously implicated in Clouston syndrome (Smith et al, 2002). The presence of a pathogenic Cx30 mutation and the lack of a pathologic molecular change in Cx26 in this patient, whose clinical features predominantly resemble KID syndrome, suggest genetic heterogeneity of KID syndrome and underscore that mutations in Cx30, similar to those in Cx26 or Cx31, can cause different phenotypes. Based on our results, connexin gene mutations should be considered in patients presenting with congenital sensorineural hearing loss and disorders of cornification, and screening of several connexin genes with known cutaneous phenotype, such as those for Cx26, Cx30, Cx30.3, and Cx31, may be required.     

A rare connexin 26 mutation in a patient with a forme fruste of keratitis–ichthyosis–deafness (KID) syndrome

Background  Keratitis–ichthyosis–deafness (KID) syndrome is a rare ectodermal dysplasia characterized by generalized erythrokeratotic plaques, sensorineural hearing loss, and vascularizing keratitis. Cutaneous changes and hearing loss typically present in early childhood, whereas ocular symptoms present later. Mutations in the connexin (Cx) 26 gene, GJB2 , are now established to underlie many of the affected cases, with the majority of patients harboring the p.D50N mutation.
Methods  A rare patient demonstrating features of incomplete KID syndrome associated with an uncommon Cx26 gene mutation is described.
Results  The patient presented late in adolescence with partial features of KID syndrome. There was limited cutaneous involvement and the rare association of cystic acne. Both hearing impairment and ophthalmic involvement were mild in severity. Genetic mutation analysis revealed a previously described, rare mutation in GJB2 , resulting in a glycine to arginine change at codon 12 (p.G12R).
Conclusions  This report describes a patient exhibiting characteristics suggestive of a late-onset, incomplete form of KID syndrome with the GJB2 mutation (p.G12R). The p.G12R mutation has only been described in one other patient with KID syndrome, whose clinical presentation was not characterized.     

Expanding the phenotypic spectrum of Cx26 disorders: Bart-Pumphrey syndrome is caused by a novel missense mutation in GJB2

Bart-Pumphrey syndrome (BPS) is an autosomal dominant disorder characterized by sensorineural hearing loss, palmoplantar keratoderma, knuckle pads, and leukonychia, which show considerable phenotypic variability. The clinical features partially overlap with Vohwinkel syndrome and Keratitis-Ichthyosis-Deafness syndrome, both disorders caused by dominant mutations in the GJB2 gene encoding the gap junction protein connexin-26, suggesting an etiological relationship. We report here a novel GJB2 mutation N54K segregating in a family with BPS, which was not detected in 110 control individuals of Northern European ancestry. This non-conservative missense mutation lies within a cluster of pathogenic GJB2 mutations affecting the evolutionary conserved first extracellular loop of Cx26 important for docking of connexin hemichannels and voltage gating. Immunostaining of Cx26 in lesional palmar and knuckle skin was weak or absent, although its adnexal expression appeared normal and the punctate membrane staining of Cx26 and other epidermal connexins was not altered. Nevertheless, the widespread immunostaining of Cx30 throughout the spinous cell layers suggested a compensatory overexpression. Our results emphasize that pleiotropic GJB2 mutations are responsible for at least 5 overlapping dermatological disorders associated with syndromic hearing loss and cover a wide range of severity and organ involvement.     

Connexin 26 (GJB2) mutations in two Swedish patients with atypical Vohwinkel (mutilating keratoderma plus deafness) and KID syndrome both extensively treated with acitretin

Neuroectodermal syndromes involving the skin and inner ear may be associated with mutations in connexin proteins, which form gap junctions important for intercellular communication. Vohwinkel syndrome (keratodermia mutilans with hearing loss) and keratitis-ichthyosis-deafness (KID) syndrome are rare ectodermal dysplasias associated with dominant mutations in the GJB2 gene encoding connexin 26. We report here two patients, one with KID and one with Vohwinkel syndrome. Both displayed unusual clinical features and responded well to long-term treatment with oral retinoid. Mutation analysis revealed a novel GJB2 mutation p.Gly59Ser in the patient with Vohwinkel syndrome, whereas a recurrent mutation p.Asp50Asn was found in the patient with KID syndrome. The clinical features, particularly a proneness to skin cancer in the patient with Vohwinkel syndrome, are discussed in relation to the identified genotypes.     

G59S mutation in the GJB2 gene in a Chinese family with classic Vohwinkel syndrome

Vohwinkel syndrome (VS) is a rare autosomal dominant condition, also known as mutilating palmoplantar keratoderma accompanied by sensorineural deafness. The LOR and GJB2 genes are reported to be responsible for VS. The GJB2 gene encodes connexin 26, a component of intercellular gap junctions expressed in various tissues. We report the case of a 31‐year‐old Chinese woman with classic VS characterized by sensorineural deafness and mutilating palmoplantar keratoderma. Further genetic studies demonstrated a nucleotide change (c.175G>A) in the GJB2 gene, leading to an amino acid alteration (G59S). This identical missense mutation (G59S) has also been reported in a patient with Bart–Pumphrey syndrome. Together with our findings and previous studies, we conclude that the identical mutation (G59S) in the GJB2 gene contributes to various manifestations.     

Connexin 26 mutation in keratitis-ichthyosis-deafness (KID) syndrome in mother and daughter with combined conductive and sensorineural hearing loss

Background Keratitis–ichthyosis–deafness (KID) syndrome most commonly results from a mutation in the gap‐junctional protein connexin 26 (Cx26) gene, GJB2. Most cases are sporadic and are associated with sensorineural hearing loss. Methods We encountered a mother and daughter with KID syndrome, and pursued genetic analysis and an extensive hearing loss evaluation. Results The analysis of genomic DNA of both affected patients revealed the mutation 148G → A in GJB2 (D50N). No mutation was found in an unaffected son. Auditory phenotype analysis showed a combined conductive and sensorineural hearing loss in both affected patients. Conclusions This is the second vertical transmission of the D50N mutation. These are the first two cases with combined sensorineural and conductive hearing loss without any significant history of middle ear disease. This points to the possibility that the Cx26 D50N mutation can cause conductive hearing loss.     

Novel mutations in GJB6 and GJB2 in Clouston syndrome

Clouston syndrome (CS; also termed hidrotic ectodermal dysplasia) is a rare autosomal dominant genetic skin disorder, characterized by alopecia, nail dystrophy, and palmoplantar hyperkeratosis. Mutations in the GJB6 gene, which encodes the gap junction protein connexin 30, have been shown to cause this disorder. To date, four mutations of GJB6 have been found in patients with CS: G11R, V37E, D50N and A88V. Mutations in GJA1 (V41L) and GJB2 (R127H) are also related to CS. We found a novel missense mutation, N14S, in GJB6 and the previously identified F191L mutation in GJB2 (Cx26) in a proband with CS in a Han Chinese pedigree; these mutations were not found in 200 ethnically matched nonconsanguineous Han Chinese controls.     

A phenotype resembling the Clouston syndrome with deafness is associated with a novel missense GJB2 mutation

Mutations in GJB2 (connexin26) are associated with skin disorders and deafness. The Clouston syndrome (MIM129500) is associated with mutations in GJB6 (connexin30). Here, we describe a patient suffering from a Clouston-syndrome-like phenotype of thin hair, deafness, nail dystrophy, and mild erythrokeratoderma, caused by a novel spontaneous missense mutation in GJB2. The heterozygous mutation in codon 42, AAC>AAG, changes asparagine to lysine (N14K). Interestingly, this asparagine is near two of the residues mutated in Keratitis-like ichthyosis deafness (KID) syndrome (G12R and S17F), yet the phenotype associated with N14K strongly differs from the KID phenotype. Instead, there is a clear phenotypic overlap with syndromes associated with connexin26 or 30 mutations. Our finding suggest that careful audiological evaluation of patients suffering from Clouston-syndrome-like phenotypes is warranted and expand the spectrum of connexin26-associated disease.     

Connexin mutations causing skin disease and deafness increase hemichannel activity and cell death when expressed in Xenopus oocytes

Mutations in the GJB2 gene-encoding connexin 26 (Cx26) have been linked to skin disorders and genetic deafness. However, the severity and type of the skin disorders caused by Cx26 mutations are heterogeneous. Here we explored the effect of Cx26 KID syndrome-associated mutations, G12R, S17F, and D50N on channel function. The Cx26 N14K mutation was also examined that is associated with deafness but has a skin disorder distinct from the KID syndrome mutations. The proteins were all expressed in Xenopus oocytes with levels equal to wild-type Cx26. The G12R, N14K, and D50N mutations resulted in larger hemichannel currents than the wild-type-expressing cells, but the S17F mutation resulted in a complete loss of hemichannel activity. Elevated hemichannel activity correlated with an increased cell death. This result could be reversed through the elevation of calcium (Ca2+) in the extracellular media. Functional gap junctions were only produced by paired N14K cells, which had a similar conductance level to wild type, even though they exhibited a complete loss of voltage sensitivity. This set of data confirms that aberrant hemichannel activity is a common feature of Cx26 mutations associated with KID syndrome, and this may contribute to a loss of cell viability and tissue integrity.     

Extending the phenotypic spectrum of keratitis-ichthyosis-deafness syndrome: report of a patient with GJB2 (G12R) Connexin 26 mutation and unusual clinical findings

Keratitis-ichthyosis-deafness (KID) syndrome is a rare ectodermal dysplasia, characterized mainly by the presence of hyperkeratotic skin lesions, neurosensory hearing loss, and vascularizing keratitis. Most mutations that have been discovered as a cause of KID syndrome are autosomal dominant, found in exon 2 of the Connexin (Cx) 26 gene. A G12R (p.Gly12Arg) is a GJB2 mutation reported in only two patients with KID syndrome to date. This article describes a patient with the G12R mutation and KID syndrome with interesting additional features, which include a porokeratotic eccrine ostial and dermal duct nevus, follicular occlusion triad, and unusual persistent oral mucosal papules. We compare this patient's phenotype with the only two other patients described with the same (G12R) mutation. The phenotypic heterogeneity of KID syndrome, inexplicable according to our current understanding of these proteins, speaks to the complexity of the connexin system and its overlapping expression patterns in different tissues.     

Erythrokeratoderma variabilis caused by a recessive mutation in GJB3

Background. Erythrokeratoderma variabilis (EKV) is a rare disorder of cornification usually associated with dominant mutations in the genes GJB3 and GJB4, which code for connexin (Cx)31 and Cx30.3, respectively, and contribute to the formation of functional gap junctions in the epidermis. Aim. To identify the molecular basis of recessive EKV in a consanguineous family of Middle Eastern origin. Methods. Direct sequencing and site‐directed mutagenesis was used to search for the disease‐causing mutation and identify its molecular consequences. Results. A novel missense mutation (c.G88A) was found in the human GJB3 gene, resulting in substitution of the amino acid isoleucine for valine at position 30 (p.V30I). Under in vitro conditions, p.V30I prevents Cx31 reaching the cell membrane and taking part in gap‐junction formation. Conclusions. Autosomal recessive inheritance should be considered when providing genetic counselling to consanguineous families at risk for EKV.     

Connexin‐26‐Mutation bei „Keratitis‐Ichthyosis‐Deafness”‐Syndrom (KID‐Syndrom)

Background: Keratitis‐ichthyosis‐deafness syndrome (KID syndrome) is an extremely rare disorder. Inheritance is autosomal dominant but many cases occur sporadically following a spontaneous mutation. The cause of KID syndrome are missense mutations of the gene GJB2, encoding connexin 26. Patients and Methods: We clinically studied two cases of KID syndrome and extracted genomic DNA from peripheral blood. Results: The patients showed different heterozygous mutations of the connexin 26 gene and had quite different clinical courses. Conclusions: Both patients showed heterozygous mutations of the connexin 26 gene; a different Cx26 dominant mutation can cause a very different clinical course.     

Genetic heterogeneity in erythrokeratodermia variabilis: novel mutations in the connexin gene GJB4 (Cx30.3) and genotype-phenotype correlations

Erythrokeratodermia variabilis is an autosomal dominant genodermatosis characterized by persistent plaque-like or generalized hyperkeratosis and transient red patches of variable size, shape, and location. The disorder maps to a cluster of connexin genes on chromosome 1p34-p35.1 and, in a subset of families, results from mutations in the gene GJB3 encoding the gap junction protein connexin-31 (Cx31). A recent report suggested the involvement of another connexin gene (GJB4) in the etiology of erythrokeratodermia variabilis. In this study, we sequenced the coding region of GJB4 in 13 unrelated erythrokeratodermia variabilis families without detectable mutations in GJB3. Mutation analysis revealed six distinct missense mutations in five families and a sporadic case of erythrokeratodermia variabilis, all of which were not found in controls. Mutation G12D, identified in an extended Dutch family, lies in the predicted amino-terminus and may interfere with the flexibility of this domain, connexin selectivity, or gating polarity of gap junction channels. Other mutations (R22H, T85P, F137L, F189Y) were located in the transmembrane domains of Cx30.3, and are predicted to hinder regulation of voltage gating or alter the kinetics of channel closure. Affected individuals of two unrelated families harbored point mutations leading to amino acid substitution F137L, which was also reported in GJB3, yet the extent and severity of hyperkeratosis was milder compared to the corresponding mutation in GJB3. Two mutations (T85P, F137L) were associated with the occurrence of rapidly changing erythematous patches with prominent, circinate, or gyrate borders in affected children but not in adults, supporting the notion that this feature is specific to Cx30.3 defects. Nevertheless, we observed highly variable intrafamilial phenotypes, suggesting the strong influence of modifying genetic and epigenetic factors. In addition to pathogenic mutations, we identified several missense mutations and a 4 bp deletion within the GJB4 coding region, which might represent either inconsequential polymorphisms or recessive mutations. In conclusion, our results demonstrate genetic heterogeneity in erythrokeratodermia variabilis, and emphasize that intercellular communication mediated by both Cx31 and Cx30.3 is crucial for epidermal differentiation.     

Specific loss of connexin 26 expression in ductal sweat gland epithelium associated with the deletion mutation del(GJB6-D13S1830)

A whole array of cutaneous syndromes is associated with distinct dominant mutations in GJB2 encoding the gap junction protein connexin 26 (C x 26), including Vohwinkel's syndrome and keratitis-ichthyosis-deafness syndrome. In contrast, recessive GJB2 mutations occur in a large proportion of individuals with hearing loss but no obvious dermatological phenotype. Recently, a large deletion of approximately 342 kb, encompassing the coding region of GJB6 encoding C x 30, but not affecting GJB2, was shown to be associated with hearing loss. From analysis of patient skin, we provide immunohistochemical and bioinformatic data to show that the expression of C x 26 is affected by del(GJB6-D13S1830) in a cell-type-specific manner within the sweat gland. This putative regulatory element of C x 26 expression may be a key factor related to the severe or profound deafness associated with del(GJB6-D13S1830).     

Connexins: a connection with the skin

The intercellular signaling system mediated by connexin channels is crucial for maintaining tissue homeostasis, growth control, development, and synchronized response of cells to stimuli. This review summarizes the structure, assembly, and properties of the components of the complex and diverse connexin system, and their biological functions in skin. The importance of gap junctional intercellular communication for normal development and differentiation of human epidermis as well as the hearing function of the inner ear is illustrated by the examples of erythrokeratodermia variabilis and palmoplantar keratoderma associated with hearing loss. These autosomal dominant inherited disorders are caused by germline mutations in the connexin genes GJB3 (encoding connexin-31) and GJB2 (encoding connexin-26), respectively. Recent functional studies of individual connexin mutations suggest that they may exert a dominant inhibitory effect on normal connexin channel function and perturb gap junctional intercellular communication, resulting in phenotypic manifestation in patients with these disorders.     

Novel and recurrent connexin 30.3 and connexin 31 mutations associated with erythrokeratoderma variabilis

Erythrokeratoderma variabilis (EKV) is characterized by fixed hyperkeratotic plaques and transient erythema. Mutations in the genes GJB3 and GJB4, which encode connexin (Cx)31 and Cx30.3, are associated with EKV. We report one novel mutation in Cx31 and one recurrent mutation in Cx30.3 in two different families. One novel rare sequence variant of unknown clinical significance was also identified. This finding extends the spectrum of known EKV-associated mutations.     

A heterozygous mutation in GJB2 (Cx26F142L) associated with deafness and recurrent skin rashes results in connexin assembly deficiencies

Mutations in GJB2 encoding Connexin 26 (CX26) are associated with hearing loss and hyperproliferative skin disorders of differing severity including keratitis-ichthyosis-deafness (KID) and Vohwinkel syndrome. A 6-year-old Caucasian girl who presented with recurrent skin rashes and sensorineural hearing loss harboured a heterozygous point mutation in GJB2 (c.424T > C; p.F142L). To characterize the impact of CX26F142L on cellular events. Plasmids CX26WT, CX26F142L, CX26G12R (KID) or CX26D66H (Vohwinkel) were transfected into HeLa cells expressing Cx26 or Cx43 or into HaCaT cells, a model keratinocyte cell line. Confocal microscopy determined protein localization. MTT assays assessed cell viability in the presence or absence of carbenoxolone, a connexin-channel blocker. Co-immunoprecipitation/Western blot analysis determined Cx43:Cx26 interactions. Quantitative real-time polymerase chain reaction assessed changes in gene expression of ER stress markers. Dye uptake assays determined Connexin-channel functionality. F142L and G12R were restricted to perinuclear areas. Collapse of the microtubule network, rescued by co-treatment with paclitaxel, occurred. ER stress was not involved. Cell viability was reduced in cells expressing F142L and G12R but not D66H. Unlike G12R that forms “leaky” hemichannels, F142L had restricted permeability. Cell viability of F142L and G12R transfected cells was greater in HeLa cells expressing Cx43 than in native Cx-free HeLa cells. Co-immunoprecipitation suggested a possible interaction between Cx43 and the three mutations. Expression of CX26F142L and G12R results in microtubule collapse, rescued by interaction with Cx43. The GJB2 mutations interacted with Cx43 suggesting that unique Cx43:Cx26 channels are central to the diverse phenotype of CX26 skin-related channelopathies.