Connexin disorders of the skin

Clinics in Dermatology (2005) 23, 23 – 32

Connexin disorders of the skin Gabriele Richard, MD* Department of Dermatology and Cutaneous Biology and the Jefferson Institute of Molecular Medicine, Jefferson Medical College, Philadelphia, PA 19107, USA

Abstract Over the past decade, the molecular basis of most disorders of cornification has been unveiled. Among these, a distinct group has emerged because of primary defects in cell-cell communication due to faulty gap junction proteins also known as connexins. This review aims to delineate the cutaneous connexin disorders and to highlight intriguing genotype-phenotype correlations and emanating clinical implications. D 2005 Elsevier Inc. All rights reserved.

Gap junctions are clusters of intercellular channels that permit the diffusional exchange of ions and small metabolites, nutrients, and signaling molecules of less than 1000 Da between adjoining cells.1,2 Gap junction intercellular communication controls and coordinates an abundance of cellular activities. It is crucial for tissue morphogenesis and homeostasis, cell growth, differentiation, response to stimuli, and fulfils many other tissue-specific functions. In humans, the gap junction system is formed by a polygenic family of more than 20 different connexin proteins named by their predicted molecular mass or alternatively by their classification into 3 or more different groups, a-, b-, cconnexins.1,2 All connexins are integral membrane proteins and share a common structure and composition consisting of 4 transmembrane domains, which separate 2 extracellular domains from 3 cytoplasmic portions (Fig. 1).3 The latter cytoplasmic domains are specific to each connexin species and control the gating behavior of connexin channels. The other sequence motifs are highly conserved. Four a-helical domains span across the cell membrane to anchor the

* Tel.: +1 215 503 8259; fax: +1 215 503 5788. E-mail address: [email protected]. 0738-081X/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.clindermatol.2004.09.010

polypeptide, form the channel pore, and determine the functional properties of connexin channels. The two extracellular domains are rigid and extend about 2 nm into the extracellular space to dock with those of a partner connexon from an apposing cell.4,5 They also determine the compatibility between different connexin species. Six connexin molecules are assembled into oligomers called connexon, which are transported to the cell membrane and aggregate into gap junction plaques. Each hemichannel can dock end-to-end with a hemichannel partner of a neighboring cell, thus forming an aqueous channel that directly connects the cytoplasms of these cells.2,6 Based on their compatibility, connexins may not only form channels by themselves but also with other connexin species, giving rise to mixed connexons and gap junction channels, whereby each of these channels has selective properties for ion permeability, charge, size, and gating. The complexity and diversity of the gap junction system are further amplified by the fact that most cell types and tissues express more than one connexin species.2,7 The epidermis, its appendages, and other ectodermderived epithelia of the inner ear and cornea share the expression of several connexin proteins, including Cx26, Cx30, Cx31, and Cx43. In each tissue, between 3 and 6

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Fig. 1 Schematic of a generic connexin polypeptide indicating structural domains and position of mutations in connexin disorders of the skin. The cytoplasmic amino terminus (NT), cytoplasmic loop (CL), and carboxy terminus (CT) are shown in yellow. The highly conserved membrane-spanning domains (M1-M4) are depicted in turquoise, and the extracellular loops (E1, E2) in light green. EKV indicates erythrokeratodermia variabilis; PPK, palmoplantar keratoderma; SNHL, sensorineural hearing loss; VS, Vohwinkel’s syndrome (keratoderma hereditaria mutilans); BPS, Bart-Pumphrey syndrome; KIDS, keratitis-ichthyosis-deafness syndrome; PC, pachyonychia congenita.

different connexin species are expressed in distinct yet overlapping patterns during embryological development and epithelial differentiation.8,9 For example, Cx43 is almost ubiquitously found in all epithelial cells of ectodermal origin. The expression of Cx26 is limited to certain cochlear cells, corneal limbal cells, palmoplantar epidermis, sweat glands and ducts, and hair follicles, and is widely paralleled by the expression pattern of Cx30, which is also found in the granular layers of the epidermis. Both Cx30.3 and Cx31 are expressed during the late stages of epidermal differentiation in the upper spinous and granular layers of the epidermis, and Cx31 is also found in the inner ear. The crucial functional importance of each of these connexins in the mentioned ectodermal tissues is reflected by the finding that genetic defects in their genes produce a wide spectrum of genetic disorders comprising sensorineural hearing loss (SNHL), disorders of cornification of the skin, hair, and nails, and keratitis. To date, mutations in the connexin genes for Cx26 (GJB2), and less frequently Cx30 (GJB6), are the principal cause of nonsyndromic deafness worldwide.10 Most of these mutations are recessive and predicted to lead to a complete loss of Cx26 (null alleles) or to compromise its function,11,12 although a few autosomal dominant mutations have also been reported. The targeted ablation of Cx26 or the expression of a dominant-negative Cx26 transgene in the inner ear of mice has been shown to result in sensorineural deafness due to the progressive degeneration of sensory hair cells and a disturbed cortilymph homeostasis.13,14 Although these findings vividly illustrate the essential function of Cx26 for the auditory process in the inner ear, the mere loss of Cx26 or Cx30 is not sufficient to produce an overt skin disorder. Nevertheless, different mutations in 5 connexin genes have been

associated with a range of heritable skin disorders, which are outlined in the following (Table 1).

Erythrokeratodermia variabilis Erythrokeratodermia variabilis (EKV), first described in the Netherlands in the early 1900s, is a rare genodermatosis characterized by a unique combination of 2 distinct morphological features, transient, bmigratoryQ erythematous patches and relatively persistent hyperkeratosis. 15,16 Reflected in the name is also the remarkable variability of its clinical appearance. Erythrokeratodermia variabilis usually manifests at birth or during infancy and persists lifelong with periods of remission and exacerbation. In childhood, sharply demarcated red patches that are shaped and distributed at random and that usually last for several hours to days often predominate. They may be surrounded by an anemic halo, coalesce into large, figurate patches or have a circinate, targetoid appearance, and can arise on normal or hyperkeratotic skin (Fig. 2). Simultaneously or over time, figurate outlined, rough, thickened, and slowly progressive plaques of hyperkeratosis develop in a strikingly symmetric distribution with predilection of extremities, buttocks, and lateral trunk.16 In a subgroup of patients with severe disease, the hyperkeratosis is generalized and can lead to an ichthyosis hystrix–like appearance. About 50% of families with EKV feature involvement of palms and soles with a patchy or diffuse keratoderma, often associated with peeling.17-19 Interestingly, both erythema and hyperkeratosis can be triggered or exaggerated by sudden temperature changes, mechanical friction, and other endogenous or environmental factors. Histopathological and ultrastructural

Connexin disorders of the skin Table 1

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Genetic basis and available diagnostic gene testing for cutaneous connexin disorders

Disorder

OMIM No. Inheritance Gene

Locus

Erythrokeratodermia variabilis

133200

GeneDx, Inc; http://www.genedx.com Institute for Human Genetics and Anthropology, Freiburg, Germany; [email protected] 13q11-q12 GeneDx, Inc; http://www.genedx.com

Diffuse palmoplantar 128350 keratoderma with sensorineural hearing loss Vohwinkel 124500 syndrome

Bart-Pumphrey 149200 syndrome Keratitis-ichthyosis- 148210 deafness syndrome

Diagnostic gene testing

ad or ar

GJB3 (Cx31) 1p35.1 GJB4 (Cx30.3) 1p35.1

ad

GJB2 (Cx26)

ad

GJB2 (Cx26)

ad

GJB2 (Cx26)

ad

GJB2 (Cx26) GJB6 (Cx30)

Hidrotic ectodermal dysplasia

129500

ad

GJB6 (Cx30)

Oculodentodigital dysplasia

164200

ad

GJA1 (Cx43)

abnormalities are not unique and include a basket-weaved orthokeratotic hyperkeratosis, acanthosis, church-spire–like papillomatosis, dilatation of superficial capillaries, and a very mild perivascular inflammation.20,21 Erythrokeratodermia variabilis belongs to the heterogeneous group of erythrokeratodermas and overlaps with palmoplantar keratodermas and progressive symmetric erythrokeratoderma (OMIM 602036 and 133200). The hyperkeratosis in progressive symmetric erythrokeratoderma is quite similar to that in EKV but usually develops on an erythematous base, affects more often palms and soles, and is very persistent and progressive. In contrast to EKV, however, there is no clinical or historical evidence for the occurrence of independent bmigratingQ red patches, the hallmark of EKV. Nevertheless, the considerable phenotypic variability of progressive symmetric erythrokeratodermia and EKV, and the observation of both phenotypes within a single family have raised the question whether they are distinct entities, or within the spectrum of a single disorder.22,23 Erythrokeratodermia variabilis is predominantly inherited in an autosomal dominant fashion, although rarely autosomal

13q11-q12 GeneDx, Inc; http://www.genedx.com University of Iowa, Molecular Otolaryngology Research Laboratories, Iowa City, Iowa, [email protected] 13q11-q12 GeneDx, Inc; http://www.genedx.com (ask for availability) 13q11-q12 University of Freiburg, Institute for Human Genetics and Anthropology, Freiburg, Germany; [email protected] 13q11-q12 GeneDx, Inc; http://www.genedx.com Children’s Hospital of Eastern Ontario, Molecular Genetics Diagnostic Laboratory, Ottawa, Ontario, Canada; [email protected] McGill University Health Centre, Montreal Children’s Hospital, Molecular Genetics Unit, Montreal, Quebec, Canada; [email protected] 6q22-q24 Johns Hopkins Hospital. DNA Diagnostic Laboratory, Baltimore, Md; [email protected] GeneDx, Inc; http://www.genedx.com Johns Hopkins Hospital, Ethylin Wang Jabs Laboratory, Baltimore, Md; [email protected] (Research)

recessive transmission occurs. Several large multigeneration families with EKV have been reported. A few of these were used to map the disease gene first near the prhesus blood group locus on the short arm of chromosome 124 and later to a narrow interval on bands 1p34-p35.1 harboring a cluster of 4 connexin genes.25 Subsequently, disease-causing mutations have been identified, first in the GJB3 gene encoding the gap junction protein h3, connexin-31 (Cx31), and 2 years later, in the GJB4 gene encoding the gap junction protein h4, connexin-30.3 (Cx30.3).17,26 This makes EKV the first human connexin disorder of the skin to be discovered. Moreover, these findings demonstrate that EKV is genetically heterogeneous and may be caused by mutations in at least 2 different connexin genes, GJB3 and GJB4. Both genes encode b-type connexins, which participate in forming the epidermal gap junction system and are crucial for normal epidermal differentiation. Connexin-31 is further expressed in the inner ear, testis, placenta, and perhaps by Schwann cells of the peripheral nervous system.27 It is therefore not surprising that Cx31 mutations can also result in other clinical phenotypes, specifically autosomal dominant or

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Fig. 2 Erythrokeratodermia variabilis. (a) Characteristic erythematous patches and generalized, brown hyperkeratosis (Cx31 mutation); (b) symmetrical erythema and hyperkeratotic plaques (Cx30.3 mutation); (c) sharply outlined, figurate erythematous patches (Cx31 mutation); (d) circinate erythematous patches (Cx30.3 mutation).

recessive SNHL and peripheral neuropathy, albeit without any apparent overlap with EKV.27-29 To date, 11 autosomal dominant mutations have been detected in 15 unrelated families and sporadic cases with EKV, all of which are heterozygous nucleotide substitutions that result in replacement of amino acid residues conserved among b-connexins.17,30,31 The nature and location of these missense mutations in GJB3 and GJB4 are remarkably similar. The majority affect residues in the transmembrane domains, which can be predicted to hinder the regulation of voltage gating or the kinetics of channel closure (Fig. 1). The remaining mutations involve a small, conserved glycine residue at position 12 of the cytoplasmic amino-terminus of Cx31 or Cx30.3 and lead to replacement with a positively (G12R) or negatively charged residue (G12D). These mutations are assumed to interfere with the flexibility of this domain, connexin selectivity, or gating polarity of gap junction channels.32 In vitro expression studies recently revealed that amino acid substitutions in Cx31 and Cx30.3 impair cytoplasmic trafficking of mutant gap junction proteins to the cell membrane, alter the activity level of gap junction communication (G12R, G12D), and/or induce cell death.33-36 Apart from the dominant connexin gene mutations described so far, 2 recent genetic studies confirmed the existence of an autosomal recessive variant of EKV.37,38 Gottfried et al found that 2 siblings with EKV carried a homozygous missense mutation, while both unaffected parents were heterozygous carriers. Similar observations were made in an Italian family with EKV segregating a different missense mutation in the cytoplasmic loop. Functional in vitro studies have suggested that the presence of a recessive Cx31 mutation results in the abnormal cytoplasmic accumulation of the mutant Cx31 protein and thus abolishes Cx31-mediated cell-cell communication.37 In general, the clinical features of patients with mutations in the Cx31 gene or the Cx30.3 gene are indistinguishable. Nevertheless, in a subset of patients (currently 3 out of 7 families) Cx30.3 gene mutations manifested with a unique

clinical feature: the occurrence of transient erythematous patches with peculiar, circinate, or gyrate borders reminiscent of erythema gyratum repens.26,39 More definitive genotypephenotype correlations will likely become available with the continuous expansion of the mutation catalogue in EKV. Finally, it seems worthwhile mentioning that not all patients with EKV harbor mutations in GJB3 or GJB4, suggesting that mutations may lie outside the coding sequences of these genes or perhaps in yet another disease gene(s).30,39,40 Likewise, there is currently no evidence for the involvement of the Cx31 or Cx30.3 gene in progressive symmetric erythrokeratoderma39 indicating that this disorder is of different etiology.

Palmoplantar keratoderma associated with SNHL Hereditary palmoplantar keratodermas (PPK) are a very heterogeneous group of disorders with diverse clinical presentations and underlying causes, and are the focus of a detailed review in this issue. In a subgroup, the bilateral thickening and scaling of the skin of palms and soles are consistently associated with or precede the onset of SNHL or prelingual deafness and other variable cutaneous and extracutaneous ectodermal abnormalities. Therefore, the presence of PPK during infancy or early childhood warrants an audiological examination, detailed pedigree evaluation and potentially genetic testing. Most of the disorders with PPK and SNHL can be attributed to autosomal dominant mutations in GJB2, the gene for the gap junction protein h2, connexin-26 (Cx26), on chromosome 13q11-q12. The exception is focal, plaquelike PPK and progressive SNHL with maternal inheritance. To date, 3 large families have been reported in which all affected individuals were maternally related and harbored a common point mutation (A7445G) in the extrachromosomal mitochondrial genome (mtDNA).41,42 The clinical spectrum of Cx26 gene mutations with a skin phenotype is very broad. It includes variable forms of

Connexin disorders of the skin PPK associated with SNHL, Vohwinkel syndrome, BartPumphrey syndrome, and keratitis-ichthyosis-deafness (KID) syndrome. In the first-named disorder, PPK may be diffuse, transgradient, with fissures and underlying erythema, or mild with accentuation over pressure points. The severity of SNHL can vary from infantile, bilateral prelingual deafness to slowly progressive, high-frequency SNHL (Fig. 3). All families studied so far were found to segregate heterozygous mutations in the Cx26 gene (GJB2), which cluster in or at the border of the first extracellular loop of Cx26 (Fig. 1).11,43-46 The amino acid sequence of this protein domain is highly conserved throughout evolution and among all connexins. The first extracellular loop is essential for gap junction functions, such as gating properties and interaction of connexin hemichannels, assembly of intercellular channels and their voltage gating behavior.6,47-49 It is therefore no surprise that some of these mutations were found to interfere with one or more of these properties, to exert a dominant negative effect on other wildtype connexins and to disturb intercellular communication.12,45,49-51 Vohwinkel syndrome, also known as keratoderma hereditaria mutilans, is a rare, autosomal dominant disorder with diffuse, often transgradient hyperkeratosis of the skin of palms and soles that has a characteristic honeycomblike surface pattern. Other cutaneous features include circular constriction bands on the distal digits, which can lead to autoamputation (pseudoainhum); knuckle pads; and linear or starfish-shaped hyperkeratotic plaques on extremities (Fig. 3). The hearing loss in Vohwinkel syndrome is usually bilateral, prelingual, and, in contrast to the disorders mentioned above, only mild to moderate.52 The nails and other ectodermal tissues are mostly not involved. Several cases associated with alopecia and neurological abnormalities might perhaps fall into the spectrum of KID syndrome. The structural and ultrastructural abnormalities in Vohwinkel syndrome are nonspecific and include massive ortho-

27 keratotic hyperkeratosis, acanthosis, and papillomatosis of the epidermis. Vohwinkel syndrome has been mapped to a cluster of connexin genes on chromosome 13q11-q1253 and is caused by a specific missense mutation in the GJB2 gene encoding Cx26. All affected individuals of 4 unrelated families from the UK, Spain, and Italy were found to carry a similar nucleotide substitution on one allele of GJB2, which replaces an invariably present aspartic acid residue at position 66 of Cx26 with histidine (designated D66H; Fig. 1).43,53 This mutation, which falls within the evolutionary conserved first extracellular domain of Cx26, is predicted to interfere with connexin channel formation and apparently gives rise to a consistent phenotype. Based on the finding of additional sequence variants in Cx26 and Cx31 in individual members of one British family, it has been hypothesized that these variants might perhaps modulate some phenotypic features of the disease, such as type and severity of hearing loss.43 Interestingly, the cutaneous features of Vohwinkel syndrome are paralleled by another type of PPK with more widespread scaling of the skin but no hearing loss, known as Camisa-type or loricrin keratoderma.54 In contrast to Vohwinkel syndrome, loricrin keratoderma is histologically characterized by extensive parakeratosis and abnormal intracellular distribution of loricrin, a major component of the cornified cell envelope. The disorder is caused by small insertion mutations in the loricrin gene resulting in a shift of the reading frame and translation of an abnormally long and aberrant carboxy-terminal tail of loricrin.55-57 As a consequence, the proper assembly of the cornified cell envelope, which normally replaces the plasma membrane during keratinocyte differentiation, is altered and the skin barrier function is disturbed.58 The striking overlap between the cutaneous presentation of Vohwinkel syndrome and loricrin keratoderma may indicate a common pathway and the direct or indirect influence of gap junction–mediated intercellular

Fig. 3 The clinical spectrum of dominant Cx26 disorders. (a) Diffuse PPK and deafness, (b) circular constriction band and verruciform hyperkeratoses on dorsum of palm in Vohwinkel syndrome, (c) leukonychia and knuckle pads in Bart-Pumphrey syndrome, (d) palmoplantar keratoderma with rough surface, (e) corneal neovascularization, and (f) symmetrical erythematous, hyperkeratotic plaques of the face in KID syndrome.

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G. Richard

signaling on the function of proteins expressed during keratinocyte differentiation. Bart-Pumphrey syndrome is another rare, autosomal dominant variant of PPK and SNHL that shares several clinical features with Vohwinkel syndrome and KID syndrome. The disorder is characterized by severe SNHL, PPK with a honeycomblike surface, knuckle pads, and leukonychia, although the clinical features often vary with age and between family members.59,60 Recently, a novel Cx26 gene mutation has been reported in a family with BartPumphrey syndrome. The 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. Keratitis-ichthyosis-deafness syndrome is the most severe cutaneous connexin disorder because of the involvement of several epithelia of ectodermal origin, including skin, appendages, nail, teeth, inner ear, and cornea.61 Most of the cases are sporadic (N90%) , but examples of autosomal dominant or potentially recessive transmission have been reported.62-65 The cutaneous features of hyperkeratosis and erythema usually manifest already at birth or during infancy but may considerably vary between patients. The majority develop symmetrical, well-circumscribed hyperkeratotic plaques with underlying erythema on the extremities and face (erythrokeratoderma). In others, the skin is thickened and has a coarse-grained appearance or shows filiform follicular hyperkeratoses without erythema. With very few exceptions, palms and soles are hyperkeratotic with a rough, stippled, or grainy-appearing surface (Fig. 3). Chronic cheilitis and perle`che are common, whereas hair and nail dystrophy, scarring alopecia, dental anomalies, and heat intolerance are less frequent. Roughly 96% of patients with KID syndrome also develop during childhood ophthalmologic problems, such as corneal neovascularization, chronic blepharitis, and conjunctivitis, which may cause progressive visual decline and blindness.61,66 Most consistent is the finding of SNHL, which is often congenital, bilateral, and severe to profound.67 Keratitis-ichthyosis-deafness syn-

Fig. 4

drome is also associated with increased susceptibility to mucocutaneous infections, which can be fatal in the neonatal period.68 Approximately 10% of affected individuals were also reported to develop squamous cell carcinoma of the skin and oral cavity.61,62,69 A clinical variant of KID syndrome has been described as hystrixlike-ichthyosis-deafness syndrome and mainly differs by generalized hyperkeratosis and some electron microscopic aspects of the disease.70-72 Keratitis-ichthyosis-deafness syndrome is genetically heterogeneous and may be caused by mutations in 2 closely related connexin genes, GJB2 (Cx26) and GJB6 (Cx30). Most of the affected individuals harbor distinct missense mutations in the Cx26 gene, GJB2.73,74 Of those, almost 80% harbor a specific nucleotide change leading to replacement of aspartic acid in codon 50 with asparagine (D50N) or another residue.72-75 Mutation D50N was also found to segregate in a family with vertical transmission of KID syndrome, thus confirming the autosomal dominant inheritance of this disorder. And as shown by van Geel et al, mutation D50N may also present with features of hystrixlike-ichthyosis-deafness syndrome.72 This common and recurrent mutation lies again in the first extracellular domain of Cx26. As discussed above, other missense mutations in the same domain can give rise to nonsyndromic SNHL, PPK and SNHL, Vohwinkel syndrome, and Bart-Pumphrey syndrome. It remains to be determined whether perhaps the specific position and character of each mutation translate into different pathophysiological consequences, which might explain such a remarkable phenotypic diversity. Besides D50N, a few other GJB2 mutations have been identified in KID syndrome, which all cluster in the cytoplasmic amino-terminus of Cx26 and are predicted to alter the charge and structure of this domain.73 Preliminary genotype-phenotype observations suggest that these mutations are often associated with atypical clinical features of KID syndrome, such as lack of PPK or corneal involvement and generalized follicular hyperkeratoses.69 Recently, Jan et al presented evidence that mutations in the Cx30 gene, GJB6, can present with features of KID

The clinical spectrum of dominant Cx30 disorders. (a) Hidrotic ectodermal dysplasia, (b,c) KID syndrome with congenital atrichia.

Connexin disorders of the skin syndrome.76 They identified a child with characteristic features of KID syndrome, including SNHL, keratitis, corneal neovascularization, PPK, and follicular, spiny hyperkeratoses who also had congenital absence of hair and nail abnormalities (Fig. 4). The disorder was due to a heterozygous missense mutation predicted to alter the sequence and charge of the first transmembrane helix of Cx30. A similar mutation had been previously implicated in Clouston syndrome without evidence for abnormal sweating, hearing, photophobia, and keratitis.77 These data illustrate a perplexing phenotypic variability of this GJB6 mutation, similar to mutations in Cx26 or Cx31, and underscore the profound influence of other genetic and epigenetic factors in modifying the clinical outcome of connexin disorders. It remains to be elucidated, however, what role Cx30 defects play in KID syndrome and what are the specific functions of these connexins in the skin and its appendages. Finally, another new and clinically ill-recognized phenotype outside the spectrum of KID syndrome has been attributed to a dominant GJB2 mutation. Brown et al recently reported a young child with congenital SNHL, periorificial and mucosal erythrokeratoderma, dental lamina cysts, enamel defects, and a tendency to develop excessive granulation tissue.78 The child carried a missense mutation of GJB2 altering the third membrane-spanning domain of Cx26, which has not been found in either parent or hundreds of unaffected controls.

Clouston syndrome Clouston syndrome (hidrotic ectodermal dysplasia) is another autosomal dominant ectodermal dysplasia with the key feature of PPK. Other characteristics include hypotrichosis with brittle, blond hair and diffuse alopecia as well as progressive nail dystrophy with thickened and shortened nails that may be easily shed.79 The function of sweat and sebaceous glands is usually normal in contrast to X-linked or autosomal hypo- and anhidrotic ectodermal dysplasia. Unlike in KID syndrome, patients with Clouston syndrome lack corneal vascularization, and association with SNHL is exceptional.80,81 The histopathological abnormalities of the skin are nonspecific, but ultrastructural studies of the hair revealed disorganization of the hair fibrils with loss of the cuticular cortex.82 Clouston syndrome is particularly common among the French-Canadian population due to a founder effect and has been mapped to a cluster of connexin genes on chromosome 13q11-q12 including GJB2 (Cx26) and GJB6 (Cx30).83 Both genes are closely related and share overlapping but not identical expression patterns in ectodermal tissues, such as the cochlea, epidermis, hair follicles, and sweat glands.8,84,85 The underlying basis of Clouston syndrome is missense mutations in the Cx30 gene, GJB6. Lamartine et al identified 2 distinct and recurrent mutations in 12 families of French Canadian and other ancestry.86 These mutations, however,

29 can also present predominantly with pachyonychia congenita–like nail dystrophy as recently reported by van Steensel et al.87 A third Cx30 gene mutation has also been associated with different phenotypes. Whereas Smith et al observed a mutation replacing valine 37 with glutamic acid in a patient with sporadic Clouston syndrome,77 a similar mutation was detected in a patient with KID syndrome and congenital atrichia (Fig. 4).76 To further add to the clinical complexity of Cx30 defects, a fourth missense mutation was associated with autosomal dominant SNHL without skin involvement,88 while a partial deletion of the Cx30 gene was found to be a common cause of autosomal recessive SNHL.89,90

Oculodentodigital dysplasia This is a rare autosomal dominant syndrome with a multitude of developmental abnormalities. The most common features are craniofacial and limbal dysmorphism, microcephaly, conductive hearing loss, and various ophthalmological and neurological anomalies, including microphthalmia, microcornea, cataracts, and ataxia. Frequent abnormalities of the cutaneous system are hypotrichosis, curly hair, and nail dystrophy, whereas mild PPK is rare.91,92 The gene for oculodentodigital dysplasia has been assigned to chromosome 6q22-q24, and pathogenic mutations have been reported in the Cx43 gene, GJA1.93 To date, 18 distinct GJA1 mutations were reported in unrelated families and sporadic cases with oculodentodigital dysplasia, most of which lead to the nonconserved substitution of amino acid residues on one copy of Cx43.93-95 The mutations are scattered across the gene and only spare the end of the gene encoding the extended cytoplasmic tail of Cx43. The pathological mechanisms leading to oculodentodigital dysplasia are still unclear. Interestingly, Cx43 is one of the major gap junction proteins of the skin, hair follicles, sweat, and sebaceous glands. Despite its widespread expression in these tissues, however, the cutaneous manifestations of oculodentodigital dysplasia are very limited and mild, suggesting that other connexins might be important for the embryonic development of skin structures and/or can substitute for the function of Cx43. In summary, advances in molecular genetics of disorders of cornification have drawn attention to a new group of disorders characterized by primary abnormalities of the gap junction system. This group of connexin disorders of the skin encompasses erythrokeratoderma, palmoplantar keratoderma, and ectodermal dysplasias. The broad clinical overlap between these genetic disorders likely stems from the shared tissue expression and function of cutaneous connexins. Molecular diagnostic and prenatal testing for these connexin disorders has become available and certainly aids in establishing a correct diagnosis, family counseling, and a better understanding of the clinical spectrum of these rare disorders, in particular erythrokeratodermia variabilis, KID syndrome, and Clouston syndrome. Nevertheless,

30 mutations in the genes encoding for Cx26, Cx30, and Cx31 may have pleiotropic effects and interfere with the function of one or many ectodermal tissues, thus producing more than one clinical phenotype. These observations make it sometimes difficult to draw clinical and prognostic predictions based on the underlying mutation alone. Based on our current knowledge, however, connexin gene mutations should be considered in patients with disorders of cornification with or without erythema associated with SNHL or involvement of other ectodermal tissues.

Acknowledgments The author is grateful for the stimulating discussions and contribution of photographic material by Drs Bale, Bijlsma, Itin, Krol, Munro, Sprecher, Sybert, Uitto, and Willoughby. The original work was supported in part by the Dermatology Foundation, The Foundation for Ichthyosis and Related Skin Disorders, the American Skin Association, the National Foundation for Ectodermal Dysplasias, and NIH/ NIAMS grants K08-AR02141 and P01-AR38923.

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