Missense mutations in GJB2 encoding connexin-26 cause the ectodermal dysplasia keratitis-ichthyosis-deafness syndrome

Abstract

Keratitis-ichthyosis-deafness syndrome (KID) is a rare ectodermal dysplasia characterized by vascularizing keratitis, profound sensorineural hearing loss (SNHL), and progressive erythrokeratoderma, a clinical triad that indicates a failure in development and differentiation of multiple stratifying epithelia. Here, we provide compelling evidence that KID is caused by heterozygous missense mutations in the connexin-26 gene, GJB2. In each of 10 patients with KID, we identified a point mutation leading to substitution of conserved residues in the cytoplasmic amino terminus or first extracellular domain of Cx26. One of these mutations was detected in six unrelated sporadic case subjects and also segregated in one family with vertical transmission of KID. These results indicate the presence of a common, recurrent mutation and establish its autosomal dominant nature. Cx26 and the closely related Cx30 showed differential expression in epidermal, adnexal, and corneal epithelia but were not significantly altered in lesional skin. However, mutant Cx26 was incapable of inducing intercellular coupling in vitro, which indicates its functional impairment. Our data reveal striking genotype-phenotype correlations and demonstrate that dominant GJB2 mutations can disturb the gap junction system of one or several ectodermal epithelia, thereby producing multiple phenotypes: nonsyndromic SNHL, syndromic SNHL with palmoplantar keratoderma, and KID. Decreased host defense and increased carcinogenic potential in KID illustrate that gap junction communication plays not only a crucial role in epithelial homeostasis and differentiation but also in immune response and epidermal carcinogenesis.

Figures

Figure 1

Clinical features of KID. A, Sharply demarcated, figurate outlined, red-brown hyperkeratotic plaques on the central face and outer rim of the ear (KID 05). B, Rarefied eyelashes and vascularizing keratitis (KID 05). C, Acanthosis of the skin with a heavy-grained leather appearance (KID 08). D, Diffuse palmar keratoderma with grainy surface (KID 03). E, Chronic Candida albicans onychia and paronychia with hypertrophy of distal digits, acanthosis, and hyperkeratosis of the skin (KID 09).

Figure 2

Mutation analysis of GJB2 in eight individuals affected with sporadic disease and one multiplex family with KID. A, Sequence chromatograms of GJB2from unaffected (left panel) and affected (right panel) family members depicting the heterozygous transition 148G→A at codon 50 encoding asparagine instead of aspartic acid (D50N) observed in seven unrelated families with KID. B, Genotyping by automated pyrosequencing of biotinylated single-stranded DNA templates (Pyrosequencing) confirmed the presence of D50N in the affected patients (top row,pyrogram; bottom row, outcome as bar graph; both generated with Pyrosequencing software), whereas it was excluded from unaffected parents and siblings and from 54 control subjects. C and D, Sequence chromatograms of GJB2 from unaffected (left panel) and affected (middle panel) family members and the mutant allele (right panel) subcloned in the pcDNA2.1 vector (Invitrogen), showing (C) the heterozygous G→C transversion in codon 12, replacing glycine with arginine (G12R) in patient KID 08, and (D) the transition 50C→T, leading to substitution of serine 17 with phenylalanine (S17F), in one allele of patient KID 09. Individuals from whom DNA was obtained are marked with an asterisk (*). E, Schematic of the Cx26 polypeptide that depicts structural motifs and location of mutations in KID. NT = amino-terminus; M1–M4 = transmembrane domains 1–4, respectively; E1 and E2 = extracellular domains 1 and 2, respectively; CL = cytoplasmic loop; and CT = carboxy-terminus.

Figure 3

Immunofluorescence (confocal microscopy) of normal (A, E, and I) and lesional (B–D, F–H, K, and L) skin in KID (KID 08) with Cx26, Cx43, and Cx30 antibodies. Immunohistochemical analyses were performed on 5-μm–thick sections of snap-frozen skin biopsies of normal control skin and lesional skin, as described elsewhere (Rouan et al. 2001). The primary monoclonal anti-Cx26 antibody (mouse [Zymed Laboratories; catalog number 33-5800]), which does not cross-react with Cx30, was detected with fluorescein-conjugated anti-mouse immunoglobin (green). The polyclonal anti-Cx43 and anti-Cx30 antibodies (rabbit [Zymed Laboratories]) were visualized with Texas Red–conjugated anti-rabbit immunoglobin (red). Areas of focal colocalization of Cx26 with either Cx30 or Cx43 are visualized in yellow. A, Normal epidermis, Cx26 (green). B–D, KID syndrome, Cx26 (green). Note the weak immunostaining of Cx26 in the granular cell layers (B, arrows) but strong expression in epithelial cells of sweat ducts (C) and outer root sheath of hair follicles (oblique cut) (D). E, Normal epidermis, Cx30 (red). F–H, KID syndrome, Cx30 (red) and Cx26 (green). Cx30 staining appears strictly compartmentalized with membranous and cytoplasmic staining in basal keratinocytes and punctate membrane staining of the upper differentiated cell layers. Note the focal colocalization of Cx26 and Cx30 in the granular epidermal layers (F, arrows), as well as in the sweat ducts (G) and hair follicles (H). I, Normal epidermis, Cx43 (red). K and L, KID syndrome, Cx43 (red) and Cx26 (green). Comparable expression of Cx43 throughout the epidermis in normal (I) and lesional skin (K) and colocalization of Cx26 with Cx43 in hair follicles (L). d = dermis; e = epidermis; sd = sweat duct; and hf = hair follicle. Scale bars = 10 μm.

Figure 4

Immunofluorescence of normal human corneal epithelium with Cx26 (A), Cx30 (B), and Cx43 (C) antibodies. All connexin proteins showed, as expected, punctate staining at the cell membranes. A, Cx26 staining was relatively weak and was limited to the basal layer of the epithelium. B, Comparatively strong Cx30 expression throughout the basal and lower wing layers, sparing the superficial wing and squamous layers. C, Similar distribution of Cx43, with highest protein levels in the basal layer but staining extended throughout the wing layers. Scale bar = 10 μm.

Figure 5

Immunolocalization of wild-type Cx26 (A) and Cx26 S17F (B). HeLa cells expressing a tTA vector were stably transfected with Cx26-MYC-cDNA/pCEP-TetP vectors, grown in selective medium to >80% confluency, fixed, and stained with a monoclonal Cx26 antibody. A, WtCx26. B, Cx26 S17F. Arrows indicate plasma membrane staining at cell-cell contacts. Scale bar = 10 μm.