Fatima Rouan

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

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.

Novel ITGB4 Mutations in Lethal and Nonlethal Variants of Epidermolysis Bullosa with Pyloric Atresia: Missense versus Nonsense

Epidermolysis bullosa with pyloric atresia (EB-PA), an autosomal recessive genodermatosis, manifests with neonatal cutaneous blistering associated with congenital pyloric atresia. The disease is frequently lethal, but nonlethal cases have also been reported. Expression of the alpha6 beta4 integrin is altered at the dermal-epidermal basement-membrane zone; recently, mutations in the corresponding genes (ITGA6 and ITGB4) have been disclosed in a limited number of patients, premature termination codons in both alleles being characteristic of lethal variants. In this study, we have examined the molecular basis of EB-PA in five families, two of them with lethal and three of them with nonlethal variants of the disease. Mutation analysis disclosed novel lesions in both ITGB4 alleles of each proband. One of the patients with lethal EB-PA was a compound heterozygote for premature termination-codon mutations (C738X/4791delCA), whereas the other patient with a lethal variant was homozygous for a missense mutation involving a cysteine residue (C61Y). The three nonlethal cases had missense mutations in both alleles (C562R/C562R, R1281W/R252C, and R1281W/R1281W). Immunofluorescence staining of skin in two of the nonlethal patients and in one of the lethal cases was positive, yet attenuated, for alpha6 and beta4 integrins. These results confirm that ITGB4 mutations underlie EB-PA and show that missense mutations may lead to nonlethal phenotypes.

Progress in epidermolysis bullosa: the phenotypic spectrum of plectin mutations

Abstract:  Plectin, a large multidomain adhesive protein with versatile binding functions, is expressed in a number of tissues and cell types. In the skin, plectin is a critical component of hemidesmosomes, interacting with keratin intermediate filaments and β4 integrin. Mutations in the plectin gene (PLEC1) result in fragility of skin, demonstrating blister formation at the level of hemidesmosomes. These blistering disorders belong to the spectrum of epidermolysis bullosa (EB) phenotypes, and three distinct variants because of plectin mutations have been identified. First, EB with muscular dystrophy, an autosomal recessive syndrome, is frequently caused by premature termination codon‐causing mutations leading to the absence of plectin both in the skin and in the muscle. Second, a heterozygous missense mutation (R2110W) in PLEC1 has been documented in patients with EB simplex of the Ogna type, a rare autosomal dominant disorder. Finally, recent studies have disclosed plectin mutations in patients with EB with pyloric atresia, an autosomal recessive syndrome, frequently with lethal consequences. Collectively, these observations attest to the phenotypic spectrum of plectin mutations, and provide the basis for accurate genetic counselling with prognostic implications, as well as for prenatal diagnosis in families at the risk of recurrence of the disease.

A Compound Heterozygous One Amino-Acid Insertion/Nonsense Mutation in the Plectin Gene Causes Epidermolysis Bullosa Simplex with Plectin Deficiency

Plectin is a cytoskeleton linker protein expressed in a variety of tissues including skin, muscle, and nerves. Mutations in its gene are associated with epidermolysis bullosa simplex with late-onset muscular dystrophy. Whereas in most of these patients the pathogenic events are mediated by nonsense-mediated mRNA decay, the consequences of an in-frame mutation are less clear. We analyzed a patient with compound heterozygosity for a 3-bp insertion at position 1287 leading to the insertion of leucine as well as the missense mutation Q1518X leading to a stop codon. The presence of plectin mRNA was demonstrated by a RNase protection assay. However, a marked reduction of plectin protein was found using immunofluorescence microscopy of the patients skin and Western blot analysis of the patients cultured keratinocytes. The loss of plectin protein was associated with morphological alterations in plectin-containing structures of the dermo-epidermal junction, in skeletal muscle, and in nerves as detected by electron microscopy. In an in vitro overlay assay using recombinant plectin peptides spanning exons 2 to 15 the insertion of leucine resulted in markedly increased self-aggregation of plectin peptides. These results describe for the first time the functional consequences of an in-frame insertion mutation in humans.

Plectin defects in epidermolysis bullosa simplex with muscular dystrophy

Epidermolysis bullosa simplex with muscular dystrophy (EBS‐MD, MIM 226670) is caused by plectin defects. We performed mutational analysis and immunohistochemistry using EBS‐MD (n = 3 cases) and control skeletal muscle to determine pathogenesis. Mutational analysis revealed a novel homozygous plectin‐exon32 rod domain mutation (R2465X). All plectin/HD1‐121 antibodies stained the control skeletal muscle membrane. However, plectin antibodies stained the cytoplasm of type II control muscle fibers (as confirmed by ATPase staining), whereas HD1‐121 stained the cytoplasm of type I fibers. EBS‐MD samples lacked membrane (n = 3) but retained cytoplasmic HD1‐121 (n = 1) and plectin staining in type II fibers (n = 3). Ultrastructurally, EBS‐MD demonstrated widening and vacuolization adjacent to the membrane and disorganization of Z‐lines (n = 2 of 3) compared to controls (n = 5). Control muscle immunogold labeling colocalized plectin and desmin to filamentous bridges between Z‐lines and the membrane that were disrupted in EBS‐MD muscle. We conclude that fiber‐specific plectin expression is associated with the desmin‐cytoskeleton, Z‐lines, and crucially myocyte membrane linkage, analogous to hemidesmosomes in skin. Muscle Nerve, 2006

Severe mucous membrane involvement in epidermolysis bullosa simplex with muscular dystrophy due to a novel plectin gene mutation

Epidermolysis bullosa simplex with muscular dystrophy (OMIM 226670) is an autosomal recessive disorder caused by mutations of the human plectin gene on chromosome 8q24. Here, we report a 3-year-old girl, offspring of a consanguineous Lebanese family, who presented with skin blistering and recurrent episodes of severe respiratory distress necessitating tracheotomy at the age of 2 years. Repeated examination did not provide any evidence of muscle involvement. Indirect immunofluorescence analysis of a diagnostic skin biopsy with four different domain specific plectin antibodies showed a complete absence of plectin staining. Mutation analysis revealed a novel homozygous single guanine insertion mutation (5588insG/5588insG) residing in the N-terminal part of exon 31 of the plectin gene. Conclusion: the complete lack of protein expression, which may be attributed to a nonsense-mediated plectin mRNA decay, is likely to cause muscular dystrophy and other multisystem involvement later in life.