non-Herlitz Junctional Epidermolysis Bullosa Research Articles

Type XVII Collagen is a Key Player in Tooth Enamel Formation

Inherited tooth enamel hypoplasia occurs due to mutations in genes that encode major enamel components. Enamel hypoplasia also has been reported in junctional epidermolysis bullosa, caused by mutations in the genes that encode type XVII collagen (COL17), a component of the epithelial-mesenchymal junction. To elucidate the pathological mechanisms of the enamel hypoplasia that arise from the deficiency of epithelial-mesenchymal junction molecules, such as COL17, we investigated tooth formation in our recently established Col17(-/-) and Col17 rescued mice. Compared with wild-type mice, the incisors of the Col17(-/-) mice exhibited reduced yellow pigmentation, diminished iron deposition, delayed calcification, and markedly irregular enamel prisms, indicating the presence of enamel hypoplasia. The molars of the Col17(-/-) mice demonstrated advanced occlusal wear. These abnormalities were corrected in the Col17 rescued humanized mice. Thus, the Col17(-/-) mice clearly reproduced the enamel hypoplasia in human patients with junctional epidermolysis bullosa. We were able to investigate tooth formation in the Col17(-/-) mice because the Col17(-/-) genotype is not lethal. Col17(-/-) mouse incisors had poorly differentiated ameloblasts that lacked enamel protein-secreting Tomes' processes and reduced mRNA expression of amelogenin, ameloblastin, and of other enamel genes. These findings indicated that COL17 regulates ameloblast differentiation and is essential for normal formation of Tomes' processes. In conclusion, COL17 deficiency disrupts the epithelial-mesenchymal interactions, leading to both defective ameloblast differentiation and enamel malformation.

Development of Animal Models to Study Basement Membranes

While inherited and acquired diseases have proven invaluable in increasing our understanding of the basement membrane zone (BMZ), the identification of the genes of various BMZ proteins and the development of advanced molecular techniques has allowed for the development of many new animal models, which present a great opportunity for increasing our knowledge of BMZ biology, the pathogenesis of disease and ultimately the treatment of these diseases. Three types of animal models have significantly advanced our understanding of inherited diseases of the BMZ and now provide in vivo paradigms to further study their pathogenesis and to test potential therapies. These animal models include transgenic mice, immunocompromised mice with transplanted human skin equivalents and spontaneously occurring basement membrane zone (BM) disease in large animals. Distinct transgenic mouse models demonstrate many of the features of inherited BM diseases, including the simplex, junctional, hemidesmosomal and dystrophic forms of epidermolysis bullosa (EB). This was first established by the Fuchs laboratory (Vassar et al., 1991), who showed that transgenic mice expressing a mutated form of keratin 14 demonstrated basal cell cytolysis and intraepithelial blistering that mimicked EB simplex to a remarkable degree. This landmark study was the first to demonstrate that a keratin gene mutation was linked to an inherited disease in humans. Since that time, other transgenic models have targeted a variety of BM components to establish transgenic models of hemidesmosomal (integrina6, integrin-b4 and plectin), junctional (laminin 5 subunits) and dystrophic (type VII collagen) forms of EB. Unfortunately, these transgenic animals result in neonatal lethality that has precluded long-term studies to test therapeutic strategies targeted at phenotypic reversion of the BM defect. To overcome this obstacle and to establish humanized animal models of inherited BM disorders, human skin equivalents have been fabricated from primary cells derived from EB patients and then grafted to immunocompromised mice. This novel approach was first used by the Khavari laboratory (Ortiz-Urda et al., 2003) to study the phenotypic reversion of a skin equivalent generated in vitro using keratinocytes from a recessive dystrophic EB patient and then transplanted to scid/scid mice. These grafts were injected with an intradermal bolus of recessive dystrophic EB fibroblasts that were engineered to correct their type VII collagen deficiency through overexpression of this protein. Strikingly, this exogenous type VII collagen was delivered to the BMZ and resulted in the prevention of sub-epidermal blistering. Subsequently, a similar animal model has been adapted by the Woodley laboratory (Woodley et al., 2004) to correct a recessive dystrophic EB phenotype by direct injection of a recombinant form of human type VII collagen. The successful reversion of this disease phenotype supports future use of protein-based therapy to treat inherited skin disorders of BM. Finally, spontaneously occurring, large animal models of inherited BM disease have been well characterized and are now available to study these diseases in an immunocompetent background. These naturally occurring diseases mimic the simplex (bovine), hemidesmosomal (dog), junctional (horse, dog) and dystrophic (sheep, dog, cat) forms of EB and have been linked to the characteristic genotypes and phenotypes seen in their human counterparts (Spirito et al., 2002; Capt et al., 2005; Hengge, 2005). Interestingly, phenotypes observed in these animals demonstrate varying degrees of severity that have been linked to levels of functional BM proteins. For example, a breed of German Pointer manifests an intronic mutation in the gene encoding the a3 chain of laminin 5 that interferes with RNA maturation, decreases levels of functional laminin 5 and results in a mild phenotype similar to that seen in non-Herlitz junctional EB. In contrast, a horse model that mimics severe Herlitz junctional EB has been linked to a base pair insertion generating a premature termination codon that leads to complete loss of laminin-a2 expression. These large animal models provide new opportunities to test delivery of therapeutic genes or gene products to correct molecular defects underlying these

Tracheolaryngeal Complications of Inherited Epidermolysis Bullosa: Cumulative Experience of the National Epidermolysis Bullosa Registry

To accurately determine the frequency with which complications arise in the ears, noses, and throats of patients with inherited epidermolysis bullosa (EB) as well as the cumulative risk of tracheolaryngeal stenosis or stricture. Cross-sectional study (3,280 patients) with a nested, randomly sampled longitudinal subcohort (n=450), representing data collection, stratified by major EB subtype, of the National EB Registry, an epidemiologic project focused on enrolling all EB patients within the continental United States from 1986 to 2002, to permit generalization of findings to the entire American EB population. Systematic epidemiologic case finding and data collection were performed throughout the continental United States, followed by subclassification of patients by EB subtype. ENT complications were quantified via contingency tables (as frequencies) and lifetable analyses. Frequencies of surgical procedures were also determined. The most important clinical ENT complication in inherited EB was tracheolaryngeal stenosis or stricture, arising during early childhood and primarily within infants and children with junctional EB (JEB) (cumulative risk of 39.8% and 12.8% in Herlitz and non-Herlitz JEB, respectively, by ages 6 and 9). Other uncommon complications included chronic otitis media, chronic otitis externa, and hearing loss. Given the potential risk for sudden airway occlusion and death, meticulous surveillance by a pediatric otolaryngologist is a critical part of the overall management of infants and children with EB, especially those with JEB and two rare subtypes of generalized EB simplex. Elective tracheostomy should be considered in EB infants and children with evidence of airway embarrassment.

A new passive transfer animal model of bullous pemphigoid

Evaluation of: Nishie W, Sawamura D, Goto M et al. Humanuzation of autoantigen. Nat. Med. 13, 378–383 (2007).The development of a conventional passive transfer model of bullous pemphigoid (BP) is made impossible by the fact that BP antibodies that react with the immunodominant and potentially pathogenic epitope within the NC16A region fail to cross-react with the murine form of COL17. A recent paper by Nishie et al. describes one of the first successful murine models of organ-specific autoimmunity based on genetic modification and ‘humanization’ of the autoantigen (COL17). A COL17 knockout mouse (a murine equivalent of non-Herlitz junctional epidermolysis bullosa) was rescued by the human ortholog, which made it susceptible to a disease resembling BP on passive transfer of BP patients’ sera. The data presented provides in vivo evidence of the pathogenicity of circulating anti-NC16A autoantibodies from patients with BP.

Oral steroid improves bullous pemphigoid-like clinical manifestations in non-Herlitz junctional epidermolysis bullosa with COL17A1 mutation

Non-Herlitz junctional epidermolysis bullosa (JEB-nH), a nonlethal variant of junctional epidermolysis bullosa (JEB), is an autosomal recessive disorder characterized by separation of the dermal-epidermal junction. JEB-nH is caused by mutations in several genes and lack of the COL17A1 gene product may lead to skin fragility. A 41-year-old Japanese man with JEB-nH, featuring mutations in the gene encoding type XVII collagen, presented with great blisters over his entire body accompanied by severe itching and eosinophilia usually observed in bullous pemphigoid (BP). To our knowledge, our patient is the first with JEB-nH to be treated successfully with an oral steroid to control his skin affliction, symptoms and eosinophilia. This suggests that in the case of JEB-nH with eosinophilia caused by some secondary immune activation, oral steroids may constitute an alternate therapy to improve aggravated skin conditions and severe itching, both of which tend to show resistance to usual dermatological treatments.

Cutaneous mosaicism: right before our eyes

Autosomal recessive cutaneous disorders, including various types of epidermolysis bullosa (EB), usually manifest shortly after birth. The clinical course of these diseases is often characterized by severe complications, limited therapeutic options, and a poor prognosis. A study by Pasmooij et al. reported in this issue of the JCI unravels the molecular mechanisms by which germline mutations in non-Herlitz junctional EB can be corrected in vivo by multiple spontaneously occurring somatic mutational events, a phenomenon known as revertant mosaicism (see the related article beginning on page 1240). These insights open new avenues of thinking for the design of future gene therapy strategies for skin diseases.

Revertant mosaicism in junctional epidermolysis bullosa due to multiple correcting second-site mutations in LAMB3

Revertant mosaicism due to in vivo reversion of an inherited mutation has been described in the genetic skin disease epidermolysis bullosa (EB) for the genes KRT14 and COL17A1. Here we demonstrate the presence of multiple second-site mutations, all correcting the germline mutation LAMB3:c.628G-->A;p.E210K, in 2 unrelated non-Herlitz junctional EB patients with revertant mosaicism. Both probands had a severe reduction in laminin-332 expression in their affected skin. Remarkably, the skin on the lower leg of patient 078-01 (c.628G-->A/c.1903C-->T) became progressively clinically healthy, with normal expression of laminin-332 on previously affected skin. In the other proband, 029-01 (c.628G-->A/c.628G-->A), the revertant patches were located at his arms, shoulder, and chest. DNA analysis showed different second-site mutations in revertant keratinocytes of distinct biopsy specimens (c.565-3T-->C, c.596G-->C;p.G199A, c.619A-->C;p.K207Q, c.628+42G-->A, and c.629-1G-->A), implying that there is not a single preferred mechanism for the correction of a specific mutation. Our data offer prospects for EB treatment in particular cases, since revertant mosaicism seems to occur at a higher frequency than expected. This opens the possibility of applying revertant cell therapy in mosaic EB of the LAMB3 gene by using autologous naturally corrected keratinocytes, thereby bypassing the recombinant gene correction phase.

Localized and generalized forms of blistering in junctional epidermolysis bullosa due to COL17A1 mutations in the Netherlands

Mutations in the gene COL17A1 coding for type XVII collagen cause non-Herlitz junctional epidermolysis bullosa (nH-JEB). Here we give an overview of the genotype-phenotype correlation in 12 patients from the Netherlands with type XVII collagen-deficient nH-JEB. Family and personal history and clinical presentation were recorded from each patient, and skin biopsies of intact and bullous skin were taken for immunofluorescence and electron microscopy. The mutations were identified by analysing the patient's DNA isolated from peripheral blood cells. DNA analysis identified five novel deletions: 1284delA, 1365delC, 3236delT, 3600-3601delCT and 4425delT. Interestingly, we identified a new patient, homozygous for 4425delT, with an exceptionally mild blistering phenotype. All together, three patients had more localized blistering confined to hands, lower legs and face, absent or very mild nail dystrophy, normal primary hair and sparse secondary hair. Nine patients had generalized blistering, nail dystrophy, sparse primary and absent secondary hair. All 12 patients had amelogenesis imperfecta (enamel pitting). Immunofluorescence (IF) antigen mapping with monoclonal antibodies 1A8C and 1D1 that bind to type XVII collagen, but not to its 97-kDa fragment was completely negative in patients with generalized blistering, whereas reduced in patients with localized blistering. Our data reveal that in patients with COL17A1 mutations a localized nH-JEB phenotype can be differentiated from a generalized nH-JEB phenotype by IF antigen mapping. The data are important for genetic counselling at early age when the clinical phenotype is not yet clear.

Urethral meatal stenosis in junctional epidermolysis bullosa: a rare complication effectively treated with a novel and simple modality

A 10-year-old boy was diagnosed with non-Herlitz junctional epidermolysis bullosa (nh-JEB) shortly after birth when he developed blisters on his feet and under the toenails. His younger brother was also affected by nh-JEB. Their parents were first cousins. Immunofluorescence mapping showed a blister in the lamina lucida and reduced staining with GB3 antibody against laminin V. Molecular analysis showed a homozygous mutation in the LAMC2 gene. The patient complained of intermittent inability to pass urine, causing significant suprapubic pain, anxiety, and general discomfort. He reported the development of blisters and erosions at the urethral meatus, which caused fusion of the meatal opening. In order to micturate, he had to tear apart the fused tissue, resulting in considerable pain. Physical examination revealed multiple healing erosions and atrophic scars, mainly on the legs (Fig. 1), and anonychia of most of the toenails. The walls of the urethral meatus were adherent (Fig. 2). The urethral meatus could not be opened using a moderate amount of pressure by the examining physicians. Prevention of re-stenosis of the urethral meatus was accomplished with the application of Mepilex (Mölnlycke, Sweden) to the urethral meatus after each micturition, where it remained until the next episode of micturition. Since the institution of this care regimen 10 months ago, there has been no recurrence of the stenosis.

Analysis of the COL17A1 in non-Herlitz junctional epidermolysis bullosa and amelogenesis imperfecta

Non-Herlitz junctional epidermolysis bullosa (nH-JEB) disease manifests with skin blistering, atrophy and tooth enamel hypoplasia. The majority of patients with nH-JEB harbor mutations in COL17A1, the gene encoding type XVII collagen. Heterozygotes with a single COL17A1 mutation, nH-JEB defect carriers, may exhibit only enamel hypoplasia. In this study, to further elucidate COL17A1 mutation phenotype/ genotype correlations, we examined two unrelated families with nH-JEB. Furthermore, we hypothesized that COL17A1 mutations might underlie or worsen the enamel hypoplasia seen in amelogenesis imperfecta (AI) patients that are characterized by defects in tooth enamel formation without other systemic manifestations. We therefore conducted COL17A1 mutational analysis in three patients from two AI families. One nH-JEB patient showed no COL17A1 expression and was a compound heterozygote for the novel premature termination codon (PTC) mutations 1285delA and Q1387X. In addition, reduced COL17A1 expression was found in a second nH-JEB patient who was homozygous for the novel PTC mutation 4335delC, the most carboxyl terminal PTC mutation thus far identified. Due to nonsense mediated mRNA decay, the position of these PTC mutations is thought not to influence the effect of COL17A1 transcript loss and hence the severity of the nH-JEB phenotype. This study is the first to suggest that type XVII collagen carboxyl PTC mutations lead to restoration of truncated polypeptide expression and to a milder clinical disease severity in nH-JEB. Conversely, we failed to detect any pathogenic COL17A1 defects in AI patients, in either exon or within the intron-exon borders of AI patients. This study furthers the understanding of mutations in COL17A1 causing nH-JEB, and clearly demonstrates that the mechanism of enamel hypoplasia differs between nH-JEB and AI diseases.

Multiple Correcting COL17A1 Mutations in Patients with Revertant Mosaicism of Epidermolysis Bullosa

Revertant mosaicism by somatic reversion of inherited mutations has been described for a number of genetic diseases. Several mechanisms can underlie this reversion process, such as gene conversion, crossing-over, true back mutation, and second-site mutation. Here, we report the occurrence of multiple corrections in two unrelated probands with revertant mosaicism of non-Herlitz junctional epidermolysis bullosa, an autosomal recessive genodermatosis due to mutations in the COL17A1 gene. Immunofluorescence microscopy and laser dissection microscopy, followed by DNA and RNA analysis, were performed on skin biopsy specimens. In patient 1, a true back mutation, 3781T-->C, was identified in the specimen from the arm, and a second-site mutation, 4463-1G-->A, which compensated for the frameshift caused by the inherited 4424-5insC mutation, was identified in the 3' splice site of exon 55 in a specimen from the middle finger. Patient 2 showed--besides two distinct gene conversion events in specimens from the arm and hand sites, both of which corrected the 1706delA mutation--a second-site mutation (3782G-->C) in an ankle specimen, which prevented the premature ending of the protein by the 3781C-->T nonsense mutation (R1226X). Thus, both inherited mutations, paternal as well as maternal, reverted at least once by different reversion events in distinct cell clusters in the described patients. The occurrence of multiple correcting mutations within the same patient indicates that in vivo reversion is less unusual than was generally thought. Furthermore, in the male patient, mosaic patterns of type XVII collagen-positive keratinocytes were present in clinically unaffected and affected skin. This latter observation makes it likely that reversion may be overlooked and may happen more often than expected.

A very mild form of non-Herlitz junctional epidermolysis bullosa: BP180 rescue by outsplicing of mutated exon 30 coding for the COL15 domain.

Mutations in the gene COL17A1 cause non-Herlitz junctional epidermolysis bullosa. Here, we describe a patient who, despite two heterozygous mutations in COL17A1, has an extremely mild form of the disease missing most of the characteristic clinical features. DNA analysis revealed a frame-shift mutation 3432delT and a nonsense mutation 2356C-->T (Q751X). cDNA analysis showed that the deleterious effect of the latter mutation was skirted by deleting the premature termination codon containing exon 30. In this way, the reading frame was restored, resulting in a 36 nucleotides shorter mRNA transcript. Immunoblot analysis showed expression of the 180-kDa bullous pemphigoid antigen (BP180) with a slightly higher SDS-PAGE mobility, in line with the deletion of 12 amino acids from the COL15 domain. Immunofluorescence of skin sections showed diminished, but correctly localised expression of BP180, and this, in concert with the mild clinical phenotype, suggests that this COL15 mutated BP180 is still partly functional.

Type XVII collagen gene mutations in junctional epidermolysis bullosa and prospects for gene therapy.

Non-Herlitz junctional epidermolysis bullosa (nH-JEB) is caused predominantly by mutations leading to premature stop codons on both alleles of the type XVII collagen gene (COL17A1). The analysis of mutations in this gene has provided a means of correlating genotype with phenotype of nH-JEB patients. The phenotype of nH-JEB is characterized by generalized blistering of skin and mucous membranes with atrophic scarring and nail dystrophy. Atrophic alopecia is a distinct feature of nH-JEB patients, but one that is not associated with the severity of the disease at other sites. Enamel hypoplasia and pitting of the teeth are also characteristic for nH-JEB and can be used to facilitate the correct diagnosis in children with a blistering skin disease. Analysis of the biological consequences of mutations in the COL17A1 gene has shown that most patients lack type XVII collagen mRNA due to nonsense-mediated mRNA decay. Patients with these mutations can therefore be a target for corrective gene therapy using vectors coding for full-length type XVII collagen. Proof of principle for this approach has recently been demonstrated. The analysis of naturally occurring phenomena of gene correction in the COL17A1 gene provides evidence for other mechanisms of gene correction in genetic diseases. For example, exclusion of an exon carrying a mutation can lead to a milder phenotype of nH-JEB than predicted by the original mutation. In addition, we have gained data suggesting that COL17A1 exons harbouring pathogenic mutations can also be repaired by trans-splicing, i.e. aligning corrected RNA sequences to introns in the vicinity of faulty exons in the COL17A1 premtRNA.

Laminin 5 mutations in junctional epidermolysis bullosa: molecular basis of Herlitz vs. non-Herlitz phenotypes.

Junctional epidermolysis bullosa (JEB) is a group of heritable blistering diseases in which tissue separation occurs within the lamina lucida of the cutaneous basement membrane zone. Clinically, two broad subcategories have been recognized: The Herlitz variant (H-JEB; OMIM 226700) is characterized by early demise of the affected individuals, usually within the first year of life, while non-Herlitz (nH-JEB; OMIM 226650) patients show a milder phenotype with life-long blistering, yet with normal lifespan. In this study, we have examined a cohort of 27 families, 15 with Herlitz and 12 with non-Herlitz JEB, for mutations in the candidate genes, LAMA3, LAMB3, and LAMC2, encoding the subunit polypeptides of laminin 5. The mutation detection strategy consisted of PCR amplification of all exons in these genes, followed by heteroduplex scanning and nucleotide sequencing. We were able to identify pathogenic mutations in both alleles of each proband, the majority of the mutations being in the LAMB3 gene. Examination of the mutation database revealed that most cases with Herlitz JEB harbored premature termination codon (PTC) mutations in both alleles. In non-Herlitz cases, the PTC mutation was frequently associated with a missense mutation or a putative splicing mutation in trans. In three cases with putative splicing mutations, RT-PCR analysis revealed a repertoire of splice variants in-frame, predicting the synthesis of either shortened or lengthened, yet partly functional, polypeptides. These observations would explain the relatively mild phenotype in cases with splicing mutations. Collectively, these findings, together with the global laminin 5 mutation database, contribute to our understanding of the genotype/phenotype correlations explaining the Herlitz vs non-Herlitz phenotypes.

Novel Mutations in the LAMC2 Gene in Non-Herlitz Junctional Epidermolysis Bullosa: Effects on Laminin-5 Assembly, Secretion, and Deposition

Laminin-5 is the major adhesion ligand of epithelial cells. Mutations in the three genes (LAMA3, LAMB3, LAMC2) encoding the laminin-5 chains cause junctional epidermolysis bullosa, a clinically and genetically heterogeneous blistering skin disease. Here, we describe a non-Herlitz junctional epidermolysis bullosa patient, compound heterozygote for two novel mutations affecting the LAMC2 gene. The mutation in the paternal allele is a de novo splice site mutation (522-1G-->A) that results in in-frame skipping of exon 4 and synthesis of a mutated gamma2 polypeptide (gamma2Delta4) carrying a 33 amino acid deletion within the N-terminal domain V. The maternal mutation is a one base pair insertion (3511insA) in the 3' terminal exon of LAMC2 resulting in a frameshift and a premature termination codon. Mutation 3511insA is predicted to lead to the synthesis of a gamma2 polypeptide (gamma2t) disrupted in its alpha-helical C-terminal structure and truncated of the last 25 amino acids. Keratinocytes isolated from the patient's skin showed a markedly decreased level of gamma2 chain mRNA and secreted scant amounts of laminin-5, which undergoes physiologic proteolytic processing. To investigate the biologic function of the laminin-5 molecules synthesized by the patient, mutant gamma2 cDNAs were transiently expressed in gamma2-null keratinocytes. Transfection of the gamma2Delta4 cDNA resulted in restoration of laminin-5 deposition onto the culture substrate, which demonstrates that the gamma2 polypeptides carrying a deletion in domain V, upstream of the gamma2 proteolytic cleavage site, are assembled into native laminin-5 that is secreted and extracellularly processed. In contrast, transfection of a mutant cDNA expressing the gamma2t chain failed to restore laminin-5 immunoreactivity, which indicates that integrity of the gamma2 C-terminal amino acid sequences is required for laminin-5 assembly. These results correlate for the first time a functional alteration in a laminin-5 domain with a mild junctional epidermolysis bullosa phenotype.

Truncated Type XVII Collagen Expression in a Patient with Non-Herlitz Junctional Epidermolysis Bullosa Caused by a Homozygous Splice-Site Mutation

Type XVII collagen (180-kDa bullous pemphigoid antigen) is a structural component of hemidesmosomes. Mutations in the type XVII collagen gene (COL17A1) have been established to be the molecular basis of non-Herlitz junctional epidermolysis bullosa (JEB-nH), an inherited skin blistering disorder. Here we report for the first time truncated type XVII collagen expression, caused by homozygosity for a COL17A1 donor splice-site mutation (4261+1 g → c), which was identified by PCR amplification on genomic DNA. By RT-PCR and sequencing of cDNA derived from mRNA from the patient's cultured keratinocytes, we provide evidence of cryptic splicing and exon skipping, most abundantly of exon 52. JEB-nH patients with COL17A1 splice-site mutations resulting in an exon skip often have no immunologically detectable type XVII collagen. However, in our patient with the generalized atrophic benign epidermolysis bullosa subtype, a small amount of type XVII collagen was detectable in the skin, and immunoblotting of cultured keratinocytes revealed that the 180-kDa protein was 10 kDa shorter. We hypothesize that the function of this truncated type XVII collagen polypeptide, which is expressed at low levels, is impaired, explaining the JEB-nH phenotype.

A Homozygous Nonsense Mutation in Type XVII Collagen Gene (COL17A1) Uncovers an Alternatively Spliced mRNA Accounting for an Unusually Mild Form of Non-Herlitz Junctional Epidermolysis Bullosa

In this study we describe six Italian patients presenting an unusually mild variant of non-Herlitz junctional epidermolysis bullosa associated with a reduced expression of type XVII collagen. All patients are homozygous for a novel nonsense mutation (R795X) within exon 33 of COL17A1 and show a common haplotype, attesting propagation of an ancestral allele within the Italian population. Analysis of patients' COL17A1 transcripts showed the presence of two mRNA species: a normal-sized mRNA carrying mutation R795X that undergoes rapid decay, and a transcript generated by in-frame skipping of exon 33. Patients keratinocytes were shown to synthesize minute amounts of type XVII collagen, which appeared correctly localized along the cutaneous basement membrane. We therefore suggest that the exon 33-deleted COL17A1 splice variant encodes for type XVII collagen molecules that maintain a functional role and account for the mild phenotype of our patients.

A disease-associated glycine substitution in BP180 (type XVII collagen) leads to a local destabilization of the major collagen triple helix.

BP180 is a homotrimeric transmembrane protein with a carboxy-terminal ectodomain that forms an interrupted collagen triple helix. Null type mutations in the BP180 gene produce a recessive subepidermal blistering disease, non-Herlitz junctional epidermolysis bullosa. Like the null mutations, a glycine substitution (G627V) within the longest BP180 collagenous domain (COL15) is also associated with the recessive skin disease; however, unlike the null mutations, this glycine substitution appears to act in a dominant fashion to give rise to a novel form of random pitting dental enamel hypoplasia. The dominant effects of this mutation were thought to be due to alterations in the assembly and/or stability of this BP180 collagenous region. To further investigate this issue, a structural analysis was performed on recombinant forms of the wild type and G627V mutant BP180 ectodomain. Both proteins were found to form collagen-like triple helices with very similar Stokes radii and melting temperatures and exhibited very similar rates of synthesis, secretion and turn-over. Tryptic digestion analysis revealed that the mutant G627V-sec180e contains an additional highly sensitive proteolytic site that maps within the region of the mutation. Thus, the disease-associated G627V mutation in BP180 does not grossly alter protein structure, but causes a local destabilization of the triple-helix that exposes sensitive residues to the in vitro effects of trypsin and possibly affects its structure-function in vivo.

Developmental defects of enamel in humans with hereditary epidermolysis bullosa

Developmental defects of enamel are often reported as a feature of the more severe forms of epidermolysis bullosa (EB). The purpose of this investigation was to determine the prevalence and character of enamel defects in each of the major hereditary EB types. Clinical evaluations were made on 237 individuals representing all of the major EB types and 58 unaffected individuals. All EB cases were categorized by phenotype, mode of inheritance and skin biopsy. The frequency of individuals having developmental enamel defects ranged from 8.6% in recessive dystrophic EB to 100% in junctional EB; 27.5% of the control population had these defects. Generalized hypoplasia characterized by either severe pitting and/or thin enamel was seen in all junctional EB cases but not in any other EB type. There was a tendency for the severe Herlitz form of junctional EB to have thin enamel while non-Herlitz junctional EB cases had less severe pitting and generally no reduction in enamel thickness. The prevalence of individuals with hypoplastic enamel bands was greater in the EB population (9.7%) than controls (1.9%). Thus individuals with simplex and dominant dystrophic EB typically have enamel defects that are similar in frequency and distribution to those of unaffected individuals. Developmental defects of enamel are a consistent feature of junctional EB, although the clinical expression is highly variable.

Kalinin is abnormally expressed in epithelial basement membranes of Herlitz's junctional epidermolysis bullosa patients.

Kalinin is an extracellular adhesion molecule specific to epithelial basement membranes (BM) identified as a component of anchoring filaments of hemidesmosomes. This heterotrimeric protein is synthesized by cultured normal human keratinocytes and is involved in cell attachment. In indirect immunofluorescence studies, the epidermal BM of patients with junctional epidermolysis bullosa (JEB) of Herlitz's type were found not to be reactive with the anti-kalinin monoclonal antibodies ka146 and K140 and displayed a decreased immunoreactivity to two anti-kalinin antibodies cross-reacting with K-laminin, an anchoring filament component recently described. The intrinsic defect of JEB keratinocytes in the synthesis of kalinin was further documented by indirect immunofluorescence on in vitro cultures of these cells. In non-Herlitz JEB patients, staining of BM was constantly detected. Impairment of expression of kalinin correlated with the lack of reactivity to the monoclonal antibody GB3, which detects the BM component nicein/BM600. These results clearly demonstrate a defect of kalinin expression in epithelial basement membranes of Herlitz JEB patients and suggest that kalinin may play a role in the pathogenesis of the disease. Further studies are in progress to define possible relationships between kalinin and nicein.