Molecular basis of recessive dystrophic epidermolysis bullosa: genotype/phenotype correlation in a case of moderate clinical severity

Novel and very rare causative variants in the COL7A1 gene of Vietnamese patients with recessive dystrophic epidermolysis bullosa revealed by whole-exome sequencing

BACKGROUND

Dystrophic epidermolysis bullosa (DEB) is a rare inherited disorder characterized by skin fragility leading to trauma-induced subepidermal blisters and healing with scarring. DEB is caused by mutations in COL7A1, the gene encoding for type VII collagen (COLVII). The DEB inheritance trait is divided into dominant dystrophic epidermolysis bullosa (DDEB) and recessive dystrophic epidermolysis bullosa (RDEB).

METHODS

Whole-exome sequencing (WES) was performed for identifying mutations in six affected individuals of five Vietnamese families.

RESULTS

Three novel variants in total of eight variants were found in five families. The first novel variant causing glycine substitution (c.8279G>A, p.G2760E), the remaining two novel variants resulted in splice site affecting (c.4518+2delT and c.5821-2A>G). Functional analysis indicated that the splice site at c.4518+2delT resulted in a skipping of exon 43, leading to an in-frame deletion of 12 amino acids.

CONCLUSION

Our finding expands the spectrum of COL7A1 mutations and reports altered splicing at c.4518+2delT during the processing of the pre-mRNA. This study provides an additional scientific basis for diagnosis, genetic counseling, and prognosis purposes of EB patients.

Bone marrow transplant with post-transplant cyclophosphamide for recessive dystrophic epidermolysis bullosa expands the related donor pool and permits tolerance of nonhaematopoietic cellular grafts

BACKGROUND

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe systemic genodermatosis lacking therapies beyond supportive care for its extensive, life-limiting manifestations.

OBJECTIVES

To report the safety and preliminary responses of 10 patients with RDEB to bone marrow transplant (BMT) with post-transplant cyclophosphamide (PTCy BMT) after reduced-intensity conditioning with infusions of immunomodulatory donor-derived mesenchymal stromal cells (median follow-up 16 months).

METHODS

BMT toxicities, donor blood and skin engraftment, skin biopsies, photographic and dynamic assessments of RDEB disease activity were obtained at intervals from pre-BMT to 1 year post-BMT.

RESULTS

Related donors varied from haploidentical (n = 6) to human leucocyte antigen (HLA)-matched (n = 3), with one HLA-matched unrelated donor. Transplant complications included graft failure (n = 3; two pursued a second PTCy BMT), veno-occlusive disease (n = 2), posterior reversible encephalopathy (n = 1) and chronic graft-versus-host disease (n = 1; this patient died). In the nine ultimately engrafted patients, median donor chimerism at 180 days after transplant was 100% in peripheral blood and 27% in skin. Skin biopsies showed stable (n = 7) to improved (n = 2) type VII collagen protein expression by immunofluorescence and gain of anchoring fibril components (n = 3) by transmission electron microscopy. Early signs of clinical response include trends toward reduced body surface area of blisters/erosions from a median of 49·5% to 27·5% at 100 days after BMT (P = 0·05), with parental measures indicating stable quality of life.

CONCLUSIONS

PTCy BMT in RDEB provides a means of attaining immunotolerance for future donor-derived cellular grafts (ClinicalTrials.gov identifier NCT02582775). What's already known about this topic? Severe, generalized recessive dystrophic epidermolysis bullosa (RDEB) is marked by great morbidity and early death. No cure currently exists for RDEB. Bone marrow transplant (BMT) is the only described systemic therapy for RDEB. What does this study add? The first description of post-transplant cyclophosphamide (PTCy) BMT for RDEB. PTCy was well tolerated and provided excellent graft-versus-host disease prophylaxis, replacing long courses of calcineurin inhibitors in patients receiving human leucocyte antigen-matched sibling BMT. What is the translational message? The PTCy BMT platform permits identification of a suitable related donor for most patients and for subsequent adoptive transfer of donor nonhaematopoietic cells after establishment of immunological tolerance.

Establishment of integration-free induced pluripotent stem cells from human recessive dystrophic epidermolysis bullosa keratinocytes

BACKGROUND

Induced pluripotent stem cell (iPSC) technology enables patient-specific pluripotent stem cells to be derived from adult somatic cells without the use of an embryonic cell source. To date, recessive dystrophic epidermolysis bullosa (RDEB)-specific iPSCs have been generated from patients using integrating retroviral vectors. However, vector integration into the host genome can endanger the biosafety and differentiation propensities of iPSCs. Although various integration-free reprogramming systems have been reported, their utility in reprogramming somatic cells from patients remains largely undetermined.

OBJECTIVE

Our study aims to establish safe iPSCs from keratinocytes of RDEB patients using non-integration vector.

METHOD

We optimized and infected non-integrating Sendai viral vectors to reprogram keratinocytes from healthy volunteers and RDEB patients.

RESULTS

Sendai vector infection led to the reproducible generation of genomic modification-free iPSCs from these keratinocytes, which was proved by immunohistochemistry, reverse transcription polymerase chain reaction, methylation assay, teratoma assay and embryoid body formation assay. Furthermore, we confirmed that these iPSCs have the potential to differentiate into dermal fibroblasts and epidermal keratinocytes.

CONCLUSION

This is the first report to prove that the Sendai vector system facilitates the reliable reprogramming of patient keratinocytes into transgene-free iPSCs, providing another pluripotent platform for personalized diagnostic and therapeutic approaches to RDEB.

Novel and emerging therapies in the treatment of recessive dystrophic epidermolysis bullosa

Epidermolysis bullosa (EB) is a clinically and genetically heterogeneous group of inherited blistering diseases that affects ∼ 500,000 people worldwide. Clinically, individuals with EB have fragile skin and are susceptible to blistering following minimal trauma, with mucous membrane and other organ involvement in some subtypes. Within the spectrum of EB, ∼ 5% of affected individuals have the clinically more severe recessive dystrophic (RDEB) variant with a prevalence of 8 per one million of the population. RDEB is caused by loss-of-function mutations in the type VII collagen gene, COL7A1, which leads to reduced or absent type VII collagen (C7) and a paucity of structurally effective anchoring fibrils at the dermal-epidermal junction (DEJ). Currently, there is no cure for RDEB, although considerable progress has been made in testing novel treatments including gene therapy (lentiviral and gamma retroviral vectors for COL7A1 supplementation in keratinocytes and fibroblasts), as well as cell therapy (use of allogeneic fibroblasts, mesenchymal stromal cells (MSCs), and bone marrow transplantation (BMT)). Here, we review current treatment modalities available as well as novel and emerging therapies in the treatment of RDEB. Clinical trials of new translational therapies in RDEB offer hope for improved clinical management of patients as well as generating broader lessons for regenerative medicine that could be applicable to other inherited or acquired abnormalities of wound healing or scarring.

Preconditioning of mesenchymal stem cells for improved transplantation efficacy in recessive dystrophic epidermolysis bullosa

Introduction

The use of hematopoietic cell transplantation (HCT) has previously been shown to ameliorate cutaneous blistering in pediatric patients with recessive dystrophic epidermolysis bullosa (RDEB), an inherited skin disorder that results from loss-of-function mutations in COL7A1 and manifests as deficient or absent type VII collagen protein (C7) within the epidermal basement membrane. Mesenchymal stem cells (MSCs) found within the HCT graft are believed to be partially responsible for this amelioration, in part due to their intrinsic immunomodulatory and trophic properties and also because they have been shown to restore C7 protein following intradermal injections in models of RDEB. However, MSCs have not yet been demonstrated to improve disease severity as a stand-alone systemic infusion therapy. Improving the efficacy and functional utility of MSCs via a pre-transplant conditioning regimen may bring systemic MSC infusions closer to clinical practice.

Methods

MSCs were isolated from 2- to 4-week-old mice and treated with varying concentrations of transforming growth factor-β (TGFβ; 5-20 ng/mL), tumor necrosis factor- α (TNFα; 10-40 ng/mL), and stromal cell-derived factor 1-α (SDF-1α; 30 ng/mL) for 24-72 hours.

Results

We demonstrate that treating murine MSCs with exogenous TGFβ (15 ng/mL) and TNFα (30 ng/mL) for 48 hours induces an 8-fold increase in Col7a1 expression and a significant increase in secretion of C7 protein, and that the effects of these cytokines are both time and concentration dependent. This cytokine treatment also promotes a 4-fold increase in Tsg-6 expression, a gene whose product is associated with improved wound-healing and immunosuppressive features. Finally, the addition of exogenous SDF-1α to this regimen induces a simultaneous upregulation of Col7a1, Tsg-6, and Cxcr4 expression.

Conclusions

These data suggest that preconditioning represents a feasible method for improving the functional utility of MSCs in the context of RDEB stem cell transplantation, and also highlight the applicability of preconditioning principles toward other cell-based therapies aimed at treating RDEB patients.

Induced pluripotent stem cells from individuals with recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is an inherited blistering skin disorder caused by mutations in the COL7A1 gene-encoding type VII collagen (Col7), the major component of anchoring fibrils at the dermal-epidermal junction. Individuals with RDEB develop painful blisters and mucosal erosions, and currently, there are no effective forms of therapy. Nevertheless, some advances in patient therapy are being made, and cell-based therapies with mesenchymal and hematopoietic cells have shown promise in early clinical trials. To establish a foundation for personalized, gene-corrected, patient-specific cell transfer, we generated induced pluripotent stem (iPS) cells from three subjects with RDEB (RDEB iPS cells). We found that Col7 was not required for stem cell renewal and that RDEB iPS cells could be differentiated into both hematopoietic and nonhematopoietic lineages. The specific epigenetic profile associated with de-differentiation of RDEB fibroblasts and keratinocytes into RDEB iPS cells was similar to that observed in wild-type (WT) iPS cells. Importantly, human WT and RDEB iPS cells differentiated in vivo into structures resembling the skin. Gene-corrected RDEB iPS cells expressed Col7. These data identify the potential of RDEB iPS cells to generate autologous hematopoietic grafts and skin cells with the inherent capacity to treat skin and mucosal erosions that typify this genodermatosis.

Bone marrow transplantation for recessive dystrophic epidermolysis bullosa

BACKGROUND

Recessive dystrophic epidermolysis bullosa is an incurable, often fatal mucocutaneous blistering disease caused by mutations in COL7A1, the gene encoding type VII collagen (C7). On the basis of preclinical data showing biochemical correction and prolonged survival in col7 −/− mice, we hypothesized that allogeneic marrow contains stem cells capable of ameliorating the manifestations of recessive dystrophic epidermolysis bullosa in humans.

METHODS

Between October 2007 and August 2009, we treated seven children who had recessive dystrophic epidermolysis bullosa with immunomyeloablative chemotherapy and allogeneic stem-cell transplantation. We assessed C7 expression by means of immunofluorescence staining and used transmission electron microscopy to visualize anchoring fibrils. We measured chimerism by means of competitive polymerase-chain-reaction assay, and documented blister formation and wound healing with the use of digital photography.

RESULTS

One patient died of cardiomyopathy before transplantation. Of the remaining six patients, one had severe regimen-related cutaneous toxicity, with all having improved wound healing and a reduction in blister formation between 30 and 130 days after transplantation. We observed increased C7 deposition at the dermal-epidermal junction in five of the six recipients, albeit without normalization of anchoring fibrils. Five recipients were alive 130 to 799 days after transplantation; one died at 183 days as a consequence of graft rejection and infection. The six recipients had substantial proportions of donor cells in the skin, and none had detectable anti-C7 antibodies.

CONCLUSIONS

Increased C7 deposition and a sustained presence of donor cells were found in the skin of children with recessive dystrophic epidermolysis bullosa after allogeneic bone marrow transplantation. Further studies are needed to assess the long-term risks and benefits of such therapy in patients with this disorder. (Funded by the National Institutes of Health; ClinicalTrials.gov number, NCT00478244.)

Natural skin surface pH is on average below 5, which is beneficial for its resident flora

Variable skin pH values are being reported in literature, all in the acidic range but with a broad range from pH 4.0 to 7.0. In a multicentre study (N = 330), we have assessed the skin surface pH of the volar forearm before and after refraining from showering and cosmetic product application for 24 h. The average pH dropped from 5.12 +/- 0.56 to 4.93 +/- 0.45. On the basis of this pH drop, it is estimated that the 'natural' skin surface pH is on average 4.7, i.e. below 5. This is in line with existing literature, where a relatively large number of reports (c. 50%) actually describes pH values below 5.0; this is in contrast to the general assumption, that skin surface pH is on average between 5.0 and 6.0. Not only prior use of cosmetic products, especially soaps, have profound influence on skin surface pH, but the use of plain tap water, in Europe with a pH value generally around 8.0, will increase skin pH up to 6 h after application before returning to its 'natural' value of on average below 5.0. It is demonstrated that skin with pH values below 5.0 is in a better condition than skin with pH values above 5.0, as shown by measuring the biophysical parameters of barrier function, moisturization and scaling. The effect of pH on adhesion of resident skin microflora was also assessed; an acid skin pH (4-4.5) keeps the resident bacterial flora attached to the skin, whereas an alkaline pH (8-9) promotes the dispersal from the skin.

Keratinocyte-/fibroblast-targeted rescue of Col7a1-disrupted mice and generation of an exact dystrophic epidermolysis bullosa model using a human COL7A1 mutation

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe hereditary bullous disease caused by mutations in COL7A1, which encodes type VII collagen (COL7). Col7a1 knockout mice (COL7(m-/-)) exhibit a severe RDEB phenotype and die within a few days after birth. Toward developing novel approaches for treating patients with RDEB, we attempted to rescue COL7(m-/-) mice by introducing human COL7A1 cDNA. We first generated transgenic mice that express human COL7A1 cDNA specifically in either epidermal keratinocytes or dermal fibroblasts. We then performed transgenic rescue experiments by crossing these transgenic mice with COL7(m+/-) heterozygous mice. Surprisingly, human COL7 expressed by keratinocytes or by fibroblasts was able to rescue all of the abnormal phenotypic manifestations of the COL7(m-/-) mice, indicating that fibroblasts as well as keratinocytes are potential targets for RDEB gene therapy. Furthermore, we generated transgenic mice with a premature termination codon expressing truncated COL7 protein and performed the same rescue experiments. Notably, the COL7(m-/-) mice rescued with the human COL7A1 allele were able to survive despite demonstrating clinical manifestations very similar to those of human RDEB, indicating that we were able to generate surviving animal models of RDEB with a mutated human COL7A1 gene. This model has great potential for future research into the pathomechanisms of dystrophic epidermolysis bullosa and the development of gene therapies for patients with dystrophic epidermolysis bullosa.

Forty-two novel COL7A1 mutations and the role of a frequent single nucleotide polymorphism in the MMP1 promoter in modulation of disease severity in a large European dystrophic epidermolysis bullosa cohort

BACKGROUND

Dystrophic epidermolysis bullosa (DEB) is a severe genetic skin blistering disorder caused by mutations in the gene COL7A1, encoding collagen VII. Recently, the MMP1 promoter single nucleotide polymorphism (SNP) rs1799750, designated as 1G 2G, was shown to be involved in modulation of disease severity in patients with recessive DEB (RDEB), and was proposed as a genetic modifier.

OBJECTIVES

To identify the molecular basis of DEB in 103 individuals and to replicate the results of the MMP1 promoter SNP analysis in an independent patient group, as verification is necessary in such a rare and heterogeneous disorder.

METHODS

To determine the molecular basis of the disease, we performed COL7A1 mutation screening, reverse transcription-polymerase chain reaction (PCR) and real-time quantitative PCR. The status of the MMP1 SNP was analysed by PCR and restriction enzyme digestion and verified by sequencing.

RESULTS

We disclosed 42 novel COL7A1 mutations, including the first large genomic deletion of 4 kb affecting only the COL7A1 gene, and three apparently silent mutations affecting splicing. Even though the frequency of the high-risk allele was increased in patients with RDEB, no statistically significant correlation between disease severity and genotype could be made. Also, no correlation was observed with development of squamous cell carcinoma, a severe complication of DEB.

CONCLUSIONS

Taken together, the results suggest that the MMP1 SNP is not the sole disease modifier in different forms of DEB, and other genetic and environmental factors contribute to the clinical phenotype.

Characterization of molecular mechanisms underlying mutations in dystrophic epidermolysis bullosa using site-directed mutagenesis

Type VII collagen (C7) is a major component of anchoring fibrils, structures that mediate epidermal-dermal adherence. Mutations in gene COL7A1 encoding for C7 cause dystrophic epidermolysis bullosa (DEB), a genetic mechano-bullous disease. The biological consequences of specific COL7A1 mutations and the molecular mechanisms leading to DEB clinical phenotypes are unknown. In an attempt to establish genotype-phenotype relationships, we generated four individual substitution mutations that have been associated with recessive DEB, G2049E, R2063W, G2569R, and G2575R, and purified the recombinant mutant proteins. All mutant proteins were synthesized and secreted as a 290-kDa mutant C7 alpha chain at levels similar to wild type C7. The G2569R and G2575R glycine substitution mutations resulted in mutant C7 with increased sensitivity to protease degradation and decreased ability to form trimers. Limited proteolytic digestion of mutant G2049E and R2063W proteins yielded aberrant fragments and a triple helix with reduced stability. These two mutations next to the 39-amino acid helical interruption hinge region caused local destabilization of the triple-helix that exposed an additional highly sensitive proteolytic site within the region of the mutation. Our functional studies demonstrated that C7 is a potent pro-motility matrix for skin human keratinocyte migration and that this activity resides within the triple helical domain. Furthermore, G2049E and R2063W mutations reduced the ability of C7 to support fibroblast adhesion and keratinocyte migration. We conclude that known recessive DEB C7 mutations perturb critical functions of the C7 molecule and likely contribute to the clinical phenotypes of DEB patients.

Genotype-phenotype correlations in six Japanese patients with recessive dystrophic epidermolysis bullosa with the recurrent p.Glu2857X mutation

BACKGROUND

General genotype-phenotype correlations have been delineated in recessive dystrophic epidermolysis bullosa (RDEB), but these remain complicated and it is still difficult to assess the clinical consequences of individual COL7A1 mutations.

OBJECTIVE

To characterize recurrent p.Glu2857X mutations and show how other COL7A1 mutations influence the phenotype in RDEB patients harboring p.Glu2857X.

METHODS

Genotype-phenotype correlations were studied in six Japanese RDEB patients with the p.Glu2857X mutation.

RESULTS

Besides the common p.Glu2857X mutation, premature termination codon (PTC) mutations were found in three patients, glycine substitution missense mutations in two patients, and a non-glycine substitution missense mutation in one patient. PTC mutations in both alleles generally cause the most severe, mutilating Hallopeau-Siemens (HS) variant of RDEB, whereas none of the PTC mutations resulted in severe phenotypes consistent with the HS subtype when coupled with p.Glu2857X. Missense glycine and non-glycine mutations caused phenotypes of differing severity, suggesting that the extent of destabilization of anchoring fibrils depends on the type of mutation.

CONCLUSION

A p.Glu2857X mutation exhibits mild pathogenic effects compared to other PTC mutations in COL7A1, and its uniqueness enables detailed analysis and comparison of the destabilizing effects of missense mutations in RDEB patients.

Dystrophic epidermolysis bullosa with one dominant and one recessive mutation of the COL7A1 gene in a child with deafness

Dystrophic epidermolysis bullosa can be inherited in autosomal dominant and recessive forms, the former usually expressed as a milder phenotype, although mild forms of recessive dystrophic epidermolysis bullosa can occur. We present a patient who was found to be a compound heterozygote, inheriting a dominant mutation from his father and a recessive mutation from his mother, resulting in a clinically severe case of dystrophic epidermolysis bullosa. Mutations in the gene for collagen VII (COL7A1) have been documented in both types of dystrophic epidermolysis bullosa. Our patient has also been diagnosed with bilateral auditory neuropathy, a disorder coincidentally also mapped to a nearby gene on chromosome 3p21 (the transmembrane inner ear expressed gene, TMIE).

A frequent functional SNP in the MMP1 promoter is associated with higher disease severity in recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is caused by mutations in the COL7A1 gene encoding type VII collagen. Variations in severity between the different clinical forms of RDEB likely depend on the nature and location of COL7A1 mutations, but observed intrafamilial phenotypic variations suggest additional genetic and/or environmental factors. Candidate modifier genes include MMP1, encoding matrix metalloproteinase 1, the first gene implicated in RDEB before its primary role in the disease was excluded. Type VII collagen is a substrate of MMP1 and an imbalance between its synthesis and degradation could conceivably worsen the RDEB phenotype. Here, we studied a previously described family with three affected siblings of identical COL7A1 genotype but displaying great sibling-to-sibling variations in disease severity. RDEB severity did not correlate with type VII collagen synthesis levels, but with protein levels at the dermal-epidermal junction, suggesting increased degradation by metalloproteinases. This was supported by the presence of increased transcript and active MMP1 levels in the most severely affected children, who carried a known SNP (1G/2G) in the MMP1 promoter. This SNP creates a functional Ets binding site resulting in transcriptional upregulation. We next studied a French cohort of 31 unrelated RDEB patients harboring at least one in-frame COL7A1 mutation, ranging from mild localized RDEB to the severe Hallopeau-Siemens form. We found a strong genetic association between the 2G variant and the Hallopeau-Siemens disease type (odds ratio: 73.6). This is the first example of a modifier gene in RDEB and has implications for its prognosis and possible new treatments.

Mutation analyses of COL7A1 gene in three Taiwanese patients with severe recessive dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is a hereditary mechanobullous disorder characterized by fragility of the skin and mucous membranes caused by abnormal anchoring fibrils. Both dominant and recessive DEB are caused by mutations in COL7A1, the gene encoding type VII collagen, the major component of anchoring fibrils. We performed mutation analysis of COL7A1 in three patients with recessive DEB. The diagnosis of DEB was based on the characteristic clinical features and confirmed histopathologically. All 118 exons and flanking intron boundaries of COL7A1 were amplified. Four novel mutations (3373insGG, 7769delG, E1535X, G2061E) and two potential splicing mutations were detected. The first three of these mutations resulted in premature termination codons, while G2061E caused a glycine substitution mutation in the triple-helical domain. This is the first report of mutation analyses of the COL7A1 gene in Taiwanese pedigrees with recessive DEB. Each patient had a heterozygous premature termination codon mutation combined with either a glycine substitution mutation in the critical triple-helical collagenous domain or a potential splicing mutation. These genotypes correlate well with the severe clinical phenotype of recessive DEB.

Epidermolysis bullosa. II. Type VII collagen mutations and phenotype-genotype correlations in the dystrophic subtypes

BACKGROUND

The dystrophic forms of epidermolysis bullosa (DEB), a group of heritable blistering disorders, show considerable phenotypic variability, and both autosomal dominant and autosomal recessive inheritance can be recognised. DEB is derived from mutations in the type VII collagen gene (COL7A1), encoding a large collagenous protein that is the predominant, if not exclusive, component of the anchoring fibrils at the dermal-epidermal junction.

METHODS

The Dystrophic Epidermolysis Bullosa Research Association Molecular Diagnostics Laboratory (Philadelphia, Pennsylvania, USA), established in 1996, has analysed more than 1000 families with different forms of epidermolysis bullosa, among them 332 families with DEB. DNA specimens were subjected to mutation analysis by polymerase chain reaction (PCR) amplification of all 118 exons and flanking intronic sequences of COL7A1, followed either by heteroduplex scanning and sequencing of the PCR products demonstrating heteroduplexes or by direct nucleotide sequencing.

RESULTS

355 mutant alleles out of the anticipated 438 (81.1%) were disclosed. Among these mutations, a total of 242 mutations were distinct and 138 were novel, previously unreported mutations. No evidence of mutations in any other gene was obtained.

DISCUSSION

Examination of the mutation database suggested phenotype-genotype correlations, contributing to the improved subclassification of DEB with prognostic implications. The mutation information also forms the basis for accurate genetic counselling and prenatal diagnosis in families at risk for recurrence.

COL7A1 mutation G2037E causes epidermal retention of type VII collagen

COL7A1 glycine substitution (GS) mutations result in dominant and recessive dystrophic epidermolysis bullosa (DDEB and RDEB). Here, we report a DDEB family in which retention of type VII collagen by epidermal keratinocytes was observed for a female proband. Mutational analysis detected a GS mutation, G2037E, in the proband and her affected father. To demonstrate direct association of G2037E and type VII collagen retention we introduced this mutated COL7A1 gene into cultured keratinocytes using retroviral methods. This mutation was dominant, so we transferred a 1:1 mixture of wild-type (unaffected) and G2037E-mutated COL7A1, together, in addition to the unaffected gene or the mutated gene alone. The increase in type VII collagen cytoplasmic staining in the G2037E/wild transfectant cell samples was compared with that for control/wild-type cells. Intracellular collagen VII staining in the G2037E (alone)-transfected cells was even stronger than for the G2037E/wild transfection sample. These results indicate that the G2037E COL7A1 mutation leads to increased epidermal retention of type VII collagen in vivo, and also suggests that homozygotes carrying this dominant GS mutation may have more severe phenotypes than heterozygotes. This study furthers our understanding of GS COL7A1 mutations in DEB.

Genetic studies of 20 Japanese families of dystrophic epidermolysis bullosa

Dystrophic EB (DEB) is clinically characterized by mucocutaneous blistering in response to minor trauma, followed by scarring and nail dystrophy, and is caused by mutations in the COL7A1 gene encoding type VII collagen. DEB is inherited in either an autosomal dominant (DDEB) or recessive (RDEB) fashion. DDEB basically results from a glycine substitution mutation within the collagenous domain on one COL7A1 allele, while a combination of mutations such as premature stop codon, missense, and splice-site mutations on both alleles causes RDEB. In this study, mutation analysis was performed in 20 distinct Japanese DEB families (16 RDEB and four DDEB). The result demonstrated 30 pathogenic COL7A1 mutations with 16 novel mutations, which included four missense, five nonsense, one deletion, two insertion, one indel, and three splice-site mutations. We confirmed that Japanese COL7A1 mutations were basically family specific, although three mutations, 5818delC, 6573 + 1G > C, and E2857X, were recurrent based on previous reports. Furthermore, the Q2827X mutation found in two unrelated families would be regarded as a candidate recurrent Japanese COL7A1 mutation. The study furthers our understanding of both the clinical and allelic heterogeneity displayed in Japanese DEB patients.

Single Amino Acid Substitutions in Procollagen VII Affect Early Stages of Assembly of Anchoring Fibrils

Procollagen VII is a homotrimer of 350-kDa pro-alpha1(VII) chains, each consisting of a central collagenous domain flanked by the noncollagenous N-terminal NC1 domain and the C-terminal NC2 domain. After secretion from cells, procollagen VII molecules form anti-parallel dimers with a C-terminal 60-nm overlap. Characteristic alignment of procollagen VII monomers forming a dimer depends on site-specific binding between the NC2 domain and the triple-helical region adjacent to Cys-2634 of the interacting procollagen VII molecules. Formation of the intermolecular disulfide bonds between Cys-2634 and either Cys-2802 or Cys-2804 is promoted by the cleavage of the NC2 domain by procollagen C-proteinase. By employing recombinant procollagen VII variants harboring G2575R, R2622Q, or G2623C substitutions previously disclosed in patients with dystrophic epidermolysis bullosa, we studied how these amino acid substitutions affect intermolecular interactions. Binding assays utilizing an optical biosensor demonstrated that the G2575R substitution increased affinity between mutant molecules. In contrast, homotypic binding between the R2622Q or G2623C molecules was not detected. In addition, kinetics of heterotypic binding of all analyzed mutants to wild type collagen VII were different from those for binding between wild type molecules. Moreover, solid-state binding assays demonstrated that R2622Q and G2623C substitutions prevent formation of stable assemblies of procollagen C-proteinase-processed mutants. These results indicate that single amino acid substitutions in procollagen VII alter its self-assembly and provide a basis for understanding the pathomechanisms leading from mutations in the COL7A1 gene to fragility of the dermal-epidermal junction seen in patients with dystrophic forms of epidermolysis bullosa.

Does the position of the premature termination codon in COL7A1 correlate with the clinical severity in recessive dystrophic epidermolysis bullosa?

Recessive dystrophic epidermolysis bullosa (RDEB) is an inherited skin disease caused by mutations in the gene encoding type VII collagen (COL7A1). The mutations are highly variable and this greatly complicates the study of the genotype-phenotype relationships. To date, three recurrent mutations, specific to Japanese RDEB patients have been reported. By comparing the phenotypes of RDEB patients with different recurrent mutations, the upstream positions of the premature termination codons (PTCs) showed strong correlation with the RDEB clinical disease severity. However, such correlations have not been supported by patients with mutations that were different from these recurrent Japanese patients mutations. In this study, we report a case of RDEB with a very mild clinical phenotype, who was a compound heterozygote harbouring both a recurrent Japanese mutation and a novel deletion mutation resulting in a more upstream PTC. The patient and his mother were shown to have a recurrent donor splice site mutation within intron 81 (6573 + 1G > C), a recurrent Japanese mutation that activates a cryptic donor splicing site and results in a downstream PTC. The patient and his father shared a single-nucleotide deletion within exon 64 (5504delA), which causes a downstream frame shift in five amino acids before creating a PTC. Occurrence of the PTCs in mRNA was confirmed by reverse transcription-polymerase chain reaction (RT)-PCR. The patient's skin showed reduced immunofluorescence staining for COL7A1 and reduced number of abnormal or short anchoring fibrils by electron microscopy. Although the position of the mutation 5504delA PTC was located upstream of the previous mutations reported in combination with the 6573 + 1G > C mutation, the two mutations together give an apparently milder clinical phenotype. Therefore, genotype-phenotype relationships in RDEB cannot be explained purely by the position of PTC.

Progress in epidermolysis bullosa: Genetic classification and clinical implications

Epidermolysis bullosa (EB), a heterogenous group of genodermatoses, is characterized by fragility and blistering of the skin associated with extracutaneous manifestations. Based on clinical severity, constellation of the phenotypic manifestations, and the level of tissue separation within the cutaneous basement membrane zone (BMZ), EB has been divided into distinct subcategories. Traditionally, these include the simplex, junctional, and dystrophic forms of EB, and recently attention has been drawn to hemidesmosomal variants demonstrating tissue separation at the level of the hemidesmosomes. Specific mutations in ten distinct genes expressed within the cutaneous BMZ have been delineated in >500 families with different variants of EB. The types of mutations, their positions along the affected genes, and their consequences at the mRNA and protein levels provide explanation for the phenotypic variability and genetic heterogeneity of this group of genodermatoses. Elucidation of mutations in different forms of EB has direct translational applications for improved diagnosis and molecularly based classification with prognostic implications as well as for genetic counseling and DNA-based prenatal testing in families with EB.

The G2028R glycine substitution mutation in COL7A1 leads to marked inter-familiar clinical heterogeneity in dominant dystrophic epidermolysis bullosa

BACKGROUND

Glycine substitution mutations in COL7A1 not only cause dominant dystrophic epidermolysis bullosa (DDEB), but can also be silent mutations which lead to recessive dystrophic epidermolysis bullosa (RDEB) in combination with additional mutations in the other allele.

OBJECTIVE

In this study, we have examined a large American Caucasian pedigree in which 10 family members from four generations presented with simple toenail dystrophy without skin fragility in autosomal dominant manner.

METHOD

We sequenced COL7A1 of this pedigree.

RESULTS

Mutational analysis indeed detected a heterozygous G-to-A transition at nucleotide position 6082 leading to G2028R in all the affected members. Surprisingly, mutation database revealed that this G2028R mutation had been previously identified in two distinct Asian families with DDEB showing apparent skin fragility and blister formation. One case was a 17-month-old Chinese female with classical phenotype of DDEB and the other was a 27-year-old Japanese female with typical epidermolysis bullosa (EB) pruriginosa. To better understand the molecular mechanisms of this marked inter-familiar clinical heterogeneity, we examined the entire sequence of all the exons and exon-intron borders as well as the promoter region of COL7A1 in all the three families. Sequence results demonstrated no significant nucleotide difference in COL7A1 among the three pedigrees.

CONCLUSION

This paper has demonstrated for the first time that identical COL7A1 glycine substitutions can cause remarkably heterogeneous clinical phenotypes extending from simple toe nail dystrophy without skin fragility to typical DDEB and EB pruriginosa. In addition, the fact of inter-familiar, not intra-familiar clinical heterogeneity associated with G2028R suggest that the other molecular mechanisms not controlled by COL7A1 coding sequence might be responsible for the clinical heterogeneity.

Procollagen VII self-assembly depends on site-specific interactions and is promoted by cleavage of the NC2 domain with procollagen C-proteinase

Procollagen VII is a homotrimer of 350-kDa proalpha1(VII) chains. Each chain has a central collagenous domain flanked by a noncollagenous amino-terminal NC1 domain and a carboxy-terminal NC2 domain. After secretion from cells, procollagen VII molecules form antiparallel dimers with a 60 nm overlap. These dimers are stabilized by disulfide bonds formed between cysteines present in the NC2 domain and cysteines present in the triple-helical domain. Electron microscopy has provided direct evidence for the existence of collagen VII dimers, but the dynamic process of dimer formation is not well understood. In the present study, we tested the hypothesis that, during dimer formation, the NC2 domain of one procollagen VII molecule specifically recognizes and binds to the triple-helical region adjacent to Cys-2625 of another procollagen VII molecule. We also investigated the role of processing of the NC2 domain by the procollagen C-proteinase/BMP-1 in dimer assembly. We engineered mini mouse procollagen VII variants consisting of intact NC1 and NC2 domains and a shortened triple helix in which the C-terminal region encompassing Cys-2625 was either preserved or substituted with the region encompassing Cys-1448 derived from the N-terminal part of the triple-helical domain. The results indicate that procollagen VII self-assembly depends on site-specific interactions between the NC2 domain and the triple-helical region adjacent to Cys-2625 and that this process is promoted by the cleavage of the NC2 by procollagen C-proteinase/BMP1.

Dilemmas in distinguishing between dominant and recessive forms of dystrophic epidermolysis bullosa

BACKGROUND

Dystrophic epidermolysis bullosa (DEB) is a heterogeneous inherited blistering skin disorder. The mode of inheritance may be autosomal dominant or recessive but all forms of DEB result from mutations in the gene encoding the anchoring fibril protein, type VII collagen, COL7A1. Consequently, in spite of careful clinical and skin biopsy examination, it may be difficult to distinguish mild recessive cases from de novo dominant disease in families with clinically normal parents and no other affected siblings; this distinction has significant implications for the accuracy of genetic counselling.

OBJECTIVES

To assess whether COL7A1 mutation analysis might help determine mode of inheritance in mild to moderate DEB.

METHODS

We performed COL7A1 screening using heteroduplex analysis and direct nucleotide sequencing in four individuals with mild to moderate "sporadic" DEB and clinically unaffected parents.

RESULTS

In each patient, we identified a heterozygous glycine substitution within the type VII collagen triple helix. However, in two cases these mutations had been inherited in trans with a non-sense mutation on the other allele (i.e. autosomal recessive DEB). In the other two cases, no additional mutation was identified and neither mutation was present in parental DNA (i.e. de novo dominant disease).

CONCLUSIONS

This study highlights the usefulness of DNA sequencing in determining the inherited basis of some sporadic cases of DEB. However, delineation of glycine substitutions should prompt comprehensive COL7A1 gene sequencing in the affected individual, as well as clinical assessment of parents and mutation screening in parental DNA, if the true mode of inheritance is to be established correctly.

Squamous cell carcinoma developing in a 12-year-old boy with nonHallopeau-Siemens recessive dystrophic epidermolysis bullosa

We report a 12-year-old boy with nonHallopeau-Siemens recessive dystrophic epidermolysis bullosa (nHS-RDEB) who developed two skin lesions of squamous cell carcinoma (SCC) on the left foot. The incidence of SCC in nHS-RDEB is much lower than in the HS-RDEB subtype. Furthermore, this boy is the youngest among 92 previously described patients with DEB to develop SCC. This study emphasizes the importance of vigilance in monitoring the possible development of SCC in DEB patients regardless of age or clinical severity.

Dominant and recessive compound heterozygous mutations in epidermolysis bullosa simplex demonstrate the role of the stutter region in keratin intermediate filament assembly

Keratin intermediate filaments are important cytoskeletal structural proteins involved in maintaining cell shape and function. Mutations in the epidermal keratin genes, keratin 5 or keratin 14 lead to the disruption of keratin filament assembly, resulting in an autosomal dominant inherited blistering skin disease, epidermolysis bullosa simplex (EBS). We investigated a large EBS kindred who exhibited a markedly heterogeneous clinical presentation and detected two distinct keratin 5 mutations in the proband, the most severely affected. One missense mutation (E170K) in the highly conserved helix initiation peptide sequence of the 1A rod domain was found in all the affected family members. In contrast, the other missense mutation (E418K) was found only in the proband. The E418K mutation was located in the stutter region, an interruption in the heptad repeat regularity, whose function as yet remains unclear. We hypothesized that this mutated stutter allele was clinically silent when combined with the wild type allele but aggravates the clinical severity of EBS caused by the E170K mutation on the other allele. To confirm this in vitro, we transfected mutant keratin 5 cDNA into cultured cells. Although only 12.7% of the cells transfected with the E170K mutation alone showed disrupted keratin filament aggregations, significantly more cells (30.0%) cotransfected with both E170K and E418K mutations demonstrated keratin aggregation (p < 0.05). These transfection assay results corresponded to the heterogeneous clinical findings of the EBS patient in this kindred. We have identified the first case of both compound heterozygous dominant (E170K) and recessive (E418K) mutations in any keratin gene and confirmed the significant involvement of the stutter region in the assembly and organization of the keratin intermediate filament network in vitro.

Generalized dystrophic epidermolysis bullosa: identification of a novel, homozygous glycine substitution, G2031S, in exon 73 of COL7A1 in monozygous triplets

We report monozygous triplets affected with dystrophic epidermolysis bullosa (DEB). The female triplets were delivered by Caesarean section and skin fragility of each child, which was partly induced by trauma, was apparent from the third to fourth day of life. Clinically, the triplets were equally affected. Mutation analysis in this family revealed a novel recessively expressed glycine substitution, G2031S, in exon 73 of the collagen VII gene COL7A1. Most glycine substitutions in this gene region encoding for the triple helical domain of collagen VII are associated with milder, dominantly inherited phenotypes. By contrast, the novel point mutation of this study is clinically silent in the heterozygous state and leads to a severe DEB subtype when homozygous.

Three homozygous PTC mutations in the collagen type VII gene of patients affected by recessive dystrophic epidermolysis bullosa: analysis of transcript levels in dermal fibroblasts

The Hallopeau-Siemens variant of recessive dystrophic epidermolysis bullosa (HS-RDEB) is a severe inherited skin disease characterized by the absence of collagen type VII (COLVII) and anchoring fibrils (AF), caused by mutations in collagen type VII gene (COL7A1). Mutations leading to the formation of premature termination codons (PTCs) of translation are the characteristic genetic lesions in HS-RDEB patients; many PTC mutations have been found to be associated with a marked reduction or complete absence of COLVII mRNA. In this article, we report homozygosity for three different mutations in the COL7A1 of HS-RDEB patients. One mutation, the R2685X, falling in exon 109, is a novel mutation, whereas the other two, the 425A-->G falling in exon 3 and the 497insA in exon 4, have been previously identified in compound heterozygosity with different mutations in other unrelated RDEB patients. Haplotype analysis in three Italian families carrying the 497insA mutation suggested a common origin of this mutation and indicated that this is an ancestral Italian mutation. All these mutations generate PTCs and are associated with the absence of COLVII expression, as detected by immunofluorescence analysis of the patient's skin. Evaluation of the levels of the mutated COLVII mRNAs in cultured skin fibroblasts of the patients and of their parents showed that all the mutated transcripts were expressed at consistent levels. Therefore, our results indicate that a marked mRNA reduction is not a constant feature associated with PTC mutations in COL7A1.

Molecular basis of dystrophic epidermolysis bullosa: mutations in the type VII collagen gene (COL7A1)

Epidermolysis bullosa (EB), a group of heritable blistering diseases characterized by tissue separation within the cutaneous basement membrane zone, is inherited either in an autosomal dominant or autosomal recessive fashion. EB has been divided into four broad categories based on the precise level of tissue separation. In the dystrophic forms of EB (DEB), tissue separation occurs below the lamina densa within the upper papillary dermis at the level of anchoring fibrils, which are frequently altered in morphology, reduced in number, or entirely absent. Since type VII collagen is the major component of anchoring fibrils, the corresponding gene, COL7A1, was proposed as the candidate for DEB. Subsequent cloning of COL7A1 and elucidation of its genomic structure have led to identification of 53 distinct mutations in COL7A1 reported thus far. These mutations consist of nonsense mutations, small insertions or deletions resulting in frameshift and premature termination codons, splice site mutations, or missense mutations, particularly glycine substitutions within the collagenous domain of the protein. The types and combinations of these mutations and their positions along the type VII collagen molecule result in a spectrum of phenotypic severity and determine the mode of inheritance. Thus, examination of the mutation database has allowed genotype/phenotype predictions, with an impact on genetic counseling in this group of genodermatoses.

Targeted inactivation of the type VII collagen gene (Col7a1) in mice results in severe blistering phenotype: a model for recessive dystrophic epidermolysis bullosa

Dystrophic forms of epidermolysis bullosa (DEB) are associated with mutations in the type VII collagen gene (Col7a1) which encodes the major component of anchoring fibrils. To develop a DEB animal model, type VII collagen deficient mice were generated by targeted homologous recombination. The targeting vector replaced exons 46–69 of Col7a1 with the neomycin-resistance gene, in reverse transcriptional orientation, resulting in elimination of most of the collagenous domain 1. Col7a1 heterozygous (+/-) mice were phenotypically normal. Mating of Col7a1 +/- mice revealed that Col7a1 null (-/-) mice, which were born with extensive cutaneous blistering, died during the first two weeks of life probably due to complications arising from the blistering. Transmission electron microscopy revealed subepidermal blistering below the lamina densa and absence of anchoring fibrils. Immunohistochemical staining with anti-human type VII collagen antibody stained the dermal-epidermal junction in control mice, but did not stain the skin of Col7a1 null mice. Collectively, the DEB mice recapitulate the clinical, genetic, immunohistochemical and ultrastructural characteristics of recessive DEB in humans. These mice provide an animal model to study the pathomechanisms of DEB and serve as a system to test therapeutic approaches, including gene replacement, towards the cure of this devastating skin disease.

Comparative mutation detection screening of the type VII collagen gene (COL7A1) using the protein truncation test, fluorescent chemical cleavage of mismatch, and conformation sensitive gel electrophoresis

Mutations in the type VII collagen gene, COL7A1, give rise to the blistering skin disease, dystrophic epidermolysis bullosa. We have developed two new mutation detection strategies for the screening of COL7A1 mutations in patients with dystrophic epidermolysis bullosa and compared them with an established protocol using conformational sensitive gel electrophoresis. The first strategy consisted of an RNA based protein truncation test that amplified the entire coding region in only four overlapping nested reverse transcriptase-polymerase chain reaction assays. These fragments were transcribed and translated in vitro and analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We have used the protein truncation test procedure to characterize 15 truncating mutations in 13 patients with severe recessive dystrophic epidermolysis bullosa yielding a detection sensitivity of 58%. The second strategy was a DNA-based fluorescent chemical cleavage of mismatch (fl-CCM) procedure that amplified the COL7A1 gene in 21 polymerase chain reaction assays. Mismatches, formed between patient and control DNA, were identified using chemical modification and cleavage of the DNA. We have compared fl-CCM with conformational sensitive gel electrophoresis by screening a total of 50 dominant and recessive dystrophic epidermolysis bullosa patients. The detection sensitivity for fl-CCM was 81% compared with 75% for conformational sensitive gel electrophoresis (p = 0.37 chi2-test). Using a combination of the three techniques we have screened 93 dystrophic epidermolysis bullosa patients yielding an overall sensitivity of 87%, detecting 79 different mutations, 57 of which have not been reported previously. Comparing all three approaches, we believe that no single method is consistently better than the others, but that the fl-CCM procedure is a sensitive, semiautomated, high throughput system that can be recommended for COL7A1 mutation detection.

A recurrent frameshift mutation in exon 19 of the type VII collagen gene (COL7A1) in Mexican patients with recessive dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is an inherited blistering skin disorder caused by mutations in the type VII collagen gene (COL7A1). In this study, we determined the molecular basis of autosomal recessive DEB in a 19-year-old Hispanic Mexican woman by PCR amplification of genomic DNA, heteroduplex analysis, and automated sequencing of heteroduplex bandshifts. This approach revealed a homozygous frameshift mutation, 2470insG, in exon 19 of COL7A1 and resulted in attenuated basement membrane zone expression of type VII collagen, a reduced number of anchoring fibrils at the dermal-epidermal junction, and a sub-lamina densa level of blister formation. Clinically, the patient had widespread trauma-induced skin fragility and complete loss of the nails, but had less pseudosyndactyly of the fingers and toes and milder mucosal involvement compared to most patients with the generalized form of this genodermatosis. We also screened 7 other Hispanic-Mexican patients with recessive DEB, none of whom were known to be related to this individual, for the mutation 2470insG using heteroduplex analysis and direct sequencing and detected this mutation on 7/14 alleles. Haplotype analysis using intragenic COL7A1 and flanking polymorphisms and microsatellite markers revealed that all the mutant alleles had arisen on similar allelic backgrounds, consistent with propagation of a common Hispanic Mexican ancestral haplotype. In view of the high allelic frequency of the mutation 2470insG in the patients studied, we recommend initial screening for this mutation when attempting to identify the molecular pathology of recessive DEB in Hispanic Mexican patients.

New insights into the immunoultrastructural organization of cutaneous basement membrane zone molecules

The epidermal basement membrane zone (BMZ) is composed of various molecules, each of which plays an important role in dermo-epidermal adhesion. Genetic abnormality of certain BMZ molecules leads to an inherited group of skin diseases collectively referred to as epidermolysis bullosa, whose hallmark is skin fragility of varying degrees. Furthermore, development of autoantibodies to certain BMZ molecules leads to the onset of a number of acquired autoimmune blistering diseases in which dermo epidermal separation occurs, including bullous pemphigoid and epidermolysis bullosa acquisita. The ultrastructural location of each BMZ molecule has been studied using a range of immunoelectron microscopy (immuno-EM) techniques. Recent technical advances in immuno-EM and in molecular engineering for production of epitope-specific antibodies have enabled a more correct and precise elucidation of the native ultrastructural molecular organization of the respective molecules and their relationship to each other. These recent studies have also revealed several misinterpretations in the previously established model of the immunoultrastructural organization of BMZ molecules. In response to these findings, this review focuses on three major BMZ-related molecules, type VII collagen, BPAG2 and laminin 5, for which recent immuno-EM studies have produced a revision in the accepted dogma on their ultrastructural distribution at the BMZ.

Transient bullous dermolysis of the newborn associated with compound heterozygosity for recessive and dominant COL7A1 mutations

The neonatal skin blistering disorder transient bullous dermolysis of the newborn (TBDN) heals spontaneously or improves dramatically within the first months and years of life. TBDN is characterized by subepidermal blisters, reduced or abnormal anchoring fibrils at the dermo-epidermal junction, and electron-dense inclusions in keratinocytes. These features are partly similar to those in dystrophic epidermolysis bullosa, which is caused by defects in COL7A1 gene encoding collagen VII, the main anchoring fibril protein. TBDN has been grouped separately from dystrophic epidermolysis bullosa based on the pronounced morphologic features and the self-limiting course of the disorder; however, it remains unclear whether it represents a distinct clinical entity with a single etiology. We now report a TBDN patient who is compound heterozygous for a recessive and a dominant glycine substitution mutation in COL7A1. Two point mutations caused amino acid substitutions G1519D and G2251E in the triple helical domain of collagen VII. In the heterozygous state G1519D was silent, and G2251E led to nail dystrophy, but not to skin blistering. In the proband, compound heterozygosity for the mutations caused massive, transitory retention of collagen VII in the epidermis, its reduced deposition at the basement membrane zone, and extensive dermo-epidermal separation at birth. Accordingly, TBDN keratinocytes in vitro accumulated collagen VII intracellularly in the rough endoplasmic reticulum.

A recurrent glycine substitution mutation, G2043R, in the type VII collagen gene (COL7A1) in dominant dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is caused by mutations in the type VII collagen gene (COL7A1). Nearly all cases of dominant DEB are caused by glycine substitution mutations occurring within the triple helical region of type VII collagen, and most of the mutations are unique to individual families. In this study, we identified a patient of Hispanic-Mexican origin with a mild form of DEB, which resulted from a de novo dominant glycine substitution, G2043R, in exon 73 of COL7A1. We also investigated a Scottish family with a three-generation pedigree of dominant DEB, in whom the same glycine to arginine substitution mutation was demonstrated. This particular mutation has also been detected previously in three other families with dominant DEB: one Italian, one Hungarian and one Norwegian. Given the widespread geographical distribution of this mutation and the demonstration of its occurrence as a de novo event, G2043R therefore represents the first example of a mutational hotspot in dominant DEB. Interestingly, although both the Mexican and Scottish families we studied had some clinical features in keeping with the Pasini form of the disorder, there was considerable interfamilial variability as well as intrafamilial diversity in the affected individuals.

Some, but not all, glycine substitution mutations in COL7A1 result in intracellular accumulation of collagen VII, loss of anchoring fibrils, and skin blistering

COL7A1 gene mutations cause dystrophic epidermolysis bullosa, a skin blistering disorder. The phenotypes result from defects of collagen VII, the major component of the anchoring fibrils at the dermo-epidermal junction; however, the molecular mechanisms underlying the phenotypes remain elusive. We investigated naturally occurring COL7A1 mutations and showed that some, but not all, glycine substitutions in collagen VII interfered with biosynthesis of the protein in a dominant-negative manner. Three point mutations in exon 73 caused glycine substitutions G2006D, G2034R, and G2015E in the triple helical domain of collagen VII and interfered with its folding and secretion. Confocal laser scanning studies and semiquantitative immunoblotting determined that dystrophic epidermolysis bullosa keratinocytes retained up to 2.5-fold more procollagen VII within the rough endoplasmic reticulum than controls. Limited proteolytic digestions of mutant procollagen VII produced aberrant fragments and revealed reduced stability of the triple helix. In contrast, the glycine substitution G1519D in another segment of the triple helix affected neither procollagen VII secretion nor anchoring fibril function and remained phenotypically silent. These data demonstrate that collagen VII presents a remarkable exception among collagens in that not all glycine substitutions within the triple helix exert dominant-negative interference and that the biological consequences of the substitutions probably depend on their position within the triple helix.

Frameshift mutations in the type VII collagen gene (COL7A1) in five Mexican cousins with recessive dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is caused by mutations in the type VII collagen gene (COL7A1). In this study, we assessed the molecular basis of recessive DEB in five affected individuals from two Mexican families. Both fathers of the affected children were first cousins. Genomic DNA was extracted from peripheral blood samples and assessed for COL7A1 mutations by polymerase chain reaction (PCR) amplification, heteroduplex analysis and direct automated sequencing of PCR products displaying heteroduplex bandshifts. In one family, we identified a homozygous 1 bp insertion of a G nucleotide in exon 19 of COL7A1, designated 2470insG, in three affected sisters. This mutation causes a frameshift and a premature termination codon on both alleles 178 bp downstream from the insertion; both parents were shown to be heterozygous carriers of this mutation. In the second family, the father of the other two affected children was also found to be a heterozygous carrier of this frameshift mutation. In addition, his unrelated partner was shown to be a heterozygous carrier of a different COL7A1 frameshift mutation, an insertion of a T nucleotide in exon 32, designated 3948insT. This mutation also results in a premature termination codon, 126 bp downstream from the insertion. Both affected children were compound heterozygotes for the 2470insG/3948insT mutations in COL7A1. Overall, these molecular findings offer a genetic explanation for the skin fragility in these related Mexican patients with recessive DEB. Immediate benefits from elucidation of the mutations include assessment of carrier status in other members of the family and the feasibility of DNA-based prenatal testing in subsequent pregnancies.