Compound heterozygosity for a recessive glycine substitution and a splice site mutation in the COL7A1 gene causes an unusually mild form of localized recessive dystrophic epidermolysis bullosa

A Novel Phenotype of Junctional Epidermolysis Bullosa with Transient Skin Fragility and Predominant Ocular Involvement Responsive to Human Amniotic Membrane Eyedrops

Junctional epidermolysis bullosa (JEB) is a clinically and genetically heterogeneous skin fragility disorder frequently caused by mutations in genes encoding the epithelial laminin isoform, laminin-332. JEB patients also present mucosal involvement, including painful corneal lesions. Recurrent corneal abrasions may lead to corneal opacities and visual impairment. Current treatments are merely supportive. We report a novel JEB phenotype distinguished by the complete resolution of skin fragility in infancy and persistent ocular involvement with unremitting and painful corneal abrasions. Biallelic LAMB3 mutations c.3052-5C>G and c.3492_3493delCG were identified as the molecular basis for this phenotype, with one mutation being a hypomorphic splice variant that allows residual wild-type laminin-332 production. The reduced laminin-332 level was associated with impaired keratinocyte adhesion. Then, we also investigated the therapeutic power of a human amniotic membrane (AM) eyedrop preparation for corneal lesions. AM were isolated from placenta donors, according to a procedure preserving the AM biological characteristics as a tissue, and confirmed to contain laminin-332. We found that AM eyedrop preparation could restore keratinocyte adhesion in an in vitro assay. Of note, AM eyedrop administration to the patient resulted in long-lasting remission of her ocular manifestations. Our findings suggest that AM eyedrops could represent an effective, non-invasive, simple-to-handle treatment for corneal lesions in patients with JEB and possibly other EB forms.

Inherited epidermolysis bullosa: update on the clinical and genetic aspects

Inherited epidermolysis bullosa is a group of genetic diseases characterized by skin fragility and blistering on the skin and mucous membranes in response to minimal trauma. Epidermolysis bullosa is clinically and genetically very heterogeneous, being classified into four main types according to the layer of skin in which blistering occurs: epidermolysis bullosa simplex (intraepidermal), junctional epidermolysis bullosa (within the lamina lucida of the basement membrane), dystrophic epidermolysis bullosa (below the basement membrane), and Kindler epidermolysis bullosa (mixed skin cleavage pattern). Furthermore, epidermolysis bullosa is stratified into several subtypes, which consider the clinical characteristics, the distribution of the blisters, and the severity of cutaneous and extracutaneous signs. Pathogenic variants in at least 16 genes that encode proteins essential for the integrity and adhesion of skin layers have already been associated with different subtypes of epidermolysis bullosa. The marked heterogeneity of the disease, which includes phenotypes with a broad spectrum of severity and many causal genes, hinders its classification and diagnosis. For this reason, dermatologists and geneticists regularly review and update the classification criteria. This review aimed to update the state of the art on inherited epidermolysis bullosa, with a special focus on the associated clinical and genetic aspects, presenting data from the most recent reclassification consensus, published in 2020.

Genomics-based treatment in a patient with two overlapping heritable skin disorders: Epidermolysis bullosa and acrodermatitis enteropathica

Next-generation sequencing (NGS) is helpful in diagnosing complex genetic disorders and phenotypes, particularly when more than one overlapping condition is present. From a large cohort of 362 families with clinical manifestations of skin and mucosal fragility, referred by several major medical centers, one patient was found by NGS to have two overlapping heritable skin diseases, recessive dystrophic epidermolysis bullosa (RDEB; COL7A1 mutations) and acrodermatitis enteropathica (AE; SLC39A4 mutations). The pathogenicity of the variants was studied at gene expression as well as ultrastructural and tissue levels. Although there is no specific treatment for RDEB except avoiding trauma, supplementation with oral zinc (3 mg·kg^-1 ·day^-1 ) for the AE resulted in rapid amelioration of the skin findings. This case demonstrates the power of NGS in identifying two genetically unlinked diseases that led to effective treatment with major clinical benefits as an example of genomics-guided treatment.

Natural Exon Skipping Sets the Stage for Exon Skipping as Therapy for Dystrophic Epidermolysis Bullosa

Dystrophic epidermolysis bullosa (DEB) is a devastating blistering disease affecting skin and mucous membranes. It is caused by pathogenic variants in the COL7A1 gene encoding type VII collagen, and can be inherited dominantly or recessively. Recently, promising proof-of-principle has been shown for antisense oligonucleotide (AON)-mediated exon skipping as a therapeutic approach for DEB. However, the precise phenotypic effect to be anticipated from exon skipping, and which patient groups could benefit, is not yet clear. To answer these questions, we studied new clinical and molecular data on seven patients from the Dutch EB registry and reviewed the literature on COL7A1 exon skipping variants. We found that phenotypes associated with dominant exon skipping cannot be distinguished from phenotypes caused by other dominant DEB variants. Recessive exon skipping phenotypes are generally relatively mild in the spectrum of recessive DEB. Therefore, for dominant DEB, AON-mediated exon skipping is unlikely to ameliorate the phenotype. In contrast, the overall severity of phenotypes associated with recessive natural exon skipping pivots toward the milder end of the spectrum. Consequently, we anticipate AON-mediated exon skipping for recessive DEB caused by bi-allelic null variants should lead to a clinically relevant improvement of this devastating phenotype.

An overview of the genetic basis of epidermolysis bullosa in Brazil: discovery of novel and recurrent disease-causing variants

Epidermolysis bullosa (EB) is a genodermatosis that encompasses a group of clinically and genetically heterogeneous disorders classified in four major types: EB simplex (EBS), junctional EB (JEB), dystrophic EB (DEB) and Kindler syndrome. Our aim was to characterize recurrent and novel mutations associated to EB in a sample of Brazilian patients. Eighty-seven patients (25 EBS, 4 JEB and 58 DEB) were studied. We performed a next-generation sequencing-based multigene panel through ion torrent technology including 11 genes: KRT5, KRT14, PLEC, TGM5, LAMA3, LAMB3, LAMC2, COL17A1, ITGB4, COL7A1, and FERMT1. A total of 72 different pathogenic or likely pathogenic variants were identified, 32 of them are novel. The causal variant was detected in 82 patients (efficiency of 94.3%). Pathogenic variants in the residue 125 of KRT14 were identified in 32% of all EBS patients. In DEB patients, four COL7A1 variants were quite frequent, some of them clustered in specific Brazilian regions. Our study extends the spectrum of known mutations in EB and describes, for the first time, the genetic profile of EB patients from Brazil.

Compound heterozygosity for novel splice site mutations of ITGA6 in lethal junctional epidermolysis bullosa with pyloric atresia

Junctional epidermolysis bullosa with pyloric atresia (PA-JEB) is a rare congenital bullous disease with gastrointestinal disturbance that has been associated with mutations in ITGA6 or ITGB4 encoding the α6 or β4 subunit of integrin, respectively. Only six ITGA6 mutations in PA-JEB have been reported while many ITGB4 mutations have been identified, and all the ITGA6 mutations were homozygous. Here, we report a case of lethal type PA-JEB, in which immunofluorescence showed the lack of both α6 and β4 integrins resulting from compound heterozygous splice site mutation in ITGA6, c.387G>T and c.2506-1G>C. Maternal c.387G>T induced the skipping of the entire exon 3 and both exons 3 and 4, resulting in premature termination codon and in-frame deletion, respectively. Paternal c.2506-1G>C caused the skipping of the exon 20 and resulted in in-frame deletion. As a reason why the present case showed lethal phenotype despite the in-frame deletion mutation, rapid degradation of neo-synthesized α6 protein and/or impaired transport of integrin were suggested from previous reports, and the lack of localization of integrin α6β4 to the epidermal basement membrane resulted in skin fragility. Our case expands the variety of integrin α6 mutations in PA-JEB.

Targeted Exon Skipping Restores Type VII Collagen Expression and Anchoring Fibril Formation in an In Vivo RDEB Model

Dystrophic epidermolysis bullosa is a group of orphan genetic skin diseases dominantly or recessively inherited, caused by mutations in COL7A1 encoding type VII collagen, which forms anchoring fibrils. Individuals with recessive dystrophic epidermolysis bullosa develop severe skin and mucosal blistering after mild trauma. The exon skipping strategy consists of modulating splicing of a pre-mRNA to induce skipping of a mutated exon. We have targeted COL7A1 exons 73 and 80, which carry recurrent mutations and whose excision preserves the open reading frame. We first showed the dispensability of these exons for type VII collagen function in vivo. We then showed that transfection of primary recessive dystrophic epidermolysis bullosa keratinocytes and fibroblasts carrying null mutations in exon 73 and/or 80, with 2'-O-methyl antisense oligoribonucleotides, led to efficient ex vivo skipping of these exons (50-95%) and resulted in a significant level (up to 36%) of type VII collagen re-expression. Finally, one or two subcutaneous injections of antisense oligoribonucleotides at doses ranging from 400 μg up to 1 mg restored type VII collagen expression and anchoring fibril formation in vivo in a xenograft model of recessive dystrophic epidermolysis bullosa skin equivalent. This work provides a proof of principle for the treatment of patients with recessive dystrophic epidermolysis bullosa by exon skipping using subcutaneous administration of antisense oligoribonucleotides.

Lethal autosomal recessive epidermolytic ichthyosis due to a novel donor splice-site mutation in KRT10

Epidermolytic ichthyosis (EI; MIM 113800), previously named bullous congenital ichthyosiform erythroderma or epidermolytic hyperkeratosis, is a rare and clinically variable defect of cornification characterized by generalized erythema, erosions, scaling and easily breaking blisters that become less frequent later in life while hyperkeratosis increases. EI is caused by dominant mutations in either KRT1 or KRT10, encoding keratin 1 (K1) and keratin 10 (K10), respectively. Usually, mutations are missense substitutions into the highly conserved α-helical rod domains of the proteins. However, three inbred pedigrees in which EI is transmitted as a recessive trait due to KRT10 null mutations have been described.

Type VII collagen gene mutations (c.8569G>T and c.4879G>A) result in the moderately severe phenotype of recessive dystrophic epidermolysis bullosa in a Korean patient

Dystrophic epidermolysis bullosa (DEB) are caused by mutations in the COL7A1 gene, which encodes type VII collagen. Even though more than 500 different COL7A1 mutations have been identified in DEB, it still remains to be under-investigated. To investigate the mutation of COL7A1 in moderately severe phenotype of recessive DEB (RDEB) in a Korean patient, the mutation detection strategy was consisted of polymerase chain reaction (PCR) amplification of genomic DNA, followed by heteroduplex analysis, nucleotide sequencing of the PCR products demonstrating altered mobility. In this study, we found that one mutation (c.8569G>T) was detected within exon 116. The mutation of c.8569G>T in exon 116 changed the GAG (Glu) to TAG, eventually resulted in premature termination of type VII collagen polypeptide. Furthermore the mother did not have the mutation c.8569G>T in exon 116. The other novel mutation (c.4879G>A) was detected within exon 51 of both patient and mother, thereby resulting in changing valine (Val) to isoleucine (Ile) in type VII collagen polypeptide. Taken together, in this study we identified compound heterozygosity for COL7A1 mutations (c.8569G>T and c.4879G>A) in moderately severe RDEB in a Korean patient. We hope that this data contribute to the expanding database on COL7A1 mutations in DEB.

Mutation analysis and characterization of COL7A1 mutations in dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is inherited in both an autosomal dominant DEB and autosomal recessive manner RDEB, both of which result from mutations in the type VII collagen gene (COL7A1). To date, 324 pathogenic mutations have been detected within COL7A1 in different variants of DEB; many mutations are clustered in exon 73 (10.74%) which is close to the 39 amino acid interruption region. Dominant dystrophic epidermolysis bullosa usually involves glycine substitutions within the triple helix of COL7A1 although other missense mutations, deletions or splice-site mutations may underlie some cases. In recessive dystrophic epidermolysis bullosa, the mutations include nonsense, splice site, deletions or insertions, 'silent' glycine substitutions within the triple helix and non-glycine missense mutations within the triple helix or non-collagenous NC-2 domain. The nature of mutations in COL7A1 and their positions correlate reasonably logically with the severity of the resulting phenotypes.

Mutations in the hairless gene underlie APL in three families of Pakistani origin

BACKGROUND

Atrichia with papular lesions (APL) (OMIM#209500) is a rare autosomal recessively inherited form of irreversible alopecia characterized by papular lesions of keratin-filled cysts on various regions of the body. Males and females are equally affected and present with a distinct pattern of total hair loss on scalp, axilla and body. It begins shortly after birth with the development of hair loss, and patients are normally devoid of eyelashes and eyebrows. Mutations in the hairless (HR) gene have been previously shown to be responsible for APL.

OBJECTIVE

In this study, we studied the molecular basis of APL in three unrelated families of Pakistani origin.

METHOD

Molecular analysis of the HR genes was performed on genomic DNA from probands and family members.

RESULTS

DNA sequencing of the HR gene in family A revealed a novel homozygous 2bp deletion in exon 6 leading to a frameshift and a downstream premature termination codon in exon 8 (1782-83delAG). In family B, we identified a novel homozygous deletion of a G nucleotide at the exon 15-intron 15 boundary, termed 3097delG. Family C carries a previously reported missense mutation consisting of an A-to-G transition at nucleotide 276 resulting in the mutation N970S in exon 14.

CONCLUSION

Two mutations identified in this study are novel mutations in the HR gene and extend the body of evidence implicating the hairless gene family in the pathogenesis of human skin disorders. The one previously reported mutation suggests it may represent a recurrent mutation, or alternatively, an allele that is widely dispersed around the world.

Molecular Basis of Kindler Syndrome in Italy: Novel and Recurrent Alu/Alu Recombination, Splice Site, Nonsense, and Frameshift Mutations in the KIND1 Gene

Kindler syndrome (KS) is a rare autosomal recessive disorder characterized by skin blistering in childhood followed by photosensitivity and progressive poikiloderma. Most cases of KS result from mutations in the KIND1 gene encoding kindlin-1, a component of focal adhesions in keratinocytes. Here, we report novel and recurrent KIND1 gene mutations in nine unrelated Italian KS individuals. A novel genomic deletion of approximately 3.9 kb was identified in four patients originating from the same Italian region. This mutation deletes exons 10 and 11 from the KIND1 mRNA leading to a truncated kindlin-1. The deletion breakpoint was embedded in AluSx repeats, specifically in identical 30-bp sequences, suggesting Alu-mediated homologous recombination as the pathogenic mechanism. KIND1 haplotype analysis demonstrated that patients with this large deletion were ancestrally related. Five additional mutations were disclosed, two of which were novel. To date, four recurrent mutations have been identified in Italian patients accounting for approximately approximately 75% of KS alleles in this population. The abundance of repetitive elements in intronic regions of KIND1, together with the identification of a large deletion, suggests that genomic rearrangements could be responsible for a significant proportion of KS cases. This finding has implications for optimal KIND1 mutational screening in KS individuals.

Targeted skipping of a single exon harboring a premature termination codon mutation: implications and potential for gene correction therapy for selective dystrophic epidermolysis bullosa patients

This study examined the feasibility of antisense oligoribonucleotide (AON) therapy for dystrophic epidermolysis bullosa (DEB). AON was designed to induce skipping of a targeted exon containing a premature termination codon mutation, resulting in restoration of the open reading frame. We targeted exon 70 of COL7A1, as a recurrent mutation 5818delC in Japanese DEB patients was localized to exon 70. We found that one AON induced effective skipping of normal exon 70 containing 16 amino acids. Attachment and migration analyses showed that recombinant collagen without contribution of exon 70 was similar in effect to normal type VII collagen. Next, we synthesized mutation-specific AON by deleting cytosine at 5818. Introduction of this AON into DEB keratinocytes harboring 5818delC showed that the AON induced skipping of exon 70 in the abnormal 5818delC allele. Furthermore, 6.2% of DEB keratinocytes started to express type VII collagen in vitro after application of the mutation-specific AON. Injection of the AON into rat model grafted with DEB keratinocytes and fibroblasts induced a low amount of type VII collagen expression. We conclude that skipping of targeted exons using mutation-specific AON may show potential for future gene therapy for DEB patients.

Denaturing HPLC-based approach for detection of COL7A1 gene mutations causing dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is a rare clinically heterogeneous genodermatosis due to genetic defects in type VII collagen gene (COL7A1). Identification of COL7A1 mutations is a challenge since this gene comprises 118 exons and more than 300 mutations scattered over the gene have been reported. Here, we describe for the first time the use of denaturing high performance liquid chromatography (DHPLC) for COL7A1 mutation detection. To validate the method, exon-specific DHPLC conditions were applied to screen DNA samples from patients carrying known COL7A1 mutations. Abnormal DHPLC profiles were obtained for all known mutations. Subsequent DHPLC analysis of 17 DEB families of unknown genotype allowed the identification of 21 distinct mutations, 9 of which were novel. The DHPLC mutation detection rate was significantly higher compared with our mutation scanning rate with conventional techniques (97% vs 86%), indicating DHPLC as the method of choice for COL7A1 molecular characterization in DEB patients.

Identification of novel type VII collagen gene mutations resulting in severe recessive dystrophic epidermolysis bullosa

In this work, we studied the proband in a small nuclear family of Chinese and Dutch/German descent and identified two novel mutations in the type VII collagen gene leading to recessive dystrophic epidermolysis bullosa, Hallopeau-Siemens variant (HS-RDEB). The maternal mutation is a single base pair deletion of a cytosine nucleotide in exon 26, designated 3472delC, resulting in a frameshift and a premature termination codon (PTC) within the same exon, 7 bp downstream of the site of the mutation. The paternal mutation is a G-->A transition located at the 5' donor splice site within intron 51, designated IVS51 + 1G-->A. This mutation leads to the activation of a cryptic splice site, 32 bp downstream of the mutation site and to subsequent aberrant out-of-frame splicing, resulting in two alternative mRNA transcripts and a downstream PTC. To our knowledge, these two mutations have not been previously reported. These findings extend the body of evidence for compound heterozygous mutations leading to HS-RDEB and provide the basis for prenatal diagnosis in this family.

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.

Genotype-phenotype correlation in italian patients with dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is a rare skin disorder that is clinically heterogeneous and is transmitted either in dominant (DDEB) or recessive (RDEB) mode. Nevertheless, all variants of DEB are caused by mutations in type VII collagen gene (COL7A1). We report an analysis of COL7A1 mutations in 51 Italian DEB patients, 27 affected with Hallopeau-Siemens RDEB, 19 with non Hallopeau-Siemens RDEB, two with DDEB, two with pretibial RDEB, and one with inversa RDEB. Forty-one mutations were identified, 18 of which are novel. Mutation consequences were analyzed at the mRNA and protein level and genotype-phenotype correlation was determined. Recessive inheritance of a new case of pretibial RDEB was also established. In RDEB patients, six recurrent mutations were identified: 7344G-->A, 425A-->G, 8441-14del21, 4783-1G-->A, 497insA, and G1664A, the last three being found only in Italian patients. Indeed, haplotype analysis supported propagation of ancestral mutated alleles within the Italian population for these particular mutations. Altogether recurrent mutations account for approximately 43% of RDEB alleles in Italian patients and therefore new DEB patients should first be screened for the presence of these mutations.

Different phenotypes in recessive dystrophic epidermolysis bullosa patients sharing the same mutation in compound heterozygosity with two novel mutations in the type VII collagen gene

BACKGROUND

Dystrophic epidermolysis bullosa (DEB) is a bullous skin disease caused by mutations in the type VII collagen gene (COL7A1).

OBJECTIVE

To elucidate the mutations shown by two patients with DEB and understand the clinical phenotypes that they displayed.

METHODS

We have characterized two patients, one affected by the severe recessive Hallopeau-Siemens variant of DEB (HS-RDEB) and the other by a milder recessive DEB form.

RESULTS

In both patients we identified the R2063W missense mutation. The second mutation, in the HS-RDEB patient, was a novel 344insG, leading to a premature termination codon of translation (PTC) in exon 3, while, in the other patient, it was a novel 4965C-->T transition, which creates a new donor splice site in exon 53. The effect of this anomalous splice site leads to the maturation of a 17-nucleotides-deleted mRNA containing a PTC. In addition to this aberrant transcript, a certain amount of full-length mRNA is also generated from the mutated pre-mRNA through splicing at the canonical site.

CONCLUSIONS

In these patients therefore the severity of the phenotype depends on the second mutation. In the patient with the 344insG mutation, leading to a PTC, type VII collagen (COLVII) molecules are exclusively composed of chains containing the R2063W substitution; as a consequence, all anchoring fibrils (AF) are abnormal and the phenotype is severe. In the other patient, the 4965C-->T splicing mutation allows the synthesis of a certain quantity of normal chains and the consequent assembly of partially functional COLVII molecules and AF, thus explaining the mild phenotype.

Esophageal stenosis in childhood: dystrophic epidermolysis bullosa without skin blistering due to collagen VII mutations

We report a 9-year-old girl who experienced recurrent dysphagia since infancy. Crohn's disease was suspected because she had aphthous ulcers of the mouth and anal dermatitis with hematochezia. After bougienages of esophageal stenoses and medication for inflammatory bowel disease proved unsuccessful, interdisciplinary re-examination revealed the cause of the symptoms to be an extracutaneous form of dystrophic epidermolysis bullosa, a genetic skin fragility disorder. Dystrophic epidermolysis bullosa is caused by mutations in the COL7A1 gene encoding collagen VII, a protein of the epidermal attachment complex, and typically manifests with trauma-induced skin blistering, scarring, nail dystrophy, and, in some cases, mucosal involvement. The present proband never developed skin blisters but had nail dystrophy and erosions of the oral, esophageal, and genitoanal mucosa, which healed with slight scarring. Mutation analysis disclosed compound heterozygosity for recessive mutations in the COL7A1 gene. The paternal mutation 425 A-->G caused abnormal splicing resulting in a premature stop codon. The maternal mutation G2775S led to the substitution of a glycine by a serine in the triple helical domain of collagen VII. This case shows that mucosal disease and esophageal strictures in childhood are not always acquired, but can also represent a genetic defect of dermal-epidermal adhesion, even in the absence of skin blistering.

Truncated typeXVII collagen expression in a patient with non-herlitz junctional epidermolysis bullosa caused by a homozygous splice-site mutation

SUMMARY

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.

Recent advances in the molecular basis of inherited skin diseases

Over the last few years the molecular basis of several inherited skin diseases has been delineated. Some discoveries have stemmed from a candidate gene approach using clinical, biochemical, immunohistochemical, and ultrastructural clues, while others have arisen from genetic linkage and positional cloning analyses. Notable advances have included elucidation of specific gene pathology in the major forms of inherited skin fragility, ichthyosis, and keratoderma. These findings have led to a better understanding of the significance of individual structural proteins and regulatory enzymes in keratinocyte adhesion and differentiation. From a clinical perspective, the advances have led to better genetic counseling in many disorders, the development of DNA-based prenatal diagnosis, and a foundation for planning newer forms of treatment, including somatic gene therapy, in selected conditions.

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.

Pretibial dystrophic epidermolysis bullosa: a recessively inherited COL7A1 splice site mutation affecting procollagen VII processing

Pretibial epidermolysis bullosa (PEB) is a rare form of localized epidermolysis bullosa dystrophica (EBD), a heterogeneous group of inherited, blistering diseases characterized by scarring, loss of dermal-epidermal adhesion and altered anchoring fibrils (AF). Mutations in the type VII collagen gene (COL7A1) underlie EBD and in a dominant PEB family a glycine substitution mutation has been identified. We report a 33-year-old man affected by PEB showing abnormal AF and reduced immunostaining for type VII collagen. Mutation search in the COL7A1 gene revealed a 14 bp deletion in the 115 exon-intron boundary (33563del14), which resulted in the in-frame skipping of exon 115 with elimination of 29 amino acids from the pro-alpha1(VII) polypeptide chain. As a consequence, procollagen VII failed to be processed to mature collagen VII and accumulated at the dermal-epidermal junction, as revealed by immunofluorescence staining using a NC-2 domain-specific antibody. The proband's father was a clinically unaffected heterozygous carrier of mutation 33563del14, whereas the maternal pathogenetic mutation has still not been identified. This represents the first report of a recessive deletion mutation in PEB and extends the range of EBD phenotypes associated with mutation 33563del14.

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.

Hereditary skin diseases of anchoring fibrils

Remarkable progress has been made in the last few years in understanding the functions of the anchoring fibrils, polymers of collagen VII, that connect the epidermal basement membrane with the dermal connective tissue. Novel insights into the biology of these fibrils have been gained from studies on dystrophic epidermolysis bullosa (DEB), a group of inherited blistering disorders caused by abnormalities of the anchoring fibrils. Mutations in the COL7A1 gene encoding collagen VII have been disclosed in a number of DEB families, and the mutation analyses and studies on genotype-phenotype correlations in DEB have revealed an unusual complexity of the gene defects and their biological consequences. In analogy to heritable disorders of other collagen genes, predictable phenotypes of COL7A1 mutations causing premature termination codons (PTC) or dominant negative interference have been observed. However, collagen VII seems to be unique among collagens in that many mutations lead to minimal phenotypes, or to no phenotype at all. Furthermore, the mild DEB phenotypes can be severely modulated by a second mutation in individuals compound heterozygous for two different COL7A1 defects. Therefore, not only definition of mutations with diagnostic analyses, but also cell biological, protein chemical and suprastructural studies of the mutated molecules are required for understanding the pathomechanisms underlying DEB.

Biology of anchoring fibrils: lessons from dystrophic epidermolysis bullosa

Anchoring fibrils are adhesive suprastructures that ensure the connection of the epidermal basement membrane with the dermal extracellular matrix. The fibrils represent polymers of collagen VII, the major structural fibril component, but may also contain other proteins. Remarkable progress has been made in the last few years in understanding the functions of skin basement membrane components including the anchoring fibrils. Novel insights into the biology of the anchoring fibrils have been gained from experimental studies on dystrophic epidermolysis bullosa (DEB), a group of inherited blistering disorders caused by mutations in the gene for collagen VII, COL7A1. Mutation analyses of DEB families have disclosed more than 100 COL7A1 gene defects so far, but the unusual complexity of the mutation constellations and their biological consequences are only beginning to emerge. In analogy to heritable disorders of other collagen genes, predictable phenotypes of COL7A1 mutations causing premature termination codons or dominant negative interference have been observed. However, collagen VII seems to represent a remarkable exception among collagens in that many mutations, including heterozygous glycine substitutions and deletions, lead to minimal phenotypes, or to no phenotype at all. In contrast to fibrillar collagens, structural abnormalities of collagen VII molecules in anchoring fibrils appear to be tolerated to a certain extent. However, the mild DEB phenotypes can be severely modulated by a second aberration in individuals compound heterozygous for two different COL7A1 mutations. Therefore, not only definition of mutation(s) but also cell biological, protein chemical and suprastructural studies of the mutated molecules yield novel insight into the molecular pathomechanisms underlying disease.

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.