Heterogeneity of severe dystrophic epidermolysis bullosa: overexpression of collagen VII by cutaneous cells from a patient with mutilating disease

Immunofluorescence mapping, electron microscopy and genetics in the diagnosis and sub-classification of inherited epidermolysis bullosa: a single-centre retrospective comparative study of 87 cases with long-term follow-up

BACKGROUND

Epidermolysis bullosa (EB) comprises a heterogeneous group of skin fragility disorders, classified in four major types based on skin cleavage level, i.e. EB simplex (EBS), junctional EB (JEB), dystrophic EB (DEB), Kindler EB, and in more than 30 subtypes defined by the combination of laboratory and clinical data, including disease course.

OBJECTIVES

Our aims were to address whether, in the age of genomics, electron microscopy (TEM) has still a role in diagnosing EB, and whether the genotype per se may be sufficient to sub-classify EB.

METHODS

A thoroughly characterized single-centre EB case series was retrospectively evaluated to compare the power of TEM with immunofluorescence mapping (IFM) in establishing the EB type, and the ability of TEM, IFM and genetics to predict selected EB subtypes, i.e. severe dominant EBS (DEBS), severe JEB, severe recessive DEB (RDEB) and DEB self-improving, using genetic and final diagnosis, respectively, as gold standard.

RESULTS

The series consisted of 87 patients, including 44 newborns, with a median follow-up of 54 months. Ninety-five mutations were identified in EB-associated genes, including 25 novel variants. Both IFM and TEM were diagnostic in about all cases of JEB (21/21 for both) and DEB (43/44 for IFM, 44/44 for TEM). TEM sensitivity was superior to IFM for EBS (19/20 vs. 16/19). As to EB subtyping, IFM performed better than genetics in identifying severe JEB cases due to laminin-332 defect (14/14 vs. 10/14) and severe RDEB (eight/nine vs. seven/nine). Genetics had no role in self-improving DEB diagnosis; it almost equalled TEM in predicting severe DEBS (eight/nine vs. nine/nine) and enabled to discriminate dominant from recessive non-severe DEB phenotypes and to identify special subtypes, e.g. DEBS with KLHL24 mutations.

CONCLUSIONS

Transmission electron microscopy remains relevant to the diagnosis of EBS. IFM and genetics are essential and complementary tools in the vast majority of EB cases.

Basement membrane zone remodeling during appendageal development in human fetal skin. The absence of type VII collagen is associated with gelatinase-A (MMP2) activity

Epithelial cell adhesion, migration, and differentiation are controlled by interactions at the basement membrane zone (BMZ). Type VII collagen is the major collagenous component of anchoring fibrils that are essential for the attachment of the epidermis to the dermis. Gelatinase A (MMP-2) is believed to be necessary for the degradation of type VII collagen. In this study we have examined the in vivo distribution of type VII collagen and gelatinase A (Gel A) in the developing human epidermis and its appendages. At 13-15 wk of gestation a marked decrease in type VII collagen immunoreactivity was seen in the BMZ surrounding invading appendageal buds; however, type VII collagen mRNA was strongly expressed in the budding epidermal keratinocytes adjacent to the BMZ. At these stages, Gel A-positive mesenchymal-like cells were found scattered throughout the stroma with numerous Gel A-containing cells in direct contact with the developing appendageal buds. In situ zymography was used to show Gel A-activity in vivo. Gel A-mediated lysis was present at the interface between the appendageal buds and the underlying BMZ. By 20-25 wk of gestational age, immunostaining for type VII collagen protein was absent from the BMZ surrounding the distal portion of invading appendageal epithelial cords of both hair follicles and sweat glands. In contrast, type VII collagen mRNA was present in the basal keratinocytes adjacent to the BMZ surrounding the distal portion of these invading appendageal epithelial cords. At these stages Gel A-positive cells were present in the stroma directly adjacent to the distal portion of developing appendageal cords that lacked type VII collagen. In situ zymography showed zones of Gel A-mediated stromal lysis at the distal portion of developing appendageal cords. Interestingly, no differences were seen in the distribution of type IV collagen in the BMZ of both budding and resting fetal epidermis. These observations suggest that the absence of type VII collagen protein correlates directly with the presence of Gel A-activity at the BMZ. Gel A appears to play a major role in appendageal development and contributes to remodeling of the BMZ during fetal skin morphogenesis.

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

Type VII collagen is the major component of anchoring fibrils, adhesion structures of stratified epithelia that span the basement membrane region and papillary dermis. Mutations in the gene COL7A1 encoding type VII collagen cause dystrophic epidermolysis bullosa, a clinically heterogeneous autosomal dominant or recessive blistering disorder of the skin and mucous membranes. In this report, we investigate three siblings affected by an unusually mild form of localized recessive dystrophic epidermolysis bullosa who were shown to be compound heterozygotes for novel mutations affecting COL7A1. The maternally inherited mutation is a G-->C transversion that converts a codon for glycine to a codon for arginine (G1347R). The paternal mutation is a neutral G-->A transition at the last base of exon 70(5820G-->A) that alters the correct splicing of COL7A1 pre-mRNA, giving rise to an aberrant mRNA carrying the in-frame skipping of exon 70 in addition to the full-length RNA transcript carrying the G-->A substitution. Consistent with the normal levels of COL7A1 mRNA transcripts detected by northern analysis, immunoblotting and immunofluorescence studies evidenced that the patient keratinocytes synthesize and secrete normal amounts of stable type VII collagen, which is correctly deposited at the dermal-epidermal junction. In addition, mutated type VII collagen molecules assemble to form numerous, normally shaped anchoring fibrils, as shown by electron microscopic examination. The combination of a recessive glycine substitution with a splice site mutation that permits partially correct splicing therefore leads to a normal expression of mutated type VII collagen molecules with marginally altered biologic activity, and to the extremely mild phenotype observed in our patients.

A combination of a common splice site mutation and a frameshift mutation in the COL7A1 gene: absence of functional collagen VII in keratinocytes and skin

We describe a patient with severe generalized dystrophic epidermolysis bullosa (EBD) and a novel combination of compound heterozygous mutations in the COL7A1 gene. The maternal mutation was an A-to-G transition (425-A --> G) at position -2 of the donor splice site within exon 3 that causes aberrant splicing of two abnormal transcripts. One includes intron 3, and one excludes both exon 3 and intron 3. Both splice variants contained a premature termination of the translation. The paternal mutation is a 25-bp deletion in exon 20 (2638de125) that leads to a frameshift and a premature termination codon 133 bp downstream from the site of deletion. This combination of mutations allowed expression of collagen VII mRNA. Immunofluorescence staining of the patient's skin and cultured keratinocytes with domain-specific collagen VII antibodies, however, demonstrated markedly reduced levels of alpha1(VII) polypeptides, and no stable collagen VII protein could be extracted from the patient's cells. Electron microscopy showed severely hypoplastic fibrils below the lamina densa, without evidence of normal anchoring fibrils. The clinically unaffected parents were heterozygous for the mutations, suggesting that both COL7A1 gene defects were recessively inherited disease-causing mutations that are "silent" in heterozygous carriers but in combination can severely interfere with the dermal-epidermal adhesion and lead to severe EBD.

An extract from cultured human keratinocytes that contains the major autoantigens related to autoimmune bullous skin diseases

Autoantibodies characteristic of autoimmune bullous skin diseases (AIBDs) can be detected by immunoblotting on epidermal, dermal, or bovine muzzle extracts. However, none of those substrates contain all the autoantigens involved in AIBDs, and the diagnosis requires the use of various substrates. Human keratinocytes were cultured under such conditions that they expressed the major autoantigens associated with AIBDs. Forty-two sera with antiepidermal antibodies were immunoblotted on the keratinocyte extract. Bands corresponding to desmoglein III, desmoglein I, BPAg2, BPAg1, and type VII collagen were found in 38 sera. Desmoplakins I and II were revealed by specific monoclonal antibodies. A review of the patients' charts showed a perfect correlation between the blots and the diagnoses of pemphigus vulgaris, pemphigus foliaceus, bullous pemphigoid, cicatricial pemphigoid, and epidermolysis bullosa acquisita. Four sera revealing no band typical of AIBD were from patients with no autoimmune skin disease. Therefore, a single extract of keratinocytes can be used for the differential diagnosis of AIBDs.

Characterization of a 50-kDa component of epithelial basement membranes using GDA-J/F3 monoclonal antibody

Using the monoclonal antibody GDA-J/F3, a 50-kDa noncollagenous component of human skin basement membrane zone was identified. Immunofluorescence stainings of normal human skin with the GDA-J/F3 antibody showed a linear fluorescence decorating the basement membrane zone. With immunoelectron microscopy, the epitope was localized to the insertion points of the anchoring fibrils into the lamina densa. The antigen is distinct from collagen VII, from the main structural protein of the anchoring fibrils, and from several other structural molecules of the basement membrane zone, because the GDA-J/F3 antibody did not react with purified basement membrane components in vitro. In serum-free cultures, the antigen was synthesized and secreted by normal and transformed human keratinocytes and to a lesser extent by normal human skin fibroblasts. Immunoprecipitation of radiolabeled epithelial cell-conditioned medium with the GDA-J/F3 antibody yielded two polypeptides that migrated on SDS-polyacrylamide gel electrophoresis with apparent molecular masses of 46 and 50 kDa under nonreducing conditions. Using reducing gels, only the 50-kDa polypeptide was observed. The antigen was resistant to digestion with bacterial collagenase but sensitive to trypsin and pepsin. It also bound to heparin and DEAE cellulose at low ionic strength and alkaline pH. These findings indicate that the GDA-J/F3 antigen is a small globular disulphide-bonded protein with a potential to interact with basement membrane proteoglycans. Integration of the GDA-J/F3 antigen into the histoarchitecture of the dermo-epidermal junction is dependent on the presence of collagen VII, because the GDA-J/F3 epitope was missing in several patients with a genetic blistering disorder of the skin, epidermolysis bullosa dystrophica, who lacked collagen VII and anchoring fibrils.

Epidermolysis bullosa: pathogenetic pathways from mutations to symptoms

Recent developments of the molecular and cell biology of the cutaneous basement membrane zone have greatly advanced our understanding of the pathomechanisms underlying skin blistering disorders. The heritable blistering diseases, the epidermolysis bullosa group, have been investigated as model diseases. Defects in genes coding for the structural proteins of the basement membrane zone have been defined in some EB subtypes and abnormal expression of the structural proteins in others. In vitro studies utilizing cutaneous cells derived from epidermolysis bullosa skin have helped to understand the pathogenetic pathways that lead from the mutation to the symptom, skin blistering. The data accumulated from analyses of the genetic disorders will yield indirect information on the normal physiology of the skin and be highly relevant for discerning the etiopathogenesis of acquired blistering diseases and for dermal-epidermal interactions required for reparative processes, such as wound healing.