Molecular Heterogeneity of Blistering Disorders: The Paradigm of Epidermolysis Bullosa

Abstract

Inherited epidermolysis bullosa (EB) represents a clinically and genetically heterogeneous group of genodermatoses characterized by skin fragility, i.e., blisters and erosions of skin and mucous membranes in response to minor friction or mechanical trauma (Figure 1a–e ). EB comprises a broad spectrum of phenotypes, ranging from severe cutaneous and extracutaneous involvement caused by severely compromised dermal-epidermal or intra-epidermal adhesion to discrete traits caused by subtle molecular defects. On the basis of our current knowledge, mutations in 17 different genes account for the genetic and allelic heterogeneity of EB (Figure 1). The EB-associated genes code for intracellular, transmembrane, or extracellular proteins involved in cytoskeleton, cell–cell or cell–matrix adhesion (Figure 1f). The main adhesive structures in the skin, i.e., desmosomes, hemidesmosomes, basement membrane, and anchoring fibrils represent supramolecular protein complexes and networks that not only assure the integrity and mechanical stability of the integument, but also regulate cellular functions by transmitting signals between the cells and their extracellular milieu. Therefore, the heterogeneity and the partial overlap between clinical and molecular EB subtypes are not surprising if one considers the high density of molecules and their complex interactions in cell–cell and cell–matrix adhesions. The term EB (implying involvement limited to epidermis) was coined by Koebner more than 120 years ago (Koebner, 1886Koebner H. Hereditäre Anlage zur Blas-enbildung (epidermolysis bullosa hereditare).Deut Med Wochenschr. 1886; 12: 21-22Crossref Scopus (51) Google Scholar). It has remained in use until today, although this group of disorders now encompasses the entire spectrum of known skin fragility disorders, a concept sometimes difficult to comprehend by the patients and some clinicians as well. In the first half of the 20th century, the major achievements in the EB field consisted of defining clinical entities and distinguishing between inherited and acquired forms of bullous diseases. In the 1960s, ultrastructural studies led to the classification of EB into three major types—simplex, junctional, and dystrophic—based on the precise level of tissue separation (Pearson, 1962Pearson R.W. Studies on the pathogenesis of epidermolysis bullosa.J Invest Dermatol. 1962; 39: 551-575Abstract Full Text PDF PubMed Scopus (203) Google Scholar, Hashimoto et al., 1975Hashimoto I. Anton-Lamprecht I. Gedde-Dahl Jr., T. et al.Ultrastructural studies in epidermolysis bullosa heriditaria. I. 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In the 80s, development of immunofluorescence techniques and generation of polyclonal and monoclonal antibodies resulted in the identification of the first molecules causally involved in EB and the establishment of first molecular criteria for diagnosis using immunofluorescence mapping of the dermal-epidermal junction ((Hintner et al., 1981Hintner H. Stingl G. Schuler G. et al.Immunofluorescence mapping of antigenic determinants within the dermal-epidermal junction in the mechanobullous diseases.J Invest Dermatol. 1981; 76: 113-118Abstract Full Text PDF PubMed Scopus (162) Google Scholar), reviewed in (Fine, 1987Fine J.D. Altered skin basement membrane antigenicity in epidermolysis bullosa.Curr Probl Dermatol. 1987; 17: 111-126Crossref PubMed Google Scholar)). This was the second milestone in our pathogenetic understanding of this group of disorders. In parallel, rapid improvements in protein biochemical methods and recombinant protein expression systems facilitated the isolation and molecular characterization of the proteins of the dermal-epidermal basement membrane zone or their functional domains. Using molecular tools, combined with immunoelectron microscopy, such pivotal adhesion proteins as laminin-332 (previously kalinin, laminin 5) and collagen VII were identified as structural components of the hemidesmosomes and the anchoring fibrils (Sakai et al., 1986Sakai L.Y. Keene D.R. Morris N.P. et al.Type VII collagen is a major structural component of anchoring fibrils.J Cell Biol. 1986; 103: 1577-1586Crossref PubMed Scopus (435) Google Scholar, Bruckner-Tuderman et al., 1987Bruckner-Tuderman L. Schnyder U.W. 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The complexity of elastic fiber biogenesis: the paradigm of cutis laxa.J Invest Dermatol. 2012; 132: E12-E14Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar). All the new knowledge and diagnostic advances acquired during this decade were reflected in the revised clinical and laboratory criteria for EB, published in 1991, which split EB into numerous clinical subtypes (Fine et al., 1991Fine J.D. Bauer E.A. Briggaman R.A. et al.Revised clinical and laboratory criteria for subtypes of inherited epidermolysis bullosa.A consensus report by the Subcommittee on Diagnosis and Classification of the National Epidermolysis Bullosa Registry. J Am Acad Dermatol. 1991; 24: 119-135Scopus (466) Google Scholar). The 1990s were marked by rapid progress in genetics, which was based on the development of molecular genetic methods, including gene cloning, linkage analyses for gene mapping, and efficient DNA sequencing. In 1991, KRT14 mutations were found to cause EB simplex, the most common EB type (Coulombe et al., 1991Coulombe P.A. Hutton M.E. Letai A. et al.Point mutations in human keratin 14 genes of epidermolysis bullosa simplex patients: genetic and functional analyses.Cell. 1991; 66: 1301-1311Abstract Full Text PDF PubMed Scopus (533) Google Scholar). Thereafter, in rapid succession, mapping and discovery of the genetic defects underlying several different EB subtypes were reported (Ryynanen et al., 1991aRyynanen M. Knowlton R.G. Parente M.G. et al.Human type VII collagen: genetic linkage of the gene (COL7A1) on chromosome 3 to dominant dystrophic epidermolysis bullosa.Am J Hum Genet. 1991; 49: 797-803PubMed Google Scholar, Ryynanen et al., 1991bRyynanen M. Knowlton R.G. Uitto J. Mapping of epidermolysis bullosa simplex mutation to chromosome 12.Am J Hum Genet. 1991; 49: 978-984PubMed Google Scholar, Ryynanen et al., 1992Ryynanen M. Ryynanen J. 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Duquesnoy P. et al.A homozygous insertion-deletion in the type VII collagen gene (COL7A1) in Hallopeau-Siemens dystrophic epidermolysis bullosa.Nat Genet. 1993; 5: 287-293Crossref PubMed Scopus (111) Google Scholar, McGrath et al., 1995McGrath J.A. Gatalica B. Christiano A.M. et al.Mutations in the 180-kD bullous pemphigoid antigen (BPAG2), a hemidesmosomal transmembrane collagen (COL17A1), in generalized atrophic benign epidermolysis bullosa.Nat Genet. 1995; 11: 83-86Crossref PubMed Scopus (328) Google Scholar, Li and Uitto, 2012Li Q. Uitto J. Heritable ectopic mineralization disorders: the paradigm of pseudoxanthoma elasticum.J Invest Dermatol. 2012; 132: E15-E19Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar). Molecular genetic diagnostics became available, but were still labor-intensive, expensive, and dependent on pre-screening. In parallel to the elucidation of new genetic defects and different kinds of mutations in the known genes, researchers started to explore the molecular mechanisms underlying keratinocyte fragility in EB simplex and dermal-epidermal destabilization in junctional and dystrophic EB. In 1997, revertant mosaicism, i.e., genetic reversion of inherited mutations was recognized clinically in patients with junctional EB and demonstrated on molecular genetic level (Jonkman et al., 1997Jonkman M.F. Scheffer H. Stulp R. et al.Revertant mosaicism in epidermolysis bullosa caused by mitotic gene conversion.Cell. 1997; 88: 543-551Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar). For many years, this phenomenon of natural healing was considered rare and described only in isolated cases. Recently, however, it has become clear that revertant mosaicism occurs in most, if not all EB types, and is more widespread than expected (Lai-Cheong et al., 2011Lai-Cheong J.E. McGrath J.A. Uitto J. Revertant mosaicism in skin: natural gene therapy.Trends Mol Med. 2011; 17: 140-148Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar, Kiritsi et al., 2012Kiritsi D. He Y. Pasmooij A.M. et al.Revertant mosaicism in a human skin fragility disorder results from slipped mispairing and mitotic recombination.J Clin Invest. 2012; 122: 1742-1746Crossref PubMed Scopus (39) Google Scholar, McLean and Irvine, 2012McLean W.H.I. Irvine A. Heritable filaggrin disorders: the paradigm of atopic dermatitis.J Invest Dermatol. 2012; 132: E20-E21Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar). In the beginning of the 21st century, a rich body of molecular genetic data on EB was already available. Mutation databases (http://www.interfil.org, https://portal.biobase-international.com/hgmd/) and patient registries (https://grenada.lumc.nl/LOVD2, http://www.deb-central.org/molgenis.do, http://www.col7.info) facilitated both the study of genotype–phenotype correlations and the genetic counselling. Together with the knowledge acquired on biochemical and cell biological disease mechanisms, these data served as prerequisite for rational disease classifications. During the first decade of the 21st century, two revisions of the EB classification were needed to reflect the complexity and significant developments in the field (Fine et al., 2000Fine J.D. Eady R.A. Bauer E.A. et al.Revised classification system for inherited epidermolysis bullosa: Report of the Second International Consensus Meeting on diagnosis and classification of epidermolysis bullosa.J Am Acad Dermatol. 2000; 42: 1051-1066Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar, Fine et al., 2008Fine J.D. Eady R.A. Bauer E.A. et al.The classification of inherited epidermolysis bullosa (EB): Report of the Third International Consensus Meeting on Diagnosis and Classification of EB.J Am Acad Dermatol. 2008; 58: 931-950Abstract Full Text Full Text PDF PubMed Scopus (713) Google Scholar). Substantial advances in understanding the molecular basis of many old and new EB forms led to the tendency to avoid splitting of the disease into too many sub-entities and to reduce the use of eponyms. However, this is in part counteracted by continuous progress relating to discovery of new genes in rare EB subtypes. The most extensive changes have concerned EB simplex. Although the vast majority of EB simplex cases is caused by keratin 5/14 mutations (Figure 1b), several new causative genes have been identified. BPAG1e (dystonin isoform 4) and plectin mutations can cause cytolysis of basal keratinocytes and mild blistering, and may account for at least some of the molecularly unsolved EB simplex cases (Rezniczek et al., 2010Rezniczek G.A. Walko G. Wiche G. Plectin gene defects lead to various forms of epidermolysis bullosa simplex.Dermatol Clin. 2010; 28: 33-41Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar, Liu et al., 2012Liu L. Dopping-Hepenstal P.J. Lovell P.A. et al.Autosomal recessive epidermolysis bullosa simplex due to loss of BPAG1-e expression.J Invest Dermatol. 2012; 132: 742-744Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar). Furthermore, the spectrum of EB simplex has extended to include subtypes with cleavage in the suprabasal epidermal layers, e.g., the plakophilin deficiency and the lethal acantholytic EB. These very rare new forms are caused by genetic defects of desmosomal proteins (McGrath et al., 1997McGrath J.A. McMillan J.R. Shemanko C.S. et al.Mutations in the plakophilin 1 gene result in ectodermal dysplasia/skin fragility syndrome.Nat Genet. 1997; 17: 240-244Crossref PubMed Scopus (312) Google Scholar, Jonkman et al., 2005Jonkman M.F. Pasmooij A.M. Pasmans S.G. et al.Loss of desmoplakin tail causes lethal acantholytic epidermolysis bullosa.Am J Hum Genet. 2005; 77: 653-660Abstract Full Text Full Text PDF PubMed Scopus (166) Google Scholar, Pigors et al., 2011Pigors M. Kiritsi D. Krumpelmann S. et al.Lack of plakoglobin leads to lethal congenital epidermolysis bullosa: a novel clinico-genetic entity.Hum Mol Genet. 2011; 20: 1811-1819Crossref PubMed Scopus (60) Google Scholar). Hence, our vision on EB has extended to include conditions with perturbed cell–cell adhesion, in which the clinical picture is dominated by skin erosions rather than blisters. The molecular basis of EB simplex superficialis has remained unclear (Fine et al., 2008Fine J.D. Eady R.A. Bauer E.A. et al.The classification of inherited epidermolysis bullosa (EB): Report of the Third International Consensus Meeting on Diagnosis and Classification of EB.J Am Acad Dermatol. 2008; 58: 931-950Abstract Full Text Full Text PDF PubMed Scopus (713) Google Scholar), as a mutation in the collagen VII gene, associated with dominant dystrophic EB, was identified in the original family (Fine et al., 1989Fine J.D. Johnson L. Wright T. Epidermolysis bullosa simplex superficialis. A new variant of epidermolysis bullosa characterized by subcorneal skin cleavage mimicking peeling skin syndrome.Arch Dermatol. 1989; 125: 633-638Crossref PubMed Scopus (37) Google Scholar, Martinez-Mir et al., 2002Martinez-Mir A. Liu J. Gordon D. et al.EB simplex superficialis resulting from a mutation in the type VII collagen gene.J Invest Dermatol. 2002; 118: 547-549Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar). In a number of patients with superficial blisters and erosions (Figure 1a), transglutaminase 5 mutations indicative of acral peeling skin syndrome have been disclosed (Kiritsi et al., 2010Kiritsi D. Cosgarea I. Franzke C.W. et al.Acral peeling skin syndrome with TGM5 gene mutations may resemble epidermolysis bullosa simplex in young individuals.J Invest Dermatol. 2010; 130: 1741-1746Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar, Pigors et al., 2012Pigors M. Kiritsi D. Cobzaru C. et al.TGM5 mutations impact epidermal differentiation in acral peeling skin syndrome.J Invest Dermatol. 2012; 132: 2422-2429Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar). We believe that it is time to classify acral peeling skin syndrome as a skin fragility disorder, rather than a form of ichthyosis as it is now (Oji, 2010Oji V. Clinical presentation and etiology of ichthyoses. Overview of the new nomenclature and classification.Hautarzt. 2010; 61: 891-902Crossref PubMed Scopus (13) Google Scholar). The classification of the junctional EB was simplified by distinction of the lethal (Herlitz) subtype from the others, which were collectively designated as junctional EB–other. Junctional EB Herlitz is defined by loss of laminin-332, and it remains one of the most severe forms of EB. Junctional EB–other encompasses diverse phenotypes, in which blistering ranges from lifelong, generalized (Figure 1c), to late onset, mild, and localized, depending on the residual expression or activity of the mutated protein. The laryngo-onycho-cutaneous syndrome was included as a junctional EB variant, as it has similar clinical features and is associated with mutations in the a3-chain of laminin-332. Very recently, integrin a3 mutations were discovered in three patients with a new complex phenotype, including junctional EB, congenital nephrotic syndrome, and interstitial lung disease (Has et al., 2012Has C. Sparta G. Kiritsi D. et al.Integrin alpha3 mutations with kidney, lung, and skin disease.N Engl J Med. 2012; 366: 1508-1514Crossref PubMed Scopus (170) Google Scholar). All dystrophic EB subtypes are caused by mutations in the gene encoding collagen VII, the main component of the anchoring fibrils. The clinical presentations vary from severe generalized blistering and scarring (Figure 1d) to sole nail dystrophy without skin blistering. Although more than 600 COL7A1 gene mutations are known to date, the genotype–phenotype correlations are understood only in part. The heterogeneity of the allelic subtypes is likely to result from the variable expression and function of mutated collagen VII, with the complex interplay of genetic and environmental modifiers. Because of the variable (mixed) level of skin cleavage, the Kindler syndrome was recognized as a distinct type of EB. It is clinically characterized by skin blistering during childhood and progressive poikiloderma, skin and mucosal scarring, and predisposition to epithelial skin cancer in adulthood (Figure 1e). Mutations in the FERMT1 gene encoding kindlin-1 underlie this rare autosomal recessive genodermatosis. From a molecular point of view, the Kindler syndrome and the EB with integrin a3 mutations may be classified separately as focal adhesion disorders, because they impair the functions of focal adhesions, adherence, and signalling platforms, which are needed for epidermal cell adhesion and migration. By the end of 2011, more than 1250 different mutations in 17 genes responsible for EB were included in the Human Gene Mutation Database Professional release (2011.4; Harel and Christiano, 2012Harel S. Christiano A.M. Genetics of structural hair disorders.J Invest Dermatol. 2012; 132: E22-E26Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar). In several countries, mutation analysis is now routinely available, every patient having the right to know his/her own mutation. This development expanded the spectrum of disorders encompassed by the term EB and represents the basis for individualized molecular therapeutic approaches (Cho et al., 2012Cho R.J. Simpson M.A. McGrath J.A. Next-generation diagnostics for genodermatoses.J Invest Dermatol. 2012; 132: E27-E28Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar, Uitto, 2012Uitto J. Molecular therapeutics for heritable skin diseases.J Invest Dermatol. 2012; 132: E29-E34Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar). Many preliminary studies regarding translational approaches in EB have been published during the last decade (Mavilio et al., 2006Mavilio F. Pellegrini G. Ferrari S. et al.Correction of junctional epidermolysis bullosa by transplantation of genetically modified epidermal stem cells.Nat Med. 2006; 12: 1397-1402Crossref PubMed Scopus (512) Google Scholar, Fritsch et al., 2008Fritsch A. Loeckermann S. Kern J.S. et al.A hypomorphic mouse model of dystrophic epidermolysis bullosa reveals mechanisms of disease and response to fibroblast therapy.J Clin Invest. 2008; 118: 1669-1679Crossref PubMed Scopus (169) Google Scholar, Wong et al., 2008Wong T. Gammon L. Liu L. et al.Potential of fibroblast cell therapy for recessive dystrophic epidermolysis bullosa.J Invest Dermatol. 2008; 128: 2179-2189Abstract Full Text Full Text PDF PubMed Scopus (195) Google Scholar, Remington et al., 2009Remington J. Wang X. Hou Y. et al.Injection of recombinant human type VII collagen corrects the disease phenotype in a murine model of dystrophic epidermolysis bullosa.Mol Ther. 2009; 17: 26-33Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar, Wagner et al., 2010Wagner J.E. Ishida-Yamamoto A. McGrath J.A. et al.Bone marrow transplantation for recessive dystrophic epidermolysis bullosa.N Engl J Med. 2010; 363: 629-639Crossref PubMed Scopus (265) Google Scholar, Cho et al., 2012Cho R.J. Simpson M.A. McGrath J.A. Next-generation diagnostics for genodermatoses.J Invest Dermatol. 2012; 132: E27-E28Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar). In the future, the implementation of novel biologically valid therapies must take into account the balance between the gain and risk. No unique solution will be available for all patients, but the success will rather come with personalized therapies adapted to the individual molecular and clinical constellation (Uitto, 2012Uitto J. Molecular therapeutics for heritable skin diseases.J Invest Dermatol. 2012; 132: E29-E34Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar). The authors state no conflict of interest. The work of the authors is supported by the Federal Ministry for Education and Research (BMBF), the Excellence Initiative of the German Federal Governments and the Freiburg Institute for Advanced Studies FRIAS, School of Life Sciences – LifeNet, and the German Research Foundation (DFG).