EBS is the type of EB that comprises the largest number of distinct clinical subgroups. In the latest classification proposed by Has et al., those authors presented a total of 14 subtypes of EBS associated with seven different genes.2Table 1 shows the classification of EBS subtypes, highlighting the altered gene and protein, as well as the pattern of inheritance.

The localized, intermediate, and severe forms are the most frequent in EBS, with a prevalence of 4.69 per 1,000,000 together, while all other forms of EBS together reach a prevalence of 1.31 per 1,000,000.4 The three main subtypes are caused by pathogenic variants in the KRT5 or KRT14 genes, which encode keratins 5 and 14, respectively, and account for 60%‒70% of the cases of EBS.1 Localized EBS, formerly known as Weber-Cockayne EBS, is characterized by blisters limited to the regions of the palms and soles of the feet. The lesions can also appear in other regions of recurrent trauma, and they tend to get worse in warmer months. The blisters are usually not present at birth, but they appear early in childhood, especially when the child begins to crawl and walk. Focal palmoplantar keratoderma (thickening of the skin of the hands and feet) may be observed in some cases during adulthood. Among the possible complications, infections secondary to blistering lesions on the feet are the most common (Fig. 2).3,11,12

Figure 2.

Clinical aspects of epidermolysis bullosa simplex (EBS). (A‒C) Plantar keratoderma, dystrophic and thick nails on the hands and feet of patients with localized EBS. (D‒F) Blisters and erosions distributed in a generalized way in patients with intermediate EBS. The presence of epidermolysis bullosa nevi is common in all subtypes of EBS.

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Severe EBS, formerly known as severe generalized or Dowling-Meara EBS, has a unique clinical phenotype, with large generalized blisters, which can occur in a group (herpetiform blisters) and can be hemorrhagic.12 During childhood, blisters develop throughout the body, most often reaching the hands and feet, the region around the mouth, and the regions of the trunk and neck. In this subtype, the involvement of the oral mucosa, the occurrence of progressive palmoplantar keratosis, and nail dystrophy are common.11 Lesions commonly heal without scarring, but inflammation can occur, especially in hemorrhagic blisters, followed by milia and hypo- and hyper-pigmentation of the skin. As in the other EBS subtypes, the blisters tend to improve with age; however, palmoplantar keratoderma is more severe in most cases (Fig. 2).3,11,12

Intermediate EBS, formerly known as intermediate generalized EBSor KoebnerEBS, often manifests at birth, with the generalized distribution of blisters, but without the formation of herpetiform clusters. The clinical presentation is milder in comparison with the severe form, and does not present extracutaneous involvement. Although the distribution is widespread, the blisters predominantly affect the hands and feet. Focal palmoplantar keratoderma can be observed, and lesions tend to get worse with heat.3,11,12

The three main subtypes of EBS, detailed above, are caused by changes in the genes encoding keratins 5 and 14 of the keratinocyte cytoskeleton, as will be further detailed in the next section. In addition to them, other forms of EBS are also noteworthy, because they represent a considerable fraction of all cases. Pathogenic variants in the PLECgene, which encodes the plectin protein, can result in different subtypes with different inheritance patterns, which together account for up to 8% of cases of EBS.1,13 The autosomal dominant form results in intermediate EBS, while the recessive form can result in intermediate EBS, intermediate EBSwith muscular dystrophy, and severe EBSwith pyloric atresia.2 IntermediateEBSwith muscular dystrophy is characterized by the occurrence of blisters since birth and progressive muscle weakness with late onset.14,15 The main characteristics of severe EBS with pyloric atresia are the occurrence of blisters in the neonatal period, the presence of pyloric atresia and, frequently, premature death.15,16 In contrast, the subtype with autosomal dominant inheritance, formerly known as Ogna EBS, which is now just intermediate EBS, is characterized by a mild clinical picture, which includes mild skin fragility and the absence of involvement of other organs.15,17,18

In 2016, two independent studies almost simultaneously identified pathogenic variants in the KLHL24gene, encoding the Kelch-like protein 24, in a total of 15 patients with the EB-compatible phenotype who were not related to each other.19,20 Subsequently, other researchers began to investigate possible variants in this gene in patients who remained until that moment without the identified genetic mutation. Several cases with typical signs of EB and mutations in KLHL24have been described since then.21–25 It is estimated that 5% of cases of EBSare associated with mutations in this gene.1 The typical clinical signs of this subtype of EBS are still being characterized, as more studies and more patients are being described. Initial reports indicated that the phenotype would almost exclusively involve skin fragility, with lesions since birth, especially in the legs, and the formation of blisters on the trunk and upper limbs.21 From additional reports, a more syndromic phenotype appears to be related, with extracutaneous involvement that would include a strong association with cardiomyopathy.23,24 Studies have demonstrated that 85% of patients with pathogenic variants in KLHL24have some cardiac involvement, evidenced by a high level of biomarkers or dilated cardiomyopathy (40%).25,26 Neurological problems, such as developmental delay, intellectual disability, memory problems, and myopathy were also described in some cases.26 Future studies will be indispensable for a complete clinical characterization of this new subtype of EBS, designated in the most recent classification of EB as intermediateEBS with cardiomyopathy.2

The other subtypes of EBS described in Table 1 are considered rare, none of which represents more than 1% of the total cases of EBS.1 Some of them have specific clinical signs, such as the presence of mottled pigmentation and circinate plaques of a migratory character in the subtypes that bear these names. Other subtypes are characterized by clinical signs with different levels of severity, which overlap with other forms of the disease.3 For these cases, the clinical examination is not sufficient for the diagnosis, and additional techniques are needed to identify the altered protein or the pathogenic genetic variant.27

Although clinical expressivity varies between different forms of EBS, almost all are caused by changes in genes that encode components of keratinocytes that are important for the organization of the cytoskeleton and for cell-cell junctions.12,28 Thus, most of the proteins associated with EBS, detailed below, have the main function of providing structural support to the keratinocytes of the epidermis (Fig. 3).12

Figure 3.

Schematic representation of the complexes and proteins associated with the different subtypes of epidermolysis bullosa simplex (EBS). Changes in structural components of hemidesmosomes, intermediate keratin filaments, and cell vesicle transport support the spectrum of EBS phenotypes. Only proteins associated with EBS are shown in the figure.

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Keratins are cytoskeletal proteins that provide mechanical stability to cells and epithelial tissues. The keratin filaments are inserted between the desmosomes and hemidesmosomes, forming a network that allows not only the cell-cell connection of the keratinocytes, but also between them and the underlying layer of the dermoepidermal junction.11,12 Keratins 14 (K14) and 5 (K5) are type I and II intermediate filament proteins, respectively, and are expressed in the basal keratinocytes in the epidermis. In the physiological state, K14 and K5 form heterodimers of type I/type II intermediate filaments; thousands of them group together to form the cytoskeleton of intermediate keratin filaments (Fig. 4A).12

Figure 4.

(A) Schematic representation of the keratin intermediate filament network. The keratin filaments connect to the hemidesmosomes to ensure attachment to the underlying basement membrane and to the desmosomes to ensure cell-cell contact in the keratinocytes. A representation of the molecular configuration of the K5/K14 heterodimer, the smallest subunit that forms the intermediate filaments, is shown at the bottom of figure A. (B) Organization of keratin domains. Keratins have a central α-helix rod domain containing four segments (1A ‒ B, 2A ‒ B), which are interrupted by three binding domains (L1, L12, and L2). The rod domain has highly conserved motifs (HIP and HTP) at its ends, often associated with the most severe cases of severe EBS. Variants in the binding domains are generally associated with localized EBS, while in the intermediate form of the disease the variants involved tend to be distributed across segments of the rod domain.

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Pathogenic variants in the KRT14and KRT5 genes, which code K14 and K5, respectively, impair the assembly of the intermediate filament network in the keratinocyte cytoplasm, resulting in the EBS phenotype.8 Amino acid substitutions (missense variants) are the main type of genetic alteration in these genes, and the pattern of inheritance is autosomal dominant, with some exceptions. The clinical severity of EBS is correlated, in most cases, to the location of the genetic changes. Thus, variants in the highly conserved ends of the α-helix rod domains, essential for the assembly of the keratin filaments, are often associated with the most severe cases of the disease (severe EBS, Fig. 4B).8,29 However, this correlation is not true for all cases, as some residues may be more or less important for the protein, and the nature of the substituted amino acids, such as their structure and polarity, also influences the phenotype.8,30–32 The milder form, localized EBS, is commonly associated with variants in the linker region, while the intermediate form (intermediate EBS) comprises variants that tend to be distributed along the K5 and K14 sequences.8,30,33

Plectin, coded by the PLECgene, is a binding protein that plays an essential role in stabilizing and organizing the keratin filament networks in the cells. Although it is able to interact with actin filaments and microtubules, the main interaction of plectin is with the filaments of keratin intermediates. It works by anchoring the intermediate filaments to strategic sites, such as focal adhesions, desmosomes, hemidesmosomes, and other adhesion structures, thus stabilizing the network and cells.34,35 There are several isoforms of PLEC, which are preferentially expressed in different specific tissues.15

Pathogenic variants in the PLECgene can lead to different subtypes of EBS: a) intermediate EBS with autosomal dominant inheritance pattern; b) intermediate EBS with autosomal dominant inheritance pattern; c) intermediate EBS with muscular dystrophy; d) severe EBS with pyloric atresia – the last two also have a recessive inheritance pattern.2 To date, no clear genotype-phenotype correlation has been established regarding the position of the variants in PLEC. Most cases of EBS with muscular dystrophy and EBS with pyloric atresia involve substitutions that lead to a premature stop codon (nonsense variants) or insertions/deletions that alter the reading phase (frameshift variants), in both cases resulting in reduction or complete absence of plectin expression.34 It is suggested that the effect of the variants on the different isoforms could explain the phenotypic difference between these two subtypes (EBS with muscular dystrophy and EBS with pyloric atresia).34 To date, the only dominant pathogenic variant known to be associated with intermediate EBS (formerly Ogna EBS) is c.5998C > T (p. Arg.2000Trp), which leads to the disturbance of plectin by a negative dominant effect.18,34,36 Simply stated, variants with a negative dominant effect are those that lead to the formation of an altered protein that negatively affects the normal wild-type protein that is coexpressed within the cell, reducing its cellular function.

Most of the seven genes associated with EBShave structural functions for organizing the layers of the skin. The DSTgene, which encodes epidermal dystonin, also known as bullous pemphigoid antigen (BP230 or BPAG1e), plays a role in the regulation of keratinocytes adhesion and in the control of the integrin subtypes expressed.37 The exophilin-5 protein, encoded by EXPH5, is not an integral component of the intermediate filaments, desmosomes, or hemidesmosomes. Although its role is not fully known, it is suggested that it contributes to the regulation of cell membrane traffic, including controlling the formation and movement of vesicles along the actin and tubulin networks.37

Since the discovery of the association of the KLHL24gene with the EBS phenotype, researchers have sought to understand the molecular mechanism involved in its pathogenicity.20,26,38 Unlike most EBS-associated proteins, the Kelch-like protein 24 is not a structural protein. All pathogenic variants identified in patients involve the start codon of the translation.26 Preliminary findings suggest that this protein would act by a new mechanism in EB, through the deregulation of its autoubiquitination, which would lead to changes in the levels of the intermediate filaments, especially keratin 14, in keratinocytes.20,25,38

JEB is classified into nine different subtypes, according to the clinical signs presented and the severity of the phenotype. The most frequent subtypes of JEB are caused by pathogenic variants in the genes encoding the laminin 332 protein and type XVII collagen. The type of genetic alteration and its consequence for protein expression are directly linked to phenotypic severity, as will be discussed in the next section. Table 2 presents the JEB subtypes with emphasis on genes and associated proteins.2

The most severe form of this group is severe JEB, previously known as generalized severe JEB or Herlitz JEB, which is characterized by a clinical presentation of extensive skin and mucous membranes fragility.2,3 Mucocutaneous blisters and erosions are present since birth and are observed throughout the body’s surface, resulting in loss of proteins, fluids, and iron, which increases susceptibility to infections (Fig. 5).9 Exuberant granulation tissue is formed, especially around the nose and mouth, on the buttocks, and on the nail folds.43 Patients suffer from extreme pain, and the long-term consequences include developmental delay, difficulty in healing wounds, anemia, respiratory complications, and infections that eventually lead to death in the first years of life.9,41,44 This JEB subtype is associated with the complete absence or profound reduction of laminin 332, as will be detailed in the subsequent section.3,9

Figure 5.

Clinical aspects of junctional epidermolysis bullosa (JEB). Dental enamel anomalies, chronic wounds, and absence of nails in a patient with severe JEB.

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The sub-type intermediate JEB, formerly known as generalized intermediate JEB or generalized non-Herlitz JEB, comprises a clinically heterogeneous group, with generalized distribution of blisters, which heal and leave subtle atrophy and hypopigmentation at the injury sites (Fig. 5).2 The severity of the disease is milder when compared with the most severe subtype, although the membranes can also be affected.9 Other manifestations include alopecia, enamel defects, and dystrophy or absence of nails, at different levels of severity. Granulation tissue, formation of chronic wounds, and extracutaneous involvement in the cornea, larynx, and urinary tract may also be associated.3,9,41,42 In this JEB subtype, the expression of laminin 332 or type XVII collagen is reduced, but not entirely absent.9,45

The localized form of JEB was considered a rare condition. The improvement of molecular analysis techniques allowed the identification of cases with atypical clinical presentation in relation to the most well-known EB subtypes and, since then, a series of new cases of localized JEB have been described.42,45–47 In this subtype, the blisters are locally distributed, mainly in the limbs. Localized JEB, as well as the intermediate subtype, is associated with dystrophy or absence of nails, hypoplasia of tooth enamel, and the tendency to develop cavities (Fig. 5).3,48 In contrast to the other subtypes, patients with this form of JEB usually do not have extensive atrophic scars, alopecia, and other extracutaneous findings such as anemia, developmental delay, ocular alterations, or respiratory and genitourinary changes.3,48 Phenotypic variability is striking in JEB; in contrast to the severe form of the disease, the localized form may present a phenotype so mild that the only clinical sign is nail dystrophy.45

Among the rarest subtypes of JEB, the inversa form stands out, in which the cutaneous manifestation occurs predominantly in intertriginous areas (areas of the skin that touch or rub) and usually more extensive than to localized JEB.48 The subtype JEB with pyloric atresia is associated with alterations in the genes encoding the α6β4 integrin and is associated with severe phenotypes, which include, in addition to pyloric atresia, generalized distribution of blisters, congenital absence of skin, and early lethality.3,49 However, JEB with pyloric atresia is clinically heterogeneous and is also associated with less severe cases.50

The dermoepidermal junction area comprises a specialized basement membrane that connects the epidermis to the dermis and thus provides the skin with integrity and resistance against external mechanical forces.9,51 The membrane is formed mainly by networks of laminin 332 and collagen IV, which overlap and connect by the action of other molecules.9,52,53 Several proteins of the cell surface of the keratinocytes and the extracellular matrix are inserted in this network and interact in a specific way with each other, ensuring the adhesion of the skin layers.9,52,54

Pathogenic variants in the genes that encode the components of the dermoepidermal junction result in the absence or disturbance of their functions and the consequent decrease in adhesion of the skin layers, which leads to the characteristic JEB phenotypes.9,52

Laminin 332, formerly known as laminin 5, is a heterotrimeric protein consisting of α3, β3, and γ2 chains, encoded by the LAMA3, LAMB3, and LAMC2genes, respectively. It is one of the essential proteins of the dermoepidermal junction, being considered a molecular bridge between the basal keratinocytes of the epidermis and the underlying dermis.9,55 To connect to the surface of basal keratinocytes, laminin 332 interacts with α6β4 integrin in hemidesmosomes and α3β1 integrin in focal adhesions. On the dermal side, the connection to the basement membrane is due to its interaction with type VII collagen in the anchoring fibrils, which ensures the adhesion of the basement membrane to the extracellular matrix of the dermis (Fig. 6). It is through these protein-protein interactions that laminin 332 acts as a support for dermoepidermal adhesion.9,41,52

Figure 6.

Schematic representation of the main structures involved in the adhesion of the skin layers. The main proteins associated with junctional epidermolysis bullosa ( to the right of the figure) interact in order to allow the connection of the keratin intermediate filaments to the anchoring fibrils (formed by type VII collagen) in the dermis.

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Pathogenic variants in the LAMA3, LAMB3, or LAMC2genes lead to the absence or reduction of laminin 332, which results in the loss of its function as a bridge between hemidesmosomes and anchoring fibrils. Consequently, the epidermis and dermis separate at the level of the lamina lucida, resulting in typical JEB phenotypes.52 The phenotypic severity and, therefore, the clinical subtype of JEB are related to the expression of laminin 332. The most severe form, severe JEB, is usually caused by loss of function variants (null variants), such as the substitutions that generate a premature stop codon (nonsense variants) and those that alter the reading phase (frameshift variants), thus resulting in the complete absence of laminin 332.9,41

In approximately 80% of severe cases, the associated gene is LAMB3, which encodes the protein's β3 chain.41 In the milder forms of JEB (intermediate and localized), at least one of the associated pathogenic variants allows the expression, even if partial, of the functional protein, such as amino acid substitutions (missense) or variants in splice sites that do not alter the reading phase (in-frame).9,39,40

Type XVII collagen, encoded by the COL17A1gene, is a type II-oriented transmembrane protein, in which the N-terminal end is cytoplasmic and the C-terminal crosses the lamina lucida towards the outer side of the cell.46 This protein is a three-chain α1 trimer, whose main function is to connect the intermediate keratin filament network to the basement membrane through interactions with the proteins plectin, BP230, and α6β4 integrin (Fig. 6).45,46,56

Pathogenic variants in the COL17A1gene result in a variable spectrum of phenotypes. Although several studies have searched for possible genotype-phenotype correlations, the only clear correlation is that collagen XVII expression levels directly influence clinical expression. Thus, patients with genetic variants that lead to the complete absence of protein in the skin, such as nonsense and frameshift, present a severe phenotype. In turn, patients with milder symptoms of the disease usually carry missense variants or variants in the in-frame splice sites, which reduce, but do not eliminate collagen XVII expression, or even alter the protein without affecting its expression.45,46,57

A6β4 integrin is a heterodimeric transmembrane polypeptide, located in the center of hemidesmosomes (Fig. 6).58 Its main function is to connect the hemidesmosomal plaque of the basal keratinocytes to the underlying basement membrane, thus providing mechanical resistance to the skin and mucous membranes.58,59 The phenotypes resulting from pathogenic variants in the ITGA6and ITGB4genes, which encode the α6β4 integrin, are also quite heterogeneous. Lethal forms of the disease usually result from null mutations (such as nonsense substitutions, and frameshift insertions and deletions) in both alleles, while the milder forms usually involve the presence of amino acid substitutions (missense variants) in at least one of the alleles.50

The gene encoding the α3 subunit of the α3β1 integrin has recently been associated with a rare form of JEB (JEB with respiratory and renal involvement).60 Although the importance of α3β1 integrin to the epithelium is known, including epidermal keratinocytes, alveolar epithelial cells, and podocytes, the molecular mechanisms involved in this JEB subtype are not yet established.61

The classification of DEB in its various subtypes takes into account the characteristic cutaneous and extracutaneous signs, the distribution of the blisters (localized or generalized), and the severity of the phenotype.2,3 Although some subtypes are characterized by specific clinical signs of different severity, there is an overlap of certain phenotypes among some groups (Fig. 7A‒I), which makes the diagnosis difficult and requires additional molecular tests.3,27 As DEB is inherited in both a dominant and a recessive manner, the classification of the type of DEB is especially important for the purposes of genetic counseling.6Table 3 presents the DEB subtypes.

Figure 7.

Clinical aspects of dystrophic epidermolysis bullosa (DEB). (A) Patient with dominant DEB, pre-tibial. (B‒C) Dystrophy of the toenails and hands of patients with dominant DEB, localized. (D‒E) Pseudosyndactyly (fusion of the fingers) and generalized distribution of blisters and extensive scarring in a patient with recessive DEB, severe. (G‒H) Blisters and generalized scarring, especially on knees, legs, feet, and hands in a patient with recessive DEB, intermediate; hyperkeratosis of the hands and feet leads to flexion contractures. (F, I) Blisters and lesions restricted to hands and feet and nail dystrophy in patients with recessive DEB, localized.

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In the DEB subtypes, the severity of the phenotypes is related, at a certain level, to the consequence of COL7A1mutations for the formation of anchoring fibrils, which may present morphological changes, have their expression reduced, or even be completely absent, as detailed in the next section.64 Although the severe forms of DEB are described as the most frequent, it is possible that there is underdiagnosis of the localized forms, given the involvement of more subtle clinical signs. From the development of new generation sequencing technologies, new studies, including large sample numbers, have enabled the establishment of genotype-phenotype correlations for the disease.34–67 The main subtypes of DEB are detailed below.

Severe RDEB, formerly known as severe generalized RDEB or Hallopeau-Siemens RDEB, constitutes the most severe form of DEB, usually associated with the complete absence of collagen VII expression.68 The classic clinical signs are the formation of generalized blisters throughout the body from birth and mutilating scars that lead to pseudosyndactyly (fusion of the fingers) in the hands and feet and that result in joint contractures. Furthermore, chronic ulcers that are difficult to heal, milia, and nail dystrophy are other typical signs (Fig. 7 D‒E). The blisters can be observed in the oral mucosa, in the corneal region, and in the gastrointestinal epithelium; esophageal stenosis is described in many cases. Anemia, developmental delay, and squamous cell carcinoma associated with early death remain components of the pattern of this subtype of RDEB.3,62,64

Intermediate RDEB, formerly known as intermediate generalized RDEB or non-Hallopeau-Siemens RDEB, is also characterized the generalized distribution blisters; however, it is less severe (Fig. 7 G‒H).69 Very frequent clinical signs of the severe form, such as esophageal constriction, corneal lesions, and pseudosyndactyly of the hands and feet still occur, but are less common in the intermediate form of the disease. Developmental delay and anemia are even more rare conditions in this subtype of RDEB.3,69

Among the dominant subtypes, the intermediate form, intermediate DDEB (formerly generalized DDEB), is characterized by the formation of blisters with generalized distribution at birth, albopapuloid lesions, milia, atrophic scarring, and nail dystrophy. In contrast to severe RDEB, developmental delay, severe anemia, and risk of squamous cell carcinoma are not characteristic of the DDEB subtypes.3,69

Amid the complexity that is the classification of DEB in its various subtypes, the localized forms of both patterns of inheritance make it even more difficult: localized DDEB and localized RDEB are clinically indistinguishable.3 That is, for typical phenotypes of localized DEB, the genetic result is essential for the determination of the associated inheritance pattern and, ultimately, for a correct genetic counseling. These are subtypes with very attenuated EB phenotypes, whose main clinical sign is the localized distribution of blisters in the hand and foot regions and nail dystrophy (Fig. 7 B, C, F, and I). Milia and atrophic scars can also occur. There is no extracutaneous involvement or pseudosyndactyly in these forms of the disease.3,69

The DEB subdivision still encompasses a number of other, rarer subtypes, which usually involve specific clinical signs. Inverse RDEB is characterized by a peculiar succession of clinical findings, which begins with the formation of generalized blisters in the neonatal period that heal in early childhood, leaving atrophic scars. Skin signs tend to improve over the years, while serious lesions develop on the oral, esophageal, anal, and genital mucosa.27,70 DEB pruriginosa is a generalized subtype that includes the complaint of severe pruritus.69,71 DEB self-improving, formerly known as transient bullous dermolysis of the newborn, is a subtype of DEB that appears at birth, or soon after, with the formation of generalized blisters. In contrast to the other subtypes, the clinical signs of this form of the disease disappear within 6 to 24 months of life.69

Type VII collagen is the main, if not the only, component of anchoring fibrils. These are large U-shaped structures that extend from the bottom of the dermal-epidermal basement membrane to the upper layer of the dermis (papillary dermis).52 Thus, the anchoring fibrils provide integrity for the association of the epidermis with the underlying dermis.62

COL7A1is a gene that has a complex arrangement, consisting of 118 exons and a size of approximately 32 kb. This gene encodes collagen polypeptides pro-α1 (VII), which will be organized into anchoring fibrils.62,74 The structure of these polypeptides includes a collagenous domain in its central portion, formed by Gly-X-Y repeats that fold to achieve the characteristic triple helix conformation of collagen molecules.62 The triple helix domain of collagen VII contains a total of 19 disruptions or imperfections, including a non-collagenous central segment of 39 amino acids.74 Such interruptions in the Gly-X-Y sequence provide flexibility for collagen VII molecules.52 The central collagenous domain is flanked by non-helical globular domains, the NC1 domain at the amino-terminal end and the NC2 domain at the carboxy-terminal end.52,62,74

Fig. 8 presents a diagram of the formation of anchoring fibrils. The pro-α1 (VII) collagen polypeptides are synthesized mainly by the keratinocytes of the epidermis, but also by the dermal fibroblasts.52 Three formed polypeptides are associated through their carboxy-terminal ends (NC2 domains), while their collagenous domains fold into a triple helix conformation. Two type VII collagen molecules in triple helix form antiparallel dimers and, after part of the carboxy-terminal end (NC2) is removed, the dimer formed is stabilized by intermolecular disulfide bonds.

Figure 8.

Schematic representation of the assembly of the anchoring fibrils from pro-a1 (VII) collagen polypeptides. Three formed polypeptides are associated through their C-terminal ends, while their collagenous domains fold into a triple helix conformation. Thereafter, type VII collagen monomers in triple helix form antiparallel dimers, which unite after part of the C-terminal end is removed. The different dimer molecules join laterally to form anchoring fibrils. The N-terminal domains of the anchoring fibrils are bound by homology to the macromolecules in the lamina densa, stabilizing its adhesion with the underlying dermis.

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Subsequently, several type VII collagen dimers are organized laterally in the anchoring fibrils. The binding of fibrils to the lamina densa occurs due to the non-collagenous amino-terminal domain NC1, which, by homology, binds to proteins of the lamina densa, stabilizing the association of this layer with the underlying dermis (Fig. 8).62,72,75–77

The importance of knowing the stages of the formation of anchoring fibrils lies in the fact that disturbances in any of these stages result in specific consequences for the structure and expression of the fibrils, which ultimately leads to the different phenotypes of DEB.62,72 As will be presented below, it can be said that the DEB phenotype is, as a rule, determined by the expression and residual function of COL7A1, although exceptions to this pattern have already been reported.78,79

Severe RDEB is often caused by premature stop codon variants (null variants) in both alleles of the COL7A1gene, resulting from nonsense substitutions, frameshift insertions or deletions, or even variants in splice sites that result in alteration in the reading phase.7,80 Such changes cause serious consequences for the protein formed, resulting in decay of mRNA mediated by nonsense mutations (transcription degradation mechanism with premature stop codon) or in the formation of truncated polypeptides that are not able to associate for the assembly of anchoring fibrils.7,62,80

In turn, intermediate RDEB is often caused by variants in compound heterozygosis: a premature stop codon variant and an amino acid substitution.7 It should be noted, however, that several other types of variants, in different combinations, have also been associated with this subtype.67,79,81–84 The intermediate form is characterized the presence of at least one variant that allows for some type VII collagen production, thus enabling the assembly, even if partial, of the anchoring fibrils. Thus, the genetic changes associated with this subtype can affect the association of polypeptides, the formation and stability of the triple helix, or cause some conformational modification in the protein.7,62 The polypeptides formed in these cases are still capable of forming a small number of anchoring fibrils, even if unstable, which explains the more attenuated phenotype in relation to the severe subtype of the disease.7,85

Similarly, localized RDEB is correlated to variants that minimally affect the formation of anchoring fibrils, either by reducing their expression or through slight conformational or stability changes.86–88 Interestingly, the most described variants in cases of localized RDEB are those that occur in splice sites.83,86,88,89 Splice variants are associated with a diversity of phenotypes, depending on their influence on the sequence of the protein formed. In the localized form, they usually result in the excision of whole exons (exon skipping), without altering the remaining protein sequence. Studies have shown that these variants do not alter the totality of the formed transcripts, but only a portion of them, allowing the formation of normal transcripts to a considerable extent. This allows the assembly of the anchoring fibrils with only small functional defects, which justifies the very mild phenotype.86,88–90

The dominant form of DEB usually involves substitutions of glycine in the collagenous triple helix domain, although other substitutions, insertions, deletions, and splice site variants have also been described.7,62,91,92 These variants affect amino acids that are essential for the structure of the triple helix and, therefore, are associated with the decreased stability of the anchoring fibrils.7,62,72 Some amino acids are positioned in segments that are more important to the structure of collagen VII, and some substitutions affect the formation of the molecule more significantly than others. Thus, the nature and position of the genetic variation are closely correlated to the resulting phenotype.7,93