Autosomal Recessive Epidermolysis Bullosa Simplex Due to Loss of BPAG1-e Expression

DST variants are responsible for neurogenic arthrogryposis multiplex congenita enlarging the spectrum of type VI hereditary sensory autonomic neuropathy

Arthrogryposis multiplex congenita (AMC) is a developmental condition characterized by multiple joint contractures resulting from reduced or absent fetal movements. Through whole-exome sequencing combined with arrayCGH from DNA of a fetus presenting with early onset AMC, we identified biallelic loss of function variants in Dystonin (DST): a stop gain variant (NM_001144769.5:c.12208G > T:p.(Glu4070Ter)) on the neuronal isoform and a 175 kb microdeletion including exons 25-96 of this isoform on the other allele [NC_000006.11:g.(56212278_56323554)_(56499398_56507586)del]. Transmission electron microscopy of the sciatic nerve revealed abnormal morphology of the peripheral nerve with severe hypomyelination associated with dramatic reduction of fiber density which highlights the critical role of DST in peripheral nerve axonogenesis during development in human. Variants in the neuronal isoforms of DST cause hereditary sensory and autonomic neuropathy which has been reported in several unrelated families with highly variable age of onset from fetal to adult onset. Our data enlarge the disease mechanisms of neurogenic AMC.

Dystonin modifiers of junctional epidermolysis bullosa and models of epidermolysis bullosa simplex without dystonia musculorum

The Lamc2jeb junctional epidermolysis bullosa (EB) mouse model has been used to demonstrate that significant genetic modification of EB symptoms is possible, identifying as modifiers Col17a1 and six other quantitative trait loci, several with strong candidate genes including dystonin (Dst/Bpag1). Here, CRISPR/Cas9 was used to alter exon 23 in mouse skin specific isoform Dst-e (Ensembl GRCm38 transcript name Dst-213, transcript ID ENSMUST00000183302.5, protein size 2639AA) and validate a proposed arginine/glutamine difference at amino acid p1226 in B6 versus 129 mice as a modifier of EB. Frame shift deletions (FSD) in mouse Dst-e exon 23 (Dst-eFSD/FSD) were also identified that cause mice carrying wild-type Lamc2 to develop a phenotype similar to human EB simplex without dystonia musculorum. When combined, Dst-eFSD/FSD modifies Lamc2jeb/jeb (FSD+jeb) induced disease in unexpected ways implicating an altered balance between DST-e (BPAG1e) and a rarely reported rodless DST-eS (BPAG1eS) in epithelium as a possible mechanism. Further, FSD+jeb mice with pinnae removed are found to provide a test bed for studying internal epithelium EB disease and treatment without severe skin disease as a limiting factor while also revealing and accelerating significant nasopharynx symptoms present but not previously noted in Lamc2jeb/jeb mice.

Epidermolysis Bullosa Simplex With Dystonin Gene Mutation: First Reported Case in Saudi Arabia

We report a case of a 3-year-old Saudi female patient as the first case reported in Saudi Arabia who is homozygous for dystonin c.3370C>T, p.(Gln1124*). At the age of 3 months, the girl started to develop numerous vesicles and bullae involving the dorsum of the feet that were not on a pressure site, with remission and aggravation, but she had no mucosal lesions or nail affection. The patient was diagnosed with epidermolysis bullosa simplex.

Seven naturally variant loci serve as genetic modifiers of Lamc2jeb induced non-Herlitz junctional Epidermolysis Bullosa in mice

Epidermolysis Bullosa (EB) is a group of rare genetic disorders that compromise the structural integrity of the skin such that blisters and subsequent erosions occur after minor trauma. While primary genetic risk of all subforms of EB adhere to Mendelian patterns of inheritance, their clinical presentations and severities can vary greatly, implying genetic modifiers. The Lamc2jeb mouse model of non-Herlitz junctional EB (JEB-nH) demonstrated that genetic modifiers can contribute substantially to the phenotypic variability of JEB and likely other forms of EB. The innocuous changes in an 'EB related gene', Col17a1, have shown it to be a dominant modifier of Lamc2jeb. This work identifies six additional Quantitative Trait Loci (QTL) that modify disease in Lamc2jeb/jeb mice. Three QTL include other known 'EB related genes', with the strongest modifier effect mapping to a region including the epidermal hemi-desmosomal structural gene dystonin (Dst-e/Bpag1-e). Three other QTL map to intervals devoid of known EB-associated genes. Of these, one contains the nuclear receptor coactivator Ppargc1a as its primary candidate and the others contain related genes Pparg and Igf1, suggesting modifier pathways. These results, demonstrating the potent disease modifying effects of normally innocuous genetic variants, greatly expand the landscape of genetic modifiers of EB and therapeutic approaches that may be applied.

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

Pathogenic DST sequence variants result in either epidermolysis bullosa simplex (EBS) or hereditary sensory and autonomic neuropathy type 6 (HSAN-VI)

DST encodes bullous pemphigoid antigen-1 (BPAG1), a protein with eight tissue-specific isoforms expressed in the skin, muscle, brain, and nerves. Accordingly, mutations in this gene are associated with epidermolysis bullosa simplex (EBS) and hereditary sensory and autonomic neuropathy type 6 (HSAN-VI). The genotypic spectrum is attested to by 19 distinct mutations but genotype-phenotype correlation for both disorders is not well established. In this study, we performed next-generation sequencing (NGS) on two families with different phenotypic presentations, one fetus (P1) with musculoskeletal and neurological malformations established by prenatal ultrasound and family history, and a 15-year-old female (P2) with skin blistering. P1 had a novel homozygous nonsense mutation, DST: NM_001144769, c.3805C>T, p.R1269* within a region of homozygosity (ROH). This mutation resides within the plakin domain of BPAG1 and ablates all isoforms of this protein, leading to novel extracutaneous phenotypes consistent with HSAN-VI in P1. P2 had a recurrent homozygous mutation DST: NM_001723.7, c.3370C>T, p.Gln1124* that presented with giant, trauma-induced skin blisters without extracutaneous involvement. This mutation is located within the coiled-coil domain present on the skin isoform of DST, BPGA1-e, associated with EBS. In summary, we report two families with pathogenic DST variants and expand the spectrum of DST genotype and phenotypes.

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Novel Point Mutation of EBSS Gene Coexisted with 1p36 Deletion

EBSS (epidermolysis bullosa simplex superficialis) is mainly caused by gene mutations which targeted protein as plakophilin1, desmoplakin and keratins. 1p36 gene deleted could cause typical clinical manifestations and might also affect the expression of functional genes in other regions. Here we reported the first case of PKP1 gene and DSP gene mutation coexisted with 1p36 deletion presented as serious EBSS and 1p36 deletion syndromes and identified a new homozygous mutation in the PKP1 gene (chr1:201292246 c.1672 T>C) and in the DSP gene (chr6:7580346 c.3923C>T).

Epidermolysis bullosa simplex due to bi-allelic DST mutations: Case series and review of the literature

BACKGROUND

Epidermolysis bullosa simplex (EBS) is a heterogeneous group of inherited disorders characterized by skin fragility due to intraepidermal separation. Most cases result from heterozygous mutations in KRT5 or KRT14; however, a minority of affected individuals carry mutations in non-keratin genes including DST encoding an epithelial isoform of dystonin. DST-associated EBS is transmitted as an autosomal recessive trait. Here, we report a series of EBS patients carrying bi-allelic DST mutations and review previously reported cases aiming to delineate phenotype-genotype correlations.

METHODS

Whole-exome and direct sequencing were used for variant analysis. Review of previously reported cases was performed.

RESULTS

Mutation analysis revealed DST mutations in five patients belonging to three families. Two variants have not been previously reported: c.7097dupA (p.Tyr2366X) and c.7429delC (p.Leu2477Serfs*13). We identified an additional six cases in the literature, bringing the total number of individuals affected with EBS due to DST variants to 11. Patients displayed distinctive phenotypes regardless of the causative variant.

CONCLUSIONS

The current study expands the clinical and genetic spectrum of DST-associated EBS subtype.

Autoantibodies to BPAG1e Trigger Experimental Bullous Pemphigoid in Mice

Bullous pemphigoid (BP) is an autoimmune blistering disease that targets the hemidesmosomal proteins BP180 and BP230/BPAG1e. Whereas the role of anti-BP180 antibodies has been extensively characterized, the pathogenicity of anti-BPAG1e antibodies remains unclear. The purpose of this study is to elucidate the role of antibodies to BPAG1e in the experimental bullous pemphigoid models. We generated Bpag1 conditional knockout mice, where the knockout of Bpag1 is restricted to keratin 5-expressing epithelial cells. Bpag1 conditional knockout mice were immunized with the C-terminal portion of BPAG1e, and the splenocytes were injected into Rag2^-/- mice intravenously. The recipient mice presented with erosion on the feet and tails. Microscopic examination showed subepidermal blisters and a linear deposition of IgG at the dermal-epidermal junction. To assess the potential role of trauma on BP development, we inflicted surface wounds on the dorsum of the Rag2^-/- recipient mice after adoptive transfer. The wounded Rag2^-/- mice had increased morbidity and severity of BP-like symptoms. Moreover, the depletion of B cells from splenocytes abolished a subepidermal blistering phenotype in vivo. These findings demonstrate that antibodies to BPAG1e might play a pathogenic role in causing subepidermal blistering, and external factors, including trauma, might be a trigger for BP development.

Isoform-specific loss of dystonin causes hereditary motor and sensory neuropathy

Objective

To determine the genetic cause of axonal Charcot-Marie-Tooth disease in a small family with 2 affected siblings, one of whom had cerebellar features on examination.

Methods

Whole-exome sequencing of genomic DNA and analysis for recessively inherited mutations; PCR-based messenger RNA/complementary DNA analysis of transcripts to characterize the effects of variants identified by exome sequencing.

Results

We identified compound heterozygous mutations in dystonin (DST), which is alternatively spliced to create many plakin family linker proteins (named the bullous pemphigoid antigen 1 [BPAG1] proteins) that function to bridge cytoskeletal filament networks. One mutation (c.250C>T) is predicted to cause a nonsense mutation (p.R84X) that only affects isoform 2 variants, which have an N-terminal transmembrane domain; the other (c.8283+1G>A) mutates a consensus splice donor site and results in a 22 amino acid in-frame deletion in the spectrin repeat domain of all BPAG1a and BPAG1b isoforms.

Conclusions

These findings introduce a novel human phenotype, axonal Charcot-Marie-Tooth, of recessive DST mutations, and provide further evidence that BPAG1 plays an essential role in axonal health.

Diverse dystonin gene mutations cause distinct patterns of Dst isoform deficiency and phenotypic heterogeneity in Dystonia musculorum mice

Loss-of-function mutations in dystonin (DST) can cause hereditary sensory and autonomic neuropathy type 6 (HSAN-VI) or epidermolysis bullosa simplex (EBS). Recently, DST-related diseases were recognized to be more complex than previously thought because a patient exhibited both neurological and skin manifestations, whereas others display only one or the other. A single DST locus produces at least three major DST isoforms: DST-a (neuronal isoform), DST-b (muscular isoform) and DST-e (epithelial isoform). Dystonia musculorum (dt) mice, which have mutations in Dst, were originally identified as spontaneous mutants displaying neurological phenotypes. To reveal the mechanisms underlying the phenotypic heterogeneity of DST-related diseases, we investigated two mutant strains with different mutations: a spontaneous Dst mutant (Dst^dt-23Rbrc mice) and a gene-trap mutant (Dst^Gt mice). The Dst^dt-23Rbrc allele possesses a nonsense mutation in an exon shared by all Dst isoforms. The Dst^Gt allele is predicted to inactivate Dst-a and Dst-b isoforms but not Dst-e. There was a decrease in the levels of Dst-a mRNA in the neural tissue of both Dst^dt-23Rbrc and Dst^Gt homozygotes. Loss of sensory and autonomic nerve ends in the skin was observed in both Dst^dt-23Rbrc and Dst^Gt mice at postnatal stages. In contrast, Dst-e mRNA expression was reduced in the skin of Dst^dt-23Rbrc mice but not in Dst^Gt mice. Expression levels of Dst proteins in neural and cutaneous tissues correlated with Dst mRNAs. Because Dst-e encodes a structural protein in hemidesmosomes (HDs), we performed transmission electron microscopy. Lack of inner plaques and loss of keratin filament invasions underneath the HDs were observed in the basal keratinocytes of Dst^dt-23Rbrc mice but not in those of Dst^Gt mice; thus, the distinct phenotype of the skin of Dst^dt-23Rbrc mice could be because of failure of Dst-e expression. These results indicate that distinct mutations within the Dst locus can cause different loss-of-function patterns among Dst isoforms, which accounts for the heterogeneous neural and skin phenotypes in dt mice and DST-related diseases.

Summary: This paper describes the mechanism by which diverse dystonin gene mutations result in phenotypic heterogeneity in neural and cutaneous tissues of Dystonia musculorum mice.

HSAN-VI: A spectrum disorder based on dystonin isoform expression

Hereditary sensory and autonomic neuropathy (HSAN-VI) is a recessive genetic disorder that arises because of mutations in the human dystonin gene (DST, previously known as bullous pemphigoid antigen 1). Although initial characterization of HSAN-VI reported it as a sensory neuropathy that was lethal in infancy, we now know of a number of heterozygous mutations in DST that result in milder forms of the disease. Akin to what we observe in the mouse model dystonia musculorum (Dst^dt), we believe that the heterogeneity of HSAN-VI can be attributed to a number of dystonin isoforms that the mutation affects. Lack of neuronal isoform dystonin-a2 is likely the universal determinant of HSAN-VI because all reported human cases are null for this isoform, as are all Dst^dt mouse alleles. Compensatory mechanisms by intact dystonin-a isoforms also likely play a role in regulating disease severity, although we have yet to determine what specific effect dystonin-a1 and dystonin-a3 have on the pathogenesis of HSAN-VI.

The spectraplakin Dystonin antagonizes YAP activity and suppresses tumourigenesis

Aberrant expression of the Spectraplakin Dystonin (DST) has been observed in various cancers, including those of the breast. However, little is known about its role in carcinogenesis. In this report, we demonstrate that Dystonin is a candidate tumour suppressor in breast cancer and provide an underlying molecular mechanism. We show that in MCF10A cells, Dystonin is necessary to restrain cell growth, anchorage-independent growth, self-renewal properties and resistance to doxorubicin. Strikingly, while Dystonin maintains focal adhesion integrity, promotes cell spreading and cell-substratum adhesion, it prevents Zyxin accumulation, stabilizes LATS and restricts YAP activation. Moreover, treating DST-depleted MCF10A cells with the YAP inhibitor Verteporfin prevents their growth. In vivo, the Drosophila Dystonin Short stop also restricts tissue growth by limiting Yorkie activity. As the two Dystonin isoforms BPAG1eA and BPAG1e are necessary to inhibit the acquisition of transformed features and are both downregulated in breast tumour samples and in MCF10A cells with conditional induction of the Src proto-oncogene, they could function as the predominant Dystonin tumour suppressor variants in breast epithelial cells. Thus, their loss could deem as promising prognostic biomarkers for breast cancer.

Tetraspanin CD151 and integrin α3β1 contribute to the stabilization of integrin α6β4-containing cell-matrix adhesions

Tetraspanin CD151 has been suggested to regulate cell adhesion through its association with laminin-binding integrins α3β1 and α6β4; however, its precise function in keratinocyte adhesion remains elusive. In this study, we investigated the role of CD151 in the formation and maintenance of laminin-associated adhesions. We show that CD151, through binding to integrin α3β1, plays a critical role in the stabilization of an adhesion structure with a distinct molecular composition of hemidesmosomes with tetraspanin features. These hybrid cell-matrix adhesions, which are formed early during cell adhesion and spreading and at later stages of cell spreading, are present in the central region of the cells. They contain the CD151-α3β1/α6β4 integrin complexes and the cytoskeletal linker protein plectin, but are not anchored to the keratin filaments. In contrast, hemidesmosomes, keratin filament-associated adhesions that contain integrin α6β4, plectin, BP180 (encoded by COL17A1) and BP230 (encoded by DST), do not require CD151 for their formation or maintenance. These findings provide new insights into the dynamic and complex regulation of adhesion structures in keratinocytes and the pathogenic mechanisms underlying skin blistering diseases caused by mutations in the gene for CD151.

Clinical subtypes and molecular basis of epidermolysis bullosa in Kuwait

BACKGROUND

Epidermolysis bullosa (EB) is a clinically and genetically heterogeneous blistering skin disease, but in countries such as Kuwait, there are very limited data on the clinical and molecular pathology of EB. To improve understanding of EB in Kuwait, we report the experience of a local tertiary referral center over a 17.5 year period (January 2000-June 2017) in establishing clinical and molecular diagnoses.

METHODS

Review of hospital records and diagnostic reports. Individual cases were diagnosed by combinations of clinical assessment, skin biopsy (immunohistochemistry and transmission electron microscopy), Sanger sequencing of EB genes, and whole exome sequencing.

RESULTS

Fifty-four families with EB were registered with the clinic over this period, 41 of whom (84 patients) participated in diagnostic studies. Thirty-seven of these 41 families had consanguineous marriages; 34 had recessive forms of EB, while only seven had dominant subtypes. Recurrent mutations were observed in epidermal dystonin, transglutaminase 5, and type VII collagen.

CONCLUSIONS

The prevalence of EB in Kuwait is approximately three times that of internationally cited rates with an over-representation of autosomal recessive variants. Establishing the molecular basis of EB in Kuwait with accurate diagnostic subtyping provides a basis for determining healthcare requirements and improving patient management of EB.

Inherited epidermolysis bullosa: New diagnostics and new clinical phenotypes

Inherited epidermolysis bullosa (EB) is a group of heterogeneous genetic disorders characterized by skin fragility. EB comprises a large spectrum of phenotypes, ranging from severe cutaneous and extracutaneous involvement caused by lack of key adhesion proteins, to mild cutaneous fragility caused by subtle molecular defects. Disease-causing variants in 20 different genes account for the genetic and allelic heterogeneity of EB. Here, we discuss the development of laboratory methods that enabled these discoveries and the clinical and molecular features of some new EB entities elucidated during the past 5-6 years.

Structural proteins of the dermal-epidermal junction targeted by autoantibodies in pemphigoid diseases

The dermal-epidermal junction consists of a network of several interacting structural proteins that strengthen adhesion and mediate signalling events. This structural network consists of hemidesmosomal-anchoring filament complexes connecting the basal keratinocytes to the basement membrane. The anchoring filaments in turn interact with the anchoring fibrils to attach the basement membrane to the underlying dermis. Several of these structural proteins are recognized by autoantibodies in pemphigoid diseases, a heterogeneous group of clinically and immunopathologically diverse entities. Targeted proteins include the two intracellular plakins, plectin isoform 1a and BP230 (also called bullous pemphigoid antigen (BPAG) 1 isoform e (BPAG1e)). Plectin 1a and BP230 are connected to the intermediate filaments and to the cell surface receptor α6β4 integrin, which in turn is connected to laminin 332, a component of the anchoring filaments. Further essential adhesion proteins are BP180, a transmembrane protein, laminin γ1 and type VII collagen. Latter protein is the major constituent of the anchoring fibrils. Mutations in the corresponding genes of these adhesion molecules lead to inherited epidermolysis bullosa emphasizing the importance of these proteins for the integrity of the dermal-epidermal junction. This review will provide an overview on the structure and function of the proteins situated in the dermal-epidermal junction targeted by autoantibodies.

Spectraplakin family proteins - cytoskeletal crosslinkers with versatile roles

The different cytoskeletal networks in a cell are responsible for many fundamental cellular processes. Current studies have shown that spectraplakins, cytoskeletal crosslinkers that combine features of both the spectrin and plakin families of crosslinkers, have a critical role in integrating these different cytoskeletal networks. Spectraplakin genes give rise to a variety of isoforms that have distinct functions. Importantly, all spectraplakin isoforms are uniquely able to associate with all three elements of the cytoskeleton, namely, F-actin, microtubules and intermediate filaments. In this Review, we will highlight recent studies that have unraveled their function in a wide range of different processes, from regulating cell adhesion in skin keratinocytes to neuronal cell migration. Taken together, this work has revealed a diverse and indispensable role for orchestrating the function of different cytoskeletal elements in vivo.

BPAG1, a distinctive role in skin and neurological diseases

Spectraplakins are multifunctional cytoskeletal linker proteins that act as important communicators, connecting cytoskeletal components with each other and to cellular junctions. Bullous pemphigoid antigen 1 (BPAG1)/dystonin is a member of spectraplakin family and expressed in various tissues. Alternative splicing of BPAG1 gene produces various isoforms with unique structure and domains. BPAG1 plays crucial roles in numerous biological processes, such as cytoskeleton organization, cell polarization, cell adhesion, and cell migration as well as signaling transduction. Genetic mutation of BPAG1 isoforms is the miscreant of epidermolysis bullosa and multifarious, destructive neurological diseases. In this review, we summarize the recent advances of BPAG1's role in various biological processes and in skin and neurological diseases.

Recently Identified Forms of Epidermolysis Bullosa

Epidermolysis bullosa (EB) comprises a collection of clinically diverse inherited blistering diseases that affect the skin and, in some subtypes, mucous membranes and other organs. Currently classified into four main subtypes (EB simplex, junctional EB, dystrophic EB, and Kindler syndrome, mainly based on the level of skin cleavage), the spectrum of EB extends to more than 30 clinical subtypes with pathogenic mutations in at least 18 distinct genes. This review focuses on three recent additions to variants of EB: all are autosomal recessive, and result from mutations in either DST-e (coding for epidermal dystonin, also known as the 230 kDa bullous pemphigoid antigen, BP230), EXPH5 (coding for exophilin-5, also known as Slac2-b), or ITGA3 (coding for the integrin alpha-3 subunit). Each of these new forms of EB is reviewed with respect to the initial gene discovery, clinical features, the current mutation database, and skin pathology. Awareness of these recently described forms of EB is helpful in the clinical evaluation of patients with EB and in defining genotype-phenotype correlation for inherited blistering skin diseases.

Molecular architecture and function of the hemidesmosome

Hemidesmosomes are multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. The mechanical stability of hemidesmosomes relies on multiple interactions of a few protein components that form a membrane-embedded tightly-ordered complex. The core of this complex is provided by integrin α6β4 and P1a, an isoform of the cytoskeletal linker protein plectin that is specifically associated with hemidesmosomes. Integrin α6β4 binds to the extracellular matrix protein laminin-332, whereas P1a forms a bridge to the cytoplasmic keratin intermediate filament network. Other important components are BPAG1e, the epithelial isoform of bullous pemphigoid antigen 1, BPAG2, a collagen-type transmembrane protein and CD151. Inherited or acquired diseases in which essential components of the hemidesmosome are missing or structurally altered result in tissue fragility and blistering. Modulation of hemidesmosome function is of crucial importance for a variety of biological processes, such as terminal differentiation of basal keratinocytes and keratinocyte migration during wound healing and carcinoma invasion. Here, we review the molecular characteristics of the proteins that make up the hemidesmosome core structure and summarize the current knowledge about how their assembly and turnover are regulated by transcriptional and post-translational mechanisms.

Founder mutation in dystonin-e underlying autosomal recessive epidermolysis bullosa simplex in Kuwait

Only two homozygous nonsense mutations in the epidermal isoform of the dystonin gene, DST-e, have been reported previously in autosomal recessive epidermolysis bullosa simplex (EBS); the affected pedigrees were Kuwaiti and Iranian. This subtype of EBS is therefore considered to be a rare clinicopathological entity. In this study, we identified four seemingly unrelated Kuwaiti families in which a total of seven individuals had predominantly acral trauma-induced blistering since infancy. All affected individuals were homozygous for the mutation p.Gln1124* in DST-e, the same mutation that was identified in the originally reported family from Kuwait. Haplotype analysis in the five pedigrees (including the previous case) revealed a shared block of ~60 kb of genomic DNA across the site of the mutation, consistent with a founder effect. Most heterozygotes had no clinical abnormalities although one subject had mild transient skin fragility during childhood, an observation noted in the previously reported Iranian pedigree, suggesting that the condition may also be semidominant in some pedigrees rather than purely autosomal recessive. Our study reveals propagation of a mutant ancestral allele in DST-e throughout Kuwait, indicating that this subtype of EBS may be more common in Kuwait, and perhaps other Middle Eastern countries, than is currently appreciated.

Blistering disease: insight from the hemidesmosome and other components of the dermal-epidermal junction

The hemidesmosome is a specialized transmembrane complex that mediates the binding of epithelial cells to the underlying basement membrane. In the skin, this multiprotein structure can be regarded as the chief adhesion unit at the site of the dermal-epidermal junction. Focal adhesions are additional specialized attachment structures located between hemidesmosomes. The integrity of the skin relies on well-assembled and functional hemidesmosomes and focal adhesions (also known as integrin adhesomes). However, if these adhesion structures are impaired, e.g., as a result of circulating autoantibodies or inherited genetic mutations, the mechanical strength of the skin is compromised, leading to blistering and/or tissue inflammation. A particular clinical presentation emerges subject to the molecule that is targeted. None of these junctional complexes are simply compounds of adhesion molecules; they also play a significant role in signalling pathways involved in the differentiation and migration of epithelial cells such as during wound healing and in tumour invasion. We summarize current knowledge about hereditary and acquired blistering diseases emerging from pathologies of the hemidesmosome and its neighbouring proteins as components of the dermal-epidermal junction.

Molecular architecture and function of the hemidesmosome

Hemidesmosomes are multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. The mechanical stability of hemidesmosomes relies on multiple interactions of a few protein components that form a membrane-embedded tightly-ordered complex. The core of this complex is provided by integrin α6β4 and P1a, an isoform of the cytoskeletal linker protein plectin that is specifically associated with hemidesmosomes. Integrin α6β4 binds to the extracellular matrix protein laminin-332, whereas P1a forms a bridge to the cytoplasmic keratin intermediate filament network. Other important components are BPAG1e, the epithelial isoform of bullous pemphigoid antigen 1, BPAG2, a collagen-type transmembrane protein and CD151. Inherited or acquired diseases in which essential components of the hemidesmosome are missing or structurally altered result in tissue fragility and blistering. Modulation of hemidesmosome function is of crucial importance for a variety of biological processes, such as terminal differentiation of basal keratinocytes and keratinocyte migration during wound healing and carcinoma invasion. Here, we review the molecular characteristics of the proteins that make up the hemidesmosome core structure and summarize the current knowledge about how their assembly and turnover are regulated by transcriptional and post-translational mechanisms.

BPAG1a and b associate with EB1 and EB3 and modulate vesicular transport, Golgi apparatus structure, and cell migration in C2.7 myoblasts

BPAG1a and BPAG1b (BPAG1a/b) constitute two major isoforms encoded by the dystonin (Dst) gene and show homology with MACF1a and MACF1b. These proteins are members of the plakin family, giant multi-modular proteins able to connect the intermediate filament, microtubule and microfilament cytoskeletal networks with each other and to distinct cell membrane sites. They also serve as scaffolds for signaling proteins that modulate cytoskeletal dynamics. To gain better insights into the functions of BPAG1a/b, we further characterized their C-terminal region important for their interaction with microtubules and assessed the role of these isoforms in the cytoskeletal organization of C2.7 myoblast cells. Our results show that alternative splicing does not only occur at the 5′ end of Dst and Macf1 pre-mRNAs, as previously reported, but also at their 3′ end, resulting in expression of additional four mRNA variants of BPAG1 and MACF1. These isoform-specific C-tails were able to bundle microtubules and bound to both EB1 and EB3, two microtubule plus end proteins. In the C2.7 cell line, knockdown of BPAG1a/b had no major effect on the organization of the microtubule and microfilament networks, but negatively affected endocytosis and maintenance of the Golgi apparatus structure, which became dispersed. Finally, knockdown of BPAG1a/b caused a specific decrease in the directness of cell migration, but did not impair initial cell adhesion. These data provide novel insights into the complexity of alternative splicing of Dst pre-mRNAs and into the role of BPAG1a/b in vesicular transport, Golgi apparatus structure as well as in migration in C2.7 myoblasts.

Interaction of plectin with keratins 5 and 14: dependence on several plectin domains and keratin quaternary structure

Plectin, a cytolinker of the plakin family, anchors the intermediate filament (IF) network formed by keratins 5 and 14 (K5/K14) to hemidesmosomes, junctional adhesion complexes in basal keratinocytes. Genetic alterations of these proteins cause epidermolysis bullosa simplex (EBS) characterized by disturbed cytoarchitecture and cell fragility. The mechanisms through which mutations located after the documented plectin IF-binding site, composed of the plakin-repeat domain (PRD) B5 and the linker, as well as mutations in K5 or K14, lead to EBS remain unclear. We investigated the interaction of plectin C terminus, encompassing four domains, the PRD B5, the linker, the PRD C, and the C extremity, with K5/K14 using different approaches, including a rapid and sensitive fluorescent protein-binding assay, based on enhanced green fluorescent protein-tagged proteins (FluoBACE). Our results demonstrate that all four plectin C-terminal domains contribute to its association with K5/K14 and act synergistically to ensure efficient IF binding. The plectin C terminus predominantly interacted with the K5/K14 coil 1 domain and bound more extensively to K5/K14 filaments compared with monomeric keratins or IF assembly intermediates. These findings indicate a multimodular association of plectin with K5/K14 filaments and give insights into the molecular basis of EBS associated with pathogenic mutations in plectin, K5, or K14 genes.

[The many facets of inherited skin fragility]

The inherited skin fragility encompasses a heterogeneous group of disorders, collectively designated as epidermolysis bullosa, characterized by recurrent mechanically induced blisters, erosions or wounds. The spectrum of clinical manifestations is broad, as well as the molecular background. Besides the skin, mucosal membranes and other organs can be affected. In real-world practice, patients with mild genetic skin fragility usually do not require medical care and often remain underdiagnosed. In contrast, the well-defined severe EB subtypes are recognized based on typical clinical features. The molecular diagnostics is usually performed in order to allow genetic counselling and prenatal diagnosis. Besides wound care and careful management of the disease complications, new experimental targeted therapies are being developed. New very rare forms of inherited skin fragility have been identified with modern sequencing methods.

The genetics of skin fragility

Genetic skin fragility manifests with diminished resistance of the skin and mucous membranes to external mechanical forces and with skin blistering, erosions, and painful wounds as clinical features. Skin fragility disorders, collectively called epidermolysis bullosa, are caused by mutations in 18 distinct genes that encode proteins involved in epidermal integrity and dermal-epidermal adhesion. The genetic spectrum, along with environmental and genetic modifiers, creates a large number of clinical phenotypes, spanning from minor localized lesions to severe generalized blistering, secondary skin cancer, or early demise resulting from extensive loss of the epidermis. Laboratory investigations of skin fragility have greatly augmented our understanding of genotype-phenotype correlations in epidermolysis bullosa and have also advanced skin biology in general. Current translational research concentrates on the development of biologically valid treatments with therapeutic genes, cells, proteins, or small-molecule compounds in preclinical settings or human pilot trials.

Inherited blistering skin diseases: underlying molecular mechanisms and emerging therapies

A key function of human skin is the formation of a structural barrier against the external environment. In part, this is achieved through the formation of a cornified cell envelope derived from a stratified squamous epithelium attached to an epithelial basement membrane. Resilient in health, the structural integrity of skin can become impaired or break down in a collection of inherited skin diseases, referred to as the blistering genodermatoses. These disorders arise from inherited gene mutations in a variety of structural and signalling proteins and manifest clinically as blisters or erosions following minor skin trauma. In some patients, blistering can be severe resulting in significant morbidity. Furthermore, a number of these conditions are associated with debilitating extra-cutaneous manifestations including gastro-intestinal, cardiac, and ocular complications. In recent years, an improved understanding of the molecular basis of the blistering genodermatoses has led to better disease classification and genetic counselling. For patients, this has also advanced translational research with the advent of new clinical trials of gene, protein, cell, drug, and small molecule therapies. Although curing inherited blistering skin diseases still remains elusive, significant improvements in patients' quality of life are already being achieved.

Plakins, a versatile family of cytolinkers: roles in skin integrity and in human diseases

The plakin family consists of giant proteins involved in the cross-linking and organization of the cytoskeleton and adhesion complexes. They further modulate several fundamental biological processes, such as cell adhesion, migration, and polarization or signaling pathways. Inherited and acquired defects of plakins in humans and in animal models potentially lead to dramatic manifestations in the skin, striated muscles, and/or nervous system. These observations unequivocally demonstrate the key role of plakins in the maintenance of tissue integrity. Here we review the characteristics of the mammalian plakin members BPAG1 (bullous pemphigoid antigen 1), desmoplakin, plectin, envoplakin, epiplakin, MACF1 (microtubule-actin cross-linking factor 1), and periplakin, highlighting their role in skin homeostasis and diseases.

BPAG1-e restricts keratinocyte migration through control of adhesion stability

Bullous pemphigoid antigen 1 (BPAG1-e, also known as BP230) is a member of the plakin family of hemidesmosome cytoskeletal linker proteins that is encoded by an isoform of the dystonin (DST) gene. Recently, we reported two unrelated families with homozygous nonsense mutations in this DST isoform that led to ultrastructural loss of hemidesmosomal inner plaques and clinical features of trauma-induced skin fragility. We now demonstrate that keratinocytes isolated from these individuals have significant defects in adhesion, as well as increased cell spreading and migration. These mutant keratinocytes also display reduced levels of β4 integrins at the cell surface but increased total protein levels of keratin-14 and β1 integrins. These alterations in cell behavior and protein expression were not seen in control keratinocytes in which BPAG1-e expression had been silenced by stable expression of short hairpin RNA to target DST. The failure of knockdown approaches to recapitulate the changes in morphology, adhesion, and migration seen in patient cells therefore suggests such approaches are not appropriate to study loss of this protein in vivo. The contrasting findings in keratinocytes harboring naturally occurring mutations, however, demonstrate a previously unappreciated key role for BPAG1-e in regulating keratinocyte adhesion and migration and suggest a requirement for this protein in controlling functional switching between integrin types in epithelial cells.

New findings in genodermatoses

New technologies are accelerating the pace at which genetic defects leading to inherited skin disease are elucidated. Translation of these genetic discoveries into new therapies for patients with inherited skin diseases has not been as rapid but the pace is now accelerating. This article summarizes recent findings in genetic skin diseases, the scope of advances being made, the role of new DNA analysis technologies in these discoveries, as well as highlighting some examples of how an understanding of the genetic cause of inherited skin diseases can lead to therapeutic interventions for patients.

[New developments in hereditary blistering skin diseases]

Hereditary blistering skin diseases were described more than hundred years ago, but only the rapid scientific developments in molecular genetics in the last years have revealed the full spectrum of these diseases, delineated disease mechanisms and pointed to novel therapeutic strategies. Not only the classic forms of epidermolysis bullosa, but also new syndromic forms with multiorgan involvement, or skin fragility disorders that manifest with erosive, crusty lesions and pigment anomalies, instead of marked skin blistering belong to the group of hereditary blistering diseases. Understanding the biological functions of skin structures that provide intraepidermal and dermo-epidermal adhesion has furthered development of novel cell- and molecule-based therapies that are currently being tested in preclinical and clinical pilot trial settings.

Spectraplakins: master orchestrators of cytoskeletal dynamics

The dynamics of different cytoskeletal networks are coordinated to bring about many fundamental cellular processes, from neuronal pathfinding to cell division. Increasing evidence points to the importance of spectraplakins in integrating cytoskeletal networks. Spectraplakins are evolutionarily conserved giant cytoskeletal cross-linkers, which belong to the spectrin superfamily. Their genes consist of multiple promoters and many exons, yielding a vast array of differential splice forms with distinct functions. Spectraplakins are also unique in their ability to associate with all three elements of the cytoskeleton: F-actin, microtubules, and intermediate filaments. Recent studies have begun to unveil their role in a wide range of processes, from cell migration to tissue integrity.