Life-long course and molecular characterization of the original Dutch family with epidermolysis bullosa simplex with muscular dystrophy due to a homozygous novel plectin point mutation

Plectin Deficiency in Fibroblasts Deranges Intermediate Filament and Organelle Morphology, Migration, and Adhesion

Plectin, a highly versatile and multifunctional cytolinker, has been implicated in several multisystemic disorders. Most sequence variations in the human plectin gene (PLEC) cause epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), an autosomal recessive skin-blistering disorder associated with progressive muscle weakness. In this study, we performed a comprehensive cell biological analysis of dermal fibroblasts from three different patients with EBS-MD, where PLEC expression analyses revealed preserved mRNA levels in all cases, whereas full-length plectin protein content was significantly reduced or completely absent. Downstream effects of pathogenic PLEC sequence alterations included massive bundling of vimentin intermediate filament networks, including the occurrence of ring-like nuclei-encasing filament bundles, elongated mitochondrial networks, and abnormal nuclear morphologies. We found that essential fibroblast functions such as wound healing, migration, or orientation upon cyclic stretch were significantly impaired in the cells of patients with EBS-MD. Finally, EBS-MD fibroblasts displayed reduced adhesion capacities, which could be attributed to smaller focal adhesion contacts. Our study not only emphasizes plectin's functional role in human skin fibroblasts, it also provides further insights into the understanding of EBS-MD-associated disease mechanisms.

Mutation update: The spectra of PLEC sequence variants and related plectinopathies

Plectin, encoded by PLEC, is a cytoskeletal linker of intermediate filaments expressed in many cell types. Plectin consists of three main domains that determine its functionality: the N-terminal domain, the Rod domain, and the C-terminal domain. Molecular defects of PLEC correlating with the functional aspects lead to a group of rare heritable disorders, plectinopathies. These multisystem disorders include an autosomal dominant form of epidermolysis bullosa simplex (EBS-Ogna), limb-girdle muscular dystrophy (LGMD), aplasia cutis congenita (ACC), and an autosomal recessive form of EBS, which may associate with muscular dystrophy (EBS-MD), pyloric atresia (EBS-PA), and/or congenital myasthenic syndrome (EBS-MyS). In this study, genotyping of over 600 Iranian patients with epidermolysis bullosa by next-generation sequencing identified 15 patients with disease-causing PLEC variants. This mutation update analyzes the clinical spectrum of PLEC in our cohort and in the literature and demonstrates the relationship between PLEC genotype and phenotypic manifestations. This study has integrated our seven novel PLEC variants and phenotypic findings with previously published data totaling 116 variants to provide the most complete overview of pathogenic PLEC variants and related disorders.

Muscle-Related Plectinopathies

Plectin is a giant cytoskeletal crosslinker and intermediate filament stabilizing protein. Mutations in the human plectin gene (PLEC) cause several rare diseases that are grouped under the term plectinopathies. The most common disorder is autosomal recessive disease epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), which is characterized by skin blistering and progressive muscle weakness. Besides EBS-MD, PLEC mutations lead to EBS with nail dystrophy, EBS-MD with a myasthenic syndrome, EBS with pyloric atresia, limb-girdle muscular dystrophy type R17, or EBS-Ogna. In this review, we focus on the clinical and pathological manifestations caused by PLEC mutations on skeletal and cardiac muscle. Skeletal muscle biopsies from EBS-MD patients and plectin-deficient mice revealed severe dystrophic features with variation in fiber size, degenerative myofibrillar changes, mitochondrial alterations, and pathological desmin-positive protein aggregates. Ultrastructurally, PLEC mutations lead to a disorganization of myofibrils and sarcomeres, Z- and I-band alterations, autophagic vacuoles and cytoplasmic bodies, and misplaced and degenerating mitochondria. We also summarize a variety of genetically manipulated mouse and cell models, which are either plectin-deficient or that specifically lack a skeletal muscle-expressed plectin isoform. These models are powerful tools to study functional and molecular consequences of PLEC defects and their downstream effects on the skeletal muscle organization.

Whole-Transcriptome Analysis by RNA Sequencing for Genetic Diagnosis of Mendelian Skin Disorders in the Context of Consanguinity

BACKGROUND

Among the approximately 8000 Mendelian disorders, >1000 have cutaneous manifestations. In many of these conditions, the underlying mutated genes have been identified by DNA-based techniques which, however, can overlook certain types of mutations, such as exonic-synonymous and deep-intronic sequence variants. Whole-transcriptome sequencing by RNA sequencing (RNA-seq) can identify such mutations and provide information about their consequences.

METHODS

We analyzed the whole transcriptome of 40 families with different types of Mendelian skin disorders with extensive genetic heterogeneity. The RNA-seq data were examined for variant detection and prioritization, pathogenicity confirmation, RNA expression profiling, and genome-wide homozygosity mapping in the case of consanguineous families. Among the families examined, RNA-seq was able to provide information complementary to DNA-based analyses for exonic and intronic sequence variants with aberrant splicing. In addition, we tested the possibility of using RNA-seq as the first-tier strategy for unbiased genome-wide mutation screening without information from DNA analysis.

RESULTS

We found pathogenic mutations in 35 families (88%) with RNA-seq in combination with other next-generation sequencing methods, and we successfully prioritized variants and found the culprit genes. In addition, as a novel concept, we propose a pipeline that increases the yield of variant calling from RNA-seq by concurrent use of genome and transcriptome references in parallel.

CONCLUSIONS

Our results suggest that "clinical RNA-seq" could serve as a primary approach for mutation detection in inherited diseases, particularly in consanguineous families, provided that tissues and cells expressing the relevant genes are available for analysis.

Four Individuals with a Homozygous Mutation in Exon 1f of the PLEC Gene and Associated Myasthenic Features

We identified the known c.1_9del mutation in the PLEC gene in four unrelated females from consanguineous families of Turkish origin. All individuals presented with slowly progressive limb-girdle weakness without any dermatological findings, and dystrophic changes observed in their muscle biopsies. Additionally, the neurological examination revealed ptosis, facial weakness, fatigability, and muscle cramps in all four cases. In two patients, repetitive nerve stimulation showed a borderline decrement and a high jitter was detected in all patients by single-fiber electromyography. Clinical improvement was observed after treatment with pyridostigmine and salbutamol was started. We further characterize the phenotype of patients with limb-girdle muscular dystrophy R17 clinically, by muscle magnetic resonance imaging (MRI) features and by describing a common 3.8 Mb haplotype in three individuals from the same geographical region. In addition, we review the neuromuscular symptoms associated with PLEC mutations and the role of plectin in the neuromuscular junction.

Epidermolysis bullosa simplex with muscular dystrophy. Review of the literature and a case report

BACKGROUND

Epidermolysis bullosa simplex associated with muscular dystrophy is a genetic skin disease caused by plectin deficiency. A case of a 19-year-old Czech patient affected with this disease and a review all previously published clinical cases are presented.

MAIN OBSERVATIONS

In our patient, skin signs of the disease developed after birth. Bilateral ptosis at the age of 8 years was considered as the first specific symptom of muscular dystrophy. Since then, severe scoliosis, urological and psychiatric complication have quickly developed. The signs of plectin deficiency were found by histopathological studies, electron microscopy and antigen mapping of the skin and muscular samples. Two autosomal recessive mutations in the plectin gene leading to premature termination codon were disclosed by mutation analysis. By review of all published clinical cases, 49 patients with this disease were found. 54 different mutations in the plectin gene were published, p.(Arg2319*) in exon 31 being the most frequently found. Median age of muscular dystrophy development was 9.5 years. Hoarseness and respiratory complications were the most often complications beside skin involvement.

CONCLUSION

Epidermolysis bullosa simplex with muscular dystrophy was diagnosed based on clinical, histopathological (skin and muscle biopsy) and mutation analysis of the plectin gene. Overview of the genetic and clinical characteristic of this disease could be presented by review of all previously published clinical cases.

Plectin-related skin diseases

Plectin has been characterized as a linker protein that is expressed in many cell types and is distinctive in various isoforms in the N-terminus and around the rod domain due to complicated alternative splicing of PLEC, the gene encoding plectin. Plectin deficiency causes autosomal recessive epidermolysis bullosa simplex (EBS) with involvement of the skin and other organs, such as muscle and gastrointestinal tract, depending on the expression pattern of the defective protein. In addition, a point mutation in the rod domain of plectin leads to autosomal dominant EBS, called as EBS-Ogna. Plectin can be targeted by circulating autoantibodies in subepidermal autoimmune blistering diseases. This review summarizes plectin-related skin diseases, from congenital to autoimmune disorders.

The many faces of plectin and plectinopathies: pathology and mechanisms

Plectin, a giant multifunctional cytolinker protein, plays a crucial role in stabilizing and orchestrating intermediate filament networks in cells. Mutations in the human plectin gene result in multiple diseases manifesting with muscular dystrophy, skin blistering, and signs of neuropathy. The most common disease caused by plectin deficiency is epidermolysis bullosa simplex (EBS)-MD, a rare autosomal-recessive skin blistering disorder with late-onset muscular dystrophy. EBS-MD patients and plectin-deficient mice display pathologic desmin-positive protein aggregates, degenerated myofibrils, and mitochondrial abnormalities, the hallmarks of myofibrillar myopathies. In addition to EBS-MD, plectin mutations have been shown to cause EBS-MD with a myasthenic syndrome, limb-girdle muscular dystrophy type 2Q, EBS with pyloric atresia, and EBS-Ogna. This review focuses on clinical and pathological manifestations of these plectinopathies. It addresses especially plectin's role in skeletal muscle, where a loss of muscle fiber integrity and profound changes of myofiber cytoarchitecture are observed in its absence. Furthermore, the highly complex genetic and molecular structure of plectin is discussed; a high number of differentially spliced exons give rise to a variety of different isoforms, which fulfill distinct functions in different cell types and tissues. Plectin's abilities to act as a dynamic organizer of intermediate filament networks and to interact with a multitude of different interaction partners are the basis for its function as a scaffolding platform for proteins involved in signaling. Finally, the article addresses a series of genetically manipulated mouse lines that were generated to serve as powerful models to study functional and molecular consequences of plectin gene defects.

Plectin expression patterns determine two distinct subtypes of epidermolysis bullosa simplex

Plectin is a cytoskeletal linker protein that has a dumbbell-like structure with a long central rod and N- and C-terminal globular domains. Mutations in the gene encoding plectin (PLEC1) cause two distinct autosomal recessive subtypes of epidermolysis bullosa (EB): EB simplex with muscular dystrophy (EBS-MD), and EB simplex with pyloric atresia (EBS-PA). Here, we demonstrate that normal human fibroblasts express two different plectin isoforms including full-length and rodless forms of plectin. We performed detailed analysis of plectin expression patterns in six EBS-MD and three EBS-PA patients. In EBS-PA, expression of all plectin domains was found to be markedly attenuated or completely lost; in EBS-MD, the expression of the N- and C-terminal domains of plectin remained detectable, although the expression of rod domains was absent or markedly reduced. Our data suggest that loss of the full-length plectin isoform with residual expression of the rodless plectin isoform leads to EBS-MD, and that complete loss or marked attenuation of full-length and rodless plectin expression underlies the more severe EBS-PA phenotype. These results also clearly account for the majority of EBS-MD PLEC1 mutation restriction within the large exon 31 that encodes the plectin rod domain, whereas EBS-PA PLEC1 mutations are generally outside exon 31.

Epidermolysis bullosa care in Scandinavia

The recessive forms of epidermolysis bullosa (EB) are common in Scandinavia, especially in the northern parts of Norway and Sweden. The daily care of EB in Scandinavia is organized around the patient via settings at the local hospital or health service. However, the diagnosis of EB and providing correct patient/family information usually require a specialized service. Specialized EB care in Scandinavia is mainly provided by dermatologists, pediatricians, and dentists working together in a team. The increasing number of EB families with foreign ethnic backgrounds and language problems is a challenge to the health service, especially in Sweden, and demands increased facilities. Also, the high expectations by parents of children with junctional EB and recessive dystrophic EB about new, revolutionizing therapies are challenges that can only be met by international collaboration and more research in specialized centers for EB. A close collaboration with patient organizations and various charity organizations will be very helpful in this respect.

Antiplectin autoantibodies in subepidermal blistering diseases

BACKGROUND

Hemidesmosomal proteins may become targets of autoimmunity in subepidermal blistering diseases. Well-known recognized autoantigens are the intracellular plaque protein BP230, the transmembrane BP180 and its shed ectodomain LAD-1.

OBJECTIVES

To establish the prevalence of autoimmunity against plectin, another intracellular plaque protein, and to investigate its antigenic sites.

METHODS

Two hundred and eighty-two patients with subepidermal blistering diseases, investigated by routine immunoblot analysis for possible antiplectin antibodies, were included in the study. Epitope mapping was performed using recombinantly produced overlapping plectin domains from the actin-binding domain to the rod domain. The COOH-terminal region of plectin was not included in the study.

RESULTS

In 11 of 282 (3.9%) patients an immunoblot staining pattern identical to that of antiplectin monoclonal antibody HD121 was found. Affinity-purified antibodies bound back to normal human skin in a pattern typical for plectin, i.e. to the epidermal basement membrane zone as well as to keratinocytes in the epidermis, and to myocytes. No binding was seen to plectin-deficient skin of a patient with epidermolysis bullosa simplex with muscular dystrophy. Epitope mapping of the plectin molecule showed that the central coiled-coil rod domain is an immunodominant hotspot as 92% of the sera with antiplectin antibodies reacted with it. Most patients with antiplectin antibodies also had antibodies to other pemphigoid antigens.

CONCLUSIONS

Plectin is a minor pemphigoid antigen with an immunodominant epitope located on the central rod domain.

Fibroblast heterogeneity in collagenolytic response to colchicine

Matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) are important in various physiological and pathological conditions, including those that involve homeostasis of collagen. Drug induced regulation of MMP-1, other MMPs and TIMPs is critical in treatment of various diseases, e.g. the use of the plant alkaloid, colchicine. One possible factor that might explain the failure in colchicine-treatment of some patients is interindividual variability on the cellular level. To investigate the possible individual heterogeneity in response to colchicine, we studied the effect of colchicine-induced synthesis of collagenase from 32 different human skin fibroblast strains derived from both healthy individuals as well as individuals with different skin diseases. We showed that colchicine induced an increased synthesis of collagenase in 22 of 32 cases. This heterogeneity occurred in fibroblasts from healthy as well as diseased individuals. To determine if colchicine also affected the fibroblast synthesis of gelatinase, stromelysin and tissue inhibitors of MMPs, we investigated several individuals from a single family. The results showed that both colchicine responsive and non-responsive fibroblasts with respect to collagenase synthesis responded to colchicine by an increased stromelysin synthesis, while the synthesis of gelatinase and TIMP-1 were unaffected. As a whole, our results indicate that individual heterogeneity in collagenase response to colchicine treatment may partly explain some of the controversial results obtained with colchicine as a drug.

Plectin deficient epidermolysis bullosa simplex with 27-year-history of muscular dystrophy

BACKGROUND

Epidermolysis bullosa simplex associated with muscular dystrophy is caused by plectin deficiency.

OBJECTIVE

To report clinical, immunohistochemical, ultrastructural and molecular features of a 52-year-old Japanese patient affected with this disease, whose muscular disease had been followed-up for 27 years.

METHODS

We performed histopathological study, immunofluorescence, electron microscopic study and mutation detection analysis for plectin.

RESULTS

The patient developed blisters and erosions followed by nail deformity on the traumatized regions from birth. The skin lesions were continuously developed to date. The histopathological study showed subepidermal blister. Electron microscopic study showed blister formation inside the basal cells at the level just above the attachment plaque of hemidesmosome. Immunofluorescence showed complete loss of staining to plectin. The mutation analysis using protein truncation test and DNA sequencing revealed a C-to-T transition at nucleotide position 7006 of the plectin cDNA sequence, which lead a novel homozygous nonsense mutation (R2319X).

CONCLUSION

From the above results, the diagnosis of epidermolysis bullosa simplex associated with muscular dystrophy was made. Slight muscular dystrophy was noticed at the age of 25 years. The muscular dystrophy gradually progressed and she could not walk at the age of 46 years. However, she can still breathe and swallow by herself. This is the patient of this disease with the longest follow-up, and may indicate the slow progress of muscular condition of this disease.