Mutations in genes encoding desmosomal proteins have been reported to cause arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D), an autosomal-dominant disease characterised by progressive myocardial atrophy with fibro-fatty replacement. We screened 112 ARVC/D probands for mutations in desmocollin-2 (DSC2) gene and detected two different amino-acid substitutions (p.E102K, p.I345T) and a frameshift variation (p.A897KfsX4) in 7 (6.2%) patients. DSC2a variant p.A897KfsX4, previously reported as a p.E896fsX900 mutation, was identified in five unrelated probands. Four of them were found to carry one or two mutations in different ARVC/D genes. Unexpectedly, p.A897KfsX4 variation was also found in 6 (1.5%) out of 400 control chromosomes. In vitro functional studies showed that, unlike wild-type DSC2a, this C-terminal mutated protein was localised in the cytoplasm. p.A897KfsX4 variation affects the last five amino acids of the DSC2a isoform but not of DSC2b. In contrast with what we found in other human tissues, in the heart DSC2b is more expressed than DSC2a, suggesting that relative deficiency of DSC2a might be compensated by isoform b. In conclusion, DSC2 gene mutations are not frequently involved in ARVC/D. The p.A897KfsX4 variation, identified in several Italian healthy control subjects, which affects only one of the two DSC2 isoforms, may be considered a rare variant, though possibly affecting phenotypic expression of concomitant ARVC/D mutations.
sudden death; ARVC/D; desmosome; DSC2; polymorphism
To define the genetic basis of arrhythmogenic right ventricular cardiomyopathy.
Arrhythmogenic right ventricular cardiomyopathy (ARVC), characterized by right ventricular fibrofatty replacement and arrhythmias, causes sudden death. Autosomal dominant Inheritance, reduced penetrance, and 7 desmosome-encoding causative genes are known. The basis of low penetrance is poorly understood.
ARVC probands and family members were enrolled, blood obtained, lymphoblastoid cell lines immortalized, DNA extracted, PCR amplification of desmosome-encoding genes performed, PCR products sequenced and diseased tissue samples studied for intercellular junction protein distribution using confocal immunofluorescence microscopy and antibodies against key proteins.
We identified 21 variants in plakophilin-2 (PKP2) in 38 of 198 probands (19%), including missense, nonsense, splice site, and deletion/insertion mutations. Pedigrees showed wide intra-familial variability (severe early-onset disease to asymptomatic individuals). In 9/38 probands, PKP2 variants were identified that were encoded in trans (compound heterozygosity). The 38 probands hosting PKP2 variants were screened for other desmosomal genes mutations; second variants (digenic heterozygosity) were identified in 16/38 subjects with PKP2 variants (42%) including desmoplakin (DSP, n=6), desmoglein-2 (DSG2, n=5), plakophilin-4 (PKP4, n=1), and desmocollin-2 (DSC2, n=1). Heterozygous mutations in non-PKP 2desmosomal genes occurred in 14/198 subjects (7%), including DSP (n=4), DSG2 (n=5), DSC2 (n=3), and junctional plakoglobin (JUP, n=2). All variants occurred in conserved regions; none were identified in 700 ethnic-matched controls.
Immunohistochemical analysis demonstrated abnormalities of protein architecture.
These data suggest that the genetic basis of ARVC includes reduced penetrance with compound and digenic heterozygosity. Disturbed junctional cytoarchitecture in subjects with desmosomal mutations confirms that ARVC is a disease of the desmosome and cell junction.
Arrhythmias; Cardiomyopathies; Desmosomes; Intercalated Disks; Genetic Mutations
Mutations in genes encoding desmosomal proteins have been reported to cause arrhythmogenic right ventricular cardiomyopathy (ARVC), an autosomal dominant disease characterised by progressive myocardial atrophy with fibro-fatty replacement.
We screened 54 ARVC probands for mutations in desmocollin-2 (DSC2), the only desmocollin isoform expressed in cardiac tissue.
Mutation screening was performed by denaturing high-performance liquid chromatography and direct sequencing.
To evaluate the pathogenic potentials of the DSC2 mutations detected in patients affected with ARVC, full-length wild-type and mutated cDNAs were cloned in eukaryotic expression vectors to obtain a fusion protein with green fluorescence protein (GFP); constructs were transfected in neonatal rat cardiomyocytes and in HL-1 cells.
We identified two heterozygous mutations (c.304G>A (p.E102K) and c.1034T>C (p.I345T)) in two probands and in four family members. The two mutations p.E102K and p.I345T map to the N-terminal region, relevant to adhesive interactions.
In vitro functional studies demonstrated that, unlike wild-type DSC2, the two N-terminal mutants are predominantly localised in the cytoplasm.
The two missense mutations in the N-terminal domain affect the normal localisation of DSC2, thus suggesting the potential pathogenic effect of the reported mutations. Identification of additional DSC2 mutations associated with ARVC may result in increased diagnostic accuracy with implications for genetic counseling.
To review recent developments in clinical aspects, molecular geneticsand pathogenesis of arrhythmogenic right ventricular cardiomyopathy (ARVC).
ARVC is a primary disease of the myocardium characterized by fibro-adipocytic replacement of myocytes, predominantly in the right ventricle.
Phenotypic expression of ARVC is variable and a significant number of patients may exhibit a subtle phenotype, particularly in the early stages of the disease. Mutations in DSP, JUP, PKP2, DSG2 and DSC2; encoding desmosomal proteins desmoplakin (DP), plakoglobin (PG), plakophilin 2 (PKP2), desmoglein 2 (DSG2), and desmocollin 2 (DSC2), respectively, cause ARVC. Thus, ARVC, at least in a subset, is a disease of desmosomes. In addition, mutations in TMEM43 and TGFB1 have been associated with ARVC. Mechanistic studies indicate that suppressed canonical Wnt signaling, imposed by nuclear PG, is the responsible mechanism for the pathogenesis of ARVC. It leads to the differentiation of a subset of second heart field cardiac progenitor cells at the epicardium to adipocytes due to enhanced expression of adipogenic factors. This mechanism explains the predominant involvement of the right ventricle in ARVC. Hence, ARVC is the first identified disease of disrupted differentiation of cardiac progenitor cells.
Advances in molecular genetics and the pathogenesis of ARVC could afford the opportunity for a genetic-based diagnosis and development of novel diagnostic markers and therapeutic targets aimed to prevent, attenuate and reverse the evolving phenotype.
Cardiomyopathy; Genetics; Sudden death; Heart failure; Stem cells
Immunoreactive signal for the desmosomal protein plakoglobin (γ-catenin) is reduced at cardiac intercalated disks in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC), a highly arrhythmogenic condition caused by mutations in genes encoding desmosomal proteins. Previously, we observed a “false positive” case in which plakoglobin signal was reduced in a patient initially thought to have ARVC but who actually had cardiac sarcoidosis. Sarcoidosis can masquerade clinically as ARVC, but has not previously been associated with altered desmosomal proteins.
Methods and Results
We observed marked reduction in immunoreactive signal for plakoglobin at cardiac myocyte junctions in patients with sarcoidosis and giant cell myocarditis, both highly arrhythmogenic forms of myocarditis associated with granulomatous inflammation. In contrast, plakoglobin signal was not depressed in lymphocytic (non-granulomatous) myocarditis. To determine whether cytokines might promote dislocation of plakoglobin from desmosomes, we incubated cultures of neonatal rat ventricular myocytes with selected inflammatory mediators. Brief exposure to low concentrations of IL-17, TNFα and IL-6, cytokines implicated in granulomatous myocarditis, caused translocation of plakoglobin from cell-cell junctions to intracellular sites, whereas other potent cytokines implicated in non-granulomatous myocarditis had no effect, even at much high concentrations. We also observed myocardial expression of IL-17 and TNFα, and elevated serum levels of inflammatory mediators including IL-6R, IL-8, MCP1 and MIP1β in ARVC patients (all p<0.0001 compared with controls).
These results suggest novel disease mechanisms involving desmosomal proteins in granulomatous myocarditis and implicate cytokines, perhaps derived in part from the myocardium, in disruption of desmosomal proteins and arrhythmogenesis in ARVC.
plakoglobin; desmosome; sarcoidosis; giant cell myocarditis; cytokines
Arrhythmogenic right ventricular Dysplasia/cardiomyopathy (ARVD/C) is an autosomal dominant inherited cardiomyopathy associated with ventricular arrhythmia, heart failure and sudden death. Genetic studies have demonstrated the central role of desmosomal proteins in this disease, where 50% of patients harbor a mutation in a desmosmal gene. However, clinical diagnosis of the disease remains difficult and molecular mechanisms appears heterogeneous and poorly understood. The aim of this study was to characterize the expression profile of desmosomal proteins in explanted ARVD/C heart samples, in order to identify common features of the disease.
Methods and Results
We examined plakophilin-2, desmoglein-2, desmocollin-2, plakoglobin and β-catenin protein expression levels from seven independent ARVD/C heart samples compared to two ischemic, five dilated cardiomyopathy and one healthy heart sample as controls. Ventricular and septum sections were examined by immunoblot analysis of total heart protein extracts and by immunostaining.
Immunoblots indicated significant decreases in desmoglein-2 and desmocollin-2, independent of any known underlying mutations, whereas immune-histochemical analysis showed normal localization of all desmosomal proteins. Quantitative RT-PCR revealed normal DSG2 and DSC2 mRNA transcript levels, suggesting increased protein turn-over rather than transcriptional down regulation.
Reduced cardiac desmoglein-2 and desmocollin-2 levels appear to be specifically associated with ARVD/C, independent of underlying mutations. These findings highlight a key role of desmosomal cadherins in the pathophysiology of ARVD/C. Whether these reductions could be considered as specific markers for ARVD/C requires replication analysis.
Adherens junctions and desmosomes are intercellular adhesive junctions and essential for the morphogenesis, differentiation, and maintenance of tissues that are subjected to high mechanical stress, including heart and skin. The different junction complexes are organized at the termini of the cardiomyocyte called the intercalated disc. Disruption of adhesive integrity via mutations in genes encoding desmosomal proteins causes an inherited heart disease, arrhythmogenic right ventricular cardiomyopathy (ARVC). Besides plakoglobin, which is shared by adherens junctions and desmosomes, other desmosomal components, desmoglein-2, desmocollin-2, plakophilin-2, and desmoplakin are also present in ultrastructurally defined fascia adherens junctions of heart muscle, but not other tissues. This mixed-type of junctional structure is termed hybrid adhering junction or area composita. Desmosomal plakophilin-2 directly interacts with adherens junction protein alphaT-catenin, providing a new molecular link between the cadherin-catenin complex and desmosome. The area composita only exists in the cardiac intercalated disc of mammalian species suggesting that it evolved to strengthen mechanical coupling in the heart of higher vertebrates. The cross-talk among different junctions and their implication in the pathogenesis of ARVC are discussed in this review.
Cellular adhesion mediated by cardiac desmosomes is a prerequisite for proper electric propagation mediated by gap junctions in the myocardium. However, the molecular principles underlying this interdependence are not fully understood.
The purpose of this study was to determine potential causes of right ventricular conduction abnormalities in a patient with borderline diagnosis of arrhythmogenic right ventricular cardiomyopathy.
To assess molecular changes, the patient's myocardial tissue was analyzed for altered desmosomal and gap junction (connexin43) protein levels and localization. In vitro functional studies were performed to characterize the consequences of the desmosomal mutations.
Loss of plakoglobin signal was evident at the cell junctions despite expression of the protein at control levels. Although the distribution of connexin43 was not altered, total protein levels were reduced and changes in phosphorylation were observed. The truncation mutant in desmocollin-2a is deficient in binding plakoglobin. Moreover, the ability of desmocollin-2a to directly interact with connexin43 was abolished by the mutation. No pathogenic potential of the desmoglein-2 missense change was identified.
The observed abnormalities in gap junction protein expression and phosphorylation, which precede an overt cardiac phenotype, likely are responsible for slow myocardial conduction in this patient. At the molecular level, altered binding properties of the desmocollin-2a mutant may contribute to the changes in connexin43. In particular, the newly identified interaction between the desmocollin-2a isoform and connexin43 provides novel insights into the molecular link between desmosomes and gap junctions.
Cardiomyopathy; Conduction; Connexin43; Desmocollin-2; Desmoglein-2; Desmosome; Functional studies; Gap junction; Mutation; Plakoglobin; ARVC, arrhythmogenic right ventricular cardiomyopathy; Cx43, connexin43; DAPI, 4′,6-diamidino-2-phenylindole; DSC2, desmocollin-2; DSG2, desmoglein-2; DSP, desmoplakin; GFP, green fluorescent protein; GST, glutathione-S-transferase; ICS, intracellular cadherin segment; PG, plakoglobin; PKP2, plakophilin-2; RV, right ventricle; YFP, yellow fluorescent protein
Mutations in the plakoglobin (JUP) gene have been identified in arrhythmogenic right ventricular cardiomyopathy (ARVC) patients. However, the mechanisms underlying plakoglobin dysfunction involved in the pathogenesis of ARVC remain poorly understood. Plakoglobin is a component of both desmosomes and adherens junctions located at the intercalated disc (ICD) of cardiomyocytes, where it functions to link cadherins to the cytoskeleton. In addition, plakoglobin functions as a signaling protein via its ability to modulate the Wnt/β-catenin signaling pathway. To investigate the role of plakoglobin in ARVC, we generated an inducible cardiorestricted knockout (CKO) of the plakoglobin gene in mice. Plakoglobin CKO mice exhibited progressive loss of cardiac myocytes, extensive inflammatory infiltration, fibrous tissue replacement, and cardiac dysfunction similar to those of ARVC patients. Desmosomal proteins from the ICD were decreased, consistent with altered desmosome ultrastructure in plakoglobin CKO hearts. Despite gap junction remodeling, plakoglobin CKO hearts were refractory to induced arrhythmias. Ablation of plakoglobin caused increase β-catenin stabilization associated with activated AKT and inhibition of glycogen synthase kinase 3β. Finally, β-catenin/TCF transcriptional activity may contribute to the cardiac hypertrophy response in plakoglobin CKO mice. This novel model of ARVC demonstrates for the first time how plakoglobin affects β-catenin activity in the heart and its implications for disease pathogenesis.
Tail–tail interactions of desmoglein 2, promoted by its C-terminal unique region, inhibit its internalization, stabilizing it at the cell surface and promoting intercellular adhesion.
Desmosomal cadherins, desmogleins (Dsgs) and desmocollins, make up the adhesive core of intercellular junctions called desmosomes. A critical determinant of epithelial adhesive strength is the level and organization of desmosomal cadherins on the cell surface. The Dsg subclass of desmosomal cadherins contains a C-terminal unique region (Dsg unique region [DUR]) with unknown function. In this paper, we show that the DUR of Dsg2 stabilized Dsg2 at the cell surface by inhibiting its internalization and promoted strong intercellular adhesion. DUR also facilitated Dsg tail–tail interactions. Forced dimerization of a Dsg2 tail lacking the DUR led to decreased internalization, supporting the conclusion that these two functions of the DUR are mechanistically linked. We also show that a Dsg2 mutant, V977fsX1006, identified in arrhythmogenic right ventricular cardiomyopathy patients, led to a loss of Dsg2 tail self-association and underwent rapid endocytosis in cardiac muscle cells. Our observations illustrate a new mechanism desmosomal cadherins use to control their surface levels, a key factor in determining their adhesion and signaling roles.
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetic disorder related to mutations in desmosomal proteins. The current study tests the hypothesis that immunohistochemical staining for desmosomal proteins is of diagnostic utility by studying autopsy-confirmed cases of ARVC.
Methods and Results:
We studied 23 hearts from patients dying suddenly with ARVC. Control subject tissues were 21 hearts from people dying from non-cardiac causes (n=15), dilated cardiomyopathy (n=3) and coronary artery disease (n=3).
Areas free of fibrofatty change or scarring were assessed on 50 sections from ARVC (24 left ventricle, 26 right ventricle) and 28 sections from controls. Immunohistochemical stains against plakoglobin, plakophilin, desmoplakin, connexin-43, and N-cadherin were applied and area expression analyzed by computerized morphometry. Desmin was stained as a control for fixation and similarly analyzed.
The mean area of desmin expression was similar in controls and ARVC (86% vs. 85%, p=0.6). Plakoglobin expression was 4.9% ± 0.3% in controls, vs. 4.6% ± 0.3% in ARVC (p=0.3). Plakophilin staining was 4.8% ± 0.3% in controls vs. 4.4% ± 03% in ARVC (p=0.3). Desmoplakin staining was 3.4% in controls vs. 3.2 ± 0.2% in ARVC (p=0.6). There were no significant differences when staining was compared between right and left ventricles (all p > 0.1).
For non-desmosomal proteins, the mean area of connexin-43 staining showed no significant difference by presence of disease.
The small and insignificant decrease in junction protein expression in ARVC suggests that immunohistochemistry is not a useful tool for the diagnosis.
ARVC; arrhytmogenic cardiomyopaty; sudden death; autopsy.
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited heart muscle disease characterized by fibrofatty replacement of the myocardium and ventricular arrhythmias, associated with mutations in the desmosomal genes. Only a missense mutation in the DES gene coding for desmin, the intermediate filament protein expressed by cardiac and skeletal muscle cells, has been recently associated with ARVC. We screened 91 ARVC index cases (53 negative for mutations in desmosomal genes and an additional 38 carrying desmosomal gene mutations) for DES mutations. Two rare missense variants were identified. The heterozygous p.K241E substitution was detected in 1 patient affected with a severe form of ARVC who also carried the p.T816RfsX10 mutation in plakophilin-2 gene. This DES substitution, showing an allele frequency of <0.01 in the control population, is predicted to cause an intolerant amino acid change in a highly conserved protein domain. Thus, it can be considered a rare variant with a possible modifier effect on the phenotypic expression of the concomitant mutation. The previously known p.A213V substitution was identified in 1 patient with ARVC who was negative for mutations in the desmosomal genes. Because a greater prevalence of p.A213V has been reported in patients with heart dilation than in control subjects, the hypothesis that this rare variant could have an unfavorable effect on cardiac remodeling cannot be ruled out. In conclusion, our data help to establish that, in the absence of skeletal muscle involvement suggestive of a desminopathy, the probability of DES mutations in ARVC is very low. These findings have important implications in the mutation screening strategy for patients with ARVC.
In the past decade, an avalanche of findings and reports has correlated arrhythmogenic ventricular cardiomyopathies (ARVC) and Naxos and Carvajal diseases with certain mutations in protein constituents of the special junctions connecting the polar regions (intercalated disks) of mature mammalian cardiomyocytes. These molecules, apparently together with some specific cytoskeletal proteins, are components of (or interact with) composite junctions. Composite junctions contain the amalgamated fusion products of the molecules that, in other cell types and tissues, occur in distinct separate junctions, i.e. desmosomes and adherens junctions. As the pertinent literature is still in an expanding phase and is obviously becoming important for various groups of researchers in basic cell and molecular biology, developmental biology, histology, physiology, cardiology, pathology and genetics, the relevant references so far recognized have been collected and are presented here in the following order: desmocollin-2 (Dsc2, DSC2), desmoglein-2 (Dsg2, DSG2), desmoplakin (DP, DSP), plakoglobin (PG, JUP), plakophilin-2 (Pkp2, PKP2) and some non-desmosomal proteins such as transmembrane protein 43 (TMEM43), ryanodine receptor 2 (RYR2), desmin, lamins A and C, striatin, titin and transforming growth factor-β3 (TGFβ3), followed by a collection of animal models and of reviews, commentaries, collections and comparative studies.
Arrhythmogenic ventricular cardiomyopathy; Carvajal disease; Composite junction; Desmosomes; Intercalated disk; Naxos disease
Desmosomes are intercellular adhesive junctions of epithelial cells that contain two major transmembrane components, the desmogleins (Dsg) and desmocollins (Dsc), which are cadherin-type cell–cell adhesion molecules and are anchored to intermediate filaments of keratin through interactions with plakoglobin and desmoplakin. Desmosomes play an important role in maintaining the proper structure and barrier function of the epidermis and mucous epithelia. Four Dsg isoforms have been identified to date, Dsg1–Dsg4, and are involved in several skin and heart diseases. Dsg1 and Dsg3 are the two major Dsg isoforms in the skin and mucous membranes, and are targeted by IgG autoantibodies in pemphigus, an autoimmune disease of the skin and mucous membranes. Dsg1 is also targeted by exfoliative toxin (ET) released by Staphylococcus aureus in the infectious skin diseases bullous impetigo and staphylococcal scalded skin syndrome (SSSS). ET is a unique serine protease that shows lock and key specificity to Dsg1. Dsg2 is expressed in all tissues possessing desmosomes, including simple epithelia and myocardia, and mutations in this gene are responsible for arrhythmogenic right ventricular cardiomyopathy/dysplasia. Dsg4 plays an important adhesive role mainly in hair follicles, and Dsg4 mutations cause abnormal hair development. Recently, an active disease model for pemphigus was generated by a unique approach using autoantigen-deficient mice that do not acquire tolerance against the defective autoantigen. Adoptive transfer of Dsg3−/− lymphocytes into mice expressing Dsg3 induces stable anti-Dsg3 IgG production with development of the pemphigus phenotype. This mouse model is a valuable tool with which to investigate immunological mechanisms of harmful IgG autoantibody production in pemphigus. Further investigation of desmoglein molecules will continue to provide insight into the unsolved pathophysiological mechanisms of diseases and aid in the development of novel therapeutic strategies with minimal side effects.
cadherin; pemphigus; impetigo; SSSS; mouse model; ELISA
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a cardiac disease characterized by the presence of fibrofatty replacement of the right ventricular myocardium, which may cause ventricular arrhythmias and sudden cardiac death. Pathogenic mutations in several genes encoding mainly desmosomal proteins have been reported. Our aim is to perform genotype-phenotype correlations to establish the diagnostic value of genetics and to assess the role of mutation type in age-related penetrance in ARVC.
Methods and Results
Thirty unrelated Spanish patients underwent a complete clinical evaluation. They all were screened for PKP2, DSG2, DSC2, DSP, JUP and TMEM43 genes. A total of 70 relatives of four families were also studied. The 30 patients fulfilled definite disease diagnostic criteria. Genetic analysis revealed a pathogenic mutation in 19 patients (13 in PKP2, 3 in DSG2, 2 in DSP, and 1 in DSC2). Nine of these mutations created a truncated protein due to the generation of a stop codon. Familial assessment revealed 28 genetic carriers among family members. Stop-gain mutations were associated to a later age of onset of ARVC, without differences in the severity of the pathology.
Familial genetic analysis helps to identify the cause responsible for the pathology. In discrepancy with previous studies, the presence of a truncating protein does not confer a worse severity. This information could suggest that truncating proteins may be compensated by the normal allele and that missense mutations may act as poison peptides.
Desmocollin 1 (Dsc1) is part of a desmosomal cell adhesion receptor formed in terminally differentiating keratinocytes of stratified epithelia. The dsc1 gene encodes two proteins (Dsc1a and Dsc1b) that differ only with respect to their COOH-terminal cytoplasmic amino acid sequences. On the basis of in vitro experiments, it is thought that the Dsc1a variant is essential for assembly of the desmosomal plaque, a structure that connects desmosomes to the intermediate filament cytoskeleton of epithelial cells. We have generated mice that synthesize a truncated Dsc1 receptor that lacks both the Dsc1a- and Dsc1b-specific COOH-terminal domains. This mutant transmembrane receptor, which does not bind the common desmosomal plaque proteins plakoglobin and plakophilin 1, is integrated into functional desmosomes. Interestingly, our mutant mice did not show the epidermal fragility previously observed in dsc1-null mice. This suggests that neither the Dsc1a- nor the Dsc1b-specific COOH-terminal cytoplasmic domain is required for establishing and maintaining desmosomal adhesion. However, a comparison of our mutants with dsc1-null mice suggests that the Dsc1 extracellular domain is necessary to maintain structural integrity of the skin.
We sought to quantify the number and length of desmosomes, gap junctions, and adherens junctions in arrhythmogenic right ventricular cardiomyopathy (ARVC) and non-ARVC dogs, and to determine if ultrastructural changes existed.
Hearts from 8 boxer dogs afflicted with histopathologically confirmed ARVC and 6 dogs without ARVC were studied.
Quantitative transmission electron microscopy (TEM) and Western blot semi-quantification of α-actinin were used to study the intercalated disc and sarcomere of the right and left ventricles.
When ARVC dogs were compared to non-ARVC dogs reductions in the number of desmosomes (P = 0.04), adherens junctions (P = 0.04) and gap junctions (P = 0.02) were found. The number of gap junctions (P = 0.04) and adherens junctions (P = 0.04) also were reduced in the left ventricle, while the number of desmosomes was not (P = 0.88). A decrease in the length of desmosomal complexes within LV samples (P=0.04) was found. These findings suggested disruption of proteins providing attachment of the cytoskeleton to the intercalated disc. Immunoblotting did not demonstrate a quantitative reduction in the amount of α-actinin in ARVC afflicted samples. All boxers with ARVC demonstrated the presence of electron dense material originating from the Z band and extending into the sarcomere, apparently at the expense of the cytoskeletal structure.
These results emphasize the importance of structural integrity of the intercalated disc in the pathogenesis of ARVC. In addition, observed abnormalities in sarcomeric structure suggest a novel link between ARVC and the actin-myosin contractile apparatus.
Canine; ARVC; Boxer; electron microscopy; desmosome; intercalated disc; cardiac ultrastructure
Arrhythmic right ventricular cardiomyopathy (ARVC) is a hereditary heart muscle disease that causes sudden cardiac death (SCD) in young people. Almost half of ARVC patients have a mutation in genes encoding cell adhesion proteins of the desmosome, including plakoglobin (JUP). We previously reported that cardiac tissue-specific plakoglobin (PG) knockout (PG CKO) mice have no apparent conduction abnormality and survive longer than expected. Importantly, the PG homolog, β-catenin (CTNNB1), showed increased association with the gap junction protein connexin43 (Cx43) in PG CKO hearts. To determine whether β-catenin is required to maintain cardiac conduction in the absence of PG, we generated mice lacking both PG and β-catenin specifically in the heart (i.e., double knockout [DKO]). The DKO mice exhibited cardiomyopathy, fibrous tissue replacement, and conduction abnormalities resulting in SCD. Loss of the cadherin linker proteins resulted in dissolution of the intercalated disc (ICD) structure. Moreover, Cx43-containing gap junction plaques were reduced at the ICD, consistent with the arrhythmogenicity of the DKO hearts. Finally, ambulatory electrocardiogram monitoring captured the abrupt onset of spontaneous lethal ventricular arrhythmia in the DKO mice. In conclusion, these studies demonstrate that the N-cadherin-binding partners, PG and β-catenin, are indispensable for maintaining mechanoelectrical coupling in the heart.
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited genetic myocardial disease characterized by fibrofatty replacement of the myocardium and a predisposition to cardiac arrhythmias and sudden death. We evaluated the cardiomyopathy gene titin (TTN) as a candidate ARVC gene because of its proximity to an ARVC locus at position 2q32 and the connection of the titin protein to the transitional junction at intercalated disks.
Methods and Results
All 312 titin exons known to be expressed in human cardiac titin and the complete 3’ untranslated region were sequenced in 38 ARVC families. Eight unique TTN variants were detected in 7 families including a prominent Thr2896Ile mutation that showed complete segregation with the ARVC phenotype in one large family. The Thr2896IIe mutation maps within a highly conserved immunoglobulin-like fold (Ig10 domain), located in titin’s spring region. Native gel electrophoresis, NMR, intrinsic fluorescence, and proteolysis assays of wildtype and mutant Ig10 domains revealed that the Thr2896IIe exchange reduces the structural stability and increases the propensity towards degradation of the Ig10 domain. The phenotype of TTN variant carriers was characterized by history of sudden death (5/7 families), progressive myocardial dysfunction causing death or heart transplant (8/14 cases), frequent conduction disease (11/14), and incomplete penetrance (86%).
Our data provide evidence that titin mutations can cause ARVC, a finding that further expands the origin of the disease beyond desmosomal proteins. Structural impairment of the titin spring is a likely cause of ARVC and constitutes a novel mechanism underlying myocardial remodeling and sudden cardiac death.
cardiomyopathy; arrhythmia; death; sudden; genetics; arrhythmogenic right ventricular cardiomyopathy
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inheritable myocardial disorder associated with fibrofatty replacement of myocardium and ventricular arrhythmia. A subset of ARVC is categorized as Naxos disease, which is characterized by ARVC and a cutaneous disorder. A homozygous loss-of-function mutation of the Plakoglobin (Jup) gene, which encodes a major component of the desmosome and the adherens junction, had been identified in Naxos patients, although the underlying mechanism remained elusive. We generated Jup mutant mice by ablating Jup in cardiomyocytes. Jup mutant mice largely recapitulated the clinical manifestation of human ARVC: ventricular dilation and aneurysm, cardiac fibrosis, cardiac dysfunction and spontaneous ventricular arrhythmias. Ultra-structural analyses revealed that desmosomes were absent in Jup mutant myocardia, whereas adherens junctions and gap junctions were preserved. We found that ventricular arrhythmias were associated with progressive cardiomyopathy and fibrosis in Jup mutant hearts. Massive cell death contributed to the cardiomyocyte dropout in Jup mutant hearts. Despite the increase of β-catenin at adherens junctions in Jup mutant cardiomyoicytes, the Wnt/β-catenin-mediated signaling was not altered. Transforming growth factor-beta-mediated signaling was found significantly elevated in Jup mutant cardiomyocytes at the early stage of cardiomyopathy, suggesting an important pathogenic pathway for Jup-related ARVC. These findings have provided further insights for the pathogenesis of ARVC and potential therapeutic interventions.
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a familial cardiac disease characterized by ventricular arrhythmias and sudden cardiac death. It is most frequently inherited as an autosomal dominant trait with incomplete and age-related penetrance and variable clinical expression. The human disease is most commonly associated with a causative mutation in one of several genes encoding desmosomal proteins.
We have previously described a spontaneous canine model of ARVC in the boxer dog. We phenotyped adult boxer dogs for ARVC by performing physical examination, echocardiogram and ambulatory electrocardiogram. Genome-wide association using the canine 50k SNP array identified several regions of association, of which the strongest resided on chromosome 17. Fine-mapping and direct DNA sequencing identified an eight base pair deletion in the 3’ untranslated region (UTR) of the striatin (STRN) gene on chromosome 17 in association with ARVC in the boxer dog. Evaluation of the secondary structure of the 3’ UTR demonstrated that the deletion affects a stem loop structure of the mRNA and expression analysis identified a reduction in striatin mRNA. Dogs that were homozygous for the deletion had a more severe form of disease based on a significantly higher number of ventricular premature complexes. Immunofluorescence studies localized striatin to the intercalated disc region of the cardiac myocyte and co-localized it to three desmosomal proteins, plakophilin- 2, plakoglobin and desmoplakin, all involved in the pathogenesis of ARVC in human beings.
We suggest that striatin may serve as a novel candidate gene for human ARVC.
Diagnosing arrhythmogenic right ventricular cardiomyopathy (ARVC) is often challenging because no single diagnostic tool is available to detect the disease. We evaluated whether analysis of plakoglobin, N-cadherin, and connexin-43 immunoreactivity can be used as a significant test in diagnosis of ARVC. We selected subjects with suspicion of ARVC (n=22) in patients who underwent endomyocardial biopsy (EMB) in Kyungpook National University Hospital (n=1326). The patients (n=22) were classified into definite ARVC patients (n=17) and borderline ARVC (n=5). We selected control subjects (n=20) who were autopsied and died of non-cardiac disease. Hematoxylin-eosin, Masson’s trichrome, and immunohistochemical stains for plakoglobin, N-cadherin, and connexin-43 were used for all specimens. Reduced immunoreactivity of plakoglobin was observed in 13 (76%) of the 17 patients with a definite ARVC and in 4 (80%) of the 5 patients with a borderline ARVC. All subjects displayed no significant reduction of the immunoreactivity for connexin-43 as well as for N-cadherin. Our investigation revealed that the immunohistochemical analysis for plakoglobin had an accuracy of 81%, 76% sensitivity, and 84% specificity in diagnosis of ARVC. Results of our study showed that the immunohistochemical analysis of plakoglobin had a relatively high sensitivity and specificity in ARVC, but immunohistochemistry for plakoglobin alone could not be relied upon as a diagnostic test for ARVC. We confirmed that N-cadherin and connexin-43 had no diagnostic value in ARVC.
Plakoglobin; N-cadherin; connexin-43; arrhythmogenic right ventricular cardiomyopathy
Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy (ARVD/C) is an inherited disorder typically caused by mutations in components of the cardiac desmosome. The prevalence and significance of desmosome mutations among ARVD/C patients in North America has not previously been described. We report comprehensive desmosome genetic analysis for 100 North Americans with clinically confirmed or suspected ARVD/C.
Methods and results
In 82 individuals with ARVD/C and 18 people with suspected ARVD/C, DNA sequence analysis was performed on PKP2, DSG2, DSP, DSC2, and JUP. In those with ARVD/C, 52% harbored a desmosome mutation. A majority of these mutations occurred in PKP2. Notably, 3 of the individuals studied have a mutation in more than one gene. Patients with a desmosome mutation were more likely to have experienced ventricular tachycardia (73% versus 44%) and they presented at a younger age (33 versus 41 years) compared to those without a desmosome mutation. Males with ARVD/C were more likely to carry a desmosome mutation than females (63% versus 38%). A mutation was identified in 5/18 (28%) patients with suspected ARVD. In this smaller subgroup there were no significant phenotypic differences identified between individuals with a desmosome mutation compared to those without a mutation.
Our study shows that in 52% of North Americans with ARVD/C a mutation in one of the cardiac desmosome genes can be identified. Compared to those without a desmosome gene mutation, individuals with a desmosome gene mutation had earlier onset ARVD/C and were more likely to have ventricular tachycardia.
Cardiomyopathy; ventricular tachycardia; sudden cardiac death; genetics; desmosome
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a myocardial disease characterized by fibro-fatty replacement of myocardium in the right ventricular free wall and frequently results in life-threatening ventricular arrhythmias and sudden cardiac death. A heterozygous missense mutation in the transmembrane protein 43 (TMEM43) gene, p.S358L, has been genetically identified to cause autosomal dominant ARVC type 5 in a founder population from the island of Newfoundland, Canada. Little is known about the function of the TMEM43 protein or how it leads to the pathogenesis of ARVC. We sought to determine the distribution of TMEM43 and the effect of the p.S358L mutation on the expression and distribution of various intercalated (IC) disc proteins as well as functional effects on IC disc gap junction dye transfer and conduction velocity in cell culture. Through Western blot analysis, transmission electron microscopy (TEM), immunofluorescence (IF), and electrophysiological analysis, our results showed that the stable expression of p.S358L mutation in the HL-1 cardiac cell line resulted in decreased Zonula Occludens (ZO-1) expression and the loss of ZO-1 localization to cell-cell junctions. Junctional Plakoglobin (JUP) and α-catenin proteins were redistributed to the cytoplasm with decreased localization to cell-cell junctions. Connexin-43 (Cx43) phosphorylation was altered, and there was reduced gap junction dye transfer and conduction velocity in mutant TMEM43-transfected cells. These observations suggest that expression of the p.S358L mutant of TMEM43 found in ARVC type 5 may affect localization of proteins involved in conduction, alter gap junction function and reduce conduction velocity in cardiac tissue.
The desmosomal cadherins (DCs), desmocollin (Dsc) and desmoglein (Dsg), are the adhesion molecules of desmosomes, intercellular adhesive junctions of epithelia and cardiac muscle. Both the DCs and desmosomes have demonstrably essential roles in mammalian development. In order to initiate their study in a more tractable developmental system we have characterised zebrafish DCs and examined their roles in early zebrafish development.
We find that zebrafish possess one Dsc, the orthologue of mammalian Dsc1, which we designate zfDsc. Unlike mammalian Dscs, zfDsc exists only as the "a" form since it lacks the alternatively-spliced mini-exon that shortens the cytoplasmic domain to produce the "b" form. Zebrafish possess two Dsgs, designated zfDsgα and zfDsgβ, orthologues of mammalian Dsg2. They show 43.8% amino acid identity and the α form has a 43 amino acid glycine-rich sequence of unknown function in its extracellular domain. Both zfDsc and zfDsgα were present as maternal and zygotic transcripts whereas zfDsgβ was first expressed from 8 hours post-fertilisation (hpf). All three transcripts were present throughout subsequent stages of development. Morpholino knockdown of both zfDsc and zfDsgα expression produced similar defects in epiboly, axis elongation and somite formation, associated with abnormal desmosomes or reduced desmosome numbers.
These results demonstrate an important role for DCs and desmosomes in the early morphogenesis of the zebrafish embryo, provide a basis for more detailed analysis of their role and raise interesting questions relating to the evolution and functional significance of DC isoforms.