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Arrhythmogenic right ventricular cardiomyopathy: moving toward mechanism
Calum A. MacRae,1 Walter Birchmeier,2 and Ludwig Thierfelder2,3
1Cardiovascular Research Center, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA. 2Max Delbrück Center for Molecular Medicine, Berlin, Germany. 3Franz Volhard Clinic, Charité, HELIOS Clinics, Berlin, Germany.
Address correspondence to: Calum A. MacRae, Massachusetts General Hospital, Cardiovascular Research Center, CNY149-4218, 149 13th Street, Charlestown, Massachusetts 02129, USA. Phone: (617) 726-4343; Fax: (617) 726-5806; E-mail: macrae@cvrc.mgh.harvard.edu . Or to: Ludwig Thierfelder, Max Delbrueck Center for Molecular Medicine, Robert Rossle Strasse 10, 13092 Berlin, Germany. Phone: 49-30-9417-2398; Fax: 49-30-9417-2540; E-mail: lthier@mdc-berlin.de .
Find articles by MacRae, C. in: JCI | PubMed | Google Scholar
1Cardiovascular Research Center, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA. 2Max Delbrück Center for Molecular Medicine, Berlin, Germany. 3Franz Volhard Clinic, Charité, HELIOS Clinics, Berlin, Germany.
Address correspondence to: Calum A. MacRae, Massachusetts General Hospital, Cardiovascular Research Center, CNY149-4218, 149 13th Street, Charlestown, Massachusetts 02129, USA. Phone: (617) 726-4343; Fax: (617) 726-5806; E-mail: macrae@cvrc.mgh.harvard.edu . Or to: Ludwig Thierfelder, Max Delbrueck Center for Molecular Medicine, Robert Rossle Strasse 10, 13092 Berlin, Germany. Phone: 49-30-9417-2398; Fax: 49-30-9417-2540; E-mail: lthier@mdc-berlin.de .
Find articles by Birchmeier, W. in: JCI | PubMed | Google Scholar
1Cardiovascular Research Center, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA. 2Max Delbrück Center for Molecular Medicine, Berlin, Germany. 3Franz Volhard Clinic, Charité, HELIOS Clinics, Berlin, Germany.
Address correspondence to: Calum A. MacRae, Massachusetts General Hospital, Cardiovascular Research Center, CNY149-4218, 149 13th Street, Charlestown, Massachusetts 02129, USA. Phone: (617) 726-4343; Fax: (617) 726-5806; E-mail: macrae@cvrc.mgh.harvard.edu . Or to: Ludwig Thierfelder, Max Delbrueck Center for Molecular Medicine, Robert Rossle Strasse 10, 13092 Berlin, Germany. Phone: 49-30-9417-2398; Fax: 49-30-9417-2540; E-mail: lthier@mdc-berlin.de .
Find articles by Thierfelder, L. in: JCI | PubMed | Google Scholar
Published July 3, 2006 - More info
J Clin Invest. 2006;116(7):1825–1828. https://doi.org/10.1172/JCI29174.
© 2006 The American Society for Clinical Investigation
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Abstract
Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVC) is a genetic disease caused by mutations in desmosomal proteins. The phenotypic hallmark of ARVC is fibroadipocytic replacement of cardiac myocytes, which is a unique phenotype with a yet-to-be-defined molecular mechanism. We established atrial myocyte cell lines expressing siRNA against desmoplakin (DP), responsible for human ARVC. We show suppression of DP expression leads to nuclear localization of the desmosomal protein plakoglobin and a 2-fold reduction in canonical Wnt/β-catenin signaling through Tcf/Lef1 transcription factors. The ensuing phenotype is increased expression of adipogenic and fibrogenic genes and accumulation of fat droplets. We further show that cardiac-restricted deletion of Dsp, encoding DP, impairs cardiac morphogenesis and leads to high embryonic lethality in the homozygous state. Heterozygous DP-deficient mice exhibited excess adipocytes and fibrosis in the myocardium, increased myocyte apoptosis, cardiac dysfunction, and ventricular arrhythmias, thus recapitulating the phenotype of human ARVC. We believe our results provide for a novel molecular mechanism for the pathogenesis of ARVC and establish cardiac-restricted DP-deficient mice as a model for human ARVC. These findings could provide for the opportunity to identify new diagnostic markers and therapeutic targets in patients with ARVC.
Authors
Eduardo Garcia-Gras, Raffaella Lombardi, Michael J. Giocondo, James T. Willerson, Michael D. Schneider, Dirar S. Khoury, Ali J. Marian
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Human genetics
ARVC is typically inherited as a dominant Mendelian disease, although recessive variants exist and the involvement of family members often can only be detected by directed screening (1). Human genetic studies over the last few years have offered insight into the potential causes of ARVC. Early work demonstrated substantial genetic heterogeneity, and at least 9 independent loci have now been identified. The discovery of cutaneous and hair follicle involvement in recessive forms of ARVC led to the identification of mutations in the desmosomal proteins plakoglobin and desmoplakin (2, 3). These findings also implicated other desmosomal proteins or their partner proteins as candidate causes of the disorder (Table 1). Subsequent work has revealed desmoplakin mutations in a small proportion of dominantly inherited ARVC cases and in arrhythmogenic cardiomyopathy localized to the left ventricle (4–6). The description of mutations in the cardiac ryanodine receptor in families with an exercise-related arrhythmia known as catecholaminergic polymorphic ventricular tachycardia has highlighted phenotypic distinctions from typical ARVC (1).
In the last year, ARVC genetics have taken a significant step forward, led by the observation that mice null for the armadillo protein plakophilin 2 (PKP2), another desmosomal component, die at around E11 with profound cardiac abnormalities (7). These mice fail to form normal cardiac desmosomes, and desmoplakin dissociates from the abnormal junctions accumulating in cytoplasmic aggregates. These findings led in turn to the discovery of dominant mutations in the PKP2 gene in a large proportion of probands with ARVC and not only established mutant desmosomal proteins as a major cause of the syndrome, but also raised the possibility of genetic testing as a diagnostic tool (8). The initial report in a series of 120 unselected European probands identified PKP2 mutations in almost one-third of these individuals (8). Recent data from more selected cohorts of index patients with evidence of familial involvement have suggested that as many as 70% of such kindreds may have mutations in PKP2 (9). Of note, these investigators also described evidence of founder effects for several PKP2 mutations in remote kindreds, implying less dramatic effects on survival than are seen in other forms of ARVC.
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Possible disease mechanisms
How do these mutant junctional proteins result in a unique, predominantly right ventricular cardiac phenotype? Desmosomal proteins are widely expressed, so the focal nature of apparent pathology in both dominant and recessive ARVC led to initial speculation on the role of mechanical stresses (1, 10, 11). Impaired desmosome function under conditions of mechanical stress was proposed to predispose to cardiomyocyte detachment and death, with subsequent inflammatory reaction and fibrofatty replacement. However, consideration of the distribution of skin lesions in recessive variants infers that this mechanism alone is unlikely to be responsible. Several areas of the body subject to substantial physical stresses are not involved, while the hair follicles are uniformly affected. In addition, the prominent adipose replacement suggests not scarring and healing, but rather a more fundamental perturbation of primary tissue architecture.
Three different types of intercellular junction are distinguished at the cardiac intercalated disc: (a) adherens junctions, which anchor actin filaments; (b) desmosomes, which anchor intermediate filaments; and (c) gap junctions, which mediate ion transfer (Figure 1). Cardiac myocytes rely on these specialized structures for both mechanical and electrical coupling of the myocardial syncytium (12). Desmosomes may protect other junctions from mechanical stress, but they also have been implicated in the structural organization of the intercalated disc. Desmosome-dependent orchestration of local membrane and cytoplasmic domains may be critical for many of the physiologic functions of the intercalated disc (12). For example, the destabilization of cell adhesion complexes may perturb the kinetics of gap junction turnover, resulting in heterogeneous conduction, a potential contributor to arrhythmogenesis in ARVC (13, 14).
Figure 1Cardiac-specific restriction of the desmosomal protein desmoplakin causes nuclear localization of plakoglobin and reduced Wnt/β-catenin signaling, recapitulating human ARVC. (A) ARVC predominantly affects the right ventricle of the heart. (B) The intercalated discs of cardiac myocytes are characterized by gap junctions, adherens junctions, and desmosomes. (C) Cell-cell adhesion is largely dependent on interaction of intracellular components of the desmosomal plaque such as desmoplakin and plakoglobin. (D) In this issue of the JCI, Garcia-Gras et al. (16) report that heterozygous cardiac desmoplakin-deficient mice show nuclear localization of plakoglobin and reduced Wnt/β-catenin signaling. This causes increased expression of adipogenic and fibrogenic genes in vitro, abnormal cardiac adipose tissue and fibrosis in vivo, and ventricular arrhythmias similar to human ARVC. Interactions between signaling defects and mechanical stresses are likely to be involved in the genesis of the final phenotype.
Desmosomes also participate in intercellular signaling networks, of which the Wnt/β-catenin pathway is the most extensively studied (15). In the archetypal pathway the cytoplasmic concentration of β-catenin is exquisitely regulated by multiple inputs, including secreted ligands of the Wnt family and recruitment of β-catenin to intercellular junctional complexes. Cytoplasmic accumulation of β-catenin leads to its nuclear translocation, association with the T cell factor/lymphoid enhancer factor (Tcf/Lef) family of transcription factors and subsequent changes in gene expression. This evolutionarily conserved pathway plays a central role in many of the most fundamental cellular behaviors and has been directly implicated in the regulation of cell fate, proliferation, and apoptosis. Importantly, the various pathway components are duplicated in higher organisms, and specific isoforms may even be employed serially for discrete functions at different times and at different sites (15). In addition, superimposed on the basic structure of the Wnt/β-catenin signaling network are many subtle feedback loops and points of cross-talk that are only beginning to be uncovered (15).
In this context, in this issue of the JCI Garcia-Gras et al. explored the effects of cardiac-restricted desmoplakin deficiency on canonical Wnt/β-catenin signaling (16). Using Cre recombinase driven by an α–myosin heavy chain gene promoter with a floxed desmoplakin allele, desmoplakin expression was eliminated in the heart. Homozygous cardiac deletion of desmoplakin results in substantial embryonic lethality, with evidence of growth arrest at around E11 and cardiac abnormalities reminiscent of the phenotypes seen with germline desmoplakin-null or PKP2-null mice (7, 17). Approximately 5% of cardiac null desmoplakin homozygotes survive to gestation, but these mice die within 6 weeks. Mice heterozygous for the conditional null allele develop age-dependent multichamber cardiac enlargement and dystrophic myocardium with many of the hallmarks of ARVC, including disorganized myocytes and areas of fibrous and adipose tissue. The authors also convincingly demonstrate — using siRNA in HL-1 cells — that inhibition of desmoplakin expression is sufficient to cause nuclear translocation of plakoglobin and upregulation of genes implicated in adipogenesis and generation of new collagen. Increased cardiomyocyte nuclear localization of plakoglobin and ventricular arrhythmias was also observed in heterozygous mice (Figure 1). These results infer that desmoplakin deficiency results in inhibition of canonical Wnt/β-catenin signaling and a consequent shift away from a myocyte fate and toward an adipocyte fate in the heart. These data support previous evidence of competition between plakoglobin and β-catenin and demonstrate that disruption of the Wnt/β-catenin axis is sufficient to push cell fate toward adipogenesis, even in myocardium (15, 18). Clearly more work will be required to reconcile these data with the proposed mechanisms in ARVC families resulting from mutations in plakoglobin itself (13).
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