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Arrhythmogenic cardiomyopathy is a rare primary myocardial disease that is clinically characterized by life-threatening ventricular arrhythmias secondary to fibrofatty replacement of ventricular myocytes.1,2,3
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In the first systematic description of 24 adult cases in 1982, Marcus et al4 outlined the profile of right ventricular (RV) dysplasia as a pathologic condition primarily affecting the right ventricle and characterized by partial or total absence of RV musculature due to substitution by fatty and fibrous tissue. The authors distinguished conditions in which the RV myocardium was almost completely absent from those in which the fatty and fibrous tissue was limited to portions of the right ventricle. The former showed cardiomegaly and clinically manifested with congestive heart failure. The latter showed mild RV remodeling and minimally impaired cardiac dysfunction. When ventricular tachycardia was the principal manifestation, the condition was termed arrhythmogenic RV dysplasia (ARVD).4 The authors additionally described 34 adult cases and provided the clinical profile of the disease:
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Clinical presentation is characterized by ventricular tachycardia, supraventricular arrhythmias, right heart failure, or asymptomatic cardiomegaly.
Electrocardiogram (ECG) shows T wave inversion in the right precordial leads.
Increased RV diastolic dimensions.
Onset is in young adult age.
Prevalence is higher in males than in females.
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In 1988, Thiene et al5 described the morphologic features of RV cardiomyopathy in 12 young people who died suddenly. Findings included:
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Normal or moderately increased heart weights
Lipomatous transformation (6 of 12 patients) or a fibrolipomatous (6 of 12 patients) transformation of the RV free wall
Substantially spared left ventricle
Occasional myocardial degeneration and necrosis, with or without inflammatory infiltrates
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In 1994, an International Task Force grouped criteria for the clinical diagnosis of arrhythmogenic RV cardiomyopathy/dysplasia (ARVC/D).6 These criteria were aimed at facilitating recognition and interpretation of the clinical and pathologic features of ARVC/D and were grouped according to:
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These features were incorporated into criteria that were subdivided into major and minor categories according to the specificity of their association with ARVC/D. These criteria revealed high specificity, but low sensitivity, for early and familial disease.6
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In 2010, task force criteria were modified to improve the diagnosis and management of ARVC/D. The modified criteria incorporated new knowledge on the genetic basis of the disease, improving diagnostic sensitivity and maintaining diagnostic specificity (Table 62–1). The structural, histologic, ECG, arrhythmic, and genetic features were structured in major and minor criteria. The task force document formally introduced the biventricular variant and the left dominant variant.7 These latter criteria support the broader new term of arrhythmogenic cardiomyopathy (ACM). Therefore, the disease is currently called ACM, ARVC/D, ARVC, or ARVD. In this chapter, the disease is named according to the task force criteria—ACM.
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The involvement of the left ventricle in ACM had been originally described by Marcus et al4 and confirmed in several additional studies. Importantly, task force criteria are easy to apply to classical and manifested forms in adults, but not in children. Recent studies report that many pediatric patients do not meet the current ACM diagnostic criteria, resulting in delays in diagnosis and treatment. According to pediatric cardiologists, the current criteria need further revision to encompass pediatric manifestations of the disease, suggesting that modified pediatric criteria would facilitate prompt diagnosis and management of ACM and structured research with the goal of improving outcomes.8
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The prevalence of ACM is estimated to range from 1 in 1000 to 1 in 5000 in the general population.1,2,3,9 A small proportion of patients progress to left ventricular (LV) dysfunction with ventricular arrhythmias remaining the clinical hallmark.10 The prevalence of clinical manifestations is higher in males than in females (3:1) and typically manifests in young adults.11,12 Of 439 index patients described by Groeneweg et al,11 only four presented before the age of 13 years and none before the age of 10 years. The reasons for delayed manifestation may be related to the time required for completion of intercalated disk maturation or the need for prolonged exposure to exercise.11
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ACM is an inherited cardiomyopathy, with autosomal dominant inheritance in the majority of cases and recessive inheritance in a minority.13 Classical right-sided ACM is a genetic disease of the intercalated disks, which are the sites of contact between adjacent myocytes.
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Intercalated disks are highly specialized cell-cell junctions; they are the units of electromechanical continuity between cardiac myocytes14 and are comprised of gap junctions, adherens junctions, and desmosomes (Fig. 62–1).
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Gap junctions (nexus, communicating junctions) are located in the lateral parts of the intercalated disks. Gap junctions mediate ionic traffic between adjacent cells and provide the basis for functional cell coupling. Key structural proteins are connexins; the connexins predominantly expressed by cardiac myocytes are Cx43, Cx40, and Cx45.
Adherens junctions (fasciae adhaerentes) are located in the transverse parts of the intercalated disks where the actin filaments of the sarcomeres are anchored and connected with the plasma membrane. This anchorage provides the intercellular “mechanical continuity” between the myocytes supporting the transmission of force between cells and synchronous contraction and relaxation. Key structural proteins are nondesmosomal cadherins that are calcium-dependent transmembrane glycoproteins.
Desmosomes (maculae adhaerentes) are located in both the transverse and lateral parts of the intercalated disks: they reinforce the adherens junctions and fix adjacent cells. Desmosomes bind desmin on the intracellular side, spans the cell membrane, and bind adjacent desmosomes on the extracellular side. Desmosome-forming proteins include plakophilin-2, desmoglein 2, desmocollin 2, plakoglobin, and desmoplakin.
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Since the first detailed clinical description of ACM,4 significant advances in imaging, epidemiology, animal models, and protein biochemistry have accelerated our understanding of disease pathogenesis and progression. However, much of our current understanding of ACM pathobiology is derived from genetic findings of rare, yet severe, autosomal recessive forms of cardiocutaneous diseases, including Naxos disease and Carvajal disease. For example, Naxos disease, characterized by fully penetrant ACM, wooly hair, and palmoplantar keratoderma,15 was the first direct evidence for dysfunction in the desmosomal gene product plakoglobin in ACM (truncation of final 56 residues of protein).16 Subsequently, desmoplakin loss-of-function variants that inhibit the ability of desmoplakin to associate with desmin (see below) were implicated in Carvajal syndrome, a cardiocutaneous disease associated with left-dominant cardiac phenotype.17 Since these original discoveries, advances in gene sequencing have revealed a host of protein pathways now linked with ACM pathogenesis. With rare exceptions, these findings illustrate a crucial role of a key cardiac membrane domain—the desmosome—in ACM pathobiology.
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The Cardiac Desmosome
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The cardiac desmosome is a central feature of the myocyte intercalated disk, a multifunctional membrane domain essential for both myocyte electrical and structural coupling. The desmosome is primarily composed of three families of proteins: cadherins, plakins, and armadillo proteins. Desmosomal cardiac cadherins are type 1 transmembrane proteins that include desmocollin (encoded by DSC2) and desmoglein (encoded by DSG2). These proteins span the extracellular space to physically link adjacent cardiomyocytes. The intracellular domains of desmocollin and desmoglein directly associate with plakins and armadillo linker proteins to integrate the membrane with the underlying intermediate filament network. In the heart, these linker proteins are primarily desmoplakin (encoded by DSP), plakophilin 2 (encoded by PKP2), and plakoglobin (encoded by JUP), whereas desmin (encoded by DES) is the central component of the intercalated disk intermediate filament network that extends across the length of the myocyte. Desmoplakin homodimers connect plakofillin 2 at the amino-terminal domain and desmin at the carboxy-terminal domain. Together, this multiprotein complex has fundamental roles in regulating mechanical force within myocytes and also between adjacent myocytes and myocyte networks.
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Notably, the role of the desmosome for normal cardiac function is clearly illustrated by human gene mutations in PKP2, JUP, DSP, DSG2, and DSC2 associated with autosomal dominant ACM.18 For example, in the United States alone, approximately 50% to 60% of ACM patients harbor a loss-of-function gene variant in at least one of five desmosomal genes.18 PKP2 remains the most common ACM gene (45%), with DSG2 loss-of-function variants present in nearly 10% of human ACM cases.19 As discussed later in this chapter, approximately 86% of ACM patients harbor a single heterozygous gene variant, 7% show compound heterozygosity, and 7% show digenic heterozygosity (> 1400 variants identified to date; see www.arvcdatabase.info).18,19 Desmosome and nondesmosome genes reported to date as being associated with ACM are routinely tested and contribute as major criteria to the diagnosis (Table 62–2).
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Based on the central role of the cardiac desmosome in regulating inter- and intracellular mechanical forces, ACM pathophysiology has historically been attributed to dysfunction in desmosomal proteins, resulting in structural instability of this critical membrane domain. Thus, in the “degeneration” or “inflammation” models for ACM pathobiology, structurally compromised desmosomes are more sensitive to mechanical strain/forces, resulting in damaged cell-cell junctions, myocyte uncoupling, and ultimately inflammation, fibrosis, necrosis, and potentially adipogenesis.20,21,22 These models are supported by multiple lines of evidence, including the following: (1) aberrant myocyte intercalated disk coupling in human ACM samples as observed by electron microscopy23; (2) abnormal myocyte intercalated disk widening in mice harboring human ACM variants as observed by electron microscopy24; (3) prevalence of ACM phenotypes in conditions of myocardial strain19; (4) accelerated ACM phenotypes and increased risk for sudden cardiac death (SCD) in athletes25; (5) impact of disease on right ventricle (thin, distensible wall) versus left ventricle19; (6) reduced adhesive strength of cells expressing small interfering RNAs that silence selective desmosomal proteins26; and (7) accelerated ACM phenotypes in animal models harboring mutant desmosome genes following mechanical stress.27,28 However, not all data necessarily support loss of cell adhesion in disease pathogenesis. For example, Hariharan et al29 indicated that although neonatal rat ventricular myocytes deficient in plakoglobin or plakophilin showed reduced cell-cell adhesion, these same cultures expressing human ACM plakoglobin or plakophilin loss-of-function mutations displayed no significant differences in cell-cell adhesion. However, these same mutant myocytes displayed sheer stress phenotypes that were reversed by inhibition of glycogen synthase kinase 3β (GSK3β).29 Further, Asimaki et al30 have consistently identified decreased plakoglobin junctional expression in ACM samples, supporting the complexity of the disease, the variability in phenotypic penetrance/expressivity,19 and the likelihood of secondary contributing pathogenic signaling mechanisms, as described below.
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More recently, a second, yet non–mutually exclusive model for ACM pathobiology has emerged that frames the desmosome as a multifunctional signaling node for the myocyte. Specifically, the second model converges on nonjunctional transcriptional pathways mediated by the Wnt (wingless-related integration site) axis. In this model, in the nondiseased heart, baseline Wnt signaling is critical for normal cardiogenic transcriptional signaling.18 This signaling is mediated by regulation of T-cell factor/lymphoid enhancer factor (Tcf/Lef) transcription factors by nuclear β-catenin (β-catenin is also found at cell junctions at baseline).18 Also central to this model is the localization of junctional plakoglobin. In nondiseased heart, plakoglobin is concentrated at the intercalated disk membrane.18 In fact, nonmembrane (cytosolic) populations of plakoglobin are maintained at low concentrations by ubiquitination and proteasomal degradation.18 Although the precise mechanisms are still being elucidated, it is now thought that in ACM, compromised junctional infrastructure increases the cytosolic pool of plakoglobin. Ultimately, diffusion of this pool into the nucleus is proposed to suppress β-catenin/Wnt signaling to transition procardiogenic transcriptional signaling to transcriptional programming favoring inflammation, apoptosis, and adipogenesis.31 Notably, expression, activity, and translocation of β-catenin are central to this pathway. Outstanding questions related to this model are the relative levels of junctional/cytosolic/nuclear plakoglobin and β-catenin at baseline and in disease, the impact of the local signaling environment (eg, activity of GSK3β on plakoglobin/β-catenin), the relative half-lives of plakoglobin/β-catenin in disease, and the specific roles/downstream targets of Tcf/Lef transcriptional signaling based on nuclear plakoglobin/β-catenin concentrations.
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The impact of altered Wnt signaling in ACM pathogenesis was first described by Lombardi et al.32 Mice lacking Dsp expression display suppression of the β-catenin/Wnt pathway favoring apoptosis, fibrosis, and adipogenesis.32 In further support of this model, altered desmoplakin expression in a human-derived cardiomyocyte cell line alters plakoglobin nuclear translocation, ultimately altering β-catenin/Tcf/Lef transcriptional signaling and adipogenesis.31 Moreover, Wnt signaling is reduced in mice expressing a mutant form of plakoglobin lacking the C-terminus, resulting in nuclear plakoglobin translocation and adipogenesis.32 Finally, in seminal work from the Marion Laboratory, fate-mapping techniques identified a select population of cardiac progenitor cells as the primary source of adipocytes in the heart.33 These foundational data have sparked a wealth of studies that have elucidated new and unanticipated signaling pathways in the heart likely related to ACM pathogenesis.34 Further, this work has suggested that ACM is a disease of multiple cardiac cell types, an observation that may further explain the variability in clinical presentation and disease progression.
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Nondesmosomal Proteins
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Although ACM pathogenesis has largely been attributed to dysfunction of the desmosome, new protein pathways associated with ACM continue to add to our understanding of disease pathophysiology. Mutations in junctional nondesmosomal proteins, intermediate filaments, nonjunctional proteins, sarcomeric proteins, membrane-anchored proteins, and adipogenic factors have been reported in patients with ACM phenotypes (often described as overlapping syndromes).
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αT-catenin3 (CTTNA3) is a cell adhesion molecule. In cardiac intercalated disks, CTNNA3 is a component of the adherens junction, or area composita. Mice with deletion of the Ctnna3 gene (αT-catenin–null mice) develop progressive cardiomyopathy. The loss of αT-catenin does not seem to affect adherens junction and desmosomal proteins; the exception is the desmosomal protein plakophilin-2. In fact, immunogold labeling at the intercalated disk demonstrates preferential reduction of plakophilin-2 at the area composita compared with the desmosome.35 The p.Val281Asp (c.281T > A) and c.2293_2295delTTG (p.del765Leu) mutations have been identified in two probands with ACM36; yeast two-hybrid and cell transfection experiments suggested a causal relationship between CTNNA3 mutations and ACM.
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The desmin (DES) gene encodes a muscle-specific class III intermediate filament. Homopolymers of this protein form an intracytoplasmic filamentous network connecting myofibrils to each other and to the plasma membrane. DES mutations are associated with restrictive cardiomyopathy, desmin-related myopathy, a familial cardiac and skeletal myopathy, and distal myopathies. Searching for the genetic cause of autosomal dominant myofibrillar myopathy with ACM in a Swedish family, Hedberg et al37 identified the p.(Pro419Ser) Ser mutation in the DES gene. In a series of 91 ACM index cases (including 53 negative and 38 positive for mutations in desmosomal genes), mutations in the DES gene were found in two cases: (1) the p.(Lys241Glu) substitution was detected in one patient who also carried the p.(Thr816ArgfsX10) mutation in the PKP2 gene; and (2) p.(Ala213Val) was found in a second patient and was not associated with mutations in the desmosomal genes.38 The LMNA gene encodes lamin A/C, which is a major structural component of the nuclear lamina. LMNA mutations are associated with different phenotypes (see Chap. 58). LMNA mutations were identified in 4 of 108 patients with ACM phenotype (borderline in 27 and definite in 81); three patients had severe RV involvement.39 During follow-up, there were two sudden deaths and one death from congestive heart failure. These three patients had conduction abnormalities on resting 12-lead ECG. Myocardial tissue from two patients showed myocyte loss and fibrofatty replacement, and one patient had reduced/absent plakophilin-2 staining of the intercalated disks in the myocardium.39 Dilated cardiomyopathy (DCM)/ACM overlapping phenotype may be observed in patients who carry LMNA mutations40 and in patients with ACM and Charcot-Marie-Tooth phenotype.41
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The transmembrane protein 43 (TMEM43) gene encodes an inner nuclear membrane protein termed LUMA that interacts with lamin A/C and emerin. The missense mutation c.1073C>T (p.(Ser358Leu)) in TMEM43 causes ACM and was identified in a founder population from Newfoundland. The p.(Ser358Leu)-associated phenotype is highly malignant and is characterized by poor R-wave progression, early ventricular ectopy, LV dilatation, heart failure, and early death.42,43 TMEM43 mutations, including p.(Ser358Leu), have been identified in individuals not from Newfoundland.44 Further studies confirmed that TMEM43 ACM subtype is a sex-influenced lethal ACM, with a unique ECG finding, LV dilatation, heart failure, and early death. The genetic presymptomatic diagnosis has the greatest clinical utility.45 TMEM43 sequencing is now incorporated into clinical genetic testing for ACM patients.46,47 TMEM43 protein is active in an adipogenic pathway regulated by Peroxisome proliferator-activated receptor γ (PPARγ; an adipogenic transcription factor); this may explain the fibrofatty replacement of the myocardium. The pathogenetic mechanisms of this mutation are poorly understood. Mutant TMEM43 exhibits normal cellular localization and does not disrupt integrity and localization of other nuclear envelope and desmosomal proteins.47
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The phospholamban (PLN) gene encodes a protein that is a major substrate for the cAMP-dependent protein kinase in cardiac muscle and inhibits cardiac muscle sarcoplasmic reticulum Ca2+-ATPase in the unphosphorylated state; inhibition is relieved by phosphorylation of the protein. Mutations in this gene cause ACM, DCM with refractory congestive heart failure, and familial hypertrophic cardiomyopathy (HCM).48,49
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The ryanodine receptor-2 (RYR2) gene encodes a receptor located in cardiac muscle sarcoplasmic reticulum. The protein is a component of a calcium channel that is constituted of a tetramer of the ryanodine receptor proteins and a tetramer of FK506-binding protein 1B proteins, which supplies calcium to cardiac muscle. Mutations in this gene are more commonly associated with stress-induced polymorphic ventricular tachycardia and less commonly with ARVD (9% of ACM probands).50,51
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The titin (TTN) gene encodes a large abundant protein of striated muscle. The protein is divided into two regions, an N-terminal I-band and a C-terminal A-band. Mutations in this gene are associated with DCM; familial HCM; limb-girdle, type 2 muscular dystrophy; early-onset myopathy with fatal cardiomyopathy; proximal myopathy with early respiratory muscle involvement; and tardive tibial muscular dystrophy. In a screening study including 39 ACM families, 13% (5 of 39 families) carried TTN rare variants (11 affected subjects).52 TTN variants were further investigated and were identified in seven families; the prominent Thr2896Ile mutation showed complete segregation with the ACM phenotype in one large family. This mutation affects a highly conserved immunoglobulin-like fold located in the spring region of titin and reduces the structural stability and increases the propensity for degradation of the immunoglobulin-like domain. Carriers of TTN mutations demonstrated history of sudden death (5 of 7 families), progressive myocardial dysfunction ending in death or heart transplantation (8 of 14 cases), frequent conduction disease (11 of 14 cases), and incomplete penetrance (86%).53
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The transforming growth factor-β3 (TGFB3) gene encodes a secreted member of the TGF-β family that is involved in embryogenesis and cell differentiation.54 Typically, TGFB3 mutations cause Loeys-Dietz syndrome type 5. However, mutation screening of the promoter and untranslated regions (UTRs) of the TGFB3 gene identified a nucleotide substitution (c.–36G>A) in the 5′ UTR of the gene invariably associated with the classical, right-sided ACM phenotype and an additional mutation (c.1723C>T) in the 3′ UTR of one proband. Both nucleotide changes were absent in control subjects. In vitro expression assays with constructs containing the mutations showed that mutated UTRs were two-fold more active than wild-types.
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ADAM17 (a disintegrin and metalloproteinase 17 or tumor necrosis factor-α converting enzyme) sheds a number of ligands by cleavage of substrates (eg, ligands for the epidermal growth factor receptor and tumor necrosis factor-α) in the juxtamembrane region, plays a role in ligand-independent Notch signaling, and sheds cell adhesion molecules including desmoglein 2.55 A homozygous loss-of-function ADAM17 mutation causes the autosomal recessive neonatal inflammatory skin and bowel disease type 1, which is characterized by severe skin inflammation, increased susceptibility to infection, bowel inflammation, and cardiomyopathy. The cardiomyopathy and other phenotypes in this syndrome may be related, in part, to impaired ADAM17-mediated DSG2 processing.56