Number 41, 2008
Taking genes to heart

Genetics of cardiomyopathy and channelopathy

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Connie R. Bezzina
Heart Failure Research Center, Department of Experimental Cardiology, AMC, Amsterdam, The Netherlands
Correspondence: C. R. Bezzina, Heart Failure Research Center, Department of Experimental Cardiology, Room L2-108-1, Academic Medical Center, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
E-mail: C.R.Bezzina@amc.uva.nl

Introduction
During the past 20 years, we have witnessed a dramatic increase in our knowledge of the genetic basis of cardiac disease. The greatest advancements have undoubtedly taken place in the understanding of the genetic basis of the cardiomyopathies and the primary electrical disorders, the latter commonly referred to as the “ion channelopathies”. Recognition of the genetic substrate in cardiomyopathy and channelopathy not only has provided clues to the underlying molecular mechanisms, but, importantly, has enabled the introduction of genetic diagnostic testing, providing new opportunities for patient management such as early, presymptomatic, identification of patients at risk of developing fatal arrhythmias. This review aims to provide an overview of the different genes linked to these disorders to date.

Cardiomyopathy
Cardiomyopathy is typically divided into several subtypes: hypertrophic cardiomyopathy, glycogen cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy, and arrhythmogenic right ventricular cardiomyopathy.

Hypertrophic cardiomyopathy
Hypertrophic cardiomyopathy (HCM) is a relatively common disorder with a prevalence of about 1 : 500 [1]. It is recognized clinically by the presence of cardiac hypertrophy in the absence of an increased external load [2]. Echocardiographically, it is associated with preserved systolic, but impaired diastolic, function. Hypertrophy is asymmetric in about two-thirds of cases, with the septum being the predominant site of involvement. Significant left ventricular outflow tract obstruction is observed in approximately one-quarter of cases.
HCM is usually inherited as an autosomal dominant trait [3,4]. The first gene linked to HCM was reported in 1990 [5]. This was the gene encoding cardiac β-myosin heavy chain (MYH7), a component of the sarcomere, the contractile unit within the cardiomyocyte. The sarcomere is an immense protein complex that is organized into thick and thin filaments that, in the presence of calcium and ATP, slide past each other, thereby generating contractile force (Figure 1). After the discovery of mutations in MYH7, mutations in another 10 sarcomeric genes were linked to HCM, leading to the proposition that HCM was a disease of the sarcomere (Table I). Most commonly affected are the MYH7 and the MYBPC3 genes, with the other genes accounting for far fewer cases. Mutations in sarcomeric genes account for about 60% of cases of HCM [6]. More recently, mutations have also been described in genes encoding sarcomere-associated proteins such as muscle LIM protein [7]. However, mutations in these genes appear to be rare.

Table I. Genes encoding sarcomeric proteins involved in hypertrophic cardiomyopathy.

Glycogen cardiomyopathy
Cardiac hypertrophy can also be triggered by defects in genes of metabolism [3]. Glycogen deposition is a shared feature of these metabolic cardiomyopathies. A number of features distinguish this form of cardiomyopathy from the sarcomere-related type. Histologic features associated with sarcomere-related hypertrophy (myocyte and myofiber disarray, myocyte hypertrophy, fibrosis) are notably absent in the glycogen cardiomyopathies, which, in contrast, contain myocyte vacuoles containing glycogen. Furthermore, patients with metabolic gene defects usually present with electrophysiological dysfunction.
Mutations in three lysosomal proteins produce such glycogen cardiomyopathy. Recessively inherited lysosomal acid α 1,4-glucosidase (GAA) deficiency causes Pompe disease, X-linked lysosome-associated membrane protein (LAMP2) deficiency causes Danon disease, and X-linked lysosomal hydrolase α galactosidase A (GLA) deficiency causes Fabry disease; these three diseases are systemic disorders. Another gene associated with glycogen storage disease is PRKAG2, encoding the γ2 subunit of AMP-activated protein kinase (AMPK) [8]. AMPK functions as a metabolite-sensing protein kinase that is activated under conditions of energy depletion, manifested by increased concentrations of cellular AMP. Pompe, Danon, and Fabry diseases. Because the γ2 subunit has cardiac-specific expression, extracardiac manifestations do not occur in PRKAG2 cardiomyopathy, distinguishing this from other forms of glycogen storage cardiomyopathies.

Dilated cardiomyopathy
Dilated cardiomyopathy (DCM) [9] is characterized by left ventricular chamber enlargement and systolic dysfunction, with normal or a modest increase in ventricular wall thickness. It has an estimated prevalence of 1 : 2500. Affected individuals gradually develop heart failure, often in association with life-threatening atrial or ventricular arrhythmias.
The disease is familial in about 35% of cases, pointing to an important role of genetics in disease pathogenesis [10]. In such cases, inheritance is most commonly autosomal dominant, but autosomal recessive, X-linked, or matrilinear (mitochondrial) inheritances also occur. DCM has been linked to 25 different chromosomal loci and genes. Known mutations affect proteins with a wide range of unrelated functions. Mutations have been identified in several components of the myocyte cytoskeleton, which, through a complex network of proteins, links the sarcomere to the sarcolemma and extracellular matrix and functions to transmit force generated during contraction (Figure 1). Cytoskeletal components found to be associated with DCM include cardiac muscle LIM protein (MLP), cypher/ZASP (LBD3), δ-sarcoglycan (SGCD), desmoplakin (DSP), desmin (DES), dystrophin (DMD), telethonin (TCAP), and vinculin (VCL).
Mutations in the ubiquitously expressed nuclear envelope protein lamin A/C (LMNA) cause DCM associated with conduction disease [11]. A myriad of phenotypes, besides DCM, are associated with lamin A/C mutation, including Emery–Dreifuss muscular dystrophy type B1 and Charcot–Marie–Tooth disease. Several genes encoding sarcomeric proteins linked to HCM, including TTN, ACTC, TPM1, MYH7 and TNNT2, may also cause DCM. Other genes linked to DCM include genes encoding ion channel subunits. One of these is SCN5A, which encodes the pore-forming subunit of the cardiac sodium channel. Conduction disease and atrial fibrillation are a common finding in patients with DCM harboring mutations in SCN5A [12]. Another ion channel gene subunit linked to DCM is the sulfonylurea receptor 2A (SUR2A), which is the regulatory subunit of K_ATP channels in the heart. DCM is also caused by mutation in phospholamban (PLN), which has an essential role in calcium metabolism by modulating calcium-ATPase activity.

Restrictive cardiomyopathy
Restrictive cardiomyopathy is a rare disorder characterized by a normal or decreased volume of both ventricles, associated with bi-atrial enlargement, impaired ventricular filling with restrictive physiology, normal myocardial wall thickness, and normal or near-normal systolic function. A mutation has been described in cardiac troponin I (TNNTI3) in a family in which carriers exhibited restrictive or hypertrophic cardiomyopathy. Additional TNNTI3 mutations were also found in unrelated patients with restrictive cardiomyopathy [2,3].

Arrhythmogenic right ventricular cardiomyopathy
Arrhythmogenic right ventricular cardiomyopathy (ARVC) involves predominantly the right ventricle, with progressive loss of myocytes and fatty or fibrofatty tissue replacement – a process that appears to begin at the epicardium and gradually extends towards the subendocardium [13]. Left ventricular involvement is now commonly recognized. Significant advances in the understanding of the genetic basis of this disorder have been made over the past few years.
Mutations in five different desmosomal components^* (plakoglobin, desmoplakin, plakophilin-2, desmoglein-2, desmocollin-2) have been described, leading to the notion that ARVC is a disease of the desmosome. Desmosomes form specialized intercellular junctions that anchor intermediate filaments to the cytoplasmic membrane in adjoining cells, imparting mechanical strength. Mutations have also been described in two extradesmosomal genes. One of these is RYR2, encoding the cardiac ryanodine receptor. However, one could argue that the phenotype of these patients more closely resembles that of catecholaminergic polymorphic ventricular tachycardia (CPVT, see below), which also is caused by mutation in RYR2. The other extradesmosomal gene in which mutations have been described is TGF-β3, encoding transforming growth factor-β3. The contribution of mutations in this gene to the overall genetic profile of ARVC is not known with certainty.

Channelopathy
The cardiac action potential is mediated by the exceptionally well orchestrated activity of a diversity of ion channels. Cardiac ion channels are protein complexes in the membrane of cardiomyocytes which, via highly regulated opening and closing (gating), conduct a selective and rapid flow of ions through a central pore. Spatial heterogeneity of ion channel expression underlies the different action potential morphology of the different parts of the heart, which in turn ensures a coordinated contraction. The maintenance of normal cardiac rhythm is dependent on the correct movement of ions mediating the action potential in each cardiac compartment. Abnormalities in ion channel function can have disastrous consequences that manifest themselves as abnormalities of the electrocardiogram (ECG) and arrhythmias. These disorders of ion channels, commonly referred to as “cardiac channelopathies”, have been brought into focus in recent years, as mutations in genes coding for specific ion channels were shown to underlie specific forms of heritable arrhythmogenic disorders occurring in the structurally normal heart. These include long-QT syndrome (LQTS), short-QT syndrome, Brugada syndrome, conduction disease, sinus node dysfunction, and CPVT, discussed here.

Long-QT syndrome
LQTS, estimated to affect 1 in 5000 individuals, is a repolarization disorder identified by prolongation of the QT interval on the ECG [14]. It has long been recognized as a familial disorder, frequently presenting in childhood with syncopal episodes and potentially lethal torsades de pointes tachyarrhythmias, which occur in a significant proportion of untreated patients. Inheritance of the disease is either autosomal dominant or recessive. The autosomal recessive form (Jervell and Lange-Nielsen syndrome) is also associated with deafness.
Mutations in eight different genes encoding potassium (K⁺), sodium (Na⁺), or calcium (Ca²⁺) ion channel subunits have been associated with the disorder (Table II). The pore-forming subunits of the slowly and rapidly activating repolarizing potassium currents (KCNQ1 and KCNH2 genes, respectively) are most often affected. Mutations affecting the potassium channel subunits (KCNQ1, KCNH2, KCNE1, KCNE2) prolong action potential repolarization – and, consequently, the QT interval on the ECG – by a net reduction in outward repolarizing K⁺ current. Mutations in SCN5A, which encodes the pore-forming subunit of the sodium channel, lead to an increased inward Na⁺ current during the action potential plateau, shifting the balance to prolonged repolarization. Mutations in SCN4B, an ancillary subunit of the sodium channel, have recently been reported in one family with LQTS [15].

Table II. Summary of genes for inherited cardiac arrhythmia syndromes.

Short-QT syndrome
The short-QT syndrome presents with a high rate of sudden death and exceptionally short QT intervals (QTc typically ≤300 ms). To date, only 30–40 patients have been described. In contrast to LQTS, in SQTS repolarization is hastened by an enhanced outward current during repolarization. Gain-of-function mutations in the KCNH2, KCNQ1, and KCNJ2 genes were identified in patients with the disorder (Table II). QT-interval shortening in the KCNJ2 subtype seems less severe than that for the other two subtypes.

Brugada syndrome
The Brugada syndrome is characterized by ST-segment elevation in the right precordial leads, with or without conduction abnormalities, and a significant risk of sudden cardiac death. The disorder is endemic in East and Southeast Asia, where it underlies the sudden unexpected death syndrome. Brugada syndrome is familial in about one-third of patients, in which case an autosomal dominant mode of inheritance is observed.
Genes involved in pathogenesis of Brugada syndrome encode the pore-forming and auxiliary subunits of the cardiac Na⁺ channel encoded, respectively, by SCN5A and SCN1B [14,19] (Table II). The functional effects of Brugada syndrome on the sodium current are opposite to those found in LQTS. Loss-of-function mutations underlie Brugada syndrome and the frequently associated (mild) conduction disorders. Mutations in GPD1L, which encodes glycerol-3-phosphate dehydrogenase 1-like protein, also lead to Brugada syndrome by attenuation of the sodium current, an effect probably caused by interference with cell membrane expression of the channel [20]. Recently, loss-of-function mutations in two subunits of the cardiac Ca²⁺ channel complex (CACNA1C and CACNB2) were associated with Brugada syndrome, in combination with somewhat shorter-than-normal QT intervals [21].

Cardiac conduction disease and sinus node dysfunction
Loss-of-function mutations in components of the cardiac Na⁺ channel complex, namely SCN5A and SCN1B, also lead to cardiac conduction disease [19]. It is unknown why such mutations leading to loss of Na⁺ channel function lead to conduction disease in some patients and Brugada syndrome (with conduction defects) in others. Mutations in SCN5A leading to loss of Na⁺ channel function also cause a recessive form of sick sinus syndrome. Mutations in HCN4, which encodes the cardiac pacemaker channel, cause autosomal dominant sinus node dysfunction [14].

Catecholaminergic polymorphic ventricular tachycardia
Arrhythmias in the setting of CPVT are, typically, bidirectional and polymorphic ventricular tachycardia, exclusively triggered by adrenergic stimuli. The phenotype often presents in early childhood. In the majority of cases, CPVT displays an autosomal dominant mode of inheritance and is caused by mutations in the gene encoding the ryanodine receptor channel (RYR2; Table II). This is an intracellular Ca²⁺-release channel on the sarcoplasmic reticulum that releases Ca²⁺ in response to the entry of Ca²⁺ through membrane Ca²⁺ channels. A recessive form of CPVT is caused by homozygous mutation in the CASQ2 gene (Table II), which encodes calsequestrin, a protein that serves as the major Ca²⁺ reservoir within the lumen of the sarcoplasmic reticulum. Symptoms are apparently more severe in CASQ2-related CPVT, including an earlier age of onset.

Concluding remarks
The identification of the mutation within a family affected by inherited cardiac disorders allows diagnosis in other family members independently of echocardiographic features, ECG features, or arrhythmic manifestations. This has led to the realization that inherited cardiac disorders exhibit variability in clinical expression [22,23]. As in the case of many other Mendelian disorders, reduced penetrance and variable expression are more the rule than the exception. Hence, not all carriers of mutations are clinically affected to the same degree by the disorders. Clinical expression is probably influenced by several factors, including age, sex, and environmental factors such as lifestyle, exercise, and blood pressure. Genetic modifiers are also expected to modulate disease penetrance and expression. Although some genetic modifiers are beginning to be uncovered [22,24], the nature of such modifiers remains, largely, unknown. The identification of genetic modifiers is regarded as the major next step in genetic studies of inherited cardiac disorders.

*See glossary for definition of this term.

REFERENCES