Pathophysiology of Cardiomyopathies

Introduction

Cardiomyopathies constitute a group of heterogenous disorders in which the muscle of the heart (myocardium) remodels and becomes structurally and functionally abnormal. This pathophysiology can potentially lead to progressive systolic and/or diastolic heart failure, thus representing a cause of morbidity and mortality during the neonatal period. ^, Cardiomyopathies are frequently caused by pathogenic gene variants, in addition to other triggers such as the presence of anatomic anomalies, dysrhythmias, infectious and environmental exposures, and underlying systemic disorders (metabolic, neuromuscular, mitochondrial). Unfortunately, not all neonates present with obvious signs and symptoms of heart failure, making the diagnosis more challenging when compared with patients at other pediatric ages. In 2006, a classification scheme included five types of cardiomyopathy: dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM), arrhythmogenic cardiomyopathy (ACM), and noncompaction cardiomyopathy (NCM); which can be further classified into genetic/inherited and acquired/noninherited diseases. ^{, ,} An estimated incidence of cardiomyopathy in children has been reported by the Pediatric Cardiomyopathy Registry as 1.13 per 100,000. However, the overall incidence of cardiomyopathy in the neonatal period has not been completely elucidated, mainly because of the high incidence of unknown diagnoses in sudden unexpected infant deaths and the lack of recognition of this disease before death. This chapter provides information regarding the molecular structure of the cardiomyocytes and describes the pathophysiology of the most common types of neonatal cardiomyopathies (DCM, HCM, RCM, NCM). ACM is a group of cardiomyopathies associated with early-onset arrhythmias (ventricular tachycardia or fibrillation, atrioventricular block, etc.) and can affect the right ventricle, the left ventricle, or have biventricular disease. Right ventricular ACM is almost always identified in late adolescence or in young adults and is rarely described before 10 years of age; thus, it will not be addressed in detail in this chapter. Additionally, transient conditions that mimic cardiomyopathy, such as diabetic cardiomyopathy, will not be covered.

Normal Cardiac Structure

Cardiac muscle fibers are composed of separate cellular units or myocytes connected in series. In contrast to skeletal muscle fibers, cardiac fibers do not assemble in parallel arrays but bifurcate and recombine to form a complex three-dimensional network. Cardiac myocytes are joined at each end to adjacent myocytes at the intercalated disk, the specialized area of interdigitating cell membrane ( Fig. 153.1 ). The intercalated disk contains gap junctions (containing connexins) and mechanical junctions, composed of adherens junctions (consisting of N-cadherin, catenins, and vinculin) and desmosomes (containing desmin, desmoplakin, desmocollin, desmoglein, and junctional plakoglobin). Cardiac myocytes are surrounded by a thin membrane (sarcolemma), and the interior of each myocyte contains bundles of longitudinally arranged myofibrils. The myofibrils are formed by repeating sarcomeres, the basic contractile units of cardiac muscle composed of interdigitating thin (actin) and thick (myosin) filaments (see Fig. 153.1 ), which give the muscle its characteristic striated appearance. ^, The thick filaments are composed primarily of myosin but additionally contain myosin-binding proteins C, H, and X. The thin filaments are composed of cardiac actin, α-tropomyosin, and troponins T, I, and C. In addition, myofibrils contain a third filament formed by the giant filamentous protein titin, which extends from the Z disk to the M line and acts as a molecular template for the layout of the sarcomere. The Z disk at the borders of the sarcomere is formed by a lattice of interdigitating proteins that maintain myofilament organization by crosslinking antiparallel titin and thin filaments from adjacent sarcomeres ( Fig. 153.2 ). Other proteins in the Z disk include α-actinin, nebulette, telethonin, muscle LIM protein (MLP; encoded by CSRP3 , and also known as cysteine- and glycine-rich protein 3 ), myopalladin, myotilin, cardiac ankyrin repeat protein (CARP; encoded by ANKRD1 , and also known as ankyrin repeat domain 1 ), cypher (encoded by LDB3 and also known as LIM domain–binding 3 and ZASP ), filamin, nexilin, and filamin, actinin, and FATZ . ^{, ,}

Finally, the extra-sarcomeric cytoskeleton, a complex network of proteins linking the sarcomere with the sarcolemma and the extracellular matrix, provides structural support for subcellular structures and transmits mechanical and chemical signals within and between cells. The extra-sarcomeric cytoskeleton has intermyofibrillar and subsarcolemmal components, with the intermyofibrillar cytoskeleton composed of intermediate filaments, microfilaments, and microtubules. Desmin intermediate filaments form a three-dimensional scaffold throughout the extra-sarcomeric cytoskeleton with desmin filaments surrounding the Z disk, allowing longitudinal connections to adjacent Z disks and lateral connections to subsarcolemmal costameres. ^, Microfilaments composed of nonsarcomeric actin (mainly γ-actin) also form complex networks linking the sarcomere (via α-actinin) to various components of the costameres. Costameres are subsarcolemmal domains located in a periodic, grid-like pattern, flanking the Z disks and overlying the I band, along the cytoplasmic side of the sarcolemma. These costameres are sites of interconnection between various cytoskeletal networks linking the sarcomere and the sarcolemma and are thought to function as anchor sites for stabilization of the sarcolemma and for integration of pathways involved in mechanical force transduction. Costameres contain three principal components: the focal adhesion–type complex, the spectrin-based complex, and the dystrophin-associated protein complex. ^, The focal adhesion–type complex, composed of cytoplasmic proteins (i.e., vinculin, talin, tensin, paxillin, and zyxin), connects with cytoskeletal actin filaments and with the transmembrane proteins α-dystroglycan, β-dystroglycan, α-sarcoglycan, β-sarcoglycan, γ-sarcoglycan, δ-sarcoglycan, dystrobrevin, and syntrophin. Several actin-associated proteins are located at sites of attachment of cytoskeletal actin filaments. The carboxyl terminus of dystrophin binds β-dystroglycan (see Fig. 153.1 ), which in turn interacts with α-dystroglycan to link to the extracellular matrix (via α ₂ -laminin). ^{, , ,} The amino terminus of dystrophin interacts with actin. Also notable, voltage-gated sodium channels colocalize with dystrophin, β-spectrin, ankyrin, and syntrophins, and potassium channels interact with the sarcomeric Z disk and intercalated disks. ^{, ,} Because arrhythmias and conduction system diseases are common in children and adults with DCM, it is likely that functional disturbance of these ion channels could play an important role in the development of the arrhythmia phenotype commonly associated with DCM. Hence, disruption of the links from the sarcolemma to the extracellular matrix at the dystrophin carboxyl terminus and those to the sarcomere and nucleus via amino-terminal dystrophin interactions could lead to a domino effect disruption of systolic function and the development of arrhythmias.

Dilated Cardiomyopathy

Clinical Features of Dilated Cardiomyopathy

DCM is characterized by LV dilation, relatively decreased LV wall thickness, and systolic dysfunction in the absence of either pressure or volume overload or coronary artery disease sufficient to explain the dysfunction ( Fig. 153.3 ). Arrhythmias, biventricular dilation, and/or diastolic dysfunction may also be appreciated. DCM is the most common form of CM, and it may be secondary to genetic, infectious, metabolic, systemic, and/or toxic etiologies. In children, the annual incidence of DCM is 0.57 cases per 100,000 but is higher in boys than in girls, higher in blacks than in whites, and higher in infants (younger than 1 year) than in older children. Most children (66%) are thought to have idiopathic disease. Although such rigorous data are not available in adult populations, it is believed that the incidence is much higher than that seen in children, possibly being as high as 1 in 250 individuals. ^, There is also an important distinction in adults not generally applicable in children: ischemic versus nonischemic DCM.

Presentation of symptomatic DCM in the neonatal and pediatric patient usually involves irritability, difficulty feeding, and poor weight gain. Physical examination findings may include tachycardia, gallop rhythm, jugular venous distension, hepatomegaly, and a murmur consistent with mitral valve regurgitation. Testing can reveal cardiomegaly and pulmonary edema on chest x-ray; sinus tachycardia, conduction system disease, ST-segment changes, and atrial or ventricular arrhythmias on electrocardiography; and a dilated, poorly functioning left ventricle on echocardiography, with or without mitral regurgitation or pericardial effusion. ^, Elevation of serum biomarkers of heart failure (troponins, soluble-ST2, BNP, and NT-ProBNP) also can be considered in the diagnosis, prognosis, and management. Patients with DCM, or any cardiomyopathy, can be further stratified by their degree of heart failure. Adult classification schemes, such as the scheme from the NYHA, are often inappropriate or useless in pediatric patients, particularly neonates. Because of this, the modified Ross classification of heart failure was developed for pediatric patients ( Table 153.1 ). Further, use of the ACC/AHA staging of heart failure is also generally applicable in pediatric cardiology ( Table 153.2 ).

Table 153.1

Modified Ross Scoring System for Heart Failure in Infants.

Adapted from Ross RD. The Ross classification for heart failure in children after 25 years: a review and an age-stratified revision. Pediatr Cardiol . 2012;33:1295–1300.

Scoring	0	1	2
Feeding History
Volume consumed (oz)	>3.5	2.5–3.5	<2.5
Time taken per feeding (min)	<40	>40	—
Physical Examination
Respiratory rate per minute	<50	50–60	>60
Heart rate per minute	<160	160–170	>170
Respiratory pattern	Normal	Abnormal	—
Peripheral perfusion	Normal	Decreased	—
S3 or diastolic rumble	Absent	Present	—
Liver edge from costal margin (cm)	<2	2–3	>3

Table 153.2

American College of Cardiology/American Heart Association Staging System of Heart Failure.

Adapted from Jessup M, Abraham WT, Casey DE, et al. 2009 focused update: ACCF/AHA Guidelines for the Diagnosis and Management of Heart Failure in Adults: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines: developed in collaboration with the International Society for Heart and Lung Transplantation . Circulation . 2009;119:1977–2016.

At Risk for Heart Failure		Heart Failure
Stage A	Stage B	Stage C	Stage D
At high risk for HF but without structural heart disease or symptoms of HF	Structural heart disease but without signs or symptoms of HF	Structural heart disease with prior or current symptoms of HF	Refractory HF

HF, Heart failure.

The overall mortality rate in the United States resulting from cardiomyopathy is greater than 10,000 deaths per annum, with DCM being the major contributor. DCM has among the highest death rates for any pediatric heart disease. Fortunately, survival appears to be improving, and since the 1990s the 1- and 5-year survival rates for all individuals with DCM have been 90% and 83%, respectively. ^{, ,} In a study of 1803 children with DCM by the Pediatric Cardiomyopathy Registry Investigators, the 5-year incidence rates were 29% for heart transplantation, 2.4% for SCD, and 12.1% for non-SCD.

Causes of Dilated Cardiomyopathy

The clinical appearance of DCM, a dilated left ventricle with poor contractile performance, has various causes. Acquired forms of disease include myocarditis; intrauterine disorders such as placental insufficiency and nonimmune hydrops, postpartum cardiomyopathy, dysfunction in the setting of systemic disease such as renal or hepatic failure, toxic exposures, neonatal myocarditis, and acute stress such as hypoxia. These disorders are not the focus of this chapter. Alternatively, DCM can develop because of genetic-based disease, either familial or nonfamilial. These genetic disorders include inherited forms of disease, cardioskeletal disease such as muscular dystrophies, and metabolic forms of disease. ^{, ,}

Clinical Genetics of Dilated Cardiomyopathy

DCM was initially believed to be an inherited disorder in a small percentage of cases, but it has been estimated that between 20% and 40% patients with idiopathic DCM have a positive family history, suggesting primary genetic etiologies, with autosomal dominant inheritance being the predominant pattern of transmission; X-linked, autosomal recessive, and mitochondrial (maternal) patterns of inheritance are less commonly encountered. ^,

Molecular Genetics of Dilated Cardiomyopathy

DCM has become a popular target of research during the past few decades, with numerous genes being identified ( Table 153.3 ). Studies have demonstrated that there is a significant amount of overlap in the genes identified as important for all types of cardiomyopathy. The advent of next-generation sequencing technologies expedited gene screening studies and brought fresh perspectives on DCM genetics. Commercially available gene testing for cardiomyopathy includes tests for more than 75 genes, with the option to analyze a nearly unlimited number of genes by means of current next-generation sequencing approaches. The genes important in DCM appear to encode two major subgroups: cytoskeletal and sarcomeric proteins. Additionally, ion channels, mitochondrial proteins, desmosomal proteins, nuclear envelope proteins, and an assortment of other protein-encoding genes also have been found associated with DCM. ^, Despite this wide genetic variation, the individuals with DCM and HCM in whom a genetic cause is identified have mutations in the genes encoding myosin-binding protein C, β-myosin heavy chain, cardiac troponin I, cardiac troponin T, and α-tropomyosin in more than 95% of cases. ^, Gene discovery efforts will continue to elucidate additional pathogenic gene variants and comprehensive assessment of transcription factors, noncoding sequencing tracks, gene copy number variants, and other key factors involved in heritable myocardial disease and epigenetics. Currently, in most cases of DCM the cause cannot be determined by genetic testing, with a diagnostic rate of approximately 40%. ^,

Acquired forms of DCM can encompass the same clinical features as genetically determined disease, including symptomatic heart failure, arrhythmias, and conduction block. ^, The most common causes of myocarditis are viral, including the enteroviruses (Coxsackie virus and echovirus), adenoviruses, parvovirus B19, and Epstein-Barr virus, among other cardiotropic viruses. ^,

Initial progress in the understanding of familial forms of DCM came from study of families with X-linked forms of the disease, XLCM, which presents in adolescence and young adults. Barth syndrome is most frequently identified in infancy, while Danon disease typically presents with HCM with later onset of DCM, although this presentation can be variable. ^,

Danon Disease

Another form of X-linked disease associated with DCM is Danon disease, which is discussed in the section entitled “Hypertrophic Cardiomyopathy Associated With Infiltrative and Systemic Diseases.”

Cytoskeletal Proteins

Most DCM-associated genes identified to date encode either cytoskeletal or sarcomeric proteins (see Table 153.3 ). In the case of cytoskeletal proteins, defects of force transmission are considered to result in the DCM phenotype, whereas defects of force generation have been speculated to cause sarcomeric protein–induced DCM. Desmin is a cytoskeletal protein that forms intermediate filaments specific for muscle. This muscle-specific 53-kDa subunit of class III intermediate filaments forms connections between the nuclear and plasma membranes of cardiac, skeletal, and smooth muscle. Desmin is found at the Z lines and intercalated disk of muscle, and its role in muscle function appears to involve attachment or stabilization of the sarcomere. Mutations in the desmin gene appear to cause abnormalities of force and signal transmission similar to those believed to occur with actin gene mutations (see later). ^,

Another DCM-causing gene encodes δ-sarcoglycan, a member of the sarcoglycan subcomplex of the dystrophin-associated protein complex. The gene encodes a protein involved in stabilization of the myocyte sarcolemma as well as signal transduction. Mutations identified in familial and sporadic cases resulted in reduction of the expression of protein within the myocardium. In the absence of δ-sarcoglycan, the remaining sarcoglycans (δ, β, γ, Σ) cannot assemble properly in the endoplasmic reticulum. Mouse models of δ-sarcoglycan deficiency demonstrate dilated HCM, sarcolemmal fragility, and disrupted vascular smooth muscle, which leads to vascular spasm, including coronary spasm. ^,

Laminins are extracellular proteins that interact with cellular integrins, allowing communication between the cell and its extracellular matrix environment. Mutations in the genes LAMA and ILK , which connect integrins and the actin cytoskeleton, lead to severe cardiac dysfunction in mice. This was the impetus to look for, and find, mutations in the human genes in patients with DCM.

Dystrophin is a vital protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. This complex is variously known as the costamere or the dystrophin-associated protein complex . Many muscle proteins, such as α-dystrobrevin, syncoilin, synemin, sarcoglycan, dystroglycan, and sarcospan, colocalize with dystrophin at the costamere. Pathogenic gene variants in the dystrophin gene causes various degrees of deficiency. When this protein is abnormally expressed or deficient, it results in a constellation of symptoms characterized by muscular dystrophy, DCM, heart failure, and arrhythmias typically seen in the first or second decades of life.

The final cytoskeletal protein-encoding gene, VCL , encodes vinculin and its splice variant metavinculin. Vinculin is ubiquitously expressed, and metavinculin is coexpressed with vinculin in heart, skeletal, and smooth muscle. The protein complex is localized to subsarcolemmal costameres in the heart, where it interacts with α-actinin, talin, and γ-actin to form a microfilamentous network linking cytoskeleton and sarcolemma. ^, In addition, these proteins are present in adherens junctions in intercalated disks and participate in cell–cell adhesion. Mutations in metavinculin have been shown to disrupt the intercalated disks and alter actin filament crosslinking.

Sarcomeric Proteins

Mutations in the sarcomere may produce HCM, DCM, or NCM. In the latter case, abnormalities in force generation or transmission are thought to contribute to the development of this phenotype. ^, Cardiac actin is a sarcomeric protein that is a member of the sarcomeric thin filament interacting with tropomyosin and the troponin complex. As previously noted, actin plays a significant role in linking the sarcomere to the sarcolemma. The mutations in actin that resulted in DCM, as described by Olson and colleagues, appear to be directly involved in the binding of dystrophin. Further, actin interacts in the sarcomere with cardiac troponin T and β-myosin heavy chain, the products of two other genes resulting in either DCM or HCM depending on the position of the mutation.

In addition to mutations in the thin filament protein actin, mutations in the thick filament protein-encoding gene β-myosin heavy chain and myosin binding protein C3 have been shown to cause DCM with associated sudden death in at least one infant, as well as DCM in older children and adults. Mutations in this gene are thought to perturb the actin-myosin interaction and force generation or alter cross-bridge movement during contraction.

Mutations in all three cardiac troponin types (C, I, and T) have been speculated to disrupt calcium-sensitive troponin C binding. ^, Recessive mutation in troponin I is thought to impair the interaction with troponin T, and α-tropomyosin mutations also have been identified and were predicted to alter the surface charge of the protein, leading to impaired interaction with actin. ^, Mutations in phospholamban have also been identified that further support calcium handling as a potentially important mechanism in the development of DCM.

An area of interest for evaluation at the molecular level has been the Z disk. Knoll and colleagues identified mutations in the CSRP3 gene and demonstrated that this results in defects in the interaction of MLP with telethonin. Mutations in CSRP3 also have been shown to result in abnormalities in the T-tubule system and Z-disk architecture by electron microscopy, which correlates with the histopathologic findings in CSRP3 -knockout mice. In addition, mutations in α-actinin 2, which is involved in crosslinking actin filaments and shares a common actin-binding domain with dystrophin, were also identified in familial DCM; these mutations in α-actinin 2 disrupt its binding to MLP. ^{, ,}

A number of other sarcomeric proteins also have been shown involved in DCM. Titin is a giant sarcomeric cytoskeletal protein that contributes to the maintenance of the sarcomere organization and myofibrillar elasticity, and interacts with these proteins at the Z disk–I band transition zone. ^{, ,} Mutations in the titin gene have been identified in familial DCM, and it is thought to be the most common gene mutated, as it is estimated to be seen in 20% of affected individuals. ^, The titin splicing factor RNA-binding motif protein 20 has a role in posttranslational modifications which appear to alter the mechanics of titin, and mutations are likewise associated with DCM. Purevjav and colleagues showed that mutations in myopalladin, an immunoglobulin-domain family member protein that works as an intermediate in sarcomere/Z-disk assembly, were present in several patients with DCM and HCM, and animal models revealed various pathways of cellular disturbance based on specific mutations. Finally, CARP, encoded by ANKRD1 , also interacts with myopalladin and titin and may be mutated in both DCM and HCM. ^,