Dilated Cardiomyopathy

Prevalence and Pathogenesis of Dilated Cardiomyopathy

Cardiomyopathies are defined as myocardial disorders in which the heart is structurally and functionally abnormal. Morphologically defined subtypes include hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), arrhythmogenic cardiomyopathy, and left ventricular (LV) noncompaction cardiomyopathy,^1,2 and each of these subtypes can be genetically mediated (Figure 1). DCM is characterized by an enlarged and poorly contractile LV. DCM can be attributed to genetic and nongenetic causes, including hypertension, valve disease, inflammatory/infectious causes, and toxins.³ Even these nongenetic forms of cardiomyopathy can be influenced by an individual’s genetic profile and, furthermore, mixed pathogeneses may be present. In DCM, the degree of LV systolic dysfunction is variable, and LV systolic dysfunction is often progressive. DCM is a major risk factor for developing heart failure (HF) as the presence of reduced systolic function does not imply symptoms. Notably, DCM is often associated with an increased risk of severe arrhythmia, indicating the pathological involvement of the cardiac conduction system.

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Randomized clinical HF trials typically report 30% to 40% of subjects with a nonischemic DCM compared with ischemic DCM.³ Clinical trials are evaluating interventions to reduce congestive heart failure symptoms, and these studies may focus on the later stages of disease. Similarly, a recent survey of hospitalized patients in the United States in which the mean age was 75 years (n=156 013) found that ischemic cardiomyopathy was more common than nonischemic (59% compared with 41%).⁴ Of nonischemic DCM, hypertension accounted for 48%, and idiopathic was the next most common at 31%. In this study, individuals with nonischemic DCM were more likely to be women, nonwhite, and younger that those with ischemic cardiomyopathy.

The true prevalence of DCM, and of genetically mediated DCM, is not fully known. An early estimate of DCM prevalence derived from a study performed from 1975 to 1984 in Olmstead County, MN.⁵ This epidemiological study relied on echocardiography, angiography, or autopsy evaluation of DCM cases and resulted in a prevalence of 36.5/100 000 individuals or 1 in 2700 with a men to women ratio of 3:4 in a European-American population.⁵ The prevalence of DCM varies likely reflecting geographic and ethnic differences, as well as the methodologies used.^6–10 Studies from England (8.3/100 000),¹¹ Italy (7.0/100 000),¹² and Japan (14/100 000)⁹ report similar DCM prevalence. However, the prevalence of DCM is likely underestimated. The prior studies relied on older less sensitive imaging modalities.¹³ More recently, Hershberger et al¹⁴ used a different approach to estimate DCM prevalence, based on the known ratio of idiopathic DCM to HCM of ≈2:1, prevalence estimates of HF, and prevalence estimates of LV dysfunction as a surrogate for DCM. With this approach, a much higher prevalence of DCM is estimated, in the range of 1:250. Similarly, estimates of familial DCM prevalence varies: a meta-analysis of 23 studies found a prevalence estimate of 23% with a range of 2% to 65%, indicating a significant heterogeneity in diagnostic criteria, and a frequency progressively increasing over time because of more systematic clinical screening.¹⁵ In the clinical practice and in the current guidelines, the prevalence of familial DCM is assumed to be ≈30% to 50%.^{1,3,13,16–18} In patients with familial DCM, ≈40% has an identifiable genetic cause.¹⁷ Also in sporadic DCM, pathogenic genetic variants can be identified although the frequency of genetic causes in this population is not well defined.¹⁷

Clinical Diagnosis of DCM

DCM has been defined by the presence of (1) fractional shortening <25% (>2 SD) or ejection fraction <45% (>2 SD), and (2) LV end-diastolic diameter >117% (>2 SD of the predicted value of 112% corrected for age and body surface area), excluding any known cause of myocardial disease.¹⁹ In the context of a familial DCM, these criteria are used to diagnose the proband in a family,¹⁹ and the strategy for evaluation is shown in Figure 2. Familial DCM is defined by the presence of (1) ≥2 affected relatives with DCM meeting the above criteria, or (2) a relative of a DCM patient with unexplained sudden death before the age of 35 years.¹⁹ Family members may be classified as affected, unaffected, or unknown when subtle cardiac abnormalities are present but not sufficient for a definitive diagnosis. Less common forms of primary cardiomyopathies are peripartum, tachycardia-induced, stress-provoked Takotsubo cardiomyopathy and myocarditis, according to the 2006 American Heart Association (AHA) definition and classification.² Interestingly, myocarditis and peripartum cardiomyopathy can occur in a familial setting and are believed to have a genetic component.^22–24 Secondary forms of cardiomyopathies, in which cardiomyopathy arises from systemic disorders, such as amyloidosis, hemochromatosis, and because of toxicity from agents like doxorubicin, are also under genetic influence or may arise due to primary genetic mutations.^25,26 Neuromuscular disease is also commonly associated with cardiomyopathy, and cardiomyopathy typically arises from the primary responsible genetic mutation exerting pathological effects directly in the myocardium (see below). With the increase in using genetic testing, especially testing in family members, it is now possible to use cardiac imaging modalities to ascertain early features of disease in gene mutation-positive individual who do not fully manifest disease. Because many of these studies examine gene positive individuals at one point in time, a full view of DCM progression has not been delineated. Definitive studies on DCM progression in genetically at-risk individuals must span many years. Imaging studies have identified chamber size dimensions, strain abnormalities, and contrast enhancement each as features of early DCM.²⁵

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Imaging: Echocardiography

To diagnose DCM, LV measurements can be determined using multiple imaging modalities. M-mode and 2-dimensional echocardiography are frequently used to determine LV internal dimensions in systole and diastole (Figure 1). It was originally thought that LV dilation occurs in response to reduced function. However, in genetic DCM, where genetic markers make it feasible to monitor LV dimensions for many years, increased LV dimensions typically precede detectable reduction in function.^27–29 This state of LV enlargement is recognized as a prodome to DCM. Enlarged LV dimensions contrast with what is seen in HCM, where the earliest findings in genetically mediated, sarcomere-positive HCM are reduced LV dimensions.³⁰ Strain and strain rate differences can be detected by echocardiography in first-degree relatives of those with newly diagnosed nonischemic DCM, indicating that LV dimensions are not the earliest detectable differences in familial DCM.^31,32

Imaging: Cardiac Magnetic Resonance

LV chamber dimensions and function, including strain measurements, are also accurately determined by cardiac magnetic resonance (CMR) imaging. Contrast agents, mainly gadolinium, are used to evaluate fibrosis and therefore provide additional information on myocardial tissue quality. In DCM, the degree of fibrosis, marked by delayed gadolinium enhancement, is a predictor of all-cause mortality and need for future hospitalization.³³ Specifically, delayed enhancement is associated with increased risk for ventricular arrhythmias.^34–36 Delayed enhancement may also reflect features beyond fibrosis, including edema and inflammatory infiltrate.³⁷

DCM can also be associated with LV noncompaction seen using either echocardiography or CMR. A ratio of >2.3:1 for the noncompacted to compacted layer of LV myocardium is considered abnormal but notably can be detected in normal hearts.³⁸ Recent studies have suggested that hypertrabeculation is seen in a high fraction (36%) of adult DCM although this was not associated with adverse outcomes compared with DCM without these noncompaction features.³⁹

Endomyocardial Biopsy

Endomyocardial biopsy (EMB) has been used to confirm diagnosis in some forms of DCM although with improved cardiac imaging, EMB is less frequently used. In some settings, for example, iron overload, amyloid, and other infiltrative processes, myocardial biopsy may still be highly useful.⁴⁰ The complication rates with EMB range from 1% to 3%, and serious complications including perforation and tamponade occur at 0.5%.⁴⁰ EMB has been used to evaluate myocarditis and in the setting of unexplained HF.^41,42 The nonuniform nature of infiltrative disease limits the sensitivity of myocardial biopsy because the right ventricular septum is targeted for sampling.⁴³ For the majority of genetic cardiomyopathies, genetic testing is favored over EMB. Of the genetic cardiomyopathies, arrhythmogenic right ventricular cardiomyopathy (ARVC) may be evaluated by EMB although more recent work suggests that alternative and more easily accessible cell types can be used to diagnose ARVC and avoid EMB.^44,45

Noninvasive Arrhythmia Monitoring

DCM is associated with an increased risk for cardiac arrhythmias and sudden cardiac death (SCD),⁴⁶ and specific genetic DCM subtypes are especially prone to arrhythmias. CMR, especially the identification of delayed enhancement, can help risk stratify for sudden death.³⁶ Because of increased risk for SCD, there is need for arrhythmia surveillance to more appropriately deploy device management, including pacemakers and implantable cardioverter defibrillators (ICDs). Symptomatic and even life-threatening bradycardia and tachycardia may occur in genetic DCM. Personal history of syncope or near syncope should be ascertained, and patient education to increase awareness of symptoms is needed. Holter monitoring, for its ease, remains a mainstay using 24- to 48-hour sampling. Other external event recorders are similarly transcutaneous and provide real-time transmitted data, as well as triggered monitoring. External loop recorders and now implantable loop recorders offer long-term information. Those with familial DCM are likely to have more findings of ventricular ectopy and ventricular tachycardia (VT) on monitoring.⁴⁷ For primary prevention of SCD in DCM, risk stratification often relies on evaluating the specific genetic contribution (see below), family history, delayed enhancement, and presence of VT on monitoring.

Clinical Manifestations Including Neuromuscular Findings

The range of clinical manifestations in DCM ranges from none to overt HF. With the increase in familial and genetic screening, it is now more common to identify the minimally to mildly affected stage of DCM in younger individuals. Using genetic markers, strain defects can be detected by echo or CMR. LGE (late gadolinium enhancement) may be present even when the heart appears still normal, suggesting that disease is ongoing. There is an emerging view that this represents an early stage of disease and one in which early institution of treatment should benefit. Although it is generally thought that arrhythmia risk scales with degree of LV dysfunction, for several subtypes of genetic cardiomyopathy, arrhythmias may be the earliest manifestation.^48–51 Specifically, in LMNA- and SCN5A-mediated cardiomyopathies, arrhythmias including atrial fibrillation with slow ventricular response or ventricular arrhythmias may be the presenting finding. There is little in the clinical evaluation that makes it possible to distinguish one genetic subtype of DCM from another. This phenocopying is what has driven gene panel testing because with this approach, multiple genes are screened at the same time. A DCM gene panel is shown in Figure 3, and a comprehensive list of DCM genes is provided in the Table.

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Neuromuscular disease may accompany cardiomyopathy, and in some forms of neuromuscular disease, the presenting feature may be irregular heart rhythms. LMNA mutations can present with or without muscle disease, and the muscle disease ranges from limb-girdle muscular dystrophy to Emery–Dreifuss muscular dystrophy, which is typically associated with contractures of the elbows and Achilles tendons.⁵⁴LMNA mutations are inherited in an autosomal manner, seen as multiple affected family members in each generation. Both X-linked and autosomal neuromuscular diseases can also associate with cardiomyopathy, and this includes Duchenne muscular dystrophy, as well as the autosomal recessive forms of sarcoglycanopathies.⁵⁵ In these disorders, skeletal muscle disease usually appears in childhood with a typical DCM arising in the teenage years or early twenties. DCM in neuromuscular disease is highly amenable to treatment and responds well to guideline-directed medical therapy. Both forms of myotonic muscular dystrophy, type 1 and type 2, can also be associated with DCM.^56–59 Atrial and ventricular arrhythmias are common in these tri- and tetra-nucleotide repeat expansion disorders and should be aggressively managed. Myotonic dystrophy type 2 usually present in older individuals, and in this case, genetic testing panels usually do not include these genes and thus the diagnosis can be easily missed especially if neuromuscular symptoms are not so pronounced.

DCM Genetics

The majority of genetic DCM is inherited in an autosomal dominant pattern with variable expressivity and penetrance (Figure 4) although specific forms of autosomal recessive, X-linked recessive, and mitochondrial inheritance each occur.^14,19,60 De novo mutations also contribute to genetic cardiomyopathy and are defined when neither biological parent carries the offspring’s mutation. De novo mutations have been described in many different genes, and the presence of a de novo variant can be used to define the pathogenic status of genetic variants because the frequency of de novo variation in each genome is exceedingly rare. Thus, a novel mutation introducing a protein-altering change in a cardiomyopathy gene is typically considered pathogenic.

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Interpreting whether genetic variants are pathogenic is increasing complex because of the vast amount of rare variation in each human genome. The emerging consensus around interpretation of genetic variation and its effect on phenotype rely on a classification system ranging from pathogenic, likely pathogenic, variant of uncertain significance, likely benign, and benign.⁶¹ The availability of large control cohorts provides invaluable information of the frequency of variants, and the largest available data set is currently Exome Aggregation Consortium (http://exac.broadinstitute.org), which collected exome sequencing data of >60 000 individuals from a series of studies including the 1000 Genomes Project and the Exome Sequencing Project (http://evs.gs.washington.edu/EVS). The current trend is to consider putative pathogenic variants as those that are either unique to the DCM patient or family,⁵² or extremely rare (minor allele frequency <1×10^–4).⁶² The recently adopted more stringent criteria for genetic testing have prompted the reclassification of variants and indicate the needs of a continuous reanalysis of data.⁵² The use of whole-exome/-genome sequencing in clinical laboratories warrants strong criteria to discriminate common variants. At present, genetic testing typically relies on self-reported ethnicity testing, and it is important to match ethnicity between the proband and testing databases. However, at this point, this integration of common and rare variation is not routinely being used in cardiomyopathy genetic testing, potentially contributing to false-positive interpretation.⁶³

DCM is genetically heterogeneous, and DCM genes encode proteins of broad cellular functions. Mutations in genes encoding cytoskeletal, sarcomeric, mitochondrial, desmosomal, nuclear membrane, and RNA-binding proteins have all been linked to DCM. Thus, the pathological mechanisms that lead to DCM are diverse. The genes below are listed in order of frequency for their contribution to genetic DCM with focus on the most commonly implicated genes and their mechanism of action if known.

TTN

The discovery of the role of TTN-truncating variants in DCM has been major advance.⁶⁴ The TTN gene encodes the giant protein titin, which is the largest known protein expressed in the heart. Titin functions as a spring, providing passive force and regulating sarcomere contraction and signaling.⁶⁵ Titin is a large ≈35 000 amino acid protein that spans half the length of the sarcomere from Z disc to M band and is referred to as a third filament with the thin and thick filaments that comprise the sarcomere. Proposed as a molecular rule for the sarcomere, titin has domains that can accommodate passive stiffness.⁶⁶ Titin’s I band region includes the proline–glutamate–valine–lysine repetitive region, which is thought to directly regulate passive tension. The I band region of the TTN gene is encoded by 220 of TTN’s 360 exons. The large size, repetitive nature, and extensive alternative splicing of TTN make it challenging for genetic analysis. The proline–glutamate–valine–lysine region is just carboxyl to the N2A and N2B regions that interact with the four and half LIM protein, identified as a modifier for HCM.⁶⁷ Notably, TTN is differentially spliced throughout heart development and adaptively to distinct physiological states, including HF.⁶⁸ The larger N2A form is associated with a more compliant ventricle (Figure 5). In contrast, the smaller N2B form lacks more of the repetitive units and is associated with stiffer heart. Deep RNA sequencing of TTN from failed hearts suggests highly variable exon usage in this region consistent with even subtler defects in cardiac elasticity that may be variable across regions of the LV.⁶⁹

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Using a TTN-specific array designed to capture all TTN exons, it was shown that truncating variants of TTN contribute to 20% to 25% of nonischemic DCM.⁶⁴ Before this, only a few missense TTN variants had been described linked to DCM.⁷⁰ Induced pluripotent stem cells (iPSC) differentiated into cardiomyocytes in culture demonstrate a paucity of sarcomeres, suggesting that force may be impaired directly through sarcomere loss in TTN truncations.⁷¹ In these studies and others, it has been shown that TTN truncations are observed at a low frequency in the general population, ranging from 1% to 3%.^64,69,72 There is a tendency for TTN truncations in DCM to distribute to the A band, rather than the I band,⁶⁹ and TTN truncations can also be associated with mild DCM.⁷³ A recent study showed that truncating variants in the general population are linked to eccentric cardiac remodeling, suggesting that TTN truncations may be at-risk alleles.⁷⁴

Peripartum cardiomyopathy is a heterogeneous syndrome of mixed pathogenesis. Yet a subset of peripartum cardiomyopathy is attributable to TTN-truncating variants.^75,76 Peripartum cardiomyopathy can be associated with recovered LV function after pregnancy. Moreover, the observation that TTN-truncating variants can be associated with recovery of function in DCM after LV assist device placement also suggests a dynamic state of TTN-truncating variants.⁷⁷ Additional genes with mutations beyond TTN have also been described in peripartum cardiomyopathy.²³ Overall, the presence of TTN-truncating variants in the general population argues for caution in interpreting these variants and again underscores the importance of familial segregation analysis. At this point, until more is known, the presence of a TTN truncation variant should trigger at least intermittent cardiac imaging and management aimed at reducing other stressors to the heart.

TTN has a high prevalence of missense variants, both rare and common.⁷⁸TTN missense variants have been reported in ARVC and other forms of cardiomyopathy.^70,78–81 In addition, TTN missense variants have also been reported in skeletal myopathy, including the common tibial myopathy.^82,83 The enormous number of TTN missense variants makes these variants exceedingly complex to interpret in the context of broad genetic testing on individuals with DCM.

Zebrafish have been used successfully to model myopathies because of TTN mutations, demonstrating both cardiac and skeletal muscle defects.⁸⁴ A mouse model with an in-frame deletion in the proline–glutamate–valine–lysine region of TTN develops diastolic dysfunction, consistent with the complex role of titin for both systolic and diastolic dysfunctions.⁸⁵ In both rats and mice with heterozygous TTN truncation mutations, additional stressors, such as transaortic constriction, are used to promote the development of DCM.^74,86

LMNA

LMNA missense and truncating mutations account for 5% to 8% of genetic DCM.^87,88 Like TTN, LMNA mutations are inherited in an autosomal dominant manner. The single LMNA gene encodes lamins A and C, and differential splicing at the 3′ end results in 2 proteins that are identical across their first 566 amino acids; mutations in LMNA lead to a constellation of diseases from premature aging to myopathies and DCM.⁸⁹ Mutations that alter processing of lamin A lead to accumulation of prelamin A (sometimes called progerin), and these have been associated with the premature aging syndrome Hutchinson–Gilford progeria.⁹⁰LMNA mutations linked to autosomal dominant DCM are both missense and frameshifting in nature, and these mutations may occur anywhere along the length of the coding region. DCM-associated mutations are not specifically associated with prelamin A accumulation; thus, the basic mechanisms underlying the premature aging syndrome versus DCM seem to be distinct. The mechanisms responsible for autosomal dominant DCM LMNA mutations may be a mix of multiple defects, including dominant-negative function and haploinsufficiency.^91,92 Lamins A and C are implicated in many different cellular processes from regulating gene expression, mechanosensing, DNA replication, and nuclear to cytoplasmic transport.

Loss of LMNA leads to a defect in mechanosignaling.^93,94 Mechanosignaling defects were observed in cell with a homozygous deletion in the mouse Lmna gene. Male mice heterozygously deleted for Lmna exhibit cardiomyopathy features in later life, suggesting that mice can be used to model laminopathy.⁹⁵ In LMNA-associated adult-onset DCM, the mTOR pathway can be activated, and in animal models, inhibition of mTOR by temsirolimus or rapamycin was able to rescue the DCM phenotype.^96,97 Mitogen-activated protein kinase signaling is increased in these models, leading to clinical trials testing compounds aimed at reducing this signaling.⁹⁸ Recently, early phase, encouraging results were reported from a phase 2 registration trial on A797 (Array Biopharma), an oral, selective p38 mitogen-activated protein kinase inhibitor in 12 patients with LMNA-associated DCM.⁹⁹

LMNA mutations associate frequently with a signature of dysrhythmias that includes sinus node dysfunction, atrial fibrillation, atrioventricular node dysfunction, VT, ventricular fibrillation, and SCD.^48,49 Notably, cardiac conduction system disease may precede the development of LV dilation and dysfunction, and the presence of early conduction system disease may suggest LMNA mutation. Aspects of the arrhythmia and LV dysfunction phenotypes are not fully replicated in the mouse models; namely, atrial fibrillation and ventricular arrhythmias are not frequently seen, and a homozygous mutation is often needed to generate DCM. Heterozygous truncating LMNA mutations have a higher arrhythmia risk than missense variants,⁴⁹ and a prolonged PR interval indicates cardiac conduction system disease in laminopathy.¹⁰⁰ The susceptibility of the cardiac conduction system to LMNA mutations is not well understood.

PLN

The PLN gene encodes phospholamban, a 52 amino acid residue transmembrane protein that, when unphosphorylated, inhibits sarcoplasmic reticulum Ca²⁺-ATPase. Several dominant mutations in PLN have been associated with DCM, including the R14del mutation that is a founder mutation in the Netherlands and Germany. Thus, in some populations, the percentage of DCM due to PLN mutations is high. The phenotype with PLN mutations is variable. Early onset DCM with lethal ventricular arrhythmias was described.^101,102 Similarly, individuals from the Netherlands with the R14del founder mutation have a severe phenotype.¹⁰³ However, other reports suggest a milder phenotype.^104,105 Identifying the same primary mutation(s) with a range of phenotype dependent on genetic background supports that other factors, including genetic factors, may modify the outcome of PLN-mediated DCM.

iPSCs with the R14del PLN mutation were generated and found to display features of cellular cardiomyopathy, including aberrant Ca²⁺ handling after caffeine and a higher percentage of irregular Ca²⁺ transients, and these features were reversed after gene editing to correct the primary mutation.¹⁰⁶ These same cells were used to engineer 3-dimensional human heart tissues, and in this setting, more clear cut cardiomyopathic features were seen including reduced developed force that was improved after genetic correction.¹⁰⁷ In iPSC-derived cardiomyocytes and in hearts from PLN mutation carriers, aggregates of phospholamban were seen in a perinuclear and cytoplasmic pattern, suggesting that aggregated phospholamban contributes to the pathology possibly through aberrant autophagy.¹⁰⁸

RBM20

RNA-binding motif 20 is an RNA-binding protein expressed highly in both atria and ventricle. Dominant mutations in the RBM20 gene were first described in DCM, where they contribute to 1% to 5% of DCM.^109,110 RBM20 is 1227 amino acid in length and contains a ribonucleic acid recognition motif domain between residues 525 and 600 amino acid. A second conserved domain is found between 650 and 725 amino acid, and the mutations originally described in nonischemic DCM fall within or near these domains. More recently, mutations in a third conserved region were identified in a glutamate-rich region.¹¹¹ As an RNA-binding protein, RBM20 is implicated in tissue-specific splicing relevant to development and adaptation to disease states. In the heart, RBM20 regulates cardiac splicing, including the splicing of TTN.^112–114 Thus, the downstream molecular consequences of RBM20 mutations may share similarities to those occurring from TTN-truncating variants.

iPSC-derived cardiomyocytes with the RBM20 R636S mutation develop a gene expression and splicing profile consistent with cardiomyopathy affecting not only TTN but also the CAMK2D and CACNA1C genes.¹¹⁵ Sarcomeres within these RBM20 mutant lines were thinner, similar to what was described for TTN mutant iPSC-cardiomyocytes.⁷¹ Recently, RBM20 was implicated in the production of circular RNAs from the TTN locus, and mice deleted for RBM20 failed to produce these Ttn-derived circular RNAs.¹¹⁶ Although the function of circular RNAs is not known, the authors described that a subset of Ttn-derived circular RNAs were misregulated in DCM.

SCN5A

SCN5A encodes the major sodium channel expressed in the heart, and heterozygous dominant mutations in SCN5A are also found in primary arrhythmia syndromes, including the long QT and Brugada syndromes. Missense mutations in SCN5A have also been described in familial DCM, and these mutations carry a higher risk for arrhythmias.^50,51 There is considerable genetic heterogeneity in the SCN5A gene in the general population, making it challenging to interpret rare variation in the SCN5A gene.⁶² Genotype–phenotype association studies may guide genotype-based therapies. For example, the SCN5A R222Q falls within the S4 voltage sensor and is thought to enhance excitability. Treatment with lidocaine was observed to suppress the bigeminy associated with cardiomyopathy.¹¹⁷

iPSCs have been used to model SCN5A mutations associated with primary human arrhythmia syndrome.^118–120 Transgenic-directed inducible expression of the Scn5A F1759A in mice leads to atrial fibrillation and persistent sodium currents in atria and ventricles.¹²¹ Along with atrial fibrillation, these mice have progressive reduced LV ejection fraction (LVEF) consistent with a model for DCM. Thus, the distinct roles of SCN5A in the myocardium and the conduction system lead to a combination of arrhythmia and myocyte dysfunction.

Cytoskeletal Genes

Genes encoding cardiac cytoskeletal proteins have been implicated in DCM (Figure 6). For example, mutations in dystrophin link to X-linked DCM and cardiomyopathy in Duchenne muscular dystrophy.^122–124 Along with dystrophin mutations, mutations in the sarcoglycan genes produce cardiomyopathy, usually associated with muscular dystrophy.^55,125 In these disorders, the gene products play a normal and essential role in managing sarcolemmal stability. Thus, in the absence of these genes, the sarcolemma becomes unstable, leading to cardiomyocytes loss and heart dysfunction. Several emerging therapies for restoring dystrophin expression are being tested or have been approved recently.^126,127 Antisense oligonucleotides are being delivered to produce internally truncated dystrophin proteins, and stop codon suppression compounds promote read through of premature stop codons. The degree to which these drugs access the human heart is not well-known, and ongoing studies will be in a position to assess this in humans.

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More recently, FLNC mutations, in the gene-encoding filamin C, have been described in DCM.^128–130 Filamin C interacts with the dystrophin complex, and deletion of Flnc from the mouse leads to skeletal myopathy.^131,132 In humans, truncating mutations lead to cardiomyopathy that is associated with a high rate of ventricular arrhythmias and SCD, suggesting that filamin C has a role in the cardiac conduction system in addition to the cardiomyocyte.

Mitochondrial Mutations

Both nuclear-encoded and mitochondrial-encoded mitochondrial genes lead to cardiomyopathy.^133–136 Mutations in the mitochondrial genomes may be difficult to identify and interpret given the role of heteroplasmy and the fact that most genetic testing relies on peripheral blood DNA, which may or may not match what occurs in the heart. Nuclear-encoded mitochondrial genes follow either autosomal dominant or recessive inheritance, whereas mitochondrial-encoded genes show maternal inheritance.

Additional Genetic Mutations in DCM

The proteins encoding the sarcomere, the unit of contraction, are also implicated in DCM. Recent data from clinical genetic testing indicate that MYH7, TNNT2, and TPM1 are the most frequent mutated sarcomere genes in DCM, ranging from ≈2% to 4%, while MYBPC3 mutations are rare.⁵² Recently, truncations of the gene-encoding obscurin have been found in both LV noncompaction and DCM phenotypes.¹³⁷ DCM may be best considered as a cardiomyopathy of mixed origin, familial in ≈30% to 50% of patients, consistent with its being a genetic disease. Nonfamilial or idiopathic DCM may still have a genetic origin albeit a complex one. Because all DCM mutations have variable expressivity, this may support a model of oligogenic contribution along with environmental or other pathogenic stimuli. As it currently stands, not all DCM genes have been discovered. Thus, the pathogenesis in nonfamilial cases could be as yet undiscovered genes, low penetrance, de novo mutations, missing heritability because of multiple genes with weaker effect, copy number variations, enhancer region mutations, and intronic variants or may be the result of the interaction between modifier genes and the environment.^13,138

Digenic/Oligogenic Pathogenesis

Occasionally, digenic variants have been reported; while it is possible that some of these variants may in fact be variant of uncertain significance or benign, in other cases the digenic condition have been found to segregate in informative kindred and associate with a more severe phenotype. This is the case of a large family cosegregating a LMNA mutation, where relatives with an additional TTN truncation showed worse outcome and distinct pathological changes.¹³⁹ With the growth of gene panels for cardiomyopathy genetic testing, it is not uncommon to identify >1 potential pathogenic variant. Segregation analysis may be helpful to clarify primary versus other variants. Compound mutations MYBPC3 lead to early onset, sometime neonatal cardiomyopathy.^140–142 Thus, in families with multiple affected members with DCM, a broad gene panel on a younger, affected proband may provide a more comprehensive view of potential variation. Segregation testing for variants of interest can then help clarify those variants with greatest effect.

Genetic Testing

Family Screening

Genetic evaluation of DCM should begin with extensive and accurate evaluations of the patient’s family history, involving at least 3 generations and including history of cardiomyopathy and history of sudden unexpected death at young age (<35 years).^3,19,20 This information will guide genetic testing, provide good care to family members, and aid in the interpretation of the results and help identifying relatives at risk of disease.

Clinical Cascade Screening

Clinical cascade screening of relatives is recommended per AHA and European Society of Cardiology (ESC) guidelines.^3,143 First-degree relatives of affected family members should be clinically evaluated (Figure 2). First line of screening usually relies on ECG and echocardiography to evaluate ventricular size and function. Clinical history should evaluate signs and symptoms for arrhythmias and any history of neuromuscular disease. Other cost-effective tools to consider in family member screening include ascertaining arrhythmia history and Holter monitoring.⁴⁷ A diagnosis of familial DCM is made where ≥2 family members are affected by DCM.¹⁹

Clinical Genetic Testing

According to the 2016 AHA Scientific Statement on DCM,³ genetic testing is recommended (with moderate level of consensus) in patients with familial (Level of Evidence A) and nonfamilial idiopathic cardiomyopathy in conjunction with genetic counseling (Level of Evidence B) although there is strong level of consensus in recommending mutation-specific genetic testing for family members after the identification of a DCM-causative mutation in the proband (Level of Evidence B). Reflecting lack of full consensus in the field, the 2016 ESC Position Statement¹⁴³ is slightly different and recommends genetic testing in all familial DCM or nonfamilial with clinical clues (such as atrioventricular block or creatine kinase elevation). Interestingly, the ESC Position Statement recommends that genetic testing should be oriented by clinical diagnostic clues and restricted to genes known to cause DCM although considering large panels of genes only when the family structure is large enough to permit segregation analysis.

Multiple commercial and academic vendors provide genetic testing gene panels under certification of the Clinical Laboratory Improvement Amendment. A typical DCM gene panel includes ≈40 to 50 genes (Figure 3).^52,144 The probability of positive genetic testing in familial DCM is overall in the range of 40% with the current next-generation sequencing panels and seems not different compared with sporadic cases.^13,52 A pancardiomyopathy panel, as opposed to a DCM panel, is chosen when the phenotype is unclear, and a more comprehensive screening is preferred. The level of evidence to support testing for some genes has been questioned, and this is largely based on a high level of genetic variation in those genes in the population at large.¹⁴⁵ Larger panels may yield greater difficulty in interpreting the results because of variant of unknown significance. In a survey of 766 patients screened in a clinical laboratory, with the introduction of next-generation sequencing technology and larger panels, while the rate of positive testing increased from 10% to 40%, the number of variant of uncertain significance increased 10-fold.^13,52,146

Genetic Counseling

Genetic counselors, especially those with experience in cardiovascular testing, provide both pre- and post-test counseling. With the increasing complexity of cardiomyopathy genetic testing, referral to specialized cardiovascular genetic clinics should be considered.²⁰ Pre-test genetic counseling should involve discussion of potential genetic results (pathogenic mutations, variants of uncertain significance, and benign genetic variants). This counseling should also discuss the impact on insurability, reproduction, and lifestyle. Post-test counseling focuses on variant interpretation/possible reinterpretation, reproductive risks to offspring, and family testing.

If a genetic mutation is identified, genetic cascade screening can be conducted with family members. Cascade genetic testing evaluates the specific family mutation rather than a gene panel. Site-specific testing is of low cost and rapid turnaround so that this can be a cost-effective strategy to eliminate the need to serially follow gene mutation-negative family members. For gene mutation-positive family members, current guidelines suggest an annual clinical follow-up with ECG and echocardiography.^3,20,143

In the absence of an informative genetic testing, asymptomatic first-degree relatives should be examined every 3 to 5 years.²⁰ This strategy may allow prompt therapeutic measures in carriers showing sign of myocardial dysfunction. Even in the absence of a positive genetic testing, longitudinal studies have shown the benefit of family screening and monitoring. In ≈10% of cases, mild myocardial dysfunction may progress into overt DCM within 5 years.^29,147 Furthermore, clinical family screening in DCM helps to identify affected relatives at earlier stages of disease, and this associates with improved survival as compared with sporadic DCM.¹⁴⁸

Management of DCM

Established Medical Therapies

Management of DCM is focused on (1) LV dimension and function, (2) arrhythmia surveillance and treatment, and (3) reducing congestive symptoms if present. Symptomatic DCM and HF with reduced LVEF is managed following current AHA/American College of Cardiology and ESC guidelines. Guideline-directed therapy includes angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, in association with β-blockers, aldosterone antagonists, and in selected cases, vasodilators.^149–151 Medications should be titrated to the dose used in clinical trials unless limited by side effects.¹⁵² Patients with DCM on optimal therapy with complete left bundle branch block may benefit from cardiac resynchronization.^{149,150,153,154} The improvement of survival with ICDs in patients with LVEF <35% is also well established.^149,150 Patients with refractory HF may require advanced therapies, including LV assist devices or cardiac transplant.^149,150

Two newer medications for those not responding to optimal medical therapy include the angiotensin receptor-neprylisin inhibitor (valsartan/sacubitril) and the sinoatrial modulator, ivabradine.^3,150,153 Updated guidelines recommend switching from angiotensin-converting enzyme inhibitor/angiotensin receptor blockers to angiotensin receptor-neprylisin inhibitor in class II to III patients who are not responding to optimal medical therapy (ESC guidelines¹⁵⁰) or even in those responding to optimal therapy, considering the evidence of superior benefit of angiotensin receptor-neprylisin inhibitor over angiotensin-converting enzyme inhibitor/angiotensin receptor blockers in terms of mortality and morbidity (AHA/American College of Cardiology/Heart Failure Society of America guidelines¹⁵⁵). Ivabradine instead can be added to optimal medical therapy to reduce morbidity in patients with sinus rhythm and a heart rate >70 bmp.^3,150 The treatment of patients with an LVEF between 40% and 50%, defined in the ESC guidelines HF with midrange ejection fraction and in the American College of Cardiology/AHA/Heart Failure Society of America guidelines HF with improved ejection fraction, remains less clear.¹⁵³

Arrhythmia Management

Arrhythmia management in genetic DCM patients follows the general recommendations for prevention of SCD and ICD implantation based on the reduced LVEF (<35%). However, there are notable exceptions. First, a subset of patients with DCM present early in the disease course with life-threatening ventricular arrhythmias (2%)¹⁵⁶ or with frequent ventricular arrhythmias (30%), which are unrelated to the severity of LV dysfunction.⁴⁷ These patients mirror the arrhythmogenic presentation of ARVC and are described as arrhythmogenic DCM.⁴⁷ Arrhythmogenic DCM patients who present with syncope, nonsustained ventricular tachycardia, and frequent premature ventricular contractions show a higher incidence of life-threatening arrhythmic events (SCD, sustained VT, and cardiac arrest) compared with the other patients with DCM while they show no difference in outcome of HF. The coexistence of a family history of SCD and the arrhythmogenic DCM phenotype predicts a high risk of SCD events (Figure 7). These recent data suggest that ventricular arrhythmias should be systematically and carefully evaluated with monitoring and that family history of ventricular arrhythmias predicts a poor prognosis and increased risk of SCD.

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Arrhythmogenic DCM can be genetically determined, and a clear association between risk of SCD and gene has been established with the LMNA gene. The 2015 ESC guidelines for the management of patients with ventricular arrhythmias and the prevention of SCD¹⁵⁷ recommend an ICD in patients with DCM and a confirmed disease-causing LMNA mutation and clinical risk factors (nonsustained ventricular tachycardia during ambulatory ECG monitoring, LVEF 45%, male sex, and truncating mutations [Class IIa; Level of Evidence B]). Likewise, the Heart Rhythm Society/American College of Cardiology/AHA Expert Consensus Statement on the use of ICD therapy inpatients highlighted the increased risk for SCD in LMNA carriers.¹⁵⁸ Risk factors for SCD identified in 2 large LMNA carrier cohorts in Europe⁴⁹ and in the United States¹⁵⁹ include nonsustained ventricular tachycardia during ambulatory ECG monitoring,⁴⁹ LVEF <45%⁴⁹ to 50%,¹⁵⁹ male sex, and truncating mutations.^49,159 Kumar et al¹⁵⁹ reported life-threatening ventricular arrhythmia rates of 3% to 7% per year, which is higher or comparable to other known groups of high-risk patients including those with LVEF<25%, ARVC, HCM, and high-risk ischemic cardiomyopathy.

DCM also carries an increased stroke risk although less is known about the specific risk in genetic cardiomyopathy. As such, anticoagulation should be considered to reduce the risk of stroke in DCM with atrial fibrillation in particular in the presence of additional risk factor for cardioembolic events, such as history of hypertension, diabetes mellitus, previous stroke or transient ischemic attack, or ≥75 years of age.¹⁴⁹

Management of Genotype-Positive/Phenotype-Negative Patients

Genetic testing identifies genotype-positive/phenotype-negative family members, and this information is useful for prevention strategies, lifestyle recommendations, including participation in competitive sports, and possible arrhythmia management. The current guidelines recommend observation in asymptomatic at-risk relatives with yearly clinical assessment.²⁰ There is less consensus on the medical management, timing, and the type of intervention in these patients. Current guidelines recommend control of risk factors in this stage, such as hypertension.¹⁴⁹ ESC and AHA suggest restriction from competitive sports in DCM genotype-positive/phenotype-negative although evidence is lacking to support these recommendations.¹⁶⁰ When initial signs of ventricular dysfunction present during follow-up, earlier institution of medical therapy can begin although the exact timing of this is not known.²⁰

Cardiac Regeneration for DCM?

DCM is usually associated with cardiomyocytes loss, and the human heart has limited regenerative capacity. Strategies for regeneration and repair include the application of a cell suspension, growth factors, miRNAs, and the implantation of an engineered tissue.^161–163 Various studies and clinical trials have tested cardiac progenitor cells, bone marrow–derived stem cells, and pluripotent stem cells.¹⁶⁴ Frustratingly, these clinical interventions demonstrated safety but often failed to prove functional improvement. The mechanism underlying the potential effects of bone marrow–derived stem cells is unclear; the injected cells do not appear to remain in the cardiac tissue but may release paracrine factors and recruit cardiac progenitor cells.¹⁶¹

Stem cells are now being used to provide cellular models of DCM. The absence of human cardiomyocyte cell lines has been a problem for advancing research in human cardiomyopathy. iPSCs can be more readily generated from patients with DCM, and these cells can be differentiated into cardiomyocytes for study. A major limitation of iPSC-derived cardiomyocytes is their relative immaturity and variability from culture to culture, but nonetheless, these cells can be used to study cellular properties reflective of DCM features. As yet, these stem cells are not yet sufficiently mature for treating cardiomyopathy¹⁶¹ although they are yielding an important platform for understanding mechanisms and testing therapies.¹⁶⁵

Conclusions and Future Directions

Better understanding of the DCM phenome and recent improvements in sequencing technology to define DCM genome will eventually improve the diagnosis, prevention, and therapy of this disease. Next-generation sequencing technology provides a cost effective and accurate diagnostic method to yield biomarkers that indicate disease risk, especially within families. With this progress, criteria for pathogenic mutations are evolving and becoming more and more stringent and may require re-evaluation of the molecular diagnosis over time. Several questions still remain in DCM management that prompt future investigations, such as the interpretation of genetic testing, the correct treatment of pre-clinical asymptomatic DCM gene carriers, and the development of gene- and mechanism-specific therapies.

Footnotes