Advances in Molecular Genetics and Management of Hypertrophic Cardiomyopathy

A 28-year-old woman was diagnosed as having obstructive hypertrophic cardiomyopathy (HCM) during a family screening 13 years ago. The patient was initially treated with atenolol and verapamil for chest pain, but after several years, she complained of increasing angina and dyspnea. Physical examination revealed a prominent apical impulse and a grade 2/6 precordial systolic murmur. The 12-lead electrocardiogram showed sinus rhythm at 79/min, PR interval of 205 milliseconds, normal QRS axis, and left ventricular (LV) hypertrophy plus associated ST-T wave changes. The echocardiogram showed a septal thickness of 19 mm (normal, ≤11 mm); LV free wall, 15 mm (normal, <11 mm); left atrium, 52 mm (normal, <40 mm); no systolic anterior motion of the mitral valve, but apposition of LV walls at the level of papillary muscles; mild mitral regurgitation; and predicted LV gradient of 85 mm Hg at rest. A cardiac magnetic resonance imaging study confirmed LV hypertrophy, most marked at the mid-LV cavity level, and an apical LV aneurysm. The pressures at cardiac catheterization were as follows: mean right atrium, 4 mm Hg; right ventricle, 36/4 mm Hg; pulmonary artery, 30/14 mm Hg; mean pulmonary arterial capillary wedge , 19 mm Hg; LV, 190/20 mm Hg; and aorta, 100/50 mm Hg. Hence, the intracavitary LV pressure gradient, measured at midcavity, was 90 mm Hg. Cardiac output was 4.7 L/min. An LV angiogram confirmed a diagnosis of midcavity obstructive HCM. Genetic studies showed that HCM in the patient's family is caused by an Met149Val mutation of a cardiac contractile protein called essential light chain of myosin.

The patient received a dual chamber (DDD) pacemaker to relieve the severe midcavity LV obstruction. Her symptoms improved, and the hemodynamic indices during DDD pacing at a 5-year follow-up cardiac catheterization were as follows: mean right atrium, 2 mm Hg; right ventricle, 32/8 mm Hg; pulmonary artery, 24/12 mm Hg; mean pulmonary arterial capillary wedge , 9 mm Hg; LV, 150/10 mm Hg; and aorta, 105/50 mm Hg (LV gradient, 45 mm Hg). Cardiac output was 4.2 L/min. With the pacemaker programmed to atrial demand mode, the pressures were as follows: mean right atrium, 2 mm Hg; right ventricle, 26/6 mm Hg; pulmonary artery, 26/14 mm Hg; mean pulmonary arterial capillary wedge, 13 mm Hg; LV, 155/10 mm Hg; and aorta, 85/48 mm Hg (LV gradient, 70 mm Hg). Cardiac output was 4.3 L/min. The DDD pacing has significantly reduced this unusual form of LV obstruction without adversely affecting filling pressures or cardiac output. Prolonged pacing has also resulted in hemodynamic changes that were evident even when DDD pacing was discontinued temporarily.

Significant advances have been made in the understanding of the pathophysiology and management of HCM in the past decade. Hypertrophic cardiomyopathy is a genetic disease with an autosomal dominant pattern of inheritance, characterized by LV wall thickening, in the absence of another cause for the increased cardiac mass. It has an estimated prevalence of 0.1% to 0.2% in the general population, but this is probably an underestimate because many people with the disease are asymptomatic and therefore undiagnosed.¹ The cardiac hypertrophy is often associated with a dynamic obstruction to LV outflow, LV filling and relaxation abnormalities (diastolic dysfunction), and exercise-induced myocardial perfusion abnormalities.^{2 - 3} Patients often have disabling symptoms and are prone to atrial and ventricular arrhythmias and sudden death.⁴

A notable feature of HCM is its phenotypic heterogeneity.² The severity and distribution of the LV hypertrophy, presence or absence of intracavitary LV obstruction, LV systolic and diastolic dysfunction and myocardial ischemia, and risk of sudden death vary significantly even in affected members of the same family. Several distinct cardiac morphologies have been identified.² In asymmetrical septal hypertrophy, the hypertrophy primarily affects the anterior septum. In the form known as idiopathic hypertrophic subaortic stenosis, the proximal septal hypertrophy obstructs LV outflow. Japanese or apical HCM primarily affects the apex of the heart. In midcavity HCM, the hypertrophy is mainly at the level of the papillary muscles, and a narrowed passage connects the proximal LV with a distal aneurysm (Figure 1).

Hypertrophic cardiomyopathy also exhibits marked genetic (allelic and nonallelic) heterogeneity. To date, HCM has been shown to be caused by mutations in 7 genes that encode for contractile proteins that are organized into repetitive units known as sarcomeres (Figure 2 and Table 1).^{5 - 9} It is probable that less than 50% of HCM cases are accounted for by these genes, and it is likely that several nonsarcomeric genes also cause HCM. In addition, a locus in chromosome 7q3 has been linked to HCM associated with Wolff-Parkinson-White syndrome.¹⁰

Although relatives with HCM caused by the identical mutation often have significantly different clinical presentations, evidence from several studies shows that the phenotype in HCM is largely determined by the genetic defect.

Gene-Specific Cardiac Morphology. Some cardiac morphologies occur more commonly in patients with certain mutations. For example, midcavity HCM, a rare form of the disease, is strongly associated with mutations of the essential and regulatory light chains of myosin (Figure 1).⁸ In some patients with this mutation, the hypertrophied papillary muscles obliterate the LV apex and give the appearance of apical HCM. Therefore, some forms of apical HCM may be earlier stages of midcavity HCM.

Mutation-Specific Penetrance and Natural History. The type of mutation also determines disease penetrance (number of patients with HCM/number of members with mutation × 100%) and prognosis (Table 1). Most β-myosin mutations are associated with almost complete disease penetrance and a high incidence of sudden death. A few β-myosin mutations are associated with low disease penetrance and a benign prognosis.^{11 - 13} The cardiac phenotype of HCM may skip 1 or more generations in these families.^{12 - 13} In contrast, some genetic defects are associated with mild LV hypertrophy but render a poor prognosis.^{6 ,11}

Mutation-Specific Etiologies of Sudden Death. The mechanism of sudden death may also be related to the type of mutation. For example, presyncope, syncope, and presumably sudden death have been strongly associated with myocardial ischemia in patients from a National Institutes of Health family with HCM caused by the Arg403Gln β-myosin mutation.¹³ The exercise-induced myocardial perfusion abnormalities may be treated successfully with verapamil and β-blocker therapy. In some families, the genetic defect causes not only HCM but also some other distinct cardiac abnormality, such as preexcitation syndrome or heart block.

Mutation-Specific Functional Abnormalities. All of the β-myosin mutations occur in different functional domains of the head or the head-rod junction of the molecule (Table 1). It is therefore likely that the mutations have different effects on the function of the molecular motor (Figure 2).¹⁴

Demonstration that some mutations are expressed in skeletal muscle¹⁵ has also permitted study of differences in several mechanical properties (isometric force, force-stiffness ratio, maximum unloaded shortening velocity, and power) of single skeletal muscle fibers from patients with HCM caused by distinct mutations.¹⁶ Fiber stiffness is proportional to the number of strongly attached cross-bridges. A force-stiffness ratio can be used as an indication of the force generated per cross-bridge in normal fibers. The maximum unloaded velocity of shortening is used to quantify the physiologic rate of cross-bridge cycling during fiber activation. The nature and extent of functional impairments varies in skeletal fibers containing different β-myosin mutations that may correlate with the severity and penetrance of HCM.

The motility test is another elegant way of studying functional abnormalities caused by mutations at a molecular level. In this assay, myosin is bound to a coverslip using an anti β-myosin C-terminal specific antibody to the tail. The myosin heads are free to translocate fluorescently labeled actin.^{15 ,17} Mutations would be expected to affect the velocity with which the myosin translocates actin. Indeed, the velocities of fluorescently labeled actin are depressed by certain mutations but are increased by others.^{8 ,17} Hence, the motility test provides further evidence that these sarcomeric mutations have specific effects on the molecular motor.

Mutation-Specific Skeletal Myopathy. Certain sarcomeric genes, such as β-myosin heavy chain, myosin light chains, and α-tropomyosin, are expressed in both myocardium and skeletal muscle. Expression of the gene in skeletal muscle may be associated with specific skeletal myopathy.^{8 ,18} Microscopic examinations of biopsy specimens of soleus muscle from patients with HCM caused by β-myosin mutations have demonstrated a skeletal myopathy similar to central core disease, characterized by predominance of type I "slow" fibers and absence of mitochondria in the center of many type I fibers.¹⁷ Hypertrophic cardiomyopathy caused by light chains of myosin is also associated with specific skeletal myopathy, notably, a ragged red fiber pattern characteristic of mitochondrial myopathies.

Despite the hyperdynamic appearances of LV contraction in HCM, there is increasing evidence that the cardiac hypertrophy is an attempt to compensate for impaired LV systolic function. The primary abnormality is a malfunctioning sarcomeric protein, at least in cases with a known genetic defect.⁶ The abnormal motility and the mechanics of single-skinned slow skeletal myofibers support this hypothesis. Magnetic resonance imaging studies provide further evidence of impaired regional LV wall contraction in HCM.¹⁹

Thus, in contrast to conditions associated with increased afterload, such as hypertension, normal systemic pressures in HCM are perceived as an excessive load by a myocardium with impaired contractile properties. Furthermore, because the hypertrophy and impairment of contraction are not uniform throughout the LV, regional differences in LV contractility may act as a stimulus for maintenance of the LV hypertrophy. Myocardial ischemia caused by excessive oxygen demand and abnormal intramyocardial blood vessels may contribute to the cardiac remodeling.

The increased LV wall thickness in HCM reduces ventricular wall tension and augments LV systolic function in the short- and medium-term. In the long-term, however, myocyte necrosis, fibrosis, myocellular energy depletion, and diastolic dysfunction develop and lead to cardiac failure. The impression of supernormal LV systolic contractility is therefore misleading: the high LV ejection fractions characteristic of HCM reflect the low-wall stress caused by the massively hypertrophied LV wall and by small LV volumes.

Clinical findings that vary significantly, even in patients with identical mutations, and phenotypic features that do not directly involve the sarcomere suggest the importance of modifying factors.^{11 - 13} Several systems that may be involved in the initiation and maintenance of cardiac hypertrophy are being investigated.

The angiotensin-converting enzyme (ACE) gene, localized to chromosome 17q23, has an insertion (I)/deletion (D) polymorphism in intron 16.²⁰ The D allele is associated with higher plasma ACE activity, probably due to tight linkage to another locus that is important in the regulation of the ACE gene. Studies have suggested that the DD genotype is associated with increased risk for sudden death and more severe LV hypertrophy in HCM.²¹ These studies, however, involved patients with HCM caused by diverse genetic defects and mostly with mutations having high disease penetrance. We have evaluated large families in which HCM is caused by β-myosin mutations associated with low disease penetrance to see if the ACE gene is responsible for modifying the phenotypic expression of the cardiac disease. It is suggested that the D allele is responsible for variations in circulating hormone or cardiac renin-angiotensin system activity and that the latter may account for some of the differences in the cardiac phenotype.²² Studies are being conducted to test the ability of ACE inhibition and angiotensin receptor blockade to reverse LV hypertrophy, myocardial ischemia, and LV diastolic dysfunction in nonobstructive HCM.

Most hypertrophic stimuli cause an increase in intracellular Ca²⁺. It has been postulated that sarcomeric mutations also increase intracellular Ca²⁺.²³ High levels of intracellular Ca²⁺ and calmodulin activate calcineurin (Ca²⁺-calmodulin dependent protein phosphatase). Activated calcineurin dephosphorylates NF-AT3 (nuclear factors of activated T cells), 1 of a family of transcription factors.²⁴ This dephosphorylation allows migration of NF-AT3 to the nucleus, where it binds with GATA4 (a cardiac zinc finger transcription factor), with consequent transcription of cardiac proteins. In transgenic animal models of cardiomyopathy in which cardiac hypertrophy was related to sarcomeric dysfunction (HCM caused by overexpression of tropomodulin, myosin light chain-2, and fetal tropomyosin), treatment with inhibitors of calcineurin (cyclosporine or tacrolimus) prevented development and progression of cardiac disease.²⁵

The therapeutic efficacy of drugs that inhibit the renin-angiotensin system or calcineurin in patients with HCM remains to be established, however.

There is a continuing effort to improve risk stratification and to elucidate mechanisms of sudden death that would be amenable to therapy.^{3 - 4 ,26} One study evaluated several investigations, including an electrophysiologic (EP) protocol in which 155 patients with HCM were referred for risk stratification.^{3 - 4} Previously, the use of different EP protocols in small numbers of patients with diverse clinical presentations had raised questions about the value of EP studies in HCM.

About two thirds of the patients were found to have abnormal sinus node function, 20% had delayed or rapid AV node conduction, and 30% had abnormalities of His-Purkinje conduction. The most commonly induced arrhythmias were atrial reentrant tachycardia and atrial fibrillation. Sustained ventricular tachycardia (VT) was induced by a standardized programmed stimulation in 43% of the patients; induced VT was polymorphic in about 75% of these patients and monomorphic in about 25%. Importantly, a significant relationship was found between induction of sustained VT and a history of cardiac arrest or syncope. These findings in patients with HCM at high risk for sudden death prompted a prospective study that evaluated a therapeutic strategy to correct the defined hemodynamic and EP abnormalities in 230 patients.²⁶ The study also examined the relationship of a number of clinical, Holter, hemodynamic, and EP parameters to subsequent cardiac events (sudden death, implantable defibrillator discharge, cardiac arrest, syncope) using multivariate logistic regression analysis.²⁶ In the study, 164 patients presented with cardiac arrest, syncope, or presyncope, and 66 patients had no symptoms of impaired consciousness. A total of 115 patients had nonsustained VT during ambulatory ECG monitoring. Two variables were significant independent predictors of subsequent cardiac events (sudden death, syncope, and implantable defibrillator discharge): (1) a history of cardiac arrest or syncope; and (2) sustained VT induced during EP study with at least 3 premature extra stimuli. The 1-year and 5-year mean (SD) cardiac event-free rates in patients with VT induced during EP study were 94% (3%) and 48% (21%), respectively, compared with 100% and 92% (6%), respectively in patients in whom VT was not induced at EP study. A less aggressive EP stimulation protocol (≤2 extra stimuli) also identified patients without symptoms of impaired consciousness who were at risk for sudden death. However, this represented a minority of the asymptomatic patients. In contrast, nonsustained VT on Holter, present in about 25% of patients during 24- to 48-hour recording, predicted outcomes only in patients who presented with presyncope, syncope, or cardiac arrest.

Such results indicate the value of EP studies in identifying patients with HCM at high and at low risk for subsequent cardiac events. These risk evaluation studies suggest that all patients with HCM who have symptoms of impaired consciousness undergo the following tests: (1) 48-hour ambulatory Holter monitoring; (2) exercise thallium scintigraphy to identify reversible myocardial ischemia; and (3) combined cardiac catheterization and EP study to determine the presence or absence of LV obstruction and arrhythmias and to assess the hemodynamic effects of any observed arrhythmias (Table 2 and Table 3). In contrast, such studies are only indicated for asymptomatic HCM patients with a special need for risk stratification, for example, in airline pilots or athletes, or in patients with a strong family history of sudden death or Wolff-Parkinson-White syndrome.

Most sudden deaths in patients with HCM are probably caused by ventricular arrhythmias. There is an important need for safe and effective antiarrhythmic therapy in affected patients. Type 1 and 2 antiarrhythmic drugs do not decrease mortality in HCM. Amiodarone therapy has been associated with a high incidence of sudden death in symptomatic HCM patients.²⁷ Amiodarone suppresses induction of sustained VT in only 30% of patients, facilitates induction of VT (proarrhythmic) in 50%, and may cause heart block in some patients.

Implantable defibrillators are a safe and effective therapy in HCM patients prone to ventricular arrhythmias.^{28 - 29} Defibrillator therapy should be considered in the following subsets: (1) patients who have survived cardiac arrest; (2) patients with nonobstructive HCM who present with syncope or presyncope and in whom a sustained VT is induced at EP study; (3) patients who do not have symptoms of impaired consciousness but in whom a sustained VT is easily induced at EP study (≤2 premature extra stimuli); and (4) young patients who despite adequate drug therapy present with recurrent syncope on the basis of myocardial ischemia.^{28 - 30} In 1 report,²⁸ defibrillator therapy based on the above risk-evaluation studies was associated with a 20% 3-year rate of appropriate and successful defibrillator discharges.

Obstruction to LV outflow occurs in about one third of patients with HCM. Asymptomatic adults probably do not require treatment. However, obstruction is an important determinant of clinical outcomes, and affected patients often complain of chest discomfort, dyspnea, palpitations, and exercise- or posture-related lightheadedness or syncope. These symptoms are initially managed with verapamil or β-blocker therapy. Patients with disabling symptoms refractory to pharmacotherapy are referred for either LV myotomy and myectomy or mitral valve replacement. However, these are major operations with significant mortality and morbidity.

Several centers have demonstrated that DDD pacing also reduces LV outflow obstruction and improves symptoms in these patients.^{31 - 32} Acutely, RV pacing alters the pattern of LV contraction that may increase systolic LV outflow tract dimensions and hence reduce the LV outflow tract blood velocities. In turn, this would diminish systolic anterior motion, further reducing LV outflow obstruction and the associated mitral regurgitation. Prolonged pacing also alters electrical and hemodynamic properties of the myocardium, changes that are evident when pacing is temporarily discontinued. The long-term results of DDD pacing have been encouraging.

Midcavity obstructive HCM is more difficult to manage than the more common LV outflow obstructive HCM. It may progress more rapidly and may be associated with ventricular arrhythmias and increased risk of sudden death. The role of cardiac surgery and DDD pacing in this subgroup of patients with HCM is less well established.

Finally, the procedure of percutaneous septal ablation is being studied in the continuing search for alternatives to cardiac surgery.^{33 - 37} The procedure involves infusion of ethanol via an angioplasty catheter into 1 or more septal perforator branches of the left anterior descending coronary artery. The aim is to cause infarction and thinning of the part of proximal interventricular septum that is implicated in the LV outflow obstruction. Preliminary studies have reported a 70% reduction in LV pressure gradients and significant improvement of symptoms and exercise tolerance. Percutaneous septal ablation has been generally well tolerated. However, it has been complicated by in-hospital conduction abnormalities and heart block requiring pacemaker implantation, ventricular arrhythmias, and death. The long-term results of this procedure are unknown. Potential adverse long-term consequences of percutaneous septal ablation may include late arrhythmias and LV dysfunction, although these complications have not been reported.

None of these procedures have been shown to prevent sudden death. Their primary objective is to relieve symptoms and to improve exercise performance. The procedures are not mutually exclusive, so patients who fail to benefit from DDD pacing may undergo cardiac surgery. Similarly, patients who continue to have symptomatic obstructive HCM following cardiac surgery have been successfully treated with DDD pacing.

Hypertrophic cardiomyopathy is often diagnosed by the presence of LV hypertrophy in the absence of another cause for the increased cardiac mass. However, it has increasingly been recognized that the term encompasses many distinct genetic diseases with certain unique phenotypic manifestations. In some cases, the disease affects skeletal muscle as well as the heart. The genotype often also determines penetrance, severity, and distribution of LV hypertrophy, and prognosis. Risk evaluation studies have provided insights into potential mechanisms of syncope and sudden death, and a number of novel therapies are being evaluated that promise to improve symptoms and clinical outcome.