Sudden cardiac death in patients with hypertrophic cardiomyopathy: from bench to bedside with an emphasis on genetic markers

Effects of triiodo-thyronine on angiotensin-induced cardiomyocyte hypertrophy: reversal of increased beta-myosin heavy chain gene expression

Thyroid hormone-induced cardiac hypertrophy is similar to that observed in physiological hypertrophy, which is associated with high cardiac contractility and increased alpha-myosin heavy chain (alpha-MHC, the high ATPase activity isoform) expression. In contrast, angiotensin II (Ang II) induces an increase in myocardial mass with a compromised contractility accompanied by a shift from alpha-MHC to the fetal isoform beta-MHC (the low ATPase activity isoform), which is considered as a pathological hypertrophy and inevitably leads to the development of heart failure. The present study is designed to assess the effect of thyroid hormone on angiotensin II-induced hypertrophic growth of cardiomyocytes in vitro. Cardiomyocytes were prepared from hearts of neonatal Wistar rats. The effects of Ang II and 3,3',5-triiodo-thyronine (T3) on incorporations of [3H]-thymine and [3H]-leucine, MHC isoform mRNA expression, PKC activity, and PKC isoform protein expression were studied. Ang II enhanced [3H]-leucine incorporation, beta-MHC mRNA expression, PKC activity, and PKCepsilon expression and inhibited alpha-MHC mRNA expression in cardiomyocytes. T3 treatment prevented Ang II-induced increases in PKC activity, PKCepsilon, and beta-MHC mRNA overexpression and favored alpha-MHC mRNA expression. Thyroid hormone appears to be able to reprogram gene expression in Ang II-induced cardiac hypertrophy, and a PKC signal pathway may be involved in such remodeling process.

Hemodynamic changes and prognosis in patients with hypertrophic cardiomyopathy and abnormal blood pressure responses during exercise

BACKGROUND

An abnormal blood pressure response (BPR) during exercise has been proposed as a risk factor for sudden cardiac death in patients with hypertrophic cardiomyopathy (HCM). Some patients with HCM show systolic dysfunction during exercise.

HYPOTHESIS

The aim of this study was to clarify the hemodynamic response during exercise and prognosis in patients with HCM and abnormal BPR.

METHODS

Sixty-five patients with HCM underwent radionuclide monitoring of left ventricular function and measurement of blood pressure during supine ergometer exercise. Thereafter, cardiac events were recorded for an average period of 76 months.

RESULTS

Seven of 65 patients had abnormal BPR, while the others had normal BPR. Changes of heart rate and systemic vascular resistance during exercise did not differ between the two groups. Stroke volume did not increase in the abnormal BPR group but did in the normal BPR group. During a mean follow-up period of 76 months, three of the seven patients (43%) with abnormal but only one patient (2%) with normal BPR suffered a malignant arrhythmia.

CONCLUSIONS

Abnormal BPR occurred in about 11% of patients with nonobstructive HCM and was associated with a high prevalence of cardiac events. The predictor of abnormal BPR during exercise may not be an abnormal response of systemic vascular resistance and heart rate, but the lack of an appropriate increase in stroke volume.

[Molecular genetics of cardiomyopathies]

In the last decade our understanding of cardiac pathophysiology has experienced significant advances linked to major advances in molecular genetics. Although many genes are associated today with cardiac diseases, the genetics of both hypertrophic cardiomyopathy and dilated cardiomyopathy have generated great interest. The familial nature of the disease in some patients has been very useful in this regard. In addition, there are also excellent experimental models to study the implications of the genetic abnormalities. Altogether the study of the molecular genetics of the cardiomyopathies should provide not only prognostic information but also new therapeutic alternatives.

Hemodynamic benefits of treatment modalities for hypertrophic cardiomyopathy: a case study

Hypertrophic cardiomyopathy is a primary disease of the cardiac muscle characterized by a hypertrophied and nondilated left ventricle in the absence of other cardiac or systemic disease. The disorder occurs twice as often in men than in women and is relatively more common in young adults. Early treatment of symptoms may improve hemodynamic benefits and prevent complications, including sudden death. This case illustrates various treatment modalities used to manage symptoms and describes the challenges in effectively maintaining hemodynamic stability.

A perspective: the new millennium dawns on a new paradigm for cardiology--molecular genetics

Western civilization had two great epochs--the sixth century B.C. and the 18th century. The 21st century is likely to be the third great epoch. Although cardiology has advanced more in the last 50 years than in the previous 2,000, it is likely to advance more in the next two or three decades than in the previous 2,000 years, including those 50 golden years. The engines of ingenuity to provide the thrust for the 21th century will come from molecular genetics and the application of recombinant deoxyribonucleic acid (DNA) techniques. Identification of all human genes (50,000 to 100,000) in the next two to three years will help link thousands of etiologies and risk factors with their respective diseases, which represents a new paradigm in medicine. This is illustrated by the implications to be drawn from familial hypertrophic cardiomyopathy and the 50 new genes already identified to be responsible for cardiac disease. The hope for prevention and treatment of human disease is unprecedented. Twenty diseases account for 80% of the deaths in the Western world and are due to 100 to 200 genes, all of which will be available in a couple of years. The Phoenician alphabet (inclusive of the Greek vowels) of 26 letters launched two millenniums of Western civilization, whereas the DNA alphabet of only four letters will launch and dominate the next millennium.

Familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is a cardiomyopathy that occurs in 0.2% of the general population. It is characterized by asymmetrical hypertrophy of the ventricle, predominantly the intraventricular septum. FHC is caused by genetic mutations in several of the sarcomeric proteins, such as myosin heavy chain, troponin T, troponin I, alpha-tropomyosin, essential and regulatory light chains of myosin, and the cardiac myosin-binding protein C. FHC is genetically heterogeneous, and, therefore, it is associated with a very diverse clinical presentation in terms of altered cardiac structure and clinical manifestations. The most severe manifestation is sudden death. The purpose of this article is to provide the reader with new insights into the genetic mutations that give rise to FHC and to discuss risk factors that are associated with severe hypertrophy and sudden death in this population.

Identification of a contractile deficit in adult cardiac myocytes expressing hypertrophic cardiomyopathy-associated mutant troponin T proteins

The direct effects of expressing hypertrophic cardiomyopathy-associated (HCM-associated) mutant troponin T (TnT) proteins on the force generation of single adult cardiac myocytes have not been established. Replication-defective recombinant adenovirus vectors were generated for gene transfer of HCM-associated I79N and R92Q mutant cardiac TnT cDNAs into fully differentiated adult cardiac myocytes in primary culture. We tested the hypothesis that the mutant TnT proteins would be expressed and incorporated into the cardiac sarcomere and would behave as dominant-negative proteins to directly alter calcium-activated force generation at the level of the single cardiac myocyte. Interestingly, under identical experimental conditions, the ectopic expression of the mutant TnTs was significantly less ( approximately 8% of total) than that obtained with expression of wild-type TnT ( approximately 35%) in the myocytes. Confocal imaging of immunolabeled TnT showed a regular periodic pattern of localization of ectopic mutant TnT that was not different than that in normal controls, suggesting that mutant TnT incorporation had no deleterious effects on sarcomeric architecture. Direct measurements of isometric force production in single cardiac myocytes demonstrated marked desensitization of submaximal calcium-activated tension, with unchanged maximum tension generation in mutant TnT-expressing myocytes compared with control myocytes. Collectively, these results demonstrate an impaired expression of the mutant protein and a disabling of cardiac contraction in the submaximal range of myoplasmic calcium concentrations. Our functional results suggest that development of new pharmacological, chemical, or genetic approaches to sensitize the thin-filament regulatory protein system could ameliorate force deficits associated with expression of I79N and R92Q in adult cardiac myocytes.

Altered cardiac excitation-contraction coupling in mutant mice with familial hypertrophic cardiomyopathy

Excitation-contraction coupling in cardiac muscle of familial hypertrophic cardiomyopathy (FHC) remains poorly understood, despite the fact that the genetic alterations are well defined. We characterized calcium cycling and contractile activation in trabeculae from a mutant mouse model of FHC (Arg403Gln knockin, alpha-myosin heavy chain). Wild-type mice of the same strain and age ( approximately 20 weeks old) served as controls. During twitch contractions, peak intracellular Ca2+ ([Ca2+]i) was higher in mutant muscles than in the wild-type (P < 0.05), but force development was equivalent in the two groups. Ca2+ transient amplitude increased dramatically in both groups as stimulation rate increased from 0.2 to 4 Hz. Nevertheless, developed force fell at the higher stimulation rates in the mutants but not in controls (P < 0.05). The steady-state force-[Ca2+]i relationship was less steep in mutants (Hill coefficient, 2.94 +/- 0.27 vs. 5.28 +/- 0.64; P > 0.003), with no changes in the [Ca2+]i required for 50% activation or maximal Ca2+-activated force. Thus, calcium cycling and myofilament properties are both altered in FHC mutant mice: more Ca2+ is mobilized to generate force, but this does not suffice to maintain contractility at high stimulation rates.

Sudden cardiac death in the young athlete

Sudden cardiac death in athletes is usually due to underlying cardiovascular disease. In the young less than 30 years of age, the most common abnormality is hypertrophic cardiomyopathy, followed by congenital coronary artery anomalies. The final common pathway is usually ventricular fibrillation. Sudden cardiac death in the young is rare but remains a source of concern. A careful screening history and physical examination, especially for potential athletes, should identify the majority of young people at risk.

[Sudden death in hypertrophic myocardiopathy]

Hypertrophic cardiomyopathy is the most common cause of sudden death in young individuals who are otherwise healthy. Risk of sudden death is highest in patients who are between 14 and 35 years old. Several mechanisms are involved in sudden death: ventricular arrhythmias, supraventricular arrhythmias leading to cardiac collapse, bradycardias and severe ischemia. Many studies have analyzed how to identify high risk patients. The factors that best identify high risk patients are: previous history of sudden death or syncope, induction in adults of sustained ventricular arrhythmias, the presence of non-sustained ventricular tachycardia in symptomatic patients, the presence of ischemia associated with hypotension in children, the presence of mutations in the beta-myosin heavy chain together with a family history of sudden death and a poor left ventricular ejection fraction. Risk stratification should be done on an individualized basis. In those patients in whom a high risk for sudden arrhythmic death is suspected, the only current effective treatment is the implantable defibrillator.

Molecular genetic basis of hypertrophic cardiomyopathy: genetic markers for sudden cardiac death

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease caused by mutations in sarcomeric proteins. The disease is characterized by left ventricular hypertrophy in the absence of an increased external load, and myofibrillar disarray. A large number of mutations in genes coding for the beta-myosin heavy chain (beta-MyHC), cardiac troponin T (cTnT), cardiac troponin I, alpha-tropomyosin, myosin binding protein C (MyBP-C), and myosin light chain 1 and 2 in patients with HCM have been identified. Genotype-phenotype correlation studies have shown that mutations carry prognostic significance. The Gly256Glu, Val606Met, and Leu908Val mutations in the beta-MyHC are associated with a benign prognosis. In contrast, Arg403Gln, Arg719Trp, and Arg453Cys mutations are associated with a high incidence of sudden cardiac death (SCD). Mutations in cTnT are associated with a mild degree of hypertrophy, but a high incidence of SCD. Mutations in MyBP-C are associated with mild hypertrophy and a benign prognosis. However, it has become evident that factors other than the underlying mutations, such as genetic background and possibly environmental factors, also modulate phenotypic expression of HCM.

Structural studies on myosin II: communication between distant protein domains

Understanding how chemical energy is converted into directed movement is a fundamental problem in biology. In higher organisms this is accomplished through the hydrolysis of ATP by three families of motor proteins: myosin, dynein and kinesin. The most abundant of these is myosin, which operates against actin and plays a central role in muscle contraction. As summarized here, great progress has been made towards understanding the molecular basis of movement through the determination of the three-dimensional structures of myosin and actin and through the establishment of systems for site-directed mutagenesis of this motor protein. It now appears that the generation of movement is coupled to ATP hydrolysis by a series of domain movements within myosin.

Familial hypertrophic cardiomyopathy: diagnostic and therapeutic implications of recent genetic studies

Familial hypertrophic cardiomyopathy is the first primary cardiomyopathy to have yielded to the techniques of modern molecular genetics. In the past few years, four genes responsible for this disease have been identified, all of which code for sarcomeric structural proteins. In addition, structure-function analysis and genotype-phenotype correlation studies have shed significant light on the molecular basis of this disease. It is hoped that within the next few years the application of molecular genetic tools will not only facilitate the diagnosis of hypertrophic cardiomyopathy but will also provide prognostic and therapeutic stratification for more definitive therapy.