Slowed conduction and ventricular tachycardia after targeted disruption of the cardiac sodium channel gene Scn5a

Voltage-gated sodium channels drive the initial depolarization phase of the cardiac action potential and therefore critically determine conduction of excitation through the heart. In patients, deletions or loss-of-function mutations of the cardiac sodium channel gene, SCN5A, have been associated with a wide range of arrhythmias including bradycardia (heart rate slowing), atrioventricular conduction delay, and ventricular fibrillation. The pathophysiological basis of these clinical conditions is unresolved. Here we show that disruption of the mouse cardiac sodium channel gene, Scn5a, causes intrauterine lethality in homozygotes with severe defects in ventricular morphogenesis whereas heterozygotes show normal survival. Whole-cell patch clamp analyses of isolated ventricular myocytes from adult Scn5a(+/-) mice demonstrate a approximately 50% reduction in sodium conductance. Scn5a(+/-) hearts have several defects including impaired atrioventricular conduction, delayed intramyocardial conduction, increased ventricular refractoriness, and ventricular tachycardia with characteristics of reentrant excitation. These findings reconcile reduced activity of the cardiac sodium channel leading to slowed conduction with several apparently diverse clinical phenotypes, providing a model for the detailed analysis of the pathophysiology of arrhythmias.     

Cardiac conduction is required to preserve cardiac chamber morphology

Electrical cardiac forces have been previously hypothesized to play a significant role in cardiac morphogenesis and remodeling. In response to electrical forces, cultured cardiomyocytes rearrange their cytoskeletal structure and modify their gene expression profile. To translate such in vitro data to the intact heart, we used a collection of zebrafish cardiac mutants and transgenics to investigate whether cardiac conduction could influence in vivo cardiac morphogenesis independent of contractile forces. We show that the cardiac mutant dco^s226 develops heart failure and interrupted cardiac morphogenesis following uncoordinated ventricular contraction. Using in vivo optical mapping/calcium imaging, we determined that the dco cardiac phenotype was primarily due to aberrant ventricular conduction. Because cardiac contraction and intracardiac hemodynamic forces can also influence cardiac development, we further analyzed the dco phenotype in noncontractile hearts and observed that disorganized ventricular conduction could affect cardiomyocyte morphology and subsequent heart morphogenesis in the absence of contraction or flow. By positional cloning, we found that dco encodes Gja3/Cx46, a gap junction protein not previously implicated in heart formation or function. Detailed analysis of the mouse Cx46 mutant revealed the presence of cardiac conduction defects frequently associated with human heart failure. Overall, these in vivo studies indicate that cardiac electrical forces are required to preserve cardiac chamber morphology and may act as a key epigenetic factor in cardiac remodeling.     

Cardiac arrhythmias: the quest for a cure: a historical perspective

During the last 40 years, much progress has been made in our understanding and management of cardiac arrhythmias. A major step in the late 1960s was to combine programmed electrical stimulation of the heart with intracardiac activation recording. This allowed: 1) localization of the site of the block in the atrioventricular conduction system in patients with bradycardia; and 2) identification of the site of origin and the mechanism of supraventricular and ventricular tachycardia. Combining information from intracardiac studies with findings on the 12-lead electrocardiogram (ECG) resulted in much better localization of conduction abnormalities and arrhythmias using the ECG. This new knowledge led to the development of new therapies, such as bradycardia and antitachycardia pacing, and surgery for supraventricular and ventricular tachycardia. A very important development in the treatment of life-threatening arrhythmias was the implantable defibrillator. Growing concern about failure to protect patients at risk for dying suddenly with antiarrhythmic drugs led to a rapid increase in their number. Cure by catheter ablation became possible for patients with different types of arrhythmias. Genetic analysis allowed the identification of different monogenic arrhythmic diseases. Several challenges remain: the epidemic of atrial fibrillation, arrhythmias in heart failure, and sudden death out-of-hospital. One-fifth of all deaths are sudden and unexpected. The important issue is how we are going to prevent these unnecessary deaths from occurring.     

Cardiac morphogenetic defects and conduction abnormalities in mice homozygously deficient for connexin40 and heterozygously deficient for connexin45

Connexin40 (Cx40) and connexin45 (Cx45) are involved in both cardiac morphogenesis and propagation of electrical activity. We found that Cx40/Cx45 double deficiency (Cx40(-/-)/Cx45(+/-)) causes a variety of cardiac defects leading to high mortality during embryonic development and at birth. The majority of Cx40(-/-)/Cx45(+/-) embryos and postnatal mice suffered from atrioventricular septal defects. Additional cardiac abnormalities, e.g., ventricular septal defects and abnormal myocardial arrangement, occurred at lower abundance. Electrocardiograms of Cx40(-/-)/Cx45(+/+) and Cx40(-/-)/Cx45(+/-) mice revealed prolongation of P-wave, PQ interval and QRS duration compared to controls. Interestingly, in Cx40(-/-)/Cx45(+/-) mice, PQ interval and QRS duration were significantly prolonged compared to Cx40(-/-)/Cx45(+/+) mice. We conclude that the gap junctional proteins Cx40 and Cx45 have overlapping and partially compensatory functions with regard to heart morphogenesis and cardiac conduction. Cx45 might be one of the genetic modifiers that can cause variations in the phenotype of connexin40-deficient animals. Our findings may be particularly relevant for understanding molecular factors contributing to human congenital cardiac diseases.     

Cardiac connexins as candidate genes for idiopathic atrial fibrillation

PURPOSE OF REVIEW: Atrial fibrillation is the most common sustained cardiac arrhythmia and may cause significant morbidity. Current management strategies offer only modest success and may be associated with intolerable drug side effects or risk of procedural complications. As with other cardiac arrhythmias, the identification of genetic determinants predisposing to atrial fibrillation may provide novel molecular targets for drug development. This review discusses the role of cardiac connexins in the heart and suggests that genetic defects in cardiac connexins may predispose to arrhythmia vulnerability. RECENT FINDINGS: Animal models deficient in cardiac connexins demonstrate abnormalities in myocardial tissue conduction and vulnerability to re-entrant arrhythmias, including ventricular tachycardia and atrial fibrillation. Atrial tissue analyses from human patients with atrial fibrillation consistently demonstrate alterations in connexin distribution and protein levels, suggesting a role of connexins in the perpetuation of the arrhythmia. Most recently, genetic studies of Cx43 and Cx40 indicate that genetic variations in these genes may predispose to arrhythmia vulnerability in humans. SUMMARY: Current data support the critical role of cardiac connexins in mediating coordinated electrical activation and conduction through myocardial tissue. Alterations in the tissue distribution or function of cardiac connexins may predispose to cardiac arrhythmias, supporting a previously proposed hypothesis that cardiac connexins should be considered a major therapeutic target in the management of atrial fibrillation.     

Modulation of cardiac gap junction expression and arrhythmic susceptibility

Connexin43 (Cx43), the predominant ventricular gap junction protein, is critical for maintaining normal cardiac electrical conduction, and its absence in the mouse heart results in sudden arrhythmic death. The mechanisms linking reduced Cx43 abundance in the heart and inducibility of malignant ventricular arrhythmias have yet to be established. In this report, we investigate arrhythmic susceptibility in a murine model genetically engineered to express progressively decreasing levels of Cx43. Progressively older cardiac-restricted Cx43 conditional knockout (CKO) mice were selectively bred to produce a heart-specific Cx43-deficient subline ("O-CKO" mice) in which the loss of Cx43 in the heart occurs more gradually. O-CKO mice lived significantly longer than the initial series of CKO mice but still died suddenly and prematurely. At 25 days of age, cardiac Cx43 protein levels decreased to 59% of control values (P<0.01), but conduction velocity was not significantly decreased and no O-CKO mice were inducible into sustained ventricular tachyarrhythmias. By 45 days of age, cardiac Cx43 abundance had decreased in a heterogeneous fashion to 18% of control levels, conduction velocity had slowed to half of that observed in control hearts, and 80% of O-CKO mice were inducible into lethal tachyarrhythmias. Enhanced susceptibility to induced arrhythmias was not associated with altered invasive hemodynamic measurements or changes in ventricular effective refractory period. Thus, moderately severe reductions in Cx43 abundance are associated with slowing of impulse propagation and a dramatic increase in the susceptibility to inducible ventricular arrhythmias.     

Electrophysiologic residua and sequelae of surgery for congenital heart defects

Postoperative arrhythmias may occur in any patient who undergoes intracardiac surgery for a congenital heart defect. The correction of certain intracardiac heart defects predisposes to a large incidence of cardiac arrhythmias. Ventricular arrhythmias and conduction disturbances are seen after correction of tetralogy of Fallot, ventricular septal defect and atrioventricular canal defect. Supraventricular arrhythmias and sinus nodal dysfunction may be seen after surgery for transposition of the great arteries or atrial septal defect. The identification, evaluation and treatment of these patients are discussed.     

Neuregulin-1 promotes formation of the murine cardiac conduction system

The cardiac conduction system is a network of cells responsible for the rhythmic and coordinated excitation of the heart. Components of the murine conduction system, including the peripheral Purkinje fibers, are morphologically indistinguishable from surrounding cardiomyocytes, and a paucity of molecular markers exists to identify these cells. The murine conduction system develops in close association with the endocardium. Using the recently identified CCS-lacZ line of reporter mice, in which lacZ expression delineates the embryonic and fully mature conduction system, we tested the ability of several endocardial-derived paracrine factors to convert contractile cardiomyocytes into conduction-system cells as measured by ectopic reporter gene expression in the heart. In this report we show that neuregulin-1, a growth and differentiation factor essential for ventricular trabeculation, is sufficient to induce ectopic expression of the lacZ conduction marker. This inductive effect of neuregulin-1 was restricted to a window of sensitivity between 8.5 and 10.5 days postcoitum. Using the whole mouse embryo culture system, neuregulin-1 was shown to regulate lacZ expression within the embryonic heart, whereas its expression in other tissues remained unaffected. We describe the electrical activation pattern of the 9.5-days postcoitum embryonic mouse heart and show that treatment with neuregulin-1 results in electrophysiological changes in the activation pattern consistent with a recruitment of cells to the conduction system. This study supports the hypothesis that endocardial-derived neuregulins may be the major endogenous ligands responsible for inducing murine embryonic cardiomyocytes to differentiate into cells of the conduction system.     

Atrial-like phenotype is associated with embryonic ventricular failure in retinoid X receptor alpha -/- mice.

We have recently characterized a cardiac model of ventricular chamber defects in retinoid X receptor alpha (RXR alpha) homozygous mutant (-/-) gene-targeted mice. These mice display generalized edema, ventricular chamber hypoplasia, and muscular septal defects, and they die at embryonic day 15. To substantiate our hypothesis that the embryos are dying of cardiac pump failure, we have used digital bright-field and fluorescent video microscopy and in vivo microinjection of fluorescein-labeled albumin to analyze cardiac function. The affected embryos showed depressed ventricular function (average left ventricular area ejection fraction, 14%), ventricular septal defects, and various degrees of atrioventricular block not seen in the RXR alpha wild-type (+/+) and heterozygous (+/-) littermates (average left ventricular area ejection fraction, 50%). The molecular mechanisms involved in these ventricular defects were studied by evaluating expression of cardiac-specific genes known to be developmentally regulated. By in situ hybridization, aberrant, persistent expression of the atrial isoform of myosin light chain 2 was identified in the ventricles. We hypothesize that retinoic acid provides a critical signal mediated through the RXR alpha pathway that is required to allow progression of development of the ventricular region of the heart from its early atrial-like form to the thick-walled adult ventricle. The conduction system disturbances found in the RXR alpha -/- embryos may reflect a requirement of the developing conduction system for the RXR alpha signaling pathway, or it may be secondary to the failure of septal development.     

Cardiac connexins and impulse propagation

Gap junctions form the intercellular pathway for cell-to-cell transmission of the cardiac impulse from its site of origin, the sinoatrial node, along the atria, the atrioventricular conduction system to the ventricular myocardium. The component parts of gap junctions are proteins called connexins (Cx), of which three main isoforms are found in the conductive and working myocardial cells: Cx40, Cx43, and Cx45. These isoforms are regionally expressed in the heart, which suggests a specific role or function of a specific connexin in a certain part of the heart. Using genetically modified mice, the function of these connexins in the different parts of the heart have been assessed in the past years. This review will follow the cardiac impulse on its path through the heart and recapitulate the role of the different connexins in the different cardiac compartments.     

Atrial fibrillation and Brugada syndrome

Brugada syndrome is characterized by right bundle branch block pattern with ST-segment elevation in leads V(1) to V(3) and a propensity for sudden cardiac death due to ventricular arrhythmias. The arrhythmogenic substrate in Brugada syndrome may not be restricted to the ventricles, and atrial arrhythmias are being increasingly reported. Incidences of spontaneous atrial arrhythmias vary from 6% to 38% and those of inducible atrial arrhythmias from 3% to 100%. Atrial fibrillation (AF) is the most common atrial arrhythmia found in Brugada syndrome. Enhanced duration of atrial action potential and increased intra-atrial conduction time may contribute to the genesis of atrial arrhythmias in Brugada syndrome. Atrial arrhythmias are an important cause of inappropriate discharge of implantable defibrillators in patients with Brugada syndrome. Hence, implantation of dual-chamber defibrillators and careful programming of single-chamber devices have been recommended. Atrial fibrillation has been associated with mutations in both the sodium and calcium channels of the heart, as well as with cases of Brugada syndrome that could not genotyped to any of the known genes associated with the disease. This observation suggests that the substrate responsible for the development of ventricular arrhythmias also may contribute to arrhythmogenesis in the atria of the heart. The presence of a prominent transient outward current in atria and the observation that episodes of AF are triggered by closely coupled atrial extrasystoles point to the possibility that a substrate similar to that responsible for ventricular arrhythmogenesis underlies the development of AF in patients with Brugada syndrome.     

Prognosis of the acute stage of myocardial infarction. Rhythm factors

At the present time, it is still difficult to determine the real prognostic significance of cardiac arrhythmias in the acute phase of myocardial infarction. The metabolic and electrogenic consequences of myocardial ischaemia are responsible for the principal mechanisms involved in the development of arrhythmias in the ICU: conduction disorders and/or ventricular arrhythmias. Based on a review of the literature, the author analyses the prognostic value of the principal arrhythmias: branch block, atrio-ventricular block, primary ventricular fibrillation and repetitive or isolated ventricular extrasystoles. A number of conclusions can be drawn from this study: isolated arrhythmias in the acute phase are a sign of metabolic and electrolyte disorders and only influence the immediate prognosis. The same arrhythmias, associated with anatomical damage, altered myocardial function or heart failure, may be the sign of severe, long term complications. The development of arrhythmias in the acute phase of infarction should not be interpreted in isolation, but together with the results of further investigations to test the value of the myocardium and the electrical instability. The patient's real risk can only be evaluated on the basis of all of these findings.     

Ventricular arrhythmias: medical therapy, device treatment, and indications for electrophysiologic study

Sustained ventricular arrhythmias occur most frequently during the first several months following MI, although the risk of arrhythmia development continues for many years. The mechanism responsible for most of these arrhythmias is a re-entrant circuit located in the border zone between the normal and scarred subendocardium. Clinical factors that identify patients at greatest risk for developing sustained ventricular arrhythmias after MI include depressed left ventricular function, acute left ventricular aneurysm formation, electrical instability, new bundle branch blocks, and residual ischemia. Attempts to lower the arrhythmic risk of these patients is a major area of clinical investigation. Although frequent asymptomatic ventricular ectopy and nonsustained ventricular tachycardia are risk factors for sudden death after infarction, it has not been demonstrated that empiric treatment of these arrhythmias with antiarrhythmic agents improves survival. Electrophysiologic studies have significantly contributed to understanding the mechanisms responsible for sustained ventricular arrhythmias. Although the role of electrophysiologic studies in guiding therapy in patients with sustained ventricular tachycardia or sudden death after infarction has been well established, their utility to identify high risk subgroups after infarction has not been conclusively determined. New treatment modalities have resulted in an improved outcome in patients with malignant ventricular arrhythmias (Table 2). These strategies include new pharmacologic therapies, arrhythmia surgery, use of automatic implantable cardioverter defibrillators or antitachycardia pacemakers, and percutaneous catheter ablation of the re-entrant circuit responsible for these arrhythmias.     

Molecular basis of arrhythmias

The characterization of single gene disorders has provided important insights into the molecular pathogenesis of cardiac arrhythmias. Primary electrical diseases including long-QT syndrome, short-QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia have been associated with mutations in a variety of ion channel subunit genes that promote arrhythmogenesis. Pathological remodeling of ionic currents and network properties of the heart critical for normal electrical propagation plays a critical role in the initiation and maintenance of acquired arrhythmias. This review focuses on the molecular and cellular basis of electrical activity in the heart under normal and pathophysiological conditions to provide insights into the fundamental mechanisms of inherited and acquired cardiac arrhythmias. Improved understanding of the basic biology of cardiac arrhythmias holds the promise of identifying new molecular targets for the treatment of cardiac arrhythmias.