Clinical Genetic Testing for Atrial Fibrillation: Are We There Yet?

Important progress has been made toward unravelling the complex genetics underlying atrial fibrillation (AF). Initial studies were aimed to identify monogenic causes; however, it has become increasingly clear that the most common predisposing genetic substrate for AF is polygenic. Despite intensive investigations, there is robust evidence for rare variants for only a limited number of genes and cases. Although the current yield for genetic testing in early onset AF might be modest, there is an increasing appreciation that genetic culprits for potentially life-threatening ventricular cardiomyopathies and channelopathies might initially present with AF. The potential clinical significance of this recognition is highlighted by evidence that suggests that identification of a pathogenic or likely pathogenic rare variant in a patient with early onset AF is associated with an increased risk of death. These findings suggest that it might be warranted to screen patients with early onset AF for these potentially more sinister cardiac conditions. Beyond facilitating the early identification of genetic culprits associated with potentially malignant phenotypes, insight into underlying AF genetic substrates might improve the selection of patients for existing therapies and guide the development of novel ones. Herein, we review the evidence that links genetic factors to AF, then discuss an approach to using genetic testing for early onset AF patients in the present context, and finally consider the potential value of genetic testing in the foreseeable future. Although further work might be necessary before recommending uniform integration of genetic testing in cases of early onset AF, ongoing research increasingly highlights its potential contributions to clinical care.

HERG K+ channel, a regulator of tumor cell apoptosis and proliferation

The human ether-a-go-go related gene (HERG) encodes K+ channel identified as a molecular target for mutations underlying some forms of the long Q-T syndrome, a lethal cardiac arrhythmia. Recent studies revealed that HERG is abundantly expressed in a variety of tumor cells. Yet, the role of HERG in tumor cells had remained unclear. Here, we show that HERG conductance markedly promotes H2O2-induced apoptosis of various tumor cells, whereas HERG expression facilitates the tumor cell proliferation caused by tumor necrosis factor (TNF) ligand (TNF-alpha). Immunostaining and immunocoprecipitation reveal coexpression of HERG and TNF receptor 1 on the cytoplasmic membrane, which is correlated with greater activities of nuclear transcription factor, nuclear factor-kappaB, in HERG-expressing tumor cells. Our data suggest that HERG K+ channel is a regulator of tumor cell proliferation and apoptosis and provide a potential new target for cancer therapy.

Effects of class III antiarrhythmic drugs on transient outward and ultra-rapid delayed rectifier currents in human atrial myocytes

A variety of class III antiarrhythmic agents have been shown to block the delayed rectifier current, but their effects on other K+ currents, particularly in human tissues, are less clear. We studied the concentration-dependent actions of the class III compounds d-sotalol, E-4031 and ambasilide on the transient outward current (I(to)) and the ultra-rapid delayed rectifier current (I(Kur)) in human atrial myocytes. d-Sotalol and E-4031 failed to alter I(to) or I(Kur) at concentrations up to 500 and 50 microM, respectively. In contrast, ambasilide produced a concentration-dependent inhibition of I(to) and I(Kur), with statistically significant effects at 10 microM and maximum effects at 100 microM. The 50% inhibitory concentration of ambasilide averaged 23 +/- 2 microM and 34 +/- 3 microM for I(to) and I(Kur) respectively. Ambasilide did not alter the voltage-dependence of activation or inactivation of I(to), or the voltage-dependence of I(Kur), and it did not affect I(to) recovery from inactivation. On the other hand, ambasilide accelerated I(to) inactivation, by introducing a more rapid component that accelerated with increasing drug concentration. Furthermore, block of both I(to) and I(Kur) developed over time after the onset of depolarization, with time constants of 5.8 +/- 0.8 msec and 2.5 +/- 0.4 msec at concentrations of 10 and 50 microM for I(to) and 6.1 +/- 0.8 msec and 2.1 +/- 0.3 msec at 10 and 50 microM for I(Kur). We conclude that neither d-sotalol nor E-4031 affects I(to) or I(Kur), whereas ambasilide produces efficacious open-channel block of both currents, in human atrial myocytes.

Experimental amitriptyline intoxication: electrophysiologic manifestations and management

Amitriptyline intoxication can result in severe ventricular arrhythmias that may be refractory to medical management. The mechanisms of these arrhythmias are unclear, and their optimal management is problematic. We studied the cardiac effects of amitriptyline infusion in anesthetized and awake dogs. Amitriptyline significantly increased heart rate, QRS duration, and AH and HV intervals. The concentration-response curves for these effects were, however, quite different, with significant changes beginning at a concentration of 1.5 +/- 0.4 mg/L for heart rate, compared with 2.4 +/- 0.4 mg/L for QRS and HV intervals and 3.7 +/- 0.5 mg/L for the AH interval. Ventricular tachyarrhythmias developed after marked QRS widening had occurred, and appeared in all six awake dogs and five of the six anesthetized dogs studied. Sodium bicarbonate was given to seven animals with ventricular tachyarrhythmias, and it rapidly reversed the arrhythmia in all instances. The benefit from sodium bicarbonate could not be attributed to changes in serum potassium or amitriptyline concentrations. It may have been due to alkalinization or changes in serum sodium concentration. These experiments suggest that: (a) amitriptyline intoxication frequently produces ventricular tachyarrhythmias, if high enough drug concentrations are achieved; (b) these arrhythmias are associated with marked slowing of intraventricular conduction; and (c) sodium bicarbonate is effective therapy for amitriptyline-induced ventricular arrhythmia.

Mechanisms of atrial remodeling and clinical relevance

PURPOSE OF REVIEW

Atrial fibrillation usually occurs in the context of an atrial substrate produced by alterations in atrial tissue properties referred to as remodeling. Remodeling can result from cardiac disease, cardiac arrhythmias, or biologic processes such as senescence. Recent advances in understanding remodeling have allowed for insights into mechanisms underlying atrial fibrillation that have been transferred from experimental models to humans. This paper reviews recent progress in understanding atrial remodeling, as well as the consequent clinical insights into atrial fibrillation pathophysiology and treatment.

RECENT FINDINGS

Two principal forms of remodeling have been described in animal models of atrial fibrillation: ionic remodeling, which affects cellular electrical properties, and structural remodeling, which alters atrial tissue architecture. Atrial tachycardias (particularly rapid tachyarrhythmias such as atrial flutter and atrial fibrillation) cause ionic remodeling, which decreases the atrial refractory period and promotes atrial reentry. Congestive heart failure produces atrial interstitial fibrosis, which promotes arrhythmogenesis by interfering with atrial conduction properties. Recent animal studies have provided insights into the pathways involved in remodeling, and have indicated the pathophysiological role of remodeling in specific contexts. In addition, work in animal models has provided information about pharmacological interventions that can prevent the development of remodeling. Clinical studies have shown that novel approaches to remodeling prevention identified in animal work have potential therapeutic value in man.

SUMMARY

Understanding atrial remodeling has the potential to improve our appreciation of the pathophysiology of clinical atrial fibrillation and to allow for the development of useful new therapeutic approaches.

Beta-blockers as antiarrhythmic agents

Drugs that suppress beta-adrenergic signaling by competitively inhibiting agonist binding to beta-adrenergic receptors ("beta-blockers") have important antiarrhythmic properties. They differ from most other antiarrhythmic agents by not directly modifying ion channel function; rather, they prevent the arrhythmia-promoting actions of beta-adrenergic stimulation. beta-Blockers are particularly useful in preventing sudden death due to ventricular tachyarrhythmias associated with acute myocardial ischemia, congenital long QT syndrome, and congestive heart failure. They are also quite valuable in controlling the ventricular rate in patients with atrial fibrillation. This chapter reviews the properties of beta-adrenoceptor signaling, the basic mechanisms of cardiac arrhythmias on which beta-blockers act, the ion channel mediators of beta-adrenergic responses, the evidence for clinical antiarrhythmic indications for beta-blocker therapy and the specific pharmacodynamic and pharmacokinetic properties of beta-blockers that differentiate the various agents of this class.

Atrial fibrillation variant-to-gene prioritization through cross-ancestry eQTL and single-nucleus multiomic analyses

Atrial fibrillation (AF) is the most common arrhythmia in the world. Human genetics can provide strong AF therapeutic candidates, but the identification of the causal genes and their functions remains challenging. Here, we applied an AF fine-mapping strategy that leverages results from a previously published cross-ancestry genome-wide association study (GWAS), expression quantitative trait loci (eQTLs) from left atrial appendages (LAAs) obtained from two cohorts with distinct ancestry, and a paired RNA sequencing (RNA-seq) and ATAC sequencing (ATAC-seq) LAA single-nucleus assay (sn-multiome). At nine AF loci, our co-localization and fine-mapping analyses implicated 14 genes. Data integration identified several candidate causal AF variants, including rs7612445 at GNB4 and rs242557 at MAPT. Finally, we showed that the repression of the strongest AF-associated eQTL gene, LINC01629, in human embryonic stem cell-derived cardiomyocytes using CRISPR inhibition results in the dysregulation of pathways linked to genes involved in the development of atrial tissue and the cardiac conduction system.

Autonomic modulation of the frequency-dependent actions of diltiazem on the atrioventricular node in anesthetized dogs

Calcium antagonists have rate-dependent effects on atrioventricular node refractoriness in autonomically blocked dogs. Autonomic reflexes can attenuate diltiazem's actions and could alter their frequency-dependence. We evaluated the effects of four steady-state drug concentrations in each of seven dogs with intact autonomic tone, six with muscarinic blockade (atropine) and eight with combined muscarinic and beta adrenergic blockade. Diltiazem depressed atrioventricular nodal function less in autonomically intact dogs than in the other two groups. The concentration-response relationship for increases in Wenckebach cycle length (compared to intact dogs) was twice as steep among dogs with muscarinic blockade (P less than .001) and three times as steep with combined blockade (P less than .001). For an equal dose of diltiazem, the slope of drug-induced refractory period prolongation vs. pacing cycle length was similar (-0.20 +/- 0.11%/msec, mean +/- S.D.) in intact dogs compared to atropinized dogs (-0.21 +/- 0.10%/msec) and dogs with combined blockade (-0.24 +/- 0.14%/msec). Amplification of diltiazem's actions by the rapid atrial rate during atrial fibrillation was associated with increases in mean RR interval which were 8.3 times as large as changes in refractory period produced by the drug at slow heart rates. We conclude: 1) autonomic reflexes reduce diltiazem's effect for any given plasma concentration; 2) changes in vagal tone can play an important role in the autonomic response to diltiazem; and 3) despite altering the magnitude of diltiazem's effects, autonomic mechanisms do not prevent frequency-dependent drug action.(ABSTRACT TRUNCATED AT 250 WORDS)

Interval-dependent effects of lidocaine on conduction in canine cardiac Purkinje fibers: experimental observations and theoretical analysis

Whereas the interval-dependence of antiarrhythmic drug effects on Vmax is known, the corresponding time-dependence of drug-induced changes in cardiac conduction is not established. The purpose of these experiments was to study the relationship between the time-dependence of lidocaine-induced changes in Vmax and in conduction time. Standard microelectrode techniques were used to monitor conduction and action potential characteristics of Purkinje fibers within free-running canine false tendons. Lidocaine-induced alterations in conduction and Vmax were related to drug concentration and to the preceding diastolic recovery time. At concentrations of 18 to 74 microM, changes in both Vmax and conduction time were an exponential function of diastolic interval, with recovery time constants averaging 123 to 150 msec for conduction time and 138 to 150 msec for Vmax. With higher lidocaine concentrations, changes in Vmax continued to be an exponential function of recovery interval, whereas changes in conduction time consistently deviated from the terminal exponential relationship at short diastolic intervals. These observations are consistent with the predictions of a model based on linear cable theory. Calculations using this model suggest that recovery from moderate drug-induced conduction slowing should proceed with a time course similar to changes in Vmax, whereas recovery from more severe conduction slowing should occur more rapidly than changes in Vmax. These observations suggest that the time dependence of drug effects on conduction in vivo can be analyzed quantitatively in relationship to observations on Vmax in vitro.

Frequency-dependent effects of antiarrhythmic drugs on action potential duration and refractoriness of canine cardiac Purkinje fibers

Antiarrhythmic drugs are known to have frequency dependent effects on Vmax and conduction. The purpose of this study was to determine whether the effects of antiarrhythmic drugs on action potential duration (APD) and refractory period depend on frequency. Standard microelectrode techniques were used to measure APD and refractory period after abrupt or sustained alterations in stimulation frequency. Increases in APD95 resulting from bretylium and quinidine and decreases resulting from lidocaine were 50 to 60% attenuated (P less than .01 for each) by sustained increases in activation rate. Unlike sustained rate increases, abrupt increases in rate enhanced quinidine-induced APD prolongation (P less than .01). Quinidine increased the time constant (222 +/- 6 to 346 +/- 36 msec, P less than .01) and decreased the magnitude of the rapid phase of APD abbreviation upon premature stimulation. In contrast, bretylium increased the magnitude of APD abbreviation upon premature stimulation, reducing the tendency of bretylium to prolong premature APD compared to basic APD. Changes in refractory period parallelled changes in APD95 with the exception of the effect of quinidine after sustained rate change. Despite attenuation of its effects oN APD95, quinidine prolonged refractory period more after sustained rate increases (P less than .01), apparently because of greater depression of maximum sodium conductance. These experiments show that the effects of antiarrhythmic drugs on APD and refractoriness depend on activation frequency and that abrupt changes in rate may alter drug effects differently from sustained rate changes.

RELATIVE CONTRIBUTIONS OF FIBROSIS AND CONNEXIN CHANGES TOTHE ATRIAL FIBRILLATION SUBSTRATE DURING THE DEVELOPMENT AND REVERSAL OF CONGESTIVEHEART FAILURE

Background: Connexin alterations occur in various atrial fibrillation (AF) paradigms, but their functional significance remains unclear. No data are available regarding the effects of CHF on atrial connexin expression and phosphorylation. We therefore analyzed connexin changes and their contribution to the AF substrate during the development and reversal ofCHF. Methods and Results: Dogs were allocated to three groups: CHF induced by 2-week ventricular tachypacing (CHF, n=15); CHF dogs allowed to recover for 4 weeks after 2-week tachypacing (REC, n=15) and non-paced shams (CTL, n=11). Left ventricular end-diastolic pressure increased with CHF (14.5±1.0*** vs.3.7±0.7, ***P < 0.001 vs. CTL) and normalized upon CHF recovery (5.1±1.0^†††, ^††† P < 0.001 vs. CHF). Real-time PCR and Western-blot analyses revealed connexin43 (Cx43) and connexin40 (Cx40) mRNA and protein expression to be unchanged by CHF and REC. However, CHF caused Cx43 dephosphorylation(by ~73%***) and increased Cx40/Cx43 protein ratio (by ~35%***), with both alterations completely reversing in REC. Immunofluorescent confocal microscopy confirmed connexin protein trends, with a reduction in phosphorylated Cx43 (by ~68%*** in CHF) that returned to control in REC. CHF caused conduction abnormalities (phasedelay-range and heterogeneity index, both P < 0.01) and burst pacing-induced AF prolongation (CTL 22±7s, CHF 1100±171s***, REC 884±220s***) which persisted in the recovery period, along with residual fibrosis (CTL 3.6±0.7%, CHF 14.7±1.5%***, REC13.3±2.3%***). Fibrosis physically interrupted muscle bundle continuity and anionically-based action potential model of canine atrium showed that fibrosiswas able to account for the observed conduction abnormalities. Conclusions: CHF causes connexin-dephosphorylation and Cx40/Cx43ratio increases. With CHF reversal, atrial connexin alterations recover completely, but tissue fibrosis, conduction abnormalities and a substrate forAF remain with fibrosis accounting for conduction abnormalities. Thus, althougha trial connexin changes occur with CHF, they are not essential for conduction disturbances and AF promotion, which appear rather to be related primarily tofibrotic interruption of muscle-bundle continuity.

New insights into the mechanisms and management of atrial fibrillation

Atrial fibrillation (AF) is a common contributor to cardiovascular morbidity and mortality. Two generally acceptable strategies exist for long-term AF management, with ongoing studies comparing the overall mortality associated with each. One strategy aims to maintain sinus rhythm, with antiarrhythmic agents if necessary, thereby preserving physiological cardiac electrical function but exposing the patient to the potential side effects of potent drugs. The second approach is to control the ventricular rate and prevent thromboembolic complications with anticoagulants, leaving the patient with AF. Both beta-blocking agents and calcium antagonists are more effective than digoxin in achieving rate control. Several nonpharmacological therapies including catheter ablation, implantable devices and surgical interventions show promise for rate control and maintenance of sinus rhythm. This paper provides an overview of new developments in pharmacological and nonpharmacological therapy. Key features of recently published clinical guidelines, including a unified classification scheme for AF and issues relating to rate control and maintenance of sinus rhythm, are considered. In addition, preliminary results from the recently presented AFFIRM study, the largest AF trial to date, are summarized. Finally, we discuss recent insights into the basic mechanisms underlying AF that have potentially significant clinical implications.