Showing papers by "Charles Antzelevitch published in 2011"

Identification of a novel loss-of-function calcium channel gene mutation in short QT syndrome (SQTS6)

Abstract: Aims Short QT syndrome (SQTS) is a genetically determined ion-channel disorder, which may cause malignant tachyarrhythmias and sudden cardiac death. Thus far, mutations in five different genes encoding potassium and calcium channel subunits have been reported. We present, for the first time, a novel loss-of-function mutation coding for an L-type calcium channel subunit. Methods and results The electrocardiogram of the affected member of a single family revealed a QT interval of 317 ms (QTc 329 ms) with tall, narrow, and symmetrical T-waves. Invasive electrophysiological testing showed short ventricular refractory periods and increased vulnerability to induce ventricular fibrillation. DNA screening of the patient identified no mutation in previously known SQTS genes; however, a new variant at a heterozygous state was identified in the CACNA2D1 gene (nucleotide c.2264G > C; amino acid p.Ser755Thr), coding for the Cavα2δ-1 subunit of the L-type calcium channel. The pathogenic role of the p.Ser755Thr variant of the CACNA2D1 gene was analysed by using co-expression of the two other L-type calcium channel subunits, Cav1.2α1 and Cavβ2b, in HEK-293 cells. Barium currents ( I Ba) were recorded in these cells under voltage-clamp conditions using the whole-cell configuration. Co-expression of the p.Ser755Thr Cavα2δ-1 subunit strongly reduced the I Ba by more than 70% when compared with the co-expression of the wild-type (WT) variant. Protein expression of the three subunits was verified by performing western blots of total lysates and cell membrane fractions of HEK-293 cells. The p.Ser755Thr variant of the Cavα2δ-1 subunit was expressed at a similar level compared with the WT subunit in both fractions. Since the mutant Cavα2δ-1 subunit did not modify the expression of the pore-forming subunit of the L-type calcium channel, Cav1.2α1, it suggests that single channel biophysical properties of the L-type channel are altered by this variant. Conclusion In the present study, we report the first pathogenic mutation in the CACNA2D1 gene in humans, which causes a new variant of SQTS. It remains to be determined whether mutations in this gene lead to other manifestations of the J-wave syndrome.

Phenotypical Manifestations of Mutations in the Genes Encoding Subunits of the Cardiac Voltage–Dependent L-Type Calcium Channel

Abstract: The L-type cardiac calcium channel (LTCC) plays a prominent role in the electric and mechanical function of the heart. Mutations in the LTCC have been associated with a number of inherited cardiac arrhythmia syndromes, including Timothy, Brugada, and early repolarization syndromes. Elucidation of the genetic defects associated with these syndromes has led to a better understanding of molecular and cellular mechanisms and the development of novel therapeutic approaches to dealing with the arrhythmic manifestations. This review provides an overview of the molecular structure and function of the LTCC, the genetic defects in these channels known to contribute to inherited disorders, and the underlying molecular and cellular mechanisms contributing to the development of life-threatening arrhythmias.

Minimum Information about a Cardiac Electrophysiology Experiment (MICEE): Standardised reporting for model reproducibility, interoperability, and data sharing

Abstract: Cardiac experimental electrophysiology is in need of a well-defined Minimum Information Standard for recording, annotating, and reporting experimental data. As a step towards establishing this, we present a draft standard, called Minimum Information about a Cardiac Electrophysiology Experiment (MICEE). The ultimate goal is to develop a useful tool for cardiac electrophysiologists which facilitates and improves dissemination of the minimum information necessary for reproduction of cardiac electrophysiology research, allowing for easier comparison and utilisation of findings by others. It is hoped that this will enhance the integration of individual results into experimental, computational, and conceptual models. In its present form, this draft is intended for assessment and development by the research community. We invite the reader to join this effort, and, if deemed productive, implement the Minimum Information about a Cardiac Electrophysiology Experiment standard in their own work.

I. Experimental analysis of the use-dependent block

Abstract: Atrial-selective inhibition of cardiac Na(+) channel current (I(Na)) and I(Na)-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study examined the basis for the atrial-selective actions of ranolazine. Whole cell I(Na) was recorded at 15°C in canine atrial and ventricular myocytes and in human embryonic kidney (HEK)-293 cells expressing SCN5A. Tonic block was negligible at holding potentials from -140 to -100 mV, suggesting minimal drug interactions with the closed state. Trains of 40 pulses were elicited over a range of holding potentials to determine use-dependent block. Guarded receptor formalism was used to analyze the development of block during pulse trains. Use-dependent block by ranolazine increased at more depolarized holding potentials, consistent with an interaction of the drug with either preopen or inactivated states, but was unaffected by longer pulse durations between 5 and 200 ms, suggesting a weak interaction with the inactivated state. Block was significantly increased at shorter diastolic intervals between 20 and 200 ms. Responses in atrial and ventricular myocytes and in HEK-293 cells displayed a similar pattern. Ranolazine is an open state blocker that unbinds from closed Na(+) channels unusually fast but is trapped in the inactivated state. Kinetic rates of ranolazine interactions with different states of atrial and ventricular Na(+) channels were similar. Our data suggest that the atrial selectivity of ranolazine is due to a more negative steady-state inactivation curve, less negative resting membrane potential, and shorter diastolic intervals in atrial cells compared with ventricular cells at rapid rates.

Mechanisms of atrial-selective block of Na⁺ channels by ranolazine: I. Experimental analysis of the use-dependent block.

Abstract: Atrial-selective inhibition of cardiac Na+ channel current (INa) and INa-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present s...

Short QT syndrome

Abstract: The short QT syndrome (SQTS) is a recently described genetic arrhythmogenic disorder, characterized by abnormally short QT intervals on surface electrocardiogram (ECG) and a high incidence of sudden death (SD) during life, including the first months of life. The inheritance of SQTS is autosomal dominant, with genetic heterogeneity. Gain-of-function mutations in 3 genes encoding potassium channels have been associated to the disease: KCNH2 encoding IKr (SQT1), KCNQ1 encoding IKs (SQT2), and KCNJ2 encoding IK1 (SQT3). Loss-of-function mutations in 3 genes encoding the cardiac L-type calcium channel, CACNA1C , CACNB2b and CACNA2D1 may underlie a mixed phenotype of Brugada pattern ECG (or non-specific repolarization changes in case of CACNA2D1 ) and shorter than normal QT intervals. Clinical presentation is often severe, as cardiac arrest represents the first clinical presentation in most subjects. Moreover, often a noticeable family history of cardiac SD is present. Atrial fibrillation may be observed, also in young individuals. At electrophysiological study, short atrial and ventricular refractory periods are found, and atrial and ventricular fibrillation are easily induced by programmed electrical stimulation. The outcome of patients with SQTS becomes relatively safe when they are identified and treated. Currently, the suggested therapeutic strategy is an implantable cardioverter- defibrillator (ICD) in patients with personal history of aborted SD or syncope. In asymptomatic adult patients from highly symptomatic families and in newborn children pharmacological treatment with hydroquinidine, which has been shown to prolong the QT interval and reduce the inducibility of ventricular arrhythmias, may be proposed.

Extracellular proton depression of peak and late Na⁺ current in the canine left ventricle.

Abstract: Cardiac ischemia reduces excitability in ventricular tissue. Acidosis (one component of ischemia) affects a number of ion currents. We examined the effects of extracellular acidosis (pH 6.6) on pea...

Mechanisms of atrial-selective block of Na⁺ channels by ranolazine: II. Insights from a mathematical model.

Abstract: Block of Na+ channel conductance by ranolazine displays marked atrial selectivity that is an order of magnitude higher that of other class I antiarrhythmic drugs. Here, we present a Markovian model...

Comparison of the Effects of a Transient Outward Potassium Channel Activator on Currents Recorded from Atrial and Ventricular Cardiomyocytes

Abstract: Effect of NS5806 on Atrial Currents. Introduction: NS5806 activates the transient outward potassium current (Ito) in canine ventricular cells. We compared the effects of NS5806 on canine atrial versus ventricular tissues and myocytes. Methods and Results: NS5806 (10 μM) was evaluated in arterially perfused canine right atrial and right ventricular wedge preparations. In ventricular wedges NS5806 (10 μM) accentuated phase 1 in epicardium (Epi), with little effect in endocardium (Endo), resulting in augmented J-waves on the ECG. In contrast, application of NS5806 (10 μM) to atrial preparations had no effect on phase 1 repolarization but significantly decreased upstroke velocity (dV/dt) and depressed excitability, consistent with sodium channel block. Current and voltage-clamp recordings were made in the absence and presence of NS5806 in (10 μM) enzymatically dissociated atrial and ventricular myocytes. In ventricular myocytes, NS5806 increased Ito magnitude by 80% and 16% in Epi and Endo, respectively (at +40 mV). In atrial myocytes, NS5806 increased peak Ito by 25% and had no effect on the sustained current, IKur. Under control conditions, INa density in atrial myocytes was nearly double that in ventricular myocytes. NS5806 caused a shift in steady-state mid-inactivation (V1/2) from –73.9 ± 0.27 to –77.3 ± 0.21 mV in ventricular and from –82.6 ± 0.12 to –85.1 ± 0.11 mV in atrial cells, resulting in reduction of INa in both cell types. Expression of mRNA encoding putative INa and Ito channel subunits was evaluated by qPCR. Conclusion: NS5806 produces a prominent augmentation of Ito with little effect on INa in the ventricles, but a potent inhibition of INa with little augmentation of Ito in atria. (J Cardiovasc Electrophysiol, Vol. 22, pp. 1057-1066, September 2011)

American Heart Association Atrial Fibrillation Research Summit A Conference Report From the American Heart Association

Abstract: Atrial fibrillation (AF) poses a major global public health challenge because it is increasing in prevalence and is associated with an increased risk of stroke, dementia, heart failure, and death.1–3 In response to the many challenges posed by AF, the American Heart Association (AHA) convened a conference in Washington, DC, on June 12–13, 2010, that included patients, nurses, physicians, and healthcare policy makers and regulators. In addition, basic, translational, population, outcomes, and clinical scientists participated (Appendix). The 22 presentations and 6 panel discussions were organized into 4 sessions: (1) Mechanisms of AF: Basic and Translational Science and Genetics; (2) Epidemiology, Outcomes, Cost, AF, and Stroke Prevention; (3) Meeting the Clinical Challenges in AF; and (4) Redefining the Therapeutic Goals of AF (Appendix). The focus of the present report is to provide an overview of the key concepts presented and the core recommendations made by the summit participants. Attempts to develop safe and effective pharmacological therapy for AF have focused on atrium-selective drugs that take advantage of electrophysiological differences between the atrium and ventricle.4–7 Heterogeneous abbreviation of the effective refractory period within the atrium provides the electric substrate for development of AF. The reduced effective refractory period results from abbreviation of the atrial action potential duration, which is caused by a decrease in the calcium channel current ( ICa ) and an increase in the potassium channel current ( IK1 ) and the constitutively active acetylcholine-sensitive current (CA IKACh ).4–7 Maintenance of AF is facilitated by structural remodeling and additional abbreviation of the effective refractory period. The principal goal of pharmacological therapy is therefore to augment the effective refractory period. Distinctions in the ion channel currents between the atrium and ventricle open the possibility for development of atrium-specific and -selective drugs for rhythm …

Unraveling the Enigma of Bangungut: Is Sudden Unexplained Nocturnal Death Syndrome (SUNDS) in the Philippines a Disease Allelic to the Brugada Syndrome?

Abstract: BACKGROUND: Sudden unexplained nocturnal death syndrome (SUNDS) has been reported worldwide. SUNDS is endemic in Southeast Asia and is colloquially known as Bangungut in the Philippines, Lai Tai in Thailand, and Pokkuri in Japan. Although SUNDS in Thailand and Japan have been determined to be phenotypically, genetically and functionally identical to the Brugada syndrome, the relationship between Bangungut/SUNDS in the Philippines and the Brugada syndrome has not been clarified. This paper explores the concordance between Bangungut/SUNDS and the Brugada syndrome. METHODS: We summarized autopsy studies on Bangungut retrieved from PubMed since 1917 and current epidemiological data on Philippine SUNDS to clarify its diagnostic features. We also reviewed current hypotheses of the pathophysiological mechanism of the Brugada syndrome to explore its applicability to Bangungut/SUNDS. RESULTS: The use of the term Bangungut is confusing as it includes many diseases that may cause SUNDS. However, our review reveals a notable subset of Bangungut, identified as Bangungut/SUNDS with no gross cardiac pathology that conforms to the clinical picture of the folk-belief of Bangungut and of the Brugada syndrome, namely: predominance among male in the 20-40 age range; sudden death during sleep or at rest, usually following ingestion of a large meal at night; and victims were in apparent good health prior to their demise. Current pathophysiological mechanisms of Brugada syndrome seemed plausible explanations for a majority of this subset of Bangungut/SUNDS. CONCLUSION: Bangungut/SUNDS and the Brugada syndrome appear closely related. Pathophysiological mechanisms of the Brugada syndrome may explain the enigma of Bangungut/SUND. Whether Bangungut/SUNDS is phenotypically, genetically and functionally an allele of the Brugada syndrome remains inconclusive due to lack of research data. We therefore proposed a research agenda including genetic testing and pharmacological challenge of probands and their family members suspected of SUNDS to conclusively establish the relationship between these two syndromes.

Biophysical and molecular characterization of a novel de novo KCNJ2 mutation associated with Andersen-Tawil syndrome and catecholaminergic polymorphic ventricular tachycardia mimicry.

Abstract: Background— Mutations in KCNJ2 , the gene encoding the human inward rectifier potassium channel Kir2.1 (IK1 or IKir2.1), have been identified in Andersen-Tawil syndrome. Andersen-Tawil syndrome is a multisystem inherited disease exhibiting periodic paralysis, cardiac arrhythmias, and dysmorphic features at times mimicking catecholaminergic polymorphic ventricular tachycardia. Methods and Results— Our proband displayed dysmorphic features including micrognathia, clinodactyly, and syndactyly and exhibited multiform extrasystoles and bidirectional ventricular tachycardia both at rest and during exercise testing. The patient's symptoms continued after administration of nadolol but subsided after treatment with flecainide. Molecular genetic screening revealed a novel heterozygous mutation (c.779G>C/p.R260P) in KCNJ2 . Whole-cell patch-clamp studies conducted in TSA201 cells transfected with wild-type human KCNJ2 cDNA (WT- KCNJ2 ) yielded robust IKir2.1 but no measurable current in cells expressing the R260P mutant. Coexpression of WT and R260P- KCNJ2 (heterozygous expression) yielded a markedly reduced inward IKir2.1 compared with WT alone (−36.5±9.8 pA/pF versus −143.5±11.4 pA/pF, n=8 for both, P <0.001, respectively, at −90 mV), indicating a strong dominant negative effect of the mutant. The outward component of IKir2.1 measured at −50 mV was also markedly reduced with the heterozygous expression versus WT (0.52±5.5 pA/pF versus 23.4±6.7 pA/pF, n=8 for both, P <0.001, respectively). Immunocytochemical analysis indicates that impaired trafficking of R260P- KCNJ2 channels. Conclusions— We report a novel de novo KCNJ2 mutation associated with classic phenotypic features of Andersen-Tawil syndrome and catecholaminergic polymorphic ventricular tachycardia mimicry. The R260P mutation produces a strong dominant negative effect leading to marked suppression of IK1 secondary to a trafficking defect.

LQT5 masquerading as LQT2: a dominant negative effect of KCNE1-D85N rare polymorphism on KCNH2 current

Abstract: Aims KCNE1 encodes an auxiliary subunit of cardiac potassium channels. Loss-of-function variations in this gene have been associated with the LQT5 form of the long QT syndrome (LQTS), secondary to reduction of I Ks current. We present a case in which a D85N rare polymorphism in KCNE1 is associated with an LQT2 phenotype. Methods and results An 11-year old competitive athlete presented with mild bradycardia and a QTc interval of 470 ms. An LQT2 phenotype, consisting of low-voltage bifid T waves, was evident in the right precordial electrocardiogram leads. During the tachycardia phase following adenosine, QTc increased to 620 ms. Genetic analysis revealed a rare heterozygous polymorphism in KCNE1 predicting the substitution of asparagine for aspartic acid at position 85 of minK (D85N). Patch clamp experiments showed that KCNE1- D85N, when co-expressed with KCNH2 in TSA201 cells, significantly reduced I Kr. Homozygous co-expression of the mutant with KCNH2 reduced I Kr tail current by 85%, whereas heterozygous co-expression reduced the current by 52%, demonstrating for the first time a dominant-negative effect of D85N to reduce I Kr. Co-expression of the mutant with KCNQ1, either homozygously or heterozygously, produced no change in I Ks. Conclusions Our results suggest that a rare polymorphism KCNE1- D85N underlies the development of an LQT2 phenotype in this young athlete by interacting with KCNH2 to cause a dominant-negative effect to reduce I Kr. Our data provide further evidence in support of the promiscuity of potassium channel β subunits in modulating the function of multiple potassium channels leading to a diversity of clinical phenotypes.

Multiple arrhythmic syndromes in a newborn, owing to a novel mutation in SCN5A

Abstract: Background. Mutations in the SCN5A gene have been linked to Brugada syndrome (BrS), conduction disease, Long QT syndrome (LQT3), atrial fibrillation (AF), and to pre- and neonatal ventricular arrhythmias. Objective. The objective of this study is to characterize a novel mutation in Nav1.5 found in a newborn with fetal chaotic atrial tachycardia, post-partum intraventricular conduction delay, and QT interval prolongation. Methods. Genomic DNA was isolated and all exons and intron borders of 15 ion-channel genes were sequenced, revealing a novel missense mutation (Q270K) in SCN5A. Nav1.5 wild type (WT) and Q270K were expressed in CHO-K1 with and without the Navβ1 subunit. Results. Patch-clamp analysis showed ∼40% reduction in peak sodium channel current (INa) density for Q270K compared with WT. Fast and slow decay of INa were significantly slower in Q270K. Steady-state activation and inactivation of Q270K channels were shifted to positive potentials, and window current was increased. The tetrodotoxin-sensit...

Antiarrhythmic effects of losartan and enalapril in canine pulmonary vein sleeve preparations.

Abstract: Introduction—Angiotensin-converting enzyme (ACE) inhibitors and angiotensin II-receptor blockers (ARBs) are prototypes of “upstream” therapy for the management of atrial fibrillation (AF). Ectopic activity arising from the PV sleeves plays a prominent role in the development of AF. Methods—Transmembrane action potentials were recorded from canine superfused left superior or inferior PV sleeves using standard microelectrode techniques. Acetylcholine (ACh, 1 µM), isoproterenol (1 µM), high calcium ([Ca 2+ ]o=5.4mM) or a combination was used to induce early or delayed afterdepolarizations (EADs or DADs) and triggered activity. Results—The ARB losartan (1 µM, n=5) and the ACE inhibitor enalapril (10 µM, n=5) produced no significant change in action potential duration, maximum rate of rise of action potential upstroke (Vmax), action potential amplitude or take-off potential at basic cycle lengths of 200 to 2000 ms. Losartan (1 µM) and enalapril (10–20 µM) markedly attenuated or suppressed EADs and DAD–induced triggered activity elicited by exposure of the PV sleeves to ACh, isoproterenol or high calcium following rapid pacing in 6 of 6 (losartan) and 4 of 5 (enalapril) PV sleeve preparations. Neither losartan or enalapril altered Ca 2+ or K+ channel currents in enzymatically-dissociated atrial myocytes at these concentrations. Conclusions—Our data suggest that in addition to their “upstream” effects to reduce atrial structural remodeling, ACE inhibitors and ARBs exert a “direct” antiarrhythmic effect by suppressing triggers responsible for the genesis of AF and other atrial arrhythmias.

Ranolazine versus amiodarone for prevention of postoperative atrial fibrillation

Abstract: Evaluation of: Miles RH, Passman R, Murdock DK. Comparison of effectiveness and safety of ranolazine versus amiodarone for preventing atrial fibrillation after coronary artery bypass grafting. Am. J. Cardiol. 108(5), 673–676 (2011). Postoperative atrial fibrillation (AF) is a major complication of cardiothoracic surgery, leading to significant consequences, including a higher rate of stroke, longer hospital stays and increased costs. Amiodarone is among the most widely used agents for prevention of postoperative AF. Ranolazine, a US FDA-approved antianginal agent, has been shown to effectively, safely prevent and terminate nonpostoperative AF in both experimental and clinical studies. In a recent publication, Miles and colleagues directly compared the efficacy and safety of amiodarone and ranolazine for prevention of postoperative AF in 393 patients. The patients were pretreated with amiodarone and ranolaizne for >1 week and 1 day, respectively, and the treatment continued for 10–14 days after surgery. Fo...

Ion channels and beating heart: the players and the music

Abstract: Soft gentle music accompanies us throughout our lifetime; it is the music of our heart beating. Although at times it is questionable as to who serves as conductor of the orchestra, there is little doubt that our ion channels are the main players. Whenever one of them plays too loudly, too softly or simply off key, disharmony results, sometimes leading to total disruption of the rate and rhythm. Ion channels can disrupt the music of our heart by different mechanisms. Sometimes their function is correct, but their expression is altered by underlying cardiac diseases ( i.e. heart failure); sometimes the defect is in their structure, because of an underlying genetic defect, and in this case a channelopathy is present.