Background— Cardiac ion channelopathies are responsible for an ever-increasing number and diversity of familial cardiac arrhythmia syndromes. We describe a new clinical entity that consists of an ST-segment elevation in the right precordial ECG leads, a shorter-than-normal QT interval, and a history of sudden cardiac death. Methods and Results— Eighty-two consecutive probands with Brugada syndrome were screened for ion channel gene mutations with direct sequencing. Site-directed mutagenesis was performed, and CHO-K1 cells were cotransfected with cDNAs encoding wild-type or mutant CACNB2b (Ca _vβ2b ), CACNA2D1 (Ca _vα2δ1 ), and CACNA1C tagged with enhanced yellow fluorescent protein (Ca _v 1.2). Whole-cell patch-clamp studies were performed after 48 to 72 hours. Three probands displaying ST-segment elevation and corrected QT intervals ≤360 ms had mutations in genes encoding the cardiac L-type calcium channel. Corrected QT ranged from 330 to 370 ms among probands and clinically affected family members. Rate adaptation of QT interval was reduced. Quinidine normalized the QT interval and prevented stimulation-induced ventricular tachycardia. Genetic and heterologous expression studies revealed loss-of-function missense mutations in CACNA1C (A39V and G490R) and CACNB2 (S481L) encoding the α ₁ - and β _2b -subunits of the L-type calcium channel. Confocal microscopy revealed a defect in trafficking of A39V Ca _v 1.2 channels but normal trafficking of channels containing G490R Ca _v 1.2 or S481L Ca _vβ2b -subunits. Conclusions— This is the first report of loss-of-function mutations in genes encoding the cardiac L-type calcium channel to be associated with a familial sudden cardiac death syndrome in which a Brugada syndrome phenotype is combined with shorter-than-normal QT intervals.

Background— Sudden cardiac death takes the lives of more than 300 000 Americans annually. Malignant ventricular arrhythmias occurring in individuals with structurally normal hearts account for a subgroup of these sudden deaths. The present study describes the genetic basis for a new clinical entity characterized by sudden death and short-QT intervals in the ECG. Methods and Results— Three families with hereditary short-QT syndrome and a high incidence of ventricular arrhythmias and sudden cardiac death were studied. In 2 of them, we identified 2 different missense mutations resulting in the same amino acid change (N588K) in the S5-P loop region of the cardiac I _Kr channel HERG (KCNH2). The mutations dramatically increase I _Kr , leading to heterogeneous abbreviation of action potential duration and refractoriness, and reduce the affinity of the channels to I _Kr blockers. Conclusions— We demonstrate a novel genetic and biophysical mechanism responsible for sudden death in infants, children, and young adults caused by mutations in KCNH2. The occurrence of sudden cardiac death in the first 12 months of life in 2 patients suggests the possibility of a link between KCNH2 gain of function mutations and sudden infant death syndrome. KCNH2 is the binding target for a wide spectrum of cardiac and noncardiac pharmacological compounds. Our findings may provide better understanding of drug interaction with KCNH2 and have implications for diagnosis and therapy of this and other arrhythmogenic diseases.

Background— Ranolazine is a novel antianginal agent capable of producing antiischemic effects at plasma concentrations of 2 to 6 μmol/L without reducing heart rate or blood pressure. The present study examines its electrophysiological effects in isolated canine ventricular myocytes, tissues, and arterially perfused left ventricular wedge preparations. Methods and Results— Transmembrane action potentials (APs) from epicardial and midmyocardial (M) regions and a pseudo-ECG were recorded simultaneously from wedge preparations. APs were also recorded from epicardial and M tissues. Whole-cell currents were recorded from epicardial and M myocytes. Ranolazine inhibited I _Kr (IC ₅₀ =11.5 μmol/L), late I _Na , late I _Ca , peak I _Ca , and I _Na-Ca (IC ₅₀ =5.9, 50, 296, and 91 μmol/L, respectively) and I _Ks (17% at 30 μmol/L), but caused little or no inhibition of I _to or I _K1 . In tissues and wedge preparations, ranolazine produced a concentration-dependent prolongation of AP duration of epicardial but abbreviation of that of M cells, leading to reduction or no change in transmural dispersion of repolarization (TDR). At [K ⁺ ] _o =4 mmol/L, 10 μmol/L ranolazine prolonged QT interval by 20 ms but did not increase TDR. Extrasystolic activity and spontaneous torsade de pointes (TdP) were never observed, and stimulation-induced TdP could not be induced at any concentration of ranolazine, either in normal or low [K ⁺ ] _o . Ranolazine (5 to 20 μmol/L) suppressed early afterdepolarizations (EADs) and reduced the increase in TDR induced by the selective I _Kr blocker d -sotalol. Conclusions— Ranolazine produces ion channel effects similar to those observed after chronic amiodarone (reduced I _Kr , I _Ks , late I _Na , and I _Ca ). The actions of ranolazine to suppress EADs and reduce TDR suggest that, in addition to its antianginal actions, the drug may possess antiarrhythmic activity.

Background— The Brugada syndrome displays an autosomal dominant mode of transmission with low penetrance. Despite equal genetic transmission of the disease, the clinical phenotype is 8 to 10 times more prevalent in males than in females. The basis for this intriguing sex-related distinction is unknown. The present study tests the hypothesis that the disparity in expression of the Brugada phenotype is a result of a more prominent I _to -mediated action potential notch in the right ventricular (RV) epicardium of males versus females. Methods and Results— We studied epicardial tissue slices, arterially perfused wedge preparations, and dissociated epicardial myocytes isolated from male and female canine hearts. RV epicardium action potential phase 1 amplitude was 64.8±2.0% of that of phase 2 in males compared with 73.8±4.4% in females ( P <0.05) at a cycle length of 2000 ms. I _to density was 26% smaller and time constant for inactivation 17% smaller at +40 mV in female versus male RV epicardial cells ( P <0.05). The other functional characteristics of I _to , including the voltage dependence of inactivation and time course of reactivation, were no different between the sexes. Pinacidil caused loss of action potential dome in male, but not female, RV epicardial tissue slices. Terfenadine (5 μmol/L) induced phase 2 reentry in 6 of 7 male but only 2 of 7 female arterially perfused wedge preparations. Two of 6 male and 1 of 2 female preparations developed polymorphic ventricular tachycardia/ventricular fibrillation. Conclusions— Our results suggest that the predominance of the Brugada phenotype in males is a result of the presence of a more prominent I _to in males versus females.

Background— The Brugada syndrome, an inherited syndrome associated with a high incidence of sudden cardiac arrest, has been linked to mutations in 4 different genes, leading to a loss of function in sodium and calcium channel activity. Although the transient outward current ( I _to ) is thought to play a prominent role in the expression of the syndrome, mutations in I _to -related genes have not been identified as yet. Methods and Results— One hundred five probands with the Brugada syndrome were screened for ion channel gene mutations using single-strand conformation polymorphism electrophoresis and direct sequencing. A missense mutation (R99H) in KCNE3 ( MiRP2 ) was detected in 1 proband. The R99H mutation was found 4/4 phenotype-positive and 0/3 phenotype-negative family members. Chinese hamster ovary-K1 cells were cotransfected using wild-type (WT) or mutant KCNE3 and either WT KCND3 or KCNQ1. Whole-cell patch clamp studies were performed after 48 hours. Interactions between Kv4.3 and KCNE3 were analyzed in coimmunoprecipitation experiments in human atrial samples. Cotransfection of R99H- KCNE3 with KCNQ1 produced no alteration in tail current magnitude or kinetics. However, cotransfection of R99H KCNE3 with KCND3 resulted in a significant increase in the I _to intensity compared with WT KCNE3 +KCND3. Using tissues isolated from the left atrial appendages of human hearts, we also demonstrate that K v 4.3 and KCNE3 can be coimmunoprecipitated. Conclusions— These results provide definitive evidence for a functional role of KCNE3 in the modulation of I _to in the human heart and suggest that mutations in KCNE3 can underlie the development of the Brugada syndrome.

Background—Brugada syndrome, characterized by ST-segment elevation in the right precordial ECG leads and the development of life-threatening ventricular arrhythmias, has been associated with mutations in 6 different genes. We identify and characterize a mutation in a new gene.Methods and Results—A 64-year-old white male displayed a type 1 ST-segment elevation in V1 and V2 during procainamide challenge. Polymerase chain reaction-based direct sequencing was performed using a candidate gene approach. A missense mutation (L10P) was detected in exon 1 ofSCN3B, the β3 subunit of the cardiac sodium channel, but not in any other gene known to be associated with Brugada syndrome or in 296 controls. Wild-type (WT) and mutant genes were expressed in TSA201 cells and studied using whole-cell patch-clamp techniques. Coexpression ofSCN5A/WT+SCN1B/WT+SCN3B/L10P resulted in an 82.6% decrease in peak sodium current density, accelerated inactivation, slowed reactivation, and a −9.6-mV shift of half-inactivation voltage compared withSCN5A/WT+SCN1B/WT+SCN3B/WT. Confocal microscopy revealed thatSCN5A/WT channels tagged with green fluorescent protein are localized to the cell surface when coexpressed with WTSCN1BandSCN3Bbut remain trapped in intracellular organelles when coexpressed withSCN1B/WT andSCN3B/L10P. Western blot analysis confirmed the presence of Na_Vβ3 in human ventricular myocardium.Conclusions—Our results provide support for the hypothesis that mutations inSCN3Bcan lead to loss of transport and functional expression of the hNa_v1.5 protein, leading to reduction in sodium channel current and clinical manifestation of a Brugada phenotype.