Background-The electrocardiographic short QT-interval syndrome forms a distinct clinical entity presenting with a high rate of sudden death and exceptionally short QT intervals. The disorder has recently been linked to gain-of-function mutation in KCNH2. The present study demonstrates that this disorder is genetically heterogeneous and can also be caused by mutation in the KCNQ1 gene. Methods and Results-A 70-year man presented with idiopathic ventricular fibrillation. Both immediately after the episode and much later, his QT interval was abnormally short without any other physical or electrophysiological anomalies. Analysis of candidate genes identified a g919c substitution in KCNQ1 encoding the K ϩ channel KvLQT1. Functional studies of the KvLQT1 V307L mutant (alone or coexpressed with the wild-type channel, in the presence of IsK) revealed a pronounced shift of the half-activation potential and an acceleration of the activation kinetics leading to a gain of function in I Ks . When introduced in a human action potential computer model, the modified biophysical parameters predicted repolarization shortening. Conclusions-We present an alternative molecular mechanism for the short QT-interval syndrome. Functional and computational studies of the KCNQ1 V307L mutation identified in a patient with this disorder favor the association of short QT with mutation in KCNQ1. Key Words: death, sudden Ⅲ genetics Ⅲ arrhythmia Ⅲ ion channels Ⅲ fibrillation, ventricular I n recent years, extensive progress has been made in unraveling the pathophysiology of the monogenic arrhythmia syndromes among which are long-QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. 1 The latest addition to this class of disorders is the description of families with a high rate of sudden death and exceptionally short QT intervals, 2 recently attributed to gain-of-function mutation in KCNH2. 3 In this study, we demonstrate that this disorder is genetically heterogeneous and can also be caused by mutation in the KCNQ1 gene that encodes the KvLQT1 K ϩ channel, which, in association with the ␤-subunit IsK, forms the slow component of the cardiac delayed rectifier K ϩ current (I Ks ). 4 Methods Patient CharacteristicsA 70-year-old man was successfully resuscitated after a ventricular fibrillation episode. He had been without complaints up until then, and his family history was unremarkable. Physical examination revealed no abnormalities. His ECG is presented in Figure 1. Sinus rhythm was present with normal conduction intervals and a QT interval of 290 ms (QTc, 302 ms). Similarly short QT intervals were observed on every ECG up to 3 years of follow-up. Biochemical analysis at the time of admission, including echocardiography, exercise testing, coronary angiography, left (LV) and right ventricular (RV) angiography, scintigraphy, and ergonovine coronary spasm test, revealed no abnormalities. Nuclear LV ejection fraction was 49%. During electrophysiological study, no arrhythmias could be induced. The electrophysiology ...

Phosphatidylinositol-4,5-bisphosphate (PIP 2 ) is a major signaling molecule implicated in the regulation of various ion transporters and channels. Here we show that PIP 2 and intracellular MgATP control the activity of the KCNQ1/KCNE1 potassium channel complex. In excised patch±clamp recordings, the KCNQ1/KCNE1 current decreased spontaneously with time. This rundown was markedly slowed by cytosolic application of PIP 2 and fully prevented by application of PIP 2 plus MgATP. PIP 2 -dependent rundown was accompanied by acceleration in the current deactivation kinetics, whereas the MgATP-dependent rundown was not. Cytosolic application of PIP 2 slowed deactivation kinetics and also shifted the voltage dependency of the channel activation toward negative potentials. Complex changes in the current characteristics induced by membrane PIP 2 was fully restituted by a model originally elaborated for ATP-regulated two transmembrane-domain potassium channels. The model is consistent with stabilization by PIP 2 of KCNQ1/KCNE1 channels in the open state. Our data suggest a striking functional homology between a six transmembrane-domain voltage-gated channel and a two transmembrane-domain ATP-gated channel.

Abstract-Although electrophysiological remodeling occurs in various myocardial diseases, the underlying molecular mechanisms are poorly understood. cDNA microarrays containing probes for a large population of mouse genes encoding ion channel subunits ("IonChips") were developed and exploited to investigate remodeling of ion channel transcripts associated with altered thyroid status in adult mouse ventricle. Functional consequences of hypo-and hyperthyroidism were evaluated with patch-clamp and ECG recordings. Hypothyroidism decreased heart rate and prolonged QTc duration. Opposite changes were observed in hyperthyroidism. Microarray analysis revealed that hypothyroidism induces significant reductions in KCNA5, KCNB1, KCND2, and KCNK2 transcripts, whereas KCNQ1 and KCNE1 expression is increased. In hyperthyroidism, in contrast, KCNA5 and KCNB1 expression is increased and KCNQ1 and KCNE1 expression is decreased. Real-time RT-PCR validated these results. Consistent with microarray analysis, Western blot experiments confirmed those modifications at the protein level. Patch-clamp recordings revealed significant reductions in I to,f and I K,slow densities, and increased I Ks density in hypothyroid myocytes. In addition to effects on K ϩ channel transcripts, transcripts for the pacemaker channel HCN2 were decreased and those encoding the ␣1C Ca 2ϩ channel (CaCNA1C) were increased in hypothyroid animals. The expression of Na ϩ , Cl Ϫ , and inwardly rectifying K ϩ channel subunits, in contrast, were unaffected by thyroid hormone status. Taken together, these data demonstrate that thyroid hormone levels selectively and differentially regulate transcript expression for at least nine ion channel ␣-and ␤-subunits. Our results also document the potential of cDNA microarray analysis for the simultaneous examination of ion channel transcript expression levels in the diseased/remodeled myocardium. Key Words: ion channel Ⅲ cDNA microarray Ⅲ repolarization Ⅲ ionic remodeling E lectrical remodeling refers to changes in cardiac electrophysiological function caused by heart disease. Various pathologies are associated with electrophysiological remodeling including cardiac hypertrophy and failure, 1 chronic arrhythmia (eg, atrial fibrillation), 2 ischemic injury, 3 or altered thyroid status, 4 and several lines of evidence suggest that the underlying molecular mechanisms are complex and may well be model-dependent. In addition, most studies in the remodeled myocardium have focused on examination of individual ionic currents and/or expression levels of the channel subunits generating these currents.The application of genomic techniques, however, holds the promise of allowing the expression levels of thousands of genes to be examined simultaneously. 5 We have developed cDNA microarrays ("IonChips") containing probes for a large subset of genes encoding ion channel subunit proteins. With the objective to validate this novel tool, we have investigated cardiac ion channel remodeling associated with altered thyroid hormone status in the mo...

QT prolongation, a classic risk factor for arrhythmias, can result from a mutation in one of the genes governing cardiac repolarization and also can result from the intake of a medication acting as blocker of the cardiac K ϩ channel human ether-a-gogo-related gene (HERG). Here, we identified the arrhythmogenic potential of a nonopioid antitussive drug, clobutinol. The deleterious effects of clobutinol were suspected when a young boy, with a diagnosis of congenital long QT syndrome, experienced arrhythmias while being treated with this drug. Using the patch-clamp technique, we showed that clobutinol dose-dependently inhibited the HERG K ϩ current with a half-maximum block concentration of 2.9 M. In the proband, we identified a novel A561P HERG mutation. Two others long QT mutations (A561V and A561T) had been reported previously at the same position. None of the three mutants led to a sizeable current in heterologous expression system. When coexpressed with wildtype (WT) HERG channels, the three Ala561 mutants reduced the trafficking of WT and mutant heteromeric channels, resulting in decreased K ϩ current amplitude (dominant-negative effects). In addition, A561P but not A561V and A561T mutants induced a ϷϪ11 mV shift of the current activation curve and accelerated deactivation, thereby partially counteracting the dominant-negative effects. A561P mutation and clobutinol effects on the human ventricular action potential characteristics were simulated using the Priebe-Beuckelmann model. Our work shows that clobutinol has limited effects on WT action potential but should be classified as a "drug to be avoided by congenital long QT patients" rather than as a "drug with risk of torsades de pointes".

In this population, two subjects with borderline QTc prolongations (438 and 443 ms) were carriers of KCNQ1 mutations leading to haploinsufficiency and are potentially at risk of developing drug-induced arrhythmia. The study provides the first demonstration of a defective cell surface localization of a KvLQT1 mutant missing one amino acid in a transmembrane domain.

Background-The basis for the unique effectiveness of long-term amiodarone treatment on cardiac arrhythmias is incompletely understood. The present study investigated the pharmacogenomic profile of amiodarone on genes encoding ion-channel subunits. Methods and Results-Adult male mice were treated for 6 weeks with vehicle or oral amiodarone at 30, 90, or 180 mg · kg Ϫ1 · d Ϫ1. Plasma and myocardial levels of amiodarone and N-desethylamiodarone increased dose-dependently, reaching therapeutic ranges observed in human. Plasma triiodothyronine levels decreased, whereas reverse triiodothyronine levels increased in amiodarone-treated animals. In ECG recordings, amiodarone dose-dependently prolonged the RR, PR, QRS, and corrected QT intervals. Specific microarrays containing probes for the complete ion-channel repertoire (IonChips) and real-time reverse transcription-polymerase chain reaction experiments demonstrated that amiodarone induced a dose-dependent remodeling in multiple ion-channel subunits. Genes encoding Na ϩ (SCN4A, SCN5A, SCN1B), connexin (GJA1), Ca 2ϩ (CaCNA1C), and K ϩ channels (KCNA5, KCNB1, KCND2) were downregulated. In patch-clamp experiments, lower expression of K ϩ and Na ϩ channel genes was associated with decreased I to,f , I K,slow , and I Na currents. Inversely, other K ϩ channel ␣-and ␤-subunits, such as KCNA4, KCNK1, KCNAB1, and KCNE3, were upregulated. Conclusions-Long-term amiodarone treatment induces a dose-dependent remodeling of ion-channel expression that is correlated with the cardiac electrophysiologic effects of the drug. This profile cannot be attributed solely to the amiodarone-induced cardiac hypothyroidism syndrome. Thus, in addition to the direct effect of the drug on membrane proteins, part of the therapeutic action of long-term amiodarone treatment is likely related to its effect on ion-channel transcripts. Key Words: antiarrhythmic agents Ⅲ ion channels Ⅲ molecular biology Ⅲ electrophysiology A miodarone, a widely used antiarrhythmic drug, has remarkable efficacy for the treatment of ventricular tachyarrhythmias and atrial fibrillation. However, the basis for its effectiveness is still poorly understood. The pharmacologic profile of this drug is complex, and much remains to be clarified about both short-and long-term actions. Amiodarone has been referred to as a class III antiarrhythmic agent, 1 but it also possesses electrophysiologic characteristics of class I and IV agents and minor class II effects. 2 The drug is also known to modify thyroid function extensively because of its iodinated nature. 3 The question arose as to whether the long-term effects of amiodarone might stem from its molecular interaction with thyroid hormone receptors or other mechanisms. In particular, it has been hypothesized that the effects of amiodarone could depend on modulation of transcript expression in addition to its direct effect on cell membrane channels. 4 Genomic techniques now bring gene expression studies to a genome scale, allowing investigators to examine simultaneous changes in th...

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The 2 -receptor agonist, ifenprodil, was suggested as an inhibitor of G protein-coupled inwardly rectifying potassium channels. Nevertheless, an analysis of the role of 2 receptors in cardiac electrophysiology has never been done. This work aims i) to identify the roles of cardiac 2 receptors in the regulation of cardiac K ϩ channel conductances and ii) to check whether 2 -receptor agonists exhibit class III antiarrhythmic properties. , and 1,3-di(2-tolyl)guanidine were used to discriminate the effects linked to 2 receptors from those of the 1 subtype, induced by (Ϯ)-N-allylnormetazocine (SKF-10,047). The 2 -receptor antagonist 3-␣-tropanyl-2(pCl-phenoxy)butyrate (SM-21) was employed to characterize 2 -mediated effects in patchclamp experiments. In rabbits, all 2 -receptor agonists reduced phenylephrine-induced cardiac arrhythmias. They prolonged action potential duration in rabbit Purkinje fibers and reduced human ether-a-go-go-related gene (HERG) K ϩ currents. (ϩ)-SKF-10,047 was completely inactive in the last two tests. The effects of threoifenprodil were not antagonized by SM-21. In HERG-transfected COS-7 cells, SM-21 potentiated the ifenprodil-induced blockade of the HERG current. These data suggest that 2 -receptor ligands block I Kr and that this effect could explain part of the antiarrhythmic properties of this ligands family. Nevertheless, an interaction with HERG channels not involving 2 receptors seems to share this pharmacological property. This work shows for the first time that particular caution has to be taken toward ligands with affinity for 2 receptors. The repolarization prolongation and the earlyafterdepolarization can be responsible for "torsades de pointe" and sudden cardiac death.