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.

Background-The SCN5A gene encoding the human cardiac sodium channel ␣ subunit plays a key role in cardiac electrophysiology. Mutations in SCN5A lead to a large spectrum of phenotypes, including long-QT syndrome, Brugada syndrome, and isolated progressive cardiac conduction defect (Lenègre disease). Methods and Results-In the present study, we report the identification of a novel single SCN5A missense mutation causing either Brugada syndrome or an isolated cardiac conduction defect in the same family. A G-to-T mutation at position 4372 was identified by direct sequencing and was predicted to change a glycine for an arginine (G1406R) between the DIII-S5 and DIII-S6 domain of the sodium channel protein. Among 45 family members, 13 were carrying the G1406R SCN5A mutation. Four individuals from 2 family collateral branches showed typical Brugada phenotypes, including ST-segment elevation in the right precordial leads and right bundle branch block. One symptomatic patient with the Brugada phenotype required implantation of a cardioverter-defibrillator. Seven individuals from 3 other family collateral branches had isolated cardiac conduction defects but no Brugada phenotype. Three flecainide test were negative. One patient with an isolated cardiac conduction defect had an episode of syncope and required pacemaker implantation. An expression study of the G1406R-mutated SCN5A showed no detectable Na ϩ current but normal protein trafficking. Conclusions-We conclude that the same mutation in the SCN5A gene can lead either to Brugada syndrome or to an isolated cardiac conduction defect. Our findings suggest that modifier gene(s) may influence the phenotypic consequences of a SCN5A mutation. Key Words: fibrillation Ⅲ heart block Ⅲ bundle-branch block Ⅲ genetics Ⅲ arrhythmia T he SCN5A gene encoding a voltage-gated Na ϩ channel is predominantly expressed in the heart, where it plays a key role in the generation and propagation of the cardiac impulse. Autosomal-dominant mutations in the SCN5A gene are responsible for distinct rhythm and conduction disorders, including the long-QT syndrome (LQT3), 1 Brugada syndrome, 2 and isolated cardiac conduction defect (ICCD; Lenègre disease). 3,4 Distinct ECG phenotypes and risks characterize these syndromes. The LQT3 phenotype is characterized by a prolonged QT interval, potentially leading to torsade de pointes arrhythmias. At the cellular level, the pathophysiology sequence for LQT3 includes slowed inactivation of the Na ϩ current, resulting in a sustained inward current (gain of function) during the plateau of the cardiac action potential. 5 The Brugada phenotype is characterized by ST-segment elevation in the right precordial leads, often accompanied (albeit not always) by right bundle branch block but a normal QT duration. 6 As originally described, Brugada syndrome is associated with a high mortality resulting from nocturnal ventricular fibrillation. 7 The pathophysiology sequence of the Brugada syndrome remains incompletely understood, although the syndrome possibly results from a...

A new SCN5A-related cardiac syndrome, MEPPC, was identified. The SCN5A mutation leads to a gain of function of the sodium channel responsible for hyperexcitability of the fascicular-Purkinje system. The MEPPC syndrome is responsive to hydroquinidine.