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 ...

Cultures of neonatal rat heart cells contain predominantly myocytes and fibroblastic cells. Most abundant are groups of synchronously contracting myocytes, which are electrically well coupled through large gap junctions. Cardiac fibroblasts may be electrically coupled to each other and to adjacent myocytes, be it with low intercellular conductances. Nevertheless, synchronously beating myocytes interconnected via a fibroblast were present, demonstrating that nonexcitable cardiac cells are capable of passive impulse conduction. In fibroblast pairs as well as in myocyte-fibroblast cell pairs, no sensitivity to junctional voltage could be detected when transjunctional conductance was > 1-2 nS. However, in pairs coupled by a conductance of < 1 nS, complex voltage-dependent gating was evident; gap junction channel open probability decreased with increasing junctional voltage but a nongated residual conductance remained at all voltages tested. Single gap junction channel conductance between fibroblasts was approximately 21 pS, very similar to an approximately 18-pS channel conductance that was found between myocytes next to the major conductance of 43 pS. Single-channel conductance in heterologous myocyte-fibroblast gap junctions was approximately 32 pS, which matches the theoretical value of 29 pS for gap junction channels composed of a fibroblast connexon and the major myocyte connexon. A site-directed antibody against rat heart gap junction protein connexin43 recognized gap junctions between neonatal cardiomyocytes, as demonstrated by immunocytochemical labeling. In contrast, junctions between fibroblasts showed no labeling, while in myocyte-fibroblast junctions labeling occasionally was present. Our results suggest the existence of two gap junction proteins between neonatal rat cardiocytes, connexin43 and another yet unidentified connexin. An alternative explanation (cell-specific regulation of the conductance of connexin43 channels) is discussed.

Background-Patients carrying the cardiac sodium channel (SCN5A) mutation 1795insD show sudden nocturnal death and signs of multiple arrhythmia syndromes including bradycardia, conduction delay, QT prolongation, and right precordial ST-elevation. We investigated the electrophysiological characteristics of a transgenic model of the murine equivalent mutation 1798insD. Methods and Results-On 24-hour continuous telemetry and surface ECG recordings, Scn5a 1798insD/ϩ heterozygous mice showed significantly lower heart rates, more bradycardic episodes (pauses Ն500 ms), and increased PQ interval, QRS duration, and QTc interval compared with wild-type mice. The sodium channel blocker flecainide induced marked sinus bradycardia and/or sinus arrest in the majority of Scn5a 1798insD/ϩ mice, but not in wild-type mice. Epicardial mapping using a multielectrode grid on excised, Langendorff-perfused hearts showed preferential conduction slowing in the right ventricle of Scn5a 1798insD/ϩ hearts. On whole-cell patch-clamp analysis, ventricular myocytes isolated from Scn5a

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) are widely used in studying basic mechanisms of cardiac arrhythmias that are caused by ion channelopathies. Unfortunately, the action potential profile of hiPSC-CMs—and consequently the profile of individual membrane currents active during that action potential—differs substantially from that of native human cardiomyocytes, largely due to almost negligible expression of the inward rectifier potassium current (IK1). In the present study, we attempted to “normalize” the action potential profile of our hiPSC-CMs by inserting a voltage dependent in silico IK1 into our hiPSC-CMs, using the dynamic clamp configuration of the patch clamp technique. Recordings were made from single hiPSC-CMs, using the perforated patch clamp technique at physiological temperature. We assessed three different models of IK1, with different degrees of inward rectification, and systematically varied the magnitude of the inserted IK1. Also, we modified the inserted IK1 in order to assess the effects of loss- and gain-of-function mutations in the KCNJ2 gene, which encodes the Kir2.1 protein that is primarily responsible for the IK1 channel in human ventricle. For our experiments, we selected spontaneously beating hiPSC-CMs, with negligible IK1 as demonstrated in separate voltage clamp experiments, which were paced at 1 Hz. Upon addition of in silico IK1 with a peak outward density of 4–6 pA/pF, these hiPSC-CMs showed a ventricular-like action potential morphology with a stable resting membrane potential near −80 mV and a maximum upstroke velocity >150 V/s (n = 9). Proarrhythmic action potential changes were observed upon injection of both loss-of-function and gain-of-function IK1, as associated with Andersen–Tawil syndrome type 1 and short QT syndrome type 3, respectively (n = 6). We conclude that injection of in silico IK1 makes the hiPSC-CM a more reliable model for investigating mechanisms underlying cardiac arrhythmias.