Timothy syndrome (TS) is a multisystem disorder that causes syncope and sudden death from cardiac arrhythmias. Prominent features include congenital heart disease, immune deficiency, intermittent hypoglycemia, cognitive abnormalities, and autism. All TS individuals have syndactyly (webbing of fingers and toes). We discovered that TS resulted from a recurrent, de novo cardiac L-type calcium channel (CaV1.2) mutation, G406R. G406 is located in alternatively spliced exon 8A, encoding transmembrane segment S6 of domain I. Here, we describe two individuals with a severe variant of TS (TS2). Neither child had syndactyly. Both individuals had extreme prolongation of the QT interval on electrocardiogram, with a QT interval corrected for heart rate ranging from 620 to 730 ms, causing multiple arrhythmias and sudden death. One individual had severe mental retardation and nemaline rod skeletal myopathy. We identified de novo missense mutations in exon 8 of CaV1.2 in both individuals. One was an analogous mutation to that found in exon 8A in classic TS, G406R. The other mutation was G402S. Exon 8 encodes the same region as exon 8A, and the two are mutually exclusive. The spliced form of CaV1.2 containing exon 8 is highly expressed in heart and brain, accounting for Ϸ80% of CaV1.2 mRNAs. G406R and G402S cause reduced channel inactivation, resulting in maintained depolarizing L-type calcium currents. Computer modeling showed prolongation of cardiomyocyte action potentials and delayed afterdepolarizations, factors that increase risk of arrhythmia. These data indicate that gain-of-function mutations of CaV1.2 exons 8 and 8A cause distinct forms of TS.long QT syndrome ͉ Timothy syndrome ͉ CACNA1C T imothy syndrome (TS) is a multisystem disorder characterized by simple syndactyly and life-threatening cardiac arrhythmias. The first cases of TS were described in 1992 and 1995 as sporadic cases of long QT syndrome, congenital heart disease, and syndactyly (1-3). With time and life-extending therapy it became clear that TS manifests major phenotypic abnormalities in multiple organ systems (4). All TS cases had QT interval prolongation on electrocardiogram, syndactyly, and abnormal teeth and were born bald. Most had arrhythmias, including bradycardia, atrio-ventricular block, torsades de pointes ventricular tachycardia (torsades), and ventricular fibrillation. Ten of 17 TS children died with an average age at death of 2.5 years. Additional common features included congenital heart disease, dysmorphic facial features, myopia, immune deficiency, recurrent infections, intermittent hypoglycemia, and hypothermia. Finally, many TS children had developmental delays, including language, motor, and generalized cognitive impairment. Some did not produce speech sounds during infancy. Significant problems in articulation, reception, and expression were identified. Five children were evaluated for autism, and three met the criteria for this disorder. One TS child met criteria for autism spectrum disorders and one had severe delays in language deve...

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One contribution of 11 to a Theme Issue 'Towards the virtual physiological human: mathematical and computational case studies'. Ongoing developments in cardiac modelling have resulted, in particular, in the development of advanced and increasingly complex computational frameworks for simulating cardiac tissue electrophysiology. The goal of these simulations is often to represent the detailed physiology and pathologies of the heart using codes that exploit the computational potential of high-performance computing architectures. These developments have rapidly progressed the simulation capacity of cardiac virtual physiological human style models; however, they have also made it increasingly challenging to verify that a given code provides a faithful representation of the purported governing equations and corresponding solution techniques. This study provides the first cardiac tissue electrophysiology simulation benchmark to allow these codes to be verified. The benchmark was successfully evaluated on 11 simulation platforms to generate a consensus gold-standard converged solution. The benchmark definition in combination with the gold-standard solution can now be used to verify new simulation codes and numerical methods in the future.

Investigating the mechanisms underlying the genesis and conduction of electrical excitation in the atria at physiological and pathological states is of great importance. To provide knowledge concerning the mechanisms of excitation, we constructed a biophysical detailed and anatomically accurate computer model of human atria that incorporates both structural and electrophysiological heterogeneities. The three-dimensional geometry was extracted from the visible female dataset. The sinoatrial node (SAN) and atrium, including crista terminalis (CT), pectinate muscles (PM), appendages (APG) and Bachmann's bundle (BB) were segmented in this work. Fibre orientation in CT, PM and BB was set to local longitudinal direction. Descriptions for all used cell types were based on modifications of the Courtemanche et al. model of a human atrial cell. Maximum conductances of Ito, IKr and ICa,L were modified for PM, CT, APG and atrioventricular ring to reproduce measured action potentials (AP). Pacemaker activity in the human SAN was reproduced by removing IK1, but including If, ICa,T, and gradients of channel conductances as described in previous studies for heterogeneous rabbit SAN. Anisotropic conduction was computed with a monodomain model using the finite element method. The transversal to longitudinal ratio of conductivity for PM, CT and BB was 1:9. Atrial working myocardium (AWM) was set to be isotropic. Simulation of atrial electrophysiology showed initiation of APs in the SAN centre. The excitation spread afterwards to the periphery near to the region of the CT and preferentially towards the atrioventricular region. The excitation extends over the right atrium along PM. Both CT and PM activated the right AWM. Earliest activation of the left atrium was through BB and excitation spread over to the APG. The conduction velocities were 0.6ms-1 for AWM, 1.2ms-1 for CT, 1.6ms-1 for PM and 1.1ms-1 for BB at a rate of 63bpm. The simulations revealed that bundles form dominant pathways for atrial conduction. The preferential conduction towards CT and along PM is comparable with clinical mapping. Repolarization is more homogeneous than excitation due to the heterogeneous distribution of electrophysiological properties and hence the action potential duration.

The description of a novel, de novo gain of function mutation in KCNQ1, responsible for atrial fibrillation and short QT syndrome in utero indicates that some of these cases may have a genetic basis and confirms a previous hypothesis that gain of function mutations in KCNQ1 channels can shorten the duration of ventricular and atrial action potentials.

Our results suggest that in rat cardiomyocytes I(TASK) makes a substantial contribution to the outward current flowing in the plateau range of potentials and that this current component can be inhibited via alpha1A-adrenergic receptors.

Although chloroquine remains an important therapeutic agent for treatment of malaria in many parts of the world, its safety margin is very narrow. Chloroquine inhibits the cardiac inward rectifier K ؉ current IK1 and can induce lethal ventricular arrhythmias. In this study, we characterized the biophysical and molecular basis of chloroquine block of Kir2.1 channels that underlie cardiac I K1. The voltage-and K ؉ -dependence of chloroquine block implied that the binding site was located within the ion-conduction pathway. Site-directed mutagenesis revealed the location of the chloroquine-binding site within the cytoplasmic pore domain rather than within the transmembrane pore. Molecular modeling suggested that chloroquine blocks Kir2.1 channels by plugging the cytoplasmic conduction pathway, stabilized by negatively charged and aromatic amino acids within a central pocket. Unlike most ionchannel blockers, chloroquine does not bind within the transmembrane pore and thus can reach its binding site, even while polyamines remain deeper within the channel vestibule. These findings explain how a relatively low-affinity blocker like chloroquine can effectively block I K1 even in the presence of high-affinity endogenous blockers. Moreover, our findings provide the structural framework for the design of safer, alternative compounds that are devoid of Kir2.1-blocking properties.IK1 ͉ ion channel ͉ KCNJ2 ͉ malaria ͉ polyamines

Activation of human ether-a-go-go-related gene 1 (hERG1) K ؉ channels mediates cardiac action potential repolarization. Drugs that activate hERG1 channels represent a mechanism-based approach for the treatment of long QT syndrome, a disorder of cardiac repolarization associated with ventricular arrhythmia and sudden death. Here, we characterize the mechanisms of action and the molecular determinants for binding of RPR260243 [(3R,4R)-4-[3-(6-methoxy-quinolin-4-yl)-3-oxo-propyl]-1-[3-(2,3,5-trifluorophenyl)-prop-2-ynyl]-piperidine-3-carboxylic acid] (RPR), a recently discovered hERG1 channel activator. Channels were heterologously expressed in Xenopus laevis oocytes, and currents were measured by using the two-microelectrode voltage-clamp technique. RPR induced a concentration-dependent slowing in the rate of channel deactivation and enhanced current magnitude by shifting the voltage dependence of inactivation to more positive potentials. This mechanism was confirmed by demonstrating that RPR slowed the rate of deactivation, but did not increase current magnitude of inactivation-deficient mutant channels. The effects of RPR on hERG1 kinetics and magnitude could be simulated by reducing three rate constants in a Markov model of channel gating. Point mutations of specific residues located in the S4 -S5 linker or cytoplasmic ends of the S5 and S6 domains greatly attenuated or ablated the effects of 3 M RPR on deactivation (five residues), inactivation (one residue), or both gating mechanisms (four residues). These findings define a putative binding site for RPR and confirm the importance of an interaction between the S4 -S5 linker and the S6 domain in electromechanical coupling of voltage-gated K ؉ channels.voltage clamp ͉ Xenopus ͉ long QT syndrome H uman ether-a-go-go-related gene 1 (hERG1) ␣-subunits coassemble to form channels that conduct I Kr (1-3), the rapid delayed rectifier K ϩ current that contributes to normal repolarization of cardiac action potentials (4). Loss-of-function mutations in hERG1 cause inherited long QT syndrome (LQTS), a disorder characterized by delayed repolarization of ventricular action potentials and prolonged QT interval of the body surface electrocardiogram (5). The acquired form of LQTS is more common and is most often caused by unintended block of hERG1 channels by a plethora of common medications (6). Inherited and acquired LQTS are associated with an increased risk of torsades de pointes, an arrhythmia that can degenerate into ventricular fibrillation and cause sudden death (7).Current treatments for inherited LQTS include the administration of ␤-adrenergic receptor blockers, left cardiac sympathetic denervation, or implantation of cardiac defibrillators for the most severe cases (8). However, pharmacologic treatment is not always effective (9) and surgery or devices are expensive and require invasive procedures. Acute episodes of drug-induced LQTS are treated with magnesium sulfate administration and discontinued use of the suspect medication. Activation of hERG1 could provide an...