Potassium currents in the heart: functional roles in repolarization, arrhythmia and therapeutics

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Mechanistic Basis for Atrial and Ventricular Arrhythmias Caused by KCNQ1 Mutations

Cardiac arrhythmias are caused by a disruption of the normal initiation or propagation of electrical impulses in the heart. Hundreds of mutations in genes encoding ion channels or ion channel regulatory proteins are linked to congenital arrhythmia syndromes that increase the risk for sudden cardiac death. This dissertation focuses on how mutations in a gene (KCNQ1) that encodes a voltage-gated K+ ion channel (Kv7.1) can disrupt proper channel function and lead to abnormal repolarization of atrial and ventricular cardiomyocytes. In the heart, Kv7.1 coassembles with a regulatory protein to conduct the slowly activating delayed rectifier K+ current (IKs). Loss-of-function KCNQ1 mutations are linked to type 1 long QT syndrome (LQT1), and typically decrease IKs, which can lead to ventricular action potential (AP) prolongation. In patients, LQT1 is often characterized by an abnormally long corrected QT (QTc) interval on an electrocardiogram (ECG), and increases the risk for polymorphic ventricular tachycardias. KCNQ1 mutations are also linked to atrial fibrillation (AF), but cause a gain-of-function phenotype that increases IKs. Surprisingly, patients diagnosed with both LQT1 and AF are increasingly identified as genotype positive for a KCNQ1 mutation. The first aim of this dissertation was to determine a unique functional phenotype of KCNQ1 mutations linked to both arrhythmia syndromes by functional analyses via the whole-cell patch clamp technique in HEK293 cells. A proportion of patients with LQT1-linked KCNQ1 mutations do not have abnormal QTc prolongation known as latent LQT1. Interestingly, exercise can reveal abnormal QTc prolongation in these patients. During exercise, beta-adrenergic activation stimulates PKA to phosphorylate Kv7.1, causing an increase in IKs to prevent ventricular AP prolongation. Therefore, the second aim of this dissertation was to determine a molecular mechanism of latent LQT1 through functional analyses in HEK293 cells while incorporating pharmacological and phosphomimetic approaches to study PKA regulation of mutant Kv7.1 channels. The findings in this dissertation provide new insight into how KCNQ1 mutations disrupt the function of Kv7.1 in a basal condition or during beta-adrenergic activation. Also, this dissertation suggests these approaches will improve patient management by identifying mutation specific risk factors for patients with KCNQ1 mutations

Effects of Taurine-Magnesium Coordination Compound on Type 2 Short QT Syndrome: A Simulation Study

Short QT Syndrome (SQTS)is an identified genetic arrhythmogenic disease associated with abnormally abbreviated QT intervals and an increased susceptibility to malignant arrhythmia and sudden cardiac death (SCD). SQT2 variant (linked to slow delayed rectifier, IKs) of SQTS, results from a gain-of-function (V307L) in the KCNQ1 subunit of the IKschannel. Pro-arrhythmogenic effects of SQT2 have been well characterized, but less is known about the pharmacological treatment of SQT2. We find that taurine-magnesium coordination compound (TMCC)exerted anti-arrhythmic effects with low toxicity. Therefore, this study aimed to assess the potential effects of TMCC on SQT2. The channel-blocking effect of TMCC on IKsin healthy and SQT2 cells were incorporated into computer models ofhuman ventricular action potential (AP) and into one dimensional transmural tissue simulations. In the single-cell model, TMCC prolonged cell AP duration at 90% repolarization (APD90). In the one dimensionalintact model, TMCC prolonged the QT interval on the pseudo-ECGs. Thus, the present study provides evidence that TMCC can extend the repolarization period and APD90and QT interval, thereby representing a therapeutic candidate for arrhythmia in SQT2

Cardiac Arrhythmias

The most intimate mechanisms of cardiac arrhythmias are still quite unknown to scientists. Genetic studies on ionic alterations, the electrocardiographic features of cardiac rhythm and an arsenal of diagnostic tests have done more in the last five years than in all the history of cardiology. Similarly, therapy to prevent or cure such diseases is growing rapidly day by day. In this book the reader will be able to see with brighter light some of these intimate mechanisms of production, as well as cutting-edge therapies to date. Genetic studies, electrophysiological and electrocardiographyc features, ion channel alterations, heart diseases still unknown , and even the relationship between the psychic sphere and the heart have been exposed in this book. It deserves to be read

Recent Advances in Short QT Syndrome

Short QT syndrome is a highly malignant inherited cardiac disease characterized by ventricular tachyarrhythmias leading to syncope and sudden cardiac death. It is responsible of lethal episodes in young people, mainly infants. International guidelines establish diagnostic criteria with the presence of a QTc ≤ 340 ms in the electrocardiogram despite clinical diagnostic values remain controversial. In last years, clinical diagnosis, risk stratification as well as preventive therapies have been improved due to identification of pathophysiological mechanisms. The only effective option is implantation of a defibrillator despite Quinidine may be at times an effective option. Currently, a limited number of rare variants have been identified in seven genes, which account for nearly 20–30% of families. However, some of these variants are associated with phenotypes showing a shorter QT interval but no conclusive diagnosis of Short QT syndrome. Therefore, an exhaustive interpretation of each variant and a close genotype-phenotype correlation is necessary before clinical translation. Here, we review the main clinical and genetic hallmarks of this rare entity

Biophysical Characterization of Three SCN5A Mutations Linked to Long QT Syndrome Type 3, Sudden Infant Death Syndrome, and Atrial Fibrillation

Le gène SCN5A encode la sous-unité principale du canal sodique cardiaque (Nav1.5). Ce canal est responsable de l'initiation et de la propagation du potentiel d'action cardiaque. Un dysfonctionnement de ce canal peut causer le syndrome du QT long de type 3 (LQT3) et la fibrillation auriculaire (AF). Les patients atteints du LQT3 sont à risques de développer des arythmies létales, particulièrement des torsades de pointes qui peuvent causer le syndrome de mort subite du nourrisson (SIDS). Objectifs : Le but de cette étude est de caractériser les propriétés biophysiques de trois mutations sur le gène SCN5A : Y1767C, S1333Y et K1493R. Ces trois mutations ont respectivement été retrouvées chez un patient souffrant du LQT3, chez un patient mort du SIDS et la dernière mutation chez un patient souffrant d'AF. Méthodes : Des cellules tsA 201 ont été transfectées avec le gène codant pour le canal sauvage et les gènes codant pour les canaux mutés. Par la suite, leurs caractéristiques biophysiques ont été étudiées par la méthode du patch-clamp en configuration cellule entière. Résultats : La mutation Y1767C est située dans le segment 6 du domaine IV (DIVS6). Cette mutation sur le canal produit un courant persistant et un courant de fenêtre augmenté, ces résultats expliquent les phénotypes cliniques des patients affectés de cette mutation. La ranolazine, un nouveau bloqueur des canaux Na+, peut bloquer efficacement le courant Na+ persistant et réduire le courant de fenêtre. Ces canaux mutés montrent aussi une augmentation de l'inhibition fréquence-dépendante ainsi qu'une réactivation lente. La mutation S1333Y est situé sur la boucle S4 et S5 du domaine III. L'étude fonctionnelle de ce canal montre un gain de fonction : un courant Na+ persistant et une augmentation du courant de fenêtre provoquée par un déplacement de -8 mV de l'activation et de +7mV de l'inactivation. La mutation K1493R est située sur la boucle entre les domaines III-IV. Cette mutation provoque un déplacement vers des potentiels plus dépolarisés de l'inactivation est entraîne une augmentation du courant de fenêtre. Conclusion : Les manifestations cliniques observées chez les patients sont probablement dues aux changements des propriétés biophysiques provoqués par les trois mutations sur Nav1.5 rapportées dans cette étude. Nous concluons donc que (1) Y1767C est une mutation provoquant le LQT3. L'effet observé par la ranolazine sur cette mutation (la ranolazine agit probablement comme un bloqueur des canaux ouverts) nous donne de nouveaux indices pour le traitement des patients porteurs de cette mutation. (2) La mort subite du nourrisson observé est probablement lié à un syndrome LQT3 associé à la mutation S1333Y. (3) La mutation K1493R provoque de la fibrillation auriculaire causée par une hyperexcitabilité des cardiomyocytes

Cardiac electrical defects in progeroid mice and Hutchinson-Gilford progeria syndrome patients with nuclear lamina alterations

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disease caused by defective prelamin A processing, leading to nuclear lamina alterations, severe cardiovascular pathology, and premature death. Prelamin A alterations also occur in physiological aging. It remains unknown how defective prelamin A processing affects the cardiac rhythm. We show age-dependent cardiac repolarization abnormalities in HGPS patients that are also present in the Zmpste24−/− mouse model of HGPS. Challenge of Zmpste24−/− mice with the β-adrenergic agonist isoproterenol did not trigger ventricular arrhythmia but caused bradycardia-related premature ventricular complexes and slow-rate polymorphic ventricular rhythms during recovery. Patch-clamping in Zmpste24−/− cardiomyocytes revealed prolonged calcium-transient duration and reduced sarcoplasmic reticulum calcium loading and release, consistent with the absence of isoproterenol-induced ventricular arrhythmia. Zmpste24−/− progeroid mice also developed severe fibrosis-unrelated bradycardia and PQ interval and QRS complex prolongation. These conduction defects were accompanied by overt mislocalization of the gap junction protein connexin43 (Cx43). Remarkably, Cx43 mislocalization was also evident in autopsied left ventricle tissue from HGPS patients, suggesting intercellular connectivity alterations at late stages of the disease. The similarities between HGPS patients and progeroid mice reported here strongly suggest that defective cardiac repolarization and cardiomyocyte connectivity are important abnormalities in the HGPS pathogenesis that increase the risk of arrhythmia and premature death

Syncytial Model of Human Pluripotent Stem Cell-Derived Cardiomyocytes for Electrophysiology Studies

Human pluripotent stem cells (hPSCs) are a valuable resource for generating human cardiomyocytes and modeling human cardiac physiology in vitro. In this thesis, the electrophysiology of hPSC derived cardiomyocytes was studied in large populations of cells in syncytial models. It was found that heterogeneity in electrophysiology, as represented by action potential variability, is common in small clusters of hPSC-derived cardiomyocytes. A waveform-based automated algorithm was used to identify groups of cardiomyocytes based on similarity of their action potentials. It was found that, unlike in small cell clusters, action potential variability in monolayer culture was relatively low, resembling mainly a single electrophysiological phenotype. The utility of a monolayer hPSC-CM model was explored in two applications: 1) modeling the monogenic disease, type 2 Long QT syndrome (LQT2), using a human induced pluripotent stem cell (hiPSC) line carrying a hERG-A422T mutation, and 2) studying responses to cardioactive drugs using hiPSC-derived cardiomyocytes. The monolayer model with LQT2 hiPSC-derived cardiomyocytes had prolonged action potentials and an increased sensitivity to IKr block compared to that of non-disease cardiomyocytes, consistent with the expected LQT2 phenotype. The prolonged action potentials could be normalized by activation of IKr with ML-T531, a compound that delays the inactivation of the hERG channel. However, ectopic activity, such as early-afterdepolarizations (EADs), were mostly absent in LQT2 monolayers, in contrast to the frequent occurrence reported in smaller cell cultures or single cells. For drug testing, monolayers of hiPSC-derived cardiomyocytes responded to a panel of eight cardioactive drugs in a manner consistent with the mechanism of the drugs: blockers of repolarizing currents prolonged the action potentials, while blockers of depolarizing currents shortened them. At a tissue level, blockers of excitatory INa slowed propagation of action potentials. Computational modeling showed that a drug can alter the repolarization gradient, a proarrhythmia biomarker, in monolayers having a defined electrophysiological gradient

Multiscale Modeling of the Ventricles: From Cellular Electrophysiology to Body Surface Electrocardiograms

This work is focused on different aspects within the loop of multiscale modeling: On the cellular level, effects of adrenergic regulation and the Long-QT syndrome have been investigated. On the organ level, a model for the excitation conduction system was developed and the role of electrophysiological heterogeneities was analyzed. On the torso level a dynamic model of a deforming heart was created and the effects of tissue conductivities on the solution of the forward problem were evaluated