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Keywords: CALM-2; calmodulin; IKS potassium channel; long QT syndrome EN CALM-2 calmodulin IKS potassium channel long QT syndrome 3695 3696 2 09/05/23 20230901 NES 230901 A genetic disorder disrupting ventricular myocardial repolarization in the heart, congenital long QT syndrome (LQTS) can lead to life-threatening arrhythmias and sudden cardiac death. Since the discovery of the three canonical LQTS-causative genes in the 1980s and 1990s, up to 14 additional, albeit minor, LQTS-susceptibility genes have been discovered by either hypothesis-driven candidate gene research or genomic triangulation strategies using whole exome sequencing (Giudicessi et al., [4]). [Extracted from the article]

Congenital long QT syndrome is a heritable family of arrhythmias caused by mutations in 13 genes encoding ion channel complex proteins. Mounting evidence has implicated the Purkinje fiber network in the genesis of ventricular arrhythmias. In this study, we explore the hypothesis that long QT mutations can demonstrate different phenotypes depending on the tissue type of expression. Using computational models of the human ventricular myocyte and the Purkinje fiber cell, the biophysical alteration in channel function in LQT1, LQT2, LQT3, and LQT7 are modeled. We identified that the plateau potential was important in LQT1 and LQT2, in which mutation led to minimal action potential prolongation in Purkinje fiber cells. The phenotype of LQT3 mutation was dependent on the biophysical alteration induced as well as tissue type. The canonical ΔKPQ mutation causes severe action potential prolongation in both tissue types. For LQT3 mutation F1473C, characterized by shifted channel availability, a more severe phenotype was seen in Purkinje fiber cells with action potential prolongation and early afterdepolarizations. The LQT3 mutation S1904L demonstrated striking effects on action potential duration restitution and more severe action potential prolongation in Purkinje fiber cells at higher heart rates. Voltage clamp simulations highlight the mechanism of effect of these mutations in different tissue types, and impact of drug therapy is explored. We conclude that arrhythmia formation in long QT syndrome may depend not only on the basis of mutation and biophysical alteration, but also upon tissue of expression. The Purkinje fiber network may represent an important therapeutic target in the management of patients with heritable channelopathies. [ABSTRACT FROM AUTHOR]

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Copyright of Journal of Physiology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: The congenital long QT syndrome (LQTS) is a heritable arrhythmia in which mutations in genes coding for ion channels or ion channel associated proteins delay ventricular repolarization and place mutation carriers at risk for serious or fatal arrhythmias. Triggers and therapeutic management of LQTS arrhythmias have been shown to differ in a manner that depends strikingly on the gene that is mutated. Additionally, beta-blockers, effective in the management of LQT-1, have been thought to be potentially proarrhythmic in the treatment of LQT-3 because of concomitant slowing of heart rate that accompanies decreased adrenergic activity. Here we report that the beta-blocker propranolol interacts with wild type (WT) and LQT-3 mutant Na+ channels in a manner that resembles the actions of local anesthetic drugs. We demonstrate that propranolol blocks Na+ channels in a use-dependent manner; that propranolol efficacy is dependent on the inactivated state of the channel; that propranolol blocks late non-inactivating current more effectively than peak sodium current; and that mutation of the local anesthetic binding site greatly reduces the efficacy of propranolol block of peak and late Na+ channel current. Furthermore our results indicate that this activity, like that of local anesthetic drugs, differs both with drug structure and the biophysical changes in Na+ channel function caused by specific LQT-3 mutations. [Copyright &y& Elsevier]

A-kinase anchoring proteins (AKAPs) recruit signaling molecules and present them to downstream targets to achieve efficient spatial and temporal control of their phosphorylation state. In the heart, sympathetic nervous system (SNS) regulation of cardiac action potential duration (APD), mediated by β-adrenergic receptor (βAR) activation, requires assembly of AKAP9 (Yotiao) with the lKs potassium channel α subunit (KCNQ1). KCNQ1 mutations that disrupt this complex cause type 1 long-QT syndrome (LQT1), one of the potentially lethal heritable arrhythmia syndromes. Here, we report identification of (i) regions on Yotiao critical to its binding to KCNQ1 and (ii) a single putative LQTS-causing mutation (S1570L) in AKAP9 (Yotiao) localized to the KCNQ1 binding domain in 1/50 (2%) subjects with a clinically robust phenotype for LQTS but absent in 1,320 reference alleles. The inherited S1570L mutation reduces the interaction between KCNQ1 and Yotiao, reduces the cAMP-induced phosphorylation of the channel, eliminates the functional response of the lKs channel to cAMP, and prolongs the action potential in a computational model of the ventricular cardiocyte. These reconstituted cellular consequences of the inherited S1570L-Yotiao mutation are consistent with delayed repolarization of the ventricular action potential observed in the affected siblings. Thus, we have demonstrated a link between genetic perturbations in AKAP and human disease in general and AKAP9 and LQTS in particular. [ABSTRACT FROM AUTHOR]

1 We studied the effects of ranolazine, an antianginal agent with promise as an antiarrhythmic drug, on wild-type (WT) and long QT syndrome variant 3 (LQT-3) mutant Na(+) channels expressed in human embryonic kidney (HEK) 293 cells and knock-in mouse cardiomyocytes and used site-directed mutagenesis to probe the site of action of the drug. 2 We find preferential ranolazine block of sustained vs peak Na(+) channel current for LQT-3 mutant (DeltaKPQ and Y1795C) channels (IC(50)=15 vs 135 microM) with similar results obtained in HEK 293 cells and knock-in myocytes. 3 Ranolazine block of both peak and sustained Na(+) channel current is significantly reduced by mutation (F1760A) of a single residue previously shown to contribute critically to the binding site for local anesthetic (LA) molecules in the Na(+) channel. 4 Ranolazine significantly decreases action potential duration (APD) at 50 and 90% repolarization by 23+/-5 and 27+/-3%, respectively, in DeltaKPQ mouse ventricular myocytes but has little effect on APD of WT myocytes. 5 Computational modeling of human cardiac myocyte electrical activity that incorporates our voltage-clamp data predicts marked ranolazine-induced APD shortening in cells expressing LQT-3 mutant channels. 6 Our results demonstrate for the first time the utility of ranolazine as a blocker of sustained Na(+) channel activity induced by inherited mutations that cause human disease and further, that these effects are very likely due to interactions of ranolazine with the receptor site for LA molecules in the sodium channel. [ABSTRACT FROM AUTHOR]

Examines how mutations in cardiac sodium and potassium channel may disrupt the precise balance of ionic currents that underlies normal cardiac excitation and relaxation. Emergence of arrhythmogenic phenotypes leading to syncope, seizures and sudden cardiac death; Congenital defects; Mutation-induced arrhythmia; Development of therapeutic approaches.

Objectives: The purpose of this study was to determine the clinical and biophysical characteristics of a novel SCN5A mutation.Background: Brugada syndrome and isolated cardiac conduction defect have been linked to SCN5A mutations.Methods: Eleven members of a western European family underwent electrophysiologic investigations and mutation analysis of the SCN5A gene. Wild-type and mutant SCN5A channels were expressed in HEK293 cells, and whole cell currents were studied using patch clamp procedures.Results: A novel mutation, R376H, in the first pore segment of SCN5A variably causes Brugada syndrome and/or conduction disease in a single family. Biophysical analysis demonstrated a significant current reduction for the mutant, a pathophysiologic profile consistent with Brugada syndrome and isolated cardiac conduction defect. Among 11 family members, 9 were carriers of the mutation. The proband's initial presentation was a saddleback Brugada ECG, atrial flutter, and diffuse conduction disturbances. He had no inducible ventricular arrhythmias but experienced sudden cardiac death. His brother was affected by atrial flutter and had a clear conduction disorder, but he did not display baseline or evocable ECG signs of Brugada syndrome. He received an implantable cardioverter-defibrillator that delivered one appropriate shock after 1 year of follow-up. The phenotype in the family members was highly variable and ranged from noninducible and inducible asymptomatic carriers of the mutations to isolated conduction disease and to symptomatic Brugada syndrome.Conclusions: We describe the functional characterization of a novel SCN5A pore mutation, R376H, with variable clinical expression in the same family. Differentiating between electrophysiologic entities (Brugada syndrome-isolated cardiac conduction defect) is more challenging. Recognition of factors modifying the clinical presentation may be important for clinical decision making. [ABSTRACT FROM AUTHOR]

The congenital long QT syndrome is a rare disorder in which mutation carriers are at risk for polymorphic ventricular tachycardia and/or sudden cardiac death. Discovery and analysis of gene mutations associated with variants of this disorder have provided novel insight into mechanisms of cardiac arrhythmia and have raised the possibility of mutation-specific therapeutic intervention. [ABSTRACT FROM AUTHOR]

Voltage-gated sodium channels (VGSCs) are critical transmembrane proteins responsible for the rapid action potential upstroke in most excitable cells. Recently discovered mutations in VGSCs, which underlie idiopathic clinical disease, have emphasized the importance of these channels in tissues such as skeletal muscle, nervous system, and myocardium. Mutations in the gene encoding the cardiac sodium channel isoform (SCN5A) have been linked to at least three abnormal phenotypes: variant 3 of the Long QT syndrome (LQT-3); Brugada's syndrome (BrS); and isolated cardiac conduction disease (ICCD). Mutations in SCN5A manifest as one or more of these clinical phenotypes - the precise distinction between these diseases is increasingly subtle. Clinical management of LQT-3 and diagnosis of BrS with the local anesthetic flecainide has proven promising. Channels associated with LQT-3 (D1790G) and BrS (Y1795H) both show more sensitivity to flecainide than wild-type (WT) channels, while lidocaine sensitivity is unchanged. One plausible explanation for differential drug sensitivity is that mutant channels may allow more access to a receptor site compared with WT through altered protein allosteric changes during an action potential. The high affinity binding site for local anesthetic block has been identified in the pore region of the channel. This region is not water accessible during the closed state, thus requiring channel opening for charged drug (flecainide and mexiletine) access and block. Channel mutations which disrupt inactivation biophysics lead to increased drug binding by altering the time the binding site is accessible during an action potential. Neutral drugs (lidocaine) which are not dependent on channel opening for binding site access will not be sensitive to mutations that alter channel inactivation properties. Interestingly another LQT-3 mutant (Y1795C) shows no change in flecainide sensitivity, suggesting that although drug effects of SCN5A mutations cross... [ABSTRACT FROM AUTHOR]

Congenital long QT syndrome is a rare disease in which the electrocardiogram QT interval is prolonged due to dysfunctional ventricular repolarization. Variant 3 (LQT-3) is associated with mutations in SCN5A, the gene coding for the heart Na(+) channel alpha subunit. Arrhythmias in LQT-3 mutation carriers are more likely to occur at rest, when heart rate is slow. Several LQT-3 Na(+) channel mutations exert their deleterious effects by promoting a mode of Na(+) channel gating wherein a fraction of channels fails to inactivate. This gating mode, termed "bursting, " results in sustained macroscopic inward Na(+) channel current (I(sus)), which can delay repolarization and prolong the QT interval. However, the mechanism of heart-rate dependence of I(sus) has been unresolved at the single-channel level. We investigate an LQT-3 mutant (Y1795C) using experimental and theoretical frameworks to elucidate the molecular mechanism of I(sus) rate dependence. Our results indicate that mutation-induced changes in the length of time mutant channels spend bursting, rather than how readily they burst, determines I(sus) inverse heart-rate dependence. Our results indicate that mutation-induced changes in the length of time mutant channels spend bursting, rather than how readily they burst, determines I(sus) inverse heart-rate dependence. These results link mutation-induced changes in Na+ channel gating mode transitions to heart rate-dependent changes in cellular electrical activity underlying a key LQT-3 clinical phenotype. [ABSTRACT FROM AUTHOR]

Understanding the basis for differential responses to drug therapies remains a challenge despite advances in genetics and genomics. Induced pluripotent stem cells (iPSCs) offer an unprecedented opportunity to investigate the pharmacology of disease processes in therapeutically and genetically relevant primary cell types in vitro and to interweave clinical and basic molecular data. We report here the derivation of iPSCs from a long QT syndrome patient with complex genetics. The proband was found to have a de novo SCN5A LQT-3 mutation (F1473C) and a polymorphism (K897T) in KCNH2, the gene for LQT-2. Analysis of the biophysics and molecular pharmacology of ion channels expressed in cardiomyocytes (CMs) differentiated from these iPSCs (iPSC-CMs) demonstrates a primary LQT-3 (Na+channel) defect responsible for the arrhythmias not influenced by the KCNH2 polymorphism. The F1473C mutation occurs in the channel inactivation gate and enhances late Na+channel current (INAL) that is carried by channels that fail to inactivate completely and conduct increased inward current during prolonged depolarization, resulting in delayed repolarization, a prolonged QT interval, and increased risk of fatal arrhythmia. We find a very pronounced rate dependence of (INAL)such that increasing the pacing rate markedly reduces (INAL) and, in addition, increases its inhibition by the Na+channel blocker mexiletine. These rate-dependent properties and drug interactions, unique to the proband's iPSC-CMs, correlate with improved management of arrhythmias in the patient and provide support for this approach in developing patient-specific clinical regimens. [ABSTRACT FROM AUTHOR]

The Ca2+ release channel ryanodine receptor 2 (RyR2) is required for excitation-contraction coupling in the heart and is also present in the brain. Mutations in RyR2 have been linked to exercise-induced sudden cardiac death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). CPVT-associated RyR2 mutations result in "leaky" RyR2 channels due to the decreased binding of the calstabin2 (FKBP12.6) subunit, which stabilizes the closed state of the channel. We found that mice heterozygous for the R2474S mutation in Ryr2 (Ryr2-R2474S mice) exhibited spontaneous generalized tonic-clonic seizures (which occurred in the absence of cardiac arrhythmias), exercise-induced ventricular arrhythmias, and sudden cardiac death. Treatment with a novel RyR2-specific compound (S107) that enhances the binding of calstabin2 to the mutant Ryr2-R2474S channel inhibited the channel leak and prevented cardiac arrhythmias and raised the seizure threshold. Thus, CPVT-associated mutant leaky Ryr2-R2474S channels in the brain can cause seizures in mice, independent of cardiac arrhythmias. Based on these data, we propose that CPVT is a combined neurocardiac disorder in which leaky RyR2 channels in the brain cause epilepsy, and the same leaky channels in the heart cause exercise-induced sudden cardiac death. [ABSTRACT FROM AUTHOR]

Electrical activity in nerve, skeletal muscle, and heart requires finely tuned activity of voltage-gated Na+ channels that open and then enter a nonconducting inactivated state upon depolarization. Inactivation occurs when the gate, the cytoplasmic loop linking domains III and IV of the α subunit, occludes the open pore. Subtle destabilization of inactivation by mutation is causally associated with diverse human disease. Here we show for the first time that the inactivation gate is a molecular complex consisting of the III-IV loop and the COOH terminus (C-T), which is necessary to stabilize the closed gate and minimize channel reopening. When this interaction is disrupted by mutation, inactivation is destabilized allowing a small, but important, fraction of channels to reopen, conduct inward current, and delay cellular repolarization. Thus, our results demonstrate for the first time that physiologically crucial stabilization of inactivation of the Na+ channel requires complex interactions of intracellular structures and indicate a novel structural role of the C-T domain in this process. [ABSTRACT FROM AUTHOR]

Mutations associated with sodium channel-linked inherited Long-QT syndrome often result in a gain of channel function by disrupting channel inactivation. A small fraction of channels fail to inactivate (burst) at depolarized potentials where normal (wild type) channels fully inactivate. These non-inactivating channels give rise to a sustained macroscopic current. We studied the effects of protein kinase A stimulation on sustained current in wild type and three disease-linked C-terminal mutant channels (D1790G, Y1795C, and Y1795H). We show that protein kinase A stimulation differentially affects gating in the mutant channels. Wild type, Y1795C, and Y1795H channels are insensitive to protein kinase A stimulation, whereas "bursting" in the D1790G mutant is markedly enhanced by protein kinase A-dependent phosphorylation. Our results suggest that the charge at position 1790 of the C terminus of the channel modulates the response of the cardiac sodium channel to protein kinase A stimulation and that phosphorylation of residue 36 in the N terminus and residue 525 in the cytoplasmic linker joining domains I and II of the channel α subunit facilitate destabilization of inactivation and thereby increase sustained current. [ABSTRACT FROM AUTHOR]

Presents a study that determined what disease-modified channel properties underlie distinct responses to the sodium ion (Na[sup+]) channel blocker flecainide in Brugada syndrome (BrS) and LQT-3, a variant of the Long QT syndrome mutations. Analysis of the molecular determinants of flecainide block; Influence of inherited mutations on the voltage dependence of flecainide use-dependent block; Impact of flecainide on Na[sup+] channel availability.