KCNJ2 mutation in short QT syndrome 3 results in atrial fibrillation and ventricular proarrhythmia. Proc Natl Acad Sci U S A. 2013;110:4291-4296. [PMID: 23440193 DOI: 10.1073/pnas.1218154110]

Congenital short QT syndrome: A review focused on electrocardiographic features

Congenital short QT syndrome is a very low prevalence inherited primary arrhythmia syndrome first reported in 2000 by Gussak et al., who described two families with a short QT interval, syncope, and sudden cardiac death. In 2004, Ramon Brugada et al. identified the first genetic type of this entity. To date, a total of nine genotypes have been described. The diagnosis is easy from the electrocardiogram (ECG), not only due to the short QT duration, but also based on other aspects covered in this review. During 24-h Holter monitoring, paroxysmal atrial fibrillation spontaneously converting to sinus rhythm may be found. Even though the T wave may appear symmetric on the ECG, the T loop of the vectorcardiogram confirms that the T wave is constantly asymmetric due to the presence of dashes closer to each other in the efferent branch. In this review, we also describe the minus-plus T wave sign that we have described in a previously published article. In addition to congenital causes, we briefly highlight the existence of numerous acquired causes of short QT interval.

Kir2.1-NaV1.5 channelosome and its role in arrhythmias in inheritable cardiac diseases

Sudden cardiac death in children and young adults is a relatively rare but tragic event whose pathophysiology is unknown at the molecular level. Evidence indicates that the main cardiac sodium channel (NaV1.5) and the strong inward rectifier potassium channel (Kir2.1) physically interact and form macromolecular complexes (channelosomes) with common partners, including adapter, scaffolding, and regulatory proteins that help them traffic together to their eventual membrane microdomains. Most important, dysfunction of either or both ion channels has direct links to hereditary human diseases. For example, certain mutations in the KCNJ2 gene encoding the Kir2.1 protein result in Andersen-Tawil syndrome type 1 and alter both inward rectifier potassium and sodium inward currents. Similarly, trafficking-deficient mutations in the gene encoding the NaV1.5 protein (SCN5A) result in Brugada syndrome and may also disturb both inward rectifier potassium and sodium inward currents. Moreover, gain-of-function mutations in KCNJ2 result in short QT syndrome type 3, which is extremely rare but highly arrhythmogenic, and can modify Kir2.1-NaV1.5 interactions in a mutation-specific way, further highlighting the relevance of channelosomes in ion channel diseases. By expressing mutant proteins that interrupt or modify Kir2.1 or NaV1.5 function in animal models and patient-specific pluripotent stem cell-derived cardiomyocytes, investigators are defining for the first time the mechanistic framework of how mutation-induced dysregulation of the Kir2.1-NaV1.5 channelosome affects cardiac excitability, resulting in arrhythmias and sudden death in different cardiac diseases.

The Kir2.1E299V mutation increases atrial fibrillation vulnerability while protecting the ventricles against arrhythmias in a mouse model of short QT syndrome type 3

AIMS

Short QT syndrome type 3 (SQTS3) is a rare arrhythmogenic disease caused by gain-of-function mutations in KCNJ2, the gene coding the inward rectifier potassium channel Kir2.1. We used a multidisciplinary approach and investigated arrhythmogenic mechanisms in an in-vivo model of de-novo mutation Kir2.1E299V identified in a patient presenting an extremely abbreviated QT interval and paroxysmal atrial fibrillation.

METHODS AND RESULTS

We used intravenous adeno-associated virus-mediated gene transfer to generate mouse models, and confirmed cardiac-specific expression of Kir2.1WT or Kir2.1E299V. On ECG, the Kir2.1E299V mouse recapitulated the QT interval shortening and the atrial-specific arrhythmia of the patient. The PR interval was also significantly shorter in Kir2.1E299V mice. Patch-clamping showed extremely abbreviated action potentials in both atrial and ventricular Kir2.1E299V cardiomyocytes due to a lack of inward-going rectification and increased IK1 at voltages positive to -80 mV. Relative to Kir2.1WT, atrial Kir2.1E299V cardiomyocytes had a significantly reduced slope conductance at voltages negative to -80 mV. After confirming a higher proportion of heterotetrameric Kir2.x channels containing Kir2.2 subunits in the atria, in-silico 3D simulations predicted an atrial-specific impairment of polyamine block and reduced pore diameter in the Kir2.1E299V-Kir2.2WT channel. In ventricular cardiomyocytes, the mutation increased excitability by shifting INa activation and inactivation in the hyperpolarizing direction, which protected the ventricle against arrhythmia. Moreover, Purkinje myocytes from Kir2.1E299V mice manifested substantially higher INa density than Kir2.1WT, explaining the abbreviation in the PR interval.

CONCLUSION

The first in-vivo mouse model of cardiac-specific SQTS3 recapitulates the electrophysiological phenotype of a patient with the Kir2.1E299V mutation. Kir2.1E299V eliminates rectification in both cardiac chambers but protects against ventricular arrhythmias by increasing excitability in both Purkinje-fiber network and ventricles. Consequently, the predominant arrhythmias are supraventricular likely due to the lack of inward rectification and atrial-specific reduced pore diameter of the Kir2.1E299V-Kir2.2WT heterotetramer.

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

Impact of rs1805127 and rs55742440 Variants on Atrial Remodeling in Hypertrophic Cardiomyopathy Patients with Atrial Fibrillation: A Romanian Cohort Study

Atrial fibrillation (AFib) is characterized by a complex genetic component. We aimed to investigate the association between variations in genes related to cardiac ion handling and AFib in a cohort of Romanian patients with hypertrophic cardiomyopathy (HCM). Forty-five unrelated probands with HCM were genotyped by targeted next-generation sequencing (NGS) for 24 genes associated with cardiac ion homeostasis. Subsequently, the study cohort was divided into two groups based on the presence (AFib+) or absence (AFiB-) of AFib detected during ECG monitoring. We identified two polymorphisms (rs1805127 located in KCNE1 and rs55742440 located in SCN1B) linked to AFib susceptibility. In AFib+, rs1805127 was associated with increased indexed left atrial (LA) maximal volume (LAVmax) (58.42 ± 21 mL/m2 vs. 32.54 ± 6.47 mL/m2, p < 0.001) and impaired LA strain reservoir (LASr) (13.3 ± 7.5% vs. 24.4 ± 6.8%, p < 0.05) compared to those without respective variants. The rs55742440 allele was less frequent in patients with AFib+ (12 out of 25, 48%) compared to those without arrhythmia (15 out of 20, 75%, p = 0.05). Also, AFib+ rs55742440 carriers had significantly lower LAVmax compared to those who were genotype negative. Among patients with HCM and AFib+, the rs1805127 variant was accompanied by pronounced LA remodeling, whereas rs55742440's presence was related to a milder LA enlargement.

Verification of the Efficacy of Mexiletine Treatment for the A1656D Mutation on Downgrading Reentrant Tachycardia Using a 3D Cardiac Electrophysiological Model

The SCN5A mutations have been long associated with long QT variant 3 (LQT3). Recent experimental and computation studies have reported that mexiletine effectively treats LQT3 patients associated with the A1656D mutation. However, they have primarily focused on cellular level evaluations and have only looked at the effects of mexiletine on action potential duration (APD) or QT interval reduction. We further investigated mexiletine's effects on cardiac cells through simulations of single-cell (behavior of alternant occurrence) and 3D (with and without mexiletine). We discovered that mexiletine could shorten the cell's APD and change the alternant's occurrence to a shorter basic cycle length (BCL) between 350 and 420 ms. The alternant also appeared at a normal heart rate under the A1656D mutation. Furthermore, the 3D ventricle simulations revealed that mexiletine could reduce the likelihood of a greater spiral wave breakup in the A1656D mutant condition by minimizing the appearance of rotors. In conclusion, we found that mexiletine could provide extra safety features during therapy for LQT3 patients because it can change the alternant occurrence from a normal to a faster heart rate, and it reduces the chance of a spiral wave breakup. Therefore, these findings emphasize the promising efficacy of mexiletine in treating LQT3 patients under the A1656D mutation.

In-silico drug trials for precision medicine in atrial fibrillation: From ionic mechanisms to electrocardiogram-based predictions in structurally-healthy human atria

Atrial fibrillation (AF) inducibility, sustainability and response to pharmacological treatment of individual patients are expected to be determined by their ionic current properties, especially in structurally-healthy atria. Mechanisms underlying AF and optimal cardioversion are however still unclear. In this study, in-silico drug trials were conducted using a population of human structurally-healthy atria models to 1) identify key ionic current properties determining AF inducibility, maintenance and pharmacological cardioversion, and 2) compare the prognostic value for predicting individual AF cardioversion of ionic current properties and electrocardiogram (ECG) metrics. In the population of structurally-healthy atria, 477 AF episodes were induced in ionic current profiles with both steep action potential duration (APD) restitution (eliciting APD alternans), and high excitability (enabling propagation at fast rates that transformed alternans into discordant). High excitability also favored 211 sustained AF episodes, so its decrease, through prolonged refractoriness, explained pharmacological cardioversion. In-silico trials over 200 AF episodes, 100 ionic profiles and 10 antiarrhythmic compounds were consistent with previous clinical trials, and identified optimal treatments for individual electrophysiological properties of the atria. Algorithms trained on 211 simulated AF episodes exhibited >70% accuracy in predictions of cardioversion for individual treatments using either ionic current profiles or ECG metrics. In structurally-healthy atria, AF inducibility and sustainability are enabled by discordant alternans, under high excitability and steep restitution conditions. Successful pharmacological cardioversion is predicted with 70% accuracy from either ionic or ECG properties, and it is optimal for treatments maximizing refractoriness (thus reducing excitability) for the given ionic current profile of the atria.

Inward rectifier potassium (Kir) channels in the retina: living our vision

Channel proteins are vital for conducting ions throughout the body and are especially relevant to retina physiology. Inward rectifier potassium (Kir) channels are a class of K+ channels responsible for maintaining membrane potential and extracellular K+ concentrations. Studies of the KCNJ gene (that encodes Kir protein) expression identified the presence of all of the subclasses (Kir 1-7) of Kir channels in the retina or retinal-pigmented epithelium (RPE). However, functional studies have established the involvement of the Kir4.1 homotetramer and Kir4.1/5.1 heterotetramer in Müller glial cells, Kir2.1 in bipolar cells, and Kir7.1 in the RPE cell physiology. Here, we propose the potential roles of Kir channels in the retina based on the physiological contributions to the brain, pancreatic, and cardiac tissue functions. There are several open questions regarding the expressed KCNJ genes in the retina and RPE. For example, why does not the Kir channel subtype gene expression correspond with protein expression? Catching up with multiomics or functional "omics" approaches might shed light on posttranscriptional changes that might influence Kir subunit mRNA translation within the retina that guides our vision.

Molecular stratification of arrhythmogenic mechanisms in the Andersen Tawil Syndrome

Andersen Tawil Syndrome (ATS) is a rare inheritable disease associated with loss-of-function mutations in KCNJ2, the gene coding the strong inward rectifier potassium channel Kir2.1, which forms an essential membrane protein controlling cardiac excitability. ATS is usually marked by a triad of periodic paralysis, life-threatening cardiac arrhythmias and dysmorphic features, but its expression is variable and not all patients with a phenotype linked to ATS have a known genetic alteration. The mechanisms underlying this arrhythmogenic syndrome are poorly understood. Knowing such mechanisms would be essential to distinguish ATS from other channelopathies with overlapping phenotypes and to develop individualized therapies. For example, the recently suggested role of Kir2.1 as a countercurrent to sarcoplasmic calcium reuptake might explain the arrhythmogenic mechanisms of ATS and its overlap with catecholaminergic polymorphic ventricular tachycardia (CPVT). Here we summarize current knowledge on the mechanisms of arrhythmias leading to sudden cardiac death in ATS. We first provide an overview of the syndrome and its pathophysiology, from the patient´s bedside to the protein, and discuss the role of essential regulators and interactors that could play a role in cases of ATS. The review highlights novel ideas related to some post-translational channel interactions with partner proteins that might help define the molecular bases of the arrhythmia phenotype. We then propose a new all-embracing classification of the currently known ATS loss-of-function mutations according to their position in the Kir2.1 channel structure and their functional implications. We also discuss specific ATS pathogenic variants, their clinical manifestations and treatment stratification. The goal is to provide a deeper mechanistic understanding of the syndrome toward the development of novel targets and personalized treatment strategies.

Arrhythmogenic influence of mutations in a myocyte-based computational model of the pulmonary vein sleeve

In the heart, electrophysiological dysregulation arises from defects at many biological levels (from point mutations in ion channel proteins to gross structural abnormalities). These defects disrupt the normal pattern of electrical activation, producing ectopic activity and reentrant arrhythmia. To interrogate mechanisms that link these primary biological defects to macroscopic electrophysiologic dysregulation most prior computational studies have utilized either (i) detailed models of myocyte ion channel dynamics at limited spatial scales, or (ii) homogenized models of action potential conduction that reproduce arrhythmic activity at tissue and organ levels. Here we apply our recent model (EMI), which integrates electrical activation and propagation across these scales, to study human atrial arrhythmias originating in the pulmonary vein (PV) sleeves. These small structures initiate most supraventricular arrhythmias and include pronounced myocyte-to-myocyte heterogeneities in ion channel expression and intercellular coupling. To test EMI's cell-based architecture in this physiological context we asked whether ion channel mutations known to underlie atrial fibrillation are capable of initiating arrhythmogenic behavior via increased excitability or reentry in a schematic PV sleeve geometry. Our results illustrate that EMI's improved spatial resolution can directly interrogate how electrophysiological changes at the individual myocyte level manifest in tissue and as arrhythmia in the PV sleeve.

Atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia despite substantial efforts to understand the pathophysiology of the condition and develop improved treatments. Identifying the underlying causative mechanisms of AF in individual patients is difficult and the efficacy of current therapies is suboptimal. Consequently, the incidence of AF is steadily rising and there is a pressing need for novel therapies. Research has revealed that defects in specific molecular pathways underlie AF pathogenesis, resulting in electrical conduction disorders that drive AF. The severity of this so-called electropathology correlates with the stage of AF disease progression and determines the response to AF treatment. Therefore, unravelling the molecular mechanisms underlying electropathology is expected to fuel the development of innovative personalized diagnostic tools and mechanism-based therapies. Moreover, the co-creation of AF studies with patients to implement novel diagnostic tools and therapies is a prerequisite for successful personalized AF management. Currently, various treatment modalities targeting AF-related electropathology, including lifestyle changes, pharmaceutical and nutraceutical therapy, substrate-based ablative therapy, and neuromodulation, are available to maintain sinus rhythm and might offer a novel holistic strategy to treat AF.

Quantitative Analysis of the Cytoskeleton's Role in Inward Rectifier K IR 2.1 Forward and Backward Trafficking

Alteration of the inward rectifier current I_K1, carried by K_IR2.1 channels, affects action potential duration, impacts resting membrane stability and associates with cardiac arrhythmias. Congenital and acquired K_IR2.1 malfunction frequently associates with aberrant ion channel trafficking. Cellular processes underlying trafficking are intertwined with cytoskeletal function. The extent to which the cytoskeleton is involved in K_IR2.1 trafficking processes is unknown. We aimed to quantify the dependence of K_IR2.1 trafficking on cytoskeleton function. GFP or photoconvertible Dendra2 tagged K_IR2.1 constructs were transfected in HEK293 or HeLa cells. Photoconversion of the Dendra2 probe at the plasma membrane and subsequent live imaging of trafficking processes was performed by confocal laser-scanning microscopy. Time constant of green fluorescent recovery (τg,s) represented recruitment of new K_IR2.1 at the plasma membrane. Red fluorescent decay (τr,s) represented internalization of photoconverted K_IR2.1. Patch clamp electrophysiology was used to quantify I_KIR2.₁. Biochemical methods were used for cytoskeleton isolation and detection of K_IR2.1-cytoskeleton interactions. Cytochalasin B (20 μM), Nocodazole (30 μM) and Dyngo-4a (10 nM) were used to modify the cytoskeleton. Chloroquine (10 μM, 24 h) was used to impair K_IR2.1 breakdown. Cytochalasin B and Nocodazole, inhibitors of actin and tubulin filament formation respectively, strongly inhibited the recovery of green fluorescence at the plasma membrane suggestive for inhibition of K_IR2.1 forward trafficking [τg,s 13 ± 2 vs. 131 ± 31* and 160 ± 40* min, for control, Cytochalasin B and Nocodazole, respectively (*p < 0.05 vs. control)]. Dyngo-4a, an inhibitor of dynamin motor proteins, strongly slowed the rate of photoconverted channel internalization, whereas Nocodazole and Cytochalasin B had less effect [τr,s 20 ± 2 vs. 87 ± 14*, 60 ± 16 and 64 ± 20 min (*p < 0.05 vs. control)]. Cytochalasin B treatment (20 μM, 24 h) inhibited I_KIR2.₁. Chloroquine treatment (10 μM, 24 h) induced intracellular aggregation of K_IR2.1 channels and enhanced interaction with the actin/intermediate filament system (103 ± 90 fold; p < 0.05 vs. control). Functional actin and tubulin cytoskeleton systems are essential for forward trafficking of K_IR2.1 channels, whereas initial backward trafficking relies on a functional dynamin system. Chronic disturbance of the actin system inhibits K_IR2.1 currents. Internalized K_IR2.1 channels become recruited to the cytoskeleton, presumably in lysosomes.

The genetic basis of sudden death in young people - Cardiac and non-cardiac

Sudden death is one of the major causes of death in young adults. Sudden death could be a result from both genetic and environmental or acquired factors. Understanding the genetic etiology is crucial to prevent preventable sudden death for those who are not aware of their genetic condition. In fact, the spectrum of causes of sudden death is complex and varied. In this study, we reviewed the genes that are associated with multiple causes of sudden death in terms of both sudden cardiac death and sudden noncardiac death. A summary of genetic risk factors of the major causes of genetic relevant sudden death is also provided. We believe this review could benefit the researchers who are interested in sudden death genetic studies or the young people who are concerning about their own risk on sudden death.

Investigation of the Effects of the Short QT Syndrome D172N Kir2.1 Mutation on Ventricular Action Potential Profile Using Dynamic Clamp

The congenital short QT syndrome (SQTS) is a cardiac condition that leads to abbreviated ventricular repolarization and an increased susceptibility to arrhythmia and sudden death. The SQT3 form of the syndrome is due to mutations to the KCNJ2 gene that encodes Kir2.1, a critical component of channels underlying cardiac inwardly rectifying K⁺ current, I_K1. The first reported SQT3 KCNJ2 mutation gives rise to the D172N Kir2.1 mutation, the consequences of which have been studied on recombinant channels in vitro and in ventricular cell and tissue simulations. The aim of this study was to establish the effects of the D172N mutation on ventricular repolarization through real-time replacement of I_K1 using the dynamic clamp technique. Whole-cell patch-clamp recordings were made from adult guinea-pig left ventricular myocytes at physiological temperature. Action potentials (APs) were elicited at 1 Hz. Intrinsic I_K1 was inhibited with a low concentration (50 µM) of Ba²⁺ ions, which led to AP prolongation and triangulation, accompanied by a ∼6 mV depolarization of resting membrane potential. Application of synthetic I_K1 through dynamic clamp restored AP duration, shape and resting potential. Replacement of wild-type (WT) I_K1 with heterozygotic (WT-D172N) or homozygotic (D172N) mutant formulations under dynamic clamp significantly abbreviated AP duration (APD₉₀) and accelerated maximal AP repolarization velocity, with no significant hyperpolarization of resting potential. Across stimulation frequencies from 0.5 to 3 Hz, the relationship between APD₉₀ and cycle length was downward shifted, reflecting AP abbreviation at all stimulation frequencies tested. In further AP measurements at 1 Hz from hiPSC cardiomyocytes, the D172N mutation produced similar effects on APD and repolarization velocity; however, resting potential was moderately hyperpolarized by application of mutant I_K1 to these cells. Overall, the results of this study support the major changes in ventricular cell AP repolarization with the D172N predicted from prior AP modelling and highlight the potential utility of using adult ventricular cardiomyocytes for dynamic clamp exploration of functional consequences of Kir2.1 mutations.

Envisioning the role of inwardly rectifying potassium (Kir) channel in epilepsy

Epilepsy is a devastating neurological disorder characterized by recurrent seizures attributed to the disruption of the dynamic excitatory and inhibitory balance in the brain. Epilepsy has emerged as a global health concern affecting about 70 million people worldwide. Despite recent advances in pre-clinical and clinical research, its etiopathogenesis remains obscure, and there are still no treatment strategies modifying disease progression. Although the precise molecular mechanisms underlying epileptogenesis have not been clarified yet, the role of ion channels as regulators of cellular excitability has increasingly gained attention. In this regard, emerging evidence highlights the potential implication of inwardly rectifying potassium (Kir) channels in epileptogenesis. Kir channels consist of seven different subfamilies (Kir1-Kir7), and they are highly expressed in both neuronal and glial cells in the central nervous system. These channels control the cell volume and excitability. In this review, we discuss preclinical and clinical evidence on the role of the several subfamilies of Kir channels in epileptogenesis, aiming to shed more light on the pathogenesis of this disorder and pave the way for future novel therapeutic approaches.

Preclinical short QT syndrome models: studying the phenotype and drug-screening

Cardiovascular diseases are the main cause of sudden cardiac death (SCD) in developed and developing countries. Inherited cardiac channelopathies are linked to 5-10% of SCDs, mainly in the young. Short QT syndrome (SQTS) is a rare inherited channelopathy, which leads to both atrial and ventricular tachyarrhythmias, syncope, and even SCD. International European Society of Cardiology guidelines include as diagnostic criteria: (i) QTc ≤ 340 ms on electrocardiogram, (ii) QTc ≤ 360 ms plus one of the follwing, an affected short QT syndrome pathogenic gene mutation, or family history of SQTS, or aborted cardiac arrest, or family history of cardiac arrest in the young. However, further evaluation of the QTc ranges seems to be required, which might be possible by assembling large short QT cohorts and considering genetic screening of the newly described pathogenic mutations. Since the mechanisms underlying the arrhythmogenesis of SQTS is unclear, optimal therapy for SQTS is still lacking. The disease is rare, unclear genotype-phenotype correlations exist in a bevy of cases and the absence of an international short QT registry limit studies on the pathophysiological mechanisms of arrhythmogenesis and therapy of SQTS. This leads to the necessity of experimental models or platforms for studying SQTS. Here, we focus on reviewing preclinical SQTS models and platforms such as animal models, heterologous expression systems, human-induced pluripotent stem cell-derived cardiomyocyte models and computer models as well as three-dimensional engineered heart tissues. We discuss their usefulness for SQTS studies to examine genotype-phenotype associations, uncover disease mechanisms and test drugs. These models might be helpful for providing novel insights into the exact pathophysiological mechanisms of this channelopathy and may offer opportunities to improve the diagnosis and treatment of patients with SQT syndrome.

Heritable arrhythmias associated with abnormal function of cardiac potassium channels

Cardiomyocytes express a surprisingly large number of potassium channel types. The primary physiological functions of the currents conducted by these channels are to maintain the resting membrane potential and mediate action potential repolarization under basal conditions and in response to changes in the concentrations of intracellular sodium, calcium, and ATP/ADP. Here, we review the diversity and functional roles of cardiac potassium channels under normal conditions and how heritable mutations in the genes encoding these channels can lead to distinct arrhythmias. We briefly review atrial fibrillation and J-wave syndromes. For long and short QT syndromes, we describe their genetic basis, clinical manifestation, risk stratification, traditional and novel therapeutic approaches, as well as insights into disease mechanisms provided by animal and cellular models.

Immediate and Delayed Response of Simulated Human Atrial Myocytes to Clinically-Relevant Hypokalemia

Although plasma electrolyte levels are quickly and precisely regulated in the mammalian cardiovascular system, even small transient changes in K⁺, Na⁺, Ca²⁺, and/or Mg²⁺ can significantly alter physiological responses in the heart, blood vessels, and intrinsic (intracardiac) autonomic nervous system. We have used mathematical models of the human atrial action potential (AP) to explore the electrophysiological mechanisms that underlie changes in resting potential (V_r) and the AP following decreases in plasma K⁺, [K⁺]_o, that were selected to mimic clinical hypokalemia. Such changes may be associated with arrhythmias and are commonly encountered in patients (i) in therapy for hypertension and heart failure; (ii) undergoing renal dialysis; (iii) with any disease with acid-base imbalance; or (iv) post-operatively. Our study emphasizes clinically-relevant hypokalemic conditions, corresponding to [K⁺]_o reductions of approximately 1.5 mM from the normal value of 4 to 4.5 mM. We show how the resulting electrophysiological responses in human atrial myocytes progress within two distinct time frames: (i) Immediately after [K⁺]_o is reduced, the K⁺-sensing mechanism of the background inward rectifier current (I_K1) responds. Specifically, its highly non-linear current-voltage relationship changes significantly as judged by the voltage dependence of its region of outward current. This rapidly alters, and sometimes even depolarizes, V_r and can also markedly prolong the final repolarization phase of the AP, thus modulating excitability and refractoriness. (ii) A second much slower electrophysiological response (developing 5-10 minutes after [K⁺]_o is reduced) results from alterations in the intracellular electrolyte balance. A progressive shift in intracellular [Na⁺]_i causes a change in the outward electrogenic current generated by the Na⁺/K⁺ pump, thereby modifying V_r and AP repolarization and changing the human atrial electrophysiological substrate. In this study, these two effects were investigated quantitatively, using seven published models of the human atrial AP. This highlighted the important role of I_K1 rectification when analyzing both the mechanisms by which [K⁺]_o regulates V_r and how the AP waveform may contribute to "trigger" mechanisms within the proarrhythmic substrate. Our simulations complement and extend previous studies aimed at understanding key factors by which decreases in [K⁺]_o can produce effects that are known to promote atrial arrhythmias in human hearts.

Functional genomics and epigenomics of atrial fibrillation

Atrial fibrillation is a progressive cardiac arrhythmia that increases the risk of hospitalization and adverse cardiovascular events. Despite years of study, we still do not have a full comprehension of the molecular mechanism responsible for the disease. The recent implementation of large-scale approaches in both patient samples, population studies and animal models has helped us to broaden our knowledge on the molecular drivers responsible for AF and on the mechanisms behind disease progression. Understanding genomic and epigenomic changes that take place during chronification of AF will prove essential to design novel treatments leading to improved patient care.

Targeting of Potassium Channels in Cardiac Arrhythmias

Cardiomyocytes are endowed with a complex repertoire of ion channels, responsible for the generation of action potentials (APs), travelling waves of electrical excitation, propagating throughout the heart and leading to cardiac contractions. Cardiac AP waveforms are shaped by a striking diversity of K⁺ channels. The pivotal role of K⁺ channels in cardiac health and disease is underscored by the dramatic impact that K⁺ channel dysfunction has on cardiac arrhythmias. The development of drugs targeted to specific K⁺ channels is expected to provide an optimized approach to antiarrhythmic therapy. Here, we review the functional roles of cardiac potassium channels under normal and diseased states. We survey current antiarrhythmic drugs (AADs) targeted to voltage-gated and Ca²⁺-activated K⁺ channels and highlight future research opportunities.

Prediction of Cardiac Mechanical Performance From Electrical Features During Ventricular Tachyarrhythmia Simulation Using Machine Learning Algorithms

In ventricular tachyarrhythmia, electrical instability features including action potential duration, dominant frequency, phase singularity, and filaments are associated with mechanical contractility. However, there are insufficient studies on estimated mechanical contractility based on electrical features during ventricular tachyarrhythmia using a stochastic model. In this study, we predicted cardiac mechanical performance from features of electrical instability during ventricular tachyarrhythmia simulation using machine learning algorithms, including support vector regression (SVR) and artificial neural network (ANN) models. We performed an electromechanical tachyarrhythmia simulation and extracted 12 electrical instability features and two mechanical properties, including stroke volume and the amplitude of myocardial tension (ampTens). We compared predictive performance according to kernel types of the SVR model and the number of hidden layers of the ANN model. In the SVR model, the prediction accuracies of stroke volume and ampTens were the highest when using the polynomial kernel and linear kernel, respectively. The predictive performance of the ANN model was better than that of the SVR model. The prediction accuracies were the highest when the ANN model consisted of three hidden layers. Accordingly, we propose the ANN model with three hidden layers as an optimal model for predicting cardiac mechanical contractility in ventricular tachyarrhythmia. The results of this study are expected to be used to indirectly estimate the hemodynamic response from the electrical cardiac map measured by the optical mapping system during cardiac surgery, as well as cardiac contractility under normal sinus rhythm conditions.

The Short QTc Is a Marker for the Development of Atrial Flutter and Atrial Fibrillation

A short QT interval has been difficult to define, and there is debate whether it exists outside of an extremely small group of individuals with inherited channelopathies and whether it predicts cardiac arrhythmias. The objective was to identify cases with short QT and their consequences. Our hospital ECG database was screened for cases with a QTc based on the Bazett formula (QTcBZT) of less than 340 ms. The QTc was recalculated using the spline (QTcRBK) formula, which more accurately adjusts for the heart rate and identifies cases based on percentile distribution of the QT interval. The exclusion criteria were presence of bundle branch block, arrhythmias, or electronic pacemakers. An age- and sex-matched cohort was obtained from individuals with normal QT intervals with the same exclusion criteria. There were 28 cases with a short QTc (QTcRBK < 380 ms). The age was 69.6 ± 14.6 years (mean ± SD) (50% males). The QT interval was 305.7 ± 61.1 ms with QTcRBK 308.4 ± 31.4 ms. Subsequent ECGs showed atrial flutter in 21%, atrial fibrillation in 18%, and atrial tachycardia in 4% of cases. Thus, atrial arrhythmias occurred in 43% of cases. This incidence was significantly (p < 0.0001) greater than the incidence of atrial arrhythmias in age- and sex-matched controls. In conclusion, a short QT interval can be readily identified based on the first percentile of the new QTc formula. A short QTc is an important marker for the development of atrial arrhythmias, including atrial flutter and atrial fibrillation, with the former predominating. It should be part of patient assessment and warrants consideration to develop strategies for detection and prevention of atrial arrhythmias.

Long noncoding RNA TCONS-00106987 promotes atrial electrical remodelling during atrial fibrillation by sponging miR-26 to regulate KCNJ2

Long noncoding RNAs (lncRNAs) have been suggested to play indispensable roles in multiple heart diseases. However, the correlations between lncRNAs and atrial fibrillation (AF) are unclear. In this study, we performed comprehensive lncRNA profiling via high-throughput RNA sequencing analysis using non-AF and AF rabbit models. Based on a series of filtering pipelines and bioinformatics analyses, TCONS-00106987 was selected for further research. TCONS-00106987 levels were increased in the atria during AF. Moreover, the atrial effective refractory period was shortened and the AF inducibility was increased in vivo in response to lentiviral-mediated up-regulation of TCONS-00106987. TCONS-00106987 repression resulted in the opposite effects. Further studies indicated that TCONS-00106987 expression was positively correlated with the expression of the protein-coding gene KCNJ2. Luciferase reporter assays and whole-cell patch-clamp recording confirmed that TCONS-00106987 promoted electrical remodelling via endogenous competition with microRNA-26 (miR-26) to induce transcription of its target gene KCNJ2, thereby increasing inward-rectifier K⁺ current (I_K1 ). In conclusion, our study reveals a pathogenic lncRNA-miRNA regulatory network specific to atrial electrical remodelling that offers potential therapeutic targets for AF.

Andersen-Tawil Syndrome: A Comprehensive Review

Andersen-Tawil syndrome (ATS) is a very rare orphan genetic multisystem channelopathy without structural heart disease (with rare exceptions). ATS type 1 is inherited in an autosomal dominant fashion and is caused by mutations in the KCNJ2 gene, which encodes the α subunit of the K+ channel protein Kir2.1 (in ≈ 50-60% of cases). ATS type 2 is in turn linked to a rare mutation in the KCNJ5-GIRK4 gene that encodes the G protein-sensitive-activated inwardly rectifying K+ channel Kir3.4 (15%), which carries the acetylcholine-induced potassium current. About 30% of cases are de novo/sporadic, suggesting that additional as-yet unidentified genes also cause the disorder. A triad of periodic muscle paralysis, repolarization changes in the electrocardiogram, and structural body changes characterize ATS. The typical muscular change is episodic flaccid muscle weakness. Prolongation of the QU/QUc intervals and normal or minimally prolonged QT/QTc intervals with a tendency to ventricular arrhythmias are typical repolarization changes. Bidirectional ventricular tachycardia is the hallmark ventricular arrhythmia, but also premature ventricular contractions, and rarely, polymorphic ventricular tachycardia of torsade de pointes type may be present. Patients with ATS have characteristic physical developmental dysmorphisms that affect the face, skull, limbs, thorax, and stature. Mild learning difficulties and a distinct neurocognitive phenotype (deficits in executive function and abstract reasoning) have been described. About 60% of affected individuals have all features of the major triad. The purpose of this review is to present historical aspects, nomenclature (observations/criticisms), epidemiology, genetics, electrocardiography, arrhythmias, electrophysiological mechanisms, diagnostic criteria/clues of periodic paralysis, prognosis, and management of ATS.

Genetics and Epigenetics of Atrial Fibrillation

Atrial fibrillation (AF) is known to be the most common supraventricular arrhythmia affecting up to 1% of the general population. Its prevalence exponentially increases with age and could reach up to 8% in the elderly population. The management of AF is a complex issue that is addressed by extensive ongoing basic and clinical research. AF centers around different types of disturbances, including ion channel dysfunction, Ca²⁺-handling abnormalities, and structural remodeling. Genome-wide association studies (GWAS) have uncovered over 100 genetic loci associated with AF. Most of these loci point to ion channels, distinct cardiac-enriched transcription factors, as well as to other regulatory genes. Recently, the discovery of post-transcriptional regulatory mechanisms, involving non-coding RNAs (especially microRNAs), DNA methylation, and histone modification, has allowed to decipher how a normal heart develops and which modifications are involved in reshaping the processes leading to arrhythmias. This review aims to provide a current state of the field regarding the identification and functional characterization of AF-related epigenetic regulatory networks

Stabilizer Cell Gene Therapy: A Less-Is-More Strategy to Prevent Cardiac Arrhythmias

BACKGROUND

In cardiac gene therapy to improve contractile function, achieving gene expression in the majority of cardiac myocytes is essential. In preventing cardiac arrhythmias, however, this goal may not be as important since transduction efficiencies as low as 40% suppressed ventricular arrhythmias in genetically modified mice with catecholaminergic polymorphic ventricular tachycardia.

METHODS

Using computational modeling, we simulated 1-, 2-, and 3-dimensional tissue under a variety of conditions to test the ability of genetically engineered nonarrhythmogenic stabilizer cells to suppress triggered activity due to delayed or early afterdepolarizations.

RESULTS

Due to source-sink relationships in cardiac tissue, a minority (20%-50%) of randomly distributed stabilizer cells engineered to be nonarrhythmogenic can suppress the ability of arrhythmogenic cells to generate delayed and early afterdepolarizations-related arrhythmias. Stabilizer cell gene therapy strategy can be designed to correct a specific arrhythmogenic mutation, as in the catecholaminergic polymorphic ventricular tachycardia mice studies, or more generally to suppress delayed or early afterdepolarizations from any cause by overexpressing the inward rectifier K channel Kir2.1 in stabilizer cells.

CONCLUSIONS

This promising antiarrhythmic strategy warrants further testing in experimental models to evaluate its clinical potential.

Molecular Basis of Atrial Fibrillation Pathophysiology and Therapy: A Translational Perspective

Atrial fibrillation (AF) is a highly prevalent arrhythmia, with substantial associated morbidity and mortality. There have been significant management advances over the past 2 decades, but the burden of the disease continues to increase and there is certainly plenty of room for improvement in treatment options. A potential key to therapeutic innovation is a better understanding of underlying fundamental mechanisms. This article reviews recent advances in understanding the molecular basis for AF, with a particular emphasis on relating these new insights to opportunities for clinical translation. We first review the evidence relating basic electrophysiological mechanisms to the characteristics of clinical AF. We then discuss the molecular control of factors leading to some of the principal determinants, including abnormalities in impulse conduction (such as tissue fibrosis and other extra-cardiomyocyte alterations, connexin dysregulation and Na+-channel dysfunction), electrical refractoriness, and impulse generation. We then consider the molecular drivers of AF progression, including a range of Ca2+-dependent intracellular processes, microRNA changes, and inflammatory signaling. The concept of key interactome-related nodal points is then evaluated, dealing with systems like those associated with CaMKII (Ca2+/calmodulin-dependent protein kinase-II), NLRP3 (NACHT, LRR, and PYD domains-containing protein-3), and transcription-factors like TBX5 and PitX2c. We conclude with a critical discussion of therapeutic implications, knowledge gaps and future directions, dealing with such aspects as drug repurposing, biologicals, multispecific drugs, the targeting of cardiomyocyte inflammatory signaling and potential considerations in intervening at the level of interactomes and gene-regulation. The area of molecular intervention for AF management presents exciting new opportunities, along with substantial challenges.

Computational Study to Identify the Effects of the KCNJ2 E299V Mutation in Cardiac Pumping Capacity

The KCNJ2 gene mutations induce short QT syndrome (SQT3) by directly increasing the I_K1 current. There have been many studies on the electrophysiological effects of mutations such as the KCNJ2 D172N that cause the SQT3. However, the KCNJ2 E299V mutation is distinguished from other representative gene mutations that can induce the short QT syndrome (SQT3) in that it increased I_K1 current by impairing the inward rectification of K⁺ channels. The studies of the electromechanical effects on myocardial cells and mechanisms of E299V mutations are limited. Therefore, we investigated the electrophysiological changes and the concomitant mechanical responses according to the expression levels of the KCNJ2 E299V mutation during sinus rhythm and ventricular fibrillation. We performed excitation-contraction coupling simulations using a human ventricular model with both electrophysiological and mechanical properties. In order to observe the electromechanical changes due to the expression of KCNJ2 E299V mutation, the simulations were performed under normal condition (WT), heterogeneous mutation condition (WT/E299V), and pure mutation condition (E299V). First, a single-cell simulation was performed in three types of ventricular cells (endocardial cell, midmyocardial cell, and epicardial cell) to confirm the electrophysiological changes and arrhythmogenesis caused by the KCNJ2 E299V mutation. In three-dimensional sinus rhythm simulations, we compared electrical changes and the corresponding changes in mechanical performance caused by the expression level of E299V mutation. Then, we observed the electromechanical properties of the E299V mutation during ventricular fibrillation using the three-dimensional reentry simulation. The KCNJ2 E299V mutation accelerated the opening of the I_K1 channel and increased I_K1 current, resulting in a decrease in action potential duration. Accordingly, the QT interval was reduced by 48% and 60% compared to the WT condition, for the WT/E299V and E299V conditions, respectively. During sustained reentry, the wavelength was reduced due to the KCNJ2 E299V mutation. Furthermore, there was almost no ventricular contraction in both WT/E299V and E299V conditions. We concluded that in both sinus rhythm and fibrillation, the KCNJ2 E299V mutation results in very low contractility regardless of the expression level of mutation and increases the risk of cardiac arrest and cardiac death.

Relationship Between Electrical Instability and Pumping Performance During Ventricular Tachyarrhythmia: Computational Study

There are representative electrical parameters for understanding the mechanism of reentrant waves in studies on tachyarrhythmia, namely the action potential duration (APD), dominant frequency, phase singularity, and filament. However, there are no studies that have directly identified the correlation between these electrophysiological parameters and cardiac contractility. Therefore, we have identified individual and integrative correlations between these electrical phenomena and contractility during tachyarrhythmia by deriving regression equations and also investigated the electrophysiological parameters affecting cardiac contractility during tachyarrhythmia. We simulated ventricular tachyarrhythmia with 48 types of electrical patterns by applying four reentry generation methods and changing the electrical conductivity of the potassium channel, which has the greatest effect on ventricular tissue. The mechanical responses reflecting electrical complexity were obtained through deterministic simulations of excitation-contraction coupling. We used the stroke volume and amplitude of myocardial tension (ampTens) as the variables representing contractility. We derived stochastic models through single- and multivariable regression analyses to identify the electrical parameters affecting contractility during tachyarrhythmia. In single-variable regression analysis, the APD, dominant frequency, and filament, excluding phase singularity, have statistically significant correlations with the stroke volume and ampTens. Among them, the APD has the maximum influence on these two mechanical parameters (standard beta coefficient: 0.859 for stroke volume, 0.930 for ampTens). The stochastic model using all four electrical parameters fails to accurately predict contractility owing to the multicollinearity between the APD and dominant frequency. We have rederived the multi-variable stochastic model using three electrical parameters without the APD. The filament has the greatest effect on the stroke volume stochastically (standard beta coefficient: 0.853 and 0.752). The dominant frequency has the greatest effect on ampTens statistically (standard beta coefficient: -0.813). We conclude that among the electrical parameters, the APD has the highest individual influence on mechanical contraction, and the filament has the highest integrative influence in both statistical terms.

Genetics of Atrial Fibrillation

Background

Atrial fibrillation (AF) is a common arrhythmia seen in clinical practice. Occasionally, no common risk factors are present in patients with this arrhythmia. This suggests the potential underlying role of genetic factors associated with predisposition to developing AF.

Methods and Results

We conducted a comprehensive review of the literature through large online libraries, including PubMed. Many different potassium and sodium channel mutations have been discussed in their relation to AF. There have also been non–ion channel mutations that have been linked to AF. Genome‐wide association studies have helped in identifying potential links between single‐nucleotide polymorphisms and AF. Ancestry studies have also highlighted a role of genetics in AF. Blacks with a higher percentage of European ancestry are at higher risk of developing AF. The emerging field of ablatogenomics involves the use of genetic profiles in their relation to recurrence of AF after catheter ablation.

Conclusions

The evidence for the underlying role of genetics in AF continues to expand. Ultimately, the role of genetics in risk stratification of AF and its recurrence is of significant interest. No established risk scores that are useful in clinical practice are present to date.

Mechanisms by Which Ranolazine Terminates Paroxysmal but Not Persistent Atrial Fibrillation

BACKGROUND

Ranolazine inhibits Na⁺ current (I_Na), but whether it can convert atrial fibrillation (AF) to sinus rhythm remains unclear. We investigated antiarrhythmic mechanisms of ranolazine in sheep models of paroxysmal (PxAF) and persistent AF (PsAF).

METHODS

PxAF was maintained during acute stretch (N=8), and PsAF was induced by long-term atrial tachypacing (N=9). Isolated, Langendorff-perfused sheep hearts were optically mapped.

RESULTS

In PxAF ranolazine (10 μmol/L) reduced dominant frequency from 8.3±0.4 to 6.2±0.5 Hz (P<0.01) before converting to sinus rhythm, decreased singularity point density from 0.070±0.007 to 0.039±0.005 cm^-2 s^-1 (P<0.001) in left atrial epicardium (LA_epi), and prolonged AF cycle length (AFCL); rotor duration, tip trajectory, and variance of AFCL were unaltered. In PsAF, ranolazine reduced dominant frequency (8.3±0.5 to 6.5±0.4 Hz; P<0.01), prolonged AFCL, increased the variance of AFCL, had no effect on singularity point density (0.048±0.011 to 0.042±0.016 cm^-2 s^-1; P=ns) and failed to convert AF to sinus rhythm. Doubling the ranolazine concentration (20 μmol/L) or supplementing with dofetilide (1 μmol/L) failed to convert PsAF to sinus rhythm. In computer simulations of rotors, reducing I_Na decreased dominant frequency, increased tip meandering and produced vortex shedding on wave interaction with unexcitable regions.

CONCLUSIONS

PxAF and PsAF respond differently to ranolazine. Cardioversion in the former can be attributed partly to decreased dominant frequency and singularity point density, and prolongation of AFCL. In the latter, increased dispersion of AFCL and likely vortex shedding contributes to rotor formation, compensating for any rotor loss, and may underlie the inefficacy of ranolazine to terminate PsAF.

Nocturnal Atrial Fibrillation Caused by Mutation in KCND2, Encoding Pore-Forming (α) Subunit of the Cardiac Kv4.2 Potassium Channel

BACKGROUND

Paroxysmal atrial fibrillation (AF) can be caused by gain-of-function mutations in genes, encoding the cardiac potassium channel subunits KCNJ2, KCNE1, and KCNH2 that mediate the repolarizing potassium currents I_k1, I_ks, and I_kr, respectively.

METHODS

Linkage analysis, whole-exome sequencing, and Xenopus oocyte electrophysiology studies were used in this study.

RESULTS

Through genetic studies, we showed that autosomal dominant early-onset nocturnal paroxysmal AF is caused by p.S447R mutation in KCND2, encoding the pore-forming (α) subunit of the Kv4.2 cardiac potassium channel. Kv4.2, along with Kv4.3, contributes to the cardiac fast transient outward K⁺ current, I_to. I_to underlies the early phase of repolarization in the cardiac action potential, thereby setting the initial potential of the plateau phase and governing its duration and amplitude. In Xenopus oocytes, the mutation increased the channel's inactivation time constant and affected its regulation: p.S447 resides in a protein kinase C (PKC) phosphorylation site, which normally allows attenuation of Kv4.2 membrane expression. The mutant Kv4.2 exhibited impaired response to PKC; hence, Kv4.2 membrane expression was augmented, enhancing potassium currents. Coexpression of mutant and wild-type channels (recapitulating heterozygosity in affected individuals) showed results similar to the mutant channel alone. Finally, in a hybrid channel composed of Kv4.3 and Kv4.2, simulating the mature endogenous heterotetrameric channel underlying I_to, the p.S447R Kv4.2 mutation exerted a gain-of-function effect on Kv4.3.

CONCLUSIONS

The mutation alters Kv4.2's kinetic properties, impairs its inhibitory regulation, and exerts gain-of-function effect on both Kv4.2 homotetramers and Kv4.2-Kv4.3 heterotetramers. These effects presumably increase the repolarizing potassium current I_to, thereby abbreviating action potential duration, creating arrhythmogenic substrate for nocturnal AF. Interestingly, Kv4.2 expression was previously shown to demonstrate circadian variation, with peak expression at daytime in murine hearts (human nighttime), with possible relevance to the nocturnal onset of paroxysmal AF symptoms in our patients. The atrial-specific phenotype suggests that targeting Kv4.2 might be effective in the treatment of nocturnal paroxysmal AF, avoiding adverse ventricular effects.

Characterization of a novel LQT3 variant with a selective efficacy of mexiletine treatment

Pathogenic variants in the human SCN5A gene encoding the a-subunit of the principle Na⁺ channel (Nav1.5) are associated with long QT syndrome (LQTS) 3. LQT3 patients display variable responses to Na⁺ channel blockers demanding for the development of variant-specific therapeutic strategies. Here we performed a combined electrophysiological analysis with in silico simulation of variant channel to elucidate mechanisms of therapeutic responsiveness. We identified a novel SCN5A variant (A1656D) in a LQTS patient with a distinct response to mexiletine resulting in suppression of non-sustained ventricular tachycardia and manifestation of premature atrial contraction. Patch clamp analysis revealed that A1656D variant exerted gain-of-function effects including hyperpolarizing shift of the voltage-dependence of activation, depolarizing shift in the voltage-dependence of inactivation, and slowing of fast inactivation. Among ranolazine, flecainide, and mexiletine, only mexiletine restored inactivation kinetics of A1656D currents. In silico simulation to assess the effect of A1656D variant on ventricular cardiac cell excitation predicted a prolonged action potential which is consistent with the prolonged QT and non-sustained ventricular tachycardia of the patient. It also predicted that only mexiletine suppressed the prolonged action potential of human ventricular myocytes expressing A1656D. These data elucidate the underlying mechanism of the distinct response to mexiletine in this patient.

Short QT Syndrome: A Comprehensive Genetic Interpretation and Clinical Translation of Rare Variants

Short QT syndrome, one of the most lethal entities associated with sudden cardiac death, is a rare genetic disease characterized by short QT intervals detected by electrocardiogram. Several genetic variants are causally linked to the disease, but there has yet to be a comprehensive analysis of variants among patients with short QT syndrome. To fill this gap, we performed an exhaustive study of variants currently catalogued as deleterious in short QT syndrome according to the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Analysis of the 32 variants described in the literature determined that only nine (28.12%) have a conclusive pathogenic role. All definitively pathogenic variants are located in KCNQ1, KCNH2, or KCNJ2; three genes encoding potassium channels. Other variants located in genes encoding calcium or sodium channels are associated with electrical alterations concomitant with shortened QT intervals but do not guarantee a diagnosis of short QT syndrome. We recommend caution regarding previously reported variants classified as pathogenic. An exhaustive re-analysis is necessary to clarify the role of each variant before routinely translating genetic findings to the clinical setting.

Understanding AF Mechanisms Through Computational Modelling and Simulations

AF is a progressive disease of the atria, involving complex mechanisms related to its initiation, maintenance and progression. Computational modelling provides a framework for integration of experimental and clinical findings, and has emerged as an essential part of mechanistic research in AF. The authors summarise recent advancements in development of multi-scale AF models and focus on the mechanistic links between alternations in atrial structure and electrophysiology with AF. Key AF mechanisms that have been explored using atrial modelling are pulmonary vein ectopy; atrial fibrosis and fibrosis distribution; atrial wall thickness heterogeneity; atrial adipose tissue infiltration; development of repolarisation alternans; cardiac ion channel mutations; and atrial stretch with mechano-electrical feedback. They review modelling approaches that capture variability at the cohort level and provide cohort-specific mechanistic insights. The authors conclude with a summary of future perspectives, as envisioned for the contributions of atrial modelling in the mechanistic understanding of AF.

Genetics of Atrial Fibrilation: In Search of Novel Therapeutic Targets

Atrial fibrillation (AF) is the most frequent arrhythmogenic disease in humans, ranging from 2% in the general population and rising up to 10-12% in 80+ years. Genetic analyses of AF familiar cases have identified a series of point mutations in distinct ion channels, supporting a causative link. However, these genetic defects only explain a minority of AF patients. Genomewide association studies identified single nucleotide polymorphisms (SNPs), close to PITX2 on 4q25 chromosome, that are highly associated to AF. Subsequent GWAS studies have identified several new loci, involving additional transcription and growth factors. Furthermore, these risk 4q25 SNPs serve as surrogate biomarkers to identify AF recurrence in distinct surgical and pharmacological interventions. Experimental studies have demonstrated an intricate signalling pathway supporting a key role of the homeobox transcription factor PITX2 as a transcriptional regulator. Furthermore, cardiovascular risk factors such as hyperthyroidism, hypertension and redox homeostasis have been identified to modulate PITX2 driven gene regulatory networks. We provide herein a state-of-the-art review of the genetic bases of atrial fibrillation, our current understanding of the genetic regulatory networks involved in AF and its plausible usage for searching novel therapeutic targets.

Functional consequences of genetic variation in sodium channel modifiers in early onset lone atrial fibrillation

Aim: We investigated the effect of variants in genes encoding sodium channel modifiers SNTA1 and GPD1L found in early onset atrial fibrillation (AF) patients. Patients & methods: Genetic screening in patients with early onset lone AF revealed three variants in GPD1L and SNTA1 in three AF patients. Functional analysis was performed by patch-clamp electrophysiology. Results: Co-expression of GPD1L or its p.A326E variant with NaV1.5 did not alter INa density or current kinetics. SNTA1 shifted the peak-current by -5 mV. The SNTA1-p.A257G variant significantly increased INa. SNTA1-p.P74L did not produce functional changes. Conclusion: Although genetic variation of sodium channel modifiers may contribute to development of AF at a molecular level, it is unlikely a monogenic cause of the disease.

Slow [Na+]i dynamics impacts arrhythmogenesis and spiral wave reentry in cardiac myocyte ionic model

Accumulation of intracellular Na⁺ is gaining recognition as an important regulator of cardiac myocyte electrophysiology. The intracellular Na⁺ concentration can be an important determinant of the cardiac action potential duration, can modulate the tissue-level conduction of excitation waves, and can alter vulnerability to arrhythmias. Mathematical models of cardiac electrophysiology often incorporate a dynamic intracellular Na⁺ concentration, which changes much more slowly than the remaining variables. We investigated the dependence of several arrhythmogenesis-related factors on [Na⁺]_i in a mathematical model of the human atrial action potential. In cell simulations, we found that [Na⁺]_i accumulation stabilizes the action potential duration to variations in several conductances and that the slow dynamics of [Na⁺]_i impacts bifurcations to pro-arrhythmic afterdepolarizations, causing intermittency between different rhythms. In long-lasting tissue simulations of spiral wave reentry, [Na⁺]_i becomes spatially heterogeneous with a decreased area around the spiral wave rotation center. This heterogeneous region forms a functional anchor, resulting in diminished meandering of the spiral wave. Our findings suggest that slow, physiological, rate-dependent variations in [Na⁺]_i may play complex roles in cellular and tissue-level cardiac dynamics.

Chloride Channels are Involved in the Development of Atrial Fibrillation - A Transcriptomic and proteomic Study

Electrical and structural remodeling processes are contributors to the self-perpetuating nature of atrial fibrillation (AF). However, their correlation has not been clarified. In this study, human atrial tissues from the patients with rheumatic mitral valve disease in either sinus rhythm or persistent AF were analyzed using a combined transcriptomic and proteomic approach. An up-regulation in chloride intracellular channel (CLIC) 1, 4, 5 and a rise in type IV collagen were revealed. Combined with the results from immunohistochemistry and electron microscope analysis, the distribution of type IV collagen and effects of fibrosis on myocyte membrane indicated the possible interaction between CLIC and type IV collagen, confirmed by protein structure prediction and co-immunoprecipitation. These results indicate that CLICs play an important role in the development of atrial fibrillation and that CLICs and structural type IV collagen may interact on each other to promote the development of AF in rheumatic mitral valve disease.

PA-6 inhibits inward rectifier currents carried by V93I and D172N gain-of-function KIR2.1 channels, but increases channel protein expression

BACKGROUND

The inward rectifier potassium current IK1 contributes to a stable resting membrane potential and phase 3 repolarization of the cardiac action potential. KCNJ2 gain-of-function mutations V93I and D172N associate with increased IK1, short QT syndrome type 3 and congenital atrial fibrillation. Pentamidine-Analogue 6 (PA-6) is an efficient (IC50 = 14 nM with inside-out patch clamp methodology) and specific IK1 inhibitor that interacts with the cytoplasmic pore region of the KIR2.1 ion channel, encoded by KCNJ2. At 10 μM, PA-6 increases wild-type (WT) KIR2.1 expression in HEK293T cells upon chronic treatment. We hypothesized that PA-6 will interact with and inhibit V93I and D172N KIR2.1 channels, whereas impact on channel expression at the plasma membrane requires higher concentrations.

METHODS

Molecular modelling was performed with the human KIR2.1 closed state homology model using FlexX. WT and mutant KIR2.1 channels were expressed in HEK293 cells. Patch-clamp single cell electrophysiology measurements were performed in the whole cell and inside-out mode of the patch clamp method. KIR2.1 expression level and localization were determined by western blot analysis and immunofluorescence microscopy, respectively.

RESULTS

PA-6 docking in the V93I/D172N double mutant homology model of KIR2.1 demonstrated that mutations and drug-binding site are >30 Å apart. PA-6 inhibited WT and V93I outward currents with similar potency (IC50 = 35.5 and 43.6 nM at +50 mV for WT and V93I), whereas D172N currents were less sensitive (IC50 = 128.9 nM at +50 mV) using inside-out patch-clamp electrophysiology. In whole cell mode, 1 μM of PA-6 inhibited outward IK1 at -50 mV by 28 ± 36%, 18 ± 20% and 10 ± 6%, for WT, V93I and D172N channels respectively. Western blot analysis demonstrated that PA-6 (5 μM, 24 h) increased KIR2.1 expression levels of WT (6.3 ± 1.5 fold), and V93I (3.9 ± 0.9) and D172N (4.8 ± 2.0) mutants. Immunofluorescent microscopy demonstrated dose-dependent intracellular KIR2.1 accumulation following chronic PA-6 application (24 h, 1 and 5 μM).

CONCLUSIONS

1) KCNJ2 gain-of-function mutations V93I and D172N in the KIR2.1 ion channel do not impair PA-6 mediated inhibition of IK1, 2) PA-6 elevates KIR2.1 protein expression and induces intracellular KIR2.1 accumulation, 3) PA-6 is a strong candidate for further preclinical evaluation in treatment of congenital SQT3 and AF.