Gain-of-function mutations in potassium channel subunit KCNE2 associated with early-onset lone atrial fibrillation

Pathophysiological Mechanisms of Psychosis-Induced Atrial Fibrillation: The Links between Mental Disorder and Arrhythmia

Atrial fibrillation (AF) is a common phenomenon of sustained arrhythmia leading to heart failure or stroke. Patients with mental disorders (MD), particularly schizophrenia and bipolar disorder, are at a high risk of AF triggered by the dysregulation of the autonomic nervous system, atrial stretch, oxidative stress, inflammation, and electrical or structural remodeling. Moreover, pathophysiological mechanisms underlying MD may also contribute to the genesis of AF. An overactivated hypothalamic-pituitary-adrenal axis, aberrant renin-angiotensin-aldosterone system, abnormal serotonin signaling, disturbed sleep, and genetic/epigenetic factors can adversely alter atrial electrophysiology and structural substrates, leading to the development of AF. In this review, we provide an update of our collective knowledge of the pathophysiological and molecular mechanisms that link MD and AF. Targeting the pathogenic mechanisms of MD-specific AF may facilitate the development of therapeutics that mitigate AF and cardiovascular mortality in this patient population.

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.

Discovery of TBX20 as a Novel Gene Underlying Atrial Fibrillation

Atrial fibrillation (AF), the most prevalent type of sustained cardiac dysrhythmia globally, confers strikingly enhanced risks for cognitive dysfunction, stroke, chronic cardiac failure, and sudden cardiovascular demise. Aggregating studies underscore the crucial roles of inherited determinants in the occurrence and perpetuation of AF. However, due to conspicuous genetic heterogeneity, the inherited defects accounting for AF remain largely indefinite. Here, via whole-genome genotyping with genetic markers and a linkage assay in a family suffering from AF, a new AF-causative locus was located at human chromosome 7p14.2-p14.3, a ~4.89 cM (~4.43-Mb) interval between the markers D7S526 and D7S2250. An exome-wide sequencing assay unveiled that, at the defined locus, the mutation in the TBX20 gene, NM_001077653.2: c.695A>G; p.(His232Arg), was solely co-segregated with AF in the family. Additionally, a Sanger sequencing assay of TBX20 in another family suffering from AF uncovered a novel mutation, NM_001077653.2: c.862G>C; p.(Asp288His). Neither of the two mutations were observed in 600 unrelated control individuals. Functional investigations demonstrated that the two mutations both significantly reduced the transactivation of the target gene KCNH2 (a well-established AF-causing gene) and the ability to bind the promoter of KCNH2, while they had no effect on the nuclear distribution of TBX20. Conclusively, these findings reveal a new AF-causative locus at human chromosome 7p14.2-p14.3 and strongly indicate TBX20 as a novel AF-predisposing gene, shedding light on the mechanism underlying AF and suggesting clinical significance for the allele-specific treatment of AF patients.

Human atrial fibrillation and genetic defects in transient outward currents: mechanistic insights from multi-scale computational models

Previous studies have linked dysfunctional Ito arising from mutations to KCND3-encoded Kv4.3 and KCND2-encoded Kv4.2 to atrial fibrillation. Using computational models, this study aimed to investigate the mechanisms underlying pro-arrhythmic effects of the gain-of-function Kv4.3 (T361S, A545P) and Kv4.2 (S447R) mutations. Wild-type and mutant Ito formulations were developed from and validated against experimental data and incorporated into the Colman et al. model of human atrial cells. Single-cell models were incorporated into one- (1D) and two-dimensional (2D) models of atrial tissue, and a three-dimensional (3D) realistic model of the human atria. The three gain-of-function mutations had similar, albeit quantitatively different, effects: shortening of the action potential duration; lowering the plateau membrane potential, abbreviating the effective refractory period (ERP) and the wavelength (WL) of atrial excitation at the tissue level. Restitution curves for the WL, the ERP and the conduction velocity were leftward shifted, facilitating the conduction of atrial excitation waves at high excitation rates. The mutations also increased lifespan and stationarity of re-entry in both 2D and 3D simulations, which further highlighted a mutation-induced increase in spatial dispersion of repolarization. Collectively, these changes account for pro-arrhythmic effects of these Kv4.3 and Kv4.2 mutations in facilitating AF. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Genetic and non-genetic risk factors associated with atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmic disorder and its prevalence in the United States is projected to increase to more than twelve million cases in 2030. AF increases the risk of other forms of cardiovascular disease, including stroke. As the incidence of atrial fibrillation increases dramatically with age, it is paramount to elucidate risk factors underlying AF pathogenesis. Here, we review tissue and cellular pathways underlying AF, as well as critical components that impact AF susceptibility including genetic and environmental risk factors. Finally, we provide the latest information on potential links between SARS-CoV-2 and human AF. Improved understanding of mechanistic pathways holds promise in preventative care and early diagnostics, and also introduces novel targeted forms of therapy that might attenuate AF progression and maintenance.

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.

Arrhythmogenic Effects of Genetic Mutations Affecting Potassium Channels in Human Atrial Fibrillation: A Simulation Study

Genetic mutations in genes encoding for potassium channel protein structures have been recently associated with episodes of atrial fibrillation in asymptomatic patients. The aim of this study is to investigate the potential arrhythmogenicity of three gain-of-function mutations related to atrial fibrillation-namely, KCNH2 T895M, KCNH2 T436M, and KCNE3-V17M-using modeling and simulation of the electrophysiological activity of the heart. A genetic algorithm was used to tune the parameters' value of the original ionic currents to reproduce the alterations experimentally observed caused by the mutations. The effects on action potentials, ionic currents, and restitution properties were analyzed using versions of the Courtemanche human atrial myocyte model in different tissues: pulmonary vein, right, and left atrium. Atrial susceptibility of the tissues to spiral wave generation was also investigated studying the temporal vulnerability. The presence of the three mutations resulted in an overall more arrhythmogenic substrate. Higher current density, action potential duration shortening, and flattening of the restitution curves were the major effects of the three mutations at the single-cell level. The genetic mutations at the tissue level induced a higher temporal vulnerability to the rotor's initiation and progression, by sustaining spiral waves that perpetuate until the end of the simulation. The mutation with the highest pro-arrhythmic effects, exhibiting the widest sustained VW and the smallest meandering rotor's tip areas, was KCNE3-V17M. Moreover, the increased susceptibility to arrhythmias and rotor's stability was tissue-dependent. Pulmonary vein tissues were more prone to rotor's initiation, while in left atrium tissues rotors were more easily sustained. Re-entries were also progressively more stable in pulmonary vein tissue, followed by the left atrium, and finally the right atrium. The presence of the genetic mutations increased the susceptibility to arrhythmias by promoting the rotor's initiation and maintenance. The study provides useful insights into the mechanisms underlying fibrillatory events caused by KCNH2 T895M, KCNH2 T436M, and KCNE3-V17M and might aid the planning of patient-specific targeted therapies.

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

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.

Atrial Fibrillation in Long QT Syndrome by Genotype

BACKGROUND

Long QT syndrome (LQTS) is caused by the abnormal function of ion channels, which may also affect atrial electrophysiology and be associated with the risk of atrial fibrillation (AF). However, large-scale studies of AF risk among patients with LQTS and its relation to LQTS manifestations are lacking. We aimed to assess the risk of AF and its relationship to the LQTS genotype and the long-term prognosis in patients with LQTS.

METHODS

Genotype-positive patients with LQTS (784 LQT1, 746 LQT2, and 233 LQT3) were compared with 2043 genotype-negative family members. Information on the occurrence of AF was based on physician-reported ECG-verified events. Multivariate Cox proportional hazards regression analyses were performed for ages 0 to 60 and after 60 years (reflecting an early and late-onset of AF) to assess the risk of incident AF by genotype and the relationship of AF to the risk of cardiac events defined as syncope, documented torsades de pointes, and aborted cardiac arrest or sudden cardiac death.

RESULTS

In patients followed from birth to 60 years of age, patients with LQT3 had an increased risk of AF compared with genotype-negative family members (hazard ratio=6.62; 95% CI, 2.04-21.49; P<0.001), while neither LQT1 nor LQT2 demonstrated increased AF risk. After the age of 60 years, patients with LQT2 had significantly lower risk of AF compared with genotype-negative controls (hazard ratio=0.07; 95% CI, 0.01-0.53, P=0.011). AF was a significant predictor of cardiac events in patients with LQT3 through the age of 60 (hazard ratio=5.38; 95% CI, 1.17-24.82; P=0.031).

CONCLUSIONS

Our data demonstrate an increased risk of early age AF in patients with LQT3 and also indicate a protective effect of the LQT2 genotype in it's association with a decreased risk of AF after the age of 60.

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.

Genomics of Atrial Fibrillation

Atrial fibrillation (AF) is a common clinical arrhythmia that appears to be highly heritable, despite representing a complex interplay of several disease processes that generally do not manifest until later in life. In this manuscript, we will review the genetic basis of this complex trait established through studies of familial AF, linkage and candidate gene studies of common AF, genome wide association studies (GWAS) of common AF, and transcriptomic studies of AF. Since AF is associated with a five-fold increase in the risk of stroke, we also review the intersection of common genetic factors associated with both of these conditions. Similarly, we highlight the intersection of common genetic markers associated with some risk factors for AF, such as hypertension and obesity, and AF. Lastly, we describe a paradigm where genetic factors predispose to the risk of AF, but which may require additional stress and trigger factors in older age to allow for the clinical manifestation of AF.

The Role of Pharmacogenetics in Atrial Fibrillation Therapeutics: Is Personalized Therapy in Sight?

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia worldwide requiring therapy. Despite recent advances in catheter-based and surgical therapy, antiarrhythmic drugs (AADs) remain the mainstay of treatment for symptomatic AF. However, response in individual patients is highly variable with over half the patients treated with rhythm control therapy experiencing recurrence of AF within a year. Contemporary AADs used to suppress AF are incompletely and unpredictably effective and associated with significant risks of proarrhythmia and noncardiac toxicities. Furthermore, this "one-size" fits all strategy for selecting antiarrhythmics is based largely on minimizing risk of adverse effects rather than on the likelihood of suppressing AF. The limited success of rhythm control therapy is in part due to heterogeneity of the underlying substrate, interindividual differences in disease mechanisms, and our inability to predict response to AADs in individual patients. Genetic studies of AF over the past decade have revealed that susceptibility to and response to therapy for AF is modulated by the underlying genetic substrate. However, the bedside application of these new discoveries to the management of AF patients has thus far been disappointing. This may in part be related to our limited understanding about genetic predictors of drug response in general, the challenges associated with determining efficacy of response to AADs, and lack of randomized genotype-directed clinical trials. Nonetheless, recent studies have shown that common AF susceptibility risk alleles at the chromosome 4q25 locus modulated response to AADs, electrical cardioversion, and ablation therapy. This monograph discusses how genetic approaches to AF have not only provided important insights into underlying mechanisms but also identified AF subtypes that can be better targeted with more mechanism-based "personalized" therapy.

The two-pore domain potassium channel, TWIK-1, has a role in the regulation of heart rate and atrial size

The two-pore domain potassium (K(+)) channel TWIK-1 (or K2P1.1) contributes to background K(+) conductance in diverse cell types. TWIK-1, encoded by the KCNK1 gene, is present in the human heart with robust expression in the atria, however its physiological significance is unknown. To evaluate the cardiac effects of TWIK-1 deficiency, we studied zebrafish embryos after knockdown of the two KCNK1 orthologues, kcnk1a and kcnk1b. Knockdown of kcnk1a or kcnk1b individually caused bradycardia and atrial dilation (p<0.001 vs. controls), while ventricular stroke volume was preserved. Combined knockdown of both kcnk1a and kcnk1b resulted in a more severe phenotype, which was partially reversed by co-injection of wild-type human KCNK1 mRNA, but not by a dominant negative variant of human KCNK1 mRNA. To determine whether genetic variants in KCNK1 might cause atrial fibrillation (AF), we sequenced protein-coding regions in two independent cohorts of patients (373 subjects) and identified three non-synonymous variants, p.R171H, p.I198M and p.G236S, that were all located in highly conserved amino acid residues. In transfected mammalian cells, zebrafish and wild-type human TWIK-1 channels had a similar cellular distribution with predominant localization in the endosomal compartment. Two-electrode voltage-clamp experiments using Xenopus oocytes showed that both zebrafish and wild-type human TWIK-1 channels produced K(+) currents that are sensitive to external K(+) concentration as well as acidic pH. There were no effects of the three KCNK1 variants on cellular localization, current amplitude or reversal potential at pH7.4 or pH6. Our data indicate that TWIK-1 has a highly conserved role in cardiac function and is required for normal heart rate and atrial morphology. Despite the functional importance of TWIK-1 in the atrium, genetic variation in KCNK1 is not a common primary cause of human AF.

Gene regulatory networks in atrial fibrillation

Atrial fibrillation (AF) is the most frequent arrhythmogenic syndrome in humans. With an estimate incidence of 1%-2% in the general population, AF raises up to almost 10%-12% in 80+ years. Thus, AF represents nowadays a highly prevalent medical problem generating a large economic burden. At the electrophysiological level, distinct mechanisms have been elucidated. Yet, despite its prevalence, the genetic and molecular culprits of this pandemic cardiac electrophysiological abnormality have remained largely obscure. Molecular genetics of AF familiar cases have demonstrated that single nucleotide mutations in distinct genes encoding for ion channels underlie the onset of AF, albeit such alterations only explain a minor subset of patients with AF. In recent years, analyses by means of genome-wide association studies have unraveled a more complex picture of the etiology of AF, pointing out to distinct cardiac-enriched transcription factors, as well as to other regulatory genes. Furthermore a new layer of regulatory mechanisms have emerged, i.e., post-transcriptional regulation mediated by non-coding RNA, which have been demonstrated to exert pivotal roles in cardiac electrophysiology. In this manuscript, we aim to provide a comprehensive review of the genetic regulatory networks that if impaired exert electrophysiological abnormalities that contribute to the onset, and subsequently, on self-perpetuation of AF.

A genome-wide association study reveals susceptibility loci for myocardial infarction/coronary artery disease in Saudi Arabs

BACKGROUND

Multiple loci have been identified for coronary artery disease (CAD) by genome-wide association studies (GWAS), but no such studies on CAD incidence has been reported yet for any Middle Eastern population.

METHODS

In this study, we performed a GWAS for CAD and myocardial infarction (MI) incidence in 5668 Saudis of Arab descent using the Affymetrix Axiom Genotyping platform.

RESULTS

We describe SNPs at 16 loci that showed significant (P < 5 × 10(-8)) or suggestive GWAS association (P < 1 × 10(-5)) with CAD or MI, in the ethnic Saudi Arab population. Among the four variants reaching GWAS significance in the present study, the rs10738607_G [0.78(0.71-0.85); p = 2.17E-08] in CDNK2A/B gene was associated with CAD. Two other SNPs on the same gene, rs10757274_G [0.79(0.73-0.86); p = 2.98E-08] and rs1333045_C [0.79(0.73-0.86); p = 1.15E-08] as well as the rs9982601_T [1.38(1.23-1.55); p = 3.49E-08] on KCNE2 were associated with MI. These variants have been previously described in other populations. Several SNPs, including the rs7421388 (PLCL1) and rs12541758 (TRPA1) displaying a suggestive GWAS association (P < 1 × 10(-5)) with CAD as well as rs41411047 (RNF13), rs32793 (PDZD2) and rs4739066 (YTHDF3), similarly showing weak association with MI, were confirmed in an independent dataset. Furthermore, our estimation of heritability of CAD and MI based on observed genome-wide sharing in unrelated Saudi Arabs was approximately 33% and 44%, respectively.

CONCLUSIONS

Our study has identified susceptibility variants for CAD/MI in ethnic Arabs. These findings provide further insights into pathways contributing to the susceptibility for CAD and will enable more comprehensive genetic studies of these diseases in Middle East populations.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

NKX2-6 mutation predisposes to familial atrial fibrillation

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia and is associated with substantially increased morbidity and mortality rates. Aggregating evidence demonstrates that genetic defects are involved in the pathogenesis of AF and a number of AF-associated genes have been identified. Nevertheless, AF is a genetically heterogeneous disorder and the genetic components underpinning AF in an overwhelming majority of patients remain unclear. In this study, the entire coding exons and splice junction sites of the NK2 homeobox 6 (NKX2-6) gene, which encodes a homeodomain transcription factor important for cardiovascular development, were sequenced in 150 unrelated patients with lone AF, and a novel heterozygous NKX2-6 mutation, p.Q175H, was identified in an index patient. Genetic analysis of the available family members of the mutation carrier revealed that the mutation co-segregated with AF transmitted in an autosomal dominant pattern. The missense mutation was absent in the 200 unrelated ethnically matched healthy individuals used as controls and the altered amino acid was completely conserved evolutionarily among species. Due to unknown transcriptional targets of NKX2-6, the functional characteristics of the mutation as regards transcriptional activity were analyzed using NKX2-5 as a surrogate. Alignment between human NKX2-6 and NKX2-5 proteins displayed that the Q175H-mutant NKX2-6 was equivalent to the Q181H-mutant NKX2-5, and the introduction of Q181H into NKX2-5 significantly decreased its transcriptional activity at the atrial natriuretic factor promoter. The present study firstly associates genetically defective NKX2-6 with enhanced susceptibility to AF, providing novel insight into the molecular mechanisms underlying AF and suggesting potential strategies for the antenatal prophylaxis and personalized treatment of AF.