Monogenic atrial fibrillation as pathophysiological paradigms

Coexistent HCN4 and GATA5 Rare Variants and Atrial Fibrillation in a Large Spanish Family

BACKGROUND

Familial association of atrial fibrillation (AF) can involve single gene variants related to known arrhythmogenic mechanisms; however, genome-wide association studies often disclose complex genetic variants in familial and nonfamilial AF, making it difficult to relate to known pathogenetic mechanisms.

METHODS

The finding of 4 siblings with AF led to studying 47 members of a family. Long-term Holter monitoring (average 298 hours) ruled out silent AF. Whole-exome sequencing was performed, and variants shared by the index cases were filtered and prioritised according to current recommendations. HCN4 currents (IHCN4) were recorded in Chinese hamster ovary cells expressing human p.P1163H or native HCN4 channels with the use of the patch-clamp technique, and topologically associating domain analyses of GATA5 variant were performed.

RESULTS

The clinical study diagnosed 2 more AF cases. Five family members carried the heterozygous p.P1163H HCN4 variant, 14 carried the intronic 20,61040536,G,A GATA5 rare variant, and 9 carried both variants (HCN4+GATA5). Five of the 6 AF cases (onset age ranging from 33 to 70 years) carried both variants and 1 carried the GATA5 variant alone. Multivariate analysis showed that the presence of HCN4+GATA5 variants significantly increased AF risk (odds ratio 32.7, 95% confidence interval 1.8-591.4) independently from age, hypertension, and overweight. Functional testing showed that IHCN4 generated by heterozygous p.P1163H were normal. Topologically associating domain analysis suggested that GATA5 could affect the expression of many genes, including those encoding microRNA-1.

CONCLUSION

The coincidence of 2 rare gene variants was independently associated with AF, but functional studies do not allow the postulation of the arrhythmogenic mechanisms involved.

A Genomic Link From Heart Failure to Atrial Fibrillation Risk: FOG2 Modulates a TBX5/GATA4-Dependent Atrial Gene Regulatory Network

BACKGROUND

The relationship between heart failure (HF) and atrial fibrillation (AF) is clear, with up to half of patients with HF progressing to AF. The pathophysiological basis of AF in the context of HF is presumed to result from atrial remodeling. Upregulation of the transcription factor FOG2 (friend of GATA2; encoded by ZFPM2) is observed in human ventricles during HF and causes HF in mice.

METHODS

FOG2 expression was assessed in human atria. The effect of adult-specific FOG2 overexpression in the mouse heart was evaluated by whole animal electrophysiology, in vivo organ electrophysiology, cellular electrophysiology, calcium flux, mouse genetic interactions, gene expression, and genomic function, including a novel approach for defining functional transcription factor interactions based on overlapping effects on enhancer noncoding transcription.

RESULTS

FOG2 is significantly upregulated in the human atria during HF. Adult cardiomyocyte-specific FOG2 overexpression in mice caused primary spontaneous AF before the development of HF or atrial remodeling. FOG2 overexpression generated arrhythmia substrate and trigger in cardiomyocytes, including calcium cycling defects. We found that FOG2 repressed atrial gene expression promoted by TBX5. FOG2 bound a subset of GATA4 and TBX5 co-bound genomic locations, defining a shared atrial gene regulatory network. FOG2 repressed TBX5-dependent transcription from a subset of co-bound enhancers, including a conserved enhancer at the Atp2a2 locus. Atrial rhythm abnormalities in mice caused by Tbx5 haploinsufficiency were rescued by Zfpm2 haploinsufficiency.

CONCLUSIONS

Transcriptional changes in the atria observed in human HF directly antagonize the atrial rhythm gene regulatory network, providing a genomic link between HF and AF risk independent of atrial remodeling.

hiPSC-derived cardiomyocytes as a model to study the role of small-conductance Ca2+-activated K+ (SK) ion channel variants associated with atrial fibrillation

Atrial fibrillation (AF), the most common arrhythmia, has been associated with different electrophysiological, molecular, and structural alterations in atrial cardiomyocytes. Therefore, more studies are required to elucidate the genetic and molecular basis of AF. Various genome-wide association studies (GWAS) have strongly associated different single nucleotide polymorphisms (SNPs) with AF. One of these GWAS identified the rs13376333 risk SNP as the most significant one from the 1q21 chromosomal region. The rs13376333 risk SNP is intronic to the KCNN3 gene that encodes for small conductance calcium-activated potassium channels type 3 (SK3). However, the functional electrophysiological effects of this variant are not known. SK channels represent a unique family of K+ channels, primarily regulated by cytosolic Ca2+ concentration, and different studies support their critical role in the regulation of atrial excitability and consequently in the development of arrhythmias like AF. Since different studies have shown that both upregulation and downregulation of SK3 channels can lead to arrhythmias by different mechanisms, an important goal is to elucidate whether the rs13376333 risk SNP is a gain-of-function (GoF) or a loss-of-function (LoF) variant. A better understanding of the functional consequences associated with these SNPs could influence clinical practice guidelines by improving genotype-based risk stratification and personalized treatment. Although research using native human atrial cardiomyocytes and animal models has provided useful insights, each model has its limitations. Therefore, there is a critical need to develop a human-derived model that represents human physiology more accurately than existing animal models. In this context, research with human induced pluripotent stem cells (hiPSC) and subsequent generation of cardiomyocytes derived from hiPSC (hiPSC-CMs) has revealed the underlying causes of various cardiovascular diseases and identified treatment opportunities that were not possible using in vitro or in vivo studies with animal models. Thus, the ability to generate atrial cardiomyocytes and atrial tissue derived from hiPSCs from human/patients with specific genetic diseases, incorporating novel genetic editing tools to generate isogenic controls and organelle-specific reporters, and 3D bioprinting of atrial tissue could be essential to study AF pathophysiological mechanisms. In this review, we will first give an overview of SK-channel function, its role in atrial fibrillation and outline pathophysiological mechanisms of KCNN3 risk SNPs. We will then highlight the advantages of using the hiPSC-CM model to investigate SNPs associated with AF, while addressing limitations and best practices for rigorous hiPSC studies.

Genetics in Ischemic Stroke: Current Perspectives and Future Directions

Ischemic stroke is a heterogeneous condition influenced by a combination of genetic and environmental factors. Recent advancements have explored genetics in relation to various aspects of ischemic stroke, including the alteration of individual stroke occurrence risk, modulation of treatment response, and effectiveness of post-stroke functional recovery. This article aims to review the recent findings from genetic studies related to various clinical and molecular aspects of ischemic stroke. The potential clinical applications of these genetic insights in stratifying stroke risk, guiding personalized therapy, and identifying new therapeutic targets are discussed herein.

Utilizing human induced pluripotent stem cells to study atrial arrhythmias in the short QT syndrome

BACKGROUND

Among the monogenic inherited causes of atrial fibrillation is the short QT syndrome (SQTS), a rare channelopathy causing atrial and ventricular arrhythmias. One of the limitations in studying the mechanisms and optimizing treatment of SQTS-related atrial arrhythmias has been the lack of relevant human atrial tissues models.

OBJECTIVE

To generate a unique model to study SQTS-related atrial arrhythmias by combining the use of patient-specific human induced pluripotent stem cells (hiPSCs), atrial-specific differentiation schemes, two-dimensional tissue modeling, optical mapping, and drug testing.

METHODS AND RESULTS

SQTS (N588K KCNH2 mutation), isogenic-control, and healthy-control hiPSCs were coaxed to differentiate into atrial cardiomyocytes using a retinoic-acid based differentiation protocol. The atrial identity of the cells was confirmed by a distinctive pattern of MLC2v downregulation, connexin 40 upregulation, shorter and triangular-shaped action potentials (APs), and expression of the atrial-specific acetylcholine-sensitive potassium current. In comparison to the healthy- and isogenic control cells, the SQTS-hiPSC atrial cardiomyocytes displayed abbreviated APs and refractory periods along with an augmented rapidly activating delayed-rectifier potassium current (IKr). Optical mapping of a hiPSC-based atrial tissue model of the SQTS displayed shortened APD and altered biophysical properties of spiral waves induced in this model, manifested by accelerated spiral-wave frequency and increased rotor curvature. Both AP shortening and arrhythmia irregularities were reversed by quinidine and vernakalant treatment, but not by sotalol.

CONCLUSIONS

Patient-specific hiPSC-based atrial cellular and tissue models of the SQTS were established, which provide examples on how this type of modeling can shed light on the pathogenesis and pharmacological treatment of inherited atrial arrhythmias.

Polygenic Risk Scores for Cardiovascular Disease: A Scientific Statement From the American Heart Association

Cardiovascular disease is the leading contributor to years lost due to disability or premature death among adults. Current efforts focus on risk prediction and risk factor mitigation' which have been recognized for the past half-century. However, despite advances, risk prediction remains imprecise with persistently high rates of incident cardiovascular disease. Genetic characterization has been proposed as an approach to enable earlier and potentially tailored prevention. Rare mendelian pathogenic variants predisposing to cardiometabolic conditions have long been known to contribute to disease risk in some families. However, twin and familial aggregation studies imply that diverse cardiovascular conditions are heritable in the general population. Significant technological and methodological advances since the Human Genome Project are facilitating population-based comprehensive genetic profiling at decreasing costs. Genome-wide association studies from such endeavors continue to elucidate causal mechanisms for cardiovascular diseases. Systematic cataloging for cardiovascular risk alleles also enabled the development of polygenic risk scores. Genetic profiling is becoming widespread in large-scale research, including in health care-associated biobanks, randomized controlled trials, and direct-to-consumer profiling in tens of millions of people. Thus, individuals and their physicians are increasingly presented with polygenic risk scores for cardiovascular conditions in clinical encounters. In this scientific statement, we review the contemporary science, clinical considerations, and future challenges for polygenic risk scores for cardiovascular diseases. We selected 5 cardiometabolic diseases (coronary artery disease, hypercholesterolemia, type 2 diabetes, atrial fibrillation, and venous thromboembolic disease) and response to drug therapy and offer provisional guidance to health care professionals, researchers, policymakers, and patients.

Genetics of atrial fibrillation-an update of recent findings

Atrial fibrillation (AF) is a common cardiac arrhythmia and a major risk factor for stroke, heart failure, and premature death. AF has a strong genetic predisposition. This review highlights the recent findings on the genetics of AF from genome-wide association studies (GWAS) and high-throughput sequencing studies. The consensus from GWAS implies that AF is both polygenic and pleiotropic in nature. With the advent of whole-genome sequencing and whole-exome sequencing, rare variants associated with AF pathogenesis have been identified. The recent studies have contributed towards better understanding of AF pathogenesis.

Identifying Atrial Fibrillation Mechanisms for Personalized Medicine

Atrial fibrillation (AF) is a major cause of heart failure and stroke. The early maintenance of sinus rhythm has been shown to reduce major cardiovascular endpoints, yet is difficult to achieve. For instance, it is unclear how discoveries at the genetic and cellular level can be used to tailor pharmacotherapy. For non-pharmacologic therapy, pulmonary vein isolation (PVI) remains the cornerstone of rhythm control, yet has suboptimal success. Improving these therapies will likely require a multifaceted approach that personalizes therapy based on mechanisms measured in individuals across biological scales. We review AF mechanisms from cell-to-organ-to-patient from this perspective of personalized medicine, linking them to potential clinical indices and biomarkers, and discuss how these data could influence therapy. We conclude by describing approaches to improve ablation, including the emergence of several mapping systems that are in use today.

Prediction of risk factors for recurrence of atrial fibrillation in patients with arterial hypertension

This work aimed to assess the significance of risk factors for atrial fibrillation recurrence and predict the significance of these factors in the development of arterial hypertension in patients. For the first time, the authors performed prognostic analysis of atrial fibrillation recurrence probability taking into account the number and significance of risk factors. The study was conducted in 2018-2019 based on the Cardiovascular Department of Military Hospital 175, Ho Chi Minh City (Vietnam). A total of 200 patients were diagnosed with arterial hypertension of I-III degree, divided into 2 groups: Group 1 (84 patients) without recurrence of atrial fibrillation, Group 2 (116 patients) with recurrences. All patients underwent echocardiographic and veloergometric studies, as well as Holter monitoring. Group 2 consisted of a significantly higher number of overweight patients (p ≤ 0.05) compared to Group 1. Also, more patients with a prolonged period of arterial hypertension (p ≤ 0.05), high blood pressure (p ≤ 0.02), and diabetes mellitus (p ≤ 0.01) were found in Group 1. A history of atrial fibrillation was more typical for Group 2 (p ≤ 0.03). Among risk factors, arterial hypertension of III degree (0.01) and left ventricular myocardial mass index (0.02) had the highest level of significance. Maximum unfavorable prognosis of atrial fibrillation recurrence is released under the action of all three factors. Under one risk factor (arterial hypertension), the degree of risk is quite high (1.4 times), but minimal compared to the other factors. With the fibrillation, the risk of recurrence increases slightly (1.6 times, p ≤ 0.05). The risk of atrial fibrillation recurrence is maximal when all three factors are combined (with atrial dilatation attached), with the risk increasing almost 2-fold (2.2-fold, p ≤ 0.01).

Human iPSC modelling of a familial form of atrial fibrillation reveals a gain of function of If and ICaL in patient-derived cardiomyocytes

AIMS

Atrial fibrillation (AF) is the most common type of cardiac arrhythmias, whose incidence is likely to increase with the aging of the population. It is considered a progressive condition, frequently observed as a complication of other cardiovascular disorders. However, recent genetic studies revealed the presence of several mutations and variants linked to AF, findings that define AF as a multifactorial disease. Due to the complex genetics and paucity of models, molecular mechanisms underlying the initiation of AF are still poorly understood. Here we investigate the pathophysiological mechanisms of a familial form of AF, with particular attention to the identification of putative triggering cellular mechanisms, using patient's derived cardiomyocytes (CMs) differentiated from induced pluripotent stem cells (iPSCs).

METHODS AND RESULTS

Here we report the clinical case of three siblings with untreatable persistent AF whose whole-exome sequence analysis revealed several mutated genes. To understand the pathophysiology of this multifactorial form of AF we generated three iPSC clones from two of these patients and differentiated these cells towards the cardiac lineage. Electrophysiological characterization of patient-derived CMs (AF-CMs) revealed that they have higher beating rates compared to control (CTRL)-CMs. The analysis showed an increased contribution of the If and ICaL currents. No differences were observed in the repolarizing current IKr and in the sarcoplasmic reticulum calcium handling. Paced AF-CMs presented significantly prolonged action potentials and, under stressful conditions, generated both delayed after-depolarizations of bigger amplitude and more ectopic beats than CTRL cells.

CONCLUSIONS

Our results demonstrate that the common genetic background of the patients induces functional alterations of If and ICaL currents leading to a cardiac substrate more prone to develop arrhythmias under demanding conditions. To our knowledge this is the first report that, using patient-derived CMs differentiated from iPSC, suggests a plausible cellular mechanism underlying this complex familial form of AF.

Microtubule polymerization state and clathrin-dependent internalization regulate dynamics of cardiac potassium channel: Microtubule and clathrin control of KV1.5 channel

Ion channel trafficking powerfully influences cardiac electrical activity as it regulates the number of available channels at the plasma membrane. Studies have largely focused on identifying the molecular determinants of the trafficking of the atria-specific K_V1.5 channel, the molecular basis of the ultra-rapid delayed rectifier current I_Kur. Besides, regulated K_V1.5 channel recycling upon changes in homeostatic state and mechanical constraints in native cardiomyocytes has been well documented. Here, using cutting-edge imaging in live myocytes, we investigated the dynamics of this channel in the plasma membrane. We demonstrate that the clathrin pathway is a major regulator of the functional expression of K_V1.5 channels in atrial myocytes, with the microtubule network as the prominent organizer of K_V1.5 transport within the membrane. Both clathrin blockade and microtubule disruption result in channel clusterization with reduced membrane mobility and internalization, whereas disassembly of the actin cytoskeleton does not. Mobile K_V1.5 channels are associated with the microtubule plus-end tracking protein EB1 whereas static K_V1.5 clusters are associated with stable acetylated microtubules. In human biopsies from patients in atrial fibrillation associated with atrial remodeling, drastic modifications in the trafficking balance occurs together with alteration in microtubule polymerization state resulting in modest reduced endocytosis and increased recycling. Consequently, hallmark of atrial K_V1.5 dynamics within the membrane is clathrin- and microtubule- dependent. During atrial remodeling, predominance of anterograde trafficking activity over retrograde trafficking could result in accumulation ok K_V1.5 channels in the plasma membrane.

DACT2 regulates structural and electrical atrial remodeling in atrial fibrillation

Background

Atrial fibrillation (AF) is the most common sustained arrhythmia. DACT2 is a novel and important mediator of signaling pathways. The aim of this study was to investigate the clinical significance and functions of DACT2 expression in AF.

Methods

Immunohistochemistry was used to detect the DACT2 expression pattern in valvular disease patients. DACT2 was overexpressed in HL-1 cells and primary atrial fibroblasts. The expression levels of the potassium channel, the L-type calcium current channel, sodium ion channel proteins and collagen proteins were detected by real-time polymerase chain reaction (RT-PCR). The proteins involved in the Wnt and TGF-β signaling pathways were detected after DACT2 overexpression by western blotting.

Results

DACT2 expression was significantly associated with AF (P=0.016). The fibrosis ratio in the strong DACT2 expression group was significantly lower than that in the weak DACT2 expression group (weak: 0.198±0.091, strong: 0.129±0.064, P=0.048), and a negative correlation between DACT2 expression levels and fibrosis severity was observed (Spearman rho =−0.476, P=0.010). DACT2 significantly increased the expression levels of KCNE5 and decreased the levels of KCNH2 and SCN5A. Overexpression of DACT2 significantly inhibited the expression of collagen I and collagen III in primary rat atrial fibroblasts. DACT2 could facilitate β-catenin accumulation by reducing its phosphorylation at Thr41/Ser45 in HL-1 cells and inhibit the TGF-β signaling pathway in primary atrial fibroblasts.

Conclusions

DACT2 played a role in AF by regulating both structural and electrical atrial remodeling and by affecting β-catenin accumulation and TGF-β signaling, and it could serve as a protective factor against AF in valvular heart disease.

The 4q25 variant rs13143308T links risk of atrial fibrillation to defective calcium homoeostasis

AIMS

Single nucleotide polymorphisms on chromosome 4q25 have been associated with risk of atrial fibrillation (AF) but the exiguous knowledge of the mechanistic links between these risk variants and underlying electrophysiological alterations hampers their clinical utility. Here, we tested the hypothesis that 4q25 risk variants cause alterations in the intracellular calcium homoeostasis that predispose to spontaneous electrical activity.

METHODS AND RESULTS

Western blotting, confocal calcium imaging, and patch-clamp techniques were used to identify mechanisms linking the 4q25 risk variants rs2200733T and rs13143308T to defects in the calcium homoeostasis in human atrial myocytes. Our findings revealed that the rs13143308T variant was more frequent in patients with AF and that myocytes from carriers of this variant had a significantly higher density of calcium sparks (14.1 ± 4.5 vs. 3.1 ± 1.3 events/min, P = 0.02), frequency of transient inward currents (ITI) (1.33 ± 0.24 vs. 0.26 ± 0.09 events/min, P < 0.001) and incidence of spontaneous membrane depolarizations (1.22 ± 0.26 vs. 0.56 ± 0.17 events/min, P = 0.001) than myocytes from patients with the normal rs13143308G variant. These alterations were linked to higher sarcoplasmic reticulum calcium loading (10.2 ± 1.4 vs. 7.3 ± 0.5 amol/pF, P = 0.01), SERCA2 expression (1.37 ± 0.13 fold, P = 0.03), and RyR2 phosphorylation at ser2808 (0.67 ± 0.08 vs. 0.47 ± 0.03, P = 0.01) but not at ser2814 (0.28 ± 0.14 vs. 0.31 ± 0.14, P = 0.61) in patients carrying the rs13143308T risk variant. Furthermore, the presence of a risk variant or AF independently increased the ITI frequency and the increase in the ITI frequency observed in carriers of the risk variants was exacerbated in those with AF. By contrast, the presence of a risk variant did not affect the amplitude or properties of the L-type calcium current in patients with or without AF.

CONCLUSIONS

Here, we identify the 4q25 variant rs13143308T as a genetic risk marker for AF, specifically associated with excessive calcium release and spontaneous electrical activity linked to increased SERCA2 expression and RyR2 phosphorylation.

AGE-RAGE Stress in the Pathophysiology of Atrial Fibrillation and Its Treatment

Atrial fibrillation (AF) is the most common of cardiac arrhythmias. Mechanisms such as atrial structural remodeling and electrical remodeling have been implicated in the pathogenesis of AF. The data to date suggest that advanced glycation end products (AGEs) and its cell receptor RAGE (receptor for AGE) and soluble receptor (sRAGE) are involved in the pathogenesis of AF. This review focuses on the role of AGE-RAGE axis in the pathogenesis of AF. Interaction of AGE with RAGE generates reactive oxygen species, cytokines, and vascular cell adhesion molecules. sRAGE is a cytoprotective agent. The data show that serum levels of AGE and sRAGE, and expression of RAGE, are elevated in AF patients. Elevated levels of sRAGE did not protect the development of AF. This might be due to greater elevation of AGE than sRAGE. Measurement of AGE-RAGE stress (AGE/sRAGE) would be appropriate as compared with measurement of AGE or RAGE or sRAGE alone in AF patients. AGE and its interaction with RAGE can induce AF through alteration in cellular protein and extracellular matrix. AGE and its interaction with RAGE induce atrial structural and electrical remodeling. The treatment strategy should be directed toward reduction in AGE levels, suppression of RAGE expression, blocking of binding of AGE to RAGE, and elevation of sRAGE and antioxidants. In conclusion, AGE-RAGE axis is involved in the development of AF through atrial structural and electrical remodeling. The treatment modalities for AF should include lowering of AGE, suppression of RAGE, elevation of sRAGE, and use of antioxidants.

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.

Monogenic and Polygenic Contributions to Atrial Fibrillation Risk: Results From a National Biobank

RATIONALE

Genome-wide association studies have identified over 100 genetic loci for atrial fibrillation (AF); recent work described an association between loss-of-function (LOF) variants in TTN and early-onset AF.

OBJECTIVE

We sought to determine the contribution of rare and common genetic variation to AF risk in the general population.

METHODS

The UK Biobank is a population-based study of 500 000 individuals including a subset with genome-wide genotyping and exome sequencing. In this case-control study, we included AF cases and controls of genetically determined white-European ancestry; analyses were performed using a logistic mixed-effects model adjusting for age, sex, the first 4 principal components of ancestry, empirical relationships, and case-control imbalance. An exome-wide, gene-based burden analysis was performed to examine the relationship between AF and rare, high-confidence LOF variants in genes with ≥10 LOF carriers. A polygenic risk score for AF was estimated using the LDpred algorithm. We then compared the contribution of AF polygenic risk score and LOF variants to AF risk.

RESULTS

The study included 1546 AF cases and 41 593 controls. In an analysis of 9099 genes with sufficient LOF variant carriers, a significant association between AF and rare LOF variants was observed in a single gene, TTN (odds ratio, 2.71, P=2.50×10^-8). The association with AF was more significant (odds ratio, 6.15, P=3.26×10^-14) when restricting to LOF variants located in exons highly expressed in cardiac tissue (TTN_LOF). Overall, 0.44% of individuals carried TTN_LOF variants, of whom 14% had AF. Among individuals in the highest 0.44% of the AF polygenic risk score only 9.3% had AF. In contrast, the AF polygenic risk score explained 4.7% of the variance in AF susceptibility, while TTN_LOF variants only accounted for 0.2%.

CONCLUSIONS

Both monogenic and polygenic factors contribute to AF risk in the general population. While rare TTN_LOF variants confer a substantial AF penetrance, the additive effect of many common variants explains a larger proportion of genetic susceptibility to AF.

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.

Atrial Fibrillation Genetics Update: Toward Clinical Implementation

Atrial fibrillation (AF) is the most common heart rhythm disorder worldwide and may have serious cardiovascular health consequences. AF is associated with increased risk of stroke, dementia, heart failure, and death. There are several known robust, clinical risk predictors for AF, such as male sex, increasing age, and hypertension; however, during the last couple of decades, a substantive genetic component has also been established. Over the last 10 years, the discovery of novel AF-related genetic variants has accelerated, increasing our understanding of mechanisms behind AF. Current studies are focusing on mapping the polygenic structure of AF, improving risk prediction, therapeutic development, and patient-specific management. Nevertheless, it is still difficult for clinicians to interpret the role of genetics in AF prediction and management. Here, we provide an overview of relevant topics within the genetics of AF and attempt to provide some guidance on how to interpret genetic advances and their implementation into clinical decision-making.

Genetic variants on chromosomes 7p31 and 12p12 are associated with abnormal atrial electrical activation in patients with early-onset lone atrial fibrillation

BACKGROUND

Abnormal P-wave morphology (PWM) has been associated with a history of atrial fibrillation (AF) in earlier studies. Although lone AF is believed to have substantial genetic basis, studies on associations between single nucleotide polymorphisms (SNP) linked to lone AF and PWM have not been reported. We aimed to assess whether SNPs previously associated with lone AF (rs2200733, rs13376333, rs3807989, and rs11047543) are also linked to P-wave abnormalities.

METHODS

Four SNPs were studied in 176 unrelated individuals with early-onset lone AF (age at onset <50 years), median age 38 years (19-63 years), 149 men. Using sinus rhythm ECG, orthogonal PWM was classified as Type 1-positive in leads X and Y and negative in lead Z, Type 2-positive in leads X and Y and biphasic (-/+) in lead Z, Type 3-positive in lead X and biphasic in lead Y (+/-), and the remaining as atypical.

RESULTS

Two SNPs were found to be significantly associated with altered P-wave morphology distribution: rs3807989 near the gene CAV1/CAV2 and rs11047543 near the gene SOX5. Both SNPs were associated with a higher risk of non-Type 1 P-wave morphology (rs3807989: OR = 4.8, 95% CI = 2.3-10.2, p < 0.001; rs11047543: OR = 4.7, 95% CI = 1.1-20.5, p = 0.04). No association was observed for rs2200733 and rs13376333.

CONCLUSION

In this study, the two variants rs3807989 and rs11047543, previously associated with PR interval and lone AF, were associated with altered P-wave morphology distribution in patients with early-onset lone AF. These findings suggest that common genetic variants may modify atrial conduction properties.

Value of multilocus genetic risk score for atrial fibrillation in end-stage kidney disease patients in a Polish population

Genetic factors play a key role in the pathogenesis of atrial fibrillation (AF). We would like to establish an association between previously described single-nucleotide polymorphisms (SNPs) and AF in haemodialysed patients with end-stage kidney disease (ESKD-HD) as well as to assess the cumulative effect of all genotyped SNPs on AF risk. Sixteen SNPs were genotyped in 113 patients with AF-ESKD-HD and in 157 controls: without AF (NAF) and with ESKD-HD. The distribution of the risk alleles was compared in both groups and between different sub-phenotypes. The multilocus genetic risk score (GRS) was calculated to estimate the cumulative risk conferred by all SNPs. Several loci showed a trend toward an association with permanent AF (perm-AF): CAV1, Cx40 and PITX2. However, GRS was significantly higher in the AF and perm-AF groups, as compared to NAF. Three of the tested variables were independently associated with AF: male sex, history of myocardial infarction (MI) and GRS. The GRS, which combined 13 previously described SNPs, showed a significant and independent association with AF in a Polish population of patients with ESKD-HD and concomitant AF. Further studies on larger groups of patients are needed to confirm the associations.

Genetics of atrial fibrillation

PURPOSE OF REVIEW

Atrial fibrillation is a common cardiac arrhythmia with a high morbidity and mortality affecting 34 million worldwide. Current therapies are inadequate and often fail to directly address molecular mechanisms of the disease. In this review, we will provide an overview of recent advances in our understanding of the genetic underpinnings of atrial fibrillation.

RECENT FINDINGS

Large-scale genetic association studies have more than doubled the number of genetic loci associated with atrial fibrillation during the last year. Studies examining how genes at or near these loci can affect the pathogenesis of atrial fibrillation are ongoing in cellular, animal, and computational models. In addition, several recent clinical studies have also demonstrated that variants at these loci can aid in risk stratification of patients.

SUMMARY

There are now over 30 genetic loci associated with atrial fibrillation. A better understanding of how these loci relate to disease pathogenesis may provide insight into novel therapeutic targets and ultimately lead to improved clinical care.

A Chromosome 4q25 Variant is Associated with Atrial Fibrillation Recurrence After Catheter Ablation: A Systematic Review and Meta-Analysis

Background

Recent studies suggested that variants on chromosome loci 4q25, 1q21, and 16q22 were associated with atrial fibrillation recurrence after catheter ablation. In this study, we performed a systematic review and meta-analysis to explore the association between variants on chromosome loci 4q25, 1q21, and 16q22 and atrial fibrillation recurrence after catheter ablation.

Methods

We comprehensively searched the databases of MEDLINE and EMBASE from inception to January 2017. Included studies were published prospective or retrospective cohort and case control studies that compared the risk of atrial fibrillation recurrence after catheter ablation in AF patients with chromosome 4q25, 1q21, and 16q22 variants versus no variants. Single-nucleotide polymorphism rs1906617, rs2106261, rs7193343, rs2200733, rs10033464, rs13376333, and rs6843082 were included in this analysis. Data from each study were combined using the random-effects, generic inverse variance method of DerSimonian and Laird to calculate the risk ratios and 95% confidence intervals.

Results

Seven studies from January 2010 to June 2017 involving 3,322 atrial fibrillation patients were included in this meta-analysis. According to the pooled analysis, there was a strong independent association between chromosome 4q25 variant (rs2200733) and the risk of atrial fibrillation recurrence after catheter ablation (risk ratio 1.45 [95% confidence interval 1.15-1.83], P = 0.002). No association was found in other variants.

Conclusion

Our meta-analysis demonstrates a statistically significant increased risk of atrial fibrillation recurrence after catheter ablation in 4q25 variant (only in rs2200733) but not in 1q21 or 16q22 variants.

Association between KCNE1 G38S gene polymorphism and risk of atrial fibrillation: A PRISMA-compliant meta-analysis

BACKGROUND

Previous case-control studies on association between KCNE1 G38S polymorphism and risk of atrial fibrillation (AF) have been published but because of the conflicting results and small sample size of individual studies, the consolidated result is still controversial.

OBJECTIVES

The aim of this study was to explore the relationship between KCNE1 G38S polymorphism and risk of AF.

METHODS

We performed a comprehensive literature search on PubMed, Embase, OVID, Web of Science, Wan Fang, and CNKI databases up to March 10, 2017 in English and Chinese languages. Two of the authors individually extracted study data and assessed the study quality using Newcastle-Ottawa scale. Odds ratios (ORs) and 95% confidence intervals (CIs) were combined in different genetic models for evaluation using a random-effect model or fixed-effect model according to interstudy heterogeneity.

RESULTS

There were totally 14 independent case-control studies of 2810 patients and 3080 healthy controls included. Significant associations were found between KCNE1 G38S polymorphism and AF in overall population under all genetic models: allelic (OR: 1.34, 95% CI: 1.24-1.45, P < .001), homozygous (OR: 1.90, 95% CI: 1.61-2.24, P < .001), heterozygous (OR: 1.43, 95% CI: 1.21-1.68, P < .001), recessive (OR: 1.42, 95% CI: 1.20-1.69, P < .001), dominant genetic model (OR: 1.62, 95% CI: 1.39-1.89, P < .001). Subgroup analyses indicated similar association in Chinese and white.

CONCLUSIONS

The G38S polymorphism in the KCNE1 gene can significantly increase the risk of AF in both Chinese and white.

Electrocardiographic intervals associated with incident atrial fibrillation: Dissecting the QT interval

BACKGROUND

Prolongation of the QT interval has been associated with an increased risk of developing atrial fibrillation (AF), but the responsible mechanism remains unknown.

OBJECTIVES

The aims of this study were to subdivide the QT interval into its components and identify the resultant electrocardiographic interval(s) responsible for the association with AF.

METHODS

Predefined QT-interval components were assessed for association with incident AF in the Atherosclerosis Risk in Communities study using Cox proportional hazards models. Hazard ratios (HRs) were calculated per 1-SD increase in each component. Among QT-interval components exhibiting significant associations, additional analyses evaluating long extremes, defined as greater than the 95^th percentile, were performed.

RESULTS

Of the 14,625 individuals, 1505 (10.3%) were diagnosed with incident AF during a mean follow-up period of 17.6 years. After multivariable adjustment, QT-interval components involved in repolarization, but not depolarization, exhibited significant associations with incident AF, including a longer ST segment (HR 1.27; 95% confidence interval [CI] 1.14-1.41; P < .001) and a prolonged T-wave onset to T-wave peak (T-onset to T-peak) (HR 1.13; 95% CI 1.07-1.20; P < .001). Marked prolongation of the ST segment (HR 1.31; 95% CI 1.04-1.64; P = .022) and T-onset to T-peak (HR 1.36; 95% CI 1.09-1.69; P = .006) was also associated with an increased risk of incident AF.

CONCLUSION

The association between a prolonged QT interval and incident AF is primarily explained by components involved in ventricular repolarization: prolongation of the ST segment and T-onset to T-peak. These observations suggest that prolongation of phases 2 and 3 of the cardiac action potential drives the association between the QT interval and AF risk.

Gain-of-function mutations in GATA6 lead to atrial fibrillation

BACKGROUND

The genetic basis of atrial fibrillation (AF) and congenital heart disease remains incompletely understood.

OBJECTIVE

We sought to determine the causative mutation in a family with AF, atrial septal defects, and ventricular septal defects.

METHODS

We evaluated a pedigree with 16 family members, 1 with an atrial septal defect, 1 with a ventricular septal defect, and 3 with AF; we performed whole exome sequencing in 3 affected family members. Given that early-onset AF was prominent in the family, we then screened individuals with early-onset AF, defined as an age of onset <66 years, for mutations in GATA6. Variants were functionally characterized using reporter assays in a mammalian cell line.

RESULTS

Exome sequencing in 3 affected individuals identified a conserved mutation, R585L, in the transcription factor gene GATA6. In the Massachusetts General Hospital Atrial Fibrillation (MGH AF) Study, the mean age of AF onset was 47.1 ± 10.9 years; 79% of the participants were men; and there was no evidence of structural heart disease. We identified 3 GATA6 variants (P91S, A177T, and A543G). Using wild-type and mutant GATA6 constructs driving atrial natriuretic peptide promoter reporter, we found that 3 of the 4 variants had a marked upregulation of luciferase activity (R585L: 4.1-fold, P < .0001; P91S: 2.5-fold, P = .0002; A177T; 1.7-fold, P = .03). In addition, when co-overexpressed with GATA4 and MEF2C, GATA6 variants exhibited upregulation of the alpha myosin heavy chain and atrial natriuretic peptide reporter activity.

CONCLUSION

Overall, we found gain-of-function mutations in GATA6 in both a family with early-onset AF and atrioventricular septal defects as well as in a family with sporadic, early-onset AF.

Ablatogenomics: can genotype guide catheter ablation for cardiac arrhythmias?

Previously confined to the management of rare inherited arrhythmia syndromes, a role for genetics within cardiac electrophysiology has begun to emerge for more common arrhythmias, including atrial fibrillation (AF). Catheter ablation for AF is an invasive procedure effective for restoring normal rhythm, however, fails in up to 40% of those undergoing their first procedure and carries a risk for serious adverse events. Recent studies have suggested that a common genetic variant within chromosome 4q25 may be a powerful predictor of procedural success, highlighting the potential of an 'ablatogenomic' strategy. Although still in its infancy, an ablatogenomic approach for AF may facilitate delivery of ablation to those most likely to benefit, while sparing those prone to fail from its risks.

Coding and non-coding variants in the SHOX2 gene in patients with early-onset atrial fibrillation

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia with a strong genetic component. Molecular pathways involving the homeodomain transcription factor Shox2 control the development and function of the cardiac conduction system in mouse and zebrafish. Here we report the analysis of human SHOX2 as a potential susceptibility gene for early-onset AF. To identify causal variants and define the underlying mechanisms, results from 378 patients with early-onset AF before the age of 60 years were analyzed and compared to 1870 controls or reference datasets. We identified two missense mutations (p.G81E, p.H283Q), that were predicted as damaging. Transactivation studies using SHOX2 targets and phenotypic rescue experiments in zebrafish demonstrated that the p.H283Q mutation severely affects SHOX2 pacemaker function. We also demonstrate an association between a 3'UTR variant c.*28T>C of SHOX2 and AF (p = 0.00515). Patients carrying this variant present significantly longer PR intervals. Mechanistically, this variant creates a functional binding site for hsa-miR-92b-5p. Circulating hsa-miR-92b-5p plasma levels were significantly altered in AF patients carrying the 3'UTR variant (p = 0.0095). Finally, we demonstrate significantly reduced SHOX2 expression levels in right atrial appendages of AF patients compared to patients with sinus rhythm. Together, these results suggest a genetic contribution of SHOX2 in early-onset AF.

Cardiac Delayed Rectifier Potassium Channels in Health and Disease

Cardiac delayed rectifier potassium channels conduct outward potassium currents during the plateau phase of action potentials and play pivotal roles in cardiac repolarization. These include IKs, IKr and the atrial specific IKur channels. In this article, we will review their molecular identities and biophysical properties. Mutations in the genes encoding delayed rectifiers lead to loss- or gain-of-function phenotypes, disrupt normal cardiac repolarization and result in various cardiac rhythm disorders, including congenital Long QT Syndrome, Short QT Syndrome and familial atrial fibrillation. We will also discuss the prospect of using delayed rectifier channels as therapeutic targets to manage cardiac arrhythmia.

Atrial fibrillation and arterial hypertension: A common duet with dangerous consequences where the renin angiotensin-aldosterone system plays an important role

Atrial fibrillation (AF) represents the most common sustained cardiac arrhythmia, as it affects 1%-2% of the general population and up to 15% of people over 80 years. High blood pressure, due to its high prevalence in the general population, is by far the most common condition associated with AF, although a variety of diseases, including valvular, coronary heart and metabolic diseases, are held to create the substrate favouring AF. Due to the concomitance of these conditions, it is quite challenging to dissect the precise role of high blood pressure in triggering/causing AF. Hence, even though the intimate association between high blood pressure and AF has been known for decades, the underlying mechanisms remain partially unknown. Accumulating evidences point to a major role of the renin-angiotensin-aldosterone system in inducing cardiac inflammation and fibrosis, and therefore electric and structural atrial and ventricular remodelling, with changes in ions and cell junctions leading to AF development. These evidences are herein reviewed with a particular emphasis to the role of the renin-angiotensin-system aldosterone system.

Expression and function of Kv1.1 potassium channels in human atria from patients with atrial fibrillation

Voltage-gated Kv1.1 channels encoded by the Kcna1 gene are traditionally regarded as being neural-specific with no known expression or intrinsic functional role in the heart. However, recent studies in mice reveal low-level Kv1.1 expression in heart and cardiac abnormalities associated with Kv1.1-deficiency suggesting that the channel may have a previously unrecognized cardiac role. Therefore, this study tests the hypothesis that Kv1.1 channels are associated with arrhythmogenesis and contribute to intrinsic cardiac function. In intra-atrial burst pacing experiments, Kcna1-null mice exhibited increased susceptibility to atrial fibrillation (AF). The atria of Kcna1-null mice showed minimal Kv1 family ion channel remodeling and fibrosis as measured by qRT-PCR and Masson's trichrome histology, respectively. Using RT-PCR, immunocytochemistry, and immunoblotting, KCNA1 mRNA and protein were detected in isolated mouse cardiomyocytes and human atria for the first time. Patients with chronic AF (cAF) showed no changes in KCNA1 mRNA levels relative to controls; however, they exhibited increases in atrial Kv1.1 protein levels, not seen in paroxysmal AF patients. Patch-clamp recordings of isolated human atrial myocytes revealed significant dendrotoxin-K (DTX-K)-sensitive outward current components that were significantly increased in cAF patients, reflecting a contribution by Kv1.1 channels. The concomitant increases in Kv1.1 protein and DTX-K-sensitive currents in atria of cAF patients suggest that the channel contributes to the pathological mechanisms of persistent AF. These findings provide evidence of an intrinsic cardiac role of Kv1.1 channels and indicate that they may contribute to atrial repolarization and AF susceptibility.

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.

IKs Gain- and Loss-of-Function in Early-Onset Lone Atrial Fibrillation

INTRODUCTION

Atrial fibrillation (AF) is the most frequent cardiac arrhythmia. The potassium current IKs is essential for cardiac repolarization. Gain-of-function mutation in KCNQ1, the gene encoding the pore-forming α-subunit of the IKs channel (KV 7.1), was the first ion channel dysfunction to be associated with familial AF. We hypothesized that early-onset lone AF is associated with a high prevalence of mutations in KCNQ1.

METHODS AND RESULTS

We bidirectionally sequenced the entire coding sequence of KCNQ1 in 209 unrelated patients with early-onset lone AF (<40 years) and investigated the identified mutations functionally in a heterologous expression system. We found 4 nonsynonymous KCNQ1 mutations (A46T, R195W, A302V, and R670K) in 4 unrelated patients (38, 31, 39, and 36 years, respectively). None of the mutations were present in the control group (n = 416 alleles). No other mutations were found in genes previously associated with AF. The mutations A46T, R195W, and A302V have previously been associated with long-QT syndrome. In line with previous reports, we found A302V to display a pronounced loss-of-function of the IKs current, while the other mutants exhibited a gain-of-function phenotype.

CONCLUSIONS

Mutations in the IKs channel leading to gain-of-function have previously been described in familial AF, yet this is the first time a loss-of-function mutation in KCNQ1 is associated with early-onset lone AF. These findings suggest that both gain-of-function and loss-of-function of cardiac potassium currents enhance the susceptibility to AF.

Cellular and molecular electrophysiology of atrial fibrillation initiation, maintenance, and progression

Atrial fibrillation (AF) is the most common clinically relevant arrhythmia and is associated with increased morbidity and mortality. The incidence of AF is expected to continue to rise with the aging of the population. AF is generally considered to be a progressive condition, occurring first in a paroxysmal form, then in persistent, and then long-standing persistent (chronic or permanent) forms. However, not all patients go through every phase, and the time spent in each can vary widely. Research over the past decades has identified a multitude of pathophysiological processes contributing to the initiation, maintenance, and progression of AF. However, many aspects of AF pathophysiology remain incompletely understood. In this review, we discuss the cellular and molecular electrophysiology of AF initiation, maintenance, and progression, predominantly based on recent data obtained in human tissue and animal models. The central role of Ca(2+)-handling abnormalities in both focal ectopic activity and AF substrate progression is discussed, along with the underlying molecular basis. We also deal with the ionic determinants that govern AF initiation and maintenance, as well as the structural remodeling that stabilizes AF-maintaining re-entrant mechanisms and finally makes the arrhythmia refractory to therapy. In addition, we highlight important gaps in our current understanding, particularly with respect to the translation of these concepts to the clinical setting. Ultimately, a comprehensive understanding of AF pathophysiology is expected to foster the development of improved pharmacological and nonpharmacological therapeutic approaches and to greatly improve clinical management.

Destruction Of Medium Already Afected By Destructive Disorder: Fibrillating Atria Conceptually Need Therapeutic Help Rather Than Surgical Or Ablative Destruction

Atrial fibrillation (AF) as the most common supraventricular arrhythmia is scarcely amenable to contemporary treatment. Due to the diverse origin and variable clinical course of AF there is a broad spectrum of therapy options. However, optimal AF management has not become a gold standard yet. In general, the recurrence rate of AF is most often clinically unacceptable despite drug, surgical and/or ablation therapy. Substrate-based approach and ongoing ablation of atrial wall in its selected areas including the vicinity of pulmonary veins can be harmful. Applied physical factors do produce total disintegration of cardiomyocites - both intra- and inter-cellular damage which, in turn, leads to functional hypo-/inactivation of atria irrespective of whether the sinus rhythm is restored or not. In fact, iatrogenic phenomenon of ablation-induced atrial incompetence did emerge. Heterogeneity in clinical results reflects the uncertainty regarding the efficacy, risks and benefits of invasive AF therapy. In this regard the overall burden of AF may increase when using current therapy methods. Applicability of destructive techniques is yet to be fully elucidated and discussed. We hypothesize that currently used ablation and/or surgical techniques are potentially harmful since the success rates are likely achieved through violation of atrial myocardium. That is why a new and well-designed therapeutic strategy is needed. Invention of highly selective curative methods producing fibrillatory/electric blockage with concomitant saving of atrial transport function is to be encouraged.

Electroanatomic mapping and late gadolinium enhancement MRI in a genetic model of arrhythmogenic atrial cardiomyopathy

INTRODUCTION

Although atrial arrhythmias may have genetic causes, very few data are available on evaluation of the arrhythmic substrate in genetic atrial diseases in humans. In this study, we evaluate the nature and evolution of the atrial arrhythmic substrate in a genetic atrial cardiomyopathy.

METHODS AND RESULTS

Repeated electroanatomic mapping and tomographic evaluations were used to investigate the evolving arrhythmic substrate in 5 patients with isolated arrhythmogenic atrial cardiomyopathy, caused by Natriuretic Peptide Precursor A (NPPA) gene mutation. Atrial fibrosis was assessed using late gadolinium enhancement magnetic resonance imaging (LGE-MRI). The substrate of atrial tachycardia (AT) and atrial fibrillation (AF) was biatrial dilatation with patchy areas of low voltage and atrial wall scarring (in the right atrium: 68.5% ± 6.0% and 22.2% ± 10.2%, respectively). The evolution of the arrhythmic patterns to sinus node disease with atrial standstill (AS) was associated with giant atria with extensive low voltage and atrial scarring areas (in the right atrium: 99.5% ± 0.7% and 57.5% ± 33.2%, respectively). LGE-MRI-proven biatrial fibrosis (Utah stage IV) was associated with AS. Atrial conduction was slow and heterogeneous, with lines of conduction blocks. The progressive extension and spatial distribution of the scarring/fibrosis were strictly associated with the different types of arrhythmias.

CONCLUSION

The evolution of the amount and distribution of atrial scarring/fibrosis constitutes the structural substrate for the different types of atrial arrhythmias in a pure genetic model of arrhythmogenic atrial cardiomyopathy.

Lone atrial fibrillation - an overview

Atrial fibrillation (AF) sometimes develops in younger individuals without any evident cardiac or other disease. To refer to these patients who were considered to have a very favourable prognosis compared with other AF patients, the term 'lone' AF was introduced in 1953. However, there are numerous uncertainties associated with 'lone' AF, including inconsistent entity definitions, considerable variations in the reported prevalence and outcomes, etc. Indeed, increasing evidence suggests a number of often subtle cardiac alterations associated with apparently 'lone' AF, which may have relevant prognostic implications. Hence, 'lone' AF patients comprise a rather heterogeneous cohort, and may have largely variable risk profiles based on the presence (or absence) of overlooked subclinical cardiovascular risk factors or genetically determined subtle alterations at the cellular or molecular level. Whether the implementation of various cardiac imaging techniques, biomarkers and genetic information could improve the prediction of risk for incident AF and risk assessment of 'lone' AF patients, and influence the treatment decisions needs further research. In this review, we summarise the current knowledge on 'lone' AF, highlight the existing inconsistencies in the field and discuss the prognostic and treatment implications of recent insights in 'lone' AF pathophysiology.

Single-nucleotide variations in cardiac arrhythmias: prospects for genomics and proteomics based biomarker discovery and diagnostics

Cardiovascular diseases are a large contributor to causes of early death in developed countries. Some of these conditions, such as sudden cardiac death and atrial fibrillation, stem from arrhythmias—a spectrum of conditions with abnormal electrical activity in the heart. Genome-wide association studies can identify single nucleotide variations (SNVs) that may predispose individuals to developing acquired forms of arrhythmias. Through manual curation of published genome-wide association studies, we have collected a comprehensive list of 75 SNVs associated with cardiac arrhythmias. Ten of the SNVs result in amino acid changes and can be used in proteomic-based detection methods. In an effort to identify additional non-synonymous mutations that affect the proteome, we analyzed the post-translational modification S-nitrosylation, which is known to affect cardiac arrhythmias. We identified loss of seven known S-nitrosylation sites due to non-synonymous single nucleotide variations (nsSNVs). For predicted nitrosylation sites we found 1429 proteins where the sites are modified due to nsSNV. Analysis of the predicted S-nitrosylation dataset for over- or under-representation (compared to the complete human proteome) of pathways and functional elements shows significant statistical over-representation of the blood coagulation pathway. Gene Ontology (GO) analysis displays statistically over-represented terms related to muscle contraction, receptor activity, motor activity, cystoskeleton components, and microtubule activity. Through the genomic and proteomic context of SNVs and S-nitrosylation sites presented in this study, researchers can look for variation that can predispose individuals to cardiac arrhythmias. Such attempts to elucidate mechanisms of arrhythmia thereby add yet another useful parameter in predicting susceptibility for cardiac diseases.

Atrial fibrillation: the role of common and rare genetic variants

Atrial fibrillation (AF) is the most common cardiac arrhythmia affecting 1-2% of the general population. A number of studies have demonstrated that AF, and in particular lone AF, has a substantial genetic component. Monogenic mutations in lone and familial AF, although rare, have been recognized for many years. Presently, mutations in 25 genes have been associated with AF. However, the complexity of monogenic AF is illustrated by the recent finding that both gain- and loss-of-function mutations in the same gene can cause AF. Genome-wide association studies (GWAS) have indicated that common single-nucleotide polymorphisms (SNPs) have a role in the development of AF. Following the first GWAS discovering the association between PITX2 and AF, several new GWAS reports have identified SNPs associated with susceptibility of AF. To date, nine SNPs have been associated with AF. The exact biological pathways involving these SNPs and the development of AF are now starting to be elucidated. Since the first GWAS, the number of papers concerning the genetic basis of AF has increased drastically and the majority of these papers are for the first time included in a review. In this review, we discuss the genetic basis of AF and the role of both common and rare genetic variants in the susceptibility of developing AF. Furthermore, all rare variants reported to be associated with AF were systematically searched for in the Exome Sequencing Project Exome Variant Server.

A novel PITX2c loss-of-function mutation associated with familial atrial fibrillation

Atrial fibrillation (AF) represents the most prevalent form of sustained cardiac arrhythmia and contributes substantially to cardiovascular morbidity and mortality. Aggregating evidence demonstrates that genetic risk factors play an important role in the pathogenesis of AF. However, AF is a genetically heterogeneous disease and the genetic defects responsible for AF in an overwhelming majority of patients remain unclear. In the present study, the whole coding region and splice junction sites of the PITX2c gene, which encodes a paired-like homeobox transcription factor essential for normal cardiovascular development, were sequenced in 160 unrelated patients with lone AF, and a novel heterozygous mutation, c.349C > T equivalent to p.P117S, was identified in a patient with positive family history of AF. The missense mutation, which co-segregated with AF in the family with complete penetrance and was absent in 700 unrelated ethnically matched healthy individuals, altered the amino acid completely conserved evolutionarily across species and was predicted to be pathogenic by MutationTaster and PolyPhen-2. Biological assays revealed that the mutant PITX2c protein was associated with significantly decreased transcriptional activity when compared with its wild-type counterpart. The findings implicate PITX2c loss-of-function mutation in familial AF for the first time, providing novel insight into the molecular pathology of AF.

PITX2C loss-of-function mutations responsible for idiopathic atrial fibrillation

OBJECTIVE

This study aimed to identify novel PITX2c mutations responsible for idiopathic atrial fibrillation.

METHODS

A cohort of 210 unrelated patients with idiopathic atrial fibrillation and 200 unrelated, ethnically matched healthy individuals used as controls were recruited. The whole coding exons and splice junctions of the PITX2c gene, which encodes a paired-like homeobox transcription factor required for normal cardiovascular morphogenesis, were sequenced in 210 patients and 200 control subjects. The causative potentials of the identified mutations were automatically predicted by MutationTaster and PolyPhen-2. The functional characteristics of the PITX2c mutations were explored using a dual-luciferase reporter assay system.

RESULTS

Two novel heterozygous PITX2c mutations (p.Q105L and p.R122C) were identified in 2 of the 210 unrelated patients with idiopathic atrial fibrillation. These missense mutations were absent in the 400 control chromosomes and were both predicted to be pathogenic. Multiple alignments of PITX2c protein sequences across various species showed that the altered amino acids were highly evolutionarily conserved. A functional analysis demonstrated that the mutant PITX2c proteins were both associated with significantly reduced transcriptional activity compared with their wild-type counterparts.

CONCLUSION

The findings of this study associate PITX2c loss-of-function mutations with atrial fibrillation, supporting the hypothesis that dysfunctional PITX2c confers enhanced susceptibility to atrial fibrillation and suggesting potential implications for early prophylaxis and allele-specific therapy for this common arrhythmia.

Mutational spectrum of the NKX2-5 gene in patients with lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia in humans and is responsible for substantial morbidity and mortality worldwide. Emerging evidence indicates that abnormal cardiovascular development is involved in the pathogenesis of AF. In this study, the coding exons and splice sites of the NKX2-5 gene, which encodes a homeodomain-containing transcription factor essential for cardiovascular genesis, were sequenced in 146 unrelated patients with lone AF as well as the available relatives of the mutation carriers. A total of 700 unrelated ethnically matched healthy individuals used as controls were genotyped. The disease-causing potential of the identified NKX2-5 variations was predicted by MutationTaster and PolyPhen-2. The functional characteristics of the mutant NKX2-5 proteins were analyzed using a dual-luciferase reporter assay system. As a result, two heterozygous NKX2-5 mutations, including a previously reported p.E21Q and a novel p.T180A mutation, were identified in two families with AF transmitted in an autosomal dominant pattern. The mutations co-segregated with AF in the families with complete penetrance. The detected substitutions, which altered the amino acids highly conserved evolutionarily across species, were absent in 700 control individuals and were both predicted to be causative. Functional analyses demonstrated that the NKX2-5 mutants were associated with significantly decreased transcriptional activity compared with their wild-type counterpart. The findings expand the spectrum of NKX2-5 mutations linked to AF and provide additional evidence that dysfunctional NKX2-5 may confer vulnerability to AF, suggesting the potential benefit for the early prophylaxis and personalized treatment of AF.

Recent molecular insights from mutated IKS channels in cardiac arrhythmia

Co-assembly of KCNQ1 with KCNE1 generates the IKS potassium current that is vital for the proper repolarization of the cardiac action potential. Mutations in either KCNQ1 or KCNE1 genes lead to life-threatening cardiac arrhythmias causing long QT syndrome, short QT syndrome, sinus bradycardia and atrial fibrillation. Findings emerging from recent studies are beginning to provide a picture of how gain-of-function and loss-of-function mutations are associated with pleiotropic cardiac phenotypes in the clinics. In this review, we discuss recent molecular insights obtained from mutations altering different structural modules of the channel complex that are essential for proper IKS function. We present the possible molecular mechanisms underlying mutations impairing the voltage sensing functions, as well as those altering the channel regulation by phosphatidylinositol-4,5-bisphosphate, calmodulin and protein kinase A. We also discuss the significance of diseased IKS channels for adequate pharmacological targeting of cardiac arrhythmias.

Atrial fibrillation from the pathologist's perspective

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia encountered in clinical practice, is associated with increased morbidity and mortality. Electrophysiologically, it is characterized by a high rate of asynchronous atrial cell depolarization causing a loss of atrial contractile function and irregular ventricular rates. For a long time, AF was considered as a pure functional disorder without any structural background. Only in recent years, have new mapping and imaging techniques identified atrial locations, which are very often involved in the initiation and maintenance of this supraventricular arrhythmia (i.e. the distal portion of the pulmonary veins and the surrounding atrial myocardium). Morphological analysis of these myocardial sites has demonstrated significant structural remodeling as well as paved the way for further knowledge of AF natural history, pathogenesis, and treatment. This architectural myocardial disarrangement is induced by the arrhythmia itself and the very frequently associated cardiovascular disorders. At the same time, the structural remodeling is also capable of sustaining AF, thereby creating a sort of pathogenetic vicious circle. This review focuses on current understanding about the structural and genetic bases of AF with reference to their classification, pathogenesis, and clinical implications.