The genetics of atrial fibrillation: from the bench to the bedside

Annotation of susceptibility SNPs associated with atrial fibrillation

OBJECTIVE

Genome-wide association studies (GWAS) and the candidate gene based association studies have identified a panel of variants associated with atrial fibrillation (AF), however, most of the identified single nucleotide polymorphisms (SNPs) were found located within intergenic or intronic genomic regions, and whether the positive SNPs have a real biological function is unknown, and the real disease causing gene need to be studied.

RESULTS

The current results of the genetic studies including common variants identified by GWAS (338 index SNPs) and candidate gene based association studies (40 SNPs) were summarized.

CONCLUSION

Our study suggests the relationship between genetic variants and possible targeted genes, and provides insight into potential genetic pathways underlying AF incidence and development. The results may provide an encyclopedia of AF susceptibility SNPs and shed light on the functional mechanisms of AF variants identified through genetic studies.

METHODS

We summarized AF susceptibility SNPs identified by GWAS and candidate gene based association studies, and give a comprehensive functional annotation of all these AF susceptibility loci. by genomic annotation, microRNA binding prediction, promoter activity analysis, enhancer activity analysis, transcription factors binding activity prediction, expression quantitative trait loci (eQTL) analysis, long-range transcriptional regulatory function analysis, gene ontology and pathway enrichment analysis.

A Novel PITX2c Gain-of-Function Mutation, p.Met207Val, in Patients With Familial Atrial Fibrillation

Genome-wide studies have associated several genetic variants upstream of PITX2 on chromosome 4q25 with atrial fibrillation (AF), suggesting a potential role of PITX2 in AF. Our study aimed at identifying rare coding variants in PITX2 predisposing to AF. The Polymerase chain reaction sequencing of PITX2c was performed in 60 unrelated patients with idiopathic AF. The p.Met207Val variant was identified in 1 of 60 French patients with early onset AF and in none of 389 French referents. This variant, located in the inhibitory domain 1 distal to the homeodomain, was evaluated by the software MutationTaster as a disease-causing mutation with a probability of 0.999. Reporter gene assays demonstrated that p.Met207Val caused a 3.1-fold increase in transactivation activity of PITX2c in HeLa cells in comparison with its wild-type counterpart. When the variant was coexpressed with wild-type PITX2c in the HL-1 immortalized mouse atrial cell line, this gain-of-function caused an increase in the mRNA level of KCNH2 (2.6-fold), SCN1B (1.9-fold), GJA5 (3.1-fold), GJA1 (2.1-fold), and KCNQ1 in the homozygous form (1.8-fold). These genes encode for the IKr channel α subunit, the β-1 Na+ channel subunit, connexin 40, connexin 43 and the IKs channel α subunit, respectively. These conditions may contribute to the propensity to AF found in patients carrying the p.Met207Val variant. In conclusion, the present report is the first to associate a gain-of-function mutation of PITX2c with increased vulnerability to AF, therefore, restoration of normal PITX2c function may be a potential therapeutic target in AF patients.

Functional Characterization of Novel Atrial Fibrillation-Linked GJA5 (Cx40) Mutants

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia. Recently, four novel heterozygous Cx40 mutations-K107R, L223M, Q236H, and I257L-were identified in 4 of 310 unrelated AF patients and a followup genetic analysis of the mutant carriers' families showed that the mutants were present in all the affected members. To study possible alterations associated with these Cx40 mutants, including their cellular localization and gap junction (GJ) function, we expressed GFP-tagged and untagged mutants in connexin-deficient model cells. All four Cx40 mutants showed clustered localization at cell-cell junctions similar to that observed of wildtype Cx40. However, cell pairs expressing Cx40 Q236H, but not the other individual mutants, displayed a significantly lower GJ coupling conductance (Gj) than wildtype Cx40. Similarly, co-expression of Cx40 Q236H with Cx43 resulted in a significantly lower Gj. Transjunctional voltage-dependent gating (Vj gating) properties were also altered in the GJs formed by Q236H. Reduced GJ function and altered Vj gating may play a role in promoting the Q236H carriers to AF.

A new cellular type in invertebrates: first evidence of telocytes in leech Hirudo medicinalis

Telocytes, a peculiar cell type, were recently found in vertebrates. Hence this cell system has been reported as ubiquitous in the bodies of mammals and interpreted as an important player in innate immunity and tissue regeneration, it is reasonable to look for it also in invertebrates, that rely their integrity solely by innate immunity. Here we describe, at morphological and functional level, invertebrate telocytes from the body of leech Hirudo medicinalis (Annelida), suggesting how these cells, forming a resident stromal 3D network, can influence or participate in different events. These findings support the concepts that leech telocytes: i) are organized in a cellular dynamic and versatile 3D network likewise the vertebrate counterpart; ii) are an evolutionarily conserved immune-neuroendocrine system; iii) form an immuno-surveillance system of resident cells responding faster than migrating immunocytes recruited in stimulated area; iv) communicate with neighbouring cells directly and indirectly, via cell-cell contacts and soluble molecules secreted by multivesicular bodies; v) present within neo-vessels, share with immunocytes the mesodermal lineage; vi) are involved in regenerative processes. In conclusion, we propose that HmTCs, integrating so different functions, might explain the innate immune memory and can be associated with several aged related diseases.

Increased TRPM6 expression in atrial fibrillation patients contribute to atrial fibrosis

BACKGROUND

Transient receptor potential (TRP) family plays important roles in cardiovascular system. We investigated the relationship between transient receptor potential channel subfamily M6 (TRPM6) and atrial fibrosis in rheumatic heart disease patients with atrial fibrillation (AF).

METHODS

The right atrial tissue samples were obtained from 64 patients with rheumatic heart diseases who underwent heart valve replacement surgery, and composed of 34 sinus rhythm (SR) patients and 30 AF patients. Hematoxylin and Eosin (HE) staining was used to observe cross-sectional area (CSA) of myocardial cell. Masson staining and measurement of connective tissue growth factor (CTGF), transforming growth factor beta 1 (TGF-β 1), and collagen type I/III (Collagen I/III) were performed to determine atrial fibrosis. The mRNA and protein levels of TRPM6 were detected by real-time quantitative polymerase chain reaction (RT-PCR) and western blotting, respectively.

RESULTS

Marked increases were observed in CSA of myocardial cell and myocardial collagen volume fraction in AF group compared with the SR group (all P<0.05). The mRNA levels of myocardial fibrosis markers (CTGF, TGF-beta 1, Collagen I/III) in AF group increased significantly compared to the SR group (all P<0.05). TRPM6 mRNA and protein levels in AF group were elevated markedly in comparison with SR group (P<0.01).

CONCLUSION

These findings revealed that increased TRPM6 mRNA and protein levels may contribute to atrial fibrosis, and suggested that TRPM6 might be involved in AF development by promoting fibrogenesis.

An atrial-fibrillation-linked connexin40 mutant is retained in the endoplasmic reticulum and impairs the function of atrial gap-junction channels

Connexin40 (Cx40)-containing gap-junction channels are expressed in the atrial myocardium and provide a low-resistance passage for rapid impulse propagation. A germline mutation in the GJA5 gene, which encodes Cx40, resulting in a truncated Cx40 (Q49X) was identified in a large Chinese family with lone (idiopathic) atrial fibrillation (AF). This mutation co-segregated with seven AF probands in an autosomal-dominant way over generations. To test the hypothesis that this Cx40 mutant affects the distribution and function of atrial gap junctions, we studied the Q49X mutant in gap-junction-deficient HeLa and N2A cells. The Q49X mutant, unlike wild-type Cx40, was typically localized in the cytoplasm and failed to form gap-junction plaques at cell-cell interfaces. When the Q49X mutant was co-expressed with Cx40 or Cx43, the mutant substantially reduced the gap-junction plaque formation of Cx40 and Cx43. Electrophysiological studies revealed no electrical coupling of cell pairs expressing the mutant alone and a significant decrease in the coupling conductance when the mutant was co-expressed with Cx40 or Cx43. Further colocalization experiments with the organelle residential proteins indicate that Q49X was retained in the endoplasmic reticulum. These findings provide evidence that the Q49X mutant is capable of impairing gap-junction distribution and function of key atrial connexins, which might play a role in the predisposition to and onset of AF.

Atrial fibrillation-linked GJA5/connexin40 mutants impaired gap junctions via different mechanisms

The gap junctions (GJs) formed by Cx40 and Cx43 provide a low resistance passage allowing for rapid propagation of action potentials. Sporadic somatic mutations in GJA5 (encoding Cx40) have been identified in lone atrial fibrillation (AF) patients. More recently germline autosomal dominantly inherited mutations in GJA5 have been found in early onset lone AF patients in several families over generations. Characterizations of these AF-linked Cx40 mutants in model cells and in patient tissues revealed that some of the mutants reduced GJ channel function due to an impaired trafficking or channel formation. While others showed a gain-of-function in hemichannels. These functional alterations in GJs or hemichannel may play an important role in the pathogenesis of AF in the mutant carriers.

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.

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.

Genetic ACE I/D polymorphism and recurrence of atrial fibrillation after catheter ablation

BACKGROUND

The angiotensin-converting enzyme (ACE) deletion allele, ACE D, is associated with increased cardiac ACE activity, cardiac fibrosis, and adverse outcomes in cardiovascular disease and has been linked with failure of antiatrial fibrillation (anti-AF) drug treatment. This study tested the hypothesis that the ACE gene insertion/deletion polymorphism associates with AF recurrence after catheter ablation.

METHODS AND RESULTS

In 238 consecutive patients (69% male; mean age, 58±11 years) undergoing catheter ablation of paroxysmal (59%) or persistent (41%) AF, the ACE insertion/deletion polymorphism was genotyped using polymerase chain reaction. After a blanking period of 3 months, AF recurrence (defined as any atrial arrhythmia lasting ≥30 s) was detected using serial 7-day Holter ECG recordings after 3, 6, and 12 months. AF recurrence was observed in 39% and was associated with persistent AF, longer history of AF, previous antiarrhythmic drug use, previous use of diuretics, increased left atrial diameter, increased left ventricular end-diastolic diameter, additional linear ablation lesions, and ACE DD polymorphism. In multivariable analysis, left atrial diameter (odds ratio, 1.111; 95% confidence interval, 1.040-1.187; P=0.002) and ACE DD genotype (odds ratio, 2.251; 95% confidence interval, 1.056-4.798; P=0.036) remained predictors for AF recurrence.

CONCLUSIONS

Left atrial enlargement and the ACE DD polymorphism are predictors for AF recurrence after catheter ablation. The association between the ACE DD polymorphism and AF recidivism supports the use of genetic data for predicting response to AF therapies and highlights the role of fibrosis in AF development.

Telocytes: ultrastructural, immunohistochemical and electrophysiological characteristics in human myometrium

Telocytes (TCs) have been described in various organs and species (www.telocytes.com) as cells with telopodes (Tps) – very long cellular extensions with an alternation of thin segments (podomers) and dilated portions (podoms). We examined TCs using electron microscopy (EM), immunohistochemistry (IHC), immunofluorescence (IF), time-lapse videomicroscopy and whole-cell patch voltage clamp. EM showed a three-dimensional network of dichotomous-branching Tps, a labyrinthine system with homocellular and heterocellular junctions. Tps release extracellular vesicles (mean diameter of 160.6±6.9 nm in non-pregnant myometrium and 171.6±4.6 nm in pregnant myometrium), sending macromolecular signals to neighbouring cells. Comparative measurements (non-pregnant and pregnant myometrium) of podomer thickness revealed values of 81.94±1.77 vs 75.53±1.81 nm, while the podoms' diameters were 268.6±8.27 vs 316.38±17.56 nm. IHC as well as IF revealed double c-kit and CD34 positive results. Time-lapse videomicroscopy of cell culture showed dynamic interactions between Tps and myocytes. In non-pregnant myometrium, patch-clamp recordings of TCs revealed a hyperpolarisation-activated chloride inward current with calcium dependence and the absence of L-type calcium channels. TCs seem to have no excitable properties similar to the surrounding smooth muscle cells (SMCs). In conclusion, this study shows the presence of TCs as a distinct cell type in human non-pregnant and pregnant myometrium and describes morphometric differences between the two physiological states. In addition, we provide a preliminary in vitro electrophysiological evaluation of the non-pregnant state, suggesting that TCs could influence timing of the contractile activity of SMCs.

Variant rs2200733 on chromosome 4q25 confers increased risk of atrial fibrillation: evidence from a meta-analysis

INTRODUCTION

Several genome-wide association studies have identified rs2200733, a single-nucleotide polymorphism (SNP) at 4q25 to be the most common chromosomal variant present in patients with atrial fibrillation (AF). We aimed to explore the association of rs2200733 with AF through a systematic review and meta-analysis.

METHOD

An extensive literature search was performed on PubMed, and other databases using the key words "genetics" and "AF." Seven case-control studies evaluating the association via multivariate analysis were identified including a total of 83,335 subjects (10,546 with AF, 72,789 referent individuals without AF). Meta-analytic estimates were derived using random effects models. Potential sources of heterogeneity were examined in sensitivity analyses, and publication biases were estimated.

RESULT

At pooled analysis, there was a strong independent association between the variant rs2200733 and the risk of AF (OR 1.89 [95% CI 1.62-2.16], P < 0.001). Minor allelic frequencies for SNP rs22000733 were significantly more prevalent in AF population than non-AF. Metaregression results revealed that country of descent (logOR 0.38, P = 0.45) or site of study (logOR: -0.16, P = 0.41) did not moderate the overall effect size.

CONCLUSION

Variant rs2200733 on chromosome 4q25 independently confers increased risk of AF. This finding will aid in improving our understanding of AF pathophysiology, risk prediction, and stratification of treatment strategy.

Experimental acute myocardial infarction: telocytes involvement in neo-angiogenesis

We used rat experimental myocardial infarction to study the ultrastructural recovery, especially neo-angiogenesis in the infarction border zone. We were interested in the possible role(s) of telocytes (TCs), a novel type of interstitial cell very recently discovered in myocardim (see http://www.telocytes.com). Electron microscopy, immunocytochemistry and analysis of several proangiogenic microRNAs provided evidence for TC involvement in neo-angiogenesis after myocardial infarction. Electron microscopy showed the close spatial association of TCs with neoangiogenetic elements. Higher resolution images provided the following information: (a) the intercellular space between the abluminal face of endothelium and its surrounding TCs is frequently less than 50 nm; (b) TCs establish multiple direct nanocontacts with endothelial cells, where the extracellular space seems obliterated; such nanocontacts have a length of 0.4–1.5 μm; (c) the absence of basal membrane on the abluminal face of endothelial cell. Besides the physical contacts (either nanoscopic or microscopic) TCs presumably contribute to neo-angiognesis via paracrine secretion (as shown by immunocytochemistry for VEGF or NOS2). Last but not least, TCs contain measurable quantities of angiogenic microRNAs (e.g. let-7e, 10a, 21, 27b, 100, 126-3p, 130a, 143, 155, 503). Taken together, the direct (physical) contact of TCs with endothelial tubes, as well as the indirect (chemical) positive influence within the ‘angiogenic zones’, suggests an important participation of TCs in neo-angiogenesis during the late stage of myocardial infarction.

Cardiac telocytes - their junctions and functional implications

Telocytes (TCs) form a cardiac network of interstitial cells. Our previous studies have shown that TCs are involved in heterocellular contacts with cardiomyocytes and cardiac stem/progenitor cells. In addition, TCs frequently establish ‘stromal synapses’ with several types of immunoreactive cells in various organs (www.telocytes.com). Using electron microscopy (EM) and electron microscope tomography (ET), we further investigated the interstitial cell network of TCs and found that TCs form ‘atypical’ junctions with virtually all types of cells in the human heart. EM and ET showed different junction types connecting TCs in a network (puncta adhaerentia minima, processus adhaerentes and manubria adhaerentia). The connections between TCs and cardiomyocytes are ‘dot’ junctions with nanocontacts or asymmetric junctions. Junctions between stem cells and TCs are either ‘stromal synapses’ or adhaerens junctions. An unexpected finding was that TCs have direct cell–cell (nano)contacts with Schwann cells, endothelial cells and pericytes. Therefore, ultrastructural analysis proved that the cardiac TC network could integrate the overall ‘information’ from vascular system (endothelial cells and pericytes), nervous system (Schwann cells), immune system (macrophages, mast cells), interstitium (fibroblasts, extracellular matrix), stem cells/progenitors and working cardiomyocytes. Generally, heterocellular contacts occur by means of minute junctions (point contacts, nanocontacts and planar contacts) and the mean intermembrane distance is within the macromolecular interaction range (10–30 nm). In conclusion, TCs make a network in the myocardial interstitium, which is involved in the long-distance intercellular signaling coordination. This integrated interstitial system appears to be composed of large homotropic zones (TC–TC junctions) and limited (distinct) heterotropic zones (heterocellular junctions of TCs).

Potassium-channel mutations and cardiac arrhythmias--diagnosis and therapy

The coordinated generation and propagation of action potentials within cardiomyocytes creates the intrinsic electrical stimuli that are responsible for maintaining the electromechanical pump function of the human heart. The synchronous opening and closing of cardiac Na(+), Ca(2+), and K(+) channels corresponds with the activation and inactivation of inward depolarizing (Na(+) and Ca(2+)) and outward repolarizing (K(+)) currents that underlie the various phases of the cardiac action potential (resting, depolarization, plateau, and repolarization). Inherited mutations in pore-forming α subunits and accessory β subunits of cardiac K(+) channels can perturb the atrial and ventricular action potential and cause various cardiac arrhythmia syndromes, including long QT syndrome, short QT syndrome, Brugada syndrome, and familial atrial fibrillation. In this Review, we summarize the current understanding of the molecular and cellular mechanisms that underlie K(+)-channel-mediated arrhythmia syndromes. We also describe translational advances that have led to the emerging role of genetic testing and genotype-specific therapy in the diagnosis and clinical management of individuals who harbor pathogenic mutations in genes that encode α or β subunits of cardiac K(+) channels.

Telocytes and putative stem cells in the lungs: electron microscopy, electron tomography and laser scanning microscopy

This study describes a novel type of interstitial (stromal) cell — telocytes (TCs) — in the human and mouse respiratory tree (terminal and respiratory bronchioles, as well as alveolar ducts). TCs have recently been described in pleura, epicardium, myocardium, endocardium, intestine, uterus, pancreas, mammary gland, etc. (see www.telocytes.com). TCs are cells with specific prolongations called telopodes (Tp), frequently two to three per cell. Tp are very long prolongations (tens up to hundreds of μm) built of alternating thin segments known as podomers (≤ 200 nm, below the resolving power of light microscope) and dilated segments called podoms, which accommodate mitochondria, rough endoplasmic reticulum and caveolae. Tp ramify dichotomously, making a 3-dimensional network with complex homo- and heterocellular junctions. Confocal microscopy reveals that TCs are c-kit- and CD34-positive. Tp release shed vesicles or exosomes, sending macromolecular signals to neighboring cells and eventually modifying their transcriptional activity. At bronchoalveolar junctions, TCs have been observed in close association with putative stem cells (SCs) in the subepithelial stroma. SCs are recognized by their ultrastructure and Sca-1 positivity. Tp surround SCs, forming complex TC-SC niches (TC-SCNs). Electron tomography allows the identification of bridging nanostructures, which connect Tp with SCs. In conclusion, this study shows the presence of TCs in lungs and identifies a TC-SC tandem in subepithelial niches of the bronchiolar tree. In TC-SCNs, the synergy of TCs and SCs may be based on nanocontacts and shed vesicles.