Familial aggregation of atrial fibrillation: a study in Danish twins

The rs3807989 G/A polymorphism in CAV1 is associated with the risk of atrial fibrillation in Chinese Han populations

BACKGROUND

A recent meta-analysis of several genome-wide association studies identified six new susceptibility single nucleotide polymorphisms (SNPs) for atrial fibrillation (AF) in individuals of the European ancestry. We aimed to replicate the associations between these SNPs and the risk of AF in a Chinese Han population.

METHODS

We genotyped six SNPs (rs3903239 in PRRX1, rs3807989 in CAV1, rs10821415 in C9orf3, rs10824026 in SYNPO2L, rs1152591 in SYNE2, and rs7164883 in HCN4) using the middle-throughput iPLEX Sequenom MassARRAY platform. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated in logistic regression models.

RESULTS

We enrolled a total of 1,593 Chinese Han origin individuals in the study, including 597 AF patients and 996 non-AF controls. Among the six SNPs analyzed in the study, the SNP rs3807989 in CAV1 on chromosome 7q31 was found to be significantly associated with a decreased risk of AF (crude OR = 0.76, 95% CI: 0.64-0.89, P = 0.001; adjusted OR = 0.75, 95% CI: 0.63-0.89, P = 0.001). There were no significant associations between the other five loci and AF risk.

CONCLUSION

Our results confirmed that CAV1 rs3807989 may contribute to a decreased AF risk in Chinese Han populations. However, further validation studies with different ethnic backgrounds and biological function analyses are warranted to confirm our finding.

Twelve-single nucleotide polymorphism genetic risk score identifies individuals at increased risk for future atrial fibrillation and stroke

BACKGROUND AND PURPOSE

Atrial fibrillation (AF) is prevalent and there is a clinical need for biomarkers to identify individuals at higher risk for AF. Fixed throughout a life course and assayable early in life, genetic biomarkers may meet this need. Here, we investigate whether multiple single nucleotide polymorphisms together as an AF genetic risk score (AF-GRS) can improve prediction of one's risk for AF.

METHODS

In 27 471 participants of the Malmö Diet and Cancer Study, a prospective, community-based cohort, we used Cox models that adjusted for established AF risk factors to assess the association of AF-GRS with incident AF and ischemic stroke. Median follow-up was 14.4 years for incident AF and 14.5 years for ischemic stroke. The AF-GRS comprised 12 single nucleotide polymorphisms that had been previously shown to be associated with AF at genome-wide significance.

RESULTS

During follow-up, 2160 participants experienced a first AF event and 1495 had a first ischemic stroke event. Participants in the top AF-GRS quintile were at increased risk for incident AF (hazard ratio, 2.00; 95% confidence interval, 1.73-2.31; P=2.7×10(-21)) and ischemic stroke (hazard ratio, 1.23; 95% confidence interval, 1.04-1.46; P=0.02) when compared with the bottom quintile. Addition of the AF-GRS to established AF risk factors modestly improved both discrimination and reclassification (P<0.0001 for both).

CONCLUSIONS

An AF-GRS can identify 20% of individuals who are at ≈2-fold increased risk for incident AF and at 23% increased risk for ischemic stroke. Targeting diagnostic or therapeutic interventions to this subset may prove clinically useful.

Global epidemiology of atrial fibrillation

Atrial fibrillation (AF) is a major public health burden worldwide, and its prevalence is set to increase owing to widespread population ageing, especially in rapidly developing countries such as Brazil, China, India, and Indonesia. Despite the availability of epidemiological data on the prevalence of AF in North America and Western Europe, corresponding data are limited in Africa, Asia, and South America. Moreover, other observations suggest that the prevalence of AF might be underestimated-not only in low-income and middle-income countries, but also in their high-income counterparts. Future studies are required to provide precise estimations of the global AF burden, identify important risk factors in various regions worldwide, and take into consideration regional and ethnic variations in AF. Furthermore, in response to the increasing prevalence of AF, additional resources will need to be allocated globally for prevention and treatment of AF and its associated complications. In this Review, we discuss the available data on the global prevalence, risk factors, management, financial costs, and clinical burden of AF, and highlight the current worldwide inadequacy of its treatment.

The clinical profile and pathophysiology of atrial fibrillation: relationships among clinical features, epidemiology, and mechanisms

Atrial fibrillation (AF) is the most common arrhythmia (estimated lifetime risk, 22%-26%). The aim of this article is to review the clinical epidemiological features of AF and to relate them to underlying mechanisms. Long-established risk factors for AF include aging, male sex, hypertension, valve disease, left ventricular dysfunction, obesity, and alcohol consumption. Emerging risk factors include prehypertension, increased pulse pressure, obstructive sleep apnea, high-level physical training, diastolic dysfunction, predisposing gene variants, hypertrophic cardiomyopathy, and congenital heart disease. Potential risk factors are coronary artery disease, kidney disease, systemic inflammation, pericardial fat, and tobacco use. AF has substantial population health consequences, including impaired quality of life, increased hospitalization rates, stroke occurrence, and increased medical costs. The pathophysiology of AF centers around 4 general types of disturbances that promote ectopic firing and reentrant mechanisms, and include the following: (1) ion channel dysfunction, (2) Ca(2+)-handling abnormalities, (3) structural remodeling, and (4) autonomic neural dysregulation. Aging, hypertension, valve disease, heart failure, myocardial infarction, obesity, smoking, diabetes mellitus, thyroid dysfunction, and endurance exercise training all cause structural remodeling. Heart failure and prior atrial infarction also cause Ca(2+)-handling abnormalities that lead to focal ectopic firing via delayed afterdepolarizations/triggered activity. Neural dysregulation is central to atrial arrhythmogenesis associated with endurance exercise training and occlusive coronary artery disease. Monogenic causes of AF typically promote the arrhythmia via ion channel dysfunction, but the mechanisms of the more common polygenic risk factors are still poorly understood and under intense investigation. Better recognition of the clinical epidemiology of AF, as well as an improved appreciation of the underlying mechanisms, is needed to develop improved methods for AF prevention and management.

Lone atrial fibrillation: does it exist?

The historical origin of the term "lone atrial fibrillation" (AF) predates by 60 years our current understanding of the pathophysiology of AF, the multitude of known etiologies for AF, and our ability to image and diagnose heart disease. The term was meant to indicate AF in patients for whom subsequent investigations could not demonstrate heart disease, but for many practitioners has become synonymous with "idiopathic AF." As the list of heart diseases has expanded and diagnostic techniques have improved, the prevalence of lone AF has fallen. The legacy of the intervening years is that definitions of lone AF in the literature are inconsistent so that studies of lone AF are not comparable. Guidelines provide a vague definition of lone AF but do not provide direction about how much or what kind of imaging and other testing are necessary to exclude heart disease. There has been an explosion in the understanding of the pathophysiology of AF in the last 20 years in particular. Nevertheless, there are no apparently unique mechanisms for AF in patients categorized as having lone AF. In addition, the term "lone AF" is not invariably useful in making treatment decisions, and other tools for doing so have been more thoroughly and carefully validated. It is, therefore, recommended that use of the term "lone AF" be avoided.

Novel genetic markers associate with atrial fibrillation risk in Europeans and Japanese

OBJECTIVES

This study sought to identify nonredundant atrial fibrillation (AF) genetic susceptibility signals and examine their cumulative relations with AF risk.

BACKGROUND

AF-associated loci span broad genomic regions that may contain multiple susceptibility signals. Whether multiple signals exist at AF loci has not been systematically explored.

METHODS

We performed association testing conditioned on the most significant, independently associated genetic markers at 9 established AF loci using 2 complementary techniques in 64,683 individuals of European ancestry (3,869 incident and 3,302 prevalent AF cases). Genetic risk scores were created and tested for association with AF in Europeans and an independent sample of 11,309 individuals of Japanese ancestry (7,916 prevalent AF cases).

RESULTS

We observed at least 4 distinct AF susceptibility signals on chromosome 4q25 upstream of PITX2, but not at the remaining 8 AF loci. A multilocus score comprised 12 genetic markers demonstrated an estimated 5-fold gradient in AF risk. We observed a similar spectrum of risk associated with these markers in Japanese. Regions containing AF signals on chromosome 4q25 displayed a greater degree of evolutionary conservation than the remainder of the locus, suggesting that they may tag regulatory elements.

CONCLUSIONS

The chromosome 4q25 AF locus is architecturally complex and harbors at least 4 AF susceptibility signals in individuals of European ancestry. Similar polygenic AF susceptibility exists between Europeans and Japanese. Future work is necessary to identify causal variants, determine mechanisms by which associated loci predispose to AF, and explore whether AF susceptibility signals classify individuals at risk for AF and related morbidity.

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.

Targeted sequencing in candidate genes for atrial fibrillation: the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Targeted Sequencing Study

BACKGROUND

Genome-wide association studies (GWAS) have identified common genetic variants that predispose to atrial fibrillation (AF). It is unclear whether rare and low-frequency variants in genes implicated by such GWAS confer additional risk of AF.

OBJECTIVE

To study the association of genetic variants with AF at GWAS top loci.

METHODS

In the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Targeted Sequencing Study, we selected and sequenced 77 target gene regions from GWAS loci of complex diseases or traits, including 4 genes hypothesized to be related to AF (PRRX1, CAV1, CAV2, and ZFHX3). Sequencing was performed in participants with (n = 948) and without (n = 3330) AF from the Atherosclerosis Risk in Communities Study, the Cardiovascular Health Study, the Framingham Heart Study, and the Massachusetts General Hospital.

RESULTS

One common variant (rs11265611; P = 1.70 × 10(-6)) intronic to IL6R (interleukin-6 receptor gene) was significantly associated with AF after Bonferroni correction (odds ratio 0.70; 95% confidence interval 0.58-0.85). The variant was not genotyped or imputed by prior GWAS, but it is in linkage disequilibrium (r(2) = .69) with the single-nucleotide polymorphism, with the strongest association with AF so far at this locus (rs4845625). In the rare variant joint analysis, damaging variants within the PRRX1 region showed significant association with AF after Bonferroni correction (P = .01).

CONCLUSIONS

We identified 1 common single-nucleotide polymorphism and 1 gene region that were significantly associated with AF. Future sequencing efforts with larger sample sizes and more comprehensive genome coverage are anticipated to identify additional AF-related variants.

Atrial Fibrillation and Cardiovascular Comorbidities, Survival and Mortality: A Real-Life Observational Study

Background

Atrial fibrillation (AF) is the most common cardiac tachyarrhythmia encountered in clinical practice affecting up to 10% of the population over 60 years old and its prevalence rises with age. The main goals were to characterize the AF patient population after the initial diagnosis of AF and to determine overall survival.

Methods

It is a real-life observational study of 269 subjects with an AF diagnosis over 60 years old randomly selected. The collected variables were: sociodemographic, cardiovascular complications/comorbidities (CVCs) included in the CHA₂DS₂-VASc and HAS_BLED scores, drug assigned as clinical treatment, mean range INR and CVCs and death dates (all-cause mortality). The survival curve and the risk of death were assessed using Kaplan-Meier survival curve and comparisons with log-rank.

Results

The average following time was 6.2 ± 3.7 years (0.2-20.4). Eleven point five percent died. Sixty-five point four percent had some CVCs. There were no differences in the overall incidence of CVCs by gender. The survival probability was 0.86 ± DE 0.03 among men and 0.90 ± DE 0.04 among women without differences. Thirty-six point eight percent (95% CI: 30.8 - 42.7) were diagnosed vascular complications before AF diagnosis, being ischemic cardiopathy (24.2%) and ischemic stroke (23.2%) the most frequent. The mortality is higher (P < 0.036) among those who suffered ≥ 3 vascular complications and significantly lower among those treated with statins (P = 0.032). After AF diagnosis, the most frequent was the cardiac heart failure (46.7%), significantly higher among women (P = 0.037). The mortality is significantly lower in those treated with OAC (P = 0.003).

Conclusions

AF is associated with ischemic heart disease, ischemic stroke and congestive heart failure, but the average mortality age is not different from the global population in Spain and Catalonia.

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.

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.

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.

A novel PITX2c loss‑of‑function mutation underlies lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia responsible for substantial morbidity and significantly increased mortality rates. A growing body of evidence documents the important role of genetic defects in the pathogenesis of AF. However, AF is a heterogeneous disease and the genetic determinants for AF in an overwhelming majority of patients remain unknown. In the present study, a cohort of 100 unrelated patients with lone AF and a total of 200 unrelated, ethnically matched healthy individuals used as controls, were recruited. The whole coding exons and splice junctions of the pituitary homeobox 2c (PITX2c) gene, which encodes a paired‑like homeobox transcription factor required for normal cardiovascular morphogenesis, were sequenced in the 100 patients and 200 control subjects. The causative potential of the identified mutation of PITX2c was predicted by MutationTaster and PolyPhen‑2. The functional characteristics of the PITX2c mutation were assayed using a dual‑luciferase reporter assay system. Based on the results, a novel heterozygous PITX2c mutation (p.T97A) was identified in a patient with AF. The missense mutation was absent in the 400 reference chromosomes and was automatically predicted to be disease‑causing. Multiple alignments of PITX2c protein sequences across species revealed that the altered amino acid was completely conserved evolutionarily. Functional analysis demonstrated that the mutant PITX2c protein was associated with significantly decreased transcriptional activity when compared with its wild‑type counterpart. The findings of the present study firstly link the PITX2c loss‑of‑function mutation to lone AF, and provide novel insight into the molecular mechanisms underlying AF, suggesting the potential implications for the early prophylaxis and allele‑specific therapy of this common type of arrhythmia.

Weighted gene coexpression network analysis of human left atrial tissue identifies gene modules associated with atrial fibrillation

BACKGROUND

Genetic mechanisms of atrial fibrillation (AF) remain incompletely understood. Previous differential expression studies in AF were limited by small sample size and provided limited understanding of global gene networks, prompting the need for larger-scale, network-based analyses.

METHODS AND RESULTS

Left atrial tissues from Cleveland Clinic patients who underwent cardiac surgery were assayed using Illumina Human HT-12 mRNA microarrays. The data set included 3 groups based on cardiovascular comorbidities: mitral valve (MV) disease without coronary artery disease (n=64), coronary artery disease without MV disease (n=57), and lone AF (n=35). Weighted gene coexpression network analysis was performed in the MV group to detect modules of correlated genes. Module preservation was assessed in the other 2 groups. Module eigengenes were regressed on AF severity or atrial rhythm at surgery. Modules whose eigengenes correlated with either AF phenotype were analyzed for gene content. A total of 14 modules were detected in the MV group; all were preserved in the other 2 groups. One module (124 genes) was associated with AF severity and atrial rhythm across all groups. Its top hub gene, RCAN1, is implicated in calcineurin-dependent signaling and cardiac hypertrophy. Another module (679 genes) was associated with atrial rhythm in the MV and coronary artery disease groups. It was enriched with cell signaling genes and contained cardiovascular developmental genes including TBX5.

CONCLUSIONS

Our network-based approach found 2 modules strongly associated with AF. Further analysis of these modules may yield insight into AF pathogenesis by providing novel targets for functional studies.

Genetic mechanisms of atrial fibrillation: impact on response to treatment

Atrial fibrillation (AF) is the most-common sustained arrhythmia observed in clinical practice, but response to therapy is highly variable between patients. Current drug therapies to suppress AF are incompletely and unpredictably effective and carry substantial risk of proarrhythmia and noncardiac toxicities. The limited success of therapy for AF is partially the result of heterogeneity of the underlying substrate, interindividual differences in disease mechanisms, and our inability to predict response to therapies in individual patients. In this Review, we discuss the evidence that variability in response to drug therapy is also conditioned by the underlying genetic substrate for AF. Increased susceptibility to AF is mediated through diverse genetic mechanisms, including modulation of the atrial action-potential duration, conduction slowing, and impaired cell-to-cell communication, as well as novel mechanisms, such as regulation of signalling proteins important in the pathogenesis of AF. However, the translation of genetic data to the care of the patients with AF has been limited because of poor understanding of the underlying mechanisms associated with common AF-susceptibility loci, a dearth of prospective, adequately powered studies, and the challenges associated with determining efficacy of antiarrhythmic drugs. What is apparent, however, is the need for appropriately designed, genotype-directed clinical trials.

Mutations of the SCN4B-encoded sodium channel β4 subunit in familial atrial fibrillation

Atrial fibrillation (AF) represents the most common form of sustained cardiac arrhythmia and accounts for substantial morbidity and mortality. Mutations in the cardiac sodium channel α, β1, β2 and β3 subunit genes (SCN5A, SCN1B, SCN2B and SCN3B) have been associated with AF, which suggests that mutations in the sodium channel β4 subunit gene, SCN4B, are also involved in the pathogenesis of AF. To examine this hypothesis, the coding exons and exon-intron boundaries of SCN4B were sequenced in 170 unrelated index patients with familial AF. The available relatives of the probands carrying the identified mutations and 200 unrelated ethnically matched healthy individuals used as the controls were subsequently genotyped. The pathogenic potential of a SCN4B sequence variation was predicted using MutationTaster. As a result, 2 novel heterozygous SCN4B mutations, p.V162G and p.I166L, were identified in 2 unrelated families with AF transmitted in an autosomal dominant pattern, respectively. In each family the mutation co-segregated with AF and was absent in the 400 control chromosomes. The mutations altered the amino acids evolutionarily highly conserved across species and were both predicted to be disease-causing. To the best of our knowledge, this is the first study to demonstrate an association of SCN4B mutations with AF, suggesting SCN4B as a novel AF susceptibility gene.

Risk of atrial fibrillation as a function of the electrocardiographic PR interval: results from the Copenhagen ECG Study

BACKGROUND

Prolongation of the PR interval has been associated with an increased risk of incident atrial fibrillation (AF).

OBJECTIVE

To determine if there was a nonlinear relation between PR interval duration and the risk of AF.

METHODS

We included 288,181 individuals, corresponding to one third of the population in the greater region of Copenhagen. These individuals had a digital electrocardiogram (ECG) recorded in a general practitioner's core facility from 2001 to 2010. Data on drug use, comorbidity, and outcomes were collected from Danish registries.

RESULTS

During a median follow-up period of 5.7 years, 11,087 developed AF. Having a PR interval ≥95th percentile (≥196 ms for women, ≥204 ms for men) was associated with an increased risk of AF as evidenced by a multivariable-adjusted hazard ratio (HR) of 1.18 (95% confidence interval [CI] 1.06-1.30, P = .001) for women and 1.30 (1.17-1.44, P < .001) for men compared with the respective reference groups (PR interval between 40th and 60th percentile). Having a short PR interval <5th percentile (≤121 ms for women, ≤129 ms for men) was also associated with an increased risk of AF for women (HR 1.32, 95% CI 1.12-1.56, P = .001), but this was not significant for men (HR 1.09, 95% CI 0.92-1.29, P = .33).

CONCLUSION

In this large ECG study, we found an increased risk of AF for longer PR intervals for both women and men. With respect to short PR intervals, we also observed an increased risk of AF for women.

Atrial fibrillation in the 21st century: a current understanding of risk factors and primary prevention strategies

Atrial fibrillation (AF) is the most common arrhythmia worldwide, and it has a significant effect on morbidity and mortality. It is a significant risk factor for stroke and peripheral embolization, and it has an effect on cardiac function. Despite widespread interest and extensive research on this topic, our understanding of the etiology and pathogenesis of this disease process is still incomplete. As a result, there are no set primary preventive strategies in place apart from general cardiology risk factor prevention goals. It seems intuitive that a better understanding of the risk factors for AF would better prepare medical professionals to initially prevent or subsequently treat these patients. In this article, we discuss widely established risk factors for AF and explore newer risk factors currently being investigated that may have implications in the primary prevention of AF. For this review, we conducted a search of PubMed and used the following search terms (or a combination of terms): atrial fibrillation, metabolic syndrome, obesity, dyslipidemia, hypertension, type 2 diabetes mellitus, omega-3 fatty acids, vitamin D, exercise toxicity, alcohol abuse, and treatment. We also used additional articles that were identified from the bibliographies of the retrieved articles to examine the published evidence for the risk factors of AF.

A novel NKX2.5 loss-of-function mutation responsible for familial atrial fibrillation

Atrial fibrillation (AF) represents the most common form of sustained cardiac arrhythmia and accounts for substantial morbidity and mortality. Increasing evidence demonstrates 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 pivotal for normal cardiovascular morphogenesis, were sequenced in 110 unrelated index patients with familial AF. The available relatives of the mutation carrier and 200 unrelated ethnically-matched healthy individuals serving as controls were subsequently genotyped. The disease-causing potential of the identified NKX2.5 variation was predicted by MutationTaster. The functional characteristics of the mutant NKX2.5 protein were analyzed using a dual-luciferase reporter assay system. As a result, a novel heterozygous NKX2.5 mutation, p.F145S, was identified in a family with AF transmitted as an autosomal dominant trait, which co-segregated with AF in the family with complete penetrance. The detected substitution, which altered the amino acid completely conserved evolutionarily across species, was absent in 400 control chromosomes and was automatically predicted to be causative. Functional analysis demonstrated that the NKX2.5 mutant was associated with significantly decreased transcriptional activity compared with its wild-type counterpart. To the best of our knowledge, this is the first report on the association of the NKX2.5 loss-of-function mutation with increased susceptibility to familial AF. The findings of the present study provide novel insights into the molecular mechanism underlying AF, suggesting the potential implications for the early prophylaxis and allele-specific therapy of AF.

Prevalence and spectrum of GJA5 mutations associated with lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia observed in clinical practice and a major contributor to cardiovascular morbidity and mortality. Accumulating evidence indicates a substantial genetic basis for AF. However, AF is genetically heterogeneous and the hereditary components responsible for AF remain to be identified in the majority of patients. The cardiac gap junction protein α 5 (GJA5) is specifically expressed in atrial myocytes and is associated with the coordinated electrical activation of the atria, providing a rationale to screen GJA5 as a logical candidate gene for AF. A cohort of 310 unrelated patients with lone AF and their available relatives were included in this study. A group of 200 unrelated healthy individuals matched for age, gender and race were also included as controls. The entire coding region and splice sites of the GJA5 gene were initially sequenced in 310 unrelated AF patients. The relatives of mutation carriers and 200 controls were subsequently genotyped for the presence of identified mutations. As a result, 4 novel heterozygous GJA5 mutations, p.K107R, p.L223M, p.Q236H and p.I257L, were identified in 4 of 310 unrelated AF patients, respectively, with a prevalence of ~1.29%. Genetic analysis of the carriers' families showed that in each family the missense mutation was present in all the affected family members. Absent in the 400 reference alleles, these mutations altered the amino acids highly conserved among various species, with the exception of p.I257L. In conclusion, this study expands the spectrum of GJA5 mutations associated with AF and provides novel insights into the molecular basis of AF, suggesting potential implications for the improved, gene-specific rhythm control strategies.

Familial Atrial Fibrillation Predicts Increased Risk of Mortality

Background—

Atrial fibrillation (AF) is a common arrhythmia. Several studies have shown association of genetic variants with AF and that familial AF increases the risk of AF. We have previously shown a substantial heritability of AF in a twin study. The objective of this study was to determine whether having a co-twin with AF influences mortality.

Methods and Results—

We identified all Danish twins with AF born during and after 1912 in the Danish Twin Registry, the National Patient Registry, and the Central Office of Civil Registration. For each twin, we randomly identified 4 twins without AF, matched on sex, zygosity, and age. We compared survival among the co-twins of the affected twins (co-cases, n=2164) and the co-twins of the unaffected twins (co-controls, n=8626). The co-cases showed increased death rates compared with the co-controls (hazard ratio, 1.20; 95% confidence interval, 1.11–1.30; P <0.0001), and this effect was more pronounced in monozygotic twins (hazard ratio, 1.30; 95% confidence interval, 1.09–1.55; P =0.003), compared with dizygotic same sex (hazard ratio, 1.16; 95% confidence interval, 1.04–1.29; P =0.006) and opposite sex twins (hazard ratio, 1.20; 95% confidence interval, 0.97–1.47; P =0.093).

Conclusions—

The mortality rate was 20% higher in twins who had a co-twin with AF than in twins without familial AF. This effect was almost doubled in monozygotic twins compared with dizygotic twins, suggesting the influence of genetic factors.

High familial risk of atrial fibrillation/atrial flutter in multiplex families: a nationwide family study in Sweden

BACKGROUND

Although the heritability of atrial fibrillation/atrial flutter (AF/AFl) has been determined, the familial risk in multiplex families is unclear. The main aim of this nationwide study was to determine the familial risk of AF/AFl in multiplex families.

METHODS AND RESULTS

We examined the familial risk of AF/AFl in the entire Swedish population. We linked Multigeneration Register data on individuals aged 0 to 76 years with Hospital Discharge Register data for 1987-2008 and Outpatient Register data for 2001-2008 to compare AF/AFl risk among relatives of all 300 586 individuals with AF/AFl with that among relatives of unaffected individuals. We used conditional logistic regression to investigate differences in exposure between cases and controls. Parents (odds ratio [OR] 1.95 [95% CI 1.89 to 2.00]) and siblings (OR=3.08 [3.00 to 3.16]) of cases had higher odds of AF/AFl than did parents and siblings of controls. AF/AFl ORs were increased in both sexes. For 2% of cases, both parents had AF/AFl, compared with only 0.7% of controls (OR=3.60 [3.30 to 3.92]). Moreover, 3% of cases had ≥2 siblings with AF/AFl, compared with 1% of controls (OR=5.72 [5.28 to 6.19]). In premature cases (diagnosed at age <50 years), the ORs were 5.04 (4.36 to 5.82) and 8.51 (6.49 to 11.15) for AF/AFl in both parents and AF/AFl in ≥2 siblings, respectively. The overall spouse OR was 1.16 (1.13 to 1.19).

CONCLUSIONS

Family history of AF/AFl increases the odds of AF/AFl in first-degree relatives. High familial risks were observed in multiplex families.

Family history as a risk factor for recurrent hospitalization for lone atrial fibrillation: a nationwide family study in Sweden

BACKGROUND

Although the heritability of atrial fibrillation (AF) has been determined, the relevance of family history of AF for the likelihood of recurrent hospitalization for AF is unknown. The aim of this nationwide study was to determine whether family history of AF is a risk factor of recurrent hospitalization for lone AF (LAF), i.e., AF with unknown etiology. The familial risk for first time LAF hospitalization was also determined and compared to the risk of recurrent hospitalization for LAF.

METHODS

We examined whether family history of AF is a risk factor for recurrent hospitalization for LAF in the whole Swedish population. We linked Multigeneration Register data on individuals aged 0-60 years to Hospital Discharge Register data for the period 1987-2009 to compare LAF recurrent hospitalization risk among individuals with and without parental or sibling history of AF. We calculated hazard ratios (HRs) to determine the familial HR of recurrent hospitalization for LAF. Odds ratios (OR) were calculated for familial risk of first time LAF hospitalization.

RESULTS

The risk of recurrent LAF hospitalization was 1.23 (95% CI 1.17-1.30) for individuals with affected parents compared to 1.30 (95% CI 1.22-1.38) for those with affected siblings. After 10 years of follow up 50% of those without and 60% of those with family history had recurrent hospitalization for LAF. The risk of recurrent LAF hospitalization in individuals with two affected parents was 1.65 (95% CI 1.44-1.90). There was an interaction between age and family history, with family history having a weaker effect on LAF hospitalization risk in older age groups. The OR for first time LAF hospitalization was 2.08 (95% CI 2.02-2.15) for offspring with affected parents and 3.23 (95% CI 3.08-3.39) for individuals with affected siblings.

CONCLUSIONS

Family history of AF is a novel risk factor for recurrent LAF hospitalization. The higher recurrence hospitalization risk in multiplex families and younger individuals suggests a genetic contribution. However, the familial risk for recurrent LAF hospitalization was much lower than the risk for first time LAF hospitalization, suggesting that familial and possibly genetic factors are more important for first time LAF hospitalization than recurrent LAF hospitalization.

Personalized medicine and atrial fibrillation: will it ever happen?

Atrial fibrillation (AF) is a common arrhythmia of substantial public health importance. Recent evidence demonstrates a heritable component underlying AF, and genetic discoveries have identified common variants associated with the arrhythmia. Ultimately one hopes that the consideration of genetic variation in clinical practice may enhance care and improve health outcomes. In this review we explore areas of potential clinical utility in AF management including those relating to pharmacogenetics and risk prediction.

The genetics of atrial fibrillation

PURPOSE OF REVIEW

A substantial genetic contribution to the etiology of atrial fibrillation has emerged in the last decade, and has bolstered links between this arrhythmia and other forms of heart disease. In this article, we will summarize the work that has defined the inherited diathesis toward atrial fibrillation, outline the genetic studies to date, and characterize the roadblocks to a complete mechanistic understanding of this common arrhythmia.

RECENT FINDINGS

Clinical genetic work has demonstrated a large heritable contribution in atrial fibrillation, with studies in lone forms of the arrhythmia suggesting a traditional monogenic syndrome with reduced penetrance. Several Mendelian loci for typical forms of atrial fibrillation have been identified but the genes have not yet been cloned. Rare forms of familial atrial fibrillation are caused by mutations in potassium channel genes, and there are single families with mutations in a nuclear pore and a natriuretic peptide gene, respectively. Common loci with small effects are now being identified in genome-wide association (GWA) studies, including a locus on chromosome 4q25. These loci explain less than 10% of the inherited contribution to the arrhythmia, suggesting there are major contributions that have not yet been uncovered.

SUMMARY

Although great strides have been made in exploring the genetics of atrial fibrillation, the paroxysmal and asymptomatic nature of the phenotype challenges investigators as they seek the mechanistic basis of the arrhythmia.

A novel GATA5 loss-of-function mutation underlies lone atrial fibrillation

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, is associated with significantly increased morbidity and mortality. Cumulative evidence highlights the importance of genetic defects in the pathogenesis of AF. However, AF is of remarkable heterogeneity and the genetic determinants of AF in a vast majority of patients remain illusive. In this study, the coding exons and splice junctions of the GATA5 gene, which encodes a zinc-finger transcription factor essential for normal cardiogenesis, were sequenced in 118 unrelated patients with lone AF. The available relatives of the index patient carrying an identified mutation and 200 unrelated ethnically-matched healthy individuals used as controls were genotyped. The functional effect of the mutant GATA5 was characterized in contrast to its wild-type counterpart using a luciferase reporter assay system. As a result, a novel heterozygous GATA5 mutation, p.W200G, was identified in a family with AF inherited as an autosomal dominant trait. The mutation was absent in 200 control individuals and the altered amino acid was completely conserved evolutionarily across species. Functional analysis showed that the mutation of GATA5 was associated with a significantly decreased transcriptional activity. These findings provide novel insight into the molecular mechanism involved in AF, suggesting potential implications for the early prophylaxis and gene-specific therapy of AF.

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.

Novel GATA6 loss-of-function mutation responsible for familial atrial fibrillation

Atrial fibrillation (AF) is the most commonly sustained cardiac arrhythmia, and confers a substantially increased risk of morbidity and mortality. Increasing evidence has indicated that hereditary defects are implicated in AF. However, AF is genetically heterogeneous and the genetic etiology of AF in a significant portion of patients remains unclear. In this study, the entire coding sequence and splice junctions of the GATA6 gene, which encodes a zinc-finger transcription factor crucial for cardiogenesis, were sequenced in 140 unrelated patients with lone AF. The available relatives of the index patient carrying an identified mutation and 200 unrelated ethnically-matched healthy individuals used as the controls were genotyped. The functional characteristics of the mutant GATA6 were assessed in contrast to its wild-type counterpart using a luciferase reporter assay system. As a result, a novel heterozygous GATA6 mutation, p.G469V, was identified in a family with AF inherited in an autosomal dominant pattern. The mutation was absent in the 200 control individuals and the altered amino acid was completely conserved across species. Functional analysis demonstrated that the GATA6 mutation was associated with a significantly decreased transcriptional activity. The findings provide novel insight into the molecular mechanism involved in the pathogenesis of AF, as well as insight into potential therapies for the prevention and treatment of AF.

GATA6 loss-of-function mutation in atrial fibrillation

Atrial fibrillation (AF) is the most common type of sustained cardiac arrhythmia and is associated with substantial morbidity and mortality. Increasing evidence demonstrates that hereditary defects are involved in the pathogenesis of AF. However, AF is of remarkable genetic heterogeneity, and the heritable components responsible for AF in the majority of patients remain unclear. In this study, the entire coding region of the GATA6 gene, which encodes a zinc-finger transcription factor crucial for cardiogenesis, was sequenced in 138 unrelated patients with lone AF, and a novel heterozygous GATA6 mutation, c.704A > C equivalent to p.Y235S, was identified in a patient. The detected substitution, which altered the amino acid highly conserved evolutionarily across species, was absent in 200 unrelated ethnically matched healthy individuals, and was predicted to be disease-causing by MutationTaster. Genetic analysis of the available relatives of the mutation carrier showed that in the family the variation co-segregated with the disease transmitted as an autosomal dominant trait, with complete penetrance. The functional analysis performed using a luciferase reporter assay system revealed that the mutant GATA6 protein resulted in significantly decreased transcriptional activity compared with its wild-type counterpart. These findings provide novel insight into the molecular pathophysiology implicated in AF, suggesting the potential implications in the prophylactic strategy and effective therapy for this common arrhythmia.

Mutation spectrum of the GATA4 gene in patients with idiopathic atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia associated with substantially increased morbidity and mortality. Growing evidence strongly implicates hereditary determinants in the pathogenesis of AF. However, AF is genetically heterogeneous and the genetic defects responsible for AF in the majority of cases remain to be identified. In this study, all the coding exons and splice junctions of GATA4, a gene encoding a zinc-finger transcription factor critical for normal cardiac morphogenesis, were sequenced in a cohort of 150 unrelated patients with idiopathic AF. The available relatives of the mutation carriers and a total of 200 unrelated ethnically matched healthy individuals used as controls were genotyped for the presence of mutations identified in index patients. The functional effect of the mutant GATA4 was characterized using a luciferase reporter assay system. As a result, two novel heterozygous GATA4 mutations (p.Y38D and p.P103A) were identified in 2 unrelated families with AF, respectively. In each family the mutation co-segregated with AF and was absent in the 400 control chromosomes. Functional analysis showed that the mutations of GATA4 were associated with a significantly decreased transcriptional activity. The findings expand the mutation spectrum of GATA4 linked to AF, and further support the notion that compromised GATA4 confers genetic susceptibility to AF.

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

Atrial fibrillation (AF) has become a growing global epidemic and a financial burden for society. The past 10 years have seen significant advances in our understanding of the genetic aspects of AF: At least 2 chromosomal loci and 17 causal genes have been identified in familial AF, and an additional 7 common variants and single-nucleotide polymorphisms in 11 different genes have been indicated in nonfamilial AF. However, the current management strategies for AF are suboptimal. The integration of genetic information into clinical practice may aid the early identification of AF patients who are at risk as well as the characterization of molecular pathways that culminate in AF, with the eventual result of better treatment. Never before has such an opportunity arisen to advance our understanding of the biology of AF through the translation of genetics findings from the bench to the bedside.

Register-based research on twins

INTRODUCTION

The Danish Twin Registry (DTR) has for more than 50 years been based on surveys and clinical investigations and over the two last decades also on register linkage. Currently these two approaches are merged within Statistics Denmark.

RESEARCH TOPICS

Here we report on three major groups of register-based research in the DTR that used the uniqueness of twinning. First, we focus on the ''long-term prognosis'' of being a twin compared with being a singleton and show that Danish twins have health trajectories in adulthood similar to singletons, which is a result of interest for twins and their families as well as a test of the fetal origins hypothesis that states that fetal growth restriction has long-term health consequences. Secondly, we summarise some of the most important register-based ''classical twin studies'', e.g. heritability studies on lifespan and exceptional longevity. Finally, we illustrate how the co-twin control method in a register setting can be used to control for the effect of rearing environment and genetic factors in studies of the association between exposures and health.

CONCLUSION

The spectrum of register-based twin studies is very wide and have changed in accordance with methodological and data resource developments.

Genetics of atrial fibrillation and possible implications for ischemic stroke

Atrial fibrillation is the most common cardiac arrhythmia mainly caused by valvular, ischemic, hypertensive, and myopathic heart disease. Atrial fibrillation can occur in families suggesting a genetic background especially in younger subjects. Additionally recent studies have identified common genetic variants to be associated with atrial fibrillation in the general population. This cardiac arrhythmia has important public health implications because of its main complications: congestive heart failure and ischemic stroke. Since atrial fibrillation can result in ischemic stroke, one might assume that genetic determinants of this cardiac arrhythmia are also implicated in cerebrovascular disease. Ischemic stroke is a multifactorial, complex disease where multiple environmental and genetic factors interact. Whether genetic variants associated with a risk factor for ischemic stroke also increase the risk of a particular vascular endpoint still needs to be confirmed in many cases. Here we review the current knowledge on the genetic background of atrial fibrillation and the consequences for cerebrovascular disease.

GATA4 loss-of-function mutations in familial atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major source of the substantially increased morbidity and mortality. Growing studies demonstrate that genetic defects play pivotal roles in a subgroup of AF. However, AF is a genetically heterogeneous disorder and the molecular basis of AF in a majority of cases remains unknown.

METHODS

The whole coding region of the GATA4 gene, which encodes a zinc-finger transcription factor essential for cardiogenesis, was analyzed in 130 unrelated probands with AF in contrast to 200 unrelated ethnically matched healthy individuals used as controls. The available family members of the probands harboring the identified mutations were genotyped. The functional effect of the mutant GATA4 was characterized using a luciferase reporter assay system.

RESULTS

Two novel heterozygous GATA4 mutations, p.S70T and p.S160T, were identified in 2 unrelated families with AF inherited as an autosomal dominant trait, respectively, which co-segregated with AF in each family with complete penetrance. Functional analysis showed that the mutations of GATA4 were associated with a significantly decreased transcriptional activity.

CONCLUSION

The findings provide new insight into the molecular mechanism involved in the pathogenesis of AF, suggesting the potential implications in the genetic diagnosis and gene-specific therapy of this common arrhythmia.

Common polymorphisms in KCNJ5 [corrected] are associated with early-onset lone atrial fibrillation in Caucasians

OBJECTIVES

The aim of this study was to screen lone atrial fibrillation (AF) patients for mutations in the genes KCNJ2, KCNJ3 and KCNJ5, all encoding potassium channels. Furthermore, we wanted to replicate the prior association of two single-nucleotide polymorphisms (SNPs) in KCNJ5, C171T (rs6590357) and G810T (rs7118824), with lone AF in Han Chinese.

METHODS

We sequenced the coding region and splice site of KCNJ2, KCNJ3 and KCNJ5 in 187 early-onset lone-AF patients screening for mutations and counting SNP frequencies for the two noted SNPs in KCNJ5.

RESULTS

No mutations were found in KCNJ2, KCNJ3 or KCNJ5. Both genotype distribution and allele frequencies of the SNPs rs6590357 and rs7118824 significantly differed between the AF and control group (p(genotype) = 0.0067, p(allele) = 0.0021 and p(genotype) = 0.014, p(allele) = 0.0101, respectively). On allele level, the OR for lone AF for rs6590357 was 1.77 (95% CI 1.16-2.73, p = 0.009) and for rs7118824 it was 1.71 (95% CI 1.13-2.57, p = 0.01) in a model adjusted for age and gender.

CONCLUSIONS

Our findings indicate that rs6590357 and rs7118824 in KCNJ5 are associated with early-onset lone AF in Caucasians. No mutations were found in the exon or splice site of KCNJ2, KCNJ3 or KCNJ5.

Lone atrial fibrillation: what is known and what is to come

Atrial fibrillation (AF) is the most prevalent sustained cardiac arrhythmia in adults, affecting >1% of general population. Atrial fibrillation is commonly associated with structural heart disease and is a major cause of significant cardiovascular morbidity and mortality. AF sometimes develops in a subset of young patients (e.g. aged ≤60 years), with no evidence of associated cardiopulmonary or other comorbid disease (including hypertension), and has been referred to as 'lone AF'. The latter generally has a favourable prognosis; the prognostic and therapeutic implications of an accurate identification of patients with truly lone AF (that is, truly at low risk of complications), if any, would be of the utmost importance. The true prevalence of lone AF is unknown, varying between 1.6% and 30%, depending on the particular study population. Nonetheless, novel risk factors for AF, including obesity, metabolic syndrome, sleep apnea, alcohol consumption, endurance sports, anger, hostility, subclinical atherosclerosis and others, have been increasingly recognised. Also, various underlying pathophysiological mechanisms predisposing to AF, including increased atrial stretch, structural and electrophysiological alterations, autonomic imbalance, systemic inflammation, oxidative stress and genetic predisposition, have been proposed. The growing evidence of these diverse (and numerous) pathogenic mechanisms and factors related to AF inevitably raises the question of whether 'lone AF' does exist at all. In this review article, we summarise the current knowledge of the epidemiology, pathophysiology, clinical course and treatment of patients with so-called 'lone AF' and outline emerging insights into its pathogenesis and the potential therapeutic implications of a diagnosis of lone AF.

Absolute and attributable risks of atrial fibrillation in relation to optimal and borderline risk factors: the Atherosclerosis Risk in Communities (ARIC) study

BACKGROUND

Atrial fibrillation (AF) is an important risk factor for stroke and overall mortality, but information about the preventable burden of AF is lacking. The aim of this study was to determine what proportion of the burden of AF in blacks and whites could theoretically be avoided by the maintenance of an optimal risk profile.

METHODS AND RESULTS

This study included 14 598 middle-aged Atherosclerosis Risk in Communities (ARIC) Study cohort members. Previously established AF risk factors, namely high blood pressure, elevated body mass index, diabetes mellitus, cigarette smoking, and prior cardiac disease, were categorized into optimal, borderline, and elevated levels. On the basis of their risk factor levels, individuals were classified into 1 of these 3 groups. The population-attributable fraction of AF resulting from having a nonoptimal risk profile was estimated separately for black and white men and women. During a mean follow-up of 17.1 years, 1520 cases of incident AF were identified. The age-adjusted incidence rates were highest in white men and lowest in black women (7.45 and 3.67 per 1000 person-years, respectively). The overall prevalence of an optimal risk profile was 5.4% but varied according to race and gender: 10% in white women versus 1.6% in black men. Overall, 56.5% of AF cases could be explained by having ≥ 1 borderline or elevated risk factors, of which elevated blood pressure was the most important contributor.

CONCLUSION

As with other forms of cardiovascular disease, more than half of the AF burden is potentially avoidable through the optimization of cardiovascular risk factors levels.

Screening of KCNN3 in patients with early-onset lone atrial fibrillation

AIMS

The aim of this study was to screen KCNN3 encoding the small-conductance calcium-activated K+ channel (SK3) in lone atrial fibrillation patients. Atrial fibrillation (AF) is the most common cardiac arrhythmia. A genome-wide association study has recently associated an intronic single-nucleotide polymorphism (SNP) in KCNN3 with lone AF.

METHODS AND RESULTS

We sequenced the coding region and splice junctions of KCNN3 in 209 early-onset lone AF patients, screening for variations. A group of 208 healthy blood donors with normal ECGs and without cardiac symptoms were used as controls. All patients and controls were of Danish ethnicity. No mutations were found in the coding regions or splice sites of KCNN3. We found one known exonic synonymous SNP (rs1131820) in KCNN3 that was associated with AF. Both the genotype distribution and allele frequencies of SNP rs1131820 were significantly different between the AF cases and controls (PGenotype=0.047 and PAllele=0.027). Being a homozygous carrier of the major allele (GG) vs. the minor allele (AA) of rs1131820 was associated with an odds ratio of 2.85 (95% CI 1.13-7.18, P=0.026) for lone AF.

CONCLUSIONS

In this study of 209 young lone AF patients, we found no mutations in the exons or splice sites of KCNN3, but we found an association between the synonymous SNP rs1131820 in KCNN3 and lone AF.

Genome-wide association studies of atrial fibrillation: past, present, and future

Genome-wide association studies (GWAS) for atrial fibrillation (AF) have identified three distinct genetic loci on chromosomes 1q21, 4q25, and 16q22 that are associated with the arrhythmia. Susceptibility loci also have been identified by GWAS for PR interval duration, a quantitative phenotype related to AF. In this review article, we have sought to summarize the latest findings for population-based genetic studies of AF, to highlight ongoing functional studies, and to explore the future directions of genetic research on AF.

Association between familial atrial fibrillation and risk of new-onset atrial fibrillation

CONTEXT

Although the heritability of atrial fibrillation (AF) is established, the contribution of familial AF to predicting new-onset AF remains unknown.

OBJECTIVE

To determine whether familial occurrence of AF is associated with new-onset AF beyond established risk factors.

DESIGN, SETTING, AND PARTICIPANTS

The Framingham Heart Study, a prospective community-based cohort study started in 1948. Original and Offspring Cohort participants were aged at least 30 years, were free of AF at the baseline examination, and had at least 1 parent or sibling enrolled in the study. The 4421 participants in this analysis (mean age, 54 [SD, 13] years; 54% women) were followed up through December 31, 2007.

MAIN OUTCOME MEASURES

Incremental predictive value of incorporating different features of familial AF (any familial AF, premature familial AF [onset ≤65 years old], number of affected relatives, and youngest age of onset in a relative) into a risk model for new-onset AF.

RESULTS

Across 11,971 examinations during the period 1968-2007, 440 participants developed AF. Familial AF occurred among 1185 participants (26.8%) and premature familial AF occurred among 351 participants (7.9%). Atrial fibrillation occurred more frequently among participants with familial AF than without familial AF (unadjusted absolute event rates of 5.8% and 3.1%, respectively). The association was not attenuated by adjustment for AF risk factors (multivariable-adjusted hazard ratio, 1.40; 95% confidence interval [CI], 1.13-1.74) or reported AF-related genetic variants. Among the different features of familial AF examined, premature familial AF was associated with improved discrimination beyond traditional risk factors to the greatest extent (traditional risk factors, C statistic, 0.842 [95% CI, 0.826-0.858]; premature familial AF, C statistic, 0.846 [95% CI, 0.831-0.862]; P = .004). Modest changes in integrated discrimination improvement were observed with premature familial AF (2.1%). Net reclassification improvement (assessed using 8-year risk thresholds of <5%, 5%-10%, and >10%) did not change significantly with premature familial AF (index statistic, 0.011; 95% CI, -0.021 to 0.042; P = .51), although categoryless net reclassification was improved (index statistic, 0.127; 95% CI, 0.064-0.189; P = .009).

CONCLUSIONS

In this cohort, familial AF was associated with an increased risk of AF that was not attenuated by adjustment for AF risk factors including genetic variants. Assessment of premature familial AF was associated with a very slight increase in predictive accuracy compared with traditional risk factors.

Mutations in sodium channel β-subunit SCN3B are associated with early-onset lone atrial fibrillation

AIMS

Atrial fibrillation (AF) is the most frequent arrhythmia. Screening of SCN5A-the gene encoding the α-subunit of the cardiac sodium channel-has indicated that disturbances of the sodium current may play a central role in the mechanism of lone AF. We tested the hypothesis that lone AF in young patients is associated with genetic mutations in SCN3B and SCN4B, the genes encoding the two β-subunits of the cardiac sodium channel.

METHODS AND RESULTS

In 192 unrelated lone AF patients, the entire coding sequence and splice junctions of SCN3B and SCN4B were bidirectionally sequenced. Three non-synonymous mutations were found in SCN3B (R6K, L10P, and M161T). Two mutations were novel (R6K and M161T). None of the mutations were present in the control group (n = 432 alleles), nor have any been previously reported in conjunction with AF. All SCN3B mutations affected residues that are evolutionarily conserved across species. Electrophysiological studies on the SCN3B mutation were carried out and all three SCN3B mutations caused a functionally reduced sodium channel current. One synonymous variant was found in SCN4B.

CONCLUSION

In 192 young lone AF patients, we found three patients with suspected disease-causing non-synonymous mutations in SCN3B, indicating that mutations in this gene contribute to the mechanism of lone AF. The three mutations in SCN3B were investigated electrophysiologically and all led to loss of function in the sodium current, supporting the hypothesis that decreased sodium current enhances AF susceptibility.

Novel connexin40 missense mutations in patients with familial atrial fibrillation

AIMS

This research was aimed at screening connexin40, a cardiac gap junction protein alpha 5, for genetic defects in patients with familial atrial fibrillation (AF).

METHODS

The subjects included 218 unrelated families with lone AF and 200 ethnically matched unrelated healthy individuals as controls. The entire coding region of the connexin40 gene was sequenced initially in 218 unrelated probands with familial AF. The relatives of mutation carriers and 200 controls were subsequently genotyped for the presence of mutations identified in probands.

RESULTS

Three novel connexin40 mutations, p.V85I, p.L221I, and p.L229M, were identified in 3 of 218 unrelated AF families, respectively. These heterozygous missense mutations co-segregated with AF in the families and were absent in the 200 unrelated control subjects. A cross-species alignment of connexin40 protein sequences revealed that the altered amino acids were completely conserved evolutionarily.

CONCLUSION

The findings expand the spectrum of mutations in connexin40 linked to AF and provide new insight into the molecular aetiology involved in the pathogenesis of AF.