Variants in ZFHX3 are associated with atrial fibrillation in individuals of European ancestry

Genomewide Association Studies in Cardiovascular Disease—An Update 2011

BACKGROUND

Genomewide association studies have led to an enormous boost in the identification of susceptibility genes for cardiovascular diseases. This review aims to summarize the most important findings of recent years.

CONTENT

We have carefully reviewed the current literature (PubMed search terms: “genome wide association studies,” “genetic polymorphism,” “genetic risk factors,” “association study” in connection with the respective diseases, “risk score,” “transcriptome”).

SUMMARY

Multiple novel genetic loci for such important cardiovascular diseases as myocardial infarction, hypertension, heart failure, stroke, and hyperlipidemia have been identified. Given that many novel genetic risk factors lie within hitherto-unsuspected genes or influence gene expression, these findings have inspired discoveries of biological function. Despite these successes, however, only a fraction of the heritability for most cardiovascular diseases has been explained thus far. Forthcoming techniques such as whole-genome sequencing will be important to close the gap of missing heritability.

Comprehensive risk reduction in patients with atrial fibrillation: emerging diagnostic and therapeutic options--a report from the 3rd Atrial Fibrillation Competence NETwork/European Heart Rhythm Association consensus conference

While management of atrial fibrillation (AF) patients is improved by guideline-conform application of anticoagulant therapy, rate control, rhythm control, and therapy of accompanying heart disease, the morbidity and mortality associated with AF remain unacceptably high. This paper describes the proceedings of the 3rd Atrial Fibrillation NETwork (AFNET)/European Heart Rhythm Association (EHRA) consensus conference that convened over 60 scientists and representatives from industry to jointly discuss emerging therapeutic and diagnostic improvements to achieve better management of AF patients. The paper covers four chapters: (i) risk factors and risk markers for AF; (ii) pathophysiological classification of AF; (iii) relevance of monitored AF duration for AF-related outcomes; and (iv) perspectives and needs for implementing better antithrombotic therapy. Relevant published literature for each section is covered, and suggestions for the improvement of management in each area are put forward. Combined, the propositions formulate a perspective to implement comprehensive management in AF.

Genome-wide association studies: Where we are heading?

We have witnessed tremendous success in genome-wide association studies (GWAS) in recent years. Since the identification of variants in the complement factor H gene on the risk of age-related macular degeneration, GWAS have become ubiquitous in genetic studies and have led to the identification of genetic variants that are associated with a variety of complex human diseases and traits. These discoveries have changed our understanding of the biological architecture of common, complex diseases and have also provided new hypotheses to test. New tools, such as next-generation sequencing, will be an important part of the future of genetics research; however, GWAS studies will continue to play an important role in disease gene discovery. Many traits have yet to be explored by GWAS, especially in minority populations, and large collaborative studies are currently being conducted to maximize the return from existing GWAS data. In addition, GWAS technology continues to improve, increasing genomic coverage for major global populations and decreasing the cost of experiments. Although much of the variance attributable to genetic factors for many important traits is still unexplained, GWAS technology has been instrumental in mapping over a thousand genes to hundreds of traits. More discoveries are made each month and the scale, quality and quantity of current work has a steady trend upward. We briefly review the current key trends in GWAS, which can be summarized with three goals: increase power, increase collaborations and increase populations.

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.

Large-scale candidate gene analysis in whites and African Americans identifies IL6R polymorphism in relation to atrial fibrillation: the National Heart, Lung, and Blood Institute's Candidate Gene Association Resource (CARe) project

BACKGROUND

The genetic background of atrial fibrillation (AF) in whites and African Americans is largely unknown. Genes in cardiovascular pathways have not been systematically investigated.

METHODS AND RESULTS

We examined a panel of approximately 50,000 common single-nucleotide polymorphisms (SNPs) in 2095 cardiovascular candidate genes and AF in 3 cohorts with participants of European (n=18,524; 2260 cases) or African American descent (n=3662; 263 cases) in the National Heart, Lung, and Blood Institute's Candidate Gene Association Resource. Results in whites were followed up in the German Competence Network for AF (n=906, 468 cases). The top result was assessed in relation to incident ischemic stroke in the Cohorts for Heart and Aging Research in Genomic Epidemiology Stroke Consortium (n=19,602 whites, 1544 incident strokes). SNP rs4845625 in the IL6R gene was associated with AF (relative risk [RR] C allele, 0.90; 95% confidence interval [CI], 0.85-0.95; P=0.0005) in whites but did not reach statistical significance in African Americans (RR, 0.86; 95% CI, 0.72-1.03; P=0.09). The results were comparable in the German AF Network replication, (RR, 0.71; 95% CI, 0.57-0.89; P=0.003). No association between rs4845625 and stroke was observed in whites. The known chromosome 4 locus near PITX2 in whites also was associated with AF in African Americans (rs4611994; hazard ratio, 1.40; 95% CI, 1.16-1.69; P=0.0005).

CONCLUSIONS

In a community-based cohort meta-analysis, we identified genetic association in IL6R with AF in whites. Additionally, we demonstrated that the chromosome 4 locus known from recent genome-wide association studies in whites is associated with AF in African Americans.

The "missing" link in atrial fibrillation heritability

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, represents a major health burden to individuals and health care system within the Western world. The lifetime risk for the development of AF at age 40 years has been estimated to be approximately 1 in 4. Atrial fibrillation is associated with substantial morbidity and a 2-fold increased risk of mortality. Given its increasing prevalence with age, coupled with the aging population, the number of Americans affected with AF is expected to increase from approximately 2.3 million in the year 2000 to nearly 16 million by 2050. This AF epidemic is further complicated by the lack of highly effective therapies. One reason for the lack of effective therapies for AF stems from incomplete understanding of the complex pathophysiology of the arrhythmia. Atrial fibrillation has often been regarded as a condition that occurs in the context of atrial electrical and structural remodeling that can result from cardiac and systemic disorders. However, up to 30% of patients have no obvious cause and are said to have idiopathic or "lone" AF. Up until recently, AF was considered to be a sporadic, nongenetic disorder, but we and others have shown that lone AF has a substantial genetic basis. Mutations in genes encoding cardiac ion channels (KCNQ1, KCNE1-5, KCNJ2, KCNA5, and SCN5A), gap junctions (GJA5), and signaling molecules (atrial natriuretic peptide, nucleoporins [NUP155]) have been reported in isolated cases and small kindreds. The advent of the human genome and HapMap projects and high-throughput genotyping has fundamentally accelerated our ability to discover the genetic contribution to common variation in human disease. In 2007, a genome-wide association study identified 2 genetic variants that associated with AF. More recently, 2 additional AF loci on chromosomes 16q22 and 1q21 have been identified. It is quite likely, however, that the effects of alleles in many genes contribute to common complex diseases such as AF. The overall AF risk associated with common variants identified by the genome-wide association study approach is small (odds ratios, 1.1-2.5) and explains less than 10% of the heritability in lone AF. This raises the possibility that rare independent variants with large effects strong effects may account for a large fraction of the risk for lone AF.

Influence of race on atrial fibrillation after cardiac surgery

BACKGROUND

Despite having fewer risk factors for atrial fibrillation (AF), white patients have a greater prevalence of AF in the community than black patients, and a genetic basis has been postulated. However, it is unknown whether occurrence of new-onset AF after cardiac surgery is different in white versus black patients, and secondarily, other non-Caucasian patients.

METHODS AND RESULTS

From 1995 through 2005, 20 282 white, 1323 black, and 1919 other non-Caucasian patients in sinus rhythm underwent coronary artery bypass grafting with or without valve surgery. To adjust for clinical and socioeconomic confounders, we performed propensity-adjusted analyses; 7093 white patients (35%) had postoperative AF, compared with 255 (22%) black patients and 550 (29%) other non-Caucasians (P<0.0001). Whites were older than black patients, had higher socioeconomic position, and greater left atrial size but were less likely to have hypertension or congestive heart failure. In 847 propensity-matched patient pairs, postoperative AF occurred more frequently in white than in black patients (odds ratio, 1.74; 95% confidence interval, 1.7-1.78). Other than higher occurrence of bradycardia requiring pacing and reintubation in white patients, occurrence of other postoperative complications, hospital mortality, and length of postoperative stay were similar. Age and valvular surgery were the strongest predictors of AF irrespective of race.

CONCLUSIONS

White patients had a markedly higher risk of postoperative AF than black and other non-Caucasian patients. The cause for racial differences of arrhythmic risk is unknown, but a genetic predisposition is plausible. Our results have implications for risk stratification and mechanistic understanding of postoperative AF.

The Rotterdam Study: 2012 objectives and design update

The Rotterdam Study is a prospective cohort study ongoing since 1990 in the city of Rotterdam in The Netherlands. The study targets cardiovascular, endocrine, hepatic, neurological, ophthalmic, psychiatric, dermatological, oncological, and respiratory diseases. As of 2008, 14,926 subjects aged 45 years or over comprise the Rotterdam Study cohort. The findings of the Rotterdam Study have been presented in over a 1,000 research articles and reports (see www.erasmus-epidemiology.nl/rotterdamstudy ). This article gives the rationale of the study and its design. It also presents a summary of the major findings and an update of the objectives and methods.

Recent advances in the molecular pathophysiology of atrial fibrillation

Atrial fibrillation (AF) is an extremely common cardiac rhythm disorder that causes substantial morbidity and contributes to mortality. The mechanisms underlying AF are complex, involving both increased spontaneous ectopic firing of atrial cells and impulse reentry through atrial tissue. Over the past ten years, there has been enormous progress in understanding the underlying molecular pathobiology. This article reviews the basic mechanisms and molecular processes causing AF. We discuss the ways in which cardiac disease states, extracardiac factors, and abnormal genetic control lead to the arrhythmia. We conclude with a discussion of the potential therapeutic implications that might arise from an improved mechanistic understanding.

Differential impact of race and risk factors on incidence of atrial fibrillation

Despite some common risk factors for atrial fibrillation (AF) being more prevalent among blacks, African Americans are increasingly being reported with lower prevalence and incidence of AF compared with whites. Contemporary studies have not provided a complete explanation for this apparent AF paradox in African Americans. Although many traditional and novel risk factors for AF have been identified, the role of ethnic-specific risk factors has not been examined. Whereas hypertension has been the most common risk factor associated with AF, coronary artery disease also plays an important role in AF pathophysiology in whites. Thereby, elucidating the role of ethnic-specific risk factors for AF may provide important insight into why African Americans are protected from AF or why whites are more prone to develop the arrhythmia. The link between AF susceptibility and genetic processes has only been recently uncovered. Polymorphisms in renin-angiotensin system genes have been characterized as predisposing to AF under certain environmental conditions. Several ion channel genes, signaling molecules, and several genetic loci have been linked with AF. Thereby, studies investigating genetic variants contributing to the differential AF risk in individuals of African American versus European ancestry may also provide important insight into the etiology of the AF paradox in blacks.

Chronic kidney disease is associated with the incidence of atrial fibrillation: the Atherosclerosis Risk in Communities (ARIC) study

BACKGROUND

Chronic kidney disease is associated with the incidence of cardiovascular disease. Chronic kidney disease may also increase the risk of atrial fibrillation (AF), but existing studies have reported inconsistent results.

METHODS AND RESULTS

We estimated cystatin C-based glomerular filtration rate (eGFR(cys)) and measured urinary albumin-to-creatinine ratio (ACR) in 10 328 men and women free of AF from the Atherosclerosis Risk in Communities (ARIC) Study in 1996 to 1998. Incidence of AF was ascertained through the end of 2007. During a median follow-up of 10.1 years, we identified 788 incident AF cases. Compared with individuals with eGFR(cys) ≥90 mL · min(-1) · 1.73 m(-2), multivariable hazard ratios and 95% confidence intervals (CIs) of AF were 1.3 (95% CI, 1.1 to 1.6), 1.6 (95% CI, 1.3 to 2.1), and 3.2 (95% CI, 2.0 to 5.0; P for trend <0.0001) in those with eGFR(cys) of 60 to 89, 30 to 59, and 15 to 29 mL · min(-1) · 1.73 m(-2), respectively. Similarly, the presence of macroalbuminuria (ACR ≥300 mg/g; hazard ratio, 3.2; 95% CI, 2.3 to 4.5) and microalbuminuria (ACR, 30 to 299 mg/g; hazard ratio, 2.0; 95% CI, 1.6 to 2.4) was associated with higher AF risk compared with those with ACR <30 mg/g. Risk of AF was particularly elevated in those with both low eGFR(cys) and macroalbuminuria (hazard ratio, 13.1; 95% CI, 6.0 to 28.6, comparing individuals with ACR ≥300 mg/g and eGFR(cys) of 15 to 29 mL · min(-1) · 1.73 m(-2) and those with ACR <30 mg/g and eGFR(cys) ≥90 mL · min(-1) · 1.73 m(-2)).

CONCLUSION

In this large population-based study, reduced kidney function and presence of albuminuria were strongly associated with the incidence of AF independently of other risk factors.

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.

Genetics of hypertension and cardiovascular disease and their interconnected pathways: lessons from large studies

Blood pressure (BP), hypertension (HT) and cardiovascular disease (CVD) are common complex phenotypes, which are affected by multiple genetic and environmental factors. This article describes recent genome-wide association studies (GWAS) that have reported causative variants for BP/HT and CVD/heart traits and analyzes the overlapping associated gene polymorphisms. It also examines potential replication of findings from the HyperGEN data on African Americans and whites. Several genes involved in BP/HT regulation also appear to be involved in CVD. A better picture is emerging, with overlapping hot-spot regions and with interconnected pathways between BP/HT and CVD. A systemic approach to full understanding of BP/HT and CVD development and their progression to disease may lead to the identification of gene targets and pathways for the development of novel therapeutic interventions.

The emerging genetic landscape underlying cardiac conduction system function

Proper function of an organized Cardiac Conduction System (CCS) is vital to the survival of metazoans ranging from fly to man. The routine use of non-invasive electrocardiogram measures in the diagnosis and monitoring of cardiovascular health has established a trove of reliable CCS functional data in both normal and diseased cardiac states. Recent combination of echocardiogram (ECG) data with genome-wide association studies has identified genomic regions implicated in ECG variability which impact CCS function. In this study, we review the substantial recent progress in this area, highlighting the identification of novel loci, confirming the importance of previously implicated loci in CCS function, and exploring potential links between genes with important roles in developmental processes and variation in function of the CCS.

From guidelines to bench: implications of unresolved clinical issues for basic investigations of atrial fibrillation mechanisms

The 2011 Canadian Cardiovascular Society Atrial Fibrillation (AF) Guidelines provide detailed recommendations for AF management, as well as extensive background information. The Guidelines documents highlight many important unresolved questions and areas of clinical need that could benefit from basic research investigations. This article discusses basic research priorities emanating from the Guidelines reflections. Topics addressed include forms of AF and their interrelations, limitations of the presently available experimental models of AF, genetic factors, determinants of drug efficacy for pharmacologic cardioversion, mechanisms of AF-related thromboembolism, ventricular rate control, drugs for rhythm control, upstream therapy, mechanisms by which catheter ablation controls AF, mechanisms of postoperative AF, and the possibility of novel patient-based surgical procedures. A guidelines-to-bench approach to research may allow for the development of important, clinically relevant new knowledge with impacts on patient management and future AF guidelines.

The pathology and treatment of cardiac arrhythmias: focus on atrial fibrillation

Atrial fibrillation (AF) is the most frequently encountered sustained cardiac arrhythmia in clinical practice and a major cause of morbidity and mortality. Effective treatment of AF still remains an unmet medical need. Treatment of AF is based on drug therapy and ablative strategies. Antiarrhythmic drug therapy is limited by a relatively high recurrence rate and proarrhythmic side effects. Catheter ablation suppresses paroxysmal AF in the majority of patients without structural heart disease but is more difficult to achieve in patients with persistent AF or with concomitant cardiac disease. Stroke is a potentially devastating complication of AF, requiring anticoagulation that harbors the risk of bleeding. In search of novel treatment modalities, targeted pharmacological treatment and gene therapy offer the potential for greater selectivity than conventional small-molecule or interventional approaches. This paper summarizes the current understanding of molecular mechanisms underlying AF. Established drug therapy and interventional treatment of AF is reviewed, and emerging clinical and experimental therapeutic approaches are highlighted.

Smoking and incidence of atrial fibrillation: results from the Atherosclerosis Risk in Communities (ARIC) study

BACKGROUND

Cigarette smoking increases the risk of coronary heart disease, but whether smoking increases atrial fibrillation (AF) is uncertain.

OBJECTIVE

The purpose of this study was to determine the association of cigarette smoking with incident AF in a population-based cohort of blacks and whites.

METHODS

We determined the risk of incident AF through December 2002 in relation to baseline (1987-1989) smoking status and cigarette-years of smoking in over 15,000 participants of the prospective Atherosclerosis Risk in Communities (ARIC) study.

RESULTS

Over a mean follow-up of 13.1 years, 876 incident AF events were identified. Compared to never smokers, the multivariable-adjusted hazard ratios (HRs) for AF were 1.32 (95% confidence interval [CI] 1.10-1.57) in former smokers, 2.05 (95% CI 1.71-2.47) in current smokers, and 1.58 (95% CI 1.35-1.85) in ever smokers. In the highest tertile of accumulated smoking amount (>675 cigarette-years), the incidence of AF was 2.10 times greater (95% CI 1.74-2.53) than in those who never smoked. Associations were similar by gender, race, type of event (AF and atrial flutter), and when only AF events identified by study exam ECGs were included. Finally, individuals who quit smoking exhibited a trend indicating a slightly lower risk of developing AF (HR 0.88, 95% CI 0.65-1.17) compared to those who continued to smoke.

CONCLUSION

Smoking was associated with the incidence of AF, with more than a two-fold increased risk of AF attributed to current smoking. In addition, a trend toward a lower incidence of AF appeared among smokers who quit compared to continued smokers.

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.

A clinical risk score for atrial fibrillation in a biracial prospective cohort (from the Atherosclerosis Risk in Communities [ARIC] study)

A risk score for atrial fibrillation (AF) has been developed by the Framingham Heart Study; however, the applicability of this risk score, derived using data from white patients, to predict new-onset AF in nonwhites is uncertain. Therefore, we developed a 10-year risk score for new-onset AF from risk factors commonly measured in clinical practice using 14,546 subjects from the Atherosclerosis Risk In Communities (ARIC) study, a prospective community-based cohort of blacks and whites in the United States. During 10 years of follow-up, 515 incident AF events occurred. The following variables were included in the AF risk score: age, race, height, smoking status, systolic blood pressure, hypertension medication use, precordial murmur, left ventricular hypertrophy, left atrial enlargement, diabetes, coronary heart disease, and heart failure. The area under the receiver operating characteristics curve (AUC) of a Cox regression model that included the previous variables was 0.78, suggesting moderately good discrimination. The point-based score developed from the coefficients in the Cox model had an AUC of 0.76. This clinical risk score for AF in the Atherosclerosis Risk In Communities cohort compared favorably with the Framingham Heart Study's AF (AUC 0.68), coronary heart disease (CHD) (AUC 0.63), and hard CHD (AUC 0.59) risk scores and the Atherosclerosis Risk In Communities CHD risk score (AUC 0.58). In conclusion, we have developed a risk score for AF and have shown that the different pathophysiologies of AF and CHD limit the usefulness of a CHD risk score in identifying subjects at greater risk of AF.

Significant association of SNP rs2106261 in the ZFHX3 gene with atrial fibrillation in a Chinese Han GeneID population

Atrial fibrillation (AF) is the most common cardiac rhythm disorder at the clinical setting and accounts for up to 15% of all strokes. Recent genome-wide association studies (GWAS) identified two single nucleotide polymorphisms (SNPs), rs2106261 and rs7193343 in ZFHX3 (zinc finger homeobox 3 gene) and rs13376333 in KCNN3 (encoding a potassium intermediate/small conductance calcium-activated channel, subfamily N, member 3) that showed significant association with AF in multiple populations of European ancestry. Here, we studied a Chinese Han, GeneID cohort consisting of 650 AF patients and 1,447 non-AF controls to test whether the GWAS findings on ZFHX3/KCNN3 and AF can be expanded to a different ethnic population. No significant association was detected for rs7193343 in ZFHX3 and rs13376333 in KCNN3. However, significant association was identified between rs2106261 in ZFHX3 and AF in the GeneID population for both allelic frequencies (P=0.001 after adjusting for covariates of age, gender, hypertension, coronary artery disease, and diabetes mellitus; OR=1.32), and genotypic frequencies assuming either an additive or recessive model (OR=1.29, P=0.001 and OR=1.77, P =0.00018, respectively). When only lone AF cases were analyzed, the association remained significant (OR=1.50, P=0.001 for allelic association; OR=1.45, P=0.001 for an additive model; OR=2.24, P=0.000043 for a recessive model). Our results indicate that rs2106261 in ZFHX3 confers a significant risk of AF in a Chinese Han population. The study expands the association between ZFHX3 and AF to a non-European ancestry population and provides the first evidence of a cross-race susceptibility of the 16q22 AF locus.

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.

Lack of replication in polymorphisms reported to be associated with atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common sustained arrhythmia and has a substantial heritable component. Numerous associations between single nucleotide polymorphisms (SNPs) and AF have been described, but few have been replicated.

OBJECTIVE

We sought to systematically replicate SNPs that are reported to be associated with AF in two large study samples of European descent.

METHODS

We searched PubMed for studies reporting associations between SNPs and AF published before July 1, 2007. SNPs were genotyped in two independent case-control samples from Germany and the United States. Associations between SNPs and AF were assessed using logistic regression models adjusting for age, sex, and hypertension. A meta-analysis of the results from the two studies was performed.

RESULTS

We identified 21 SNPs and the angiotensin-converting enzyme insertion/deletion polymorphism that were reported to be associated with AF in the literature. Nine of these genetic variants were not represented on common genome-wide SNP arrays. We successfully genotyped 21 of these 22 variants in 2,145 cases with AF from the German Competence Network for Atrial Fibrillation and 4,073 controls from the KORA S4 study and 16 variants in 790 cases and 1,330 controls from the Massachusetts General Hospital. None of the SNPs replicated in independent populations with AF.

CONCLUSION

Our results suggest that previously reported associations to AF were likely false positives and highlight the need for systematic replication of genetic associations in large, independent cohorts to accurately detect variants associated with disease.

Hemostatic markers are associated with the risk and prognosis of atrial fibrillation: the ARIC study

BACKGROUND

Various hemostatic markers are associated with the risk of cardiovascular disease; however, limited information exists on their relationship with the occurrence and prognosis of atrial fibrillation (AF).

OBJECTIVES

To assess whether hemostatic markers are associated with the incidence and prognosis of AF.

METHODS

We studied 14,858 men and women in the Atherosclerosis Risk in Communities cohort, aged 45-64 and free of AF at baseline (1987-1989). Fibrinogen, von Willebrand factor (vWf), factor VII activity (VIIc), factor VIII activity (VIIIc), protein C, antithrombin III (ATIII), and activated partial thromboplastin time (aPTT) were measured in blood samples at baseline. AF and other cardiovascular outcomes through 2005 were determined following standardized protocols.

RESULTS

During a median follow-up of 16.8 years, 1209 cases of AF were identified. In multivariable Cox models, the hazard ratios (HR) and 95% confidence intervals (CI) of incident AF associated with a 1-standard deviation (SD) increase in each marker were 1.13 (1.07-1.20) for fibrinogen, 1.17 (1.11-1.23) for vWf, 1.17 (1.11-1.24) for factor VIIIc, 0.93 (0.88-1.00) for factor VIIc, 0.98 (0.92-1.04) for protein C, 1.00 (0.94-1.06) for aPTT and 1.00 (0.95-1.06) for ATIII. Greater factor VIIIc, fibrinogen and vWf were consistently associated with a higher risk of cardiovascular outcomes and mortality in those with and without incident AF, while greater protein C was associated with a lower risk of ischemic stroke.

CONCLUSION

Several hemostatic markers are associated with the incidence of AF independently of other cardiovascular risk factors. Their role in the risk stratification of AF patients should be further studied.

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.

The ZFHX3 (ATBF1) transcription factor induces PDGFRB, which activates ATM in the cytoplasm to protect cerebellar neurons from oxidative stress

Ataxia telangiectasia (A-T) is a neurodegenerative disease caused by mutations in the large serine-threonine kinase ATM. A-T patients suffer from degeneration of the cerebellum and show abnormal elevation of serum alpha-fetoprotein. Here, we report a novel signaling pathway that links ATM via cAMP-responsive-element-binding protein (CREB) to the transcription factor ZFHX3 (also known as ATBF1), which in turn promotes survival of neurons by inducing expression of platelet-derived growth factor receptor β (PDGFRB). Notably, AG1433, an inhibitor of PDGFRB, suppressed the activation of ATM under oxidative stress, whereas AG1433 did not inhibit the response of ATM to genotoxic stress by X-ray irradiation. Thus, the activity of a membrane-bound tyrosine kinase is required to trigger the activation of ATM in oxidative stress, independent of the response to genotoxic stress. Kainic acid stimulation induced activation of ATM in the cerebral cortex, hippocampus and deep cerebellar nuclei (DCN), predominately in the cytoplasm in the absence of induction of γ-H2AX (a marker of DNA double-strand breaks). The activation of ATM in the cytoplasm might play a role in autophagy in protection of neurons against oxidative stress. It is important to consider DCN of the cerebellum in the etiology of A-T, because these neurons are directly innervated by Purkinje cells, which are progressively lost in A-T.

Independent susceptibility markers for atrial fibrillation on chromosome 4q25

BACKGROUND

Genetic variants on chromosome 4q25 are associated with atrial fibrillation (AF). We sought to determine whether there is more than 1 susceptibility signal at this locus.

METHODS AND RESULTS

Thirty-four haplotype-tagging single-nucleotide polymorphisms (SNPs) at the 4q25 locus were genotyped in 790 case and 1177 control subjects from Massachusetts General Hospital and tested for association with AF. We replicated SNPs associated with AF after adjustment for the most significantly associated SNP in 5066 case and 30 661 referent subjects from the German Competence Network for Atrial Fibrillation, Atherosclerosis Risk In Communities Study, Cleveland Clinic Lone AF Study, Cardiovascular Health Study, and Rotterdam Study. All subjects were of European ancestry. A multimarker risk score composed of SNPs that tagged distinct AF susceptibility signals was constructed and tested for association with AF, and all results were subjected to meta-analysis. The previously reported SNP, rs2200733, was most significantly associated with AF (minor allele odds ratio 1.80, 95% confidence interval 1.50 to 2.15, P=1.2 x 10(-20)) in the discovery sample. Adjustment for rs2200733 genotype revealed 2 additional susceptibility signals marked by rs17570669 and rs3853445. A graded risk of AF was observed with an increasing number of AF risk alleles at SNPs that tagged these 3 susceptibility signals.

CONCLUSIONS

We identified 2 novel AF susceptibility signals on chromosome 4q25. Consideration of multiple susceptibility signals at chromosome 4q25 identifies individuals with an increased risk of AF and may localize regulatory elements at the locus with biological relevance in the pathogenesis of AF.

Different aspects of atrial fibrillation genetics

Atrial fibrillation (AF) is a consequence of a complex interplay of genetic, epigenetic and environmental factors. In addition, AF is a major contributor to stroke, heart failure, and mortality. Several family studies have shown a strong polygenetic predisposition for AF but, so far, most of the linkage analysis and candidate gene studies have discovered only monogenic, rare, deleterious mutations. While research in human genetics has moved from monogenic to oligogenic to complex diseases, its pharmacogenetics branch has followed, usually a few years behind. The present paper reviews the potential contributions of genetic approaches to AF.

Birth weight is a significant risk factor for incident atrial fibrillation

BACKGROUND

Few if any studies have assessed the relationship between birth weight and incident atrial fibrillation (AF).

METHODS AND RESULTS

From 1993 to 2009, we prospectively followed 27 982 women who were >45 years of age and free of cardiovascular disease and AF at baseline. Information on birth weight was categorized into 5 different categories: <2.5, 2.5 to 3.2, 3.2 to 3.9, 3.9 to 4.5, and >4.5 kg. The primary outcome was time to incident AF. During 14.5 years of follow-up, 735 AF events occurred. Age-adjusted incidence rates for incident AF from the lowest to the highest birth weight category were 1.45, 1.82, 1.88, 2.57, and 2.55 events per 1000 person-years of follow-up. After multivariable adjustment, hazard ratios for incident AF across increasing birth weight categories were 1.0, 1.30 (95% confidence interval [CI], 0.96 to 1.75), 1.28 (95% CI, 0.96 to 1.69), 1.70 (95% CI, 1.23 to 2.37), and 1.71 (95% CI, 1.12 to 2.61) (P for linear trend=0.002). Adding body mass index, blood pressure, and diabetes mellitus at study entry did not have a large effect on these estimates (P for linear trend=0.004). In contrast, including height in the multivariable model substantially attenuated the relationship between birth weight and AF (P for linear trend=0.17), and additional adjustment for maximum weight in young adulthood further attenuated this association (multivariable-adjusted hazard ratio across birth weight categories, 1.0, 1.27 [95% CI, 0.94 to 1.71], 1.10 [95% CI, 0.83 to 1.46], 1.41 [95% CI, 1.01 to 1.96], and 1.29 [95% CI, 0.84 to 1.98]; P for linear trend=0.23).

CONCLUSIONS

Birth weight is significantly associated with incident AF among women, suggesting that early life determinants may play an important role in the pathogenesis of AF.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00000479.

Challenges in the classification of atrial fibrillation

The incidence and prevalence of atrial fibrillation (AF) are increasing worldwide. AF is of public health importance because it accounts for substantial morbidity, mortality, and health-care costs. AF may be transient initially, but many patients have progressive disease marked by increasing frequency and duration of episodes. Various classification schemes for AF have been proposed, although current guidelines are based on temporal rhythm-based patterns. We discuss existing schemes for the classification of AF, focusing on the advantages and limitations of the pattern-based scheme, in the context of new knowledge about AF pathophysiology, AF patterns, and clinical outcomes. Furthermore, we address gaps in knowledge that present opportunities to re-examine the current pattern-based classification of AF. A future classification scheme should ideally combine elements such as the risk of stroke, an assessment of symptoms, and the degree of impairment of the atrial substrate.

Evaluation of non-synonymous NPPA single nucleotide polymorphisms in atrial fibrillation

AIMS

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, is an important cause of morbidity and mortality. A genetic mutation in the NPPA gene, which encodes the atrial natriuretic peptide, has been identified as the putative causative factor in a family with an autosomal dominant pattern of inheritance for AF. Two common single nucleotide polymorphisms (SNPs) in NPPA, rs5063 and rs5065, result in amino acid changes of the primary peptide and have been previously implicated in conditions associated with AF, including stroke and hypertension. Recently, the rs5063 SNP has been reported to confer an increased risk of AF development in a Chinese population. We sought to examine the associations of both rs5063 and rs5065 with AF in two separate North American cohorts of European ancestry.

METHODS AND RESULTS

Patients with early-onset AF, along with healthy controls, were recruited at the University of Ottawa Heart Institute (UOHI) and the Massachusetts General Hospital (MGH). Study participants were genotyped for rs5063 and rs5065 using a combination of restriction fragment length polymorphism analysis and DNA microarrays. The study genotyped a total of 620 AF cases and 2446 healthy controls. The UOHI arm of the study identified an odds ratio (OR) of 0.72 [95% confidence interval (CI): 0.42-1.24] for rs5063, whereas an OR of 1.33 (95% CI: 0.80-2.21) was observed in the MGH arm. The combined OR approximated unity (OR 0.99; 95% CI: 0.54-1.80). Analysis of rs5065 revealed an OR of 1.12 (95% CI: 0.84-1.48) in UOHI, 1.08 (95% CI 0.80-1.45) in MGH, and 1.10 (95% CI 0.90-1.35) when combined.

CONCLUSION

Common non-synonymous genetic variants within NPPA in these two large North American cohorts of European ancestry are not associated with the development of AF.

Cardiovascular genomics: outcomes and implications

The application of genomics technology to clinical cardiovascular research is producing fundamentally new insights concerning the etiology of cardiovascular disease phenotypes. Recent genome-wide association studies demonstrate clear associations between single nucleotide polymorphisms and important cardiovascular phenotypes. However, risk alleles for the single nucleotide polymorphisms in question do not explain a sufficient portion of individual risk to be useful for screening purposes. Therefore, clinicians should continue to make use of family history to augment risk stratification and emphasize established forms of prevention for their patients with, or at risk for, cardiovascular disease.

Metabolic syndrome and incidence of atrial fibrillation among blacks and whites in the Atherosclerosis Risk in Communities (ARIC) Study

BACKGROUND

The metabolic syndrome (MetSyn) has been implicated in the development of atrial fibrillation (AF); however, knowledge of this association among blacks is limited.

METHODS

We determined the risk of incident AF through December 2005 in relation to baseline (1987-1989) MetSyn status in 15,094 participants of the Atherosclerosis Risk in Communities study.

RESULTS

Over a mean follow-up of 15.4 years, 1,238 incident AF events were identified. The hazard ratio (HR) for AF among individuals with, compared to those without, the MetSyn was 1.67 (95% CI 1.49-1.87), and associations did not differ by race (P for interaction = .73). The population attributable risk of AF from the MetSyn was 22%. The multivariable-adjusted HRs (95% CI) for each MetSyn component were 1.95 (1.72-2.21) (elevated blood pressure), 1.40 (1.23-1.59) (elevated waist circumference), 1.20 (1.06-1.37) (low high-density lipoprotein cholesterol), 1.16 (1.03-1.31) (impaired fasting glucose), and 0.95 (0.84-1.09) (elevated triglycerides). A monotonically increasing risk of AF with increasing number of MetSyn components was observed, with an HR of 4.40 (95% CI 3.25-5.94) for those with all 5 MetSyn components compared to those with 0 components.

CONCLUSION

In this large cohort, the MetSyn and most of its components were associated with a higher risk of AF in both blacks and whites. Given the high prevalence of the MetSyn, strategies to prevent its development or to control individual components may reduce the burden of AF.

Genome-wide association studies in cardiac electrophysiology: recent discoveries and implications for clinical practice

Genome-wide association studies have been increasingly used to study the genetics of complex human diseases. Within the field of cardiac electrophysiology, this technique has been applied to conditions such as atrial fibrillation, and several electrocardiographic parameters including the QT interval. While these studies have identified multiple genomic regions associated with each trait, questions remain, including the best way to explore the pathophysiology of each association and the potential for clinical utility. This review will summarize recent genome-wide association study results within cardiac electrophysiology and discuss their broader implications in basic science and clinical medicine.