Variants conferring risk of atrial fibrillation on chromosome 4q25

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.

PITX2c is expressed in the adult left atrium, and reducing Pitx2c expression promotes atrial fibrillation inducibility and complex changes in gene expression

BACKGROUND

Intergenic variations on chromosome 4q25, close to the PITX2 transcription factor gene, are associated with atrial fibrillation (AF). We therefore tested whether adult hearts express PITX2 and whether variation in expression affects cardiac function.

METHODS AND RESULTS

mRNA for PITX2 isoform c was expressed in left atria of human and mouse, with levels in right atrium and left and right ventricles being 100-fold lower. In mice heterozygous for Pitx2c (Pitx2c(+/-)), left atrial Pitx2c expression was 60% of wild-type and cardiac morphology and function were not altered, except for slightly elevated pulmonary flow velocity. Isolated Pitx2c(+/-) hearts were susceptible to AF during programmed stimulation. At short paced cycle lengths, atrial action potential durations were shorter in Pitx2c(+/-) than in wild-type. Perfusion with the β-receptor agonist orciprenaline abolished inducibility of AF and reduced the effect on action potential duration. Spontaneous heart rates, atrial conduction velocities, and activation patterns were not affected in Pitx2c(+/-) hearts, suggesting that action potential duration shortening caused wave length reduction and inducibility of AF. Expression array analyses comparing Pitx2c(+/-) with wild-type, for left atrial and right atrial tissue separately, identified genes related to calcium ion binding, gap and tight junctions, ion channels, and melanogenesis as being affected by the reduced expression of Pitx2c.

CONCLUSIONS

These findings demonstrate a physiological role for PITX2 in the adult heart and support the hypothesis that dysregulation of PITX2 expression can be responsible for susceptibility to AF.

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.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Effect of race on the frequency of postoperative atrial fibrillation following coronary artery bypass grafting

Atrial fibrillation (AF) commonly complicates the postoperative course after coronary artery bypass grafting (CABG). Among the general population, African Americans have been shown to have a lower prevalence of AF than European Americans. Although many factors have been identified to predict risk for postoperative AF, race has not been examined. All patients aged ≥18 years who underwent CABG at Henry Ford Hospital during a 5-year period from January 1, 2004, to December 31, 2008, were included. Patients were excluded for any previous diagnosis of AF or if they had concomitant valve surgery at the time of CABG. The incidence of AF was determined by International Classification of Diseases, Ninth Revision, coding from postoperative hospitalization records. Overall, 1,001 patients were eligible for analysis. Of these, 731 (73%) were European American and 270 (27%) were African American. The African American group had a higher prevalence of hypertension (75.6% vs 58.8%, p <0.001) and heart failure (22.6% vs 15.7%, p = 0.01) and a trend toward a higher prevalence of diabetes mellitus (38.1% vs 33.4%, p = 0.159). Postoperative AF was diagnosed in 214 European Americans (29.3%) and 50 African Americans (18.5%) (p = 0.001). In multivariate analysis adjusting for age strata, gender, hypertension, diabetes, and heart failure, African Americans had less postoperative AF than European Americans, with an adjusted odds ratio of 0.539 (95% confidence interval 0.374 to 0.777, p = 0.001). In conclusion, African Americans have a significantly reduced incidence of AF compared to European Americans after CABG.

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.

Incomplete right bundle branch block: a novel electrocardiographic marker for lone atrial fibrillation

AIMS

P-wave morphology and PR interval have both been previously associated with atrial fibrillation (AF). We hypothesized that incomplete right bundle branch block (IRBBB) would be associated with early-onset lone AF.

METHODS AND RESULTS

We conducted a case-control study comparing electrocardiographic (ECG) markers from patients with early-onset lone AF and from a healthy control population. We included 187 patients with early-onset lone AF and 383 healthy controls. Sixty-two lone AF patients were excluded from the study because of AF at the time of enrolment or because of the use of antiarrhythmic drugs. For the remaining 125 patients with paroxysmal or persistent lone AF (84% males, mean age 37), controls were matched on a 1:1 basis on the parameters gender and age. A significantly higher proportion of the lone AF population had an IRBBB compared with the subjects in the control group (33.6 vs. 10.4%; P<0.001). In multivariable analysis adjusted for conventional risk factors, IRBBB was strongly associated with lone AF [odds ratio (OR) 5.43; 95% confidence interval (CI) 2.30-13.02; P<0.001]. Lone AF patients had a significantly longer PR interval than the control group (175.1 vs. 160.9 ms; P<0.001), but in multivariable analysis, every 10 ms increase in the PR interval was only borderline significantly associated with an OR of 1.15 (95% CI 0.99-1.32; P=0.060) for lone AF.

CONCLUSION

We are the first to report that IRBBB is strongly and independently associated with early-onset lone AF.

Guidelines for the primary prevention of stroke: a guideline for healthcare professionals from the American Heart Association/American Stroke Association

BACKGROUND AND PURPOSE

This guideline provides an overview of the evidence on established and emerging risk factors for stroke to provide evidence-based recommendations for the reduction of risk of a first stroke.

METHODS

Writing group members were nominated by the committee chair on the basis of their previous work in relevant topic areas and were approved by the American Heart Association (AHA) Stroke Council Scientific Statement Oversight Committee and the AHA Manuscript Oversight Committee. The writing group used systematic literature reviews (covering the time since the last review was published in 2006 up to April 2009), reference to previously published guidelines, personal files, and expert opinion to summarize existing evidence, indicate gaps in current knowledge, and when appropriate, formulate recommendations using standard AHA criteria (Tables 1 and 2). All members of the writing group had the opportunity to comment on the recommendations and approved the final version of this document. The guideline underwent extensive peer review by the Stroke Council leadership and the AHA scientific statements oversight committees before consideration and approval by the AHA Science Advisory and Coordinating Committee.

RESULTS

Schemes for assessing a person's risk of a first stroke were evaluated. Risk factors or risk markers for a first stroke were classified according to potential for modification (nonmodifiable, modifiable, or potentially modifiable) and strength of evidence (well documented or less well documented). Nonmodifiable risk factors include age, sex, low birth weight, race/ethnicity, and genetic predisposition. Well-documented and modifiable risk factors include hypertension, exposure to cigarette smoke, diabetes, atrial fibrillation and certain other cardiac conditions, dyslipidemia, carotid artery stenosis, sickle cell disease, postmenopausal hormone therapy, poor diet, physical inactivity, and obesity and body fat distribution. Less well-documented or potentially modifiable risk factors include the metabolic syndrome, excessive alcohol consumption, drug abuse, use of oral contraceptives, sleep-disordered breathing, migraine, hyperhomocysteinemia, elevated lipoprotein(a), hypercoagulability, inflammation, and infection. Data on the use of aspirin for primary stroke prevention are reviewed.

CONCLUSIONS

Extensive evidence identifies a variety of specific factors that increase the risk of a first stroke and that provide strategies for reducing that risk.

Genetics for the Electrophysiologist: Take Home Messages for the Clinician

Syncope and risk of sudden death caused by ventricular tachyarrhythmia are the common manifestations of several inherited disorders. The abnormalities of the genetic makeup may directly affect proteins controlling cardiac excitability in a structurally normal heart. Other diseases manifest primarily with ventricular arrhythmias even if the genetic mutations cause structural abnormalities of the myocardium, such as arrhythmogenic right ventricular cardiomyopathy and hypertrophic cardiomyopathy. The groundbreaking discoveries that began in the 1990s and continued until the beginning of the current decade gathered fundamental knowledge about the major genes controlling cardiac excitability and conferring an increased risk of severe arrhythmias. Stemming from such knowledge is the availability of genetic diagnosis, genotype-phenotype correlation, and genotype-based risk stratification schemes. This article provides a concise description of the known genes and key mechanisms involved in the pathogenesis of inherited arrhythmias and outlines the possibilities, limitations, advantages, and potential threats of genetic testing for inherited arrhythmogenic syndromes.

Monogenic atrial fibrillation as pathophysiological paradigms

Atrial fibrillation (AF) is the most common cardiac rhythm abnormality and represents a major burden, both to patients and to health-care systems. In recent years, increasing evidence from population-based studies has demonstrated that AF is a heritable condition. Although familial forms of AF have been recognized for many years, they represent a rare subtype of the arrhythmia. However, despite their limited prevalence, the identification of mutations in monogenic AF kindreds has provided valuable insights into the molecular pathways underlying the arrhythmia and a framework for investigating AF encountered in the general population. In contrast to these rare families, the typical forms of AF occurring in the community are likely to be multigenic and have significant environmental influences. Recently, genome-wide association studies have uncovered common sequence variants that confer increased susceptibility to the arrhythmia. In the future, the elucidation of the genetic substrate underlying both familial and more typical forms of AF will hopefully lead to the development of novel diagnostic tools as well as more targeted rhythm control strategies. In this article, we will focus on monogenic forms of AF and also provide an overview of case-control association studies for 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.

Mutational analysis of the PITX2 and NKX2-5 genes in patients with idiopathic atrial fibrillation

Atrial fibrillation (AF) is the most frequently encountered arrhythmia in clinical practice. In a subgroup of patients, AF is regarded as idiopathic when no signs of structural heart disease or other causes of the arrhythmia can be identified during conventional clinical work-up. Recent studies have demonstrated that AF has a substantial genetic basis in a number of cases. The entire coding sequences, including intron-exon boundaries, of the genes PITX2 and NKX2-5 were screened for genetic variants by means of initial polymerase chain reaction followed by DNA sequencing in 96 patients with idiopathic AF. Although we detected a number of variants, our candidate gene approach did not result in identification of mutations associated with AF in the coding regions of PITX2 or NKX2-5 in our well characterized AF cohort.

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.

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.

European bone mineral density loci are also associated with BMD in East-Asian populations

Most genome-wide association (GWA) studies have focused on populations of European ancestry with limited assessment of the influence of the sequence variants on populations of other ethnicities. To determine whether markers that we have recently shown to associate with Bone Mineral Density (BMD) in Europeans also associate with BMD in East-Asians we analysed 50 markers from 23 genomic loci in samples from Korea (n = 1,397) and two Chinese Hong Kong sample sets (n = 3,869 and n = 785). Through this effort we identified fourteen loci that associated with BMD in East-Asian samples using a false discovery rate (FDR) of 0.05; 1p36 (ZBTB40, P = 4.3×10⁻⁹), 1p31 (GPR177, P = 0.00012), 3p22 (CTNNB1, P = 0.00013), 4q22 (MEPE, P = 0.0026), 5q14 (MEF2C, P = 1.3×10⁻⁵), 6q25 (ESR1, P = 0.0011), 7p14 (STARD3NL, P = 0.00025), 7q21 (FLJ42280, P = 0.00017), 8q24 (TNFRSF11B, P = 3.4×10⁻⁵), 11p15 (SOX6, P = 0.00033), 11q13 (LRP5, P = 0.0033), 13q14 (TNFSF11, P = 7.5×10⁻⁵), 16q24 (FOXL1, P = 0.0010) and 17q21 (SOST, P = 0.015). Our study marks an early effort towards the challenge of cataloguing bone density variants shared by many ethnicities by testing BMD variants that have been established in Europeans, in East-Asians.

Calcium handling in zebrafish ventricular myocytes

The zebrafish is an important model for the study of vertebrate cardiac development with a rich array of genetic mutations and biological reagents for functional interrogation. The similarity of the zebrafish (Danio rerio) cardiac action potential with that of humans further enhances the relevance of this model. In spite of this, little is known about excitation-contraction coupling in the zebrafish heart. To address this issue, adult zebrafish cardiomyocytes were isolated by enzymatic perfusion of the cannulated ventricle and were subjected to amphotericin-perforated patch-clamp technique, confocal calcium imaging, and/or measurements of cell shortening. Simultaneous recordings of the voltage dependence of the L-type calcium current (I(Ca,L)) amplitude and cell shortening showed a typical bell-shaped current-voltage (I-V) relationship for I(Ca,L) with a maximum at +10 mV, whereas calcium transients and cell shortening showed a monophasic increase with membrane depolarization that reached a plateau at membrane potentials above +20 mV. Values of I(Ca,L) were 53, 100, and 17% of maximum at -20, +10, and +40 mV, while the corresponding calcium transient amplitudes were 64, 92, and 98% and cell shortening values were 62, 95, and 96% of maximum, respectively, suggesting that I(Ca,L) is the major contributor to the activation of contraction at voltages below +10 mV, whereas the contribution of reverse-mode Na/Ca exchange becomes increasingly more important at membrane potentials above +10 mV. Comparison of the recovery of I(Ca,L) from acute and steady-state inactivation showed that reduction of I(Ca,L) upon elevation of the stimulation frequency is primarily due to calcium-dependent I(Ca,L) inactivation. In conclusion, we demonstrate that a large yield of healthy atrial and ventricular myocytes can be obtained by enzymatic perfusion of the cannulated zebrafish heart. Moreover, zebrafish ventricular myocytes differed from that of large mammals by having larger I(Ca,L) density and a monophasically increasing contraction-voltage relationship, suggesting that caution should be taken upon extrapolation of the functional impact of mutations on calcium handling and contraction in zebrafish cardiomyocytes.

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 zebrafish: scalable in vivo modeling for systems biology

The zebrafish offers a scalable vertebrate model for many areas of biologic investigation. There is substantial conservation of genetic and genomic features and, at a higher order, conservation of intermolecular networks, as well as physiologic systems and phenotypes. We highlight recent work demonstrating the extent of this homology, and efforts to develop high-throughput phenotyping strategies suited to genetic or chemical screening on a scale compatible with in vivo validation for systems biology. We discuss the implications of these approaches for functional annotation of the genome, elucidation of multicellular processes in vivo, and mechanistic exploration of hypotheses generated by a broad range of 'unbiased' 'omic technologies such as expression profiling and genome-wide association. Finally, we outline potential strategies for the application of the zebrafish to the systematic study of phenotypic architecture, disease heterogeneity and drug responses.

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.

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.

The genetics of congestive heart failure

The heart failure syndrome is known to represent a final common pathway for a broad range of etiologies, but there is tremendous variation in the propensity to develop congestive heart failure after a given insult. This variation is thought to result in part from inherited differences in myocardial, vascular or systemic responses, but the nature of the underlying traits responsible ultimately for the development of heart failure has remained elusive. There has been limited progress in the genetic exploration of the key clinical phenotype itself: heart failure. In this article, the author attempts to place the results of genetic studies of cardiomyopathy in the broader context of the clinical syndrome of heart failure, highlighting some of the key questions for future study.

Genetics of atrial fibrillation

Recent studies of atrial fibrillation (AF) have identified mutations in a series of ion channels; however, these mutations appear to be relatively rare causes of AF. A genome-wide association study has identified novel variants on chromosome 4 associated with AF, although the mechanism of action for these variants remains unknown. Ultimately, a greater understanding of the genetics of AF should yield insights into novel pathways, therapeutic targets, and diagnostic testing for this common arrhythmia.

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.

Genomics of cardiovascular disease

As the leading cause of death worldwide and a major cause of disability, cardiovascular disease remains a central focus of basic research, pharmacological treatment, surgical interventions, and long-term care. Inherited, monogenic syndromes have provided insight into pathophysiological mechanisms across the range of cardiovascular diseases. With the advent of post-Human Genome Project resources and technology, there has been a flood of research aimed at genome-wide predisposition markers, pharmacogenetics, and genomic signatures in complex cardiovascular disorders. Genomic research has both further elucidated the impact of genes previously identified in cardiovascular disease development and progression and discovered genomic regions as yet unknown to be associated with cardiovascular outcomes. The promise of personalized medicine lies in combining this genetic information with other biomarkers to tailor preventive and therapeutic strategies to individual patients for effective management, fewer adverse events, and preventive care.

Lone AF - Etiologic Factors and Genetic Insights into Pathophysiolgy

Ever since atrial fibrillation (AF) was first recognized in young people (so called "lone" AF) over 4 decades ago, there has been increasing focus on determining the underlying pathophysiology of the condition. Although lone AF is presumed to be a highly heterogeneous disease, recent studies have identified novel risk factors such as inflammation, oxidative stress, endurance sports and genetics, for the arrhythmia. This monograph aims to highlight some of the recent advances in our understanding of the molecular pathophysiology of lone AF especially insight provided by contemporary genetic studies. These insights may provide novel therapeutic targets for treatment of this challenging arrhythmia in young patients.

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.

Pitx2 prevents susceptibility to atrial arrhythmias by inhibiting left-sided pacemaker specification

Atrial fibrillation (AF), the most prevalent sustained cardiac arrhythmia, often coexists with the related arrhythmia atrial flutter (AFL). Limitations in effectiveness and safety of current therapies make an understanding of the molecular mechanism underlying AF more urgent. Genome-wide association studies implicated a region of human chromosome 4q25 in familial AF and AFL, approximately 150 kb distal to the Pitx2 homeobox gene, a developmental left-right asymmetry (LRA) gene. To investigate the significance of the 4q25 variants, we used mouse models to investigate Pitx2 in atrial arrhythmogenesis directly. When challenged by programmed stimulation, Pitx2(null+/-) adult mice had atrial arrhythmias, including AFL and atrial tachycardia, indicating that Pitx2 haploinsufficiency predisposes to atrial arrhythmias. Microarray and in situ studies indicated that Pitx2 suppresses sinoatrial node (SAN)-specific gene expression, including Shox2, in the left atrium of embryos and young adults. In vivo ChIP and transfection experiments indicated that Pitx2 directly bound Shox2 in vivo, supporting the notion that Pitx2 directly inhibits the SAN-specific genetic program in left atrium. Our findings implicate Pitx2 and Pitx2-mediated LRA-signaling pathways in prevention of atrial arrhythmias.

Early management of atrial fibrillation: from imaging to drugs to ablation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and is responsible for the highest number of rhythm-related disorders and cardioembolic strokes worldwide. Early management of this condition will lower the risk of AF-associated morbidity and mortality. Targeted drug therapy has an important role in preventing the progression of AF through modification of the substrate. Discovery of the role of pulmonary veins as a trigger has been an important breakthrough, leading to the development of pulmonary vein ablation-an established curative therapy for drug-resistant AF. Identifying the underlying reasons for the abnormal firing of venous cardiomyocytes and the widespread progressive alterations of atrial tissue found in persistent AF are challenges for the future. Novel imaging techniques may help to determine the right time for intervention, provide specific targets for ablation, and judge the efficacy of treatment. If new developments can successfully address these issues, the knowledge acquired as a result will have a vital role in preclinical and early management 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.

Impact of limited population diversity of genome-wide association studies

The article describes the limited population diversity of genome-wide association studies and its resulting impact on the development of commercial genetic tests with restricted applicability and usefulness to certain groups, potentially increasing existing disparities. To enable development of new clinical tools applicable to all groups, much more focus is needed to engage minority communities to enroll in genetics or genomic research studies and on investigators to reach out to underrepresented communities.

Robust replication of genotype-phenotype associations across multiple diseases in an electronic medical record

Large-scale DNA databanks linked to electronic medical record (EMR) systems have been proposed as an approach for rapidly generating large, diverse cohorts for discovery and replication of genotype-phenotype associations. However, the extent to which such resources are capable of delivering on this promise is unknown. We studied whether an EMR-linked DNA biorepository can be used to detect known genotype-phenotype associations for five diseases. Twenty-one SNPs previously implicated as common variants predisposing to atrial fibrillation, Crohn disease, multiple sclerosis, rheumatoid arthritis, or type 2 diabetes were successfully genotyped in 9483 samples accrued over 4 mo into BioVU, the Vanderbilt University Medical Center DNA biobank. Previously reported odds ratios (OR(PR)) ranged from 1.14 to 2.36. For each phenotype, natural language processing techniques and billing-code queries were used to identify cases (n = 70-698) and controls (n = 808-3818) from deidentified health records. Each of the 21 tests of association yielded point estimates in the expected direction. Previous genotype-phenotype associations were replicated (p < 0.05) in 8/14 cases when the OR(PR) was > 1.25, and in 0/7 with lower OR(PR). Statistically significant associations were detected in all analyses that were adequately powered. In each of the five diseases studied, at least one previously reported association was replicated. These data demonstrate that phenotypes representing clinical diagnoses can be extracted from EMR systems, and they support the use of DNA resources coupled to EMR systems as tools for rapid generation of large data sets required for replication of associations found in research cohorts and for discovery in genome science.

Identification of NPPA variants associated with atrial fibrillation in a Chinese GeneID population

BACKGROUND

A frameshift mutation in the NPPA gene was identified in 1 family with atrial fibrillation (AF), however, further studies are needed to establish unequivocally the genetic association between NPPA and AF.

METHODS

A case control association study and mutational analysis of NPPA were performed with 384 sporadic AF patients and 844 controls from a Chinese GeneID population. Genotyping was performed using High-Resolution Melt analysis. Mutational analysis was performed using direct DNA sequencing analysis.

RESULTS

Significant allelic association was detected between single nucleotide polymorphism (SNP) rs5063 and lone AF (p=0.015, OR=1.63; adjusted p=0.003). Genotypic association was significant assuming an additive or dominant model (adjusted p=0.005 and 0.007, respectively). Six new variants were identified in NPPA, including 2 in the 5'-UTR, 2 in the 3'-UTR, and 2 missense substitutions. Variants c.413T>C, c.*48G>A and c.*133G>T were not present in 844 controls, and the others were identified in controls.

CONCLUSIONS

Variants in NPPA confer risk of lone AF in a Chinese population. Thus, in addition to being a disease-causing gene with mutations identified in familial AF cases, NPPA is a susceptibility gene for lone AF.

Atrial fibrillation in congestive heart failure

Atrial fibrillation and congestive heart failure are morbid conditions that have common risk factors and frequently coexist. Each condition predisposes to the other, and the concomitant presence of the two identifies individuals at increased risk for mortality. Recent data have emerged that help elucidate the complex genetic and nongenetic pathophysiological mechanisms that contribute to the development of atrial fibrillation in individuals with congestive heart failure. Clinical trial results offer insights into the noninvasive prevention and management of these conditions, although newer technologies, such as catheter ablation for atrial fibrillation, have yet to be studied extensively in patients with congestive heart failure.