Genomics in sudden cardiac death

Appropriate time for ejection fraction reassessment after revascularization in patients with left ventricular dysfunction for risk stratification of sudden cardiac death

BACKGROUND

Appropriate time for ejection fraction (EF) reassessment after revascularization in patients with left ventricular dysfunction has not been investigated comprehensively, although 3 months after revascularization is recommended to stratify the risk of sudden cardiac death (SCD).

HYPOTHESIS

EF reassessed within different timeframe after revascularization may have incosistent contribution for risk stratification of SCD.

METHODS

Patients who had EF ≤ 40% before revascularization and had EF reassessment at least once during follow-up were included. The role of early (<3 months) versus late (3-12 months) EF measurements in prediction of all-cause mortality and SCD were compared.

RESULTS

A total of 1589 patients were identified. EF reassessed <3 months was lower than EF reassessed within 3-12 months (42.1 ± 9.7% vs. 45.8 ± 10.8%; p < .01). Among 1069 patients who had EF reassessed <3 months, EF ≤ 35% was associated with a higher risk of all-cause mortality (hazard ratio [HR], 1.67; 95% confidence interval [CI], 1.22-2.29; p < .01), but had no association with the risk of SCD (HR, 1.44; 95% CI, 0.84-2.48; p = .18). By contrast, among 595 patients who had EF reassessed within 3-12 months, EF ≤ 35% was associated with higher risks of both all-cause death (HR, 1.81; 95% CI, 1.06-3.10; p = .03) and SCD (HR, 2.71; 95% CI, 1.31-5.61; p < .01). The relative contribution of SCD to all-cause death was higher in patients with EF ≤ 35% than patients with EF > 35% when EF was reassessed within 3-12 months (p = .04). However, when EF was reassessed <3 months, the mode of death was similar in patients with EF ≤ 35% versus >35% (p = .85).

CONCLUSIONS

3 to 12 months after revascularization may be appropriate for cardiac function reassessment and SCD risk stratification.

Role of induced pluripotent stem cells in diagnostic cardiology

Ethical concerns about stem cell-based research have delayed important advances in many areas of medicine, including cardiology. The introduction of induced pluripotent stem cells (iPSCs) has supplanted the need to use human stem cells for most purposes, thus eliminating all ethical controversies. Since then, many new avenues have been opened in cardiology research, not only in approaches to tissue replacement but also in the design and testing of antiarrhythmic drugs. This methodology has advanced to the point where induced human cardiomyocyte cell lines can now also be obtained from commercial sources or tissue banks. Initial studies with readily available iPSCs have generally confirmed that their behavioral characteristics accurately predict the behavior of beating cardiomyocytes in vivo. As a result, iPSCs can provide new ways to study arrhythmias and heart disease in general, accelerating the development of new, more effective antiarrhythmic drugs, clinical diagnoses, and personalized medical care. The focus on producing cardiomyocytes that can be used to replace damaged heart tissue has somewhat diverted interest in a host of other applications. This manuscript is intended to provide non-specialists with a brief introduction and overview of the research carried out in the field of heart rhythm disorders.

Hyperthyroidism precipitating cardiac arrest in a patient with Brugada pattern

A 38-year-old man previously healthy suffered an out-of-hospital cardiac arrest; he was resuscitated successfully and admitted to the intensive care unit. His initial ECG suggested a Brugada pattern; other laboratory tests revealed low potassium level, low Thyroid Stimulating Hormone (TSH) and high FT4. He was started on carbimazole for hyperthyroidism, along with other supportive care. A comprehensive cardiac evaluation was done, including ajmaline and flecainide tests, results were inconclusive. An implantable cardioverter defibrillator device (ICD) was inserted to prevent such catastrophic events in the future. After discharge and on follow-up, our patient was doing well. His thyroid function test (TFT) was normal; moreover, a follow-up ICD interrogation did not record any arrhythmias. This case report highlighted asymptomatic hyperthyroidism as a precipitant for Brugada pattern resulting in sudden cardiac arrest.

Seasonality of ventricular fibrillation at first myocardial infarction and association with viral exposure

AIMS

To investigate seasonality and association of increased enterovirus and influenza activity in the community with ventricular fibrillation (VF) risk during first ST-elevation myocardial infarction (STEMI).

METHODS

This study comprised all consecutive patients with first STEMI (n = 4,659; aged 18-80 years) admitted to the invasive catheterization laboratory between 2010-2016, at Copenhagen University Hospital, Rigshospitalet, covering eastern Denmark (2.6 million inhabitants, 45% of the Danish population). Hospital admission, prescription, and vital status data were assessed using Danish nationwide registries. We utilized monthly/weekly surveillance data for enterovirus and influenza from the Danish National Microbiology Database (2010-2016) that receives copies of laboratory tests from all Danish departments of clinical microbiology.

RESULTS

Of the 4,659 consecutively enrolled STEMI patients, 581 (12%) had VF before primary percutaneous coronary intervention. In a subset (n = 807), we found that VF patients experienced more generalized fatigue and flu-like symptoms within 7 days before STEMI compared with the patients without VF (OR 3.39, 95% CI 1.76-6.54). During the study period, 2,704 individuals were diagnosed with enterovirus and 19,742 with influenza. No significant association between enterovirus and VF (OR 1.00, 95% CI 0.99-1.02), influenza and VF (OR 1.00, 95% CI 1.00-1.00), or week number and VF (p-value 0.94 for enterovirus and 0.89 for influenza) was found.

CONCLUSION

We found no clear seasonality of VF during first STEMI. Even though VF patients had experienced more generalized fatigue and flu-like symptoms within 7 days before STEMI compared with patients without VF, no relationship was found between enterovirus or influenza exposure and occurrence of VF.

Multiple SCN5A variant enhancers modulate its cardiac gene expression and the QT interval

The rationale for genome-wide association study (GWAS) results is sequence variation in cis-regulatory elements (CREs) modulating a target gene's expression as the major cause of trait variation. To understand the complete molecular landscape of one of these GWAS loci, we performed in vitro reporter screens in cardiomyocyte cell lines for CREs overlapping nearly all common variants associated with any of five independent QT interval (QTi)-associated GWAS hits at the SCN5A-SCN10A locus. We identified 13 causal CRE variants using allelic reporter activity, cardiomyocyte nuclear extract-based binding assays, overlap with human cardiac tissue DNaseI hypersensitive regions, and predicted impact of sequence variants on DNaseI sensitivity. Our analyses identified at least one high-confidence causal CRE variant for each of the five sentinel hits that could collectively predict SCN5A cardiac gene expression and QTi association. Although all 13 variants could explain SCN5A gene expression, the highest statistical significance was obtained with seven variants (inclusive of the five above). Thus, multiple, causal, mutually associated CRE variants can underlie GWAS signals.

An insertion/deletion polymorphism within 3'UTR of RYR2 modulates sudden unexplained death risk in Chinese populations

Sudden unexplained death (SUD) constitutes a part of the overall sudden death that can not be underestimated. Over the last years, genetic testing on SUD has revealed that inherited channelopathies might play important roles in the pathophysiology of this disease. Ryanodine receptor type-2 (RYR2) is a kind of ion channel extensively distributed in the sarcoplasmic reticulum (SR) of myocardium. Studies on RYR2 have suggested that either dysfunction or abnormal expression of it could lead to arrhythmia, which may cause cardiac arrest. In this study, we conducted a case-control study to evaluate the association of a 4-base pair (4-bp) Indel polymorphism (rs10692285) in the 3'UTR of RYR2 with the risk of SUD and sudden cardiac death induced by coronary heart disease (SCD-AS) in a Chinese population. Logistic regression analysis showed that the insertion allele of rs10692285 had significantly increased the risk of SUD [OR=2.03; 95% confidence interval (CI)=1.08-3.77; P=0.0161; statistical power=0.743]. No relevance was observed between rs10692285 and SCD-AS. Further genotype-phenotype association analysis suggested that the expression level of RYR2 in human myocardium tissues with the insertion allele was higher than that with the deletion allele at both mRNA and protein levels. Dual-Luciferase activity assay system was used to detect the effect of rs10692285 on the transcription activity of RYR2. As expected, the result indicated that the transcription activity of RYR2 with the ins/ins genotype was higher than that with the del/del genotype. Finally, in-silico prediction revealed that different alleles of rs10692285 could alter the local structure of RYR2 mRNA and microRNA (miRNA) binding. In summary, our findings provided evidence that rs10692285 might contribute to SUD susceptibility through affecting the expression of RYR2, which suggest that abnormal ion channel activity is very likely to be the underlying mechanism of SUD, but not for SCD-AS. Thus, rs10692285 may become a potential marker for molecular diagnosis and genetic counseling of SUD.

Role of risk stratification and genetics in sudden cardiac death

Sudden cardiac death (SCD) is a major public health issue due to its increasing incidence in the general population and the difficulty in identifying high-risk individuals. Nearly 300 000 - 350 000 patients in the United States and 4-5 million patients in the world die annually from SCD. Coronary artery disease and advanced heart failure are the main etiology for SCD. Ischemia of any cause precipitates lethal arrhythmias, and ventricular tachycardia and ventricular fibrillation are the most common lethal arrhythmias precipitating SCD. Pulseless electrical activity, bradyarrhythmia, and electromechanical dissociation also result in SCD. Most SCDs occur outside of the hospital setting, so it is difficult to estimate the public burden, which results in overestimating the incidence of SCD. The insufficiency and limited predictive value of various indicators and criteria for SCD result in the increasing incidence. As a result, there is a need to develop better risk stratification criteria and find modifiable variables to decrease the incidence. Primary and secondary prevention and treatment of SCD need further research. This critical review is focused on the etiology, risk factors, prognostic factors, and importance of risk stratification of SCD.

Cumulative effects of common genetic variants on risk of sudden cardiac death

BACKGROUND

Genome-wide association studies and candidate-gene based approaches have identified multiple common variants associated with increased risk of sudden cardiac death (SCD). However, the independent contribution of these individual loci to disease risk is modest.

OBJECTIVE

To investigate the cumulative effects of genetic variants previously associated with SCD risk.

METHODS

A total of 966 SCD cases from the Oregon-Sudden Unexpected Death Study and 1,926 coronary artery disease controls from the Wellcome Trust Case-Control Consortium were investigated. We generated genetic risk scores (GRS) for each trait composed of variants previously associated with SCD or with abnormalities in specific electrocardiographic traits such as QRS duration, QTc interval and heart rate. GRSs were calculated using a weighted approach based on the number of risk alleles weighted by the beta coefficients derived from the original studies. We also compared the highest and lowest quintiles for the GRS composed of SCD SNPs.

RESULTS

Increased cumulative risk was observed for a GRS composed of 14 SCD-SNPs (OR=1.17 [1.05-1.29], P = 0.002). The risk for SCD was 1.5 fold higher in the highest quintile when compared to the lowest quintile (OR = 1.46[1.11-1.92]). We did not observe significant associations with SCD for SNPs that determine electrocardiographic traits.

CONCLUSIONS

A modest but significant effect on SCD risk was identified for a GRS composed of 14 previously associated SCD SNPs. While next generation sequencing methodology will continue to identify additional novel variants, these findings represent proof of concept for the additive effects of gene variants on SCD risk.

Identifying potential functional impact of mutations and polymorphisms: linking heart failure, increased risk of arrhythmias and sudden cardiac death

Researchers and clinicians have discovered several important concepts regarding the mechanisms responsible for increased risk of arrhythmias, heart failure, and sudden cardiac death. One major step in defining the molecular basis of normal and abnormal cardiac electrical behavior has been the identification of single mutations that greatly increase the risk for arrhythmias and sudden cardiac death by changing channel-gating characteristics. Indeed, mutations in several genes encoding ion channels, such as SCN5A, which encodes the major cardiac Na⁺ channel, have emerged as the basis for a variety of inherited cardiac arrhythmias such as long QT syndrome, Brugada syndrome, progressive cardiac conduction disorder, sinus node dysfunction, or sudden infant death syndrome. In addition, genes encoding ion channel accessory proteins, like anchoring or chaperone proteins, which modify the expression, the regulation of endocytosis, and the degradation of ion channel a-subunits have also been reported as susceptibility genes for arrhythmic syndromes. The regulation of ion channel protein expression also depends on a fine-tuned balance among different other mechanisms, such as gene transcription, RNA processing, post-transcriptional control of gene expression by miRNA, protein synthesis, assembly and post-translational modification and trafficking. The aim of this review is to inventory, through the description of few representative examples, the role of these different biogenic mechanisms in arrhythmogenesis, HF and SCD in order to help the researcher to identify all the processes that could lead to arrhythmias. Identification of novel targets for drug intervention should result from further understanding of these fundamental mechanisms.

Novel loci associated with increased risk of sudden cardiac death in the context of coronary artery disease

Background

Recent genome-wide association studies (GWAS) have identified novel loci associated with sudden cardiac death (SCD). Despite this progress, identified DNA variants account for a relatively small portion of overall SCD risk, suggesting that additional loci contributing to SCD susceptibility await discovery. The objective of this study was to identify novel DNA variation associated with SCD in the context of coronary artery disease (CAD).

Methods and Findings

Using the MetaboChip custom array we conducted a case-control association analysis of 119,117 SNPs in 948 SCD cases (with underlying CAD) from the Oregon Sudden Unexpected Death Study (Oregon-SUDS) and 3,050 controls with CAD from the Wellcome Trust Case-Control Consortium (WTCCC). Two newly identified loci were significantly associated with increased risk of SCD after correction for multiple comparisons at: rs6730157 in the RAB3GAP1 gene on chromosome 2 (P = 4.93×10⁻¹², OR = 1.60) and rs2077316 in the ZNF365 gene on chromosome 10 (P = 3.64×10⁻⁸, OR = 2.41).

Conclusions

Our findings suggest that RAB3GAP1 and ZNF365 are relevant candidate genes for SCD and will contribute to the mechanistic understanding of SCD susceptibility.

Common genetic variation modulating cardiac ECG parameters and susceptibility to sudden cardiac death

Sudden cardiac death (SCD) is a prevalent cause of death in Western societies. Genome-wide association studies (GWAS) conducted over the last few years have uncovered common genetic variants modulating risk of SCD. Furthermore, GWAS studies uncovered several loci impacting on heart rate and ECG indices of conduction and repolarization, as measures of cardiac electrophysiological function and likely intermediate phenotypes of SCD risk. We here review these recent developments and their implications for the identification of novel molecular pathways underlying normal electrophysiological function and susceptibility to SCD.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Antidepressants and the risk of sudden cardiac death and ventricular arrhythmia

PURPOSE

To examine the association between exposure to antidepressants and emergency department or inpatient admission for sudden cardiac death and ventricular arrhythmia (SD/VA), and to examine the impact of dose and cytochrome P-450 inhibition.

METHODS

A cohort study was conducted using 1999-2003 Medicaid claims data from beneficiaries of five large states, supplemented with Medicare claims for dually eligible individuals. Exposures were prescription claims for antidepressants of interest or a reference antidepressant. Outcomes were incident first-listed emergency department or principal inpatient diagnoses indicative of SD/VA originating in the outpatient setting, an outcome previously found to have a positive predictive value of 85%.

RESULTS

In 1.3 million person-years of antidepressant exposure, we identified 4222 SD/VA outcomes for a rate of 3.3/1000 person-years (95%CI, 3.2-3.4). Compared with paroxetine (a referent with a putatively favorable cardiovascular risk profile), adjusted hazard ratios (HRs) were 0.80 (0.67-0.95) for bupropion, 1.24 (0.93-1.65) for doxepin, 0.79 (0.55-1.15) for lithium, and 1.26 (1.11-1.42) for mirtazapine. HRs for amitriptyline, citalopram, fluoxetine, nefazodone, nortriptyline, sertraline, trazodone, and venlafaxine were near unity. For antidepressants having nonnull risks (bupropion and mirtazapine), we observed no relationship with antidepressant dose and some relationships with concomitant cytochrome P-450 inhibition.

CONCLUSIONS

Of antidepressants studied, only mirtazapine had a statistically significantly greater SD/VA risk versus paroxetine. However, baseline differences between these users suggest that this finding may be attributable to residual confounding. Eleven other antidepressants had SD/VA risks no greater than that of paroxetine, thereby providing reassurance regarding the comparative cardiovascular safety of antidepressants.

Informatic and functional approaches to identifying a regulatory region for the cardiac sodium channel

RATIONALE

Although multiple lines of evidence suggest that variable expression of the cardiac sodium channel gene SCN5A plays a role in susceptibility to arrhythmia, little is known about its transcriptional regulation.

OBJECTIVE

We used in silico and in vitro experiments to identify possible noncoding sequences important for transcriptional regulation of SCN5A. The results were extended to mice in which a putative regulatory region was deleted.

METHODS AND RESULTS

We identified 92 noncoding regions highly conserved (>70%) between human and mouse SCN5A orthologs. Three conserved noncoding sequences (CNS) showed significant (>5-fold) activity in luciferase assays. Further in vitro studies indicated one, CNS28 in intron 1, as a potential regulatory region. Using recombinase-mediated cassette exchange (RMCE), we generated mice in which a 435-base pair region encompassing CNS28 was removed. Animals homozygous for the deletion showed significant increases in SCN5A transcripts, Na(V)1.5 protein abundance, and sodium current measured in isolated ventricular myocytes. ECGs revealed a significantly shorter QRS (10.7±0.2 ms in controls versus 9.7±0.2 ms in knockouts), indicating more rapid ventricular conduction. In vitro analysis of CNS28 identified a short 3' segment within this region required for regulatory activity and including an E-box motif. Deletion of this segment reduced reporter activity to 3.6%±0.3% of baseline in CHO cells and 16%±3% in myocytes (both P<0.05), and mutation of individual sites in the E-box restored activity to 62%±4% and 57%±2% of baseline in CHO cells and myocytes, respectively (both P<0.05).

CONCLUSIONS

These findings establish that regulation of cardiac sodium channel expression modulates channel function in vivo, and identify a noncoding region underlying this regulation.

Genetic Basis of Ventricular Arrhythmias

Sudden cardiac death (SCD) is a leading cause of total and cardiovascular mortality, and ventricular fibrillation is the underlying arrhythmia in the majority of cases. In the young, where the incidence of SCD is low, a great proportion of SCDs occur in the context of inherited disorders such as cardiomyopathy or primary electrical disease, where a monogenic hereditary component is a strong determinant of risk. Marked advancement has been made over the past 15 years in the understanding of the genetic basis of the primary electrical disorders, and this has had an enormous impact on the management of these patients. At older ages, the great majority of SCDs occur in the context of acute myocardial ischemia and infarction. Although epidemiologic studies have shown that heritable factors also determine risk in these cases, inheritance is likely complex and multifactorial, and progress in understanding the genetic and molecular mechanisms that determine susceptibility to these arrhythmias, affecting a greater proportion of the population, has been very limited. We review the most recent insights gained into the genetic basis of both the monogenic and the more complex ventricular arrhythmias.

Genome-wide association study identifies a susceptibility locus at 21q21 for ventricular fibrillation in acute myocardial infarction

Sudden cardiac death from ventricular fibrillation during acute myocardial infarction is a leading cause of total and cardiovascular mortality. To our knowledge, we here report the first genome-wide association study for this trait, conducted in a set of 972 individuals with a first acute myocardial infarction, 515 of whom had ventricular fibrillation and 457 of whom did not, from the Arrhythmia Genetics in The Netherlands (AGNES) study. The most significant association to ventricular fibrillation was found at 21q21 (rs2824292, odds ratio = 1.78, 95% CI 1.47-2.13, P = 3.3 x 10(-10)). The association of rs2824292 with ventricular fibrillation was replicated in an independent case-control set consisting of 146 out-of-hospital cardiac arrest individuals with myocardial infarction complicated by ventricular fibrillation and 391 individuals who survived a myocardial infarction (controls) (odds ratio = 1.49, 95% CI 1.14-1.95, P = 0.004). The closest gene to this SNP is CXADR, which encodes a viral receptor previously implicated in myocarditis and dilated cardiomyopathy and which has recently been identified as a modulator of cardiac conduction. This locus has not previously been implicated in arrhythmia susceptibility.

Early identification of risk factors for sudden cardiac death

Sudden cardiac death (SCD) is a global health issue. The unexpected nature of this devastating condition compounds the urgency of discovering methods for early detection of risk, which will lead to more effective prevention. However, the complex and dynamic nature of SCD continues to present a considerable challenge for the early identification of risk factors. Measurement of the left ventricular ejection fraction (LVEF) is currently the only major risk factor used for stratification in clinical practice. Severely decreased LVEF is likely to manifest late in the natural history of SCD, however, and may only affect a small subgroup of patients who will suffer SCD. A growing body of literature describes novel risk markers and predictors of SCD, such as high-risk phenotypes, genetic variants and biomarkers. This Review will discuss the potential utility of these markers as early identifiers of risk, and suggests a framework for the conduct of future studies for the discovery, validation, and deployment of novel SCD risk factors.

Genome-wide association study identifies GPC5 as a novel genetic locus protective against sudden cardiac arrest

Background

Existing studies indicate a significant genetic component for sudden cardiac arrest (SCA) and genome-wide association studies (GWAS) provide an unbiased approach for identification of novel genes. We performed a GWAS to identify genetic determinants of SCA.

Methodology/Principal Findings

We used a case-control design within the ongoing Oregon Sudden Unexpected Death Study (Oregon-SUDS). Cases (n = 424) were SCAs with coronary artery disease (CAD) among residents of Portland, OR (2002–07, population ∼1,000,000) and controls (n = 226) were residents with CAD, but no history of SCA. All subjects were of White-European ancestry and GWAS was performed using Affymetrix 500K/5.0 and 6.0 arrays. High signal markers were genotyped in SCA cases (n = 521) identified from the Atherosclerosis Risk in Communities Study (ARIC) and the Cardiovascular Health Study (CHS) (combined n = 19,611). No SNPs reached genome-wide significance (p<5×10⁻⁸). SNPs at 6 loci were prioritized for follow-up primarily based on significance of p<10⁻⁴ and proximity to a known gene (CSMD2, GPR37L1, LIN9, B4GALNT3, GPC5, and ZNF592). The minor allele of GPC5 (GLYPICAN 5, rs3864180) was associated with a lower risk of SCA in Oregon-SUDS, an effect that was also observed in ARIC/CHS whites (p<0.05) and blacks (p<0.04). In a combined Cox proportional hazards model analysis that adjusted for race, the minor allele exhibited a hazard ratio of 0.85 (95% CI 0.74 to 0.98; p<0.01).

Conclusions/Significance

A novel genetic locus for SCA, GPC5, was identified from Oregon-SUDS and successfully validated in the ARIC and CHS cohorts. Three other members of the Glypican family have been previously implicated in human disease, including cardiac conditions. The mechanism of this specific association requires further study.

Etiology of sudden death in the community: results of anatomical, metabolic, and genetic evaluation

BACKGROUND

Identifying persons at risk for sudden cardiac death (SCD) is challenging. A comprehensive evaluation may reveal clues about the clinical, anatomical, genetic, and metabolic risk factors for SCD.

METHODS

Seventy-one patients who had SCD (25-60 years old) without an initially apparent cause of death were evaluated at the Hennepin County Medical Examiner's office (Minneapolis, MN) from August 2001 to July 2004. We reviewed their clinic records conducted next-of-kin interviews and performed autopsy, laboratory testing, and genetic analysis for mutations in genes associated with the long QT syndrome.

RESULTS

Mean age was 49.5 +/- 7 years, 86% were male, and 2 subjects had history of coronary heart disease (CHD). Coronary risk factors were highly prevalent in comparison to individuals of the same age group in this community (eg, smoking 61%, hypertension 27%, hyperlipidemia 25%) but inadequately treated. On autopsy, 80% of the subjects had high-grade coronary stenoses. Acute coronary lesions and previous silent myocardial infarction (MI) were found in 27% and 34%, respectively. Furthermore, 32% of the subjects had recently smoked cigarettes, and 50% had ingested analgesics. Possible deleterious mutations of the ion channel genes were detected in 5 subjects (7%). Of these, 4 were in the sodium channel gene SCN5A.

CONCLUSIONS

Most of the persons who had SCD in the community had severe subclinical CHD, including undetected previous MI. Traditional coronary risk factors were prevalent and undertreated. Mutations in the long QT syndrome genes were detected in a few subjects. These findings imply that improvements in the detection and treatment of subclinical CHD in the community are needed to prevent SCD.

The Changing Epidemiology of Sudden Cardiac Death

Sudden cardiac death (SCD) is a devastating complication of myocardial infarction. The global incidence of coronary artery disease and heart failure has been increasing greatly in recent years. As a consequence, there is expected to be an increase in the incidence of SCD manifesting as a shared worldwide public health problem. This article summarizes SCD epidemiology, with a focus on the anticipated global rise in incidence.

Rationale and design of the Duke Electrophysiology Genetic and Genomic Studies (EPGEN) biorepository

BACKGROUND

Disturbances in cardiac rhythm can lead to significant morbidity and mortality. Many arrhythmias are known to have a heritable component, but the degree to which genetic variation contributes to disease risk and morbidity is poorly understood.

METHODS AND RESULTS

The EPGEN is a prospective single-center repository that archives DNA, RNA, and protein samples obtained at the time of an electrophysiologic evaluation or intervention. To identify genes and molecular variants that are associated with risk for arrhythmic phenotypes, EPGEN uses unbiased genomic screening; candidate gene analysis; and both unbiased and targeted transcript, protein, and metabolite profiling. To date, EPGEN has successfully enrolled >1,500 subjects. The median age of the study population is 62.9 years; 35% of the subjects are female and 21% are black. To this point, the study population has been composed of patients who had undergone defibrillator (implantable cardioverter-defibrillator or cardiac resynchronization therapy defibrillator) implantation (45%), electrophysiology studies or ablation procedures (35%), and pacemaker implantation or other procedures (20%). The cohort has a high prevalence of comorbidities, including diabetes (33%), hypertension (73%), chronic kidney disease (26%), and peripheral vascular disease (13%).

CONCLUSIONS

We have established a biorepository and clinical database composed of patients with electrophysiologic diseases. EPGEN will seek to (1) improve risk stratification, (2) elucidate mechanisms of arrhythmogenesis, and (3) identify novel pharmacologic targets for the treatment of heart rhythm disorders.

Genetic influence on electrocardiogram time intervals and heart rate in aging mice

Understanding the genetic influence on ECG time intervals and heart rate (HR) is important for identifying the genes underlying susceptibility to cardiac arrhythmias. The objective of this study was to determine the genetic influence on ECG parameters and their age-related changes in mice. ECGs were recorded in lead I on 8 males and 8 females from each of 28 inbred strains at the ages of 6, 12, and 18 mo. Significant interstrain differences in the P-R interval, QRS complex duration, and HR were found. Age-related changes in the P-R interval, QRS complex duration, and HR differed among strains. The P-R interval increased with age in 129S1/SvlmJ females. The QRS complex duration decreased with age in C57BR/J males and DBA2/J females but increased in NON/ShiLtJ females. HR decreased in C57L/J females and SM/J and P/J males but increased in BALB/cByJ males. Differences between males and females were found for HR in SJL/J mice and in the P-R interval in 129S1/SvlmJ mice. Broad-sense heritability estimates of ECG time intervals and HR ranged from 0.31 for the QRS complex duration to 0.52 for the P-R interval. Heritability estimates decreased with age for the P-R interval. Our study revealed that genetic factors play a significant role on cardiac conduction activity and age-related changes in ECG time intervals and HR.

Common variants at ten loci modulate the QT interval duration in the QTSCD Study

The QT interval, a measure of cardiac repolarization, predisposes to ventricular arrhythmias and sudden cardiac death (SCD) when prolonged or shortened. A common variant in NOS1AP is known to influence repolarization. We analyze genome-wide data from five population-based cohorts (ARIC, KORA, SardiNIA, GenNOVA and HNR) with a total of 15,842 individuals of European ancestry, to confirm the NOS1AP association and identify nine additional loci at P < 5 x 10(-8). Four loci map near the monogenic long-QT syndrome genes KCNQ1, KCNH2, SCN5A and KCNJ2. Two other loci include ATP1B1 and PLN, genes with established electrophysiological function, whereas three map to RNF207, near LITAF and within NDRG4-GINS3-SETD6-CNOT1, respectively, all of which have not previously been implicated in cardiac electrophysiology. These results, together with an accompanying paper from the QTGEN consortium, identify new candidate genes for ventricular arrhythmias and SCD.

Sudden cardiac arrest in patients with preserved left ventricular systolic function: a clinical dilemma

Stratifying the risk for sudden cardiac arrest (SCA) in individuals with preserved systolic function remains a pressing public health problem. Current guidelines for the implantation of cardiac defibrillators largely ignore patients with preserved systolic function, even though they account for the majority of cases of SCA. Risk stratification for such individuals may be increasingly feasible. Notably, most individuals who experience SCA have structural heart disease, even if undiagnosed. Thus, clinical risk scores have been developed to identify individuals at high risk. Moreover, there are now promising data that T-wave alternans, alone and in combination with other indices, effectively predicts SCA in this population. This article presents our current understanding of SCA caused by ventricular arrhythmias in patients with preserved left ventricular systolic function, and attempts to build a framework to predict risk in this population.

Primary prevention with the ICD in clinical practice: not as straightforward as the guidelines suggest?

At first sight, guidelines for implantation of an implantable cardioverter defibrillator (ICD) for primary prevention of sudden cardiac death in patients with left ventricular systolic dysfunction seem unambiguous. There are clear cut-off values for ejection fraction, and functional class. However, determination of the ejection fraction itself is not unambiguous, and other risk factors for sudden death that may have a profound effect on risk are not used for decision-making. Furthermore, to obtain a clinically significant impact on survival, expected longevity is important as it can greatly compromise the benefit in elderly patients but underestimate the long-term potential of ICD therapy in younger patients. (Neth Heart J 2009;17:107-10.).

Genetic variations in nitric oxide synthase 1 adaptor protein are associated with sudden cardiac death in US white community-based populations

BACKGROUND

The ECG QT interval is associated with risk of sudden cardiac death (SCD). A previous genome-wide association study demonstrated that allelic variants (rs10494366 and rs4657139) in the nitric oxide synthase 1 adaptor protein (NOS1AP), which encodes a carboxy-terminal PDZ ligand of neuronal nitric oxide synthase, are associated with the QT interval in white adults. The present analysis was conducted to validate the association between NOS1AP variants and the QT interval and to examine the association with SCD in a combined population of 19 295 black and white adults from the Atherosclerosis Risk In Communities Study and the Cardiovascular Health Study.

METHODS AND RESULTS

We examined 19 tagging single-nucleotide polymorphisms in the genomic blocks containing rs10494366 and rs4657139 in NOS1AP. SCD was defined as a sudden pulseless condition of cardiac origin in a previously stable individual. General linear models and Cox proportional hazards regression models were used. Multiple single-nucleotide polymorphisms in NOS1AP, including rs10494366, rs4657139, and rs16847548, were significantly associated with adjusted QT interval in whites (P<0.0001). In whites, after adjustment for age, sex, and study, the relative hazard of SCD associated with each C allele at rs16847548 was 1.31 (95% confidence interval 1.10 to 1.56, P=0.002), assuming an additive model. In addition, a downstream neighboring single-nucleotide polymorphism, rs12567209, which was not correlated with rs16847548 or QT interval, was also independently associated with SCD in whites (relative hazard 0.57, 95% confidence interval 0.39 to 0.83, P=0.003). Adjustment for QT interval and coronary heart disease risk factors attenuated but did not eliminate the association between rs16847548 and SCD, and such adjustment had no effect on the association between rs12567209 and SCD. No significant associations between tagging single-nucleotide polymorphisms in NOS1AP and either QT interval or SCD were observed in blacks.

CONCLUSIONS

In a combined analysis of 2 population-based prospective cohort studies, sequence variations in NOS1AP were associated with baseline QT interval and the risk of SCD in white US adults.

Computational modeling of cardiac disease: potential for personalized medicine

Cardiovascular diseases are leading causes of death, reduce life quality and consume almost half a trillion dollars in healthcare expenses in the USA alone. Cardiac modeling and simulation technologies hold promise as important tools to improve cardiac care and are already in use to elucidate the fundamental mechanisms of cardiac physiology and pathophysiology. However, the emphasis has been on simulating average or exemplar cases. This report describes two classes of cardiac modeling efforts. First, electrophysiological models of channelopathies simulate the altered electrical activity that is thought to promote arrhythmias. Second, electromechanical models attempt to capture both the electrophysiological and mechanical aspects of heart function. One goal of the community is to develop models with sufficient patient customization to assist in personalized treatment planning. Some model aspects can be customized with existing data collection techniques to more closely represent individual patients while other model aspects will likely remain based on generic data. Despite important challenges, cardiac models hold promise to be important enablers of personalized medicine.

Epidemiology of sudden cardiac death: clinical and research implications

The current annual incidence of sudden cardiac death in the United States is likely to be in the range of 180,000 to 250,000 per year. Coinciding with the decreased mortality from coronary artery disease, there is evidence pointing toward a significant decrease in rates of sudden cardiac death in the United States during the second half of the 20th century. However, the alarming rise in prevalence of obesity and diabetes in the first decade of the new millennium both in the United States and worldwide, would indicate that this favorable trend is unlikely to persist. We are likely to witness a resurgence of coronary artery disease and heart failure, as a result of which sudden cardiac death will have to be confronted as a shared and indiscriminate, worldwide public health problem. There is also increasing recognition of the fact that discovery of meaningful and relevant risk stratification and prevention methodologies will require careful prospective community-wide analyses, with access to large archives of DNA, serum, and tissue that link with well-phenotyped databases. The purpose of this review is to summarize current knowledge of sudden cardiac death epidemiology. We will discuss the significance and strengths of community-wide evaluations of sudden cardiac death, summarize recent observations from such studies, and finally highlight specific potential predictors that warrant further evaluation as determinants of sudden cardiac death in the general population.

Sudden cardiac death: influence of diabetes

This article reviews current thinking on the problem of sudden cardiac death (SCD) within community settings, highlighting progress in understanding risks and mechanisms. Information available on the influence of diabetes as a risk factor for SCD and the question of whether this disease enhances susceptibility to ventricular arrhythmias is summarized as are central strategies in risk stratification and mortality prevention.

Gender and effects of a common genetic variant in the NOS1 regulator NOS1AP on cardiac repolarization in 3761 individuals from two independent populations

BACKGROUND

A longer heart-rate corrected QT interval (QTc) is associated with increased risk of ventricular arrhythmias. Women have longer resting QTc and are more likely than men to develop drug-induced QT prolongation. Recent studies have shown association between resting QTc and a common variant (rs10494366) of the NOS1 regulator, NOS1AP. We investigated the association between rs10494366 in NOS1AP and QTc, and assessed gender-specific NOS1AP associations with QTc during rest and after exercise.

METHODS

We investigated the SNP associations with resting QTc in 919 women and 918 men from 504 representative families in the UK GRAPHIC study, and with QTc at rest and at 3 min recovery after exercise in 699 women and 1225 men referred for exercise testing in the Finnish FINCAVAS study.

RESULTS

In the GRAPHIC study the minor allele (G) of the NOS1AP SNP rs10494366 prolonged QTc by 4.59 ms (95% CI 2.77-6.40; P = 7.63/10(7)) in women, but only by 1.62 ms (95% CI -0.15 to 3.38; P = 0.073) in men (gender-SNP interaction term P = 0.025). In the FINCAVAS study the G allele significantly prolonged QTc in both women (P = 0.0063) and men (P = 0.0043) at 3 min recovery after exercise, but at rest an association was only seen in women (P = 0.020 excluding outliers).

CONCLUSIONS

A common NOS1AP variant prolongs QTc with a difference between genders. Further studies should aim to confirm this finding and to assess whether NOS1AP genotype influences the risk of drug-induced QT prolongation and risk of consequent arrhythmias.

Network properties of genes harboring inherited disease mutations

By analyzing, in parallel, large literature-derived and high-throughput experimental datasets we investigate genes harboring human inherited disease mutations in the context of molecular interaction networks. Our results demonstrate that network properties influence the likelihood and phenotypic consequences of disease mutations. Genes with intermediate connectivities have the highest probability of harboring germ-line disease mutations, suggesting that disease genes tend to occupy an intermediate niche in terms of their physiological and cellular importance. Our analysis of tissue expression profiles supports this view. We show that disease mutations are less likely to occur in essential genes compared with all human genes. Disease genes display significant functional clustering in the analyzed molecular network. For about one-third of known disorders with two or more associated genes we find physical clusters of genes with the same phenotype. These clusters are likely to represent disorder-specific functional modules and suggest a framework for identifying yet-undiscovered disease genes.

Sudden cardiac death: prevalence, pathogenesis, and prevention

Sudden cardiac death (SCD), also known as sudden arrest, is a major health problem worldwide. It is usually defined as an unexpected death from a cardiac cause occurring within a short time in a person with or without preexisting heart disease. The pathogenesis of SCD is complex and multifaceted. A dynamic triggering factor usually interacts with an underlying heart disease, either genetically determined or acquired, and the final outcome is the development of lethal tachyarrhythmias or, less frequently, bradycardia. It has increasingly been highlighted that a reliable clinical and diagnostic approach might be effective to unmask the most important genetic and environmental factors, allowing the construction of a rational personalized medicine framework that can be applied in both the preclinical and clinical settings of SCD. The aim of the present article is to provide a concise overview of prevalence, pathogenesis, clinical presentation, and diagnostic approach to this challenging disorder.

Genomics, haplotypes and cardiovascular disease

Cardiovascular disease has a complex genetic and environmental origin. Single-gene mutations have been identified for a variety of disorders, including several forms of sudden cardiac death, atrial fibrillation, hypertrophic cardiomyopathy and coronary artery disease. The recent availability of haplotype data has further enabled genomic approaches to mapping genetic variants associated with the more common polygenic forms of cardiovascular disease. Genome-wide association studies have identified single nucleotide polymorphisms associated with coronary artery disease and are being applied to a variety of clinical problems such as in-stent restenosis. The combination of high-throughput genomic tools such as high density microarrays, genomic information such as sequence and haplotype data, and the careful clinical definition of phenotypes provides the framework for realizing the goals of personalized medicine.

Preventing tomorrow's sudden cardiac death today: part I: Current data on risk stratification for sudden cardiac death

Accurate and timely prediction of sudden cardiac death (SCD) is a necessary prerequisite for effective prevention and therapy. Although the largest number of SCD events occurs in patients without overt heart disease, there are currently no tests that are of proven predictive value in this population. Efforts in risk stratification for SCD have focused primarily on predicting SCD in patients with known structural heart disease. Despite the ubiquity of tests that have been purported to predict SCD vulnerability in such patients, there is little consensus on which test, in addition to the left ventricular ejection fraction, should be used to determine which patients will benefit from an implantable cardioverter defibrillator. On July 20 and 21, 2006, a group of experts representing clinical cardiology, cardiac electrophysiology, biostatistics, economics, and health policy were joined by representatives of the US Food and Drug administration, Centers for Medicare Services, Agency for Health Research and Quality, the Heart Rhythm Society, and the device and pharmaceutical industry for a round table meeting to review current data on strategies of risk stratification for SCD, to explore methods to translate these strategies into practice and policy, and to identify areas that need to be addressed by future research studies. The meeting was organized by the Duke Center for the Prevention of SCD at the Duke Clinical Research Institute and was funded by industry participants. This article summarizes the presentations and discussions that occurred at that meeting.

Personalized genomic medicine: a future prerequisite for the prevention of coronary artery disease

Within the next 10-15 years, medicine will be personalized in large part on the basis of the individual's genomic variants. Coronary artery disease remains the number one cause of morbidity and mortality in the Western world and is predicted to become the number one cause worldwide by 2010. It has been stated that treating the risk factors of coronary artery disease has made it a preventable disease that should be eliminated in the 21st century. It is postulated that about 50% of susceptibility to coronary artery disease is genetic, involving known and occult risk factors. Thus, comprehensive prevention will require identification of genetic susceptibility. The recent technology of a chip with 500,000 DNA markers makes genome-wide scanning to identify the genes contributing to coronary artery disease possible. Multislice CT will provide the high-throughput coronary arteriograms required for this research and for prevention in asymptomatic individuals with a family history of heart disease.

Whole blood RNA offers a rapid, comprehensive approach to genetic diagnosis of cardiovascular diseases

PURPOSE

Long QT Syndrome, Marfan Syndrome, hypertrophic and dilated cardiomyopathy are caused by mutations in large, multi-exon genes that are principally expressed in cardiovascular tissues. Genetic testing for these disorders is labor-intensive and expensive. We sought to develop a more rapid, comprehensive, and cost-effective approach.

METHODS

Paired whole blood samples were collected into tubes with or without an RNA-preserving solution, and harvested for whole blood RNA or leukocyte DNA, respectively. Large overlapping cDNA fragments from KCNQ1 and KCNH2 (Long QT Syndrome), MYBPC3 (hypertrophic and dilated cardiomyopathy), or FBN1 (Marfan Syndrome) were amplified from RNA and directly sequenced. Variants were confirmed in leukocyte DNA.

RESULTS

All 4 transcripts were amplified and sequenced from whole blood mRNA. Six known and 2 novel mutations were first identified from RNA of 10 probands, and later confirmed in genomic DNA, at considerable savings in time and cost. In one patient with MFS, RNA sequencing directly identified a splicing mutation. Results from RNA and DNA were concordant for single nucleotide polymorphisms at the same loci.

CONCLUSION

Taking advantage of new whole blood RNA stabilization methods, we have designed a cost-effective, comprehensive method for mutation detection that should significantly facilitate clinical genetic testing in four lethal cardiovascular disorders.