Common variants at ten loci modulate the QT interval duration in the QTSCD Study. Nat Genet. 2009;41:407-414. [PMID: 19305409 DOI: 10.1038/ng.362]

Drug-gene interactions and the search for missing heritability: a cross-sectional pharmacogenomics study of the QT interval

Variability in response to drug use is common and heritable, suggesting that genome-wide pharmacogenomics studies may help explain the 'missing heritability' of complex traits. Here, we describe four independent analyses in 33 781 participants of European ancestry from 10 cohorts that were designed to identify genetic variants modifying the effects of drugs on QT interval duration (QT). Each analysis cross-sectionally examined four therapeutic classes: thiazide diuretics (prevalence of use=13.0%), tri/tetracyclic antidepressants (2.6%), sulfonylurea hypoglycemic agents (2.9%) and QT-prolonging drugs as classified by the University of Arizona Center for Education and Research on Therapeutics (4.4%). Drug-gene interactions were estimated using covariable-adjusted linear regression and results were combined with fixed-effects meta-analysis. Although drug-single-nucleotide polymorphism (SNP) interactions were biologically plausible and variables were well-measured, findings from the four cross-sectional meta-analyses were null (Pinteraction>5.0 × 10(-8)). Simulations suggested that additional efforts, including longitudinal modeling to increase statistical power, are likely needed to identify potentially important pharmacogenomic effects.

Pitx2, an atrial fibrillation predisposition gene, directly regulates ion transport and intercalated disc genes

BACKGROUND

Pitx2 is the homeobox gene located in proximity to the human 4q25 familial atrial fibrillation (AF) locus. When deleted in the mouse germline, Pitx2 haploinsufficiency predisposes to pacing-induced AF, indicating that reduced Pitx2 promotes an arrhythmogenic substrate. Previous work focused on Pitx2 developmental functions that predispose to AF. Although Pitx2 is expressed in postnatal left atrium, it is unknown whether Pitx2 has distinct postnatal and developmental functions.

METHODS AND RESULTS

To investigate Pitx2 postnatal function, we conditionally inactivated Pitx2 in the postnatal atrium while leaving its developmental function intact. Unstressed adult Pitx2 homozygous mutant mice display variable R-R interval with diminished P-wave amplitude characteristic of sinus node dysfunction, an AF risk factor in human patients. An integrated genomics approach in the adult heart revealed Pitx2 target genes encoding cell junction proteins, ion channels, and critical transcriptional regulators. Importantly, many Pitx2 target genes have been implicated in human AF by genome-wide association studies. Immunofluorescence and transmission electron microscopy studies in adult Pitx2 mutant mice revealed structural remodeling of the intercalated disc characteristic of human patients with AF.

CONCLUSIONS

Our findings, revealing that Pitx2 has genetically separable postnatal and developmental functions, unveil direct Pitx2 target genes that include channel and calcium handling genes, as well as genes that stabilize the intercalated disc in postnatal atrium.

Personalized medicine to treat arrhythmias

The efficacy of antiarrhythmic drug therapy is incomplete, with responses ranging from efficacy to no effect to severe adverse effects, including paradoxical drug-induced arrhythmia. Most antiarrhythmic drugs were developed at a time when the mechanisms underlying arrhythmias were not well understood. In the last decade, a range of experimental approaches have advanced our understanding of the molecular and genomic contributors to the generation of an arrhythmia-prone heart, and this information is directly informing targeted therapy with existing drugs or the development of new ones. The development of inexpensive whole genome sequencing holds the promise of identifying patients susceptible to arrhythmias in a presymptomatic phase, and thus implementing preventive therapies.

From GWAS to function: genetic variation in sodium channel gene enhancer influences electrical patterning

The electrical activity of the heart depends on the correct interplay between key transcription factors and cis-regulatory elements, which together regulate the proper heterogeneous expression of genes encoding for ion channels and other proteins. Genome-wide association studies of ECG parameters implicated genetic variants in the genes for these factors and ion channels modulating conduction and depolarization. Here, we review recent insights into the regulation of localized expression of ion channel genes and the mechanism by which a single-nucleotide polymorphism (SNP) associated with alterations in cardiac conduction patterns in humans affects the transcriptional regulation of the sodium channel genes, SCN5A and SCN10A. The identification of regulatory elements of electrical activity genes helps to explain the impact of genetic variants in non-coding regulatory DNA sequences on regulation of cardiac conduction and the predisposition for cardiac arrhythmias.

Recent genetic discoveries implicating ion channels in human cardiovascular diseases

The term 'channelopathy' refers to human genetic disorders caused by mutations in genes encoding ion channels or their interacting proteins. Recent advances in this field have been enabled by next-generation DNA sequencing strategies such as whole exome sequencing with several intriguing and unexpected discoveries. This review highlights important discoveries implicating ion channels or ion channel modulators in cardiovascular disorders including cardiac arrhythmia susceptibility, cardiac conduction phenotypes, pulmonary and systemic hypertension. These recent discoveries further emphasize the importance of ion channels in the pathophysiology of human disease and as important druggable targets.

Genetics of sudden cardiac death caused by ventricular arrhythmias

Sudden cardiac death (SCD) resulting from ventricular tachyarrhythmia is a major contributor to mortality. Clinical management of SCD, currently based on clinical markers of SCD risk, can be improved by integrating genetic information. The identification of multiple disease-causing gene variants has already improved patient management and increased our understanding of the rare Mendelian diseases associated with SCD risk in the young, but marked variability in disease severity suggests that additional genetic modifiers exist. Next-generation DNA sequencing could be crucial to the discovery of SCD-associated genes, but large data sets can be difficult to interpret. SCD usually occurs in patients with an average age of 65 years who have complex cardiac disease stemming from multiple, common, acquired disorders. Heritable factors are largely unknown, but are likely to have a role in determining the risk of SCD in these patients. Numerous genetic loci have been identified that affect electrocardiogram indices, which are regarded as intermediate phenotypes for tachyarrhythmia. These loci could help to identify new molecules and pathways affecting cardiac electrical function. These loci are often located in intergenic regions, so our evolving understanding of the noncoding regulatory regions of the genome are likely to aid in the identification of novel genes that are important for cardiac electrical function and possibly SCD.

Genetic modifier of the QTc interval associated with early-onset atrial fibrillation

BACKGROUND

Both shortening and prolongation of the QTc interval have been associated with atrial fibrillation (AF). We investigated whether 8 single nucleotide polymorphisms (SNPs) at loci previously shown to affect QTc interval duration were associated with lone AF.

METHODS

We included 358 patients diagnosed with lone AF (defined as onset of AF at < 50 years of age in the absence of traditional cardiovascular risk factors) and a control group consisting of 751 individuals free of AF. The 8 loci were genotyped using TaqMan assays. Genotype frequencies in lone AF cases and controls were compared using an additive logistic regression model.

RESULTS

Risk of the development of early-onset lone AF in individuals homozygous for the variant rs2968863 (7q36.1) was higher than in individuals with no copies of the risk allele (odds ratio [OR], 2.40; P = 0.001). The association was also significant after Bonferroni correction (P = 0.016). This polymorphism has been shown to decrease the QTc interval by 1.4 ms in genome-wide association studies (GWAS). The genetic variant is situated close to the long QT syndrome (LQTS) type 2 gene KCNH2 that encodes the potassium channel Kv11.1 (hERG). Sanger sequencing of KCNH2 confirmed the known high linkage disequilibrium between rs2968863 and the nonsynonymous variant K897T in KCNH2. No novel mutations were found in the gene.

CONCLUSIONS

The variant rs2968863 (7q36.1), reported in GWAS to shorten the QTc interval, was found to be associated with early-onset lone AF. This may have implications for the pathophysiological understanding of AF.

Genome wide analysis of drug-induced torsades de pointes: lack of common variants with large effect sizes

Marked prolongation of the QT interval on the electrocardiogram associated with the polymorphic ventricular tachycardia Torsades de Pointes is a serious adverse event during treatment with antiarrhythmic drugs and other culprit medications, and is a common cause for drug relabeling and withdrawal. Although clinical risk factors have been identified, the syndrome remains unpredictable in an individual patient. Here we used genome-wide association analysis to search for common predisposing genetic variants. Cases of drug-induced Torsades de Pointes (diTdP), treatment tolerant controls, and general population controls were ascertained across multiple sites using common definitions, and genotyped on the Illumina 610k or 1M-Duo BeadChips. Principal Components Analysis was used to select 216 Northwestern European diTdP cases and 771 ancestry-matched controls, including treatment-tolerant and general population subjects. With these sample sizes, there is 80% power to detect a variant at genome-wide significance with minor allele frequency of 10% and conferring an odds ratio of ≥2.7. Tests of association were carried out for each single nucleotide polymorphism (SNP) by logistic regression adjusting for gender and population structure. No SNP reached genome wide-significance; the variant with the lowest P value was rs2276314, a non-synonymous coding variant in C18orf21 (p  =  3×10⁻⁷, odds ratio = 2, 95% confidence intervals: 1.5–2.6). The haplotype formed by rs2276314 and a second SNP, rs767531, was significantly more frequent in controls than cases (p  =  3×10⁻⁹). Expanding the number of controls and a gene-based analysis did not yield significant associations. This study argues that common genomic variants do not contribute importantly to risk for drug-induced Torsades de Pointes across multiple drugs.

Genome-wide association study identifies 8 novel loci associated with blood pressure responses to interventions in Han Chinese

BACKGROUND

Blood pressure (BP) responses to dietary sodium and potassium intervention and cold pressor test vary considerably among individuals. We aimed to identify novel genetic variants influencing individuals' BP responses to dietary intervention and cold pressor test.

METHODS AND RESULTS

We conducted a genome-wide association study of BP responses in 1881 Han Chinese and de novo genotyped top findings in 698 Han Chinese. Diet-feeding study included a 7-day low-sodium (51.3 mmol/d), a 7-day high-sodium (307.8 mmol/d), and a 7-day high-sodium plus potassium supplementation (60 mmol/d). Nine BP measurements were obtained during baseline observation and each intervention period. The meta-analyses identified 8 novel loci for BP phenotypes, which physically mapped in or near PRMT6 (P=7.29 × 10(-9)), CDCA7 (P=3.57 × 10(-8)), PIBF1 (P=1.78 × 10(-9)), ARL4C (P=1.86 × 10(-8)), IRAK1BP1 (P=1.44 × 10(-10)), SALL1 (P=7.01 × 10(-13)), TRPM8 (P=2.68 × 10(-8)), and FBXL13 (P=3.74 × 10(-9)). There was a strong dose-response relationship between the number of risk alleles of these independent single-nucleotide polymorphisms and the risk of developing hypertension during the 7.5-year follow-up in the study participants. Compared with those in the lowest quartile of risk alleles, odds ratios (95% confidence intervals) for those in the second, third, and fourth quartiles were 1.39 (0.97, 1.99), 1.72 (1.19, 2.47), and 1.84 (1.29, 2.62), respectively (P=0.0003 for trend).

CONCLUSIONS

Our study identified 8 novel loci for BP responses to dietary sodium and potassium intervention and cold pressor test. The effect size of these novel loci on BP phenotypes is much larger than those reported by the previously published studies. Furthermore, these variants predict the risk of developing hypertension among individuals with normal BP at baseline.

Association between KCNQ1 genetic variants and QT interval in a Chinese population

AIM

There is a close link between electrocardiographic ventricular repolarization QT parameters and Type 2 diabetes. The aim of the present study was to assess the effects of QT-related and diabetes-related variants in KCNQ1 on QT interval in a Chinese population.

METHODS

We recruited 2415 patients with Type 2 diabetes and 1163 subjects with normal glucose regulation in the present study. QT interval was obtained and the heart rate-corrected QT interval (QTc) was calculated using Bazett's formula. Four single nucleotide polymorphisms in KCNQ1 were selected (rs12296050, rs12576239, rs2237892 and rs2237895) and genotyped.

RESULTS

In participants with normal glucose regulation, the minor allele T of rs12296050 was associated with a 3.46-ms QTc prolongation under an additive model (P = 0.0109, empirical P = 0.0498). In patients with Type 2 diabetes, we did not find any association for the single nucleotide polymorphisms.

CONCLUSIONS

Our findings indicate that KCNQ1 is associated with QT interval in a Chinese population with normal glucose regulation.

Common variants at SCN5A-SCN10A and HEY2 are associated with Brugada syndrome, a rare disease with high risk of sudden cardiac death

Brugada syndrome is a rare cardiac arrhythmia disorder, causally related to SCN5A mutations in around 20% of cases. Through a genome-wide association study of 312 individuals with Brugada syndrome and 1,115 controls, we detected 2 significant association signals at the SCN10A locus (rs10428132) and near the HEY2 gene (rs9388451). Independent replication confirmed both signals (meta-analyses: rs10428132, P = 1.0 × 10(-68); rs9388451, P = 5.1 × 10(-17)) and identified one additional signal in SCN5A (at 3p21; rs11708996, P = 1.0 × 10(-14)). The cumulative effect of the three loci on disease susceptibility was unexpectedly large (Ptrend = 6.1 × 10(-81)). The association signals at SCN5A-SCN10A demonstrate that genetic polymorphisms modulating cardiac conduction can also influence susceptibility to cardiac arrhythmia. The implication of association with HEY2, supported by new evidence that Hey2 regulates cardiac electrical activity, shows that Brugada syndrome may originate from altered transcriptional programming during cardiac development. Altogether, our findings indicate that common genetic variation can have a strong impact on the predisposition to rare diseases.

Genomics of cardiac electrical function

Proper generation and conduction of the cardiac electrical impulse is essential for the continuous coordinated contraction of the heart. Dysregulation of cardiac electrical function may lead to cardiac arrhythmias, which constitute a huge medical and social burden. Identifying the genetic factors underlying cardiac electrical activity serves the double purpose of allowing the early identification of individuals at risk for arrhythmia and discovering new potential therapeutic targets for prevention. The aim of this review is to provide an overview of the genes and genetic loci linked thus far to cardiac electrical function and arrhythmia. These genes and loci have been primarily uncovered through studies on the familial rhythm disorders and through genome-wide association studies on electrocardiographic parameters in large sets of the general population. An overview of all genes and loci with their respective effect is given.

Identification of a KCNQ1 polymorphism acting as a protective modifier against arrhythmic risk in long-QT syndrome

BACKGROUND

Long-QT syndrome (LQTS) is characterized by such striking clinical heterogeneity that, even among family members carrying the same mutation, clinical outcome can range between sudden death and no symptoms. We investigated the role of genetic variants as modifiers of risk for cardiac events in patients with LQTS.

METHODS AND RESULTS

In a matched case-control study including 112 patient duos with LQTS from France, Italy, and Japan, 25 polymorphisms were genotyped based on either their association with QTc duration in healthy populations or on their role in adrenergic responses. The duos were composed of 2 relatives harboring the same heterozygous KCNQ1 or KCNH2 mutation: 1 with cardiac events and 1 asymptomatic and untreated. The findings were then validated in 2 independent founder populations totaling 174 symptomatic and 162 asymptomatic patients with LQTS, and a meta-analysis was performed. The KCNQ1 rs2074238 T-allele was significantly associated with a decreased risk of symptoms 0.34 (0.19-0.61; P<0.0002) and with shorter QTc (P<0.0001) in the combined discovery and replication cohorts.

CONCLUSIONS

We provide evidence that the KCNQ1 rs2074238 polymorphism is an independent risk modifier with the minor T-allele conferring protection against cardiac events in patients with LQTS. This finding is a step toward a novel approach for risk stratification in patients with LQTS.

Long QT syndrome: beyond the causal mutation

Congenital long QT syndrome (LQTS) is caused by single autosomal-dominant mutations in a gene encoding for a cardiac ion channel or an accessory ion channel subunit. These single mutations can cause life-threatening arrhythmias and sudden death in heterozygous mutation carriers. This recognition has been the basis for world-wide staggering numbers of subjects and families counselled for LQTS and treated based on finding (putative) disease-causing mutations. However, prophylactic treatment of patients is greatly hampered by the growing awareness that simple carriership of a mutation often fails to predict clinical outcome: many carriers never develop clinically relevant disease while others are severely affected at a young age. It is still largely elusive what determines this large variability in disease severity, where even within one pedigree, an identical mutation can cause life-threatening arrhythmias in some carriers while in other carriers no disease becomes clinically manifested. This suggests that additional factors modify the clinical manifestations of a particular disease-causing mutation. In this article, potential demographic, environmental and genetic factors are reviewed, which, in conjunction with a mutation, may modify the phenotype in LQTS, and thereby determine, at least partially, the large variability in disease severity.

Nitric Oxide Synthase 1 Adaptor Protein, an Emerging New Genetic Marker for QT Prolongation and Sudden Cardiac Death

Sudden cardiac death (SCD) is defined as sudden unexplained death due to cardiac causes with an acute change in cardiovascular status within 1 hour of onset of symptoms. Alternatively, in unwitnessed cases, SCD can also be defined as a person last seen functionally normal 24 hours before being found dead. Despite significant advances in understanding the pathophysiology of cardiovascular diseases and the resultant improvement in resuscitation science, SCD remains a major healthcare challenge worldwide. Although the most pronounced risk factor for SCD is the presence of coronary artery disease in the setting of a depressed left ventricular function, most deaths occur in the larger, lower-risk subgroups where genetic variations and other conditions may be the precipitating factors in triggering SCD. Recently, a common genetic variation in a neuronal nitric oxide synthase regulator, nitric oxide synthase 1 adaptor protein (NOS1AP) also known as carboxyl-terminal PDZ ligand of neuronal nitric oxide synthase protein (CAPON) gene, has been identified as a new genetic marker in modulating QT interval prolongation and SCD in general populations. Animal study revealed that NOS1AP is expressed in the heart and interacts with NOS1-NO pathways to modulate cardiac repolarization via suppressing the sarcolemmal L-type calcium current and enhancing the IKr current. This important genetic implication was soon replicated in other racial/ethnic populations and extended to a variety of clinical settings including diabetes mellitus, coronary artery disease, myocardial infarction, and congenital or drug-induced long QT syndrome. The purpose of this review aims to provide up-to-date information about the emerging new genetic marker, NOS1AP, in relation to QT prolongation and SCD.

KEY WORDS

NOS1AP; QT interval; Sudden cardiac death.

Genetics of congenital and drug-induced long QT syndromes: current evidence and future research perspectives

The long QT syndrome (LQTS) is a condition characterized by abnormal prolongation of the QT interval with an associated risk of ventricular arrhythmias and sudden cardiac death. Congenital forms of LQTS arise due to rare and highly penetrant mutations that segregate in a Mendelian fashion. Over the years, multiple mutations in genes encoding ion channels and ion channel binding proteins have been reported to underlie congenital LQTS. Drugs are by far the most common cause of acquired forms of LQTS. Emerging evidence suggests that drug-induced LQTS also has a significant heritable component. However, the genetic substrate underlying drug-induced LQTS is presently largely unknown. In recent years, advances in next-generation sequencing technology and molecular biology techniques have significantly enhanced our ability to identify genetic variants underlying both monogenic diseases and more complex traits. In this review, we discuss the genetic basis of congenital and drug-induced LQTS and focus on future avenues of research in the field. Ultimately, a detailed characterization of the genetic substrate underlying congenital and drug-induced LQTS will enhance risk stratification and potentially result in the development of tailored genotype-based therapies.

Personalized medicine: genetic diagnosis for inherited cardiomyopathies/channelopathies

Major advances in the field of molecular genetics have expanded our ability to identify genetic substrates underlying the pathogenesis of various disorders that follow Mendelian inheritance patterns. Included among these disorders are the potentially lethal and heritable channelopathies and cardiomyopathies for which the underlying genetic basis has been identified and is now better understood. Clinical and genetic heterogeneity are hallmark features of these disorders, with thousands of gene mutations being implicated within these divergent cardiovascular diseases. Genetic testing for several of these heritable channelopathies and cardiomyopathies has matured from discovery to research-based genetic testing to clinically/commercially available diagnostic tests. The purpose of this review is to provide the reader with a basic understanding of human medical genetics and genetic testing in the context of cardiovascular diseases of the heart. We review the state of clinical genetic testing for the more common channelopathies and cardiomyopathies, discuss some of the pertinent issues that arise from genetic testing, and discuss the future of personalized medicine in cardiovascular disease.

SNPs identified as modulators of ECG traits in the general population do not markedly affect ECG traits during acute myocardial infarction nor ventricular fibrillation risk in this condition

Background

Ventricular fibrillation (VF) in the setting of acute ST elevation myocardial infarction (STEMI) is a leading cause of mortality. Although the risk of VF has a genetic component, the underlying genetic factors are largely unknown. Since heart rate and ECG intervals of conduction and repolarization during acute STEMI differ between patients who do and patients who do not develop VF, we investigated whether SNPs known to modulate these ECG indices in the general population also impact on the respective ECG indices during STEMI and on the risk of VF.

Methods and Results

The study population consisted of participants of the Arrhythmia Genetics in the NEtherlandS (AGNES) study, which enrols patients with a first STEMI that develop VF (cases) and patients that do not develop VF (controls). SNPs known to impact on RR interval, PR interval, QRS duration or QTc interval in the general population were tested for effects on the respective STEMI ECG indices (stage 1). Only those showing a (suggestive) significant association were tested for association with VF (stage 2). On average, VF cases had a shorter RR and a longer QTc interval compared to non-VF controls. Eight SNPs showed a trend for association with the respective STEMI ECG indices. Of these, three were also suggestively associated with VF.

Conclusions

RR interval and ECG indices of conduction and repolarization during acute STEMI differ between patients who develop VF and patients who do not. Although the effects of the SNPs on ECG indices during an acute STEMI seem to be similar in magnitude and direction as those found in the general population, the effects, at least in isolation, are too small to explain the differences in ECGs between cases and controls and to determine risk of VF.

RNA decay machines: deadenylation by the Ccr4-not and Pan2-Pan3 complexes

Shortening and removal of the 3' poly(A) tail of mature mRNA by poly(A)-specific 3' exonucleases (deadenylases) is the initial and often rate-limiting step in mRNA degradation. The majority of cytoplasmic deadenylase activity is associated with the Ccr4-Not and Pan2-Pan3 complexes. Two distinct catalytic subunits, Caf1/Pop2 and Ccr4, are associated with the Ccr4-Not complex, whereas the Pan2 enzymatic subunit forms a stable complex with Pan3. In this review, we discuss the composition and activity of these two deadenylases. In addition, we comment on generic and specific mechanisms of recruitment of Ccr4-Not and Pan2-Pan3 to mRNAs. Finally, we discuss specialised and redundant functions of the deadenylases and review the importance of Ccr4-Not subunits in the regulation of physiological processes. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Genetics can contribute to the prognosis of Brugada syndrome: a pilot model for risk stratification

Brugada syndrome is an inherited arrhythmogenic disorder leading to sudden death predominantly in the 3-4 decade. To date the only reliable treatment is the implantation of a cardioverter defibrillator; however, better criteria for risk stratification are needed, especially for asymptomatic subjects. Brugada syndrome genetic bases have been only partially understood, accounting for <30% of patients, and have been poorly correlated with prognosis, preventing inclusion of genetic data in current guidelines. We designed an observational study to identify genetic markers for risk stratification of Brugada patients by exploratory statistical analysis. The presence of genetic variants, identified by SCN5A gene analysis and genotyping of 73 candidate polymorphisms, was correlated with the occurrence of major arrhythmic events in a cohort of 92 Brugada patients by allelic association and survival analysis. In all, 18 mutations were identified in the SCN5A gene, including 5 novel, and statistical analysis indicated that mutation carriers had a significantly increased risk of major arrhythmic events (P=0.024). In addition, we established association of five polymorphisms with major arrhythmic events occurrence and consequently elaborated a pilot risk stratification algorithm by calculating a weighted genetic risk score, including the associated polymorphisms and the presence of SCN5A mutation as function of their odds ratio. This study correlates for the first time the presence of genetic variants with increased arrhythmic risk in Brugada patients, representing a first step towards the design of a new risk stratification model.

Cardiovascular pharmacogenomics: the future of cardiovascular therapeutics?

Responses to drug therapy vary from benefit to no effect to adverse effects which can be serious or occasionally fatal. Increasing evidence supports the idea that genetic variants can play a major role in this spectrum of responses. Well-studied examples in cardiovascular therapeutics include predictors of steady-state warfarin dosage, predictors of reduced efficacy among patients receiving clopidogrel for drug eluting stents, and predictors of some serious adverse drug effects. This review summarizes contemporary approaches to identifying and validating genetic predictors of variability in response to drug treatment. Approaches to incorporating this new knowledge into clinical care, and the barriers to this concept, are addressed.

Determinants of incomplete penetrance and variable expressivity in heritable cardiac arrhythmia syndromes

Mutations in genes encoding ion channel pore-forming α-subunits and accessory β-subunits as well as intracellular calcium-handling proteins that collectively maintain the electromechanical function of the human heart serve as the underlying pathogenic substrate for a spectrum of sudden cardiac death (SCD)-predisposing heritable cardiac arrhythmia syndromes, including long QT syndrome (LQTS), short QT syndrome (SQTS), Brugada syndrome (BrS), and catecholaminergic polymorphic ventricular tachycardia (CPVT). Similar to many Mendelian disorders, the cardiac "channelopathies" exhibit incomplete penetrance, variable expressivity, and phenotypic overlap, whereby genotype-positive individuals within the same genetic lineage assume vastly different clinical courses as objectively assessed by phenotypic features such electrocardiographic abnormalities and number/type of cardiac events. In this Review, we summarize the current understanding of the global architecture of complex electrocardiographic traits such as the QT interval, focusing on the role of common genetic variants in the modulation of ECG parameters in health and the environmental and genetic determinants of incomplete penetrance and variable expressivity in the heritable cardiac arrhythmia syndromes most likely to be encountered in clinical practice.

[The Heinz Nixdorf Recall study]

The Heinz Nixdorf Recall Study is a population-based study that aims to improve the prediction of cardiovascular events by integrating new imaging and non-imaging modalities in risk assessment. One focus of the study is the evaluation of the quantification of subclinical coronary artery calcifications (coronary artery calcification, CAC) as a prognostic factor in predicting cardiac events. Primary endpoints are myocardial infarction and sudden cardiac death. The study was initiated in the late 1990s and enrolled a total of 4,814 participants aged 45-75 years between December 2000 and August 2003. A 5-year follow-up examination took place between 2006 and 2008. Currently, the 10-year follow-up is under way and is estimated to be finished in July 2013. Extending the original aims of the study, serial CAC measurements will allow the characterization of the natural history of CAC dynamics, the identification of its determinants and an understanding of the impact of CAC progression on the primary endpoints. The Heinz Nixdorf Recall Study will significantly extend our knowledge about new modalities in the prediction of cardiac events.

Integration of genetics into a systems model of electrocardiographic traits using HumanCVD BeadChip

BACKGROUND

Electrocardiographic traits are important, substantially heritable determinants of risk of arrhythmias and sudden cardiac death.

METHODS AND RESULTS

In this study, 3 population-based cohorts (n=10,526) genotyped with the Illumina HumanCVD Beadchip and 4 quantitative electrocardiographic traits (PR interval, QRS axis, QRS duration, and QTc interval) were evaluated for single-nucleotide polymorphism associations. Six gene regions contained single nucleotide polymorphisms associated with these traits at P<10(-6), including SCN5A (PR interval and QRS duration), CAV1-CAV2 locus (PR interval), CDKN1A (QRS duration), NOS1AP, KCNH2, and KCNQ1 (QTc interval). Expression quantitative trait loci analyses of top associated single-nucleotide polymorphisms were undertaken in human heart and aortic tissues. NOS1AP, SCN5A, IGFBP3, CYP2C9, and CAV1 showed evidence of differential allelic expression. We modeled the effects of ion channel activity on electrocardiographic parameters, estimating the change in gene expression that would account for our observed associations, thus relating epidemiological observations and expression quantitative trait loci data to a systems model of the ECG.

CONCLUSIONS

These association results replicate and refine the mapping of previous genome-wide association study findings for electrocardiographic traits, while the expression analysis and modeling approaches offer supporting evidence for a functional role of some of these loci in cardiac excitation/conduction.

Novel loci associated with PR interval in a genome-wide association study of 10 African American cohorts

BACKGROUND

The PR interval, as measured by the resting, standard 12-lead ECG, reflects the duration of atrial/atrioventricular nodal depolarization. Substantial evidence exists for a genetic contribution to PR, including genome-wide association studies that have identified common genetic variants at 9 loci influencing PR in populations of European and Asian descent. However, few studies have examined loci associated with PR in African Americans.

METHODS AND RESULTS

We present results from the largest genome-wide association study to date of PR in 13 415 adults of African descent from 10 cohorts. We tested for association between PR (ms) and ≈2.8 million genotyped and imputed single-nucleotide polymorphisms. Imputation was performed using HapMap 2 YRI and CEU panels. Study-specific results, adjusted for global ancestry and clinical correlates of PR, were meta-analyzed using the inverse variance method. Variation in genome-wide test statistic distributions was noted within studies (λ range: 0.9-1.1), although not after genomic control correction was applied to the overall meta-analysis (λ: 1.008). In addition to generalizing previously reported associations with MEIS1, SCN5A, ARHGAP24, CAV1, and TBX5 to African American populations at the genome-wide significance level (P<5.0 × 10(-8)), we also identified a novel locus: ITGA9, located in a region previously implicated in SCN5A expression. The 3p21 region harboring SCN5A also contained 2 additional independent secondary signals influencing PR (P<5.0 × 10(-8)).

CONCLUSIONS

This study demonstrates the ability to map novel loci in African Americans as well as the generalizability of loci associated with PR across populations of African, European, and Asian descent.

Impact of ancestry and common genetic variants on QT interval in African Americans

BACKGROUND

Ethnic differences in cardiac arrhythmia incidence have been reported, with a particularly high incidence of sudden cardiac death and low incidence of atrial fibrillation in individuals of African ancestry. We tested the hypotheses that African ancestry and common genetic variants are associated with prolonged duration of cardiac repolarization, a central pathophysiological determinant of arrhythmia, as measured by the electrocardiographic QT interval.

METHODS AND RESULTS

First, individual estimates of African and European ancestry were inferred from genome-wide single-nucleotide polymorphism (SNP) data in 7 population-based cohorts of African Americans (n=12,097) and regressed on measured QT interval from ECGs. Second, imputation was performed for 2.8 million SNPs, and a genome-wide association study of QT interval was performed in 10 cohorts (n=13,105). There was no evidence of association between genetic ancestry and QT interval (P=0.94). Genome-wide significant associations (P<2.5 × 10(-8)) were identified with SNPs at 2 loci, upstream of the genes NOS1AP (rs12143842, P=2 × 10(-15)) and ATP1B1 (rs1320976, P=2 × 10(-10)). The most significant SNP in NOS1AP was the same as the strongest SNP previously associated with QT interval in individuals of European ancestry. Low probability values (P<10(-5)) were observed for SNPs at several other loci previously identified in genome-wide association studies in individuals of European ancestry, including KCNQ1, KCNH2, LITAF, and PLN.

CONCLUSIONS

We observed no difference in duration of cardiac repolarization with global genetic indices of African American ancestry. In addition, our genome-wide association study extends the association of polymorphisms at several loci associated with repolarization in individuals of European ancestry to include individuals of African ancestry.

Drug-induced arrhythmia: pharmacogenomic prescribing?

Drug-induced Torsades de Pointes is a rare, unpredictable, and life-threatening serious adverse event. It can be caused by both cardiac and non-cardiac drugs and has become a major issue in novel drug development and for the regulatory authorities. This review describes the problem, predisposing factors, and the underlying genetic predisposition as it is understood currently. The future potential for pharmacogenomic-guided and personalized prescription to prevent drug-induced Torsades de Pointes is discussed. Database searches utilized reports from www.qtdrugs.org up to January 2012, case reports and articles from www.pubmed.com up to January 2012, and the British National Formulary edition at www.bnf.org.

A meta-analysis of genome-wide association studies of the electrocardiographic early repolarization pattern

BACKGROUND

The early repolarization pattern (ERP) is common and associated with risk of sudden cardiac death. ERP is heritable, and mutations have been described in syndromatic cases.

OBJECTIVE

To conduct a meta-analysis of genome-wide association studies to identify common genetic variants influencing ERP.

METHODS

We ascertained ERP on the basis of electrocardiograms in 3 large community-based cohorts from Europe and the United States: the Framingham Heart Study, the Health 2000 Study, and the KORA F4 Study. We analyzed genome-wide association studies in participants with and without ERP by logistic regression assuming an additive genetic model and meta-analyzed individual cohort results. We then sought to strengthen support for findings that reached P ≤ 1 × 10(-5) in independent individuals by direct genotyping or in-silico analysis of genome-wide data. We meta-analyzed the results from both stages.

RESULTS

Of 7482 individuals in the discovery stage, 452 showed ERP (ERP positive: mean age 46.9 ± 8.9 years, 30.3% women; ERP negative: 47.5 ± 9.4 years, 54.2% women). After meta-analysis, 8 single nucleotide polymorphisms reached P ≤ 1 × 10(-5): The most significant finding was intergenic rs11653989 (odds ratio 0.47; 95% confidence interval 0.36-0.61; P = 6.9 × 10(-9)). The most biologically relevant finding was intronic to KCND3: rs17029069 (odds ratio 1.46; 95% confidence interval 1.25-1.69; P = 8.5 × 10(-7)). In the replication step (7151 individuals), none of the 8 variants replicated, and combined meta-analysis results failed to reach genome-wide significance.

CONCLUSIONS

In a genome-wide association study, we were not able to reliably identify genetic variants predisposing to ERP, presumably due to insufficient statistical power and phenotype heterogeneity. The reported heritability of ERP warrants continued investigation in larger well-phenotyped populations.

Common variation in the NOS1AP gene is associated with drug-induced QT prolongation and ventricular arrhythmia

OBJECTIVES

This study sought to determine whether variations in NOS1AP affect drug-induced long QT syndrome (LQTS).

BACKGROUND

Use of antiarrhythmic drugs is limited by the high incidence of serious adverse events including QT prolongation and torsades de pointes. NOS1AP gene variants play a role in modulating QT intervals in healthy subjects and severity of presentation in LQTS.

METHODS

This study carried out an association study using 167 single nucleotide polymorphisms (SNP) spanning the NOS1AP gene in 58 Caucasian patients experiencing drug-induced LQTS (dLQTS) and 87 Caucasian controls from the DARE (Drug-Induced Arrhythmia Risk Evaluation) study.

RESULTS

The rs10800397 SNP was significantly associated with dLQTS (odds ratio [OR]: 3.3, 99.95% confidence interval [CI]: 1.0 to 10.8, p = 3.7 × 10(-4)). The associations were more pronounced in the subgroup of amiodarone users, in which 3 SNPs, including rs10800397, were significantly associated (most significant SNP: rs10919035: OR: 5.5, 99.95% CI: 1.1 to 27.9, p = 3.0 × 10(-4)). We genotyped rs10919035 in an independent replication cohort of 28 amiodarone dLQTS cases versus 173 control subjects (meta-analysis of both studies: OR: 2.81, 99.95% CI: 1.62 to 4.89, p = 2.4 × 10(-4)). Analysis of corrected QT interval among 74 control subjects from our dataset showed a similar pattern of significance over the gene region as the case-control analysis. This pattern was confirmed in 1,480 control subjects from the BRIGHT (British Genetics of Hypertension Study) cohort (top SNP from DARE: rs12734991 in meta-analysis: increase in corrected QT interval per C allele: 9.1 ± 3.2 ms, p = 1.7 × 10(-4)).

CONCLUSIONS

These results provide the first demonstration that common variations in the NOS1AP gene are associated with a significant increase in the risk of dLQTS. This study suggests that common variations in the NOS1AP gene may have relevance for future pharmacogenomic applications in clinical practice permitting safer prescription of drugs for vulnerable patients.

QT interval and long-term mortality risk in the Framingham Heart Study

BACKGROUND

The association between QT interval and mortality has been demonstrated in large, prospective population-based studies, but the strength of the association varies considerably based on the method of heart rate correction. We examined the QT-mortality relationship in the Framingham Heart Study (FHS).

METHODS

Participants in the first (original cohort, n = 2,365) and second generation (offspring cohort, n = 4,530) cohorts were included in this study with a mean follow up of 27.5 years. QT interval measurements were obtained manually using a reproducible digital caliper technique.

RESULTS

Using Cox proportional hazards regression adjusting for age and sex, a 20 millisecond increase in QTc (using Bazett's correction; QT/RR(1/2) interval) was associated with a modest increase in risk of all-cause mortality (HR 1.14, 95% CI 1.10-1.18, P < 0.0001), coronary heart disease (CHD) mortality (HR 1.15, 95% CI 1.05-1.26, P = 0.003), and sudden cardiac death (SCD, HR 1.19, 95% CI 1.03-1.37, P = 0.02). However, adjustment for heart rate using RR interval in linear regression attenuated this association. The association of QT interval with all-cause mortality persisted after adjustment for cardiovascular risk factors, but associations with CHD mortality and SCD were no longer significant.

CONCLUSION

In FHS, there is evidence of a graded relation between QTc and all-cause mortality, CHD death, and SCD; however, this association is attenuated by adjustment for RR interval. These data confirm that using Bazett's heart rate correction, QTc, overestimates the association with mortality. An association with all-cause mortality persists despite a more complete adjustment for heart rate and known cardiovascular risk factors.

Fine-mapping and initial characterization of QT interval loci in African Americans

The QT interval (QT) is heritable and its prolongation is a risk factor for ventricular tachyarrhythmias and sudden death. Most genetic studies of QT have examined European ancestral populations; however, the increased genetic diversity in African Americans provides opportunities to narrow association signals and identify population-specific variants. We therefore evaluated 6,670 SNPs spanning eleven previously identified QT loci in 8,644 African American participants from two Population Architecture using Genomics and Epidemiology (PAGE) studies: the Atherosclerosis Risk in Communities study and Women's Health Initiative Clinical Trial. Of the fifteen known independent QT variants at the eleven previously identified loci, six were significantly associated with QT in African American populations (P≤1.20×10(-4)): ATP1B1, PLN1, KCNQ1, NDRG4, and two NOS1AP independent signals. We also identified three population-specific signals significantly associated with QT in African Americans (P≤1.37×10(-5)): one at NOS1AP and two at ATP1B1. Linkage disequilibrium (LD) patterns in African Americans assisted in narrowing the region likely to contain the functional variants for several loci. For example, African American LD patterns showed that 0 SNPs were in LD with NOS1AP signal rs12143842, compared with European LD patterns that indicated 87 SNPs, which spanned 114.2 Kb, were in LD with rs12143842. Finally, bioinformatic-based characterization of the nine African American signals pointed to functional candidates located exclusively within non-coding regions, including predicted binding sites for transcription factors such as TBX5, which has been implicated in cardiac structure and conductance. In this detailed evaluation of QT loci, we identified several African Americans SNPs that better define the association with QT and successfully narrowed intervals surrounding established loci. These results demonstrate that the same loci influence variation in QT across multiple populations, that novel signals exist in African Americans, and that the SNPs identified as strong candidates for functional evaluation implicate gene regulatory dysfunction in QT prolongation.

Increased Expression of Rififylin in A < 330 Kb Congenic Strain is Linked to Impaired Endosomal Recycling in Proximal Tubules

Cell surface proteins are internalized into the cell through endocytosis and either degraded within lysosomes or recycled back to the plasma membrane. While perturbations in endosomal internalization are known to modulate renal function, it is not known whether similar alterations in recycling affect renal function. Rififylin is a known regulator of endocytic recycling with E3 ubiquitin protein ligase activity. In this study, using two genetically similar strains, the Dahl Salt-sensitive rat and an S.LEW congenic strain, which had allelic variants within a < 330 kb segment containing rififylin, we tested the hypothesis that alterations in endosomal recycling affect renal function. The congenic strain had 1.59-fold higher renal expression of rififylin. Transcriptome analysis indicated that components of both endocytosis and recycling were upregulated in the congenic strain. Transcription of Atp1a1 and cell surface content of the protein product of Atp1a1, the alpha subunit of Na⁺K⁺ATPase were increased in the proximal tubules from the congenic strain. Because rififylin does not directly regulate endocytosis and it is also a differentially expressed gene within the congenic segment, we reasoned that the observed alterations in the transcriptome of the congenic strain constitute a feedback response to the primary functional alteration of recycling caused by rififylin. To test this, recycling of transferrin was studied in isolated proximal tubules. Recycling was significantly delayed within isolated proximal tubules of the congenic strain, which also had a higher level of polyubiquitinated proteins and proteinuria compared with S. These data provide evidence to suggest that delayed endosomal recycling caused by excess of rififylin indirectly affects endocytosis, enhances intracellular protein polyubiquitination and contributes to proteinuria.

The metabochip, a custom genotyping array for genetic studies of metabolic, cardiovascular, and anthropometric traits

Genome-wide association studies have identified hundreds of loci for type 2 diabetes, coronary artery disease and myocardial infarction, as well as for related traits such as body mass index, glucose and insulin levels, lipid levels, and blood pressure. These studies also have pointed to thousands of loci with promising but not yet compelling association evidence. To establish association at additional loci and to characterize the genome-wide significant loci by fine-mapping, we designed the "Metabochip," a custom genotyping array that assays nearly 200,000 SNP markers. Here, we describe the Metabochip and its component SNP sets, evaluate its performance in capturing variation across the allele-frequency spectrum, describe solutions to methodological challenges commonly encountered in its analysis, and evaluate its performance as a platform for genotype imputation. The metabochip achieves dramatic cost efficiencies compared to designing single-trait follow-up reagents, and provides the opportunity to compare results across a range of related traits. The metabochip and similar custom genotyping arrays offer a powerful and cost-effective approach to follow-up large-scale genotyping and sequencing studies and advance our understanding of the genetic basis of complex human diseases and traits.

Genetic testing of inherited arrhythmias

Syncope and risk of sudden death due to ventricular tachyarrhythmia are the common manifestations of several inherited disorders. 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 though the genetic mutations cause structural abnormalities of the myocardium. This is the case of arrhythmogenic right ventricular cardiomyopathy and hypertrophic cardiomyopathy. Groundbreaking discoveries, starting from the 1990s until the beginning of the current decade, have provided fundamental knowledge on the major genes that confer an increased risk of arrhythmias and sudden death. Stems of such knowledge are the availability of genetic diagnosis, genotype-phenotype correlation, and genotype-based risk stratification schemes currently used in the clinical practice. This review provides a concise description of the known genes and key mechanisms involved in the pathogenesis of inherited arrhythmias. In addition, we outline possibilities, limitations, advantages, and potential threats of genetically screening for these genes.

Mapping genetic variants associated with beta-adrenergic responses in inbred mice

β-blockers and β-agonists are primarily used to treat cardiovascular diseases. Inter-individual variability in response to both drug classes is well recognized, yet the identity and relative contribution of the genetic players involved are poorly understood. This work is the first genome-wide association study (GWAS) addressing the values and susceptibility of cardiovascular-related traits to a selective β₁-blocker, Atenolol (ate), and a β-agonist, Isoproterenol (iso). The phenotypic dataset consisted of 27 highly heritable traits, each measured across 22 inbred mouse strains and four pharmacological conditions. The genotypic panel comprised 79922 informative SNPs of the mouse HapMap resource. Associations were mapped by Efficient Mixed Model Association (EMMA), a method that corrects for the population structure and genetic relatedness of the various strains. A total of 205 separate genome-wide scans were analyzed. The most significant hits include three candidate loci related to cardiac and body weight, three loci for electrocardiographic (ECG) values, two loci for the susceptibility of atrial weight index to iso, four loci for the susceptibility of systolic blood pressure (SBP) to perturbations of the β-adrenergic system, and one locus for the responsiveness of QTc (p<10⁻⁸). An additional 60 loci were suggestive for one or the other of the 27 traits, while 46 others were suggestive for one or the other drug effects (p<10⁻⁶). Most hits tagged unexpected regions, yet at least two loci for the susceptibility of SBP to β-adrenergic drugs pointed at members of the hypothalamic-pituitary-thyroid axis. Loci for cardiac-related traits were preferentially enriched in genes expressed in the heart, while 23% of the testable loci were replicated with datasets of the Mouse Phenome Database (MPD). Altogether these data and validation tests indicate that the mapped loci are relevant to the traits and responses studied.

Gene regulatory networks in cardiac conduction system development

The cardiac conduction system is a specialized tract of myocardial cells responsible for maintaining normal cardiac rhythm. Given its critical role in coordinating cardiac performance, a detailed analysis of the molecular mechanisms underlying conduction system formation should inform our understanding of arrhythmia pathophysiology and affect the development of novel therapeutic strategies. Historically, the ability to distinguish cells of the conduction system from neighboring working myocytes presented a major technical challenge for performing comprehensive mechanistic studies. Early lineage tracing experiments suggested that conduction cells derive from cardiomyocyte precursors, and these claims have been substantiated by using more contemporary approaches. However, regional specialization of conduction cells adds an additional layer of complexity to this system, and it appears that different components of the conduction system utilize unique modes of developmental formation. The identification of numerous transcription factors and their downstream target genes involved in regional differentiation of the conduction system has provided insight into how lineage commitment is achieved. Furthermore, by adopting cutting-edge genetic techniques in combination with sophisticated phenotyping capabilities, investigators have made substantial progress in delineating the regulatory networks that orchestrate conduction system formation and their role in cardiac rhythm and physiology. This review describes the connectivity of these gene regulatory networks in cardiac conduction system development and discusses how they provide a foundation for understanding normal and pathological human cardiac rhythms.

Common genetic variants associated with sudden cardiac death: the FinSCDgen study

Background

Sudden cardiac death (SCD) accounts for up to half of cardiac mortality. The risk of SCD is heritable but the underlying genetic variants are largely unknown. We investigated whether common genetic variants predisposing to arrhythmia or related electrocardiographic phenotypes, including QT-interval prolongation, are associated with increased risk of SCD.

Methodology/Principal Findings

We studied the association between 28 candidate SNPs and SCD in a meta-analysis of four population cohorts (FINRISK 1992, 1997, 2002 and Health 2000, n = 27,629) and two forensic autopsy series (The Helsinki Sudden Death Study and The Tampere Autopsy Study, n = 694). We also studied the association between established cardiovascular risk factors and SCD. Causes of death were reviewed using registry-based health and autopsy data. Cox regression and logistic regression models were adjusted for age, sex, and geographic region. The total number of SCDs was 716. Two novel SNPs were associated with SCD: SCN5A rs41312391 (relative risk [RR] 1.27 per minor T allele, 95% CI 1.11–1.45, P = 3.4×10⁻⁴) and rs2200733 in 4q25 (RR 1.28 per minor T allele, 95% CI 1.11–1.48, P = 7.9×10⁻⁴). We also replicated the associations for 9p21 (rs2383207, RR 1.13 per G allele, 95% CI 1.01–1.26, P = 0.036), as well as for male sex, systolic blood pressure, diabetes, cigarette smoking, low physical activity, coronary heart disease, and digoxin use (P<0.05).

Conclusions/Significance

Two novel genetic variants, one in the cardiac sodium channel gene SCN5A and another at 4q25 previously associated with atrial fibrillation, are associated with SCD.

A common variant in SLC8A1 is associated with the duration of the electrocardiographic QT interval

Prolongation of the electrocardiographic QT interval, a measure of cardiac repolarization, predisposes one to ventricular arrhythmias and sudden cardiac death. Since NOS1AP, a regulator of neuronal nitric oxide synthase, was discovered in a genome-wide association study (GWAS) as a novel target that modulates cardiac repolarization, several loci have been linked to the QT interval in studies (QTGEN and QTSCD) of European descendents. However, there has been no GWAS of the QT interval in Asian populations. We conducted a GWAS with regard to the QT interval in Korea Association Resource (KARE [n = 6,805]) cohorts. Replication studies in independent populations of Korean (n = 4,686) and Japanese (n = 2,687) groups validated the association between a SNP, rs13017846, which maps to near SLC8A1 (sodium/calcium exchanger 1 precursor, overall p = 8.0 × 10(-14)), and the QT interval. The minor allele frequency (MAF) of rs13017846 varies widely between ethnicities-0.053 in Europeans (HapMap CEU [Utah residents with ancestry from northern and western Europe from the Centre d'Étude du Polymorphisme Humain collection] samples) versus 0.080 in Africans (HapMap YRI [Yoruba in Ibadan, Nigeria] samples)-whereas a MAF of 0.500 has been reported in Asians (HapMap HCB [Han Chinese in Beijing, China] and JPT [Japanese in Tokyo, Japan] samples). This might explain why this locus has not been identified in Europeans in previous studies.

Genetic predisposition to sudden cardiac death

PURPOSE OF REVIEW

Sudden cardiac death (SCD) is a major public health burden, and evidence from family history and from molecular studies on inherited arrhythmogenic syndromes indicates that genetic factors are important contributors to the risk of SCD. This review discusses recent advances on the genetic predisposition to SCD, with a specific focus on primary ventricular fibrillation and channelopathies.

RECENT FINDINGS

Coronary artery disease is the major determinant of SCD, and its predisposing genetic background is complex. Very recently, a first genome-wide association study on primary ventricular fibrillation was published but the results are not conclusive and further studies with greater numbers are needed. Among channelopathies, long QT syndrome and Brugada syndrome are those in which more significant advances have been reported in the last year. Of note is the recently described early repolarization syndrome and the proposed classification of J wave syndromes. Revision of current guidelines for autopsy investigation has introduced molecular autopsy as a standard requirement for adequate assessment of SCD.

SUMMARY

Interesting data on the genetic basis of sudden cardiac death have been published in the past year, and, whereas in the field of channelopathies research findings have been partially recognized by current guidelines and translated into clinical practice, in the field of coronary artery disease further advances are still needed.

The genetics of sudden cardiac death

Sudden cardiac death (SCD), a sudden pulseless condition due to cardiac arrhythmia, remains a major public health problem despite recent progress in the treatment and prevention of overall coronary heart disease. In this review, we examine the evidence for genetic susceptibility to SCD in order to provide biological insight into the pathogenesis of this devastating disease and to explore the potential for genetics to impact clinical management of SCD risk. Both candidate gene approaches and unbiased genome-wide scans have identified novel biological pathways contributing to SCD risk. Although risk stratification in the general population remains an elusive goal, several studies point to the potential utility of these common genetic variants in high-risk individuals. Finally, we highlight novel methodological approaches to deciphering the molecular mechanisms involved in arrhythmogenesis. Although further epidemiological and clinical applications research is needed, it is increasingly clear that genetic approaches are yielding important insights into SCD that may impact the public health burden imposed by SCD and its associated outcomes.

Novel rare variants in congenital cardiac arrhythmia genes are frequent in drug-induced torsades de pointes

Marked prolongation of the QT interval and polymorphic ventricular tachycardia following medication (drug-induced long QT syndrome, diLQTS) is a severe adverse drug reaction (ADR) that phenocopies congenital long QT syndrome (cLQTS) and is one of the leading causes for drug withdrawal and relabeling. We evaluated the frequency of rare non-synonymous variants in genes contributing to the maintenance of heart rhythm in cases of diLQTS using targeted capture coupled to next-generation sequencing. Eleven of 31 diLQTS subjects (36%) carried a novel missense mutation in genes with known congenital arrhythmia associations or with a known cLQTS mutation. In the 26 Caucasian subjects, 23% carried a highly conserved rare variant predicted to be deleterious to protein function in these genes compared with only 2-4% in public databases (P<0.003). We conclude that the rare variation in genes responsible for congenital arrhythmia syndromes is frequent in diLQTS. Our findings demonstrate that diLQTS is a pharmacogenomic syndrome predisposed by rare genetic variants.

Elements of 'missing heritability'

PURPOSE OF REVIEW

To discuss the basis of 'missing heritability', which has emerged as an enigma in the post-genome-wide association studies (GWAS) era.

RECENT FINDINGS

Alleles identified through GWAS account for a relatively small fraction of heritability of the complex phenotypes. Accordingly, a significant part of heritability of the complex traits remains unaccounted for ('missing heritability'). Recent findings offer several explanations, including overestimation of heritability of the complex traits and underestimation of the effects of alleles identified through GWAS. In addition, yet-to-be identified common as well as rare alleles might in part explain the 'missing heritability'. Moreover, gene-gene (epistasis) and gene-environmental interactions might explain another fraction of heritability of complex traits. Moreover, transgenerational epigenetic changes, regulated in part by microRNAs, might also contribute to the 'missing heritability'.

SUMMARY

The new findings suggest a multifarious nature of the 'missing heritability'. The findings de-emphasize the focus on delineating the basis of 'missing heritability' and shift the focus to elucidation of the molecular mechanisms by which genomic and genetic factors govern the pathogenesis of the complex phenotypes.