RATIONALE:Genome-wide association studies previously identified an association of rs9388451 at chromosome 6q22.3 (near HEY2) with Brugada syndrome (BrS). The causal gene and underlying mechanism ...remain unresolved.
OBJECTIVE:We used an integrative approach entailing transcriptomic studies in human hearts and electrophysiological studies in Hey2 heterozygous knockout mice (Hey2) to dissect the underpinnings of the 6q22.31 association with BrS.
METHODS AND RESULTS:We queried expression quantitative trait locus (eQTL) data acquired in 190 human left ventricular (LV) samples from the Genotype-Tissue Expression (GTEx) consortium for cis-eQTL effects of rs9388451 which revealed an association between BrS risk allele dosage and HEY2 expression (β=+0.159; P=0.0036). In the same transcriptomic data, we conducted genome-wide co-expression analysis for HEY2 which uncovered KCNIP2, encoding the β-subunit of the channel underlying the transient outward current (Ito), as the transcript most robustly correlating with HEY2 expression (β=+1.47; P=2X10-). Transcript abundance of HEY2 and the Ito subunits Kcnip2 and Kcnd2, assessed by qRT-PCR, was higher in subepicardium (epi) vs. subendocardium (endo) in both left (LV) and right (RV) ventricles, with lower levels in HEY2 heterozygous knockout (Hey2) mice compared to wildtype (WT). Surface ECG measurements showed less prominent J-waves in Hey2 mice compared to WT. In WT mice, patch-clamp electrophysiological studies on cardiac myocytes from RV demonstrated a shorter AP duration and a lower Vmax in epi compared to endo cardiac myocytes, which was paralleled by a higher Ito and a lower INa density in epi vs. endo. These transmural differences were diminished in Hey2 mice due to changes in subepicardial cardiac myocytes.
CONCLUSIONS:This study uncovers a role of HEY2 in the normal transmural electrophysiological gradient in the ventricle and provides compelling evidence that genetic variation at 6q22.31 (rs9388451) is associated with BrS through a HEY2-dependent alteration of ion channel expression across the cardiac ventricular wall.
Abstract Background Mutations in SCN5A , the gene encoding the α-subunit of the cardiac sodium channel (NaV1.5), are associated with a broad spectrum of inherited cardiac arrhythmia disorders. The ...purpose of this study was to identify the genetic and functional determinants underlying a Dutch family that presented with a combined phenotype of ventricular arrhythmias with a likely adrenergic component, either in isolation or in combination with a mildly decreased heart function and early onset (< 55 years) atrial fibrillation. Methods and results We performed next generation sequencing in the proband of a two-generation Dutch family and demonstrated a novel missense mutation in SCN5A-(p.M1851V) which co-segregated with the clinical phenotype in the family. We functionally evaluated the putative genetic defect by patch clamp electrophysiological studies in human embryonic kidney cells transfected with mutant or wild-type Nav1.5. The current inactivation was slower and recovery from inactivation was faster in SCN5A-M1851V channels. The voltage dependence of inactivation was shifted towards more positive potentials and consequently, a larger TTX-sensitive window current was observed in SCN5A-M1851V channels. Furthermore, a higher upstroke velocity was observed for the SCN5A-M1851V channels, while the depolarization voltage was more negative, both indicating increased excitability. Conclusions This mutation leads to a gain-of-function mechanism based on increased channel availability and increased window current, fitting the observed clinical phenotype of (likely adrenergic-induced) ventricular arrhythmias and atrial fibrillation. These findings further expand the range of cardiac arrhythmias associated with mutations in SCN5A.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Objectives The aim of this study was to investigate the modulatory effect of the coxsackie and adenovirus receptor (CAR) on ventricular conduction and arrhythmia vulnerability in the setting of ...myocardial ischemia. Background A heritable component in the risk of ventricular fibrillation during myocardial infarction has been well established. A recent genome-wide association study of ventricular fibrillation during acute myocardial infarction led to the identification of a locus on chromosome 21q21 (rs2824292) in the vicinity of the CXADR gene. CXADR encodes the CAR, a cell adhesion molecule predominantly located at the intercalated disks of the cardiomyocyte. Methods The correlation between CAR transcript levels and rs2824292 genotype was investigated in human left ventricular samples. Electrophysiological studies and molecular analyses were performed using CAR haploinsufficient (CAR+/− ) mice. Results In human left ventricular samples, the risk allele at the chr21q21 genome-wide association study locus was associated with lower CXADR messenger ribonucleic acid levels, suggesting that decreased cardiac levels of CAR predispose to ischemia-induced ventricular fibrillation. Hearts from CAR+/− mice displayed slowing of ventricular conduction in addition to an earlier onset of ventricular arrhythmias during the early phase of acute myocardial ischemia after ligation of the left anterior descending artery. Expression and distribution of connexin 43 were unaffected, but CAR+/− hearts displayed increased arrhythmia susceptibility on pharmacological electrical uncoupling. Patch-clamp analysis of isolated CAR+/− myocytes showed reduced sodium current magnitude specifically at the intercalated disk. Moreover, CAR coprecipitated with NaV 1.5 in vitro, suggesting that CAR affects sodium channel function through a physical interaction with NaV 1.5. Conclusions CAR is a novel modifier of ventricular conduction and arrhythmia vulnerability in the setting of myocardial ischemia. Genetic determinants of arrhythmia susceptibility (such as CAR) may constitute future targets for risk stratification of potentially lethal ventricular arrhythmias in patients with coronary artery disease.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract
Aims
Cardiac arrhythmias comprise a major health and economic burden and are associated with significant morbidity and mortality, including cardiac failure, stroke, and sudden cardiac death ...(SCD). Development of efficient preventive and therapeutic strategies is hampered by incomplete knowledge of disease mechanisms and pathways. Our aim is to identify novel mechanisms underlying cardiac arrhythmia and SCD using an unbiased approach.
Methods and results
We employed a phenotype-driven N-ethyl-N-nitrosourea mutagenesis screen and identified a mouse line with a high incidence of sudden death at young age (6–9 weeks) in the absence of prior symptoms. Affected mice were found to be homozygous for the nonsense mutation Bcat2p.Q300*/p.Q300* in the Bcat2 gene encoding branched chain amino acid transaminase 2. At the age of 4–5 weeks, Bcat2p.Q300*/p.Q300* mice displayed drastic increase of plasma levels of branch chain amino acids (BCAAs—leucine, isoleucine, valine) due to the incomplete catabolism of BCAAs, in addition to inducible arrhythmias ex vivo as well as cardiac conduction and repolarization disturbances. In line with these findings, plasma BCAA levels were positively correlated to electrocardiogram indices of conduction and repolarization in the German community-based KORA F4 Study. Isolated cardiomyocytes from Bcat2p.Q300*/p.Q300* mice revealed action potential (AP) prolongation, pro-arrhythmic events (early and late afterdepolarizations, triggered APs), and dysregulated calcium homeostasis. Incubation of human pluripotent stem cell-derived cardiomyocytes with elevated concentration of BCAAs induced similar calcium dysregulation and pro-arrhythmic events which were prevented by rapamycin, demonstrating the crucial involvement of mTOR pathway activation.
Conclusions
Our findings identify for the first time a causative link between elevated BCAAs and arrhythmia, which has implications for arrhythmogenesis in conditions associated with BCAA metabolism dysregulation such as diabetes, metabolic syndrome, and heart failure.
Graphical Abstract
Graphical Abstract
In this chapter, we will use the example of the identification of Tnni3k as a modulator of cardiac conduction to introduce you to the use of a murine F2-generation intercross as a powerful method for ...the identification of novel genes relevant for cardiovascular traits. Murine F2-progeny is a genetically diverse panel of mice with differences in phenotype manifestations, e.g. cardiovascular traits such as cardiomyopathy and ECG parameters. This chapter discusses the best strategies for using F2-mice for genetic mapping. Moreover, we provide an example of the feasibility of identification of new genes modulating cardiac function utilizing the technique of mapping quantitative trait loci (QTLs) and a systems genetics integration of available genetic, gene expression, and phenotypic data.
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