CHD4 variants are associated with childhood idiopathic epilepsy with sinus arrhythmia.

AIMS: CHD4 gene, encoding chromodomain helicase DNA-binding protein 4, is a vital gene for fetal development. In this study, we aimed to explore the association between CHD4 variants and idiopathic epilepsy. METHODS: Trios-based whole-exome sequencing was performed in a cohort of 482 patients with childhood idiopathic epilepsy. The Clinical Validity Framework of ClinGen and an evaluating method from five clinical-genetic aspects were used to determine the association between CHD4 variants and epilepsy. RESULTS: Four novel heterozygous missense mutations in CHD4, including two de novo mutations (c.1597A>G/p.K533E and c.4936G>A/p.E1646K) and two inherited mutations with co-segregation (c.856C>G/p.P286A and c.4977C>G/p.D1659E), were identified in four unrelated families with eight individuals affected. Seven affected individuals had sinus arrhythmia. From the molecular sub-regional point of view, the missense mutations located in the central regions from SNF2-like region to DUF1087 domain were associated with multisystem developmental disorders, while idiopathic epilepsy-related mutations were outside this region. Strong evidence from ClinGen Clinical Validity Framework and evidences from four of the five clinical-genetic aspects suggested an association between CHD4 variants and epilepsy. CONCLUSIONS: CHD4 was potentially a candidate pathogenic gene of childhood idiopathic epilepsy with arrhythmia. The molecular sub-regional effect of CHD4 mutations helped explaining the mechanisms underlying phenotypic variations.

ATAC-Seq Reveals an Isl1 Enhancer That Regulates Sinoatrial Node Development and Function.

RATIONALE: Cardiac pacemaker cells (PCs) in the sinoatrial node (SAN) have a distinct gene expression program that allows them to fire automatically and initiate the heartbeat. Although critical SAN transcription factors, including Isl1 (Islet-1), Tbx3 (T-box transcription factor 3), and Shox2 (short-stature homeobox protein 2), have been identified, the cis-regulatory architecture that governs PC-specific gene expression is not understood, and discrete enhancers required for gene regulation in the SAN have not been identified. OBJECTIVE: To define the epigenetic profile of PCs using comparative ATAC-seq (assay for transposase-accessible chromatin with sequencing) and to identify novel enhancers involved in SAN gene regulation, development, and function. METHODS AND RESULTS: We used ATAC-seq on sorted neonatal mouse SAN to compare regions of accessible chromatin in PCs and right atrial cardiomyocytes. PC-enriched assay for transposase-accessible chromatin peaks, representing candidate SAN regulatory elements, were located near established SAN genes and were enriched for distinct sets of TF (transcription factor) binding sites. Among several novel SAN enhancers that were experimentally validated using transgenic mice, we identified a 2.9-kb regulatory element at the Isl1 locus that was active specifically in the cardiac inflow at embryonic day 8.5 and throughout later SAN development and maturation. Deletion of this enhancer from the genome of mice resulted in SAN hypoplasia and sinus arrhythmias. The mouse SAN enhancer also directed reporter activity to the inflow tract in developing zebrafish hearts, demonstrating deep conservation of its upstream regulatory network. Finally, single nucleotide polymorphisms in the human genome that occur near the region syntenic to the mouse enhancer exhibit significant associations with resting heart rate in human populations. CONCLUSIONS: (1) PCs have distinct regions of accessible chromatin that correlate with their gene expression profile and contain novel SAN enhancers, (2) cis-regulation of Isl1 specifically in the SAN depends upon a conserved SAN enhancer that regulates PC development and SAN function, and (3) a corresponding human ISL1 enhancer may regulate human SAN function.

Long noncoding RNA GAS5 attenuates cardiac fibroblast proliferation in atrial fibrillation via repressing ALK5.

OBJECTIVE: Recently, long noncoding RNAs (lncRNAs) have caught more attention for their role in the progression of many diseases. Among them, lncRNA GAS5 (Growth Inhibition Specificity 5) was studied in this research to identify how it affects the progression of atrial fibrillation (AF). PATIENTS AND METHODS: In 40 patients with AF and 30 patients with sinus rhythm (SR), the GAS5 expression of the right atrial appendage (RAA) tissues was detected by the quantitative Real Time-Polymerase Chain Reaction (qRT-PCR). Moreover, the cell proliferation assay was conducted in AC16 cells transfected with GAS5 inhibitor and mimics, respectively. Furthermore, the qRT-PCR was performed to uncover the mechanism. RESULTS: In the research, the expression of GAS5 in RAA tissues was decreased significantly in AF patients than that in SR ones. Moreover, overexpression of GAS5 inhibited cell growth in AC16 cells, while knockdown of GAS5 promoted cell growth in AC16 cells. In addition, further experiments revealed that ALK5 was a target of GAS5 and its expression in AF tissues negatively correlated to GAS5 expression. CONCLUSIONS: These results indicate that GAS5 could inhibit cell proliferation of AF via suppressing ALK5, which may offer a new vision for interpreting the mechanism of AF development.

Whole Blood Gene Expression Differentiates between Atrial Fibrillation and Sinus Rhythm after Cardioversion.

BACKGROUND: Treatment to restore sinus rhythm among patients with atrial fibrillation (AF) has limited long-term success rates. Gene expression profiling may provide new insights into AF pathophysiology. OBJECTIVE: To identify biomarkers and improve our understanding of AF pathophysiology by comparing whole blood gene expression before and after electrical cardioversion (ECV). METHODS: In 46 patients with persistent AF that underwent ECV, whole blood samples were collected 1-2 hours before and 4 to 6 weeks after successful cardioversion. The paired samples were sent for microarray and plasma biomarker comparison. RESULTS: Of 13,942 genes tested, expression of SLC25A20 and PDK4 had the strongest associations with AF. Post-cardioversion, SLC25A20 and PDK4 expression decreased by 0.8 (CI 0.7-0.8, p = 2.0x10-6) and 0.7 (CI 0.6-0.8, p = 3.0x10-5) fold respectively. Median N-terminal pro B-type natriuretic peptide (NT-proBNP) concentrations decreased from 127.7 pg/mL to 44.9 pg/mL (p = 2.3x10-13) after cardioversion. AF discrimination models combining NT-proBNP and gene expression (NT-proBNP + SLC25A20 area under the curve = 0.88, NT-proBNP + PDK4 AUC = 0.86) had greater discriminative capacity as compared with NT-proBNP alone (AUC = 0.82). Moreover, a model including NT-proBNP, SLC25A20 and PDK4 significantly improved AF discrimination as compared with other models (AUC = 0.87, Net Reclassification Index >0.56, p<5.8x10-3). We validated the association between SLC25A20 and PDK4 with AF in an independent sample of 17 patients. CONCLUSION: This study demonstrates that SLC25A20, PDK4, and NT-proBNP have incremental utility as biomarkers discriminating AF from sinus rhythm. Elevated SLC25A20 and PDK4 expression during AF indicates an important role for energy metabolism in AF.

Associations of MMP-2, BAX, and Bcl-2 mRNA and Protein Expressions with Development of Atrial Fibrillation.

BACKGROUND To examine changes of mRNA and protein expressions of MMP-2, Bcl-2, and BAX in atrial fibrillation (AF) patients, and investigate the correlations among these 3 biomarkers. MATERIAL AND METHODS Rheumatic heart disease patients (n=158) undergoing cardiac surgical procedures for mitral valve repair or replacement were included as the AF group (n=123), containing paroxysmal AF (n=42), persistent AF (n=36), and permanent AF (n=45). Rheumatic heart disease patients with sinus rhythm (SR) (n=35) were enrolled as the SR group (control group). Immunohistochemistry, Western blot, and real-time polymerase chain reaction (PCR) were applied to detect the protein and mRNA expression levels of MMP-2, Bcl-2, and BAX. Apoptosis was observed with light and electron microscopes and detected by TdT-mediated dUTP nick-end labeling (TUNEL). RESULTS Compared with the SR group, the left atrial diameters (LADs), protein and mRNA expression levels of MMP-2 and BAX, apoptotic index (AI), and Bcl-2/BAX ratio were evidently increased in the 3 AF groups, but protein and mRNA expression levels of Bcl-2 decreased in the AF groups (all P<0.05). Correlation analysis found that MMP-2 protein expression levels was positively correlated with BAX expression, but negatively correlated with Bcl-2 expression levels. CONCLUSIONS Our study results suggest that elevated MMP-2 expression and disturbance balance of Bcl-2/BAX expressions may be associated with the development and maintenance of AF. MMP-2 may be involved in the development of AF through promoting BAX expressions and inhibiting Bcl-2.

High-fat diet increases vulnerability to atrial arrhythmia by conduction disturbance via miR-27b.

BACKGROUND: Lifestyle-related diseases, such as obesity and dyslipidemia are important risk factors for atrial fibrillation (AF). However, the underlying mechanism linking these diseases and AF has not been fully investigated. METHODS: Adult male mice were fed a high-fat diet (HFD) or vehicle (NC) for 2 months. Electrocardiography and in vivo electrophysiological study were performed. Mice were then sacrificed for quantification of mRNA, microRNA, and protein in atria, in addition to histological analysis. Conduction velocity (CV) in right atrium was measured by optical mapping in Langendorff perfused hearts. Cultured atrial cardiomyocytes were treated with palmitate with or without a specific microRNA inhibitor. Twelve hours after stimulation, cells were lysed, and subjected to analysis with qPCR and Western blotting. RESULTS: HFD mice showed prolonged P wave duration, increased inducibility of sustained atrial tachycardia, and reduced atrial CV than NC mice. HFD mice also showed increased expression in inflammatory cytokines, whereas fibrotic area and signals relating fibrosis were not changed. HFD mice demonstrated reduced expression of Cx40 in mRNA and protein levels, and its lateralized expression in atria. MicroRNA array analysis revealed that miR-27b expression was up-regulated in HFD mice, and luciferase assay confirmed the direct interaction between miR-27b and Cx40 3'UTR. In palmitate-stimulated atrial cardiomyocytes, miR-27b up-regulation and Cx40 down-regulation were observed, while expression of inflammatory cytokines was not altered. Inhibition of miR-27b with antisense oligonucleotides reversed the alteration caused by palmitate stimulation. CONCLUSION: HFD may increase the vulnerability to atrial arrhythmia by down-regulation of Cx40 via miR-27b, rather than fibrosis, which is independent of inflammation.

Increased TRPM6 expression in atrial fibrillation patients contribute to atrial fibrosis.

BACKGROUND: Transient receptor potential (TRP) family plays important roles in cardiovascular system. We investigated the relationship between transient receptor potential channel subfamily M6 (TRPM6) and atrial fibrosis in rheumatic heart disease patients with atrial fibrillation (AF). METHODS: The right atrial tissue samples were obtained from 64 patients with rheumatic heart diseases who underwent heart valve replacement surgery, and composed of 34 sinus rhythm (SR) patients and 30 AF patients. Hematoxylin and Eosin (HE) staining was used to observe cross-sectional area (CSA) of myocardial cell. Masson staining and measurement of connective tissue growth factor (CTGF), transforming growth factor beta 1 (TGF-ß 1), and collagen type I/III (Collagen I/III) were performed to determine atrial fibrosis. The mRNA and protein levels of TRPM6 were detected by real-time quantitative polymerase chain reaction (RT-PCR) and western blotting, respectively. RESULTS: Marked increases were observed in CSA of myocardial cell and myocardial collagen volume fraction in AF group compared with the SR group (all P<0.05). The mRNA levels of myocardial fibrosis markers (CTGF, TGF-beta 1, Collagen I/III) in AF group increased significantly compared to the SR group (all P<0.05). TRPM6 mRNA and protein levels in AF group were elevated markedly in comparison with SR group (P<0.01). CONCLUSION: These findings revealed that increased TRPM6 mRNA and protein levels may contribute to atrial fibrosis, and suggested that TRPM6 might be involved in AF development by promoting fibrogenesis.

Functional roles of Ca(v)1.3, Ca(v)3.1 and HCN channels in automaticity of mouse atrioventricular cells: insights into the atrioventricular pacemaker mechanism.

The atrioventricular node controls cardiac impulse conduction and generates pacemaker activity in case of failure of the sino-atrial node. Understanding the mechanisms of atrioventricular automaticity is important for managing human pathologies of heart rate and conduction. However, the physiology of atrioventricular automaticity is still poorly understood. We have investigated the role of three key ion channel-mediated pacemaker mechanisms namely, Ca(v)1.3, Ca(v)3.1 and HCN channels in automaticity of atrioventricular node cells (AVNCs). We studied atrioventricular conduction and pacemaking of AVNCs in wild-type mice and mice lacking Ca(v)3.1 (Ca(v)3.1(-/-)), Ca(v)1.3 (Ca(v)1.3(-/-)), channels or both (Ca(v)1.3(-/-)/Ca(v)3.1(-/-)). The role of HCN channels in the modulation of atrioventricular cells pacemaking was studied by conditional expression of dominant-negative HCN4 channels lacking cAMP sensitivity. Inactivation of Ca(v)3.1 channels impaired AVNCs pacemaker activity by favoring sporadic block of automaticity leading to cellular arrhythmia. Furthermore, Ca(v)3.1 channels were critical for AVNCs to reach high pacemaking rates under isoproterenol. Unexpectedly, Ca(v)1.3 channels were required for spontaneous automaticity, because Ca(v)1.3(-/-) and Ca(v)1.3(-/-)/Ca(v)3.1(-/-) AVNCs were completely silent under physiological conditions. Abolition of the cAMP sensitivity of HCN channels reduced automaticity under basal conditions, but maximal rates of AVNCs could be restored to that of control mice by isoproterenol. In conclusion, while Ca(v)1.3 channels are required for automaticity, Ca(v)3.1 channels are important for maximal pacing rates of mouse AVNCs. HCN channels are important for basal AVNCs automaticity but do not appear to be determinant for ß-adrenergic regulation.

A novel mutation in the HCN4 gene causes symptomatic sinus bradycardia in Moroccan Jews.

OBJECTIVES: to conduct a clinical, genetic, and functional analysis of 3 unrelated families with familial sinus bradycardia (FSB). BACKGROUND: mutations in the hyperpolarization-activated nucleotide-gated channel (HCN4) are known to be associated with FSB. METHODS AND RESULTS: three males of Moroccan Jewish descent were hospitalized: 1 survived an out-of-hospital cardiac arrest and 2 presented with weakness and presyncopal events. All 3 had significant sinus bradycardia, also found in other first-degree relatives, with a segregation suggesting autosomal-dominant inheritance. All had normal response to exercise and normal heart structure. Sequencing of the HCN4 gene in all patients revealed a C to T transition at nucleotide position 1,454, which resulted in an alanine to valine change (A485V) in the ion channel pore found in most of their bradycardiac relatives, but not in 150 controls. Functional expression of the mutated ion channel in Xenopus oocytes and in human embryonic kidney 293 cells revealed profoundly reduced function and synthesis of the mutant channel compared to wild-type. CONCLUSIONS: we describe a new mutation in the HCN4 gene causing symptomatic FSB in 3 unrelated individuals of similar ethnic background that may indicate unexplained FSB in this ethnic group. This profound functional defect is consistent with the symptomatic phenotype.

The genetic and environmental etiology of sympathetic and parasympathetic activity in children.

The present study examines the genetic and environmental etiology of the associations among respiratory sinus arrhythmia (RSA), heart rate (HR), skin conductance level (SCL), and non-specific skin conductance responses (NS-SCR)-measures that purportedly index the parasympathetic and sympathetic branches of the autonomic nervous system. The sample was drawn from a cohort of 1,219 preadolescent twins (aged 9-10). Multivariate analyses of the data were conducted using structural equation modeling. Almost all genetic and environmental influences on the measures acted through two latent factors. The first latent factor was largely responsible for the variance in heart rate, SCL and NS-SCR, reflecting sympathetic activity, and its proportions of variance due to genetic and shared environmental influences were 27 and 28% in males, and 31 and 41% in females, respectively. The second latent factor accounted for the variance in RSA and heart rate, reflecting parasympathetic activity; genetic and shared environmental factors explained 27 and 23% of the variance in males, respectively, and 35 and 18% of the variance in females. Measurement-specific genetic effects accounted for 14-27% of the total variance in RSA and SCL, and measurement-specific shared environmental effects accounted for 10-12% in SCL. In general, the validity of separate sympathetic and parasympathetic constructs was supported.

Mouse models for studying pacemaker channel function and sinus node arrhythmia.

Pacemaker activity of the heart is generated by a small group of cells forming the sinoatrial node (SAN). Cells of the SAN are spontaneously active and generate action potentials with remarkable regularity and stability under all physiological conditions. The exact molecular mechanisms underlying pacemaker potentials in the SAN have not yet been fully elucidated. Several voltage-dependent ion channels as well as intracellular calcium cycling processes are thought to contribute to the pacemaker activity. Hyperpolarization-activated cation channels, which generate the I(f) current, have biophysical properties which seem ideally suited for the initiation of spontaneous electrical activity. This review describes recent work on several transgenic mice lacking different cardiac HCN channel subtypes. The role of I(f) for normal pacemaking and sinus node arrhythmia as revealed by these genetic models will be discussed. In addition, a new mouse line is described which enables gene targeting in a temporally-controlled manner selectively in SAN cells. Elucidating the function of HCN and other ion channels in well-controlled mouse models should ultimately lead to a better understanding of the mechanisms underlying human sinoatrial arrhythmias.

Altered expression of gap junction connexin proteins may partly underlie heart rhythm disturbances in the streptozotocin-induced diabetic rat heart.

Previous studies in isolated perfused heart and in atrial preparations have demonstrated significant reductions in beating rate in STZ-induced diabetic rats, which suggests that sinus arrhythmias in diabetes mellitus may be partly caused by intrinsic alteration of sino-atrial node (SAN) function. The effects of diabetes on electrical activity and expression levels of mRNA for gap junction proteins in the SAN have been investigated. Diabetes was induced by a single intraperitoneal injection of STZ (60 mg/kg) administered to young male Wistar rats (200-250 g). Experiments were performed 8-10 weeks after treatment. Conduction time and pacemaker cycle length were measured in sino-atrial node preparations with extracellular electrodes. Expression levels of mRNA for Gja5 (Cx40), Gja1 (Cx43) and Gja7 (Cx45) were measured in SAN and compared with right atrium and right ventricle with real-time quantitative reverse transcription-polymerase chain reaction. Diabetes was confirmed by a significant elevation of blood glucose (356+/-21 mg/dl) compared to age-matched controls (66+/-2 mg/dl). Pacemaker cycle length was significantly prolonged in diabetic heart (415+/-43 ms, n=6) compared to controls (255+/-7 ms, n=6). Sino-atrial conduction time was also significantly prolonged in diabetic hearts (12+/-2 ms) compared to controls (7+/-1 ms). Expression levels of mRNA for Gja5 (Cx40) and Gja1 (Cx43) were moderately increased and for Gja7 (Cx45) was significantly increased in SAN from diabetic heart compared to controls. Expression levels for gap junction connexin proteins were not significantly altered in right atrium or right ventricle from diabetic heart compared to controls. Structural remodelling of gap junction connexin proteins may partly underlie electrophysiological defects in STZ-induced diabetic rat SAN.

Sex differences and heritability of two indices of heart rate dynamics: a twin study.

We investigated whether women show larger heart rate variability (HRV) than men after controlling for a large number of health-related covariates, using two indices of HRV, namely respiratory sinus arrhythmia (RSA) and approximate entropy (ApEn). In a twin design, the heritability of both indices was examined. The covariation between RSA and ApEn, a measure of heart rate dynamics derived from nonlinear dynamical systems theory, was decomposed into genetic and environmental components. Subjects were 196 male and 210 female middle-aged twins. Females showed larger HRV than men before (ApEn: p < .001; RSA: p = .052) and after adjustment for covariates (ApEn: p < .001; RSA: p = .015). This sex difference was confirmed by significant intrapair differences in the opposite-sex twin pairs for both ApEn (p < .001) and RSA (p = .03). In addition to sex, only heart period and age (both p < .001) were found to be independent predictors of ApEn, whereas RSA was also influenced by respiration rate and smoking (both p < .001). Age explained 16% and 6% of the variance in RSA and ApEn, respectively. Oral contraceptive use and menopausal status had no effect on HRV. Genetic model fitting yielded moderate heritability estimates for RSA (30%) and ApEn (40%) for both males and females. The correlation between RSA and ApEn (r = .60) could be attributed to genetic factors (48%), environmental factors (36%) and age (16%). The present study found support for a gender difference in HRV with women having greater HRV than men even after controlling for a large number of potential confounders. Indices of heart rate dynamics derived from nonlinear dynamical systems theory are moderately heritable and may be more sensitive than traditional indices of HRV to reveal subtle sex differences with important implications for health and disease.

Bivariate genetic modeling of cardiovascular stress reactivity: does stress uncover genetic variance?

OBJECTIVE: To test the existence of gene-by-stress interaction by assessing cardiovascular stress reactivity in monozygotic and dizygotic twins. METHODS: We studied 160 adolescent (mean age 16.7 +/- 2.0 years; range 13-22 years) and 212 middle-aged twin pairs (mean age 44.2 +/- 6.7 years; range 34-63 years). Systolic (SBP) and diastolic (DBP) blood pressure, heart rate (HR), pre-ejection period (PEP), and respiratory sinus arrhythmia (RSA) were measured at rest and during a choice reaction time and a mental arithmetic task. We used a bivariate analysis of the resting and mean stress levels to test for gene-by-stress interaction, which can be caused by the emergence of new genetic variance specific to stress or by stress-induced amplification of the existing genetic variance at rest. RESULTS: Genetic factors significantly contributed to individual differences in resting SBP, DBP, HR, PEP, and RSA levels in the adolescent (heritability range 0.31-0.70) and middle-aged (heritability range 0.32-0.64) cohorts. The effect of these genetic factors was amplified by stress for all variables in the adolescent cohort, and for SBP in the middle-aged cohort. In addition, stress-specific genetic variation emerged for HR in both cohorts and for PEP and SBP in the adolescent cohort. Heritability of stress levels of SBP, DBP, HR, PEP, and RSA ranged from 0.54 to 0.74 in the adolescents and from 0.44 to 0.64 in the middle-aged cohort. CONCLUSIONS: Stress uncovers genetic variance in BP, HR, and cardiac sympathovagal balance through the emergence of new stress-specific genetic effects and the amplification of existing genetic effects that also affect the resting values.

Ischaemic stroke, factor V Leiden heterozygosity and left atrial thrombosis in sinus rhythm: a case report.

We describe a 63-year-old man in sinus rhythm (SR) with an ischaemic stroke involving basal ganglia region on the right side. The patient was known to be heterozygous for factor V Leiden (FVL) mutation. On diagnostic work-up, no arterial sources of embolism were found. Transoesophageal echocardiography evidenced a left atrial (LA) thrombosis without relevant cardiopathies. LA thrombosis is generally associated to atrial fibrillation, atrial enlargement, mitral valve stenosis and left ventricular dysfunction, whereas mitral regurgitation is considered protective. To our knowledge, this is the first report of cardioembolic stroke related to a LA thrombosis in a patient in SR without risk factors for thrombus formation except for FVL heterozygosity.

Familial sinus bradycardia associated with a mutation in the cardiac pacemaker channel.

We found that sinus bradycardia in members of a large family was associated with a mutation in the gene coding for the pacemaker HCN4 ion channel. Pacemaker channels of the sinoatrial node generate spontaneous activity and mediate cyclic AMP (cAMP)-dependent autonomic modulation of the heart rate. The mutation associated with bradycardia is located near the cAMP-binding site; functional analysis found that mutant channels respond normally to cAMP but are activated at more negative voltages than are wild-type channels. These changes, which mimic those of mild vagal stimulation, slow the heart rate by decreasing the inward diastolic current. Thus, diminished function of pacemaker channels is linked to familial bradycardia.

Pacemaker channels and sinus node arrhythmia.

Cardiac pacemaker activity is regulated by at least five different classes of ion channels and by the opposing influence of sympathetic and parasympathetic stimulation. Inactivation of several genes, including a subunit coding for the potassium channel activated by the muscarinic receptor, I(KACh); the calcium channel, I(Ca,); and the hyperpolarization-activated channel, I(f), results in sinus node arrhythmia. Inactivation of the gene for the hyperpolarization-activated, cyclic nucleotide-gated channel isoform HCN2 or HCN4 and the use of pacemaker channel blockers show that (a) HCN2 prevents the diastolic membrane potential from becoming too negative, (b) HCN4 is the major channel mediating sympathetic stimulation of the pacemaker activity, and (3) complete blockage of the I(f) current is compatible with slow sinus node rhythm.

Genetic correlation of exercise with heart rate and respiratory sinus arrhythmia.

PURPOSE: A twin design was used to test whether the association between exercise behavior and heart rate and the association between exercise behavior and respiratory sinus arrhythmia (RSA) derive from a common genetic factor. METHODS: Data were available from 157 adolescent (aged 13-22) and 208 middle-aged twin pairs (aged 35-62), divided into five sex by zygosity groups (male and female monozygotic twin pairs, and dizygotic twin pairs of same or opposite sex). Exercise behavior was assessed as the average weekly METs spent on sports activities or other vigorous activities in leisure time (sportMETS) in the last 3 months. RSA and heart period (HP) were assessed in the time domain from the combined ECG and respiration signals. RESULTS: Heritability estimates were 16% and 29% for RSA, 64% and 68% for HP, and 79% and 41% for sportMETS in young and middle-aged twins, respectively. A significant association was found between RSA and sportMETS (0.17) in the adolescent twins that derived entirely from a common genetic factor. No association was found between sportMETS and RSA in the older twins. A significant association was found between HP and sportMETS in both adolescent (0.35) and middle-aged (0.18) twins. A large contribution of common genetic factors to these associations was found amounting to 84% and 88% in the young and middle-aged twins, respectively. CONCLUSIONS: Although the results of this study do not preclude causal effects of exercise on RSA and heart rate, they show that the association between exercise and these cardiovascular risk factors largely derives from a common genetic factor.

Pacemaker channel dysfunction in a patient with sinus node disease.

The cardiac pacemaker current I(f) is a major determinant of diastolic depolarization in sinus nodal cells and has a key role in heartbeat generation. Therefore, we hypothesized that some forms of "idiopathic" sinus node dysfunction (SND) are related to inherited dysfunctions of cardiac pacemaker ion channels. In a candidate gene approach, a heterozygous 1-bp deletion (1631delC) in exon 5 of the human HCN4 gene was detected in a patient with idiopathic SND. The mutant HCN4 protein (HCN4-573X) had a truncated C-terminus and lacked the cyclic nucleotide-binding domain. COS-7 cells transiently transfected with HCN4-573X cDNA indicated normal intracellular trafficking and membrane integration of HCN4-573X subunits. Patch-clamp experiments showed that HCN4-573X channels mediated I(f)-like currents that were insensitive to increased cellular cAMP levels. Coexpression experiments showed a dominant-negative effect of HCN4-573X subunits on wild-type subunits. These data indicate that the cardiac I(f) channels are functionally expressed but with altered biophysical properties. Taken together, the clinical, genetic, and in vitro data provide a likely explanation for the patient's sinus bradycardia and the chronotropic incompetence.