Mutant KCNJ3 and KCNJ5 Potassium Channels as Novel Molecular Targets in Bradyarrhythmias and Atrial Fibrillation.

BACKGROUND: Bradyarrhythmia is a common clinical manifestation. Although the majority of cases are acquired, genetic analysis of families with bradyarrhythmia has identified a growing number of causative gene mutations. Because the only ultimate treatment for symptomatic bradyarrhythmia has been invasive surgical implantation of a pacemaker, the discovery of novel therapeutic molecular targets is necessary to improve prognosis and quality of life. METHODS: We investigated a family containing 7 individuals with autosomal dominant bradyarrhythmias of sinus node dysfunction, atrial fibrillation with slow ventricular response, and atrioventricular block. To identify the causative mutation, we conducted the family-based whole exome sequencing and genome-wide linkage analysis. We characterized the mutation-related mechanisms based on the pathophysiology in vitro. After generating a transgenic animal model to confirm the human phenotypes of bradyarrhythmia, we also evaluated the efficacy of a newly identified molecular-targeted compound to upregulate heart rate in bradyarrhythmias by using the animal model. RESULTS: We identified one heterozygous mutation, KCNJ3 c.247A>C, p.N83H, as a novel cause of hereditary bradyarrhythmias in this family. KCNJ3 encodes the inwardly rectifying potassium channel Kir3.1, which combines with Kir3.4 (encoded by KCNJ5) to form the acetylcholine-activated potassium channel ( IKACh channel) with specific expression in the atrium. An additional study using a genome cohort of 2185 patients with sporadic atrial fibrillation revealed another 5 rare mutations in KCNJ3 and KCNJ5, suggesting the relevance of both genes to these arrhythmias. Cellular electrophysiological studies revealed that the KCNJ3 p.N83H mutation caused a gain of IKACh channel function by increasing the basal current, even in the absence of m2 muscarinic receptor stimulation. We generated transgenic zebrafish expressing mutant human KCNJ3 in the atrium specifically. It is interesting to note that the selective IKACh channel blocker NIP-151 repressed the increased current and improved bradyarrhythmia phenotypes in the mutant zebrafish. CONCLUSIONS: The IKACh channel is associated with the pathophysiology of bradyarrhythmia and atrial fibrillation, and the mutant IKACh channel ( KCNJ3 p.N83H) can be effectively inhibited by NIP-151, a selective IKACh channel blocker. Thus, the IKACh channel might be considered to be a suitable pharmacological target for patients who have bradyarrhythmia with a gain-of-function mutation in the IKACh channel.

A molecular mechanism for adrenergic-induced long QT syndrome.

OBJECTIVES: This study sought to explore molecular mechanisms underlying the adrenergic-induced QT prolongation associated with KCNQ1 mutations. BACKGROUND: The most frequent type of congenital long QT syndrome is LQT1, which is caused by mutations in the gene (KCNQ1) that encodes the alpha subunit of the slow component of delayed rectifier K(+) current (IKs) channel. We identified 11 patients from 4 unrelated families that are heterozygous for KCNQ1-G269S. Most patients remained asymptomatic, and their resting corrected QT intervals ranged from normal to borderline but were prolonged significantly during exercise. METHODS: Wild-type (WT) KCNQ1 and/or KCNQ1-G269S (G269S) were expressed in mammalian cells with KCNE1. IKs-like currents were measured in control conditions or after isoproterenol or protein kinase A (PKA) stimulation using the patch-clamp technique. Additionally, experiments that incorporated the phosphomimetic KCNQ1 substitution, S27D, in WT or KCNQ1-G269S were also performed. RESULTS: The coexpression of WT-KCNQ1 with varying amounts of G269S decreased IKs, shifted the current-voltage I-V relation of IKs to more positive potentials, and accelerated the IKs deactivation rates in a concentration-dependent manner. In addition, the coexpression of G269S and WT blunted the activation of IKs in response to isoproterenol or PKA stimulation. Lastly, a phosphomimetic substitution in G269S did not show an increased IKs. CONCLUSIONS: G269S modestly affected IKs in control conditions, but it almost completely blunted IKs responsiveness in conditions that simulate or mimic PKA phosphorylation of KCNQ1. This insensitivity to PKA stimulation may explain why patients with G269S mutation showed an excessive prolongation of QT intervals on exercise.

P-pulmonale and the development of atrial fibrillation.

BACKGROUND: P wave ≥0.25mV in inferior leads (P pulmonale) occurs in chronic lung diseases that underlie atrial fibrillation (AF). The purpose of this study was to elucidate the prognostic value of P pulmonale for development of AF. METHODS AND RESULTS: Digital analysis of 12-lead electrocardiogram (ECG) was conducted to enroll patients with P pulmonale from among a database containing 308,391 ECGs. In a total of 591 patients (382 men; 56.4±14.8 years) with P pulmonale (follow-up, 46.7±65.6 months), AF occurred in 61 patients (AF group), but did not occur in 530 patients (non-AF group). Male gender was significantly more prevalent in the AF group than in the non-AF group (80.3% vs. 62.8%, P=0.0047). P-wave duration and PQ interval were significantly longer in the AF group than in the non-AF group (115.4±17.2ms vs. 107.0±17.2ms, P=0.0003 and 166.3±23.9ms vs. 153.2±25.4ms, P=0.0001, respectively). In the total patient group, multivariate Cox proportional-hazards analysis confirmed that male gender (hazard ratio [HR], 2.24; 95% confidence interval [CI]: 1.02-5.49; P=0.045), PQ interval >150ms (HR, 6.89; 95% CI: 2.39-29.15; P<0.0001), and P-wave axis <74° (HR, 2.55; 95% CI: 1.20-5.41; P=0.016) were associated with AF development. In medication-free patients (n=400), only PQ interval >150ms (HR, 9.26; 95% CI: 1.75-170.65; P=0.0055) was independently and significantly associated with AF development. CONCLUSIONS: PQ interval is the strongest stratifier for AF development in P pulmonale.

Cardiac channelopathies associated with infantile fatal ventricular arrhythmias: from the cradle to the bench.

BACKGROUND: Fatal ventricular arrhythmias in the early period of life have been associated with cardiac channelopathies for decades, and postmortem analyses in SIDS victims have provided evidence of this association. However, the prevalence and functional properties of cardiac ion channel mutations in infantile fatal arrhythmia cases are not clear. METHODS AND RESULTS: Seven infants with potentially lethal arrhythmias at age < 1 year (5 males, age of onset 44.1 ± 72.1 days) were genetically analyzed for KCNQ1, KCNH2, KCNE1-5, KCNJ2, SCN5A, GJA5, and CALM1 by using denaturing high-performance liquid chromatography and direct sequencing. Whole-cell currents of wildtype and mutant channels were recorded and analyzed in Chinese hamster ovary cells transfected with SCN5A and KCNH2 cDNA. In 5 of 7 patients, we identified 4 mutations (p.N1774D, p.T290fsX53, p.F1486del and p.N406K) in SCN5A, and 1 mutation (p.G628D) in KCNH2. N1774D, F1486del, and N406K in SCN5A displayed tetrodotoxin-sensitive persistent late Na(+) currents. By contrast, SCN5A-T290fsX53 was nonfunctional. KCNH2-G628D exhibited loss of channel function. CONCLUSION: Genetic screening of 7 patients was used to demonstrate the high prevalence of cardiac channelopathies. Functional assays revealed both gain and loss of channel function in SCN5A mutations, as well as loss of function associated with the KCNH2 mutation.

Electrical storm in idiopathic ventricular fibrillation is associated with early repolarization.

OBJECTIVES: This study sought to characterize patients with idiopathic ventricular fibrillation (IVF) who develop electrical storms. BACKGROUND: Some IVF patients develop ventricular fibrillation (VF) storms, but the characteristics of these patients are poorly known. METHODS: Ninety-one IVF patients (86% male) were selected after the exclusion of structural heart diseases, primary electrical diseases, and coronary spasm. Electrocardiogram features were compared between the patients with and without electrical storms. A VF storm was defined as VF occurring ≥3 times in 24 h and J waves >0.1 mV above the isoelectric line in contiguous leads. RESULTS: Fourteen (15.4%) patients had VF storms occurring out-of-hospital at night or in the early morning. J waves were more closely associated with VF storms compared to patients without VF storms: 92.9% versus 36.4% (p < 0.0001). VF storms were controlled by intravenous isoproterenol, which attenuated the J-wave amplitude. After the subsidence of VF storms, the J waves decreased to the nondiagnostic level during the entire follow-up period. Implantable cardioverter-defibrillator therapy was administered to all patients during follow-up. Quinidine therapy was limited, but the patients on disopyramide (n = 3), bepridil (n = 1), or isoprenaline (n = 1) were free from VF recurrence, while VF recurred in 5 of the 9 patients who were not given antiarrhythmic drugs. CONCLUSIONS: The VF storms in the IVF patients were highly associated with J waves that showed augmentation prior to the VF onset. Isoproterenol was effective in controlling VF and attenuated the J waves, which diminished to below the diagnostic level during follow-up. VF recurred in patients followed up without antiarrhythmic agents.

Genetic background of catecholaminergic polymorphic ventricular tachycardia in Japan.

BACKGROUND: The genetic background of catecholaminergic polymorphic ventricular tachycardia (CPVT) has been extensively investigated for the last decade in Western countries, but it remains unstudied in the Asian population. METHODS AND RESULTS: In 50 Japanese probands from unrelated families who satisfied clinical criteria for CPVT, genetic testing was conducted in all exons on 3 CPVT-related genes: cardiac ryanodine receptor 2 (RYR2), calsequestrin 2 (CASQ2) and inward rectifier potassium channel 2 (KCNJ2), and the clinical features between RYR2-genotyped and -non-genotyped patient groups were compared. Genetic and clinical evaluation was also done in 46 family members. In the genetic screening, 28 (18 novel) RYR2 (56.0%), 1 compound heterozygous CASQ2 (2.0%) and 1 KCNJ2 (2.0%) mutation carriers were identified. In the RYR2 mutation-positive group, the frequency of bidirectional ventricular tachycardia and the use of ß-blockers were significantly higher than in the mutation-negative group. In contrast, there was no significant difference in supraventricular arrhythmias between the 2 groups. With regard to disease penetrance, the number of family members of RYR2-genotyped probands with a clinical diagnosis of CPVT was high. CONCLUSIONS: Thirty gene mutation carriers were found for 3 genes in 50 probands clinically diagnosed as having CPVT. The penetrance of CPVT phenotype was significantly higher in RYR2 mutation carriers, thus RYR2 gene screening in CPVT patients would be indispensable to prevent unexpected cardiac sudden death of young family members.

Prognostic implications of progressive cardiac conduction disease.

BACKGROUND: Progressive cardiac conduction disease (PCCD), characterized by temporal increase in PR interval and QRS duration, may be attributed to diverse pathophysiological mechanisms. This study aimed to investigate whether PCCD is associated with increased risk of cardiovascular morbidity and mortality. METHODS AND RESULTS: Digital analysis of 12-lead ECG was performed to select patients with PCCD from among a database containing 359,737 ECGs. Long-term prognosis of PCCD was assessed in a large hospital-based population: 458 patients (341 males; mean age, 57.9 ± 14.7 years) with PCCD were enrolled. During a mean follow-up of 13.3 ± 6.4 years, 109 patients were hospitalized for heart failure (HF), and there were 16 and 59 deaths from cardiovascular diseases and all causes, respectively. Multivariate Cox proportional hazards analysis confirmed (1) a significant association of temporal incremental rate of PR interval (≥ 2 ms/year) and QRS duration (≥ 3 ms/year) with HF hospitalization (hazard ratio [HR], 2.34; 95% confidence interval [CI], 1.36-4.05; P=0.002 and HR, 2.08; 95% CI, 1.25-3.53; P=0.01, respectively) and (2) a significant association of temporal incremental rate of PR interval (≥ 4 ms/year) and QRS duration (≥ 5 ms/year) with cardiovascular mortality (HR, 6.9; 95% CI, 1.47-36.96; P=0.02 and HR, 4.31; 95% CI, 1.19-16.5; P=0.03, respectively). CONCLUSIONS: The severity of PCCD was independently and significantly associated with HF hospitalization and cardiovascular mortality.

Spontaneous coronary artery spasm accidentally detected by 320-row multi-detector computed tomography.

A 64-year-old woman was introduced to our hospital suspected of having angina pectoris. A 12-lead electrocardiogram showed negative T wave in inferior leads. An echocardiography revealed no abnormality. In 320-row multi-detector computed tomography (MDCT), a severe narrowing of the right coronary artery was detected with collateral vessels between the distal portion of the obstructed right coronary artery and the left anterior descending artery. Coronary angiography showed no stenosis of the right coronary artery and non-obstructive plaques in the left coronary artery. To provoke coronary artery spasm, acetylcholine was infused into the left coronary artery. But, no narrowing was inducible in the left coronary artery. After that, a spasm of the right coronary artery was observed without the injection of acetylcholine into the right coronary artery. The spasm was located in the same site as the narrowing on MDCT imaging. A calcium channel antagonist (benidipine hydrochloride 8 mg/day) and a nitrate (isosorbide dinitrate 80 mg) were effective for coronary artery spasm. .

Phenotype variability in patients carrying KCNJ2 mutations.

BACKGROUND: Mutations of KCNJ2, the gene encoding the human inward rectifier potassium channel Kir2.1, cause Andersen-Tawil syndrome (ATS), a disease exhibiting ventricular arrhythmia, periodic paralysis, and dysmorphic features. However, some KCNJ2 mutation carriers lack the ATS triad and sometimes share the phenotype of catecholaminergic polymorphic ventricular tachycardia (CPVT). We investigated clinical and biophysical characteristics of KCNJ2 mutation carriers with "atypical ATS." METHODS AND RESULTS: Mutational analyses of KCNJ2 were performed in 57 unrelated probands showing typical (≥2 ATS features) and atypical (only 1 of the ATS features or CPVT) ATS. We identified 24 mutation carriers. Mutation-positive rates were 75% (15/20) in typical ATS, 71% (5/7) in cardiac phenotype alone, 100% (2/2) in periodic paralysis, and 7% (2/28) in CPVT. We divided all carriers (n=45, including family members) into 2 groups: typical ATS (A) (n=21, 47%) and atypical phenotype (B) (n=24, 53%). Patients in (A) had a longer QUc interval [(A): 695 ± 52 versus (B): 643 ± 35 ms] and higher U-wave amplitude (0.24 ± 0.07 versus 0.18 ± 0.08 mV). C-terminal mutations were more frequent in (A) (85% versus 38%, P<0.05). There were no significant differences in incidences of ventricular tachyarrhythmias. Functional analyses of 4 mutations found in (B) revealed that R82Q, R82W, and G144D exerted strong dominant negative suppression (current reduction by 95%, 97%, and 96%, respectively, versus WT at -50 mV) and T305S moderate suppression (reduction by 89%). CONCLUSIONS: KCNJ2 gene screening in atypical ATS phenotypes is of clinical importance because more than half of mutation carriers express atypical phenotypes, despite their arrhythmia severity.

KCNE5 (KCNE1L) variants are novel modulators of Brugada syndrome and idiopathic ventricular fibrillation.

BACKGROUND: Brugada syndrome (BrS) has a significantly higher incidence among the male sex. Among genes coding ion channels and their modulatory proteins, KCNE5 (KCNE1L) is located in the X chromosome and encodes an auxiliary ß-subunit for K channels. KCNE5 has been shown to modify the transient outward current (I(to)), which plays a key role in determining the repolarization process in the myocardium. This study investigated whether KCNE5 mutations could be responsible for BrS and other idiopathic ventricular fibrillation (IVF). METHODS AND RESULTS: In 205 Japanese patients with BrS or IVF who tested negative for SCN5A mutation, we conducted a genetic screen for KCNE5 variants. We identified 2 novel KCNE5 variants: p.Y81H in 3 probands and p.[D92E;E93X] in 1 proband from 4 unrelated families. Y81H was identified in 1 man and 2 women; D92E;E93X was found in a 59-year-old man. All probands received implantable cardioverter-defibrillators. Functional consequences of the KCNE5 variants were determined through biophysical assay using cotransfection with KCND3 or KCNQ1. In the experiments with KCND3, which encodes Kv4.3, I(to) was significantly increased for both KCNE5 variants compared to wild type. In contrast, there were no significant changes in current properties reconstructed by KCNQ1+ wild type KCNE5 and the 2 variants. With the simulation model, both variants demonstrated notch-and-dome or loss-of-dome patterns. CONCLUSIONS: KCNE5 modulates I(to), and its novel variants appeared to cause IVF, especially BrS, in male patients through gain-of-function effects on I(to). Screening for KCNE5 variants is relevant for BrS or IVF.

Churg-Strauss syndrome presenting with massive pericardial effusion.

We describe a case of Churg-Strauss syndrome (CSS) presenting with a massive pericardial effusion without overt myocardial dysfunction. A 60-year-old man was referred to our hospital because of exertional dyspnea and fever. Initial chest multidetector-row computed tomography showed a massive pericardial effusion. The presence of eosinophilia, infiltrates of both lungs, pathological evidence of necrotizing vasculitis associated with eosinophilic infiltration, and history of asthma fulfilled the criteria of CSS. Massive pericardial effusion can be the first manifestation of cardiac involvement in CSS.