Takeru Makiyama

The E1784K mutation in SCN5A is associated with mixed clinical phenotype of type 3 long QT syndrome

Phenotypic overlap of type 3 long QT syndrome (LQT3) with Brugada syndrome (BrS) is observed in some carriers of mutations in the Na channel SCN5A. While this overlap is important for patient management, the clinical features, prevalence, and mechanisms underlying such overlap have not been fully elucidated. To investigate the basis for this overlap, we genotyped a cohort of 44 LQT3 families of multiple ethnicities from 7 referral centers and found a high prevalence of the E1784K mutation in SCN5A. Of 41 E1784K carriers, 93% had LQT3, 22% had BrS, and 39% had sinus node dysfunction. Heterologously expressed E1784K channels showed a 15.0-mV negative shift in the voltage dependence of Na channel inactivation and a 7.5-fold increase in flecainide affinity for resting-state channels, properties also seen with other LQT3 mutations associated with a mixed clinical phenotype. Furthermore, these properties were absent in Na channels harboring the T1304M mutation, which is associated with LQT3 without a mixed clinical phenotype. These results suggest that a negative shift of steady-state Na channel inactivation and enhanced tonic block by class IC drugs represent common biophysical mechanisms underlying the phenotypic overlap of LQT3 and BrS and further indicate that class IC drugs should be avoided in patients with Na channels displaying these behaviors.

A Novel SCN5A Gain-of-Function Mutation M1875T Associated With Familial Atrial Fibrillation

OBJECTIVES This study describes a novel heterozygous gain-of-function mutation in the cardiac sodium (Na+) channel gene, SCN5A, identified in a Japanese family with lone atrial fibrillation (AF). BACKGROUND SCN5A mutations have been associated with a variety of inherited arrhythmias, but the gain-of-function type modulation in SCN5A is associated with only 1 phenotype, long-QT syndrome type 3 (LQTS3). METHODS We studied a Japanese family with autosomal dominant hereditary AF, multiple members of which showed an onset of AF or frequent premature atrial contractions at a young age. RESULTS The 31-year-old proband received radiofrequency catheter ablation, during which time numerous ectopic firings and increased excitability throughout the right atrium were documented. Mutational analysis identified a novel missense mutation, M1875T, in SCN5A. Further investigations revealed the familial aggregation of this mutation in all of the affected individuals. Functional assays of the M1875T Na(+) channels using a whole-cell patch-clamp demonstrated a distinct gain-of-function type modulation; a pronounced depolarized shift (+16.4 mV) in V(1/2) of the voltage dependence of steady-state inactivation; and no persistent Na+ current, which is a defining mechanism of LQTS3. These biophysical features of the mutant channels are potentially associated with increased atrial excitability and normal QT interval in all of the affected individuals. CONCLUSIONS We identified a novel SCN5A mutation associated with familial AF. The mutant channels displayed a gain-of-function type modulation of cardiac Na+ channels, which is a novel mechanism predisposing to increased atrial excitability and familial AF. This is a new phenotype resulting from the SCN5A gain-of-function mutations and is distinct from LQTS3.

High prevalence of early repolarization in short QT syndrome

BACKGROUND Short QT syndrome (SQTS) is characterized by an abnormally short QT interval and sudden death. Due to the limited number of cases, the characteristics of SQTS are not well understood. It has been reported recently that early repolarization is associated with idiopathic ventricular fibrillation and the QT interval is short in patients with early repolarization. OBJECTIVE The purpose of this study was to study the association between early repolarization and arrhythmic events in SQTS. METHODS The study consisted of three cohorts: SQTS cohort (N = 37), control cohort with short QT interval and no arrhythmic events (N = 44), and control cohort with normal QT interval (N = 185). ECG parameters were compared among the study cohorts. RESULTS Heart rate, PR interval, and QRS duration were similar among the three study cohorts. Early repolarization was more common in the SQTS cohort (65%) than in the short QT control cohort (30%) and the normal QT control cohort (10%). Duration from T-wave peak to T-wave end was longer in the SQTS cohort than in the short QT control cohort, although QT and corrected QT intervals were similar. In the SQTS cohort, there were more males among patients with arrhythmic events than in those with a family history but without arrhythmic events. In multivariate models, early repolarization was associated with arrhythmic events in the SQTS cohort. ECG parameters including QT and QTc intervals were not associated with arrhythmic events in the SQTS cohort. CONCLUSION There is a high prevalence of early repolarization in patients with SQTS. Early repolarization may be useful in identifying risk of cardiac events in SQTS.

D85N, a KCNE1 Polymorphism, Is a Disease-Causing Gene Variant in Long QT Syndrome

OBJECTIVES This study aims to address whether D85N, a KCNE1 polymorphism, is a gene variant that causes long QT syndrome (LQTS) phenotype. BACKGROUND KCNE1 encodes the beta-subunit of cardiac voltage-gated K(+) channels and causes LQTS, which is characterized by the prolongation of the QT interval and torsades de pointes, a lethal arrhythmia. D85N, a KCNE1 polymorphism, is known to be a functional variant associated with drug-induced LQTS. METHODS In order to elucidate the prevalence and clinical significance of this polymorphism, we performed genetic screening in 317 LQTS probands. For comparison, we examined its presence in 496 healthy control subjects. We also conducted biophysical assays for the D85N variant in mammalian cells. RESULTS The allele frequency for D85N carriers was 0.81% in healthy people. In contrast, among LQTS probands, there were 1 homozygous and 23 heterozygous carriers (allele frequency 3.9%). Seven of 23 heterozygous carriers had additional mutations in LQTS-related genes, and 3 female subjects had documented factors predisposing to the symptom. After excluding these probands, the D85N prevalence was significantly higher compared with control subjects (allele frequency 2.1%, p < 0.05). In a heterologous expression study with Chinese hamster ovarian cells, KCNE1-D85N was found to exert significant loss-of-function effects on both KCNQ1- and KCNH2-encoded channel currents. CONCLUSIONS The KCNE1-D85N polymorphism was significantly more frequent in our LQTS probands. The functional variant is a disease-causing gene variant of LQTS phenotype that functions by interacting with KCNH2 and KCNQ1. Since its allele frequency was approximately 1% among control individuals, KCNE1-D85N may be a clinically important genetic variant.

Electrocardiographic Characteristics and SCN5A Mutations in Idiopathic Ventricular Fibrillation Associated With Early Repolarization

Background— Recently, we and others reported that early repolarization (J wave) is associated with idiopathic ventricular fibrillation. However, its clinical and genetic characteristics are unclear. Methods and Results— This study included 50 patients (44 men; age, 45±17 years) with idiopathic ventricular fibrillation associated with early repolarization, and 250 age- and sex-matched healthy controls. All of the patients had experienced arrhythmia events, and 8 (16%) had a family history of sudden death. Ventricular fibrillation was inducible by programmed electric stimulation in 15 of 29 patients (52%). The heart rate was slower and the PR interval and QRS duration were longer in patients with idiopathic ventricular fibrillation than in controls. We identified nonsynonymous variants in SCN5A (resulting in A226D, L846R, and R367H) in 3 unrelated patients. These variants occur at residues that are highly conserved across mammals. His-ventricular interval was prolonged in all of the patients carrying an SCN5A mutation. Sodium channel blocker challenge resulted in an augmentation of early repolarization or development of ventricular fibrillation in all of 3 patients, but none was diagnosed with Brugada syndrome. In heterologous expression studies, all of the mutant channels failed to generate any currents. Immunostaining revealed a trafficking defect in A226D channels and normal trafficking in R367H and L846R channels. Conclusions— We found reductions in heart rate and cardiac conduction and loss-of-function mutations in SCN5A in patients with idiopathic ventricular fibrillation associated with early repolarization. These findings support the hypothesis that decreased sodium current enhances ventricular fibrillation susceptibility.

Long QT syndrome with compound mutations is associated with a more severe phenotype: A Japanese multicenter study

BACKGROUND Long QT syndrome (LQTS) can be caused by mutations in the cardiac ion channels. Compound mutations occur at a frequency of 4% to 11% among genotyped LQTS cases. OBJECTIVE The purpose of this study was to determine the clinical characteristics and manner of onset of cardiac events in Japanese patients with LQTS and compound mutations. METHODS Six hundred three genotyped LQTS patients (310 probands and 293 family members) were divided into two groups: those with a single mutation (n = 568) and those with two mutations (n = 35). Clinical phenotypes were compared between the two groups. RESULTS Of 310 genotyped probands, 26 (8.4%) had two mutations in the same or different LQTS-related genes (compound mutations). Among the 603 LQTS patients, compound mutation carriers had significantly longer QTc interval (510 ± 56 ms vs 478± 53 ms, P = .001) and younger age at onset of cardiac events (10 ± 8 years vs 18 ± 16 years, P = .043) than did single mutation carriers. The incidence rate of cardiac events before age 40 years and use of beta-blocker therapy among compound mutation carriers also were different than in single mutation carriers. Subgroup analysis showed more cardiac events in LQTS type 1 (LQT1) and type 2 (LQT2) compound mutations compared to single LQT1 and LQT2 mutations. CONCLUSION Compound mutation carriers are associated with a more severe phenotype than single mutation carriers.

Latent Genetic Backgrounds and Molecular Pathogenesis in Drug-induced Long QT Syndrome

Background—Drugs with IKr-blocking action cause secondary long-QT syndrome. Several cases have been associated with mutations of genes coding cardiac ion channels, but their frequency among patients affected by drug-induced long-QT syndrome (dLQTS) and the resultant molecular effects remain unknown. Methods and Results—Genetic testing was carried out for long-QT syndrome–related genes in 20 subjects with dLQTS and 176 subjects with congenital long-QT syndrome (cLQTS); electrophysiological characteristics of dLQTS-associated mutations were analyzed using a heterologous expression system with Chinese hamster ovary cells together with a computer simulation model. The positive mutation rate in dLQTS was similar to cLQTS (dLQTS versus cLQTS, 8 of 20 [40%] versus 91 of 176 [52%] subjects, P=0.32). The incidence of mutations was higher in patients with torsades de pointes induced by nonantiarrhythmic drugs than by antiarrhythmic drugs (antiarrhythmic versus others, 3 of 14 [21%] versus 5 of 6 [83%] subjects, P<0.05). When reconstituted in Chinese hamster ovary cells, KCNQ1 and KCNH2 mutant channels showed complex gating defects without dominant negative effects or a relatively mild decreased current density. Drug sensitivity for mutant channels was similar to that of the wild-type channel. With the Luo-Rudy simulation model of action potentials, action potential durations of most mutant channels were between those of wild-type and cLQTS. Conclusions—dLQTS had a similar positive mutation rate compared with cLQTS, whereas the functional changes of these mutations identified in dLQTS were mild. When IKr-blocking agents produce excessive QT prolongation (dLQTS), the underlying genetic background of the dLQTS subject should also be taken into consideration, as would be the case with cLQTS; dLQTS can be regarded as a latent form of long-QT syndrome.

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 &bgr;-subunit for K channels. KCNE5 has been shown to modify the transient outward current (Ito), 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, Ito 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 Ito, and its novel variants appeared to cause IVF, especially BrS, in male patients through gain-of-function effects on Ito. Screening for KCNE5 variants is relevant for BrS or IVF.

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.

Long-Term Follow-Up of a Pediatric Cohort With Short QT Syndrome

OBJECTIVES The purpose of this study was to define the clinical characteristics and long-term follow-up of pediatric patients with short QT syndrome (SQTS). BACKGROUND SQTS is associated with sudden cardiac death. The clinical characteristics and long-term prognosis in young patients have not been reported. METHODS This was an international case series involving 15 centers. Patients were analyzed for electrocardiography characteristics, genotype, clinical events, Gollob score, and efficacy of medical or defibrillator (implantable cardioverter-defibrillator [ICD]) therapy. To assess the possible prognostic value of the Gollob score, we devised a modified Gollob score that excluded clinical events from the original score. RESULTS Twenty-five patients 21 years of age or younger (84% males, median age: 15 years, interquartile range: 9 to 18 years) were followed up for 5.9 years (interquartile range: 4 to 7.1 years). Median corrected QT interval for heart rate was 312 ms (range: 194 to 355 ms). Symptoms occurred in 14 (56%) of 25 patients and included aborted sudden cardiac death in 6 patients (24%) and syncope in 4 patients (16%). Arrhythmias were common and included atrial fibrillation (n = 4), ventricular fibrillation (n = 6), supraventricular tachycardia (n = 1), and polymorphic ventricular tachycardia (n = 1). Sixteen patients (84%) had a familial or personal history of cardiac arrest. A gene mutation associated with SQTS was identified in 5 (24%) of 21 probands. Symptomatic patients had a higher median modified Gollob score (excluding points for clinical events) compared with asymptomatic patients (5 vs. 4, p = 0.044). Ten patients received medical treatment, mainly with quinidine. Eleven of 25 index cases underwent ICD implantation. Two patients had appropriate ICD shocks. Inappropriate ICD shocks were observed in 64% of patients. CONCLUSIONS SQTS is associated with aborted sudden cardiac death among the pediatric population. Asymptomatic patients with a Gollob score of <5 remained event free, except for an isolated episode of supraventricular tachycardia, over an average 6-year follow-up. A higher modified Gollob score of 5 or more was associated with the likelihood of clinical events. Young SQTS patients have a high rate of inappropriate ICD shocks.