Torsades de Pointes electrical storm in children with KCNH2 mutations.

Congenital long QT syndrome (LQTS) is a genetic heart disorder, which may lead to life-threatening arrhythmias, especially in children. Here, we reported two children who were initially misdiagnosed with epilepsy and experienced Torsades de Pointes (TdP) cardiac electrical storm (ES). Through whole exome sequencing (WES), we identified two Potassium voltage-gated channel subfamily H member 2 (KCHN2) mutations (c.1841 C > T and c.1838 C > T) respectively in a 6-year-old boy and a 13-year-old girl. Clinical data indicated that the QT interval was significantly prolonged, the T-wave pattern of chest V5-V6 leads and limb leads were inverted. Our study suggests that patients with epilepsy, especially those refractory epilepsy with atypical features, need comprehensive evaluation of cardiovascular function. KCNH2 mutation in pore region, QT interval prolongation and T wave inversion are high risk factors for ES. For LQT2 patients with ES, Nadolol and left cardiac sympathetic denervation are indicated, sometimes with an ICD.

A polygenic risk score for the QT interval is an independent predictor of drug-induced QT prolongation.

Drug-induced QT prolongation (diLQTS), and subsequent risk of torsade de pointes, is a major concern with use of many medications, including for non-cardiac conditions. The possibility that genetic risk, in the form of polygenic risk scores (PGS), could be integrated into prediction of risk of diLQTS has great potential, although it is unknown how genetic risk is related to clinical risk factors as might be applied in clinical decision-making. In this study, we examined the PGS for QT interval in 2500 subjects exposed to a known QT-prolonging drug on prolongation of the QT interval over 500ms on subsequent ECG using electronic health record data. We found that the normalized QT PGS was higher in cases than controls (0.212±0.954 vs. -0.0270±1.003, P = 0.0002), with an unadjusted odds ratio of 1.34 (95%CI 1.17-1.53, P<0.001) for association with diLQTS. When included with age and clinical predictors of QT prolongation, we found that the PGS for QT interval provided independent risk prediction for diLQTS, in which the interaction for high-risk diagnosis or with certain high-risk medications (amiodarone, sotalol, and dofetilide) was not significant, indicating that genetic risk did not modify the effect of other risk factors on risk of diLQTS. We found that a high-risk cutoff (QT PGS ≥ 2 standard deviations above mean), but not a low-risk cutoff, was associated with risk of diLQTS after adjustment for clinical factors, and provided one method of integration based on the decision-tree framework. In conclusion, we found that PGS for QT interval is an independent predictor of diLQTS, but that in contrast to existing theories about repolarization reserve as a mechanism of increasing risk, the effect is independent of other clinical risk factors. More work is needed for external validation in clinical decision-making, as well as defining the mechanism through which genes that increase QT interval are associated with risk of diLQTS.

Mechanisms of fever-induced QT prolongation and torsades de pointes in patients with KCNH2 mutation.

AIMS: Patients with particular mutations of type-2 long QT syndrome (LQT2) are at an increased risk for malignant arrhythmia during fever. This study aimed to determine the mechanism by which KCNH2 mutations cause fever-induced QT prolongation and torsades de pointes (TdP). METHODS AND RESULTS: We evaluated three KCNH2 mutations, G584S, D609G, and T613M, in the Kv11.1 S5-pore region, identified in patients with marked QT prolongation and TdP during fever. We also evaluated KCNH2 M124T and R269W, which are not associated with fever-induced QT prolongation. We characterized the temperature-dependent changes in the electrophysiological properties of the mutant Kv11.1 channels by patch-clamp recording and computer simulation. The average tail current densities (TCDs) at 35°C for G584S, WT+D609G, and WT+T613M were significantly smaller and less increased with rising temperature from 35°C to 40°C than those for WT, M124T, and R269W. The ratios of the TCDs at 40°C to 35°C for G584S, WT+D609G, and WT+T613M were significantly smaller than for WT, M124T, and R269W. The voltage dependence of the steady-state inactivation curve for WT, M124T, and R269W showed a significant positive shift with increasing temperature; however, that for G584S, WT+D609G, and WT+T613M showed no significant change. Computer simulation demonstrated that G584S, WT+D609G, and WT+T613M caused prolonged action potential durations and early afterdepolarization formation at 40°C. CONCLUSION: These findings indicate that KCNH2 G584S, D609G, and T613M in the S5-pore region reduce the temperature-dependent increase in TCDs through an enhanced inactivation, resulting in QT prolongation and TdP at a febrile state in patients with LQT2.

The genetics of drug-induced QT prolongation: evaluating the evidence for pharmacodynamic variants.

Drug-induced long QT syndrome (diLQTS) is an adverse effect of many commonly prescribed drugs, and it can increase the risk for lethal ventricular arrhythmias. Genetic variants in pharmacodynamic genes have been associated with diLQTS, but the strength of the evidence for each of those variants has not yet been evaluated. Therefore, the purpose of this review was to evaluate the strength of the evidence for pharmacodynamic genetic variants associated with diLQTS using a novel, semiquantitative scoring system modified from the approach used for congenital LQTS. KCNE1-D85N and KCNE2-T8A had definitive and strong evidence for diLQTS, respectively. The high level of evidence for these variants supports current consideration as risk factors for patients that will be prescribed a QT-prolonging drug.

A Novel Mutation in the KCNH2 Gene Associatedwith Long QT Syndrome: A Case Report.

OBJECTIVE: Long QT syndrome is a cardiovascular disease with a prolonged QT interval. CASE REPORT: We report a 22-year-old woman presenting with frequent syncopal episodes two months after childbirth. Electrocardiography showed a sinus rhythm, QT interval prolongation, and Torsade de Pointes. Her mother had experienced an episode of syncope, but her father had not. Genetic analyses revealed that a new mutation in the KCNH2 gene, the c.2108dupA mutation (p.H703Qfs*20, exon8, M_000238), was found in the patient and in her mother and sister. CONCLUSION: The c.2108dupA mutation (p.H703Qfs*20, exon8, M_000238) is the first reported case of a KCNH2 mutation at this site.

A SPRY1 domain cardiac ryanodine receptor variant associated with short-coupled torsade de pointes.

Idiopathic ventricular fibrillation (IVF) causes sudden death in young adult patients without structural or ischemic heart disease. Most IVF cases are sporadic and some patients present with short-coupled torsade de pointes, the genetics of which are poorly understood. A man who had a first syncope at the age of 35 presented with frequent short-coupled premature ventricular beats with bursts of polymorphic ventricular tachycardia and then died suddenly. By exome sequencing, we identified three rare variants: p.I784F in the SPRY1 of the ryanodine receptor 2 (RyR2), p.A96S in connexin 40 (Cx40), reported to affect electrical coupling and cardiac conduction, and a nonsense p.R244X in the cardiac-specific troponin I-interacting kinase (TNNI3K). We assessed intracellular Ca2+ handling in WT and mutant human RYR2 transfected HEK293 cells by fluorescent microscopy and an enhanced store overload-induced Ca2+ release in response to cytosolic Ca2+ was observed in RyR2-I784F cells. In addition, crystal structures and thermal melting temperatures revealed a conformational change in the I784F-SPRY1 domain compared to the WT-domain. The novel RyR2-I784F variant in SPRY1 domain causes a leaky channel under non-stress conditions. The presence of several variants affecting Ca2+ handling and cardiac conduction suggests a possible oligogenic origin for the ectopies originating from Purkinje fibres.

SCN5A mutation identified in a patient with short-coupled variant of torsades de pointes.

BACKGROUND: Short-coupled variant of torsades de pointes (scTdP) is a disease characterized by TdP without QT prolongation, which is initiated by extremely short-coupled ventricular extra-systoles. Its genetic background remains rarely unveiled. OBJECTIVE: We aimed to identify genetic variations in patients with scTdP and to analyze the functional change of the mutant Na+ channel identified in a scTdP patient. METHODS AND RESULTS: We performed genetic analysis for inherited arrhythmia-related 45 genes using next-generation sequencer (MiSeq, Illumina) among seven consecutive scTdP patients. We identified an SCN5A mutation R800H in a 38-year-old male who suffered ventricular fibrillation during dinner and was resuscitated. Two months later, he lost his consciousness at work. His Holter electrocardiogram showed scTdP. He had no family history of sudden cardiac death or heart disease. Functional analysis of the SCN5A-R800H channels showed a significantly shortened recovery time from inactivation. Peak sodium current densities in SCN5A-R800H were larger than those in wild type but the difference was not statistically significant. CONCLUSIONS: We identified an SCN5A mutation in a scTdP patient and confirmed that the mutant channel caused the shortness of recovery time from inactivation. SCN5A might be a candidate gene for scTdP.

The long-QT syndrome and exercise practice: The never-ending debate.

Today, understanding the true risk of adverse events in long-QT syndrome (LQTS) populations may be extremely complex and potentially dependent on many factors such as the affected gene, mutation location, degree of QTc prolongation, age, sex, and other yet unknown factors. In this context, risk stratification by genotype in LQTS patients has been extremely difficult, also during exercise practice, especially due to the lack of studies that would lead to a better understanding of the natural history of each mutation and its impact upon athletes. The creation of individualized guidelines for sport participation is a goal yet to be achieved not only due to the complexity of genotype effect on the phenotype in this patient population, but also due to penetrance in genotype-positive patients. This article summarizes current knowledge and raises questions concerning the difficult relationship between exercise practice and LQTS.

A type 2 ryanodine receptor variant associated with reduced Ca2+ release and short-coupled torsades de pointes ventricular arrhythmia.

BACKGROUND: Ventricular fibrillation may be caused by premature ventricular contractions (PVCs) whose coupling intervals are <300 ms, a characteristic of the short-coupled variant of torsades de pointes (scTdP). OBJECTIVE: The purpose of this study was to analyze the underlying cardiac ryanodine receptor (RyR2) variants in patients with scTdP. METHODS: Seven patients with scTdP (mean age 34 ± 12 years; 4 men and 3 women) were enrolled in this study. The RyR2 gene was screened by targeted gene sequencing methods; variant minor allele frequency was confirmed in 3 databases; and the pathogenicity was investigated in silico analysis using multiple tools. The activity of wild-type and mutant RyR2 channels was evaluated by monitoring Ca2+ signals of HEK293 cells with a [3H]ryanodine binding assay. RESULTS: The mean coupling interval of PVCs was 282 ± 13 ms. The 12-lead electrocardiogram had no specific findings except PVCs with an extremely short-coupling interval. Genetic analysis revealed 3 novel RyR2 variants and 1 polymorphism, all located in the cytoplasmic region. p.Ser4938Phe was not detected in 3 databases, and in silico analysis indicated its pathogenicity. In functional analysis, p.Ser4938Phe demonstrated loss of function and impaired RyR2 channel Ca2+ release, while 2 other variants, p.Val1024Ile and p.Ala2673Val, had mild gain-of-function effects but were similar to the polymorphism p.Asn1551Ser. CONCLUSION: We identified an RyR2 variant associated with reduced Ca2+ release and short-coupled torsades de pointes ventricular arrhythmia. The mechanisms of arrhythmogenesis remain unclear.

Efavirenz Inhibits the Human Ether-A-Go-Go Related Current (hERG) and Induces QT Interval Prolongation in CYP2B6*6*6 Allele Carriers.

BACKGROUND: Efavirenz (EFV) has been associated with torsade de pointes despite marginal QT interval lengthening. Since EFV is metabolized by the cytochrome P450 (CYP) 2B6 enzyme, we hypothesized that EFV would lengthen the rate-corrected QT (QTcF) interval in carriers of the CYP2B6*6 decreased functional allele. OBJECTIVE: The primary objective of this study was to evaluate EFV-associated QT interval changes with regard to CYP2B6 genotype and to explore mechanisms of QT interval lengthening. METHODS: EFV was administered to healthy volunteers (n = 57) as a single 600 mg dose followed by multiple doses to steady-state. Subjects were genotyped for known CYP2B6 alleles and ECGs and EFV plasma concentrations were obtained serially. Whole-cell, voltage-clamp experiments were performed on cells stably expressing hERG and exposed to EFV in the presence and absence of CYP2B6 expression. RESULTS: EFV demonstrated a gene-dose effect and exceeded the FDA criteria for QTcF interval prolongation in CYP2B6*6/*6 carriers. The largest mean time-matched differences ∆∆QTcF were observed at 6 hours (14 milliseconds; 95% CI [1; 27]), 12 hours (18 milliseconds; 95% CI [-4; 40]), and 18 hours (6 milliseconds; 95% CI [-1; 14]) in the CYP2B6*6/*6 genotype. EFV concentrations exceeding 0.4 µg/mL significantly inhibited outward hERG tail currents (P < 0.05). CONCLUSIONS: This study demonstrates that homozygous carriers of CYP2B6*6 allele may be at increased risk for EFV-induced QTcF interval prolongation via inhibition of hERG.

QT interval and drug therapy.

The QT interval is an important component of the electrocardiogram, which when prolonged can predict the risk of developing the potentially fatal cardiac arrhythmia, torsades de pointes. There is growing understanding of the genetics associated with cardiac arrhythmias and an increasing number of drugs that can prolong the QT interval. Consequently, assessment of the effect of drugs on the QT interval has become a significant aspect of drug development, regulatory assessment and clinical care. Here, we review the QT interval and the risks associated with drug-induced prolongation of the QT interval.

The In Vitro Pharmacological Profile of Drugs as a Proxy Indicator of Potential In Vivo Organ Toxicities.

The potential of a drug to cause certain organ toxicities is somehow implicitly contained in its full pharmacological profile, provided the drug reaches and accumulates at the various organs where the different interacting proteins in its profile, both targets and off-targets, are expressed. Under this assumption, a computational approach was implemented to obtain a projected anatomical profile of a drug from its in vitro pharmacological profile linked to protein expression data across 47 organs. It was observed that the anatomical profiles obtained when using only the known primary targets of the drugs reflected roughly the intended organ targets. However, when both known and predicted secondary pharmacology was considered, the projected anatomical profiles of the drugs were able to clearly highlight potential organ off-targets. Accordingly, when applied to sets of drugs known to cause cardiotoxicity and hepatotoxicity, the approach is able to identify heart and liver, respectively, as the organs where the proteins in the pharmacological profile of the corresponding drugs are specifically expressed. When applied to a set of drugs linked to a risk of Torsades de Pointes, heart is again the organ clearly standing out from the rest and a potential protein profile hazard is proposed. The approach can be used as a proxy indicator of potential in vivo organ toxicities.

Pediatric Cohort With Long QT Syndrome　- KCNH2 Mutation Carriers Present Late Onset But Severe Symptoms.

BACKGROUND: In children with long QT syndrome (LQTS), risk factors for cardiac events have been reported, but age-, gender- and genotype-related differences in prognosis remain unknown in Asian countries. METHODS AND RESULTS: The study examined clinical prognosis at age between 1 and 20 years in 496 LQTS patients who were genotyped as either of LQT1-3 (male, n=206). Heterozygous mutations were observed in 3 major responsible genes:KCNQ1in271,KCNH2in 192, andSCN5Ain 33 patients. LQTS-associated events were classified into 3 categories: (1) syncope (n=133); (2) repetitive torsade de pointes (TdP, n=3); and (3) cardiopulmonary arrest (CPA, n=4). The risk of cardiac events was significantly lower in LQT1 girls than boys≤12 years (HR, 0.55), whereas LQT2 female patients ≥13 years had the higher risk of cardiac events than male patients (HR, 4.60). Patients in the repetitive TdP or CPA group included 1 LQT1 female patient, 1 LQT2 male patient, and 5 LQT2 female patients. All LQT2 patients in these groups had TdP repeatedly immediately after the antecedent event. In addition, all 5 female LQT2 patients in these groups had the event after or near puberty. CONCLUSIONS: Female LQT2 children might have repeated TdP shortly after prior events, especially after puberty. (Circ J 2016; 80: 696-702).

Predicting drug-induced QT prolongation and torsades de pointes.

Drugs used to treat cardiovascular disease as well as those used in the treatment of multiple other conditions can occasionally produce exaggerated prolongation of the QT interval on the electrocardiogram and the morphologically distinctive polymorphic ventricular tachycardia ('torsades de pointes'). This syndrome of drug-induced long QT syndrome has moved from an interesting academic exercise to become a key element in the development of any new drug entity. The prevailing view, which has driven both clinical care and drug regulation, holds that cardiac repolarization represents a balance between inward currents (primarily through calcium and sodium channels) and outward currents (primarily through rapid and slowed delayed rectifier potassium channels) and that block of the rapid delayed rectifier (IKr ) is the primary mechanism whereby drugs prolong individual action potentials, manifest on the surface electrocardiogram as QT interval prolongation. Such marked action potential prolongation in individual cardiac cells, in turn, is accompanied by arrhythmogenic afterdepolarizations thought to trigger torsades de pointes. This review describes the evidence in support of this construct, and describes the way in which clinical and whole heart experiments have informed molecular mechanisms and vice versa. New data that challenge these views and that may, as a result, lead to new clinical care and drug screening paradigms, are discussed.

Abnormal repolarization dynamics in a patient with KCNE1(G38S) who presented with torsades de pointes.

Risk of G38S, major KCNE1 polymorphism [KCNE1(G38S)], for long QT syndrome (LQTS) remains unclear. A 72-year-old woman was admitted with recurrent torsades de pointes (TdP). She had remarkable QT prolongation (corrected QT interval 568 ms) under conditions of hypokalemia and hypomagnesemia. After correction of this electrolytic imbalance, TdP was suppressed and metoprolol was started. The QT-RR slope in 24-hour Holter electrocardiogram was steep and this enhanced bradycardia-dependent QT prolongation was similar to that in LQTS. She carried KCNE1(G38S). Patients with KCNE1(G38S) could have similar potential risk of ventricular arrhythmia as with LQTS. Analysis of QT-RR relationship could also evaluate the latent arrhythmogenicity of KCNE1(G38S).

Molecular basis of hERG potassium channel blockade by the class Ic antiarrhythmic flecainide.

The class Ic antiarrhythmic drug flecainide inhibits KCNH2-encoded "hERG" potassium channels at clinically relevant concentrations. The aim of this study was to elucidate the underlying molecular basis of this action. Patch clamp recordings of hERG current (IhERG) were made from hERG expressing cells at 37°C. Wild-type (WT) IhERG was inhibited with an IC50 of 1.49µM and this was not significantly altered by reversing the direction of K(+) flux or raising external [K(+)]. The use of charged and uncharged flecainide analogues showed that the charged form of the drug accesses the channel from the cell interior to produce block. Promotion of WT IhERG inactivation slowed recovery from inhibition, whilst the N588K and S631A attenuated-inactivation mutants exhibited IC50 values 4-5 fold that of WT IhERG. The use of pore-helix/selectivity filter (T623A, S624A V625A) and S6 helix (G648A, Y652A, F656A) mutations showed <10-fold shifts in IC50 for all but V625A and F656A, which respectively exhibited IC50s 27-fold and 142-fold their WT controls. Docking simulations using a MthK-based homology model suggested an allosteric effect of V625A, since in low energy conformations flecainide lay too low in the pore to interact directly with that residue. On the other hand, the molecule could readily form π-π stacking interactions with aromatic residues and particularly with F656. We conclude that flecainide accesses the hERG channel from the cell interior on channel gating, binding low in the inner cavity, with the S6 F656 residue acting as a principal binding determinant.

Ankyrin-B syndrome: a case of sinus node dysfunction, atrial fibrillation and prolonged QT in a young adult.

Ankyrin-B protein is involved in regulating expression and localisation of cardiac ion channels and transporters. Mutations of the ANK2 gene in the rare condition Ankyrin-B syndrome result in loss of function of the ankyrin-B protein which in turn leads to abnormal regulation of intracellular sodium and calcium and a predisposition to cardiac arrhythmia including torsades de pointes. We describe a rare case of this condition characterised by sinus node dysfunction, atrial fibrillation and prolonged QT syndrome in a young patient with a family history of sudden death. The management of Ankyrin-B syndrome may include avoidance of QT prolonging medications, insertion of a permanent pacemaker for sinus node dysfunction, or a cardioverter defibrillator for those at high-risk of sudden death from torsades de pointes.

Exome sequencing implicates an increased burden of rare potassium channel variants in the risk of drug-induced long QT interval syndrome.

OBJECTIVES: The aim of this study was to test the hypothesis that rare variants are associated with drug-induced long QT interval syndrome (diLQTS) and torsades de pointes. BACKGROUND: diLQTS is associated with the potentially fatal arrhythmia torsades de pointes. The contribution of rare genetic variants to the underlying genetic framework predisposing to diLQTS has not been systematically examined. METHODS: We performed whole-exome sequencing on 65 diLQTS patients and 148 drug-exposed control subjects of European descent. We used rare variant analyses (variable threshold and sequence kernel association test) and gene-set analyses to identify genes enriched with rare amino acid coding (AAC) variants associated with diLQTS. Significant associations were reanalyzed by comparing diLQTS patients with 515 ethnically matched control subjects from the National Heart, Lung, and Blood Grand Opportunity Exome Sequencing Project. RESULTS: Rare variants in 7 genes were enriched in the diLQTS patients according to the sequence kernel association test or variable threshold compared with drug-exposed controls (p < 0.001). Of these, we replicated the diLQTS associations for KCNE1 and ACN9 using 515 Exome Sequencing Project control subjects (p < 0.05). A total of 37% of the diLQTS patients also had 1 or more rare AAC variants compared with 21% of control subjects (p = 0.009), in a pre-defined set of 7 congenital long QT interval syndrome (cLQTS) genes encoding potassium channels or channel modulators (KCNE1, KCNE2, KCNH2, KCNJ2, KCNJ5, KCNQ1, AKAP9). CONCLUSIONS: By combining whole-exome sequencing with aggregated rare variant analyses, we implicate rare variants in KCNE1 and ACN9 as risk factors for diLQTS. Moreover, diLQTS patients were more burdened by rare AAC variants in cLQTS genes encoding potassium channel modulators, supporting the idea that multiple rare variants, notably across cLQTS genes, predispose to diLQTS.