Faculty Bibliography

BACKGROUND: A rapidly growing number of long-QT syndrome (LQTS) patients are being treated with an implantable cardioverter-defibrillator (ICD). ICDs may pose problems, especially in the young. We sought to determine the characteristics of the LQTS patients receiving an ICD, the indications, and the aftermath. METHODS AND RESULTS: The study population included 233 patients. Beginning in 2002, data were collected prospectively. Female patients (77%) and LQT3 patients (22% of genotype positive) were overrepresented; mean QTc was 516+/-65 milliseconds; mean age at implantation was 30+/-17 years; and genotype was known in 59% of patients. Unexpectedly, 9% of patients were asymptomatic before implantation. Asymptomatic patients, almost absent among LQT1 and LQT2 patients, represented 45% of LQT3 patients. Patients with cardiac symptoms made up 91% of all study participants, but only 44% had cardiac arrest before ICD implantation. In addition, 41% of patients received an ICD without having first been on LQTS therapy. During follow-up, 4.6+/-3.2 years, at least 1 appropriate shock was received by 28% of patients, and adverse events occurred in 25%. Appropriate ICD therapies were predicted by age <20 years at implantation, a QTc >500 milliseconds, prior cardiac arrest, and cardiac events despite therapy; within 7 years, appropriate shocks occurred in no patients with none of these factors and in 70% of those with all factors. CONCLUSIONS: Reflecting previous concepts, ICDs were implanted in some LQTS patients whose high risk now appears questionable. Refined criteria for implantation, reassessment of pros and cons, ICD reprogramming, and consideration for other existing therapeutic options are necessary

This article serves as an introductory overview to a thematic review series that will present the latest advancements in the field of inherited arrhythmias. This area of cardiac electrophysiology started approximately 15 years ago thanks to the contribution of Mark Keating and coworkers, who discovered the molecular basis of long QT syndrome. The field rapidly expanded when clinicians, molecular biologists, geneticists, and cellular electrophysiologists, who undertook an impressive collaborative effort to clarify the genetic basis of 'cardiac channelopathies.' As a result of this hard work, the paradigms for diagnosis and management of patients with inherited arrhythmogenic diseases were substantially modified, demonstrating once more the value of 'translational research.' As more and more genes have been implicated in the genesis of inherited arrhythmias, we keep broadening our understanding of the complexity of ion channels and their multifaceted regulatory processes. Despite the fact that several discoveries have already been made, the field is facing new challenges that are attracting young investigators who share with the pioneers the ambitious goal of finding new therapies and even a cure for these conditions

Rationale: The Purkinje fiber network has been proposed as the source of arrhythmogenic Ca(2+) release events in catecholaminergic polymorphic ventricular tachycardia (CPVT), yet evidence supporting this mechanism at the cellular level is lacking. Objective: We sought to determine the frequency and severity of spontaneous Ca(2+) release events and the response to the antiarrhythmic agent flecainide in Purkinje cells and ventricular myocytes from RyR2(R4496C/+) CPVT mutant mice and littermate controls. Methods and Results: We crossed RyR2(R4496C/+) knock-in mice with the newly described Cntn2-EGFP BAC transgenic mice, which express a fluorescent reporter gene in cells of the cardiac conduction system, including the distal Purkinje fiber network. Isolated ventricular myocytes (EGFP(-)) and Purkinje cells (EGFP(+)) from wild-type hearts and mutant hearts were distinguished by epifluorescence and intracellular Ca(2+) dynamics recorded by microfluorimetry. Both wild-type and RyR2(R4496C/+) mutant Purkinje cells displayed significantly slower kinetics of activation and relaxation compared to ventricular myocytes of the same genotype, and tau(decay) in the mutant Purkinje cells was significantly slower than that observed in wild-type Purkinje cells. Of the 4 groups studied, RyR2(R4496C/+) mutant Purkinje cells were also most likely to develop spontaneous Ca(2+) release events, and the number of events per cell was also significantly greater. Furthermore, with isoproterenol treatment, although all 4 groups showed increases in the frequency of arrhythmogenic Ca(2+(i)) events, the RyR2(R4496C/+) Purkinje cells responded with the most profound abnormalities in intracellular Ca(2+) handling, including a significant increase in the frequency of unstimulated Ca(2+(i)) events and the development of alternans, as well as isolated and sustained runs of triggered beats. Both Purkinje cells and ventricular myocytes from wild-type mice showed suppression of spontaneous Ca(2+) release events with flecainide, whereas in RyR2(R4496C/+) mice, the Purkinje cells were preferentially responsive to drug. In contrast, the RyR2 blocker tetracaine was equally efficacious in mutant Purkinje cells and ventricular myocytes. Conclusions: Purkinje cells display a greater propensity to develop abnormalities in intracellular Ca(2+) handling than ventricular myocytes. This proarrhythmic behavior is enhanced by disease-causing mutations in the RyR2 Ca(2+) release channel and greatly exacerbated by catecholaminergic stimulation, with the development of arrhythmogenic triggered beats. These data support the concept that Purkinje cells are critical contributors to arrhythmic triggers in animal models and humans with CPVT and suggest a broader role for the Purkinje fiber network in the genesis of ventricular arrhythmias

BACKGROUND: Inherited arrhythmias can be caused by mutations in the cardiac ryanodine receptor (RyR2). The cellular source of these arrhythmias is unknown. Isolated RyR2(R4496C) mouse ventricular myocytes display arrhythmogenic activity related to spontaneous Ca(2+) release during diastole. On the other hand, recent whole-heart epicardial and endocardial optical mapping data demonstrate that ventricular arrhythmias in the RyR2(R4496C) mouse model of catecholaminergic polymorphic ventricular tachycardia (CPVT) originate in the His-Purkinje system, suggesting that Purkinje cells, and not ventricular myocytes, may be the cellular source of arrhythmogenic activity. The relative effect of the RyR2(R4496C) mutation on calcium homeostasis in ventricular myocytes versus Purkinje cells is unknown. OBJECTIVE: This study sought to determine which cardiac cell type is more severely affected, in terms of calcium handling, by expression of the RyR2(R4496C) mutant channel: the ventricular myocytes or the Purkinje cells. METHODS AND RESULTS: To discriminate Purkinje cells from ventricular myocytes, we crossed the RyR2(R4496C) mouse model of CPVT with the Cx40(EGFP/+) transgenic mouse. This genetic cross yields Purkinje cells that express eGFP, and therefore fluoresce green when excited by the appropriate wavelength; ventricular myocytes, which do not express connexin 40, are not green. Intracellular calcium was measured in each cell type using calcium-sensitive probes. Purkinje cells of the RyR2(R4496C) mouse model of CPVT show an approximately 2x greater rate (P < .05) and approximately 2x to 3x greater amplitude (P < .000001) of spontaneous calcium release events than ventricular myocytes isolated from the same heart. CONCLUSION: These results demonstrate that focally activated arrhythmias originate in the specialized electrical conducting cells of the His-Purkinje system in the RyR2(R4496C) mouse model of CPVT

AIMS: Mutations in the cardiac ryanodine receptor Ca(2+) release channel, RyR2, underlie catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited life-threatening arrhythmia. CPVT is triggered by spontaneous RyR2-mediated sarcoplasmic reticulum (SR) Ca(2+) release in response to SR Ca(2+) overload during beta-adrenergic stimulation. However, whether elevated SR Ca(2+) content--in the absence of protein kinase A activation--affects RyR2 function and arrhythmogenesis in CPVT remains elusive. METHODS AND RESULTS: Isolated murine ventricular myocytes harbouring a human RyR2 mutation (RyR2(R4496C+/-)) associated with CPVT were investigated in the absence and presence of 1 micromol/L JTV-519 (RyR2 stabilizer) followed by 100 micromol/L ouabain intervention to increase cytosolic [Na(+)] and SR Ca(2+) load. Changes in membrane potential and intracellular [Ca(2+)] were monitored with whole-cell patch-clamping and confocal Ca(2+) imaging, respectively. At baseline, action potentials (APs), Ca(2+) transients, fractional SR Ca(2+) release, and SR Ca(2+) load were comparable in wild-type (WT) and RyR2(R4496C+/-) myocytes. Ouabain evoked significant increases in diastolic [Ca(2+)], peak systolic [Ca(2+)], fractional SR Ca(2+) release, and SR Ca(2+) content that were quantitatively similar in WT and RyR2(R4496C+/-) myocytes. Ouabain also induced arrhythmogenic events, i.e. spontaneous Ca(2+) waves, delayed afterdepolarizations and spontaneous APs, in both groups. However, the ouabain-induced increase in the frequency of arrhythmogenic events was dramatically larger in RyR2(R4496C+/-) when compared with WT myocytes. JTV-519 greatly reduced the frequency of ouabain-induced arrhythmogenic events. CONCLUSION: The elevation of SR Ca(2+) load--in the absence of beta-adrenergic stimulation--is sufficient to increase the propensity for triggered arrhythmias in RyR2(R4496C+/-) cardiomyocytes. Stabilization of RyR2 by JTV-519 effectively reduces these triggered arrhythmias

OBJECTIVES: We investigated the role of nitric oxide 1 adaptor protein (NOS1AP) as a genetic modifier of long QT syndrome (LQTS). BACKGROUND: LQTS risk stratification is complicated by the phenotype variability that limits prediction of life-threatening arrhythmic events based on available metrics. Thus, the identification of new markers is desirable. Recent studies have shown that NOS1AP variations in the gene modulate QT interval in healthy and 1 LQTS kindred, and occurrence of cardiac events in healthy subjects. METHODS: The study included 901 patients enrolled in a prospective LQTS registry. Three NOS1AP marker SNPs (rs4657139, rs16847548, and rs10494366) were genotyped to assess the effect of variant alleles on QTc and on the incidence of cardiac events. We quantified the association between variant alleles, QTc, and outcomes to assess whether NOS1AP is a useful risk stratifier in LQTS. RESULTS: Variant alleles tagged by SNPs rs4657139 and rs16847548 were associated with an average QTc prolongation of 7 and 8 ms, respectively (p < 0.05; p < 0.01); whereas rs4657139 and rs10494366 were associated with increased incidence of cardiac events (25.2% vs. 18.0%, p < 0.05 and 24.8% vs. 17.8% p < 0.05). Cox multivariate analysis identified rs10494366 minor allele as an independent prognostic marker among patients with QTc <500 ms (hazard ratio: 1.63; 95% confidence interval: 1.06 to 2.5; p < 0.05) but not in the entire cohort. CONCLUSIONS: Our results provide the first demonstration, to our knowledge, of a risk-conferring genetic modifier in a large LQTS cohort. Subject to confirmation in additional cohorts, we suggest that the NOS1AP tag SNP genotype may provide an additional clinical dimension, which helps assess risk and choice of therapeutic strategies in LQTS

RATIONALE: Sodium channel blockers are used as gene-specific treatments in long-QT syndrome type 3, which is caused by mutations in the sodium channel gene (SCN5A). Response to treatment is influenced by biophysical properties of mutations. OBJECTIVE: We sought to investigate the unexpected deleterious effect of mexiletine in a mutation combining gain-of- function and trafficking abnormalities. METHODS AND RESULTS: A long-QT syndrome type 3 child experienced paradoxical QT prolongation and worsening of arrhythmias after mexiletine treatment. The SCN5A mutation F1473S expressed in HEK293 cells presented a right-ward shift of steady-state inactivation, enlarged window current, and huge sustained sodium current. Unexpectedly, it also reduced the peak sodium current by 80%. Immunostaining showed that mutant Nav1.5 is retained in the cytoplasm. Incubation with 10 micromol/L mexiletine rescued the trafficking defect of F1473S, causing a significant increase in peak current, whereas sustained current was unchanged. Using a Markovian model of the Na channel and a model of human ventricular action potential, we showed that simulated exposure of F1473S to mexiletine paradoxically increased action potential duration, mimicking QT prolongation seen in the index patient on mexiletine treatment. CONCLUSIONS: Sodium channel blockers are largely used to shorten QT intervals in carriers of SCN5A mutations. We provided evidence that these agents may facilitate trafficking of mutant proteins, thus exacerbating QT prolongation. These data suggest that caution should be used when recommending this class of drugs to carriers of mutations with undefined electrophysiological properties

OBJECTIVE: To investigate whether a significantly aberrant expression of circulating placental mRNA genes related with cardiogenesis can be detected at the second trimester of pregnancy. METHODS: The study was performed in two stages. First stage (development model group): match of 14 placental tissues at delivery of fetuses with congenital heart disease versus 20 controls. Second stage (validation model group): mRNA amplification of abnormal expressed genes in maternal blood samples from 26 women bearing a fetus with a congenital heart disease matched with 28 controls. RESULTS: We identified four functional categories of genes possibly involved in abnormal heart development: cardiac morphogenesis: tenascin, thioredoxin, salvador homolog 1 protein; extracellular matrix (ECM) and valvular tissue biosynthesis; placental-associated plasma protein, collagen, type I, alpha 2, fibulin-1, heparanase, procollagen-proline, 2-oxoglutarate 4-dioxygenase, alpha polypeptide II, Jumonji, AT rich interactive domain 1B RBP2-like; normal contractile activity: actinin, alpha 4, fascin homolog 1, actin-bundling protein; and congestive heart failure. CONCLUSION: Altered placental genetic expression was found at term delivery in affected fetuses. The aberration was also confirmed in maternal blood at the second trimester of women bearing a fetus with congenital heart disease. Sensitivity for the most aberrant genes ranged between 42% and 95% at a false positive rate (FPR) of 10%