Faculty Bibliography

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

PURPOSE OF REVIEW: In this article, we will review the appropriate use of genetic testing in those patients suspected to have inherited arrhythmogenic diseases, with specific focus on the indications for testing and the expected probability of positive genotyping. RECENT FINDINGS: Important advances have been made in the identification of new genes, associated mutations, and polymorphisms that modulate susceptibility of acquired arrhythmias. We will examine the most recent advances relevant to the rational application of genetic analysis, guided by genotype-phenotype correlations derived from disease and patient-specific evaluation, as well as discussing novel technologies and recently published cost-effectiveness data. SUMMARY: Genetic analysis can be performed to identify the molecular substrate in those patients suspected to be affected by an inherited arrhythmogenic disease; however, the clinical usefulness of this information is often not straightforward. We hope to emphasize the concept that there is a significant difference in the impact of genetic testing within the various arrhythmogenic disorders, and the benefit of accessing genetic testing is not the same in all patients. The resultant integration between the expected yield of genetic screening and cost may allow the formation of criteria to prioritize access for those who could derive the most clinical benefit

Three decades of ongoing research into the identification of genes responsible for both cardiomyopathies and ion channel diseases has facilitated a progressive understanding of the pathophysiology of inherited arrhythmogenic diseases. Recent discoveries in the area of genetics promise to significantly change the current clinical practice of cardiology, as rapid advances in technology and a coincident reduction of costs associated with sequencing have pushed the 'translation' of genomic information from bench to bedside. In turn, clinicians have at their disposal new tools for more accurate diagnosis of diseases, as well as for better calculation of health risks for affected families. It is clear, however, that the integration of genetic analysis into frontline clinical cardiology has not yet occurred, especially for heritable cardiomyopathic processes; no one simplified method exists for diagnosing these complex cardiac disease states. It therefore is important to assess the present and future roles of genetic analysis and counseling in clinical practice and how to assist the transition of genetic screening into current care to ensure the appropriate practical use of genetic tests in the routine clinical setting. The purpose of this discussion is to provide a concise review of recent developments in the field of heritable cardiomyopathies, with specific regard to genetic testing and genetic counseling

In the 8 years since the discovery of the genetic bases of catecholaminergic polymorphic ventricular tachycardia (CPVT), we have witnessed a remarkable improvement of knowledge on arrhythmogenic mechanisms involving disruption of cardiac Ca(2+) homeostasis. Studies on the consequences of RyR2 and CASQ2 mutations in cellular systems and mouse models have shed new light on pathways that are also implicated in arrhythmias occurring in highly prevalent diseases, such as heart failure. This research track has also led to the identification of therapeutic targets of potential clinical impact to abate the burden of sudden death in CPVT. Here, we review the current knowledge on the pathophysiology of CPVT also highlighting the existing controversies and possible future development

AIMS: Recent data suggest that sub-clinical structural abnormalities may be part of the Brugada syndrome (BrS) phenotype, a disease traditionally thought to occur in the structurally normal heart. In this study, we carried out detailed assessment of cardiac morphology and function using cardiac magnetic resonance imaging (CMRI). METHODS AND RESULTS: Thirty consecutive patients with BrS were compared with 30 sex- (26/4 male/female), body surface area- (+/-0.2 m(2)), and age-matched (+/-5 years) normal volunteers. CMRI exam included long- and short-axis ECG-gated breath-hold morphological T1-TSE sequences for fatty infiltration and cine-SSFP sequences for kinetic assessment. Fatty infiltration was not found in any subject. Patients with BrS compared with normal subjects showed higher incidence of mild right ventricle (RV) wall-motion abnormalities [15 (50%) vs. 5 (17%) subjects (P = 0.006) with reduced radial fractional shortening in more than two segments], reduction of outflow tract ejection fraction (49 +/- 11% vs. 55 +/- 10%; P = 0.032), enlargement of the inflow tract diameter (46 +/- 4 vs. 41 +/- 5 mm, P < 0.001 in short-axis; 46 +/- 4 vs. 42 +/- 5 mm, P = 0.001 in four-chamber long-axis view) and area (22 +/- 2 vs. 20 +/- 3 cm(2); P = 0.050), and of global RV end-systolic volume (34 +/- 10 vs. 30 +/- 6 mL/m(2); P = 0.031) but comparable outflow tract dimensions, global RV end-diastolic volume, left ventricle parameters, and atria areas. CONCLUSION: CMRI detects a high prevalence of mild structural changes of the RV, and suggests further pathophysiological complexity in BrS. Prospective studies to assess the long-term evolution of such abnormalities are warranted