Brugada syndrome: an update

Genetics of Brugada syndrome

In 1992, the Brugada syndrome (BrS) was recognized as a disease responsible for sudden cardiac death, characterized by a right bundle-branch block with ST segment elevation in the leads V1 and V2. This syndrome is highly associated with sudden cardiac death, especially in young males. BrS is currently diagnosed in patients with ST-segment elevation showing type 1 morphology ≥ 2 mm in ≥1 leads among the right precordial leads V1 or V2 positioned in the 2nd, 3rd, or 4th intercostal space, and occurring either spontaneously or after a provocative drug test by the intravenous administration of Class I antiarrhythmic drugs. With accumulated findings, the BrS inheritance model is believed to be an autosomal dominant inheritable model with incomplete penetrance, although most patients with BrS were sporadic cases. SCN5A, which was identified as the first BrS-associated gene in 1998, has emerged as the most common gene associated with BrS, and more than 10 BrS-associated genes have been identified thereafter. Mutation-specific genetic testing is recommended for the family members and appropriate relatives following the identification of BrS-causative mutations in an index patient. In addition, comprehensive or BrS1 (SCN5A) targeted genetic testing could be useful for patients in whom a cardiologist has established a clinical index of suspicion for BrS based on the patient׳s clinical history, family history, and the expressed electrocardiographic (resting 12-lead ECGs and/or provocative drug challenge testing) phenotype.

Over the past 20 years, extensive research in this field has allowed better understanding of the pathophysiology, genetic background, and management of BrS even though controversies still exist. In this review article, a background of genetics, the genetic background of BrS, the genotype and phenotype relationship, the role of genetic screening in clinical practice, and the interpretation of the identified genetic variants have been addressed based on this understanding.

New approaches to predicting the risk of sudden death

In this review article, we will explore some of the contemporary methods for predicting sudden cardiac death (SCD). These include experimental methods yet to be adopted in the clinical setting, and methods that have been extrapolated from observational data in those with a history of SCD. We will discuss how these relate to the different aetiologies and disease processes. We will also explore how these may be used in the clinical setting to decide on management.

Brugada syndrome: clinical and genetic findings

Brugada syndrome is a rare, inherited cardiac disease leading to ventricular fibrillation and sudden cardiac death in structurally normal hearts. Clinical diagnosis requires a Brugada type I electrocardiographic pattern in combination with other clinical features. The most effective approach to unmasking this diagnostic pattern is the use of ajmaline and flecainide tests, and the most effective intervention to reducing the risk of death is the implantation of a cardioverter defibrillator. To date, 18 genes have been associated with the disease, with the voltage-gated sodium channel α type V gene (SCN5A) being the most common one to date. However, only 30-35% of diagnosed cases are attributable to pathogenic variants in known genes, emphasizing the need for further genetic studies. Despite recent advances in clinical diagnoses and genetic testing, risk stratification and clinical management of patients with Brugada syndrome remain challenging.Genet Med 18 1, 3-12.

Translating advances in cardiogenetics into effective clinical practice

In this article we describe a qualitative research study in which we explored individuals' subjective experiences of both genetic testing and cardiogenetic disorders. Using a grounded theory approach, we coded and analyzed interview and focus group transcripts from 50 participants. We found that just under half of the participants who received their diagnosis during the study reported difficulty understanding information about both the purpose of genetic testing and their cardiac disease. A high level of anxiety about genetic testing and cardiac symptoms exacerbated individuals' cognitive confusion. Participants reported both positive and negative interactions with the medical community, depending on health care professionals' knowledge of cardiogenetic disorders. Overall, participants expressed a range of attitudes--positive, negative, and ambivalent--toward genetic testing. We conclude with a discussion of the barriers to achieving effective clinical care for genetic conditions and offer suggestions for improving collaborative decision making between physicians and patients.

Feasibility and outcomes of ajmaline provocation testing for Brugada syndrome in children in a specialist paediatric inherited cardiovascular diseases centre

Objectives

Brugada syndrome (BrS) is an inherited arrhythmia syndrome that causes sudden cardiac death in the young. The class Ia antiarrhythmic ajmaline can be used to provoke the diagnostic ECG pattern. Its use has been established in adults, but little data exist on the ajmaline provocation test in children. This study aims to determine the safety and feasibility of ajmaline provocation testing in a large paediatric cohort in a specialist paediatric inherited cardiac diseases centre.

Methods

98 consecutive ajmaline tests were performed in 95 children between September 2004 and July 2012 for family history of BrS (n=46 (48%)); family history of unexplained sudden cardiac death (n=39 (41%); symptoms with suspicious ECG abnormalities (n=9 (10%)). Three patients were retested with age, due to the possibility of age-related penetrance. ECG parameters were measured at baseline and during maximal ajmaline effect.

Results

The mean patient age was 12.55 years, 43% were female. Nineteen patients (20%) had a positive ajmaline test. There were no arrhythmias or adverse events during testing. Ajmaline provoked significant prolongation of the PR, QRS and QTc in all patients. Mean follow-up was 3.62 years with no adverse outcomes reported in any patients with BrS. There were no predictors of a positive ajmaline provocation test on multivariable analysis. One patient who tested negative at 12 years of age, subsequently tested positive at 15 years of age.

Conclusions

Ajmaline testing appears safe and feasible in children when performed in an appropriate setting by an experienced team. Test positivity may change with age in individuals, suggesting that the test should be repeated in the late teenage years or early adulthood.

Sodium channels, cardiac arrhythmia, and therapeutic strategy

Cardiac sodium channels are transmembrane proteins distributed in atrial and ventricular myocytes and Purkinje fibers. A large and rapid Na(+) influx through these channels initiates action potential and thus excitation-contraction coupling of cardiac cells. Cardiac sodium channel is composed of a pore-forming α-subunit and one or two accessory β-subunits. The cardiac α-subunit is encoded by gene SCN5A located on chromosome 3p21. There are four types of β-subunits identified so far, and β1 is the primary β-subunit in cardiac Na(+) channels. The gene responsible for β1 subunits is SCNB. The expression of β-subunits together with α subunits enhances the Na(+) current and modifies the channel activities. In addition, interactions of the cardiac Na(+) channel with other proteins may facilitate the channel activity and membrane expression of the channel. Over the past two decades, molecular genetic studies have identified the linkage of gene mutations of the Na(+) channel proteins and other regulatory proteins to many inherited arrhythmogenic diseases. The most common cardiac arrhythmogenic diseases associated with Na(+) channelopathies are long QT syndrome (LQT3) and Brugada syndromes (BrSs). This chapter intends to summarize the current understanding of the normal sodium-channel structure and function, the gene mutation-associated cardiac arrhythmias, and the current diagnosis and management of these diseases.

The Brugada syndrome revisited

The Brugada syndrome is a rare but well-defined cause of sudden cardiac death. The key underlying abnormality is a decrease in net depolarising current due to a genetic defect, though recent evidence also implicates structural abnormalities in some patients. Diagnosis requires a Brugada-type ECG as well as typical clinical features: such clinical considerations are currently key in guiding risk stratification and hence management. Whilst pharmacological therapies are under investigation, the only intervention with a robust evidence base remains insertion of an implantable cardioverter defibrillator. Further research will be required to allow more effective risk stratification and hence more rational therapy.

Genetic biomarkers in Brugada syndrome

Brugada syndrome is an inherited arrhythmia syndrome predisposing to sudden cardiac death. Six years after its initial description as a clinical entity, the first mutations in SCN5A encoding the cardiac sodium channel Nav1.5 were reported. Over 300 mutations in SCN5A have since been described in addition to mutations in genes encoding Nav1.5 auxiliary units, potassium and calcium channels. This review summarizes the current knowledge on the genetics of Brugada syndrome, focusing on SCN5A, and discusses its use as a biomarker for diagnosis, prognosis and treatment.