Brugada Syndrome 1

A number sign (#) is used with this entry because of evidence that Brugada syndrome-1 (BRGDA1) is caused by heterozygous mutation in the SCN5A gene (600163) on chromosome 3p22.

Description

Brugada syndrome is characterized by an ST segment elevation in the right precordial electrocardiogram leads (so-called type 1 ECG) and a high incidence of sudden death in patients with structurally normal hearts. The syndrome typically manifests during adulthood, with a mean age of sudden death of 41 +/- 15 years, but also occurs in infants and children (summary by Antzelevitch et al., 2005).

Genetic Heterogeneity of Brugada Syndrome

Brugada syndrome-2 (611777) is caused by mutation in the GPD1L gene (611778) on chromosome 3p22. Brugada syndrome-3 (611875) and Brugada syndrome-4 (611876), the phenotypes of which include a shortened QT interval on ECG, are caused by mutation in the CACNA1C gene (114205) on chromosome 12p13 and CACNB2 gene (600003) on chromosome 10p12, respectively. Brugada syndrome-5 (612838) is caused by mutation in the SCN1B gene (600235) on chromosome 19q13. Brugada syndrome-6 (613119) is caused by mutation in the KCNE3 gene (604433) on chromosome 11q13. Brugada syndrome-7 (613120) is caused by mutation in the SCN3B gene (608214) on chromosome 11q24. Brugada syndrome-8 (613123) is caused by mutation in the HCN4 gene (605206) on chromosome 15q24. Brugada syndrome-9 (616399) is caused by mutation in the KCND3 gene (605411) on chromosome 1p13.

Antzelevitch et al. (2007) screened 82 consecutive probands with a clinical diagnosis of Brugada syndrome for mutations in 16 ion channel genes. Seven probands were found to have mutations in the CACNA1C (114205) or CACNB2 (600003) genes, including 3 Brugada probands with shortened QTc intervals (see 611875 and 611876). Fifteen percent of probands harbored a pathogenic mutation in the SCN5A gene.

Delpon et al. (2008) screened 14 ion channel genes in 105 probands with Brugada syndrome and detected SCN5A mutations in 14.3%, CACNA1C mutations in 6.7%, and CACNB2 mutations in 4.8% of the probands.

Hu et al. (2009) analyzed 9 'Brugada susceptibility' genes, including SCN5A, GPD1L (611778), CACNB2, CACNA1C, SCN1B (600235), KCNE2 (603796), KCNE3 (604433), KCNE4 (607775), and IRX5 (606195), as well as the sodium channel beta subunit SCN3B (608214), in 179 probands with Brugada syndrome; they noted that 129 (72.07%) of the probands were negative for mutation in all of the genes tested.

Crotti et al. (2012) analyzed 12 Brugada syndrome susceptibility genes in 129 unrelated patients with possible or probable Brugada syndrome and identified SCN5A mutations in 21 (16.3%) of the patients; only 6 (4.6%) of the patients carried a mutation in 1 of the other 11 genes.

In a cohort of 91 SCN5A-negative Brugada syndrome patients and 91 European controls from the 1000 Genomes Project database, Di Resta et al. (2015) analyzed 158 arrhythmia- and cardiac defect-associated genes. A significant enrichment in Brugada syndrome samples was found only for the DSG2 gene (125671), with 6 (6%) of 91 patients having a rare functional variant compared to none of the 91 controls (p = 0.029). In addition, borderline significance was detected for the MYH7 gene (160760) (5 patients versus 0 controls; p = 0.059). Analysis of phenotype correlations yielded statistical significance only between the presence of a DSG2 variant and syncope, documented ventricular tachycardia/fibrillation, and/or cardiac arrest (p = 0.034). Di Resta et al. (2015) noted the possible genetic overlap between different cardiac disorders, suggesting common pathogenetic pathways.

Clinical Features

Martini et al. (1989) described 6 patients with apparently idiopathic ventricular fibrillation, 3 of whom had a distinctive ECG pattern characterized by an upsloping ST segment in the right precordial leads ('early repolarization') in association with right bundle branch block and T-wave inversion. In these patients, they documented subtle structural abnormalities of the right ventricle after detailed clinical investigation. Brugada and Brugada (1992) described 8 additional patients with the same ECG changes who experienced cardiac arrest due to ventricular fibrillation. They introduced the term 'right bundle branch block, ST segment elevation, and sudden death syndrome' to describe a seemingly new clinical entity. Brugada et al. (2001) discussed the prognostic value of electrophysiologic studies in Brugada syndrome.

Alings and Wilde (1999) suggested that Brugada syndrome accounts for up to 40 to 60% of cases of ventricular fibrillation previously classified as idiopathic. In 5 to 10% of survivors of cardiac arrest due to ventricular arrhythmia, no cause, such as coronary artery disease or a structural abnormality of the heart, is found. There are no stringent diagnostic criteria for Brugada syndrome (Grace, 1999; Gussak et al., 1999). The electrocardiogram usually suggests the diagnosis; the pattern of the right precordial leads resembles those seen in right bundle branch block (RBBB) with variable ST segment elevation and a coved or saddle-type appearance. The ECG changes may not be apparent unless an agent that inhibits the cardiac sodium channel, such as flecainide or procainamide, is administered. Intravenous flecainide may be the drug of choice to be used as a diagnostic channel in patients presenting with a defect of ventricular fibrillation whose resting ECG is normal or in whom doubt about the diagnosis remains. Conversely, ST segment elevation may disappear after intravenous isoprenaline or exercise, whereas beta-blockade may exaggerate its appearance (Kasanuki et al., 1997).

Alings and Wilde (1999) stated that only about 200 cases of Brugada syndrome had been reported. Over 90% of these cases had been in male patients, the mean age at first arrhythmic event ranging between 22 and 65 years. Brugada syndrome seems to be most prevalent in Southeast Asia and Japan (Wong et al., 1992; Nademanee et al., 1997). Symptoms occur mostly at night, and the folklore of many of these countries is replete with stories of young men with 'Lai Tai' (Thailand), 'Bangungut' (Philippines), or 'Pokkuri' (Japan), thrashing, screaming, and then dying suddenly in their beds. This disorder may be the leading cause of natural death among young men in the poverty-stricken northeast of Thailand. The annual mortality rate in this group was said by Nademanee et al. (1997) to be as high as 26-38 per 100,000.

Priori et al. (2002) presented clinical data from 130 probands with Brugada syndrome and 70 affected family members. Overall, SCN5A mutations were identified in 28 probands; the remaining individuals fulfilled accepted diagnostic ECG criteria. Multivariate Cox regression analysis showed that, after adjusting for sex, a family history of sudden cardiac death, and mutation in SCN5A, the cooccurrence of spontaneous ST segment elevation in the anterior chest leads of a resting 12-lead ECG and a personal history of syncope identified persons at risk for cardiac arrest. The authors were unable to demonstrate a relationship between inducibility of arrhythmia during programmed electrical stimulation and subsequent spontaneous occurrence of ventricular fibrillation, suggesting that this clinical investigation was a poor predictor of cardiac risk. Further, Priori et al. (2002) suggested a risk stratification scheme designed to target the use of implantable cardioverter-defibrillator devices in patients with Brugada syndrome.

Diagnosis

In a consensus report from the Arrhythmia Working Group of the European Society of Cardiology, Wilde et al. (2002) proposed diagnostic criteria for the Brugada syndrome. In a report from the second consensus conference on Brugada syndrome, Antzelevitch et al. (2005) stated that a definitive diagnosis can be made when a type 1 ST segment elevation is observed in greater than 1 right precordial lead (V1 to V3) in the presence or absence of a sodium channel-blocking agent, and in conjunction with one of the following: documented ventricular fibrillation (VF), polymorphic ventricular tachycardia (VT), a family history of sudden cardiac death at less than 45 years of age, coved-type ECGs in family members, inducibility of VT with programmed electrical stimulation, syncope, or nocturnal agonal respiration. They noted that confounding factors that could account for the ECG abnormality or syncope, including arrhythmogenic right ventricular dysplasia (see ARVD1, 107970), should be excluded.

Sarquella-Brugada et al. (2016) reviewed Brugada syndrome and reported that the diagnosis of Brugada syndrome was accepted at that time in those patients with a type 1 ECG pattern and any of the following clinical features: documented VF; polymorphic VT; inducibility of VT with programmed electrical stimulation; a family history of sudden cardiac death at younger than age 45 years; coved-type ECGs in family members; unexplained syncope; or nocturnal agonal respiration.

Chevallier et al. (2011) analyzed ECG tracings of 38 patients with either type 2 or type 3 Brugada patterns who were undergoing antiarrhythmic drug (AAD) challenge to unmask the classic type 1 pattern. Measurement of the alpha angle (between a vertical line and the downslope of the r-prime wave) and the beta angle (between the upslope of the S wave and the downslope of the r-prime wave) revealed that the mean beta angle was significantly smaller in the 14 patients with negative results on AAD compared to the 24 patients with positive results. Using 58 degrees as a cutoff point, the beta angle had a positive predictive value of 73% and a negative predictive value of 87% for conversion to a type 1 Brugada pattern on AAD. The alpha angle was slightly less sensitive and specific compared to the beta angle, and combining angle information with QRS duration improved discrimination. Chevallier et al. (2011) concluded that in patients with suspected Brugada syndrome, simple ECG criteria can enable discrimination between incomplete RBBB and type 2 or 3 Brugada patterns. Brugada (2011) commented that the alpha angle relates to repolarization, whereas the beta angle is related to both depolarization and repolarization. He stated that an ECG pattern with incomplete RBBB with a broad r-prime wave suggests an underlying channelopathy, and that if the beta angle is 58 degrees or more, a combined depolarization and repolarization abnormality should be suspected, likely associated with a mutation in the cardiac sodium channel gene or somatic mutations of the cardiac neural crest cells.

Molecular Genetics

Screening of some families with the Brugada phenotype has revealed distinct mutations in the SCN5A gene, which encodes the pore-forming alpha-subunit of the cardiac sodium channel (Chen et al., 1998). As pointed out by Rook et al. (1999), pharmacologic sodium channel blockade elicits or worsens the electrocardiographic features associated with Brugada syndrome, thus making SCN5A a plausible candidate gene. In patients with this syndrome, they found missense mutations in SCN5A and assessed the functional significance of these mutations by expression of the mutant sodium channel proteins in Xenopus oocytes. Significant effects on cardiac sodium channel characteristics were observed. Alterations seemed to be associated with an increase in inward sodium current during the action potential upstroke.

Bezzina et al. (1999) presented a large, 8-generation Dutch family with a history of sudden death, most of which had occurred at night. One individual was thought to have died suddenly as the result of carotid sinus pressure while he was being shaved. Some living members of this family demonstrated ECG features compatible with Brugada syndrome and QT prolongation characteristic of long QT syndrome-3 (LQT3; 603830). SSCP analysis revealed an aberrant conformer corresponding to a novel mutation in the C terminal of the SCN5A protein (1795insD; 600163.0013). This family demonstrated that the long QT syndrome type-3 and Brugada syndrome appear to lie on a spectrum of cardiac electrophysiologic pathology caused by SCN5A mutation.

Kyndt et al. (2001) reported a missense mutation (600163.0026) in the SCN5A gene in a large French family segregating both isolated cardiac conduction defect and Brugada syndrome in a dominant manner.

In a patient with Brugada syndrome, Rivolta et al. (2001) identified a tyr1795-to-his mutation mutation in the SCN5A gene (Y1795H; 600163.0030). In a patient with Long QT syndrome-3, they identified a different mutation at the same codon (Y1795C; 600163.0029). They concluded that these findings provided further evidence of the close interrelationship between Brugada syndrome and long QT syndrome type 3 at the molecular level.

Veldkamp et al. (2003) studied the effect of the 1795insD SCN5A mutation on sinoatrial (SA) pacemaking. Activity of 1795insD channels during SA node pacemaking was confirmed by action potential (AP) clamp experiments, and the previously characterized persistent inward current (I-pst) and negative shift were implemented into SA node (AP) models. The -10 mV shift decreased the sinus rate by decreasing the diastolic depolarization rate, whereas the I-pst decreased the sinus rate by AP prolongation, despite a concomitant increase in the diastolic depolarization rate. In combination, a moderate I-pst (1 to 2%) and the shift reduced the sinus rate by about 10%. Veldkamp et al. (2003) concluded that sodium channel mutations displaying an I-pst or a negative shift in inactivation may account for the bradycardia seen in LQT3 patients, whereas SA node pauses or arrest may result from failure of SA node cells to repolarize under conditions of extra net inward current.

Sudden unexplained nocturnal death syndrome (SUNDS), a disorder found in southeast Asia, is characterized by an abnormal electrocardiogram with ST segment elevation in leads V1 to V3 and sudden death due to ventricular fibrillation, identical to that seen in Brugada syndrome. Vatta et al. (2002) found mutations in the SCN5A gene in 3 of 10 Asian SUNDS patients. In a sporadic Asian SUNDS patient, the authors identified a 1100G-A transition in SCN5A (600163.0021). The mutation is predicted to result in an arg367-to-his (R367H) substitution, which lies in the first P segment of the pore-lining region between the DIS5 and DIS6 transmembrane segments. In transfected Xenopus oocytes, the R367H mutant channel did not express any current. The authors hypothesized that the likely effect of this mutation is to depress peak current due to the loss of one functional allele.

Makita et al. (2008) studied 41 individuals from 15 LQT3 kindreds who carried the E1784K mutation in the SCN5A gene (600163.0008); the diagnoses in these individuals included LQT3 syndrome, Brugada syndrome, and/or sinus node dysfunction (see 608567). In vitro functional characterization of E1784K compared to properties reported for other LQT3 variants suggested that a negative shift of steady-state Na channel inactivation and enhanced tonic block in response to Na channel blockers represent common biophysical mechanisms underlying the phenotypic overlap of LQT3 and Brugada syndromes, and further indicated that class IC drugs should be avoided in patients with Na channels displaying these behaviors.

Hedley et al. (2009) reviewed the diagnostic criteria for Brugada syndrome and the pathogenic mechanisms of mutations in the 7 genes known to date to cause disease.

Lidocaine-Induced Brugada Syndrome 1

In a 45-year-old black man with no history of cardiac disease who developed monomorphic wide-complex ventricular tachycardia with right precordial ST segment elevation after the administration of lidocaine, Barajas-Martinez et al. (2008) identified 2 mutations in the SCN5A gene, V232I and L1308F (600163.0040). A slight right precordial ST elevation remained 1 year after discontinuation of lidocaine. The patient's parents were unavailable for study, but given the severity of his clinical manifestations, the authors strongly suspected that both mutations were on the same allele (Dumaine, 2009). Using patch-clamp techniques in mammalian TSA201 cells, Barajas-Martinez et al. (2008) observed use-dependent inhibition of I(Na) by lidocaine that was more pronounced in double-mutant channels than in wildtype; the authors concluded that the double mutation in SCN5A alters the affinity of the cardiac sodium channel for lidocaine such that the drug assumes class IC characteristics with potent use-dependent block of the sodium channel.

Associations Pending Confirmation

See 601327.0003 for discussion of a possible association between variation in the SCN2B gene and Brugada syndrome.

See 600935.0001 for discussion of a possible association between variation in the KCNJ8 gene and Brugada syndrome.

By performing a genomewide association study of 312 individuals with Brugada syndrome and 1,115 controls, Bezzina et al. (2013) detected 2 significant association signals at the SCN10A locus (604427) and near the HEY2 gene (604674). Independent replication confirmed both signals (metaanalyses: SCN10A, p = 1.0 x 10(-68); HEY2, p = 5.1 x 10(-17)) and identified 1 additional signal in SCN5A at 3p21 (p = 1.0 x 10(-14)). The cumulative effect of the 3 loci on disease susceptibility was unexpectedly large (p trend = 6.1 x 10(-81)). Bezzina et al. (2013) concluded that the association signals at SCN5A-SCN10A demonstrated that genetic polymorphisms modulating cardiac conduction can also influence susceptibility to cardiac arrhythmia. The implication of association with HEY2, supported by evidence that HEY2 regulates cardiac electrical activity, shows that Brugada syndrome may originate from altered transcriptional programming during cardiac development.