Genetic basis of Brugada syndrome

Sudden Cardiac Death in Brugada Syndrome

The Brugada syndrome is an inherited channelopathy that alters the main transmembrane ion currents that constitute the cardiac action potential. These changes not only modify the resting electrocardiogram but also predispose patients to develop malignant ventricular tachyarrhythmias that can lead to syncope, cardiac arrest, and sudden cardiac death. This syndrome is responsible for nearly 20% of all sudden cardiac deaths in patients with structurally normal hearts and up to 12% of all sudden cardiac deaths. Brugada syndrome is diagnosed by its characteristic electrocardiogram consisting of a coved-type ST-segment elevation of at least 2 mm followed by a negative T wave in either one of the right precordial leads. These changes can be observed spontaneously or after administration of a sodium channel blocker. While our understanding of this disease has increased tremendously since its first description in 1992, the primary therapeutic option remains implantation of an implantable cardioverter-defibrillator to avoid sudden cardiac death. Therefore, tremendous effort is being made to effectively risk stratify patients to determine who would benefit from implantable cardioverter-defibrillator implantation.

Adolescent Seizure in the Emergency Department Due to Concomitant Brugada Syndrome

Brugada syndrome is an increasingly discussed entity in the emergency medicine and cardiology literature. However, there are few cases reported in the pediatric population. Seizure is a presentation common to pediatric emergency departments. This case report describes the clinical course, workup, and differential diagnosis of a 15-year-old male adolescent with first-time seizure and Brugada pattern on electrocardiogram. A brief review of the literature follows the case presentation.

An R1632C variant in the SCN5A gene causing Brugada syndrome

Brugada syndrome (BS) is an electrical disease, inherited in an autosomal dominant manner. BS is caused by mutations in up to 13 different genes. SCN5A is the gene most frequently mutated in BS, although this presents an incomplete penetrance. The present case study investigated the SCN5A gene in a family exhibiting BS. Direct sequencing of the SCN5A gene was performed to identify mutations and a familial investigation was performed. A novel variant was identified in the voltage‑sensing domain of the SCN5A protein. This familial investigation revealed one novel asymptomatic carrier in the family. Genetic investigations are useful to classify individuals who require more frequent clinical monitoring and to stratify the risk of developing the disease.

High prevalence of genetic variants previously associated with Brugada syndrome in new exome data

More than 300 variants in 12 genes have been associated with Brugada syndrome (BrS) which has a prevalence ranging between 1:2000 and 1:100,000. Until recently, there has been little knowledge regarding the distribution of genetic variations in the general population. This problem was partly solved, when exome data from the NHLI GO Exome Sequencing Project (ESP) was published. In this study, we aimed to report the prevalence of previously BrS-associated variants in the ESP population. We performed a search in ESP for variants previously associated with BrS. In addition, four variants in ESP were genotyped in a second Danish control population (n = 536) with available electrocardiograms. In ESP, we identified 38 of 355 (10%) variants, distributed on 272 heterozygote carriers and two homozygote carriers. The genes investigated were on average screened in 6258 individuals. This corresponds to a surprisingly high genotype prevalence of 1:23 (274:6258). Genotyping the four common ESP-derived variants CACNA2D1 S709N, SCN5A F2004L, CACNB2 S143F, and CACNB2 T450I in the Danish controls, we found a genotype prevalence comparable with that found in ESP. We suggest that exome data are used in research, as an additive tool to predict the pathogenicity of variants in patients suspected for BrS.

Genetics of Brugada syndrome

PURPOSE OF REVIEW

The Brugada syndrome has been investigated in depth since its description in 1992 both on a clinical and on a basic research level. Since the discovery of the first genetic defect in 1998, several genes have been subsequently identified. However, to date all these genes together explain only 30% of the cases, indicating that there is still an important amount of work to be done to totally unravel the genetic basis of this lethal disease. In the present study, we will focus on recent achievements in the genetic basis of this disease.

RECENT FINDINGS

In 2009, several additional genetic mutations have been associated with the disease. Additionally, a common variant has been described as a genetic modulator of Brugada syndrome among carriers of a SCN5A mutation.

SUMMARY

The number of scientific publications dealing with the syndrome has continued to increase substantially in recent years. New polymorphisms, mutations and genes associated with the disease have been described. However, despite the advances, knowledge of the genetic determinants of the Brugada syndrome remains limited.

A study of the SCN5A gene in a cohort of 76 patients with Brugada syndrome

We aim to study the SCN5A gene in a cohort of Brugada syndrome (BS) patients and evaluate the genotype-phenotype correlation. BS is caused by mutations in up to 10 different genes, SCN5A being the most frequently involved. Large genomic rearrangements in SCN5A have been associated with conduction disease, but its prevalence in BS is unknown. Seventy-six non-related patients with BS were studied. Clinical characteristics and family risk profile were recorded. Direct sequencing and multiplex ligation-dependent probe amplification (MLPA) of the SCN5A gene for identification of mutations and larger rearrangements were performed, respectively. Eight patients (10.5%) had point mutations (R27H, E901K, G1743R (detected in three families), V728I, N1443S and E1152X). Patients with mutations had a trend toward a higher proportion of spontaneous type I Brugada electrocardiogram (ECG) (87.5% vs 52.9%, p = 0.06) and had evidence of familial disease (62.5%, vs 23.5%, p = 0.03). The symptoms and risk profile of the carriers were not different from wild-type probands. There were non-significant differences in the prevalence of type I ECG, syncope and history of arrhythmia in carriers of selected polymorphisms. None of the patients had any deletion/duplication in the SCN5A gene. In conclusion, 10.5% of our patients had mutations in the SCN5A gene. Patients with mutations seemed to have more spontaneous type I ECG, but no differences in syncope or arrhythmic events compared with patients without mutations. Larger studies are needed to evaluate the role of polymorphisms in the SCN5A in the expression of the phenotype and prognosis. Large rearrangements were not identified in the SCN5A gene using the MLPA technique.

Sudden unexplained nocturnal death syndrome in Southern China: an epidemiological survey and SCN5A gene screening

Based on autopsy data collected in Southern China from 2001-2006, 975 cases of sudden unexplained nocturnal death syndrome (SUNDS) were surveyed. Genetic screening of SCN5A gene encoding the voltage dependent cardiac sodium channel was performed in 74 SUNDS cases. The annual occurrence rate of SUNDS in the area was estimated to be about 1 per 100,000 people. About 80.6% of deaths occurred between the ages of 21 to 40 years and the case number peaked at age 30 years. In 75.4% of cases with witnesses, victims died asleep between 11 PM and 4 AM and they showed predominantly abrupt respiratory distress shortly preceding death. The monthly distribution of emergency fever cases in the area during the same period was positively correlated to that of SUNDS cases (r(s) = 0.611, P = 0.035). Four polymorphisms in SCN5A were identified in both SUNDS and control groups. Compared with controls, the allele frequency of C5457 and C3666 + 69 were significant higher in SUNDS (P < 0.005) while the genotypes of both 5457CC (P = 0.012, OR = 2.0, 95% CI = 1.3-3.2) and 3666+69CC (P = 0.004, OR = 2.1, 95% CI = 1.3-3.3) in SUNDS cases were significantly higher. This is the first report of an epidemiological survey and SCN5A gene screening in SUNDS in the Han population of China. The genotypes of 5457CC and 3666+69CC in SCN5A gene may be Chinese SUNDS susceptible polymorphisms.

An atypical case of Brugada syndrome

Polymorphic ventricular tachycardia and ventricular fibrillation are the most common arrhythmias in Brugada syndrome causing syncope or sudden death. Sustained monomorphic ventricular tachycardias are rare in this context. We report of a patient with syncopal episodes due to episodes of sustained ventricular tachycardia, where a Type-I Brugada pattern was revealed after pharmacological provocation with procainamide.

Genetic and clinical aspects of Brugada syndrome: an update

The Brugada Syndrome (BS) is a "channellopathy," characterized by ion (e.g., sodium, calcium, and potassium) channel dysfunction and typical ECG alterations, originally described by Osher and Wolff in 1953 and further elucidated by Josep and Pedro Brugada in 1991. BS is typically associated with a high risk for sudden cardiac death (SCD) in young and otherwise healthy adults. Although in several patients the heart is structurally normal, subtle structural abnormalities in the right ventricular outflow tract are increasingly been reported. The worldwide prevalence of this disorder is still uncertain, and there are some significant regional differences. The syndrome is characterized by a strong genetic basis, and several mutations have been identified in genes encoding subunits of cardiac sodium, potassium, and calcium channels, as well as in genes involved in the trafficking or regulation of these channels. Accordingly, eight types of BS (from BS1 to BS8) have already been described, involving mutations in SCN5A, GPD1-L, CACNA1c, CACNB2b, SCN1B, KCNE3, SCN3B, and HCN4 genes. The vast majority (i.e., up to two-third) of BS patients is asymptomatic, whereas the leading clinical manifestation is polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation (VF) and SCD. The diagnosis is still challenging, and ECG abnormalities represent one component of the diagnostic criteria which also include clinical and demographic data. Although molecular genetic testing is effective in detecting mutations in 20-38% of BS patients, it represents an appealing option for stratifying the risk of adverse events as well as for prenatal testing.

Gain-of-function mutation S422L in the KCNJ8-encoded cardiac K(ATP) channel Kir6.1 as a pathogenic substrate for J-wave syndromes

BACKGROUND

J-wave syndromes have emerged conceptually to encompass the pleiotropic expression of J-point abnormalities including Brugada syndrome (BrS) and early repolarization syndrome (ERS). KCNJ8, which encodes the cardiac K(ATP) Kir6.1 channel, recently has been implicated in ERS following identification of the functionally uncharacterized missense mutation S422L.

OBJECTIVE

The purpose of this study was to further explore KCNJ8 as a novel susceptibility gene for J-wave syndromes.

METHODS

Using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing, comprehensive open reading frame/splice site mutational analysis of KCNJ8 was performed in 101 unrelated patients with J-wave syndromes, including 87 with BrS and 14 with ERS. Six hundred healthy individuals were examined to assess the allelic frequency for all variants detected. KCNJ8 mutation(s) was engineered by site-directed mutagenesis and coexpressed heterologously with SUR2A in COS-1 cells. Ion currents were recorded using whole-cell configuration of the patch-clamp technique.

RESULTS

One BrS case and one ERS case hosted the identical missense mutation S422L, which was reported previously. KCNJ8-S422L involves a highly conserved residue and was absent in 1,200 reference alleles. Both cases were negative for mutations in all known BrS and ERS susceptibility genes. K(ATP) current of the Kir6.1-S422L mutation was increased significantly over the voltage range from 0 to 40 mV compared to Kir6.1-WT channels (n = 16-21; P <.05).

CONCLUSION

These findings further implicate KCNJ8 as a novel J-wave syndrome susceptibility gene and a marked gain of function in the cardiac K(ATP) Kir6.1 channel secondary to KCNJ8-S422L as a novel pathogenic mechanism for the phenotypic expression of both BrS and ERS.

An international compendium of mutations in the SCN5A-encoded cardiac sodium channel in patients referred for Brugada syndrome genetic testing

BACKGROUND

Brugada syndrome (BrS) is a common heritable channelopathy. Mutations in the SCN5A-encoded sodium channel (BrS1) culminate in the most common genotype.

OBJECTIVE

This study sought to perform a retrospective analysis of BrS databases from 9 centers that have each genotyped >100 unrelated cases of suspected BrS.

METHODS

Mutational analysis of all 27 translated exons in SCN5A was performed. Mutation frequency, type, and localization were compared among cases and 1,300 ostensibly healthy volunteers including 649 white subjects and 651 nonwhite subjects (blacks, Asians, Hispanics, and others) that were genotyped previously.

RESULTS

A total of 2,111 unrelated patients (78% male, mean age 39 +/- 15 years) were referred for BrS genetic testing. Rare mutations/variants were more common among BrS cases than control subjects (438/2,111, 21% vs. 11/649, 1.7% white subjects and 31/651, 4.8% nonwhite subjects, respectively, P <10(-53)). The yield of BrS1 genetic testing ranged from 11% to 28% (P = .0017). Overall, 293 distinct mutations were identified in SCN5A: 193 missense, 32 nonsense, 38 frameshift, 21 splice-site, and 9 in-frame deletions/insertions. The 4 most frequent BrS1-associated mutations were E1784K (14x), F861WfsX90 (11x), D356N (8x), and G1408R (7x). Most mutations localized to the transmembrane-spanning regions.

CONCLUSION

This international consortium of BrS genetic testing centers has added 200 new BrS1-associated mutations to the public domain. Overall, 21% of BrS probands have mutations in SCN5A compared to the 2% to 5% background rate of rare variants reported in healthy control subjects. Additional studies drawing on the data presented here may help further distinguish pathogenic mutations from similarly rare but otherwise innocuous ones found in cases.

Spectrum and prevalence of mutations from the first 2,500 consecutive unrelated patients referred for the FAMILION long QT syndrome genetic test

BACKGROUND

Long QT syndrome (LQTS) is a potentially lethal, highly treatable cardiac channelopathy for which genetic testing has matured from discovery to translation and now clinical implementation.

OBJECTIVES

Here we examine the spectrum and prevalence of mutations found in the first 2,500 unrelated cases referred for the FAMILION LQTS clinical genetic test.

METHODS

Retrospective analysis of the first 2,500 cases (1,515 female patients, average age at testing 23 +/- 17 years, range 0 to 90 years) scanned for mutations in 5 of the LQTS-susceptibility genes: KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), and KCNE2 (LQT6).

RESULTS

Overall, 903 referral cases (36%) hosted a possible LQTS-causing mutation that was absent in >2,600 reference alleles; 821 (91%) of the mutation-positive cases had single genotypes, whereas the remaining 82 patients (9%) had >1 mutation in > or =1 gene, including 52 cases that were compound heterozygous with mutations in >1 gene. Of the 562 distinct mutations, 394 (70%) were missense, 428 (76%) were seen once, and 336 (60%) are novel, including 92 of 199 in KCNQ1, 159 of 226 in KCNH2, and 70 of 110 in SCN5A.

CONCLUSION

This cohort increases the publicly available compendium of putative LQTS-associated mutations by >50%, and approximately one-third of the most recently detected mutations continue to be novel. Although control population data suggest that the great majority of these mutations are pathogenic, expert interpretation of genetic test results will remain critical for effective clinical use of LQTS genetic test results.

Study of the extent of the information of cardiologists from São Paulo city, Brazil, regarding a low-prevalence entity: Brugada syndrome

OBJECTIVE

To determine the degree of knowledge that cardiologists from São Paulo, Brazil, have regarding a low-prevalent entity associated with a high rate of sudden death-Brugada syndrome.

METHODS

Two hundred forty-four cardiologists were interviewed by an instrument divided in two parts: in the first, we recorded gender, age, and data related to academic profile. The second--answered only by the professionals that manifested having some degree of knowledge on the syndrome--had 28 questions that evaluated their knowledge. The answers were spontaneous and they did not have a chance to consult. We used uni- and multivariate analysis on the average percentage of right and wrong answers, and the influence of the academic profile.

RESULTS

The predominant gender was the male gender (61.1%), the average age was 44.32 +/- 10.83 years, 40% with more than 20 years after obtaining their degree, 44% were educated in public institutions, 69% had a residency in cardiology, 20% had overseas practice, 12% had postdegree, 41% were linked to an educational institution, 24% with publication(s) in an indexed journal, 17.2% were authors of chapters in books, 2.5% had edited books, and 10% were linked to the Brazilian Society of Cardiac Arrhythmias. The average percentage of right answers was 45.7%.

CONCLUSION

The sample studied revealed a little knowledge on the entity. A residency in cardiology was the factor of greater significance in the percentage of right answers. Other significant factors were the link of the interviewed person to an educational institution, or the Brazilian Society of Cardiac Arrhythmias, and having a specialist degree.

Monomorphic ventricular tachycardia in 'Brugada syndrome': clinical case and literature review

A 20-year-old white judoka was admitted for severe palpitations during exercise followed by syncope. The electrocardiogram on admission revealed a wide-complex monomorphic tachycardia at a rate of 260 beats/min, with right bundle brunch block morphology and right axis deviation. Following electrical cardioversion, the electrocardiogram showed sinus rhythm with type 1 pattern of Brugada syndrome. We describe in detail the clinical course, the results of electrophysiological study, and therapeutic management. We reviewed literature data concerning a few cases of 'atypical Brugada syndrome' characterized by monomorphic ventricular tachycardia as clinical arrhythmia.

Molecular genetics of sudden cardiac death

Sudden cardiac death (SCD) is one of the most common causes of death. An important number of sudden deaths, especially in the young, are due to genetic heart disorders, both with structural and arrhythmogenic abnormalities. In recent years, significant advances have been made in understanding the genetic basis of SCD. Identification of the genetic causes of sudden death is important because close relatives are also at potential risk of having a fatal cardiac condition. A comprehensive post-mortem investigation is vital to determine the cause and manner of death and provides the opportunity to assess the potential risk to the family after appropriate genetic counselling. In this paper, we present an update of the different genetic causes of sudden death, emphasizing their importance for the forensic pathologist due to his relevant role in the diagnosis and prevention of SCD.

SCN5A channelopathies--an update on mutations and mechanisms

Voltage-gated Na+ channels mediate the rapid upstroke of the action potential in excitable tissues. Na(v)1.5, encoded by the SCN5A gene, is the predominant isoform in the heart. Mutations in SCN5A are associated with distinct cardiac excitation disorders often resulting in life-threatening arrhythmias. This review outlines the currently known SCN5A mutations linked to three distinct cardiac rhythm disorders: long QT syndrome subtype 3 (LQT3), Brugada syndrome (BS), and cardiac conduction disease (CCD). Electrophysiological properties of the mutant channels are summarized and discussed in terms of Na+ channel structure-function relationships and regarding molecular mechanisms underlying the respective cardiac dysfunction. Possible reasons for less convincing genotype-phenotype correlations are suggested.