Idiopathic ventricular fibrillation: is it a case for genetic testing?

Idiopathic ventricular fibrillation (IVF) is a diagnosis of exclusion in sudden cardiac arrest (SCA) survivors. Although there are clear guidelines on the clinical work-up of SCA survivors, less than one in five patients receives a complete work-up. This increases the chances of erroneously labelling these patients as having IVF, while 10-20% of them have an inherited cardiac condition (ICC). Diagnoses of ICC increase over time due to (additional) deep phenotyping or as a result of spontaneous expression of ICC over time. As SCA survivors can also harbor (likely) pathogenic variants in cardiomyopathy-associated genes in the absence of a phenotype, or can have another ICC without a clear cardiac phenotype, the question arises as to whether genetic testing in this group should be routinely performed. Family history (mainly in the case of sudden death) can increase suspicion of an ICC in an SCA victim, but does not add great value when adults underwent a complete cardiological work-up. The diagnosis of ICC has treatment consequences not only for the patient but also for their family. Genetic diagnostic yield does not appear to increase with larger gene panels, but variants of unknown significance (VUS) do. Although VUS can be confusing, careful and critical segregation analysis in the family can be performed when discussed in a multidisciplinary team at a center of expertise with at least a cardiologist as well as a clinical and laboratory geneticist, thereby degrading or promoting VUS. When to introduce genetic testing in SCA survivors remains a matter of debate, but the combination of quick, deep phenotyping with additional genetic testing for the unidentifiable phenotypes, especially in the young, seems preferable.

Identification of concealed cardiomyopathy using next-generation sequencing-based genetic testing in Korean patients initially diagnosed with idiopathic ventricular fibrillation

AIMS

Idiopathic ventricular fibrillation (IVF) is a disease in which the cause of ventricular fibrillation cannot be identified despite comprehensive clinical evaluation. This study aimed to investigate the clinical yield and implications of genetic testing for IVF.

METHODS AND RESULTS

This study was based on the multi-centre inherited arrhythmia syndrome registry in South Korea from 2014 to 2017. Next-generation sequencing-based genetic testing was performed that included 174 genes previously linked to cardiovascular disease. A total of 96 patients were clinically diagnosed with IVF. The mean age of the onset was 41.2 ± 12.7 years, and 79 patients were males (82.3%). Of these, 74 underwent genetic testing and four (5.4%) of the IVF probands had pathogenic or likely pathogenic variants (each having one of MYBPC3, MYH7, DSP, and TNNI3). All pathogenic or likely pathogenic variants were located in genes with definite evidence of a cardiomyopathy phenotype, either hypertrophic cardiomyopathy or arrhythmogenic right ventricular cardiomyopathy.

CONCLUSION

Next-generation sequencing-based genetic testing identified pathogenic or likely pathogenic variants in 5.4% of patients initially diagnosed with IVF, suggesting that genetic testing with definite evidence genes of cardiomyopathy may enable molecular diagnosis in a minority of patients with IVF. Further clinical evaluation and follow-up of patients with IVF with positive genotypes are needed to unveil concealed phenotypes, such as the pre-clinical phase of cardiomyopathy.

The genetic basis of apparently idiopathic ventricular fibrillation: a retrospective overview

AIMS

During the diagnostic work-up of patients with idiopathic ventricular fibrillation (VF), next-generation sequencing panels can be considered to identify genotypes associated with arrhythmias. However, consensus for gene panel testing is still lacking, and variants of uncertain significance (VUS) are often identified. The aim of this study was to evaluate genetic testing and its results in idiopathic VF patients.

METHODS AND RESULTS

We investigated 419 patients with available medical records from the Dutch Idiopathic VF Registry. Genetic testing was performed in 379 (91%) patients [median age at event 39 years (27-51), 60% male]. Single-gene testing was performed in 87 patients (23%) and was initiated more often in patients with idiopathic VF before 2010. Panel testing was performed in 292 patients (77%). The majority of causal (likely) pathogenic variants (LP/P, n = 56, 15%) entailed the DPP6 risk haplotype (n = 39, 70%). Moreover, 10 LP/P variants were found in cardiomyopathy genes (FLNC, MYL2, MYH7, PLN (two), TTN (four), RBM20), and 7 LP/P variants were identified in genes associated with cardiac arrhythmias (KCNQ1, SCN5A (2), RYR2 (four)). For eight patients (2%), identification of an LP/P variant resulted in a change of diagnosis. In 113 patients (30%), a VUS was identified. Broad panel testing resulted in a higher incidence of VUS in comparison to single-gene testing (38% vs. 3%, P < 0.001).

CONCLUSION

Almost all patients from the registry underwent, albeit not broad, genetic testing. The genetic yield of causal LP/P variants in idiopathic VF patients is 5%, increasing to 15% when including DPP6. In specific cases, the LP/P variant is the underlying diagnosis. A gene panel specifically for idiopathic VF patients is proposed.

Investigation of Unexplained Cardiac Arrest: Phenotyping and Genetic Testing

Unexplained cardiac arrest (UCA) is a working diagnosis that should be replaced by a final diagnosis once evaluation is completed. Complete evaluation of UCA should include high-yield tests like cardiac magnetic resonance imaging, exercise treadmill test, and sodium-channel blocker challenge to identify latent causes of UCA. If no clear etiology is revealed after complete evaluation, idiopathic ventricular fibrillation may be diagnosed, and the strength of its diagnosis can be divided into definitive, probable, and possible based on the number of high-yield tests performed. Care should be provided by a multidisciplinary team with expertise in this area.

Generation of an induced pluripotent stem cell line from a patient with conduction disease and recurrent ventricular fibrillation with a sodium voltage-gated channel alpha subunit 5 (SCN5A) gene c.392 + 3A > G splice-site variant

Variants in the sodium voltage-gated channel alpha subunit 5 gene (SCN5A) produce variable cardiac phenotypes including Brugada syndrome, conduction disease and cardiomyopathy. These phenotypes can lead to life-threatening arrhythmias, heart failure, and sudden cardiac death. Novel variants in splice-site regions of SCN5A require functional studies to characterise their pathogenicity as they are poorly understood. The generation of an induced pluripotent stem cell line provides a valuable resource to investigate the functional effects of potential splice-disrupting variants in SCN5A.

The Utility of Sodium Channel Provocation in Unexplained Cardiac Arrest Survivors and Electrocardiographic Predictors of Ventricular Fibrillation Recurrence

BACKGROUND

The implications of a drug-induced type 1 Brugada ECG pattern following sodium channel blocker provocation (SCBP) are not fully understood.

METHODS

Baseline clinical and ECG data were obtained from consecutive unexplained cardiac arrest survivors undergoing SCBP at 3 centers. A further 15 SCBP positive (SCBP+) unexplained cardiac arrest survivors were recruited from 3 additional centers to explore ventricular fibrillation recurrence.

RESULTS

A total of 121 consecutive unexplained cardiac arrest survivors underwent SCBP. The yield of the drug-induced type 1 Brugada ECG pattern was 17%. A baseline type 2/3 Brugada pattern (T2/3BP) (adjusted odds ratio, 19.36 [2.74-136.61]; P=0.003) and PR interval (odds ratio, 1.03 [1.01-1.05] per ms; P=0.017) were independent predictors of SCBP+ response. A pathogenic SCN5A variant was identified in 36% of the SCBP+ group versus 0% in the SCBP- group (P<0.001). Amongst SCBP+ patients, a spontaneous type 1 Brugada pattern was identified in 19% during follow up and in 24% a type 1 Brugada pattern was identified in a relative. Prior syncope (adjusted hazard ratio, 3.83 [1.36-10.78]; P=0.011) and the presence of global early repolarization (hazard ratio, 7.91 [3.22-19.44]; P<0.001) were independent predictors of 5-year ventricular fibrillation recurrence. There was a nonsignificant trend toward greater 5-year ventricular fibrillation recurrence in the SCBP- group (23/95 [24%] versus 3/34 [9%]; P=0.055).

CONCLUSIONS

The yield of the drug-induced type 1 Brugada ECG pattern in consecutive unexplained cardiac arrest survivors undergoing SCBP is 17%. A baseline T2/3BP and PR interval were independent predictors of the drug-induced type 1 Brugada ECG pattern. Greater heritability of BrS phenotype in this group was evidenced by a greater prevalence of pathogenic SCN5A variants and relatives with a type 1 Brugada pattern. A history of prior syncope and the presence of global early repolarization were independent predictors of ventricular fibrillation recurrence.

A SPRY1 domain cardiac ryanodine receptor variant associated with short-coupled torsade de pointes

Idiopathic ventricular fibrillation (IVF) causes sudden death in young adult patients without structural or ischemic heart disease. Most IVF cases are sporadic and some patients present with short-coupled torsade de pointes, the genetics of which are poorly understood. A man who had a first syncope at the age of 35 presented with frequent short-coupled premature ventricular beats with bursts of polymorphic ventricular tachycardia and then died suddenly. By exome sequencing, we identified three rare variants: p.I784F in the SPRY1 of the ryanodine receptor 2 (RyR2), p.A96S in connexin 40 (Cx40), reported to affect electrical coupling and cardiac conduction, and a nonsense p.R244X in the cardiac-specific troponin I-interacting kinase (TNNI3K). We assessed intracellular Ca2+ handling in WT and mutant human RYR2 transfected HEK293 cells by fluorescent microscopy and an enhanced store overload-induced Ca2+ release in response to cytosolic Ca2+ was observed in RyR2-I784F cells. In addition, crystal structures and thermal melting temperatures revealed a conformational change in the I784F-SPRY1 domain compared to the WT-domain. The novel RyR2-I784F variant in SPRY1 domain causes a leaky channel under non-stress conditions. The presence of several variants affecting Ca2+ handling and cardiac conduction suggests a possible oligogenic origin for the ectopies originating from Purkinje fibres.

Inhibition of N-type calcium channels in cardiac sympathetic neurons attenuates ventricular arrhythmogenesis in heart failure

AIMS

Cardiac sympathetic overactivation is an important trigger of ventricular arrhythmias in patients with chronic heart failure (CHF). Our previous study demonstrated that N-type calcium (Cav2.2) currents in cardiac sympathetic post-ganglionic (CSP) neurons were increased in CHF. This study investigated the contribution of Cav2.2 channels in cardiac sympathetic overactivation and ventricular arrhythmogenesis in CHF.

METHODS AND RESULTS

Rat CHF was induced by surgical ligation of the left coronary artery. Lentiviral Cav2.2-α shRNA or scrambled shRNA was transfected in vivo into stellate ganglia (SG) in CHF rats. Final experiments were performed at 14 weeks after coronary artery ligation. Real-time polymerase chain reaction and western blot data showed that in vivo transfection of Cav2.2-α shRNA reduced the expression of Cav2.2-α mRNA and protein in the SG in CHF rats. Cav2.2-α shRNA also reduced Cav2.2 currents and cell excitability of CSP neurons and attenuated cardiac sympathetic nerve activities (CSNA) in CHF rats. The power spectral analysis of heart rate variability (HRV) further revealed that transfection of Cav2.2-α shRNA in the SG normalized CHF-caused cardiac sympathetic overactivation in conscious rats. Twenty-four-hour continuous telemetry electrocardiogram recording revealed that this Cav2.2-α shRNA not only decreased incidence and duration of ventricular tachycardia/ventricular fibrillation but also improved CHF-induced heterogeneity of ventricular electrical activity in conscious CHF rats. Cav2.2-α shRNA also decreased susceptibility to ventricular arrhythmias in anaesthetized CHF rats. However, Cav2.2-α shRNA failed to improve CHF-induced cardiac contractile dysfunction. Scrambled shRNA did not affect Cav2.2 currents and cell excitability of CSP neurons, CSNA, HRV, and ventricular arrhythmogenesis in CHF rats.

CONCLUSIONS

Overactivation of Cav2.2 channels in CSP neurons contributes to cardiac sympathetic hyperactivation and ventricular arrhythmogenesis in CHF. This suggests that discovering purely selective and potent small-molecule Cav2.2 channel blockers could be a potential therapeutic strategy to decrease fatal ventricular arrhythmias in CHF.

Unusual Overlapping Cardiac Sarcoidosis and Long-QT Type 3 Induced Ventricular Fibrillation

A 54-year-old woman had been resuscitated after ventricular fibrillation and her electrocardiogram showed a QT prolongation (QTc=510 ms), and genetic screening revealed a missense variant, R1644C, in the SCN5A gene. She was therefore diagnosed with congenital long-QT syndrome (LQTS) type 3. However, the patient had left ventricular dysfunction, and based on the findings of cardiac magnetic resonance imaging, positron emission tomography and pathological examinations, she was diagnosed with cardiac sarcoidosis. Although both are rare diseases, their overlapping presence in this case may have led to an increased cardiovascular risk compared with either alone. Thus, not only genetic but comprehensive clinical examinations are important for making a correct diagnosis.

Update on Genetic Basis of Brugada Syndrome: Monogenic, Polygenic or Oligogenic?

Brugada syndrome is a rare inherited arrhythmogenic disease leading to ventricular fibrillation and high risk of sudden death. In 1998, this syndrome was linked with a genetic variant with an autosomal dominant pattern of inheritance. To date, rare variants identified in more than 40 genes have been potentially associated with this disease. Variants in regulatory regions, combinations of common variants and other genetic alterations are also proposed as potential origins of Brugada syndrome, suggesting a polygenic or oligogenic inheritance pattern. However, most of these genetic alterations remain of questionable causality; indeed, rare pathogenic variants in the SCN5A gene are the only established cause of Brugada syndrome. Comprehensive analysis of all reported genetic alterations identified the origin of disease in no more than 40% of diagnosed cases. Therefore, identifying the cause of this rare arrhythmogenic disease in the many families without a genetic diagnosis is a major current challenge in Brugada syndrome. Additional challenges are interpretation/classification of variants and translation of genetic data into clinical practice. Further studies focused on unraveling the pathophysiological mechanisms underlying the disease are needed. Here we provide an update on the genetic basis of Brugada syndrome.

Late INa Blocker GS967 Supresses Polymorphic Ventricular Tachycardia in a Transgenic Rabbit Model of Long QT Type 2

BACKGROUND

Long QT syndrome has been associated with sudden cardiac death likely caused by early afterdepolarizations (EADs) and polymorphic ventricular tachycardias (PVTs). Suppressing the late sodium current (INaL) may counterbalance the reduced repolarization reserve in long QT syndrome and prevent EADs and PVTs.

METHODS

We tested the effects of the selective INaL blocker GS967 on PVT induction in a transgenic rabbit model of long QT syndrome type 2 using intact heart optical mapping, cellular electrophysiology and confocal Ca2+ imaging, and computer modeling.

RESULTS

GS967 reduced ventricular fibrillation induction under a rapid pacing protocol (n=7/14 hearts in control versus 1/14 hearts at 100 nmol/L) without altering action potential duration or restitution and dispersion. GS967 suppressed PVT incidences by reducing Ca2+-mediated EADs and focal activity during isoproterenol perfusion (at 30 nmol/L, n=7/12 and 100 nmol/L n=8/12 hearts without EADs and PVTs). Confocal Ca2+ imaging of long QT syndrome type 2 myocytes revealed that GS967 shortened Ca2+ transient duration via accelerating Na+/Ca2+ exchanger (INCX)-mediated Ca2+ efflux from cytosol, thereby reducing EADs. Computer modeling revealed that INaL potentiates EADs in the long QT syndrome type 2 setting through (1) providing additional depolarizing currents during action potential plateau phase, (2) increasing intracellular Na+ (Nai) that decreases the depolarizing INCX thereby suppressing the action potential plateau and delaying the activation of slowly activating delayed rectifier K+ channels (IKs), suggesting important roles of INaL in regulating Nai.

CONCLUSIONS

Selective INaL blockade by GS967 prevents EADs and abolishes PVT in long QT syndrome type 2 rabbits by counterbalancing the reduced repolarization reserve and normalizing Nai. Graphic Abstract: A graphic abstract is available for this article.

Arrhythmias due to Inherited and Acquired Abnormalities of Ventricular Repolarization

Several acquired and congenital disease conditions and many cardiac and noncardiac drugs affect ventricular repolarization and increase susceptibility to ventricular arrhythmias. Abnormal ventricular repolarization can be reflected on the surface ECG by prolonged or shortened QT interval, early repolarization, and abnormal T-wave configuration. Reduced outward K+ currents and abnormal or increased sodium or calcium currents increase the vulnerability to ventricular arrhythmias. Multiple mechanisms give rise to ventricular arrhythmias in conditions of congenital or acquired abnormal ventricular repolarization. Ventricular arrhythmias associated with abnormalities of ventricular repolarization typically are rapid, usually polymorphic, ventricular tachycardia or torsades de pointes, often degenerating into ventricular fibrillation.

A Heterozygous Missense hERG Mutation Associated with Early Repolarization Syndrome

BACKGROUND/AIMS

Early repolarization syndrome (ERS) has been recently recognized as early repolarization pattern with idiopathic ventricular fibrillation. However, the genetic background of ERS has not been fully understood.

METHODS

A Chinese family with sudden cardiac death associated with ERS was investigated. Direct sequencing of ERS susceptibility genes was performed on the proband and family members. Whole-cell patch-clamp methods were used to characterize the mutant channel expressed in HEK 293 cells.

RESULTS

One missense mutation (p. K801T) was found in the hERG (KCNH2 gene) by the direct sequencing of candidate genes. Whole cell voltage clamp studies of the K801T mutation in HEK 293 cells demonstrated a 1.5-fold increase in maximum steady state current (37.2±7.3 vs 20.3±4.4 pA/pF) that occurred at a 20 mV more positive potential compared to the wild type channels. The voltage dependence of inactivation was significantly shifted in the positive voltage direction (WT -59.5±1.4 vs K801T -44.3±1.2 mV). Kinetic analysis revealed slower inactivation rates of K801T, but faster rates of activation and deactivation. The hERG channel blockers tested inhibited K801T-hERG channel in concentration response, and the potencies of these drugs can be rank-ordered as follows: quinidine> disopyramide> sotalol> flecainide.

CONCLUSION

Our study indicated that the K801T mutation caused the gain of function of hERG channels that may account for the clinical phenotype of ERS. Quinidine and disopyramide could improve the function of K801T-hERG mutant channel, and may be therapeutic options for patients with the K801T hERG mutation.

Detection of Variants in Patients with Idiopathic Ventricular Fibrillation by Whole-exome Sequencing

AIMS AND BACKGROUND

Idiopathic ventricular fibrillation (IVF) is a cause of sudden cardiac death (SCD). The frequency of mutations in disease-causing genes ranges, on average, between 16 and 48% in SCD cases. This study aimed to identify novel mutations in IVF patients without KCNQ1, KCNH2, and SCN5A mutations using whole-exome sequencing (WES).

METHODS

Genomic DNA extracted from peripheral blood samples obtained from five patients with IVF and WES was used to identify mutations associated with IVF. Candidate variants were validated by Sanger sequencing.

RESULTS

Four patients harbored suspected mutations in 100 inherited cardiomyopathy-and channelopathy-associated genes (e.g., TCAP, TTN, MYPN, CACNA1C, and TNNT2). All of these genetic variants have been given a dbSNP rs number; however, their clinical significance remains unknown. Bioinformatics tools predicted severe functional disruptions in the loci harboring these suspected mutations, suggesting their pivotal roles in IVF.

CONCLUSIONS

This study revealed the effectiveness of WES for IVF patients without KCNQ1, KCNH2, and SCN5A mutations. Although it is difficult to interpret broad WES results, the analysis can provide insight into the etiology of a heterogeneous disease.

A novel mutation of dipeptidyl aminopeptidase-like protein-6 in a family with suspicious idiopathic ventricular fibrillation

BACKGROUND

Sudden cardiac death (SCD) occurs in a broad spectrum of cardiac pathologies and is an important cause of mortality in the general population. Idiopathic ventricular fibrillation (IVF) is a rare but important factor resulting in SCD. It is diagnosed in a resuscitated cardiac arrest victim underlying unknown cause, with documented ventricular fibrillation. Previous studies have demonstrated that mutations in dipeptidyl aminopeptidase-like protein-6 (DPP6) and cardiac sodium channel Nav1.5 (SCN5A) are the most important genetic factors involve in IVF.

AIM

By using whole sequencing to identify the genetic lesion of a family with suspicious idiopathic ventricular fibrillation.

DESIGN

Prospective genetic study.

METHODS

In this study, we employed whole-exome sequencing in combination with arrhythmia-related gene filtering to identify the genetic lesion for a family suffering from suspicious IVF, syncope and SCD. We then generated the plasmids of DPP6-pcDNA3.1+ (WT and c.1578G>C/p.Q526H). Kv4.3-pcDNA3.1+ was co-transfected together with/without DPP6-pcDNA3.1+ (WT and/or c.1578G>C/p.Q526H) into HEK293 cells to perform the patch clamp experiments.

RESULTS

A novel missense mutation (c.1578G>C/p.Q526H) of DPP6 was identified and co-segregated with affected patients in this family. Patch clamp experiments suggested that this novel mutation might result in a gain of function and disturb the efflux of potassium ion.

CONCLUSION

Our study not only reported the second missense mutation of DPP6 in heart disease and expanded the spectrum of DPP6 mutations, but also contribute to the genetic diagnosis and counseling of families with suspicious IVF, syncope and SCD.

Characteristics and clinical assessment of unexplained sudden cardiac arrest in the real-world setting: focus on idiopathic ventricular fibrillation

Aims

Recent studies have shown that in more than half of apparently unexplained sudden cardiac arrests (SCA), a specific aetiology can be unmasked by a careful evaluation. The characteristics and the extent to which such cases undergo a systematic thorough investigation in real-life practice are unknown.

Methods and results

Data were analysed from an ongoing study, collecting all cases of out-of-hospital cardiac arrest in Paris area. Investigations performed during the index hospitalization or planned after discharge were gathered to evaluate the completeness of assessment of unexplained SCA. Between 2011 and 2016, among the 18 622 out-of-hospital cardiac arrests, 717 survivors (at hospital discharge) fulfilled the definition of cardiac SCA. Of those, 88 (12.3%) remained unexplained after electrocardiogram, echocardiography, and coronary angiography. Cardiac magnetic resonance imaging yielded the diagnosis in 25 (3.5%) cases, other investigations accounted for 14 (2.4%) additional diagnoses, and 49 (6.8%) patients were labelled as idiopathic ventricular fibrillation (IVF) (48.7 ± 15 years, 69.4% male). Among those labelled IVF, only 8 (16.3%) cases benefited from a complete workup (including pharmacological testing). Younger patients [odds ratio (OR) 6.00, 95% confidence interval (CI) 1.80-22.26] and those admitted to university centres (OR 3.60, 95% CI 1.12-12.45) were more thoroughly investigated. Genetic testing and family screening were initiated in only 9 (18.4%) and 12 (24.5%) cases, respectively.

Conclusion

Our findings suggest that complete investigations are carried out in a very low proportion of unexplained SCA. Standardized, systematic approaches need to be implemented to ensure that opportunities for specific therapies and preventive strategies (including relatives) are not missed.

IDENTIFYING NEW SUDDEN DEATH GENES

Inherited conditions that lead to cardiac arrhythmias and sudden cardiac death remain an important cause of morbidity and mortality. Identifying the genes responsible for these rare conditions can provide insights into the more common and heritable forms of sudden cardiac death seen in patients with structural heart disease. We and others have used candidate gene approaches and positional cloning in large families to show that mutations in ion channels and ion channel related proteins cause familial arrhythmia syndromes including long QT and Brugada syndromes. The genes responsible for many familial arrhythmia syndromes and the vast majority of the predisposition to common arrhythmias remain unknown. Using whole exome sequencing in families with Brugada syndrome and idiopathic ventricular fibrillation, we now seek to identify mutations in genes previously not thought to play a significant role in the heart.

Optical Mapping Approaches on Muscleblind-Like Compound Knockout Mice for Understanding Mechanistic Insights Into Ventricular Arrhythmias in Myotonic Dystrophy

Background

Cardiac arrhythmias are common causes of death in patients with myotonic dystrophy (dystrophia myotonica [DM]). Evidence shows that atrial tachyarrhythmia is an independent risk factor for sudden death; however, the relationship is unclear.

Methods and Results

Control wild‐type (Mbnl1^+/+; Mbnl2^+/+) and DM mutant (Mbnl1^−/−; Mbnl2^+/−) mice were generated by crossing double heterozygous knockout (Mbnl1^+/−; Mbnl2^+/−) mice. In vivo electrophysiological study and optical mapping technique were performed to investigate mechanisms of ventricular tachyarrhythmias. Transmission electron microscopy scanning was performed for myocardium ultrastructural analysis. DM mutant mice were more vulnerable to anesthesia medications and program electrical pacing: 2 of 12 mice had sudden apnea and cardiac arrest during premedication of general anesthesia; 9 of the remaining 10 had atrial tachycardia and/or atrioventricular block, but none of the wild‐type mice had spontaneous arrhythmias; and 9 of 10 mice had pacing‐induced ventricular tachyarrhythmias, but only 1 of 14 of the wild‐type mice. Optical mapping studies revealed prolonged action potential duration, slower conduction velocity, and steeper conduction velocity restitution curves in the DM mutant mice than in the wild‐type group. Spatially discordant alternans was more easily inducible in DM mutant than wild‐type mice. Transmission electron microscopy showed disarranged myofibrils with enlarged vacuole‐occupying mitochondria in the DM mutant group.

Conclusions

This DM mutant mouse model presented with clinical myofibril ultrastructural abnormality and cardiac arrhythmias, including atrial tachyarrhythmias, atrioventricular block, and ventricular tachyarrhythmias. Optical mapping studies revealed prolonged action potential duration and slow conduction velocity in the DM mice, leading to vulnerability of spatially discordant alternans and ventricular arrhythmia induction to pacing.

A Common Variant in SCN5A and the Risk of Ventricular Fibrillation Caused by First ST-Segment Elevation Myocardial Infarction

BACKGROUND

Several common genetic variants have been associated with either ventricular fibrillation (VF) or sudden cardiac death (SCD). However, replication efforts have been limited. Therefore, we aimed to analyze whether such variants may contribute to VF caused by first ST-elevation myocardial infarction (STEMI).

METHODS

We analyzed 27 single nucleotide polymorphisms (SNP) previously associated with SCD/VF in other cohorts, and examined whether these SNPs were associated with VF caused by first STEMI in the GEnetic causes of Ventricular Arrhythmias in patients with first ST-elevation Myocardial Infarction (GEVAMI) study on ethnical Danes. The GEVAMI study is a prospective case-control study involving 257 cases (STEMI with VF) and 537 controls (STEMI without VF).

RESULTS

Of the 27 candidate SNPs, one SNP (rs11720524) located in intron 1 of SCN5A which was previously associated with SCD was significantly associated with VF caused by first STEMI. The major C-allele of rs11720524 was present in 64% of the cases and the C/C genotype was significantly associated with VF with an odds ratio (OR) of 1.87 (95% CI: 1.12-3.12; P = 0.017). After controlling for clinical differences between cases and controls such as age, sex, family history of sudden death, alcohol consumption, previous atrial fibrillation, statin use, angina, culprit artery, and thrombolysis in myocardial infarction (TIMI) flow, the C/C genotype of rs11720524 was still significantly associated with VF with an OR of 1.9 (95% CI: 1.05-3.43; P = 0.032). Marginal associations with VF were also found for rs9388451 in HEY2 gene. The CC genotype showed an insignificant risk for VF with OR = 1.50 (95% CI: 0.96-2.40; P = 0.070).

CONCLUSION

One common intronic variant in SCN5A suggested an association with VF caused by first STEMI. Further studies into the functional abnormalities associated with the noncoding variant in SCN5A may lead to important insights into predisposition to VF during STEMI.

Variants in the SCN5A Promoter Associated With Various Arrhythmia Phenotypes

BACKGROUND

Mutations in the coding sequence of SCN5A, which encodes the cardiac Na(+) channel α subunit, have been associated with inherited susceptibility to various arrhythmias. Variable expression of SCN5A is a possible mechanism responsible for this pleiotropic effect; however, it is unknown whether variants in the promoter and regulatory regions of SCN5A also modulate the risk of arrhythmias.

METHODS AND RESULTS

We resequenced the core promoter region of SCN5A and the regulatory regions of SCN5A transcription in 1298 patients with arrhythmia phenotypes (atrial fibrillation, n=444; sinus node dysfunction, n=49; conduction disease, n=133; Brugada syndrome, n=583; and idiopathic ventricular fibrillation, n=89). We identified 26 novel rare variants in the SCN5A promoter in 29 patients affected by various arrhythmias (atrial fibrillation, n=6; sinus node dysfunction, n=1; conduction disease, n=3; Brugada syndrome, n=14; idiopathic ventricular fibrillation, n=5). The frequency of rare variants was higher in patients with arrhythmias than in controls. In the alignment with chromatin immunoprecipitation sequencing data, the majority of variants were located at regions bound by transcription factors. Using a luciferase reporter assay, 6 variants (Brugada syndrome, n=3; idiopathic ventricular fibrillation, n=2; conduction disease, n=1) were functionally characterized, and each displayed decreased promoter activity compared with the wild-type sequences. We also identified rare variants in the regulatory region that were associated with atrial fibrillation, and the variant decreased promoter activity.

CONCLUSIONS

Variants in the core promoter region and the transcription regulatory region of SCN5A were identified in multiple arrhythmia phenotypes, consistent with the idea that altered SCN5A transcription levels modulate susceptibility to arrhythmias.

Ventricular fibrillation and sudden cardiac death during myocardial infarction

In this PhD thesis, we report that VF is still a common complication of STEMI, with an incidence of 11.6% in the population of Danish STEMI patients who survive to reach the hospital. In this STEMI population, we identified several risk factors associated with VF independent of MI. We identified and confirmed findings from several previous studies and found several risk factors, such as younger age, a family history of sudden death, a TIMI flow grade of 0, the absence of angina, anterior infarction (i.e., VF before PPCI), and inferior infarction (i.e., VF during PPCI) that were associated with VF in a Danish cohort. Furthermore, a history of atrial fibrillation and alcohol intake were identified as novel risk factors for VF. To the best of our knowledge, this study contains data on the largest VF cohort with the longest reported follow-up published; we found that VF mortality is significantly higher within the first 30 days for patients who experience VF before and during PPCI compared with STEMI patients without VF. However, the long-term mortality rates of the three groups are the same. Importantly, our results contradict the previous understanding that VF during PPCI is "benign"; the mortality rate within the first 30 days was as high for patients with VF during PPCI as the mortality rate of patients with VF before PPCI. Finally, although it is difficult to draw clinical implications from a descriptive study, due to the comprehensiveness of Danish death certificates, we reported a high incidence of cardiac symptoms and contact with healthcare professionals based on cardiac symptoms in young SCD patients who died due to CAD, although death was not avoided.

A novel lamin A/C gene missense mutation (445 V > E) in immunoglobulin-like fold associated with left ventricular non-compaction

AIMS

Two LMNA mutations (R644C and R190W) have been associated with familial and sporadic left ventricular non-compaction (LVNC). However, the mechanisms underlying these associations have not been elucidated.

METHODS AND RESULTS

Genomic DNA was isolated from peripheral blood leucocytes and analysed by direct sequencing. Human embryonic kidney 293 cells were transfected with either wild type or mutant LMNA and SCN5A for whole-cell patch-clamp experiment and fluorescence microscopy. Point mutation modeling for mutant LMNA was also performed. One novel LVNC-associated mutation (V445E) in β2 sheet of immunoglobulin (Ig)-like fold was found in the proband and his father. We also found that the peak current of sodium channel was markedly reduced in mutant LMNA compared with WT while the activation, inactivation, and recovery curves were not significantly altered. The mutant lamin A/C were aggregated into multiple highlighted particles. Three β sheets and multiple side chains in Ig-like fold were altered due to the replacement of a valine by glutamic acid.

CONCLUSION

Our data associated a novel lamin A/C mutation (V445E) with a sudden death form of familial LVNC. The reduced sodium current in mutant LMNA may account for the advent of malignant ventricular arrhythmias. The altered structures of three β sheets and side chains may partially explain the aggregation of lamin A/C protein subjacent to the nuclear envelope.

Regional ion channel gene expression heterogeneity and ventricular fibrillation dynamics in human hearts

RATIONALE

Structural differences between ventricular regions may not be the sole determinant of local ventricular fibrillation (VF) dynamics and molecular remodeling may play a role.

OBJECTIVES

To define regional ion channel expression in myopathic hearts compared to normal hearts, and correlate expression to regional VF dynamics.

METHODS AND RESULTS

High throughput real-time RT-PCR was used to quantify the expression patterns of 84 ion-channel, calcium cycling, connexin and related gene transcripts from sites in the LV, septum, and RV in 8 patients undergoing transplantation. An additional eight non-diseased donor human hearts served as controls. To relate local ion channel expression change to VF dynamics localized VF mapping was performed on the explanted myopathic hearts right adjacent to sampled regions. Compared to non-diseased ventricles, significant differences (p<0.05) were identified in the expression of 23 genes in the myopathic LV and 32 genes in the myopathic RV. Within the myopathic hearts significant regional (LV vs septum vs RV) expression differences were observed for 13 subunits: Nav1.1, Cx43, Ca3.1, Cavα2δ2, Cavβ2, HCN2, Na/K ATPase-1, CASQ1, CASQ2, RYR2, Kir2.3, Kir3.4, SUR2 (p<0.05). In a subset of genes we demonstrated differences in protein expression between control and myopathic hearts, which were concordant with the mRNA expression profiles for these genes. Variability in the expression of Cx43, hERG, Na(+)/K(+) ATPase ß1 and Kir2.1 correlated to variability in local VF dynamics (p<0.001). To better understand the contribution of multiple ion channel changes on VF frequency, simulations of a human myocyte model were conducted. These simulations demonstrated the complex nature by which VF dynamics are regulated when multi-channel changes are occurring simultaneously, compared to known linear relationships.

CONCLUSIONS

Ion channel expression profile in myopathic human hearts is significantly altered compared to normal hearts. Multi-channel ion changes influence VF dynamic in a complex manner not predicted by known single channel linear relationships.

Poor prognosis of rare sarcomeric gene variants in patients with dilated cardiomyopathy

BACKGROUND

In dilated cardiomyopathy (DCM), the clinical and prognostic implications of rare variants in sarcomeric genes remain poorly understood. To address this question, we analyzed the outcome of rare sarcomeric gene variants in patients enrolled in our Familial Cardiomyopathy Registry.

METHODS

DCM families harboring rare sarcomeric variants in MYH6, MYH7, MYBPC3, TNNT2, and TTN were identified. Genotype-phenotype association analysis was performed, and long-term survival-free from death or heart transplant was compared between carriers and noncarriers.

RESULTS

We found 24 rare variants (3 in MYH6, 3 in MYH7, 3 in MYBPC3, 2 in TNNT2, and 13 in TTN) affecting 52 subjects in 25 families. The phenotypes of variant carriers were severe (3 sudden deaths, 6 heart failure deaths, 8 heart transplants, 2 ventricular fibrillations). There was no difference in the overall long-term survival between carriers and the 33 noncarriers (p = 0.322). However after 50 years of age, the combined endpoint of death or transplant was decreased in carriers as compared to noncarriers (p = 0.026).

CONCLUSIONS

Patients with DCM carrying rare variants in sarcomeric genes manifest a poorer prognosis as compared to noncarriers after the age of 50 years. These data further support the role of genetic testing in DCM for risk stratification.

Genetic mutation in Korean patients of sudden cardiac arrest as a surrogating marker of idiopathic ventricular arrhythmia

Mutation or common intronic variants in cardiac ion channel genes have been suggested to be associated with sudden cardiac death caused by idiopathic ventricular tachyarrhythmia. This study aimed to find mutations in cardiac ion channel genes of Korean sudden cardiac arrest patients with structurally normal heart and to verify association between common genetic variation in cardiac ion channel and sudden cardiac arrest by idiopathic ventricular tachyarrhythmia in Koreans. Study participants were Korean survivors of sudden cardiac arrest caused by idiopathic ventricular tachycardia or fibrillation. All coding exons of the SCN5A, KCNQ1, and KCNH2 genes were analyzed by Sanger sequencing. Fifteen survivors of sudden cardiac arrest were included. Three male patients had mutations in SCN5A gene and none in KCNQ1 and KCNH2 genes. Intronic variant (rs2283222) in KCNQ1 gene showed significant association with sudden cardiac arrest (OR 4.05). Four male sudden cardiac arrest survivors had intronic variant (rs11720524) in SCN5A gene. None of female survivors of sudden cardiac arrest had SCN5A gene mutations despite similar frequencies of intronic variants between males and females in 55 normal controls. Common intronic variant in KCNQ1 gene is associated with sudden cardiac arrest caused by idiopathic ventricular tachyarrhythmia in Koreans.

A nonsynonymous polymorphism in semaphorin 3A as a risk factor for human unexplained cardiac arrest with documented ventricular fibrillation

Unexplained cardiac arrest (UCA) with documented ventricular fibrillation (VF) is a major cause of sudden cardiac death. Abnormal sympathetic innervations have been shown to be a trigger of ventricular fibrillation. Further, adequate expression of SEMA3A was reported to be critical for normal patterning of cardiac sympathetic innervation. We investigated the relevance of the semaphorin 3A (SEMA3A) gene located at chromosome 5 in the etiology of UCA. Eighty-three Japanese patients diagnosed with UCA and 2,958 healthy controls from two different geographic regions in Japan were enrolled. A nonsynonymous polymorphism (I334V, rs138694505A>G) in exon 10 of the SEMA3A gene identified through resequencing was significantly associated with UCA (combined P = 0.0004, OR 3.08, 95%CI 1.67-5.7). Overall, 15.7% of UCA patients carried the risk genotype G, whereas only 5.6% did in controls. In patients with SEMA3A(I334V), VF predominantly occurred at rest during the night. They showed sinus bradycardia, and their RR intervals on the 12-lead electrocardiography tended to be longer than those in patients without SEMA3A(I334V) (1031±111 ms versus 932±182 ms, P = 0.039). Immunofluorescence staining of cardiac biopsy specimens revealed that sympathetic nerves, which are absent in the subendocardial layer in normal hearts, extended to the subendocardial layer only in patients with SEMA3A(I334V). Functional analyses revealed that the axon-repelling and axon-collapsing activities of mutant SEMA3A(I334V) genes were significantly weaker than those of wild-type SEMA3A genes. A high incidence of SEMA3A(I334V) in UCA patients and inappropriate innervation patterning in their hearts implicate involvement of the SEMA3A gene in the pathogenesis of UCA.

A common missense variant in the neuregulin 1 gene is associated with both schizophrenia and sudden cardiac death

BACKGROUND

Both schizophrenia and epilepsy have been linked to increased risk of sudden cardiac death (SCD). We hypothesized that DNA variants within genes previously associated with schizophrenia and epilepsy may contribute to an increased risk of SCD.

OBJECTIVE

To investigate the contribution to SCD susceptibility of DNA variants previously implicated in schizophrenia and epilepsy.

METHODS

From the ongoing Oregon Sudden Unexpected Death Study, comparisons were performed among 340 SCD cases presenting with ventricular fibrillation and 342 controls. We tested for the association between 17 single-nucleotide polymorphisms (SNPs) mapped to 14 loci previously implicated in schizophrenia and epilepsy by using logistic regression and assuming additive, dominant, and recessive genetic models.

RESULTS

The minor allele of the nonsynonymous SNP rs10503929 within the neuregulin 1 gene was associated with SCD under all 3 investigated models, with the strongest association for the recessive genetic model (recessive P = 4.01 × 10(-5), odds ratio [OR] 4.04; additive P = 2.84 × 10(-7), OR 1.9; and dominant P = 9.01 × 10(-6), OR 2.06). To validate our findings, we further explored the association of this variant in the Harvard Cohort SCD study. The SNP rs10503929 was associated with an increased risk of SCD under the recessive genetic model (P = .0005, OR 2.7). This missense variation causes a methionine to threonine change and functional effects are currently unknown.

CONCLUSIONS

The observed association between a schizophrenia-related neuregulin 1 gene variant and SCD may represent the first evidence of coexisting genetic susceptibility between 2 conditions that have an established clinical overlap. Further investigation is warranted to explore the molecular mechanisms of this variant in the pathogenesis of SCD.

SNPs identified as modulators of ECG traits in the general population do not markedly affect ECG traits during acute myocardial infarction nor ventricular fibrillation risk in this condition

Background

Ventricular fibrillation (VF) in the setting of acute ST elevation myocardial infarction (STEMI) is a leading cause of mortality. Although the risk of VF has a genetic component, the underlying genetic factors are largely unknown. Since heart rate and ECG intervals of conduction and repolarization during acute STEMI differ between patients who do and patients who do not develop VF, we investigated whether SNPs known to modulate these ECG indices in the general population also impact on the respective ECG indices during STEMI and on the risk of VF.

Methods and Results

The study population consisted of participants of the Arrhythmia Genetics in the NEtherlandS (AGNES) study, which enrols patients with a first STEMI that develop VF (cases) and patients that do not develop VF (controls). SNPs known to impact on RR interval, PR interval, QRS duration or QTc interval in the general population were tested for effects on the respective STEMI ECG indices (stage 1). Only those showing a (suggestive) significant association were tested for association with VF (stage 2). On average, VF cases had a shorter RR and a longer QTc interval compared to non-VF controls. Eight SNPs showed a trend for association with the respective STEMI ECG indices. Of these, three were also suggestively associated with VF.

Conclusions

RR interval and ECG indices of conduction and repolarization during acute STEMI differ between patients who develop VF and patients who do not. Although the effects of the SNPs on ECG indices during an acute STEMI seem to be similar in magnitude and direction as those found in the general population, the effects, at least in isolation, are too small to explain the differences in ECGs between cases and controls and to determine risk of VF.

Ventricular arrhythmias and sudden cardiac death

Management strategies for ventricular arrhythmias are guided by the risk of sudden death and severity of symptoms. Patients with a substantial risk of sudden death usually need an implantable cardioverter defibrillator (ICD). Although ICDs effectively end most episodes of ventricular tachycardia or ventricular fibrillation and decrease mortality in specific populations of patients, they have inherent risks and limitations. Generally, antiarrhythmic drugs do not provide sufficient protection from sudden death, but do have a role in reducing arrhythmias that cause symptoms. Catheter ablation is likewise important for reducing the frequency of spontaneous arrhythmias and is curative for some patients, usually those with idiopathic arrhythmias and no heart disease. Arrhythmia surgery is now infrequent, offered by only a few specialised centres for refractory arrhythmias. Advances in understanding of genetic arrhythmia syndromes and in technology for mapping and ablation of ventricular arrhythmias, and enhanced algorithms in implantable devices for rhythm management, have contributed to improved outcomes.

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.

Genetics of sudden cardiac death syndromes

PURPOSE OF REVIEW

To survey recent developments in the field of genetics encompassing discovery of new candidate genes, new diagnostic strategies, and new therapies for sudden cardiac death (SCD) syndromes.

RECENT FINDINGS

In addition to new mutations in known SCD genes, several novel genes not previously implicated in SCD causation have been found, particularly in long QT syndrome (e.g., KCNJ5, AKAP9, SNTA1), idiopathic ventricular fibrillation (e.g., DPP6, KCNJ8), dilated cardiomyopathy (e.g., NEBL), and hypertrophic cardiomyopathy (HCM; e.g., NEXN). Genetic SCD animal models have provided novel insights into the cellular mechanism and pathogenesis of nearly all the major SCD syndromes, which has led to several new drug therapies for patients with genetic arrhythmia syndromes (e.g., flecainide in catecholaminergic polymorphic ventricular tachycardia). Furthermore, genetic contributions to acquired heart diseases are increasingly being recognized. For example, a 21q21 locus is strongly associated with ventricular fibrillation after myocardial infarction. Near this locus is CXADR, a gene encoding a viral receptor implicated in myocarditis and dilated cardiomyopathy. Finally, common variants in cardiac ion channels and proteins likely contribute to common cardiac phenotypes.

SUMMARY

Major strides have been made in uncovering new genes, mechanisms, and syndromes that have significantly advanced the diagnosis and treatment of genetic SCD disorders.

[Premature sudden death--consider serious familial heart rhythm disturbances]

We describe 3 patients from a region in the centre of the Netherlands with several relatives who died prematurely from sudden cardiac arrest. These premature deaths appeared to be caused by a unique familial sudden death syndrome. These patients and their relatives did not present any distinguishable signs, symptoms or abnormalities on further examinations apart from premature cardiac arrest occurring in about 50% of the affected family members before the age of 60 years. Genetic analysis appeared to be the only means to identify family members at risk, carrying lethal changes in their DNA that presumably involve the DPP6-gene. Patients who survive a premature sudden cardiac arrest and relatives of patients who died prematurely from sudden cardiac arrest should be referred to a cardiogenetics outpatient clinic. Timely recognition of persons affected allows appropriate treatment and may implicate an implantable cardioverter defibrillator.

[Malignant form of familial hypertrophic cardiomyopathy complicated with ventricular fibrillation in siblings. Electrocardiogram in hypertrophic cardiomyopathy - a review]

A family with hypertrophic cardiomyopathy (HCM) (15-year-old boy, his 17-year-old sister and 45-year-old father) is described. The first sign of HCM was cardiac arrest (CA) due to ventricular fibrillation (VF) in a boy. A few months later sister of proband had the episode of CA due to VF. Both had implanted cardioverter-defibrillator (ICD). Echocardiography revealed HCM in both cases and in a father, who also received prophylactic ICD. During 24-month follow-up fast VT was observed in sister of proband. We review electrocardiographic changes in HCM and their correlation with magnetic resonance.

[Ventricular fibrillation in acute myocardial infarction - do genes play a role?]

We present two patients with ventricular fibrillation (VF) during acute myocardial infarction (AMI). First patient had torsade de pointes ventricular tachycardia episodes degenerating into VF 10 days after AMI treated with primary angioplasty. Second patient had multiple episodes of VF during the first day of AMI. He showed ST-segment elevation resembling Brugada pattern. We hypothesise that shape of ST-segment elevation during AMI might be important in assessing risk for VF. We propose that such 'arrhythmogenic' ST elevation might result more from systolic rather than diastolic current of injury during AMI. We discuss genetic predispositions (latent channellopathies) for VF during AMI.

Electrical heart disease: Genetic and molecular basis of cardiac arrhythmias in normal structural hearts

Purely electrical heart diseases, defined by the absence of any structural cardiac defects, are responsible for a large number of sudden, unexpected deaths in otherwise healthy, young individuals. These conditions include the long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia and the short QT syndrome. Collectively, these conditions have been referred to as channelopathies. Ion channels provide the molecular basis for cardiac electrical activity. These channels have specific ion selectivity and are responsible for the precise and timely regulation of the passage of charged ions across the cell membrane in myocytes, and the summation of their activity in cardiac muscle defines the surface electrocardiogram. Impairment in the flow of these ions in heart cells may mean the difference between a normal, prosperous life and the tragedy of a sudden, unexpected death due to ventricular arrhythmia. The present paper reviews the current clinical and molecular understanding of the electrical diseases of the heart associated with sudden cardiac death.

Novel mutation in the SCN5A gene associated with arrhythmic storm development during acute myocardial infarction

BACKGROUND

Ventricular tachycardia (VT) and ventricular fibrillation (VF) complicating Brugada syndrome, a genetic disorder linked to SCN5A mutations, and VF complicating acute myocardial infarction (AMI) both have been linked to phase 2 reentry.

OBJECTIVE

Given the mechanistic similarities in arrhythmogenesis, the purpose of this study was to examine the contribution of SCN5A mutations to VT/VF complicating AMI.

METHODS

Nineteen consecutive patients developing VF during AMI were enrolled in the study. Wild-type (WT) and mutant SCN5A genes were coexpressed with SCN1B in TSA201 cells and studied using whole-cell patch clamp techniques.

RESULTS

Among the cohort of 19 patients, one missense mutation (G400A) in SCN5A was detected in a conserved region. An H558R polymorphism was detected on the same allele. Unlike the other 18 patients, who each developed 1-2 VF episodes during AMI, the mutation carrier developed six episodes of VT/VF within the first 12 hours. All VT/VF episodes were associated with ST-segment changes and were initiated by short-coupled extrasystoles. Flecainide and adenosine challenge performed to unmask Brugada and long QT syndromes both were negative. Peak G400A and G400A+H558R current were 70.7% and 88.4% less than WT current at -35 mV (P </=.001). G400A current decay was accelerated and steady-state inactivation was shifted -6.39 mV (V(1/2) = -98.9 +/- 0.1 mV vs -92.5 +/- 0.1 mV, P </=.001). No mutations were detected in KCNH2, KCNQ1, KCNE1, or KCNE2 in the G400A patient.

CONCLUSION

We describe the first sodium channel mutation to be associated with the development of an arrhythmic storm during acute ischemia. These findings suggest that a loss of function in SCN5A may predispose to ischemia-induced arrhythmic storm.

Loss-of-function mutations in the cardiac calcium channel underlie a new clinical entity characterized by ST-segment elevation, short QT intervals, and sudden cardiac death

BACKGROUND

Cardiac ion channelopathies are responsible for an ever-increasing number and diversity of familial cardiac arrhythmia syndromes. We describe a new clinical entity that consists of an ST-segment elevation in the right precordial ECG leads, a shorter-than-normal QT interval, and a history of sudden cardiac death.

METHODS AND RESULTS

Eighty-two consecutive probands with Brugada syndrome were screened for ion channel gene mutations with direct sequencing. Site-directed mutagenesis was performed, and CHO-K1 cells were cotransfected with cDNAs encoding wild-type or mutant CACNB2b (Ca(v beta2b)), CACNA2D1 (Ca(v alpha2delta1)), and CACNA1C tagged with enhanced yellow fluorescent protein (Ca(v)1.2). Whole-cell patch-clamp studies were performed after 48 to 72 hours. Three probands displaying ST-segment elevation and corrected QT intervals < or = 360 ms had mutations in genes encoding the cardiac L-type calcium channel. Corrected QT ranged from 330 to 370 ms among probands and clinically affected family members. Rate adaptation of QT interval was reduced. Quinidine normalized the QT interval and prevented stimulation-induced ventricular tachycardia. Genetic and heterologous expression studies revealed loss-of-function missense mutations in CACNA1C (A39V and G490R) and CACNB2 (S481L) encoding the alpha1- and beta2b-subunits of the L-type calcium channel. Confocal microscopy revealed a defect in trafficking of A39V Ca(v)1.2 channels but normal trafficking of channels containing G490R Ca(v)1.2 or S481L Ca(v beta2b)-subunits.

CONCLUSIONS

This is the first report of loss-of-function mutations in genes encoding the cardiac L-type calcium channel to be associated with a familial sudden cardiac death syndrome in which a Brugada syndrome phenotype is combined with shorter-than-normal QT intervals.

Genetic and biophysical basis for bupivacaine-induced ST segment elevation and VT/VF. Anesthesia unmasked Brugada syndrome

BACKGROUND

Brugada syndrome is an inherited disease associated with sudden cardiac death. The electrocardiographic pattern associated with Brugada syndrome has been linked to the use of sodium channel blockers, including antiarrhythmics, trycyclics and anesthetics.

OBJECTIVE

We report a case of bupivacaine-induced Brugada syndrome, in which we investigated the genetic, biophysical and path physiological mechanism involved.

METHODS AND RESULTS

The patient developed a Brugada-like electrocardiographic pattern twice under the influence of bupivacaine. The first occurrence was accompanied by ventricular tachycardia (VT) which subsided after withdrawal of the anesthetic. The VT was also observed during co-administration of diltiazem and isosorbide-5-mononitrate, agents thought to facilitate ST segment elevation in the Brugada syndrome. Genetic analysis revealed a missense mutation in the alpha subunit of the cardiac sodium channel, SCN5A. Biophysical analysis by whole-cell patch-clamping revealed a reduction in sodium current as a result of the mutation. The study of bupivacaine in the wedge model revealed use-dependent changes in conduction, heterogeneous loss of the action potential dome in RV epicardium and phase 2 re-entry when the preparations were pretreated with low concentrations of the calcium channel blocker verapamil.

CONCLUSION

Our findings indicate that bupivacaine may induce the electrocardiographic and arrhythmic manifestations of the Brugada syndrome in silent carriers of SCN5A mutations. The data have important implications in the management of patients who develop ST segment elevation when under the influence of anesthetics such as bupivacaine.

A novel SCN5A mutation, F1344S, identified in a patient with Brugada syndrome and fever-induced ventricular fibrillation

OBJECTIVE

Brugada syndrome (BS) is an inherited electrical cardiac disorder characterized by right bundle branch block pattern and ST segment elevation in leads V1 to V3 on surface electrocardiogram that can potentially lead to malignant ventricular tachycardia and sudden cardiac death. About 20% of patients have mutations in the only so far identified gene, SCN5A, which encodes the alpha-subunit of the human cardiac voltage-dependent sodium channel (hNa(v)1.5). Fever has been shown to unmask or trigger the BS phenotype, but the associated molecular and the biophysical mechanisms are still poorly understood. We report on the identification and biophysical characterization of a novel heterozygous missense mutation in SCN5A, F1344S, in a 42-year-old male patient showing the BS phenotype leading to ventricular fibrillation during fever.

METHODS

The mutation was reproduced in vitro using site-directed mutagenesis and characterized using the patch clamp technique in the whole-cell configuration.

RESULTS

The biophysical characterization of the channels carrying the F1344S mutation revealed a 10 mV mid-point shift of the G/V curve toward more positive voltages during activation. Raising the temperature to 40.5 degrees C further shifted the mid-point activation by 18 mV and significantly changed the slope factor in Na(v)1.5/F1344S mutant channels from -6.49 to -10.27 mV.

CONCLUSIONS

Our findings indicate for the first time that the shift in activation and change in the slope factor at a higher temperature mimicking fever could reduce sodium currents' amplitude and trigger the manifestation of the BS phenotype.

Right ventricular fibrosis and conduction delay in a patient with clinical signs of Brugada syndrome: a combined electrophysiological, genetic, histopathologic, and computational study

BACKGROUND

The mechanism of ECG changes and arrhythmogenesis in Brugada syndrome (BS) patients is unknown.

METHODS AND RESULTS

A BS patient without clinically detected cardiac structural abnormalities underwent cardiac transplantation for intolerable numbers of implantable cardioverter/defibrillator discharges. The patient's explanted heart was studied electrophysiologically and histopathologically. Whole-cell currents were measured in HEK293 cells expressing wild-type or mutated sodium channels from the patient. The right ventricular outflow tract (RVOT) endocardium showed activation slowing and was the origin of ventricular fibrillation without a transmural repolarization gradient. Conduction restitution was abnormal in the RVOT but normal in the left ventricle. Right ventricular hypertrophy and fibrosis with epicardial fatty infiltration were present. HEK293 cells expressing a G1935S mutation in the gene encoding the cardiac sodium channel exhibited enhanced slow inactivation compared with wild-type channels. Computer simulations demonstrated that conduction slowing in the RVOT might have been the cause of the ECG changes.

CONCLUSIONS

In this patient with BS, conduction slowing based on interstitial fibrosis, but not transmural repolarization differences, caused the ECG signs and was the origin of ventricular fibrillation.

Spectrum and prevalence of cardiac ryanodine receptor (RyR2) mutations in a cohort of unrelated patients referred explicitly for long QT syndrome genetic testing

BACKGROUND

Mutations in the RyR2-encoded cardiac ryanodine receptor/calcium release channel cause type 1 catecholaminergic polymorphic ventricular tachycardia (CPVT1).

OBJECTIVES

Because CPVT and concealed long QT syndrome (LQTS) phenotypically mimic one other, we sought to determine the spectrum and prevalence of RyR2 mutations in a cohort of unrelated patients who were referred specifically for LQTS genetic testing.

METHODS

Using denaturing high-performance liquid chromatography and direct DNA sequencing, targeted mutational analysis of 23 RyR2 exons previously implicated in CPVT1 was performed on genomic DNA from 269 unrelated patients (180 females, average age at diagnosis 24 +/- 17 years) who were referred to Mayo Clinic's Sudden Death Genomics Laboratory for LQTS genetic testing. Previously, comprehensive mutational analysis of the five LQTS-associated cardiac channel genes proved negative for this entire subset of patients now designated as "genotype-negative" LQTS referrals.

RESULTS

Fifteen distinct RyR2 mutations (14 missense, 1 duplication/insertion, 12 novel) were found in 17 (6.3%) of 269 patients. None of these mutations were present in 400 reference alleles. Two mutations localized to the calstabin-2 (FKBP12.6) binding domain. Upon review of the clinical records, the referral diagnosis for all 17 patients was "atypical" or "borderline" LQTS.

CONCLUSION

Putative pathogenic CPVT1-causing mutations in RyR2 were detected in 6% of unrelated, genotype-negative LQTS referrals. These findings suggest that CPVT may be underrecognized among physicians referring patients because of a suspected channelopathy. A diagnosis of "atypical LQTS" may warrant consideration of CPVT and analysis of RyR2 if the standard cardiac channel gene screen for LQTS is negative.