Expression of skeletal muscle sodium channel (Nav1.4) or connexin32 prevents reperfusion arrhythmias in murine heart

AIMS

acute myocardial ischaemia induces a decrease in resting membrane potential [which leads to reduction of action potential (AP) V(max)] and intracellular acidification (which closes gap junctions). Both contribute to conduction slowing. We hypothesized that ventricular expression of the skeletal muscle Na(+) channel, Nav1.4 (which activates fully at low membrane potentials), or connexin32 (Cx32, which is less pH-sensitive than connexin43) would support conduction and be antiarrhythmic. We tested this hypothesis in a murine model of ischaemia and reperfusion arrhythmias.

METHODS AND RESULTS

empty adenovirus (Sham) or adenoviral constructs expressing either SkM1 (gene encoding Nav1.4) or Cx32 genes were injected into the left ventricular wall. Four days later, ventricular tachycardia (VT) occurred during reperfusion following a 5 min coronary occlusion. In Nav1.4- and Cx32-expressing mice, VT incidence and duration were lower than in Sham (P < 0.05). In vitro multisite microelectrode mapping was performed in the superfused right ventricular wall. To simulate ischaemic conditions, [K(+)] in solution was increased to 10 mmol/L and/or pH was decreased to 6.0. Western blots revealed Cx32 and Nav1.4 expression in both ventricles. Nav1.4 APs showed higher V(max) and conduction velocity (CV) than Shams at normal and elevated [K(+)]. Exposure of tissue to acid solution reduced intracellular pH to 6.4. There was no difference in CV between Sham and Cx32 groups in control solution. Acid solution slowed CV in Sham (P < 0.05) but not in Cx32.

CONCLUSION

Nav1.4 or Cx32 expression preserved normal conduction in murine hearts and decreased the incidence of reperfusion VT.

Beta2-adrenoceptor gene variant Arg16Gly is associated with idiopathic ventricular outflow-tract tachycardia

BACKGROUND

Imbalance of the sympathetic nervous system was involved in the pathogenesis of idiopathic ventricular outflow-tract tachycardia (IVOT). We aimed to investigate whether the major genetic variants in β(1)- and β(2)-adrenoceptors and GNB3 C825T were associated with IVOT and verapamil sensitive idiopathic left ventricular tachycardia (ILVT).

METHODS

Patients with IVOT and ILVT from December 2005 to December 2007 were consecutively enrolled into this study. Controls were randomly selected from the community-based inhabitants. Five genetic variants, Ser49Gly and Gly389Arg in the β(1)-adrenoceptor, Arg16Gly and Gln27Glu in the β(2)-adrenoceptor and GNB3 C825T, were genotyped by polymerase chain reaction-restriction fragment length polymorphism analysis.

RESULTS

A total of 227 patients with IVOT and 110 patients with ILVT were included. Genotyping revealed that the 16Gly allele of Arg16Gly variant of β(2)-adrenoceptor was associated with a higher risk of IVOT (OR: 1.40, 95%CI: 1.12 - 1.75, P = 0.003 in the addictive model and OR: 1.62, 95%CI: 1.14 - 2.31, P = 0.007 in the dominant model). Patients with Gly16Gln27 haplotype also had a higher risk of IVOT (OR: 1.38, 95%CI: 1.11 - 1.73, P = 0.012). Other four variants, including Ser49Gly and Arg389Gly in β(1)-adrenoceptor, Gln27Glu in β(2)-adrenoceptor and GNB3 C825T, did not differ between patients with IVOT and controls. In patients with ILVT, no significant difference was found in these five variants compared with controls.

CONCLUSIONS

Arg16Gly in β(2)-adrenoceptor is significantly associated with IVOT in Chinese Han population. Major genetic variants in β(1)- and β(2)-adrenoceptor and GNB3 C825T may not be associated with ILVT. These data suggest a different arrhythmogenic mechanism in IVOT and ILVT.

Primary electrical diseases diagnosis, genetic and management

Primary electrical diseases or channelopathies are inherited genetic alterations of the cell ionic and electrical behavior leading to various cardiac arrhythmias carrying the risk of sudden death. A descriptive review of the successively described channelopathies is made in this article, with emphasis on the clinical manifestations, the genetic background and the currently accepted therapeutic options.

Ryanodine receptor channelopathies

Ryanodine receptors (RyR) are intracellular Ca2+-permeable channels that provide the sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contractions. RyR1 underlies skeletal muscle contraction, and RyR2 fulfills this role in cardiac muscle. Over the past 20 years, numerous mutations in both RyR isoforms have been identified and linked to skeletal and cardiac diseases. Malignant hyperthermia, central core disease, and catecholaminergic polymorphic ventricular tachycardia have been genetically linked to mutations in either RyR1 or RyR2. Thus, RyR channelopathies are both of interest because they cause significant human diseases and provide model systems that can be studied to elucidate important structure-function relationships of these ion channels.

Purkinje cell calcium dysregulation is the cellular mechanism that underlies catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Inherited arrhythmias can be caused by mutations in the cardiac ryanodine receptor (RyR2). The cellular source of these arrhythmias is unknown. Isolated RyR2(R4496C) mouse ventricular myocytes display arrhythmogenic activity related to spontaneous Ca(2+) release during diastole. On the other hand, recent whole-heart epicardial and endocardial optical mapping data demonstrate that ventricular arrhythmias in the RyR2(R4496C) mouse model of catecholaminergic polymorphic ventricular tachycardia (CPVT) originate in the His-Purkinje system, suggesting that Purkinje cells, and not ventricular myocytes, may be the cellular source of arrhythmogenic activity. The relative effect of the RyR2(R4496C) mutation on calcium homeostasis in ventricular myocytes versus Purkinje cells is unknown.

OBJECTIVE

This study sought to determine which cardiac cell type is more severely affected, in terms of calcium handling, by expression of the RyR2(R4496C) mutant channel: the ventricular myocytes or the Purkinje cells.

METHODS AND RESULTS

To discriminate Purkinje cells from ventricular myocytes, we crossed the RyR2(R4496C) mouse model of CPVT with the Cx40(EGFP/+) transgenic mouse. This genetic cross yields Purkinje cells that express eGFP, and therefore fluoresce green when excited by the appropriate wavelength; ventricular myocytes, which do not express connexin 40, are not green. Intracellular calcium was measured in each cell type using calcium-sensitive probes. Purkinje cells of the RyR2(R4496C) mouse model of CPVT show an approximately 2x greater rate (P < .05) and approximately 2x to 3x greater amplitude (P < .000001) of spontaneous calcium release events than ventricular myocytes isolated from the same heart.

CONCLUSION

These results demonstrate that focally activated arrhythmias originate in the specialized electrical conducting cells of the His-Purkinje system in the RyR2(R4496C) mouse model of CPVT.

[Postmortem genetic testing in sudden cardiac death due to ion channelopathies]

Sudden cardiac death accounts for majority of deaths in human. Evident cardiac lesions that may explain the cause of death can be detected in comprehensive postmortem investigation in most sudden cardiac death. However, no cardiac morphological abnormality is found in a considerable number of cases although the death is highly suspected from cardiac anomaly. With the advances in the modern molecular biology techniques, it has been discovered that many of these sudden deaths are caused by congenital ion channelopathies in myocardial cell, i.e., Brugada syndrome, long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and short QT syndrome, etc. This article presents the molecular genetics, electrocardiographic abnormalities, clinical manifestations, and mechanisms leading to sudden cardiac death with emphasis on the role of postmortem genetic testing in certification of cause of death. It may provide helpful information in investigating sudden cardiac death due to ion channelopathies in medico-legal practice.

Comparison of Uhl's anomaly, right ventricular outflow tract ventricular tachycardia (RVOT VT) & arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) with an insight into genetics of ARVD/C

Among the right ventricular conditions, Uhl's anomaly, arrhythmogenic right ventricular dysplasia / cardiomyopathy (ARVD/C) and right ventricular outflow tract ventricular tachycardia (RVOT VT) are disorders that exhibit pathogenic changes involving the right ventricular (RV) myocardium, and are expected to be severe or milder forms of the same condition. The review focuses on the aspect whether the three RV disorders are a spectrum of the same disease. ARVD/C is the only condition among these to be genetically well characterized. Also, variations in the clinical expression of ARVD/C due to the genetic heterogeneity are examined. Based on clinical manifestations, age at onset, gender ratio and the possible molecular mechanisms implicated, Uhl's anomaly, ARVD/C and RVOT VT may be considered as separate entities. Further, to differentiate between the three RV disorders, the molecular studies on ARVD/C might be helpful. An attempt was made to differentiate between the eleven different types of ARVD/Cs based on clinical symptoms presented including the progression of the disease to the left ventricle, ventricular arrhythmias and clinical characteristics like ECG, SAECG, ECHO and histopathological studies.

Sudden cardiac death in young competitive athletes due to genetic cardiac abnormalities

Sudden cardiac death (SCD) in young athletes is generally caused by inherited cardiac disorders. While these events are relatively few compared to other cardiac deaths, they are tragic in that death occurs in a young, otherwise healthy person. The genetic abnormalities most associated with SCD are hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, long QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. As a result of growing awareness that these deaths can be prevented, guidelines have been issued in both Europe and the United States to help screen and determine qualification for young persons who want to participate in competitive athletics. There remains debate on the how extensive screening should be, in particular over the use of the 12-lead electrocardiogram (ECG), with European guidelines mandating ECG and United States guidelines not recommending routine use of the ECG.

[Genetic of catecholaminergic polymorphic ventricular tachycardia: basic concepts]

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a cardiac channelopathy characterized by altered intracellular calcium handling resulting in ventricular arrhythmias and high risk of cardiac sudden death in young cases with normal structural hearts. Patients present with exertional syncope and the trademark dysrhythmia is polymorphic and/or bidirectional ventricular tachycardia during exercise or adrenergic stimulation. Early detection of CPVT is crucial because opportune medical intervention prevents sudden cardiac death. Mutations in the ryanodine receptor RYR2 explain nearly 70% of the CPVT cases and cause the autosomic dominant form of the disease. Mutations in calsequestrin 2 causes a recessive form and explain less than 5% of all cases. Genetic screening in CPVT, besides providing early detection of asymptomatic carriers at risk, has provided important insights in the mechanism underlying the disease. Mutational analysis of RYR2 has been a challenge due to the large size of the gene, 105 exons encoded for 4,967 amino-acids. In this review we analyze general concepts of the disease, differential diagnosis and strategies for genetic screening.

Current review of Brugada syndrome: from epidemiology to treatment

Brugada syndrome is a genetic cause of sudden cardiac arrest characterized by abnormal electrocardiographic (ECG) pattern in the right precordial leads either at rest or after provocation. In this condition, sudden death may occur due to polymorphic ventricular tachycardia or ventricular fibrillation. In approximately 30% of patients, sudden cardiac arrest is the initial clinical manifestation of Brugada syndrome. Treatment strategies for Brugada syndrome are evolving. Currently, the implanted cardioverter defibrillator (ICD) is the only proven treatment for Brugada syndrome. Candidates for ICD include patients include those with the type 1 ECG pattern or who have been successfully resuscitated from sudden death or have had unexplained syncope.

The RYR2-encoded ryanodine receptor/calcium release channel in patients diagnosed previously with either catecholaminergic polymorphic ventricular tachycardia or genotype negative, exercise-induced long QT syndrome: a comprehensive open reading frame mutational analysis

OBJECTIVES

This study was undertaken to determine the spectrum and prevalence of mutations in the RYR2-encoded cardiac ryanodine receptor in cases with exertional syncope and normal corrected QT interval (QTc).

BACKGROUND

Mutations in RYR2 cause type 1 catecholaminergic polymorphic ventricular tachycardia (CPVT1), a cardiac channelopathy with increased propensity for lethal ventricular dysrhythmias. Most RYR2 mutational analyses target 3 canonical domains encoded by <40% of the translated exons. The extent of CPVT1-associated mutations localizing outside of these domains remains unknown as RYR2 has not been examined comprehensively in most patient cohorts.

METHODS

Mutational analysis of all RYR2 exons was performed using polymerase chain reaction, high-performance liquid chromatography, and deoxyribonucleic acid sequencing on 155 unrelated patients (49% females, 96% Caucasian, age at diagnosis 20 +/- 15 years, mean QTc 428 +/- 29 ms), with either clinical diagnosis of CPVT (n = 110) or an initial diagnosis of exercise-induced long QT syndrome but with QTc <480 ms and a subsequent negative long QT syndrome genetic test (n = 45).

RESULTS

Sixty-three (34 novel) possible CPVT1-associated mutations, absent in 400 reference alleles, were detected in 73 unrelated patients (47%). Thirteen new mutation-containing exons were identified. Two-thirds of the CPVT1-positive patients had mutations that localized to 1 of 16 exons.

CONCLUSIONS

Possible CPVT1 mutations in RYR2 were identified in nearly one-half of this cohort; 45 of the 105 translated exons are now known to host possible mutations. Considering that approximately 65% of CPVT1-positive cases would be discovered by selective analysis of 16 exons, a tiered targeting strategy for CPVT genetic testing should be considered.

[Hypertrophic cardiomyopathy and sudden cardiac death--ICD indications in the margin of uncertainty]

Hypertrophic cardiomyopathy (HCM) represents the most common genetic cardiovascular disease with sudden cardiac death being it's most feared and devastating complication. Due to the availability of an effective prevention method from sudden cardiac death since the development of the implantable cardioverter defibrillator (ICD), identification of high-risk patients has become a prominent objective. For a subgroup of patients clear evidence is lacking and the indication for an ICD is bound to a considerable uncertainty, due to the absence of randomized, prospective trials. In trying to guide and facilitate the decision making process, the following article provides an overview of the current available evidence.

A case of catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial cardiac arrhythmia that is related to RYR2 or CASQ2 gene mutation. It occurs in patients with structurally normal heart and causes exercise-emotion-triggered syncope and sudden cardiac death. We experienced a case of CPVT in an 11 year-old female patient who was admitted for sudden cardiovascular collapse. The initial electrocardiogram (ECG) on emergency department revealed ventricular fibrillation. After multiple defibrillations, sinus rhythm was restored. However, recurrent ventricular fibrillation occurred during insertion of nasogastric tube without sedation in coronary care unit. On ECG monitoring, bidirectional ventricular tachycardia occurred with sinus tachycardia and then degenerated into ventricular fibrillation. To our knowledge, there has been no previous case report of CPVT triggered by sinus tachycardia in Korea. Therefore, we report the case as well as a review of the literature.

A novel SCN5A mutation V1340I in Brugada syndrome augmenting arrhythmias during febrile illness

BACKGROUND

Mutations in the SCN5A gene, which encodes the cardiac sodium channel, have been implicated in the pathogenesis of Brugada syndrome (BrS). Febrile illnesses have been recognized to unmask and/or trigger the BrS phenotype. However, the pathophysiological mechanism has not been fully elucidated.

OBJECTIVE

A novel SCN5A missense mutation, V1340I, was identified in a patient with BrS suffering from frequent episodes of polymorphic ventricular tachycardia (VT) and syncope associated with fever. The biophysical modifications of hNa(v)1.5 by V1340I were studied.

METHODS

The effects of the V1340I mutation were studied in the 2 splice variants, SCN5A and SCN5A-Q1077del (delQ), using patch-clamp techniques at various temperatures between 22 degrees C and 40 degrees C.

RESULTS

At 22 degrees C, V1340I-SCN5A generated markedly diminished sodium currents compared to the wild-type (WT) SCN5A. On the contrary, V1340I-delQ generated almost identical current density compared to the WT-delQ. However, V1340I-delQ significantly attenuated the peak current density compared to the WT-delQ at 32 degrees C, 37 degrees C and 40 degrees C. The voltage dependency of steady-state activation was leftward shifted both in WT-delQ and V1340I-delQ at 40 degrees C. In addition, the V1340I-delQ accelerated the recovery time course from fast inactivation compared to the WT-delQ at 40 degrees C. Immunohistochemical staining showed that both V1340I-SCN5A and V1340I-dQ were expressed in the plasma membrane.

CONCLUSION

Our study supports the concept that febrile illness predisposes individuals who carry a loss of function SCN5A mutation, such as V1340I, to fever-induced ventricular arrhythmias in BrS by significantly reducing the sodium currents in the hyperthermic state.

Catecholaminergic polymorphic ventricular tachycardia from bedside to bench and beyond

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a primary electrical myocardial disease characterized by exercise- and stress-related ventricular tachycardia manifested as syncope and sudden death. The disease has a heterogeneous genetic basis, with mutations in the cardiac Ryanodine Receptor channel (RyR2) gene accounting for an autosomal-dominant form (CPVT1) in approximately 50% and mutations in the cardiac calsequestrin gene (CASQ2) accounting for an autosomal-recessive form (CPVT2) in up to 2% of CPVT cases. Both RyR2 and calsequestrin are important participants in the cardiac cellular calcium homeostasis. We review the physiology of the cardiac calcium homeostasis, including the cardiac excitation contraction coupling and myocyte calcium cycling. The pathophysiology of cardiac arrhythmias related to myocyte calcium handling and the effects of different modulators are discussed. The putative derangements in myocyte calcium homeostasis responsible for CPVT, as well as the clinical manifestations and therapeutic options available, are described.

Increased Ca2+ sensitivity of the ryanodine receptor mutant RyR2R4496C underlies catecholaminergic polymorphic ventricular tachycardia

Cardiac ryanodine receptor (RyR2) mutations are associated with autosomal dominant catecholaminergic polymorphic ventricular tachycardia, suggesting that alterations in Ca(2+) handling underlie this disease. Here we analyze the underlying Ca(2+) release defect that leads to arrhythmia in cardiomyocytes isolated from heterozygous knock-in mice carrying the RyR2(R4496C) mutation. RyR2(R4496C-/-) littermates (wild type) were used as controls. [Ca(2+)](i) transients were obtained by field stimulation in fluo-3-loaded cardiomyocytes and viewed using confocal microscopy. In our basal recording conditions (2-Hz stimulation rate), [Ca(2+)](i) transients and sarcoplasmic reticulum Ca(2+) load were similar in wild-type and RyR2(R4496C) cells. However, paced RyR2(R4496C) ventricular myocytes presented abnormal Ca(2+) release during the diastolic period, viewed as Ca(2+) waves, consistent with the occurrence of delayed afterdepolarizations. The occurrence of this abnormal Ca(2+) release was enhanced at faster stimulation rates and by beta-adrenergic stimulation, which also induced triggered activity. Spontaneous Ca(2+) sparks were more frequent in RyR2(R4496C) myocytes, indicating increased RyR2(R4496C) activity. When permeabilized cells were exposed to different cytosolic [Ca(2+)](i), RyR2(R4496C) showed a dramatic increase in Ca(2+) sensitivity. Isoproterenol increased [Ca(2+)](i) transient amplitude and Ca(2+) spark frequency to the same extent in wild-type and RyR2(R4496C) cells, indicating that the beta-adrenergic sensitivity of RyR2(R4496C) cells remained unaltered. This effect was independent of protein expression variations because no difference was found in the total or phosphorylated RyR2 expression levels. In conclusion, the arrhythmogenic potential of the RyR2(R4496C) mutation is attributable to the increased Ca(2+) sensitivity of RyR2(R4496C), which induces diastolic Ca(2+) release and lowers the threshold for triggered activity.

Physiological consequences of the P2328S mutation in the ryanodine receptor (RyR2) gene in genetically modified murine hearts

Aim

To explore the physiological consequences of the ryanodine receptor (RyR2)-P2328S mutation associated with catecholaminergic polymorphic ventricular tachycardia (CPVT).

Methods

We generated heterozygotic (RyR2^p/s) and homozygotic (RyR2^s/s) transgenic mice and studied Ca²⁺ signals from regularly stimulated, Fluo-3-loaded, cardiac myocytes. Results were compared with monophasic action potentials (MAPs) in Langendorff-perfused hearts under both regular and programmed electrical stimulation (PES).

Results

Evoked Ca²⁺ transients from wild-type (WT), heterozygote (RyR2^p/s) and homozygote (RyR2^s/s) myocytes had indistinguishable peak amplitudes with RyR2^s/s showing subsidiary events. Adding 100 nm isoproterenol produced both ectopic peaks and subsidiary events in WT but not RyR2^p/s and ectopic peaks and reduced amplitudes of evoked peaks in RyR2^s/s. Regularly stimulated WT, RyR2^p/s and RyR2^s/s hearts showed indistinguishable MAP durations and refractory periods. RyR2^p/s hearts showed non-sustained ventricular tachycardias (nsVTs) only with PES. Both nsVTs and sustained VTs (sVTs) occurred with regular stimuli and PES with isoproterenol treatment. RyR2^s/s hearts showed higher incidences of nsVTs before but mainly sVTs after introduction of isoproterenol with both regular stimuli and PES, particularly at higher pacing frequencies. Additionally, intrinsically beating RyR2^s/s showed extrasystolic events often followed by spontaneous sVT.

Conclusion

The RyR2-P2328S mutation results in marked alterations in cellular Ca²⁺ homeostasis and arrhythmogenic properties resembling CPVT with greater effects in the homozygote than the heterozygote demonstrating an important gene dosage effect.

[A Novel mutation of F189L in CASQ2 in families with catecholaminergic polymorphic ventricular tachycardia]

OBJECTIVE

To identify mutations and variants in CASQ2 gene in 27 CPVT patients/family members.

METHODS

Mutational analysis was performed with direct DNA sequence analysis. The frequency of an identified CASQ2 variant was determined using the Taqman genotyping assay.

RESULTS

A novel heterozygous mutation, F189L, in CASQ2 gene was identified in one family with CPVT. This mutation occurred at the evolutionarily, highly conserved phenylalanine residue at codon 189, and was not present in 1400 control individuals. No other disease-causing mutations were identified in the CASQ2 gene.

CONCLUSION

A novel mutation of F189L in the CASQ2 gene was identified.

[Short QT syndrome--a case report]

We present two new cases of the short QT syndrome--a 23-year-old male and his 42-year-old mother. Invasive electrophysiological study was negative in both patients. Due to polymorphic ventricular tachycardia recorded during Holter ECG monitoring, an ICD was implanted in the male patient for primary prevention. Blood samples were collected for further genetic studies. Diagnosis and management of patients with the short QT syndrome are discussed.

Catecholaminergic polymorphic ventricular tachycardia in a child: a case report

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare arrythmogenic disease characterized by exercise--or stress--induced ventricular tachyarrythmias, syncope, or sudden death, usually in the pediatric age group. Familial occurrence has been noted in about 30% of cases. Inheritance may be autosomal dominant or recessive, usually with high penetrance. The causative genes have been mapped to chromosome 1. Mutations of the cardiac ryanodine receptor gene (RyR2) have been identified in autosomal dominant pedigrees, while calsequestrin gene (CASQ2) mutations are seen in recessive cases.

CONCLUSION

Due to its potential lethal outcome, exclusion or confirmation of catecholaminergic polymorphic ventricular tachycardia in children with physical and emotional syncope is mandatory. We report a case of catecholaminergic polymorphic ventricular tachycardia in a three-year-old child only diagnosed by genetic mapping.

A novel SCN5A deletion mutation in a child with ventricular tachycardia, recurrent aborted sudden death, and Brugada electrocardiographic pattern

A novel SCN5A mutation was found in a child with congenital sick sinus disease, a Brugada-like electrocardiogram and recurrent aborted sudden death. The mutation (L1821fs/10) is a 4 base pair deletion (TCTG) at position 5464-5467 in exon 28 of the gene. The novel mutation is predicted to produce a frameshift leading to a premature stop codon after ten missense amino acids upstream that did not allow the generation of the complete protein, and probably producing an incomplete and therefore non functional protein. The resulting alteration in sodium current could explain the clinical phenotype observed in this patient.

A Novel Early Onset Lethal Form of Catecholaminergic Polymorphic Ventricular Tachycardia Maps to Chromosome 7p14-p22

INTRODUCTION

Previously, autosomal dominant catecholaminergic polymorphic ventricular tachycardia (CPVT [1]) was mapped to chromosome 1q42-43 with identification of pathogenic mutations in RYR2. Autosomal recessive CPVT (2) was mapped to chromosome 1p13-21, leading to the identification of mutations in CASQ2. In this study, we aimed to elucidate clinical phenotypes of a new variant of CPVT (3) in an inbred Arab family and also delineate the chromosomal location of the gene causing CPVT (3).

METHODS AND RESULTS

In a highly inbred family, clinical symptoms of CPVT appeared early in childhood (7-12 years) and in three of the four cases, the first appearance of symptoms turned into a fatal outcome. Parents of the affected children were first-degree cousins and without any symptoms. Segregation analysis suggested an autosomal recessive inheritance. A genome-wide search using polymorphic DNA markers mapped the disease locus to a 25-Mb interval on chromosome 7p14-p22. A maximal multipoint LOD score of 3.17 was obtained at marker D7S493. Sequencing of putative candidate genes, SP4, NPY, FKBP9, FKBP14, PDE1C, and TBX20, in and around this locus, did not reveal any mutation.

CONCLUSIONS

We have identified a novel highly malignant autosomal recessive form of CPVT and mapped this disorder to a 25-Mb interval on chromosome 7p14-p22.

Chain-reaction Ca(2+) signaling in the heart

Mutations in Ca(2+) -handling proteins in the heart have been linked to exercise-induced sudden cardiac death. The best characterized of these have been mutations in the cardiac Ca(2+) release channel known as the ryanodine receptor type 2 (RyR2). RyR2 mutations cause "leaky" channels, resulting in diastolic Ca(2+) leak from the sarcoplasmic reticulum (SR) that can trigger fatal cardiac arrhythmias during stress. In this issue of the JCI, Song et al. show that mutations in the SR Ca(2+)-binding protein calsequestrin 2 (CASQ2) in mice result not only in reduced CASQ2 expression but also in a surprising, compensatory elevation in expression of both the Ca(2+)-binding protein calreticulin and RyR2, culminating in premature Ca(2+) release from cardiac myocytes and stress-induced arrhythmia (see the related article beginning on page 1814). In the context of these findings and other recent reports studying CASQ2 mutations, we discuss how CASQ2 influences the properties of Ca(2+)-dependent regulation of RyR2 and how this contributes to cardiac arrhythmogenesis.

Expanding spectrum of human RYR2-related disease: new electrocardiographic, structural, and genetic features

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia is a disease characterized by ventricular arrhythmias elicited exclusively under adrenergic stress. Additional features include baseline bradycardia and, in some patients, right ventricular fatty displacement. The clinical spectrum is expanded by the 2 families described here.

METHODS AND RESULTS

Sixteen members from 2 separate families have been clinically evaluated and followed over the last 15 years. In addition to exercise-related ventricular arrhythmias, they showed abnormalities in sinoatrial node function, as well as atrioventricular nodal function, atrial fibrillation, and atrial standstill. Left ventricular dysfunction and dilatation was present in several affected individuals. Linkage analysis mapped the disease phenotype to a 4-cM region on chromosome 1q42-q43. Conventional polymerase chain reaction-based screening did not reveal a mutation in either the Ryanodine receptor 2 gene (RYR2) or ACTN2, the most plausible candidate genes in the region of interest. Multiplex ligation-dependent probe amplification and long-range polymerase chain reaction identified a genomic deletion that involved RYR2 exon-3, segregated in all the affected family members (n=16) in these 2 unlinked families. Further investigation revealed that the genomic deletion occurred in both families as a result of Alu repeat-mediated polymerase slippage.

CONCLUSIONS

This is the first report on a large genomic deletion in RYR2, which leads to extended clinical phenotypes (eg, sinoatrial node and atrioventricular node dysfunction, atrial fibrillation, atrial standstill, and dilated cardiomyopathy). These features have not previously been linked to RYR2.

Modest Reductions of Cardiac Calsequestrin Increase Sarcoplasmic Reticulum Ca 2+ Leak Independent of Luminal Ca 2+ and Trigger Ventricular Arrhythmias in Mice

Cardiac calsequestrin–null mice ( Casq2 −/− ) display catecholaminergic ventricular tachycardia akin to humans with CASQ2 mutations. However, the specific contribution of Casq2 deficiency to the arrhythmia phenotype is difficult to assess because Casq2 −/− mice also show significant reductions in the sarcoplasmic reticulum (SR) proteins junctin and triadin-1 and increased SR volume. Furthermore, it remains unknown whether Casq2 regulates SR Ca 2+ release directly or indirectly by buffering SR luminal Ca 2+ . To address both questions, we examined heterozygous ( Casq2 +/− ) mice, which have a 25% reduction in Casq2 but no significant decrease in other SR proteins. Casq2 +/− mice (n=35) challenged with isoproterenol displayed 3-fold higher rates of ventricular ectopy than Casq2 +/+ mice (n=31; P <0.05). Programmed stimulation induced significantly more ventricular tachycardia in Casq2 +/− mice than in Casq2 +/+ mice. Field-stimulated Ca 2+ transients, cell shortening, L-type Ca 2+ current, and SR volume were not significantly different in Casq2 +/− and Casq2 +/+ myocytes. However, in the presence of isoproterenol, SR Ca 2+ leak was significantly increased in Casq2 +/− myocytes ( Casq2 +/− 0.18±0.02 F ratio versus Casq2 +/+ 0.11±0.01 F ratio , n=57, 60; P <0.01), resulting in a significantly higher rate of spontaneous SR Ca 2+ releases and triggered beats. SR luminal Ca 2+ measured using Mag-Fura-2 was not altered by Casq2 reduction. As a result, the relationship between SR Ca 2+ leak and SR luminal Ca 2+ was significantly different between Casq2 +/− and Casq2 +/+ myocytes ( P <0.01). Thus, even modest reductions in Casq2 increase SR Ca 2+ leak and cause ventricular tachycardia susceptibility under stress. The underlying mechanism is likely the direct regulation of SR Ca 2+ release channels by Casq2 rather than altered luminal Ca 2+ .

Arrhythmogenic mechanisms in a mouse model of catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (VT) is a lethal familial disease characterized by bidirectional VT, polymorphic VT, and ventricular fibrillation. Catecholaminergic polymorphic VT is caused by enhanced Ca2+ release through defective ryanodine receptor (RyR2) channels. We used epicardial and endocardial optical mapping, chemical subendocardial ablation with Lugol's solution, and patch clamping in a knockin (RyR2/RyR2(R4496C)) mouse model to investigate the arrhythmogenic mechanisms in catecholaminergic polymorphic VT. In isolated hearts, spontaneous ventricular arrhythmias occurred in 54% of 13 RyR2/RyR2(R4496C) and in 9% of 11 wild-type (P=0.03) littermates perfused with Ca2+and isoproterenol; 66% of 12 RyR2/RyR2(R4496C) and 20% of 10 wild-type hearts perfused with caffeine and epinephrine showed arrhythmias (P=0.04). Epicardial mapping showed that monomorphic VT, bidirectional VT, and polymorphic VT manifested as concentric epicardial breakthrough patterns, suggesting a focal origin in the His-Purkinje networks of either or both ventricles. Monomorphic VT was clearly unifocal, whereas bidirectional VT was bifocal. Polymorphic VT was initially multifocal but eventually became reentrant and degenerated into ventricular fibrillation. Endocardial mapping confirmed the Purkinje fiber origin of the focal arrhythmias. Chemical ablation of the right ventricular endocardial cavity with Lugol's solution induced complete right bundle branch block and converted the bidirectional VT into monomorphic VT in 4 anesthetized RyR2/RyR2(R4496C) mice. Under current clamp, single Purkinje cells from RyR2/RyR2(R4496C) mouse hearts generated delayed afterdepolarization-induced triggered activity at lower frequencies and level of adrenergic stimulation than wild-type. Overall, the data demonstrate that the His-Purkinje system is an important source of focal arrhythmias in catecholaminergic polymorphic VT.

Calcium channel blockers and beta-blockers versus beta-blockers alone for preventing exercise-induced arrhythmias in catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

The mainstay of therapy for catecholaminergic polymorphic ventricular tachycardia (CPVT) is maximal doses of beta-blockers. However, although beta-blockers prevent exercise-induced ventricular tachycardia (VT), most patients continue to have ventricular ectopy during exercise, and some studies report high mortality rates despite beta-blockade.

OBJECTIVE

The purpose of this study was to investigate whether combining a calcium channel blocker with beta-blockers would prevent ventricular arrhythmias during exercise better than beta-blockers alone since the mutations causing CPVT lead to intracellular calcium overload.

METHODS

Five patients with CPVT and one with polymorphic VT (PVT) and hypertrophic cardiomyopathy who had exercise-induced ventricular ectopy despite beta-blocker therapy were studied. Symptom-limited exercise was first performed during maximal beta-blocker therapy and repeated after addition of oral verapamil.

RESULTS

When comparing exercise during beta-blockers with exercise during beta-blockers + verapamil, exercise-induced arrhythmias were reduced: (1) Three patients had nonsustained VT on beta-blockers, and none of them had VT on combination therapy. (2) The number of ventricular ectopics during the whole exercise test went down from 78 +/- 59 beats to 6 +/- 8 beats; the ratio of ventricular ectopic to sinus beats during the 10-second period recorded at the time of the worst ventricular arrhythmia went down from 0.9 +/- 0.4 to 0.2 +/- 0.2. One patient with recurrent spontaneous VT leading to multiple shocks from her implanted cardioverter-defibrillator (ICD) despite maximal beta-blocker therapy (14 ICD shocks over 6 months while on beta-blockers) has remained free of arrhythmias (for 7 months) since the addition of verapamil therapy.

CONCLUSIONS

This preliminary evidence suggests that beta-blockers and calcium blockers could be better than beta-blockers alone for preventing exercise-induced arrhythmias in CPVT.

Characterization of human cardiac calsequestrin and its deleterious mutants

Mutations of conserved residues of human cardiac calsequestrin (hCSQ2), a high-capacity, low-affinity Ca2+-binding protein in the sarcoplasmic reticulum, have been associated with catecholamine-induced polymorphic ventricular tachycardia (CPVT). In order to understand the molecular mechanism and pathophysiological link between these CPVT-related missense mutations of hCSQ2 and the resulting arrhythmias, we generated three CPVT-causing mutants of hCSQ2 (R33Q, L167H, and D307H) and two non-pathological mutants (T66A and V76M) and investigated the effect of these mutations. In addition, we determined the crystal structure of the corresponding wild-type hCSQ2 to gain insight into the structural effects of those mutations. Our data show clearly that all three CPVT-related mutations lead to significant reduction in Ca2+-binding capacity in spite of the similarity of their secondary structures to that of the wild-type hCSQ2. Light-scattering experiments indicate that the Ca2+-dependent monomer-polymer transitions of the mutants are quite different, confirming that the linear polymerization behavior of CSQ is linked directly to its high-capacity Ca2+ binding. R33Q and D307H mutations result in a monomer that appears to be unable to form a properly oriented dimer. On the other hand, the L167H mutant has a disrupted hydrophobic core in domain II, resulting in high molecular aggregates, which cannot respond to Ca2+. Although one of the non-pathological mutants, T66A, shares characteristics with the wild-type, the other null mutant, V76M, shows significantly altered Ca2+-binding and polymerization behaviors, calling for careful reconsideration of its status.

A novel C-terminal truncation SCN5A mutation from a patient with sick sinus syndrome, conduction disorder and ventricular tachycardia

OBJECTIVES

Individual mutations in the SCN5A-encoding cardiac sodium channel alpha-subunit cause single cardiac arrhythmia disorders, but a few cause multiple distinct disorders. Here we report a family harboring an SCN5A mutation (L1821fs/10) causing a truncation of the C-terminus with a marked and complex biophysical phenotype and a corresponding variable and complex clinical phenotype with variable penetrance.

METHODS AND RESULTS

A 12-year-old male with congenital sick sinus syndrome (SSS), cardiac conduction disorder (CCD), and recurrent monomorphic ventricular tachycardia (VT) had mutational analysis that identified a 4 base pair deletion (TCTG) at position 5464-5467 in exon 28 of SCN5A. The mutation was also present in six asymptomatic family members only two of which showed mild ECG phenotypes. The deletion caused a frame-shift mutation (L1821fs/10) with truncation of the C-terminus after 10 missense amino acid substitutions. When expressed in HEK-293 cells for patch-clamp study, the current density of L1821fs/10 was reduced by 90% compared with WT. In addition, gating kinetic analysis showed a 5-mV positive shift in activation, a 12-mV negative shift of inactivation and enhanced intermediate inactivation, all of which would tend to reduce peak and early sodium current. Late sodium current, however, was increased in the mutated channels.

CONCLUSIONS

The L1821fs/10 mutation causes the most severe disruption of SCN5A structure for a naturally occurring mutation that still produces current. It has a marked loss-of-function and unique phenotype of SSS, CCD and VT with incomplete penetrance.

Dominant Negative Suppression of Rad Leads to QT Prolongation and Causes Ventricular Arrhythmias via Modulation of L-type Ca 2+ Channels in the Heart

Disorders of L-type Ca 2+ channels can cause severe cardiac arrhythmias. A subclass of small GTP-binding proteins, the RGK family, regulates L-type Ca 2+ current ( I Ca,L ) in heterologous expression systems. Among these proteins, Rad (Ras associated with diabetes) is highly expressed in the heart, although its role in the heart remains unknown. Here we show that overexpression of dominant negative mutant Rad (S105N) led to an increase in I Ca,L and action potential prolongation via upregulation of L-type Ca 2+ channel expression in the plasma membrane of guinea pig ventricular cardiomyocytes. To verify the in vivo physiological role of Rad in the heart, a mouse model of cardiac-specific Rad suppression was created by overexpressing S105N Rad, using the α-myosin heavy chain promoter. Microelectrode studies revealed that action potential duration was significantly prolonged with visible identification of a small plateau phase in S105N Rad transgenic mice, when compared with wild-type littermate mice. Telemetric electrocardiograms on unrestrained mice revealed that S105N Rad transgenic mice had significant QT prolongation and diverse arrhythmias such as sinus node dysfunction, atrioventricular block, and ventricular extrasystoles, whereas no arrhythmias were observed in wild-type mice. Furthermore, administration of epinephrine induced frequent ventricular extrasystoles and ventricular tachycardia in S105N Rad transgenic mice. This study provides novel evidence that the suppression of Rad activity in the heart can induce ventricular tachycardia, suggesting that the Rad-associated signaling pathway may play a role in arrhythmogenesis in diverse cardiac diseases.

Calsequestrin 2 (CASQ2) mutations increase expression of calreticulin and ryanodine receptors, causing catecholaminergic polymorphic ventricular tachycardia

Catecholamine-induced polymorphic ventricular tachycardia (CPVT) is a familial disorder caused by cardiac ryanodine receptor type 2 (RyR2) or calsequestrin 2 (CASQ2) gene mutations. To define how CASQ2 mutations cause CPVT, we produced and studied mice carrying a human D307H missense mutation (CASQ(307/307)) or a CASQ2-null mutation (CASQ(DeltaE9/DeltaE9)). Both CASQ2 mutations caused identical consequences. Young mutant mice had structurally normal hearts but stress-induced ventricular arrhythmias; aging produced cardiac hypertrophy and reduced contractile function. Mutant myocytes had reduced CASQ2 and increased calreticulin and RyR2 (with normal phosphorylated proportions) but unchanged calstabin levels, as well as reduced total sarcoplasmic reticulum (SR) Ca(2+), prolonged Ca(2+) release, and delayed Ca(2+) reuptake. Stress further diminished Ca(2+) transients, elevated cytosolic Ca(2+), and triggered frequent, spontaneous SR Ca(2+) release. Treatment with Mg(2+), a RyR2 inhibitor, normalized myocyte Ca(2+) cycling and decreased CPVT in mutant mice, indicating RyR2 dysfunction was critical to mutant CASQ2 pathophysiology. We conclude that CPVT-causing CASQ2 missense mutations function as null alleles. In the absence of CASQ2, calreticulin, a fetal Ca(2+)-binding protein normally downregulated at birth, remains a prominent SR component. Adaptive changes to CASQ2 deficiency (increased posttranscriptional expression of calreticulin and RyR2) maintained electrical-mechanical coupling, but increased RyR2 leakiness, a paradoxical response further exacerbated by stress. The central role of RyR2 dysfunction in CASQ2 deficiency unifies the pathophysiologic mechanism underlying CPVT due to RyR2 or CASQ2 mutations and suggests a therapeutic approach for these inherited cardiac arrhythmias.

Molecular and Electrophysiological Bases of Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disorder characterized by adrenergically mediated polymorphic ventricular tachyarrhythmias. Genetic investigations have identified two variants of the disease: an autosomal dominant form associated with mutations in the gene encoding the cardiac ryanodine receptor (RyR2) and a recessive form associated with homozygous mutations in the gene encoding the cardiac isoform of calsequestrin (CASQ2). Functional characterization of mutations identified in the RyR2 and CASQ2 genes has demonstrated that CPVT are caused by derangements of the control of intracellular calcium. Investigations in a knock-in mouse model have shown that CPVT arrhythmias are initiated by delayed afterdepolarizations and triggered activity. In the present article, we review clinical and molecular understanding of CPVT and discuss the most recent approaches to develop novel therapeutic strategies for the disease.

Calsequestrin mutation and catecholaminergic polymorphic ventricular tachycardia: a simulation study of cellular mechanism

OBJECTIVES

Patients with a missense mutation of the calsequestrin 2 gene (CASQ2) are at risk for catecholaminergic polymorphic ventricular tachycardia. This mutation (CASQ2(D307H)) results in decreased ability of CASQ2 to bind Ca2+ in the sarcoplasmic reticulum (SR). In this theoretical study, we investigate a potential mechanism by which CASQ2(D307H) manifests its pro-arrhythmic consequences in patients.

METHODS

Using simulations in a model of the guinea pig ventricular myocyte, we investigate the mutation's effect on SR Ca2+ storage, the Ca2+ transient (CaT), and its indirect effect on ionic currents and membrane potential. We model the effects of isoproterenol (ISO) on Ca(V)1.2 (the L-type Ca2+ current, I(Ca(L))) and other targets of beta-adrenergic stimulation.

RESULTS

ISO increases I(Ca(L)), prolonging action potential (AP) duration (Control: 172 ms, +ISO: 207 ms, at cycle length of 1500 ms) and increasing CaT (Control: 0.79 microM, +ISO: 1.61 microM). ISO increases I(Ca(L)) by reducing the fraction of channels which undergo voltage-dependent inactivation and increasing transitions from a non-conducting to conducting mode of channel gating. CASQ2(D307H) reduces SR storage capacity, thereby reducing the magnitude of CaT (Control: 0.79 microM, CASQ2(D307H): 0.52 microM, at cycle length of 1500 ms). The combined effect of CASQ2(D307H) and ISO elevates SR free Ca2+ at a rapid rate, leading to store-overload-induced Ca2+ release and delayed afterdepolarization (DAD). If resting membrane potential is sufficiently elevated, the Na+-Ca2+ exchange-driven DAD can trigger I(Na) and I(Ca(L)) activation, generating a triggered arrhythmogenic AP.

CONCLUSIONS

The CASQ2(D307H) mutation manifests its pro-arrhythmic consequences due to store-overload-induced Ca2+ release and DAD formation due to excess free SR Ca2+ following rapid pacing and beta-adrenergic stimulation.

Homozygous SCN5A mutation in Brugada syndrome with monomorphic ventricular tachycardia and structural heart abnormalities

AIMS

To describe a patient showing monomorphic ventricular tachycardia, ECG aspect of Brugada syndrome, and structural heart abnormalities due to a homozygous missense mutation in SCN5A.

METHODS AND RESULTS

Thirteen subjects (six males, seven females, mean age 46 +/- 22 years) belonging to the same family underwent physical examination, basal biochemical marker detection, 12-lead ECG, Holter ECG, signal-averaged ECG, echocardiogram and genetic analysis. The proband underwent a stress test together with left and right ventricular angiography and electrophysiological study. Three subjects (the proband, his mother, and one brother) showed on ECG an ST-segment elevation in the right precordial leads with coved type aspect. Moreover, the proband presented a sustained monomorphic ventricular tachycardia (left bundle branch block aspect with superior axis), whereas all other family members were asymptomatic. Imaging techniques documented right ventricular structural abnormalities only in the proband. Mutation screening in SCN5A gene was performed in the proband and in available family members. The proband carries a novel SCN5A mutation, R814Q, in homozygous, whereas the parents and four siblings were heterozygous carriers of the same mutation.

CONCLUSION

This study provides the first evidence of a homozygous missense mutation in SCN5A associated with atypical ventricular arrhythmias and right structural abnormalities.