Optical mapping of ventricular arrhythmias in LQTS mice with SCN5A mutation N1325S

Theoretical Models and Computational Analysis of Action Potential Dispersion for Cardiac Arrhythmia Risk Stratification

Reentrant cardiac arrhythmias such as atrial fibrillation (AF) and ventricular fibrillation (VF) are common cardiac arrhythmias that account for substantial morbidity and mortality throughout the world. However, the mechanisms and optimal ablation treatment strategies for such arrhythmias are still unclear. Using 2D optical mapping of a mouse model with AF and VF, we have identified regional heterogeneity of the action potential duration (APD) in the atria and ventricles of the heart as key drivers for the initiation and persistence of reentry. The purpose of this paper is to discuss theoretical patterns of dispersion, demonstrate patterns of dispersion seen in our mouse model and discuss the computational analysis of APD dispersion patterns. These analyses and discussions may lead to better understanding of dispersion patterns in patients with these arrhythmias, as well as help comprehend whether and how reducing dispersion can lead to arrhythmia risk stratification and treatment strategies for arrhythmias.

TBX5 mutations contribute to early-onset atrial fibrillation in Chinese and Caucasians

AIMS

Atrial fibrillation (AF) is a common arrhythmia with an important heritable aspect. The genetic factors underlying AF have not been fully elucidated.

METHODS AND RESULTS

We screened six candidate genes (CAV1, KCNJ2, KCNQ1, NKX2.5, PITX2, and TBX5) for novel mutations in 139 patients of Chinese descent with early-onset AF and 576 controls. Four missense TBX5 mutations, p.R355C, p.Q376R, p.A428S, and p.S372L, were identified in evolutionarily conserved regions. We did not find any mutations in CAV1, KCNJ2, KCNQ1, NKX2.5, and PITX2. These mutations increased the expression of atrial natriuretic peptide (ANP) and connexin-40 (CX40) in the primarily cultured rat atrial myocytes but did not alter the expression of cardiac structural genes, atrial myosin heavy chain-α (MHC-α) and myosin light chain-2α (MLC-2α). Overexpression of p.R355C developed an atrial arrhythmia suggestive of paroxysmal AF in the zebrafish model. To replicate our findings, we screened TBX5 in 527 early-onset AF cases from the Massachusetts General Hospital AF study. A novel TBX5 deletion (ΔAsp118, p.D118del) was identified, while no TBX5 mutations were identified in 1176 control subjects.

CONCLUSION

Our results provide both genetic and functional evidence to support the contribution of TBX5 gene in the pathogenesis of AF. The potential mechanism of arrhythmia may be due in part to the disturbed expression of ANP and CX40.

Cardiac sodium channel regulator MOG1 regulates cardiac morphogenesis and rhythm

MOG1 was initially identified as a protein that interacts with the small GTPase Ran involved in transport of macromolecules into and out of the nucleus. In addition, we have established that MOG1 interacts with the cardiac sodium channel Nav1.5 and regulates cell surface trafficking of Nav1.5. Here we used zebrafish as a model system to study the in vivo physiological role of MOG1. Knockdown of mog1 expression in zebrafish embryos significantly decreased the heart rate (HR). Consistently, the HR increases in embryos with over-expression of human MOG1. Compared with wild type MOG1 or control EGFP, mutant MOG1 with mutation E83D associated with Brugada syndrome significantly decreases the HR. Interestingly, knockdown of mog1 resulted in abnormal cardiac looping during embryogenesis. Mechanistically, knockdown of mog1 decreases expression of hcn4 involved in the regulation of the HR, and reduces expression of nkx2.5, gata4 and hand2 involved in cardiac morphogenesis. These data for the first time revealed a novel role that MOG1, a nucleocytoplasmic transport protein, plays in cardiac physiology and development.

The SCN5A mutation A1180V is associated with electrocardiographic features of LQT3

Mutations of the SCN5A gene are associated with several arrhythmic syndromes including the Brugada syndrome, conduction disease, long QT syndrome type 3 (LQT3), atrial fibrillation, and dilated cardiomyopathy. We report LQT3 associated with an A1180V cardiac sodium channel mutation, previously associated with cardiac conduction block, and dilated cardiomyopathy in three generations of a Chinese family. Clinical, electrocardiographic (ECG), and echocardiographic examination was followed by direct sequencing of SCN5A and HERG to screen genomic DNA from blood samples. The proband presented with multiple syncopes from the age of 7 years and was found to share a mutation with two other members of his family. Continuous ECG monitoring after presentation showed prolonged QTc and biphasic T waves, multiple episodes of ventricular tachycardia and torsades de pointes. The other two mutation carriers showed ECG features of LQT3 without clinical symptoms. Transthoracic echocardiography showed normal cardiac structure in all three mutation carriers. This study shows LQT3 features associated with an A1180V cardiac sodium channel mutation, expanding the spectrum of phenotypes resulting from this mutation in which biophysical study has shown a persistent late Na(+) current.

LQTS mutation N1325S in cardiac sodium channel gene SCN5A causes cardiomyocyte apoptosis, cardiac fibrosis and contractile dysfunction in mice

OBJECTIVE

Mutations in the cardiac sodium channel gene SCN5A cause long QT syndrome (LQTS). We previously generated an LQTS mouse model (TG-NS) that overexpresses the LQTS mutation N1325S in SCN5A. The TG-NS mice manifested the clinical features of LQTS including spontaneous VT, syncope and sudden death. However, the long-term prognosis of LQTS on the structure of the heart has not been investigated in this or any other LQTS models and human patients.

METHODS AND RESULTS

Impaired systolic function and reduced left ventricular fractional shortening were detected by echocardiography, morphological and histological examination in two lines of adult mutant transgenic mice. Histological and TUNEL analyses of heart sections revealed fibrosis lesions and increased apoptosis in an age-dependent manner. Cardiomyocyte apoptosis was associated with the increased activation of caspases 3 and 9 in TG-NS hearts. Western blot analysis showed a significantly increased expression of the key Ca(2+) handling proteins L-type Ca(2+) channel, RYR2 and NCX in TG-NS hearts. Increased apoptosis and an altered expression of Ca(2+) handling proteins could be detected as early as 3months of age when echocardiography showed little or no alterations in TG-NS mice.

CONCLUSIONS

Our findings revealed for the first time that the LQTS mutation N1325S in SCN5A causes cardiac fibrosis and contractile dysfunction in mice, possibly through cellular mechanisms involving aberrant cardiomyocyte apoptosis. Therefore, we provide the experimental evidence supporting the notion that some LQTS patients have an increased risk of structural and functional cardiac damage in a prolonged disease course.

Spiral reentry waves in confluent layer of HL-1 cardiomyocyte cell lines

Cardiac excitation waves that arise in heart tissues have long been an important research topic because they are related to various cardiac arrhythmia. Investigating their properties based on intact animal whole hearts is important but quite demanding and expensive. Subsequently, dissociated cardiac cell cultures have been used as an alternative. Here, we access the usefulness of cardiomyocyte cell line HL-1 in studying generic properties of cardiac waves. Spontaneous wave activities in confluent populations of HL-1 cells are monitored using a phase-contrast optical mapping system and a microelectrode array recording device. We find that high-density cultures of HL-1 cells can support well-defined reentries. Their conduction velocity and rotation period both increase over few days. The increasing trend of rotation period is opposite to the case of control experiments using primary cultures of mouse atrial cells. The progressive myolysis of HL-1 seems responsible for this difference.

SCN5A channelopathies--an update on mutations and mechanisms

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

Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome

Aim

To perform an empirical, pharmacological, separation of early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in a genetically modified mouse heart modelling human long QT syndrome (LQT) 3.

Methods

Left ventricular endocardial and epicardial monophasic action potentials and arrhythmogenic tendency were compared in isolated wild type (WT) and Scn5a+/Δ hearts perfused with 0.1 and 1 μm propranolol and paced from the right ventricular epicardium.

Results

All spontaneously beating bradycardic Scn5a+/Δ hearts displayed EADs, triggered beats and ventricular tachycardia (VT; n = 7), events never seen in WT hearts (n = 5). Perfusion with 0.1 and 1 μm propranolol suppressed all EADs, triggered beats and episodes of VT. In contrast, triggering of VT persisted following programmed electrical stimulation in 6 of 12 (50%), one of eight (12.5%), but six of eight (75%) Scn5a+/Δ hearts perfused with 0, 0.1 and 1 μm propranolol respectively in parallel with corresponding alterations in repolarization gradients, reflected in action potential duration (ΔAPD₉₀) values. Thus 0.1 μm propranolol reduced epicardial but not endocardial APD₉₀ from 54.7 ± 1.6 to 44.0 ± 2.0 ms, restoring ΔAPD₉₀ from −3.8 ± 1.6 to 3.5 ± 2.5 ms (all n = 5), close to WT values. However, 1 μm propranolol increased epicardial APD₉₀ to 72.5 ± 1.2 ms and decreased endocardial APD₉₀ from 50.9 ± 1.0 to 24.5 ± 0.3 ms, increasing ΔAPD₉₀ to −48.0 ± 1.2 ms.

Conclusion

These findings empirically implicate EADs in potentially initiating spontaneous arrhythmogenic phenomena and transmural repolarization gradients in the re-entrant substrate that would sustain such activity when provoked by extrasystolic activity in murine hearts modelling human LQT3 syndrome.

Induction of high STAT1 expression in transgenic mice with LQTS and heart failure

Cardiac-specific expression of the N1325S mutation of SCN5A in transgenic mouse hearts (TG-NS) resulted in long QT syndrome (LQTS), ventricular arrhythmias (VT), and heart failure. In this study we carried out oligonucleotide mircoarray analysis to identify genes that are differentially expressed in the TG-NS mouse hearts. We identified 33 genes in five different functional groups that showed differential expression. None of the 33 genes are ion channel genes. STAT1, which encodes a transcription factor involved in apoptosis and interferon response, showed the most significant difference of expression between TG-NS and control mice (a nearly 10-fold increase in expression, P=4x10(-6)). The results were further confirmed by quantitative real-time PCR and Western blot analyses. Accordingly, many interferon response genes also showed differential expression in TG-NS hearts. This study represents the first microarray analysis for LQTS and implicates STAT1 in the pathogenesis and progression of LQTS and heart failure.

Cardiac-specific overexpression of SCN5A gene leads to shorter P wave duration and PR interval in transgenic mice

The Cardiac sodium channel gene SCN5A plays a critical role in cardiac electrophysiology and its mutations, either gain- or loss-of-functions, are associated with lethal arrhythmias. In this study, we investigated the effect of overexpression of SCN5A on the cardiac phenotype in a transgenic mouse model (TG-WT L10). Compared to NTG mice, heart rate, QRS duration, and QT intervals remained unchanged in TG-WT mice. Moreover, no spontaneous ventricular arrhythmias were detected in TG-WT hearts. Despite these results, a mild, irregular cardiac phenotype was observed in TG-WT mice. The P wave and PR interval were significantly shorter in TG-WT compared with NTG mice (P, 8.8+/-0.8 ms vs. 12.6+/-0.9 ms; PR, 12.5+/-2 ms vs. 33.5+/-0.7 ms). Furthermore, spontaneous premature atrial contractions were often detected in TG-WT mice. These results suggest that the expression level of the SCN5A gene is a determinant for the length of the P wave duration and PR interval on electrocardiograms (ECG).