The genetic and clinical features of cardiac channelopathies

Discovery of TBX20 as a Novel Gene Underlying Atrial Fibrillation

Atrial fibrillation (AF), the most prevalent type of sustained cardiac dysrhythmia globally, confers strikingly enhanced risks for cognitive dysfunction, stroke, chronic cardiac failure, and sudden cardiovascular demise. Aggregating studies underscore the crucial roles of inherited determinants in the occurrence and perpetuation of AF. However, due to conspicuous genetic heterogeneity, the inherited defects accounting for AF remain largely indefinite. Here, via whole-genome genotyping with genetic markers and a linkage assay in a family suffering from AF, a new AF-causative locus was located at human chromosome 7p14.2-p14.3, a ~4.89 cM (~4.43-Mb) interval between the markers D7S526 and D7S2250. An exome-wide sequencing assay unveiled that, at the defined locus, the mutation in the TBX20 gene, NM_001077653.2: c.695A>G; p.(His232Arg), was solely co-segregated with AF in the family. Additionally, a Sanger sequencing assay of TBX20 in another family suffering from AF uncovered a novel mutation, NM_001077653.2: c.862G>C; p.(Asp288His). Neither of the two mutations were observed in 600 unrelated control individuals. Functional investigations demonstrated that the two mutations both significantly reduced the transactivation of the target gene KCNH2 (a well-established AF-causing gene) and the ability to bind the promoter of KCNH2, while they had no effect on the nuclear distribution of TBX20. Conclusively, these findings reveal a new AF-causative locus at human chromosome 7p14.2-p14.3 and strongly indicate TBX20 as a novel AF-predisposing gene, shedding light on the mechanism underlying AF and suggesting clinical significance for the allele-specific treatment of AF patients.

The Role of Tbx20 in Cardiovascular Development and Function

Tbx20 is a member of the Tbx1 subfamily of T-box-containing genes and is known to play a variety of fundamental roles in cardiovascular development and homeostasis as well as cardiac remodeling in response to pathophysiological stresses. Mutations in TBX20 are widely associated with the complex spectrum of congenital heart defects (CHDs) in humans, which includes defects in chamber septation, chamber growth, and valvulogenesis. In addition, genetic variants of TBX20 have been found to be associated with dilated cardiomyopathy and heart arrhythmia. This broad spectrum of cardiac morphogenetic and functional defects is likely due to its broad expression pattern in multiple cardiogenic cell lineages and its critical regulation of transcriptional networks during cardiac development. In this review, we summarize recent findings in our general understanding of the role of Tbx20 in regulating several important aspects of cardiac development and homeostasis and heart function.

Risk assessment of drug-induced QT prolongation

Drugs can cause prolongation of the QT interval, alone or in combination, potentially leading to fatal arrhythmias such as torsades de pointes. When prescribing drugs that prolong the QT interval, the balance of benefit versus harm should always be considered. Readouts from automated ECG machines are unreliable. The QT interval should be measured manually. Changes in heart rate influence the absolute QT interval. Heart rate correction formulae are inaccurate, particularly for fast and slow heart rates. The QT nomogram, a plot of QT interval versus heart rate, can be used as a risk assessment tool to detect an abnormal QT interval.

Negative autopsy and sudden cardiac death

Forensic medicine defines the unexplained sudden death as a death with a non-conclusive diagnosis after autopsy. Molecular diagnosis is being progressively incorporated in forensics, mainly due to improvement in genetics. New genetic technologies may help to identify the genetic cause of death, despite clinical interpretation of genetic data remains the current challenge. The identification of an inheritable defect responsible for arrhythmogenic syndromes could help to adopt preventive measures in family members, many of them asymptomatic but at risk of sudden death. This multidisciplinary translational research requires a specialized team.

Synergistic restoring effects of isoproterenol and magnesium on KCNQ1-inhibited bradycardia cell models cultured in microelectrode array

OBJECTIVES

Bradycardia is caused by loss-of-function mutations in potassium channels that regulate phase 3 repolarization of the cardiac action potential. The purpose of this study is to monitor the effects of potassium channel (KCNQ1) inhibition and to evaluate the effects of isoproterenol (ISO) and MgSO4 in restoring sinus rhythm in atrial cells.

METHODS

Microelectrode array was used to analyze conduction velocity, voltage amplitude and cycle length of atrial cells (HL-1). A combination of ISO and MgSO4 was used to restore sinus rhythm in these cells.

RESULTS

mRNA expression levels of KCNQ1 (42.2 vs. 100%, p < 0.0001), connexin 43 (29.6 vs. 100%, p = 0.0033), atrial natriuretic peptide (31.0 vs. 100%, p = 0.0030), cardiac actin (38.2 vs. 100%, p < 0.0001) and α-myosin heavy chain (31.2 vs. 100%, p = 0.00254) were significantly lower in the KCNQ1 gene-inhibited group compared to the control group. When treated with MgSO4 (1 mM) and ISO (10 μM), conduction velocity (0.0208 ± 0.0036 vs. 0.0086 ± 0.0014 m/s, p = 0.0004) and voltage amplitude (1,210.78 ± 65.81 vs. 124.1 ± 13.30 μV, p < 0.0001) were higher, and cycle length (431.55 ± 2.05 vs. 1,015.15 ± 4.31 ms, p < 0.0001) was shorter than in the gene-inhibited group.

CONCLUSION

Inhibition of sinus rhythm in the bradycardia cell model was recovered by treatment with ISO and MgSO4, demonstrating the potency of combination therapy in the treatment of bradycardia.

Drug induced QT prolongation: the measurement and assessment of the QT interval in clinical practice

There has been an increasing focus on drug induced QT prolongation including research on drug development and QT prolongation, following the removal of drugs due to torsades de pointes (TdP). Although this has improved our understanding of drug-induced QT prolongation there has been much less research aimed at helping clinicians assess risk in individual patients with drug induced QT prolongation. This review will focus on assessment of drug-induced QT prolongation in clinical practice using a simple risk assessment approach. Accurate measurement of the QT interval is best done manually, and not using the measurement of standard ECG machines. Correction for heart rate (HR) using correction formulae such as Bazett's is often inaccurate. These formulae underestimate and overestimate the duration of cardiac repolarization at low and high heart rates, respectively. Numerous cut-offs have been suggested as an indicator of an abnormal QT, but are problematic in clinical practice. An alternative approach is the QT nomogram which is a plot of QT vs. HR. The nomogram has an 'at risk' line and QT-HR pairs above this line have been shown in a systematic study to be associated with TdP and the line is more sensitive and specific than Bazett's QTc of 440 ms or 500 ms. Plotting the QT-HR pair for patients on drugs suspected or known to cause QT prolongation allows assessment of the QT interval based on normal population QT variability. This risk assessment then allows the safer commencement of drugs therapeutically or management of drug induced effects in overdose.

Functional suppression of Kcnq1 leads to early sodium channel remodelling and cardiac conduction system dysmorphogenesis

AIMS

Ion channel remodelling and ventricular conduction system (VCS) alterations play relevant roles in the generation of cardiac arrhythmias, but the interaction between ion channel remodelling and cardiac conduction system dysfunctions in an arrhythmogenic context remain unexplored.

METHODS AND RESULTS

We have used a transgenic mouse line previously characterized as an animal model of Long QT Syndrome (LQTS) to analyse ion channel remodelling and VCS configuration. Reverse transcriptase-PCR and immunohistochemistry analysis showed early cardiac sodium channel upregulation at embryonic stages prior to the onset of Kv potassium channel remodelling, and cardiac hypertrophy at foetal stages. In line with these findings, patch-clamp assays demonstrated changes in sodium current density and a slowing of recovery from inactivation. Functional analysis by optical mapping revealed an immature ventricular activation pattern as well as an increase in the total left ventricle activation time in foetal transgenic hearts. Morphological analysis of LQTS transgenic mice in a Cx40(GFP/+)background demonstrated VCS dysmorphogenesis during heart development.

CONCLUSIONS

Our data demonstrate early sodium channel remodelling secondary to IKs blockage in a mouse model of LQTS leading to morphological and functional anomalies in the developing VCS and cardiac hypertrophy. These results provide new insights into the mechanisms underlying foetal and neonatal cardiac electrophysiological disorders, which might help understand how molecular, functional, and morphological alterations are linked to clinical pathologies such as cardiac congenital anomalies, arrhythmias, and perinatal sudden death.

Differential expression of genes encoding neuronal ion-channel subunits in major depression, bipolar disorder and schizophrenia: implications for pathophysiology

Evidence concerning ion-channel abnormalities in the pathophysiology of common psychiatric disorders is still limited. Given the significance of ion channels in neuronal activity, neurotransmission and neuronal plasticity we hypothesized that the expression patterns of genes encoding different ion channels may be altered in schizophrenia, bipolar and unipolar disorders. Frozen samples of striatum including the nucleus accumbens (Str-NAc) and the lateral cerebellar hemisphere of 60 brains from depressed (MDD), bipolar (BD), schizophrenic and normal subjects, obtained from the Stanley Foundation Brain Collection, were assayed. mRNA of 72 different ion-channel subunits were determined by qRT-PCR and alteration in four genes were verified by immunoblotting. In the Str-NAc the prominent change was observed in the MDD group, in which there was a significant up-regulation in genes encoding voltage-gated potassium-channel subunits. However, in the lateral cerebellar hemisphere (cerebellum), the main change was observed in schizophrenia specimens, as multiple genes encoding various ion-channel subunits were significantly down-regulated. The impaired expression of genes encoding ion channels demonstrates a disease-related neuroanatomical pattern. The alterations observed in Str-NAc of MDD may imply electrical hypo-activity of this region that could be of relevance to MDD symptoms and treatment. The robust unidirectional alteration of both excitatory and inhibitory ion channels in the cerebellum may suggests cerebellar general hypo-transcriptional activity in schizophrenia.

Ion channel drug discovery: challenges and future directions

Ion channels are targets of many therapeutically useful agents, and worldwide sales of ion channel-targeted drugs are estimated to be approximately US$12 billion. Nevertheless, considering that over 400 genes encoding ion channel subunits have been identified, ion channels remain significantly under-exploited as therapeutic targets. This is at least partly due to limitations in high-throughput assay technologies that support screening and lead optimization. Will the recent developments in automated electrophysiology rectify this situation? What are the other major limitations and can they be overcome? In this article, we review the status of ion channel drug discovery, discuss current challenges and propose alternative approaches that may facilitate the discovery of new drugs in the future.

Models of excitation-contraction coupling in cardiac ventricular myocytes

Excitation-contraction coupling describes the processes relating to electrical excitation through force generation and contraction in the heart. It occurs at multiple levels from the whole heart, to single myocytes and down to the sarcomere. A central process that links electrical excitation to contraction is calcium mobilization. Computational models that are well grounded in experimental data have been an effective tool to understand the complex dynamics of the processes involved in excitation-contraction coupling. Presented here is a summary of some computational models that have added to the understanding of the cellular and subcellular mechanisms that control ventricular myocyte calcium dynamics. Models of cardiac ventricular myocytes that have given insight into termination of calcium release and interval-force relations are discussed in this manuscript. Computational modeling of calcium sparks, the elementary events in cardiac excitation-contraction coupling, has given insight into mechanism governing their dynamics and termination as well as their role in excitation-contraction coupling and is described herein.

Tbx20 regulates a genetic program essential to adult mouse cardiomyocyte function

Human mutations in or variants of TBX20 are associated with congenital heart disease, cardiomyopathy, and arrhythmias. To investigate whether cardiac disease in patients with these conditions results from an embryonic or ongoing requirement for Tbx20 in myocardium, we ablated Tbx20 specifically in adult cardiomyocytes in mice. This ablation resulted in the onset of severe cardiomyopathy accompanied by arrhythmias, with death ensuing within 1 to 2 weeks of Tbx20 ablation. Accounting for this dramatic phenotype, we identified molecular signatures that posit Tbx20 as a central integrator of a genetic program that maintains cardiomyocyte function in the adult heart. Expression of a number of genes encoding critical transcription factors, ion channels, and cytoskeletal/myofibrillar proteins was downregulated consequent to loss of Tbx20. Genome-wide ChIP analysis of Tbx20-binding regions in the adult heart revealed that many of these genes were direct downstream targets of Tbx20 and uncovered a previously undescribed DNA-binding site for Tbx20. Bioinformatics and in vivo functional analyses revealed a cohort of transcription factors that, working with Tbx20, integrated multiple environmental signals to maintain ion channel gene expression in the adult heart. Our data provide insight into the mechanisms by which mutations in TBX20 cause adult heart disease in humans.

Acute consequences of hypoglycaemia in diabetic patients

Strict glycaemic control is a major concern in many people with diabetes, hypoglycaemia being the most limiting factor in the daily management of patients with diabetes. Acute consequences of hypoglycaemic attacks are not precisely evaluated. Acute cardiovascular (CV) complications as myocardial ischaemia or stroke seem to be rare, but possibly ignored mainly in older frail patients. Recent large trials in type 2 diabetic patients have not shown the anticipated mortality benefits of strict glycaemic control, and reported a higher frequency of severe hypoglycaemia in the intensive treatment arms with an excess of CV deaths. The authors of these trials persist to deny a direct link between CV deaths and hypoglycaemia. In young type 1 diabetics "dead in bed" syndrome represents a rare but devastating consequence probably due to arrhythmia and prolonged QTc interval. Driving mishaps represent another complication but with a controversial frequency. Neurologic syndromes are frequent during severe hypoglycaemia but usually reversible. Major brain damages are scarce, but cognitive defects or dementia should be underestimated in older and frail type 2 diabetics. Thus, iatrogenic hypoglycaemia due to insulin or sulphonylureas may cause recurrent morbidity in type 1 and type 2 diabetic subjects, and should be prevented by a reevaluation of glycaemic targets in some patients, patient education and the use of new antidiabetic drugs without hypoglycaemic risk.