Novel mechanism for sudden infant death syndrome: persistent late sodium current secondary to mutations in caveolin-3

The genetic basis for inherited forms of sinoatrial dysfunction and atrioventricular node dysfunction

The sinoatrial node (SAN) and the atrioventricular node (AVN) are the anatomical and functional regions of the heart which play critical roles in the generation and conduction of the electrical impulse. Their functions are ensured by peculiar structural cytological properties and specific collections of ion channels. Impairment of SAN and AVN activity is generally acquired,but in some cases familial inheritance has been established and therefore a genetic cause is involved. In recent years, combined efforts of clinical practice and experimental basic science studies have identified and characterized several causative gene mutations associated with the nodal syndromes. Channelopathies, i.e., diseases associated with defective ion channels, remain the major cause of genetically determined nodal arrhythmias; however, it is becoming increasingly evident that mutations in other classes of regulatory and structural proteins also have profound pathophysiological roles. In this review, we will present some aspects of the genetic identification of the molecular mechanism underlying both SAN and AVN dysfunctions with a particular focus on mutations of the Na, pacemaker (HCN), and Ca channels. Genetic defects in regulatory proteins and calcium-handling proteins will be also considered. In conclusion, the identification of the genetic defects associated with familial nodal dysfunction is an essential step for implementing an appropriate therapeutic treatment.

Protein assemblies of sodium and inward rectifier potassium channels control cardiac excitability and arrhythmogenesis

The understanding of how cardiac ion channels function in the normal and the diseased heart has greatly increased over the last four decades thanks to the advent of patch-clamp technology and, more recently, the emergence of genetics, as well as cellular and molecular cardiology. However, our knowledge of how these membrane-embedded proteins physically interact with each other within macromolecular complexes remains incomplete. This review focuses on how the main cardiac inward sodium channel (NaV1.5) and the strong inward rectifier potassium channel (Kir2.1) function within macromolecular complexes to control cardiac excitability. It has become increasingly clear that these two important ion channel proteins physically interact with multiple other protein partners and with each other from early stages of protein trafficking and targeting through membrane anchoring, recycling, and degradation. Recent findings include compartmentalized regulation of NaV1.5 channel expression and function through a PDZ (postsynaptic density protein, Drosophila disc large tumor suppressor, and zonula occludens-1 protein) domain-binding motif, and interaction of caveolin-3 with Kir2.1 and ankyrin-G as a molecular platform for NaV1.5 signaling. At the cardiomyocyte membrane, NaV1.5 and Kir2.1 interact through at least two distinct PDZ domain-scaffolding proteins (synapse-associated protein-97 and α1-syntrophin), thus modulating reciprocally their cell-surface expression at two different microdomains. Emerging evidence also shows that inheritable mutations in plakophilin-2, ankyrin-G, dystrophin, syntrophin, synapse-associated protein-97, and caveolin-3, among others, modify functional expression and/or localization in the cardiac cell of NaV1.5, Kir2.1 or both to give rise to arrhythmogenic diseases. Unveiling the mechanistic underpinnings of macromolecular interactions should increase our understanding of inherited and acquired arrhythmogenic cardiac diseases and may lead to advances in therapy.

Cardiac abnormalities and sudden infant death syndrome

Many factors have been implicated in SIDS cases including environmental influences such as sleeping arrangements and smoking. Most recently, cardiac abnormalities have been hypothesised to play a role in some cases, particularly the primary genetic arrhythmogenic disorders such as familial long QT syndrome (LQTS). Both post-mortem and clinical studies of SIDS cases have provided supporting evidence for the involvement of cardiac genetic disorders in SIDS. This review provides a summary of this evidence focussing particularly on the primary hypothesis related to underlying familial LQTS. In addition, the current literature relating to other cardiac genetic conditions such as Brugada syndrome (BrS) and structural heart diseases such as hypertrophic cardiomyopathy (HCM) is briefly presented. Finally, the implications of a possible cardiac genetic cause of SIDS is discussed with reference to the need for genetic testing in SIDS cases and subsequent clinical and genetic testing in family members.

Diseases caused by mutations in Nav1.5 interacting proteins

Sodium current in the heart flows principally through the pore protein NaV1.5, which is part of a complex of interacting proteins that serve both to target and localize the complex in the membrane, and to modulate function by such post-translational modifications as phosphorylation and nitrosylation. Multiple mutations in seven different NaV1.5 interacting proteins have been associated with dysfunctional sodium current and inherited cardiac diseases, including long QT syndrome, Brugada syndrome, atrial fibrillation, and cardiomyopathy, as well as sudden infant death syndrome (SIDS). Mutations in as yet unidentified interacting proteins may account for cardiac disease for which a genetic basis has not yet been established. Characterizing the mechanisms by which these mutations cause disease may give insight into etiologies and treatments of more common acquired cardiac disease, such as ischemia and heart failure.

Potential of caveolae in the therapy of cardiovascular and neurological diseases

Caveolae are membrane micro-domains enriched in cholesterol, sphingolipids and caveolins, which are transmembrane proteins with a hairpin-like structure. Caveolae participate in receptor-mediated trafficking of cell surface receptors and receptor-mediated signaling. Furthermore, caveolae participate in clathrin-independent endocytosis of membrane receptors. On the one hand, caveolins are involved in vascular and cardiac dysfunction. Also, neurological abnormalities in caveolin-1 knockout mice and a link between caveolin-1 gene haplotypes and neurodegenerative diseases have been reported. The aim of this article is to present the rationale for considering caveolae as potential targets in cardiovascular and neurological diseases.

Caveolin-1 modulates cardiac gap junction homeostasis and arrhythmogenecity by regulating cSrc tyrosine kinase

BACKGROUND

Genome-wide association studies have revealed significant association of caveolin-1 (Cav1) gene variants with increased risk of cardiac arrhythmias. Nevertheless, the mechanism for this linkage is unclear.

METHODS AND RESULTS

Using adult Cav1(-/-) mice, we revealed a marked reduction in the left ventricular conduction velocity in the absence of myocardial Cav1, which is accompanied with increased inducibility of ventricular arrhythmias. Further studies demonstrated that loss of Cav1 leads to the activation of cSrc tyrosine kinase, resulting in the downregulation of connexin 43 and subsequent electric abnormalities. Pharmacological inhibition of cSrc mitigates connexin 43 downregulation, slowed conduction, and arrhythmia inducibility in Cav1(-/-) animals. Using a transgenic mouse model with cardiac-specific overexpression of angiotensin-converting enzyme (ACE8/8), we demonstrated that, on enhanced cardiac renin-angiotensin system activity, Cav1 dissociated from cSrc because of increased Cav1 S-nitrosation at Cys(156), leading to cSrc activation, connexin 43 reduction, impaired gap junction function, and subsequent increase in the propensity for ventricular arrhythmias and sudden cardiac death. Renin-angiotensin system-induced Cav1 S-nitrosation was associated with increased Cav1-endothelial nitric oxide synthase binding in response to increased mitochondrial reactive oxidative species generation.

CONCLUSIONS

The present studies reveal the critical role of Cav1 in modulating cSrc activation, gap junction remodeling, and ventricular arrhythmias. These data provide a mechanistic explanation for the observed genetic link between Cav1 and cardiac arrhythmias in humans and suggest that targeted regulation of Cav1 may reduce arrhythmic risk in cardiac diseases associated with renin-angiotensin system activation.

Identification and functional analysis of a new putative caveolin-3 variant found in a patient with sudden unexplained death

Background

Sudden cardiac death (SCD) is the clinical outcome of a lethal arrhythmia that can develop on the background of unrecognized channelopathies or cardiomyopathies. Several susceptibility genes have been identified for the congenital forms of these cardiac diseases, including caveolin-3 (Cav-3) gene. In the heart Cav-3 is the main component of caveolae, plasma membrane domains that regulate multiple cellular processes highly relevant for cardiac excitability, such as trafficking, calcium homeostasis, signal transduction and cellular response to injury. Here we characterized a new putative Cav-3 variant, Cav-3 V82I, found in a patient with SCD.

Results

In heterologous systems Cav-3 V82I was expressed at significantly higher level than Cav-3 WT and accumulated within the cells. Cells expressing Cav-3 V82I exhibited a decreased activation of extracellular-signal-regulated kinases (ERKs) and were more vulnerable to sub-lethal osmotic stress.

Conclusion

Considering that abnormal loss of myocytes can play a mechanistic role in lethal cardiac diseases, we suggest that the detrimental effect of Cav-3 V82I variant on cell viability may participate in determining the susceptibility to cardiac death.

Sudden unexplained death in infants and children: the role of undiagnosed inherited cardiac conditions

Sudden unexplained death in childhood is a traumatic event for both the immediate family and medical professionals. This is termed sudden unexplained or arrhythmic death syndrome (SUDS/SADS) for children over 1 year of age while sudden unexplained death in infancy or sudden infant death syndrome (SUDI/SIDS) refers to unexplained deaths in the first year of life. There is increasing evidence for the role of undiagnosed inherited cardiac conditions, particularly channelopathies, as the cause of these deaths. This has far-reaching implications for the family regarding the potential risk to other family members and future pregnancies, providing a challenge not only in the counselling but also in the structured assessment and management of immediate relatives. This review will discuss the cardiac risk involved in sudden unexplained deaths of infants and children, the role of molecular autopsy, family cardiological screening, current management strategies, and future directions in this area.

Congenital Long QT Syndrome: An Update and Present Perspective in Saudi Arabia

Primary cardiac arrhythmias are often caused by defects, predominantly in the genes responsible for generation of cardiac electrical potential, i.e., cardiac rhythm generation. Due to the variability in underlying genetic defects, type, and location of the mutations and putative modifiers, clinical phenotypes could be moderate to severe, even absent in many individuals. Clinical presentation and severity could be quite variable, syncope, or sudden cardiac death could also be the first and the only manifestation in a patient who had previously no symptoms at all. Despite usual familial occurrence of such cardiac arrhythmias, disease causal genetic defects could also be de novo in significant number of patients. Long QT syndrome (LQTS) is the most eloquently investigated primary cardiac rhythm disorder. A genetic defect can be identified in ∼70% of definitive LQTS patients, followed by Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) and Brugada syndrome (BrS), where a genetic defect is found in <40% cases. In addition to these widely investigated hereditary arrhythmia syndromes, there remain many other relatively less common arrhythmia syndromes, where researchers also have unraveled the genetic etiology, e.g., short QT syndrome (SQTS), sick sinus syndrome (SSS), cardiac conduction defect (CCD), idiopathic ventricular fibrillation (IVF), early repolarization syndrome (ERS). There exist also various other ill-defined primary cardiac rhythm disorders with strong genetic and familial predisposition. In the present review we will focus on the genetic basis of LQTS and its clinical management. We will also discuss the presently available genetic insight in this context from Saudi Arabia.

The role of CAV3 in long-QT syndrome: clinical and functional assessment of a caveolin-3/Kv11.1 double heterozygote versus caveolin-3 single heterozygote

BACKGROUND

Mutations in CAV3, coding for caveolin-3, the major constituent scaffolding protein of cardiac caveolae, have been associated with skeletal muscle disease, cardiomyopathy, and most recently long-QT syndrome (LQTS) and sudden infant death syndrome. We examined the occurrence of CAV3 mutations in a large cohort of patients with LQTS.

METHODS AND RESULTS

Probands with LQTS (n=167) were screened for mutations in CAV3 using direct DNA sequencing. A single proband (0.6%) was found to be a heterozygous carrier of a previously described missense mutation, caveolin-3:p.T78M. The proband was also a heterozygous carrier of the trafficking-deficient Kv11.1:p.I400N mutation. The caveolin-3:p.T78M mutation was found isolated in 3 family members, none of whom had a prolonged QTc interval. Coimmunoprecipitations of caveolin-3 and the voltage-gated potassium channel subunit (Kv11.1) were performed, and the electrophysiological classification of the Kv11.1 mutant was carried out by patch-clamp technique in human embryonic kidney 293 cells. Furthermore, the T-wave morphology was assessed in mutation carriers, double mutation carriers, and nonmutation carriers by applying a morphology combination score. The morphology combination score was normal for isolated caveolin-3:p.T78M carriers and of LQT2 type in double heterozygotes.

CONCLUSIONS

Mutations in CAV3 are rare in LQTS. Furthermore, caveolin-3:p.T78M did not exhibit a LQTS phenotype. Because no association has ever been found between LQTS and isolated CAV3 mutations, we suggest that LQTS9 is considered a provisional entity.

Caveolin-3 suppresses late sodium current by inhibiting nNOS-dependent S-nitrosylation of SCN5A

AIMS

Mutations in CAV3-encoding caveolin-3 (Cav3) have been implicated in type 9 long QT syndrome (LQT9) and sudden infant death syndrome (SIDS). When co-expressed with SCN5A-encoded cardiac sodium channels these mutations increased late sodium current (INa) but the mechanism was unclear. The present study was designed to address the mechanism by which the LQT9-causing mutant Cav3-F97C affects the function of caveolar SCN5A.

METHODS AND RESULTS

HEK-293 cells expressing SCN5A and LQT9 mutation Cav3-F97C resulted in a 2-fold increase in late INa compared to Cav3-WT. This increase was reversed by the neural nitric oxide synthase (nNOS) inhibitor L-NMMA. Based on these findings, we hypothesized that an nNOS complex mediated the effect of Cav3 on SCN5A. A SCN5A macromolecular complex was established in HEK-293 cells by transiently expressing SCN5A, α1-syntrophin (SNTA1), nNOS, and Cav3. Compared with Cav3-WT, Cav3-F97C produced significantly larger peak INa amplitudes, and showed 3.3-fold increase in the late INa associated with increased S-nitrosylation of SCN5A. L-NMMA reversed both the Cav3-F97C induced increase in late and peak INa and decreased S-nitrosylation of SCN5A. Overexpression of Cav3-F97C in adult rat cardiomyocytes caused a significant increase in late INa compared to Cav3-WT, and prolonged the action potential duration (APD90) in a nNOS-dependent manner.

CONCLUSIONS

Cav3 is identified as an important negative regulator for cardiac late INa via nNOS dependent direct S-nitrosylation of SCN5A. This provides a molecular mechanism for how Cav3 mutations increase late INa to cause LQT9. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Channelopathies from mutations in the cardiac sodium channel protein complex

The cardiac sodium current underlies excitability in heart, and inherited abnormalities of the proteins regulating and conducting this current cause inherited arrhythmia syndromes. This review focuses on inherited mutations in non-pore forming proteins of sodium channel complexes that cause cardiac arrhythmia, and the deduced mechanisms by which they affect function and dysfunction of the cardiac sodium current. Defining the structure and function of these complexes and how they are regulated will contribute to understanding the possible roles for this complex in normal and abnormal physiology and homeostasis. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Adrenergic regulation of cardiac ionic channels: role of membrane microdomains in the regulation of kv4 channels

The heart must constantly adapt its activity to the needs of the body. In any potentially dangerous or physically demanding situation the activated sympathetic nervous system leads a very fast cardiac response. Under these circumstances, α1-adrenergic receptors activate intracellular signaling pathways that finally phosphorylate the caveolae-located subpopulation of Kv4 channels and reduce the transient outward K(+) current (Ito) amplitude. This reduction changes the shape of the cardiac action potential and makes the plateau phase to start at higher voltages. This means that there are more calcium ions entering the myocyte and the result is an increase in the strength of the contraction. However, an excessive reduction of Ito could dangerously prolong action potential duration and this could cause arrhythmias when the heart rate is high. This excessive current reduction does not occur because there is a second population of Ito channels located in non-caveolar membrane rafts that are not accessible for α1-AR mediated regulation. Thus, the location of the components of a given transduction signaling pathway in membrane domains determines the correct and safe behavior of the heart. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Defective interactions of protein partner with ion channels and transporters as alternative mechanisms of membrane channelopathies

The past twenty years have revealed the existence of numerous ion channel mutations resulting in human pathology. Ion channels provide the basis of diverse cellular functions, ranging from hormone secretion, excitation-contraction coupling, cell signaling, immune response, and trans-epithelial transport. Therefore, the regulation of biophysical properties of channels is vital in human physiology. Only within the last decade has the role of non-ion channel components come to light in regard to ion channel spatial, temporal, and biophysical regulation in physiology. A growing number of auxiliary components have been determined to play elemental roles in excitable cell physiology, with dysfunction resulting in disorders and related manifestations. This review focuses on the broad implications of such dysfunction, focusing on disease-causing mutations that alter interactions between ion channels and auxiliary ion channel components in a diverse set of human excitable cell disease. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

Palmitate diet-induced loss of cardiac caveolin-3: a novel mechanism for lipid-induced contractile dysfunction

Obesity is associated with an increased risk of cardiomyopathy, and mechanisms linking the underlying risk and dietary factors are not well understood. We tested the hypothesis that dietary intake of saturated fat increases the levels of sphingolipids, namely ceramide and sphingomyelin in cardiac cell membranes that disrupt caveolae, specialized membrane micro-domains and important for cellular signaling. C57BL/6 mice were fed two high-fat diets: palmitate diet (21% total fat, 47% is palmitate), and MCT diet (21% medium-chain triglycerides, no palmitate). We established that high-palmitate feeding for 12 weeks leads to 40% and 50% increases in ceramide and sphingomyelin, respectively, in cellular membranes. Concomitant with sphingolipid accumulation, we observed a 40% reduction in systolic contractile performance. To explore the relationship of increased sphingolipids with caveolins, we analyzed caveolin protein levels and intracellular localization in isolated cardiomyocytes. In normal cardiomyocytes, caveolin-1 and caveolin-3 co-localize at the plasma membrane and the T-tubule system. However, mice maintained on palmitate lost 80% of caveolin-3, mainly from the T-tubule system. Mice maintained on MCT diet had a 90% reduction in caveolin-1. These data show that caveolin isoforms are sensitive to the lipid environment. These data are further supported by similar findings in human cardiac tissue samples from non-obese, obese, non-obese cardiomyopathic, and obese cardiomyopathic patients. To further elucidate the contractile dysfunction associated with the loss of caveolin-3, we determined the localization of the ryanodine receptor and found lower expression and loss of the striated appearance of this protein. We suggest that palmitate-induced loss of caveolin-3 results in cardiac contractile dysfunction via a defect in calcium-induced calcium release.

Congenital central hypoventilation syndrome and sudden infant death syndrome: disorders of autonomic regulation

Long considered a rare and unique disorder of respiratory control, congenital central hypoventilation syndrome has recently been further distinguished as a disorder of autonomic regulation. Similarly, more recent evidence suggests that sudden infant death syndrome is also a disorder of autonomic regulation. Congenital central hypoventilation syndrome typically presents in the newborn period with alveolar hypoventilation, symptoms of autonomic dysregulation and, in a subset of cases, Hirschsprung disease or tumors of neural crest origin or both. Genetic investigation identified PHOX2B, a crucial gene during early autonomic development, as disease defining for congenital central hypoventilation syndrome. Although sudden infant death syndrome is most likely defined by complex multifactorial genetic and environmental interactions, it is also thought to result from central deficits in the control of breathing and autonomic regulation. The purpose of this article is to review the current understanding of these autonomic disorders and discuss the influence of this information on clinical practice and future research directions.

Genetic complexity in hypertrophic cardiomyopathy revealed by high-throughput sequencing

Background

Clinical interpretation of the large number of rare variants identified by high throughput sequencing (HTS) technologies is challenging. The aim of this study was to explore the clinical implications of a HTS strategy for patients with hypertrophic cardiomyopathy (HCM) using a targeted HTS methodology and workflow developed for patients with a range of inherited cardiovascular diseases. By comparing the sequencing results with published findings and with sequence data from a large-scale exome sequencing screen of UK individuals, we sought to quantify the strength of the evidence supporting causality for detected candidate variants.

Methods and results

223 unrelated patients with HCM (46±15 years at diagnosis, 74% males) were studied. In order to analyse coding, intronic and regulatory regions of 41 cardiovascular genes, we used solution-based sequence capture followed by massive parallel resequencing on Illumina GAIIx. Average read-depth in the 2.1 Mb target region was 120. Rare (frequency<0.5%) non-synonymous, loss-of-function and splice-site variants were defined as candidates. Excluding titin, we identified 152 distinct candidate variants in sarcomeric or associated genes (89 novel) in 143 patients (64%). Four sarcomeric genes (MYH7, MYBPC3, TNNI3, TNNT2) showed an excess of rare single non-synonymous single-nucleotide polymorphisms (nsSNPs) in cases compared to controls. The estimated probability that a nsSNP in these genes is pathogenic varied between 57% and near certainty depending on the location. We detected an additional 94 candidate variants (73 novel) in desmosomal, and ion-channel genes in 96 patients (43%).

Conclusions

This study provides the first large-scale quantitative analysis of the prevalence of sarcomere protein gene variants in patients with HCM using HTS technology. Inclusion of other genes implicated in inherited cardiac disease identifies a large number of non-synonymous rare variants of unknown clinical significance.

Digenic inheritance novel mutations in SCN5a and SNTA1 increase late I(Na) contributing to LQT syndrome

SCN5A and SNTA1 are reported susceptible genes for long QT syndrome (LQTS). This study was designed to elucidate a plausible pathogenic arrhythmia mechanism for the combined novel mutations R800L-SCN5A and A261V-SNTA1 on cardiac sodium channels. A Caucasian family with syncope and marginally prolonged QT interval was screened for LQTS-susceptibility genes and found to harbor the R800L mutation in SCN5A and A261V mutation in SNTA1, and those with both mutations had the strongest clinical phenotype. The mutations were engineered into the most common splice variant of human SCN5A and SNTA1 cDNA, respectively, and sodium current (INa) was characterized in human embryonic kidney 293 cells cotransfected with neuronal nitric oxide synthase (nNOS) and the cardiac isoform of the plasma membrane Ca-ATPase (PMCA4b). Peak INa densities were unchanged for wild-type (WT) and for mutant channels containing R800L-SCN5A, A261V-SNTA1, or R800L-SCN5A plus A261V-SNTA1. However, late INa for either single mutant was moderately increased two- to threefold compared with WT. The combined mutations of R800L-SCN5A plus A261V-SNTA1 significantly enhanced the INa late/peak ratio by 5.6-fold compared with WT. The time constants of current decay of combined mutant channel were markedly increased. The gain-of-function effect could be blocked by the N(G)-monomethyl-l-arginine, a nNOS inhibitor. We conclude that novel mutations in SCN5A and SNTA1 jointly exert a nNOS-dependent gain-of-function on SCN5A channels, which may consequently prolong the action potential duration and lead to LQTS phenotype.

Gene mutations in cardiac arrhythmias: a review of recent evidence in ion channelopathies

Over the past 15 years, molecular genetic studies have linked gene mutations to many inherited arrhythmogenic disorders, in particular, “ion channelopathies”, in which mutations in genes encode functional units of ion channels and/or their transporter-associated proteins in patients without primary cardiac structural abnormalities. These disorders are exemplified by congenital long QT syndrome (LQTS), short QT syndrome, Brugada syndrome (BrS) and catecholaminergic polymorphic ventricular tachycardia (CPVT). Functional and pathophysiological studies have led to better understanding of the clinical spectrum, ion channel structures and cellular electrophysiology involving dynamics of intracellular calcium cycling in many subtypes of these disorders and more importantly, development of potentially more effective pharmacological agents and even curative gene therapy. In this review, we have summarized (1) the significance of unveiling mutations in genes encoding transporter-associated proteins as the cause of congenital LQTS, (2) the technique of catheter ablation applied at the right ventricular outflow tract may be curative for severely symptomatic BrS, (3) mutations with channel function modulated by protein Kinase A-dependent phosphorylation can be the culprit of CPVT mimicry in Andersen-Tawil syndrome (LQT7), (4) ablation of the ion channel anchoring protein may prevent arrhythmogenesis in Timothy syndrome (LQT8), (5) altered intracellular Ca2+ cycling can be the basis of effective targeted pharmacotherapy in CPVT, and (6) the technology of induced pluripotent stem cells is a promising diagnostic and research tool as it has become a new paradigm for pathophysiological study of patient- and disease-specific cells aimed at screening new drugs and eventual clinical application of gene therapy. Lastly, we have discussed (7) genotype-phenotype correlation in relation to risk stratification of patients with congenital LQTS in clinical practice.

Cardiac ion channelopathies and the sudden infant death syndrome

The sudden infant death syndrome (SIDS) causes the sudden death of an apparently healthy infant, which remains unexplained despite a thorough investigation, including the performance of a complete autopsy. The triple risk model for the pathogenesis of SIDS points to the coincidence of a vulnerable infant, a critical developmental period, and an exogenous stressor. Primary electrical diseases of the heart, which may cause lethal arrhythmias as a result of dysfunctioning cardiac ion channels (“cardiac ion channelopathies”) and are not detectable during a standard postmortem examination, may create the vulnerable infant and thus contribute to SIDS. Evidence comes from clinical correlations between the long QT syndrome and SIDS as well as genetic analyses in cohorts of SIDS victims (“molecular autopsy”), which have revealed a large number of mutations in ion channel-related genes linked to inheritable arrhythmogenic syndromes, in particular the long QT syndrome, the short QT syndrome, the Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Combining data from population-based cohort studies, it can be concluded that at least one out of five SIDS victims carries a mutation in a cardiac ion channel-related gene and that the majority of these mutations are of a known malignant phenotype.

Cardiac sodium channel NaV1.5 distribution in myocytes via interacting proteins: the multiple pool model

The cardiac sodium current (INa) is responsible for the rapid depolarization of cardiac cells, thus allowing for their contraction. It is also involved in regulating the duration of the cardiac action potential (AP) and propagation of the impulse throughout the myocardium. Cardiac INa is generated by the voltage-gated Na(+) channel, NaV1.5, a 2016-residue protein which forms the pore of the channel. Over the past years, hundreds of mutations in SCN5A, the human gene coding for NaV1.5, have been linked to many cardiac electrical disorders, including the congenital and acquired long QT syndrome, Brugada syndrome, conduction slowing, sick sinus syndrome, atrial fibrillation, and dilated cardiomyopathy. Similar to many membrane proteins, NaV1.5 has been found to be regulated by several interacting proteins. In some cases, these different proteins, which reside in distinct membrane compartments (i.e. lateral membrane vs. intercalated disks), have been shown to interact with the same regulatory domain of NaV1.5, thus suggesting that several pools of NaV1.5 channels may co-exist in cardiac cells. The aim of this review article is to summarize the recent works that demonstrate its interaction with regulatory proteins and illustrate the model that the sodium channel NaV1.5 resides in distinct and different pools in cardiac cells. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Ca2+/calmodulin-dependent protein kinase II-based regulation of voltage-gated Na+ channel in cardiac disease

BACKGROUND

Human gene variants affecting ion channel biophysical activity and/or membrane localization are linked to potentially fatal cardiac arrhythmias. However, the mechanism for many human arrhythmia variants remains undefined despite more than a decade of investigation. Posttranslational modulation of membrane proteins is essential for normal cardiac function. Importantly, aberrant myocyte signaling has been linked to defects in cardiac ion channel posttranslational modifications and disease. We recently identified a novel pathway for posttranslational regulation of the primary cardiac voltage-gated Na(+) channel (Na(v)1.5) by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). However, a role for this pathway in cardiac disease has not been evaluated.

METHODS AND RESULTS

We evaluated the role of CaMKII-dependent phosphorylation in human genetic and acquired disease. We report an unexpected link between a short motif in the Na(v)1.5 DI-DII loop, recently shown to be critical for CaMKII-dependent phosphorylation, and Na(v)1.5 function in monogenic arrhythmia and common heart disease. Experiments in heterologous cells and primary ventricular cardiomyocytes demonstrate that the human arrhythmia susceptibility variants (A572D and Q573E) alter CaMKII-dependent regulation of Na(v)1.5, resulting in abnormal channel activity and cell excitability. In silico analysis reveals that these variants functionally mimic the phosphorylated channel, resulting in increased susceptibility to arrhythmia-triggering afterdepolarizations. Finally, we report that this same motif is aberrantly regulated in a large-animal model of acquired heart disease and in failing human myocardium.

CONCLUSIONS

We identify the mechanism for 2 human arrhythmia variants that affect Na(v)1.5 channel activity through direct effects on channel posttranslational modification. We propose that the CaMKII phosphorylation motif in the Na(v)1.5 DI-DII cytoplasmic loop is a critical nodal point for proarrhythmic changes to Na(v)1.5 in congenital and acquired cardiac disease.

Voltage-gated sodium channels: biophysics, pharmacology, and related channelopathies

Voltage-gated sodium channels (VGSC) are multi-molecular protein complexes expressed in both excitable and non-excitable cells. They are primarily formed by a pore-forming multi-spanning integral membrane glycoprotein (α-subunit) that can be associated with one or more regulatory β-subunits. The latter are single-span integral membrane proteins that modulate the sodium current (I_Na) and can also function as cell adhesion molecules. In vitro some of the cell-adhesive functions of the β-subunits may play important physiological roles independently of the α-subunits. Other endogenous regulatory proteins named “channel partners” or “channel interacting proteins” (ChiPs) like caveolin-3 and calmodulin/calmodulin kinase II (CaMKII) can also interact and modulate the expression and/or function of VGSC. In addition to their physiological roles in cell excitability and cell adhesion, VGSC are the site of action of toxins (like tetrodotoxin and saxitoxin), and pharmacologic agents (like antiarrhythmic drugs, local anesthetics, antiepileptic drugs, and newly developed analgesics). Mutations in genes that encode α- and/or β-subunits as well as the ChiPs can affect the structure and biophysical properties of VGSC, leading to the development of diseases termed sodium “channelopathies”.  This review will outline the structure, function, and biophysical properties of VGSC as well as their pharmacology and associated channelopathies and highlight some of the recent advances in this field.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

Sarcomeric gene mutations in sudden infant death syndrome (SIDS)

In developed countries, sudden infant death syndrome (SIDS) represents the most prevalent cause of death in children between 1 month and 1 year of age. SIDS is a diagnosis of exclusion, a negative autopsy which requires the absence of structural organ disease. Although investigators have confirmed that a significant percentage of SIDS cases are actually channelopathies, no data have been made available as to whether other sudden cardiac death-associated diseases, such as hypertrophic cardiomyopathy (HCM), could be responsible for some cases of SIDS. The presence of a genetic mutation in the sarcomeric protein usually affects the force of contraction of the myocyte, whose weakness is compensated with progressive hypertrophy and disarray. However, it is unclear whether in the most incipient forms, that is, first years of life, the lack of these phenotypes still confers a risk of arrhythmogenesis. The main goal of the present study is to wonder whether genetic defects in the sarcomeric proteins, previously associated with HCM, could be responsible for SIDS. We have analysed 286 SIDS cases for the most common genes implicated in HCM in adults. A total of 680 mutations localised in 16 genes were analysed by semi-automated matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDITOF-MS) using the Sequenom MassARRAY(®) System. Ten subjects with completely normal hearts showed mutated alleles at nine of the genetic variants analysed, and one additional novel mutation was detected by conventional sequencing. Therefore, a genetic mutation associated with HCM may cause sudden cardiac death in the absence of an identifiable phenotype.

Rippling muscle disease and facioscapulohumeral dystrophy-like phenotype in a patient carrying a heterozygous CAV3 T78M mutation and a D4Z4 partial deletion: Further evidence for "double trouble" overlapping syndromes

We report the first case of a heterozygous T78M mutation in the caveolin-3 gene (CAV3) associated with rippling muscle disease and proximal myopathy. The patient displayed also bilateral winged scapula with limited abduction of upper arms and marked asymmetric atrophy of leg muscles shown by magnetic resonance imaging. Immunohistochemistry on the patient’s muscle biopsy demonstrated a reduction of caveolin-3 staining, compatible with the diagnosis of caveolinopathy. Interestingly, consistent with the possible diagnosis of FSHD, the patient carried a 35 kb D4Z4 allele on chromosome 4q35. We discuss the hypothesis that the two genetic mutations may exert a synergistic effect in determining the phenotype observed in this patient.

Caveolins and heart diseases

Caveolins serve as a platform in plasma membrane associated caveolae to orchestrate various signaling molecules to effectively communicate extracellular signals into the interior of cell. All three types of caveolin, Cav-1, Cav-2 and Cav-3 are expressed throughout the cardiovascular system especially by the major cell types involved including endothelial cells, cardiac myocytes, smooth muscle cells and fibroblasts. The functional significance of caveolins in the cardiovascular system is evidenced by the fact that caveolin loss leads to the development of severe cardiac pathology. Caveolin gene mutations are associated with altered expression of caveolin protein and inherited arrhythmias. Altered levels of caveolins and related downstream signaling molecules in cardiomyopathies validate the integral participation of caveolin in normal cardiac physiology. This chapter will provide an overview of the role caveolins play in cardiovascular disease. Furthering our understanding of the role for caveolins in cardiovascular pathophysiology has the potential to lead to the manipulation of caveolins as novel therapeutic targets.

Pathophysiology of the cardiac late Na current and its potential as a drug target

A pathological increase in the late component of the cardiac Na(+) current, I(NaL), has been linked to disease manifestation in inherited and acquired cardiac diseases including the long QT variant 3 (LQT3) syndrome and heart failure. Disruption in I(NaL) leads to action potential prolongation, disruption of normal cellular repolarization, development of arrhythmia triggers, and propensity to ventricular arrhythmia. Attempts to treat arrhythmogenic sequelae from inherited and acquired syndromes pharmacologically with common Na(+) channel blockers (e.g. flecainide, lidocaine, and amiodarone) have been largely unsuccessful. This is due to drug toxicity and the failure of most current drugs to discriminate between the peak current component, chiefly responsible for single cell excitability and propagation in coupled tissue, and the late component (I(NaL)) of the Na(+) current. Although small in magnitude as compared to the peak Na(+) current (~1-3%), I(NaL) alters action potential properties and increases Na(+) loading in cardiac cells. With the increasing recognition that multiple cardiac pathological conditions share phenotypic manifestations of I(NaL) upregulation, there has been renewed interest in specific pharmacological inhibition of I(Na). The novel antianginal agent ranolazine, which shows a marked selectivity for late versus peak Na(+) current, may represent a novel drug archetype for targeted reduction of I(NaL). This article aims to review common pathophysiological mechanisms leading to enhanced I(NaL) in LQT3 and heart failure as prototypical disease conditions. Also reviewed are promising therapeutic strategies tailored to alter the molecular mechanisms underlying I(Na) mediated arrhythmia triggers.

LQTS-associated mutation A257G in α1-syntrophin interacts with the intragenic variant P74L to modify its biophysical phenotype

The SNTA1-encoded α1-syntrophin (SNTA1) missense mutation, p.A257G, causes long QT syndrome (LQTS) by pathogenic accentuation of Nav1.5's sodium current (I_Na). Subsequently, we found p.A257G in combination with the SNTA1 polymorphism, p.P74L in 4 victims of sudden infant death syndrome (SIDS) as well as in 3 adult controls. We hypothesized that p.P74L-SNTA1 could functionally modify the pathogenic phenotype of p.A257G-SNTA1, thus explaining its occurrence in non-LQTS populations. The SNTA1 variants p.P74L, p.A257G, and the combination variant p.P74L/p.A257G were engineered using PCR-based overlap-extension and were co-expressed heterologously with SCN5A in HEK293 cells. I_Na was recorded using the whole-cell method. Compared to wild-type (WT), the significant increase in peak I_Na and window current found with p.A257G was reversed by the intragenic variant p.P74L (p.P74L/p.A257G). These results report for the first time the intragenic rescue of an LQT-associated SNTA1 mutation when found in combination with the SNTA1 polymorphism p.P74L, suggesting an ever-increasing picture of complexity in terms of genetic risk stratification for arrhythmia.

Molecular Investigations of Sudden Unexplained Deaths

Sudden unexplained deaths in apparently healthy individuals (newborn through adult) pose a vexing challenge to medical examiners, law enforcement and society as a whole. Recent advances in “molecular autopsies” have begun to uncover the mystery surrounding sudden unexplained deaths by identifying mutations that can result in or predispose an apparently healthy individual to sudden death. Genetic risks of sudden unexplained deaths have been studied from several different perspectives, and categorized generally by systems, including: cardiac, nervous, immune, and metabolic. This article reviews the genetic risks in sudden unexplained deaths, presents the current state and challenges of molecular investigations, and sheds light on future directions in sudden unexplained death investigations.

Cardiac channelopathies and sudden infant death syndrome

Sudden infant death syndrome (SIDS) is always a devastating and unexpected occurrence. SIDS is the leading cause of death in the first 6 months after birth in the industrialized world. Since the discovery in 1998 of long QT syndrome as an underlying substrate for SIDS, around 10-20% of SIDS cases have been proposed as being caused by genetic variants in either ion channel or ion channel-associated proteins. Until now, 10 cardiac channelopathy susceptibility genes have been found to be implicated in the pathogenesis of SIDS. Four of the genes encode cardiac ion channel α-subunits, 3 genes encode ion channel β-subunits, and 3 genes encode other channel-interacting proteins. All 10 genes have been associated with primary electrical heart diseases. SIDS may hereby be the initial symptom of rare primary electric channelopathies such as long QT, short QT and Brugada syndrome, as well as catecholaminergic polymorphic ventricular tachycardia. In this review we describe the functional role of sodium, potassium and calcium channels in propagation, depolarization and repolarization in the context of the 4 arrhythmogenic diseases reported to be associated with SIDS. Lastly, the possibility of postmortem genetic testing and potential recommendations on how to deal with family members are discussed.

Genetics of heart failure and sudden death

Since the sentinel discovery of long QT syndrome as a channelopathy in 1995, many significant strides have been made related to exposing the pathogenic mechanisms underlying sudden cardiac death. However, elucidating the most influential genetic and environmental determinants that underlie the variable penetrance and expressivity of the primary syndrome-associated mutation remains a daunting task.

Caveolinopathies: translational implications of caveolin-3 in skeletal and cardiac muscle disorders

Caveolae are specialized lipid rafts localized on the cytoplasmic surface of the sarcolemmal membrane. Caveolae contribute to the maintenance of plasma membrane integrity, constitute specific macromolecular complexes that provide highly localized regulation of ion channels, and regulate vesicular trafficking and signal transduction. In skeletal muscle, the main structural assembly of caveolae is mediated by caveolin-3. Another family of adapter proteins, the cavins, is involved in the regulation of caveolae function and in the trafficking of caveolin-derived structures. Caveolin-3 defects lead to four distinct skeletal muscle disease phenotypes: limb-girdle muscular dystrophy, rippling muscle disease, distal myopathy, and hyperCKemia. Many patients show an overlap of these symptoms, and the same mutation can be linked to different clinical phenotypes. An ever-growing interest is also focused on the association between caveolin-3 mutations and heart disorders. Indeed, caveolin-3 mutants have been described in a patient with hypertrophic cardiomyopathy and two patients with dilated cardiomyopathy, and mutations in the caveolin-3 gene (CAV3) have been identified in patients affected by congenital long QT syndrome. Although caveolin-3 deficiency represents the primary event, multiple secondary molecular mechanisms lead to muscle tissue damage. Among these, sarcolemmal membrane alterations, disorganization of skeletal muscle T-tubule network, and disruption of distinct cell signaling pathways have been determined.

LQTS gene LOVD database

The Long QT Syndrome (LQTS) is a group of genetically heterogeneous disorders that predisposes young individuals to ventricular arrhythmias and sudden death. LQTS is mainly caused by mutations in genes encoding subunits of cardiac ion channels (KCNQ1, KCNH2,SCN5A, KCNE1, and KCNE2). Many other genes involved in LQTS have been described recently(KCNJ2, AKAP9, ANK2, CACNA1C, SCNA4B, SNTA1, and CAV3). We created an online database(http://www.genomed.org/LOVD/introduction.html) that provides information on variants in LQTS-associated genes. As of February 2010, the database contains 1738 unique variants in 12 genes. A total of 950 variants are considered pathogenic, 265 are possible pathogenic, 131 are unknown/unclassified, and 292 have no known pathogenicity. In addition to these mutations collected from published literature, we also submitted information on gene variants, including one possible novel pathogenic mutation in the KCNH2 splice site found in ten Chinese families with documented arrhythmias. The remote user is able to search the data and is encouraged to submit new mutations into the database. The LQTS database will become a powerful tool for both researchers and clinicians.

Genomic risk factors in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is a major contributor to postneonatal infant death, and is the third leading cause of infant mortality in the USA. While public health efforts have reduced these deaths in recent years, the pathogenesis of SIDS remains unclear. Epidemiological data on SIDS-related deaths have suggested genetic factors, and many studies have attempted to identify SIDS-associated genes. This has resulted in a large body of literature implicating various genes and their encoded proteins and signaling pathways in numerous cohorts of various sizes and ethnicities. This review has undertaken a systematic evaluation of these studies, identifying the pathways that have been implicated in these studies, including central nervous system pathways, cardiac channelopathies, immune dysfunction, metabolism/energy pathways, and nicotine response. This review also explores how new genomic techniques will aid in advancing our knowledge of the genomic risk factors associated with SIDS, including SNPs and copy number variation. Last, this review explores how the current information can be applied to aid in our assessment of the at risk infant population.