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

Caveolin-3 and Caveolin-1 Interaction Decreases Channel Dysfunction Due to Caveolin-3 Mutations

Caveolae constitute membrane microdomains where receptors and ion channels functionally interact. Caveolin-3 (cav-3) is the key structural component of muscular caveolae. Mutations in CAV3 lead to caveolinopathies, which result in both muscular dystrophies and cardiac diseases. In cardiomyocytes, cav-1 participates with cav-3 to form caveolae; skeletal myotubes and adult skeletal fibers do not express cav-1. In the heart, the absence of cardiac alterations in the majority of cases may depend on a conserved organization of caveolae thanks to the expression of cav-1. We decided to focus on three specific cav-3 mutations (Δ62-64YTT; T78K and W101C) found in heterozygosis in patients suffering from skeletal muscle disorders. We overexpressed both the WT and mutated cav-3 together with ion channels interacting with and modulated by cav-3. Patch-clamp analysis conducted in caveolin-free cells (MEF-KO), revealed that the T78K mutant is dominant negative, causing its intracellular retention together with cav-3 WT, and inducing a significant reduction in current densities of all three ion channels tested. The other cav-3 mutations did not cause significant alterations. Mathematical modelling of the effects of cav-3 T78K would impair repolarization to levels incompatible with life. For this reason, we decided to compare the effects of this mutation in other cell lines that endogenously express cav-1 (MEF-STO and CHO cells) and to modulate cav-1 expression with an shRNA approach. In these systems, the membrane localization of cav-3 T78K was rescued in the presence of cav-1, and the current densities of hHCN4, hKv1.5 and hKir2.1 were also rescued. These results constitute the first evidence of a compensatory role of cav-1 in the heart, justifying the reduced susceptibility of this organ to caveolinopathies.

Congenital Long QT Syndrome (LQTS) in Infancy: A Challenging Case

Long QT syndrome (LQTS), is an arrhythmia disorder, related to ventricular myocardial repolarization characterized by a prolonged QT interval on the electrocardiogram that can lead to symptomatic ventricular arrhythmias and increase the mortality rate. The prevalence of congenital LQTS is about 1 in 2000 live births. Here, we report the case of a two-month-old female, with a significant family history of early death, who was brought to our emergency with an episode of blueish discoloration. The initial workup was positive for COVID-19 in the respiratory panel, so she was admitted as a case of bronchiolitis. It was a challenge because of the overlapping presentation between a serious condition and other common pediatric illnesses. Furthermore, the paper aims to increase awareness of this condition among physicians and emphasizes the importance of obtaining a complete medical history, physical examination, and family screening in selected cases to facilitate early diagnosis and timely management.

Caveolin-3 and Caveolae regulate ventricular repolarization

RATIONALE

Flask-shaped invaginations of the cardiomyocyte sarcolemma called caveolae require the structural protein caveolin-3 (Cav-3) and host a variety of ion channels, transporters, and signaling molecules. Reduced Cav-3 expression has been reported in models of heart failure, and variants in CAV3 have been associated with the inherited long-QT arrhythmia syndrome. Yet, it remains unclear whether alterations in Cav-3 levels alone are sufficient to drive aberrant repolarization and increased arrhythmia risk.

OBJECTIVE

To determine the impact of cardiac-specific Cav-3 ablation on the electrophysiological properties of the adult mouse heart.

METHODS AND RESULTS

Cardiac-specific, inducible Cav3 homozygous knockout (Cav-3KO) mice demonstrated a marked reduction in Cav-3 expression by Western blot and loss of caveolae by electron microscopy. However, there was no change in macroscopic cardiac structure or contractile function. The QTc interval was increased in Cav-3KO mice, and there was an increased propensity for ventricular arrhythmias. Ventricular myocytes isolated from Cav-3KO mice exhibited a prolonged action potential duration (APD) that was due to reductions in outward potassium currents (Ito, Iss) and changes in inward currents including slowed inactivation of ICa,L and increased INa,L. Mathematical modeling demonstrated that the changes in the studied ionic currents were adequate to explain the prolongation of the mouse ventricular action potential. Results from human iPSC-derived cardiomyocytes showed that shRNA knockdown of Cav-3 similarly prolonged APD.

CONCLUSION

We demonstrate that Cav-3 and caveolae regulate cardiac repolarization and arrhythmia risk via the integrated modulation of multiple ionic currents.

Precision medicine for long QT syndrome: patient-specific iPSCs take the lead

Long QT syndrome (LQTS) is a detrimental arrhythmia syndrome mainly caused by dysregulated expression or aberrant function of ion channels. The major clinical symptoms of ventricular arrhythmia, palpitations and syncope vary among LQTS subtypes. Susceptibility to malignant arrhythmia is a result of delayed repolarisation of the cardiomyocyte action potential (AP). There are 17 distinct subtypes of LQTS linked to 15 autosomal dominant genes with monogenic mutations. However, due to the presence of modifier genes, the identical mutation may result in completely different clinical manifestations in different carriers. In this review, we describe the roles of various ion channels in orchestrating APs and discuss molecular aetiologies of various types of LQTS. We highlight the usage of patient-specific induced pluripotent stem cell (iPSC) models in characterising fundamental mechanisms associated with LQTS. To mitigate the outcomes of LQTS, treatment strategies are initially focused on small molecules targeting ion channel activities. Next-generation treatments will reap the benefits from development of LQTS patient-specific iPSC platform, which is bolstered by the state-of-the-art technologies including whole-genome sequencing, CRISPR genome editing and machine learning. Deep phenotyping and high-throughput drug testing using LQTS patient-specific cardiomyocytes herald the upcoming precision medicine in LQTS.

Caveolin-3 and Arrhythmias: Insights into the Molecular Mechanisms

Caveolin-3 is a muscle-specific protein on the membrane of myocytes correlated with a variety of cardiovascular diseases. It is now clear that the caveolin-3 plays a critical role in the cardiovascular system and a significant role in cardiac protective signaling. Mutations in the gene encoding caveolin-3 cause a broad spectrum of clinical phenotypes, ranging from persistent elevations in the serum levels of creatine kinase in asymptomatic humans to cardiomyopathy. The influence of Caveolin-3(CAV-3) mutations on current density parallels the effect on channel trafficking. For example, mutations in the CAV-3 gene promote ventricular arrhythmogenesis in long QT syndrome 9 by a combined decrease in the loss of the inward rectifier current (I_K1) and gain of the late sodium current (I_Na-L). The functional significance of the caveolin-3 has proved that caveolin-3 overexpression or knockdown contributes to the occurrence and development of arrhythmias. Caveolin-3 overexpression could lead to reduced diastolic spontaneous Ca²⁺ waves, thus leading to the abnormal L-Type calcium channel current-induced ventricular arrhythmias. Moreover, CAV-3 knockdown resulted in a shift to more negative values in the hyperpolarization-activated cyclic nucleotide channel 4 current (I_HCN4) activation curve and a significant decrease in I_HCN4 whole-cell current density. Recent evidence indicates that caveolin-3 plays a significant role in adipose tissue and is related to obesity development. The role of caveolin-3 in glucose homeostasis has attracted increasing attention. This review highlights the underlining mechanisms of caveolin-3 in arrhythmia. Progress in this field may contribute to novel therapeutic approaches for patients prone to developing arrhythmia.

Rare Variation in Drug Metabolism and Long QT Genes and the Genetic Susceptibility to Acquired Long QT Syndrome

BACKGROUND

Acquired long QT syndrome (aLQTS) is a serious unpredictable adverse drug reaction. Pharmacogenomic markers may predict risk.

METHODS

Among 153 aLQTS patients (mean age 58 years [range, 14-88], 98.7% White, 85.6% symptomatic), computational methods identified proteins interacting most significantly with 216 QT-prolonging drugs. All cases underwent sequencing of 31 candidate genes arising from this analysis or associating with congenital LQTS. Variants were filtered using a minor allele frequency <1% and classified for susceptibility for aLQTS. Gene-burden analyses were then performed comparing the primary cohort to control exomes (n=452) and an independent replication aLQTS exome sequencing cohort.

RESULTS

In 25.5% of cases, at least one rare variant was identified: 22.2% of cases carried a rare variant in a gene associated with congenital LQTS, and in 4% of cases that variant was known to be pathogenic or likely pathogenic for congenital LQTS; 7.8% cases carried a cytochrome-P450 (CYP) gene variant. Of 12 identified CYP variants, 11 (92%) were in an enzyme known to metabolize at least one culprit drug to which the subject had been exposed. Drug-drug interactions that affected culprit drug metabolism were found in 19% of cases. More than one congenital LQTS variant, CYP gene variant, or drug interaction was present in 7.8% of cases. Gene-burden analyses of the primary cohort compared to control exomes (n=452), and an independent replication aLQTS exome sequencing cohort (n=67) and drug-tolerant controls (n=148) demonstrated an increased burden of rare (minor allele frequency<0.01) variants in CYP genes but not LQTS genes.

CONCLUSIONS

Rare susceptibility variants in CYP genes are emerging as potentially important pharmacogenomic risk markers for aLQTS and could form part of personalized medicine approaches in the future.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Remodeling of Ion Channel Trafficking and Cardiac Arrhythmias

Both inherited and acquired cardiac arrhythmias are often associated with the abnormal functional expression of ion channels at the cellular level. The complex machinery that continuously traffics, anchors, organizes, and recycles ion channels at the plasma membrane of a cardiomyocyte appears to be a major source of channel dysfunction during cardiac arrhythmias. This has been well established with the discovery of mutations in the genes encoding several ion channels and ion channel partners during inherited cardiac arrhythmias. Fibrosis, altered myocyte contacts, and post-transcriptional protein changes are common factors that disorganize normal channel trafficking during acquired cardiac arrhythmias. Channel availability, described notably for hERG and K_V1.5 channels, could be another potent arrhythmogenic mechanism. From this molecular knowledge on cardiac arrhythmias will emerge novel antiarrhythmic strategies.

Investigating LMNA-Related Dilated Cardiomyopathy Using Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

LMNA-related dilated cardiomyopathy is an inherited heart disease caused by mutations in the LMNA gene encoding for lamin A/C. The disease is characterized by left ventricular enlargement and impaired systolic function associated with conduction defects and ventricular arrhythmias. We hypothesized that LMNA-mutated patients' induced Pluripotent Stem Cell-derived cardiomyocytes (iPSC-CMs) display electrophysiological abnormalities, thus constituting a suitable tool for deciphering the arrhythmogenic mechanisms of the disease, and possibly for developing novel therapeutic modalities. iPSC-CMs were generated from two related patients (father and son) carrying the same E342K mutation in the LMNA gene. Compared to control iPSC-CMs, LMNA-mutated iPSC-CMs exhibited the following electrophysiological abnormalities: (1) decreased spontaneous action potential beat rate and decreased pacemaker current (I_f) density; (2) prolonged action potential duration and increased L-type Ca²⁺ current (I_Ca,L) density; (3) delayed afterdepolarizations (DADs), arrhythmias and increased beat rate variability; (4) DADs, arrhythmias and cessation of spontaneous firing in response to β-adrenergic stimulation and rapid pacing. Additionally, compared to healthy control, LMNA-mutated iPSC-CMs displayed nuclear morphological irregularities and gene expression alterations. Notably, KB-R7943, a selective inhibitor of the reverse-mode of the Na⁺/Ca²⁺ exchanger, blocked the DADs in LMNA-mutated iPSC-CMs. Our findings demonstrate cellular electrophysiological mechanisms underlying the arrhythmias in LMNA-related dilated cardiomyopathy.

Sudden Infant Death Syndrome: Beyond Risk Factors

Sudden infant death syndrome (SIDS) is defined as "the sudden death of an infant under 1 year of age which remains unexplained after thorough investigation including a complete autopsy, death scene investigation, and detailed clinical and pathological review". A significant decrease of SIDS deaths occurred in the last decades in most countries after the beginning of national campaigns, mainly as a consequence of the implementation of risk reduction action mostly concentrating on the improvement of sleep conditions. Nevertheless, infant mortality from SIDS still remains unacceptably high. There is an urgent need to get insight into previously unexplored aspects of the brain system with a special focus on high-risk groups. SIDS pathogenesis is associated with a multifactorial condition that comprehends genetic, environmental and sociocultural factors. Effective prevention of SIDS requires multiple interventions from different fields. Developing brain susceptibility, intrinsic vulnerability and early identification of infants with high risk of SIDS represents a challenge. Progress in SIDS research appears to be fundamental to the ultimate aim of eradicating SIDS deaths. A complex model that combines different risk factor data from biomarkers and omic analysis may represent a tool to identify a SIDS risk profile in newborn settings. If high risk is detected, the infant may be referred for further investigations and follow ups. This review aims to illustrate the most recent discoveries from different fields, analyzing the neuroanatomical, genetic, metabolic, proteomic, environmental and sociocultural aspects related to SIDS.

Comprehensive Analysis of Genes Associated With Sudden Infant Death Syndrome

Background: Sudden infant death syndrome (SIDS) is a tragic incident which remains a mystery even after post-mortem investigation and thorough researches. Methods: This comprehensive review is based on the genes reported in the molecular autopsy studies conducted on SIDS so far. A total of 20 original studies and 7 case reports were identified and included in this analysis. The genes identified in children or adults were not included. Most of the genes reported in these studies belonged to cardiac channel and cardiomyopathy. Cardiac channel genes in SIDS were scrutinized for further analysis. Results: After screening and removing the duplicates, 42 unique genes were extracted. When the location of these genes was assessed, it was observed that most of these belonged to Chromosomes 11, 1 and 3 in sequential manner. The pathway analysis shows that these genes are involved in the regulation of heart rate, action potential, cardiac muscle cell contraction and heart contraction. The protein-protein interaction network was also very big and highly interactive. SCN5A, CAV3, ALG10B, AKAP9 and many more were mainly found in these cases and were regulated by many transcription factors such as MYOG C2C1 and CBX3 HCT11. Micro RNA, "hsa-miR-133a-3p" was found to be prevalent in the targeted genes. Conclusions: Molecular and computational approaches are a step forward toward exploration of these sad demises. It is so far a new arena but seems promising to dig out the genetic cause of SIDS in the years to come.

Genetic cause of heterogeneous inherited myopathies in a cohort of Greek patients

Inherited muscle disorders are caused by pathogenic changes in numerous genes. Herein, we aimed to investigate the etiology of muscle disease in 24 consecutive Greek patients with myopathy suspected to be genetic in origin, based on clinical presentation and laboratory and electrophysiological findings and absence of known acquired causes of myopathy. Of these, 16 patients (8 females, median 24 years-old, range 7 to 67 years-old) were diagnosed by Whole Exome Sequencing as suffering from a specific type of inherited muscle disorder. Specifically, we have identified causative variants in 6 limb-girdle muscular dystrophy genes (6 patients; ANO5, CAPN3, DYSF, ISPD, LAMA2, SGCA), 3 metabolic myopathy genes (4 patients; CPT2, ETFDH, GAA), 1 congenital myotonia gene (1 patient; CLCN1), 1 mitochondrial myopathy gene (1 patient; MT-TE) and 3 other myopathy-associated genes (4 patients; CAV3, LMNA, MYOT). In 6 additional family members affected by myopathy, we reached genetic diagnosis following identification of a causative variant in an index patient. In our patients, genetic diagnosis ended a lengthy diagnostic process and, in the case of Multiple acyl-CoA dehydrogenase deficiency and Pompe's disease, it enabled specific treatment to be initiated. These results further expand the genotypic and phenotypic spectrum of inherited myopathies.

A Role for Caveolin-3 in the Pathogenesis of Muscular Dystrophies

Caveolae are the cholesterol-rich small invaginations of the plasma membrane present in many cell types including adipocytes, endothelial cells, epithelial cells, fibroblasts, smooth muscles, skeletal muscles and cardiac muscles. They serve as specialized platforms for many signaling molecules and regulate important cellular processes like energy metabolism, lipid metabolism, mitochondria homeostasis, and mechano-transduction. Caveolae can be internalized together with associated cargo. The caveolae-dependent endocytic pathway plays a role in the withdrawal of many plasma membrane components that can be sent for degradation or recycled back to the cell surface. Caveolae are formed by oligomerization of caveolin proteins. Caveolin-3 is a muscle-specific isoform, whose malfunction is associated with several diseases including diabetes, cancer, atherosclerosis, and cardiovascular diseases. Mutations in Caveolin-3 are known to cause muscular dystrophies that are collectively called caveolinopathies. Altered expression of Caveolin-3 is also observed in Duchenne’s muscular dystrophy, which is likely a part of the pathological process leading to muscle weakness. This review summarizes the major functions of Caveolin-3 in skeletal muscles and discusses its involvement in the pathology of muscular dystrophies.

Post-mortem genetic investigation of cardiac disease-associated genes in sudden infant death syndrome (SIDS) cases

The sudden infant death syndrome (SIDS) is one of the leading causes of postneonatal infant death. It has been shown that there exists a complex relationship between SIDS and inherited cardiac disease. Next-generation sequencing and surveillance of cardiac channelopathy and cardiomyopathy genes represent an important tool for investigating the cause of death in SIDS cases. In the present study, targeted sequencing of 80 genes associated with genetic heart diseases in a cohort of 31 SIDS cases was performed. To determine the spectrum and prevalence of genetic heart disease associated mutations as a potential monogenic basis for SIDS, a stringent variant classification was applied and the percentage of rare (minor allele frequency ≤ 0.2%) and ultra-rare variants (minor allele frequency ≤ 0.005%) in these genes was assessed. With a minor allele frequency of ≤ 0.005%, about 20% of the SIDS cases exhibited a variant of uncertain significance (VUS), but in only 6% of these cases, gene variants proved to be “potentially informative.” The present study shows the importance of careful variant interpretation. Applying stringent criteria misinterpretations are avoided, as the results of genetic analyses may have an important impact of the family members involved.

Arrhythmogenic mechanisms of obstructive sleep apnea in heart failure patients

Heart failure (HF) affects 23 million people worldwide and results in 300000 annual deaths. It is associated with many comorbidities, such as obstructive sleep apnea (OSA), and risk factors for both conditions overlap. Eleven percent of HF patients have OSA and 7.7% of OSA patients have left ventricular ejection fraction <50% with arrhythmias being a significant comorbidity in HF and OSA patients. Forty percent of HF patients develop atrial fibrillation (AF) and 30%-50% of deaths from cardiac causes in HF patients are from sudden cardiac death. OSA is prevalent in 32%-49% of patients with AF and there is a dose-dependent relationship between OSA severity and resistance to anti-arrhythmic therapies. HF and OSA lead to various downstream arrhythmogenic mechanisms, including metabolic derangement, remodeling, inflammation, and autonomic imbalance. (1) Metabolic derangement and production of reactive oxidative species increase late Na+ currents, decrease outward K+ currents and downregulate connexin-43 and cell-cell coupling. (2) remodeling also features downregulated K+ currents in addition to decreased Na+/K+ ATPase currents, altered Ca2+ homeostasis, and increased density of If current. (3) Chronic inflammation leads to downregulation of both Nav1.5 channels and K+ channels, altered Ca2+ homeostasis and reduced cellular coupling from alterations of connexin expression. (4) Autonomic imbalance causes arrhythmias by evoking triggered activity through increased Ca2+ transients and reduction of excitation wavefront wavelength. Thus, consideration of these multiple pathophysiological pathways (1-4) will enable the development of novel therapeutic strategies that can be targeted against arrhythmias in the context of complex disease, such as the comorbidities of HF and OSA.

Cardiac Genetic Predisposition in Sudden Infant Death Syndrome

BACKGROUND

Sudden infant death syndrome (SIDS) is a leading cause of postneonatal mortality. Genetic heart diseases (GHDs) underlie some cases of SIDS.

OBJECTIVES

This study aimed to determine the spectrum and prevalence of GHD-associated mutations as a potential monogenic basis for SIDS.

METHODS

A cohort of 419 unrelated SIDS cases (257 male; average age 2.7 ± 1.9 months) underwent whole exome sequencing and a targeted analysis of 90 GHD-susceptibility genes. The yield of "potentially informative," ultra-rare variants (minor allele frequency <0.00005) in GHD-associated genes was assessed.

RESULTS

Overall, 53 of 419 (12.6%) SIDS cases had ≥1 "potentially informative," GHD-associated variant. The yield was 14.9% (21 of 141) for mixed-European ancestry cases and 11.5% (32 of 278) for European ancestry SIDS cases. Infants older than 4 months were more likely to host a "potentially informative" GHD-associated variant. There was significant overrepresentation of ultra-rare nonsynonymous variants in European SIDS cases (18 of 278 [6.5%]) versus European control subjects (30 of 973 [3.1%]; p = 0.013) when combining all 4 major cardiac channelopathy genes (KCNQ1, KCNH2, SCN5A, and RYR2). According to the American College of Medical Genetics guidelines, only 18 of 419 (4.3%) SIDS cases hosted a "pathogenic" or "likely pathogenic" variant.

CONCLUSIONS

Less than 15% of more than 400 SIDS cases had a "potentially informative" variant in a GHD-susceptibility gene, predominantly in the 4- to 12-month age group. Only 4.3% of cases possessed immediately clinically actionable variants. Consistent with previous studies, ultra-rare, nonsynonymous variants within the major cardiac channelopathy-associated genes were overrepresented in SIDS cases in infants of European ethnicity. These findings have major implications for the investigation of SIDS cases and families.

Exome-Wide Rare Variant Analyses in Sudden Infant Death Syndrome

OBJECTIVE

To determine whether a monogenic basis explains sudden infant death syndrome (SIDS) using an exome-wide focus.

STUDY DESIGN

A cohort of 427 unrelated cases of SIDS (257 male; average age = 2.7 ± 1.9 months) underwent whole-exome sequencing. Exome-wide rare variant analyses were carried out with 278 SIDS cases of European ancestry (173 male; average age = 2.7 ± 1.98 months) and 973 ethnic-matched controls based on 6 genetic models. Ingenuity Pathway Analysis also was performed. The cohort was collected in collaboration with coroners, medical examiners, and pathologists by St George's University of London, United Kingdom, and Mayo Clinic, Rochester, Minnesota. Whole-exome sequencing was performed at the Genomic Laboratory, Kings College London, United Kingdom, or Mayo Clinic's Medical Genome Facility, Rochester, Minnesota.

RESULTS

Although no exome-wide significant (P < 2.5 × 10-6) difference in burden of ultra-rare variants was detected for any gene, 405 genes had a greater prevalence (P < .05) of ultra-rare nonsynonymous variants among cases with 17 genes at P < .005. Some of these potentially overrepresented genes may represent biologically plausible novel candidate genes for a monogenic basis for a portion of patients with SIDS. The top canonical pathway identified was glucocorticoid biosynthesis (P = .01).

CONCLUSIONS

The lack of exome-wide significant genetic associations indicates an extreme heterogeneity of etiologies underlying SIDS. Our approach to understanding the genetic mechanisms of SIDS has far reaching implications for the SIDS research community as a whole and may catalyze new evidence-based SIDS research across multiple disciplines. Perturbations in glucocorticoid biosynthesis may represent a novel SIDS-associated biological pathway for future SIDS investigative research.

Caveolin-3 Microdomain: Arrhythmia Implications for Potassium Inward Rectifier and Cardiac Sodium Channel

In human cardiac ventricular myocytes, caveolin-3 functions as a scaffolding and regulatory protein for signaling molecules and compartmentalizes ion channels. Our lab has recently explored this sub-cellular microdomain and found that potassium inward rectifier Kir2.x is found in association with caveolin-3. The three cardiac Kir2.x isoforms (Kir2.1, Kir2.2, and Kir2.3) are the molecular correlates of I_K1 in the heart, of which Kir2.1 is the dominant isoform in the ventricle. Kir2.1 channels assemble with Kir2.2 and Kir2.3 forming hetero-tetramers that modulate I_K1. I_K1 sets the resting membrane potential and assists with terminal phase 3 ventricular repolarization. In our studies using native human ventricular tissue, Kir2.x co-localizes with caveolin-3 and significance of the association between Kir2.x and caveolin-3 is emphasized in relation to mutations in the gene which encodes caveolin-3, CAV3, associated with Long QT Syndrome 9 (LQT9). LQT9-associated CAV3 mutations cause decreased current density in Kir2.1 and Kir2.2 as homomeric and heteromeric channels, which affects repolarization and membrane potential stability. A portion of Kir2.1 cardiac localization parallels that of the cardiac sodium channel (Nav1.5). This may have implications for Long QT9 in which CAV3 mutations cause an increase in the late current of Nav1.5 (I_Na-L) via nNOS mediated nitrosylation of Nav1.5. In iPS-CMs, expression of LQT9 CAV3 mutations resulted in action potential duration (APD) prolongation and early-after depolarizations (EADs), supporting the arrhythmogenicity of LQT9. To evaluate the combined effect of the CAV3 mutants on I_Na-L and I_K1, we studied both ventricular and Purkinje myocyte mathematical modeling. Interestingly, mathematical ventricular myocytes, similar to iPS-CMs, demonstrated EADs but no sustained arrhythmia. In contrast, Purkinje modeling demonstrated delayed-after depolarizations (DADs) driven mechanism for sustained arrhythmia, dependent on the combined loss of I_K1 and gain of I_Na-L. This finding changes the overall assumed arrhythmia phenotype for LQT9. In future studies, we are exploring caveolar micro-domain disruption in heart failure and how this effects Kir2.x and Nav1.5. Here we review the caveolae cardiac microdomain of Kir2.x and Nav1.5 and explore some of the downstream effects of caveolin-3 and caveolae disruption in specific clinical scenarios.

The Cardiac Sodium Channel and Its Protein Partners

Activation of the electrical signal and its transmission as a depolarizing wave in the whole heart requires highly organized myocyte architecture and cell-cell contacts. In addition, complex trafficking and anchoring intracellular machineries regulate the proper surface expression of channels and their targeting to distinct membrane domains. An increasing list of proteins, lipids, and second messengers can contribute to the normal targeting of ion channels in cardiac myocytes. However, their precise roles in the electrophysiology of the heart are far from been extensively understood. Nowadays, much effort in the field focuses on understanding the mechanisms that regulate ion channel targeting to sarcolemma microdomains and their organization into macromolecular complexes. The purpose of the present section is to provide an overview of the characterized partners of the main cardiac sodium channel, Na_V1.5, involved in regulating the functional expression of this channel both in terms of trafficking and targeting into microdomains.

Long QT Syndrome and Sinus Bradycardia-A Mini Review

Congenital long-QT syndrome (LQTS) is an inherited cardiac disorder characterized by the prolongation of ventricular repolarization, susceptibility to Torsades de Pointes (TdP), and a risk for sudden death. Various types of congenital LQTS exist, all due to specific defects in ion channel-related genes. Interestingly, almost all of the ion channels affected by the various types of LQTS gene mutations are also expressed in the human sinoatrial node (SAN). It is therefore not surprising that LQTS is frequently associated with a change in basal heart rate (HR). However, current data on how the LQTS-associated ion channel defects result in impaired human SAN pacemaker activity are limited. In this mini-review, we provide an overview of known LQTS mutations with effects on HR and the underlying changes in expression and kinetics of ion channels. Sinus bradycardia has been reported in relation to a large number of LQTS mutations. However, the occurrence of both QT prolongation and sinus bradycardia on a family basis is almost completely limited to LQTS types 3 and 4 (LQT3 and Ankyrin-B syndrome, respectively). Furthermore, a clear causative role of this sinus bradycardia in cardiac events seems reserved to mutations underlying LQT3.

The expression of the rare caveolin-3 variant T78M alters cardiac ion channels function and membrane excitability

Aims

Caveolinopathies are a family of genetic disorders arising from alterations of the caveolin-3 (cav-3) gene. The T78M cav-3 variant has been associated with both skeletal and cardiac muscle pathologies but its functional contribution, especially to cardiac diseases, is still controversial. Here, we evaluated the effect of the T78M cav-3 variant on cardiac ion channel function and membrane excitability.

Methods and results

We transfected either the wild type (WT) or T78M cav-3 in caveolin-1 knock-out mouse embryonic fibroblasts and found by immunofluorescence and electron microscopy that both are expressed at the plasma membrane and form caveolae. Two ion channels known to interact and co-immunoprecipitate with the cav-3, hKv1.5 and hHCN4, interact also with T78M cav-3 and reside in lipid rafts. Electrophysiological analysis showed that the T78M cav-3 causes hKv1.5 channels to activate and inactivate at more hyperpolarized potentials and the hHCN4 channels to activate at more depolarized potentials, in a dominant way. In spontaneously beating neonatal cardiomyocytes, the expression of the T78M cav-3 significantly increased action potential peak-to-peak variability without altering neither the mean rate nor the maximum diastolic potential. We also found that in a small cohort of patients with supraventricular arrhythmias, the T78M cav-3 variant is more frequent than in the general population. Finally, in silico analysis of both sinoatrial and atrial cell models confirmed that the T78M-dependent changes are compatible with a pro-arrhythmic effect.

Conclusion

This study demonstrates that the T78M cav-3 induces complex modifications in ion channel function that ultimately alter membrane excitability. The presence of the T78M cav-3 can thus generate a susceptible substrate that, in concert with other structural alterations and/or genetic mutations, may become arrhythmogenic.

Functional Invalidation of Putative Sudden Infant Death Syndrome-Associated Variants in the KCNH2-Encoded Kv11.1 Channel

BACKGROUND

Heterologous functional validation studies of putative long-QT syndrome subtype 2-associated variants clarify their pathological potential and identify disease mechanism(s) for most variants studied. The purpose of this study is to clarify the pathological potential for rare nonsynonymous KCNH2 variants seemingly associated with sudden infant death syndrome.

METHODS

Genetic testing of 292 sudden infant death syndrome cases identified 9 KCNH2 variants: E90K, R181Q, A190T, G294V, R791W, P967L, R1005W, R1047L, and Q1068R. Previous studies show R181Q-, P967L-, and R1047L-Kv11.1 channels function similar to wild-type Kv11.1 channels, whereas Q1068R-Kv11.1 channels accelerate inactivation gating. We studied the biochemical and biophysical properties for E90K-, G294V-, R791W-, and R1005W-Kv11.1 channels expressed in human embryonic kidney 293 cells; examined the electronic health records of patients who were genotype positive for the sudden infant death syndrome-linked KCNH2 variants; and simulated their functional impact using computational models of the human ventricular action potential.

RESULTS

Western blot and voltage-clamping analyses of cells expressing E90K-, G294V-, R791W-, and R1005W-Kv11.1 channels demonstrated these variants express and generate peak Kv11.1 current levels similar to cells expressing wild-type-Kv11.1 channels, but R791W- and R1005W-Kv11.1 channels accelerated deactivation and activation gating, respectively. Electronic health records of patients with the sudden infant death syndrome-linked KCNH2 variants showed that the patients had median heart rate-corrected QT intervals <480 ms and none had been diagnosed with long-QT syndrome or experienced cardiac arrest. Simulating the impact of dysfunctional gating variants predicted that they have little impact on ventricular action potential duration.

CONCLUSIONS

We conclude that these rare Kv11.1 missense variants are not long-QT syndrome subtype 2-causative variants and therefore do not represent the pathogenic substrate for sudden infant death syndrome in the variant-positive infants.

Multiple genetic variations in sodium channel subunits in a case of sudden infant death syndrome

BACKGROUND

Dysfunction of Na_V 1.5 encoded by SCN5A accounts for approximately half of the channelopathic SIDS cases. We investigated the functional effect of two gene variants identified in the same patient, one in SCN5A and one in SCN1Bb. The aim of the study was to risk stratify the proband's family.

METHODS

The family was referred for cardiovascular genetic evaluation to assess familial risk of cardiac disease. Functional analysis of the identified variants was performed with patch-clamp electrophysiology in HEK293 cells.

RESULTS

A 16-month-old healthy boy died suddenly in the context of nonspecific illness and possible fever. Postmortem genetic testing revealed variants in the SCN5A and SCN1Bb genes. The proband's father carries the same variants but is asymptomatic. Electrophysiological analysis of the Na_V 1.5_1281X truncation revealed complete loss-of-function of the channel. Coexpression of Na_V 1.5 with Na_V β1b significantly increased I_Na density when compared to Na_V 1.5 alone. The Na_V β1b _V268I variant abolished this I_Na density increase. Moreover, it shifted the activation curve toward more depolarized potentials.

CONCLUSIONS

Genetic variation of both sodium channel and its modifiers may contribute to sudden unexplained death in childhood. However, the asymptomatic father suggests that genetic variation of these genes is not sufficient to cause sudden death or clinically detectable SCN5A phenotypes.

Distinguishing pathogenic mutations from background genetic noise in cardiology: The use of large genome databases for genetic interpretation

Advances in clinical genetic testing have led to increased insight into the human genome, including how challenging it is to interpret rare genetic variation. In some cases, the ability to detect genetic mutations exceeds the ability to understand their clinical impact, limiting the advantage of these technologies. Obstacles in genomic medicine are many and include: understanding the level of certainty/uncertainty behind pathogenicity determination, the numerous different variant interpretation-guidelines used by clinical laboratories, delivering the certain or uncertain result to the patient, helping patients evaluate medical decisions in light of uncertainty regarding the consequence of the findings. Through publication of large publicly available exome/genome databases, researchers and physicians are now able to highlight dubious variants previously associated with different cardiac traits. Also, continuous efforts through data sharing, international collaborative efforts to develop disease-gene-specific guidelines, and computational analyses using large data, will indubitably assist in better variant interpretation and classification. This article discusses the current, and quickly changing, state of variant interpretation resources within cardiovascular genetic research, e.g., publicly available databases and ways of how cardiovascular genetic counselors and geneticists can aid in improving variant interpretation in cardiology.

Sudden infant death syndrome and inherited cardiac conditions

Sudden infant death syndrome (SIDS) is the leading cause of post-neonatal infant mortality in developed countries, characterized by the death of infants for no obvious reason and without prior warning. The complex interaction of multiple factors in the pathogenesis of SIDS is illustrated by the 'triple risk hypothesis', which proposed that SIDS results from a convergence of three overlapping risk factors: a critical developmental period, an exogenous stressor, and underlying genetic and/or nongenetic vulnerability in the infant. Rare variants in genes associated with inherited arrhythmia syndromes and cardiomyopathies have been proposed as the substrate for an infant's critical vulnerability in a small subset of SIDS cases. Given the potential risk of inherited cardiac disease, current guidelines recommend post-mortem genetic testing (molecular autopsy) and cardiological investigation of the surviving family, complemented by targeted genetic testing if appropriate. In this Review, we highlight the latest developments in understanding the spectrum and prevalence of cardiac-mediated SIDS, and discuss the clinical implications of SIDS in the surviving family and the general population.

Sudden cardiac death: focus on the genetics of channelopathies and cardiomyopathies

Sudden cardiac death (SCD) describes a natural and unexpected death from cardiac causes occurring within a short period of time (generally within 1 h of symptom onset) in the absence of any other potentially lethal condition. Most SCD-related diseases have a genetic basis; in particular congenital cardiac channelopathies and cardiomyopathies have been described as leading causes of SCD. Congenital cardiac channelopathies are primary electric disorders caused by mutations affecting genes encoding cardiac ion channels or associated proteins, whereas cardiomyopathies are related to mutations in genes encoding several categories of proteins, including those of sarcomeres, desmosomes, the cytoskeleton, and the nuclear envelope. The purpose of this review is to provide a general overview of the main genetic variants that have been linked to the major congenital cardiac channelopathies and cardiomyopathies. Functional alterations of the related proteins are also described.

Infection: the neglected paradigm in SIDS research

Despite decades of investigation and millions of dollars spent, the cause of sudden infant death syndrome (SIDS) eludes researchers. It is timely therefore to reconsider the reasons for this failure and to explore how research might go forward with better prospects. This review assesses SIDS research in the context of clinicopathological and epidemiological features and determines that only infection attains congruence.

Landscape and variation of novel retroduplications in 26 human populations

Retroduplications come from reverse transcription of mRNAs and their insertion back into the genome. Here, we performed comprehensive discovery and analysis of retroduplications in a large cohort of 2,535 individuals from 26 human populations, as part of 1000 Genomes Phase 3. We developed an integrated approach to discover novel retroduplications combining high-coverage exome and low-coverage whole-genome sequencing data, utilizing information from both exon-exon junctions and discordant paired-end reads. We found 503 parent genes having novel retroduplications absent from the reference genome. Based solely on retroduplication variation, we built phylogenetic trees of human populations; these represent superpopulation structure well and indicate that variable retroduplications are effective population markers. We further identified 43 retroduplication parent genes differentiating superpopulations. This group contains several interesting insertion events, including a SLMO2 retroduplication and insertion into CAV3, which has a potential disease association. We also found retroduplications to be associated with a variety of genomic features: (1) Insertion sites were correlated with regular nucleosome positioning. (2) They, predictably, tend to avoid conserved functional regions, such as exons, but, somewhat surprisingly, also avoid introns. (3) Retroduplications tend to be co-inserted with young L1 elements, indicating recent retrotranspositional activity, and (4) they have a weak tendency to originate from highly expressed parent genes. Our investigation provides insight into the functional impact and association with genomic elements of retroduplications. We anticipate our approach and analytical methodology to have application in a more clinical context, where exome sequencing data is abundant and the discovery of retroduplications can potentially improve the accuracy of SNP calling.

Sudden infant death syndrome due to long QT syndrome: a brief review of the genetic substrate and prevalence

The pathophysiological mechanisms which lead to sudden infant death syndrome (SIDS) are not completely understood. Cardiac channelopathies are a well-established causative factor with long QT syndrome (LQTS) being the most frequent one, accounting for approximately 12% of SIDS cases. The genetic substrate of the above arrhythmogenic syndrome has been thoroughly described but only specific gene mutations or polymorphisms have been identified as SIDS causative. The review will focus on the prevalence of LQTS-induced SIDS or near-SIDS cases and the mutations held responsible. A literature search was performed in PubMed and Scopus electronic databases. Search terms used were: long QT syndrome, channelopathies, QT prolongation, cardiac ion channels. The above-mentioned search terms were always combined with the term: sudden infant death syndrome. Study types considered eligible were: case–control, family pedigree analysis, case reports. The prevalence of LQTS-induced SIDS according to six broad genetic studies ranges from 3.9 to 20.6%, with an average of 12%. Since LQTS can be effectively managed, LQTS-related SIDS cases could be prevented, provided that a screening method is efficient enough to detect all the affected infants.

The Caveolin-3 P104L mutation of LGMD-1C leads to disordered glucose metabolism in muscle cells

Caveolin-3 (CAV3) is a muscle specific protein that plays an important role in maintaining muscle health and glucose homeostasis in vivo. A novel autosomal dominant form of LGMD-1C in humans is due to a P104L mutation within the coding sequence of the human CAV3 gene. The mechanism by which the LGMD-1C mutation leads to muscle weakness remains unknown. Our objective was to determine whether muscle weakness was related to the imbalance of glucose metabolism. We found that when the P104L mutation was transiently transfected into C2C12 cells, there was decreased glucose uptake and glycogen synthesis after insulin stimulation. Immunoblotting analysis showed that the P104L mutation resulted in decreased expression of CAV3, CAV1 and pAkt. Confocal immunomicroscopy indicated that the P104L mutation reduced CAV3 and GLUT4 in the cell membrane, which accumulated mainly near the nucleus. This work is the first report of an association between muscle weakness due to LGMD-1C and energy metabolism. The P104L mutation led to a decrease in C2C12 muscle glucose uptake and glycogen synthesis and may be involved in the pathogenesis of LGMD-1C.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

PTRF/Cavin-1 Deficiency Causes Cardiac Dysfunction Accompanied by Cardiomyocyte Hypertrophy and Cardiac Fibrosis

Mutations in the PTRF/Cavin-1 gene cause congenital generalized lipodystrophy type 4 (CGL4) associated with myopathy. Additionally, long-QT syndrome and fatal cardiac arrhythmia are observed in patients with CGL4 who have homozygous PTRF/Cavin-1 mutations. PTRF/Cavin-1 deficiency shows reductions of caveolae and caveolin-3 (Cav3) protein expression in skeletal muscle, and Cav3 deficiency in the heart causes cardiac hypertrophy with loss of caveolae. However, it remains unknown how loss of PTRF/Cavin-1 affects cardiac morphology and function. Here, we present a characterization of the hearts of PTRF/Cavin-1-null (PTRF^−/−) mice. Electron microscopy revealed the reduction of caveolae in cardiomyocytes of PTRF^−/− mice. PTRF^−/− mice at 16 weeks of age developed a progressive cardiomyopathic phenotype with wall thickening of left ventricles and reduced fractional shortening evaluated by echocardiography. Electrocardiography revealed that PTRF^−/− mice at 24 weeks of age had low voltages and wide QRS complexes in limb leads. Histological analysis showed cardiomyocyte hypertrophy accompanied by progressive interstitial/perivascular fibrosis. Hypertrophy-related fetal gene expression was also induced in PTRF^−/− hearts. Western blotting analysis and quantitative RT-PCR revealed that Cav3 expression was suppressed in PTRF^−/− hearts compared with that in wild-type (WT) ones. ERK1/2 was activated in PTRF^−/− hearts compared with that in WT ones. These results suggest that loss of PTRF/Cavin-1 protein expression is sufficient to induce a molecular program leading to cardiomyocyte hypertrophy and cardiomyopathy, which is partly attributable to Cav3 reduction in the heart.

Sudden Unexpected Death in Fetal Life Through Early Childhood

In March 2015, the Eunice Kennedy Shriver National Institute of Child Health and Human Development held a workshop entitled "Sudden Unexpected Death in Fetal Life Through Early Childhood: New Opportunities." Its objective was to advance efforts to understand and ultimately prevent sudden deaths in early life, by considering their pathogenesis as a potential continuum with some commonalities in biological origins or pathways. A second objective of this meeting was to highlight current issues surrounding the classification of sudden infant death syndrome (SIDS), and the implications of variations in the use of the term "SIDS" in forensic practice, and pediatric care and research. The proceedings reflected the most current knowledge and understanding of the origins and biology of vulnerability to sudden unexpected death, and its environmental triggers. Participants were encouraged to consider the application of new technologies and "omics" approaches to accelerate research. The major advances in delineating the intrinsic vulnerabilities to sudden death in early life have come from epidemiologic, neural, cardiac, metabolic, genetic, and physiologic research, with some commonalities among cases of unexplained stillbirth, SIDS, and sudden unexplained death in childhood observed. It was emphasized that investigations of sudden unexpected death are inconsistent, varying by jurisdiction, as are the education, certification practices, and experience of death certifiers. In addition, there is no practical consensus on the use of "SIDS" as a determination in cause of death. Major clinical, forensic, and scientific areas are identified for future research.

Spectrum of clinical manifestations in two young Turkish patients with congenital generalized lipodystrophy type 4

Congenital generalized lipodystrophy type 4 is an extremely rare autosomal recessive disorder. We report our clinical experience on two unrelated Turkish patients with congenital generalized lipodystrophy type 4. A 13-year-old girl (patient-1) presented with generalized lipodystrophy and myopathy. Further tests revealed ventricular and supraventricular arrhythmias, gastrointestinal dysmotility, atlantoaxial instability, lumbosacral scoliosis, and metabolic abnormalities associated with insulin resistance. A 16-year-old girl (patient-2) with congenital generalized lipodystrophy type 4 was previously reported. Here, we report on her long term clinical follow-up. She received several course of anti-arrhythmic treatments for catecholaminergic polymorphic ventricular tachycardia and rapid atrial fibrillation. An implantable cardioverter defibrillator was also placed. A homozygous PTRF mutation, c.259C > T (p.Gln87*), was identified in patient-1. Congenital generalized lipodystrophy type 4 was caused by homozygous PTRF c.481-482insGTGA (p.Lys161Serfs*41) mutation in patient-2. Our data indicate that patients with congenital generalized lipodystrophy type 4 should be meticulously evaluated for cardiac, neuromuscular, gastrointestinal and skeletal diseases, as well as metabolic abnormalities associated with insulin resistance.

α1-Syntrophin Variant Identified in Drug-Induced Long QT Syndrome Increases Late Sodium Current

Drug-induced long-QT syndrome (diLQTS) is often due to drug block of I_Kr, especially in genetically susceptible patients with subclinical mutations in the I_Kr-encoding KCHN2. Few variants in the cardiac Na_V1.5 Na⁺ channel complex have been associated with diLQTS. We tested whether a novel SNTA1 (α1-syntrophin) variant (p.E409Q) found in a patient with diLQTS increases late sodium current (I_Na-L), thereby providing a disease mechanism. Electrophysiological studies were performed in HEK293T cells co-expressing human Na_V1.5/nNOS/PMCA4b with either wild type (WT) or SNTA1 variants (A390V-previously reported in congenital LQTS; and E409Q); and in adult rat ventricular cardiomyocytes infected with SNTA1 expressing adenoviruses (WT or one of the two SNTA1 variants). In HEK293T cells and in cardiomyocytes, there was no significant difference in the peak I_Na densities among the SNTA1 WT and variants. However, both variants increased I_Na-L (% of peak current) in HEK293T cells (0.58±0.10 in WT vs. 0.90±0.11 in A390V, p = 0.048; vs. 0.88±0.07 in E409Q, p = 0.023). In cardiomyocytes, I_Na-L was significantly increased by E409Q, but not by A390V compared to WT (0.49±0.14 in WT vs.0.94±0.23 in A390V, p = 0.099; vs. 1.12±0.24 in E409Q, p = 0.019). We demonstrated that a novel SNTA1 variant is likely causative for diLQTS by augmenting I_Na-L. These data suggest that variants within the Na_V1.5-interacting α1-syntrophin are a potential mechanism for diLQTS, thereby expanding the concept that variants within congenital LQTS loci can cause diLQTS.

Electrophysiology and metabolism of caveolin-3-overexpressing mice

Caveolin-3 (Cav-3) plays a critical role in organizing signaling molecules and ion channels involved in cardiac conduction and metabolism. Mutations in Cav-3 are implicated in cardiac conduction abnormalities and myopathies. Additionally, cardiac-specific overexpression of Cav-3 (Cav-3 OE) is protective against ischemic and hypertensive injury, suggesting a potential role for Cav-3 in basal cardiac electrophysiology and metabolism involved in stress adaptation. We hypothesized that overexpression of Cav-3 may alter baseline cardiac conduction and metabolism. We examined: (1) ECG telemetry recordings at baseline and during pharmacological interventions, (2) ion channels involved in cardiac conduction with immunoblotting and computational modeling, and (3) baseline metabolism in Cav-3 OE and transgene-negative littermate control mice. Cav-3 OE mice had decreased heart rates, prolonged PR intervals, and shortened QTc intervals with no difference in activity compared to control mice. Dobutamine or propranolol did not cause significant changes between experimental groups in maximal (dobutamine) or minimal (propranolol) heart rate. Cav-3 OE mice had an overall lower chronotropic response to atropine. The expression of Kv1.4 and Kv4.3 channels, Nav1.5 channels, and connexin 43 were increased in Cav-3 OE mice. A computational model integrating the immunoblotting results indicated shortened action potential duration in Cav-3 OE mice linking the change in channel expression to the observed electrophysiology phenotype. Metabolic profiling showed no gross differences in VO2, VCO2, respiratory exchange ratio, heat generation, and feeding or drinking. In conclusion, Cav-3 OE mice have changes in ECG intervals, heart rates, and cardiac ion channel expression. These findings give novel mechanistic insights into previously reported Cav-3 dependent cardioprotection.

Sudden infant death syndrome caused by cardiac arrhythmias: only a matter of genes encoding ion channels?

Sudden infant death syndrome is the unexpected demise of a child younger than 1 year of age which remains unexplained after a complete autopsy investigation. Usually, it occurs during sleep, in males, and during the first 12 weeks of life. The pathophysiological mechanism underlying the death is unknown, and the lethal episode is considered multifactorial. However, in cases without a conclusive post-mortem diagnosis, suspicious of cardiac arrhythmias may also be considered as a cause of death, especially in families suffering from any cardiac disease associated with sudden cardiac death. Here, we review current understanding of sudden infant death, focusing on genetic causes leading to lethal cardiac arrhythmias, considering both genes encoding ion channels as well as structural proteins due to recent association of channelopathies and desmosomal genes. We support a comprehensive analysis of all genes associated with sudden cardiac death in families suffering of infant death. It allows the identification of the most plausible cause of death but also of family members at risk, providing cardiologists with essential data to adopt therapeutic preventive measures in families affected with this lethal entity.

Underdiagnosis of Conditions Associated with Sudden Cardiac Death in Children--Is it the Absence of a Comprehensive Screening Program or a True Low Prevalence?

This study aimed to assess the prevalence of conditions associated with sudden cardiac death (SCD) among all children and children with sudden infant death syndrome (SIDS) in the State of Hawai'i, where no comprehensive screening program is conducted for such conditions. A retrospective chart review was conducted from the single tertiary pediatric hospital in Hawai'i, from offices of all pediatric cardiologists in Hawai'i, and the Hawai'i State Department of Health from 1/1/2000 to 12/31/2013. Children aged 0-18 years were included in the study. A subset of the study analyzed records of infants aged 0-12 months. SIDS rate was calculated and compared to national data. Prevalence was calculated for known conditions associated with SCD. The identified prevalence was compared to the established prevalence of conditions associated with SCD. In Hawai'i, the infant SIDS rate (66.4/100,000) was similar to the national rate (54.4/100,000). Over 14 years, only 51 children were diagnosed with a condition associated with SCD; 28 with a cardiomyopathy and 21 with a channelopathy. A 14-year retrospective analysis in the State of Hawai'i revealed that less than 1 in 30 children, who are expected to harbor a SCD-associated condition, had been appropriately diagnosed. The underdiagnosis of conditions associated with SCD reflects that in the absence of a comprehensive screening program, conditions without obvious signs and symptoms are difficult to diagnose. Many children with these conditions will remain at risk of SCD.

Cardiac voltage-gated calcium channel macromolecular complexes

Over the past 20 years, a new field of research, called channelopathies, investigating diseases caused by ion channel dysfunction has emerged. Cardiac ion channels play an essential role in the generation of the cardiac action potential. Investigators have largely determined the physiological roles of different cardiac ion channels, but little is known about the molecular determinants of their regulation. The voltage-gated calcium channel Ca(v)1.2 shapes the plateau phase of the cardiac action potential and allows the influx of calcium leading to cardiomyocyte contraction. Studies suggest that the regulation of Ca(v)1.2 channels is not uniform in working cardiomyocytes. The notion of micro-domains containing Ca(v)1.2 channels and different calcium channel interacting proteins, called macro-molecular complex, has been proposed to explain these observations. The objective of this review is to summarize the currently known information on the Ca(v)1.2 macromolecular complexes in the cardiac cell and discuss their implication in cardiac function and disorder. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Caveolae: One Function or Many?

Caveolae are small, bulb-shaped plasma membrane invaginations. Mutations that ablate caveolae lead to diverse phenotypes in mice and humans, making it challenging to uncover their molecular mechanisms. Caveolae have been described to function in endocytosis and transcytosis (a specialized form of endocytosis) and in maintaining membrane lipid composition, as well as acting as signaling platforms. New data also support a model in which the central function of caveolae could be related to the protection of cells from mechanical stress within the plasma membrane. We present evidence for these diverse roles and consider in vitro and in vivo experiments confirming a mechanoprotective role. We conclude by highlighting current gaps in our knowledge of how mechanical signals may be transduced by caveolae.

Ion channel macromolecular complexes in cardiomyocytes: roles in sudden cardiac death

The movement of ions across specific channels embedded on the membrane of individual cardiomyocytes is crucial for the generation and propagation of the cardiac electric impulse. Emerging evidence over the past 20 years strongly suggests that the normal electric function of the heart is the result of dynamic interactions of membrane ion channels working in an orchestrated fashion as part of complex molecular networks. Such networks work together with exquisite temporal precision to generate each action potential and contraction. Macromolecular complexes play crucial roles in transcription, translation, oligomerization, trafficking, membrane retention, glycosylation, post-translational modification, turnover, function, and degradation of all cardiac ion channels known to date. In addition, the accurate timing of each cardiac beat and contraction demands, a comparable precision on the assembly and organizations of sodium, calcium, and potassium channel complexes within specific subcellular microdomains, where physical proximity allows for prompt and efficient interaction. This review article, part of the Compendium on Sudden Cardiac Death, discusses the major issues related to the role of ion channel macromolecular assemblies in normal cardiac electric function and the mechanisms of arrhythmias leading to sudden cardiac death. It provides an idea of how these issues are being addressed in the laboratory and in the clinic, which important questions remain unanswered, and what future research will be needed to improve knowledge and advance therapy.

[Long QT syndrome. History, genetics, clinical symptoms, causes and therapy]

The long QT syndrome is caused by a change in cardiac repolarization due to functional ion channel defects. A differentiation is made between a congenital (cLQTS) and an acquired (aLQTS) form of the disease. The disease results in the name-giving prolongation of the QT interval in the electrocardiogram and represents a predisposition for cardiac arrhythmia and sudden cardiac death. This article summarizes the current knowledge on the history, pathophysiology, clinical symptoms and therapy of cLQTS and aLQTS. This knowledge of pathophysiological features of the symptoms allows the underlying anesthesiological approach for individualized perioperative concepts for patients suffering from LQTS to be derived.