Molecular biology and the prolonged QT syndromes

Discovery and characterization of a monogenetic insult, caveolin-3-V37L, that precipitated oligo-proteomic perturbations governing repolarization reserve

BACKGROUND

Caveolin-3 (Cav-3) is an essential scaffolding protein for caveolae formation in cardiomyocytes and targets multiple long QT syndrome (LQTS)-associated ion channels. Mutations in CAV3 have caused an LQT3-like accentuation in late sodium current, I_Na (Nav1.5). Here, we characterize a novel CAV3-V37L variant and determine whether it is the substrate for the patient's LQTS.

METHODS

The proband was a 39-year-old female with drug-induced, sudden cardiac arrest (SCA) with profound QT prolongation (QTc > 600 ms). Genetic testing revealed a rare CAV3-V37L variant of uncertain significance (VUS). Whole-cell patch clamp technique was used to measure I_Ks, I_Kr, I_Na, and I_{Ca, L} currents co-expressed with either CAV3-WT or CAV3-V37L in TSA201 cells and to measure the action potential duration (APD) in control human induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) overexpressed with CAV3-WT or CAV3-V37L.

RESULTS

CAV3-V37L did not affect Nav1.5 late current. Instead, CAV3-V37L resulted in 1) I_{Ca, L} with slower inactivation, a 1.5 fold increase in peak I_{Ca, L} current density and a 1.1 fold increase in I_{Ca, L} persistent current, 2) dramatically reduced I_Ks peak current density by 74.9%, 3) significantly reduced I_Kr peak current density by 31.1%, and 4) significantly prolonged the APD in hiPSC-CMs.

CONCLUSIONS

These functional validation assays enabled the promotion of CAV3-V37L from VUS status to a likely pathogenic variant. Although Nav1.5 was spared, this monogenetic insult precipitated an oligo-proteomic impact with a concomitant gain-of-function of I_{Ca, L} and loss-of-function of both I_Ks and I_Kr culminating in a marked prolongation of the cardiomyocyte's action potential duration.

Insulin Signaling as a Key Moderator in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is an autosomal dominant genetic disease characterized by multi-system involvement. Affected organ system includes skeletal muscle, heart, gastro-intestinal system and the brain. In this review, we evaluate the evidence for alterations in insulin signaling and their relation to clinical DM1 features. We start by summarizing the molecular pathophysiology of DM1. Next, an overview of normal insulin signaling physiology is given, and evidence for alterations herein in DM1 is presented. Clinically, evidence for involvement of insulin signaling pathways in DM1 is based on the increased incidence of insulin resistance seen in clinical practice and recent trial evidence of beneficial effects of metformin on muscle function. Indirectly, further support may be derived from certain CNS derived symptoms characteristic of DM1, such as obsessive-compulsive behavior features, for which links with altered insulin signaling has been demonstrated in other diseases. At the basic scientific level, several pathophysiological mechanisms that operate in DM1 may compromise normal insulin signaling physiology. The evidence presented here reflects the importance of insulin signaling in relation to clinical features of DM1 and justifies further basic scientific and clinical, therapeutically oriented research.

Bufalin, a bufanolide steroid from the parotoid glands of the Chinese toad, suppresses hERG K+ currents expressed in HEK293 cells

In this study, we investigated the effect of bufalin on the human ether-à-go-go-related gene (hERG) K⁺ channels using the perforated patch recording technique. We measured a half-maximal inhibitory concentration (IC₅₀ ) of 24.83 μM and maximal inhibitory effect of 39.45 ± 1.14% with bufalin. These findings suggest that bufalin is a potent hERG K⁺ channel blocker and may provide a new way for understanding Chan Su-induced arrhythmia.

Long-QT syndrome-associated caveolin-3 mutations differentially regulate the hyperpolarization-activated cyclic nucleotide gated channel 4

Background

Caveolin-3 (cav-3) mutations are linked to the long-QT syndrome (LQTS) causing distinct clinical symptoms. Hyperpolarization-activated cyclic nucleotide channel 4 (HCN4) underlies the pacemaker current If. It associates with cav-3 and both form a macromolecular complex.

Methods

To examine the effects of human LQTS-associated cav-3 mutations on HCN4-channel function, HEK293-cells were cotransfected with HCN4 and wild-type (WT) cav-3 or a LQTS-associated cav-3 mutant (T78M, A85T, S141R, or F97C). HCN4 currents were recorded using the whole-cell patch-clamp technique.

Results

WT cav-3 significantly decreased HCN4 current density and shifted midpoint of activation into negative direction. HCN4 current properties were differentially modulated by LQTS-associated cav-3 mutations. When compared with WT cav-3, A85T, F97C, and T78M did not alter the specific effect of cav-3, but S141R significantly increased HCN4 current density. Compared with WT cav-3, no significant modifications of voltage dependence of steady-state activation curves were observed. However, while WT cav-3 alone had no significant effect on HCN4 current activation, all LQTS-associated cav-3 mutations significantly accelerated HCN4 activation kinetics.

Conclusions

Our results indicate that HCN4 channel function is modulated by cav-3. LQTS-associated mutations of cav-3 differentially influence pacemaker current properties indicating a pathophysiological role in clinical manifestations.

Effects of salvianolic acid B on L-type calcium channels and myocardial contractility in isolated rat ventricular myocytes and hERG K+ channels expressed in HEK293 cells

Salvianolic acid B (Sal B), one of the chief water-soluble constituents in Radix Salviae Milthiorrhizae, has often been reported to possess considerable cardiovascular regulatory effects. However, the underlying biochemical and cellular mechanisms of its cardioprotection remain unclear. This study was designed to evaluate the role of Sal B regulation in L-type Ca²⁺ channel currents (I_Ca,L) and cell contractility in rat cardiomyocytes and hERG K⁺ channels expressed in HEK293 cells with the patch-clamp and Ca²⁺ imaging techniques to clarify its underlying cardioprotective mechanisms. Exposure to Sal B blocked I_Ca,L with IC₅₀ of 2.07 × 10^-5 M, shifted the curves of current and voltage upwards, shifted the curves of activation and inactivation to the left, and significantly inhibited the amplitude of the cell shortening, but the regulatory effects of Sal B on the expressed rapidly activating delayed rectifier potassium current (I_Kr) did not reach a significant level. These results indicate that the cardioprotective mechanisms of Sal B may be related to the attenuation of calcium overload by directly inhibiting I_Ca,L and consequently decreasing myocardial contractility without causing drug-induced long QT syndrome (LQTS).

Utility of hERG Assays as Surrogate Markers of Delayed Cardiac Repolarization and QT Safety

HERG (human-ether-a-go-go-related gene) encodes for a cardiac potassium channel that plays a critical role in defining ventricular repolarization. Noncardiovascular drugs associated with a rare but potentially lethal ventricular arrhythmia (Torsades de Pointes) have been linked to delayed cardiac repolarization and block of hERG current. This brief overview will discuss the role of hERG current in cardiac electrophysiology, its involvement in drug-induced delayed repolarization, and approaches used to define drug effects on hERG current. In addition, examples of hERG blocking drugs acting differently (i.e., overt and covert hERG blockade due to multichannel block) together with the utility and limitations of hERG assays as tools to predict the risk of delayed repolarization and proarrhythmia are discussed.

Filamin C: a novel component of the KCNE2 interactome during hypoxia

Aim

KCNE2 encodes for the potassium voltage-gated channel, KCNE2. Mutations in KCNE2 have been associated with long-QT syndrome (LQTS). While KCNE2 has been extensively studied, the functions of its C-terminal domain remain inadequately described. Here, we aimed to elucidate the functions of this domain by identifying its protein interactors using yeast two-hybrid analysis.

Methods

The C-terminal domain of KCNE2 was used as bait to screen a human cardiac cDNA library for putative interacting proteins. Co-localisation and co-immunoprecipitation analyses were used for verification.

Results

Filamin C (FLNC) was identified as a putative interactor with KCNE2. FLNC and KCNE2 co-localised within the cell, however, a physical interaction was only observed under hypoxic conditions.

Conclusion

The identification of FLNC as a novel KCNE2 ligand not only enhances current understanding of ion channel function and regulation, but also provides valuable information about possible pathways likely to be involved in LQTS pathogenesis.

Role of Genetic Testing in Patients with Ventricular Arrhythmias in Apparently Normal Hearts

Ventricular arrhythmias without structural heart disease are responsible for ∼35% of patients who have sudden cardiac death before the age of 40 years. Molecular autopsy and/or cardiological investigation of nearby family members often reveals the diagnosis and genetic testing can be helpful in family screening and risk stratification in disease carriers. Extended gene panels can be screened in a short period of time at low cost. A multidisciplinary team of (genetically) specialized clinicians is necessary to judge all the available details and to decide on the significance of the variant and further strategies.

Lmod2 piggyBac mutant mice exhibit dilated cardiomyopathy

BACKGROUND

Leiomodin proteins, Lmod1, Lmod2 and Lmod3, are key regulators of the thin filament length in muscles. While Lmod1 is specifically expressed in smooth muscles, both Lmod2 and Lmod3 are expressed in striated muscles including both cardiac and skeletal muscles. We and others have previously shown that Lmod3 mainly function in skeletal muscles and the mutant mice display disorganized sarcomere. Lmod2 protein has been found to act as an actin filament nucleator in both cell-free assays and in cultured rat and chicken cardiomyocytes.

RESULTS

To better understand the function of Lmod2 in vivo, we have identified and characterized a piggyBac (PB) insertional mouse mutant. Our analysis revealed that the PB transposon inserts in the first exon of the Lmod2 gene and severely disrupts its expression. We found that Lmod2 (PB/PB) mice exhibit typical dilated cardiomyopathy (DCM) with ventricular arrhythmias and postnatal lethality. Electron microscope reveals that the Lmod2 (PB/PB) hearts carry disordered sarcomere, disarrayed thin filaments, and distorted intercalated discs (ICDs). Those ICDs display not only decreased convolutions, but also reduced electron-dense staining, indicating less ICDs component proteins in Lmod2 (PB/PB) hearts. Consistent with the phenotype, the expression of the ICD component genes, β-catenin and Connexin43, are down-regulated.

CONCLUSIONS

Taken together, our data reveal that Lmod2 is required in heart thin filaments for integrity of sarcomere and ICD and deficient mice exhibit DCM with ventricular arrhythmias and postnatal lethality. The Lmod2 (PB/PB) mutant offers a valuable resource for interrogation of pathogenesis and development of therapeutics for DCM.

Cardiac Delayed Rectifier Potassium Channels in Health and Disease

Cardiac delayed rectifier potassium channels conduct outward potassium currents during the plateau phase of action potentials and play pivotal roles in cardiac repolarization. These include IKs, IKr and the atrial specific IKur channels. In this article, we will review their molecular identities and biophysical properties. Mutations in the genes encoding delayed rectifiers lead to loss- or gain-of-function phenotypes, disrupt normal cardiac repolarization and result in various cardiac rhythm disorders, including congenital Long QT Syndrome, Short QT Syndrome and familial atrial fibrillation. We will also discuss the prospect of using delayed rectifier channels as therapeutic targets to manage cardiac arrhythmia.

An investigation of the association of the H558R polymorphism of the SCN5A gene with idiopathic cardiac conduction disorders

AIM

The aim of this study was to investigate the predictive value of the rs1805124 polymorphism of the SCN5A gene with regard to idiopathic cardiac conduction disorders.

RESULTS

The AG genotype frequency was significantly higher in patients with an atrioventricular block (61,2%±6,0%) compared with healthy control subjects (34,8%±2,3%), p<0.0001. The AG genotype frequencies among patients with only idiopathic complete right bundle-branch block (CRBBB) (54,2%±5,5%) and those with both CLBBB and LAH (50%±5,1) were significantly higher than in the control group (34,8%±2,3%), p<0.005.

CONCLUSIONS

The AG genotype of the H558R (rs1805124) polymorphism of the SCN5A gene is a genetic predictor of idiopathic disorders of atrioventricular and intraventricular conduction.

Effects of external stimuli on the pacemaker function of the sinoatrial node in sodium channel gene mutations models

Loss of function and gain of function mutations of the sodium channel were investigated using an intact two-dimensional rabbit sinoatrial node (SAN) and atrial cell model. The effects of three external stimuli (acetylcholine secretion by the vagal nerve, acid-base concentration, and tissue temperature) on cardiac pacemaker function and conduction were studied. Our results show that these two groups of mutations have different effects on pacemaker function and conduction. Furthermore, we found that the negative effects of these mutations could be altered by external stimuli. The bradycardic effects of mutations were magnified by an increase in acetylcholine level. Changes in acid-base concentration and tissue temperature increased the ability of the SAN to recover its pacemaker function. The results of this study increase our understanding of sodium channel disorders, and help to advance research on the treatment of these conditions.

Acquired long QT syndrome: frequency, onset, and risk factors in intensive care patients

BACKGROUND

Acquired long QT syndrome is a reversible condition that can lead to torsades de pointes and sudden cardiac death.

OBJECTIVE

To determine the frequency, onset, frequency of medications, and risk factors for the syndrome in intensive care patients.

METHODS

In a retrospective chart review of 88 subjects, hourly corrected QT intervals calculated by using the Bazett formula were collected. Acquired long QT syndrome was defined as a corrected QT of 500 milliseconds or longer or an increase in corrected QT of 60 milliseconds or greater from baseline level. Risk factors and medications administered were collected from patients' medical records.

RESULTS

The syndrome occurred in 46 patients (52%); mean time of onset was 7.4 hours (SD, 9.4) from time of admission. Among the 88 patients, 52 (59%) received a known QTc-prolonging medication. Among the 46 with the syndrome, 23 (50%) received a known QT-prolonging medication. No other risk factor studied was significantly predictive of the syndrome.

CONCLUSIONS

Acquired long QT syndrome occurs in patients not treated with a known QT-prolonging medication, indicating the importance of frequent QT monitoring of all intensive care patients.

Age-and sex-dependent mRNA expression of KCNQ1 and HERG in patients with long QT syndrome type 1 and 2

INTRODUCTION

The main goal of this study was to examine the patient age and sex dependent expression of KCNQ1 and HERG genes that encode potassium channels responsible for the occurrence of long QT syndrome (LQTS).

MATERIAL AND METHODS

The study enrolled 43 families whose members suffered from LQTS type 1 (LQTS1) or 2 (LQTS2) or were healthy. The study attempted to prove that β-actin is a good endogenous control when determining the expression of the studied genes. Examination of gene expression was achieved with quantitative real-time PCR (QRT-PCR). Expression of the investigated genes was inferred from the analysis of the number of mRNA copies per 1 μg total RNA isolated from whole blood.

RESULTS

Significantly lower KCNQ1 and KCNH2 mRNA levels in healthy females than healthy males were observed (p = 0.032; p = 0.02). In male patients both transcripts were expressed at a lower level (p = 0.0084; p = 0.035). The comparison of transcriptional activity of KCNQ1 and KCNH2 in healthy adults and children revealed higher KCNQ1 and lower KCNH2 mRNA levels in healthy adults (p = 0.033; p = 0.04), higher KCNQ1 and lower KCNH2 mRNA levels in adult patients below 55 years old than in adults over 55 years old (p=0.036; p = 0.044), and significantly higher KCNQ1 and lower KCNH2 mRNA levels in adult patients (over 55 years) than in paediatric patients (below 15 years) (p=0.047; p = 0.08).

CONCLUSIONS

The results support the hypothesis that KCNQ1 and HERG gene expression is influenced by age and gender in human patients with long QT syndrome and in healthy subjects.

New type 2 diabetes risk genes provide new insights in insulin secretion mechanisms

Type 2 diabetes results from the inability of beta cells to increase insulin secretion sufficiently to compensate for insulin resistance. Insulin resistance is thought to result mainly from environmental factors, such as obesity. However, there is compelling evidence that the decline of both insulin sensitivity and insulin secretion have also a genetic component. Recent genome-wide association studies identified several novel risk genes for type 2 diabetes. The vast majority of these genes affect beta cell function by molecular mechanisms that remain unknown in detail. Nevertheless, we and others could show that a group of genes affect glucose-stimulated insulin secretion, a group incretin-stimulated insulin secretion (incretin sensitivity or secretion) and a group proinsulin-to-insulin conversion. The most important so far type 2 diabetes risk gene, TCF7L2, interferes with all three mechanisms. In addition to advancing knowledge in the pathophysiology of type 2 diabetes, the discovery of novel genetic determinants of diabetes susceptibility may help understanding of gene-environment, gene-therapy and gene-gene interactions. It was also hoped that it could make determination of the individual risk for type 2 diabetes feasible. However, the allelic relative risks of most genetic variants discovered so far are relatively low. Thus, at present, clinical criteria assess the risk for type 2 diabetes with greater sensitivity and specificity than the combination of all known genetic variants.

Clinically relevant drug interactions between anticancer drugs and psychotropic agents

Drug interactions are commonly seen in the treatment of cancer patients. Psychotropics are often indicated for these patients since they may also suffer from pre-existing psychological disorders or experience insomnia and anxiety associated with cancer therapy. Thus, the risk of anticancer drug (ACD)-psychotropic drug-drug interactions (DDIs) is high. Drug interactions were compiled from the British National Formulary (53rd edn), Lexi-Comp's Drug Information Handbook (15th edn), Micromedex (v5.1), Hansten & Horn's Drug Interactions (2000) and Drug Interaction Facts (2008 edn). Product information of the individual drugs, as well as documented literature on ACD-psychotropic interactions from PubMed and other databases was also incorporated. This paper identifies clinically important ACD-psychotropic DDIs that are frequently observed. Pharmacokinetic DDIs were observed for tyrosine kinase inhibitors, corticosteroids and antimicrotubule agents due to their inhibitory or inductive effects on cytochrome P450 isoenzymes. Pharmacodynamic DDIs were identified for thalidomide with central nervous system depressants, procarbazine with antidepressants, myelosuppressive ACDs with clozapine and anthracyclines with QT-prolonging psychotropics. Clinicians should be vigilant when psychotropics are prescribed concurrently with ACDs. Close monitoring of plasma drug levels should be carried out to avoid toxicity in the patient, as well as to ensure adequate chemotherapeutic and psychotropic coverage.

Genetic variants affecting incretin sensitivity and incretin secretion

Recent genome-wide association studies identified several novel risk genes for type 2 diabetes. The majority of these type 2 diabetes risk variants confer impaired pancreatic beta cell function. Though the molecular mechanisms by which common genetic variation within these loci affects beta cell function are not completely understood, risk variants may alter glucose-stimulated insulin secretion, proinsulin conversion, and incretin signals. In humans, the incretin effect is mediated by the secretion and insulinotropic action of two peptide hormones, glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1. This review article aims to give an overview of the type 2 diabetes risk loci that were found to associate with incretin secretion or incretin action, paying special attention to the potential underlying mechanisms.

hERG-related drug toxicity and models for predicting hERG liability and QT prolongation

BACKGROUND

hERG K(+) channels have been recognized as a primary antitarget in safety pharmacology. Their blockade, caused by several drugs with different therapeutic indications, may lead to QT prolongation and, eventually, to potentially fatal arrhythmia, namely torsade de pointes. Therefore, a number of preclinical models have been developed to predict hERG liability early in the drug development process.

OBJECTIVE

The aim of this review is to outline the present state of the art on drug-induced hERG blockade, providing insights on the predictive value of in vitro and in silico models for hERG liability.

METHODS

On the basis of latest reports, high-throughput preclinical models have been discussed outlining advantages and limitations.

CONCLUSION

Although no single model has an absolute value, an integrated risk assessment is recommended to predict the pro-arrhythmic risk of a given drug. This prediction requires expertise from different areas and should encompass emerging issues such as interference with hERG trafficking and QT shortening.

Association of type 2 diabetes candidate polymorphisms in KCNQ1 with incretin and insulin secretion

OBJECTIVE

KCNQ1 gene polymorphisms are associated with type 2 diabetes. This linkage appears to be mediated by altered beta-cell function. In an attempt to study underlying mechanisms, we examined the effect of four KCNQ1 single nucleotide polymorphisms (SNPs) on insulin secretion upon different stimuli.

RESEARCH DESIGN AND METHODS

We genotyped 1,578 nondiabetic subjects at increased risk of type 2 diabetes for rs151290, rs2237892, rs2237895, and rs2237897. All participants underwent an oral glucose tolerance test (OGTT); glucagon-like peptide (GLP)-1 and gastric inhibitory peptide secretion was measured in 170 participants. In 519 participants, a hyperinsulinemic-euglycemic clamp was performed, in 314 participants an intravenous glucose tolerance test (IVGTT), and in 102 subjects a hyperglycemic clamp combined with GLP-1 and arginine stimuli.

RESULTS

rs151290 was nominally associated with 30-min C-peptide levels during OGTT, first-phase insulin secretion, and insulinogenic index after adjustment in the dominant model (all P < or = 0.01). rs2237892, rs2237895, and rs2237897 were nominally associated with OGTT-derived insulin secretion indexes (all P < 0.05). No SNPs were associated with beta-cell function during intravenous glucose or GLP-1 administration. However, rs151290 was associated with glucose-stimulated gastric inhibitory polypeptide and GLP-1 increase after adjustment in the dominant model (P = 0.0042 and P = 0.0198, respectively). No associations were detected between the other SNPs and basal or stimulated incretin levels (all P > or = 0.05).

CONCLUSIONS

Common genetic variation in KCNQ1 is associated with insulin secretion upon oral glucose load in a German population at increased risk of type 2 diabetes. The discrepancy between orally and intravenously administered glucose seems to be explained not by altered incretin signaling but most likely by changes in incretin secretion.

Functional studies on three novel HCNH2 mutations in Taiwan: identification of distinct mechanisms of channel defect and dissociation between glycosylation defect and assembly defect

Mutations of the KCNH2 with decreased channel activity lead to congenital long QT syndrome (LQTS). We studied the electrophysiological, glycosylation, trafficking and assembly properties of three novel KCNH2 mutations identified in Taiwanese patients with LQTS (p.N633D, p.R744fs, and p.P923fs). When expressed in HEK293T cells, p.N633D and p.R744fs HERG channels displayed no HERG current while p.P923fs HERG channel showed HERG current with significantly lower (34%) current density and faster inactivation kinetics. In Western blot analysis, pR744fs was the only one with glycosylation defect, which was in consistence with the confocal microscopic findings showing that p.R744fs-GFP was the only one with trafficking defect. However, p.R744fs-GFP differed from pR744fs in being fully glycosylated while p.R744fs fusion with GFP at the N-terminus revealed glycosylation defect. To access the assembly capacity of each mutant, co-immunoprecipitation was conducted. Wild type (WT), p.N633D, and p.P923fs HERG protein showed assembly ability while p.R744fs failed to assemble with neither WT nor itself. In conclusion, we identified three novel LQTS-related KCNH2 mutations and each had a distinct mechanism of channel defect. For p.R744fs mutant, adding GFP to the C-terminus rescued the glycosylation defect but the channel was still assembly defective indicating a dissociation between glycosylation and assembly defects.

The hERG K+ channel: target and antitarget strategies in drug development

The human ether-à-go-go related gene (hERG) K+ channel is of great interest for both basic researchers and clinicians because its blockade by drugs can lead to QT prolongation, which is a risk factor for torsades de pointes, a potentially life-threatening arrhythmia. A growing list of agents with "QT liability" have been withdrawn from the market or restricted in their use, whereas others did not even receive regulatory approval for this reason. Thus, hERG K+ channels have become a primary antitarget (i.e. an unwanted target) in drug development because their blockade causes potentially serious side effects. On the other hand, the recent identification and functional characterization of hERG K+ channels not only in the heart, but also in several other tissues (e.g. neurons, smooth muscle and cancer cells) may have far reaching implications for drug development for a possible exploitation of hERG as a target, especially in oncology and cardiology.

Defining the Cellular Phenotype of “Ankyrin-B Syndrome” Variants

Background— Mutations in the ankyrin-B gene ( ANK2 ) cause type 4 long-QT syndrome and have been described in kindreds with other arrhythmias. The frequency of ANK2 variants in large populations and molecular mechanisms underlying the variability in the clinical phenotypes are not established. More importantly, there is no cellular explanation for the range of severity of cardiac phenotypes associated with specific ANK2 variants.

Methods and Results— We performed a comprehensive screen of ANK2 in populations (control, congenital arrhythmia, drug-induced long-QT syndrome) of different ethnicities to discover unidentified ANK2 variants. We identified 7 novel nonsynonymous ANK2 variants; 4 displayed abnormal activity in cardiomyocytes. Including the 4 new variants, 9 human ANK2 loss-of-function variants have been identified. However, the clinical phenotypes associated with these variants vary strikingly, from no obvious phenotype to manifest long-QT syndrome and sudden death, suggesting that mutants confer a spectrum of cellular phenotypes. We then characterized the relative severity of loss-of-function properties of all 9 nonsynonymous ANK2 variants identified to date in primary cardiomyocytes and identified a range of in vitro phenotypes, including wild-type, simple loss-of-function, and severe loss-of-function activity, seen with the variants causing severe human phenotypes.

Conclusions— We present the first description of differences in cellular phenotypes conferred by specific ANK2 variants. We propose that the various degrees of ankyrin-B loss of function contribute to the range of severity of cardiac dysfunction. These data identify ANK2 variants as modulators of human arrhythmias, provide the first insight into the clinical spectrum of “ankyrin-B syndrome,” and reinforce the role of ankyrin-B–dependent protein interactions in regulating cardiac electrogenesis.

Prolonged QT interval in pediatric sickle cell disease

We recently cared for a patient with sickle cell disease (SCD) who presented with cardiac arrest and was found to have a prolonged corrected QT interval (QTc). This prompted us to perform a retrospective review of all electrocardiograms performed in patients with SCD during the last two years. Among 142 patients with ECG results, we identified 12 patients (8 males and 4 females), who had one or more documented measurements of prolonged QTc intervals. The relatively high prevalence of borderline or moderately prolonged QTc intervals in our patient population warrants further investigation.

Mutant caveolin-3 induces persistent late sodium current and is associated with long-QT syndrome

BACKGROUND

Congenital long-QT syndrome (LQTS) is a primary arrhythmogenic syndrome stemming from perturbed cardiac repolarization. LQTS, which affects approximately 1 in 3000 persons, is 1 of the most common causes of autopsy-negative sudden death in the young. Since the sentinel discovery of cardiac channel gene mutations in LQTS in 1995, hundreds of mutations in 8 LQTS susceptibility genes have been identified. All 8 LQTS genotypes represent primary cardiac channel defects (ie, ion channelopathy) except LQT4, which is a functional channelopathy because of mutations in ankyrin-B. Approximately 25% of LQTS remains unexplained pathogenetically. We have pursued a "final common pathway" hypothesis to elicit novel LQTS-susceptibility genes. With the recent observation that the LQT3-associated, SCN5A-encoded cardiac sodium channel localizes in caveolae, which are known membrane microdomains whose major component in the striated muscle is caveolin-3, we hypothesized that mutations in caveolin-3 may represent a novel pathogenetic mechanism for LQTS.

METHODS AND RESULTS

Using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing, we performed open reading frame/splice site mutational analysis on CAV3 in 905 unrelated patients referred for LQTS genetic testing. CAV3 mutations were engineered by site-directed mutagenesis and the molecular phenotype determined by transient heterologous expression into cell lines that stably express the cardiac sodium channel hNa(v)1.5. We identified 4 novel mutations in CAV3-encoded caveolin-3 that were absent in >1000 control alleles. Electrophysiological analysis of sodium current in HEK293 cells stably expressing hNa(v)1.5 and transiently transfected with wild-type and mutant caveolin-3 demonstrated that mutant caveolin-3 results in a 2- to 3-fold increase in late sodium current compared with wild-type caveolin-3. Our observations are similar to the increased late sodium current associated with LQT3-associated SCN5A mutations.

CONCLUSIONS

The present study reports the first CAV3 mutations in subjects with LQTS, and we provide functional data demonstrating a gain-of-function increase in late sodium current.

Effect of ciprofloxacin and levofloxacin on the QT interval: is this a significant "clinical" event?

BACKGROUND

The widespread use of the fluoroquinolones has raised the question of the cardiac safety of these medications. This widespread use of this class of antibiotics has displayed their safety profile, which is actually more favorable than many other drug classes. The cardiac toxicity issue at the center of this discussion is the prolongation of the QT interval leading to torsade de pointes. Ciprofloxacin and levofloxacin, two of the more commonly used fluoroquinolones, are considered less likely than other fluoroquinolones to prolong the QT interval. The authors set out to evaluate the effect on the QT interval of patients after administration of ciprofloxacin and levofloxacin.

METHODS

A prospective evaluation of 38 consecutive patients evaluated by the infectious disease service and receiving either ciprofloxacin or levofloxacin was undertaken. Twelve-lead electrocardiograms were obtained at baseline and at least 48 hours after the first dose of the antibiotic was administered. Both the longest QT interval and the mean QT interval were evaluated. To account for variations in heart rate, the corrected QT interval was calculated by using Bazett's formula (QTc = QT(square root of) R-R). Statistical analysis was undertaken to assess for the presence of a change after the administration of the antibiotic.

RESULTS

Thirty-eight patients (mean age, 65 +/- 19 years), 23 women and 15 men, were studied. There was a small but significant increase in the longest QTc intervals over baseline in patients receiving levofloxacin; there was no significant change in the mean QTc interval. However, one patient who received levofloxacin was, statistically, an outlier and, on retrospective analysis, had demonstrated severe electrolyte disturbances at the time of the study. When this patient was excluded, the increase in the longest QTc interval was not significant. Patients receiving ciprofloxacin did not demonstrate any significant change in the longest QTc interval or mean QTc interval.

CONCLUSIONS

Neither levofloxacin nor ciprofloxacin significantly prolonged the mean QTc interval over baseline. When electrolyte deficiencies in one of the patients evaluated were taken into account, this also held true for the longest QTc interval. There is, therefore, evidence that taking ciprofloxacin or levofloxacin, assuming that there are not any concurrent risk factors, will not cause a significant prolongation in the QT interval.

Evaluating cardiovascular presentations: what does an electrocardiogram have to offer?

The general pediatrician remains the key to the timely recognition and treatment of cardiovascular disorders, particularly those that present acutely and may require immediate attention. In the evaluation of these cardiovascular urgencies, ancillary studies such as the electrocardiogram continue to be important, readily available tools that can aide in the diagnostic process. It is thus incumbent on the general practitioner to foster the skill necessary to employ such tools reliably, particularly in the setting of urgent evaluations. At the same time, recognition of the limitations of such testing will help both in the acute setting and in the understanding of their application in population settings.

Severe syncope and sudden death in children with inborn salt-losing hypokalaemic tubulopathies

BACKGROUND

Potassium deficiency may cause cardiac arrhythmias culminating in syncope or sudden death.

METHODS

An inquiry performed among physicians caring for a total of 249 patients with inborn salt-losing tubulopathies revealed that acute cardiac complications occurred in seven children.

RESULTS

Four patients died suddenly and three had severe syncope. These episodes occurred in the context of severe chronic hypokalaemia (< or =2.5 mmol/l) or were precipitated by acute diseases, which exacerbated hypokalaemia (< or =2.0 mmol/l).

CONCLUSIONS

In conclusion, severe chronic or acute hypokalaemia is hazardous in inborn salt-losing tubulopathies.

Mutations in the genes KCND2 and KCND3 encoding the ion channels Kv4.2 and Kv4.3, conducting the cardiac fast transient outward current (ITO,f), are not a frequent cause of long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is a hereditary cardiac arrhythmogenic disorder characterized by prolongation of the QT interval in the electrocardiogram, torsades de pointes arrhythmia, and syncopes and sudden death. LQTS is caused by mutations in ion channel genes. However, only in half of the families is it possible to identify mutations in one of the seven known LQTS genes, why further genetic heterogeneity is expected. The genes KCND2 and KCND3, encoding the alpha-subunits of the voltage-gated potassium channels Kv4.2 and Kv4.3 conducting the fast transient outward current (I(TO,f)) of the cardiac action potential (AP) in the myocardium, have been associated with prolongation of AP duration and QT prolongation in murine models.

METHODS

KCND2 and KCND3 were examined for mutations using single-strand conformation polymorphism (SSCP) analysis in 43 unrelated LQTS patients, where mutations in the coding regions of known LQTS genes had been excluded.

RESULTS

Seven single nucleotide polymorphismsm (SNPs) were found, two exonic SNPs in KCND2 and three exonic and two intronic in KCND3. None of the five exonic SNPs had coding effect. All seven SNPs are considered normal variants.

CONCLUSION

The data suggest that mutations in KCND2 and KCND3 are not a frequent cause of long QT syndrome.

QTc interval prolongation in patients on long-term methadone maintenance therapy

OBJECTIVE

The aim of the present study was to assess the incidence of abnormal QTc interval values in a population of subjects on a long-term methadone maintenance treatment, as a single therapy, and with methadone dosages ranging between 10 and 600 mg/daily (mean+/-SD=87+/-76).

METHOD

Basal ECG recordings were carried out in 83 former heroin addicts on long-term successful methadone maintenance therapy for at least 6 months, while no other known QT-prolonging agent was being administered.

RESULTS

Eighty-three percent of the subjects had a more prolonged QT interval than the reference values for persons of the same sex and age. Only 2 patients displayed a QTc interval of >500 ms. No correlation emerged between QTc values and methadone dosages.

CONCLUSION

Patients on long-term methadone maintenance treatment show longer than expected QTc interval values. This data, associated with the finding that methadone is a rather potent inhibitor of HERG potassium channels and that it may induce torsade de pointes in predisposed subjects, supports the recommendation that patients entering methadone treatment (MT) are screened for cardiac risk factors. ECG might be considered in ongoing MT patients especially before starting QT-prolonging medications.

Prolongation of the QT interval in primary aldosteronism

1. Only limited information is available concerning the changes in the electrocardiogram in primary aldosteronism. The aim of the present study was to determine factors influencing the QTc interval in patients with primary aldosteronism. 2. Nineteen patients with primary aldosteronism caused by a Conn's adenoma and 69 patients with essential hypertension, in whom all possible causes of secondary hypertension had been excluded, were included in the present study. Before and 10-20 days after adrenalectomy, blood and electrocardiographic examinations were conducted. 3. Systolic and diastolic blood pressures in primary aldosteronism were found to be comparable to those in essential hypertension. However, the QTc interval was significantly prolonged and serum potassium concentrations were significantly decreased in patients with primary aldosteronism compared with patients with essential hypertension (492.7+/-20.3 vs 428.5+/-3.1 msec for QTc interval, respectively (P<0.01); 3.07+/-0.12 vs 4.07+/-0.05 mEq/L for serum potassium concentrations, respectively (P<0.01)). The QTc interval was significantly correlated with serum potassium concentrations in primary aldosteronism (P=0.0011; r=-0.6902), but not in patients with essential hypertension. 4. Blood pressure significantly decreased after adrenalectomy. Furthermore, serum potassium concentrations increased significantly and did not correlate with the QTc interval after adrenalectomy (P=0.54; r=-0.1500). 5. Our results indicate that the QTc interval is prolonged in patients with primary aldosteronism, probably owing to hypokalaemia.

Genomic Imprinting and Kinship: How Good is the Evidence?

The kinship theory of genomic imprinting proposes that parent-specific gene expression evolves at a locus because a gene's level of expression in one individual has fitness effects on other individuals who have different probabilities of carrying the maternal and paternal alleles of the individual in which the gene is expressed. Therefore, natural selection favors different levels of expression depending on an allele's sex-of-origin in the previous generation. This review considers the strength of evidence in support of this hypothesis for imprinted genes in four "clusters," associated with the imprinted loci Igf2, Igf2r, callipyge, and Gnas. The clusters associated with Igf2 and Igf2r both contain paternally expressed transcripts that act as enhancers of prenatal growth and maternally expressed transcripts that act as inhibitors of prenatal growth. This is consistent with predictions of the kinship theory. However, the clusters also contain imprinted genes whose phenotypes as yet remain unexplained by the theory. The principal effects of imprinted genes in the callipyge and Gnas clusters appear to involve lipid and energy metabolism. The kinship theory predicts that maternally expressed transcripts will favor higher levels of nonshivering thermogenesis (NST) in brown adipose tissue (BAT) of animals that huddle for warmth as offspring. The phenotypes of reciprocal heterozygotes for Gnas knockouts provide provisional support for this hypothesis, as does some evidence from other imprinted genes (albeit more tentatively). The diverse effects of imprinted genes on the development of white adipose tissue (WAT) have so far defied a unifying hypothesis in terms of the kinship theory.

The molecular basis of cardiac arrhythmias in patients with cardiomyopathy

Cardiac arrhythmias are a leading cause of mortality and morbidity in Western society. In some specific instances, these arrhythmias are caused by abnormalities of cardiac ion channels, such as sodium, calcium, and potassium channels, which carry ionic currents and are fundamental determinants of cardiac excitability. Abnormalities of these ion channels are attributed to mutations in the genes encoding the channel protein and cause altered function of channels, which can predispose to arrhythmias. During heart failure, many channels also malfunction because of altered expression, resulting in lethal arrhythmias.

Does KCNE5 play a role in long QT syndrome?

BACKGROUND

Long QT syndrome [LQTS] is a congenital cardiac disease characterised by prolonged QTC-time, syncopes and sudden cardiac death. LQTS is caused by mutations in genes coding for ion channels involved in the action potential. KCNE5 codes for a novel beta-subunit of the ion channel conducting the delayed rectifier repolarizing current IKs. As KCNE5 is expressed in the human heart and suppresses the IKs current in heterologous systems, it is a candidate gene that may be mutated in LQTS families where no causative mutations in known LQTS associated genes have been found. We examined whether this was the case.

METHODS

Genomic DNA from LQTS patients [n=88] and normal controls [n=90] was screened for mutations in KCNE5 by endonuclease-enhanced single strand conformation polymorphism analysis [EE-SSCP], and DNA sequencing of aberrant conformers. Mutations in other LQTS associated ion channels were excluded by SSCP.

RESULTS

No mutations were found in the coding region of the KCNE5 gene in LQTS patients. One polymorphism, a T-to-C transition at nucleotide 97, causing an amino acid polymorphism P33S, was present in 16 persons, nine heterozygotes and seven homozygotes. The T-allele frequency was 0.13 in LQTS patients and 0.10 in controls.

A cardiac arrhythmia syndrome caused by loss of ankyrin-B function

220-kDa ankyrin-B is required for coordinated assembly of Na/Ca exchanger, Na/K ATPase, and inositol trisphosphate (InsP(3)) receptor at transverse-tubule/sarcoplasmic reticulum sites in cardiomyocytes. A loss-of-function mutation of ankyrin-B identified in an extended kindred causes a dominantly inherited cardiac arrhythmia, initially described as type 4 long QT syndrome. Here we report the identification of eight unrelated probands harboring ankyrin-B loss-of-function mutations, including four previously undescribed mutations, whose clinical features distinguish the cardiac phenotype associated with loss of ankyrin-B activity from classic long QT syndromes. Humans with ankyrin-B mutations display varying degrees of cardiac dysfunction including bradycardia, sinus arrhythmia, idiopathic ventricular fibrillation, catecholaminergic polymorphic ventricular tachycardia, and risk of sudden death. However, a prolonged rate-corrected QT interval was not a consistent feature, indicating that ankyrin-B dysfunction represents a clinical entity distinct from classic long QT syndromes. The mutations are localized in the ankyrin-B regulatory domain, which distinguishes function of ankyrin-B from ankyrin-G in cardiomyocytes. All mutations abolish ability of ankyrin-B to restore abnormal Ca(2+) dynamics and abnormal localization and expression of Na/Ca exchanger, Na/K ATPase, and InsP(3)R in ankyrin-B(+/-) cardiomyocytes. This study, considered together with the first description of ankyrin-B mutation associated with cardiac dysfunction, supports a previously undescribed paradigm for human disease due to abnormal coordination of multiple functionally related ion channels and transporters, in this case the Na/K ATPase, Na/Ca exchanger, and InsP(3) receptor.

Electrocardiographic Abnormalities and Serum Magnesium in Patients With Subarachnoid Hemorrhage

Background and Purpose— ECG abnormalities and hypomagnesemia frequently occur after aneurysmal subarachnoid hemorrhage (SAH). Because hypomagnesemia is associated with several ECG abnormalities, we studied whether hypomagnesemia mediates ECG abnormalities after SAH.

Methods— We prospectively studied a consecutive series of 62 patients admitted within 72 hours after aneurysmal SAH. A standard 12-lead ECG and serum magnesium measurement were routinely performed at admission. The relationship between serum magnesium and ECG abnormalities was assessed with linear regression analysis and the Mann-Whitney test in case of dichotomized ECG abnormalities.

Results— Hypomagnesemia was present in 23 patients (37%), and 38 patients (61%) had a long QTc duration. Low serum magnesium was related to a long PR interval ( P =0.001) and a shorter QTc interval ( P =0.004). Adjustment for World Federation of Neurological Surgeons score, hydrocephalus, and the amount of cisternal and ventricular blood did not influence these relations.

Conclusions— In patients with SAH, lower serum magnesium levels are related to less pronounced increase in the QTc interval. Although the direction of the relation was unexpected, decreased serum magnesium might be the missing link between SAH and ECG abnormalities.

VKCDB: voltage-gated potassium channel database

Background

The family of voltage-gated potassium channels comprises a functionally diverse group of membrane proteins. They help maintain and regulate the potassium ion-based component of the membrane potential and are thus central to many critical physiological processes. VKCDB (Voltage-gated potassium [K] Channel DataBase) is a database of structural and functional data on these channels. It is designed as a resource for research on the molecular basis of voltage-gated potassium channel function.

Description

Voltage-gated potassium channel sequences were identified by using BLASTP to search GENBANK and SWISSPROT. Annotations for all voltage-gated potassium channels were selectively parsed and integrated into VKCDB. Electrophysiological and pharmacological data for the channels were collected from published journal articles. Transmembrane domain predictions by TMHMM and PHD are included for each VKCDB entry. Multiple sequence alignments of conserved domains of channels of the four Kv families and the KCNQ family are also included. Currently VKCDB contains 346 channel entries. It can be browsed and searched using a set of functionally relevant categories. Protein sequences can also be searched using a local BLAST engine.

Conclusions

VKCDB is a resource for comparative studies of voltage-gated potassium channels. The methods used to construct VKCDB are general; they can be used to create specialized databases for other protein families. VKCDB is accessible at .

Phosphatidylinositol-4,5-bisphosphate, PIP2, controls KCNQ1/KCNE1 voltage-gated potassium channels: a functional homology between voltage-gated and inward rectifier K+ channels

Phosphatidylinositol-4,5-bisphosphate (PIP(2)) is a major signaling molecule implicated in the regulation of various ion transporters and channels. Here we show that PIP(2) and intracellular MgATP control the activity of the KCNQ1/KCNE1 potassium channel complex. In excised patch-clamp recordings, the KCNQ1/KCNE1 current decreased spontaneously with time. This rundown was markedly slowed by cytosolic application of PIP(2) and fully prevented by application of PIP(2) plus MgATP. PIP(2)-dependent rundown was accompanied by acceleration in the current deactivation kinetics, whereas the MgATP-dependent rundown was not. Cytosolic application of PIP(2) slowed deactivation kinetics and also shifted the voltage dependency of the channel activation toward negative potentials. Complex changes in the current characteristics induced by membrane PIP(2) was fully restituted by a model originally elaborated for ATP-regulated two transmembrane-domain potassium channels. The model is consistent with stabilization by PIP(2) of KCNQ1/KCNE1 channels in the open state. Our data suggest a striking functional homology between a six transmembrane-domain voltage-gated channel and a two transmembrane-domain ATP-gated channel.

QT prolongation through hERG K+ channel blockade: Current knowledge and strategies for the early prediction during drug development

Prolongation of the QT interval of the electrocardiogram is a typical effect of Class III antiarrhythmic drugs, achieved through blockade of potassium channels. In the past decade, evidence has accrued that several classes of drugs used for non-cardiovascular indications may prolong the QT interval with the same mechanism (namely, human ether-a-go-go-related gene (hERG) K(+) channel blockade). The great interest in QT prolongation is because of several reasons. First, drug-induced QT prolongation increases the likelihood of a polymorphous ventricular arrhythmia (namely, torsades de pointes, TdP), which may cause syncope and degenerate into ventricular fibrillation and sudden death. Second, the fact that several classes of drugs, such as antihistamines, fluoroquinolones, macrolides, and neuroleptics may cause the long QT syndrome (LQTS) raises the question whether this is a class effect (e.g., shared by all agents of a given pharmacological class) or a specific effect of single agents within a class. There is now consensus that, in most cases, only a few agents within a therapeutic class share the ability to significantly affect hERG K(+) channels. These compounds should be identified as early as possible during drug development. Third, QT prolongation and interaction with hERG K(+) channels have become surrogate markers of cardiotoxicity and have received increasing regulatory attention. This review briefly outlines the mechanisms leading to QT prolongation and the different strategies that can be followed to predict this unwanted effect. In particular, it will focus on the approaches recently proposed for the in silico screening of new compounds.

Zebrafish embryos express an orthologue of HERG and are sensitive toward a range of QT-prolonging drugs inducing severe arrhythmia☆

A wide range of drugs has been shown to prolong the QT interval of the electrocardiogram by blocking the pore-forming subunit of the rapidly activating delayed rectifier K+ channel, HERG (ether-à-go-go-related gene), sometimes leading to life-threatening arrhythmia. In this paper we describe cloning, sequence, and expression of the zebrafish orthologue of HERG, Zerg. Further, we studied effects of Zerg inhibition in zebrafish embryos caused by drugs or by an antisense approach. Zerg is expressed specifically in both heart chambers of zebrafish embryos, is composed of six transmembrane domains, and shows an especially high degree of amino acid conservation in the S6 and pore domain (99% identity). Several QT-prolonging drugs added to the bathing medium elicited bradycardia and arrhythmia in zebrafish embryos. The arrhythmia induced ranged from an atrioventricular 2:1 block, the ventricle beating half as often as the atrium, to more severe irregular arrhythmia with higher concentrations of the drugs. These effects were highly specific, reproducible, and rapid, e.g., 10 microM astemizole caused a 2:1 heartbeat within a minute after addition of the compound in all the embryos studied. Morpholino antisense oligonucleotides targeting Zerg were injected into zebrafish embryos and elicited similar dose-sensitive and specific arrhythmia as the QT-prolonging drugs, suggesting an evolutionarily conserved role for Erg in regulating heartbeat rate and rhythm. Further, we identified a mutation in the Per-Arnt-Sim domain of the Zerg channel in the breakdance mutant, also characterized by a 2:1 atrioventricular block. In conclusion, the zebrafish could be a tractable model organism for the study of Erg function and modulation but might also have a value in the field of cardiovascular pharmacology, e.g., as an early preclinical model for testing drugs under development for potential QT prolongation.

Management of ventricular fibrillation or unstable ventricular tachycardia in patients with congenital long-QT syndrome: a suggested modification to ACLS guidelines

Prolongation of the QT interval is a known risk factor for syncope, seizures and sudden cardiac death. Most patients with QT prolongation have an acquired cause, but congenital forms of QT prolongation are being increasingly recognized. However, existing advanced cardiac life support (ACLS) treatment algorithms for prolonged QT mediated ventricular fibrillation pertains to acquired long-QT syndrome (LQTS). Here, a young patient with out-of-hospital cardiac arrest secondary to congenital LQTS illustrates critical exceptions to the current ACLS treatment algorithms for ventricular fibrillation and unstable ventricular tachycardia when QT prolongation is congenital in origin. A clarified ACLS algorithm is proposed.

K+ channel structure-activity relationships and mechanisms of drug-induced QT prolongation

Pharmacological intervention, often for the purpose of treating syndromes unrelated to cardiac disease, can increase the vulnerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from genetic defects or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proven useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K(+) currents and prolong the QT interval on the electrocardiogram. Understanding the structural determinants of K(+) channel blockade may provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.

Cardiac retention of [11C]HED in genotyped long QT patients: a potential amplifier role for severity of the disease

Although mutations in cardiac sodium and potassium channel genes are associated with congenital long QT syndrome (LQTS), a "modifier" role of the sympathetic nervous system was proposed to explain the distinct severity of the disease. We evaluated cardiac sympathetic innervation using [11C]hydroxyephedrine ([11C]HED) and positron emission tomography (PET) in genotyped LQTS patients. H215O and [11C]HED PET studies were performed in 11 patients (5 symptomatic) and 8 controls. Perfusion and [11C]HED images were depicted as 36-sector polar maps. Sectorial values of perfusion (H2O%), absolute (HEDRet) and relative retention (HED%Ret) of [11C]HED, and the ratio of HED%Ret to H2O% (HED%Ret/H2O%) were calculated. Normal databases were obtained from controls. Sectorial values below 2SD database values were defined as "outside sectors." Controls and patients showed similar sectorial perfusion. Sectorial HEDRet did not differ between groups, but means of HED%Ret were lower in three sectors for patients (P < 0.05). Three sectors from 3 controls had HED%Ret below 2SD, whereas 36 sectors in 9 patients were outside sectors (P < 0.01). In patients, average HED%Ret/H2O% was lower in 9 sectors (P < 0.05 vs. controls); 2 outside sectors were found in controls, but 43 outside sectors were found in patients (P < 0.01), 77% of them in the 5 symptomatic patients. Heterogeneous [11C]HED retention was localized in the septal, anterior, and lateral walls. Most LQTS patients showed a localized and decreased pattern of [11C]HED retention. The larger number of heterogeneous sectors in symptomatic patients suggests that sympathetic function could play an amplifier role for severity of the disease.

Structural and functional basis for the long QT syndrome: relevance to veterinary patients

Long QT syndrome (LQTS) is a condition characterized by prolongation of ventricular repolarization and is manifested clinically by lengthening of the QT interval on the surface ECG. Whereas inherited forms of LQTS associated with mutations in the genes that encode ion channel proteins are identified only in humans, the acquired form of LQTS occurs in humans and companion animal species. Often, acquired LQTS is associated with drug-induced block of the cardiac K+ current designated I(Kr). However, not all drugs that induce potentially fatal ventricular arrhythmias antagonize I(Kr), and not all drugs that block I(Kr), are associated with ventricular arrhythmias. In clinical practice, the extent of QT interval prolongation and risk of ventricular arrhythmia associated with antagonism of I(Kr) are modulated by pharmacokinetic and pharmacodynamic variables. Veterinarians can influence some of the potential risk factors (eg, drug dosage, route of drug administration, presence or absence of concurrent drug therapy, and patient electrolyte status) but not all (eg, patient gender/genetic background). Veterinarians need to be aware of the potential for acquired LQTS during therapy with drugs identified as blockers of HERG channels and I(Kr).

Unusual presentation of long QT syndrome

We describe the case of a 9-yr-old child with undiagnosed long QT syndrome who experienced an intraoperative cardiac arrest after accidental intravascular injection of bupivacaine with epinephrine via a misplaced epidural catheter.