The proarrhythmogenic role of autonomics and emerging neuromodulation approaches to prevent sudden death in cardiac ion channelopathies

Ventricular arrhythmias in cardiac channelopathies are linked to autonomic triggers, which are sub-optimally targeted in current management strategies. Improved molecular understanding of cardiac channelopathies and cellular autonomic signalling could refine autonomic therapies to target the specific signalling pathways relevant to the specific aetiologies as well as the central nervous system centres involved in the cardiac autonomic regulation. This review summarizes key anatomical and physiological aspects of the cardiac autonomic nervous system and its impact on ventricular arrhythmias in primary inherited arrhythmia syndromes. Proarrhythmogenic autonomic effects and potential therapeutic targets in defined conditions including the Brugada syndrome, early repolarization syndrome, long QT syndrome, and catecholaminergic polymorphic ventricular tachycardia will be examined. Pharmacological and interventional neuromodulation options for these cardiac channelopathies are discussed. Promising new targets for cardiac neuromodulation include inhibitory and excitatory G-protein coupled receptors, neuropeptides, chemorepellents/attractants as well as the vagal and sympathetic nuclei in the central nervous system. Novel therapeutic strategies utilizing invasive and non-invasive deep brain/brain stem stimulation as well as the rapidly growing field of chemo-, opto-, or sonogenetics allowing cell-specific targeting to reduce ventricular arrhythmias are presented.

Novel Gain-of-Function Mutation in the Kv11.1 Channel Found in the Patient with Brugada Syndrome and Mild QTc Shortening

Brugada syndrome (BrS) is an inherited disease characterized by right precordial ST-segment elevation in the right precordial leads on electrocardiograms (ECG), and high risk of life-threatening ventricular arrhythmia and sudden cardiac death (SCD). Mutations in the responsible genes have not been fully characterized in the BrS patients, except for the SCN5A gene. We identified a new genetic variant, c.1189C>T (p.R397C), in the KCNH2 gene in the asymptomatic male proband diagnosed with BrS and mild QTc shortening. We hypothesize that this variant could alter IKr-current and may be causative for the rare non-SCN5A-related form of BrS. To assess its pathogenicity, we performed patch-clamp analysis on IKr reconstituted with this KCNH2 mutation in the Chinese hamster ovary cells and compared the phenotype with the wild type. It appeared that the R397C mutation does not affect the IKr density, but facilitates activation, hampers inactivation of the hERG channels, and increases magnitude of the window current suggesting that the p.R397C is a gain-of-function mutation. In silico modeling demonstrated that this missense mutation potentially leads to the shortening of action potential in the heart.

Systemic primary carnitine deficiency induces severe arrhythmia due to shortening of QT interval

BACKGROUND

Systemic primary carnitine deficiency (PCD) is characterized by cardiomyopathy and arrhythmia. Without carnitine supplementation, progression is usually towards fatal cardiac decompensation. While the cardiomyopathy is most likely secondary to energy deficiency, the mechanism of arrhythmia is unclear, and may be related to a short QT interval.

OBJECTIVE

We aim to describe rhythmic manifestations at diagnosis and with carnitine supplementation.

METHODS

French patients diagnosed for PCD were retrospectively included. Clinical and para clinical data at diagnosis and during follow-up were collected. Electrocardiograms with QT interval measurements were blinded reviewed by two paediatric cardiologists.

RESULTS

Nineteen patients (median age at diagnosis 2.3 years (extremes 0.3-28.9)) followed in 8 French centres were included. At diagnosis, 21% of patients (4/19) had arrhythmia (2 ventricular fibrillations, 1 ventricular tachycardia and 1 sudden death), and 84% (16/19) had cardiomyopathy. Six electrocardiograms before treatment out of 11 available displayed a short QT (QTc < 340 ms). Median corrected QTc after carnitine supplementation was 404 ms (extremes 341-447) versus 350 ms (extremes 282-421) before treatment (p < 0.001). The whole QTc was prolonged, and no patient reached the criterion of short QT syndrome with carnitine supplementation. Three patients died, probably from rhythmic cause without carnitine supplementation (two extra-hospital sudden deaths and one non-recoverable rhythmic storm before carnitine supplementation), whereas no rhythmic complication occurred in patients with carnitine supplementation.

CONCLUSION

PCD is associated with shortening of the QT interval inducing severe arrhythmia. A potential explanation would be a toxic effect of accumulated fatty acid and metabolites on ionic channels embedded in the cell membrane. Carnitine supplementation normalizes the QTc and prevents arrhythmia. Newborn screening of primary carnitine deficiency would prevent avoidable deaths.

Utilizing human induced pluripotent stem cells to study atrial arrhythmias in the short QT syndrome

BACKGROUND

Among the monogenic inherited causes of atrial fibrillation is the short QT syndrome (SQTS), a rare channelopathy causing atrial and ventricular arrhythmias. One of the limitations in studying the mechanisms and optimizing treatment of SQTS-related atrial arrhythmias has been the lack of relevant human atrial tissues models.

OBJECTIVE

To generate a unique model to study SQTS-related atrial arrhythmias by combining the use of patient-specific human induced pluripotent stem cells (hiPSCs), atrial-specific differentiation schemes, two-dimensional tissue modeling, optical mapping, and drug testing.

METHODS AND RESULTS

SQTS (N588K KCNH2 mutation), isogenic-control, and healthy-control hiPSCs were coaxed to differentiate into atrial cardiomyocytes using a retinoic-acid based differentiation protocol. The atrial identity of the cells was confirmed by a distinctive pattern of MLC2v downregulation, connexin 40 upregulation, shorter and triangular-shaped action potentials (APs), and expression of the atrial-specific acetylcholine-sensitive potassium current. In comparison to the healthy- and isogenic control cells, the SQTS-hiPSC atrial cardiomyocytes displayed abbreviated APs and refractory periods along with an augmented rapidly activating delayed-rectifier potassium current (IKr). Optical mapping of a hiPSC-based atrial tissue model of the SQTS displayed shortened APD and altered biophysical properties of spiral waves induced in this model, manifested by accelerated spiral-wave frequency and increased rotor curvature. Both AP shortening and arrhythmia irregularities were reversed by quinidine and vernakalant treatment, but not by sotalol.

CONCLUSIONS

Patient-specific hiPSC-based atrial cellular and tissue models of the SQTS were established, which provide examples on how this type of modeling can shed light on the pathogenesis and pharmacological treatment of inherited atrial arrhythmias.

Genetic analysis identifies the SLC4A3 anion exchanger as a major gene for short QT syndrome

BACKGROUND

A variant in the SLC4A3 anion exchanger has been identified as a novel cause of short QT syndrome (SQTS), but the clinical importance of SLC4A3 as a cause of SQTS or sudden cardiac death remains unknown.

OBJECTIVE

The purpose of this study was to investigate the prevalence of potential disease-causing variants in SQTS patients using gene panels including SLC4A3.

METHODS

In this multicenter study, genetic testing was performed in 34 index patients with SQTS. The pathogenicity of novel SLC4A3variants was validated in a zebrafish embryo heart model.

RESULTS

Potentially disease-causing variants were identified in 9 (26%) patients and were mainly (15%) located in SLC4A3: 4 patients heterozygous for novel nonsynonymous SLC4A3 variants-p.Arg600Cys, p.Arg621Trp, p.Glu852Asp, and p.Arg952His-and 1 patient with the known p.Arg370His variant. In other SQTS genes, potentially disease-causing variants were less frequent (2× in KCNQ1, 1× in KCNJ2, and CACNA1C each). SLC4A3 variant carriers (n = 5) had a similar heart rate but shorter QT and J point to T wave peak intervals than did noncarriers (n = 29). Knockdown of slc4a3 in zebrafish resulted in shortened heart rate-corrected QT intervals (calculated using the Bazett formula) that could be rescued by overexpression of the native human SLC4A3-encoded protein (AE3), but neither by the mutated AE3 variants p.Arg600Cys, p.Arg621Trp, p.Glu852Asp nor by p.Arg952His, suggesting pathogenicity of these variants. Dysfunction in slc4a3/AE3 was associated with alkaline cytosol and shortened action potential of cardiomyocytes.

CONCLUSION

In about a quarter of patients with SQTS, a potentially disease-causing variant can be identified. Nonsynonymous variants in SLC4A3 represent the most common cause of SQTS, underscoring the importance of including SLC4A3 in the genetic screening of patients with SQTS or sudden cardiac death.

Epigenetic mechanism of L-type calcium channel β-subunit downregulation in short QT human induced pluripotent stem cell-derived cardiomyocytes with CACNB2 mutation

AIMS

A loss-of-function mutation in L-type calcium (Ca2+) channel subunit gene CACNB2 has been reported to cause short QT syndrome subtype 5 (SQT5). However, the mechanism underlying the loss-of-function of the Ca2+ channel has not been clarified. In the present study, we aim to explore the DNA methylation mechanism of L-type Ca2+ channel downregulation in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) of SQT5.

METHODS AND RESULTS

The hiPSC-CMs were generated from a healthy donor and a SQT5 patient carrying the CACNB2 variant c.1439C > T/p.S480L. The variant was genetically corrected using ribonucleoprotein-based CRISPR/Cas9 technique to obtain an isogenic control cell line. The action potential (AP) and Ca2+ current were measured by patch clamp. Protein expression levels were determined by western blotting. Dot blotting and bisulfite sequence were performed for epigenetic study. Our results showed that AP durations at 10% repolarization (APD10) and 50% repolarization (APD50) were significantly shortened in SQT5 cells and both the expression level of the β-subunit and channel current of L-type Ca2+ channel were reduced. Besides, an increased level of whole-genome DNA methylation and DNA methylation of CpG island in the promoter region of CACNB2 gene was detected. Overexpression of demethylation enzyme could rescue the decreased expression of CACNB2 and the L-type Ca2+ current.

CONCLUSION

In SQT5 hiPSC-CMs carrying the CACNB2-S480L variant, the decreased L-type Ca2+ current resulting from decreased CACNB2 protein expression was caused by enhanced methylation in the promoter region of the CACNB2 gene and upregulation of DNA methyltransferases might be one of the mechanisms.

Case Report

Short QT syndrome (SQTS) represents a diagnosis challenge where the symptoms may vary from palpitations in an otherwise asymptomatic patient to sudden death. Is a recently discovered rare channelopathy, identified by Gussak in 2000, characterized by short QT intervals on the electrocardiogram and a tendency to develop atrial and ventricular arrhythmias in the absence of structural heart disease, hyperkalemia, hypercalcemia, hyperthermia, acidosis and endocrine disorders. We present the case of a 16-year-old patient with short QT-type channelopathy, who presented with sinus arrest and junctional rhythm, who later developed atrial tachycardia and atrial flutter.

Evaluation of gene validity for CPVT and short QT syndrome in sudden arrhythmic death

AIMS

Catecholaminergic polymorphic ventricular tachycardia (CPVT) and short QT syndrome (SQTS) are inherited arrhythmogenic disorders that can cause sudden death. Numerous genes have been reported to cause these conditions, but evidence supporting these gene-disease relationships varies considerably. To ensure appropriate utilization of genetic information for CPVT and SQTS patients, we applied an evidence-based reappraisal of previously reported genes.

METHODS AND RESULTS

Three teams independently curated all published evidence for 11 CPVT and 9 SQTS implicated genes using the ClinGen gene curation framework. The results were reviewed by a Channelopathy Expert Panel who provided the final classifications. Seven genes had definitive to moderate evidence for disease causation in CPVT, with either autosomal dominant (RYR2, CALM1, CALM2, CALM3) or autosomal recessive (CASQ2, TRDN, TECRL) inheritance. Three of the four disputed genes for CPVT (KCNJ2, PKP2, SCN5A) were deemed by the Expert Panel to be reported for phenotypes that were not representative of CPVT, while reported variants in a fourth gene (ANK2) were too common in the population to be disease-causing. For SQTS, only one gene (KCNH2) was classified as definitive, with three others (KCNQ1, KCNJ2, SLC4A3) having strong to moderate evidence. The majority of genetic evidence for SQTS genes was derived from very few variants (five in KCNJ2, two in KCNH2, one in KCNQ1/SLC4A3).

CONCLUSIONS

Seven CPVT and four SQTS genes have valid evidence for disease causation and should be included in genetic testing panels. Additional genes associated with conditions that may mimic clinical features of CPVT/SQTS have potential utility for differential diagnosis.

Identification through action potential clamp of proarrhythmic consequences of the short QT syndrome T618I hERG 'hotspot' mutation

The T618I KCNH2-encoded hERG mutation is the most frequently observed mutation in genotyped cases of the congenital short QT syndrome (SQTS), a cardiac condition associated with ventricular fibrillation and sudden death. Most T618I hERG carriers exhibit a pronounced U wave on the electrocardiogram and appear vulnerable to ventricular, but not atrial fibrillation (AF). The basis for these effects is unclear. This study used the action potential (AP) voltage clamp technique to determine effects of the T618I mutation on hERG current (I_hERG) elicited by APs from different cardiac regions. Whole-cell patch-clamp recordings were made at 37 °C of I_hERG from hERG-transfected HEK-293 cells. Maximal I_hERG during a ventricular AP command was increased ∼4-fold for T618I I_hERG and occurred much earlier during AP repolarization. The mutation also increased peak repolarizing currents elicited by Purkinje fibre (PF) APs. Maximal wild-type (WT) I_hERG current during the PF waveform was 87.2 ± 4.5% of maximal ventricular repolarizing current whilst for the T618I mutant, the comparable value was 47.7 ± 2.7%. Thus, the T618I mutation exacerbated differences in repolarizing I_hERG between PF and ventricular APs; this could contribute to heterogeneity of ventricular-PF repolarization and consequently to the U waves seen in T618I carriers. The comparatively shorter duration and lack of pronounced plateau of the atrial AP led to a smaller effect of the T618I mutation during the atrial AP, which may help account for the lack of reported AF in T618I carriers. Use of a paired ventricular AP protocol revealed an alteration to protective I_hERG transients that affect susceptibility to premature excitation late in AP repolarization/early in diastole. These observations may help explain altered arrhythmia susceptibility in this form of the SQTS.

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T618I is a ‘hotspot’ hERG potassium channel mutation in the congenital short QT syndrome.

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Differences in hERG current during ventricular and Purkinje fibre action potentials are exacerbated by the T618I mutation.

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T618I has more modest effects on current during atrial action potentials.

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T618I modifies the protective response of hERG to premature ventricular excitation.

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These alterations to hERG function help explain ECG changes reported in T618I-hERG carriers.

Preclinical short QT syndrome models: studying the phenotype and drug-screening

Cardiovascular diseases are the main cause of sudden cardiac death (SCD) in developed and developing countries. Inherited cardiac channelopathies are linked to 5-10% of SCDs, mainly in the young. Short QT syndrome (SQTS) is a rare inherited channelopathy, which leads to both atrial and ventricular tachyarrhythmias, syncope, and even SCD. International European Society of Cardiology guidelines include as diagnostic criteria: (i) QTc ≤ 340 ms on electrocardiogram, (ii) QTc ≤ 360 ms plus one of the follwing, an affected short QT syndrome pathogenic gene mutation, or family history of SQTS, or aborted cardiac arrest, or family history of cardiac arrest in the young. However, further evaluation of the QTc ranges seems to be required, which might be possible by assembling large short QT cohorts and considering genetic screening of the newly described pathogenic mutations. Since the mechanisms underlying the arrhythmogenesis of SQTS is unclear, optimal therapy for SQTS is still lacking. The disease is rare, unclear genotype-phenotype correlations exist in a bevy of cases and the absence of an international short QT registry limit studies on the pathophysiological mechanisms of arrhythmogenesis and therapy of SQTS. This leads to the necessity of experimental models or platforms for studying SQTS. Here, we focus on reviewing preclinical SQTS models and platforms such as animal models, heterologous expression systems, human-induced pluripotent stem cell-derived cardiomyocyte models and computer models as well as three-dimensional engineered heart tissues. We discuss their usefulness for SQTS studies to examine genotype-phenotype associations, uncover disease mechanisms and test drugs. These models might be helpful for providing novel insights into the exact pathophysiological mechanisms of this channelopathy and may offer opportunities to improve the diagnosis and treatment of patients with SQT syndrome.

Heritable arrhythmias associated with abnormal function of cardiac potassium channels

Cardiomyocytes express a surprisingly large number of potassium channel types. The primary physiological functions of the currents conducted by these channels are to maintain the resting membrane potential and mediate action potential repolarization under basal conditions and in response to changes in the concentrations of intracellular sodium, calcium, and ATP/ADP. Here, we review the diversity and functional roles of cardiac potassium channels under normal conditions and how heritable mutations in the genes encoding these channels can lead to distinct arrhythmias. We briefly review atrial fibrillation and J-wave syndromes. For long and short QT syndromes, we describe their genetic basis, clinical manifestation, risk stratification, traditional and novel therapeutic approaches, as well as insights into disease mechanisms provided by animal and cellular models.

Novel electrocardiographic criteria for short QT syndrome in children and adolescents

AIMS

Although shortening of the corrected QT interval (QTc) is a key finding in the diagnosis of short QT syndrome (SQTS), there may be overlap of the QTc between SQTS patients and normal subjects in childhood and adolescence. We aimed to investigate electrocardiographic findings for differentiation of SQTS patients.

METHODS AND RESULTS

The SQTS group comprised 34 SQTS patients <20 years old, including 9 from our institutions and 25 from previous reports. The control group comprised 61 apparently healthy subjects with an QTc of <360 ms who were selected from 13 314 participants in a school-based screening programme. We compared electrocardiographic findings, including QT and Jpoint-Tpeak intervals (QT and J-Tpeak, respectively), those corrected by using the Bazett's and Fridericia's formulae (cB and cF, respectively) and early repolarization (ER) between the groups. QT, QTc by using Bazett's formula (QTcB), QTc by using Fridericia's formula (QTcF), J-Tpeak, J-Tpeak cB, and J-Tpeak cF were significantly shorter in the SQTS group than in the control group. On receiver operating characteristic curve analysis, the area under the curve (AUC) was largest for QTcB (0.888) among QT, QTcB, and QTcF, with a cut-off value of 316 ms (sensitivity: 79.4% and specificity: 96.7%). The AUC was largest for J-Tpeak cB (0.848) among J-Tpeak, J-Tpeak cB, and J-Tpeak cF, with a cut-off value of 181 ms (sensitivity: 80.8% and specificity: 91.8%). Early repolarization was found more frequently in the SQTS group than in the control group (67% vs. 23%, P = 0.001).

CONCLUSION

A QTcB <316 ms, J-Tpeak cB < 181 ms, and the presence of ER may indicate SQTS patients in childhood and adolescence.

Atrial Fibrillation in Inherited Channelopathies

Atrial fibrillation (AF), the common sustained arrhythmia in clinical practice, has major public health implications due to its associated morbidity and increased mortality. The AF epidemic is due to the burgeoning elderly population and the identification of novel risk factors, for example, genetics. Since the diagnosis of AF has a major impact on the clinical assessment and management of patients with inherited arrhythmia syndromes, improved understanding of the cause and pathogenesis of AF has provided important insights into the underlying pathophysiological mechanisms of this common arrhythmia and identified potential mechanism-based therapies.

Long-term prognosis of short QT interval in Korean patients: a multicenter retrospective cohort study

Background

Short QT syndrome is a rare, inherited channelopathy associated with sudden cardiac arrest (SCA) but the characteristics and prognosis of short QT interval (SQTI) in Korean patients remain unclear. This study aimed to determine the clinical characteristics and outcomes of SQTI in a Korean population.

Methods

Consecutive patients with SQTI from January 1999 to March 2019 in three university hospitals in South Korea were recruited. SQTI was defined as a Bazett’s formula-corrected QT interval (QTc) ≤ 340 ms in serial electrocardiograms. Age- and sex-matched patients with a normal QTc and without overt cardiovascular disease were included in a 1:4 ratio. Clinical and ECG features and outcomes were compared between patients with and without SQTI.

Results

34 patients with SQTI [age, 23.5 (21–30.5) years; 31 male] were followed up for 4.8 (2.0–7.8) years. Early repolarization, tall T wave, and U wave were significantly more frequent in patients with SQTI than in those without SQTI. QT dispersion [44.0 (28.0–73.0) vs. 20.0 (12.0–35.0) ms, P < 0.001] was significantly wider and heart rate [52.0 (47.0–58.0) vs. 70.0 (62.3–84.0)/min, P < 0.001] was significantly slower in patients with SQTI than in those without. Atrial fibrillation (AF, 11.8% vs. 2.2%, P = 0.030) and ventricular arrhythmia (VA)/SCA (8.7% vs. 0%, P = 0.007) were significantly more frequent in patients with SQTI than in those without. SQTI was significantly associated with AF [odds ratio, 5.911; 95% confidence interval, 1.257–27.808; P = 0.025] and VA/SCA.

Conclusions

In this subset of Korean population, SQTI was associated with AF and VA/SCA.

SARS-CoV-2, COVID-19, and inherited arrhythmia syndromes

Ever since the first case was reported at the end of 2019, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the associated coronavirus disease 2019 (COVID-19) has become a serious threat to public health globally in short time. At this point in time, there is no proven effective therapy. The interactions with concomitant disease are largely unknown, and that may be particularly pertinent to inherited arrhythmia syndrome. An arrhythmogenic effect of COVID-19 can be expected, potentially contributing to disease outcome. This may be of importance for patients with an increased risk of cardiac arrhythmias, either secondary to acquired conditions or comorbidities or consequent to inherited syndromes. Management of patients with inherited arrhythmia syndromes such as long QT syndrome, Brugada syndrome, short QT syndrome, and catecholaminergic polymorphic ventricular tachycardia in the setting of the COVID-19 pandemic may prove particularly challenging. Depending on the inherited defect involved, these patients may be susceptible to proarrhythmic effects of COVID-19–related issues such as fever, stress, electrolyte disturbances, and use of antiviral drugs. Here, we describe the potential COVID-19–associated risks and therapeutic considerations for patients with distinct inherited arrhythmia syndromes and provide recommendations, pending local possibilities, for their monitoring and management during this pandemic.

Inherited Cardiac Arrhythmias and Channelopathies

With recent advances in genetic diagnostics, many inherited diseases, which can cause life-threatening arrhythmias, are being better characterized. Many of these diseases are caused by genetic disorders that affect the function of the ion channels that regulate the action potential or the function of important cardiac muscle regulatory proteins. This article summarizes the diseases that we have learned about, such as the long QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. The article examines the diagnosis, genetic screening of patients and their relatives, management, and referral to a specialist for further therapy.

Long-term follow-up of implantable cardioverter-defibrillators in Short QT syndrome

BACKGROUND

Short QT syndrome (SQTS) is associated with sudden cardiac death and implantable cardioverter-defibrillator (ICD) implantation is recommended in this rare disease. However, only a few SQTS families have been reported in literature with limited follow-up data.

OBJECTIVES

In the recent study, we describe the outcome data of 57 SQTS patients receiving ICD implantation. This includes seven SQTS families consecutively admitted to our hospital between 2002 and 2017 as well as patients reported in published literature.

METHODS

Seven SQTS patients admitted to our hospital were followed up. Additionally, 7 studies out of a total of 626 researched articles were identified through systematic database search (PubMed, Web of Science, Cochrane Library, and Cinahl) and their data analyzed according to our model.

RESULTS

Complications during a median follow-up time of 67.4 months (IQR 6-162 months) were documented in 31 (54%) patients. Inappropriate shocks were seen in 33% due to T wave oversensing (8.7%), supraventricular tachycardia (19%), lead failure and fracture (21%). Further complications were infection (10%), battery depletion (7%) and psychological distress (3.5%). Appropriate shocks were documented in 19%. Three patients (5%) were treated with s-ICD due to recurrent complications of transvenous ICD.

CONCLUSION

ICD therapy is an effective therapy in SQTS patients. However, it is also associated with significant risk of device-related complications.

Use of microminipigs for unveiling unknown mechanisms of azithromycin-induced cardiovascular death

Although azithromycin can suppress cardiac I_Na, I_Kr, I_Ks, I_Ca,L and I_K1, its onset mechanisms for cardiovascular death have not been fully investigated. We examined electropharmacological effects of azithromycin in intravenous doses of 0.3, 3 and 30 mg/kg using microminipigs under the halothane anesthesia (n = 4), which provided plasma concentrations of 3.1, 11.2 and 120.4 μg/mL, respectively. The low dose did not alter any of the cardiohemodynamic or electrocardiographic variables. The middle dose significantly shortened QT interval for 10-20 min and QTc for 10-30 min. The high dose significantly decreased mean blood pressure for 5-60 min, prolonged QRS width at 20 min, but shortened QT interval for 15-20 min and QTc for 15-30 min (n = 3). Cardiohemodynamic collapse occurred in 1 animal after the start of the high dose infusion, which might be associated with the cardiovascular death in patients with vasomotor dysfunction. Prolongation of QRS width indicates that azithromycin may suppress ventricular I_Na in vivo, which may unmask latent type of Brugada electrocardiographic genotype. Meanwhile, abbreviation of the QTc might cause potentially lethal, short QT-related, cardiac arrhythmia syndrome. These findings with microminipigs suggest the possible entry point for analyzing the mechanisms of cardiovascular death clinically seen with this antibiotic.

[Syncopes and channelopathies]

Syncope can be the first manifestation of cardiac channelopathies, namely Brugada syndrome, long QT syndrome, short QT syndrome and catecholaminergic polymorphic ventricular tachycardia (CPVT). Patients affected by these rare diseases are at increased risk for sudden cardiac death due to ventricular tachyarrhythmias and require specific therapy and follow-up. As syncope is common in the general population, only few cases are caused by an underlying channelopathy. Nevertheless, the diagnosis should be considered in young patients with structurally normal hearts, especially if the history of syncope is typical for an arrhythmogenic cause, in the presence of characteristic echocardiogram (ECG) patterns, and if there is a family history of channelopathies or sudden cardiac death. On the other hand, syncope plays an important role in the management of patients with diagnosed channelopathies, as they may indicate an increased risk for sudden cardiac death. The predictive value and consequences for treatment vary between the different channelopathies. However, data on this issue are scarce due to the low prevalence of these diseases. This review highlights typical findings in the medical history and diagnostic tests that may point towards an underlying channelopathy in patients with syncope. It also discusses the prognostic and therapeutic implications of a history of syncope in patients with known channelopathies.

Inherited primary arrhythmia disorders: cardiac channelopathies and sports activity

Sudden cardiac death (SCD) in an apparently healthy individual is a tragedy. It is important to identify the cause of death and to prevent SCD in potentially at-risk family members. Inherited primary arrhythmia disorders are associated with exercise-related SCD. Despite the well-known benefits of exercise, exercise restriction has been a historical mainstay of therapy for these conditions. However, since familiarity with inherited arrhythmia conditions has increased and patients are often children and young adults, it is necessary to reassess the treatment guidelines regarding exercise constraints. The aim of this review is to analyze the risk of exercise-induced SCD in patients with inherited cardiac conditions and explore the challenges faced when advising patients about exercise limitations. We searched for publications on cardiac channelopathies in PubMed with the following medical subject headings (MeSH): "long QT syndrome"; "short QT syndrome"; "Brugada syndrome"; and "catecholaminergic polymorphic ventricular tachycardia". The abstracts of these articles were scanned, and articles of relevance, along with pertinent references, were read in full. The analysis was restricted to reports published in English. The findings of this analysis suggest that exercise with low-to-moderate cardiovascular demand may be possible under regular clinical follow-up in inherited primary arrhythmia disorders. Recent data show that patients with inherited primary arrhythmia disorders are at low risk for events once a comprehensive treatment program has been established. Recreational activity is likely safe for these individuals, with personalized management based on individual patient preferences and priorities.

Drug-induced life-threatening arrhythmias and sudden cardiac death: A clinical perspective of long QT, short QT and Brugada syndromes

Sudden cardiac death is a major public health challenge, which can be caused by genetic or acquired structural or electrophysiological abnormalities. These abnormalities include hereditary channelopathies: long QT, short QT and Brugada syndromes. These syndromes are a notable concern, particularly in young people, due to their high propensity for severe ventricular arrhythmias and sudden cardiac death. Current evidence suggests the involvement of an increasing number of drugs in acquired forms of long QT and Brugada syndromes. However, drug-induced short QT syndrome is still a rarely reported condition. Therefore, there has been speculation on its clinical significance, since few fatal arrhythmias and sudden cardiac death cases have been described so far. Drug-induced proarrhythmia is a growing challenge for physicians, regulatory agencies and the pharmaceutical industry. Physicians should weigh the risks of potentially fatal outcomes against the therapeutic benefits, when making decisions about drug prescriptions. Growing concerns about its safety and the need for more accurate predictive models for drug-induced fatal outcomes justify further research in these fields. The aim of this article is to comprehensively and critically review the recently published evidence with regard to drug-induced life-threatening arrhythmias and sudden cardiac death. This article will take into account the provision of data to physicians that are useful in the identification of the culprit drugs, and thus, contribute to the prompt recognition and management of these serious clinical conditions.

PA-6 inhibits inward rectifier currents carried by V93I and D172N gain-of-function KIR2.1 channels, but increases channel protein expression

BACKGROUND

The inward rectifier potassium current IK1 contributes to a stable resting membrane potential and phase 3 repolarization of the cardiac action potential. KCNJ2 gain-of-function mutations V93I and D172N associate with increased IK1, short QT syndrome type 3 and congenital atrial fibrillation. Pentamidine-Analogue 6 (PA-6) is an efficient (IC50 = 14 nM with inside-out patch clamp methodology) and specific IK1 inhibitor that interacts with the cytoplasmic pore region of the KIR2.1 ion channel, encoded by KCNJ2. At 10 μM, PA-6 increases wild-type (WT) KIR2.1 expression in HEK293T cells upon chronic treatment. We hypothesized that PA-6 will interact with and inhibit V93I and D172N KIR2.1 channels, whereas impact on channel expression at the plasma membrane requires higher concentrations.

METHODS

Molecular modelling was performed with the human KIR2.1 closed state homology model using FlexX. WT and mutant KIR2.1 channels were expressed in HEK293 cells. Patch-clamp single cell electrophysiology measurements were performed in the whole cell and inside-out mode of the patch clamp method. KIR2.1 expression level and localization were determined by western blot analysis and immunofluorescence microscopy, respectively.

RESULTS

PA-6 docking in the V93I/D172N double mutant homology model of KIR2.1 demonstrated that mutations and drug-binding site are >30 Å apart. PA-6 inhibited WT and V93I outward currents with similar potency (IC50 = 35.5 and 43.6 nM at +50 mV for WT and V93I), whereas D172N currents were less sensitive (IC50 = 128.9 nM at +50 mV) using inside-out patch-clamp electrophysiology. In whole cell mode, 1 μM of PA-6 inhibited outward IK1 at -50 mV by 28 ± 36%, 18 ± 20% and 10 ± 6%, for WT, V93I and D172N channels respectively. Western blot analysis demonstrated that PA-6 (5 μM, 24 h) increased KIR2.1 expression levels of WT (6.3 ± 1.5 fold), and V93I (3.9 ± 0.9) and D172N (4.8 ± 2.0) mutants. Immunofluorescent microscopy demonstrated dose-dependent intracellular KIR2.1 accumulation following chronic PA-6 application (24 h, 1 and 5 μM).

CONCLUSIONS

1) KCNJ2 gain-of-function mutations V93I and D172N in the KIR2.1 ion channel do not impair PA-6 mediated inhibition of IK1, 2) PA-6 elevates KIR2.1 protein expression and induces intracellular KIR2.1 accumulation, 3) PA-6 is a strong candidate for further preclinical evaluation in treatment of congenital SQT3 and AF.

Pro-arrhythmogenic Effects of the V141M KCNQ1 Mutation in Short QT Syndrome and Its Potential Therapeutic Targets: Insights from Modeling

Gain-of-function mutations in the pore-forming subunit of I_Ks channels, KCNQ1, lead to short QT syndrome (SQTS) and lethal arrhythmias. However, how mutant I_Ks channels cause SQTS and the possibility of I_Ks-specific pharmacological treatment remain unclear. V141M KCNQ1 is a SQTS associated mutation. We studied its effect on I_Ks gating properties and changes in the action potentials (AP) of human ventricular myocytes. Xenopus oocytes were used to study the gating mechanisms of expressed V141M KCNQ1/KCNE1 channels. Computational models were used to simulate human APs in endocardial, mid-myocardial, and epicardial ventricular myocytes with and without β-adrenergic stimulation. V141M KCNQ1 caused a gain-of-function in I_Ks characterized by increased current density, faster activation, and slower deactivation leading to I_Ks accumulation. V141M KCNQ1 also caused a leftward shift of the conductance-voltage curve compared to wild type (WT) I_Ks (V_1/2 = 33.6 ± 4.0 mV for WT, and 24.0 ± 1.3 mV for heterozygous V141M). A Markov model of heterozygous V141M mutant I_Ks was developed and incorporated into the O’Hara–Rudy model. Compared to the WT, AP simulations demonstrated marked rate-dependent shortening of AP duration (APD) for V141M, predicting a SQTS phenotype. Transmural electrical heterogeneity was enhanced in heterozygous V141M AP simulations, especially under β-adrenergic stimulation. Computational simulations identified specific I_K1 blockade as a beneficial pharmacologic target for reducing the transmural APD heterogeneity associated with V141M KCNQ1 mutation. V141M KCNQ1 mutation shortens ventricular APs and enhances transmural APD heterogeneity under β-adrenergic stimulation. Computational simulations identified I_K1 blockers as a potential antiarrhythmic drug of choice for SQTS.

Effects of amiodarone on short QT syndrome variant 3 in human ventricles: a simulation study

Background

Short QT syndrome (SQTS) is a newly identified clinical disorder associated with atrial and/or ventricular arrhythmias and increased risk of sudden cardiac death (SCD). The SQTS variant 3 is linked to D172N mutation to the KCNJ2 gene that causes a gain-of-function to the inward rectifier potassium channel current (I_K1), which shortens the ventricular action potential duration (APD) and effective refractory period (ERP). Pro-arrhythmogenic effects of SQTS have been characterized, but less is known about the possible pharmacological treatment of SQTS. Therefore, in this study, we used computational modeling to assess the effects of amiodarone, class III anti-arrhythmic agent, on human ventricular electrophysiology in SQT3.

Methods

The ten Tusscher et al. model for the human ventricular action potentials (APs) was modified to incorporate I_K1 formulations based on experimental data of Kir2.1 channels (including WT, WT-D172N and D172N conditions). The modified cell model was then implemented to construct one-dimensional (1D) and 2D tissue models. The blocking effects of amiodarone on ionic currents were modeled using IC₅₀ and Hill coefficient values from literatures. Effects of amiodarone on APD, ERP and pseudo-ECG traces were computed. Effects of the drug on the temporal and spatial vulnerability of ventricular tissue to genesis and maintenance of re-entry were measured, as well as on the dynamic behavior of re-entry.

Results

Amiodarone prolonged the ventricular cell APD and decreased the maximal voltage heterogeneity (δV) among three difference cells types across transmural ventricular wall, leading to a decreased transmural heterogeneity of APD along a 1D model of ventricular transmural strand. Amiodarone increased cellular ERP, prolonged QT interval and decreased the T-wave amplitude. It reduced tissue’s temporal susceptibility to the initiation of re-entry and increased the minimum substrate size necessary to sustain re-entry in the 2D tissue.

Conclusions

At the therapeutic-relevant concentration of amiodarone, the APD and ERP at the single cell level were increased significantly. The QT interval in pseudo-ECG was prolonged and the re-entry in tissue was prevented. This study provides further evidence that amiodarone may be a potential pharmacological agent for preventing arrhythmogenesis for SQT3 patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-017-0369-0) contains supplementary material, which is available to authorized users.

Short QT syndrome in pediatrics

Short QT syndrome is a malignant cardiac disease characterized by the presence of ventricular tachyarrhythmias leading to syncope and sudden cardiac death. Currently, international guidelines establish diagnostic criteria when QTc is below 340 ms. This entity is one of the main diseases responsible for sudden cardiac death in the pediatric population. In recent years, clinical, genetic and molecular advances in pathophysiological mechanisms related to short QT syndrome have improved diagnosis, risk stratification, and preventive measures. Despite these advances, automatic implantable cardiac defibrillator remains the most effective measure. Currently, six genes have been associated with short QT syndrome, which account for nearly 60% of clinically diagnosed families. Here, we review the main clinical hallmarks of the disease, focusing on the pediatric population.

Electrophysiological mechanisms of long and short QT syndromes

The QT interval on the human electrocardiogram is normally in the order of 450 ms, and reflects the summated durations of action potential (AP) depolarization and repolarization of ventricular myocytes. Both prolongation and shortening in the QT interval have been associated with ventricular tachy-arrhythmias, which predispose affected individuals to sudden cardiac death. In this article, the molecular determinants of the AP duration and the causes of long and short QT syndromes (LQTS and SQTS) are explored. This is followed by a review of the recent advances on their arrhythmogenic mechanisms involving reentry and/or triggered activity based on experiments conducted in mouse models. Established and novel clinical risk markers based on the QT interval for the prediction of arrhythmic risk and cardiovascular mortality are presented here. It is concluded by a discussion on strategies for the future rational design of anti-arrhythmic agents.

Genomic-based diagnosis of arrhythmia disease in a personalized medicine era

INTRODUCTION

Although thousands of potentially disease-causing mutations have been identified in a handful of genes, the genetic heterogeneity has led to diagnostic confusions, stemming directly from the limitations in our arsenal of genetic tools.

AREAS COVERED

We discuss the genetic basis of cardiac ion channelopathies, the gaps in our knowledge and how Next-generation sequencing technology (NGS) and can be used to bridge them, and how induced pluripotent stem cell (iPSC) derived-cardiomyocytes can be used for drug discovery.

EXPERT COMMENTARY

Univariate, arrhythmogenic arrhythmias can explain some congenital arrhythmias, however, it is far from a comprehensive understanding of the complexity of many arrhythmias. Mutational screening is a critical step in personalized medicine and is critical to the management of patients with arrhythmias. The success of personalized medicine requires a more efficient way to identify a high number of genetic variants potentially implicated in cardiac arrhythmogenic diseases than traditional sequencing methods (eg, Sanger sequencing). Next-generation sequencing technology provides us with unprecedented opportunities to achieve high-throughput, rapid, and cost-effective detection of congenital arrhythmias in patients. Moreover, in personalized medicine era, IPSC derived-cardiomyocytes can be used as 'cardiac arrhythmia in a dish' model for drug discovery, and help us improve management of arrhythmias in patients by developing patient-specific drug therapies with target specificity.

Assessment of drug-induced proarrhythmia: The importance of study design in the rabbit left ventricular wedge model

In the present study, we investigated an impact of the stimulation rate on the detection of the proarrhythmic potential of 10 reference compounds with effects on different cardiac ion channels in the isolated arterially-perfused rabbit left ventricular wedge preparation. The compounds were tested in the wedge model using two distinct protocols; including baseline stimulation at 1-Hz followed by a brief period at 0.5-Hz, either without an additional brief period of 2-Hz stimulation (i.e. Protocol 1) or with 2-Hz stimulation (i.e. Protocol 2). As expected, QT-prolonging drugs (ibutilide and quinidine) prolonged the QT interval, similarly increased the Torsades de Pointes (TdP) score, and elicited early afterdepolarizations (EADs) in both protocols. HMR1556 and JNJ-303 (IKs blockers) also prolonged the QT interval up to 1μM similarly in both protocols. Nifedipine (Ca(2+) antagonist) shortened the QT interval, and reduced force of contraction similarly in both protocols. However, Na(+) channel blockers (Ia, Ib, Ic) widened the QRS duration more in Protocol 2 than in Protocol 1. Furthermore, it was only possible to detect non-TdP-like ventricular tachycardia/fibrillation (VT/VF) induced by Na(+) blockers and by QT-shortening drugs (levcromakalim and mallotoxin) using the 2-Hz stimulation (Protocol 2). Our data suggest that the inclusion of a brief period of fast stimulation at 2Hz is critical for detecting drug-induced slowing of conduction (QRS widening), QT shortening and associated (non-TdP-like) VT/VF, which are distinct from the QT prolongation/TdP proarrhythmia in isolated, arterially-perfused rabbit left ventricular wedges.

Clinical Features of Genetic Cardiac Diseases Related to Potassium Channelopathies

Genetic cardiac diseases related to potassium channelopathies are a group of relatively rare syndromes that includes long QT syndrome, short QT syndrome, Brugada syndrome, and early repolarization syndrome. Patients with these syndromes share a propensity for the development of life-threatening ventricular arrhythmias in the absence of significant cardiac structural abnormalities. Familial atrial fibrillation has also been associated with potassium channel dysfunction but differs from the other syndromes by being a rare cause of a common condition. This article focuses on the clinical features, diagnosis, and management of these syndromes.

Genetic Control of Potassium Channels

Approximately 80 genes in the human genome code for pore-forming subunits of potassium (K(+)) channels. Rare variants (mutations) in K(+) channel-encoding genes may cause heritable arrhythmia syndromes. Not all rare variants in K(+) channel-encoding genes are necessarily disease-causing mutations. Common variants in K(+) channel-encoding genes are increasingly recognized as modifiers of phenotype in heritable arrhythmia syndromes and in the general population. Although difficult, distinguishing pathogenic variants from benign variants is of utmost importance to avoid false designations of genetic variants as disease-causing mutations.

Sinus bradycardia, junctional rhythm, and low-rate atrial fibrillation in Short QT syndrome during 20 years of follow-up: three faces of the same genetic problem

We describe the case of an asymptomatic girl with sinus bradycardia and short QT interval at birth, junctional bradycardia in infancy requiring single-chamber pacemaker, atrial fibrillation in adolescence, and V141M mutation in the KCNQ1 gene. Atrial fibrillation recurred and became unresponsive to electrical or anti-arrhythmic therapy. During 20 years of follow-up, a progressive evolution from sinus node dysfunction to low-rate atrial fibrillation was observed.

Atrial arrhythmias in inherited arrhythmogenic disorders

Atrial arrhythmias are being increasingly recognized in inherited arrhythmogenic disorders particularly in patients with Brugada syndrome and short QT syndrome. Atrial arrhythmias in inherited arrhythmogenic disorders have significant epidemiologic, clinical, and prognostic implications. There has been progress in the understanding of underlying genetic characteristics and the mechanistic link between atrial arrhythmias and inherited arrhythmogenic disorders. Appropriate management of these patients is of paramount importance.

Use of a subcutaneous ICD in a patient with short QT syndrome

Short QT syndrome carries the risk for inappropriate therapies using transvenous ICD because of overdetection of the ample T wave. SQT syndrome may also benefit from subcutaneous ICD, although additional cases are needed to affirm the safety of such device in this setting.

Role of pharmacotherapy in cardiac ion channelopathies

In the last decade, there have been considerable advances in the understanding of the pathophysiology of malignant ventricular tachyarrhythmias (VT) and sudden cardiac death (SCD). Over 80% of SCD occurs in patients with organic heart disease. However, approximately 10%-15% of SCD occurs in the presence of structurally normal heart, and the majority of these patients are young. In this group of patients, changes in genes encoding cardiac ion channels produce modifications of the function of the channel resulting in an electrophysiological substrate of VT and SCD. Collectively, these disorders are referred to as cardiac ion channelopathies. The four major syndromes in this group are: the long QT syndrome (LQTS), the Brugada syndrome (BrS), the short QT syndrome (SQTS), and the catecholaminergic polymorphic ventricular tachycardia (CPVT). Each of these syndromes includes multiple subtypes with different and sometimes complex cardiac ion channel genetic abnormalities. Many are associated with other somatic and neurological abnormalities besides the risk of VT and SCD. The current management of cardiac ion channelopathies can be summarized as follows: (1) in symptomatic patients, the implantable cardioverter defibrillator (ICD) is the only viable option; (2) in asymptomatic patients, risk stratification is necessary, followed by either the ICD, pharmacotherapy, or a combination of both. A genotype-specific approach to pharmacotherapy requires a thorough understanding of the molecular-cellular basis of arrhythmogenesis in cardiac ion channelopathies as well as the specific drug profile.

Characterization of a Chinese KCNQ1 mutation (R259H) that shortens repolarization and causes short QT syndrome 2

Objectives

To evaluate the association between a KCNQ1 mutation, R259H, and short QT syndrome (SQTS) and to explore the electrophysiological mechanisms underlying their association.

Methods

We performed genetic screening of SQTS genes in 25 probands and their family members (63 patients). We used direct sequencing to screen the exons and intron-exon boundaries of candidate genes that encode ion channels which contribute to the repolarization of the ventricular action potential, including KCNQ1, KCNH2, KCNE1, KCNE2, KCNJ2, CACNA1c, CACNB2b and CACNA2D1. In one of the 25 SQTS probands screened, we discovered a KCNQ1 mutation, R259H. We cloned R259H and transiently expressed it in HEK-293 cells; then, currents were recorded using whole cell patch clamp techniques.

Results

R259H-KCNQ1 showed significantly increased current density, which was approximately 3-fold larger than that of wild type (WT) after a depolarizing pulse at 1 s. The steady state voltage dependence of the activation and inactivation did not show significant differences between the WT and R259H mutation (P > 0.05), whereas the time constant of deactivation was markedly prolonged in the mutant compared with the WT in terms of the test potentials, which indicated that the deactivation of R259H was markedly slower than that of the WT. These results suggested that the R259H mutation can effectively increase the slowly activated delayed rectifier potassium current (I_Ks) in phase 3 of the cardiac action potential, which may be an infrequent cause of QT interval shortening.

Conclusions

R259H is a gain-of-function mutation of the KCNQ1 channel that is responsible for SQTS2. This is the first time that the R259H mutation was detected in Chinese people.

Carnitine deficiency induces a short QT syndrome

BACKGROUND

Short QT syndrome is associated with an increased risk of cardiac arrhythmias and unexpected sudden death. Until now, only mutations in genes encoding the cardiac potassium and calcium channels have been implicated in early T-wave repolarization.

OBJECTIVE

The purpose of this study was to confirm a relationship between a short QT syndrome and carnitine deficiency.

METHODS

We report 3 patients affected by primary systemic carnitine deficiency and an associated short QT syndrome. Ventricular fibrillation during early adulthood was the initial symptom in 1 case. To confirm the relationship between carnitine, short QT syndrome, and arrhythmias, we used a mouse model of carnitine deficiency induced by long-term subcutaneous perfusion of MET88.

RESULTS

MET88-treated mice developed cardiac hypertrophy associated with a remodeling of the mitochondrial network. The continuous monitoring of electrocardiograms confirmed a shortening of the QT interval, which was negatively correlated with the plasma carnitine concentration. As in humans, such alterations coincided with the genesis of ventricular premature beats and ventricular tachycardia and fibrillation.

CONCLUSION

Altogether, these results suggest that long-chain fatty acid metabolism influence the morphology and the electrical function of the heart.

A new KCNQ1 mutation at the S5 segment that impairs its association with KCNE1 is responsible for short QT syndrome

AIMS

KCNQ1 and KCNE1 encode Kv7.1 and KCNE1, respectively, the pore-forming and the accessory subunits of the slow delayed rectifier potassium current, IKs. KCNQ1 mutations are associated with long and short QT syndrome. The aim of this study was to characterize the biophysical and cellular phenotype of a KCNQ1 missense mutation, F279I, found in a 23-year-old man with a corrected QT interval (QTc) of 356 ms and a family history of sudden cardiac death.

METHODS AND RESULTS

Experiments were performed using perforated patch-clamp, western blot, co-immunoprecipitation, biotinylation, and immunocytochemistry techniques in HEK293, COS7 cells and in cardiomyocytes transfected with WT Kv7.1/KCNE1 or F279I Kv7.1/KCNE1 channels. In the absence of KCNE1, F279I Kv7.1 current exhibited a lesser degree of inactivation than WT Kv7.1. Also, functional analysis of F279I Kv7.1 in the presence of KCNE1 revealed a negative shift in the activation curve and an acceleration of the activation kinetics leading to a gain of function in IKs. The co-assembly between F279I Kv7.1 channels and KCNE1 was markedly decreased compared with WT Kv7.1 channels, as revealed by co-immunoprecipitation and Föster Resonance Energy Transfer experiments. All these effects contribute to the increase of IKs when channels incorporate F279I Kv7.1 subunits, as shown by a computer model simulation of these data that predicts a shortening of the action potential (AP) consistent with the patient phenotype.

CONCLUSION

The F279I mutation induces a gain of function of IKs due to an impaired gating modulation of Kv7.1 induced by KCNE1, leading to a shortening of the cardiac AP.

Optogenetics-enabled dynamic modulation of action potential duration in atrial tissue: feasibility of a novel therapeutic approach

AIMS

Diseases that abbreviate the cardiac action potential (AP) by increasing the strength of repolarizing transmembrane currents are highly arrhythmogenic. It has been proposed that optogenetic tools could be used to restore normal AP duration (APD) in the heart under such disease conditions. This study aims to evaluate the efficacy of an optogenetic treatment modality for prolonging pathologically shortened APs in a detailed computational model of short QT syndrome (SQTS) in the human atria, and compare it to drug treatment.

METHODS AND RESULTS

We used a human atrial myocyte model with faster repolarization caused by SQTS; light sensitivity was inscribed via the presence of channelrhodopsin-2 (ChR2). We conducted simulations in single cells and in a magnetic resonance imaging-based model of the human left atrium (LA). Application of an appropriate optical stimulus to a diseased cell dynamically increased APD, producing an excellent match to control AP (<1.5 mV deviation); treatment of a diseased cell with an AP-prolonging drug (chloroquine) also increased APD, but the match to control AP was worse (>5 mV deviation). Under idealized conditions in the LA (uniform ChR2-expressing cell distribution, no light attenuation), optogenetics-based therapy outperformed chloroquine treatment (APD increased to 87% and 81% of control). However, when non-uniform ChR2-expressing cell distribution and light attenuation were incorporated, optogenetics-based treatment was less effective (APD only increased to 55%).

CONCLUSION

This study demonstrates proof of concept for optogenetics-based treatment of diseases that alter atrial AP shape. We identified key practical obstacles intrinsic to the optogenetic approach that must be overcome before such treatments can be realized.

New echocardiographic insights in short QT syndrome: More than a channelopathy? Heart Rhythm. 2015;12:2096-2105. [PMID: 26001507 DOI: 10.1016/j.hrthm.2015.05.024]

BACKGROUND

Short QT syndrome (SQTS) is a congenital ion channel disease characterized by an increased risk of sudden cardiac death. Little is known about the possibility that accelerated repolarization alters mechanical function in SQTS.

OBJECTIVES

The study investigated the presence of left ventricular dysfunction and mechanical dispersion, assessed by tissue Doppler imaging (TDI) and speckle tracking echocardiography (STE), and their correlation with QT interval duration and genetics.

METHODS

Fifteen SQTS patients (7 with HERG and 3 with KCNQ1 mutation) were studied. Electrocardiographic and echocardiographic parameters were compared with age- and sex-matched healthy controls.

RESULTS

When compared to the control group, SQTS patients showed reduced left ventricular contraction (global longitudinal strain: -16.0% ± 3.4% vs -22.6% ± 1.7%, P < .001; myocardial performance index 0.59 ± 0.17 vs 0.34 ± 0.08, P < .001) and a higher incidence of ejection fraction <55% (odds ratio 11, 95% confidence interval 1.045-374, P = .04). Mechanical dispersion assessed by TDI (P < .01) and STE (P < .001) was higher in the SQTS group than in controls; each parameter showed a significant inverse correlation with QT interval but not with QT dispersion.

CONCLUSION

This study showed that in SQTS systolic function may also be affected. SQTS patients presented a significant dispersion of myocardial contraction. TDI and STE could become part of the evaluation of this rare disease.

PQ segment depression in patients with short QT syndrome: a novel marker for diagnosing short QT syndrome?

BACKGROUND

Patients with short QT syndrome (SQTS) have an increased risk for atrial tachyarrhythmias, ventricular tachyarrhythmias, and/or sudden cardiac death. PQ segment depression (PQD) is related to atrial fibrillation and carries a poor prognosis in the setting of acute inferior myocardial infarction and is a well-defined electrocardiographic (ECG) marker of acute pericarditis.

OBJECTIVE

To evaluate the prevalence of PQD in SQTS and to analyze the association with atrial arrhythmias.

METHODS

Digitalized 12-lead ECGs of SQTS patients were evaluated for PQD in all leads and for QT intervals in leads II and V5. PQD was defined as ≥0.05 mV (0.5 mm) depression from the isoelectric line.

RESULTS

A total of 760 leads from 64 SQTS patients (mean age 36 ± 18 years; 48 [75%] men) were analyzed. PQD was seen in 265 (35%) leads from 52 (81%) patients and was more frequent in leads II, V3, aVF, V4, and I (n = 43 [67%], n = 30 [47%], n = 27 [42%], n = 25 [39%], and n = 25 [39%], respectively). Nine of 64 (14%) patients presented with atrial tachyarrhythmias, and all of them had PQD.

CONCLUSION

Fifty-two of 64 (81%) patients with SQTS reveal PQD. As PQD is rarely observed in healthy individuals, this ECG stigma may constitute a novel marker for SQTS in addition to a short QT interval.

Mechanisms of drug-induced proarrhythmia in clinical practice

Drug-induced proarrhythmia represents a great challenge for those involved in the development of novel pharmaceuticals and in the regulatory bodies for drug approval as well as for the prescribing clinicians. Our understanding of the mechanisms that underlie drug-induced proarrhythmia has grown dramatically over the last two decades. A growing number of cardiac and non-cardiac agents have been shown to alter cardiac repolarization predisposing to fatal cardiac arrhythmias such as ventricular tachycardia or ventricular fibrillation and sudden cardiac death. These agents may induce the phenotype of long QT syndrome and less commonly of short QT syndrome and Brugada syndrome (BS). Although, genetic susceptibility underlie drug-induced proarrhythmia in certain cases, current data are limited regarding this topic. The present review surveys the current published literature on the mechanisms and the offending medical agents that predispose to drug-induced long QT syndrome, short QT syndrome and BS. Drug-induced proarrhythmia should be considered as a predictor of sudden cardiac death and should prompt critical re-evaluation of the risks and benefits of the suspicious medication. Survivors of drug-induced proarrhythmia and family members require careful examination and possibly genetic testing for the presence of a channelopathy. Treating physicians are advised to follow the lists of agents implicated in drug-induced proarrhythmia in order to minimize the risk of arrhythmia and sudden cardiac death.