Unraveling Complexities in Genetically Elusive Long QT Syndrome.

Genetic testing has become standard of care for patients with long QT syndrome (LQTS), providing diagnostic, prognostic, and therapeutic information for both probands and their family members. However, up to a quarter of patients with LQTS do not have identifiable Mendelian pathogenic variants in the currently known LQTS-associated genes. This absence of genetic confirmation, intriguingly, does not lessen the severity of LQTS, with the prognosis in these gene-elusive patients with unequivocal LQTS mirroring genotype-positive patients in the limited data available. Such a conundrum instigates an exploration into the causes of corrected QT interval (QTc) prolongation in these cases, unveiling a broad spectrum of potential scenarios and mechanisms. These include multiple environmental influences on QTc prolongation, exercise-induced repolarization abnormalities, and the profound implications of the constantly evolving nature of genetic testing and variant interpretation. In addition, the rapid advances in genetics have the potential to uncover new causal genes, and polygenic risk factors may aid in the diagnosis of high-risk patients. Navigating this multifaceted landscape requires a systematic approach and expert knowledge, integrating the dynamic nature of genetics and patient-specific influences for accurate diagnosis, management, and counseling of patients. The role of a subspecialized expert cardiogenetic clinic is paramount in evaluation to navigate this complexity. Amid these intricate aspects, this review outlines potential causes of gene-elusive LQTS. It also provides an outline for the evaluation of patients with negative and inconclusive genetic test results and underscores the need for ongoing adaptation and reassessment in our understanding of LQTS, as the complexities of gene-elusive LQTS are increasingly deciphered.

International Cohort of Neonatal Timothy Syndrome.

INTRODUCTION: Timothy syndrome (TS) is an extremely rare, multisystem disorder classically associated with long QT, syndactyly, ventricular arrhythmias, and hypoglycaemia. A neonatal diagnosis allows maximal medical and device therapy to be implemented to avoid malignant arrhythmias and sudden cardiac death. METHODS: This was a retrospective case series study of type I TS (TS1) patients using data from the Timothy Syndrome Foundation's international registry, encompassing patients with a genetic diagnosis (CACNA1C variant G406R in exon 8A) recruited over a 28-year period. RESULTS: Forty-four cases of TS1 were included (26 male; 60%). Mean gestational age (GA) was 35.6 weeks (range 28 weeks - term), with 43% of patients born less than 37 weeks GA. In TS1 patients presenting with foetal bradycardia, mean GA was significantly lower (34.2 weeks, p < 0.05). Foetal bradycardia secondary to atrioventricular block was present in 20 patients (45%), resulting in premature delivery in 14 patients (32%). Fifteen patients (34%) were diagnosed with TS1 as neonates. Long QT at birth helped secure a diagnosis in 25 patients (57%). Syndactyly was seen in most patients (n = 40, 91%). Twenty patients died, with an average age of death of 2.3 years (range 1 month-6 years). Of the 7 patients who died before the first year of life (16%), the average age of death was 2.5 months. CONCLUSION: TS is associated with high early mortality. TS should be considered in paediatric patients presenting with long QT and syndactyly. Recognition of TS in the neonatal period allows for early intervention to prevent life-threatening arrhythmias.

Torsade de pointes secondary to long QT syndrome after intragastric balloon placement. A rare but severe complication.

The obesity pandemic is becoming one of the most prevalent diseases nowadays. There is a wide spectrum of treatment, ranging from hygienic-dietary measures to bariatric surgery. Endoscopic intragastric balloon placement is becoming increasingly more frequent, due to its technical simplicity, safety and short-term success(1). Although complications are rare some can be severe, so pre-endoscopic evaluation must be carried out carefully. A 43-year-old woman with a history of grade I obesity (BMI 32.7) had an Orbera® intragastric balloon implanted successfully. After the procedure she presented frequent nausea and vomiting, partially controlled with antiemetics. She attended the Emergency Department(ED) with a persistent emetic syndrome - oral intolerance and short-term loss of consciousness(syncope), for which she was admitted. Lab tests showed metabolic alkalosis with severe hypokalemia(K+ 1.8mmol/L), so fluid therapy was initiated for hydroelectrolytic replacement. During the patient's stay in the ED, two episodes of polymorphic ventricular tachychardia "Torsades de Pointes" (PVT-TDP) occurred, leading to cardiac arrest and requiring electrical cardioversion to restore sinus rhythm, in addition to a temporary pacemaker placement. Telemetry showed a corrected QT interval of >500ms, compatible with Long QT Syndrome(LQTS). Once the patient was hemodynamically stabilized a gastroscopy was performed. The intragastric balloon located in the fundus was removed using an extraction kit, puncturing and aspirating 500ml of saline solution, and extracting the collapsed balloon without any complications. The patient achieved an adequate oral intake afterwards, and no recurrence of emetic episodes were noticed. Previous ECGs revealed a prolonged QT interval and a genetic study confirmed a congenital type 1 LQTS. Treatment was initiated with beta-blockers and a bicameral automatic defibrillator was implanted in order to prevent recurrences. Intragastric balloon placement is generally a safe procedure, serious complications present in 0.70% of cases(2). It is essential to have a proper pre-endoscopic evaluation, including patient's medical history and comorbidities. Episodes of PVT-TDP may present precipitated by certain medications (eg. metoclopramide) or hydroelectrolytic imbalances (eg, hypokalemia)(3). A standardized evaluation of ECG before intragastric balloon placement may be useful to prevent these rare but serious complications.

In Vivo Base Editing of Scn5a Rescues Type 3 Long QT Syndrome in Mice.

BACKGROUND: Pathogenic variants in SCN5A can result in long QT syndrome type 3, a life-threatening genetic disease. Adenine base editors can convert targeted A T base pairs to G C base pairs, offering a promising tool to correct pathogenic variants. METHODS: We generated a long QT syndrome type 3 mouse model by introducing the T1307M pathogenic variant into the Scn5a gene. The adenine base editor was split into 2 smaller parts and delivered into the heart by adeno-associated virus serotype 9 (AAV9-ABEmax) to correct the T1307M pathogenic variant. RESULTS: Both homozygous and heterozygous T1307M mice showed significant QT prolongation. Carbachol administration induced Torsades de Pointes or ventricular tachycardia for homozygous T1307M mice (20%) but not for heterozygous or wild-type mice. A single intraperitoneal injection of AAV9-ABEmax at postnatal day 14 resulted in up to 99.20% Scn5a transcripts corrected in T1307M mice. Scn5a mRNA correction rate >60% eliminated QT prolongation; Scn5a mRNA correction rate <60% alleviated QT prolongation. Partial Scn5a correction resulted in cardiomyocytes heterogeneity, which did not induce severe arrhythmias. We did not detect off-target DNA or RNA editing events in ABEmax-treated mouse hearts. CONCLUSIONS: These findings show that in vivo AAV9-ABEmax editing can correct the variant Scn5a allele, effectively ameliorating arrhythmia phenotypes. Our results offer a proof of concept for the treatment of hereditary arrhythmias.

Torsade de pointes secondary to long QT syndrome after intragastric balloon placement. A rare but severe complication

The obesity pandemic is becoming one of the most prevalent diseases nowadays. There is a wide spectrum of treatment, ranging from hygienic-dietary measures to bariatric surgery. Endoscopic intragastric balloon placement is becoming increasingly more frequent, due to its technical simplicity, safety and short-term success(1). Although complications are rare some can be severe, so pre-endoscopic evaluation must be carried out carefully. A 43-year-old woman with a history of grade I obesity (BMI 32.7) had an Orbera® intragastric balloon implanted successfully. After the procedure she presented frequent nausea and vomiting, partially controlled with antiemetics. She attended the Emergency Department(ED) with a persistent emetic syndrome - oral intolerance and short-term loss of consciousness(syncope), for which she was admitted. Lab tests showed metabolic alkalosis with severe hypokalemia(K+ 1.8mmol/L), so fluid therapy was initiated for hydroelectrolytic replacement. During the patient’s stay in the ED, two episodes of polymorphic ventricular tachychardia “Torsades de Pointes” (PVT-TDP) occurred, leading to cardiac arrest and requiring electrical cardioversion to restore sinus rhythm, in addition to a temporary pacemaker placement. Telemetry showed a corrected QT interval of >500ms, compatible with Long QT Syndrome(LQTS). Once the patient was hemodynamically stabilized a gastroscopy was performed. The intragastric balloon located in the fundus was removed using an extraction kit, puncturing and aspirating 500ml of saline solution, and extracting the collapsed balloon without any complications. The patient achieved an adequate oral intake afterwards, and no recurrence of emetic episodes were noticed. Previous ECGs revealed a prolonged QT interval and a genetic study confirmed a congenital type 1 LQTS. Treatment was initiated with beta-blockers and a bicameral automatic defibrillator was implanted in order to prevent recurrences. ... (AU)

Proof of concept for monoclonal antibody therapy in a cellular model of acquired long QT syndrome type 3.

Long QT syndrome (LQTS) type 3 although less common than the first two forms, differs in that arrhythmic events are less likely triggered by adrenergic stimuli and are more often lethal. Effective pharmacological treatment is challenged by interindividual differences, mutation dependence, and adverse effects, translating into an increased use of invasive measures (implantable cardioverter-defibrillator, sympathetic denervation) in patients with LQTS type 3. Previous studies have demonstrated the therapeutic potential of polyclonal KCNQ1 antibody for LQTS type 2. Here, we sought to identify a monoclonal KCNQ1 antibody that preserves the electrophysiological properties of the polyclonal form. Using hybridoma technology, murine monoclonal antibodies were generated, and patch clamp studies were performed for functional characterization. We identified a monoclonal KCNQ1 antibody able to normalize cardiac action potential duration and to suppress arrhythmias in a pharmacological model of LQTS type 3 using human-induced pluripotent stem cell-derived cardiomyocytes.NEW & NOTEWORTHY Long QT syndrome is a leading cause of sudden cardiac death in the young. Recent research has highlighted KCNQ1 antibody therapy as a new treatment modality for long QT syndrome type 2. Here, we developed a monoclonal KCNQ1 antibody that similarly restores cardiac repolarization. Moreover, the identified monoclonal KCNQ1 antibody suppresses arrhythmias in a cellular model of long QT syndrome type 3, holding promise as a first-in-class antiarrhythmic immunotherapy.

The 2023 Canadian Cardiovascular Society Clinical Practice Update on Management of the Patient With a Prolonged QT Interval.

A prolonged QT interval on the electrocardiogram is associated with an increased risk of the torsades de pointes form of ventricular arrhythmia resulting in syncope, sudden cardiac arrest or death, or misdiagnosis as a seizure disorder. The cause of QT prolongation can be congenital and inherited as an autosomal dominant variant, or it can be transient and acquired, often because of QT-prolonging drugs or electrolyte abnormalities. Automated measurement of the QT interval can be inaccurate, especially when the baseline electrocardiogram is abnormal, and manual verification is recommended. In this clinical practice update we provide practical tips about measurement of the QT interval, diagnosis, and management of congenital long QT syndrome and acquired prolongation of the QT interval. For congenital long QT syndrome, certain ß-adrenergic-blocking drugs are highly effective, and implantable defibrillators are infrequently required. Many commonly prescribed drugs such as antidepressants and antibiotics can prolong the QT interval, and recommendations are provided on their safe use.

Risk stratification of sudden cardiac death: a review.

Sudden cardiac death (SCD) is responsible for several millions of deaths every year and remains a major health problem. To reduce this burden, diagnosing and identification of high-risk individuals and disease-specific risk stratification are essential. Treatment strategies include treatment of the underlying disease with lifestyle advice and drugs and decisions to implant a primary prevention implantable cardioverter-defibrillator (ICD) and perform ablation of the ventricles and novel treatment modalities such as left cardiac sympathetic denervation in rare specific primary electric diseases such as long QT syndrome and catecholaminergic polymorphic ventricular tachycardia. This review summarizes the current knowledge on SCD risk according to underlying heart disease and discusses the future of SCD prevention.

Personalized Care in Long QT Syndrome: Better Management, More Sports, and Fewer Devices.

Long QT Syndrome (LQTS) is a potentially life-threatening yet highly treatable inherited cardiac channelopathy. When evaluating these patients, it is important to consider patient-specific as well as genotype-specific factors in order to adequately encompass the many nuances to care that exist in its management. The tendency to follow a "one-size-fits-all" approach needs to be replaced by treatment strategies that embrace the unique considerations of the individual patient in the context of their genotype. Herein, the authors aim to review the spectrum of LQTS, including the considerations when tailoring a personalized, genotype-tailored treatment program for a patient's LQTS.

Secular trends of health care resource utilization and costs between Brugada syndrome and congenital long QT syndrome: A territory-wide study.

BACKGROUND: Health care resource utilization (HCRU) and costs are important metrics of health care burden, but they have rarely been explored in the setting of cardiac ion channelopathies. HYPOTHESIS: This study tested the hypothesis that attendance-related HCRUs and costs differed between patients with Brugada syndrome (BrS) and congenital long QT syndrome (LQTS). METHODS: This was a retrospective cohort study of consecutive BrS and LQTS patients at public hospitals or clinics in Hong Kong, China. HCRUs and costs (in USD) for Accident and Emergency (A&E), inpatient, general outpatient and specialist outpatient attendances were analyzed between 2001 and 2019 at the cohort level. Comparisons were made using incidence rate ratios (IRRs [95% confidence intervals]). RESULTS: Over the 19-year period, 516 BrS (median age of initial presentation: 51 [interquartile range: 38-61] years, 92% male) and 134 LQTS (median age of initial presentation: 21 [9-44] years, 32% male) patients were included. Compared to LQTS patients, BrS patients had lower total costs (2 008 126 [2 007 622-2 008 629] vs. 2 343 864 [2 342 828-2 344 900]; IRR: 0.857 [0.855-0.858]), higher costs for A&E attendances (83 113 [83 048-83 177] vs. 70 604 [70 487-70 721]; IRR: 1.177 [1.165-1.189]) and general outpatient services (2,176 [2,166-2,187] vs. 921 [908-935]; IRR: 2.363 [2.187-2.552]), but lower costs for inpatient stay (1 391 624 [1 391 359-1 391 889] vs. 1 713 742 [1 713 166-1 714 319]; IRR: 0.812 [0.810-0.814]) and lower costs for specialist outpatient services (531 213 [531 049-531 376] vs. 558 597 [558268-558926]; IRR: 0.951 [0.947-0.9550]). CONCLUSIONS: Overall, BrS patients consume 14% less health care resources compared to LQTS patients in terms of attendance costs. BrS patients require more A&E and general outpatient services, but less inpatient and specialist outpatient services than LQTS patients.

Precision Medicine and Cardiac Channelopathies: Human iPSCs Take the Lead.

Sudden cardiac death (SCD) is one of the leading causes of death worldwide, usually involving young people. SCD remains a critical public health problem accounting for 185,000-450,000 deaths annually, representing around 7%-18% of all deaths globally. As per evidence, ∼2%-54% of sudden unexpected deaths in people under the age of 35 years fail to show evidence of structural cardiac abnormalities at autopsy, making ion channelopathies the probable causes in such cases. The most generally recognized cardiac ion channelopathies with genetic testing are long QT syndrome (LQTS), Brugada syndrome (BrS), short QT syndrome (SQTS), and catecholaminergic polymorphic ventricular tachycardia (CPVT). The substantial progress in understanding the genetics of ion channelopathies in the last 2 decades has obliged the early diagnosis and prevention of SCD to a certain extent. In this review, we analyze the critical challenges and recent advancements in the identification, risk stratification, and clinical management of potentially fatal cardiac ion channel disorders. We also emphasize the application of precision medicine (PM) and artificial intelligence (AI) for comprehending the underlying genetic mechanisms, especially the role of human induced pluripotent stem cell (iPSC) based platforms to unravel the primary refractory clinical problems associated with channelopathies.

hERG stereoselective modulation by mexiletine-derived ureas: Molecular docking study, synthesis, and biological evaluation.

Long QT syndrome (LQTS) is a disorder of cardiac electrophysiology resulting in life-threatening arrhythmias; nowadays, only a few drugs are available for the management of LQTS. Focusing our attention on LQT2, one of the most common subtypes of LQTS caused by mutations in the human ether-à-go-go-related gene (hERG), in the present work, the stereoselectivity of the recently discovered mexiletine-derived urea 8 was investigated on the hERG potassium channel. According to preliminary in silico predictions, in vitro studies revealed a stereoselective behavior, with the meso form showing the greatest hERG opening activity. In addition, functional studies on guinea pig isolated left atria, aorta, and ileum demonstrated that 8 does not present any cardiac or intestinal liability in our ex vivo studies. Due to its overall profile, (R,S)-8 paves the way for the design and development of a new series of compounds potentially useful in the treatment of both congenital and drug-induced forms of LQTS.

Same Gene, Different Story (a Case Report of Congenital Long QT Syndrome Subtype 8 With a Novel Mutation).

Long QT syndrome (LQTS) 8 is a rare inherited channelopathy caused by CACNA1C gene mutations that affects calcium channels, and when combined with congenital heart defects, musculoskeletal defects, and neurodevelopmental defects, it is referred to as Timothy syndrome. A female patient, aged 17 years, presented with a witnessed episode of syncope secondary to ventricular fibrillation that was successfully cardioverted. Electrocardiogram showed sinus bradycardia 52/min, normal axis, and a QTc of 626 ms. In the hospital, she had another episode of asystole and Torsade de pointes and underwent successful cardiopulmonary resuscitation. Echocardiogram showed severely reduced left ventricular systolic function from postcardiac arrest myocardial dysfunction and no congenital heart defects. Long QT genetic test detected a missense mutation in the CACNA1C gene (NM_199460.3, variant c.2573G>A, p Arg858His, heterozygous, autosomal dominant), resulting in replacement of arginine with histidine at position 858(R858H), leading to the gain of function in the L-type calcium channel. Given the absence of congenital cardiac defects, musculoskeletal deformities, or neurodevelopmental delay a final diagnosis of LQTS subtype 8 was made. A cardioverter defibrillator was implanted. In conclusion, our case highlights the importance of genetic testing in the diagnosis of LQTS. Some CACNA1C mutations, such as R858H described here, cause LQTS without the extracardiac manifestations observed in classic Timothy syndrome and should be included in the genetic testing for LQTS. To the best of our knowledge, our case is the first one from United States with the R585H mutation. Three cases with similar mutations have been reported from Japan and one from New Zealand.

Current updates on arrhythmia within Timothy syndrome: genetics, mechanisms and therapeutics.

Timothy syndrome (TS), characterised by multiple system malfunction especially the prolonged corrected QT interval and synchronised appearance of hand/foot syndactyly, is an extremely rare disease affecting early life with devastating arrhythmia. In this work, firstly, the various mutations in causative gene CACNA1C encoding cardiac L-type voltage-gated calcium channel (LTCC), regard with the genetic pathogeny and nomenclature of TS are reviewed. Secondly, the expression profile and function of CACNA1C gene encoding Cav1.2 proteins, and its gain-of-function mutation in TS leading to multiple organ disease phenotypes especially arrhythmia are discussed. More importantly, we focus on the altered molecular mechanism underlying arrhythmia in TS, and discuss about how LTCC malfunction in TS can cause disorganised calcium handling with excessive intracellular calcium and its triggered dysregulated excitation-transcription coupling. In addition, current therapeutics for TS cardiac phenotypes including LTCC blockers, beta-adrenergic blocking agents, sodium channel blocker, multichannel inhibitors and pacemakers are summarised. Eventually, the research strategy using patient-specific induced pluripotent stem cells is recommended as one of the promising future directions for developing therapeutic approaches. This review updates our understanding on the research progress and future avenues to study the genetics and molecular mechanism underlying the pathogenesis of devastating arrhythmia within TS, and provides novel insights for developing therapeutic measures.

Deciphering HERG mutation in long QT syndrome type 2 using antisense oligonucleotide-mediated techniques: Lessons from cystic fibrosis.

Long QT syndrome type 2 (LQT2) is a genetic disorder caused by mutations in the KCNH2 gene, also known as the human ether-a-go-go-related gene (HERG). More than 30% of HERG mutations result in a premature termination codon that triggers a process called nonsense-mediated messenger RNA (mRNA) decay (NMD), where the mRNA transcript is degraded. NMD is a quality control mechanism that removes faulty mRNA to prevent the translation of truncated proteins. Recent advances in antisense oligonucleotide (ASO) technology in the field of cystic fibrosis (CF) have yielded significant progress, including the ASO-mediated comprehensive characterization of key NMD factors and exon-skipping therapy. These advances have contributed to our understanding of the role of premature termination codon-containing mutations in disease phenotypes and have also led to the development of potentially useful therapeutic strategies. Historically, studies of CF have provided valuable insights for the research on LQT2, particularly concerning increasing the expression of HERG. In this article, we outline the current state of knowledge regarding ASO, NMD, and HERG and discuss the introduction of ASO technology in the CF to elucidate the pathogenic mechanisms through targeting NMD. We also discuss the potential clinical therapeutic benefits and limitations of ASO for the management of LQT2. By drawing on lessons learned from CF research, we explore the potential translational values of these advances into LQT2 studies.

Implantable cardioverter-defibrillator in patients with inherited arrhythmia syndromes: A systematic review.

BACKGROUND: The potential benefit of implantable cardioverter-defibrillator (ICD) therapy in individuals with inherited arrhythmia syndromes is well known. However, it is not deprived of morbidity, in the form of inappropriate therapies and other ICD-related complications. OBJECTIVE: The aim of this systematic review is to estimate the rate of appropriate and inappropriate therapy, as well as other ICD-related complications, in individuals with inherited arrhythmia syndromes. METHODS: A systematic review was performed, regarding appropriate and inappropriate therapy, and other ICD-related complications, in individuals with inherited arrhythmia syndromes (Brugada Syndrome, Catecholaminergic Polymorphic Ventricular Tachycardia, Early Repolarization Syndrome, Long QT Syndrome and Short QT syndrome). Studies were identified by searching published papers in PubMed and Embase up to August 23rd, 2022. RESULTS: From data gathered of 36 studies, with a total of 2750 individuals, during a mean follow-up time of 69 months, appropriate therapies occurred in 21% of the individuals and inappropriate therapies in 20% of the individuals. Concerning the other ICD-related complications, 456 complications were observed, amongst 2084 individuals (22%), with the most frequent being lead malfunction (46%), followed by infectious complications (13%). CONCLUSIONS: ICD-related complications are not uncommon, especially when one considers the exposure time of young individuals. The incidence of inappropriate therapies was 20%, although lower rates were reported in recent publications. S-ICD is an effective alternative to transvenous ICD for sudden death prevention. The decision to implant an ICD should be individualized, taking into account the risk profile of each patient, as well as the possibility of complications.

A rare case with fetal autoimmune heart block and KNCH2 variant-induced long QT syndrome: a controversial opinion on prenatal management strategy.

BACKGROUND: Among all fetal heart block patients, > 50% cases are associated with maternal autoimmune diseases, and such patients should receive treatment. However, nearly half of fetal heart block cases involve a mother with negative results following autoimmune antibody screening. A few studies have reported long QT syndrome (LQTS) can also present as a severe fetal bradycardia, which does not respond to fetal treatment. Herein, we reported a rare case of an infant who presented with high-degree autoimmune-mediated fetal atrioventricular block (AVB) with LQTS induced by a novel KCNH2 variant. This case led us to review our prenatal therapeutic strategy. CASE PRESENTATION: A 1-year-old boy presented to our heart center having experienced syncope 5 times in the past year. He had previously presented with fetal bradycardia during the fetal stage from 27 + 3 gestational weeks. The fetal echocardiography demonstrated AVB (2:1 transmission). As the maternal autoimmune antibody results were positive, his mother had received dexamethasone treatment during pregnancy; subsequently, the fetal AVB had changed from 2:1 to 4:3 transmission with elevated ventricular beating rates. However, this patient was identified to have complete AVB after birth. The initial electrocardiogram and Holter measurements at hospital administration showed complete AVB, pleomorphic ventricular tachycardia, a prolonged QT interval (QT = 602 ms, corrected QT = 538 ms), and wide and deep inverted T-waves. Meanwhile, torsades de pointes could be observed in several transit ventricular tachycardias based on Holter monitoring review. Genetic testing revealed KCNH2 c.2483G > A variant-induced LQTS. An implantable cardioverter defibrillator device and permanent pacemaker were both considered as therapeutic alternations; his parents ultimately accepted the implantation of a permanent pacemaker. CONCLUSIONS: For fetuses with autoimmune-mediated AVB, intrauterine treatment should still be pursued immediately. However, once the treatment outcomes are deemed unacceptable or unexpected, other genetic variant-related channelopathies should be highly suspected. If the fetus lacks a positive family history, fetal genetic testing should be recommended to improve the prognosis of such patients by introducing integrative therapeutic strategies between the prenatal and postnatal phases.