Differences in Short QT Syndrome Subtypes: A Systematic Literature Review and Pooled Analysis

Human Genetics of Cardiac Arrhythmias

Inherited forms of cardiac arrhythmias mostly are rare diseases (prevalence <1:2000) and considered to be either "primary electrical heart disorders" due to the absence of structural heart abnormalities or "cardiac ion channel disorders" due to the myocellular structures involved. Precise knowledge of the electrocardiographic features of these diseases and their genetic classification will enable early disease recognition and prevention of cardiac events including sudden cardiac death.The genetic background of these diseases is complex and heterogeneous. In addition to the predominant "private character" of a mutation in each family, locus heterogeneity involving many ion channel genes for the same familial arrhythmia syndrome is typical. Founder pathogenic variants or mutational hot spots are uncommon. Moreover, phenotypes may vary and overlap even within the same family and mutation carriers. For the majority of arrhythmias, the clinical phenotype of an ion channel mutation is restricted to cardiac tissue, and therefore, the disease is nonsyndromic.Recent and innovative methods of parallel DNA analysis (so-called next-generation sequencing, NGS) will enhance further mutation and other variant detection as well as arrhythmia gene identification.

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.

CLINICAL CASES OF LIFE-THREATENING ARRHYTHMIAS: LONG AND SHORT QT SYNDROMES

OBJECTIVE

The aim: To present clinical cases of sudden cardiac death in patients with prolonged and shortened QT interval.

PATIENTS AND METHODS

Materials and methods: The study includes description of two different clinical cases with prolonged and shortened QT interval after sudden cardiac death. Verification of the diagnosis was performed using the criteria recommended by the European Society of Cardiology (ESC) and European Heart Rhythm Association (EHRA).

RESULTS

Clinical case: Two clinical cases of syncopе with life-threatening arrhythmias, confirmed by electrocardiographic and clinical diagnostic criteria, indicating a change in the dispersion of the QT interval, are presented. The first case represents a patient with intermittent syncope. The patient had previously had attacks of sudden palpitations with fainting. The patient came after another episode of syncope. Further follow-up revealed clinical and electrocardiographic signs of ventricular tachycardia paroxysm. Than the prolongation of the QT interval is set. In this clinical case, verification of QT prolongation syndrome was established in the elderly. Another clinical case is associated with QT syndrome, which remains difficult to diagnose. Such cases have been described relatively recently. The clinical picture of the syndrome of short QT interval in the presented clinical case was characterized by the appearance of syncopal states. The patient showed changes in the adjusted QT interval <320 ms. The causes of syncope in a patient with a short QT interval were paroxysms of atrial fibrillation (AF) or ventricular arrhythmias. At the same time the anatomical structure of a myocardium remains normal and unchanged. The hereditary nature of the disease in the patient has been proven.

CONCLUSION

Conclusions: Timely diagnosis of prolongation (LQTS) or shortening (SQTS) of the QT interval after ECG and Holter monitoring allows you to identify a group of patients with an increased risk of developing ventricular arrhythmias, syncope and sudden cardiac death. Implantation of a cardioverter-defibrillator is an effective and safe method of preventing sudden cardiac death in patients with long and short QT syndromes.

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.

Transcriptomic uniqueness and commonality of the ion channels and transporters in the four heart chambers

Myocardium transcriptomes of left and right atria and ventricles from four adult male C57Bl/6j mice were profiled with Agilent microarrays to identify the differences responsible for the distinct functional roles of the four heart chambers. Female mice were not investigated owing to their transcriptome dependence on the estrous cycle phase. Out of the quantified 16,886 unigenes, 15.76% on the left side and 16.5% on the right side exhibited differential expression between the atrium and the ventricle, while 5.8% of genes were differently expressed between the two atria and only 1.2% between the two ventricles. The study revealed also chamber differences in gene expression control and coordination. We analyzed ion channels and transporters, and genes within the cardiac muscle contraction, oxidative phosphorylation, glycolysis/gluconeogenesis, calcium and adrenergic signaling pathways. Interestingly, while expression of Ank2 oscillates in phase with all 27 quantified binding partners in the left ventricle, the percentage of in-phase oscillating partners of Ank2 is 15% and 37% in the left and right atria and 74% in the right ventricle. The analysis indicated high interventricular synchrony of the ion channels expressions and the substantially lower synchrony between the two atria and between the atrium and the ventricle from the same side.

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.

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.