Ventricular fibrillation with prominent early repolarization associated with a rare variant of KCNJ8/KATP channel

Cardiac Channelopathies: Clinical Diagnosis and Promising Therapeutics

Cardiac channelopathies, also known as primary electrical heart diseases, are inherited genetic abnormalities of cardiomyocyte electrical behavior. Notable for their absence of structural heart diseases, they include a diverse group of diseases such as long QT syndrome, short QT syndrome, Brugada syndrome, early repolarization syndrome, catecholaminergic polymorphic ventricular tachycardia, and idiopathic ventricular fibrillation, and carry the risk of malignant arrhythmias leading to sudden cardiac death. The genetic and molecular foundations of these diseases are diverse and complex, with evolving research highlighting the multifactorial nature of their pathophysiology and the intricate interplay of various genes in the manifestation of arrhythmias. While advances in diagnostic techniques, such as genetic testing and electrophysiological studies, have improved the identification and management of these conditions, the relationship between specific genetic mutations and sudden cardiac death remains incompletely understood. This review provides an overview of the molecular and genetic mechanisms underlying those inherited arrhythmias, exploring both well-established and emerging data. Additionally, it discusses current diagnostic approaches and management strategies, aiming to enhance the understanding of these conditions and contribute to better sudden cardiac death prevention.

Early Repolarization Augmentation Mimicking Pseudo-Infarction in a Patient With Diabetic Ketoacidosis and Normokalemia

Early repolarization (ER) changes, characterized by J point elevation with or without ST-segment elevation, are dynamic in their presentation and can be exacerbated by factors such as hypothermia, hypercalcemia, vagotonia, and certain medications. There is limited research regarding the mechanism of these changes and the dynamic changes of ER secondary to diabetic ketoacidosis (DKA). This case report highlights the augmentation of early repolarization changes resembling ST-segment elevation myocardial infarction (STEMI) in a patient with DKA that resolved with the treatment of acidosis. The misinterpretation of ER changes on electrocardiogram (ECG) as STEMI or pericarditis may result in the inappropriate utilization of resources, increased patient risk, and elevated morbidity and mortality. Recognition of the potential of DKA to cause ER changes can potentially avoid these unfavorable outcomes.

Mechanisms underlying the antiarrhythmic effect of ARumenamide-787 in experimental models of the J wave syndromes and hypothermia

BACKGROUND

Brugada (BrS) and early repolarization syndromes (ERS), the so-called J wave syndromes (JWS), are associated with life-threatening ventricular arrhythmias. Pharmacologic approaches to therapy are currently limited. In this study, we examine the effects of ARumenamide-787 (AR-787) to suppress the electrocardiographic and arrhythmic manifestations of JWS and hypothermia.

METHODS

We studied the effects of AR-787 on INa and IKr in HEK-293 cells stably expressing the α- and β1-subunits of the cardiac (NaV1.5) sodium channel and hERG channel, respectively. In addition, we studied its effect on Ito, INa and ICa in dissociated canine ventricular myocytes along with action potentials and ECG from coronary-perfused right (RV) and left (LV) ventricular wedge preparations. The Ito agonist, NS5806 (5-10 μM), ICa blocker, verapamil (2.5 μM), and INa blocker, ajmaline (2.5 μM), were used to mimic the genetic defects associated with JWS and to induce the electrocardiographic and arrhythmic manifestations of JWS (prominent J waves/ST segment elevation, phase 2 reentry and polymorphic VT/VF) in canine ventricular wedge preparations.

RESULTS

AR-787 (1, 10 and 50 μM) exerted pleiotropic effects on cardiac ion channels. The predominant effect was inhibition of the transient outward current (Ito) and enhancement of the sodium channel current (INa), with lesser effects to inhibit IKr and augment calcium channel current (ICa). AR-787 diminished the electrocardiographic J wave and prevented and/or suppressed all arrhythmic activity in canine RV and LV experimental models of BrS, ERS and hypothermia.

CONCLUSIONS

Our findings point to AR-787 as promising candidate for the pharmacologic treatment of JWS and hypothermia.

Chinese herbal medicine for the treatment of cardiovascular diseases ─ targeting cardiac ion channels

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality, imposing an increasing global health burden. Cardiac ion channels (voltage-gated Na_V, Ca_V, K_Vs, and others) synergistically shape the cardiac action potential (AP) and control the heartbeat. Dysfunction of these channels, due to genetic mutations, transcriptional or post-translational modifications, may disturb the AP and lead to arrhythmia, a major risk for CVD patients. Although there are five classes of anti-arrhythmic drugs available, they can have varying levels of efficacies and side effects on patients, possibly due to the complex pathogenesis of arrhythmias. As an alternative treatment option, Chinese herbal remedies have shown promise in regulating cardiac ion channels and providing anti-arrhythmic effects. In this review, we first discuss the role of cardiac ion channels in maintaining normal heart function and the pathogenesis of CVD, then summarize the classification of Chinese herbal compounds, and elaborate detailed mechanisms of their efficacy in regulating cardiac ion channels and in alleviating arrhythmia and CVD. We also address current limitations and opportunities for developing new anti-CVD drugs based on Chinese herbal medicines.

Sudden Cardiac Death Risk Stratification of the Early Repolarization Syndrome: An Updated Review of the Literature

PURPOSE OF REVIEW

Early repolarization syndrome (ERS), once thought to be innocuous, has now been shown to be responsible for ventricular arrythmias and sudden cardiac death occurrence. This review will help the clinician to identify the patients who are most at risk for arrhythmias, so that they can adopt a preventive or secondary treatment approach, either of which is still poorly defined.

RECENT FINDINGS

Patients at high risk are found to be particularly young men, with a personal or family history of syncope or sudden death, who have an inferolateral ER on the electrocardiogram, more elevated J-point, longer J-wave duration and wider J angle, a horizontal or descending ST segment, lower T/R ratio, and small and/or inverted waves. The association with a structural heart disease or another channelopathy potentiates this arrhythmic risk. Taken together, these parameters allow prediction of the malignancy of ER pattern with a certain reliability. Further research is however needed to develop concrete risk stratification algorithms and the therapeutic strategies taken in function of it.

Personalized Therapeutics for KATP-Dependent Pathologies

Ubiquitously expressed throughout the body, ATP-sensitive potassium (KATP) channels couple cellular metabolism to electrical activity in multiple tissues; their unique assembly as four Kir6 pore-forming subunits and four sulfonylurea receptor (SUR) subunits has resulted in a large armory of selective channel opener and inhibitor drugs. The spectrum of monogenic pathologies that result from gain- or loss-of-function mutations in these channels, and the potential for therapeutic correction of these pathologies, is now clear. However, while available drugs can be effective treatments for specific pathologies, cross-reactivity with the other Kir6 or SUR subfamily members can result in drug-induced versions of each pathology and may limit therapeutic usefulness. This review discusses the background to KATP channel physiology, pathology, and pharmacology and considers the potential for more specific or effective therapeutic agents.

J wave syndromes: What's new?

Among the inherited ion channelopathies associated with potentially life-threatening ventricular arrhythmia syndromes in nominally structurally normal hearts are the J wave syndromes, which include the Brugada (BrS) and early repolarization (ERS) syndromes. These ion channelopathies are responsible for sudden cardiac death (SCD), most often in young adults in the third and fourth decade of life. Our principal goal in this review is to briefly outline the clinical characteristics, as well as the molecular, ionic, cellular, and genetic mechanisms underlying these primary electrical diseases that have challenged the cardiology community over the past two decades. In addition, we discuss our recently developed whole-heart experimental model of BrS, providing compelling evidence in support of the repolarization hypothesis for the BrS phenotype as well as novel findings demonstrating that voltage-gated sodium and transient outward current channels can modulate each other's function via trafficking and gating mechanisms with implications for improved understanding of the genetics of both cardiac and neuronal syndromes.

Why Is There an Increased Risk for Sudden Cardiac Death in Patients With Early Repolarization Syndrome?

The last two decades have changed the viewpoint on early repolarization syndrome (ERS). The prevalence of the early repolarization pattern is variable and ranges between 3-24% depending upon age, gender, and criteria used for J-point upliftment from baseline (0.05mV vs. 1 mV). While this pattern was previously linked with a benign result, multiple recent investigations have found a link between early repolarization and Sudden Cardiac Death (SCD) by causing life-threatening arrhythmias like Ventricular tachycardia/Ventricular fibrillation, a condition known as early repolarization syndrome. The syndrome falls under a broader bracket of J wave syndromes, which can be caused by early repolarization or depolarization abnormalities. The characteristics of early repolarization that are considered high risk for Sudden Cardiac Death include the amplitude of J-point upliftment from baseline ( > 0.2 mV), Inferior-lateral location of Early Repolarization pattern, and horizontal and downsloping ST-segment. Patients with symptomatic early repolarisation patterns on ECG are more likely to have repeated cardiac episodes. Implantable Cardioverter-Defibrillator (ICD) implantation and isoproterenol are the recommended treatments in symptomatic patients. On the other hand, asymptomatic patients with early repolarization patterns are prevalent and have a better outcome. Risk categorization is still obscure in asymptomatic early repolarization patterns. This traditional review outlines the known knowledge of pathophysiology behind the increased risk of sudden cardiac death, risk stratification of patients with ERS, and the treatment guidelines for patients with ERS. Further prospective studies are recommended to elucidate the exact mechanism for ventricular arrhythmogenesis in ERS patients and to risk stratifying asymptomatic patients with ERS.

Case Report: Loss-of-Function ABCC9 Genetic Variant Associated With Ventricular Fibrillation

Genetic variants in the ABCC9 gene, encoding the SUR2 auxiliary subunit from KATP channels, were previously linked with various inherited diseases. This wide range of congenital disorders includes multisystem and cardiovascular pathologies. The gain-of-function mutations result in Cantu syndrome, acromegaloid facial appearance, hypertrichosis, and acromegaloid facial features. The loss-of-function mutations in the ABCC9 gene were associated with the Brugada syndrome, early repolarization syndrome, and dilated cardiomyopathy. Here, we reported a patient with a loss-of-function variant in the ABCC9 gene, identified by target high-throughput sequencing. The female proband presented with several episodes of ventricular fibrillation and hypokalemia upon emotional stress. This case sheds light on the consequences of KATP channel dysfunction in the cardiovascular system and underlines the complexity of the clinical presentation of ABCC9-related diseases.

Early Repolarization Syndrome, Epilepsy and Atrial Fibrillation in a young girl with novel KCND3 mutation managed with quinidine

A 6-year-old girl presented with a difficult to control epilepsy syndrome. On evaluation, additional presyncope episodes associated with polymorphic ventricular tachycardia were also noted. A diagnosis of early repolarization syndrome was made with early repolarization pattern on ECG, documented VT episodes and clinical presyncope (Proposed Shanghai score 7). Paroxysmal atrial fibrillation was also noted on 24-hour Holter recordings. The child was stabilized with isoprenaline infusion and was later discharged with arrhythmia control on quinidine and cilostazol. Genetic evaluation revealed a potassium channel KCND3 gene missense mutation. The case highlights the association of epilepsy syndrome and atrial fibrillation with early repolarization syndrome; possible association of KCND3 gene mutation with a malignant phenotype; and management issues in a small child. This article is protected by copyright. All rights reserved.

A descriptive report on short QT interval in Kherameh branch of the PERSIAN cohort study

Short QT-interval is a condition that bear the suspicion of short QT syndrome (SQTS). SQTS is known to increase risk of life-threatening arrythmias and sudden cardiac death (SCD). Due to the insufficient population-based studies and use of various QT cut-off values, it accounts for as an undiagnosed condition. In this study, we sought for prevalence of short QT interval in Kherameh cohort study, one of the southern branches of the Prospective Epidemiological Research Studies in Iran (PERSIAN). Data of 4363 adult subjects were analyzed from phase 1 of the cohort during 2014–2017. The corrected QT (QTc) intervals were calculated and electrocardiograms (ECGs) with QTc of less than 370 ms (msec) were reanalyzed for bradycardia, early repolarization, atrial fibrillation (AF), arrhythmias, and other electrical conduction abnormalities. Seventy-two subjects (1.65%) had a QTc of less than 370 ms (mean QTc of 360.72 ± 11.72). A male predominance and a lower mean heart rate observed in SQTS susceptible group (M/F of 1/0.26 vs. 1/1.145, p-value < 0.0001; 58.389 ± 9.787 vs. 70.899 ± 11.775; p-value < 0.0001) compare to the subjects with normal QTc. At least, 2 subjects with high-probability SQTS and 3 with intermediate-probability SQTS identified. The frequency of AF, syncope, bradycardia, early repolarization, low voltage ECG, and infantile SCD in first- and second-degree relatives were 16.67, 4.17, 33.33, 11.11, 11.11, 11.11%, respectively. The prevalence of short QT interval in our cohort was in line with previous studies. The incidence of cardiac symptoms/events, familial SCDs and ECG derived specific findings were high amongst SQTS-susceptible index persons. However, these variables could not predict the symptomatic subjects, which emphasizes gene studies and family screening.

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.

Two siblings with early repolarization syndrome: clinical and genetic characterization by whole-exome sequencing

Aims 

The early repolarization syndrome (ERS) can cause ventricular fibrillation (VF) and sudden death in young, otherwise healthy individuals. There are limited data suggesting that ERS might be heritable. The aim of this study was to characterize the clinical phenotype and to identify a causal variant in an affected family using an exome-sequencing approach.

Methods and results 

Early repolarization syndrome was diagnosed according to the recently proposed Shanghai ERS Score. After sequencing of known ERS candidate genes, whole-exome sequencing (WES) was performed. The index patient (23 years, female) showed a dynamic inferolateral early repolarization (ER) pattern and electrical storm with intractable VF. Isoproterenol enabled successful termination of electrical storm with no recurrence on hydroquinidine therapy during 33 months of follow-up. The index patient’s brother (25 years) had a persistent inferior ER pattern with malignant features and a history of syncope. Both parents were asymptomatic and showed no ER pattern. While there was no pathogenic variant in candidate genes, WES detected a novel missense variant affecting a highly conserved residue (p. H2245R) in the ANK3 gene encoding Ankyrin-G in the two siblings and the father.

Conclusion 

We identified two siblings with a malignant ERS phenotype sharing a novel ANK3 variant. A potentially pathogenic role of the novel ANK3 variant is suggested by the direct interaction of Ankyrin-G with the cardiac sodium channel, however, more patients with ANK3 variants and ERS would be required to establish ANK3 as novel ERS susceptibility gene. Our study provides additional evidence that ERS might be a heritable condition.

The Pharmacology of ATP-Sensitive K+ Channels (KATP)

ATP-sensitive K⁺ channels (K_ATP) are inwardly-rectifying potassium channels, broadly expressed throughout the body. K_ATP is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels thus playing an important physiological role by coupling cellular metabolism with membrane excitability. The hetero-octameric channel complex is formed of 4 pore-forming inward rectifier Kir6.x subunits (Kir6.1 or Kir6.2) and 4 regulatory sulfonylurea receptor subunits (SUR1, SUR2A, or SUR2B). These subunits can associate in various tissue-specific combinations to form functional K_ATP channels with distinct electrophysiological and pharmacological properties. K_ATP channels play many important physiological roles and mutations in channel subunits can result in diseases such as disorders of insulin handling, cardiac arrhythmia, cardiomyopathy, and neurological abnormalities. The tissue-specific expression of K_ATP channel subunits coupled with their rich and diverse pharmacology makes K_ATP channels attractive therapeutic targets in the treatment of endocrine and cardiovascular diseases.

Clinical and Functional Genetic Characterization of the Role of Cardiac Calcium Channel Variants in the Early Repolarization Syndrome

Background: Early repolarization syndrome (ERS) is an inherited sudden cardiac death (SCD) syndrome. The present study investigates the role of genetic variants in cardiac calcium-channel genes in the pathogenesis of ERS and probes the underlying mechanisms. Methods: Polymerase chain reaction-based next-generation sequencing was carried out using a targeted gene approach. Unrelated ERS probands carrying calcium-channel variants were evaluated clinically and compared with matched healthy controls. Wild-type (WT) and mutant CACNA1C genes were coexpressed with CACNB2b and CACNA2D1 in HEK293 cells and studied using whole-cell patch-clamp techniques and confocal fluorescence microscope. Results: Among 104 ERS probands, 16 carried pathogenic variants in calcium-channel genes (32.2 ± 14.6 years old, 87.5% male). The symptoms at diagnosis included syncope (56.3%), ventricular tachycardia/fibrillation (62.5%), and SCD (56.3%). Three cases (18.8%) had a family history of SCD or syncope. Eight patients (50.0%) had a single calcium gene rare variant. The other half carried rare variants in other ERS-susceptible genes. Compared with controls, the heart rate was slower (72.7 ± 8.9 vs. 65.6 ± 16.1 beats/min, * p < 0.05), QTc interval was shorter (408.2 ± 21.4 vs. 386.8 ± 16.9 ms, ** p < 0.01), and Tp-e/QT was longer (0.22 ± 0.05 vs. 0.28 ± 0.04, *** p < 0.001) in single calcium mutation carriers. Electrophysiological analysis of one mutation, CACNA1C-P817S (c.2449C>T), revealed that the density of whole-cell calcium current (I Ca) was reduced by ~84.61% compared to WT (-3.17 ± 2.53 vs. -20.59 ± 3.60 pA/pF, n = 11 and 15, respectively, ** p < 0.01). Heterozygous expression of mutant channels was associated with a 51.35% reduction of I Ca. Steady-state inactivation was shifted to more negative potentials and significantly accelerated as well. Confocal microscopy revealed trafficking impairment of CACNA1C-P817S (peripheral/central intensity: 0.94 ± 0.10 in WT vs. 0.33 ± 0.12 in P817S, n = 10 and 9, respectively, ** p < 0.01). Conclusions: ERS associated with loss-of-function (LOF) genetic defects in genes encoding the cardiac calcium channel represents a unique clinical entity characterized by decreased heart rate and QTc, as well as increased transmural dispersion of repolarization. In the case of CACNA1C-P817S, impaired trafficking of the channel to the membrane contributes to the LOF.

Inherited arrhythmia syndrome predisposing to sudden cardiac death

Inherited arrhythmia (IA) is one of the main causes of sudden cardiac death (SCD) in young people, and is reported to be a more prevalent cause of SCD in Asia than in Western countries. IAs are a group of genetic disorders caused by mutations in genes encoding cardiac ion channels, leading to electrophysiological characteristics that often occur in the absence of structural abnormalities. Channelopathies, such as long QT syndrome and Brugada syndrome, carry a potential risk of life-threatening ventricular tachyarrhythmias that predispose to SCD, although early prediction and prevention of the risk remain challenging. Recent advances in genetic testing have facilitated risk stratification as well as a precise diagnosis for IA, despite ongoing debates about the implications. Herein, we provide epidemiological data, a pathophysiological overview, and the current clinical approach to IAs related to SCD. In addition, we review the general issues arising from genetic testing for IAs.

Targeting of Potassium Channels in Cardiac Arrhythmias

Cardiomyocytes are endowed with a complex repertoire of ion channels, responsible for the generation of action potentials (APs), travelling waves of electrical excitation, propagating throughout the heart and leading to cardiac contractions. Cardiac AP waveforms are shaped by a striking diversity of K⁺ channels. The pivotal role of K⁺ channels in cardiac health and disease is underscored by the dramatic impact that K⁺ channel dysfunction has on cardiac arrhythmias. The development of drugs targeted to specific K⁺ channels is expected to provide an optimized approach to antiarrhythmic therapy. Here, we review the functional roles of cardiac potassium channels under normal and diseased states. We survey current antiarrhythmic drugs (AADs) targeted to voltage-gated and Ca²⁺-activated K⁺ channels and highlight future research opportunities.

Omega-3 polyunsaturated fatty acids and hypertension: a review of vasodilatory mechanisms of docosahexaenoic acid and eicosapentaenoic acid

Hypertension is often characterised by impaired vasodilation involving dysfunction of multiple vasodilatory mechanisms. ω-3 polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) can reduce blood pressure and vasodilation. In the endothelium, DHA and EPA improve function including increased NO bioavailability. However, animal studies show that DHA- and EPA-mediated vasodilation persists after endothelial removal, indicating a role for vascular smooth muscle cells (VSMCs). The vasodilatory effects of ω-3 PUFAs on VSMCs are mediated via opening of large conductance calcium-activated potassium channels (BK_Ca ), ATP-sensitive potassium channels (K_ATP ) and possibly members of the K_v 7 family of voltage-activated potassium channels, resulting in hyperpolarisation and relaxation. ω-3 PUFA actions on BK_Ca and voltage-gated ion channels involve electrostatic interactions that are dependent on the polyunsaturated acyl tail, cis-geometry of these double bonds and negative charge of the carboxyl headgroup. This suggests structural manipulation of ω-3 PUFA could generate novel, targeted, therapeutic leads.

Physiological function and molecular composition of ATP-sensitive K+ channels in human gastric smooth muscle

Gastric motility is controlled by slow waves. In general, the activation of the ATP-sensitive K⁺ (K_ATP) channels in the smooth muscle opposes the membrane excitability and produces relaxation. Since metabolic inhibition and/or diabetes mellitus are accompanied by dysfunctions of gastric smooth muscle, we examined the possible roles of K_ATP channels in human gastric motility. We used human gastric corpus and antrum smooth muscle preparations and recorded the mechanical activities with a conventional contractile measuring system. We also identified the subunits of the K_ATP channels using Western blot. Pinacidil (10 μM), a K_ATP channel opener, suppressed contractions to 30% (basal tone to −0.2 g) of the control. The inhibitory effect of pinacidil on contraction was reversed to 59% of the control by glibenclamide (20 μM), a K_ATP channel blocker. The relaxation by pinacidil was not affected by a pretreatment with L-arginine methyl ester, tetraethylammonium, or 4-aminopyridine. Pinacidil also inhibited the acetylcholine (ACh)-induced tonic and phasic contractions in a glibenclamide-sensitive manner (42% and 6% of the control, respectively). Other K_ATP channel openers such as diazoxide, cromakalim and nicorandil also inhibited the spontaneous and ACh-induced contractions. Calcitonin gene-related peptide (CGRP), a gastric neuropeptide, induced muscle relaxation by the activation of K_ATP channels in human gastric smooth muscle. Finally, we have found with Western blot studies, that human gastric smooth muscle expressed K_ATP channels which were composed of Kir 6.2 and SUR2B subunits.

Susceptibility to Ventricular Arrhythmias Resulting from Mutations in FKBP1B, PXDNL, and SCN9A Evaluated in hiPSC Cardiomyocytes

BACKGROUND

We report an inherited cardiac arrhythmia syndrome consisting of Brugada and Early Repolarization Syndrome associated with variants in SCN9A, PXDNL, and FKBP1B. The proband inherited the 3 mutations and exhibited palpitations and arrhythmia-mediated syncope, whereas the parents and sister, who carried one or two of the mutations, were asymptomatic.

METHODS AND RESULTS

We assessed the functional impact of these mutations in induced pluripotent stem cell cardiomyocytes (hiPSC-CMs) derived from the proband and an unaffected family member. Current and voltage clamp recordings, as well as confocal microscopy analysis of Ca2+ transients, were evaluated in hiPSC-CMs from the proband and compared these results with hiPSC-CMs from undiseased controls. Genetic analysis using next-generation DNA sequencing revealed heterozygous mutations in SCN9A, PXDNL, and FKBP1B in the proband. The proband displayed right bundle branch block and exhibited episodes of syncope. The father carried a mutation in FKBP1B, whereas the mother and sister carried the SCN9A mutation. None of the 3 family members screened developed cardiac events. Action potential recordings from control hiPSC-CM showed spontaneous activity and a low upstroke velocity. In contrast, the hiPSC-CM from the proband showed irregular spontaneous activity. Confocal microscopy of the hiPSC-CM of the proband revealed low fluorescence intensity Ca2+ transients that were episodic in nature. Patch-clamp measurements in hiPSC-CM showed no difference in I Na but reduced I Ca in the proband compared with control. Coexpression of PXDNL-R391Q with SCN5A-WT displayed lower I Na density compared to PXDNL-WT. In addition, coexpression of PXDNL-R391Q with KCND3-WT displayed significantly higher I to density compared to PXDNL-WT.

CONCLUSION

SCN9A, PXDNL, and FKBP1B variants appeared to alter spontaneous activity in hiPSC-CM. Only the proband carrying all 3 mutations displayed the ERS/BrS phenotype, whereas one nor two mutations alone did not produce the clinical phenotype. Our results suggest a polygenic cause of the BrS/ERS arrhythmic phenotype due to mutations in these three gene variants caused a very significant loss of function of I Na and I Ca and gain of function of I to.

The relationship between J wave and ventricular tachycardia during Takotsubo cardiomyopathy

Background and objectives

Takotsubo cardiomyopathy (TTC) occasionally causes life-threatening ventricular arrhythmia. J wave on surface electrocardiography (sECG) has also been associated with idiopathic ventricular fibrillation and cardiac events; therefore, we investigated whether the presence of J wave on sECG is a potential risk factor for ventricular arrhythmia in patients with TTC.

Subjects and methods

We performed a retrospective study in 79 patients who were diagnosed with TTC from 2010 to 2014. Among them, 20 (25.3%) were diagnosed with ventricular tachycardia (VT). The J wave on the sECG was defined as J point elevation manifested through QRS notching or slurring at least 1 mm above the baseline in at least two leads.

Results

A higher prevalence of ventricular tachycardia was observed in patients with J wave. The corrected QT interval (QTc) was significantly longer in the VT group than in the non-VT group. In a multivariate analysis, the presence of J wave appeared to be the only independent predictors of VT [Hazard Ratio (HR) 3.5, p = 0.019].

Conclusion

Our results suggest that the presence of J wave on the sECG is significantly associated with VT, and appear to indicate that the presence of J wave is a strong and independent predictor of VT in patients with TTC.

KCND3 potassium channel gene variant confers susceptibility to electrocardiographic early repolarization pattern

BACKGROUNDThe presence of an early repolarization pattern (ERP) on the surface ECG is associated with risk of ventricular fibrillation and sudden cardiac death. Family studies have shown that ERP is a highly heritable trait, but molecular genetic determinants are unknown.METHODSTo identify genetic susceptibility loci for ERP, we performed a GWAS and meta-analysis in 2,181 cases and 23,641 controls of European ancestry.RESULTSWe identified a genome-wide significant (P < 5 × 10-8) locus in the potassium voltage-gated channel subfamily D member 3 (KCND3) gene that was successfully replicated in additional 1,124 cases and 12,510 controls. A subsequent joint meta-analysis of the discovery and replication cohorts identified rs1545300 as the lead SNP at the KCND3 locus (OR 0.82 per minor T allele, P = 7.7 × 10-12) but did not reveal additional loci. Colocalization analyses indicate causal effects of KCND3 gene expression levels on ERP in both cardiac left ventricle and tibial artery.CONCLUSIONSIn this study, we identified for the first time to our knowledge a genome-wide significant association of a genetic variant with ERP. Our findings of a locus in the KCND3 gene provide insights not only into the genetic determinants but also into the pathophysiological mechanism of ERP, discovering a promising candidate for functional studies.FUNDINGThis project was funded by the German Center for Cardiovascular Research (DZHK Shared Expertise SE081 - STATS). For detailed funding information per study, see the Supplemental Acknowledgments.

Update on Brugada Syndrome 2019

Brugada syndrome (BrS) was first described in 1992 as an aberrant pattern of ST segment elevation in right precordial leads with a high incidence of sudden cardiac death (SCD) in patients with structurally normal heart. It represents 4% ∼ 12% of all SCD and 20% of SCD in patients with structurally normal heart. The extremely wide genetic heterogeneity of BrS and other inherited cardiac disorders makes this new area of genetic arrhytmology a fascinating one. This review shows the state of art in diagnosis, management, and treatment of BrS focusing all the aspects regarding genetics and Preimplant Genetic Diagnosis (PGD) of embryos, overlapping syndromes, risk stratification, familial screening, and future perspectives. Moreover the review analyzes key points like electrocardiogram (ECG) criteria, the role of electrophysiological study (the role of ventricular programmed stimulation and the need of universal accepted protocol) and the importance of a correct risk stratification to clarify when implantable cardioverter defibrillator or a close follow-up is needed. In recent years, cardiovascular studies have been focused on personalized risk assessment and to determine the most optimal therapy for an individual. The BrS syndrome has also benefited of these advances although there remain several key points to be elucidated. We will review the present knowledge, progress made, and future research directions on BrS.

A de novo gain-of-function KCND3 mutation in early repolarization syndrome

BACKGROUND

Early repolarization syndrome (ERS) is characterized by J-point elevation on electrocardiograms and ventricular fibrillation (VF). Early repolarization arises from augmentation of the transmural electrical gradient in the cardiac action potential; therefore, the transient outward potassium current (I_to) has been regarded as a key candidate current for elucidating the mechanism of ERS. KCND3 encoding Kv4.3, an α-subunit of the I_to channel, is considered as one of target genes.

OBJECTIVE

The purpose of this study was to search for novel KCND3 mutations associated with ERS and to clarify the pathogenesis.

METHODS

We performed genetic screening for 11 unrelated probands with ERS and analyzed the electrophysiological properties of detected mutations by patch-clamp methods.

RESULTS

A novel de novo KCND3 heterozygous mutation, Gly306Ala (c.917g>c), was found in 1 proband. The proband was a 12-year-old boy, who suffered VF storm and showed significant J-point elevation in multiple leads. Intravenous isoproterenol and subsequent administration of quinidine were effective in preventing VF recurrence and restored the J-point elevation. In electrophysiological analysis, cultured cells expressing mutant Kv4.3 showed significantly increased current densities, slow inactivation, and slow recovery from inactivation compared to wild type. Extracellular application of quinidine significantly restored the inactivation time course in mutant Kv4.3. A simulation study confirmed the relationship between the novel KCND3 mutation and early repolarization on electrocardiograms.

CONCLUSION

A novel KCND3 heterozygous mutation was found to be associated with ERS. The pathogenesis can be explained by the increased I_to. Genetic screening for KCND3 could be useful for understanding the pathogenesis and selecting effective treatment.

Genetic Discovery of ATP-Sensitive K+ Channels in Cardiovascular Diseases

The ATP-sensitive K+ (KATP) channels are hetero-octameric protein complexes comprising 4 pore-forming (Kir6.x) subunits and 4 regulatory sulfonylurea receptor (SURx) subunits. They are prominent in myocytes, pancreatic β cells, and neurons and link cellular metabolism with membrane excitability. Using genetically modified animals and genomic analysis in patients, recent studies have implicated certain ATP-sensitive K+ channel subtypes in physiological and pathological processes in a variety of cardiovascular diseases. In this review, we focus on the causal relationship between ATP-sensitive K+ channel activity and pathophysiology in the cardiovascular system, particularly from the perspective of genetic changes in human and animal models.

Early Repolarization Syndrome: Diagnostic and Therapeutic Approach

An early repolarization pattern can be observed in 1% up to 13% of the overall population. Whereas, this pattern was associated with a benign outcome for many years, several more recent studies demonstrated an association between early repolarization and sudden cardiac death, so-called early repolarization syndrome. In early repolarization syndrome patients, current imbalances between epi- and endo-cardial layers result in dispersion of de- and repolarization. As a consequence, J waves or ST segment elevations can be observed on these patients' surface ECGs as manifestations of those current imbalances. Whereas, an early repolarization pattern is relatively frequently found on surface ECGs in the overall population, the majority of individuals presenting with an early repolarization pattern will remain asymptomatic and the isolated presence of an early repolarization pattern does not require further intervention. The mismatch between frequently found early repolarization patterns in the overall population, low incidences of sudden cardiac deaths related to early repolarization syndrome, but fatal, grave consequences in affected patients remains a clinical challenge. More precise tools for risk stratification and identification of this minority of patients, who will experience events, remain a clinical need. This review summarizes the epidemiologic, pathophysiologic and diagnostic background and presents therapeutic options of early repolarization syndrome.

Electrophysiological Basis for Early Repolarization Syndrome

During last centuries, Early Repolarization pattern has been interpreted as an ECG manifestation not linked to serious cardiovascular events. This view has been challenged on the basis of sporadic clinical observations that linked the J-wave with ventricular arrhythmias and sudden cardiac death. The particular role of this characteristic pattern in initiating ventricular fibrillation has been sustained by clinical descriptions of a marked and consistent J-wave elevation preceding the onset of the ventricular arrhythmia. Until now, Early Repolarization syndrome patients have been evaluated using ECG and theorizing different interpretations of the findings. Nonetheless, ECG analysis is not able to reveal all depolarization and repolarization properties and the explanation for this clinical events. Recent studies have characterized the epicardial substrate in these patients on the basis of high-resolution data, in an effort to provide insights into the substrate properties that support arrhythmogenicity in these patients. An overview for the current evidence supporting different theories explaining Early Repolarization Syndrome is provided in this review. Finally, future developments in the field directed toward individualized treatment strategies are examined.

The narrow-sense and common single nucleotide polymorphism heritability of early repolarization

BACKGROUND

Early repolarization (ER) is a risk marker for sudden cardiac death. Higher risk is associated with horizontal/descending ST-segment ER in the inferior or inferolateral ECG leads. Studies in family cohorts have demonstrated substantial heritability for the ER pattern, but genome-wide association studies (GWAS) have failed to identify statistically significant and replicable genetic signals.

METHODS AND RESULTS

We assessed the narrow-sense and common single nucleotide polymorphism (SNP) heritability of ER and ER subtypes using ECG data from 5829 individuals (TwinsUK, BRIGHT and GRAPHIC cohorts). ER prevalence was 8.3%. In 455 monozygous vs 808 dizygous twin pairs, concordances and twin correlations for ER subtypes (except horizontal/descending ST-segment ER) were higher and familial resemblance (except notched ER) was significant. Narrow-sense heritability estimates derived from 1263 female twin pairs using the structural equation program Mx ranged from 0.00-0.47 and common SNP heritability estimates derived from 4009 unrelated individuals of both sexes using Genome-wide Restricted Maximum Likelihood (GREML) ranged from 0.00-0.36, but none were statistically significant.

CONCLUSION

From our data, ER shows limited genetic predisposition. There appears to be significant environmental influence and these modest narrow-sense and common SNP heritability estimates may explain why previous GWAS have been unsuccessful.

J wave syndromes as a cause of malignant cardiac arrhythmias

The J wave syndromes, including the Brugada (BrS) and early repolarization (ERS) syndromes, are characterized by the manifestation of prominent J waves in the electrocardiogram appearing as an ST segment elevation and the development of life-threatening cardiac arrhythmias. BrS and ERS differ with respect to the magnitude and lead location of abnormal J waves and are thought to represent a continuous spectrum of phenotypic expression termed J wave syndromes. Despite over 25 years of intensive research, risk stratification and the approach to therapy of these two inherited cardiac arrhythmia syndromes are still rapidly evolving. Our objective in this review is to provide an integrated synopsis of the clinical characteristics, risk stratifiers, as well as the molecular, ionic, cellular, and genetic mechanisms underlying these two syndromes that have captured the interest and attention of the cardiology community over the past two decades.

Medium-term outcomes of idiopathic ventricular fibrillation survivors and family screening: a multicentre experience

Aims

Early repolarization (ER) has been linked to poorer outcomes in idiopathic ventricular fibrillation (IVF). The role of family screening in IVF is not clear. Our aim was to review predictors for poorer outcomes and evaluate the role of family screening in IVF.

Methods and results

This was a retrospective multicentre cohort study including all patients diagnosed with IVF. Data were collected on baseline characteristics, ECG findings, and recurrence of ventricular arrhythmia (VA) during follow-up. Electrocardiogram findings were reviewed in first-degree relatives that were screened. A total of 66 patients were included with male predominance (42/66, 64%) and Caucasian ethnicity (47/66, 71%). Mean age at cardiac arrest was 38 years ± 11. Thirty-one patients had ER (47%) predominantly with J-point amplitude ≥2 mm and horizontal ST segments (18/31, 58%). Recurrent VA was seen in 13 patients (20%). Horizontal ST segments were associated with increased rates of VA recurrence (OR 11, 95% CI 2.7-43.7; P = 0.0007). Early repolarization was seen in 20% of the 72 first-degree relatives and was more common if the proband had persistent ER pattern (OR 10.7, 95% CI 2.2-51.5; P = 0.003).

Conclusion

Ventricular arrhythmia recurrence was lower than previously reported. Early repolarization was common in this IVF cohort, and horizontal ST segments were suggestive predictor for poorer outcomes. Persistent ER in proband was associated with ER in first-degree relatives. With better understanding of its predictive value and the relationship to IVF, this information could potentially be used to guide family screening and identify new mutations using family members with persistent ER.

Genetic analysis of sick sinus syndrome in a family harboring compound CACNA1C and TTN mutations

Sick sinus syndrome (SSS) is a sinus node dysfunction characterized by severe sinus bradycardia. SSS results in insufficient blood supply to the brain, heart, kidneys, and other organs and is associated with the increased risk of sudden cardiac death. Bradyarrhythmia appears in the absence of any associated cardiac pathology and displays a genetic legacy. The present study identified a family with primary manifestation of sinus bradycardia (five individuals) along with early repolarization (four individuals) and atrial fibrillation (one individual). Targeted exome sequencing was used to screen exons and adjacent splice sites of 61 inherited arrhythmia‑associated genes, to detect pathogenic genes and variant sites in the proband. Family members were sequenced by Sanger sequencing and protein functions predicted by Polyphen‑2 software. A total of three rare variants were identified in the family, including two missense variants in calcium voltage‑gated channel subunit alpha1 C (CACNA1C) (gi:193788541, NM_001129843), c.1786G>A (p.V596M) and c.5344G>A (p.A1782T), and one missense variant in titin (TTN) c.49415G>A (p.R16472H) (gi:291045222, NM_003319). The variants p.V596M and p.R16472H were predicted to be deleterious and resulted in alterations in the amino acid type and sequence of the polypeptide chain, which may partially or completely inactivate the encoded protein. The comparison of literature, gene database, and pedigree phenotype analysis suggests that p.V596M or p.R16472H variants are pathogenic. The complex overlapping variants at three loci lead to a more severe phenotype in the proband, and may increase the susceptibility of individuals to atrial fibrillation. The simultaneous occurrence of V596M and R16472H may increase the severity of early repolarization. Various family members may have carried heterozygous mutants of p.A1782T and p.R16472H due to genetic heterogeneity, however did not exhibit clinical signs of cardiac electrophysiological alterations, potentially attributable to the low vagal tone. To the best of the author's knowledge, this is the first study to suggest the involvement of the novel missense CACNA1C c.1786G>A and TTN c.49415G>A variants in the inheritance of symptomatic bradycardia and development of SSS.

Genetic variations involved in sudden cardiac death and their associations and interactions

Although the mechanism of sudden cardiac death (SCD) in heart failure is not completely known, genetic variations are known to play key roles in this process. Increasing numbers of mutations and variants are being discovered through genome-wide association studies. The genetic variations involved in the mechanisms of SCD have aroused widespread concern. Comprehensive understanding of the genetic variations involved in SCD may help prevent it. To this end, we briefly reviewed the genetic variations involved in SCD and their associations and interactions, and observed that cardiac ion channels are the core molecules involved in this process. Genetic variations involved in cardiac structure, cardiogenesis and development, cell division and differentiation, and DNA replication and transcription are all speculated to be loci involved in SCD. Additionally, the systems involved in neurohumoral regulation as well as substance and energy metabolism are also potentially responsible for susceptibility to SCD. They form an elaborate network and mutually interact with each other to govern the fate of SCD-susceptible individuals.

The Early Repolarization ECG Pattern – An Update

Early repolarization pattern (ERP), a form of J-wave syndromes, was considered long time a benign ECG phenomenon. However, recent data confirmed that certain phenotypes of ERP are related to an increased risk of sudden cardiac death (idiopathic ventricular fibrillation). Our paper gives a short and practical update regarding the main issues related to ERP: epidemiological data, molecular and electrophysiological background, clinical significance and risk stratification. At the end, the future directions of research and clinical management related to ERP are presented.

Electrophysiological analyses of transgenic mice overexpressing KCNJ8 with S422L mutation in cardiomyocytes

Genetic analysis of KCNJ8 has pointed a mutation (S422L) as a susceptible link to J wave syndrome (JWS). In vitro expression study indicated that the ATP-sensitive K⁺ (K_ATP) channel with the S422L mutation has the gain-of-function with reduced sensitivity to ATP. However, the electrophysiological impact of KCNJ8 has not been elucidated in vivo. Transgenic mouse strains overexpressing KCNJ8 S422L variant (TGmt) or WT (TGWT) in cardiomyocytes have been created to investigate the influence of KCNJ8 in cardiomyocytes and the JWS-related feature of the S422L variant on the cardiac electrophysiology. These TG strains demonstrated distinct changes in the J-ST segment of ECG with marked QT prolongation, which might be ascribed to the action potential prolongation resulting from the reduction of voltage-dependent K⁺ currents in ventricular cells. The pinacidil-induced K_ATP current was decreased in these TG myocytes and no obvious difference between TG and non-TG (WT) myocytes in the ATP sensitivity of the K_ATP channel was observed although the open probability of the K_ATP channels was significantly lower in TG myocytes than WT. These transgenic mouse strains with distinct ECG changes suggested that the S422L mutation in KCNJ8 gene is not a direct cause of JWS.

A mutation in the CACNA1C gene leads to early repolarization syndrome with incomplete penetrance: A Chinese family study

BACKGROUND

Early repolarization syndrome (ERS) may be a near-Mendelian or an oligogenic disease; however, no direct evidence has been provided to support this theory.

METHODS AND RESULTS

We described a large Chinese family with nocturnal sudden cardiac death induced by ERS in most of the young male adults. One missense mutation (p.Q1916R) was found in the major subunit of the L-type calcium channel gene CACNA1C by the direct sequencing of candidate genes. A concomitant gain-of-function variant in the sodium channel gene SCN5A (p.R1193Q) was found to rescue the phenotype of the female CACNA1C-Q1916R mutation carriers, which led to the incomplete penetrance. The functional studies, via the exogenous expression approach, revealed that the CACNA1C-Q1916R mutation led to a decreasing L-type calcium current and the protein expression defect. The decreased calcium current produced by the mutant channel was improved by isoproterenol but exacerbated by testosterone. The effects of CACNA1C-Q1916R mutation and testosterone on cellular electrophysiology were further confirmed by the human ventricular action potential simulation.

CONCLUSIONS

Our results demonstrated that the loss-of-function CACNA1C-Q1916R mutation contributed to ERS-related sudden cardiac death, and the phenotypic incomplete penetrance was modified by the SCN5A-R1193Q variant and sex. These findings suggest that phenotypes of ERS are modified by multiple genetic factors, which supports the theory that ERS may be an oligogenic disease.

Novel trigenic CACNA1C/DES/MYPN mutations in a family of hypertrophic cardiomyopathy with early repolarization and short QT syndrome

Background

Hypertrophic cardiomyopathy (HCM) patients with early repolarization (ER) pattern are at higher risk of ventricular arrhythmia, yet the genetic background of this situation has not been well investigated. Here we report novel trigenic mutations detected in a Chinese family of obstructive HCM with ER and short QT syndrome (SQTS).

Methods

Proband and family members underwent detailed medical assessments. DNAs were extracted from peripheral blood leukocytes for genetic screening with next generation method. The functional characterization of the mutation was conducted in TSA201 cells with patch-clamp experiment.

Results

The proband was a 52-year-old male who had a ER pattern ECG in inferioral-lateral leads with atrioventricular block and QTc of 356 ms. He also suffered from severe left ventricular hypertrophy and dysfunction. Targeted sequencing revealed trigenic mutations: c.700G>A/p.E234K in DES, c.2966G>A/p.R989H in MYPN, and c.5918G>C/p.R1973P in CACNA1C. All mutations were also detected in his daughter with ER and mild myocardium hypertrophy. The CACNA1C-R1973P mutation caused significant reduction (68.4%) of I_Ca compared to CACNA1C-WT (n = 14 and 14, P < 0.05). The computer modeling showed that all 3 mutations were highly disease-causing. The proband received the CRT-D (cardiac resynchronizing therapy) implantation, which lowered the left ventricular outflow tract gradient (LVOTG, 124 mmHg pre vs. 27 mmHg post) and restored the LV function (LVEF 40% pre vs. 63% post).

Conclusions

The study reveals a novel CACNA1C mutation underlying the unique ER pattern ECGs with SQTS. It also shows the rare trigenic mutations are the pathogenic substrates for the complicated clinical manifestation in HCM patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1180-1) contains supplementary material, which is available to authorized users.

Early repolarization is associated with a significantly increased risk of ventricular arrhythmias and sudden cardiac death in patients with structural heart diseases

BACKGROUND

Early repolarization pattern (ERP) has been proved to increase risk of arrhythmia death in the general population, but its prognostic significance in patients with structural heart disease (SHD) is controversial.

OBJECTIVE

The purpose of this study was to conduct a meta-analysis of studies assessing the association between ERP and risk of ventricular arrhythmias (VTAs) and sudden cardiac death (SCD) in patients with SHD.

METHODS

We performed a literature search using MEDLINE (January 1, 1966, to September 25, 2016) and EMBASE (January 1, 1980, to September 25, 2016) with no restrictions. Studies that reported odds ratio (OR) estimates with 95% confidence intervals (CIs) for the associations of interest were included.

RESULTS

The search yielded 19 observational studies, involving 7268 patients that reported 1127 cases of VTAs or SCD. In the selected studies, the point estimates of the ORs were consistently greater than 1. Compared with those without ERP, patients with ERP experienced a significantly increased risk of developing VTAs or SCD (OR 4.76; 95% CI 3.62-6.26), ventricular fibrillation (OR 7.14; 95% CI 4.31-11.82), and SCD (OR 4.07; 95% CI 1.58-10.51). The results were consistent and statistically significant in all subgroups. ERP with J-point elevation in inferior leads, notching configuration, and horizontal or descending ST segment connote higher risk.

CONCLUSION

ERP is associated with a significant increased risk of VTAs or SCD in patients with SHD. Future research should attempt to understand the exact mechanisms for the arrhythmia risk and to introduce ERP in the risk stratification in this patient group.

High Prevalence and Clinical Implication of Myocardial Bridging in Patients with Early Repolarization

PURPOSE

Recent evidence suggests that early repolarization (ER) is related with myocardial ischemia. Compression of coronary artery by a myocardial bridging (MB) can be associated with clinical manifestations of myocardial ischemia. This study aimed to evaluate the associations of MB in patients with ER.

MATERIALS AND METHODS

In consecutive patients (n=1303, age, 61±12 years) who had undergone coronary angiography, we assessed the prevalence and prognostic implication of MB in those with ER (n=142) and those without ER (n=1161).

RESULTS

MB was observed in 54 (38%) and 196 (17%) patients in ER and no-ER groups (p<0.001). In multivariate analysis, MB was independently associated with ER (odd ratio: 2.9, 95% confidence interval: 1.98-4.24, p<0.001). Notched type ER was more frequently observed in MB involving the mid portion of left anterior descending coronary artery (LAD) (69.8% vs. 30.2%, p=0.03). Cardiac event was observed in nine (6.3%) and 22 (1.9%) subjects with and without ER, respectively. MB was more frequently observed in sudden death patients with ER (2 out of 9, 22%) than in those without ER (0 out of 22).

CONCLUSION

MB was independently associated with ER in patients without out structural heart disease who underwent coronary angiography. Notched type ER was closely related with MB involving the mid portion of the LAD. Among patients who had experienced cardiac events, a higher prevalence of MB was observed in patients with ER than those without ER. Further prospective studies on the prognosis of MB in ER patients are required.

Brugada Syndrome and Early Repolarisation: Distinct Clinical Entities or Different Phenotypes of the Same Genetic Disease?

Brugada and early repolarisation (ER) syndromes are currently considered two distinct inherited electrical disorders with overlapping clinical and electrocardiographic features. A considerable number of patients diagnosed with ER syndrome have a genetic mutation related to Brugada syndrome (BrS). Due to the high variable phenotypic manifestation, patients with BrS may present with inferolateral repolarisation abnormalities only, resembling the ER pattern. Moreover, the complex genotype-phenotype interaction in BrS can lead to the occurrence of mixed phenotypes with ER syndrome. The first part of this review focuses on specific clinical and electrocardiographic features of BrS and ER syndrome, highlighting the similarity shared by the two primary electrical disorders. The genetic background, with emphasis on the complexity of genotype-phenotype interaction, is explored in the second part of this review.