Molecular, Subcellular, and Arrhythmogenic Mechanisms in Genetic RyR2 Disease

Drowning incidents precipitated by unusual causes (DIPUCs): A narrative review of their diagnoses, evaluation and management

Drowning is a cause of significant morbidity and mortality worldwide. In most circumstances, the proximate cause is attributable to human factors, such as inexperience, fatigue, intoxication, or hazardous water conditions. The phenomenon of drowning incidents precipitated by unusual circumstances (DIPUCs) - either fatal or nonfatal - involving otherwise healthy individuals under generally safe conditions has not been comprehensively addressed in the medical and drowning literature to date. In this review, we discuss etiologies of DIPUCs, diagnostic clues, suggested workup, suggested postmortem testing, and implications for surviving patients and families. Identifying the cause of a drowning incident can be extremely challenging for the initially treating physician, relying perforce on historical context, environmental clues, physical exam, medical history, eyewitness accounts or video recordings. If no clear explanation for a drowning incident emerges despite a thorough investigation, clinicians should consider some of the less common diagnoses we describe in this paper, and, when appropriate, refer for an autopsy with postmortem molecular genetic testing. While time-consuming, these efforts can prove life-saving for some non-fatal drowning victims and the families of all victims of DIPUCs.

Structural basis for ryanodine receptor type 2 leak in heart failure and arrhythmogenic disorders

Heart failure, the leading cause of mortality and morbidity in the developed world, is characterized by cardiac ryanodine receptor 2 channels that are hyperphosphorylated, oxidized, and depleted of the stabilizing subunit calstabin-2. This results in a diastolic sarcoplasmic reticulum Ca2+ leak that impairs cardiac contractility and triggers arrhythmias. Genetic mutations in ryanodine receptor 2 can also cause Ca2+ leak, leading to arrhythmias and sudden cardiac death. Here, we solved the cryogenic electron microscopy structures of ryanodine receptor 2 variants linked either to heart failure or inherited sudden cardiac death. All are in the primed state, part way between closed and open. Binding of Rycal drugs to ryanodine receptor 2 channels reverts the primed state back towards the closed state, decreasing Ca2+ leak, improving cardiac function, and preventing arrhythmias. We propose a structural-physiological mechanism whereby the ryanodine receptor 2 channel primed state underlies the arrhythmias in heart failure and arrhythmogenic disorders.

Evolution of ion channels in cetaceans: a natural experiment in the tree of life

Cetaceans represent a natural experiment within the tree of life in which a lineage changed from terrestrial to aquatic habitats. This shift involved phenotypic modifications, representing an opportunity to explore the genetic bases of phenotypic diversity. Among the different molecular systems that maintain cellular homeostasis, ion channels are crucial for the proper physiological functioning of all living species. This study aims to explore the evolution of ion channels during the evolutionary history of cetaceans. To do so, we created a bioinformatic pipeline to annotate the repertoire of ion channels in the genome of the species included in our sampling. Our main results show that cetaceans have, on average, fewer protein-coding genes and a higher percentage of annotated ion channels than non-cetacean mammals. Signals of positive selection were detected in ion channels related to the heart, locomotion, visual and neurological phenotypes. Interestingly, we predict that the NaV1.5 ion channel of most toothed whales (odontocetes) is sensitive to tetrodotoxin, similar to NaV1.7, given the presence of tyrosine instead of cysteine, in a specific position of the ion channel. Finally, the gene turnover rate of the cetacean crown group is more than three times faster than that of non-cetacean mammals.

Esketamine Exposure Impairs Cardiac Development and Function in Zebrafish Larvae

Esketamine is a widely used intravenous general anesthetic. However, its safety, particularly its effects on the heart, is not fully understood. In this study, we investigated the effects of esketamine exposure on zebrafish embryonic heart development. Zebrafish embryos were exposed to esketamine at concentrations of 1, 10, and 100 mg/L from 48 h post-fertilization (hpf) to 72 hpf. We found that after exposure, zebrafish embryos had an increased hatching rate, decreased heart rate, stroke volume, and cardiac output. When we exposed transgenic zebrafish of the Tg(cmlc2:EGFP) strain to esketamine, we observed ventricular dilation and thickening of atrial walls in developing embryos. Additionally, we further discovered the abnormal expression of genes associated with cardiac development, including nkx2.5, gata4, tbx5, and myh6, calcium signaling pathways, namely ryr2a, ryr2b, atp2a2a, atp2a2b, slc8a3, slc8a4a, and cacna1aa, as well as an increase in acetylcholine concentration. In conclusion, our findings suggest that esketamine may impair zebrafish larvae's cardiac development and function by affecting acetylcholine concentration, resulting in weakened cardiac neural regulation and subsequent effects on cardiac function. The insights garnered from this research advocate for a comprehensive safety assessment of esketamine in clinical applications.

A novel RyR2 mutation associated with co-morbid catecholaminergic polymorphic ventricular tachycardia (CPVT) and benign epilepsy with centrotemporal spikes (BECTS)

In this case report, we describe a 14-year-old patient with a novel RyR2 gene mutation (c.6577G > T/p.Val2193Leu), identified through a comprehensive review of medical history, examination findings, and follow-up data. The pathogenic potential of this mutation, which results in the loss of some interatomic forces and compromises the closure of the RyR2 protein pore leading to calcium leakage, was analyzed using the I-TASSER Suite to predict the structural changes in the protein. This mutation manifested clinically as co-morbid catecholaminergic polymorphic ventricular tachycardia (CPVT) and benign epilepsy with centrotemporal spikes (BECTS), a combination not previously documented in the same patient. While seizures were successfully managed with levetiracetam, the patient's exercise-induced syncope episodes could not be controlled with metoprolol, highlighting the complexity and challenge in managing CPVT associated with this novel RyR2 variation.

A missense variant in MYOF is associated with ARVC and sudden cardiac death

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is rare autosomal dominant genetic disorder that leads to severe arrhythmia and sudden cardiac death. Although previous studies in clinical, pathological and genetics of ARVC established consensus diagnostic criteria and expanded the spectrum of pathogenic genes, there is still a proportion of patients with unclear causative factors. Here, whole-exome sequencing was employed to investigate the genetic etiology of a 15-year-old sudden cardiac death female caused by ARVC. A novel variant of MYOF (NM_013451.3: c.4723G > C: p.D1575H) was identified, which is a member of the Ferlin family of proteins is associated with cardiomyopathy. And the bioinformatics analysis predicted the pathogenicity of this variant. We report the first variant of MYOF in ARVC, which imply a vital role of MYOF in cardiomyopathy.

RYR2 receptor gene mutation associated with catecholaminergic polymorphic ventricular tachycardia in children: a case report & literature review

Background

Ryanodine receptor 2 (RYR2) gene mutation causing catecholaminergic polymorphic ventricular tachycardia (CPVT) is one of the identified causes of sudden death in adults and children.

Case Description

We report a case of RYR2 gene mutation presented with cardiac arrest and recurrent syncopal attack with accidental finding of cardiac tumour. For the systematic review, we used four databases (Scopus, PubMed, Ovid and Google Scholar) to search articles with the terms "RYR2 gene mutation" and "catecholaminergic polymorphic ventricular tachycardia (CPVT)". Fourteen studies were chosen and reviewed together with our reported patient. Most of the patients presented initially with syncopal attack and developed cardiac arrest later. Some of them presented with both syncopal attack and seizures precipitated by exercise or stress. We found that 43.8% of patients shared similar variants or coding effects in RYR2 gene mutation. Demographically, the mean age at presentation is 11 years old with 53% of reported cases were male.

Conclusions

Refractory arrhythmias cardiac arrest not responding to adrenaline should raise the suspicion towards RYR2 gene mutations. Recognition of this condition is important as it affects the outcome of resuscitation. Untimely diagnosis of RYR2 gene mutations with appropriate use of pharmacological agents during resuscitation is important to ensure a better outcome.

The ryanodine receptor microdomain in cardiomyocytes

The ryanodine receptor type 2 (RyR) is a key player in Ca2+ handling during excitation-contraction coupling. During each heartbeat, RyR channels are responsible for linking the action potential with the contractile machinery of the cardiomyocyte by releasing Ca2+ from the sarcoplasmic reticulum. RyR function is fine-tuned by associated signalling molecules, arrangement in clusters and subcellular localization. These parameters together define RyR function within microdomains and are subject to disease remodelling. This review describes the latest findings on RyR microdomain organization, the alterations with disease which result in increased subcellular heterogeneity and emergence of microdomains with enhanced arrhythmogenic potential, and presents novel technologies that guide future research to study and target RyR channels within specific microdomains.

RYR2-ryanodinopathies: from calcium overload to calcium deficiency

The sarcoplasmatic reticulum (SR) cardiac ryanodine receptor/calcium release channel RyR2 is an essential regulator of cardiac excitation-contraction coupling and intracellular calcium homeostasis. Mutations of the RYR2 are the cause of rare, potentially lethal inherited arrhythmia disorders. Catecholaminergic polymorphic ventricular tachycardia (CPVT) was first described more than 20 years ago and is the most common and most extensively studied cardiac ryanodinopathy. Over time, other distinct inherited arrhythmia syndromes have been related to abnormal RyR2 function. In addition to CPVT, there are at least two other distinct RYR2-ryanodinopathies that differ mechanistically and phenotypically from CPVT: RYR2 exon-3 deletion syndrome and the recently identified calcium release deficiency syndrome (CRDS). The pathophysiology of the different cardiac ryanodinopathies is characterized by complex mechanisms resulting in excessive spontaneous SR calcium release or SR calcium release deficiency. While the vast majority of CPVT cases are related to gain-of-function variants of the RyR2 protein, the recently identified CRDS is linked to RyR2 loss-of-function variants. The increasing number of these cardiac 'ryanodinopathies' reflects the complexity of RYR2-related cardiogenetic disorders and represents an ongoing challenge for clinicians. This state-of-the-art review summarizes our contemporary understanding of RYR2-related inherited arrhythmia disorders and provides a systematic and comprehensive description of the distinct cardiac ryanodinopathies discussing clinical aspects and molecular insights. Accurate identification of the underlying type of cardiac ryanodinopathy is essential for the clinical management of affected patients and their families.

Molecular Aspects Implicated in Dantrolene Selectivity with Respect to Ryanodine Receptor Isoforms

Dantrolene is an intra-cellularly acting skeletal muscle relaxant used for the treatment of the rare genetic disorder, malignant hyperthermia (MH). In most cases, MH susceptibility is caused by dysfunction of the skeletal ryanodine receptor (RyR1) harboring one of nearly 230 single-point MH mutations. The therapeutic effect of dantrolene is the result of a direct inhibitory action on the RyR1 channel, thus suppressing aberrant Ca2+ release from the sarcoplasmic reticulum. Despite the almost identical dantrolene-binding sequence exits in all three mammalian RyR isoforms, dantrolene appears to be an isoform-selective inhibitor. Whereas RyR1 and RyR3 channels are competent to bind dantrolene, the RyR2 channel, predominantly expressed in the heart, is unresponsive. However, a large body of evidence suggests that the RyR2 channel becomes sensitive to dantrolene-mediated inhibition under certain pathological conditions. Although a consistent picture of the dantrolene effect emerges from in vivo studies, in vitro results are often contradictory. Hence, our goal in this perspective is to provide the best possible clues to the molecular mechanism of dantrolene’s action on RyR isoforms by identifying and discussing potential sources of conflicting results, mainly coming from cell-free experiments. Moreover, we propose that, specifically in the case of the RyR2 channel, its phosphorylation could be implicated in acquiring the channel responsiveness to dantrolene inhibition, interpreting functional findings in the structural context.

HIV-Tat Exacerbates the Actions of Atazanavir, Efavirenz, and Ritonavir on Cardiac Ryanodine Receptor (RyR2)

The incidence of sudden cardiac death (SCD) in people living with HIV infection (PLWH), especially those with inadequate viral suppression, is high and the reasons for this remain incompletely characterized. The timely opening and closing of type 2 ryanodine receptor (RyR2) is critical for ensuring rhythmic cardiac contraction-relaxation cycles, and the disruption of these processes can elicit Ca²⁺ waves, ventricular arrhythmias, and SCD. Herein, we show that the HIV protein Tat (HIV-Tat: 0-52 ng/mL) and therapeutic levels of the antiretroviral drugs atazanavir (ATV: 0-25,344 ng/mL), efavirenz (EFV: 0-11,376 ng/mL), and ritonavir (RTV: 0-25,956 ng/mL) bind to and modulate the opening and closing of RyR2. Abacavir (0-14,315 ng/mL), bictegravir (0-22,469 ng/mL), Rilpivirine (0-14,360 ng/mL), and tenofovir disoproxil fumarate (0-18,321 ng/mL) did not alter [³H]ryanodine binding to RyR2. Pretreating RyR2 with low HIV-Tat (14 ng/mL) potentiated the abilities of ATV and RTV to bind to open RyR2 and enhanced their ability to bind to EFV to close RyR2. In silico molecular docking using a Schrodinger Prime protein-protein docking algorithm identified three thermodynamically favored interacting sites for HIV-Tat on RyR2. The most favored site resides between amino acids (AA) 1702-1963; the second favored site resides between AA 467-1465, and the third site resides between AA 201-1816. Collectively, these new data show that HIV-Tat, ATV, EFV, and RTV can bind to and modulate the activity of RyR2 and that HIV-Tat can exacerbate the actions of ATV, EFV, and RTV on RyR2. Whether the modulation of RyR2 by these agents increases the risk of arrhythmias and SCD remains to be explored.

Defective ryanodine receptor N-terminus inter-subunit interaction is a common mechanism in neuromuscular and cardiac disorders

The ryanodine receptor (RyR) is a homotetrameric channel mediating sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contraction. Mutations in RyR1 and RyR2 lead to life-threatening malignant hyperthermia episodes and ventricular tachycardia, respectively. In this brief report, we use chemical cross-linking to demonstrate that pathogenic RyR1 R163C and RyR2 R169Q mutations reduce N-terminus domain (NTD) tetramerization. Introduction of positively-charged residues (Q168R, M399R) in the NTD-NTD inter-subunit interface normalizes RyR2-R169Q NTD tetramerization. These results indicate that perturbation of NTD-NTD inter-subunit interactions is an underlying molecular mechanism in both RyR1 and RyR2 pathophysiology. Importantly, our data provide proof of concept that stabilization of this critical RyR1/2 structure-function parameter offers clear therapeutic potential.