Abstract

The cardiac ryanodine receptor (RyR2) is a critical calcium release channel essential for normal cardiac contraction. Dysregulation of RyR2 function is a key pathogenic factor in various cardiac diseases, notably arrhythmias and heart failure. The clinical spectrum of RyR2-related diseases (ryanodinopathy) has expanded beyond the well-established gain-of-function (GOF) mutations causing catecholaminergic polymorphic ventricular tachycardia (CPVT). Recent research has now delineated two additional distinct clinical entities: exon 3 deletion syndrome (E3DS), which presents with CPVT along with structural abnormalities, and calcium release deficiency syndrome (CRDS), caused by loss-of-function (LOF) mutations. CRDS is characterized by impaired calcium release from the sarcoplasmic reticulum and fundamentally differs from the classical CPVT phenotype, necessitating distinct approaches to its diagnosis, clinical management, and therapeutic intervention. This review provides a comprehensive overview of the current understanding of CRDS. We discuss its clinical manifestations, scrutinize the underlying molecular mechanisms, evaluate available diagnostic strategies, and explore potential therapeutic avenues. By synthesizing latest findings, this review aims to illuminate the complexities of RyR2-mediated pathologies and foster further progress in this evolving field.

1 Introduction

Calcium functions as a critical second messenger that exerts multifaceted and hierarchical control over cardiac function at both the organ and cellular levels. Its most fundamental role lies in triggering cardiac contraction. This process is central to excitation-contraction coupling (ECC): membrane depolarization prompts a minor influx of extracellular Ca2+ through L-type calcium channels, which in turn activates the release of a larger Ca2+ store from the sarcoplasmic reticulum (SR) via cardiac ryanodine receptor (RyR2). This concerted action generates a rapid surge in cytosolic Ca2+ concentration, known as the Ca2+ transient (Bers, 2002). The elevated cytosolic Ca2+ then binds to troponin C (TnC) on the myofilaments, inducing a conformational shift that initiates cross-bridge cycling and sarcomere shortening, thereby culminating in myocardial contraction.

For relaxation to occur, cytosolic Ca2+ must be rapidly removed. This is achieved primarily through its reuptake into the SR by the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) and its extrusion from the cell via the Na+-Ca2+ exchanger (NCX) (Bers, 2002; Eisner et al., 2017). The consequent decline in cytosolic Ca2+ prompts its dissociation from TnC, leading to myocardial relaxation and the completion of the contraction-relaxation cycle. The precision of this cyclical Ca2+ handling is a primary determinant of both the force of cardiac contraction and the efficiency of diastolic relaxation.

Furthermore, Ca2+ serves as a key modulator of cardiac electrical activity. The influx of Ca2+ through L-type calcium channel (LTCC) dynamically balances the efflux of K+, a process essential for the formation and maintenance of the action potential plateau in cardiomyocytes. This delicate balance ensures the proper duration of the action potential plateau and timely repolarization, which is essential for preventing afterdepolarizations and allowing complete relaxation and subsequent ventricular filling during diastole (Tonko and Lambiase, 2024; Zhong and Karma, 2024). Aside from working myocardium, the depolarization of pacemaker cells in the sinoatrial node is critically dependent on both T-type and L-type calcium channels. The inward Ca2+ current during phase-4 drives the slow diastolic depolarization, which is fundamental to the generation and modulation of the normal rhythm of the heart (Namekata et al., 2022).

In summary, through its regulation of myocardial contraction, electrical activity, and intracellular signaling pathways, Ca2+ emerges as a central orchestrator of cardiac function. The precise maintenance of Ca2+ homeostasis is, therefore, a critical mechanism underpinning normal cardiac pumping efficiency, electrical rhythm stability, and metabolic adaptation.

RyR2 is a Ca2+ release channel protein embedded in the SR membrane. Composed of four identical subunits and interacting with various modulators to form an intricate regulatory supercomplex (Peng et al., 2016; Gong et al., 2019; Marks, 2023; Hadiatullah et al., 2025), RyR2 opens in response to extracellular Ca2+ influx. This mediates the release of Ca2+ from the SR into the cytoplasm, a pivotal step in cardiac ECC (Bers, 2002). Beyond its physiological role, RyR2 dysfunction is implicated in various cardiac pathologies, including arrhythmias and heart failure (HF) (Belevych et al., 2013; Zaffran et al., 2023; Miotto et al., 2024). Recent advances have further expanded the spectrum of RyR2-related diseases, demonstrating that RyR2 dysfunction also contributes to the pathogenesis of dilated cardiomyopathy (DCM) (Xie et al., 2026), and is also implicated in metabolic heart diseases such as diabetes and prediabetes (Federico et al., 2017; Tow et al., 2022), thus broadening the clinical relevance of this calcium channel beyond inherited arrhythmias.

The association between RyR2 dysfunction and Ca2+ cycling disorder has been uncovered previously (Keefe et al., 2023). Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a typical ventricular arrhythmia (VA) linked to RyR2 dysfunction. The most common form, CPVT1, is caused by RyR2 gain-of-function (GOF) mutations (Priori et al., 2001). CPVT can also result from mutations in RyR2-associated proteins such as calsequestrin 2 (CASQ2) and calmodulin (CaM) (Priori et al., 2002; Crotti et al., 2023). The estimated prevalence of classical CPVT is probably 1:10,000 (Priori et al., 2021). So far, CPVT has been characterized by sudden cardiac death (SCD) in the absence of structural heart disease, as well as bidirectional and/or polymorphic ventricular tachycardia (VT) that can be reproducibly induced by exercise stress testing (EST) (Priori and Chen, 2011). Notably, exon 3 deletion syndrome (E3DS) is an established, distinct entity of RyR2 channelopathy (ryanodinopathy), where CPVT with additional features/phenotypes e.g., dilated cardiomyopathy, atrial standstill has been described (Bhuiyan et al., 2007; Steinberg et al., 2023), mechanism of which is almost similar to CPVT alone, but in E3DS exon 3 deletion destabilizes the N-terminal structure of the RyR2 channel, facilitating premature pore opening and diastolic calcium leak for a longer period (Lobo et al., 2011). The incidence of E3DS is approximately 1:100,000 (Marjamaa et al., 2009; Steinberg et al., 2023).

However, an increasing number of patients harbouring RyR2 mutations with negative EST were reported (Roston et al., 2017b; Santiago and Priori, 2022). In vitro functional studies have identified that the overwhelming majority of these “atypical” CPVT individuals exhibited RyR2 loss-of-function (LOF) mutations, a condition termed cardiac ryanodine receptor calcium release deficiency syndrome (CRDS) (Sun et al., 2021). Accordingly, CRDS is regarded as an emerging ryanodinopathy entity distinguished from CPVT (Figure 1). Here, we will summarize the current understanding of the manifestation, mechanism, diagnosis and therapies of CRDS.

Apprehending CRDS through varying lenses. The four sections in the figure respectively illustrate the currently known pathological characteristics of CRDS, exhibited across various levels: the individual, the intact heart, the cardiac myocyte, and the RyR2 single channel. EST, exercise stress testing. LBLPS, long-burst, long-pause and short-coupled. ECG, electrocardiogram. EAD, early afterdepolarization. SR, sarcoplasmic reticulum.

2 Clinical manifestations and genetic mapping of CRDS

Genetic research and testing conducted on CRDS patients’ families suggested that mutations in the RYR2 gene (encoding RyR2) may be responsible for the inheritance of CRDS (Roston et al., 2017a; 2022; Sun et al., 2021). All the probands in these families suffered from SCD or aborted SCD (aSCD) and had rare RyR2 mutations functionally identified as RyR2 LOF variants. Notably, a subset of CRDS patients exhibited more complex life-threatening syndromes, including short-coupled variant of torsades de pointes (scTdP) and cardiomyopathy (Fujii et al., 2017). Despite the consensus that a disease entity should not be defined by DNA sequencing without concern for the phenotype in clinical practice, the gold standard for identifying CRDS individuals has been RYR2 gene mutation detection thus far, since they typically do not exhibit clearly distinguishable phenotypes on routine clinical testing. Nevertheless, the unique clinical and molecular features associated with CRDS still indicate that it is a distinct inherited arrhythmogenic syndrome from the known ones.

The genetic mutations accounting for CRDS were located in the RYR2 gene, and have been functionally evaluated to result in RyR2 LOF (Priori et al., 2002; Jiang et al., 2007; Medeiros-Domingo et al., 2009; Nof et al., 2011; Tester et al., 2012; 2020; Paech et al., 2014; Kron et al., 2015; Fujii et al., 2017; Roston et al., 2017a; 2022; Blancard et al., 2021; Li et al., 2021; Shauer et al., 2021; Sun et al., 2021; Zhong et al., 2021; Hirose et al., 2022; Hopton et al., 2022; Ormerod et al., 2022; Tian et al., 2023). For a summary of the documented CRDS-associated RyR2 LOF mutations, see Table 1. The hotspot regions of RyR2 mutations linked to CRDS were concentrated in the central domain and the transmembrane domain located at the C-terminal (Figure 2), both of which are essential to the gating of the RyR2 pore and its activity (Peng et al., 2016; Uchinoumi et al., 2025).

CRDS-linked RyR2 LOF genotypesMutation-located RyR2 domainsPatient phenotypesReferencesA4860G*Transmembrane domainSyncope, cIVF(Jiang et al., 2007; Priori et al., 2002)]I4855M*Transmembrane domainSCA, SCD, LVNCRoston et al. (2017a)RYR2-DUPPromoter region and exons 1–4Syncope, SCA, SCDTester et al. (2020)Q3774LCentral domainaSCDSun et al. (2021)I3995VCentral domainSyncope, SCD, aSCDD4112NCentral domainSCD, aSCDT4196ICentral domainSyncope, SCD, aSCDD4646A*Transmembrane domainSCD, aSCDQ4879HTransmembrane domainaSCDI2075T/K4594RHandle domain and transmembrane domainSeizures, SCD, aSCD, IVF[(Paech et al., 2014)]G3118RHelical domainSCA, SCD, aSCD, VFShauer et al. (2021)E4146KCentral domainSCA, SCD(Zhong et al., 2021; Tester et al., 2012)]G4935RC-terminal domainSeizures, SCDD3291VHelical domainSyncope, SCD, aSCDBlancard et al. (2021)G570DN-terminal domainSCA, SCDLi et al. (2021)Q3925ECentral domainSCD[(Medeiros-Domingo et al., 2009)]M4109RCentral domainSCA, SCD, VF[(Nof et al., 2011)]R4147KCentral domainSyncope, SCA, SCDA4203VCentral domainSeizures, IVFA4204VCentral domainSCA, VF[(Kron et al., 2015)]A4142TCentral domainSCD, aSCDOrmerod et al. (2022)Q2275HHelical domainSyncope, SCA, SVTRoston et al. (2022)E4451delCentral domainSCD, IRBBBF4499CTransmembrane domainSyncope, SCA, AFV4606ETransmembrane domainSyncope, seizures, SVT, AT, HF, DCMR4608QTransmembrane domainPalpitations, syncope, SCA, SCDR4608WTransmembrane domainSCAE4146DCentral domainSyncope, LQTS, scPVC, VFHirose et al. (2022)S4168PCentral domainLQTS, bradycardiaK4594QTransmembrane domainSyncope, LQTSS4938FC-terminal domainSyncope, scTdP, VF due to PVCs[(Fujii et al., 2017)]Y4591TerC-terminal truncatingSCA, NSVTTian et al. (2023)R4663TerC-terminal truncatingSeizures, cognitive impairmentN4717 + 15TerC-terminal truncatingSCD, syncope, bradycardiaR4790TerC-terminal truncatingSCA, syncopeHopton et al. (2022)K3311fsFrameshift mutationVAsClinVar ID: 3065287

Clinical features and genetic analysis of CRDS RyR2 LOF cases. The literature cited in square brackets indicates the origin of the cases, which were subsequently confirmed as CRDS RyR2 LOF variants in later studies.

*Mutations for which mouse models have been generated.

AF, atrial fibrillation; aSCD, aborted sudden cardiac death; AT, atrial tachycardia; cIVF, catecholaminergic idiopathic ventricular fibrillation; DCM, dilated cardiomyopathy; IRBBB, incomplete right bundle branch block; IVF, idiopathic ventricular fibrillation; LQTS, long-QT, syndrome; LVNC, left ventricular non-compaction; NSVT, non-sustained Ventricular Tachycardia; PVC, premature ventricular complex. RYR2-DUP, RYR2 homozygous multiexon duplication. SCA, sudden cardiac arrest; SCD, sudden cardiac death; scPVC, short coupled premature ventricular contract. scTdP, short-coupled variant of torsades de pointes. SVT, supraventricular tachycardia; VA, ventricular arrhythmia; VF, ventricular fibrillation.

The identified CRDS mutant variants and their localization of mutation sites on RyR2. The structural domain division of RyR2 is shown on the left of the number axis, and the reported CRDS RyR2 LOF variants are displayed on the right. Orange circles indicate hotspots clusters of RyR2 mutations.

EST for CRDS pedigree members, whether the probands or their affected relatives, showed normal outcomes or merely isolated PVCs (Li et al., 2021; Sun et al., 2021; Ormerod et al., 2022; Roston et al., 2022). In addition, there were no abnormalities in the cardiac structure or ejection fraction of patients with CRDS (Sun et al., 2021; Roston et al., 2022). Given the small number of cases associated with CRDS and the heterogeneity of presentation across patients (Hirose et al., 2022; Roston et al., 2022), larger clinical studies are urgently needed to provide better treatment and management strategies for patients with CRDS. Furthermore, prognostic assessment and follow-up of patients with CRDS are crucial for timely detection and management of possible arrhythmias and other complications.

3 CRDS and CPVT: a systematic comparison

The most classic RyR2-ryanodinopathy is CPVT, characterized by polymorphic VT induced by excitation of the sympathoadrenal system. The majority of CPVT patients experienced syncope or cardiac arrest induced by exercise or emotional stress with onset in childhood or adolescence. CPVT phenotypes, however, were seldom inducible by programmed fast pacing electrical stimulation during electrophysiological examination, suggesting that this arrhythmia is dependent on catecholamines rather than accelerated heart rates alone (Chang et al., 2025).

EST, typically performed using a bicycle ergometer or treadmill, is a well-established tool for detecting exercise-induced arrhythmias. In the context of ryanodinopathy, EST plays a fundamental diagnostic role: a key criterion for CPVT is the reproducible induction of bidirectional or polymorphic VT during EST (Chang et al., 2025). However, the EST results of CRDS patients are mostly negative, meaning that EST cannot reliably induce arrhythmias in individuals with CRDS. Another difference from CPVT patients is the lack of a significant effect of adrenergic stimulation on inducing arrhythmic episodes in CRDS individuals (Roston et al., 2022; Zheng et al., 2022), which may be attributed to the LOF nature of RyR2 mutations, thereby attenuating the response to adrenergic stimulation.

The genetic basis of type 1 CPVT has been established as RyR2 GOF mutations, and the underlying arrhythmogenic mechanisms have been extensively studied, forming a well-established theoretical framework. RyR2 GOF mutations increase the propensity for spontaneous Ca2+ release from the SR during diastole in cardiomyocytes, activating the Na+/Ca2+ exchanger (NCX) to clear cytosolic Ca2+. This generates an inward current, thereby inducing delayed afterdepolarizations (DADs). Once DADs reach the threshold potential, they can trigger new action potentials, leading to triggered activity that ultimately progresses to malignant ventricular arrhythmias (VAs). Therefore, DADs induced by spontaneous Ca2+ release are recognized as the primary cellular electrophysiological basis for arrhythmogenesis in CPVT (Priori and Chen, 2011).

In terms of channel function, CRDS RyR2 LOF mutations suppressed the propensity for spontaneous and caffeine-induced Ca2+ release, reduced the sensitivity of RyR2 channels to Ca2+ concentration, but conversely facilitated Ca2+ alternans and prolonged the refractoriness of Ca2+ transients in intact hearts (Jiang et al., 2007; Zhong et al., 2016; 2021; Sun et al., 2021; Ormerod et al., 2022). For a comparison of CPVT and CRDS, see Table 2. The molecular pathogenic mechanisms of CRDS will be discussed in detail in Section 4.

FeatureCPVTCRDSClinical triggers• Exercise or emotional
stress (sympathoadrenal
activation)
• Reproducibly triggers
syncope or cardiac
arrest, often in
childhood/adolescence• Not clearly associated
with adrenergic
stimulation
• Patients may present
with SCD or aborted
SCD without typical
exercise-induced
syncopeEST and ECG findings• EST reliably induces
bidirectional or
polymorphic ventricular
VT
• Hallmark ECG feature
during EST is reproducible
VT• EST mostly negative
(normal or only
isolated PVCs)
• No reproducible
induction of complex
arrhythmiasMolecular mechanismRyR2 GOF mutations (CPVT1)
→ increased propensity for
diastolic spontaneous
Ca2+ release from SR
→ activation of NCX →
inward current →DADs →
triggered activity → VTRyR2 LOF mutations
→ reduced spontaneous
and caffeine-induced
Ca2+ release,
decreased Ca2+
sensitivity of RyR2,
but enhanced Ca2+
alternans propensity

Comparison between CRDS and CPVT.

EST, exercise stress test; SCD, sudden cardiac death; VT, ventricular tachycardia; DAD, delayed afterdepolarization.

4 Molecular pathogenic mechanisms of CRDS

The occurrence of CRDS is attributed to RyR2 LOF mutations, which reduce Ca2+ release from the SR during systole. This primary defect in RyR2 function serves as the fundamental basis for all subsequent alterations in membrane potential and ion currents discussed in this section. Every change in membrane potential and ion flux ultimately stems from, and is secondary to, the impaired RyR2-mediated Ca2+ release. The diminished Ca2+ release has two major consequences. First, it weakens Ca2+-dependent inactivation (CDI) of LTCC. Under normal conditions, the systolic Ca2+ release triggers CDI, which helps terminate ICaL. In CRDS, the reduced Ca2+ release relieves this negative feedback, leading to increased ICaL amplitude (Sun et al., 2021). The enhanced ICaL prolongs the action potential duration (APD) and creates a substrate for early afterdepolarizations (EADs). Second, the impaired Ca2+ release function of RyR2 leads to an excessive Ca2+ storage in the SR. When the activation threshold for store-overload-induced Ca2+ release (SOICR) is reached, the RyR2s open spontaneously and collectively, bringing about a drastic Ca2+ release within a single heartbeat. Under these circumstances, NCX engages in the removal of excess cytosolic Ca2+, further elevating membrane potential and elongating APD (Jiang et al., 2007; Zhao et al., 2015). It is important to distinguish these baseline alterations from triggered events. The large Ca2+ transients that precede VAs are typically provoked by specific challenges, which cause transient SR Ca2+ overload, as discussed in Section 5. In summary, EAD is the initial anomalous electrophysiological event in CRDS.

4.1 Ca2+ alternans and arrhythmogenesis

Ca2+ alternans can be taken to refer to abnormal, periodic alternations in intracellular Ca2+ concentration within cardiac myocytes throughout the cardiac cycle, which serves not merely as an indicator of dysregulated Ca2+ handling, but also as a harbinger of impending heart disease (Qu and Weiss, 2023). The emergence of Ca2+ alternans is robustly linked to serious pathological conditions, such as heart failure and myocardial ischemia (Varró et al., 2021). Functional studies have shown an increased tendency for Ca2+ alternans in intact hearts expressing RyR2 LOF mutations compared to RyR2 WT (Zhong et al., 2016; Sun et al., 2021). By promoting the instability of membrane potential during repolarization and disrupting the normal electrical activity of cardiomyocytes, Ca2+ alternans can trigger EADs and, subsequently, arrhythmias.

EADs are transient voltage oscillations that occur during the plateau or repolarization phase of the action potential, before full repolarization is achieved. When EADs reach threshold potential, they can trigger premature action potentials, giving rise to triggered activity, a focal arrhythmia mechanism (Wit, 2018). While EADs themselves serve as triggers, the progression to sustained arrhythmias such as VT or ventricular fibrillation (VF) often requires a vulnerable tissue substrate. For instance, spatial heterogeneity in repolarization created by EAD-mediated APD prolongation can establish conduction blocks and reentrant circuits (Stein et al., 2025). Thus, in the setting of CRDS, EADs may initiate premature beats that, in the presence of a susceptible substrate, degenerate into life-threatening arrhythmias and SCD.

4.2 L-type Ca2+ channel

In CRDS, arrhythmogenesis stems from malfunctioning Ca2+ fluxes due to RyR2 LOF mutations (Steinberg et al., 2023). Owing to CRDS RyR2 LOF mutations followed by reduced SR Ca2+ release, cytosolic Ca2+ concentration becomes inadequate compared to that in RyR2 WT myocytes. As a consequence, LTCC is required to undertake a greater amount of Ca2+ influx to ensure calcium homeostasis and effective cardiac contractile function, thereby prolonging the action potential duration and increasing electrical instability during repolarization (Jiang et al., 2007; Zhao et al., 2015). However, the expression of LTCC (Cav1.2) is not significantly altered in RyR2 LOF variants. Functional studies have shown that ICaL current density is enhanced in RyR2 LOF cardiomyocytes (Sun et al., 2021), contributing to action potential prolongation and arrhythmogenesis, although the efficiency of ECC remains reduced due to impaired RyR2-mediated Ca2+ release (Zhao et al., 2015). The prolonged action potential duration predisposes the cells to phase-2 EADs, which may propagate into triggered activity upon termination of the effective refractory period (ERP) (Figure 3). It is worth noting that the role of LTCC can only partially compensate for the lack of intracellular Ca2+ release. In severe cases of RyR2 dysfunction, the amount of Ca2+ inflow via LTCC cannot meet the physiological demand for contraction, which is a precursor of mutation-induced heart failure (Belevych et al., 2013).

Changes in the action potential of cardiomyocytes in CRDS RyR2 LOF conditions. The black curve shows the physiological APD of cardiomyocytes, while the red curve depicts the alterations in membrane potential attributed to CRDS RyR2 LOF. The orange regions show the enhanced process, and the blue regions indicate the occurrence of EADs. ERP, effective refractory period. Ito, transient outward potassium current. ICaL, L-type calcium current. INCX, sodium-calcium exchange current. EAD, early afterdepolarization.

4.3 Na+/Ca2+ exchanger

Also of interest is the pivotal role of NCX in calcium homeostasis. The dual-directional-operational NCX serves as an efficient Ca2+ extrusion system in cardiac myocytes. Returning to the source of RyR, ryanodine alkaloid is capable of locking the RyR channels in a sub-conductance state, resulting in sluggish and extended Ca2+ leakage from the SR, ultimately leading to macroscopic muscle paralysis. Curiously, ryanodine alkaloid elicits rigid paralysis in skeletal muscles, whereas cardiac muscles exhibit flaccid paralysis in response to the compound (Fill and Copello, 2002). This contrasting response highlights the unique Ca2+ handling strategy in the heart. The excessive Ca2+ leak via RyR after exposure to ryanodine alkaloid exceeds the reuptake capacity of SERCA, resulting in the accumulation of cytosolic Ca2+, which is the cause of rigid paralysis in skeletal muscles. In contrast, the presence of NCX in cardiomyocytes has tremendous potential to remove Ca2+ from the intracellular to the extracellular side, and therefore the calcium pool (referring to SR luminal [Ca2+]) will be exhausted over time. As a result, cardiac muscles are trapped in flaccid paralysis, since there is not enough intracellular Ca2+ ([Ca2+]i) available for systole anymore (Scranton et al., 2024).

Although CRDS inhibits SOICR and increases activation threshold for spontaneous Ca2+ release, there is no significant change in SR Ca2+ store capacity (Li et al., 2021; Sun et al., 2021; Ormerod et al., 2022). In other words, the initiation of RyR2-mediated Ca2+ release is simply more difficult rather than eliminated under CRDS. Paradoxically, when SOICR does occur, the magnitude of Ca2+ release becomes more pronounced due to an increased difference between the activation and termination thresholds of SOICR, which is elevated in CRDS-associated mutants (Ormerod et al., 2022). This increased difference between the activation and termination thresholds means that, when SOICR is triggered spontaneously, the resulting release event is more robust, producing larger and more widespread cytosolic Ca2+ elevations. These stochastic Ca2+ release events can activate NCX and generate depolarizing currents, potentially contributing to arrhythmic risk. To maintain Ca2+ electrochemical equilibrium, NCX operates in forward mode (3 Na+ influx for 1 Ca2+ efflux) at this moment. Similar to the multiple effects of LTCC, the inward current from NCX activation (INCX) also leads to both membrane depolarization and APD prolongation. In CRDS, these systolic NCX activities increase the risk of phase-3 EADs, as evidenced by observable lengthening of APD90 (APD at 90% repolarization) (Sun et al., 2021). This mechanism is fundamentally different from that underlying CPVT, where diastolic Ca2+ waves activate NCX to generate DADs.

Taken together, although LTCC and NCX mediate Ca2+ fluxes in opposite directions, both generate inward currents (ICaL and INCX) from the plateau phase onward, thereby increasing the net inward current and promoting a substrate for EADs (Kettlewell et al., 2019; Lotteau et al., 2021). Furthermore, given that EADs occur during a period when membrane potential is not fully repolarized, ion channels other than mediating Ca2+ flux, particularly voltage-gated Na+ and K+ channels subjected to electrophysiological remodeling in CRDS, may also influence the substrate for EADs (Figure 3).

4.4 Na+ and K+ channels

Electrophysiological studies have shown variations in the Na+ current (INa) and the transient outward K+ current (Ito) in RyR2-D4646A+/− variants of a confirmed CRDS genotype, compared to those observed in RyR2 WT. Specifically, Ito density of RyR2-D4646A+/− cardiomyocytes was increased, and INa exhibited a hyperpolarizing shift in its voltage-dependent activation (i.e., activation at more negative potentials) (Sun et al., 2021). Although this shift does not imply earlier activation during a normal action potential, it may modulate INa availability during the upstroke. Together with the enhanced Ito, these changes promote faster early repolarization, leading to a shortened APD50. The abbreviated APD50 may modestly accelerate Na+ channel recovery from inactivation, potentially contributing to the observed reduction in the ERP/APD ratio, although this effect is likely minor compared to other electrophysiological changes in CRDS. In summary, the increase in ICaL and INCX activity is responsible for the prolongation of APD90, while the shortening of APD50 (APD at 50% repolarization) observed in CRDS models is associated with alterations in Ito and INa, though the precise contribution of INa remodeling to early repolarizat