RYR2-ryanodinopathies: from calcium overload to calcium deficiency

Abstract

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.

Keywords: Calcium-release deficiency syndrome; Cardiac ryanodinopathy; Catecholaminergic polymorphic ventricular tachycardia; RYR2; RYR2 exon-3 deletion syndrome.

Graphical abstract

Figure 1

Cardiac ryanodinopathies. CPVT, catecholaminergic polymorphic ventricular tachycardia; CRDS, calcium release deficiency syndrome; E3DS, RYR2 exon-3 deletion syndrome.

Figure 2

RyR2 molecule. (A) Three-dimensional structure of RyR2 (Protein Data Bank identification code 6JI0). (B) Regulation of the RyR2 complex. CaM, calmodulin; CaMKII, Ca²⁺/calmodulin-dependent protein kinase II; FKBP, FK506-binding protein; mAKAP, muscle-specific A kinase–anchoring protein; PKA, protein kinase A; PM, plasma membrane; PP1, protein phosphatase 1; PP2A, protein phosphatase 2A; PR130, regulatory subunit of PP2A. (C) Schematic organization of the linear sequence of RyR2 with major structural domains of RyR2 (blue boxes). Orange boxes indicate four disease-associated variant clusters (variant hotspots). Adapted with permission from Zhong et al.

Figure 3

Catecholaminergic polymorphic ventricular tachycardia phenotype. (A) Exercise treatment test of a 32-year-old female patient with RYR2-related CPVT. Inducible ventricular ectopy with monomorphic bigeminal PVCs at Stage 2 (4:22 min) and a heart rate of 134 b.p.m. (B) Same patient as (A): at Stage 3 (6:11 min), intermittent manifestation of bidirectional PVCs. (C) 67-year-old male with RYR2-related CPVT. Run of non-sustained polymorphic ventricular tachycardia at peak Stage 4 of an exercise treadmill test (11:00 min). (D) Shown are intermittent runs of non-sustained bidirectional ventricular tachycardia.

Central Illustration

Proposed mechanisms for RyR2-associated catecholaminergic polymorphic ventricular tachycardia (CPVT), exon 3 deletion syndrome (E3DS), and calcium release deficiency syndrome (CRDS). The different thresholds for store overload-induced Ca²⁺ release (SOICR) and Ca²⁺ release termination and the free sarcoplasmic reticulum (SR) luminal Ca²⁺ levels in CPVT, E3DS, or CRDS associated with RyR2 mutations are illustrated in the resting state (Rest, left panels) and in the stress states (Stress, right panels). The normal thresholds for SOICR and Ca²⁺ release termination are depicted as red and yellow dashed bars, respectively. The reduced or elevated SOICR thresholds as a consequence of CPVT, E3DS, or CRDS RyR2 mutations are depicted as solid red bars. The reduced threshold for Ca²⁺ release termination as a consequence of E3DS RyR2 mutations are depicted as solid yellow bars. The SR free luminal Ca²⁺ level is represented as a blue area. The yellow areas above the blue areas in the right panels represent an elevation, even if only transient, in the free SR luminal Ca²⁺ levels, which, we propose, will occur when sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase (SERCA) activity is enhanced by catecholamines or during the long-burst, long-pause, short-coupled (LBLPS) programed electrical stimulation. When the SR-free luminal Ca²⁺ level surpasses, even transiently, the reduced SOICR threshold in the case of CPVT and E3DS, SOICR occurs, leading to a spillover of SR Ca²⁺ that can trigger spontaneous Ca²⁺ release, delayed afterdepolarizations (DADs) and ventricular arrhythmias (VAs). The reduced termination threshold in the case of E3DS will increase the fractional Ca²⁺ release, resulting in large Ca²⁺ transients at rest (left panels) and stress (right panels) that may promote cardiomyopathies in addition to cardiac arrhythmia. In the case of CRDS, the elevated SOICR threshold prevents spontaneous SR Ca²⁺ leak, leading to markedly elevated SR Ca²⁺ load upon LBLPS electrical stimulation and subsequently large Ca²⁺ transients that promote early afterdepolarizations (EADs), reentrant activity, and ventricular arrhythmias.

Figure 4

Exon 3 deletion syndrome clinical phenotype. (A) Alu repeat–mediated RYR2 exon 3 deletion. The diagram represents the Alu-Alu recombination. Alu sequences are located in intron 2, 190 bp upstream from exon 3 and also 536 bp downstream in intron 3. Adapted with permission from Bhuiyan et al. (B) 34-year-old female patient with E3DS. Resting ECG showing marked junctional bradycardia at 42 b.p.m. The patient had symptomatic sinus node disease with chronotropic insufficiency. (C) 48-year-old female patient with E3DS. Exercise treadmill testing showed inducible polymorphic ventricular ectopy with intermittent bidirectional ventricular PVCs (arrows). (D) Same patient as (C). Induction of supraventricular tachycardia at peak exercise. The echocardiogram showed a normal sized left ventricular with preserved ejection fraction. However, there was evidence of marked apical and apico-lateral trabeculation meeting criteria for LVNC. (E) and (F) Cardiac magnetic resonance imaging of a 52-year-old female patient with E3DS. The patient developed dilated cardiomyopathy with LVNC and marked systolic dysfunction requiring the insertion of a cardiac resynchronization therapy defibrillator. Note the marked left ventricular apical and apico-lateral hypertrabeculations (arrows).

Figure 5

Exon 3 deletion syndrome phenotype diversity. Shown is the pedigree of a large family with RYR2 exon-3 deletion syndrome; [-] indicates gene-negative individuals; filled circles or squares indicates heterozygous carriers with E3DS phenotype; AF, atrial fibrillation; CRT-D, cardiac resynchronization therapy with implantable cardioverter defibrillator; CRT-P, biventricular pacemaker; DCM–LCNV, dilated cardiomyopathy with left ventricular non-compaction; SCD, sudden cardiac death.

Figure 6

Ventricular arrhythmia induction in CRDS. (A) Ventricular fibrillation induction in CRDS using programmed ventricular stimulation. Shown are the ECG tracings of a CRDS proband carrying the RYR2-T4196I LOF variant. Ventricular arrhythmia is reproducibly precipitated using the LBLPS stimulation protocol (adapted with permission from Sun et al.). (B) Spontaneous VF initiation in CRDS. Implantable cardioverter defibrillator tracings from a CRDS proband harbouring a RYR2 LOF variant. Shown is an episode of ventricular fibrillation preceded by sinus tachycardia. Ventricular tachycardia is then initiated by a sequence of two premature ventricular complexes, a long pause, a sinus beat, and a shorter coupled PVC (adapted with permission from Roston et al.). (C) Effects of quinidine and flecainide on VA in D4646A +/− mutant mice. The tracing in the top row displays typical VF induction using a LBLPS stimulation (mice treated with H₂O control). Tracings from the second and third row demonstrate a dose-dependent (10 mg/kg vs. 40 mg/kg per day) suppression of inducible ventricular arrhythmia after pretreatment with quinidine sulphate for 6 days. Similar effects on VA inducibility were also observed in mice after pretreatment with different doses (4 mg/kg vs. 20 mg/kg per day) of flecainide for 6 days. LBLPS, long-burst, long-pause, and short-coupled ventricular extra stimulus; PVC, premature ventricular contraction; VA, ventricular arrhythmia; VF, ventricular fibrillation (adapted with permission from Sun et al.).