Clinical and Functional Characterization of Ryanodine Receptor 2 Variants Implicated in Calcium-Release Deficiency Syndrome

Importance

Calcium-release deficiency syndrome (CRDS), which is caused by loss-of-function variants in cardiac ryanodine receptor 2 (RyR2), is an emerging cause of ventricular fibrillation. However, the lack of complex polymorphic/bidirectional ventricular tachyarrhythmias during exercise stress testing (EST) may distinguish it from catecholaminergic polymorphic ventricular tachycardia (CPVT). Recently, in the first clinical series describing the condition, mouse and human studies showed that the long-burst, long-pause, short-coupled ventricular extra stimulus (LBLPS) electrophysiology protocol reliably induced CRDS ventricular arrhythmias. Data from larger populations with CRDS and its associated spectrum of disease are lacking.

Objective

To further insight into CRDS through international collaboration.

Design, Setting, and Participants

In this multicenter observational cohort study, probands with unexplained life-threatening arrhythmic events and an ultrarare RyR2 variant were identified. Variants were expressed in HEK293 cells and subjected to caffeine stimulation to determine their functional impact. Data were collected from September 1, 2012, to March 6, 2021, and analyzed from August 9, 2015, to March 6, 2021.

Main Outcomes and Measures

The functional association of RyR2 variants found in putative cases of CRDS and the associated clinical phenotype(s).

Results

Of 10 RyR2 variants found in 10 probands, 6 were loss-of-function, consistent with CRDS (p.E4451del, p.F4499C, p.V4606E, p.R4608Q, p.R4608W, and p.Q2275H) (in 4 [67%] male and 2 [33%] female probands; median age at presentation, 22 [IQR, 8-34] years). In 5 probands with a documented trigger, 3 were catecholamine driven. During EST, 3 probands with CRDS had no arrhythmias, 1 had a monomorphic couplet, and 2 could not undergo EST (deceased). Relatives of the decedents carrying the RyR2 variant did not have EST results consistent with CPVT. After screening 3 families, 13 relatives were diagnosed with CRDS, including 3 with previous arrhythmic events (23%). None had complex ventricular tachyarrhythmias during EST. Among the 19 confirmed cases with CRDS, 10 had at least 1 life-threatening event at presentation and/or during a median follow-up of 7 (IQR, 6-18) years. Two of the 3 device-detected ventricular fibrillation episodes were induced by a spontaneous LBLPS-like sequence. β-Blockers were used in 16 of 17 surviving patients (94%). Three of 16 individuals who were reportedly adherent to β-blocker therapy (19%) had breakthrough events.

Conclusions and Relevance

The results of this study suggest that calcium-release deficiency syndrome due to RyR2 loss-of-function variants mechanistically and phenotypically differs from CPVT. Ventricular fibrillation may be precipitated by a spontaneous LBLPS-like sequence of ectopy; however, CRDS remains difficult to recognize clinically. These data highlight the need for better diagnostic tools and treatments for this emerging condition.

An International Multicenter Evaluation of Inheritance Patterns, Arrhythmic Risks, and Underlying Mechanisms of CASQ2-Catecholaminergic Polymorphic Ventricular Tachycardia

BACKGROUND

Genetic variants in calsequestrin-2 (CASQ2) cause an autosomal recessive form of catecholaminergic polymorphic ventricular tachycardia (CPVT), although isolated reports have identified arrhythmic phenotypes among heterozygotes. Improved insight into the inheritance patterns, arrhythmic risks, and molecular mechanisms of CASQ2-CPVT was sought through an international multicenter collaboration.

METHODS

Genotype-phenotype segregation in CASQ2-CPVT families was assessed, and the impact of genotype on arrhythmic risk was evaluated using Cox regression models. Putative dominant CASQ2 missense variants and the established recessive CASQ2-p.R33Q variant were evaluated using oligomerization assays and their locations mapped to a recent CASQ2 filament structure.

RESULTS

A total of 112 individuals, including 36 CPVT probands (24 homozygotes/compound heterozygotes and 12 heterozygotes) and 76 family members possessing at least 1 presumed pathogenic CASQ2 variant, were identified. Among CASQ2 homozygotes and compound heterozygotes, clinical penetrance was 97.1% and 26 of 34 (76.5%) individuals had experienced a potentially fatal arrhythmic event with a median age of onset of 7 years (95% CI, 6-11). Fifty-one of 66 CASQ2 heterozygous family members had undergone clinical evaluation, and 17 of 51 (33.3%) met diagnostic criteria for CPVT. Relative to CASQ2 heterozygotes, CASQ2 homozygote/compound heterozygote genotype status in probands was associated with a 3.2-fold (95% CI, 1.3-8.0; P=0.013) increased hazard of a composite of cardiac syncope, aborted cardiac arrest, and sudden cardiac death, but a 38.8-fold (95% CI, 5.6-269.1; P<0.001) increased hazard in genotype-positive family members. In vitro turbidity assays revealed that p.R33Q and all 6 candidate dominant CASQ2 missense variants evaluated exhibited filamentation defects, but only p.R33Q convincingly failed to dimerize. Structural analysis revealed that 3 of these 6 putative dominant negative missense variants localized to an electronegative pocket considered critical for back-to-back binding of dimers.

CONCLUSIONS

This international multicenter study of CASQ2-CPVT redefines its heritability and confirms that pathogenic heterozygous CASQ2 variants may manifest with a CPVT phenotype, indicating a need to clinically screen these individuals. A dominant mode of inheritance appears intrinsic to certain missense variants because of their location and function within the CASQ2 filament structure.

Linking the heart and the brain: Neurodevelopmental disorders in patients with catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an uncommon inherited arrhythmia disorder characterized by adrenergically evoked ventricular arrhythmias. Mutations in the cardiac calcium release channel/ryanodine receptor gene (RYR2) are identified in the majority of patients with CPVT. RyR2 is also the major RyR isoform expressed in the brain.

OBJECTIVE

The purpose of this study was to estimate the prevalence of intellectual disability (ID) and other neurodevelopmental disorders (NDDs) in RYR2-associated CPVT (CPVT1) and to study the characteristics of these patients.

METHODS

We reviewed the medical records of all CPVT1 patients from 12 international centers and analyzed the characteristics of all CPVT1 patients with concomitant NDDs. We functionally characterized the mutations to assess their response to caffeine activation. We did not correct for potential confounders.

RESULTS

Among 421 CPVT1 patients, we identified 34 patients with ID (8%; 95% confidence interval 6%-11%). Median age at diagnosis was 9.3 years (interquartile range 7.0-14.5). Parents for 24 of 34 patients were available for genetic testing, and 13 of 24 (54%) had a de novo mutation. Severity of ID ranged from mild to severe and was accompanied by other NDDs in 9 patients (26%). Functionally, the ID-associated mutations showed a markedly enhanced response of RyR2 to activation by caffeine. Seventeen patients (50%) also had supraventricular arrhythmias. During median follow-up of 8.4 years (interquartile range 1.8-12.4), 15 patients (45%) experienced an arrhythmic event despite adequate therapy.

CONCLUSION

Our study indicates that ID is more prevalent among CPVT1 patients (8%) than in the general population (1%-3%). This subgroup of CPVT1 patients reveals a malignant cardiac phenotype with marked supraventricular and ventricular arrhythmias.