Provocation Testing and Therapeutic Response in a Newly Described Channelopathy: RyR2 Calcium Release Deficiency Syndrome

Abstract

Background: A novel familial arrhythmia syndrome, RyR2 calcium release deficiency syndrome (CRDS), has recently been described. We evaluated a large and well characterized family to assess provocation testing, risk factor stratification and response to therapy in CRDS. Methods: We present a family with multiple unheralded sudden cardiac deaths and aborted cardiac arrests, primarily in children and young adults, with no clear phenotype on standard clinical testing. Results: Genetic analysis, including whole genome sequencing, firmly established that a missense mutation in RYR2 , Ala4142Thr, was the underlying cause of disease in the family. Functional study of the variant in a cell model showed RyR2 loss-of-function, indicating that the family was affected by CRDS. EPS (Electrophysiological Study) was undertaken in 9 subjects known to carry the mutation, including a survivor of aborted sudden cardiac death, and the effects of flecainide alone and in combination with metoprolol were tested. There was a clear gradation in inducibility of nonsustained and sustained ventricular arrhythmia between subjects at EPS, with the survivor of aborted sudden cardiac death being the most inducible subject. Administration of flecainide substantially reduced arrhythmia inducibility in this subject and abolished arrhythmia in all others. Finally, the effects of additional metoprolol were tested; it increased inducibility in 4/9 subjects. Conclusions: The Ala4142Thr mutation of RYR2 causes the novel heritable arrhythmia syndrome CRDS, which is characterized by familial sudden death in the absence of prior symptoms or a recognizable phenotype on ambulatory monitoring or exercise stress testing. We increase the experience of a specific EPS protocol in human subjects and show that it is helpful in establishing the clinical status of gene carriers, with potential utility for risk stratification. Our data provide evidence that flecainide is protective in human subjects with CRDS, consistent with the effect previously shown in a mouse model.

How does flecainide impact RyR2 channel function?

“…They should perhaps be considered a separate familial RyR2-dependent, noncatecholaminergic arrhythmic syndrome. Indeed, many LOF mutants linked to sudden cardiac death or aborted cardiac death do not respond to exercise stress testing or produce the characteristic bidirectional ventricular tachycardia, hallmarks of CPVT ( Sun et al, 2021 ; Zhong et al, 2021 ; Ormerod et al, 2021 ). Instead LOF mutations lead to a CRDS, in which the general effect of the mutations on RyR2 is to reduce or abolish CICR indicated in [ 3 H] ryanodine binding and single channel experiments ( Zhao et al, 2015a ; Zhao et al, 2015b ; Jiang et al, 2007 ; Hirose et al, 2021 ).…”

“…At the cellular level, cells expressing these mutations lack detectable Ca 2+ oscillations and display a reduced sensitivity to caffeine and store-overload–induced Ca 2+ release (SOICR), suggesting they may not respond as fully to β-adrenergic stimulation ( Ormerod et al, 2021 ; Sun et al, 2021 ; Hirose et al, 2021 ; Zhao et al, 2015b ; Fujii et al, 2017 ). SR Ca 2+ content may be increased because of the lack of SOICR.…”

“…The same study also revealed a novel higher affinity flecainide-mediated increase of isolated RyR2 channel activity, raising the possibility that RyR2 might be involved in the proarrhythmic actions of the drug. Contrastingly, the activating effect might also contribute to the counterintuitive action of the drug in protecting against arrhythmia in loss-of-function (LOF) RyR2 mutants associated with calcium release deficient syndrome (CRDS) seen in a murine model and in patients ( Sun et al, 2021 ; Ormerod et al, 2021 ; Hirose et al, 2021 ). These varying anti- and proarrhythmic consequences underscore the importance of understanding the molecular mechanisms of flecainide’s actions, in order to maximize its therapeutic potential, while limiting potentially fatal consequences.…”

The Antiarrhythmic Mechanisms of Flecainide in Catecholaminergic Polymorphic Ventricular Tachycardia

“…It is logical to infer that flecainide might exacerbate the CRDS phenotype if it can inhibit RyR2. To date, flecainide has proven to be a promising therapeutic agent for CRDS ( Tester et al, 2020 ; Ormerod et al, 2021 ). The programmed electrical stimulation protocol with a pattern of long-burst, long-pause, and short-coupled (LBLPS) can induce ventricular arrhythmias in transgenic mice with RyR2 LOF mutations.…”

“…In the induced pluripotent stem cell cardiomyocytes carrying homozygous RYR2 duplication, which presented LOF, Tester et al (2020) reported that flecainide significantly reduced arrhythmic activity caused by isopropanol. Ormerod et al (2021) tested flecainide in nine CRDS patients and found that the administration of flecainide substantially reduced arrhythmia inducibility in one subject and abolished arrhythmia in all others. Sun et al (2021) proposed that the therapeutic mechanisms of flecainide in CRDS are attributable to its multiple blocking of membrane channels.…”