Non-ventricular, clinical, and functional features of the RyR2R420Q mutation causing catecholaminergic polymorphic ventricular tachycardia

Introduction and objectives: Catecholaminergic polymorphic ventricular tachycardia is a malignant disease, due to mutations in proteins controlling Ca2+ homeostasis. While the phenotype is characterized by polymorphic ventricular arrhythmias under stress, supraventricular arrhythmias may occur and are not fully characterized. Methods: Twenty-five relatives from a Spanish family with several sudden deaths were evaluated with electrocardiogram, exercise testing, and optional epinephrine challenge. Selective RyR2 sequencing in an affected individual and cascade screening in the rest of the family was offered. The RyR2R420Q mutation was generated in HEK-293 cells using site-directed mutagenesis to conduct in vitro functional studies. Results: The exercise testing unmasked catecholaminergic polymorphic ventricular tachycardia in 8 relatives (sensitivity = 89%; positive predictive value = 100%; negative predictive value = 93%), all of them carrying the heterozygous RyR2R420Q mutation, which was also present in the proband and a young girl without exercise testing, a 91% penetrance at the end of the follow-up. Remarkably, sinus bradycardia, atrial and junctional arrhythmias, and/or giant post-effort U-waves were identified in patients. Upon permeabilization and in intact cells, the RyR2R420Q expressing cells showed a smaller peak of Ca2+ release than RyR2 wild-type cells. However, at physiologic intracellular Ca2+ concentration, equivalent to the diastolic cytosolic concentration, the RyR2R420Q released more Ca2+ and oscillated faster than RyR2 wild-type cells. Conclusions: The missense RyR2R420Q mutation was identified in the N-terminus of the RyR2 gene in this highly symptomatic family. Remarkably, this mutation is associated with sinus bradycardia, atrial and junctional arrhythmias, and giant U-waves. Collectively, functional heterologous expression studies suggest that the RyR2R420Q behaves as an aberrant channel, as a loss- or gain-of-function mutation depending on cytosolic intracellular Ca2+ concentration.