Background: Familial polymorphic ventricular tachycardia (FPVT) is characterized by exercise-induced arrhythmias and sudden cardiac death due to missense mutations in the cardiac ryanodine receptor (RyR2), an intracellular Ca2+ release channel required for excitation-contraction coupling in the heart.
Methods and results: Three RyR2 missense mutations, P2328S, Q4201R, and V4653F, which occur in Finnish families, result in similar mortality rates of approximately 33% by age 35 years and a threshold heart rate of 130 bpm, above which exercise induces ventricular arrhythmias. Exercise activates the sympathetic nervous system, increasing cardiac performance as part of the fight-or-flight stress response. We simulated the effects of exercise on mutant RyR2 channels using protein kinase A (PKA) phosphorylation. All 3 RyR2 mutations exhibited decreased binding of calstabin2 (FKBP12.6), a subunit that stabilizes the closed state of the channel. After PKA phosphorylation, FPVT-mutant RyR2 channels showed a significant gain-of-function defect consistent with leaky Ca2+ release channels and a significant rightward shift in the half-maximal inhibitory Mg2+ concentration (IC50). Treatment with the experimental drug JTV519 enhanced binding of calstabin2 to RyR2 and normalized channel function.
Conclusions: Sympathetic activation during exercise induces ventricular arrhythmias above a threshold heart rate in RyR2 mutation carriers. Simulating the downstream effects of the sympathetic activation by PKA phosphorylation of RyR2 channels containing these FPVT missense mutations produced a consistent gain-of-function defect. RyR2 function and calstabin2 depletion were rescued by JTV519, suggesting stabilization of the RyR2 channel complex may represent a molecular target for the treatment and prevention of exercise-induced arrhythmias and sudden death in these patients.