Abnormal Ca²⁺ homeostasis, atrial arrhythmogenesis, and sinus node dysfunction in murine hearts modeling RyR2 modification

Ryanodine receptor type 2 (RyR2) mutations are implicated in catecholaminergic polymorphic ventricular tachycardia (CPVT) thought to result from altered myocyte Ca²⁺ homeostasis reflecting inappropriate "leakiness" of RyR2-Ca²⁺ release channels arising from increases in their basal activity, alterations in their phosphorylation, or defective interactions with other molecules or ions. The latter include calstabin, calsequestrin-2, Mg²⁺, and extraluminal or intraluminal Ca²⁺. Recent clinical studies additionally associate RyR2 abnormalities with atrial arrhythmias including atrial tachycardia (AT), fibrillation (AF), and standstill, and sinus node dysfunction (SND). Some RyR2 mutations associated with CPVT in mouse models also show such arrhythmias that similarly correlate with altered Ca²⁺ homeostasis. Some examples show evidence for increased Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2. A homozygotic RyR2-P2328S variant demonstrates potential arrhythmic substrate resulting from reduced conduction velocity (CV) in addition to delayed afterdepolarizations (DADs) and ectopic action potential (AP) firing. Finally, one model with an increased RyR2 activity in the sino-atrial node (SAN) shows decreased automaticity in the presence of Ca²⁺-dependent decreases in ICa, L and diastolic sarcoplasmic reticular (SR) Ca²⁺ depletion.