Abstract 184: Calsequestrin Mutation Causes Arrhythmia Related to the Intraluminal Ca2+ Transient Abnormality
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial arrhythmogenic disorder associated with mutations in the cardiac ryanodine receptor (RyR2) and cardiac calsequestrin (CASQ2) genes. To study the mechanism by which the CASQ2 mutations lead to the CPVT phenotype we generated a knock-in mouse model that carries D307H, a missense mutation identified in CPVT families. Mutant mice were subjected to exercise and in vivo EP studies (n=14). Calcium transients were analyzed in isolated adult ventricular cardiomyocytes (38 cells, 6 mice). All homozygous knockin (designated CASQ2307/307) mice had inducible ventricular arrhythmia with exercise or during epinephrine stimulation during EP studies. Arrhythmias included frequent premature ventricular beats non-sustained ventricular tachycardia (VT) and 3/6 mice had sustained VT with bidirectional pattern that is typically found in human CPVT. No wildtype or the heterozygous mice had exercise- or epinephrine-induced VT. Calcium transients studied during electrical pacing revealed normal baseline and peak Ca2+ levels but delayed Ca2+ reuptake, lower caffeine induced Ca2+ transients in the CASQ2307/ 307 cells compare to the WT. Epinephrine treatment in CASQ2307/307 cells produced significantly higher baseline and lower peak calcium levels, which resulted in substantially smaller calcium transients. Frequent spontaneous Ca2+ transients were also identified in epinephrine-treated CASQ2307/307 cells but not WT cells. We conclude that the D307H mutation resulted in abnormal Ca2+ reuptake into the SR and reduced the luminal Ca2+ content. While the luminal Ca2+ quantity in mutant cells was adequate to preserve the normal calcium transient at rest and to maintain the electrical-mechanical coupling, it was insufficient to produce higher Ca2+ release need in stress conditions, such as after catecholamine exposure. In these stressed conditionsCASQ2307/307 myocytes had rapid depletion of the SR Ca2+ and elevation of the cytosolic Ca2+, a scenario that can produce inappropriate Ca2+ transients and cardiac arrhythmia.