Clinical Phenotype and Functional Characterization of CASQ2 Mutations Associated With Catecholaminergic Polymorphic Ventricular Tachycardia

Abstract

Background— Four distinct mutations in the human cardiac calsequestrin gene ( CASQ2 ) have been linked to catecholaminergic polymorphic ventricular tachycardia (CPVT). The mechanisms leading to the clinical phenotype are still poorly understood because only 1 CASQ2 mutation has been characterized in vitro. Methods and Results— We identified a homozygous 16-bp deletion at position 339 to 354 leading to a frame shift and a stop codon after 5aa (CASQ2 ^G112+5X ) in a child with stress-induced ventricular tachycardia and cardiac arrest. The same deletion was also identified in association with a novel point mutation (CASQ2 ^L167H ) in a highly symptomatic CPVT child who is the first CPVT patient carrier of compound heterozygous CASQ2 mutations. We characterized in vitro the properties of CASQ2 mutants: CASQ2 ^G112+5X did not bind Ca ²⁺ , whereas CASQ2 ^L167H had normal calcium-binding properties. When expressed in rat myocytes, both mutants decreased the sarcoplasmic reticulum Ca ²⁺ -storing capacity and reduced the amplitude of I _Ca -induced Ca ²⁺ transients and of spontaneous Ca ²⁺ sparks in permeabilized myocytes. Exposure of myocytes to isoproterenol caused the development of delayed afterdepolarizations in CASQ2 ^G112+5X . Conclusions— CASQ2 ^L167H and CASQ2 ^G112+5X alter CASQ2 function in cardiac myocytes, which leads to reduction of active sarcoplasmic reticulum Ca ²⁺ release and calcium content. In addition, CASQ2 ^G112+5X displays altered calcium-binding properties and leads to delayed afterdepolarizations. We conclude that the 2 CASQ2 mutations identified in CPVT create distinct abnormalities that lead to abnormal intracellular calcium regulation, thus facilitating the development of tachyarrhythmias.