The human CASQ2 mutation K206N is associated with hyperglycosylation and altered cellular calcium handling

Mutations in the human cardiac calsequestrin gene (CASQ2) are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT-2). This inherited disorder is characterized by life-threatening arrhythmias induced by physical and emotional stress in young patients. Here we identified a novel heterozygous missense mutation (K206N) in the CASQ2 gene in a symptomatic family in which one member died of cardiac arrest. The functional properties of CSQK206N were investigated in comparison to the wild-type form of CASQ2 (CSQWT) by expression in eukaryotic cell lines and neonatal mouse myocytes. The mutation created an additional N-glycosylation site resulting in a higher molecular weight form of the recombinant protein on immunoblots. The mutation reduced the Ca2+ binding capacity of the protein and exhibited an altered aggregation state. Consistently, CSQK206N-expressing myocytes exhibited an impaired response to caffeine administration, suggesting a lower Ca2+ load of the sarcoplasmic reticulum (SR). The interaction of the mutated CSQ with triadin and the protein levels of the ryanodine receptor were unchanged but the maximal specific [3H]ryanodine binding was increased in CSQK206N-expressing myocytes, suggesting a higher opening state of the SR Ca2+ release channel. Myocytes with expression of CSQK206N showed a higher rate of spontaneous SR Ca2+ releases under basal conditions and after β-adrenergic stimulation. We conclude that CSQK206N caused a reduced Ca2+ binding leading to an abnormal regulation of intracellular Ca2+ in myocytes. This may then contribute to the increased propensity to trigger spontaneous Ca2+ transients in CSQK206N-expressing myocytes.