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.

Abstract-Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disordercharacterized by life threatening arrhythmias elicited by physical and emotional stress in young individuals. The recessive form of CPVT is associated with mutation in the cardiac calsequestrin gene (CASQ2). We engineered and characterized a homozygous CASQ2 R33Q/R33Q mouse model that closely mimics the clinical phenotype of CPVT patients. CASQ2R33Q/R33Q mice develop bidirectional VT on exposure to environmental stress whereas CASQ2 R33Q/R33Q myocytes show reduction of the sarcoplasmic reticulum (SR) calcium content, adrenergically mediated delayed (DADs) and early (EADs) afterdepolarizations leading to triggered activity. Furthermore triadin, junctin, and CASQ2-R33Q proteins are significantly decreased in knock-in mice despite normal levels of mRNA, whereas the ryanodine receptor (RyR2), calreticulin, phospholamban, and SERCA2a-ATPase are not changed. Trypsin digestion studies show increased susceptibility to proteolysis of mutant CASQ2. Despite normal histology, CASQ2 R33Q/R33Q hearts display ultrastructural changes such as disarray of junctional electron-dense material, referable to CASQ2 polymers, dilatation of junctional SR, yet normal total SR volume. Based on the foregoings, we propose that the phenotype of the CASQ2 R33Q/R33Q CPVT mouse model is portrayed by an unexpected set of abnormalities including (1) reduced CASQ2 content, possibly attributable to increased degradation of CASQ2-R33Q, (2) reduction of SR calcium content, (3) dilatation of junctional SR, and (4) 4 The identification of the genes underlying CPVT has had implications that extend beyond those impacting clinical management of patients inasmuch as it stimulated fundamental research targeted to understand the links between intracellular calcium regulation and arrhythmogenesis. We recently developed 5 a knock-in mouse model carrying the R4496C RyR2 mutation identified in the first genotyped CPVT family and demonstrated that the RyR2 R4496C mice develop bidirectional and polymorphic VT similar to those observed in patients. In this model we also demonstrated 6 the occurrence of delayed after depolarizations (DADs) induced by adrenergic stimulation in isolated myocytes from the heart of heterozygous mice, suggesting that arrhythmias are elicited by triggered activity. Recently 2 mutants CASQ2 knock-in mice models were developed by Song et al 7 : the first strain carries the homozygous point mutation discovered by Lahat et al 3 in the first recessive CPVT family (D307H), and the second strain carries a homozygous deletion ⌬E9/⌬E9; in analogy with RyR2 mice, 5 both models develop bidirectional-polymorphic Materials and MethodsDetailed methods for mouse generation, electrophysiological measurements, immunofluorescence, real-time PCR, microarray, protein and electron microscopy analysis are reported in the online supplement (available online at http://circres.ahajournals.org). Generation of Knock-In of R33Q CASQ2 in Mouse ModelThe kno...

Sphingosylphosphocholine (SPC) modulates Ca2+ release from isolated cardiac sarcoplasmic reticulum membranes; 50 microM SPC induces the release of 70 80% of the accumulated calcium. SPC release calcium from cardiac sarcoplasmic reticulum through the ryanodine receptor, since the release is inhibited by the ryanodine receptor channel antagonists ryanodine. Ruthenium Red and sphingosine. In intact cardiac myocytes, even in the absence of extracellular calcium. SPC causes a rise in diastolic Ca2+, which is greatly reduced when the sarcoplasmic reticulum is depleted of Ca2+ by prior thapsigargin treatment. SPC action on the ryanodine receptor is Ca(2+)-dependent. SPC shifts to the left the Ca(2+)-dependence of [3H]ryanodine binding, but only at high pCa values, suggesting that SPC might increase the sensitivity to calcium of the Ca(2+)-induced Ca(2+)-release mechanism. At high calcium concentrations (pCa 4.0 or lower), where [3H]ryanodine binding is maximally stimulated, no effect of SPC is observed. We conclude that SPC releases calcium from cardiac sarcoplasmic reticulum membranes by activating the ryanodine receptor and possibly another intracellular Ca(2+)-release channel, the sphingolipid Ca(2+)-release-mediating protein of endoplasmic reticulum (SCaMPER) [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc.Natl.Acad.Sci. U.S.A 93, 1993-1996], which we have identified for the first time in cardiac tissue.