Not all sudden death is the same.

Sudden death as a result of cardiac arrhythmia is probably the most common symptom associated with cardiac disease. It occurs not only in people with known cardiac disease, most notably congestive heart failure, but also in young, apparently healthy individuals who have no apparent structural heart disease. Frequently, in this latter group, these fatal arrhythmias are associated with exercise and increased -adrenergic stimulation. One possible mechanism for how these arrhythmias could occur in otherwise “normal” individuals is an aberrant release of Ca from the sarcoplasmic reticulum (SR), which in turn could cause delayed afterdepolarizations1 that can trigger potentially fatal ventricular arrhythmias. Unlike skeletal muscle, where excitation-contraction coupling (EC coupling) is intermittent and mediated through a mechanical coupling between the slow voltage-gated Ca channel (dihydropyridine receptor, DHPR) in the sarcolemma and the skeletal isoform of the large-conductance calcium release channel in the SR (ryanodine receptor, RyR), RyR1 in cardiac muscle EC coupling is rhythmic and the cardiac isoform of RyR (RyR2) is activated by the inward Ca influx through the cardiac DHPR via Ca -induced Ca release (CICR).2 In the heart, RyR2 does not act in a vacuum but rather is part of a macromolecular complex containing the immunophilin FKBP12.6, phosphorylases, and phosphatases,3 in addition to the DHPR and several other proteins including calsequestrin, triadin, junctin, and junctophilin, to name only a few, that make up the calcium release unit (CRU).4 Heart failure has been associated with disruption of this macromolecular complex secondary to hyperphosphorylation of RyR2 and the associated dissociation of FKBP12.6.5 Interestingly, mice that carry two null alleles for FKBP12.6 have been shown to have exerciseand catecholamineinduced fatal ventricular arrhythmias suggesting that this is a crucial subunit for controlling ventricular Ca homeostasis.6 Recently, 11 missense mutations of RyR2 and one missense mutation of calsequestrin have been associated with a group of closely associated cardiomyopathies that are characterized by early sudden death: catecholaminergic polymorphic ventricular tachycardia (CPVT), arrhythmogenic right ventricular cardiomyopathy (ARVD2), and familial polymorphic ventricular tachycardia.7–10 Interestingly, the RyR2 mutations associated with these cardiomyopathies are clustered in the same hot spots as the more than 50 missense mutations in RyR1 that are associated with malignant hyperthermia (MH) and central core disease (CCD).11,12 Like CPVT and ARVD2, MH individuals have normal muscle histology and have a “normal” phenotype until triggered by exposure to a triggering agent or stress. MH mutations are associated with a high resting free myoplasmic Ca , increased sensitivity to caffeine and halothane, reduced internal Ca stores, and a reduced sensitivity to Ca and Mg inhibition. This has led to the hypothesis that the cardiac RyR channelopathies are likely to result in an increased diastolic Ca , slowed relaxation after an action potential, and arrhythmogenic Ca waves. It has been previously shown by Jiang et al13 that, when studied in lipid bilayers, one CPVT RyR2 mutation (R4496C) expressed in HEK293 cells has an increased open probability at low (5 nmol/L) Ca concentrations. However, at normal and elevated Ca concentrations, there was no difference in the open probability between wt or mutant channel. Their findings, at Ca concentrations above 150 nmol/L, were confirmed by Wehrens et al,6 who studied three CPVT mutations (S2246L, R2474S, and R4497C) expressed in the same heterologous cell line. In the latter study, it was demonstrated that the CPVD RyR2s were more sensitive to protein kinase A (PKA) phosphorylation, and it was suggested, based on an in vitro binding study, that CPVT RyR2s had a lower affinity for FKBP12.6. In this issue of Circulation Research, George et al14 report for the first time the effects that the expression of CPVT RyR2s have on cardiac cells. Using HL-I cardiomyocytes transfected with wt or three CPVT (S2246L, N4104K, and R4497C) RyR2 cDNAs, they overexpressed RyR2 by 2-fold. This did not suppress native wtRyR2 expression, and thus the mix of RyRs expressed mimics the clinical heterozygous situation. Interestingly, they demonstrated that in unstimulated cells, beating frequency was not increased in CPVT RyR2-expressing cultures; furthermore, the endoplasmic reticulum Ca load was increased in all transfected cells, suggesting that, if CPVT RyR2s were leaky, there was a mechanism to completely compensate for such leakiness. As would be expected, they demonstrated that caffeine and 4-chloro-m-cresol (4CmC) sensitivity was shifted to the left in CPVT RyR2-expressing cells. They also showed that the Ca release amplitude was higher and relaxation time was longer in these cells after exposure to these direct RyR agonists. They did not show a decrease in the amount of FKBP12.6 associated with the membrane fractions of these cells. This finding would have been predicted by Tiso et al15 from yeast two-hybrid studies but is the opposite of what has been suggested from studies of some of the same heterologously expressed CPVT RyR2s.6 They did find that CPVT RyR2-expressing cells were more sensiThe opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Department of Anesthesia, Brigham and Women’s Hospital, Boston, Mass. Correspondence to P.D. Allen, MD, PhD, Professor of Anesthesia, Department of Anesthesia, Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115. E-mail [email protected] (Circ Res. 2003;93:484-486.) © 2003 American Heart Association, Inc.