A CACNA1C Mutation that Causes a Subset of Timothy Syndrome Phenotypes Correlates

1745 appears to mimic important features seen in large mammals and may become a simple and reliable tool for pacing-related studies. LEADING REENTRY CIRCUIT(S) IN THE RIGHT ATRIAL FREE WALL UNDERLIES ADENOSINE-INDUCED ATRIAL FIBRILLATION IN CORONARY PERFUSED HUMAN HEARTS B.J. Hansen1, T.A. Csepe1, A. Kalyanasundaram1, G. Kamalov1, J. Zhao2, P.M.L. Janssen1, P.J. Mohler1, E.A. Davies1, R. Weiss1, V.V. Fedorov1 1 The Ohio State University, Columbus, OH, 2University of Auckland, Auckland, New Zealand. Background: Atrial fibrillation (AF) is the most common arrhythmia, yet the mechanism in human hearts remains to be elucidated. Clinical data connect adenosine to the induction of AF. We aimed to characterize the mechanism of adenosine-induced AF in explanted human hearts by using high-resolution optical and anatomic mapping. Methods: Explanted human hearts (n ¼ 12) with various chronic cardiac diseases were obtained immediately after cardioplegic arrest. Coronaryperfused atrial preparations of the right and left atrial appendages, free walls (RAFW, LAFW), intraatrial septum (IAS), and tricuspid and mitral valve rings were optically mapped from the epi- and/or endocardium simultaneously by two high-resolution CMOS cameras (100  100 pixels), positioned for whole atrial (95  95 mm) and specific (33  33 mm) fields of view. Activation, action potential duration (APD80%), and dominant frequency (DF) were analyzed during pacing and AF with correlation to atrial anatomy (histology). Results: At baseline, a stepwise pacing protocol found the functional refractory period (FRP) to be 252 ⫾ 80 ms, which was not short enough to induce AF. Adenosine (10–100 mM) led to spontaneous (n ¼ 1) and pacing-induced AF (n ¼ 5, 3/5 had clinical AF history) by shortening FRP to 183 ⫾ 61 ms (P ¼ .008 vs baseline) and heterogeneously shortening APD with maximum changes in RAFW. AF-susceptible hearts exhibited more pronounced RAFW APD shortening, from 244 ⫾ 55 ms to 177 ⫾ 45 vs nonsusceptible and from 294 ⫾ 37 ms to 252 ⫾ 38 ms (P ¼ .01) paced at 2 Hz. The principal mechanism of AF episodes (48 seconds) was a leading reentrant circuit(s) anchored to the pectinate muscles and interstitial fibrosis strands in the areas of shortest APD during adenosine treatment (superior and middle RAFW). The atrial regions outside the leading reentrant circuit(s) had slower activation due to intermittent failure to respond 1:1. As such, DF during AF was fastest in the RAFW (8.7 ⫾ 2.8 Hz) vs 4.1 ⫾ 0.8 in the IAS/LAFW (P ¼ .003). DF patterns remained similar during each AF episode in the same heart. Conclusions: This study demonstrates that in the human heart, adenosine primarily induces RAFW reentrant AF with leading reentrant circuits (rotors) anchored to the regions of shortest APD and anatomic architecture. A CACNA1C MUTATION THAT CAUSES A SUBSET OF TIMOTHY SYNDROME PHENOTYPES CORRELATES J.A. Hennessey1, Y. Jiang1, J.D. Miller2, H.A. Stadt1, W. Patrick1, R. Pfeiffer3, C. Antzelevitch3, R. Kanter1, G.S. Pitt1 1 Duke University, Durham, NC, 2Virginia Tech Carilion School of Medicine, Roanoke, VA, 3Masonic Medical Research Laboratory, Utica, NY. Background: Timothy syndrome (TS) is a rare congenital long QT syndrome (LQTS) associated with extracardiac manifestations including craniofacial dysmorphia and dental abnormalities. The locus for TS is CACNA1C, which encodes the CaV1.2 L-type Ca2þ channel, for which canonical mutations lead to a decrease in voltage-dependent inactivation (VDI). However, a recent report of a patient with LQTS in isolation and a CACNA1C mutation that did not affect VDI raised the question whether altered VDI is necessary for extracardiac phenotypes. In a patient with a maternally inherited microdeletion with a chromosomal translocation who presented with LQTS and associated ventricular tachyarrhythmias (Figure A), a subset of TS phenotypes, and a skeletal myopathy not readily explained by the translocation, we sought to identify a causative mutation for the TS phenotypes. Methods: A candidate gene approach identified a mutation in CACNA1C that was absent in the mother. We performed electrophysiologic studies on the mutant and characterized CACNA1C expression in skeletal muscle with a mouse CACNA1C reporter line. Results: We identified a glycine to arginine mutation at position 1911 (G1911R) in CaV1.2. Functional studies revealed that G1911R increased

CaV1.2 channel availability (Figures B and C) and decreased VDI (Figure D). The CACNA1C reporter mouse showed no CaV1.2 expression in skeletal muscle. Conclusions: We describe a CACNA1C mutation that leads to a subset of TS phenotypes. In the context of a recently described CACNA1C mutation that does not affect VDI in an LQTS patient without extracardiac phenotypes, these data suggest that the extracardiac phenotypes seen in TS require effects on VDI.

NOVEL PARAMETERS TO IMPROVE QUANTIFICATION OF REPOLARIZATION RESERVE AND ARRHYTHMOGENESIS USING A DOFETILIDE CHALLENGE T.R.G. Stams, S.K.G. Winckels, A. Oros, R. Varkevisser, P. Oosterhoff, R. van der Nagel, H.D.M. Beekman, M.A. Vos University Medical Center Utrecht, Utrecht, Netherlands. Background: In the canine complete AV block (AVB) model with bradycardia, inducibility of drug-induced torsades de pointes (TdP) is usually the primary outcome parameter, whereas short-term variability (STV) of the left ventricular monophasic action potential duration (LV MAPD) is used as primary surrogate parameter. The dichotomous parameter inducibility is cumbersome because it requires Z3 TdP episodes and does not take into account (multiple) ectopic beats or severity of TdP episodes. In this study, we introduce arrhythmia score (AS) and T25, using dofetilide (D) to improve quantification of repolarization reserve. Methods: Experiments performed under general anesthesia in unremodeled, acute AVB dogs (aAVB, n ¼ 13) and 3 groups of AVB dogs with chronic bradycardia but differences in remodeling due to alterations in chronic activation pattern were analyzed: spontaneous idioventricular rhythm (IVR, n ¼ 19), more physiologic high-septal pacing (HSP, n ¼ 10), and right ventricular apex pacing (RVA, n ¼ 8). AS was calculated as 1 þ mean number of beats of the 3 most severe arrhythmias after D (0.025 mg/kg/5 min), with scores of 50, 75, and 100, for 1, 2, and Z3 cardioversions, respectively. T25 was defined as the time required to obtain 25-ms increase of LV MAPD after D. Results: For groups aAVB, IVR, HSP and RVA, TdP inducibility was 0%, 74% (P o.001 vs aAVB), 30%, and 75% (P o.01 vs aAVB), respectively, with a similar pattern in AS: 3.8 ⫾ 6.5 (mean ⫾ SD), 41 ⫾ 28 (P o.001 vs aAVB), 14 ⫾ 22 (P o.05 vs IVR), and 28 ⫾ 24. Serial analysis in aAVB dogs revealed that AS was increased by D (from 1.3 ⫾ 0.4 to 4.3 ⫾ 7.4; n ¼ 10; P o.05), although 3 or more TdPs were not induced in any dog. T25 values were 2.1 ⫾ 0.4 minutes 1.4 ⫾ 0.4 minutes (P o.001 vs aAVB), 1.9 ⫾ 0.4 minutes (P o.01 vs IVR), and 1.5 ⫾ 0.3 minutes (P o.01 vs aAVB), respectively. Unexpectedly, STV before or after D was not significantly higher in any group vs aAVB. ”Predicting” inducibility after pooling all data