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(Abst. 2.052|A.08), 2015

A mutation in SCN8A results in arrhythmogenic events with altered intracellular calcium handling in a mouse model of early-infantile epileptic encephalopathy with SUDEP.
Authors: Chad R. Frasier, Jacy Wagnon, Yangyang Bao, Jack Parent, Miriam Meisler, Lori L. Isom
Content: Rationale: Early-infantile epileptic encephalopathies (EIEEs) associated with mutations in voltage-gated Na+ channels (VGSCs) are associated with increased risk for Sudden Unexpected Death in Epilepsy, or SUDEP. While the underlying mechanisms that ultimately lead to SUDEP remain unknown, growing evidence suggests that susceptibility to cardiac arrhythmias may play an important role. SCN8A, encoding the VGSC Nav1.6, is expressed in heart in addition to brain. In heart, Nav1.6 is preferentially localized at the T-tubules where it contributes to tetrodotoxin-sensitive (TTX-S) Na+ current (INa).Methods: We have generated a knock-in mouse model carrying a human SCN8A mutation from a patient with EIEE and SUDEP, SCN8AN1768D/+. Previous work in heterologous systems suggested that the N1768D mutation causes gain-of-function in both transient and persistent INa in neurons. To determine whether SCN8AN1768D/+ mice are susceptible to cardiac arrhythmias we performed in-vivo ECGs as well as experiments in isolated mouse ventricular myocytes.Results: In anesthetized mice, ECG recordings showed that SCN8AN1768D/+ mice have significantly reduced heart rate vs. littermate controls (P = 0.03; N ≥ 5 animals/group). Following intraperitoneal (i.p.) administration of 2 mg/kg norepinephrine, this difference in heart rate was no longer apparent. However, following i.p. administration of 120 mg/kg caffeine (to simulate a catecholaminergic surge), 3 of 5 SCN8AN1768D/+ mice, but 0 of 6 WT animals, showed ECG morphology similar to accelerated idioventricular rhythm. At the individual myocyte level, we observed increased incidence of delayed afterdepolarizations (8 of 10 SCN8AN1768D/+ myocytes vs. 1 of 8 WT myocytes). In addition, while the total action potential duration (APD) was not different between genotypes, the early repolarization phases of the AP (APD30 and APD50) were significantly prolonged in SCN8AN1768D/+ myocytes vs. WT. Given the important role of calcium during the early stages of the cardiac AP, we investigated if the calcium transient was altered in SCN8AN1768D/+ mice. Preliminary results show increased duration of the calcium in SCN8AN1768D/+ mice (P = 0.08). In addition, we observed diastolic calcium release events in a significantly larger proportion of SCN8AN1768D/+ myocytes (96% of SCN8AN1768D/+ myocytes vs. 46% of WT).Conclusions: Taken together, our results suggest that EIEE mutations in SCN8A may increase SUDEP risk by creating an environment in which the heart has an increased susceptibility to arrhythmias. We propose that increased susceptibility to cardiac arrhythmia in this model results from increased persistent INa at the T-tubules, with subsequent increases in calcium transient duration and elevated probability of diastolic calcium release and delayed afterdepolarizations. Supported by NIH grants T32HL007853 and UL1TR000433 to CRF, R01-NS-076752 to LLI, U01-NS-090364 to JP and LLI, and R01-NS-034509 to MHM.