Abstracts

Rationale: Dravet Syndrome (DS) is a severe pediatric epileptic encephalopathy in which febrile seizures typically appear in the first 6-12 months of life. Patients with DS exhibit developmental delay, intellectual disability, and frequent drug resistant seizures. DS patients have a high risk of sudden unexpected death in epilepsy (SUDEP), widely believed to involve cardiac mechanisms. DS is most commonly linked to heterozygous variants in the gene SCN1A (>70%), encoding the voltage-gated sodium channel (VGSC) a subunit Na_V1.1, resulting in haploinsufficiency. Homozygous recessive loss-of-function variants in SCN1B, encoding VGSC ß1/ ß1B subunits, are associated with DS in a subset of patients. We have shown that Scn1b deletion in mice phenocopies the severe seizures and SUDEP observed in DS patients. In addition, Scn1b null ventricular cardiomyocytes (CMs) have increased peak I_Na and persistent I_Na (I_NaP), increased incidence of delayed afterdepolarizations (DADs), and disrupted intracellular Ca²⁺ homeostasis, with the development of ventricular arrhythmias. However, how Scn1b deletion results in cardiac arrhythmia is not understood. Methods: Here, we tested Scn1b null mouse ventricular CMs using whole cell patch clamp recording combined with confocal Ca²⁺ imaging to investigate cardiac arrhythmogenic mechanisms in DS. Results: Whole cell patch clamp recording combined with confocal Ca²⁺ imaging in single Scn1b null mouse ventricular CMs showed decreased cell capacitance and Ca²⁺ current (I_Ca); however, the amplitude of I_Ca triggered Ca²⁺ transient released by intracellular Ca²⁺ channel ryanodine receptors (RyRs) was unchanged. Thus, the excitation-contraction coupling gain, defined as the ratio of Ca²⁺ transient amplitude to I_Ca, was increased in null CMs compared to controls. Sarcoplasmic reticulum (SR) Ca²⁺ load, measured as the peak of Ca²⁺ transient amplitude induced by acutely perfused of 20 mM caffeine, was increased in Scn1b null mouse ventricular CMs.  Conclusions: These results suggest increased RyR Ca²⁺ release ability in Scn1b null mouse ventricular CMs, at least partially due to increased SR Ca²⁺ load. We propose that increased SR Ca²⁺ load, combined with increased I_NaP, predisposes Scn1b null mouse CMs to a substrate favorable for DADs, and in turn, arrhythmias. Our results predict that SCN1B-DS patients may have increased susceptibility to ventricular arrhythmias and thus may require cardiac care. Funding: R37 NS076752 (LLI) and U01 NS090364 (LLI and JMP)