Physiological and pharmacological switches combine to uniquely modulate the most common cardiac sodium channel mutant, E1784K

The SCN5a gene encodes the cardiac voltage-gated sodium channel (NaV1.5) mainly expressed in cardiac muscle cells. The inward sodium current (INa) conducted by NaV1.5 triggers depolarization in the cardiac action potential. Mutations in SCN5a predominantly give rise to Long-QT syndrome 3 (LQT3), Brugada syndrome 1 (BrS1), and their overlapping phenotypes (mixed syndrome). The most common SCN5a mutation, expressed as E1784K in the NaV1.5 C-terminal domain (CTD), mainly displays LQT3 and sometimes mixed syndromes. E1784K causes mixed channel defects by decreasing the inward peak INa and increasing late INa, thought to underlie BrS1 and LQT3 pathogeneses, respectively. Very little is known, however, on how physiological and pharmacological switches modulate E1784K channel properties. These triggers may often govern phenotypes in SCN5a mutation carriers. The goal of my thesis is to study how exercise-related physiological triggers and pharmacological agents modulate E1784K ion channel properties. I used the whole-cell patch clamp technique to study elevated temperature, elevated cytosolic calcium, and their combined effects with ranolazine, on E1784K. Ranolazine is an antianginal drug with preferential selectivity for blocking late INa versus peak INa. My main results show that E1784K is uniquely altered by the triggers studied, compared to other NaV1.5 mutants: (1) Elevated temperature augments late INa in E1784K. (2) Elevated cytosolic calcium, which correlates with exercise-ameliorated LQT3, effectively blocks late INa in most NaV1.5 mutants. However, E1784K is resistant to the native calcium-induced block on late INa. (3) When temperature and cytosolic calcium are combined, they decrease ranolazine efficacy to suppress late INa in E1784K. The calcium-sensitivity in E1784K is clearly affected due to the mutant-induced instability in the CTD, which may cause a steric clash between the channel and ranolazine. To predict E1784K effects on arrhythmogenesis, I simulated a dynamic action potential model to account for the frequency-dependent elevations in cytosolic calcium. Alternans is observed at high heart rates in E1784K and is exacerbated by febrile temperatures and ranolazine. This work demonstrates the importance of personalized medicine since NaV1.5 mutants like E1784K display unique sensitivity to physiological triggers that potentially govern antiarrhythmic efficacy