A Versatile Chemical-Genetic Approach to Determine Bases for Arrhythmogenesis and Sodium Channelopathies - Abstract The voltage-gated sodium channel NaV1.5 controls cardiac excitability and is an established therapeutic target. Mutations in the SCN5A gene, which encodes NaV1.5, are associated with inherited arrhythmia syndromes (long QT syndrome, Brugada syndrome, congenital heart block) and dilated cardiomyopathy. While gain of function mutations that disrupt NaV1.5 inactivation explain action potential duration (APD) and QTc prolongation, the mechanisms by which loss of function NaV1.5 mutations cause the other diverse pathogenic outcomes are unresolved. The physiological significance of other Na+ channel genes expressed in the heart are also uncertain. Rodent models with gene-targeted Scn5a mutations can recapitulate some clinical features of disease, but their use is complicated by compensatory mechanisms that may occur early in development. In addition, the available pharmacological blockers of NaV1.5 block brain Na+ channels and other potential cardiac Na+ channels with equal or greater potency, limiting their utility. In order to advance our understanding of NaV1.5-related biology, we have developed a chemical-genetic model to achieve acute and reversible silencing of NaV1.5 in situ. We engineered a NaV1.5 channel that contains a high-affinity, isoform-specific binding site for acylsulfonamide (GX) drugs, enabling chemical strategies to pharmacologically drive nonconducting channel conformations. The NaV1.5-GX channel has WT voltage-dependent gating and, unlike WT NaV1.5 and most other putative cardiac Na+ channels, is blocked by nanomolar concentrations of GX compounds. We have used CRISPR gene-editing to replace the endogenous Scn5a locus with the GX binding site in mice, creating a novel NaV1.5GX strain. Homozygous NaV1.5GX/GX mice have normal cardiac phenotypes, yet the acute application of nanomolar GX compounds to NaV1.5GX/GX isolated cardiac myocytes ablates Na+ current (INa). Systemic drug application in vivo results in conduction slowing in NaV1.5GX/WT mice, and conduction block and sudden death in NaV1.5GX/GX mice, thus providing a facile means to study NaV1.5 function and SCN5A-mediated disease. We propose first to examine the effects of acute Nav1.5 blockade by GX compounds on gene expression, Ca2+ handling, ROS production, fibrosis, cardiac function and arrythmias will be studied using NaV1.5GX/WT and NaV1.5GX/GX cardiac myocytes and mice, and compared to chronic Nav1.5 blockade using Scn5a+/- heterozygous knockout mice. We will then identify the effects of Na+ channel blockade on structural and electrophysiological remodeling, and on arrhythmia susceptibility following Transverse Aortic Constriction (TAC). Lastly, we will develop in vivo and ex vivo platforms to study SCN5A mutations identified in patients. The Scn5aGX mouse presents a unique opportunity to examine the phenotypes of human SCN5A mutations in a cardiac environment. In total, we anticipate these efforts will reveal novel molecular mechanisms of genotype-phenotype coupling stemming from SCN5A's role in controlling cardiac excitability.
