Abstract P3136: Alternative Polyadenylation Of Scn5a Generates A Novel N-terminal Nav1.5 Microprotein

Abstract

SCN5A encodes the cardiac voltage-gated Na+ channel, NaV1.5, that initiates the cardiac action potential. Its expression and activity are highly regulated, and variants can cause arrhythmias and increased heart failure risk. Mechanisms controlling NaV1.5 expression and activity are not fully understood. We recently discovered a well-conserved alternative polyadenylation (APA) signal downstream of the first coding exon in SCN5A. The APA signal yields a short SCN5A transcript, resulting in the presence of mRNA-seq reads mapping to this intronic region in heart tissues from several species (e.g. human, pig, and cat; rodents lack the upstream APA). Analysis of cardiac APA data revealed a decrease in upstream APA signal usage in humans with dilated cardiomyopathy (~30%, p=0.08). Reanalysis of human cardiac RNA-seq data indicated the ratio of short/full-length SCN5A mRNA is reduced ~25% in failing hearts (p<0.05, versus non-failing hearts). Notably, the short transcript isoform has the potential to produce a novel microprotein (referred to as SCN5A-NT), which is composed of an N-terminus identical to NaV1.5 and a unique C-terminus derived from “intronic” sequence. Western blot of human heart tissue using an N-terminal NaV1.5 antibody shows a band migrating identically to transgene-derived SCN5A-NT, supporting its expression in human hearts. SCN5A-NT is predicted to contain a mitochondrial targeting sequence, and immunostaining of transgene-derived SCN5A-NT in neonatal rat cardiomyocytes colocalizes with mitochondria. These findings were corroborated in mice treated with AAV9 encoding SCN5A-NT, which showed myocardial SCN5A-NT expression by western blot and immunostaining that colocalized to cardiomyocyte mitochondria. We are currently devising cell culture and mouse experiments to test if SCN5A-NT alters mitochondrial functions and/or NaV1.5 activity, perhaps by modulating mitochondria ROS or NAD+/NADH. We also generated knock-in mice harboring the human APA signal to investigate how upstream APA influences NaV1.5 expression and whether upstream APA usage is dynamically regulated during cardiac stress. Overall, this work describes a new regulatory mechanism of NaV1.5 expression that yields an undescribed microprotein.