PROJECT SUMMARY/ABSTRACT In the past fifty years, great advancements in the diagnosis, treatment and prevention of cardiovascular disease have been made, yet cardiovascular disease remains the leading cause of death in the U.S. Notably, over half of these deaths are associated with arrhythmias. Unfortunately, we still lack a fundamental understanding of the molecular mechanisms underlying complex human arrhythmias. We assert that the study of congenital arrhythmias offers significant and novel insight into pathways underlying familial and acquired forms of cardiovascular disease. Further, combining this clinical and genetic data with targeted animal models, electrophysiology, and molecular approaches provides exciting opportunities to identify fundamental new information linking human disease with specific molecular mechanisms. Our preliminary data support a new molecular mechanism for human arrhythmia based on dysfunction in the cardiac αII spectrin-based pathway. Specifically, our preliminary findings identify putative loss-of- function variants in the αII spectrin pathway in patients with Brugada syndrome arrhythmias. Brugada syndrome is a cardiac arrhythmia syndrome which causes potentially fatal ventricular arrhythmias in young adults. It affects 1 in 2,000 adults and often presents with sudden death or syncope. The pathophysiology of Brugada syndrome is thought to be due to the reduced action potential duration, often due to reduced sodium outflow through the cardiac voltage gated sodium channel, Nav1.5. Spectrins are adapter molecules critical for membrane biogenesis, organization, and signaling in neurons and erythrocytes. Recently, we identified two unrelated families with Brugada syndrome, each harboring a missense point mutation in SPTAN1, which encodes αII spectrin. Notably, both variants have negligible minor allele frequencies and are localized within three residues of each other within the C-terminal EF hand motif, an area essential for α/β spectrin ternary complex integration with actin. However, the role of αII spectrin for cardiac excitability is unknown and essentially unstudied, particularly for arrhythmia. Our preliminary findings in arrhythmia patients as well as in a new animal model of cardiac αII spectrin-deficiency support new and unexpected roles for this molecule in heart. Based on our data, we hypothesize that αII spectrin is an unanticipated, submembrane “node” critical for ion channel and cytoskeletal regulation at the intercalated disc. Specifically, we hypothesize that αII spectrin, through interactions with βIV spectrin, actin, and ankyrin-G, facilitates the proper organization and function of the intercalated disc and the localization of the sodium channel Nav1.5. Further, we predict that dysfunction in the αII spectrin pathway results in severe electrical and structural phenotypes. We anticipate that these findings will uncover novel fundamental pathways governing excitable cell biology.