TAX1BP3 Loss Is an Autosomal Recessive Cause of Arrhythmogenic Cardiomyopathy and TRPV4-mediated Arrhythmogenesis

Abstract

Arrhythmogenic cardiomyopathy (ACM) is one of the leading causes of sudden cardiac death in children and is characterized by the fibrofatty replacement of the myocardium, predominantly of the right ventricle. Sixty percent of patients with ACM have a known genetic cause, but for the remainder, the etiology is unknown. This lack of mechanistic understanding has slowed development of disease-modifying therapies, and children with ACM have a high degree of morbidity and mortality. We identified a kindred with multiple members affected by ACM co-segregating with biallelic variants in the gene TAX1BP3, which encodes tax1-binding protein 3 (TAX1BP3). IPSC-CMs derived from this kindred demonstrated increased intracellular lipid droplets, increased transient receptor potential vanilloid type 4 (TRPV4) expression, and inducible TRPV4 current. This was associated with a change in TRPV4 trafficking, depletion of the intracellular sarcoplasmic reticulum Ca2+ store and increased ryanodine receptor 2 (RyR2)-mediated store Ca2+ leak and delayed afterdepolarizations—a known mechanism of Ca2+-mediated arrhythmogenesis. Similarly, Tax1bp3 cardiac-specific knockout mice had increased Ca2+ leak and were predisposed to ventricular arrhythmias compared with control mice. Ca2+ leak in both the iPSC-CMs and isolated mouse ventricular myocytes was rescued by small molecule TRPV4 inhibition. This strategy also effectively reduced Ca2+ leak in a Plakophilin 2 (PKP2) p.His773AlafsX8 iPSC-CM model of ACM. We conclude that TAX1BP3 is associated with rare autosomal recessive ACM through TRPV4-mediated Ca2+ leak from RyR2. Further, TRPV4 current inhibition has the potential to be a new therapeutic target for ACM.Lastly, in order to more effectively test potential therapeutics in vivo, we sought to develop an animal model with a higher phenotypic penetrance. Because exercise is a risk factor for arrhythmia induction and can accelerate ACM disease progression, we chose to develop an exercise model with the Tax1bp3-/- mice. We found that consistent swimming exercise over a period of 6 weeks increased the percentage of mice with inducible ventricular tachycardia but did not cause cardiac remodeling. Since ACM is characterized by an arrhythmogenic phase that precedes fibrofatty infiltration, this mouse model could serve as a means of studying the effects of exercise on electrical remodeling and the early stages of ACM. In summary, this dissertation describes 1) the identification of a novel gene associated with ACM, TAX1BP3, 2) the development of new in vitro and in vivo ACM models and 3) identifies a new potential ACM disease mechanism: calcium leak that is mediated by TRPV4. These findings implicate TRPV4 inhibition as a possible treatment for ACM—a deadly disease with limited therapies.