Novel BAG3 Variants in African American Patients With Cardiomyopathy: Reduced β-Adrenergic Responsiveness in Excitation–Contraction

Abstract

BackgroundWe reported 3 novel nonsynonymous single nucleotide variants of Bcl2-associated athanogene 3 (BAG3) in African Americans with heart failure (HF) that are associated with a 2-fold increase in cardiac events (HF hospitalization, heart transplantation, or death). Methods and ResultsWe expressed BAG3 variants (P63A, P380S, and A479V) via adenovirus-mediated gene transfer in adult left ventricular myocytes isolated from either wild-type (WT) or cardiac-specific BAG3 haploinsufficient (cBAG3+/−) mice: the latter to simulate the clinical situation in which BAG3 variants are only found on 1 allele. Compared with WT myocytes, cBAG3+/− myocytes expressed approximately 50% of endogenous BAG3 levels and exhibited decreased [Ca2+]i and contraction amplitudes after isoproterenol owing to decreased L-type Ca2+ current. BAG3 repletion with WT BAG3 but not P380S, A479V, or P63A/P380S variants restored contraction amplitudes in cBAG3+/− myocytes to those measured in WT myocytes, suggesting excitation–contraction abnormalities partly account for HF in patients harboring these mutants. Because P63A is near the WW domain (residues 21–55) and A479V is in the BAG domain (residues 420–499), we expressed BAG3 deletion mutants (Δ1–61 and Δ421–575) in WT myocytes and demonstrated that the BAG but not the WW domain was involved in enhancement of excitation–contraction by isoproterenol. ConclusionsThe BAG3 variants contribute to HF in African American patients partly by decreasing myocyte excitation–contraction under stress, and that both the BAG and PXXP domains are involved in mediating β-adrenergic responsiveness in myocytes.