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Novel BAG3 Variants in African American Patients With Cardiomyopathy: Reduced β-Adrenergic Responsiveness in
Arthur M Feldman1, Jennifer Gordon2, Jufang Wang3
1Department of Medicine, Lewis Katz School of Medicine of Temple University, Philadelphia, Pennsylvania.
Insights
Novel Bcl2-associated athanogene 3 (BAG3) variants in African Americans with heart failure (HF) impair myocyte function. These genetic changes reduce cardiac contractility under stress, increasing cardiac event risk.
Area of Science:
- Cardiovascular Genetics
- Molecular Cardiology
- Genetic Variants and Disease
Background:
- Identified three novel nonsynonymous single nucleotide variants in Bcl2-associated athanogene 3 (BAG3) in African Americans with heart failure (HF).
- These BAG3 variants are associated with a twofold increase in cardiac events, including HF hospitalization, heart transplantation, or death.
Purpose of the Study:
- To investigate the functional impact of novel BAG3 variants found in heart failure patients.
- To determine the role of BAG3 domains in myocyte excitation-contraction coupling and response to stress.
Main Methods:
- Expressed BAG3 variants (P63A, P380S, A479V) in mouse ventricular myocytes using adenovirus-mediated gene transfer.
- Utilized cardiac-specific BAG3 haploinsufficient (cBAG3+/-) mice to mimic clinical conditions of heterozygous variant carriers.
- Assessed myocyte contractility and calcium handling, and analyzed the function of BAG3 deletion mutants (BAG and WW domains).
Main Results:
- cBAG3+/- myocytes showed reduced endogenous BAG3 levels, decreased intracellular calcium ([Ca2+]i), and lower contraction amplitudes after isoproterenol stimulation.
- Wild-type BAG3 repletion restored contraction in cBAG3+/- myocytes, while variants P380S, A479V, and P63A/P380S did not fully restore function.
- The BAG domain, but not the WW domain, was crucial for enhancing excitation-contraction coupling by isoproterenol.
Conclusions:
- BAG3 variants contribute to heart failure in African American patients by impairing myocyte excitation-contraction coupling under stress.
- Both the BAG and PXXP domains of BAG3 are essential for mediating beta-adrenergic responsiveness in cardiac myocytes.
Background:
We 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 Results:
We 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.
Conclusions:
The 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.
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