Arthur M. Feldman

In one of the earliest reports from China during COVID-19, it was noted that over 20% of patients hospitalized with the disease had significant elevations of troponin, a marker of myocardial tissue damage, that put them at a higher risk. In a hypothesis-independent whole exome sequencing (WES) study in hospitalized COVID-19 patients of diverse ancestry, we observed putative enrichment in pathogenic variants in genes known to be involved in the pathogenesis of cardiomyopathy. This observation led us to hypothesize that the observed high morbidity and mortality in these patients might be due to the presence of rare genetic factors that had previously been silent but became relevant as a consequence of the severe stress inflicted by an infection with SARS-CoV-2. To test this hypothesis, we analyzed our WES data generated from a cohort of 325 patients sequentially admitted for COVID-19 infection. In this predominantly minority population (53.9% African ancestry and 37.9% Hispanic/Latin ancestry), our initial analysis screen identified 263 variants that were identified as highly deleterious (HD) from a total of 26,661 variants of interest that represented 215 genes. Of those, we identified 46 genes (in 58 patients) harboring rare HD coding variants that were previously implicated in dilated cardiomyopathy and were considered as disease initiators for the severe COVID-19 in this study. These findings offer valuable insights into the molecular mechanisms and genetic susceptibility to heart injury in severe COVID-19. Key messages: COVID-19 may cause cardiac damage in some affected patients without a plausible biological explanation. Our study reveals an enrichment of highly deleterious variants linked to cardiomyopathy in severe COVID-19 patients. Genetic profiling unveils the molecular basis of severe COVID-19-related heart injury, potentially aiding in patient stratification.

Bcl-2-associated athanogene 3 (BAG3) plays an important function in cellular protein quality control (PQC) maintaining proteome stability. Mutations in the BAG3 gene result in cardiomyopathies. Due to its roles in cardiomyopathies and the complexity of BAG3–protein interactions, it is important to understand these protein interactions given the importance of the multifunctional cochaperone BAG3 in cardiomyocytes, using an in vitro cardiomyocyte model. The experimental assay was conducted using high pressure liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) in the human AC16 cardiomyocyte cell line with BioID technology. Proteins with BAG3-interaction were identified in all the 28 hallmark gene sets enriched in idiopathic cardiomyopathies and/or ischemic disease. Among the 24 hallmark gene sets enriched in both idiopathic cardiomyopathies and ischemic disease, 15 gene sets had at least 3 proteins with BAG3-interaction. This study highlights BAG3 protein interactions, unveiling the key gene sets affected in cardiomyopathies, which help to explain the molecular mechanisms of the cardioprotective effects of BAG3. In addition, this study also highlighted the complexity of proteins with BAG3 interactions, implying unwanted effects of BAG3.

Background: Mutations to the co‐chaperone protein BAG3 (B‐cell lymphoma‐2–associated athanogene‐3) are a leading cause of dilated cardiomyopathy (DCM). These mutations often impact the C‐terminal BAG domain (residues 420–499), which regulates heat shock protein 70‐dependent protein turnover via autophagy. While mutations in other regions are less common, previous studies in patients with DCM found that co‐occurrence of 2 BAG3 variants (P63A, P380S) led to worse prognosis. However, the underlying mechanism for dysfunction is not fully understood. Methods and Results: In this study, we used proteomics, Western blots, and myofilament functional assays on left ventricular tissue from patients with nonfailing, DCM, and DCM with BAG3 ^63/380 to determine how these mutations impact protein quality control and cardiomyocyte contractile function. We found dysregulated autophagy and increased protein ubiquitination in patients with BAG3 ^63/380 compared with nonfailing and DCM, suggesting impaired protein turnover. Expression and myofilament localization of BAG3‐binding proteins were also uniquely altered in the BAG3, ^63/380 including abolished localization of the small heat shock protein CRYAB (alpha‐crystallin B chain) to the sarcomere. To determine whether these variants impacted sarcomere function, we used cardiomyocyte force‐calcium assays and found reduced maximal calcium‐activated force in DCM and BAG3 ^63/380 . Interestingly, myofilament calcium sensitivity was increased in DCM but not with BAG3 ^63/380 , which was not explained by differences in troponin I phosphorylation. Conclusions: Together, our data support that the disease‐enhancing mechanism for BAG3 variants outside of the BAG domain is through disrupted protein turnover leading to compromised sarcomere function. These findings suggest a shared mechanism of disease among pathogenic BAG3 variants, regardless of location.

BAG3 is a 575 amino acid protein that is found throughout the animal kingdom and homologs have been identified in plants. The protein is expressed ubiquitously but is most prominent in cardiac muscle, skeletal muscle, the brain and in many cancers. We describe BAG3 as a quintessential multi-functional protein. It supports autophagy of both misfolded proteins and damaged organelles, inhibits apoptosis, maintains the homeostasis of the mitochondria, and facilitates excitation contraction coupling through the L-type calcium channel and the beta-adrenergic receptor. High levels of BAG3 are associated with insensitivity to chemotherapy in malignant cells whereas both loss of function and gain of function variants are associated with cardiomyopathy.

Bcl2-associated athanogene-3 (BAG3) is expressed ubiquitously in humans, but its levels are highest in the heart, the skeletal muscle, and the central nervous system; it is also elevated in many cancers. BAG3's diverse functions are supported by its multiple protein-protein binding domains, which couple with small and large heat shock proteins, members of the Bcl2 family, other antiapoptotic proteins, and various sarcomere proteins. In the heart, BAG3 inhibits apoptosis, promotes autophagy, couples the β-adrenergic receptor with the L-type Ca2+ channel, and maintains the structure of the sarcomere. In cancer cells, BAG3 binds to and supports an identical array of prosurvival proteins, and it may represent a therapeutic target. However, the development of strategies to block BAG3 function in cancer cells may be challenging, as they are likely to interfere with the essential roles of BAG3 in the heart. In this Review, we present the current knowledge regarding the biology of this complex protein in the heart and in cancer and suggest several therapeutic options.