Overexpression of human BAG3P209L in mice causes restrictive cardiomyopathy due to sarcomere disruption and protein aggregate formation

An amino acid exchange (P209L) in the HSPB8 binding site of the human cochaperone Bcl2-associated athanogene 3 (BAG3) gives rise to severe dominant childhood cardiomyopathy. To phenocopy the human disease in mouse and gain insight into its mechanisms, we have generated humanized transgenic mouse models. Expression of human BAG3P209L-eGFP in mice caused Z-disc disintegration and formation of protein aggregates. This was accompanied by massive fibrosis resulting in a severe, early-onset restrictive cardiomyopathy with increased mortality, as observed in patients. RNA-Seq and proteomic analysis revealed changes in the protein quality control system and increased autophagy in hearts from hBAG3P209L-eGFP overexpressing mice. Also, the mutation renders hBAG3P209L less soluble in vivo and induces protein aggregation, but does not abrogate hBAG3 binding properties. Thus, we have established a mouse model recapitulating mimicing the human disease and found that the disease mechanism is due to accumulation of hBAG3P209L and mouse BAG3, causing sequestering of components of the protein quality control system and the autophagy machinery leading to the reduction of the stability and functional organization of sarcomeres. We have also tested a gene therapy approach utilizing expression of shRNA against hBAG3 in cardiac muscle via AAV and demonstrate that this halted and even partially reversed major phenotypic features of the disease. Comparative analysis of the single cell mRNA-sequencing profile of transgenic CMs (PGKcre:BAG3P209L_homo_GFP_bright, PGKcre:BAG3P209L_homo_GFP_dim, PGKcre:BAG3P209L_het_GFP_bright, PGKcre:BAG3P209L_het_GFP_dim) and control CMs.