Directed Evolution of AAV9 for Efficient Gene Expression in Cardiomyocytes In Vitro and In Vivo

Adeno-associated viral (AAV) vectors are increasingly used for preclinical and clinical cardiac gene therapy approaches. However, gene transfer to cardiomyocytes poses a challenge due to differences between AAV serotypes in terms of expression efficiency in vitro and in vivo. For example, AAV9 vectors work well in rodent heart muscle cells in vivo but not in cultivated neonatal rat ventricular cardiomyocytes (NRVCMs), necessitating the use of AAV6 vectors for in vitro studies. Therefore, we aimed to develop an AAV that could efficiently express genes in NRVCMs, human engineered heart tissue (hEHT), and mammalian hearts. The production of AAV6 vectors results in lower yields compared with AAV9. Hence, we used random AAV9 peptide libraries and selected variants on NRVCMs at the vector genome and RNA levels in parallel. The enriched library variants were characterized using high-throughput analysis of barcoded variants, followed by individual validation of the most promising candidates. Interestingly, we found striking differences in NRVCM transduction and gene expression patterns of the AAV capsid variants depending on the selection strategy. AAV variants selected based on the vector genome level enabled the highest transduction but were outperformed by AAVs selected on the RNA level in terms of expression efficiency. In addition, we identified a new AAV9 capsid variant that not only allowed significantly higher gene expression in NRVCMs compared with AAV6 but also enabled similar gene expression in murine hearts as AAV9 wild-type vectors after being intravenously injected into mice. Moreover, the novel variant facilitated significantly higher gene expression in hEHT compared with AAV9. Therefore, this AAV variant could streamline preclinical gene therapy studies of myocardial diseases by eliminating the need for using different AAVs for NRVCMs, hEHT, and mice.