Adeno-associated viral vector integration: implications for long-term efficacy and safety

Adeno-associated virus (AAV) vector gene therapy provides a promising platform for treatment of monogenic inherited disorders. Clinical studies have demonstrated long-term expression with reduction in bleeding using this approach for the treatment of hemophilia. Despite these advances, there are unknowns surrounding the natural history of recombinant AAV (rAAV) vectors and the cellular mechanisms mediating vector persistence. These unknowns underpin questions regarding long-term efficacy and safety. The predominant mechanism via which AAV is proposed to persist is in circular double-stranded extrachromosomal DNA structures (episomes) within the nucleus. Studies of wild-type AAV (WT-AAV) and rAAV have demonstrated that AAV also persists via integration into a host cell's DNA. It is important to determine whether these integration events can mediate expression or could result in any long-term safety concerns. WT-AAV infection affects a large proportion of the general population, which is thought to have no long-term sequelae. Recent studies have highlighted that this WT-AAV has been detected in cases of acute hepatitis in children and in a minority of cases of hepatocellular carcinoma. Integration following treatment using rAAV has also been reported in preclinical and clinical studies. There have been variable reports on the potential implications of integration for rAAV vectors, with data in some murine studies demonstrating recurrent integration with development of hepatocellular carcinoma. These findings have not been seen in other preclinical or clinical studies. In this review, we will summarize current understanding of the natural history of AAV (wild-type and recombinant) with a focus on genomic integration and cellular implications.