Lack of Site-Specific Integration of the Recombinant Adeno-Associated Virus 2 Genomes in Human Cells

Abstract

The adeno-associated virus 2 (AAV)-based vector system has been suggested for its potential use in human gene therapy because the wild-type (wt) AAV genome appears to integrate into the human chromosomal DNA in a site-specific manner. We systematically investigated the integration patterns of the recombinant AAV genomes lacking one or both the viral coding sequences. Four recombinant AAV genomes were constructed containing the genes for resistance to tetracycline (Tc^R) and the herpesvirus thymidine kinase (TK) promoter-driven gene for resistance to neomycin (neo^R; vTcNeo), the genes for resistance to ampicillin (Ap^R) and TK-neo^R (vAp.Neo), the genes for AAV replication (rep) genes and TK-neo^R (vRep.Neo), and the AAV capsid (cap) genes and TK-neo^R (vCap.Neo). The integration pattern of each of the recombinant AAV genomes in individual clonal isolates of the human nasopharyngeal carcinoma cell line (KB) analyzed on Southern blots using a neo-specific DNA probe was distinctly different. In addition, in none of the clones examined was the proviral genome covalently linked to the previously described AAV right-junction (Rt.Jn.) human chromosomal DNA fragment, the putative specific-site of integration for the wt AAV genome. Furthermore, whereas a 276-bp DNA fragment could be readily amplified from each of these clones, using a neo-specific primer-pair by polymerase chain reaction (PCR), no amplified DNA product was obtained usign the neo- and the Rt.Jn. primer-pair under identical conditions. Fluorescence in situ hybridization (FISH) analyses further revealed the lack of integration of the recombinant AAV into human chromosome 19, even in the presence of a functional rep gene as determined by rescue of the recombinant AAV genome in the presence of adenovirus. These data suggest that the recombinant AAV genomes integrate at sites that are different from that characterized for the wt AAV genome. These studies may have implications in the development of the AAV-based vector system for its potential use in human gene therapy. The potential application of adeno-associated virus (AAV) as a vector for human gene therapy has been suggested in view of the remarkable site-specificity of integration of the wild-type (wt) AAV genome in human cells. To ascertain the nature of integration of the recombinant AAV genomes, Ponnazhagan et al. constructed several different recombinant AAV-based vectors and observed that their integration patterns were different among clonal populations of human cells, and that their site of integration was distinctly different from that characterized for the wt AAV genome. On the basis of these results, Ponnazhagan et al. underscore the need to evaluate further the nature of interaction of the recombinant AAV vectors with primary human diploid cells.