Site-specific transgene integration in the human genome catalyzed by phiBT1 phage integrase

The Streptomyces phage phiBT1 integrase catalyzes recombination between phage attP and bacterial attB sites (att, attachment), resulting in phage DNA integration into the bacterial host genome in a unidirectional manner. Multiple pseudo-attB and -attP sites are present serendipitously in mammalian genomes and can recombine with wild-type attP and attB sequences. The phiBT1 system has been used previously to achieve site-specific integration of murine phenylalanine hydroxylase cDNA into hepatocytes of mice with phenylketonuria, which led to the complete and permanent correction of the disease phenotypes without apparent toxicities. Here we report the identification of three pseudo-attP and two pseudo-attB sites in human cells, which are located in intergenic regions of five different chromosomes. There are no microdeletions of human genomic sequences at the insertional junctions and the integrated transgenes are expressed. Human cells expressing phiBT1 integrase showed normal karyotypes without chromosomal translocations between the pseudo-attB and -attP sites. Polymerase chain reaction analyses were performed on genomic DNA isolated from various human cell types expressing phiBT1 integrase, using primers flanking the pseudo-attB and -attP sites from mismatched human chromosomes. No chromosomal translocation events were detected in normal human hepatocytes, peripheral blood mononuclear cells, vascular microendothelial cells, and two other transformed human cell lines, although one such event was observed in a human melanoma cell line. The results suggest that the occurrence of chromosomal translocations is human cell type dependent, and that the phiBT1 system for site-specific integration of transgenes into the human genome can be used in selected applications.