Site-specific, Rep-mediated integration of the intact beta-globin locus in the human erythroleukaemic cell line K562.

The stable, regulated and tissue-specific expression of a therapeutic transgene can be best achieved by the transfer of a complete genomic locus, which will include the short- and long-range regulatory elements that are critical for the accurate control of gene expression. However, when techniques that rely on the random integration of exogenous DNA into the human genome are used for gene transfer, the risk of insertional mutagenesis remains a major issue. Using components derived from the adeno-associated virus (AAV), we have successfully targeted the integration of 200 kb bacterial artificial chromosomes containing the entire beta-globin locus into the AAVS1 site on human chromosome 19. We show that transient expression of the AAV Rep proteins in K562 cells facilitated site-specific transgene integration in 17% (6 of 36) of all analysed integration sites. Southern blot analysis revealed the locus had integrated into AAVS1 as an intact, functional unit in five of the six clones generated. Furthermore, each of the site-specific integrants exhibited sustained and appropriately regulated transgene gene expression over a period of 8 months of continuous culture in the absence of selective pressure. We anticipate that the approach developed in this study may be suitable for facilitating targeted integration of intact genomic loci in adult and embryonic stem cells, and therefore provide a powerful tool not just for functional studies but in establishing model systems for the ex vivo correction of genetic disorders.