Genomic context vectors and artificial chromosomes for cystic fibrosis gene therapy

Cystic fibrosis (CF) is caused by mutations of the CF transmembrane conductance regulator (CFTR) gene, which encodes a cAMP dependent chloride channel whose expression is finely tuned in space and time. Gene therapy approaches to CF lung disease have demonstrated partial efficacy and short-lived CFTR expression in the airways. Drawbacks in the use of classical gene transfer vectors include immune response to viral proteins or to unmethylated CpG motifs contained in bacterially-derived vector DNA, and shut-off of viral promoters. These limitations could be overcome by providing stable maintenance and expression of the CFTR gene inside the defective cells. This strategy makes use of large fragments of DNA of various sizes containing the CFTR transgene and its relevant regulatory regions, (genomic context vectors [GCVs], reaching ultimate complexity in the form of an artificial chromosome [AC]) as vector for the transgene. Appropriate regulation in space and time would be achieved by the presence of the endogenous promoter and other control elements, while retention in daughter cells could be ensured by the presence of sequences which guarantee episomal replication. In this review, we describe recent advances in GCVs and ACs and the technology underlying their construction. These vectors have been shown to be suitable for delivery and expression of therapeutically relevant genes, including CFTR. The major issue which now limits their routine use is delivery inefficiency. Once this issue is resolved, we will be closer to achieving the goal of regulated gene therapy for CF.