Barriers to and new approaches for gene therapy and gene delivery in cystic fibrosis

Abstract

Clinical trials of gene therapy for cystic fibrosis suggest that current levels of gene transfer efficiency are probably too low to result in clinical benefit, largely as a result of the barriers faced by gene transfer vectors within the airways. The respiratory epithelium has evolved a complex series of extracellular barriers (mucus, lack of receptors, immune surveillance, etc.) aimed at preventing penetration of lumenally delivered materials, including gene therapy vectors. In addition, once in the cell, further hurdles have to be overcome, including DNA degradation, nuclear import and the ability to maintain long-term transgene expression. Strategies to overcome these barriers will be addressed in this review and include the use of: (i) clinically relevant adjuncts to overcome the extra- and intracellular barriers; (ii) less-conventional delivery routes, such as intravenous or in utero administration; (iii) more efficient non-viral vectors and 'stealth' viruses which can be re-administered; and (iv) new approaches to prolong transgene expression by means of alternative promoters or integrating vectors. These advances have the potential to improve the efficiency of gene delivery to the airway epithelium, thus making gene therapy a more realistic option for cystic fibrosis.

Fig. 1

Extracellular and intracellular barriers limit gene transfer efficiency of viral and non-viral vectors to airway epithelial cells. Extracellular barriers include the presence of infected mucus and sputum, mucociliary clearance and tight junctions between the cells, which limit the access of viral vectors to receptors localised on the basolateral membrane. Alternatively, gene transfer agents could be delivered intravenously (I/V route), even if it is unlikely that airway epithelial cells will be targeted. Inflammation, CTL-mediated degradation of transduced cells and neutralizing antibodies can further limit transgene expression or re-administration of the gene transfer vectors. Hidden stem or progenitor cells could be targeted by lentiviral vectors. Once inside the cell, the genetic material has to overcome endosome and cytoplasmic degradation and to get into the nucleus. CE, ciliated cell; B, basal cell; G, goblet cell; TJ, tight junctions; C, capillary; SMG, submucosal gland.