CFTR gene transfer to lung epithelium--on the trail of a target cell

Cystic fibrosis (CF) is a lethal inherited disease that afflicts up to 1 in 2,500 people in the western world. Since 1989, when mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene were identified as responsible for the disease, intense effort has been applied to the development of replacement gene therapy strategies to cure CF. Problems with basic gene delivery techniques along with limited knowledge of the pathogenesis of CF have hindered progress so far. However, recent insights into the expression patterns and functions of CFTR in developing and adult lungs are now advancing our understanding of this disease. It is becoming apparent that progress in gene delivery to cure CF may be best served by identification of a target cell(s) around which gene transfer strategies can be specifically tailored to most closely reproduce the effects of normal CFTR expression. In fact, accurate restoration of endogenous expression patterns may be crucial, not only for disease reversal, but also to avoid potentially deleterious effects of inappropriate expression. This approach is in turn confounded however, by ill-defined stem and progenitor cell pathways within the lung epithelium. Nonetheless, studies to date suggest that these pathways are relatively plastic and may respond differently during homeostasis compared with repair following injury. It may therefore be feasible to target the lung epithelium in a non-cell specific manner and allow endogenous differentiation pathways to subsequently establish correct CFTR distribution patterns. In this review, emerging information on CFTR expression and function in developing and adult lungs is discussed in the context of putative stem cell populations and their potential for current gene delivery approaches.