Airway Basal Cells Are the Key for Cystic Fibrosis Gene Therapy

Efforts to develop gene therapies for cystic fibrosis (CF) helped to drive vector innovations more than two decades ago, including the first use of recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) for in vivo gene therapy and for human use. ^{1 –5} These early-generation vectors failed to progress in clinical trials, however, because of immune responses and the failure of these episomal vectors to result in stable transduction of the airway progenitor cells, particularly basal cells in the conducting airways. Two articles in the current issue of Human Gene Therapy address this issue directly by using newer vector systems: helper-dependent adenovirus (HD-Ad) and lentivirus (LV) vectors. ^6,7

In the study by Cao et al. from Dr. James Hu's laboratory, the ability of HD-Ad vectors to transduce airway cells in mice and pigs in vivo and human primary epithelial cells ex vivo was tested. ⁷ Reporter gene vectors (GFP and lacZ) demonstrated widespread airway epithelial cell transduction in mice, including the more cuboidal distal airway basal cells within the lung, as well as tracheal keratin-5 (KF)-expressing basal cells, which are more similar to those found in larger animals. Porcine studies utilized bronchoscopy-based vector instillation to test HD-Ad transduction of basal cells, and likewise demonstrated efficient gene transfer in basal cells. Finally, human primary cultures of basal cells were isolated from CF patients, and the HD-Ad system was used to express CFTR in those cells, effecting correction of the CF defect.

In the paper by Farrow et al., the focus is on vesicular stomatitis virus–pseudotyped lentivirus vectors, which this group has previously shown to be effective for airway epithelial cells gene transfer. ⁶ In the current study, the investigative team from the laboratory of Dr. David Parsons studied LV-mediated lacZ gene transfer following lysophosphatidylcholine conditioning in mice. They subsequently demonstrated that when they stimulated airway regeneration with polidocanol, lacZ-transduced airway cells repopulated the epithelium in linear and spotted clusters, essentially proving the stem-ness of the transduced cells.

In both cases, the investigative teams are revisiting the prospect of human gene therapy for CF patients. Ironically, although CF was one of the first diseases targeted for direct in vivo gene therapy, clinical efficacy has been achieved in other conditions much sooner, as with RPE-65 related inherited retinal dystrophy, spinal muscular atrophy, and hemophilia. The emergence of small molecule drugs that show dramatic benefit for CF patients have obviated the need for such therapy in patients with many common CFTR mutations. However, a substantial subset of patients remain for whom gene therapy would be a welcome additional option. The key to the success of the efforts to provide that option will be the further development of HD-Ad, LV, and other newer systems for gene replacement and gene editing of the CFTR gene.