GENE THERAPY FOR CYSTIC FIBROSIS

Cystic fibrosis (CF) is an autosomal recessive disease affecting about 1 in 3000 Caucasian births. In 1989, CF mutations were mapped to the gene on chromosome 7 encoding the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-mediated chloride channel protein which also regulates several other ion channels.The major cause of morbidity and mortality in CF is pulmonary disease characterized by viscous mucus secretion, chronic bacterial infection, airway inflammation and premature death at around 29 years of age. The lung is therefore the primary target organ in CF gene therapy, whereby investigators seek to restore the normal phenotype by introduction of the wild-type (normal) CFTR gene into airway epithelial cells.

• Rationale for CF gene therapy. The feasibility of gene replacement therapy for CF was first shown in experiments which demonstrated that the introduction of the wild-type CFTR gene into cultured CF airway epithelial cells lead to restoration of normal chloride ion transport.(At the organ level, CFTR has been localized to both the surface epithelium and submucosal glands.) Although it has been reported that expression of CFTR in as few as 6% to 10% of CF airway epithelial cells can restore normal chloride transport properties to an entire epithelial sheet, gene transfer to all the cells may be necessary to correct the sodium hyperabsorption characteristic of CF.These concern has been confirmed with a murine model in which the CFTR gene was delivered by an adenoviral vector to human CF bronchial xenografts: transfection of 5% of the cells completely restored chloride transport but had a variable effect on sodium hyperabsorption.Thus, if sodium hyperabsorption across surface epithelia plays a major role in the pathogenesis of CF-related lung disease, highly efficient gene transfer may be necessary to restore normal airway function.

  These studies have provided the rationale for the delivery of the CFTR gene to intact airways, first in animal models and, subsequently, in human clinical trials based on nasal and/or lung-directed gene transfer. The goal of this work is the efficient delivery of the CFTR gene to the appropriate target cells without inducing any associated toxicity or inflammation. In addition, because the airway surface epithelium regenerates slowly over time, long-term expression of the CFTR gene will either require integration into a population of stem cells or readministration of the delivery vector. The preclinical studies and human CF gene therapy trials have been described extensively elsewhere and will therefore not be reviewed in detail here.

• Vector delivery systems: Challenges to CFTR gene transfer. Current approaches to CF gene therapy have focused on the use of adenoviruses, adeno-associated viruses and liposomes to achieve gene transfer via the airway epithelia. Each of these vectors can transfer the CFTR gene to airway epithelia in vivo, resulting in at least partial correction of the chloride transport defect. However, several problems have been identified with these systems, which will need to be overcome if gene therapy is to become of clinical utility in the treatment of CF.

Initial human trials have revealed that the efficiency of gene transfer to uninjured airway epithelia by the current generation of vectors is low. In the case of adenoviral vectors, the inefficiency of gene transfer to fully differentiated epithelial cells has been correlated with a paucity of the cellular receptors required for adenovirus internalization.A second major concern relates to the safety of the vectors. A number of studies have shown that the number of adenoviral particles required for efficient gene transfer is associated with direct viral toxicity, manifesting as both local and systemic inflammation.A further problem with the use of adenoviral vectors for CF gene therapy is that host cellular and humoral immune responses result in transient expression of the delivered gene and preclude readministration of the vector.Several groups are developing strategies to overcome these immune responses to adenoviral vectors. One approach has been to modify the adenoviral vector itself to minimize the expression of viral proteins. Various immunomodulatory regimens are also being investigated as strategies to prolong expression of the therapeutic gene and/or permit readministration of the adenoviral vector. Thus, further advances with CF gene therapy will be dependent on improvements to the efficiency and safety profiles of the vectors used to deliver the CFTR gene.