M.J. Welsh and A.E. Smith, RAC Application 
observed: a) a complete loss of cAMP-mediated transepithelial chloride secreuon, and b) a 
two to three fold increase in the rate of Na+ absorption, c AMP-stimulated chloride 
secretion requires a chloride channel in the apical membrane (62). The discovery that 
CFTR is a phosphorylation-regulated chloride channel and that the properties of the CFTR 
chloride channel are the same as those of the chloride channels in the apical membrane, 
indicate that CFTR itself mediates transepithelial chloride secretion. This conclusion was 
supported by studies localizing CFTR in lung tissue: CFTR is located in the apical 
membrane of airway epithelia (32) and has been reported to be present in the submucosal 
glands (63,64). As a consequence of loss of CFTR function, there is a loss of cAMP- 
regulated transepithelial chloride secretion. At this time it is uncertain how dysfunction of 
CFTR produces an increase in the rate of Na + absorption. However, it is thought that the 
defective chloride secretion and increased Na + absorption lead to an alteration of the 
respiratory tract fluid and hence, to defective mucociliary clearance, a normal pulmonary 
defense mechanism. As a result, clearance of inhaled material from the lung is impaired 
and repeated infections ensue. Although the presumed abnormalities in respiratory tract 
fluid and mucociliary clearance provide a plausible explanation for the disease, a precise 
understanding of the pathogenesis is still lacking. 
Correction of the genetic defect in the airway epithelial cells is likely to reverse the CF 
pulmonary phenotype. The identity of the specific cells in the airway epithelium that 
express CFTR cannot be accurately determined by immunocytochemical means, because of 
the low abundance of protein. However, functional studies suggest that the ciliated 
epithelial cells and perhaps nonciliated cells of the surface epithelium are among the main 
cell types involved in electrolyte transport (62). Thus in practical terms, the present 
preferred target cell for gene therapy would appear to be the mature cells that line the 
pulmonary airways. These are not rapidly dividing cells; rather, most of them are 
nonproliferating and many may be terminally differentiated (65). The identification of the 
progenitor cells in the airway is uncertain. Although CFTR may also be present in 
submucosal glands (63,64), there is no data as to its function at that site; furthermore, such 
glands appear to be relatively inaccessible. 
Because we are unaware of any method of removing and reimplanting airway epithelial 
cells that is likely to be possible in humans, a therapeutic strategy based on removal of the 
target cell from the patient, treatment by gene therapy in vitro, and then reimplantation into 
the patient, seems impractical. Instead, gene therapy for CF appears to require in vivo 
treatment of the airway epithelia. 
3.2 Delivery of CFTR cDNA 
The airway epithelium provides two main advantages for gene therapy, first, access to the 
airway epithelium can be relatively nonin vasive. This is a significant advantage in the 
development of delivery strategies and it will allow investigators to monitor the therapeutic 
response. Second, the epithelium forms a barrier between the airway lumen and the 
interstitium. Thus, application of the vector to the lumen will allow access to the target cell 
yet, at least to some extent, limit movement through the epithelial barrier to the interstitium 
and from there to the rest of the body. 
3.3 Efficiency of Gene Delivery Required to Correct the Genetic Defect 
It is unlikely that any gene therapy protocol will correct 100% of the cells that normally 
express CFTR. However, several observations suggest that correction of a small percent of 
the involved cells or expression of a fraction of the normal amount of CFTR may be of 
therapeutic benefit. 
Recombinant DNA Research, Volume 16 
[861] 
