accepted, however, that the protein is present in cell membranes of epithelial 
tissues (37,41). CFTR has been localized to the apical membranes of pancreatic 
ducts, intestinal epithelia, sweat ducts, and airway epithelia. There is in- 
direct functional data suggesting CFTR may also be localized to organelle mem- 
branes, including the Golgi apparatus (42). There is convincing evidence that 
CFTR can function as a cAMP regulatable Cl' channel (21) . There is also evi- 
dence that CFTR may have other functions, such as recycling of vesicles (43). 
The regulation of CFTR as a Cl' channel is not completely defined, but it re- 
quires phosphorylation of the R-domain mediated by cAMP activation of protein 
kinase A (44 , 45) . 
1.6 Molecular Pathogenesis of Cystic Fibrosis. In normal cells expressing the 
CFTR gene, the CFTR protein is produced as an unglycosylated polypeptide that 
subsequently undergoes core glycosylation in the rough endoplasmic reticulum, 
translocation to the Golgi, modification of the carbohydrate side chains to 
their mature form, and final transfer to the apical membrane (37). In the com- 
mon aF 508 mutation, there is a deletion of Phe S08 in the first NBF. For unknown 
reasons, glycosylation of this protein is incomplete, and normal translocation 
to the apical membrane does not occur (37) . Cells homozygous for the aF508 
mutation do not store the mutated form of CFTR, and it is likely degraded 
(37). Other CFTR mutations such as aI 507 and S549I have a similar pattern of 
incomplete glycosylation, but other mutations of CFTR code for a CFTR protein 
glycosylated in a normal fashion (37,46). For the common aF 508 mutation, the 
central abnormality appears to be the lack of translocation of the mutant 
protein, since delivery of the aF 508 protein to the surface of the cell con- 
veys to the cell the ability to secrete Cl' in response to cAMP, although the 
aF 508 Cl' channels have somewhat reduced activity compared to normal (47,48). 
There is evolving evidence that CFTR may also function within intracytoplasmic 
organelles (42,49). One consequence of a lack of CFTR function at these sites 
is a higher pH within the organelle, causing dysfunction of intraorganelle 
components such as enzymes that modify carbohydrate side chains of proteins 
such as mucins (42) . 
1.7 Cell Targets for Gene Transfer in Cystic Fibrosis. The abnormal CFTR gene 
is expressed in airway epithelial cells, but it is not known which airway epi- 
thelial cells play a critical role in the pathogenesis of the disease i.e., it 
may be a subset of cells or it may be all airway epithelial cells. The alveo- 
lar epithelium is not a primary site of the clinical manifestations of muta- 
tions of the CFTR gene. As the disease progresses, the mucus obstruction, in- 
fection and neutrophil-dominated inflammation takes its toll on the normal 
epithelial architecture. While the nasal epithelium is similar in CF and nor- 
mals, in CF there are significant changes in the proportions of epithelial 
cell types in the large bronchi, with fewer ciliated cells, and increased pro- 
portions of basal, secretory and undifferentiated columnar cells. In the final 
stages of the disease, small airways become completely obstructed with secre- 
tions. There are bronchiectatic cysts occupying as much as 50% of the cross- 
sectional area of the lung (1) and the bronchioles are stenosed and obliter- 
ated. Bronchiectasis with marked derangements of the epithelial surface and 
the bronchial wall is common. 
The CFTR gene is expressed in the epithelium of the human nose, trachea and 
bronchi (50). On the surface epithelium, expression is very low, averaging 1-2 
mRNA transcripts per cell (50). Consistent with this observation, the sequence 
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Recombinant DNA Research, Volume 20 
