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 (Cheng et al., 1990). In the 
common aF508 mutation, there is a deletion of Phe 506 in the first NBF. For 
unknown reasons, glycosylation of this protein is incomplete, and normal 
translocation to the apical membrane does not occur (Cheng et al., 1990). 
Cells homozygous for the aF508 mutation do not store the mutated form of 
CFTR, and it is likely degraded (Cheng et al. , 1990). Other CFTR mutations 
such as aI507 and S549I have a similar pattern of incomplete glycosylation, 
but other mutations of CFTR code for a CFTR protein glycosylated in a 
normal fashion (Cheng et al., 1990; Gregory et al., 1991). 
For the common aF508 mutation, the central abnormality appears to be the 
lack of translocation of the mutant protein, since delivery of the aF508 
protein to the surface of the cell conveys to the cell the ability to 
secrete Cl - in response to cAMP, although the aF508 Cl" channels have 
somewhat reduced activity compared to normal (Dalemens et al., 1991; Drumm 
et al. , 1991) . 
There is evolving evidence that CFTR may also function within intracyto- 
plasmic organelles (Barasch et al. , 1991; Van Dyke et al. , 1992). One 
consequence of a lack of CFTR function at these sites may be a higher pH 
within the organelle, causing dysfunction of intraorganelle components such 
as enzymes that modify carbohydrate side chains of proteins such as mucins 
(Barasch et al., 1991). 
1.9 Cell Targets for Gene Transfer in CF 
The central purpose of this protocol is to transfer the normal CFTR cDNA in 
vivo to the airway epithelial cells of individuals with CF. Before con- 
sidering how the gene will be transferred, it is important to consider 
which cells should be the targets for the gene transfer. While all evidence 
suggests the abnormal CFTR gene is expressed in airway epithelial cells, it 
is not known specifically which airway epithelial 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. Since the ultimate goal is gene 
therapy to compensate for the underlying genetic abnormality of CF, it is 
important to define the current concepts of the biology of airway epitheli- 
al cells. All evidence suggests that the alveolar epithelium is not a 
primary site of the clinical manifestations of mutations of the CFTR gene, 
and thus the following discussion focuses only on airway epithelium. 
1.9.1 General Airway Morphology 
The airways of the normal human lung begin at the trachea and end in the 
alveoli, the site of gas exchange (see Wiebel, 1991 for general review). 
The total surface area in the human lung is approximately 140 m 2 . Most of 
this surface is in the alveoli, with the airway surface representing ap- 
proximately 1-2% of the total (E. Weibel, personal communication). The 
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