that this is feasible only through the air side of the epithelium. Unlike the 
lower respiratory tract which gets its blood supply from the pulmonary capil- 
laries, the airways are supplied by the bronchial circulation, an arterial 
system comprised of multiple branches derived from the aorta (31) . Although it 
is feasible to place catheters into the bronchial circulation, their multi- 
plicity and variability make this approach very cumbersome. Further, even if 
the transfer vector could be delivered to the airways via the bronchial circu- 
lation, the cDNA would have to cross the endothelium, the endothelial basement 
membrane, the interstitial space, and the epithelial basement membrane before 
entering the basolateral surface of the epithelium; this possibility is un- 
likely . 
1.4 Normal and Abnormal Cystic Fibrosis Transmembrane Conductance Regulator 
(CFTR) Gene. The gene responsible for CF, the CFTR gene, is a 27 exon gene 
spanning over 250 kb on the long arm of human chromosome 7 at q31-q32 
(17,18,32). CFTR mRNA transcripts are about 6.5 kb in length; of this, 4.5 kb 
code for the CFTR protein. The structure of the putative CFTR gene product is 
a 1480 residue glycoprotein. There is (N- to C-terminal) a membrane -spanning 
domain with six membrane -spanning segments, a nucleotide (ATP) -binding fold 
(NBF) , a large polar R (regulatory) domain which contains multiple potential 
phosphorylation sites, a second similar membrane -spanning domain and a second 
NBF (17). More than 220 sequence variations of the CFTR gene have been identi- 
fied, of which about 170 are associated with the clinical manifestations of 
CF. The mutations include missense, nonsense, frame -shift, and splicing muta- 
tions, and small deletions and insertions (28). Most of these mutations are 
scattered throughout the coding region of the gene. Many different mutations 
have been found at the highly conserved region of the first NBF (28) , suggest- 
ing that the first NBF is important to CFTR function. The most common mutation 
is AF508 , a deletion of 3 nucleotides resulting in loss of phenylalanine at 
residue 508, a region coded by sequences in exon 10 in NBF1 . AF508 accounts 
for 60 to 70% of the CF mutant alleles (33). G551D, S549I, A455E, and G542X 
account for 10-20% of the non-delta F508 mutations (33). Most other mutant 
alleles are rare, with some represented by only a single example. Studies of 
correlations between specific mutations and severity of disease have shown 
some variation in severity even in individuals with the same genotype. For 
example, the severity of the respiratory disease in AF508 homozygotes can be 
variable although most of these individuals have relatively severe pancreatic 
insufficiency (33). "Null" mutations, including frameshift, nonsense, and 
splicing mutations, have been found in compound heterozygotes [e.g., G542X/ 
S1255X or R553X/W1316X] . Interestingly, despite the fact that these indivi- 
duals cannot produce CFTR, they only have mild pulmonary disease, suggesting 
that the absence of CFTR is not incompatible with life (34,35). 
1.5 CFTR Protein and Its Function. The CFTR predicted protein is a 1480 single 
chain glycosylated polypeptide. Although the R- domain is unique to CFTR, the 
two membrane spanning domains and the two nucleotide -binding folds that bind 
and cleave ATP are also features of proteins in the traffic ATPase/ABC trans- 
porter super family that includes prokaryotic periplasmic permeases, the STE6 
yeast mating factor, and the P-glycoprotein multidrug resistance protein (36). 
There are two predicted carbohydrate side chains, both in an external loop of 
the C-terminal membrane spanning domain. Localization of the CFTR protein has 
been difficult because many of the antibodies generated against predicted 
peptides cross react with unidentified cellular components. It is generally 
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