highly conserved region of the first NBF (Collins, 1992), suggesting 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 70X of the CF mutant alleles (Kerem et al . , 1990). G551D, S549I, 
A455E, and G542X account for 10-20X of the non-delta F508 mutations (Kerem 
et al. , 1990). 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 rela- 
tively severe pancreatic insufficiency (Kerem et al., 1990). Frameshift, 
nonsense, and splicing mutations have been found in compound heterozygotes 
[e.g., G542X/S1255X or R553X/W1316X] . Interestingly, despite the fact that 
these individuals cannot produce CFTR, they only have mild pulmonary dis- 
ease, suggesting that the absence of CFTR is not incompatible with life 
(Cutting et al. , 1990; Hamosh et al. , 1991). 
1.7 CFTR Protein and Its Function 
The CFTR protein predicted from the CFTR gene is a 1480 single chain 
glycosylated polypeptide (Figure 1.6-A, 1.6-B). 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 transporter super family that includes prokaryotic 
periplasmic permeases, the STE6 yeast mating factor, and the P-glycoprotein 
multidrug resistance protein (Hyde et al., 1990). There are two predicted 
carbohydrate side chains, both in an external loop of the C- terminal mem- 
brane spanning domain. 
Localization of the CFTR protein has been difficult because many of the 
antibodies generated against predicted peptides cross react with uniden- 
tified cellular components. It is generally accepted, however, that the 
protein is present in epithelial tissues and that it is present in cell 
membranes (Cheng et al., 1990; Crawford et al. 1991; Denning et al., 1992; 
Marino et al . , 1991; Sarkadi et al . , 1992). CFTR has been localized to the 
apical membranes of pancreatic ducts, intestinal epithelia, sweat ducts, 
and airway epithelia. There is indirect functional data suggesting CFTR may 
also be localized to organelle membranes, including the Golgi apparatus 
(Barasch et al., 1991). 
There is convincing evidence that CFTR can function as a cAMP regulatable 
Cl - channel (Bear et al. , 1992). There is also evidence that CFTR may have 
other functions, such as recycling of vesicles (Bradbury et al., 1992). The 
regulation of CFTR as a Cl - channel is not completely defined, but it 
requires phosphorylation of the R-domain mediated by cAMP activation of 
protein kinase A (Cheng et al . , 1991; Picciotto et al., 1992). 
1.8 Molecular Pathogenesis of Cystic Fibrosis 
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Recombinant DNA Research, Volume 16 
