M.J. Welsh and A.E. Smith, RAC Application 
purified from human plasma has been aerosolized to deliver enzyme activity to the lung of 
CF patients (15). Another approach would be the use of agents to inhibit the action of 
oxidants derived from neutrophils. Although biochemical parameters have been 
successfully measured, the long term beneficial effects of these treatments have not been 
established. 
Using a different rationale, other investigators have attempted to use pharmacological 
agents to reverse the abnormally decreased chloride secretion and increased sodium 
absorption in CF airways (16). Defective electrolyte transport by airway epithelia is 
thought to alter the composition of the respiratory secretions and mucus (1,8,16). Hence, 
pharmacological treatments aimed at correcting the abnormalities in electrolyte transport 
could be beneficial. Trials are in progress with aerosolized versions of the drug amiloride; 
amiloride is a diuretic that inhibits sodium channels, thereby inhibiting sodium absorption. 
Initial results indicate that the drug is safe and suggest a slight change in the rate of disease 
progression, as measured by lung function tests (17,18). Nucleotides, such as ATP or UTP, 
stimulate purinergic receptors in the airway epithelium. As a result, they open a class of 
chloride channel that is different from CFTR chloride channels. In vitro studies indicate 
that ATP and UTP can stimulate chloride secretion (19). Preliminary trials to test the 
ability of nucleotides to stimulate secretion in vivo, and thereby correct the electrolyte 
transport abnormalities are underway. 
Despite progress in therapy, CF remains a lethal disease, and no current therapy treats the 
basic defect. 
2.3 The CF Gene and Gene Product 
Three areas of knowledge now allow new therapies to be developed for CF: a) knowledge 
about the gene which encodes CFTR; b) knowledge about the function and biochemistry of 
the protein product, CFTR; and c) knowledge about the molecular basis of the disease, i.e., 
how mutations in the gene cause dysfunction of CFTR. 
2.3.1 The gene encoding CFTR. 
The gene encoding CFTR is located in region q31 of chromosome 7 (4,5). It comprises 
about 240 kb and contains 27 exons. The mRNA is about 6.5 kb. The gene encodes a 
protein named cystic fibrosis transmembrane conductance regulator (CFTR). The protein 
comprises 1,480 amino acids. 
Based on amino acid sequence similarity and similar predicted topology, CFTR appears to 
belong to a family of proteins named the traffic ATPases (20) or ABC (ATP Binding 
Cassette) transporters (21). CFTR is predicted to contain five domains (5): two membrane 
spanning domains, each composed of six transmembrane segments; a unique R (regulatory) 
domain, which contains several consensus phosphorylation sequences; and two nucleotide 
binding domains, which were predicted to interact with ATP. 
2.3.2 CFTR function. 
Availability of the cDNA encoding CFTR has allowed the protein to be expressed in cells. 
In the first functional and biochemical studies, we expressed CFTR cDNA in primary 
cultures of CF airway epithelia (9,22) and Drumm and coworkers expressed CFTR in a 
pancreatic epithelial cell line (23). Expression of wild-type CFTR corrected the defect in 
c AMP-regulated chloride permeability. This result demonstrated a causal relationship 
between mutations in the CFTR gene and the CF phenotype. More importantly, this 
[858] 
Recombinant DNA Research, Volume 16 
