Additive effect of multiple pharmacological chaperones on maturation of CFTR processing mutants

The most common cause of CF (cystic fibrosis) is the deletion of Phe(508) (Delta F508) in the CFTR [CF TM (transmembrane) conductance regulator] chloride channel. One major problem with Delta F508 CFTR is that the protein is defective in folding so that little mature protein is delivered to the cell surface. Expression of Delta F508 CFTR in the presence of small molecules known as correctors or pharmacological chaperones can increase the level of mature protein. Unfortunately, the efficiency of corrector-induced maturation of Delta F508 CFTR is probably too low to have therapeutic value and approaches are needed to increase maturation efficiency. We postulated that expression of Delta F508 CFTR in the presence of multiple correctors that bound to different sites may have an additive effect on maturation. In support of this mechanism, we found that expression of P-glycoprotein (CFTR's sister protein) processing mutants in the presence of two compounds that bind to different sites (rhodamine B and Hoechst 33342) had an additive effect on maturation. Therefore we tested whether expression of Delta F508 CFTR in the presence of combinations of three different classes of corrector molecules would increase its maturation efficiency. It was found that the combination of the quinazoline VRT-325 together with the thiazole corr-2b or bisaminomethylbithiazole corr-4a doubled the steady-state maturation efficiency of Delta F508 CFTR (approx. 40 % of total CFTR was mature protein) compared with expression in the presence of a single compound. The additive effect of the correctors on Delta F508 CFTR maturation suggests that they directly interact at different sites of the protein.