Mechanistic Approaches to Improve Correction of the Most Common Disease-Causing Mutation in Cystic Fibrosis

The most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene leads to deletion of the phenylalanine at position 508 (Delta F508) in the CFTR protein and causes multiple folding and functional defects. Contrary to large-scale efforts by industry and academia, no significant therapeutic benefit has been achieved with a single "corrector". Therefore, investigations concentrate on drug combinations. Orkambi (Vertex Pharmaceuticals), the first FDA-approved drug for treatment of cystic fibrosis (CF) caused by this mutation, is a combination of a corrector (VX-809) that facilitates Delta F508 CFTR biogenesis and a potentiator (VX-770), which improves its function. Yet, clinical trials utilizing this combination showed only modest therapeutic benefit. The low efficacy Orkambi has been attributed to VX-770-mediated destabilization of VX-809-rescued Delta F508 CFTR. Here we report that the negative effects of VX-770 can be reversed by increasing the half-life of the endoplasmic reticulum (ER) form (band B) of Delta F508 CFTR with another corrector (Corr-4a.) Although Corr-4a alone has only minimal effects on Delta F508 CFTR rescue, it increases the half-life of Delta F508 CFTR band B when it is present during half-life measurements. Our data shows that stabilization of band B Delta F508 CFTR with Corr-4a and simultaneous rescue with VX-809, leads to a >2-fold increase in cAMP-activated, CFTRinh-172-inhibited currents compared to VX-809 alone, or VX-809+VX-770. The negative effects of VX-770 and the Corr-4a protection are specific to the native I507-ATT Delta F508 CFTR without affecting the inherently more stable, synonymous variant I507-ATC Delta F508 CFTR. Our studies emphasize that stabilization of Delta F508 CFTR band B in the ER might improve its functional rescue by Orkambi.