Exome & General Genetics
The most common disease-causing mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene is the out-of-frame deletion of 3 nucleotides (CTT). This mutation leads to the loss of phenylalanine-508 (?F508) and a silent codon change (SCC) for isoleucine-507 (I507-ATC?ATT). ?F508 CFTR is misfolded and degraded by endoplasmic reticulum-associated degradation (ERAD). We have demonstrated that the I507-ATC?ATT SCC alters ?F508 CFTR mRNA structure and translation dynamics. By comparing the biochemical and functional properties of the I507-ATT and I507-ATC ?F508 CFTR, we establish that the I507-ATC?ATT SCC contributes to the cotranslational misfolding, ERAD, and to the functional defects associated with ?F508 CFTR. We demonstrate that the I507-ATC ?F508 CFTR is less susceptible to the ER quality-control machinery during translation than the I507-ATT, although 27°C correction is necessary for sufficient cell-surface expression. Whole-cell patch-clamp recordings indicate sustained, thermally stable cAMP-activated Cl(-) transport through I507-ATC and unstable function of the I507-ATT ?F508 CFTR. Single-channel recordings reveal improved gating properties of the I507-ATC compared to I507-ATT ?F508 CFTR (NPo=0.45±0.037 vs. NPo=0.09±0.002; P<0.001). Our results signify the role of the I507-ATC?ATT SCC in the ?F508 CFTR defects and support the importance of synonymous codon choices in determining the function of gene products.