Genetic, cell biological, and clinical interrogation of the CFTR mutation c.3700 A>G (p.Ile1234Val) informs strategies for future medical intervention.

Purpose: The purpose of this study was to determine the molecular consequences of the variant c.3700 A>G in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, a variant that has been predicted to cause a missense mutation in the CFTR protein (p.Ile1234Val).

Methods: Clinical assays of CFTR function were performed, and genomic DNA from patients homozygous for c.3700 A>G and their family members was sequenced. Total RNA was extracted from epithelial cells of the patients, transcribed into complementary DNA, and sequenced. CFTR complementary DNA clones containing the missense mutation p.Ile1234Val or a truncated exon 19 (p.Ile1234_Arg1239del) were constructed and heterologously expressed to test CFTR protein synthesis and processing.

Results: In vivo functional measurements revealed that the individuals homozygous for the variant c.3700 A>G exhibited defective CFTR function. We show that this mutation in exon 19 activates a cryptic donor splice site 18 bp upstream of the original donor splice site, resulting in deletion of six amino acids (r.3700_3717del; p.Ile1234_Arg1239del). This deletion, similar to p.Phe508del, causes a primary defect in folding and processing. Importantly, Lumacaftor (VX-809), currently in clinical trial for cystic fibrosis patients with the major cystic fibrosis-causing mutation, p.Phe508del, partially ameliorated the processing defect caused by p.Ile1234_Arg1239del.

Conclusion: These studies highlight the need to verify molecular and clinical consequences of CFTR variants to define possible therapeutic strategies.