Multicenter Characterization and Validation of the Intron-8 Poly(T) Tract (IVS8-T) Status in 25 Coriell Cell Repository Cystic Fibrosis Reference Cell Lines for Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Gene Mutation Assays (Technical Briefs‪)‬

Cystic fibrosis (CF) is the most common life-limiting recessive genetic disorder in Caucasians, with a carrier frequency of ~1 in 25 and incidence of ~1 in 2500-3300 live births (1). CF is caused by mutations affecting the transmembrane conductance regulator (CFTR) gene localized on the long arm of chromosome 7 (7831.2). CFTR contains 27 exons and encodes a protein of 1480 amino acids that functions as a CAMP-regulated chloride channel in the apical membrane of epithelial cells (2, 3). Mutations in the CFTR gene lead to dysfunction of the lungs, sweat glands, testes, ovaries, intestines, and pancreas. More than 1000 mutations in this gene have been identified to date (4). The clinical manifestations of the disease are variable, ranging from severe pulmonary disease with pancreatic insufficiency to mild pulmonary disease and pancreatic sufficiency (1). Moreover, mutations in the CFTR gene have also been found in patients who have normal lung function but show other clinical signs, such as congenital bilateral absence of the vas deferens (CBAVD), nasal polyposis, bronchiectasis, and bronchopulmonary allergic aspergillosis (5, 6). Some of the variability in the CF phenotype has been attributed to the influence of the 5T allele at a polymorphic poly(T) tract in intron 8 (IVS8-T) of the CFTR gene. Genotype-phenotype correlations have shown that there is a strong association of the 5T allele with male infertility caused by congenital CBAVD and with other monosymptomatic forms of CF, such as bronchiectasis and chronic idiopathic pancreatitis (5-7). At the IVS8-T locus, which functions as a splice acceptor site, three variants designated 5T, 7T, and 9T have been identified. The 5T variant is a poor splice acceptor site and gives rise to skipping of exon 9 in a high percentage of CFTR mRNA transcripts (7-11). CFTR mRNA missing exon 9 does not produce a functional protein. A CFTR gene with the 5T allele produces only 5% of the normal concentration of normal mRNA, and when coupled with a CF mutation, this can have a clinical effect (9).