Validation of CFTR intronic variants identified during cystic fibrosis population screening by a minigene splicing assay
Cystic fibrosis, caused by mutations of the CFTR gene, is the most common autosomal recessive condition in the European population and there are specific screening programs aimed at investigating healthy carriers. They are usually articulated in two steps: initially individuals are screened with a panel of the 20-50 most common CFTR mutations; the second step is offered to partners of carriers who were found negative at the first test and consists in the analysis of the entire CFTR gene. This strategy provides high sensitivity, however, it often identifies novel variants (especially in introns) of unknown significance. Establishing the pathogenicity of these variants of the CFTR gene is not a simple task. We have examined five CFTR intronic variants of unclear significance (c.274-6T>C, c.744-6T>G, c.1117-64G>A, c.2620-26A>G, and c.3468+51C>A) using a functional splicing assay based on hybrid minigenes. Four out of five variants (including c.2620-26A>G which was previously reported as a possible splice-site mutation) did not alter the correct splicing of the minigene and are likely to be neutral polymorphisms, whereas c.744-6T>G caused complete skipping of CFTR exon 7 and should be therefore regarded as a pathogenic CFTR mutation. Hybrid minigenes assay are a simple and rapid tool to evaluate the effects of intronic variants without the need of analyzing patient's mRNA, and are particularly suited to analyze variants identified during population screenings.