Systematic Evaluation of a Cluster of Nonsense Mutations in Exon 22 of CFTR to Inform Treatment

PI: Gary Cutting, MD

Institution: Johns Hopkins University School of Medicine

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Cystic fibrosis (CF) is a genetic disease caused by the alteration in the sequence of the gene named CFTR resulting in defective or deficient chloride channel activity. More than 1700 different gene alterations have been reported in CFTR from 80,000 individuals with CF worldwide. While there has been great success in developing therapies for alterations that allow the synthesis of CFTR protein, over half of the alterations affect processing of the RNA and we do not know if protein is made in these cases. Nonsense mutations that introduce a signal to stop protein production are the second most common form of RNA processing defect in CFTR. We have systematically explored nonsense mutations located in a region of CFTR which is thought to allow production of shortened but functional CFTR protein. We have used expression minigenes (EMGs) that mimic the RNA and protein processing steps that native CFTR undergoes. Nine nonsense mutations were introduced to these EMGs to assess effects on the RNA and protein of CFTR. We determined that 7 of the nine-nonsense mutation allowed production of shortened CFTR, finds its way to the cell surface, and possess residual channel function. Two nonsense mutations caused CFTR to be misfolded, not reach the cell surface and possess no function. Next, we tested response to the drugs that have already been developed to activate defective CFTR protein. We found that nonsense mutations that allowed residual CFTR function could activated by VX-770 (ivacaftor). Corrector compounds, VX-809 and VX-661 either alone or in combination, further enhanced the activation by VX-770. Our data show that CFTR-targeted therapy could be used for a subset of nonsense mutations that allow protein production.