Clinical presentation and basic defect of the CFTR genotype p.Phe508del / p.Arg117His in a mother and her monozygous twin daughters.

A non-consanguineous two-generation family of parent and monozygous twins who all three share the same CFTR mutation genotype p.Phe508del / p.Arg117His, was examined in clinical features, sweat test, nasal potential difference and intestinal current measurements. As expected the twins were very much alike in anthropometry and appearance and shared the clinical manifestation of CFTR dysfunction albeit at different intensity but unexpectedly like in comparison to their mother they were discordant in their CFTR-mediated basic defect and the response thereof to CFTR potentiation by ivacaftor. This case report illustrates the strong impact of non-inherited factors on the electrophysiological phenotype of the most common CFTR mutation genotype of variable clinical significance.