Background The effect of complex alleles in cystic fibrosis (CF) is poorly defined for the lack of functional studies. Objectives To describe the genotype-phenotype correlation and the results of either in vitro and ex vivo studies performed on nasal epithelial cells (NEC) in a cohort of patients with CF carrying cystic fibrosis transmembrane conductance regulator (CFTR) complex alleles. Methods We studied 70 homozygous, compound heterozygous or heterozygous for CFTR mutations: p. [Arg74Trp;Val201Met;Asp1270Asn], n=8; p.[Ile148Thr; Ile1023_Val1024del], n=5; p.[Arg117Leu;Leu997Phe], n=6; c.[1210-34TG[12];1210-12T[5];2930C>T], n=3; p. [Arg74Trp;Asp1270Asn], n=4; p.Asp1270Asn, n=2; p. Ile148Thr, n=6; p.Leu997Phe, n=36. In 39 patients, we analysed the CFTR gating activity on NEC in comparison with patients with CF (n=8) and carriers (n=4). Finally, we analysed in vitro the p.[Arg74Trp;Val201Met;Asp1270Asn] complex allele. Results The p.[Ile148Thr;Ile1023_Val1024del] caused severe CF in five compound heterozygous with a class I-II mutation. Their CFTR activity on NEC was comparable with patients with two class I-II mutations (mean 7.3% vs 6.9%). The p.[Arg74Trp;Asp1270Asn] and the p. Asp1270Asn have scarce functional effects, while p. [Arg74Trp;Val201Met;Asp1270Asn] caused mild CF in four of five subjects carrying a class I-II mutation in trans, or CFTR-related disorders (CFTR-RD) in three having in trans a class IV-V mutation. The p.[Arg74Trp;Val201Met; Asp1270Asn] causes significantly (pT] and a class I-II mutation had mild CF or CFTR-RD (gating activity: 18.5-19.0%). Conclusions The effect of complex alleles partially depends on the mutation in trans. Although larger studies are necessary, the CFTR activity on NEC is a rapid contributory tool to classify patients with CFTR dysfunction
