Genotype-phenotype correlation and functional studies in patients with cystic fibrosis bearing CFTR complex alleles

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 (p<0.001) higher CFTR activity compared with compound heterozygous for class I-II mutations. Furthermore, five of six compounds heterozygous with the p.[Arg117Leu;Leu997Phe] had mild CF, whereas the p.Leu997Phe, in trans with a class I-II CFTR mutation, caused CFTR-RD or a healthy status (CFTR activity: 21.3-36.9%). Finally, compounds heterozygous for the c.[1210-34TG[12];1210-12T[5];2930C>T] 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.

Impact of distant charge reversals within a robust beta-barrel protein pore

Among all beta-barrel pores, staphylococcal alpha-hemolysin (alphaHL), a heptameric transmembrane protein of known high-resolution crystal structure, features a high stability in planar lipid bilayers under a wide range of harsh experimental conditions. Here, we employed single-channel electrical recordings and standard protein engineering to explore the impact of two distant charge reversals within the interior of the beta-barrel part of the pore. The charge reversals were replacements of lysines with aspartic acids. A charge reversal within the structurally stiff region of the beta barrel near the pore constriction reduced the open-state current of the pore, but produced a quiet pore, showing current fluctuation-free channel behavior. In contrast, a charge reversal on the trans entrance, within the structurally flexible glycine-rich turn of the beta barrel, increased the open-state current and produced gating activity of the pore in the form of large-amplitude and frequent current fluctuations. Remarkably, cumulative insertion of the two distant charge reversals resulted in a large-amplitude permanent blockade of the beta barrel, as judged by both single-channel and macroscopic current measurements. The results from this work suggest that these distant charge reversals are energetically coupled, producing different impacts on the ionic transport, the unitary conductance and the open-state probability of the pore.