Towards personalized medicine for cystic fibrosis patients with rare mutations

The autosomal recessive disease cystic fibrosis (CF) is caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR), a phosphorylation-regulated, ATP-gated anion channel present in the apical membrane of epithelial cells of the lung, intestine, pancreatic duct and sweat duct. Cystic fibrosis presents with impaired salt and water transport across all the above epithelia and manifests as a devastating multi-organ disease that reduces the quality of life and the life expectancy of affected individuals (O'Sullivan & Freedman, 2009). The several hundreds of CF-associated CFTR mutations identified in patients have been classified based on their functional consequences, such as the disruption of full-length polypeptide synthesis (class I), protein folding/processing (class II), channel gating (class III) or anion permeation through the open channel pore (class IV) (de Boeck & Amaral, 2016). The most prevalent CF mutation, deletion of phenylalanine 508 (F508del), and several other mutations have been shown to belong to more than one class. In recent years, a major breakthrough in the treatment of CF was achieved through the development of two classes of small-molecule drugs: 'correctors' that improve folding/processing of class II CFTR mutants, and 'potentiators' that stimulate gating of class III mutant channels. As of today, ∼90% of CF patients benefit from effective causative treatment, either by the potentiator VX-770 (Ivacaftor) alone or by a combination of VX-770 with a cocktail of corrector drugs (Trikafta) (Barry et al., 2021). However, no causative treatment has yet been approved for ∼10% of CF patients who carry either nonsense (class I) mutations or rare, uncharacterized missense mutations. Studying the molecular mechanisms of such rare missense mutations has strong translational potential for two reasons. First, some patients who carry uncharacterized mutations are currently not eligible for pharmacotherapy even though the tools for treating them might be at hand. Second, even among patients who receive pharmacotherapy, the response to treatment is variable and mutation specific. Thus, a better understanding of the underlying molecular pathologies might promote the development of more tailored treatment recommendations. In a recent study, Li and colleagues investigated the molecular properties of two rare CFTR mutants, S1159F and S1159P, both of which affect a single position that lines the ion permeation pathway of the channel (Li et al., 2023). Earlier studies had classified those variants as class II mutations (Han et al., 2018), but single-molecule data were unavailable. Detailed investigation using single-channel recordings first of all revealed, for both mutants, a strong reduction in the fraction of time the pore is open (open probability, Po). Compared with wild-type CFTR, Po was reduced by ∼10-fold and ∼4-fold, respectively, for S1159F and S1159P, categorizing both mutations into class III. From a kinetic point of view, the reduced Po was a consequence of both a reduction in opening rate and an acceleration of closing rate. In addition, both mutations slightly reduced the rate of chloride ion flow through the open pore, revealing that they both fall also into class IV. Although that reduction in unitary conductance was larger for the proline (∼20%) compared with the phenylalanine (∼10%) variant, overall the gating defects dominate the phenotypes. Correspondingly, the more severe gating phenotype of the phenylalanine variant is well correlated with the severity of the disease observed in patients carrying the latter substitution. Based on the large reduction in Po, a strong stimulation by the potentiator drug VX-770 was expected. Surprisingly, however, the enhancement of Po by VX-770 was only ∼2-fold for both mutants, which is far less than that documented for the more prevalent gating mutations G551D or F508del (∼10-fold). That partial effect of the drug on the S1159F/P variants, attributable to an isolated rescue of channel opening rate but a lack of effect on channel closing rate, failed to stimulate gating to near wild-type levels. Altogether, the new data suggest that VX-770, the potentiator component of the Trikafta combination, is far from optimal for the S1159 variants. A number of alternative potentiator drugs are currently in preclinical trials. Future research will be required to identify other potentiators that either stimulate these mutants more efficiently than VX-770 or act additively with it. Such in vitro drug profiling might offer further clinical benefit to these patients and to CF patients who carry other rare CF alleles. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. No competing interests declared. Sole author. EU Horizon 2020 Research and Innovation Program: László Csanády, 739593; Nemzeti Kutatási Fejlesztési és Innovációs Hivatal (NKFI): László Csanády, 144199.