Unusually common cystic fibrosis mutation in Portugal encodes a misprocessed protein

Extensive CFTR sequencing through NGS in Brazilian individuals with cystic fibrosis: unravelling regional discrepancies in the country

BACKGROUND

The Brazilian population has a tri-hybrid composition with a high degree of ethnic admixture. We hypothesized that Brazilian individuals with CF from different Brazilian regions have a specific distribution of CFTR variants.

METHODS

Individuals with CF with data available in the Patient Registry and without an established genotype were submitted to CFTR sequencing by Next Generation Sequencing (NGS) methodology, and results were anonymously incorporated to the Registry Database. Genotyping results were expressed as 'positive', 'inconclusive' or 'negative'. Logistic regression models were performed to investigate the association between demographic/clinical variables and genotyping results. Mediation analysis was conducted to estimate direct and indirect effects of Brazilian region on a binary positive genotyping response.

RESULTS

In October 2017, data from 4,654 individuals with CF were available, and 3,104(66.7%) of them had a genotyping result. A total of 236 variants (114 new variants) were identified, with F508del identified in 46% of the alleles tested. Genotyping revealed 2,002(64.5%) individuals positive, 757(24.4%) inconclusive and 345(11.1%) negative. Distribution of genotype categories was markedly different across Brazilian Regions, with greater proportions of negative individuals in the North (45%) and Northeast (26%) regions. Newborn screening (CF-NBS) and age at diagnosis were identified as mediators of the effect of Brazilian region on a positive genotyping result.

CONCLUSIONS

This large initiative of CFTR genotyping showed significant regional discrepancies in Brazil, probably related to socio-economic conditions, lack of adequate CF-NBS and poor access to reliable sweat testing. These results may be useful to indicate Regions where CF care demands more attention.

Cystic Fibrosis: Overview of the Current Development Trends and Innovative Therapeutic Strategies

Cystic Fibrosis (CF), an autosomal recessive genetic disease, is caused by a mutation in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). This mutation reduces the release of chloride ions (Cl-) in epithelial tissues, and hyperactivates the epithelial sodium channels (ENaC) which aid in the absorption of sodium ions (Na+). Consequently, the mucus becomes dehydrated and thickened, making it a suitable medium for microbial growth. CF causes several chronic lung complications like thickened mucus, bacterial infection and inflammation, progressive loss of lung function, and ultimately, death. Until recently, the standard of clinical care in CF treatment had focused on preventing and treating the disease complications. In this review, we have summarized the current knowledge on CF pathogenesis and provided an outlook on the current therapeutic approaches relevant to CF (i.e., CFTR modulators and ENaC inhibitors). The enormous potential in targeting bacterial biofilms using antibiofilm peptides, and the innovative therapeutic strategies in using the CRISPR/Cas approach as a gene-editing tool to repair the CFTR mutation have been reviewed. Finally, we have discussed the wide range of drug delivery systems available, particularly non-viral vectors, and the optimal properties of nanocarriers which are essential for successful drug delivery to the lungs.

R560S: A class II CFTR mutation that is not rescued by current modulators

BACKGROUND

New therapies modulating defective CFTR have started to hit the clinic and others are in trial or under development. The endeavour of drug discovery for CFTR protein rescue is however difficult one since over 2000 mutations have been reported. For most of these, especially the rare ones, the associated defects, the respective functional class and their responsiveness to available modulators are still unknown. Our aim here was to characterize the rare R560S mutation using patient-derived materials (rectal biopsies and intestinal organoids) from one CF individual homozygous for this mutation, in parallel with cellular models expressing R560S-CFTR and to assess the functional and biochemical responses to CFTR modulators.

METHODS

Intestinal organoids were prepared from rectal biopsies and analysed by RT-PCR (to assess CFTR mRNA), by Western blot (to assess CFTR protein) and by forskolin-induced swelling (FIS) assay. A novel cell line expressing R560S-CFTR was generated by stably transducing the CFBE parental cell line and used to assess R560S-CFTR processing and function. Both intestinal organoids and the cellular model were used to assess efficacy of CFTR modulators in rescuing this mutation.

RESULTS

Our results show that: R560S does not affect CFTR mRNA splicing; R560S affects CFTR protein processing, totally abrogating the production of its mature form; R560S-CFTR evidences no function as a Cl^- channel; and none of the modulators tested rescued R560S-CFTR processing or function.

CONCLUSION

Altogether, these results indicate that R560S is a class II mutation. However, unlike F508del, it cannot be rescued by any of the CFTR modulators tested.

Partial rescue of F508del-cystic fibrosis transmembrane conductance regulator channel gating with modest improvement of protein processing, but not stability, by a dual-acting small molecule

BACKGROUND AND PURPOSE

Rescue of F508del-cystic fibrosis (CF) transmembrane conductance regulator (CFTR), the most common CF mutation, requires small molecules that overcome protein processing, stability and channel gating defects. Here, we investigate F508del-CFTR rescue by CFFT-004, a small molecule designed to independently correct protein processing and channel gating defects.

EXPERIMENTAL APPROACH

Using CFTR-expressing recombinant cells and CF patient-derived bronchial epithelial cells, we studied CFTR expression by Western blotting and channel gating and stability with the patch-clamp and Ussing chamber techniques.

KEY RESULTS

Chronic treatment with CFFT-004 improved modestly F508del-CFTR processing, but not its plasma membrane stability. By contrast, CFFT-004 rescued F508del-CFTR channel gating better than C18, an analogue of the clinically used CFTR corrector lumacaftor. Subsequent acute addition of CFFT-004, but not C18, potentiated F508del-CFTR channel gating. However, CFFT-004 was without effect on A561E-CFTR, a CF mutation with a comparable mechanism of CFTR dysfunction as F508del-CFTR. To investigate the mechanism of action of CFFT-004, we used F508del-CFTR revertant mutations. Potentiation by CFFT-004 was unaffected by revertant mutations, but correction was abolished by the revertant mutation G550E. These data suggest that correction, but not potentiation, by CFFT-004 might involve nucleotide-binding domain 1 of CFTR.

CONCLUSIONS AND IMPLICATIONS

CFFT-004 is a dual-acting small molecule with independent corrector and potentiator activities that partially rescues F508del-CFTR in recombinant cells and native airway epithelia. The limited efficacy and potency of CFFT-004 suggests that combinations of small molecules targeting different defects in F508del-CFTR might be a more effective therapeutic strategy than a single agent.

CFTR founder mutation causes protein trafficking defects in Chinese patients with cystic fibrosis

Background

Cystic fibrosis (CF) is a rare condition in Asians. Since 1985, only about 30 Chinese patients have been reported with molecular confirmation.

Method

Using our in‐house next‐generation sequencing (NGS) pipeline for childhood bronchiectasis, we identified disease‐causing CFTR mutations in CF patients in Hong Kong. After identifying p.I1023R in multiple patients, haplotype analysis was performed with genome‐wide microarray to ascertain the likelihood of this being a founder mutation. We also assessed the processing and gating activity of the mutant protein by Western hybridization and patch‐clamp test.

Results

Molecular diagnoses were confirmed in four patients, three of whom shared a missense mutation: CFTR:c.3068T>G:p.I1023R. The results suggested that p.I1023R is a founder mutation in southern Han Chinese. In addition, the processing and gating activity of the mutant protein was assessed by gel electrophoresis and a patch‐clamp test. The mutant protein exhibited trafficking defects, suggesting that the dysfunction is caused by reduced cell surface expression of the fully glycosylated proteins.

Conclusion

Together with other previously reported mutations, the specific founder mutation presented herein suggests a unique CFTR mutation spectrum in the southern Chinese populations, and this finding has vital implications for improving molecular testing and mutation‐specific treatments for Chinese patients with CF.

DCTN4 as a modifier of chronic Pseudomonas aeruginosa infection in cystic fibrosis

BACKGROUND AND AIMS

Pseudomonas aeruginosa (Pa) infection in cystic fibrosis (CF) patients is associated with worse long-term pulmonary disease and shorter survival, and chronic Pa infection (CPA) is associated with reduced lung function, faster rate of lung decline, increased rates of exacerbations and shorter survival. By using exome sequencing and extreme phenotype design, it was recently shown that isoforms of dynactin 4 (DCTN4) may influence Pa infection in CF, leading to worse respiratory disease. The purpose of this study was to investigate the role of DCTN4 missense variants on Pa infection incidence, age at first Pa infection and chronic Pa infection incidence in a cohort of adult CF patients from a single centre.

METHODS

Polymerase chain reaction and direct sequencing were used to screen DNA samples for DCTN4 variants.

RESULTS

A total of 121 adult CF patients from the Cochin Hospital CF centre have been included, all of them carrying two CFTR defects: 103 developed at least 1 pulmonary infection with Pa, and 68 patients of them had CPA. DCTN4 variants were identified in 24% (29/121) CF patients with Pa infection and in only 17% (3/18) CF patients with no Pa infection. Of the patients with CPA, 29% (20/68) had DCTN4 missense variants vs 23% (8/35) in patients without CPA. Interestingly, p.Tyr263Cys tend to be more frequently observed in CF patients with CPA than in patients without CPA (4/68 vs 0/35), and DCTN4 missense variants tend to be more frequent in male CF patients with CPA bearing two class II mutations than in male CF patients without CPA bearing two class II mutations (P = 0.06).

CONCLUSIONS

Our observations reinforce that DCTN4 missense variants, especially p.Tyr263Cys, may be involved in the pathogenesis of CPA in male CF.

Measurements of Functional Responses in Human Primary Lung Cells as a Basis for Personalized Therapy for Cystic Fibrosis

Background

The best investigational drug to treat cystic fibrosis (CF) patients with the most common CF-causing mutation (F508del) is VX-809 (lumacaftor) which recently succeeded in Phase III clinical trial in combination with ivacaftor. This corrector rescues F508del-CFTR from its abnormal intracellular localization to the cell surface, a traffic defect shared by all Class II CFTR mutants. Our goal here is to test the efficacy of lumacaftor in other Class II mutants in primary human bronchial epithelial (HBE) cells derived from CF patients.

Methods

The effect of lumacaftor was investigated in primary HBE cells from non-CF and CF patients with F508del/F508del, A561E/A561E, N1303K/G542X, F508del/G542X and F508del/Y1092X genotypes by measurements of Forskolin plus Genistein-inducible equivalent short-circuit current (I_{eq-SC-Fsk + Gen}) in perfused open-circuit Ussing chambers. Efficacy of corrector C18 was also assessed on A561E/A561E and F508del/F508del cells.

Results

Our data indicate that A561E (when present in both alleles) responds positively to lumacaftor treatment at equivalent efficacy of F508del in primary HBE cells. Similarly, lumacaftor has a positive impact on Y1092X, but not on N1303K. Our data also show that cells with only one copy of F508del-CFTR respond less to VX-809. Moreover, there is great variability in lumacaftor responses among F508del-homozygous cells from different donors. Compound C18 failed to rescue A561E-CFTR but not in F508del-CFTR, thus plausibly it has a different mechanism of action distinct from lumacaftor.

Conclusions

CF patients with A561E (and likely also those with Y1029X) can potentially benefit from lumacaftor. Moreover, the methodology used here exemplifies how ex vivo approaches may apply personalized therapies to CF and possibly other respiratory diseases.

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A561E and Y1092X CFTR mutations (but not N1303K) respond positively to lumacaftor treatment.

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One copy of F508del-CFTR responds less to lumacaftor than 2 copies and F508del-responses vary greatly.

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CF patients with A561E (and likely also those with Y1029X) can potentially benefit from lumacaftor.

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The methodology used here exemplifies how ex vivo approaches may apply personalized therapies to CF and possibly other respiratory diseases.

CFTR potentiators partially restore channel function to A561E-CFTR, a cystic fibrosis mutant with a similar mechanism of dysfunction as F508del-CFTR

BACKGROUND AND PURPOSE

Dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel causes the genetic disease cystic fibrosis (CF). Towards the development of transformational drug therapies for CF, we investigated the channel function and action of CFTR potentiators on A561E, a CF mutation found frequently in Portugal. Like the most common CF mutation F508del, A561E causes a temperature-sensitive folding defect that prevents CFTR delivery to the cell membrane and is associated with severe disease.

EXPERIMENTAL APPROACH

Using baby hamster kidney cells expressing recombinant CFTR, we investigated CFTR expression by cell surface biotinylation, and function and pharmacology with the iodide efflux and patch-clamp techniques.

KEY RESULTS

Low temperature incubation delivered a small proportion of A561E-CFTR protein to the cell surface. Like F508del-CFTR, low temperature-rescued A561E-CFTR exhibited a severe gating defect characterized by brief channel openings separated by prolonged channel closures. A561E-CFTR also exhibited thermoinstability, losing function more quickly than F508del-CFTR in cell-free membrane patches and intact cells. Using the iodide efflux assay, CFTR potentiators, including genistein and the clinically approved small-molecule ivacaftor, partially restored function to A561E-CFTR. Interestingly, ivacaftor restored wild-type levels of channel activity (as measured by open probability) to single A561E- and F508del-CFTR Cl(-) channels. However, it accentuated the thermoinstability of both mutants in cell-free membrane patches.

CONCLUSIONS AND IMPLICATIONS

Like F508del-CFTR, A561E-CFTR perturbs protein processing, thermostability and channel gating. CFTR potentiators partially restore channel function to low temperature-rescued A561E-CFTR. Transformational drug therapy for A561E-CFTR is likely to require CFTR correctors, CFTR potentiators and special attention to thermostability.

Protein folding in the endoplasmic reticulum

In this article, we will cover the folding of proteins in the lumen of the endoplasmic reticulum (ER), including the role of three types of covalent modifications: signal peptide removal, N-linked glycosylation, and disulfide bond formation, as well as the function and importance of resident ER folding factors. These folding factors consist of classical chaperones and their cochaperones, the carbohydrate-binding chaperones, and the folding catalysts of the PDI and proline cis-trans isomerase families. We will conclude with the perspective of the folding protein: a comparison of characteristics and folding and exit rates for proteins that travel through the ER as clients of the ER machinery.

The effect of NO-donors on chloride efflux, intracellular Ca(2+) concentration and mRNA expression of CFTR and ENaC in cystic fibrosis airway epithelial cells

Since previous studies showed that the endogenous bronchodilator, S-nitrosglutathione (GSNO), caused a marked increase in CFTR-mediated chloride (Cl(-)) efflux and improved the trafficking of CFTR to the plasma membrane, and that also the nitric oxide (NO)-donor GEA3162 had a similar, but smaller, effect on Cl(-) efflux, it was investigated whether the NO-donor properties of GSNO were relevant for its effect on Cl(-) efflux from airway epithelial cells. Hence, the effect of a number of other NO-donors, sodium nitroprusside (SNP), S-nitroso-N-acetyl-DL-penicillamine (SNAP), diethylenetriamine/nitric oxide adduct (DETA-NO), and diethylenetriamine/nitric oxide adduct (DEA-NONOate) on Cl(-) efflux from CFBE (∆F508/∆F508-CFTR) airway epithelial cells was tested. Cl(-) efflux was determined using the fluorescent N-(ethoxycarbonylmethyl)-6-methoxyquinoliniu bromide (MQAE)-technique. Possible changes in the intracellular Ca(2+) concentration were tested by the fluorescent fluo-4 method in a confocal microscope system. Like previously with GSNO, after 4 h incubation with the NO-donor, an increased Cl(-) efflux was found (in the order SNAP>DETA-NO>SNP). The effect of DEA-NONOate on Cl(-) efflux was not significant, and the compound may have (unspecific) deleterious effects on the cells. Again, as with GSNO, after a short (5 min) incubation, SNP had no significant effect on Cl(-) efflux. None of the NO-donors that had a significant effect on Cl(-) efflux caused significant changes in the intracellular Ca(2+) concentration. After 4 h preincubation, SNP caused a significant increase in the mRNA expression of CFTR. SNAP and DEA-NONOate decreased the mRNA expression of all ENaC subunits significantly. DETA-NO caused a significant decrease only in α-ENaC expression. After a short preincubation, none of the NO-donors had a significant effect, neither on the expression of CFTR, nor on that of the ENaC subunits in the presence and absence of L-cysteine. It can be concluded that the effect of GSNO on Cl(-) efflux is, at least in part, due to its properties as an NO-donor, and the effect is likely to be mediated by CFTR, not by Ca(2+)-activated Cl(-) channels.

Antagonistic regulation of cystic fibrosis transmembrane conductance regulator cell surface expression by protein kinases WNK4 and spleen tyrosine kinase

Members of the WNK (with-no-lysine [K]) subfamily of protein kinases regulate various ion channels involved in sodium, potassium, and chloride homeostasis by either inducing their phosphorylation or regulating the number of channel proteins expressed at the cell surface. Here, we describe findings demonstrating that the cell surface expression of the cystic fibrosis transmembrane conductance regulator (CFTR) is also regulated by WNK4 in mammalian cells. This effect of WNK4 is independent of the presence of kinase and involves interaction with and inhibition of spleen tyrosine kinase (Syk), which phosphorylates Tyr512 in the first nucleotide-binding domain 1 (NBD1) of CFTR. Transfection of catalytically active Syk into CFTR-expressing baby hamster kidney cells reduces the cell surface expression of CFTR, whereas that of WNK4 promotes it. This is shown by biotinylation of cell surface proteins, immunofluorescence microscopy, and functional efflux assays. Mutation of Tyr512 to either glutamic acid or phenylalanine is sufficient to alter CFTR surface levels. In human airway epithelial cells, downregulation of endogenous Syk and WNK4 confirms their roles as physiologic regulators of CFTR surface expression. Together, our results show that Tyr512 phosphorylation is a novel signal regulating the prevalence of CFTR at the cell surface and that WNK4 and Syk perform an antagonistic role in this process.

Signaling pathways of proteostasis network unraveled by proteomic approaches on the understanding of misfolded protein rescue

Attempts to promote normal processing and function of F508del-CFTR, the most common mutant in cystic fibrosis (CF), have been described as potential therapeutic strategies in the management of this disease. Here we described the proteomic approaches, namely two-dimensional electrophoresis (2DE), mass spectrometry (MS), and bioinformatics tools used in our recent studies to gain insight into the proteins potentially involved in low-temperature or mutagenic treatment-induced rescue process of F508del-CFTR. The proteins identified are part of the proteostasis network, such as the unfolded protein response (UPR) signaling pathways that may regulate the processing of CF transmembrane conductance regulator (CFTR) through the folding and trafficking progression. The complete characterization of these signaling pathways and their regulators in CF will certainly contribute to the development of novel therapeutic strategies against CF.

Proteomics uncovering possible key players in F508del-CFTR processing and trafficking

The achievement and maintenance of a protein native conformation is a very complex cellular process involving a multitude of key factors whose contribution to a successful folding remains to be elucidated. On top of this, it is known that correct folding is crucial for proteins to play their normal role and, consequently, for the maintenance of cellular homeostasis or proteostasis. If the folding process is affected, the protein is unable to achieve its native conformation, compromising its life and function, and a pathological condition may arise. Protein-misfolding diseases are characterized by either formation of protein aggregates that are toxic to the cell (gain-of-toxic-function diseases) or by an incorrect processing of proteins, which leads to a deficiency in protein activity (loss-of-function diseases). In this article we have focused on proteomics advances in the molecular knowledge of protein-misfolding diseases with direct impact on possible key players in F508del-CFTR processing and trafficking.

Low temperature restoring effect on F508del-CFTR misprocessing: A proteomic approach

To gain insight into the proteins potentially involved in the low temperature-induced F508del-CFTR rescue process, we have explored by two-dimensional electrophoresis (2DE) the proteome of BHK cell lines expressing wt or F508del-CFTR, grown at 37 degrees C or 26 degrees C/24h or 26 degrees C/48h followed by 3h of metabolic labelling with [(35)S]-methionine. A set of 139 protein spots (yielding 125 mass spectrometry identifications) was identified as differentially expressed (p ANOVA<0.05) among the six phenotypic groups analysed. The data analysis suggests that the unfolded protein response (UPR) induction and some cell-metabolism repression are the major cold-shock responses that may generate a favourable cellular environment to promote F508del-CFTR rescue. Down-regulation of proteasome regulatory PA28 and/or COP9 signalosome subunit, both involved in CFTR degradation, could also be a relevant cold-shock-induced condition for F508de-CFTR rescue. Moreover, cold-shock may promote the reestablishment of some proteostasis imbalance associated with over-expression of F508del-CFTR. In BHK-F508del cells, the deregulation of RACK1, a protein described to be important for stable expression of CFTR in the plasma membrane, is partially repaired after low temperature treatment. Together these findings give new insights about F508del-CFTR rescue by low temperature treatment and the proteins involved could ultimately constitute potential therapeutic targets in CF disease.

N-terminal CFTR missense variants severely affect the behavior of the CFTR chloride channel

Over 1,500 cystic fibrosis transmembrane conductance regulator (CFTR) gene sequence variations have been identified in patients with cystic fibrosis (CF) and related disorders involving an impaired function of the CFTR chloride channel. However, detailed structure-function analyses have only been established for a few of them. This study aimed evaluating the impact of eight N-terminus CFTR natural missense changes on channel behavior. By site-directed mutagenesis, we generated four CFTR variants in the N-terminal cytoplasmic tail (p.P5L, p.S50P, p.E60K, and p.R75Q) and four in the first transmembrane segment of membrane-spanning domain 1 (p.G85E/V, p.Y89C, and p.E92K). Immunoblot analysis revealed that p.S50P, p.E60K, p.G85E/V, and p.E92K produced only core-glycosylated proteins. Immunofluorescence and whole cell patch-clamp confirmed intracellular retention, thus reflecting a defect of CFTR folding and/or trafficking. In contrast, both p.R75Q and p.Y89C had a glycosylation pattern and a subcellular distribution comparable to the wild-type CFTR, while the percentage of mature p.P5L was considerably reduced, suggesting a major biogenesis flaw on this channel. Nevertheless, whole-cell chloride currents were recorded for all three variants. Single-channel patch-clamp analyses revealed that the channel activity of p.R75Q appeared similar to that of the wild-type CFTR, while both p.P5L and p.Y89C channels displayed abnormal gating. Overall, our results predict a major impact of the CFTR missense variants analyzed, except p.R75Q, on the CF phenotype and highlight the importance of the CFTR N-terminus on channel physiology.

A missense mutation in the sodium phosphate co-transporter Slc34a1 impairs phosphate homeostasis

The sodium phosphate co-transporters Npt2a and Npt2c play important roles in the regulation of phosphate homeostasis. Slc34a1, the gene encoding Npt2a, resides downstream of the gene encoding coagulation factor XII (f12) and was inadvertently modified while generating f12(-/-) mice. In this report, the renal consequences of this modification are described. The combined single allelic mutant Slc34a1m contains two point mutations in exon 13: A499V is located in intracellular loop 5, and V528M is located in transmembrane domain 11. In addition to the expected coagulopathy of the f12(-/-) phenotype, mice homozygous for the double allelic modification (f12(-/-)/slc34a1(m/m)) displayed hypophosphatemia, hypercalcemia, elevated levels of alkaline phosphatase, urolithiasis, and hydronephrosis. Strategic cross-breedings demonstrated that the kidney-related pathology was associated only with autosomal recessive transmission of the slc34a1(m) gene and was not influenced by the simultaneous inactivation of f12. Npt2a[V528M] could be properly expressed in opossum kidney cells, but Npt2a[A499V] could not. These results suggest that a single amino acid substitution in Npt2a can lead to improper translocation of the protein to the cell membrane, disturbance of phosphate homeostasis, and renal calcification. Whether point mutations in the SLC34A1 gene can lead to hypophosphatemia and nephrolithiasis in humans remains unknown.

Revertant mutants G550E and 4RK rescue cystic fibrosis mutants in the first nucleotide-binding domain of CFTR by different mechanisms

The revertant mutations G550E and 4RK [the simultaneous mutation of four arginine-framed tripeptides (AFTs): R29K, R516K, R555K, and R766K] rescue the cell surface expression and function of F508del-cystic fibrosis (CF) transmembrane conductance regulator (-CFTR), the most common CF mutation. Here, we investigate their mechanism of action by using biochemical and functional assays to examine their effects on F508del and three CF mutations (R560T, A561E, and V562I) located within a conserved region of the first nucleotide-binding domain (NBD1) of CFTR. Like F508del, R560T and A561E disrupt CFTR trafficking. G550E rescued the trafficking defect of A561E but not that of R560T. Of note, the processing and function of V562I were equivalent to that of wild-type (wt)-CFTR, suggesting that V562I is not a disease-causing mutation. Biochemical studies revealed that 4RK generates higher steady-state levels of mature CFTR (band C) for wt- and V562I-CFTR than does G550E. Moreover, functional studies showed that the revertants rescue the gating defect of F508del-CFTR with different efficacies. 4RK modestly increased F508del-CFTR activity by prolonging channel openings, whereas G550E restored F508del-CFTR activity to wt levels by altering the duration of channel openings and closings. Thus, our data suggest that the revertants G550E and 4RK might rescue F508del-CFTR by distinct mechanisms. G550E likely alters the conformation of NBD1, whereas 4RK allows F508del-CFTR to escape endoplasmic reticulum retention/retrieval mediated by AFTs. We propose that AFTs might constitute a checkpoint for endoplasmic reticulum quality control.

Activation of chloride transport in CF airway epithelial cell lines and primary CF nasal epithelial cells by S-nitrosoglutathione

Background

It has been suggested that low μM concentrations of S-nitrosoglutathione (GSNO), an endogenous bronchodilator, may promote maturation of the defective cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Because nitric oxide (NO) and GSNO levels appear to be low in the CF airway, there is an interest in the possibility that GSNO replacement could be of therapeutic benefit in CF.

Methods

The effect of GSNO on chloride (Cl^-) transport was investigated in primary nasal epithelial cells obtained from CF patients homozygous for the delF508 mutation, as well as in two CF cell lines (CFBE and CFSME), using both a fluorescent Cl^- indicator and X-ray microanalysis. Maturation of delF508 CFTR was determined by immunoblotting.

Results

Treatment with 60 μM GSNO for 4 hours increased cAMP-induced chloride efflux in nasal epithelial cells from 18 out of 21 CF patients, but did not significantly affect Cl^- efflux in cells from healthy controls. This Cl^- efflux was confirmed by measurements with a fluorescent Cl^- indicator in the CFBE and CFSME cell lines. The effect of GSNO on Cl^- efflux in CFBE cells could be inhibited both by a specific thiazolidinone CFTR inhibitor (CFTR_inh-172) and by 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H₂DIDS). X-ray microanalysis showed that, following 4 hours incubation with 60 μM GSNO, cAMP agonists caused a decrease in the cellular Cl^- concentration in CFBE cells, corresponding to Cl^- efflux. GSNO exposure resulted in an increase in the protein expression and maturation, as shown by immunoblot analysis. GSNO did not increase the cytosolic Ca²⁺ concentration in cultured airway epithelial cells.

Conclusion

Previous studies have suggested that treatment with GSNO promotes maturation of delF508-CFTR, consistent with our results in this study. Here we show that GSNO increases chloride efflux, both in the two CF cell lines and in primary nasal epithelial cells from delF508-CF patients. This effect is at least in part mediated by CFTR. GSNO may be a candidate for pharmacological treatment of the defective chloride transport in CF epithelial cells.

Insight in eukaryotic ABC transporter function by mutation analysis

With regard to structure-function relations of ATP-binding cassette (ABC) transporters several intriguing questions are in the spotlight of active research: Why do functional ABC transporters possess two ATP binding and hydrolysis domains together with two ABC signatures and to what extent are the individual nucleotide-binding domains independent or interacting? Where is the substrate-binding site and how is ATP hydrolysis functionally coupled to the transport process itself? Although much progress has been made in the elucidation of the three-dimensional structures of ABC transporters in the last years by several crystallographic studies including novel models for the nucleotide hydrolysis and translocation catalysis, site-directed mutagenesis as well as the identification of natural mutations is still a major tool to evaluate effects of individual amino acids on the overall function of ABC transporters. Apart from alterations in characteristic sequence such as Walker A, Walker B and the ABC signature other parts of ABC proteins were subject to detailed mutagenesis studies including the substrate-binding site or the regulatory domain of CFTR. In this review, we will give a detailed overview of the mutation analysis reported for selected ABC transporters of the ABCB and ABCC subfamilies, namely HsCFTR/ABCC7, HsSUR/ABCC8,9, HsMRP1/ABCC1, HsMRP2/ABCC2, ScYCF1 and P-glycoprotein (Pgp)/MDR1/ABCB1 and their effects on the function of each protein.

Cystic Fibrosis--clinical features of a sample of Portuguese patients

Even though there have been rapid advances in the comprehension of molecular determinants of Cystic Fibrosis, this disease continues to be one of the most common lethal recessive diseases in the Caucasian population worldwide. The reality of Portuguese patients is still greatly unknown, due to the lack of studies published in reference to our population. The objectives of this study were: clinical evaluation of a group of Portuguese patients with Cystic Fibrosis, with two identified mutations; comparing the clinical presentation of a group of homozygous patients for the F508del mutation with patients that are not homozygous for this mutation. A group of patients, followed in Pediatric Pneumology Consultations of S. João Hospital, were characterised in terms of phenotype and were classified according to criteria of severity. All of the patients in this group presented class I and/or II mutations, classically associated with a more severe phenotype. In conformity with the severe genotype, all patients presented a phenotype of pancreatic insufficiency but with greater variability of pulmonary manifestations. Significant variations were not found in terms of age at diagnosis, presenting forms and disease severity between F508del homozygous patients and the other patients. On the other hand, patients with the same genotype (homozygous F508del) presented different spectrums of clinical manifestations and phenotype severity. Just as much, or even more than the genotype characterisation, the time period of evolution of the disease and external factors, namely being subjected to infectious stimulus, interfere in the severity of the phenotype, at a certain moment in time.

CFTR Cl- channel function in native human colon correlates with the genotype and phenotype in cystic fibrosis

BACKGROUND & AIMS

Cystic fibrosis (CF) is caused by over 1000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and presents with a widely variable phenotype. Genotype-phenotype studies identified CFTR mutations that were associated with pancreatic sufficiency (PS). Residual Cl- channel function was shown for selected PS mutations in heterologous cells. However, the functional consequences of most CFTR mutations in native epithelia are not well established.

METHODS

To elucidate the relationships between epithelial CFTR function, CFTR genotype, and patient phenotype, we measured cyclic adenosine monophosphate (cAMP)-mediated Cl- secretion in rectal biopsy specimens from 45 CF patients who had at least 1 non-DeltaF508 mutation carrying a wide spectrum of CFTR mutations. We compared CFTR genotypes and clinical manifestations of CF patients who expressed residual CFTR-mediated Cl- secretion with patients in whom Cl- secretion was absent.

RESULTS

Residual anion secretion was detected in 40% of CF patients, and was associated with later disease onset (P < 0.0001), higher frequency of PS (P < 0.0001), and less severe lung disease (P < 0.05). Clinical outcomes correlated with the magnitude of residual CFTR activity, which was in the range of approximately 12%-54% of controls.

CONCLUSIONS

Specific CFTR mutations confer residual CFTR function to rectal epithelia, which is related closely to a mild disease phenotype. Quantification of rectal CFTR-mediated Cl- secretion may be a sensitive test to predict the prognosis of CF disease and identify CF patients who would benefit from therapeutic strategies that would increase residual CFTR activity.