Functional analysis of the p.[Arg74Trp;Val201Met;Asp1270Asn]/p.Phe508del CFTR mutation genotype in human native colon.

BACKGROUND: The impact of complex alleles on CFTR processing and function has yet not been investigated in native human tissue. METHODS: Intestinal current measurements (ICM) followed by CFTR immunoblot were performed on rectal biopsies taken from two siblings who are compound heterozygous for the CFTR mutations p.Phe508del and the complex allele p.[Arg74Trp;Val201Met;Asp1270Asn]. RESULTS: Normal and subnormal chloride secretory responses in the ICM were associated with normal and fourfold reduced amounts of the mature glycoform band C CFTR, respectively, consistent with the unequal clinical phenotype of the siblings. CONCLUSION: The combined use of bioassay and protein analysis is particularly meaningful to resolve the CFTR phenotype of "indeterminate" borderline CFTR genotypes on a case-to-case basis.

The CF-modifying gene EHF promotes p.Phe508del-CFTR residual function by altering protein glycosylation and trafficking in epithelial cells.

The three-base-pair deletion c.1521_1523delCTT (p.Phe508del, F508del) in the cystic fibrosis transmembrane conductance regulator (CFTR) is the most frequent disease-causing lesion in cystic fibrosis (CF). The CFTR gene encodes a chloride and bicarbonate channel at the apical membrane of epithelial cells. Altered ion transport of CFTR-expressing epithelia can be used to differentiate manifestations of the so-called CF basic defect. Recently, an 11p13 region has been described as a CF modifier by the North American CF Genetic Modifier Study Consortium. Selecting the epithelial-specific transcription factor EHF (ets homologous factor) as the likely candidate gene on 11p13, we have genotyped two intragenic microsatellites in EHF to replicate the 11p13 finding in the patient cohort of the European CF Twin and Sibling Study. We could observe an association of rare EHF haplotypes among homozygotes for c.1521_1523delCTT in CFTR, which exhibit a CF-untypical manifestation of the CF basic defect such as CFTR-mediated residual chloride secretion and low response to amiloride. We have reviewed transcriptome data obtained from intestinal epithelial samples of homozygotes for c.1521_1523delCTT in CFTR, which were stratified for their EHF genetic background. Transcripts that were upregulated among homozygotes for c.1521_1523delCTT in CFTR, who carry two rare EHF alleles, were enriched for genes that alter protein glycosylation and trafficking, both mechanisms being pivotal for the effective targeting of fully functional p.Phe508del-CFTR to the apical membrane of epithelial cells. We conclude that EHF modifies the CF phenotype by altering capabilities of the epithelial cell to correctly process the folding and trafficking of mutant p.Phe508del-CFTR.

Immunochemical analysis of mutant CFTR in lung explants.

BACKGROUND/AIMS: Knowledge about the abundance and distribution of CFTR protein glycoforms in native lung tissue is scarce. For upcoming studies with correctors and potentiators for CFTR it is important to get more information about mutant CFTR protein biochemistry. Target for novel treatment is the most afflicted organ in cystic fibrosis (CF), the lung. METHODS: Lung tissue sampled from patients with CF and non-CF donors prior to lung transplantation was examined for CFTR-immunoreactive signals by immunoblot. Quantitation of the immunoreactive signals was carried out by densitometry. RESULTS: The complex-glycosylated and mannose-rich CFTR isoforms were present in all non-CF specimens, whereas no or only the immature CFTR isoform was visible in CF samples. Whereas some complex-glycosylated CFTR was often present in rectal biopsies of F508del homozygous subjects, no mature CFTR was detectable in CF lungs at the stage of terminal respiratory insufficiency. CONCLUSION: Immunoblot analysis of CFTR in lung tissue is feasible, but in context of the upcoming studies of CFTR correctors and potentiators rectal biopsies seem to be a more appropriate choice because of their safe and repeatable excision.

Functional analysis of F508del CFTR in native human colon.

The major cystic fibrosis mutation F508del has been classified by experiments in animal and cell culture models as a temperature-sensitive mutant defective in protein folding, processing and trafficking, but literature data on F508del CFTR maturation and function in human tissue are inconsistent. In the present study the molecular pathology of F508del CFTR was characterized in freshly excised rectal mucosa by bioelectric measurement of the basic defect and CFTR protein analysis by metabolic labelling or immunoblot. The majority of investigated F508del homozygous subjects expressed low amounts of complex-glycosylated mature F508del CFTR and low residual F508del CFTR-mediated chloride secretory activity in the rectal mucosa. The finding that some F508del CFTR escapes the ER quality control in vivo substantiates the hope that the defective processing and trafficking of F508del CFTR can be corrected by pharmacological agents.

Multiple roles of Pseudomonas aeruginosa TBCF10839 PilY1 in motility, transport and infection.

Polymorphonuclear neutrophils are the most important mammalian host defence cells against infections with Pseudomonas aeruginosa. Screening of a signature tagged mutagenesis library of the non-piliated P. aeruginosa strain TBCF10839 uncovered that transposon inactivation of its pilY1 gene rendered the bacterium more resistant against killing by neutrophils than the wild type and any other of the more than 3000 tested mutants. Inactivation of pilY1 led to the loss of twitching motility in twitching-proficient wild-type PA14 and PAO1 strains, predisposed to autolysis and impaired the secretion of quinolones and pyocyanin, but on the other hand promoted growth in stationary phase and bacterial survival in murine airway infection models. The PilY1 population consisted of a major full-length and a minor shorter PilY1* isoform. PilY1* was detectable in small extracellular quinolone-positive aggregates, but not in the pilus. P. aeruginosa PilY1 is not an adhesin on the pilus tip, but assists in pilus biogenesis, twitching motility, secretion of secondary metabolites and in the control of cell density in the bacterial population.

CFTR protein analysis of splice site mutation 2789+5 G-A.

Ex vivo biochemical analysis of rectal biopsies of a carrier of the mild 2789+5 G-A CFTR frameshift splice site mutation revealed mutant truncated CFTR of expected size and an imbalance of more core-glycosylated and less mature full-length CFTR. This first immunoblot analysis of a non-F508del CFTR mutant protein derived from human tissue demonstrates that splice site mutations should not only be investigated at the mRNA, but also at the protein level to properly interpret the associations between genotype, molecular pathology and disease.

Ex vivo biochemical analysis of CFTR in human rectal biopsies.

This report describes the first biosynthetic analysis of the cystic fibrosis transmembrane conductance regulator (CFTR) in freshly excised human rectal biopsies. Expression of functional CFTR was assessed by intestinal current measurement (ICM) prior to biosynthetic studies. Several structural features of CFTR are found to be comparable to those established in CFTR-expressing cell lines. Interestingly, maturation of CFTR increases substantially in tissue incubated at 26 degrees C. Our data provide a solid basis for future studies on the characterisation of CFTR in pathological cases.