The CFTR is found in the epithelial cells of many organs including thelung,liver,pancreas,digestive tract, and the female reproductive tract[12] and malereproductive tract including the testis, Sertoli cells, spermatozoa.[13] epididymis,[14] and the vas deferens.[15]
In the airways of the lung, CFTR is most highly expressed by rare specialized cells calledpulmonary ionocytes.[16][17][18] In the skin, CFTR is strongly expressed in thesebaceous andeccrine sweat glands.[19] In the eccrine glands, CFTR is located on the apical membrane of the epithelial cells that make up the duct of these sweat glands.[19]
Normally, the protein allows movement ofchloride,bicarbonate andthiocyanate[20]ions (with a negative charge) out of an epithelial cell into the airway surface liquid andmucus. Positively charged sodium ions follow passively, increasing the totalelectrolyte concentration in the mucus, resulting in the movement of water out of the cell viaosmosis.
In epithelial cells with motile cilia lining the bronchus and the oviduct, CFTR is located on the apical cell membrane but not on cilia.[12] In contrast,ENaC (Epithelial sodium channel) is located along the entire length of the cilia.[12]
Insweat glands, defective CFTR results in reduced transport of sodium chloride and sodiumthiocyanate[21] in the resorptive duct and therefore saltier sweat. This is the basis of a clinically importantsweat test forcystic fibrosis often used diagnostically with genetic screening.[22]
Each individual inherits two copies of theCFTR (cystic fibrosis transmembrane conductance regulator) gene. However, some of the inherited copies have been altered. So far, theCFTR gene has been associated with over 700 distinct mutations. An individual with CF inherits two defective copies of theCFTR gene. These mutations might be heterozygous, meaning they include two different mutations, and homozygous, meaning they involve the same mutation.[25] Delta F508 is the most common mutation, accounting for more than 70% of all mutations. Those who are homozygous for Delta F508 are commonly affected by pancreatic insufficiency.[26]
Nearly 1000 cystic fibrosis-causingmutations have been described.[28] The most common mutation, DeltaF508 (ΔF508) primarily known as a processing mutation which results from a deletion (Δ) of three nucleotides which results in a loss of the amino acidphenylalanine (F) at the 508th position on the protein.[29] As a result, the protein does notfold normally and is more quickly degraded. The vast majority of mutations are infrequent. The distribution and frequency of mutations varies among different populations which has implications for genetic screening and counseling.
Drug discovery for therapeutics to address CF in all patients is complicated due to a large number of disease-causing mutations. Ideally, a library of cell lines and cell-based assays corresponding to all mutants is required to screen for broadly-active drug candidates. Cell engineering methods including fluorogenic oligonucleotide signaling probes may be used to detect and isolate clonal cell lines for each mutant.[30]
Mutations consist of replacements, duplications, deletions or shortenings in theCFTR gene. This may result in proteins that may not function, work less effectively, are more quickly degraded, or are present in inadequate numbers.[31]
It has been hypothesized that mutations in theCFTR gene may confer a selective advantage to heterozygous individuals. Cells expressing a mutant form of the CFTR protein are resistant to invasion by theSalmonella typhi bacterium, the agent oftyphoid fever, and mice carrying a single copy of mutantCFTR are resistant to diarrhea caused by cholera toxin.[32]
The most common mutations that cause cystic fibrosis and pancreatic insufficiency in humans are:[33]
DeltaF508 (ΔF508), full nameCFTRΔF508 orF508del-CFTR (rs113993960), is a specific mutation within the CFTR gene involvingdeletion of threenucleotides spanning codons for amino acid positions 507 and 508 of the CFTR gene on chromosome 7, which ultimately results in the loss of a singlecodon for theamino acidphenylalanine (F). A person with the CFTRΔF508 mutation will produce an abnormal CFTR protein that lacks this phenylalanine residue and which cannotfold properly. Most of this mutated protein does not escape theendoplasmic reticulum for further processing. The small amounts that reach the plasma membrane are destabilized and the anion channel opens infrequently. Having two copies of this mutation (one inherited from each parent) is by far the most common cause ofcystic fibrosis (CF), responsible for nearly two-thirds of mutations worldwide.[34]
The CFTR protein is largely expressed in cells of the pancreas, intestinal and respiratory epithelia, and all exocrine glands. When properly folded, it is shuttled to the cell membrane, where it becomes a transmembrane protein that forms aqueous channels allowing the flow ofchloride andbicarbonate ions out of cells; it also simultaneously inhibits the uptake ofsodium ions by another channel protein. Both of these functions help to maintain anion gradient that causesosmosis to draw water out of the cells.[35] The ΔF508 mutation leads to the misfolding of CFTR and its eventualdegradation in the ER. In organisms with two complements of the mutation, the protein is almost entirely absent from the cell membrane, and these critical ion transport functions are not performed.[36]
Having ahomozygous pair of genes with the ΔF508 mutation prevents the CFTR protein from assuming its normal position in the cell membrane. This causes increased water retention in cells, corresponding dehydration of the extracellular space, and an associated cascade of effects on various parts of the body. These effects include: thickermucous membranes in the epithelia of afflicted organs; obstruction of narrow respiratory airways as a result of thicker mucus and inhibition of the free movement of muco cilia;congenital absence of the vas deferens due to increased mucus thickness during fetal development; pancreatic insufficiency due to blockage of the pancreatic duct with mucus; and increased risk of respiratory infection due to build-up of thick, nutrient-rich mucus where bacteria thrive. Mucus thickening is also hypothesized to disrupt innate immune-system mechanisms due to changes in immune-cell and molecular profiles (including decreased alveolar macrophage activity and increased cytokine and neutrophil presence). This results in an aggravated systemic inflammatory response and impaired phagocytosis, reducing the body's ability to fight infection. These are the symptoms ofcystic fibrosis, a genetic disorder; however, ΔF508 is not the only mutation that causes this disorder.[37]
Being aheterozygouscarrier (having a single copy of ΔF508) results in decreased water loss duringdiarrhea because malfunctioning or absent CFTR proteins cannot maintain stable ion gradients across cell membranes. Typical nucleotide-binding-up of both Cl− and Na+ ions inside affected cells, creating ahypotonic solution outside the cells and causing water to diffuse into the cells by osmosis. Several studies indicate that heterozygous carriers are at increased risk for various symptoms. For example, it has been shown that heterozygosity for cystic fibrosis is associated with increased airway reactivity, and heterozygotes may be at risk for poor pulmonary function. Heterozygotes with wheeze have been shown to be at higher risk for poor pulmonary function or development and progression of chronicobstructive lung disease. One gene for cystic fibrosis is sufficient to produce mild lung abnormalities even in the absence of infection.[38]
TheCFTR gene is located on the long arm of chromosome 7, at position q31.2, and ultimately codes for a sequence of 1,480 amino acids. Normally, the threeDNAbase pairs A-T-C (paired with T-A-G on the opposite strand) at the gene's 507th position form the template for the mRNA codon A-U-C forisoleucine, while the three DNA base pairs T-T-T (paired with A-A-A) at the adjacent 508th position form the template for the codon U-U-U forphenylalanine.[39] The ΔF508 mutation is a deletion of the C-G pair from position 507 along with the first two T-A pairs from position 508, leaving the DNA sequence A-T-T (paired with T-A-A) at position 507, which istranscribed into the mRNA codon A-U-U. Since A-U-U also codes for isoleucine, position 507's amino acid does not change, and the mutation's net effect is equivalent to a deletion ("Δ") of the sequence resulting in the codon for phenylalanine at position 508.[40]
ΔF508 is present on at least one copy of chromosome 7 in approximately one in 30Caucasians. Presence of the mutation on both copies causes theautosomal recessive disease cystic fibrosis. Scientists have estimated that the original mutation occurred over 52,000 years ago in NorthernEurope thoughcystic fibrosis patients of other ethnicities are also known to harbor the mutation. The youngallele age may be a consequence of past selection. One hypothesis as to why the otherwise detrimental mutation has been maintained by natural selection is that a single copy may present a positive effect by reducing water loss duringcholera, though the introduction of pathogenicVibrio cholerae into Europe did not occur until the late 18th century.[41] Another theory posits that CF carriers (heterozygotes for ΔF508) are more resistant totyphoid fever, since CFTR has been shown to act as a receptor forSalmonella typhi bacteria to enter intestinal epithelial cells.[42]
Cystic fibrosis ΔF508 heterozygotes may be overrepresented among individuals withasthma and may have poorer lung function than non-carriers.[43][44] Carriers of a single CF mutation have a higher prevalence of chronicrhinosinusitis than the general population.[45] Approximately 50% of cystic fibrosis cases inEurope are due to homozygous ΔF508 mutations (this varies widely by region),[46] while the allele frequency of ΔF508 is about 70%.[47] The remaining cases are caused by over 1,500 other mutations, including R117H, 1717-1G>A, and 2789+56G>A. These mutations, when combined with each other or even a single copy of ΔF508, may cause CF symptoms. The genotype is not strongly correlated with severity of the CF, though specific symptoms have been linked to certain mutations.
The Overall Structure of Human CFTR in the Dephosphorylated, ATP-Free Conformation. Domains are labeled. Made from PDB 5UAK.[48]
TheCFTR gene is approximately 189kb in length, with 27exons and 26introns.[49] CFTR is a glycoprotein and is found on the surface of many epithelial cells in the body.[50] CFTR consists of five domains, which include two transmembrane or membrane-spanning domains, two nucleotide-binding domains and a regulatory domain.[51] The transmembrane domains are each connected to anucleotide binding domain (NBD) in the cytoplasm. The first NBD is connected to the second transmembrane domain by a regulatory "R" domain that is a unique feature of CFTR, not present in otherABC transporters which carries 19 predicted sites for protein kinase A(PKA). Six of these have been reported to be phosphorylated in vivo.[52] The ion channel only opens when its R-domain has been phosphorylated by PKA andATP is bound at the NBDs. Phosphorylation displaces the disordered R domain from positions preventing NBD dimerization and opening.[53][54] Theamino-terminus is part of the lasso motif which anchors into the cell membrane.[52] Thecarboxyl terminal of the protein is anchored to thecytoskeleton by aPDZ-interacting domain.[55]
The CFTR protein is a channel protein that controls the flow of H2O and Cl− ions in and out of cells inside the lungs. When the CFTR protein is working correctly, as shown in Panel 1, ions freely flow in and out of the cells. However, when the CFTR protein is malfunctioning as in Panel 2, these ions cannot flow out of the cell due to blocked CFTR channels. This occurs incystic fibrosis, characterized by the buildup of thick mucus in the lungs.
TheCFTR gene is made up of 27 exons that encode its gene makeup and is found on the long (q) arm of chromosome 7 at locus 31.2. Exons are DNA fragments that provide the code for a protein structure.[50] CFTR functions asphosphorylation andATP-gatedanionchannel, increasing theconductance for certainanions (e.g. Cl−) to flow down theirelectrochemical gradient. ATP-drivenconformational changes in CFTR open and close a gate to allow the transmembrane flow of anions down theirelectrochemical gradient.[5] This in contrast to otherABC proteins, in which ATP-driven conformational changes fuel uphill substrate transport across cellular membranes. Essentially, CFTR is an ion channel that evolved as a 'broken'ABC transporter that leaks when in the openconformation.
CFTRs consist of five domains including two trans-membrane domains, each linked to a nucleotide-binding domain. CFTR also contains another domain called the regulatory domain. Other members of the ABC transporter superfamily are involved in the uptake of nutrients in prokaryotes, or in the export of a variety of substrates in eukaryotes. ABC transporters have evolved to transduce the free energy of ATP hydrolysis to the uphill movement of substrates across the cell membrane. They have two main conformations, one where the cargo binding site is facing the cytosol or inward facing (ATP free), and one where it is outward facing (ATP bound). ATP binds to each nucleotide-binding domain, which results in the subsequent NBD dimerization, leading to the rearrangement of the transmembrane helices. This changes the accessibility of the cargo binding site from an inward-facing position to an outward facing one. ATP binding, and the hydrolysis that follows, drives the alternative exposure of the cargo binding site, ensuring a unidirectional transport of cargo against anelectrochemical gradient. In CFTR, alternating between an inward-facing conformation to an outward-facing one results in channel gating. In particular, NBD dimerization (favored by ATP binding) is coupled to transition to an outward-facing conformation in which an open transmembrane pathway for anions is formed.[56] Subsequent hydrolysis (at the canonical active site, site 2, including Walker motifs of NBD2) destabilizes the NBD dimer and favors return to the inward-facing conformation, in which the anion permeation pathway is closed off.[5]
DifferentCFTR mutations can lead to varying degrees of cystic fibrosis severity. Common symptoms include chronic lung infections, pancreatic insufficiency, and high sweat chloride levels. Mutation-specific therapies, such as CFTR modulators, have been developed to address these specific genetic defects.[57]
Congenital bilateral absence of vas deferens: Males with congenital bilateral absence of thevas deferens most often have a mildmutation (a change that allows partial function of the gene) in one copy of the CFTR gene and a cystic fibrosis-causing mutation in the other copy of CFTR.
Cystic fibrosis: More than 1,800 mutations in the CFTR gene have been found[71] but the majority of these have not been associated with cystic fibrosis.[72] Most of these mutations either substitute oneamino acid (a building block of proteins) for another amino acid in the CFTR protein or delete a small amount ofDNA in the CFTR gene. The most common mutation, called ΔF508, is a deletion (Δ) of one amino acid (phenylalanine) at position 508 in the CFTR protein. This altered protein never reaches the cell membrane because it is degraded shortly after it is made. All disease-causing mutations in the CFTR gene prevent the channel from functioning properly, leading to a blockage of the movement of salt and water into and out of cells. As a result of this blockage, cells that line the passageways of the lungs, pancreas, and other organs produce abnormally thick, sticky mucus. This mucus obstructs the airways and glands, causing the characteristic signs and symptoms of cystic fibrosis. In addition, only thin mucus can be removed bycilia; thick mucus cannot, so it traps bacteria that give rise to chronic infections.
Cholera:ADP-ribosylation caused bycholera toxin results in increased production ofcyclic AMP which in turn opens the CFTR channel which leads to Over secretion of Cl−. Na+ and H2O follow Cl− into the small intestine, resulting in dehydration and loss of electrolytes.[73]
CFTR has been adrug target in efforts to find treatments for related conditions.Ivacaftor (brand name Kalydeco, developed as VX-770) is amedication approved by the FDA in 2012, for people withcystic fibrosis who have specific CFTR mutations.[74][75] Ivacaftor was developed byVertex Pharmaceuticals in conjunction with theCystic Fibrosis Foundation and is the first medication that treats the underlying cause rather than the symptoms of the disease.[76][77] and "a wonder drug"[78]
^Riordan JR, Rommens JM, Kerem B, Alon N, Rozmahel R, Grzelczak Z, et al. (September 1989). "Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA".Science.245 (4922):1066–1073.doi:10.1016/0168-9525(89)90155-8.PMID2475911.
^Marcorelles P, Gillet D, Friocourt G, Ledé F, Samaison L, Huguen G, et al. (March 2012). "Cystic fibrosis transmembrane conductance regulator protein expression in the male excretory duct system during development".Human Pathology.43 (3):390–397.doi:10.1016/j.humpath.2011.04.031.PMID21840567.
^Bregman T, Fride E (June 2011). "Treatment with tetrahydrocannabinol (THC) prevents infertility in male cystic fibrosis mice".Journal of Basic and Clinical Physiology and Pharmacology.22 (1–2):29–32.doi:10.1515/jbcpp.2011.004.PMID22865360.S2CID19335113.
^abcEnuka Y, Hanukoglu I, Edelheit O, Vaknine H, Hanukoglu A (March 2012). "Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways".Histochemistry and Cell Biology.137 (3):339–353.doi:10.1007/s00418-011-0904-1.PMID22207244.S2CID15178940.
^Sharma S, Hanukoglu A, Hanukoglu I (April 2018). "Localization of epithelial sodium channel (ENaC) and CFTR in the germinal epithelium of the testis, Sertoli cells, and spermatozoa".Journal of Molecular Histology.49 (2):195–208.doi:10.1007/s10735-018-9759-2.PMID29453757.S2CID3761720.
^Sharma S, Hanukoglu I (April 2019). "Mapping the sites of localization of epithelial sodium channel (ENaC) and CFTR in segments of the mammalian epididymis".Journal of Molecular Histology.50 (2):141–154.doi:10.1007/s10735-019-09813-3.PMID30659401.S2CID58026884.
^Sharma S, Kumaran GK, Hanukoglu I (February 2020). "High-resolution imaging of the actin cytoskeleton and epithelial sodium channel, CFTR, and aquaporin-9 localization in the vas deferens".Mol Reprod Dev.87 (2):305–319.doi:10.1002/mrd.23317.PMID31950584.
^Yonei Y, Tanaka M, Ozawa Y, Miyazaki K, Tsukada N, Inada S, et al. (April 1992). "Primary hepatocellular carcinoma with severe hypoglycemia: involvement of insulin-like growth factors".Liver.12 (2):90–93.doi:10.1111/j.1600-0676.1992.tb00563.x.PMID1320177.
^Davies R, Conroy SJ, Davies WL, Potter IC, Trezise AE (19–23 June 2005)."Evolution and Regulation of the Cystic Fibrosis Gene"(conference paper).Molecular Biology and Evolution (MBE05) Conference. Retrieved28 July 2014.
^Welsh MJ, Smith AE (July 1993). "Molecular mechanisms of CFTR chloride channel dysfunction in cystic fibrosis".Cell.73 (7):1251–1254.doi:10.1016/0092-8674(93)90353-r.PMID7686820.
^"Genetics and CF".The Cystic Fibrosis Center at Stanford (in Samoan). Retrieved23 October 2022.
^Rowe SM, Miller S, Sorscher EJ (May 2005). "Cystic fibrosis".The New England Journal of Medicine.352 (19):1992–2001.doi:10.1056/NEJMra043184.PMID15888700.
^Verkman AS, Song Y, Thiagarajah JR (January 2003). "Role of airway surface liquid and submucosal glands in cystic fibrosis lung disease".American Journal of Physiology. Cell Physiology.284 (1): C2-15.doi:10.1152/ajpcell.00417.2002.PMID12475759.
^Boyle MP, De Boeck K (April 2013). "A new era in the treatment of cystic fibrosis: correction of the underlying CFTR defect".The Lancet. Respiratory Medicine.1 (2):158–163.doi:10.1016/s2213-2600(12)70057-7.PMID24429096.
^Dahl M, Tybjaerg-Hansen A, Lange P, Nordestgaard BG (June 1998). "DeltaF508 heterozygosity in cystic fibrosis and susceptibility to asthma".Lancet.351 (9120):1911–1913.doi:10.1016/s0140-6736(97)11419-2.PMID9654257.S2CID22970136.
^Morral N, Bertranpetit J, Estivill X, Nunes V, Casals T, Giménez J, et al. (June 1994). "The origin of the major cystic fibrosis mutation (delta F508) in European populations".Nature Genetics.7 (2):169–175.doi:10.1038/ng0694-169.PMID7920636.S2CID38005421.
^Boyle MP, De Boeck K (April 2013). "A new era in the treatment of cystic fibrosis: correction of the underlying CFTR defect".The Lancet. Respiratory Medicine.1 (2):158–163.doi:10.1016/s2213-2600(12)70057-7.PMID24429096.
^Hegedüs T, Sessler T, Scott R, Thelin W, Bakos E, Váradi A, et al. (March 2003). "C-terminal phosphorylation of MRP2 modulates its interaction with PDZ proteins".Biochemical and Biophysical Research Communications.302 (3):454–461.Bibcode:2003BBRC..302..454H.doi:10.1016/S0006-291X(03)00196-7.PMID12615054.
^McPhail GL, Clancy JP (April 2013). "Ivacaftor: the first therapy acting on the primary cause of cystic fibrosis".Drugs of Today.49 (4):253–260.doi:10.1358/dot.2013.49.4.1940984.PMID23616952.
T. Kockar Kizilirmak, H. Yin, A. Garrison, J. Browne, E. Bruscia, M. Egan, C. Britto, 219 “CFTR dysfunction shapes airway immune cell compositions contributing to lung pathogenesis in children with cystic fibrosis”, Journal of Cystic Fibrosis, Volume 23, Supplement 2, 2024, Page S119, ISSN 1569-1993,https://doi.org/10.1016/S1569-1993(24)01059-2
Cohn JA, Mitchell RM, Jowell PS (March 2005). "The impact of cystic fibrosis and PSTI/SPINK1 gene mutations on susceptibility to chronic pancreatitis".Clinics in Laboratory Medicine.25 (1):79–100.doi:10.1016/j.cll.2004.12.007.PMID15749233.
Cohn JA, Noone PG, Jowell PS (September 2002). "Idiopathic pancreatitis related to CFTR: complex inheritance and identification of a modifier gene".Journal of Investigative Medicine.50 (5):247S –255S.doi:10.1136/jim-50-suppl5-01.PMID12227654.S2CID34017638.
Dahan D, Evagelidis A, Hanrahan JW, Hinkson DA, Jia Y, Luo J, et al. (2001). "Regulation of the CFTR channel by phosphorylation".Pflügers Archiv.443 (Suppl 1):S92 –S96.doi:10.1007/s004240100652.PMID11845311.S2CID8144727.
Kerem B, Kerem E (1996). "The molecular basis for disease variability in cystic fibrosis".European Journal of Human Genetics.4 (2):65–73.doi:10.1159/000472174 (inactive 1 July 2025).PMID8744024.S2CID41476164.{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link)
Kulczycki LL, Kostuch M, Bellanti JA (January 2003). "A clinical perspective of cystic fibrosis and new genetic findings: relationship of CFTR mutations to genotype-phenotype manifestations".American Journal of Medical Genetics. Part A.116A (3):262–267.doi:10.1002/ajmg.a.10886.PMID12503104.S2CID9245855.
Marcet B, Boeynaems JM (December 2006). "Relationships between cystic fibrosis transmembrane conductance regulator, extracellular nucleotides and cystic fibrosis".Pharmacology & Therapeutics.112 (3):719–732.doi:10.1016/j.pharmthera.2006.05.010.PMID16828872.
Wong LJ, Alper OM, Wang BT, Lee MH, Lo SY (July 2003). "Two novel null mutations in a Taiwanese cystic fibrosis patient and a survey of East Asian CFTR mutations".American Journal of Medical Genetics. Part A.120A (2):296–298.doi:10.1002/ajmg.a.20039.PMID12833420.S2CID41060230.
Savant A, Lyman B, Bojanowski C, Upadia J (August 2024)."Cystic Fibrosis". In Adam MP, Feldman J, Mirzaa GM, Pagon RA, Wallace SE, Amemiya A (eds.).GeneReviews [Internet]. Seattle (WA): University of Washington, Seattle.PMID20301428.
Overview of all the structural information available in thePDB forUniProt:P13569 (Human Cystic fibrosis transmembrane conductance regulator) at thePDBe-KB.
Overview of all the structural information available in thePDB forUniProt:P26361 (Mouse Cystic fibrosis transmembrane conductance regulator) at thePDBe-KB.
