Cystic fibrosis transmembrane conductance regulator
|Cystic fibrosis transmembrane conductance regulator (ATP-binding cassette sub-family C, member 7)|
NBD1 of human CFTR complexed with ATP. PDB rendering based on .
|Symbols||; ABC35; ABCC7; CF; CFTR/MRP; MRP7; TNR-CFTR; dJ760C5.1|
|External IDs||IUPHAR: ChEMBL: GeneCards:|
CFTR is an ABC transporter-class ion channel that transports chloride and thiocyanate ions across epithelial cell membranes. Mutations of the CFTR gene affect functioning of the chloride ion channels in these cell membranes, leading to cystic fibrosis and congenital absence of the vas deferens.
The gene that encodes the CFTR protein is found on the human chromosome 7, on the long arm at position q31.2. from base pair 116,907,253 to base pair 117,095,955. CFTR orthologs  have also been identified in all mammals for which complete genome data are available.
The CFTR gene has been used in animals as a nuclear DNA phylogenetic marker. Large genomic sequences of this gene have been used to explore the phylogeny of the major groups of mammals, and confirmed the grouping of placental orders into four major clades: Xenarthra, Afrotheria, Laurasiatheria, and Euarchonta plus Glires.
Well over one thousand mutations have been described that can affect the CFTR gene. Such mutations can cause two genetic disorders, congenital bilateral absence of vas deferens and the more widely known disorder cystic fibrosis. Both disorders arise from the blockage of the movement of ions and, therefore, water into and out of cells. In congenital bilateral absence of vas deferens, the protein may be still functional but not at normal efficiency, this leads to the production of thick mucus, which blocks the developing vas deferens. In people with mutations giving rise to cystic fibrosis, the blockage in ion transport occurs in epithelial cells that line the passageways of the lungs, pancreas, and other organs. This leads to chronic dysfunction, disability, and a reduced life expectancy.
The most common mutation, ΔF508 results from a deletion (Δ) of three nucleotides which results in a loss of the amino acid phenylalanine (F) at the 508th position on the protein. As a result the protein does not fold normally and is more quickly degraded.
The vast majority of mutations are quite rare. The distribution and frequency of mutations varies among different populations which has implications for genetic screening and counseling.
Mutations consist of replacements, duplications, deletions or shortenings in the CFTR gene. This may result in proteins that may not function, work less effectively, are more quickly degraded, or are present in inadequate numbers.
It has been hypothesized that mutations in the CFTR gene may confer a selective advantage to heterozygous individuals. Cells expressing a mutant form of the CFTR protein are resistant to invasion by the Salmonella typhi bacterium, the agent of typhoid fever, and mice carrying a single copy of mutant CFTR are resistant to diarrhea caused by cholera toxin.
List of common mutations
The CFTR gene is approximately 189 kb in length, with 27 exons and 26 introns. CFTR is a glycoprotein with 1480 amino acids. The protein consists of five domains. There are two transmembrane domains, each with six spans of alpha helices. These are each connected to a nucleotide 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 other ABC transporters. The ion channel only opens when its R-domain has been phosphorylated by PKA and ATP is bound at the NBDs. The carboxyl terminal of the protein is anchored to the cytoskeleton by a PDZ-interacting domain.
Location and function
CFTR functions as a cAMP-activated ATP-gated anion channel, increasing the conductance for certain anions (e.g. Cl–) to flow down their electrochemical gradient. ATP-driven conformational changes in CFTR open and close a gate to allow transmembrane flow of anions down their electrochemical gradient. This in contrast to other ABC 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 open conformation.
The CFTR is found in the epithelial cells of many organs including the lung, liver, pancreas, digestive tract, reproductive tract, and skin. Normally, the protein moves chloride and thiocyanate ions (with a negative charge) out of an epithelial cell to the covering mucus. Positively charged sodium ions follow these anions out of the cell to maintain electrical balance. This increases the total electrolyte concentration in the mucus, resulting in the movement of water out of cell by osmosis.
In epithelial cells with motile cilia lining the bronchus and the oviduct, CFTR is located on cell membrane but not on cilia. In contrast to CFTR, ENaC is located along the entire length of the cilia. These findings contradict a previous hypothesis that CFTR normally downregulates ENaC by direct interaction and that in CF patients, CFTR cannot downregulate ENaC causing hyper-absorption in the lungs and recurrent lung infections.
In sweat glands, CFTR defects result in reduced transport of sodium chloride and sodium thiocyanate in the reabsorptive duct and saltier sweat. This was the basis of a clinically important sweat test for cystic fibrosis before genetic screening was available.
Cystic fibrosis transmembrane conductance regulator has been shown to interact with:
It is inhibited by the anti-diarrhoea drug crofelemer.
- Congenital bilateral absence of vas deferens: Males with congenital bilateral absence of the vas deferens most often have a mild mutation (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. As a result of these mutations, the movement of water and salt into and out of cells is disrupted. This disturbance leads to the production of a large amount of thick mucus that blocks the developing vas deferens (a tube that carries sperm from the testes) and causes it to degenerate, resulting in infertility.
- Cystic fibrosis: More than 1,800 mutations in the CFTR gene have been found but the majority of these have not been associated with cystic fibrosis. Most of these mutations either substitute one amino acid (a building block of proteins) for another amino acid in the CFTR protein or delete a small amount of DNA 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 by cilia, thick mucus cannot, so it traps bacteria that give rise to chronic infections.
- Cholera: the CFTR channel is up-regulated by cAMP. Cholera toxin permanently activates adenylyl cyclase, resulting in increased cAMP, resulting in oversecretion of Cl−. Na+ and H2O follow Cl− resulting in dehydration and loss of electrolytes. Treatment: Oral Rehydration Therapy (ORT) = hydration (normal saline) and glucose.
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- GeneReviews/NCBI/NIH/UW entry on CFTR-Related Disorders - Cystic Fibrosis (CF, Mucoviscidosis) and Congenital Absence of the Vas Deferens (CAVD)
- The Cystic Fibrosis Transmembrane Conductance Regulator Protein
- The Human Gene Mutation Database - CFTR Records
- Cystic Fibrosis Mutation Database
- Oak Ridge National Laboratory CFTR Information
- CFTR at OMIM (National Center for Biotechnology Information)