The gene that encodes the human CFTR protein is found on
chromosome 7, on the long arm at position q31.2. occur in the
jawed vertebrates. Each individual inherits two copies of the
CFTR (cystic fibrosis transmembrane conductance regulator) gene. However, some of the inherited copies have been altered. So far, the
CFTR gene has been associated with over 700 distinct mutations. An individual with CF inherits two defective copies of the
CFTR gene. These mutations might be heterozygous, meaning they include two different mutations, and homozygous, meaning they involve the same mutation. 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. The
CFTR gene has been used in animals as a
nuclear DNA phylogenetic marker. and confirmed the grouping of
placental orders into four major clades:
Xenarthra,
Afrotheria,
Laurasiatheria, and
Euarchonta plus
Glires.
Mutations Nearly 1000 cystic fibrosis-causing
mutations have been described. 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 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 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. 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. The most common mutations that cause cystic fibrosis and pancreatic insufficiency in humans are:
DeltaF508 DeltaF508 (
ΔF508), full name
CFTRΔF508 or
F508del-CFTR (rs113993960), is a specific mutation within the CFTR gene involving
deletion of three
nucleotides spanning codons for amino acid positions 507 and 508 of the CFTR gene on chromosome 7, which ultimately results in the loss of a single
codon for the
amino acid phenylalanine (F). A person with the CFTRΔF508 mutation will produce an abnormal CFTR protein that lacks this phenylalanine residue and which cannot
fold properly. Most of this mutated protein does not escape the
endoplasmic 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 of
cystic fibrosis (CF), responsible for nearly two-thirds of mutations worldwide.
Effects 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 of
chloride and
bicarbonate ions out of cells; it also simultaneously inhibits the uptake of
sodium ions by another channel protein. Both of these functions help to maintain an
ion gradient that causes
osmosis to draw water out of the cells. The ΔF508 mutation leads to the misfolding of CFTR and its eventual
degradation 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. Having a
homozygous 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: thicker
mucous 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 of
cystic fibrosis, a genetic disorder; however, ΔF508 is not the only mutation that causes this disorder. Being a
heterozygous carrier (having a single copy of ΔF508) results in decreased water loss during
diarrhea 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 a
hypotonic 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 chronic
obstructive lung disease. One gene for cystic fibrosis is sufficient to produce mild lung abnormalities even in the absence of infection.
Mechanism The
CFTR 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 three
DNA base 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 for
isoleucine, 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 for
phenylalanine. 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 is
transcribed 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.
Prevalence ΔF508 is present on at least one copy of chromosome 7 in approximately one in 30
Caucasians. Presence of the mutation on both copies causes the
autosomal recessive disease cystic fibrosis. Scientists have estimated that the original mutation occurred over 52,000 years ago in Northern
Europe though
cystic fibrosis patients of other ethnicities are also known to harbor the mutation. The young
allele 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 during
cholera, though the introduction of pathogenic
Vibrio cholerae into Europe did not occur until the late 18th century. Another theory posits that CF carriers (heterozygotes for ΔF508) are more resistant to
typhoid fever, since CFTR has been shown to act as a receptor for
Salmonella typhi bacteria to enter intestinal epithelial cells. Cystic fibrosis ΔF508 heterozygotes may be overrepresented among individuals with
asthma and may have poorer lung function than non-carriers. Carriers of a single CF mutation have a higher prevalence of chronic
rhinosinusitis than the general population. Approximately 50% of cystic fibrosis cases in
Europe are due to homozygous ΔF508 mutations (this varies widely by region), while the allele frequency of ΔF508 is about 70%. 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. == Structure ==