Cystic fibrosis (CF) is a chronic inherited disease that affects the secretory glands, causing excessive production of mucus and salty sweat. Normal mucus is a watery and slippery substance, but people with CF produce mucus that is unusually thick and sticky. The accumulation of this thick mucus damages the lungs, pancreas, digestive tract, and reproductive system.
The most common feature of CF is progressive damage to the respiratory system. The thick mucus clogs the airways, causing breathing difficulties and raising the likelihood of bacterial infections in the lungs. Over time, it can cause chronic coughing, wheezing, and inflammation, and develop into permanent lung damage, the formation of scar tissue (fibrosis), and cysts in the lungs.
What causes CF?
CF is caused by a mutation in a gene called cystic fibrosis transmembrane conductance regulator, or CFTR. This gene provides cells with the instructions for building a protein, also called CFTR.
Normally, the CTFR protein is present on membranes of certain cells. At the cell membrane, CFTR acts like a gate, regulating the movement of chloride ions (negatively charged salt particles) and water in and out of cells. The movement of chloride and water is essential for the production of mucus, which is important for the functioning of various organs, such as the lungs and digestive tract.
More than 2,000 different mutations in CFTR have been reported. There are different types of CFTR mutations with different effects, but in all cases of CF, mutations lead to the production of a CTFR protein that is unable to properly regulate the flow of chloride ions and water through the cell membrane. The impaired function or smaller amount of this protein leads to the production of abnormally thick and sticky mucus.
Different types of mutations have different effects on CFTR production. The most common mutation is known as F508del. About nine in 10 CF patients have at least one copy of the CFTR gene with this mutation.
How is CF inherited?
CF runs in families in an autosomal recessive pattern. As is the case with most genes, everyone has two copies of CFTR — one from each biological parent. A person will develop CF only if both copies of CFTR are mutated.
Individuals who carry only one mutant copy of the gene do not develop CF, but they can pass the disease-causing gene to their biological children. As such, people with only one mutated copy of CFTR are known as carriers.
If two people who are both carriers have biological children, their child has a 25% chance of inheriting two mutated copies of the CFTR gene and have CF. Inversely, the biological child of two carrier parents has a 25% chance of inheriting two healthy copies of the CFTR gene, in which case they would neither develop CF nor be a carrier. Lastly, the child of two carriers has a 50% chance of inheriting one mutated copy of CFTR and also be a carrier.
It is thought that about one in 35 Americans — more than 10 million people — is a CF carrier, but because it doesn’t lead to symptoms, most carriers are not aware of their genetic status. Although it is found in all ethnic groups, CF is more common among whites than other groups. However, research has suggested that newborn screening panels do not effectively detect CF mutations present in nonwhite populations.
Last updated: July 21, 2021
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