Cystic fibrosis (CF) is a chronic inherited disease that affects the secretory glands, causing excessive production of mucus and salty sweat. In healthy people mucus is a watery and slippery substance, but in CF patients it is thick and sticky. The production and accumulation of this 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 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.
The symptoms and severity of the disease vary depending on the patient’s characteristics and age.
What causes CF?
CF is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The gene provides instructions to build the CTFR protein, a channel in the cell membrane that regulates the movements of negatively charged particles, known as chloride ions, in and out of cells. This process is essential for the production of mucus, sweat, saliva, and tears. Mucus is needed for the proper functioning of several organs.
There are more than 1,700 known mutations in the CFTR gene that can cause CF and disrupt the normal function of the chloride channels. As a result of these mutations, the CTFR protein is unable to properly regulate the flow of chloride ions and water through the cells. Because of that, the cells near the lungs, pancreas, and other organs end up producing thick and sticky mucus instead of the normal watery and slippery kind.
How CF is inherited
CF runs in families in an autosomal recessive pattern. Each individual has two copies of every gene, one that is inherited from the mother and the other from the father. CF manifests only if both copies of the CFTR gene are mutated.
Individuals who carry only one mutant copy of the gene do not develop CF. They are called carriers and can pass the mutant gene to their children with a 50 percent chance of the child developing the disease.
If both parents are carriers, a child has a 25 percent risk of inheriting two mutated copies of the CFTR gene, in which case they will develop CF. Inversely, the child of two carrier parents has a 25 percent chance of inheriting two healthy copies of the CFTR gene and not developing the disease or being a carrier.
It is thought that about one in 35 Americans is a CF carrier with many of them not knowing their carrier status. Caucasian Americans have a higher risk to be a carrier with about one in 29 caucasian Americans carrying a mutated copy of the CFTR gene.
The type of mutation is, to a certain extent, correlated with the symptoms of the disease. Because there are so many known mutations, not every single mutation and its associated phenotype are well-studied.
One very common and well-studied mutation is the so-called F508del mutation. Individuals in which both CFTR genes have the F508del mutation usually have pancreatic insufficiency, but the severity of the symptoms may vary. This suggests that other factors influence the severity of the disease.
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