Cystic Fibrosis Causes

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Cystic fibrosis is an inherited disease caused by mutations in a genes called the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The CFTR gene provides instructions for the CFTR protein. The CFTR protein is located in every organ of the body that makes mucus, including the lungs, liver, pancreas, and intestines, as well as sweat glands. The CFTR protein has also been found in other cells in the body, such as cells of the heart and the immune system. The mutations in the CFTR gene cause the CFTR protein to not work properly. This causes thick, sticky mucus and blockages in the lungs and digestive system.

Normally, mucus coats tiny hair-like structures called cilia in the airways of your lungs, which sweep the mucus particles up to the nose and mouth where your body can get rid of them. In people who have cystic fibrosis, this process does not work properly. Learn more in our How the Lungs Work Health Topic.

There are almost 2,000 known disease-causing mutations of the CFTR gene. Different mutations have different effects on how the CFTR protein is made and how it works. In the most common gene mutation, part of the CFTR gene is missing, resulting in a protein that does not work properly.

Normally, the CFTR protein controls the movement of ions from inside the cell to outside the cell. In people who have cystic fibrosis, the mutated gene causes the protein to not work properly, which, in turn, affects the movement of sodium and water. When this happens, the sweat glands make sweat that is saltier than it is in people who do not have cystic fibrosis. To diagnose cystic fibrosis, your doctor will assess your symptoms and recommend some tests, including a sweat test for high sweat chloride. Also, there is less water in the mucus, which makes the mucus thick and sticky. The thick mucus creates blockages in the lungs and digestive system.

Every person inherits two CFTR genes, one gene from each parent. Children who inherit a CFTR gene with a mutation from both parents will have cystic fibrosis. When a mutated CFTR gene is inherited from only one parent and a normal CFTR gene is inherited from the other, the person will be a cystic fibrosis carrier. CF carriers are generally healthy, but they can pass the mutated CFTR gene on to their children.

The image below shows how two parents who are both CF carriers can pass a CFTR gene mutation on to their children.

A person inherits two copies of the CFTR gene, one from each parent. If each parent has a normal CFTR gene and a mutated CFTR gene, each child has a 25% chance of inheriting two normal genes, a 50% chance of inheriting one normal gene and one gene with a mutation and being a cystic fibrosis carrier, and a 25% chance of inheriting two genes with mutations and having cystic fibrosis.