Cystic Fibrosis

Cystic Fibrosis Causes

What causes cystic fibrosis?

Cystic fibrosis is an  inherited  disease caused by  mutations  in a  gene called the cystic fibrosis transmembrane conductance regulator (CFTR). The CFTR gene provides instructions for the CFTR protein .

People who inherit two copies of a mutated CFTR gene (one copy from each biological parent) will have cystic fibrosis. Having two mutated CFTR genes means that the body makes a CFTR protein that doesn’t work like it should.

In cystic fibrosis, the mutated CFTR gene changes a protein that regulates the movement of salt in and out of cells. These changes cause thick and sticky  mucus  and increase the amount of salt in sweat. The abnormal mucus can lead to blockages in and damage to the lungs, the digestive system, and other organs of the body.

Normally, mucus coats tiny hairlike structures called  cilia  in the airways of your lungs. The cilia sweep the mucus particles up to the nose and mouth so your body can get rid of them. In people who have cystic fibrosis, this process does not work well.

Learn more in How the Lungs Work.


How do CFTR mutations cause cystic fibrosis?

More than 2,000 mutations of the CFTR gene can cause cystic fibrosis. Different mutations affect how much CFTR protein the cells make and how well the protein works. With the most common gene mutation, part of the CFTR gene is missing. The mutation makes a CFTR protein that can’t stay in the correct shape. Some CFTR mutations cause cells to make hardly any CFTR protein at all.

Normally, CFTR protein controls the movement of ions from inside the cell to outside the cell. In people who have cystic fibrosis, the CFTR protein does not work properly. The faulty CFTR protein affects how sodium and water move in and out of cells. Mucus becomes thick and sticky, because it contains less water. Also, sweat glands in the skin make extra-salty sweat.

How is cystic fibrosis inherited?

Every person inherits two CFTR genes: one gene from each parent. Children who inherit a CFTR gene with a mutation from each parent will have cystic fibrosis.

When a mutated CFTR gene is inherited from one parent and a normal CFTR gene is inherited from the other, the person will be a cystic fibrosis carrier. Cystic fibrosis carriers can pass the mutated CFTR gene on to their children. Carriers are generally healthy, but they may have some mild symptoms of cystic fibrosis.

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


Inheritance pattern for cystic fibrosis

If both parents have a normal CFTR gene and a mutated CFTR gene, each of their children has a:

  • 25% (1 in 4) chance of inheriting two normal CFTR genes
  • 50% (1 in 2) chance of being a cystic fibrosis carrier, because they inherit one normal gene and one mutated gene
  • 25% (1 in 4) chance of inheriting two genes with mutations and having cystic fibrosis
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