What is cystic fibrosis, and what causes it?

[Anonymous]

What is cystic fibrosis, and what causes it?

[Genetic Counselor]

Cystic fibrosis is a genetic condition that makes the body produce thick, sticky mucus instead of the thin, slippery mucus that normally lines and protects many organs. This thick mucus builds up and blocks passageways in the lungs, the pancreas, the liver, the intestines, and the reproductive tract. Over time, these blockages cause repeated chest infections, trouble digesting food, and damage to several organs.

The cause lies in a single gene called CFTR. This gene carries the instructions for building a protein that sits on the surface of cells and works like a tiny gate, letting salt (chloride) and water move in and out of the cell. When this gate works properly, the right amount of water flows into the mucus and keeps it thin. When both copies of the CFTR gene carry a pathogenic variant, meaning a harmful change, the gate is missing, faulty, or stays shut. Salt and water get trapped inside the cells, and the mucus outside becomes dry, thick, and hard to clear.

More than 2,000 different changes in the CFTR gene have been described, though not all of them cause disease. They affect the gate in different ways. Some changes stop the protein from being made at all. Others produce a protein that is folded incorrectly and never reaches the cell surface; the most common change worldwide, called F508del, works this way. Still others let the protein reach the surface but leave the gate unable to open properly. This matters for families because the specific changes a person carries help predict how severe the condition may be and, importantly, which newer medications are likely to work.

The same faulty gate also explains a well-known sign of the condition: sweat that is unusually salty. In sweat glands, the CFTR protein normally pulls salt back into the body. When it does not work, too much salt is lost in sweat, which is the basis of the sweat test used for diagnosis.

Cystic fibrosis is not caused by anything a parent did during pregnancy, and it is not contagious. It results entirely from the combination of CFTR gene copies a child inherits from each parent.

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