What causes cystic fibrosis? 

Cystic fibrosis is caused by a specific change in the genetic code that an individual inherits from their parents at the moment of conception. This genetic alteration affects how cells in various organs, particularly the lungs and digestive system, transport salt and water across their membranes. Because it is a hereditary condition, it cannot be caught from someone else and it does not develop later in life due to environmental factors. Understanding the underlying cause involves looking closely at the role of proteins and the patterns of inheritance that allow the condition to be passed through generations. 

What We’ll Discuss in This Article 

• The role of the CFTR gene in the human body. 

• How salt and water balance is disrupted in cystic fibrosis. 

• The specific inheritance pattern known as autosomal recessive. 

• What it means to be a carrier of the cystic fibrosis gene. 

• Common genetic mutations found in the UK population. 

• Why genetic testing is used in diagnosis and family planning. 

The role of the CFTR gene 

At the core of cystic fibrosis is a fault in a single gene called the Cystic Fibrosis Transmembrane Conductance Regulator, or CFTR. This gene is responsible for providing the instructions to create a protein that acts as a channel on the surface of cells. According to the NHS, this protein controls the movement of salt and water in and out of cells, which is essential for keeping the mucus on the surface of organs thin and slippery. When the gene is faulty, the protein channel does not work correctly or is missing entirely, leading to the production of thick, sticky mucus that clogs the lungs and the pancreas. 

Autosomal recessive inheritance 

Cystic fibrosis is passed from parents to children through a specific genetic pathway called autosomal recessive inheritance. This means that for a child to be born with the condition, they must inherit two copies of the faulty CFTR gene, one from each parent. If a child inherits only one copy of the faulty gene and one healthy copy, they will be a carrier of the condition. NICE guidance states that carriers usually do not have any symptoms of the condition themselves but can pass the faulty gene to their own children. 

Understanding the risk for families 

In the UK, it is estimated that approximately 1 in 25 people are carriers of the faulty CFTR gene. Most carriers are unaware of their status because they do not experience any health problems related to the condition. When two people who are both carriers have a baby together, there is a specific set of probabilities for each pregnancy. There is a 25 percent, or one in four, chance that the child will have cystic fibrosis. There is a 50 percent chance that the child will be a carrier like their parents, and a 25 percent chance that the child will not have the condition and will not be a carrier. 

The impact of different mutations 

While all cases of cystic fibrosis are caused by faults in the CFTR gene, there are more than 2,000 different known mutations that can occur within that gene. The specific type of mutation can sometimes influence how severe the symptoms are and which organs are most affected. In the UK, the most common mutation is known as Delta F508. The majority of people with cystic fibrosis in the UK carry at least one copy of this specific mutation, which is the target for many modern treatments known as CFTR modulators. These medications are designed to help the faulty protein work more effectively, addressing the cause of the condition at a cellular level. 

How the cause affects the whole body 

Because the CFTR protein is found in cells throughout the body, the faulty gene has wide-ranging effects. In the lungs, the lack of water in the mucus makes it difficult for the tiny hairs, called cilia, to clear away bacteria, leading to chronic infections. In the digestive system, the thick mucus blocks the narrow tubes that carry digestive enzymes from the pancreas to the small intestine. This prevents food from being properly broken down, which is why people with cystic fibrosis often have trouble gaining weight. The fault also affects the sweat glands, causing the body to lose too much salt when sweating, which can lead to dehydration or salt imbalances in hot weather. 

Inheritance Outcome	Probability per Pregnancy	Health Implication
Affected (CF)	25% (1 in 4)	Individual has two faulty genes and has cystic fibrosis.
Carrier	50% (2 in 4)	Individual has one faulty gene, no symptoms, but can pass it on.
Non-carrier	25% (1 in 4)	Individual has two healthy genes and cannot pass on the condition.

Conclusion 

Cystic fibrosis is caused by inheriting two copies of a faulty CFTR gene, which disrupts the normal balance of salt and water in the body’s cells. This genetic error leads to the characteristic build-up of thick mucus that affects the respiratory and digestive systems. While the condition is lifelong and rooted in an individual’s DNA, understanding the specific mutations involved is becoming increasingly important for accessing targeted treatments. 

If you experience severe, sudden, or worsening symptoms, call 999 immediately. 

Authority Snapshot (E-E-A-T Block) 

This article provides a factual explanation of the genetic causes of cystic fibrosis, aligning with the standards set by the NHS and NICE. The information has been compiled by a medical content team and reviewed by a UK-trained physician to ensure clinical accuracy and relevance to patients in the United Kingdom. It is designed to help patients and families understand the biological basis of the condition without offering a personal diagnosis.