Cystic fibrosis is a relatively common genetic condition in which the lungs and digestive system become clogged with thick, sticky mucus. An illustration to show normal and mutant CFTR channels, with cross-sections of the effect on the airway is shown in Figure 1.
Figure 1. An illustration to show normal and mutant CFTR channels, with cross-sections of the effect on the airway. Image credit: Genome Research Limited
Genetics of cystic fibrosis
- Cystic fibrosis is caused by various mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene on chromosome 7.
- The most common genetic abnormality causing cystic fibrosis involves the deletion of just three DNA bases (a codon) from the CFTR gene.
- Cystic fibrosis is a recessive genetic disease, which means that both copies of a person’s CFTR gene must contain the mutation for cystic fibrosis to occur.
- If an individual has just one copy of the mutated gene they are said to be a carrier. About one in every 25 people is a carrier for cystic fibrosis.
- Cystic fibrosis has the highest incidence of any recessive disease in Caucasian populations (approximately 1 in 2,000) but is less common in other ethnic groups.
Biology of cystic fibrosis
- The CFTR gene provides instructions for making a protein called the cystic fibrosis transmembrane conductance regulator (CFTR).
- The CFTR protein acts as a channel across the membrane of cells that are specialised to produce mucus, sweat, saliva, tears and digestive enzymes.
- This protein channel normally transports chloride ions in and out of these cells which helps control the movement of water in tissues, ensuring that the mucus in our airways and various body systems remains thin and free-flowing.
- The most common genetic abnormality that causes cystic fibrosis results in a CFTR protein channel missing one amino acid.
- This change disrupts the function of the CFTR chloride channel, inhibiting the flow of chloride ions and water in and out of the cells.
- This means mucus-producing cells secrete mucus that is abnormally thick and sticky.
- This mucus can obstruct the airways and glands, resulting in the symptoms of cystic fibrosis.
- The most common symptoms of cystic fibrosis are coughing and chronic breathing difficulties caused by the build-up of sticky mucus in the lungs and respiratory tubes.
- Recurring lung infections are very common and in the long term this can result in lung scarring, called fibrosis.
- The damage caused by this fibrosis and thick mucus damages the airways of the lungs. In turn, this leads to a tendency toward more infections and decreased lung function.
- In 85 per cent of cystic fibrosis cases, sufferers also have a deficiency in their pancreas caused by thick mucus blocking the pancreatic ducts. This prevents the pancreas from delivering enzymes needed for digestion, and from producing insulin, which can lead to diabetes. Children with this aspect of the disease suffer from malnutrition and struggle to put on weight.
- Almost all males with cystic fibrosis are infertile because the tubes that transport sperm (vasa deferentia) are usually missing.
Diagnosis of cystic fibrosis
- In families where there is a history of cystic fibrosis, a prenatal test can also be offered which tests for the mutated form of the CFTR gene.
- Without a family history, clues to the disease can include:
- sweat tasting very salty
- persistent lung infections
- slow to growth rate.
- Since individuals with cystic fibrosis have very salty sweat, a simple test that measures the amount of salt in sweat can be used to diagnose the disease.
Management of cystic fibrosis
- There is no cure for cystic fibrosis but there are therapies and medicines that make it easier to live with the condition.
- Daily physiotherapy can help patients with the condition. Drumming and patting on the patient’s back and chest helps to loosen the mucus and clear it from the lungs. This reduces the risk of developing lung infections.
- Mucus-thinning drugs may also be administered.
- Careful monitoring is required to identify lung infections.
- Antibiotics such as tobramysin (often inhaled as a vapour) are prescribed to help manage infections, and antibiotics are often given to prevent infections throughout life.
- Supplementing the diet with enzymes normally produced by the pancreas to help break down food can help patients whose pancreas is blocked with mucus.
- Gene therapy aims to repair defective CFTR genes or replace them with working copies. Although these approaches have not yet been effective at treating cystic fibrosis it is an exciting area of research.
- Originally, average life expectancy once someone had been diagnosed with cystic fibrosis was 14 years. However, life expectancy has improved dramatically over the last few decades with many people with cystic fibrosis living well into adulthood with relatively good health.