What Is CF?
Cystic Fibrosis (CF) is one of the most common genetic (inherited) diseases in America. It is also one of the most serious. It mainly affects the lungs and the digestive systems in the body, causing breathing problems and problems digesting foods. It is a chronic disease that currently has no cure.
Glands in the body that usually produce thin, slippery secretions (like sweat, mucous, tears, saliva, or digestive juices) produce thick, sticky secretions. These thick, sticky secretions plug up the ducts (small tubes) that should carry the secretions either outside of the body or into a hollow organ such as the lungs or the intestines. This can affect vital body functions such as breathing or digestion.
CF is present at birth because both parents carried a CF gene, and their infant inherited a CF gene from each parent. Not every child from this family will necessarily have CF. Other children could inherit a single CF gene from just one parent, and thus become a carrier for CF, or they could inherit no CF gene and be completely free from CF. Since 1989, when the CF gene was first discovered, research has made great progress in understanding CF.
How is CF diagnosed?
A suspicion of CF occurs when some of these symptoms are present:
- Persistent cough, wheezing, or recurrent pneumonia
- Good appetite, but poor weight gain
- Loose, bad-smelling bowel movements
- A salty taste to the skin
- Clubbing (enlarging) of the fingertips
A simple, painless test called a sweat chloride test can then be done. CF causes a large amount of salt to be lost in the sweat. Measuring the amount of salt in the sweat can determine whether or not a person has CF.
Genetics and CF
What is a gene?
A gene is the basic unit of heredity. Genes are responsible for the physical characteristics that each person has (like eye color, facial features, and many health conditions). Each gene occupies a certain location on a chromosome (a thread-like material that is located in the nucleus of every single cell in the body). Chromosomes come in 23 pairs, and each chromosome carries thousands of genes.
Each gene has a specific role in determining how a person's body is put together and how it functions. The role of a gene is determined by its individual DNA code (deoxyribonucleic acid, the chemical coding for a gene). DNA is made up of four building blocks called bases. These bases are joined in a specific order for each gene. When a change occurs in the arrangement of the bases, it can cause the gene not to work properly.
What are genetic disorders?
A structural gene change which can cause a disease or a birth defect is called a mutation. Genes are inherited in pairs, with one gene inherited from each parent to make the pair. Cystic fibrosis occurs when both genes in the pair have a mutation. A person with cystic fibrosis inherits one CF gene from each parent. Cystic fibrosis is a genetic disorder caused by inheriting a pair of genes that are mutated or not working properly.
The Cystic Fibrosis Gene
Everyone inherits two copies of the CFTR (cystic fibrosis transmembrane conductance regulator) gene. However, some of the inherited copies are mutations. To date, over 700 mutations of the CFTR gene have been identified. A person with CF inherits two mutated copies of the CFTR gene. These mutations can either be homozygous, the same, or heterozygous, different mutations. The most common mutation is delta F508, accounting for approximately 70% of all mutations. Those homozygous for this mutation tend to be pancreatic insufficient.
What Does the Mutation Do?
The CFTR gene is a protein that functions as a chloride channel. A chloride channel helps maintain the proper balance of salt and water within a cell. A mutation in CFTR causes a dysfunction of the salt and water balance. This causes dehydration of the secretions (thick mucous) and excessive loss of salt in sweat.
What is a carrier?
A carrier is a person who only has one copy of the mutated gene. The parents of a child with CF each carry one CF gene and one normal gene. They have no symptoms and no disease.
How does CF occur?
When each of the parents contributes a gene to their child, they could pass on either their CF gene or their non-CF gene. Each pregnancy could result in one of three outcomes: