What causes cystic fibrosis?
Cystic fibrosis is a genetic disease, meaning it is caused by a defect in the person's genes. This means that both parents must pass on an abnormal gene for the child to be affected. One in 25 adults carries one copy of the cystic fibrosis gene. If both parents carry the gene, each child has a 25% risk of cystic fibrosis. If one parent has gene with no history of cystic
fibrosis and the other parent has the disease, there is a 50% chance of the child being affected.
Genes, found in the nucleus of all the body's cells, control cell function by serving as the blueprint for the production of proteins. The defective gene that is responsible for causing cystic fibrosis is on chromosome 7. Both copies of the defective gene are necessary for a person to develop the disease. If both parents are cystic fibrosis carriers (i.e., they each have one copy of the defective gene), the child has a 25% chance of inheriting both defective copies and developing cystic fibrosis, and a 50% chance of inheriting one defective copy and being a carrier.
In its most common form, a single amino acid mutation leads to the production of an abnormal transmembrane CFTR protein (cystic fibrosis transmembrane conductance regulator) which functions in transporting chloride ions across epithelial cells found in the lung and intestinal tract. Since water follows ions by osmosis, this results in water depletion and viscous mucus. The CFTR protein helps to produce mucus. Mucus is a complex mixture of salts, water, sugars, and proteins that cleanses, lubricates, and protects many passageways in the body, including those in the lungs and pancreas. The role of the CFTR protein is to allow chloride ions to exit the mucus-producing cells. When the chloride ions leave these cells, water follows, thinning the mucus. In this way, the CFTR protein helps to keep mucus from becoming thick and sluggish, thus allowing the mucus to be moved steadily along the passageways to aid in cleansing.
In cystic fibrosis, the CFTR protein cannot allow chloride ions out of the mucus-producing cells. With less chloride leaving, less water leaves, and the mucus becomes thick and sticky. It can no longer move freely through the passageways, so they become clogged. In the pancreas, clogged passageways prevent secretion of digestive enzymes into the intestine, causing serious impairment of digestion -- especially of fat -- which may lead to malnutrition. Mucus in the lungs may plug the airways, preventing good air exchange and, ultimately, leading to emphysema. The mucus is also a rich source of nutrients for bacteria, leading to frequent infections.
Recent medical research is beginning to show that an imbalance of essential fatty acids may play a role in cystic fibrosis. Tissue samples from both mice, and more recently humans, with cystic fibrosis show an excess of arachidonic acid (AA) and a deficiency of docosahexaenoic acid (DHA). Research has also indicated that healthy individuals with one copy of the cystic fibrosis gene and one copy of the normal gene have fatty acid levels in between those of cystic fibrosis patients and people with no CFTR gene mutations. Further research is needed to show how this is linked to the CFTR gene defect and what implications this may have on treatment of cystic fibrosis.