Overview of Cystic Fibrosis
Cystic fibrosis (CF) is the most common fatal, inherited disease in the United States (see Incidence). CF causes the body to produce abnormally thick and sticky mucus in several different parts of the body, most prominently in the lungs and other parts of the respiratory system. It also affects the pancreas, leading to serious digestive problems.
Cystic fibrosis alters the mucus secretions of the body's epithelial cells. Epithelial cells make up the outside layer of tissue that lines every open surface of the body, inside and out, including the various tunnels and cavities in the lungs, urinary tract, liver, and reproductive tract. In patients with CF, the mucus that the epithelial cells secrete is much thicker and stickier than normal. It clogs the airways of the lungs, blocking the flow of air and making the tissue vulnerable to continual lung infections. It blocks the flow of pancreatic juices in the pancreas, impeding digestion and the absorption of fats and fat-soluble vitamins, leading to nutrition deficiencies and intestinal complications.
CF is a genetic disease resulting from the inheritance of a defective autosomal recessive gene. A recessive gene is one that is expressed only when both copies of the gene are present, so a person with CF must have inherited the gene that causes CF from both parents. A person who has only one copy of the gene is known as a "CF carrier."
The defective gene that causes CF codes for a protein responsible for salt (sodium chloride) transport by the epithelial cells. There are several hundred possible mutations that can lead to CF symptoms. The severity of symptoms varies depending on the particular mutation. Patients with milder symptoms live longer.
Due to vast improvements in treatment, the expected life span of people with CF has increased dramatically. Before the mid-1960s, the median age of death of children with CF was 2 years. That means that one-half of babies born with CF died by the age of two. By the early 1990s, the median age of death had increased to 25 years. Today many people with the disease live into their 30s, 40s and beyond.
Incidence and Prevalence of Cystic Fibrosis
According to the Cystic Fibrosis Foundation, about 30,000 Americans, 20,000 Europeans, and 3000 Canadians have CF. In the United States, about 12 million people are carriers, and every year 2500 babies are born with CF.
The disease affects all racial and ethnic groups, though it affects Caucasians with northern European ancestry more often than other populations. It is the most common autosomal recessive genetic disorder in Caucasians, occurring in about 1 out of every 1600 births. In African Americans, CF affects 1 in 13,000 babies; in Asian Americans, 1 in 50,000 are affected.
One in 22 Caucasians are carriers. Genetic testing can identify carriers, but the tests are only 80-85 percent accurate because not all of the several hundred mutations that can cause CF are detectable.
Genetics & Cystic Fibrosis
Everything our bodies do, from making a decision to blinking our eyes, to digesting our dinner, is governed by our DNA. DNA is packaged in genes, units of genetic material located within our cells' chromosomes. Humans have 46 chromosomes: 23 pairs, one of each pair inherited from the mother, the other from the father. Just as the chromosomes come in pairs, so do the genes: one copy of the gene on one chromosome is from the mother, the other copy on the other chromosome from the father.
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 CF carriers (i.e., they each have one copy of the defective gene), the child has a 25 percent chance of inheriting both defective copies and developing CF, and a 50 percent chance of inheriting one defective copy and being a carrier.
The defective gene that causes CF was identified in 1989. It causes a biochemical abnormality in a protein known as CFTR (cystic fibrosis transmembrane regulator). CFTR is an essential protein found in the epithelial cells that line the body's internal passageways, including the lungs, pancreas, colon, and genitourinary tract. The abnormality keeps the protein from carrying out its normal role of moving chloride ions (Cl-) through the cell membrane, from inside to outside the cell.
In patients with CF, Cl- ions do not move properly, and consequently the cells do not secrete normal mucus. Defective chloride transport also causes deficient water transport, so there is not enough water to wash the mucus away from the surface it is being secreted from. The accumulation of abnormally thick and sticky mucus causes obstruction and inflammation in the glands and ducts, eventually leading to serious tissue damage.
How exactly is the gene damaged? In 70 to 80 percent of all CF cases, three of the base pairs in the gene are deleted and the amino acid that the triad codes for (phenylalanine) does not get made. The abnormal protein is called deltaF508 CFTR. The other 20 to 30 percent of CF cases result from one of hundreds of other possible mutations that can lead to abnormal CFTR proteins.
Variations in the severity of CF symptoms probably result from variation in the particular mutation that causes the protein abnormality and (as of yet unknown) environmental factors that may influence how the genetic defect is manifested. Environmental factors include those that exist outside the person (e.g., their diet) and internal factors that a person may not have any control over (e.g., other factors about the biology of a person that affect how a protein works). When the environment affects how a gene is expressed, biologists call this "genotype by environment" interaction.
Almost all genetic expression is affected by genotype by environment interaction. In other words, genes do not operate in isolation, how a gene is expressed depends in large part on its environment. Genotype by environment interaction may partially explain the variation in symptoms among CF patients, even those who have exactly the same genetic mutation.