A single mistake on a single gene can cause serious medical problems. In fact, it is just such a mistake, the omission of a three-letter word from the genetic code, that causes cystic fibrosis, a congenital lung disease that also affects the digestive system, in 90 percent of the about 30,000 cases in the United States. This deletion, or any of the approximately 1,500 other mutations that can cause cystic fibrosis, can be inherited from other parent, but only causes disease when it is inherited from both. All these mutations have similar effects, mucus forms in the lungs, interfering with breathing and leaving the person prone to infection. Cystic fibrosis also causes poor growth, infertility, and a reduced lifespan.
In fact, some researchers are suggesting that the mutation is associated with two or more different illnesses. One is the disease commonly thought of as cystic fibrosis, in which unusually thick mucus is produced, coating the lungs and causing the respiratory symptoms associated with the disease; another, often considered harmless, is now believed to cause problems with the pancreas and digestive system, with the reproductive system, with the sinuses, or elsewhere in the body.
Thickened mucus is not the only reason people with cystic fibrosis are especially prone to lung infections. People with cystic fibrosis also have high rates of an infection called cepacia syndrome, which disrupts the immune response. Cepacia syndrome is also a disease in its own right, causing fever, pneumonia, and, often, death. However, there is new evidence emerging that cystic fibrosis itself causes damage to the immune system. People with cystic fibrosis are missing a crucial immune system molecule that makes it possible for the immune cells to identify pathogens, in order to be able to hunt them down and destroy them. When the pathogens go unrecognized, they are allowed into the body to cause harm.
Fortunately, there is good news. Scientists are reporting that cystic fibrosis patients born in 2010 have a longer expected lifespan than their predecessors. Overall, only about half of cystic fibrosis patients live to 40—itself an improvement on the days when few reached elementary school age—but improvements in treatment and management mean those born this decade are expected to make it well into their 50s.