The persistent problem of cystic fibrosis: why are people with this disease plagued by lung infections?
In the United States, 30,000 people have cystic fibrosis. More than 12 years after researchers identified the mutated gene responsible for the disease (SN: 9/2/89, p. 149), scientists are still struggling to determine how its function relates to the pathology of cystic fibrosis.
Understanding exactly what goes wrong in people who have the mutation has been difficult in part because the normal protein from the gene seems to do different things in the different tissues affected by the disease and in part because it's hard to study what's happening on the surface of a person's lungs.
Using recent research findings, scientists are piecing together different ideas--some competing, some complementary--about why people with cystic fibrosis are so vulnerable to deadly lung infections. "The consensus is that people are finding a lot of functions for [the normal protein] that have been neglected," says Day. "The challenge is, can we link these definitely to the lung disease and pathology of cystic fibrosis?"
If so, researchers, physicians, and patients hope that this knowledge will stimulate the development of better therapies. However, the road ahead for cystic fibrosis patients may be--like taking a breath--more complicated than it sounds.
IS SALT THE SOLUTION? Most researchers trying to explain the symptoms of cystic fibrosis have looked first at salt transport in and out of cells. Even before the gene was isolated, scientists speculated that the protein defective in cystic fibrosis somehow regulated the transport of ions across cell membranes. The due had been that unusually salty sweat is among the many symptoms of cystic fibrosis. Sure enough, researchers discovered, the protein made by the identified gene transports chloride ions, one of salt's two components, in and out of cells. Because the movement of sodium ions, salt's other component, is naturally linked to the transport of chloride, the protein mutated in cystic fibrosis regulates sodium transport indirectly.
How this defect in ions transport triggers the lung infections of cystic fibrosis is still hotly debated. Abnormalities in salt transport may underlie the thick mucus and difficulties in clearing mucus from people's lungs. Recently, researchers have suspected that the normal protein, called cystic fibrosis transmembrane conductance regulator (CFTR), may also directly or indirectly regulate the passage of various other molecules into and out of cells.
In a variation on this idea, Richard C. Boucher of the University of North Carolina at Chapel Hill School of Medicine suggests that the initial malfunction in cystic fibrosis disturbs regulation of the fluid lining the respiratory tract. This in turn prevents normal clearance of bacteria from the lungs.
As a person breathes in, air flows into a branching series of tubes in the lungs. In each person, the cells lining these tiny airways have a combined surface area about the size of a tennis court. This surface is covered with a thin--about 7-micron--layer of watery liquid, upon which rests a layer of mucus about 20 microns thick. The normally sticky mucus traps bacteria entering the lung with each breath. In people without lung disease, hundreds of tiny cilia on each cell beat in unison through the liquid layer to move the mucus along the lung surface to the back of the throat, where it's swallowed.
Some research suggests that defective versions of the protein linked to cystic fibrosis impair clearance of mucus from the lung, permitting bacteria to replicate within the cells lining the lung and in the mucus itself. In the July 2001 Molecular Cell, Boucher and his colleagues reported measuring the thickness of the liquid underlying the mucus layer in tissue taken from mouse noses. Although mice with defective cystic fibrosis genes don't have the lung infections associated with the disease in people, Boucher showed that the liquid layer underlying the mucus in mutated-mouse tissue was only about two-thirds the height of the layer in normal-mouse tissue.
Boucher links this defect to the abnormal CFTR protein, which can't properly regulate sodium transport by lung cells. He reasons that cells keep drawing in sodium--and water, pulled by osmotic pressure, follows. The liquid layer coating the lungs thins as its water is absorbed by cells, he says. The cilia, in turn, can't beat effectively, and mucus clogs the lungs.
The abnormally thin liquid layer creates another problem, Boucher hypothesizes. When a person is ill, coughing helps to clear the lungs of bacteria-laden mucus. Without enough lubricating liquid, coughing isn't effective, Boucher says. The reduced capability to move mucus and cough effectively serves as "a double hit ... that makes cystic fibrosis such a severe disease," he says.
Boucher's hypothesis implies that adding salt to airways will pull water out of cells and increase the depth of the liquid layer that underlies mucus, which would boost a patient's rearing of mucus. This conflicts with a hypothesis proposed several years ago suggesting that the abnormal chloride channel in cystic fibrosis results in airway-surface fluid loaded with salt (SN: 5/4/96, p. 279). Jeffrey J. Smith of the University of Iowa College of Medicine in Iowa City and his colleagues found that defensins, some of the compounds the body uses to kill off invading microbes, are less effective in salty solutions. Further, the team showed that reducing the salt concentration of the culture medium for lung cells taken from people with cystic fibrosis and grown in the lab boosted the cells' ability to fight off bacteria. Smith's idea implies that adding salt could be harmful and that reducing the concentration of salt in airway fluid would help defensins attack infections.
The latest studies suggest that the salt concentrations in airway fluid produced by cystic fibrosis and that in normal lung tissue are similar. In the July 3, 2001 Proceedings of the National Academy of Sciences, Jeffrey J. Wine of Stanford University and his colleagues report no difference in the salt concentration of airway fluid produced by tissue sections removed during lung transplant surgery from healthy or unhealthy lungs.
A FREE RADICAL CONNECTION Abnormal handling of salt by lung cells isn't the only possible explanation of cystic fibrosis. Another one focuses on the action of glutathione, a compound found throughout the body. Normally, glutathione acts as an antioxidant, soaking up highly charged oxygen molecules, called free radicals, which are byproducts of some chemical reactions and are released by immune cells as they work to destroy invading infectious agents. If antioxidants don't control them, free radicals can damage the body's own cells and DNA.
For many years, scientists have known that people with cystic fibrosis tend to have unusually low concentrations of glutathione in airway fluid. Recently, Valerie M. Hudson of Brigham Young University in Provo, Utah, proposed that glutathione deficiencies attributable to the cystic fibrosis-associated protein might underlie or exacerbate many of the chronic problems of the disease. She drew on research suggesting that glutathione plays a role in immune system function and in keeping mucus at its normal consistency. Hudson--who is a political scientist and has two young children with cystic fibrosis--published her theory in the June 2001 Free Radical Biology and Medicine.
Though her hypothesis remains controversial, it's beginning to be tested in animal models. For example, in the July 2001 American Journal of Physiology: Lung, Cellular, and Molecular Physiology, Day reported that compared with normal mice, mice with mutations in the cystic fibrosis-associated gene have less glutathione in the layer of fluid lining the airways. The mice also show increased activity of antioxidants and related damage.
"We weren't expecting that," Day says, in part because the engineered mice don't develop chronic lung infections.
Preliminary data from Day's lab suggest that the normal protein may help transport glutathione into mitochondria, the energy-producing organelles of the cell. Mitochondria produce free radicals along with energy.
The lack of a normal cystic fibrosis-related gene first hampers natural antioxidant defenses in the lung, Day proposes. Then, when a patient gets an infection, the resulting release of free radicals from swarming immune cells "is another hit ... that might then set up the devastating lung disease that just gets worse and worse as you get older," he says.
FIGHTING OFF INFECTION A third emerging theory to explain the pathology of cystic fibrosis links abnormally acidic conditions inside cystic fibrosis lung cells to the ease with which bacteria can grow and live inside the respiratory tract. All cells attach sugars to proteins on their surface. These sugars serve as molecular flags to identify cells. The cystic fibrosis lung cells attach some atypical sugars, which P. aeruginosa and other bacteria can use as footholds while they attack the lung cells, says Vojo Deretic of the University of New Mexico in Albuquerque.
In the Nov. 20, 2001 Proceedings of the National Academy of Sciences, he and his colleagues show that higher-than-normal acidity within cells throws off the normal process of attaching sugars to proteins. Excess acidity may result from abnormal transport of bicarbonate through cell channels disrupted in cystic fibrosis, Deretic says.
He and his colleagues find that dosing human lung cells in culture with low concentrations of ammonia to decrease acidity reduced the sticking of P. aeruginosa by about 20 percent.
Scientists have proposed several additional explanations for P. aeruginosa infections in people with cystic fibrosis. One idea is that the normal CFTR protein boosts a cell's ingestion and destruction of the bacteria. Another hypothesis suggests that the thick mucus in the lungs of cystic fibrosis patients is low in oxygen. To adapt to this environment, the bacteria form persistent--and difficult to treat--biofilms (SN: 7/14/01, p. 28).
Other research may help physicians directly combat the P. aeruginosa infections that plague cystic fibrosis patients. Scientists have recently determined the nucleotide sequence of each gene in the bacterium. That information may lead to a vaccine against P. aeruginosa, says William B. Guggino of the Johns Hopkins Medical Institutions in Baltimore.
THE NEED TO DO SOMETHING Answers may not come fast enough for many people, says Hudson. "My oldest son is 5. Almost 25 percent of his life is gone," she says. "I just don't have time or patience for these wonderful things to come down the technological pipeline."
Hudson gives her two children glutathione supplements that are widely available as a dietary supplement. "Sure, the ultimate prize is gene therapy to cure the disease," she says, "but in the meantime you have to do something."
Although the desire of patients to embrace ideas that seem to offer help immediately is fully understandable, it's important to carefully test new therapies, says Deretic. "What we need to do is work fast, but also work carefully," he says.
Deretic is planning a trial of a drug that may, when inhaled, lower the acidity of the lungs. The drug, chloroquine, is currently used to treat malaria and has few side effects. Deretic says, "We are very hopeful this will keep these bugs from getting a foothold."
Even a solid molecular understanding of the cause of cystic fibrosis won't necessarily lead to effective new therapies, researchers warn. Though their understanding of cystic fibrosis is increasing, scientists agree there's still far to go.
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|Title Annotation:||ongoing research to understand cystic fibrosis|
|Date:||Jan 26, 2002|
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