UCSF. US: A team led by UC San Francisco professor of medicine John Fahy,
MD, has discovered why mucus in the lungs of people with cystic
fibrosis (CF) is thick, sticky and difficult to cough up, leaving these
patients more vulnerable to lung infection.
Fahy and his team found that in CF – contrary to previous belief –
inflammation causes new molecular bonds to form within mucus,
transforming it from a liquid to an elastic sludge.
The scientists also made headway in the lab in exploring a potential
new therapeutic approach to dissolve those bonds and return the mucus to
a liquid that is easier for the lungs to clear.
CF is a lifelong inherited disease that affects the lungs and
digestive system. There is no cure. About 30,000 children and adults in
the United States have CF.
Fahy said that the research, a collaborative effort between UCSF,
University College Dublin (UCD), in Ireland, and the Cleveland Clinic
(CC), in Ohio, has implications for other lung conditions characterized
by thickened mucus, such as chronic obstructive pulmonary disorder
(COPD) and asthma.
The work was reported in the February 25 issue of Science Translational Medicine
Polymers – naturally-occurring molecules in mucus that form long chains – are the key to the discovery.
Until now, scientists had thought that CF mucus is thicker than
healthy mucus because it has a greater concentration of DNA polymers. To
test that idea, Fahy and his group exposed mucus samples taken from CF
patients to two current CF medications: Pulmozyme, a drug that breaks up
DNA polymers, and N-acetylcysteine (NAC), which targets disulfide bonds
between mucin polymers. Mucin is a protein that is the major
constituent of mucus.
“We thought Pulmozyme would be more effective than NAC in liquefying
the mucus, because CF sputum contains lots of DNA,” said Fahy. “But to
our surprise, NAC worked much better.”
Using confocal microscopy, the scientists learned why: CF mucus
consists of a dense core of mucin with a layer of DNA wrapped around it,
like a thin blanket draped over a solid pillow. Thus, while Pulmozyme
makes mucus less stiff by eliminating DNA, NAC succeeds in liquefying it
by breaking up the mucin.
Fahy and his team then investigated why mucin in CF is so compacted.
They found that mucin polymers become linked together crosswise by
newly-forged disulfide bonds. Fahy likened the polymers to logs floating
down a river. “The logs can float down the river as long as they are
floating independently,” he said. “But if you bolt them together side to
side, they will clog the river.”
The researchers found that inflammation causes the extra disulfide
bonds to form, when mucin polymers are exposed to highly reactive oxygen
molecules released by inflammatory cells in a process called oxidative
stress.
This observation was confirmed by a device invented by lead investigator Leo Shaopeng Yuan, of the UCSF Cardiovascular Research Institute.
In separate chambers, mucus from healthy volunteers was exposed to pure
oxygen and pure nitrogen. The mucus exposed to oxygen became thick and
elastic within seconds. The mucus exposed to nitrogen remained liquid.
“This qualitative change, driven by oxidation, happens with other
natural polymers,” said Fahy. “Think of latex, which starts out as
liquid tree sap. When it’s vulcanized – a process of chemical
cross-linking – it turns into the solid rubber we use in tires.”
Fahy noted that patients who are treated with pure oxygen in hospital
intensive care units have long been known to develop sticky mucus.
“This could be a function of the oxygen that’s used to treat them,” he
said.
Finally, the research team turned its attention to the possibility of
creating new treatments for CF that would target disulfide bonds in
mucin polymers directly and efficiently.
NAC, which targets mucin polymer bonds, is already an approved
medication used to break up mucus. “However,” said Fahy, “there are
problems with it. It’s a relatively weak drug, and it smells like rotten
eggs.”
Team member Stefan Oscarson, PhD, a medicinal chemist from UCD,
designed TDG, an experimental compound that targets disulfide bonds. TDG
liquefied mucus samples from CF patients much more efficiently than
NAC.
Fahy cautioned that TDG cannot yet be given to human beings. He noted
that while the team has applied for funding to develop their promising
new therapeutic approach, “there are at least five years of testing
ahead before we can say we have a new medication.”
Fahy predicted that the new finding will explain the reason for thick
mucus in other lung diseases known to be associated with oxidative
stress, including COPD and asthma. “We’re very confident that we’ve
uncovered a ubiquitous mechanism here,” he said.
Co-authors of the study are Martin Hollinger, PhD, of UCD; Marrah E. Lachowicz-Scroggins,
PhD, Sheena C. Kerr, PhD, Eleanor M. Duncan, MD and Brian M. Daniel,
RRT, of UCSF; Sudakshina Ghosh PhD, Serpel C. Erzurum, MD, Belinda
Willard, PhD and Stanley L. Hazen, MD, PhD, of the Cleveland Clinic;
Xiaozhu Huang, MD, of UCSF; and Stephen D. Carrington, PhD, of UCD.
The study was supported by funds from the National Institutes of Health and Genentech.