Scientists Identify New Strategy to Fight Deadly Infection in Cystic Fibrosis
David Cohen, M.D.·Published 25 January 2013·7 min read
Key points
New research suggests that lowering excessive levels of a protein in immune system cells could be a strategy to clear an infection that is deadly to patients with cystic fibrosis (CF).
Cystic fibrosis is an inherited chronic disease that affects the lungs and digestive system of about 30,000 children and adults in the United States (70,000 worldwide).
very salty-tasting skin; persistent coughing, at times with phlegm; frequent lung infections; wheezing or shortness of breath; poor growth/weight gain in spite of a good appetite; and frequent greasy, bulky stools or difficulty in bowel movements.
About 1,000 new cases of cystic fibrosis are diagnosed each year.
New research suggests that lowering excessive levels of a protein in immune system cells could be a strategy to clear an infection that is deadly to patients with cystic fibrosis (CF).
Researchers determined that normalizing levels of the protein, called p62, in cells from mice carrying the most common mutation that causes CF will jump-start a natural cellular process that clears away the offending bacteria.
The scientists had previously determined that in cells from mice and humans carrying the CF mutation, the bacteria that cause this infection interfere with an important survival process in immune system cells; they also attributed this interference to elevated levels of p62.
The survival process, called autophagy, allows a cell to digest parts of itself to produce energy when it is experiencing starvation. In many infections, autophagy also helps digest pathogens and clear them away.
The bacterium, Burkholderia cenocepacia, causes a severe and persistent lung infection in patients with CF and is resistant to nearly all known antibiotics. Various types of chronic lung infection are responsible for about 85 percent of deaths in CF patients.
“Autophagy also controls inflammation, so when you decrease p62 levels in a CF mouse model and that improves autophagy, you are controlling inflammation produced by Burkholderia cenocepacia. And that’s what we are trying to do for patients – save them from inflammation,” said Amal Amer, associate professor of microbial infection and immunity and internal medicine at Ohio State University and senior author of the study.
What Is Cystic Fibrosis?
Cystic fibrosis is an inherited chronic disease that affects the lungs and digestive system of about 30,000 children and adults in the United States (70,000 worldwide). A defective gene and its protein product cause the body to produce unusually thick, sticky mucus that:
clogs the lungs and leads to life-threatening lung infections; and obstructs the pancreas and stops natural enzymes from helping the body break down and absorb food.
In the 1950s, few children with cystic fibrosis lived to attend elementary school. Today, advances in research and medical treatments have further enhanced and extended life for children and adults with CF. Many people with the disease can now expect to live into their 30s, 40s and beyond.
Symptoms of Cystic Fibrosis
People with CF can have a variety of symptoms, including:
very salty-tasting skin; persistent coughing, at times with phlegm; frequent lung infections;
Written and revised by
David Cohen, M.D.
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poor growth/weight gain in spite of a good appetite; and
frequent greasy, bulky stools or difficulty in bowel movements.
Statistics
About 1,000 new cases of cystic fibrosis are diagnosed each year. More than 70% of patients are diagnosed by age two. More than 45% of the CF patient population is age 18 or older. The predicted median age of survival for a person with CF is in the late 30s.
While relatively rare, B. cenocepacia infection is highly transmissible in patients with cystic fibrosis. By causing either severe sepsis or massive inflammation that damages lung tissue, the infection amounts to a death sentence for CF patients.
To lower p62 levels, the researchers introduced a small interfering RNA molecule, or siRNA, to silence a specific gene and reduce the protein’s activation. Amer plans to next test this protein-lowering technique in mice that are models for cystic fibrosis. Designing a similar strategy in humans would require many years of additional study, she noted.
The study is published in the current issue of the Journal of Biological Chemistry.
What is Cystic Fibrosis?
Cystic Fibrosis (CF) is one of the UK's most common life-threatening inherited diseases.
It is caused by a faulty gene that controls the movement of salt and water in and out of the cells within the body.
Cystic Fibrosis affects the internal organs, especially the lungs and digestive system, by clogging them with thick sticky mucus. This makes it hard to breathe and digest food.
Over 9,000 people in the UK have Cystic Fibrosis.
If two carriers have a child, the baby has a 1 in 4 chance of having Cystic Fibrosis.
Over two million people in the UK carry the faulty gene that causes Cystic Fibrosis - around 1 in 25 of the population.
Over 95% of the UK CF population is Caucasian, but CF affects many ethnic groups.
Each week, five babies are born with Cystic Fibrosis.
Each week, two young lives are lost to Cystic Fibrosis.
Currently half of the CF population will live past 41 years of age, and improvements in treatments mean a baby born today is expected to live even longer.
The cells that can use autophagy to clear infection are macrophages, which are first responders in the immune system that consume offending pathogens.
In previous work, Amer and former Ohio State doctoral student Basant Abdulrahman showed that in macrophages isolated from both mice and humans that carried the most common CF mutation, the bacterium would invade the macrophage and thrive instead of being digested and cleared away as it was in cells without the mutation.
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The research group showed that rapamycin, an existing drug known to stimulate autophagy, helped control B. cenocepacia infection in mice that serve as a model for cystic fibrosis.
“Rapamycin worked well as a proof of concept, but it has so many side effects that it’s hard to imagine giving it to small children with CF for an extended period of time. That’s why we looked for another method,” said Amer, also an investigator in Ohio State’s Center for Microbial Interface Biology (CMIB).
For this study, the researchers conducted experiments in macrophage cells derived from mice carrying the CF mutation and compared them to macrophages from normal, healthy mice.
The researchers observed in macrophages with the mutation that when p62 is elevated, other cell components clump together, causing disruption to the autophagy process.
How common is CF?
Cystic fibrosis is the most common genetically-inherited life-shortening chronic illness affecting young Australians today. A baby is born with cystic fibrosis every four days.
In Australia, one in 25 people are carriers of the CF gene. Carriers of the CF gene do not have any symptoms of the condition. If two people carry the gene and have a child, each pregnancy will have:
A one in four chance that the child will have CF A two in four chance that the child will not have CF but will carry the gene A one in four chance that the child will not have CF and will not be a carrier.
One in every 2,500 births produces a child with CF. Approximately 3,000 people in Australia have CF.
CF is usually diagnosed at birth
In most Australian States all babies are screened at birth for CF through the newborn screening test. This involves collection of a blood sample. If the results of the screening test reveal very high levels of a substance called immunoreactive trypsin (IRT), CF is suspected and the DNA in the blood is then analysed for the most common mutation-causing CF.
A sweat test may be done to measure the amount of salt (sodium chloride) in the sweat and confirm the diagnosis.
Some babies may also be diagnosed shortly after birth as a result of an intestinal blockage called meconium ileus. Most babies who have CF are now diagnosed within the first two months of life.
“p62 is a sticky protein, so high levels of it lead to the formation of aggregates. Once we get rid of that sticky protein – the glue – these protein aggregates will be able to go where they are supposed to go and allow the autophagy process to work properly,” Amer said.
Abdulrahman observed that in cells with the CF mutation, a key molecule gets caught up in those clumps. This molecule, beclin1, has a critical autophagy job, essentially escorting foreign particles to the cell parts that digest them and clear them away.
“Our hypothesis was that if we downregulate p62, this will release beclin1 from the aggregates. Once it’s available, we will have active autophagic machinery that is able to control the infection,” said Abdulrahman, first author on the paper.
In contrast, lowering p62 in macrophages from normal mice allowed the B. cenocepacia bacteria to grow. This confirmed that p62 actually controls the infection in cells from healthy animals but has the opposite effect when the CF mutation is present, she said.
This work is supported by the National Institutes of Health, Cystic Fibrosis Canada and an Egyptian Bureau of Education fellowship.
Additional co-authors are Arwa Abu Khweek, Kyle Caution, Mia Tazi, Hoda Hassan, Yucheng Zhang; Sankalp Malhotra and Patrick Rowland of the Department of Microbial Infection and Immunity, Department of Internal Medicine and CMIB; and Famke Aeffner and Ian Davis of the Medical Scientist Training Program and Department of Veterinary Biosciences, all at Ohio State; and Miguel Valvano of the University of Western Ontario and Queen’s University.
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