Thirty years ago, cystic fibrosis (CF), formerly also known as mucoviscidosis, was considered to be a disease that only affected children. Hardly any children with this hereditary metabolic disease ever reached early adulthood. Since then, both the therapy and the life expectancy of CF patients have improved considerably. The microbiologist Dr. Gerd Döring from Tübingen is investigating the occurrence of respiratory tract defects that are common in CF patients, with the aim of developing new treatment strategies.
Children with cystic fibrosis tend to have an excessively salty taste to the skin. Newborn babies fail to pass meconium in the first few days after birth. As time goes on, other symptoms include heavy diarrhoea, poor growth and poor weight gain, and liver and pancreas inflammation.
Chronic respiratory tract infections and repeated pneumonias gradually destroy the lung tissue. This explains why hardly any CF sufferers thirty years ago reached adulthood.
"In the field of medicine, the causes of the complex disease picture of cystic fibrosis long remained a mystery," said Professor Döring from the Interfaculty Institute for Microbiology and Infection Research at the University Hospital in Tübingen. In 1983 it was finally discovered that the hereditary metabolic disease was the result of defective cellular chloride transport. This discovery was confirmed in 1989 when the cystic fibrosis gene was discovered. Mutations in the so-called CFTR (cystic fibrosis transmembrane conductance regulator) gene lead to the expression of a defective protein that is rapidly degraded. Non-mutated CFTR genes generate proteins that act as a chloride channel in the cell membrane of epithelial cells. "This discovery has enabled considerable advances to be made in CF research," said Döring.
It is now known that the absence of chloride channels prevents the transport of chloride ions from the cell. This leads to the osmotic retention of water in the cells, which in turn leads to a dramatic reduction in the water content of body secretions. “These secretions then become very tough and viscous,” said the researcher, going on to add that this can lead to a blockage in the tube-like canals, thereby preventing the secretion of sufficient quantities of digestive enzymes into the intestine. This makes it difficult to effectively utilise food.
However, the effect of tough secretions on the respiratory organs is even more dramatic. CF patients have difficulties in coughing up the tough mucus, which then accumulates in the bronchia where it forms an ideal breeding ground for bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa. The latter particularly likes such conditions. "Pseudomonas can also survive in anaerobic environments such as those created by tough mucus," explained the scientist.
When the pathogen comes into contact with oxygen once again, it proliferates so rapidly that massive inflammatory reactions in the tissue occur. At the same time, cystic fibrosis sufferers find it extremely difficult to fight off the pathogen with the body's own weapons. On the one hand, the tough mucus prevents the cleaning of the cilia which then stick together and are no longer able to transport other intruders from the respiratory organs. On the other hand, the tough mucus also prevents immune defence cells from reaching the site of infection.
It is now known that early antibiotics therapy can provide effective help. “Previously, one tended to wait for too long before starting antibiotics therapy,” explains Döring adding, “once the infection has become chronic, treatment is generally not possible.” This is down to the Pseudomonas bacteria’s ability to form a biofilm. “The bacteria wrap themselves in a sugar molecule coating, which prevents standard antibiotics from getting to them,” said Döring. The microbiologist and his team of researchers are now working on the discovery of the signals and metabolic pathways that trigger the development of these protective coatings in Pseudomonas bacteria. “Once we understand the molecular mechanisms that lead to the formation of biofilm, we will be able to look for specific countermeasures,” said the scientist who has been working on the pathogenic bacteria for almost 30 years.However, the tough mucus is not the only problem associated with cystic fibrosis. The lack of chloride channels results in pH shifts in the respiratory cells. This in turn prevents many enzymes from working as well as they should. In a sensational publication that appeared in Nature Medicine in 2008, Döring and several other scientists were able to show that pH shifts in the respiratory tract of CFTR-deficient mice leads to the formation of higher quantities of ceramide, a lipid constituent of cell membranes.
Ceramide build-up increases people’s susceptibility to bacterial infections, as it triggers cell death in the affected cells. “The destruction of the cells leads to the deposition of DNA, which in turn is an excellent adhesion material for bacteria,” said Döring. The administration of an enzyme that is able to degrade the DNA deposits, DNase, led to the prevention of DNA deposits in CF mouse lungs and was also able to reduce the number of Pseudomonas bacteria in these animals. The administration of the antidepressant amitriptyline would appear to indicate an interesting new therapeutic approach. “Ceramide can also activate cytokines with a pro-inflammatory effect,” said the scientist. “These cytokines lead to inflammation that is not linked to the presence of bacteria,” said Döring highlighting that it is of paramount importance to develop suitable anti-inflammatory substances for CF patients.
Another light at the end of the tunnel can be seen in the approaches that focus on activating the residual function of defective chloride channels. Excellent results have already been obtained with mutations that lead to milder forms of CF. Certain chemical substances are able to stabilise the CFTR protein to such a degree that chloride secretion returns to normal. This does not yet work in patients who do not produce any CFTR protein. However Döring himself states that “knowing that it works in principle, gives us hope for the future.”Progress in the development of more effective CF therapies has more than doubled the life expectancy of CF patients. “CF patients can now live as long as 40,” said the scientist. In order to improve treatment for CF patients and increase the research efforts, the University Children’s Hospital in Tübingen and the Stuttgart-based Robert-Bosch Hospital’s lung clinic in Schillerhöhe have established the Southwest German Mucoviscidosis Centre Tübingen-Stuttgart. This centre now offers an age-based treatment of CF patients. The Interfaculty Institute for Microbiology and Infection Research at the University Hospital in Tübingen, and a number of other Stuttgart- and Tübingen-based research institutions are also involved in the work done by the centre. Working together, the doctors and scientists hope to decipher the disease mechanisms of cystic fibrosis in detail. Döring is sure that “this information will have a decisive effect on the development of effective new treatment strategies.”