Brussels, 31 Aug 2005
An international team of researchers has succeeded in designing a biochemical technique that allows the reliable detection of malformed proteins that cause Creutzfeldt-Jakob Disease (CJD) in blood samples taken from hamsters.
The automated test takes only a few days to complete. If the method works in humans, it could help to minimise the spread of prions in blood transfusions and organ transplants, and facilitate the diagnosis of prion diseases. Such blood tests would also allow the screening of animals before they enter the human food chain.
For years, experts have feared that thousands of people are unknowingly carrying and transmitting the human form of bovine spongiform encephalopathy (BSE), or 'mad cow disease': new-variant Creutzfeldt-Jakob disease (vCJD). This new blood test could give researchers their first chance to establish how many people may be incubating the disease. So far, blood banks do no have an effective detection method for the infectious proteins responsible for brain-destroying disorders. Some victims of vCJD in Britain are thought to have picked up the disease from transfusions.
The disease is thought to be caused by the formation of abnormal proteins in the brain known as prions. These proteins apparently multiply by changing the conformation of normal proteins that they come into contact with, eventually leading to a fatal neurodegenerative illness. Prions concentrate in the brain, and at the moment, in order to find out whether an animal is infected with the disease, experts must kill it to obtain brain tissue for tests.
Some scientists tried extracting blood from live subjects and injecting this into another animal's brain. They then wait, typically for months, to see whether the animal receiving the blood develops the disease. But this method only succeeds in picking up on an infection 31 per cent of the time that it is present, according to lead researcher Claudio Soto. Moreover, it is not even clear whether blood taken from humans can cross the species barrier to reproduce the illness in test animals such as hamsters. Professor Soto's team decided to develop another option: amplifying the negligible levels of the misshapen proteins in blood to a detectable level.
'The concentration of infectious prion protein in blood is far too small to be detected by the methods used to detect it in the brain, but we know it's still enough to spread the disease,' said Professor Soto. 'The key to our success was developing a technique that would amplify the quantity of this protein more than 10 million-fold, raising it to a detectable level.'
Professor Soto, from University of Chile in Santiago, and his colleagues from the Universidad Autónoma, Madrid - working at the University of Texas Medical Branch - overcame the first barrier to this goal four years ago, when they managed to replicate deadly prions from hamster brains. The technique involves mixing normal proteins with tiny amounts of the infectious version in a test tube, causing the abnormal molecules to multiply and clump together over a period of about half an hour. By breaking the clumps and freeing the misshapen proteins through a pulse of sound waves, the process is accelerated and the misshapen proteins in blood are raised to a detectable level. The team has now improved and automated this process, making it a viable test. A microwave-sized machine can run 140 of these cycles in about 70 hours.
The technique, called protein misfolding cyclic amplification (PMCA), was tested with blood samples taken from 18 prion-infected hamsters that had developed clinical symptoms of prion disease, and 12 healthy hamsters. The result was that the method could detect prions 50 per cent of the time they were present after two 140-cycle runs. After six runs this was boosted to 89 per cent The test did not give any false positives on the healthy hamsters.
Professor Soto says that adapting the technique to run tests on human blood should not take longer than six months. Besides the fact it will need much more testing, he warns there might be ethical issues about screening for a disease that has no known cure. However, solving the question of how many silent vCJD carriers there are may provide strong arguments for investing in finding a treatment.
To download the abstract of the Nature Medicine paper, please: click here
Remarks: Reference document: Joaquín Castilla, Paula Saá and Claudio Soto, Detection of prions in blood. Nature Medicine. Published online: 28 August 2005