A disease similar to the fatal human illness CJD can be prevented and reversed in mice, scientists say.
Depleting normal prions in mice halted the disease
All CJD-like illnesses occur when normal nerve cell proteins - prions - convert into an abnormal form, causing dementia and death.
Mutant mice that lacked normal prions did not get sick, researchers found, even if abnormal prions were present.
This suggests that the disease is caused by the conversion process - not the abnormal prions.
The findings are published in the journal Science.
When a prion becomes abnormal or "rogue", scientists believe it changes all the surrounding prions into an abnormal form as well.
The result of this process is a crippling disease which, in humans, causes a slow descent into a vegetative state and, ultimately, death.
It has been the natural assumption that these symptoms are due to the build up of abnormal prions. It was thought something about them - their odd shape perhaps - caused the brain to degenerate.
For this reason, most research has focused on finding ways to prevent the accumulation of abnormal prions in nerve tissue.
But, when tried out on laboratory animals, these therapies were disappointing.
Now researchers, led by John Collinge and Giovanna Mallucci of the MRC prion unit, University College London, UK, believe previous experiments focused on the wrong step in the process.
The researchers bred a strain of mutant mice which, after 12 weeks of age, lost their normal prions.
When these mice and ordinary mice were both infected with abnormal prions they all began to develop a CJD-like disease.
But after 12 weeks, when the mutant mice lost their normal prions, the disease reversed - but continued to progress in the ordinary mice. Although abnormal prions carried on building up in their brains, the mutant mice remained healthy, whereas the control mice all died within two weeks.
These results suggest that abnormal prions themselves are not the problem - the problem is something to do with the conversion process between a normal prion and an abnormal prion.
Dr Mallucci and her colleagues conclude that there is a toxic intermediate stage that is the ultimate cause of CJD-like illnesses.
"The conversion process might produce a toxic intermediate stage, or it might somehow cause the nerve tissue to break down - but at the moment we just don't know," Dr Mallucci told BBC News Online.
They suggest that, in the future, CJD might be treated by therapies that target normal prions within the brain.
Dr Mallucci said: "A therapeutic strategy could involve developing a drug that binds to normal prions, causing their depletion."