Symptoms of prion diseases, such as the human form of mad cow disease vCJD, can be reversed, a study of mice suggests.
Prion diseases affect the brain
Medical Research Council experts found memory and behaviour problems could be tackled by stopping production of the proteins corrupted in such diseases.
However, writing in Neuron, they warn the usefulness of the work for humans depends on having a test for vCJD (variant Creutzfeldt-Jakob disease).
A UK expert said it was "potentially very important work".
vCJD, BSE in cattle and scrapie in sheep are all caused by a build up of abnormally shaped versions of proteins called prions in the brain.
Normal prions have a very brief lifespan. But in prion diseases, they become malformed and start to accumulate.
They then damage the synapses, which pass messages between nerve cells in the brain, and eventually the cells themselves die.
The MRC team looked at mice which were in the early stages of a vCJD-like disease and having problems with memory and behaviour.
In some of these mice, scientists then used an enzyme to switch off the gene that makes the normal prion protein when the rodents are about nine weeks old - equivalent to adolescence.
The animals were seen to experience a reversal in their symptoms.
The animals regained memory of certain objects and restarted normal behaviours, such as burrowing, which had stopped as their symptoms emerged.
However, in mice in which the gene was not switched off, no reversal of symptoms was observed, and the animals went on to develop severe symptoms and die.
The researchers, led by Dr Giovanna Mallucci, say it may be possible to treat people in the early stages of prion disease.
But they accept that this relies on the development of a test to diagnose prion diseases before the brain is permanently damaged.
Dr Mallucci said: "It's very exciting that nerve cell function is recovered and early disease symptoms are reversed.
"The challenge now is to be able to detect early disease in humans and to develop treatments that can remove normal prion protein."
Further research is being planned.
Roger Morris, professor of molecular biology at King's College London, said: "This is potentially very important work because it shows something which we didn't expect and which could be extremely significant."
He said it appeared that in the early stages of prion disease in the brain, synapses were frozen, rather than being destroyed immediately; and that halting the process then allowed function to be restored.
"It means that the prospect of being able to do something for people may be much better than we thought. We may be able to reverse the behavioural effects."