BSE 'link to different CJD types'

The cause of sporadic CJD is unknown

Eating BSE-infected meat could lead to people developing different types of CJD, researchers have suggested.

Until now, it had been thought that BSE was only linked to the variant form of Creutzfeldt-Jakob Disease

But Medical Research Council experts say BSE may also manifest itself as sporadic CJD, or a new form of the disease not yet seen in humans.

The study, in Science, raises the possibility that more people than previously thought may be at risk.

CREUTZFELDT-JAKOB DISEASE vCJD - caused by the transmission via BSE-infected meat It tends to affect younger people, with the average age of onset around 27 Patients initially display psychiatric or behavioural symptoms and it may not be clear that the individual has neurological illness for some time. Sporadic or "classic" CJD - tends to affect middle-aged and elderly individuals. It has clear neurological symptoms and progresses very rapidly

The research focused on a gene which makes the normal version of the rogue "prion" protein involved in vCJD.

The gene encodes for two different types of amino acid - methionine (M) or valine (V).

Amino acids are the building blocks of proteins.

Everyone has two copies of the gene, so they can be MM, MV or VV.

Scientists had been puzzled by the fact that vCJD seemed only to develop in people who were MM, a genotype carried by 38% of the population.

So far there has only been one exception. A MV patient who developed the disease following a blood transfusion.

'Type 5'

The research was carried out on mice which had been genetically modified to produce human proteins.

Our work strongly suggests that we can not assume only those with one genetic profile are vulnerable to BSE infection Professor John Collinge, MRC Prion Unit

The mice were infected with BSE.

It was found that the type of prion disease seen depended on a mouse's genetic type.

VV type mice developed a form of the disease where the prions looked significantly different to those seen in vCJD, and which appears to be the new type of disease, and has been dubbed "type 5".

This has not yet been seen in humans, and the MRC scientists say they do not know what pattern of disease it would cause.

Mice carrying the MM strain developed disease which either resembled human vCJD, or sporadic CJD.

The researchers say their work suggest that type 4 prions, which are the type associated with vCJD, can only propagate themselves in people who make the M form of the protein.

It appears that the V form of the protein cannot adopt the particular molecular shape that characterises type 4.

While this could mean people with that genetic type are unlikely to develop vCJD, all forms of prion disease seen so far have proved fatal for humans.

'Vital clues'

Cases of the sporadic form of CJD have been increasing over recent years. Experts say this is largely due to better monitoring, but Dr Jonathan Wadsworth, who led the MRC research, said this study suggested some of those cases could be due to BSE infection.

The scientists say their work also suggests that if these prions were to pass from person to person - perhaps through a blood transfusion - the form of the disease would depend on the infected person's genetic type. Someone who was VV would not develop vCJD even if the person who had infected them did.

Dr Wadsworth added: "The real implications of this paper are that the results show we wouldn't expect to see just one form of BSE-related disease in the UK population."

Professor John Collinge, director of the MRC Prion Unit, based at University College London, said: "These mouse studies give us vital clues about the behaviour of prions and how they appear to modify and adapt depending on the genetic make-up of the individual they are infecting.

"We are always cautious about making direct comparison to the human condition, but our work strongly suggests that we can not assume only those with one genetic profile are vulnerable to BSE infection."

Professor Collinge added: "At this stage, it is not possible to say how this should alter estimates of those likely to become ill, but our findings do suggest we should be taking steps to draw up a more sophisticated system of categorising the disease so we don't mistake BSE related infection for a version of sporadic CJD."