麻豆传媒

Is a new wave of the human form of mad cow disease about to sweep through the UK? That鈥檚 the question raised by what could be the first case of vCJD in someone genetically distinct from all previous cases.

So far, 160 people in the UK are known to have died from variant Creutzfeldt-Jakob disease, but all of them have had the MM variant of the gene that makes prions 鈥� the proteins which become misshapen when the disease takes hold and fatally clog up the brain. About 40 per cent of the UK population carry the MM variant, which occurs when two copies of the amino acid methionine are present at a specific site in the prion gene.

Now, a similar but not identical case of vCJD has killed a 39-year-old woman with the VV variant of the gene, which carries two valine amino acids at the same position in the gene (). The big question is whether this case is genuinely a new form of vCJD, resulting from exposure to BSE-infected beef, or whether it is simply a rare form of 鈥渟poradic鈥� CJD, which occurs purely by chance.

鈥淭he big question is whether this case is genuinely a new form of vCJD or whether it is simply a rare form of 鈥榮poradic鈥� CJD鈥�

If it is the former, it could signal a new wave of cases in VV individuals, who make up 10 per cent of the UK population, though the researchers who analysed the woman鈥檚 brain say there鈥檚 no cause for alarm yet. 鈥淭he final conclusion remains open,鈥� says Simon Mead of the Medical Research Council鈥檚 Prion Unit at University College London, who examined the woman鈥檚 brain. 鈥淚t is waving the flag for neurologists to watch for other cases.鈥�

The case shares similarities with MM cases of vCJD, including the relatively young age of the woman. By contrast, most cases of sporadic CJD are in people over 50. Another similarity is that, as in the MM cases, the misshapen protein carries two sugar molecules, not one as is found in sporadic CJD.

The VV case shows patterns of damage to the brain that fail to correspond exactly to either MM cases of vCJD or to sporadic CJD. Mead says this is to be expected, because the pattern of brain disease is known to differ according to the prion gene variant an individual possesses, whether human or animal.

Worryingly, animal experiments and studies of people with CJD have demonstrated that the gene variant also alters the time it takes for the disease to set in. So in humans, the first wave could be in MM individuals, the second in those with the VV variant and the third in those who are MV, who comprise half the UK population.

This is what happened with kuru, a disease similar to BSE affecting members of a tribe in Papua New Guinea who ate deceased people鈥檚 brains as part of their funeral ceremonies. The disease struck last in individuals with the MV variant. Mead says experiments in mice are under way to see what sort of disease is caused by prions taken from the latest case. 鈥淭he consensus is that it鈥檚 more likely sporadic, but we can鈥檛 conclude either way until further experiments are done,鈥� says Chris Higgins, chairman of the UK government鈥檚 Spongiform Encephalopathy Advisory Committee, which discussed the new results on 14 December.

BSE and vCJD 鈥� Learn more in our comprehensive special report.