杏吧原创

FOUR years ago, Terry Singeltary watched his mother die horribly from a
degenerative brain disease. Doctors told him it was Alzheimer鈥檚, but Singeltary
was suspicious. The diagnosis didn鈥檛 fit her violent symptoms, and he demanded
an autopsy. It showed she had died of sporadic Creutzfeldt-Jakob disease.

Most doctors believe that sCJD is caused by a prion protein deforming by
chance into a killer. But Singeltary thinks otherwise. He is one of a number of
campaigners who say that some sCJD, like the variant CJD related to BSE, is
caused by eating meat from infected animals. Their suspicions have focused on
sheep carrying scrapie, a BSE-like disease that is widespread in flocks across
Europe and North America.

Now scientists in France have stumbled across new evidence that adds weight
to the campaigners鈥� fears. To their complete surprise, the researchers found
that one strain of scrapie causes the same brain damage in mice as sCJD.

鈥淭his means we cannot rule out that at least some sCJD may be caused by some
strains of scrapie,鈥� says team member Jean-Philippe Deslys of the French Atomic
Energy Commission鈥檚 medical research laboratory in Fontenay-aux-Roses,
south-west of Paris. Hans Kretschmar of the University of G枚ttingen, who
coordinates CJD surveillance in Germany, is so concerned by the findings that he
now wants to trawl back through past sCJD cases to see if any might have been
caused by eating infected mutton or lamb.

Scrapie has been around for centuries and until now there has been no
evidence that it poses a risk to human health. But if the French finding means
that scrapie can cause sCJD in people, countries around the world may have
overlooked a CJD crisis to rival that caused by BSE.

Deslys and colleagues were originally studying vCJD, not sCJD. They injected
the brains of macaque monkeys with brain from BSE cattle, and from French and
British vCJD patients. The brain damage and clinical symptoms in the monkeys
were the same for all three. Mice injected with the original sets of brain
tissue or with infected monkey brain also developed the same symptoms.

As a control experiment, the team also injected mice with brain tissue from
people and animals with other prion diseases: a French case of sCJD; a French
patient who caught sCJD from human-derived growth hormone; sheep with a French
strain of scrapie; and mice carrying a prion derived from an American scrapie
strain. As expected, they all affected the brain in a different way from BSE and
vCJD. But while the American strain of scrapie caused different damage from
sCJD, the French strain produced exactly the same pathology.

鈥淭he main evidence that scrapie does not affect humans has been
epidemiology,鈥� says Moira Bruce of the neuropathogenesis unit of the Institute
for Animal Health in Edinburgh, who was a member of the same team as Deslys.
鈥淵ou see about the same incidence of the disease everywhere, whether or not
there are many sheep, and in countries such as New Zealand with no scrapie.鈥� In
the only previous comparisons of sCJD and scrapie in mice, Bruce found they were
dissimilar.

But there are more than 20 strains of scrapie, and six of sCJD. 鈥淵ou would
not necessarily see a relationship between the two with epidemiology if only
some strains affect only some people,鈥� says Deslys. Bruce is cautious about the
mouse results, but agrees they require further investigation. Other trials of
scrapie and sCJD in mice, she says, are in progress.

People can have three different genetic variations of the human prion
protein, and each type of protein can fold up two different ways. Kretschmar has
found that these six combinations correspond to six clinical types of sCJD: each
type of normal prion produces a particular pathology when it spontaneously
deforms to produce sCJD.

But if these proteins deform because of infection with a disease-causing
prion, the relationship between pathology and prion type should be different, as
it is in vCJD. 鈥淚f we look at brain samples from sporadic CJD cases and find
some that do not fit the pattern,鈥� says Kretschmar, 鈥渢hat could mean they were
caused by infection.鈥�

There are 250 deaths per year from sCJD in the US, and a similar incidence
elsewhere. Singeltary and other US activists think that some of these people
died after eating contaminated meat or 鈥渘utritional鈥� pills containing dried
animal brain. Governments will have a hard time facing activists like Singeltary
if it turns out that some sCJD isn鈥檛 as spontaneous as doctors have
insisted.

Deslys鈥檚 work on macaques also provides further proof that the human disease
vCJD is caused by BSE. And the experiments showed that vCJD is much more
virulent to primates than BSE, even when injected into the bloodstream rather
than the brain. This, says Deslys, means that there is an even bigger risk than
we thought that vCJD can be passed from one patient to another through
contaminated blood transfusions and surgical instruments.