杏吧原创

FOR some people, beef stamped 鈥淕uaranteed BSE-free鈥� would have a strong
allure. There鈥檚 a chance that it might save their lives鈥攅ven if it鈥檚 an
exceedingly slim one. But how much would shoppers be willing to pay for that
peace of mind? Several research teams are hoping the answer is 鈥渁 lot鈥�. They鈥檙e
using genetic manipulation and cloning technology in a bid to create cows that
are devoid of the prion gene (PrP), the deformed version of which causes BSE in
cattle and variant CJD in humans
(see 鈥淓at me, I鈥檓 not mad鈥�).

So just how big a benefit will we reap from this high-tech approach?
Curiously, there is already at least one low-tech way to achieve effectively the
same result: choose animals from BSE-free herds that have never been fed meat
and bone meal, the agent that spread the abnormal prion around the world. Unless
BSE occurs spontaneously in herds鈥攚hich is not thought to
happen鈥攖hese animals will stay BSE-free. It will be a real test of
consumers鈥� foibles to see if they will pay more for sirloin that could not
possibly contain PrP than for steak which all our present thinking tells us is
free of abnormal PrP.

As far as Britain is concerned, PrP-free cows are strolling into view too
late. Had they arrived nine years ago, the public鈥檚 appetite might have been
insatiable鈥攂ut today the peak of the BSE epidemic is past. So long as
other affected countries stop feeding animal protein to their cattle, the twin
threats of BSE and vCJD should also rapidly recede.

If the benefits of prion-free meat look meagre, what about the risks that
would come with it? Knocking out a single prion protein in a cow seems unlikely
to have a big impact on human health, but we can鈥檛 take that possibility for
granted. We could find ourselves wondering 10 years from now where some other
neurological disease emerged from. The other big fear about genetic
engineering鈥攖hat a foreign gene could spread widely鈥攊s not
applicable in this case.

The one group of organisms that will definitely be affected by expunging the
gene are the cattle themselves. While mice without the PrP gene appear
superficially normal, close scrutiny reveals that communication between their
nerve cells is disrupted, as are their circadian rhythms and sleep patterns.
What all this would mean for cattle isn鈥檛 clear, but most reasonable people
would agree that farm animals should not suffer any more than they do
already.

One of the more bizarre consequences of removing PrP is that farmers could
resume feeding ground-up cattle to their herds, without the slightest risk that
the animals could contract BSE. In many countries, feeding animal protein to
herbivores has been made illegal, but the wholesale introduction of PrP-free
cattle would make those laws obsolete.

Of course, farming animals for food is not the only reason for creating
PrP-free cattle. There鈥檚 also pharming鈥攅ngineering animals to act as
bioreactors, to cook up complex proteins for curing human ills. The first
products of this new industry are nearing the market. PPL Therapeutics in
Scotland, for example, has started phase 3 clinical trials on a human protein
expressed in sheep鈥檚 milk. How much would such companies pay for a cow鈥檚 egg
that came without a PrP gene? Not a lot, probably. When PPL set up its flock it
needed to avoid scrapie, the prion disease that afflicts sheep, so it imported
animals from New Zealand, which is completely free of the disease.

All of which makes you wonder why researchers have chosen BSE as their
target, when eradicating other conditions would bring clearer benefits. Animals
resistant to foot and mouth disease would be a real boon. True, no human lives
would be saved directly, but disease-free cattle would open up export markets
for rich and poor countries alike, and save them the huge costs of vaccination
or slaughtering infected animals. The problem here is that several genes would
probably have to be altered, a far trickier prospect than the single change
needed to make cattle immune to BSE. Another good target would be sleeping
sickness, which is a huge problem for people and cattle in sub-Saharan Africa.
But, as we know too well from the drugs industry, the ills of tropical countries
aren鈥檛 considered lucrative enough for profit-making companies.

Taking all this together, it smacks of a group of researchers who are all
dressed up with nowhere to go. Their technology is still missing that 鈥渒iller
ap鈥�. Pharming may prove itself in years to come, but PrP-free cattle certainly
isn鈥檛 it.