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By James Meek
Scientists fighting malaria are preparing
the ground for one of the
most audacious attempts ever to wipe out disease:
genetically modifying an entire animal species in the wild.
In laboratories around the world, there
is increasing confidence that scientists will acquire the
ability to spread a synthetic gene throughout the populations
of dangerous mosquitoes, making it impossible for them to
pass malaria on to humans.
Until now, spreading genes throughout
a species was something only evolution was capable of, over
millions of years of natural selection.
But scientists think it might be possible
to transform
the malaria-carrying mosquito into a subtly different species
- still a bloodsucking nuisance,
but no longer a killer - within two to 25 years
of releasing the first GM insects.
In a sign of how fast research is moving,
specialists in the field are gathering in London next week
for a conference to discuss the risks and benefits of releasing
GM mosquitoes into the wild.
"We're not talking about one to one
replacement of lab mosquitoes for wild mosquitoes," said
Tony James, of the University of California in Irvine, who
is attending the conference at Imperial College.
"There's no question of competition
between transgenic and non-transgenic insects. What we're
talking about is actually driving the gene through a population.
It's an ambitious idea."
In the lab, Dr James's team has already
inserted a gene into mosquitoes which makes it impossible
for the parasite that causes malaria to gain a foothold.
Last year, a joint British-German team,
partly led by one of the organisers of next week's conference,
Andrea Crisanti of Imperial College, created a transgenic
mosquito - a GM mosquito whose offspring would also carry
the inserted gene.
"For the past decade, our efforts
have been rather esoteric, trying to get to a certain stage.
We are at that stage now," said Dr James. "We're
able to put genes into animals in a stable way."
But there are concerns. Luke Alphey, a
specialist in the field at Oxford University, supports the
release of GM insects into the wild to combat disease. But
he is wary of the idea of genetically
modifying an entire species. "I have a rather
negative view of this strategy," he said.
"One of my concerns is that once
you've let such a thing go, you can never recall it."
Supporters of the approach point out that
it is not necessary to modify every single dangerous mosquito
to stop the disease. But the nature of the technique is such
that this could well be the end result.
Normally, a new gene will spread to cover
an entire species only if it gives animals who have it some
survival or reproductive advantage over animals that do not.
But scientists have found two ingenious ways to drive a non-advantageous
gene through mosquito populations so that eventually all mosquitoes
inherit it.
One is to attach the gene to a bacterium
called wolbachia, which can be made to infect mosquitoes,
becoming effectively a part of the insect.
When GM females mate with males, they
produce GM offspring, whether the males are GM or not. But
because of the peculiar properties of wolbachia, non-GM females
cannot have offspring with GM males. In other words, GM females
will always have more children, eventually crowding out their
non-GM rivals completely.
Freakish
The other method attaches the gene to
a freakish chunk of DNA called a transposable element, which
hops between chromosomes during reproduction.
Normally, mating between parents with
different genes gives the offspring a 50% chance of inheriting
either gene. Because of the transposable elements moving around,
however, the GM mosquito will always pass on the added gene
to more than 50%of its offspring - again, eventually covering
an entire species.
Sixty of the 380 mosquito species can
transmit malaria, although one, Anopheles gambiae, is responsible
for a large part of the 2.7m deaths caused by the disease
each year. In order to transform a single species, GM insects
would have to be released in many locations to spread the
gene through different populations of that species.
Steven Sinkins, of the Liverpool School
of Tropical Medicine, who has done extensive research into
mosquitoes and wolbachia, said tests of a complete system
were unlikely in the next two years, but progress had been
rapid.
"From the theoretical point of view,
there's no reason why either approach should not be successful,"
he said.
Malaria is transmitted by female mosquitoes
who harbour a parasite called plasmodium. The parasite infects
humans from the insect's saliva when it drinks the person's
blood.
The World Health Organisation estimates
that there are 500million cases of malaria each year, with
plasmodium becoming resistant to drugs and mosquitoes becoming
resistant to insecticides.
Dr Sinkins argued against the idea that
human intervention in a wild species on such a scale was unnatural
or wrong. The species would live on: it would just be more
human-friendly.
"It doesn't have to be anything too
unnatural," he said. "Within a mosquito population
there will always be some with a natural inability to transmit
the parasite. All you're doing is increasing the percentage
of individuals with those genes. They'll still be biting."
Dr James said the genetic approach was
no more unnatural than the massive, failing effort of drugs
and insecticides.
"The last thing anybody wants to
be known for is irrevocably
screwing up mankind or the environment. The whole
idea is to figure out how one conducts experiments, what's
going to be safe, and what's not.
"The problem of infectious disease
is going to be an eternal struggle. What we are looking for
is the next wave of useful tools that's going to buy us time."
The
Guardian September 3, 2001
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