http://www.guardian.co.uk/Print/0,3858,4249413,00.html
Scientists
plan to wipe out malaria with GM mosquitoes
Special
report: the ethics of genetics
James Meek, science
correspondent
Monday September 3, 2001
The Guardian
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 con cerns 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 500m 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."
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