LONDON - Genetic engineering on mosquitoes to make them resistant to infection with malaria parasites, or that will cause the insects to bite animals instead of humans, could soon become a reality. Research conducted by scientists in the UK, Germany and Greece shows that it is now feasible to insert genes into the genome of the mosquito, a feat that until now eluded the scientific community.

Andrea Crisanti, professor of molecular parasitology at Imperial College of Science, Technology and Medicine in London, told BioWorld International, "In the long term this discovery may represent a fantastic alternative to insecticides, which are toxic, expensive and have a detrimental effect on the environment."

Malaria is one of the most common infectious diseases to affect the human race. An estimated 100 million people suffer from it each year, and 1.5 million children a year die as a result. It is caused by a species of the parasite Plasmodium and transmitted by the bite of mosquitoes belonging to the genus Anopheles.

Despite the millions of dollars that have been spent on studying malaria, very little is known about what happens to the parasite while it is inside the mosquito. This failure has been partly due to the inability to introduce genes into the genome of anopheline mosquitoes, a problem that the technique described by Crisanti and his colleagues has now resolved.

The results of their study are presented in a letter to the June 22, 2000, Nature titled, "Stable germline transformation of the malaria mosquito Anopheles stephensi."

One reason why scientists have found it so difficult to carry out genetic manipulation on mosquitoes is that the outer layer of the egg hardens very quickly. Working with A. stephensi mosquitoes, Crisanti and colleagues developed a technique that allowed them to slow down hardening of the eggs, by allowing the female insects to lay their eggs in a solution of a chemical that inhibits an enzyme that plays an important role in the hardening process. The eggs remained soft and transparent for up to three hours, giving the researchers time to perform microinjections.

To carry out the gene transfer, Crisanti's group used a DNA carrier called the Minos transposable element. He said, "This element allows the gene to jump from the carrier DNA and position itself on the mosquito genome. The integrated DNA then has the potential to be transmitted to future generations. Because we used a gene encoding green fluorescent protein, we were able to spot the mosquitoes which were carrying the gene very easily. But you can use any gene you want, and out of every 100 mosquito eggs injected you will get about one transgenic mosquito."

The commercialization of the group's findings is being undertaken by the biotechnology company Implyx plc, of London, a subsidiary of Biogeny plc, which is in the process of seeking stock market flotation.

Crisanti predicted that the discovery could have enormous potential. "This technology represents formidable tools with which we can study the biology of the malaria vector and the malaria parasite," he said. "It is as though we have opened a box that was previously impossible to open - and we have found it full of objects with unknown functions."

He is campaigning for a project to sequence the genomes of the anopheline mosquitoes, "because we now have all the tools to exploit that information."

Next, Crisanti plans to try to generate mosquitoes that are resistant to malaria. "We hope to introduce a gene or genes that will interfere with the parasite's development in the mosquito. This should be feasible because already there are resistant mosquitoes in the environment which do not transmit malaria because their immune systems are able to eliminate the parasite," he said.

Alternatively, he added, it may be possible to generate mosquitoes that have a different host, by introducing a gene that makes the mosquito attracted to animal odors rather than human ones. If this strain were able to compete successfully in the environment with wild-type mosquitoes, and fed on the blood of animals rather than humans, this could reduce transmission of malaria.

Crisanti said such an approach would take a long time to set up. "You would need to make sure that it was absolutely safe, that the transgenic mosquitoes could not transmit malaria, that they could not transmit the transgene to animals or humans, and that guidelines were in place which everyone found acceptable."

Craig Coates, of the Center for Advanced Invertebrate Molecular Sciences at Texas A&M University in College Station, commenting on the paper in a News and Views article titled "A mosquito transformed," said the approach described by Crisanti and colleagues should speed up understanding of the physiology of the mosquitoes that carry malaria.

He wrote: "Advances in understanding the molecular basis of the mechanisms involved [in producing refractory strains] will provide molecules that can be introduced into a receptive strain to make it refractory. Furthermore, advances in understanding insect immune systems are providing potential target genes that can be transformed into a mosquito strain to prevent the development and transmission of malaria parasites."

One possible strategy, he added, would be to clone DNA sequences encoding the binding domains of antibodies that recognize malaria parasites and incorporate these into the mosquito genome.