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New Fronts for Battling Malaria
In Publication of Mosquito Genome
New mosquito repellants, insecticides, and mosquito vaccines are some of the malaria-fighting tools that it may be possible to build using information from the newly-sequenced mosquito genome. The sequence of Anopheles gambiae, the primary mosquito species that transmits the malaria parasite to humans, appears in the journal Science, published by the American Association for the Advancement of Science.
Malaria is thought to afflict over 500 million people and cause nearly three million deaths each year, more than 90 percent of which occur in sub-Saharan Africa, according to the Science study, whose authors are from the United States, France, Israel, Spain, Germany, the United Kingdom, Russia, Italy, and Greece. A. gambiae is the most common mosquito species in Africa, and passes the malaria parasite, Plasmodium falciparum, on to humans when it feeds on their blood.
"Malaria in Africa is on the rise, as malaria parasites have developed resistance to anti-malarial drugs and mosquitoes have developed resistance to insecticides. Knowing the mosquito genome may help researchers identify genes involved in the insect's ability to host the parasite, or to locate a human to infect," said Don Kennedy, Editor-in-Chief of Science.
"New malaria control techniques are desperately needed in Africa, and the Anophelesgenome has an important part to play in fighting this disease," said lead author Robert A. Holt of Celera Genomics, Inc.
To sequence the A. gambiae genome Holt and colleagues used the "shotgun" method, which involves randomly sequencing segments of DNA from all over the genome and then connecting the segments by matching their overlaps. The assembled genome sequence is 278 megabases long. (Each megabase equals one million nucleotides, the basic units of DNA.)
Using software programs to identify likely genes in the sequence, Holt's team made a "first approximation" of the genes' general functions. The entire Anophelesgenome assembly was submitted to the publicly-available database, Genbank.
After the female mosquito feeds on blood, certain proteins and lipids from her meal travel to her ovaries, helping the eggs to develop in two to three days. After she has laid the eggs, the cycle of host-finding, blood feeding, digestions, and egg development begins once again.
Holt and his colleagues did a special study of the genes that were activated or deactivated when the female mosquito feeds on blood. They compared bits of gene-coding DNA (called expressed sequence tags, or ESTs) from blood-fed and non-blood-fed mosquitoes, and found that blood feeding activated a number of genes involved in cellular and nuclear signaling, digestive processes, lipid synthesis and transport, and egg production. Blood feeding also deactivated a variety of genes, including some involved in certain aspects of muscle contraction, vision, and metabolism.
"Those are the pathways that are likely to be useful in finding points of intervention for developing new insecticides or transmission-blocking vaccines," Holt said.
"I think the most important thing the genome will facilitate in the immediate future is understanding the molecular basis of resistance to insecticides, and finding new insecticide targets," he said.
Insecticide resistance can emerge when the expression of detoxifying genes increases, or through mutations in genes that encode the protein targeted by insecticides. The genome offers a catalog of both these kinds of genes, as well as variations in single nucleotides, known as "SNPs," found throughout the genome. This information should help researchers develop insecticides that kill mosquitoes via new targets or that don't elicit a detoxifying response. It should also be helpful in monitoring the spread of resistance to existing pesticides, according to Holt.
Possible "transmission-blocking vaccines" might target specific interactions between the malaria parasite and the mosquito, as the parasite progresses through its complex life cycle inside the insect. Holt speculated that one way this might work is by giving a vaccine to humans that results in the production and circulation in the blood of antibodies to specific mosquito proteins. The antibodies would then be transmitted to the mosquito when the insect feeds on human blood.
Another possible strategy might subvert the mosquitoes' ability to find human blood, which they need to produce viable eggs. Scientists generally agree that mosquitoes must be able to "sniff out" humans by recognizing human-specific odors. Holt's team, as well as another research team in the same Science issue (see the report by Catherine Hill and colleagues) described possible A. gambiae odorant receptors. A mosquito repellant that worked by blocking such receptors may prevent the spread of malaria, simply by making it harder for mosquitoes to find their prey, according to Holt.
Two additional Research Articles, ten Viewpoints, four Reports, and an Editorial accompany the genome report.
From a public health perspective, A. gambiae is the most important insect in the world, say Carlos M. Morel and colleagues. More than one million children, most of them in sub-Saharan Africa, die from malaria each year. Together with the completed human and malaria genomes, the Anophelesgenome "provides unprecedented opportunities for improving public health," say the authors.
The time is ripe for a new global effort to control malaria, says Jeffrey D. Sachs. Although earlier campaigns showed that complete global eradication of the disease was not feasible, these campaigns did make significant inroads against the disease, suggesting that malaria can be controlled by significantly restricting its transmission. Global control efforts will also receive a boost from the new Anophelesand Plasmodium genomes, but will require a sustained effort and increased funding for the next two to three decades to succeed.
A combination of drug treatment, vaccines, and mosquito control is necessary to combat malaria in Africa, and new approaches in each of these areas are "desperately needed," say Louis H. Miller and Brian Greenwood. They discuss clinical trials of vaccines in Mozambique and East Africa, the development and testing of new anti-malarial drugs such as artemisinins, and how information from the Anophelesgenome may help researchers alter the genetic characteristics that make the mosquitoes prime vectors for the disease.
Some researchers have suggested that genetically modified mosquitoes might be an effective way to combat malaria. Under this scenario, GM mosquitoes, resistant to the malaria parasite, would be released into natural populations to slow or eliminate malaria's transmission to humans. Such genetic modification has already been accomplished in Anopheles. But, more information is needed about mosquito population ecology before scientists can evaluate how well a GM mosquito strategy might work. Thomas W. Scott and colleagues discuss how further studies of mosquito ecology will complement genome studies in determining the best use, if any, for GM mosquitoes.
Recent recommendations suggest that GM work should involve public health specialists, scientists, and the general public whenever possible in areas where malaria is endemic. Contained laboratory testing, and assurance of that the release will produce significant health benefits, should both be completed before the release of any GM mosquitoes, say Luke Alphey and colleagues.
Comparisons between the genomes and proteomes of Anophelesand Drosophila reveal both considerable similarity and numerous differences, according to an analysis by Evgeny M. Zdobnov and colleagues. Both organisms began their separate evolutionary tracks about 250 million years ago, and share about 56 percent of their genetic sequence. This percentage is slightly lower than the percentage of shared sequence between pufferfish and humans (which parted evolutionary ways about 450 million years ago), suggesting that insects may have diverged "considerably faster" than vertebrates, say the researchers. Many of the notable proteome differences between the two insects related to their specialized ecological niches, in particular the mosquito's blood-feeding habit.
Insect genomics, particularly the comparative genomics of Anophelesand Drosophila, will play an increasingly important role in developing new hormonal, neuronal, and molecular targets for insecticides, according to Janet Hemingway and colleagues. Genome studies may also point the way toward methods that attack existing insecticide resistance and that lengthen the useful life span of currently used insecticides, the authors say. A report by Hilary Ranson and colleagues in this issue tracks the evolution of the supergene families related to insecticide resistance in Anophelesand Drosophila.
The Anophelesand Drosophila genomes now form a foundation for studying the comparative genetics of other insects--not just the pests that plague us, but the insects that are important to our well-being, such as the honeybee. Thomas C. Kaufman and colleagues note that the Anophelesgenome should spur genomic studies of other species of mosquitoes that carry diseases such as yellow fever and West Nile virus. The honeybee genome has already been selected for sequencing by the National Institutes of Health. As the number of sequenced genomes grows, scientists will uncover more information about the behavior, development, evolution, and often complex and highly adapted body plans of insects.
Researchers have identified different molecular and chromosomal forms within the A. gambiae species. These forms may occupy different ecological niches, and gene flow between the forms may be restricted in some instances. Scientists need to clarify this genetic substructure of the species in order to determine which forms are most often malarial vectors, say A. della Torre and colleagues. Current data suggest that differences between these forms may represent the very earliest stages of speciation, according to the researchers. Also in this issue, Igor Sharakhov and colleagues compare chromosome inversions and extensive gene shuffling in the genomes of A. gambiae and another mosquito, A. funestus.
In both its mosquito and human hosts, the malaria parasite must elude specific immune defenses. George K. Christophides and colleagues examine the set of immunity-related genes in Anophelesand compare them to the set of immunity genes in Drosophila. They found that gene families involved in immune recognition, signal modulation (sending either a "danger" or "false alarm" signal to the rest of the immune system), and response systems are characterized by extreme gene expansions in both insects. In Anopheles, many immune gene family features appear to be especially tuned to Plasmodium invasion.
Invasion of human red blood cells by Plasmodium is key to the progression of malaria. Proteins on malaria's surface aid the parasite in burrowing into and positioning itself within the blood cells. Alan D. Cowman and Brendan S. Crabb discuss how the Plasmodium genome will help researchers better characterize these proteins, leading to new targets for drug and vaccine development. A report by Sarah K. Volkman and colleagues discusses a high-throughput array technique that helps identify variations in these critical cell membrane proteins.
Cheap, easy, and safe, chloroquine was the drug of choice against malaria for much of the 20th century, distributed around the globe in massive quantities and even included in the salt supply of some countries. Now, it appears that chloroquine resistance exists in all four human malaria parasite species, and the search continues for a comparable replacement antimalarial drug. Thomas E. Wellems discusses the genetics of chloroquine resistance, and how the newly mapped genome of the Plasmodium falciparum malaria parasite may aid in the search for a replacement drug.
Recent research suggests that chloroquine resistance is associated with point mutations in the Plasmodium gene pfcrt, and in a report in this same issue, Amar Bir Singh Sidhu and colleagues provide conclusive evidence that pfrct mutations confer chloroquine resistance in malaria from Asia, Africa, and South America.
Could mosquitoes resist malaria? Natural resistance is the norm among free-living Anopheles gambiae inAfrica, but the mechanics of this are unclear. Oumou Niaré and colleagues report the first field tests of mosquito molecular resistance to malaria, and identify loci on Anopheles' chromosome 2 that may relate to resistance.
On the flip side, there is some evidence to suggest that malaria may manipulate insulin-like peptides in A. gambiae, potentially affecting the mosquito's growth and reproduction. In a Report from Michael A. Riehle and colleagues, the researchers identify some of the genes for these important peptides.
The study was supported in part by the National Institutes of Health.
This research is being published simultaneously with another paper describing the genomic sequence of Plasmodium falciparum. Contact Jo Webber, at +44 20 7843 4571, or J.Webber@nature.com.
2 October 2002