Molecular geneticists have opened a new front in the decades-long war against the world's greatest serial killer, malaria.
In the cross-hairs of their future weaponry is Anophelesgambiae, fiercest of the mosquito vectors that spread theplasmodial parasites of malaria from victim to victim.
In Africa, where A. gambiae, was discovered and described atthe turn of the century, malaria kills 1 million people a year,most of them children.
"The mosquito is not just a hypodermic syringe with wings thatexports one person's blood to another," said molecular anddevelopmental geneticist Fotis Kofatos of Harvard Universityand the University of Crete. "It is a complex organism, insidewhich the parasite undergoes a veritable metamorphosis afterit gets picked up by the mosquito from an infected person."
That very complexity offers a number of Achille's heels to thescientists now training their genetic guns on the insect. Kofatosis the principal author of a paper in last week's Science titled"A Detailed Genetic Map for the X Chromosome of the MalariaVector, Anopheles gambiae."
He and his first co-author, Liangbiao Zheng, are searching forthe gene or genes that the parasite activates or deactivates onits long march from the mosquito's mouth parts (where it sucksup plasmodium in infected blood) to its gut, across the gut wallinto the blood stream, and to the salivary glands, where itawaits injection into the next human casualty.
"The general idea of our strategy," Kofatos told BioWorld, "is togenetically engineer the mosquito to make it refractory to theparasite." He added, "The plasmodium is an organism that hasto find its way inside the mosquito, evade the insect's defensesand survive as it goes from tissue to tissue."
This complex process, Kofatos said, "can be very easily blockedbecause there are strains of A. gambiae in which the parasitewill not be able to survive."
To find such refractory strains and compare their genes withthose of parasite-susceptible mosquitoes, the Harvardresearchers enlisted vector biologist Frank Collins, chief of theMalaria Entomology Activity at the Centers for Disease Control.Collins and malariologist Lou Miller of the National Institutes ofHealth were the first to isolate such parasite-refractory strainsof A. gambiae. in the mid-1980s.
"Our paper describes the technique for mapping characters ofinterest in genomes not well-studied (such as the mosquito),"Collins, a co-author of the report in Science, told BioWorld. "Itrapidly accumulates genetic markers, to map gene loci thatbear on the ability of this particular mosquito to support thedevelopment of malarial parasites."
The question, said Kofatos, is, "What are these genes that blocktransmission of the parasite? There we are absolutely in thedark."
He and Zheng have pinpointed polymorphic microsatellitemarkers (molecular guideposts) all along A. gambiae's Xchromosome, so that Collins can cross-breed parasite-refractoryand susceptible strains of the insect.
From analyzing the recombination patterns of the tandem-repeat DNA sequences on the X chromosomes in their second-and third-generation progeny, the researchers expect to locateand clone genes that block parasite progress through themosquito.
"At that level," said Kofatos, "it's very close to the mappingtechniques being used in human medicine, to establishmolecular and genetic markers, and thereby identify diseasegenes, such as Huntington's and cystic fibrosis, for example."
By the end of the year, Kofatos expects to finish up detailedlinkage maps of the mosquito genome's other twochromosomes, to go with the X chromosome, which is nowvirtually completed. The X, smallest of the three, comprisesabout a fourth of the total genome's 260 million or so bases.
Chromosomally, said Collins, "mosquitoes are just like people.The female has two X's; male mosquitoes have one X, one Y."And the X is the sex-determining chromosome." The A. gambiaemosquito itself, he noted, "is bigger than a fruit fly, but muchsmaller than a house fly."
Once the Harvard/CDC team has located anti-parasite genes andcreated a race of transgenic plasmodium-refractory mosquitoesin their laboratory, what then? How can they fulfill Collins' goalof releasing them in the wild, to render A. gambiae no longer asuitable vector for malaria.?
"That's one of the tough questions," Collins said. "It's a problemthat people have considered only theoretically at this point."The least-effective scenario, he suggested, would be to simplyrelease masses of genetically engineered insects in hopes ofinundating natural populations.
One of his preferred strategies, "in the realm of imagination,"involves a genetic construct carrying a tiny artificial geneencoding a product toxic to the parasite or mimicking one of itsvital receptors. It would reach the mosquito perhaps via avirus that spreads rapidly throughout the population.
Or else the gene carrier might be an obligate symbioticorganism, something like a bacterium, that dwells inside themosquito's cells.
"At a more exquisite level," Collins visualizes a transposon, orjumping gene, transfected with the parasite-resisting DNAsequence. Such a mobile gene element would conferrefractoriness by lateral spread, infecting every member of thespecies in the wild, much as bacteria acquire antibioticresistance from other microbes.
"All this may seem a little far-fetched," Collins recalled, "butwhen one considers that it is now within the realm of day-to-day reality to genetically engineer a whole field of tomatoes,it's not that big a step to conceive of engineering a whole wildpopulation of mosquitoes." He surmised that "it's somethingthat will be done in the foreseeable future."
Kofatos made clear that "this is not a magic-bullet project, butone that has a longer perspective. I think it's quite excitingbecause it does represent an effort to apply the advances inmolecular genetics to an organism of tremendous humanimportance."
Right after World War II, Collins noted, a blitz began to wipeout the disease worldwide by deploying DDT insecticide andanti-malaria medicines. But it failed. For the past couple ofdecades, advances in molecular biology have encouragedefforts to perfect anti-malaria vaccines. So far these have notsucceeded. Even when they do, he said, "vaccines may not be asufficiently potent tool in isolation to take care of malaria."
The CDC entomologist said that current trends are broadeningthe research focus from parasite to mosquito, from killingplasmodia inside their human hosts to rendering their A.gambiae hosts inhospitable to the pathogen's transmission.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.