Don't let the bugs bite: can genetic engineering defeat diseases spread by insects?
All in the name of entomology.
Such an elaborate and carefully contained field site is needed for her research on Chagas' disease, says Cordon-Rosales, who works at the Universidad del Valle de Guatemala in Guatemala City. Chagas' is one of several insectborne diseases that she and other researchers are aiming to take on through genetic engineering. Given the potential problems with releasing genetically modified organisms--either microbes or the insect vectors that carry them--scientists are incorporating extraordinary precautions into their research plans.
People contract Chagas' disease via infection with Trypanosoma cruzi. These single-celled protozoa get shuttled among people and animals by several species of insects called kissing bugs or assassin bugs. The bugs feed on the blood of many mammals and leave behind feces laden with parasites that can then enter the body.
The insects, which get one of their nicknames from a tendency to bite people near the mouth, infest huts and other rudimentary dwellings throughout much of Latin America. Kissing bugs and the trypanosomes that they carry range as far north as the United States, although Chagas' infections north of Mexico occur primarily in wild animals.
Of more than 90 million people living where Chagas' disease is endemic, an estimated 12 million to 18 million are infected. Between 10 and 30 percent of those infected subsequently develop heart failure or other chronic, life-threatening symptoms, and about 50,000 people die from the infection each year. There is no vaccine or cure for Chagas' disease.
Cordon-Rosales and her U.S. collaborators are also targeting malaria, sleeping sickness, and dengue and yellow fevers.
The goal for some of these efforts is to genetically alter the disease-spreading insects, while other efforts seek to manipulate organisms that live within the bugs. It will take a lot of basic scientific research before a single infection can be prevented. But even if researchers can develop genetic approaches to preventing infections, they'll face another, perhaps tougher, challenge. Will governments permit the release of modified insects or bacteria, even in regions of the world where they might do the most good?
While nobody plans to release mutant insects or microbes for at least several years, recent progress has transformed fanciful visions of doing so into feasible projects.
RESILIENT SCOURGES In the mid-20th century, insecticides and other measures eliminated malaria from the United States and Europe, and many public health workers were optimistic that spraying chemicals could greatly reduce the global burden of many vectorborne diseases.
In retrospect, says Frank H. Collins of the University of Notre Dame in Indiana, "it was a little bit naive to think of it that way." Many insect populations, particularly in tropical regions, proved too hardy. Chagas' disease, once nearly eliminated from the southern reaches of South America, is making a comeback there. Dengue fever and yellow fever, both spread by mosquitoes, occur more widely and with more frequency today than they have in recent decades.
"We've conceded that we're not really going to get rid of the mosquitoes," says molecular biologist Anthony James of the University of California, Irvine. But augmenting conventional measures with genetic engineering and other innovative approaches might pare down some vector populations and leave others incapable of spreading sickness, he says.
Some approaches disrupt insect reproduction. In a technique already in use, millions of factory-raised bugs are sterilized with radiation or chemicals and then released within a target area. The sterilized insects compete for mates with wild counterparts, but reproductive abnormalities make the resulting eggs nonviable. If sufficient numbers of sterilized insects are continually released, successive generations of wild bugs will produce fewer and fewer offspring.
While controversial, this technique, known as the sterile-insect technique (SIT), has worked against disease vectors and agricultural pests. Tsetse flies, which spread the fatal condition known as sleeping sickness, were eradicated from the Tanzanian island of Zanzibar in the 1990s. Entomologists also used the technique to clear California citrus groves of the invasive Mediterranean fruit fly and to eradicate a livestock parasite, the New World screw-worm fly, from North and Central America.
However, SIT is unlikely to work for controlling many insect populations. For instance, on the open expanse of the African mainland, it's doubtful that authorities using SIT could roll back tsetse flies faster than the flies can repopulate cleared areas, says Paul Coleman of the London School of Hygiene and Tropical Medicine and the company Oxitec in Oxford, England. In other species, the sterilization process damages males to such an extent that they have difficulty competing against wild males for mates.
Coleman and his Oxitec colleagues advocate the use of genetic engineering as part of an approach that's similar to SIT. Rather than randomly scrambling insect DNA with radiation, the researchers intend to selectively alter genes to reduce the bugs' fertility.
Coleman is focused on Aedes aegypti, an urban mosquito that spreads both dengue fever and yellow fever. His goal is to fashion male insects that are genetically unable to sire female offspring in the wild. In theory, mutant males reared in captivity, as well as any male progeny that inherited the mutation, would steal mating opportunities from fully fertile males in the wild, leading to a preponderance of male offspring. More important, the number of female mosquitoes available to spread disease and reproduce would dwindle with each generation and with each batch of mutants released, and the mosquito species might eventually disappear.
So far, researchers have demonstrated that this approach can stifle reproduction in laboratory fruit flies. Coleman and others are now working to identify and alter genes in A. aegypti that will have a similar effect. However, some modifications recently tested at the University of California, Riverside make it harder for male mosquitoes to both survive and breed. That leaves them at a disadvantage against wild competitors, according to research described in the Jan. 20 Proceedings of the National Academy of Sciences.
Some species present more fundamental obstacles to those who would control or eradicate them. In the case of malaria-transmitting Anopheles mosquitoes, it's difficult to distinguish and separate the sexes. And because females bite, accidentally releasing even sterilized ones could contribute to the spread of disease rather than slow it down. Furthermore, there's such genetic variation among Anopheles mosquitoes that no single engineered strain would be capable of mating--and thereby disrupting reproduction--in all wild populations, Coleman says.
ASSISTED EVOLUTION Where genetic engineering can't inhibit insect reproduction, it could block insects from spreading disease, researchers hypothesize. These scientists advocate altering the biology of wild insects so that they live, bite, and breed but don't transmit pathogens.
Researchers working with Anopheles mosquitoes, for example, have produced strains with several pathogen-hobbling genes. Malaria parasites inside the engineered mosquitoes either can't mature or can't spread to new hosts when the insects feed.
Serap Aksoy and her colleagues at Yale University, meanwhile, have engineered bacteria found in the guts of wild tsetse flies and reestablished them in flies in the lab. The modified bacteria manufacture chemicals that are harmless to the flies and yet deadly to the parasite that causes sleeping sickness.
One of the next hurdles in these efforts against mosquitoes and flies is to confer on each of the modified genes some evolutionary trait that would lead it to proliferate in wild populations.
Designing such a gene "driver" is a "very knotty problem," says Notre Dame's Collins. Some scientists anticipate making a driver from self-propagating chunks of DNA called transposons. Others are angling to have bacteria called Wolbachia carry modified genes throughout the targeted species. Those bacteria spread inexorably through many insect populations (SN: 11/16/96, p. 318).
The genetic control effort for Chagas' disease hinges on a different bacterium, Rhodococcus rhodnii, which typically lives in soil and the guts of a certain species of kissing bugs. Because those bugs can't live without nutrients made by the bacteria, R. rhodnii enjoys a symbiotic relationship with the Chagas' vector. Young kissing bugs acquire it by eating the pathogen-riddled feces of older ones.
In the July 2003 Infection, Genetics and Evolution, Cordon-Rosales' collaborators at the Centers for Disease Control and Prevention (CDC) in Atlanta and at Yale University reported that eight of nine engineered strains of R. rhodnii passed down an inserted test gene through at least 100 generations.
In prior experiments, the U.S. researchers had inserted into the bacteria another gene, which encodes a peptide that's harmless to kissing bugs but toxic to the Chagas' parasite. Feeding the modified bacteria to the insects eliminates most or all of the bugs' parasites, CDC'S Ellen Dotson and her colleagues found.
More recently, in a mock hut built inside Dotson's CDC laboratory, the researchers deposited fake kissing bug feces containing engineered bacteria. The researchers then released some kissing bugs within the enclosure. A majority of the insects picked up the engineered bacteria, Dotson says. She and her colleagues are now collaborating with Cordon-Rosales' team in Guatemala to refine this approach for bugs living in the wild.
Only one Guatemalan species of kissing bug appears to naturally acquire R. rhodnii, so the Guatemalan team set out to determine whether other Chagas' vectors could pick up the engineered R. rhodnii when exposed to them. Recent experiments suggest that this single engineered microbe may be broadly effective. Cordon-Rosales' team presented those findings in New Orleans at the May meeting of the American Society for Microbiology.
WORKING OUT THE BUGS Scientific progress notwithstanding, public distrust of genetic engineering is likely to become an obstacle to implementing genetics-based vector-control measures. Genetically modified crops have been banned in parts of Africa, Europe, and elsewhere, and mobile insects carrying engineered genes might prove even more unpalatable to many people.
On the other hand, Collins argues that compared with the importance of controlling human diseases, the social value of engineering crops is "not particularly compelling." The enormous public health benefits of genetic engineering make a more forceful moral case, he says.
In the United States, jurisdiction over genetically modified insects is poorly defined under the split authority of the Department of Agriculture, the Environmental Protection Agency, and the Food and Drug Administration, according to a report released in January. The Pew Initiative on Food and Biotechnology, a Washington, D.C.-based nonprofit organization also noted that in many developing countries that are the logical settings for releases of engineered insects, it's even less clear where relevant authority lies. Lawmakers bear the onus to provide timely guidance to researchers intent on deploying engineered bugs, the Pew report says.
In the meantime, scientists are anticipating an injection of research funds from the Seattle-based Bill & Melinda Gates Foundation, which last year identified genetic control of vectorborne diseases as one of 14 "grand challenges in global health." In June, the foundation received numerous grant requests--including one for building Cordon-Rosales' adobe-hut laboratory for studying Chagas' disease.
Even if genetic disease control someday becomes reality, scientists know they'll have to continue monitoring the modified organisms after releasing them.
Says Yale's Aksoy, "Our job won't be finished by releasing these insects and then going home."
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|Date:||Aug 14, 2004|
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