www.czu.cz ; www.raupp.info

Celia Cordón-Rosales wants to build a ghost town. A dozen small thatch and
adobe huts would stand in several clusters. A few pigs would occupy nearby
pens, insects would buzz to and fro, and bacteria would live out
unremarkable lives. But the mock hamlet would be devoid of human residents.
It would also be enclosed in nets of mesh so fine that nothing as large as a
bug could escape. And a ditch would encircle everything to collect any
runoff water that might permit microorganisms to leave the site, August 2004
by Ben Harder.

All in the name of entomology.

Such an elaborate and carefully contained field site is needed for her
research on Chagas' disease, says Cordón-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.

Cordón-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 na?ve 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
screwworm 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: [sciencenews.org]).

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, Cordón-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 Cordón-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. Cordón-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 Cordón-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."

[www.sciencenews.org]

------------------------------------------
Posted to Phorum via PhorumMail