Genetic trickery probes tropical parasites.

Genetic trickery probes tropical parasites

When it comes to genetic sleight-of-hand, the one-celled trypanosomatids surely rank as masters. Some of these pathogenic protozoans sneak past their host's immune surveillance by cloaking themselves in an ever-changing wardrobe of surface proteins. Others escape detection by hiding in the very immune cells that would normally attack them. In each case, the covert maneuvers spring from a complex and cryptic chain of genetic commands.

Scientists have now devised a genetic ruse of their own for deciphering those commands. By slipping a foreign gene between DNA sequences identical to those in the parasites, and then inserting the engineered material into the organisms, they have tricked two types of trypanosomatids into substituting the foreign material for the coding sequence of a naturally occurring gene.

"The basic idea is that we can figure out the role of a gene by deleting it and studying the biology of the [altered] organism," says parasitologist Angela Cruz, who coauthored one of two reports on this work in the Nov. 8 NATURE.

Investigators say the gene-replacement technique could eventually reveal the function of a slew of trypanosomatid genes. Moreover, they suggest that the ability to decipher and disrupt genetic functions might lead to more effective drugs to treat the many tropical diseases caused by these parasites, and might enable researchers to create weakened strains that could serve as vaccines against some of the parasites.

Working separately, two research teams used a gene-insertion technique that mimics a natural mechanism for replacing damaged genes. Although this technique is becoming standard in studies of yeast and mice, it had never been tried with trypanosomatids. For their engineered material, both groups chose a bacterial gene for resistance against the antibiotic neomycin so that they could confirm insertions by exposing the parasites to the drug.

At Harvard Medical School in Boston, Cruz and Stephen M. Beverley hid the gene for neomycin resistance between certain genetic sequences taken from the trypanosomatid Leishmania major. By themselves, these sequences cannot code for proteins, but the researchers used them as homing devices to pinpoint the site of a particular L. major gene that codes for an enzyme involved in building nucleotides. After inserting the genetic sandwich into the parasites, the researchers found that 45 percent of their L. major colonies substituted the neomycin "marker" gene for the critical enzyme-encoding gene.

Piet Borst and his colleagues at the Netherlands Cancer Institute in Amsterdam report similar success in using the neomycin gene to replace a targeted gene in Trypanosoma brucei, which causes African sleeping sickness.

The new work opens up the long-range prospect of deleting trypanosomatid genes that confer virulence and using the weakened organisms as the basis for vaccines, asserts Mario Capecchi, a molecular geneticist at the University of Utah in Salt Lake City, in a commentary accompanying the reports. Borst told SCIENCE NEWS that while such an approach might work with Leishmania species, he doubts it could yield a vaccine for Trypanosoma parasites, in part because the variety of strains and the changeability of their surface proteins would likely enable them to evade vaccine-triggered antibodies. Borst adds, however, that new drugs for sleeping sickness and related diseases might ultimately emerge from studies identifying the genes that regulate the unique feeding habits of Trypanosoma parasites and that allow these organisms to change their protein cloaks.