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Three deadly parasites share common genes
15/7/2005 11:05

An international group of scientists reported Thursday to have determined genetic blueprints for parasites causing three deadly diseases--African sleeping sickness, leishmaniasis and Chagas disease.
By comparing the genomic information of the three parasites-- Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, scientists found in surprise that the pathogens share a core of about 6,200 conserved genes.
The genetic similarities among the parasites far outweigh their differences, the researchers said in three papers in the July 15 issue of Science journal.
This common core of genes is extremely important because it may provide targets for a new generation of drugs that might fight all three parasites, which threaten millions of people worldwide, according to Najib El-Sayed, a lead investigator at the Institute for Genomic Research.
Analyzing the relatively smaller ways in which the organisms diverge genetically can also help researchers design vaccines, drugs and improved diagnostics targeting each of the three parasites.
"At the moment, there are no vaccines and only a few inadequate drugs to fight these devastating and neglected diseases, " he said.
All three diseases are spread by insects. T. brucei, which causes sleeping sickness, is spread by the tsetse fly and is found in sub-Saharan Africa. If left untreated, sleeping sickness is fatal. The World Health Organization estimates there may be as many as 500,000 cases of sleeping sickness each year.
T. cruzi causes Chagas disease and is spread through the infected feces of an insect sometimes called the "kissing bug" for its habit of biting near a person's mouth. Found throughout Central and South America, Chagas disease is particularly prevalent among the poor and claims 50,000 lives each year.
Various forms of leishmaniasis are spread by L. major and are endemic in 88 countries on five continents. Visceral leishmaniasis, also known as kala azar, is the most severe form of the disease and causes high fever, a swollen spleen and severe weight loss before killing its victims.
By determining the shared genes and focusing on those that differ, the comparative study sheds new light on the genetic basis for the differences between the parasites, including how they infect people, how they cause human disease, and why they are carried by different insects.
The comparison found common genes found in all three parasites that may have been acquired from bacteria through lateral gene transfer. In addition, studies of the parasites' Variant Surface Glycoproteins (VSGs) found T. brucei to have the most complex genetic apparatus for avoiding host immune systems and, at the same time, to be the most dependent of the parasites on the host's metabolism.
Scientists also discovered that many of the genes specific to each species are found in so-called sub-telomeric regions near the ends of chromosomes - areas where the genome tends to be more changeable.
An important finding in the T. cruzi genome study was the discovery of a novel and large set of 1,300 genes, called the " cumin-associated surface protein" genes, that may play a role in the parasite's evasion of the human immune system or in its ability to survive in the variety of hosts that it infects.
"Although relatively unfamiliar in the United States, the collective misery caused by these diseases throughout the world is considerable. Having these gnome in hand will give us many new targets for drug and vaccine development," commented Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases.

 Xinhua news