LONDON ¿ Therapies for parasitic diseases such as sleeping sickness and malaria have inched closer with new insights into how parasites manage to change their protein coats to stay one step ahead of the immune system.
Researchers in the UK have pinpointed how the organism that causes sleeping sickness selectively expresses just one protein out of hundreds of potential variants.
Keith Gull, professor of molecular biology at the University of Manchester, told BioWorld International: ¿The strategy of switching the proteins on the organism¿s surface is seen in many parasites. It seems to be a way for parasites to elude the immune response over long periods. The organism that causes malaria also exhibits antigenic switching and our discovery may provide a new model for looking at malaria and certain other parasitic diseases.¿
Gull, together with Miguel Navarro, now at the Instituto de Parasitologia y Biomedicina in Granada, Spain, reports his findings in a letter to Nature titled: ¿A pol I transcriptional body associated with VSG mono-allelic expression in Trypanosoma brucei.¿
African sleeping sickness, caused by the protozoan parasite T. brucei, occurs in a broad band across central Africa. About 60 million people are at risk of contracting the disease, which is spread by the bite of the tsetse fly, and there are about 500,000 cases each year. The chronic form of the disease can last for many years, while the acute form is rapidly fatal once the parasites breach the blood-brain barrier. The only drugs available are old and frequently toxic.
T. brucei, Gull said, has a ¿wonderful way of outwitting the immune system.¿ Its entire surface is covered with a single variant of a highly variable glycosylated protein¿hence the name variable surface glycoprotein (VSG). Each variant is one of many hundreds of copies of the same protein, all encoded by individual genes in the T. brucei genome.
More than 100 years ago, people realized that those who caught sleeping sickness suffered waves of infection, Gull said. ¿Any one parasite expresses only one of the VSG genes at any one time. As this parasite multiplies and parasitemia ¿ the number of parasites in the blood ¿ increases, the immune system starts to deal with the infection and kills most of the parasites, so that parasitemia falls. But because a few parasites will, at random, have switched to expressing another VSG gene, these will then begin to multiply and parasitemia will rise again. Then the cycle repeats itself again.¿
Gull and Navarro began studying how this mechanism operates. Earlier work by other groups had shown that T. brucei has 20 possible expression sites for the family of VSG genes. These are located on the telomeres of each of the trypanosome¿s 20 chromosomes.
¿It is a conundrum that there are many expression sites in the cell, but only one of them is ever in use at any one time,¿ Gull said. ¿How come there are multiple expression sites in the cell but only one is expressing a gene at any one time and all the others are inactive?¿
The two researchers knew that the polymerase responsible for transcribing the VSG genes was RNA polymerase I (pol I), which normally works on transcribing the ribosomal genes in the nucleolus, a spherical body found in the nucleus. So they decided to find out where pol I was when it was transcribing the VSG genes.
They observed that in the cells of trypanosomes taken from the bloodstream of humans (but not those taken from tsetse flies), there is a small dot on the nucleus, which they called the expression body. They then set out to prove that the one telomere that has the active expression site on it is present in this expression body.
¿The paper in Nature shows that we tagged the active expression site and showed that it was in the expression body,¿ Gull said. ¿We also showed that pol I was in the expression body and that it was active inside it. So the answer to our question is that the active gene is in an active expression site in a single unit structure like a small transcription factory in the nucleus.¿
In malaria, Gull said, the parasite similarly switches between about 50 var genes, only a few of which are expressed at any one time.
The discovery could have important implications for the development of new therapies. ¿Since the immune response is often capable of dealing with the infection, if you could develop an inhibitor to stop this factory working or stop the process of switching, then the immune system would probably be able to cope,¿ Gull said.
Gull now wants to find out which proteins are present in the expression body and how it works. ¿If we can do this we may be able to design effective ways of stopping the switching mechanism from operating,¿ he said. n