The life cycle of Trypanosoma brucei involves adaptation to a variety of conditions in the host and Tsetse fly. The successive changes in morphology, biochemistry and plasma membrane proteins, some of which also involve cell cycle arrest, are still very poorly understood. Many of these changes are likely to be directed by changes in mRNA abundance and translation, and over two decades of effort have now been expended in the identification of stage-specific mRNAs. Because of technical limitations, however, most of the transcripts identified have been rather abundant, and the regulation studied has mainly been restricted to the rapidly-dividing long slender bloodstream and procyclic forms.

Given the relatively small size of the T. brucei genome, there is a good prospect of a complete exploration of its genome by microarray analyses. This will allow identification of lower-abundance regulated transcripts, and (using amplification methods) the study of the transcriptome of the less accessible forms found in the Tsetse fly. One format for the analysis is to perform genome-wide expression studies on genomic instead of gene-specific fragments. Such arrays have several intrinsic advantages. Overall genome representation is usually good in shotgun libraries with comparatively little variation across the genome. Thus, even if randomly selected clone inserts are used as probes, there should be good coverage and relatively little redundancy. Also, not only coding but also intergenic regions can be studied, for example in chromatin immunoprecipitation experiments. Insert amplification can be performed with a single primer pair and functional analyses can actually precede sequencing.

Human African Trypanosomiasis (HAT) is a disease that exists in two forms. The acute form is caused by T. b. rhodesiense (observed in East Africa) while the chronic form is due to T. b. gambiense (found in West and central Africa). Around 60 million persons are exposed to this disease with some 500,000 current infections. Despite a relentless fight against HAT during the last century, it is currently resurgent in epidemic form in several countries.

The low sensitivity of the diagnostic techniques associated to the low parasitaemia that characterizes T. b. gambiense infections lead permanently to residual human reservoir of trypanosomes in HAT foci after medical surveys. Moreover, the treatment of T. b. gambiense infections requires toxic drugs that are also less effective in the late stage of the disease. This ineffective treatment is strengthens by the emergency of parasite strains resistant to the most available drug (Melarsoprol). Since treatment of HAT is harsh and sometimes ineffective, the vaccine approach is probably a good perspective for trypanosomiasis prevention. Until now, there is no prospective vaccine for HAT despite the fact that investigations on vaccine have been a goal for nearly a century.

With the resurgence of HAT, it is important to improve the control of this disease by undertaking studies that may lead to the discovery of new genes essential for the survival of trypanosomes. These genes could be targeted further for drugs, vaccine or diagnostic tools. The considerable differences between T. b. gambiense and the others T. brucei sub-species should be most likely the result of changes at the nucleotide sequences or in gene expression. We are analysing gene expression in T. b. gambiense using DNA-microarrays, which were produced on the basis of the shotgun clones made for sequencing. The identification of sub-species specific gene expression variations could greatly facilitate the generation and testing of hypotheses on the mechanism of human serum resistance in T. b. gambiense, the key factors to its ability to infect human.

Simo et al. (2009) Infect. Genet. Evol., in press.