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Malaria kills a child every 30 seconds in Africa, according to the "Roll
Back Malaria" program (WHO, UNDP, Unicef). Two thousand million people,
that is 40% of the world's population, are at risk and it is estimated that
there are 300 million clinical cases each year. Today there is no vaccine against
malaria and the parasite responsible (Plasmodium) is becoming more and
more resistant to anti-malaria drugs. The fight against malaria depends largely
on the fight against the carrier, the Anopheles mosquito, which transfers
the parasite to humans. This mosquito, however, is becoming resistant to present-day
insecticides.
It is therefore crucial to understand the mosquito's biology and its interactions with the parasite in order to develop new weapons for the struggle.
In this context, the uncovering of the mosquito's genome sequence and the study of the genes provides a mine of information that will allow researchers to understand the biology of the Anopheles, and then, eventually, to develop methods to control the transmission of the parasite and so fight malaria more effectively.
During the 1990s, several teams around the world had undertaken genetic studies of the Anopheles mosquito, and especially its interactions with Plasmodium, which lives part of its life cycle in the insect. Identification of the genes by traditional methods is, however, a long-drawn-out process, although Anopheles specialists were then looking at the sequencing of a large part or even the whole of the insect's genome. Thanks to the progress resulting from the human genome sequencing program, such a project was no longer unrealistic.
France was the first country to undertake a large-scale sequencing program for the Anopheles genome. As early as 1998, Genoscope and the Unit of Insect Biochemistry and Molecular Biology at the Institut Pasteur sequenced and analyzed the ends of 12,000 large genome fragments from a "bank" set up by Frank Collins at the University of Notre-Dame in the United States. This first glimpse of the anopheline genome yielded many chromosome markers and allowed the identification of new genes. Additionally this data was useful from the point of view of sequencing the entire genome.
So it was that in March 2001 an international consortium* was set up for sequencing the Anopheles genome. Genoscope took part in the project, taking on part of the work of the sequencing itself. The greater part of the sequencing fell to the American enterprise Celera Genomics, with 9 million dollars' financing from NIAID (National Institute of Allergy and Infectious Diseases).
The strategy selected for the sequencing of the Anopheles' 280 million bases (Megabases - Mb) was the "whole genome shotgun" method already used in 2000 by Celera to sequence the Drosophila genome. This consists of determining the sequence of small random fragments (about 5 million, of which 500,000 were sequenced at Genoscope) taken from the genome as a whole and representing several times its total length. The fragments are assembled on the basis of their overlaps but, owing to the risks of sampling, these contig groupings do not cover the entire genome, being separated by gaps, here amounting to about 20,000. Other sequence information, including the preliminary data from Genoscope, allowed these contig groupings of fragments to be arranged in frameworks, or scaffolds. In this way Celera covered a great part of the mosquito's genome with such scaffolds.
Tying the scaffolds to the genome was particularly helped by a map established on the basis of the chromosomes of mosquitoes, which offer the advantage of being "giants" in some tissues. The outline genome that resulted has been freely available since March 2002. The genes were delimited using computer methods by Celera and the "EnsEMBL Bioinformatics Group" (European Bioinformatics Institute/ Wellcome Trust), in the United Kingdom. Nevertheless, this "annotation" work is far from definitive. Besides, the high degree of polymorphism in the sequenced stock of Anopheles caused difficulty in the grouping. As a result, substantial modifications may have to be made when the finishing work is undertaken. The Institut Pasteur and Genoscope are pursuing their collaboration with a view to improving the quality and interpretation of the sequence data.
Barely three years ago, the data banks contained fewer than 10 complete Anopheles genes. Now, the researchers estimate that this outline genome makes 14,000 genes available. This considerable mass of information will allow them to gain a better understanding of anopheline biology. Identification of the mosquito genes involved in the parasite's transmission, resistance to insecticides, the mosquito's olfactory system, its immunity, etc should eventually lead to the development of ways to control the transmission of malaria by this vector.
Analysis of the sequence has already made it possible to specify or orientate particular research work projects. In the Insect Biochemistry and Molecular Biology Unit at the Institut Pasteur, for example, researchers are studying certain proteins which may be implicated in the mosquito's resistance to insecticides, the ABC carriers. Four of these had been identified, but as of now we already know that there are some fifty of them! And analysis of the genome sequence has already made it possible to identify some of the gene families to work on.
Moreover, comparisons between the genome sequence of Drosophila fruit fly (obtained in year 2000) and that of the mosquito are extremely useful. For example, a mechanism capable of blocking development of the parasite in the mosquito has also been found in Drosophila, and a key gene involved in the mechanism had been identified in this little fly. Thanks to the Anopheles genome sequence, the Institut Pasteur researchers were extremely quickly able to discover the mosquito's equivalent gene, a potential target for preventing the development of the parasite; it would have taken much longer to locate it without this new data.
Many other possible applications will be accelerated. The mosquito's smell receptors, for example, are probably implicated in the female Anopheles' attraction to humans. A whole range of genes associated with smell has been discovered. This will considerably facilitate research on these receptors, and will probably result in the development of new repellents or new attractants. Furthermore, the possibility of a better understanding of the metabolism of the mosquito's resistance to current insecticides could allow a more ecological use of these products.
It must also be pointed out that knowledge of the Anopheles genome will increase the number of potential target genes that could be introduced into the mosquito by genetic engineering techniques. "I think that transgenic mosquitoes are a formidable laboratory tool, in a confined space, as they will allow us to identify the significant genes. On the other hand, transforming mosquitoes and releasing them into the wild is at present in my eyes irresponsible. We do not know enough about the ecology of Anopheles in the wild, we have not the least idea of the way in which a population of transgenic mosquitoes would be able to supplant the natural populations. We would be taking the risk of modifying something without knowing if it would go in the direction we want", stresses Paul Brey, head of the Insect Biochemistry and Molecular Biology Unit. "The most rapid applications which are emerging will, in my opinion, concern the most rational use of insecticides and the development of new mosquito-repellents."
To gain the fullest benefit from the data available today, the Institut Pasteur is to launch an Anopheles Research Program : 11 teams at the Paris campus will take part, along with two institutions in the International Network of Instituts Pasteur and associated institutions (Dakar and Madagascar). This program will be centered particularly on identifying and characterizing the genes implicated in interactions between Anopheles and the Plasmodium falciparum parasite, whose genome sequence has also just been published in the journal Nature.
Genoscope - French National Sequencing Center - is a large facility dedicated to genomic research. It puts its sequencing capacity at the research community's disposal within the framework of collaborative projects devoted to various organisms (microbes, plants, animals) of scientific, medical, or economic interest. Involved in the Human Genome Project, the Genoscope has determined the sequence of the human chromosome 14. It also pursue its own projects, such as the sequencing of the fish Tetraodon nigroviridis, whose compact genome provides a tool for annotation of the human genome. Genoscope is based at Evry, in Essonne, France.
Press contact:
Yann Esnault - Tel: +33 (0)1 60 87 84 50 / yesnault@genoscope.cns.fr
The Institut Pasteur is a private non-profit foundation. Ever since it was founded, the Institut Pasteur has had three principal missions: biological research, chiefly directed towards infectious diseases, public health applications, and education. Since September 1999, the Institut Pasteur has been part of the national network of centers for genetic study (Génopoles) set up by the French Ministry of Research and Technology. Several teams specializing in the study of model or pathogenic organisms are engaged in the complete sequencing of these organisms' genomes and in the large-scale exploration of the structure and function of their genes.
Press Contacts:
Nadine Peyrolo - Tel: +33 (0)1 45 68 81 47 / presse@pasteur.fr
Corinne Jamma - Tel: +33 (0)1 40 61 33 41 / presse@pasteur.fr