A method to stop multiplication of the malaria parasite
two genes of a malaria parasite that are responsible for its multiplication have been silenced by scientists at the International Centre for Genetic Engineering and Biotechnology (icgeb), New Delhi. The scientists say their work, carried out on mice, may provide a new approach to treat the disease in humans.
A gene is said to be silenced when its expression is stopped. Gene silencing -- also known as rna interference (rnai) -- allows almost any gene of known sequence to be shut down. This is achieved through the specific destruction of the gene's messenger rna (mrna), the intermediary molecule between dna and protein. When double-stranded rna (dsrna) with identical sequence to a specific mrna is introduced into cells, the mrna is recognised and degraded by a multi-protein body called the rna -induced silencing complex. Destruction of the target mrna leads to a decrease in the amount of the protein it encodes. In other words, the gene is silenced.
In nematode worms and fruitflies, where rnai is readily demonstrable, dsrnas that are hundreds of nucleotides long can be used to target a gene. However, in mammalian cells, long dsrnas induce a potent antiviral response, which shuts down the synthesis of all proteins of the body. More sophisticated strategies are therefore required, such as the use of the small interfering rna (sirna). These sirnas, which are about 21 nucleotides long, can be used by the rna-induced silencing complex but are too short to activate a full-blown antiviral dsrna response.
The sirna can either be made in vitro and subsequently introduced into cells, or they can be made directly in cells through the expression of short hairpin rnas (shrnas), which can be processed 'enzymatically' to generate a mature sirna. Expression of sirnas can be used to induce rnai in transgenic mice as well as in cell lines, so the technique can be applied to investigate gene function in whole animals.
This was what was done by the icgeb team. Its goal was "to find out whether it was possible to carry out rnai on the malaria parasites in vivo," says Virendar Chauhan, director general of the centre. Working with Chauhan were Pawan Malhotra, Raj Bhatnagar, Palakodeti Dasaradhi and Asif Mohammed.
The team targeted 'cysteine protease' genes (berghepain-1 and -2) of Plasmodium berghei, the parasite that causes malaria in mice. These two genes play a vital role in the multiplication of the parasite, by coding for enzymes that break down the haemoglobin -- an essential component of human blood. To trigger rnai in P berghei, the team injected synthetic sirnas corresponding to berghepain-1 and -2 into animals infected by P berghei. Expression of these genes was inhibited in parasites treated in this way, implying that the treatment blocked the haemoglobin breakdown by silencing the genes.
The study builds on an earlier work in which the researchers silenced two genes of the human malaria parasite P falciparum by adding their respective dsrnas. "We had previously shown that rnai is effective in controlling P falciparum in vitro," says Chauhan, adding: "By in vivo experiments on mice, we have now provided evidence that rnai holds the potential for a therapeutic strategy." But he warns that many caveats still exist for the therapeutic use of si rnas in the control of parasitic diseases.
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