Genetic Approaches to Controlling Malaria Mosquitoes

A dangerous remedy that may bring malaria back worse than before. Prof. Joe Cummins

We first reviewed the use of transgenic mosquitoes in 2001, when they appeared ready to be released in the field [1] (Two takes on Malaria -Transgenic Mosquitoes Coming-Rolling Back Malaria*, SiS 13/14). But alas, they have yet to leave the confines of the laboratory, and may not do so until well after malaria has been eradicated. The use of genetically engineered mosquitoes to eradicate the parasite-transmitting mosquitoes have not yet advanced to a state that would  allow their safe and effective  release to the environment.  

The strategy involves introducing transgenes into wild mosquito populations that will render the mosquitoes resistant to infection by the malaria parasite plasmodium. The transgenic mosquitoes must be assimilated into the wild malaria-transmitting mosquito population by a process called gene drive. Gene drive requires introgression of antiparasite genes into the insect vector populations. Such genes, if present at high enough frequencies, will impede transmission of the target parasites and result in reduced human sickness or death [2].   

Transgenic mosquitoes

Transgenic malaria-resistant mosquitoes, modified with a single gene inhibiting parasite growth in the mosquitoes, were found to have a fitness advantage when feeding on parasite-infected blood [3]. This might allow the transgenic mosquitoes to spread in a wild population. The main problem encountered is not so much in finding clever genetic methods to control the parasite in the laboratory as it is to develop gene drive systems that spread the genetically modified malaria mosquitoes safely throughout the entire population of wild malaria mosquitoes. That has been the main hang up.

An example of a potentially viable gene drive system is a gene unit called Medea, discovered in the flour beetle and enhanced synthetically [4]. The synthetic Medea element works by encoding both a maternally expressed toxin and a zygotically expressed antidote. The toxin expressed in the female causes the death of all her progeny that lack the Medea allele, and the antidote rescues Medea-bearing progeny from an otherwise imminent death. In this way, the proportion of Medea-bearing individuals is increased with each generation; and an attached gene conferring resistance to malaria could come along for the ride. Medea is thus able to rapidly spread through a population. Medea is an acronym for maternal-effect dominant embryonic arrest, also alluding to the mythological Greek mother who murdered her children. Medea may produce an effective malaria mosquito gene drive device but that will not be known for some time to come.

Medea is tightly associated with a very large composite transposon Tc1/mariner which is capable of jumping into human genomes. We have previously warned against the release of transgenic insects with transposons that are widely distributed across kingdoms in the living world as a gene drive mechanism [5] (Stop Release of GM Insects!, ISIS News 9/10). It is tantamount to providing potentially disruptive genome-invaders with wings as well as sharp mouthparts.

Bacterial symbionts of malaria mosquitoes

A Wolbachia symbiont in Aedes aegypti limits infection with Dengue, Chikungunya (a dengue like virus disease), and Plasmodium  Wolbachia are maternally inherited intracellular bacterial symbionts estimated to infect more than 60 percent of all insect species. While Wolbachia is commonly found in many mosquitoes it is absent from the species considered to be of major importance for the transmission of human pathogens. The successful introduction of a life-shortening strain of Wolbachia into the dengue vector Aedes aegypti that halves adult lifespan has been reported [6]. This same Wolbachia infection also directly inhibits the ability of a range of pathogens and parasites to infect this mosquito species. The effect is Wolbachia strain-specific, and relates to Wolbachia priming of the mosquito innate immune system, and potential competition for limiting cellular resources required for pathogen replication. By shortening  the mosquito life-span Wolbachia also reduces or prevents the full development of the parasites.The virulent Wolbachia strain wMelPop can survive and replicate when injected into adult female Anopheles gambiae (the most important vectors of malaria in Sub-Saharan Africa), but the somatic infections established are not virulent. These data suggest that stable Wolbachia infections of Anopheles may be possible. The researchers propose using the avirulent Wolbachia  modified with transgenes inhibiting parasite development to drive the inhibitor genes through malaria mosquito populations [7]. The term ‘paratransgenesis’ has been coined to describe the genetic manipulation of insect symbiotic microorganisms  

Bacteria of the genus Asaia have been proposed as a paratransgenic weapon against malaria.  Those symbiotic bacteria are capable of colonizing all of the organs necessary for Anopheles mosquito reproduction.  The bacteria are capable of horizontal transfer through an oral route in both adult and larval stages and by venereal route during mating in adults. Asaia  bacteria are vertically transmitted from mothers to progeny showing that they may rapidly spread through a population [8]. Paratransgenic  Asaia bacteria and other symbiotic bacteria have not yet faced  regulatory agencies and the special properties of paratransgenic symbionts require special consideration by the regulators because they may be transmitted again both horizontally and vertically and across taxonomic boundaries.

Are transgenic malaria mosquitoes a remedy for the disease?

A mathematical treatment concluded that Medea and Wolbachia bacteria  present the best compromise between invasiveness and containment of the gene drives being considered for the control of mosquito born diseases [9]. Another view was put forward by Johns Hopkins University researcher Jason L. Rasgon, who stated [10]: “Replacement of wild-type mosquito populations with genetically modified versions is being explored as a potential strategy to control vector-borne diseases. Due to lower expected relative fitness of transgenic individuals, transgenes must be driven into populations for these scenarios to be successful. Several gene drive mechanisms exist in a theoretical sense but none are currently workable in mosquitoes. Even if strategies were workable, it would be very difficult to recall released transgenes in the event of unforeseen consequences.”

However, commenting on a method called ‘Multi-locus assortment’ that spreads transgenes into vector populations by the release of genetically-modified mosquitoes carrying multiple stable transgene inserts, Rasgon said [10]: “Simulations indicate that insects do not have to carry transgenes at more than 4 loci,  transgenes can be maintained at high levels by sequential small releases, the frequency of which depends on the construct fitness cost, and  in the case of unforeseen negative non-target effects, transgenes can be eliminated from the population by halting transgenic releases and/or mass releases of wild-type insects.” The remedy for unforeseen negative non-target effects of the transgenes is that the transgenic mosquitoes can be eliminated from the population through the mass releases of wild type mosquitoes! A remedy that unfortunately for the human residents,  may bring malaria back worse than ever before. This perhaps summarises the usefulness of the entire approach; even before serious safety concerns have been taken on board.

Article first published 09/06/10

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References

1. Cummins  J and Burcher S. Two takes on malaria: Transgenic mosquitoes coming, and Rolling back malaria i-sis news13/14, 28-29, 2002.
2. James AA. Gene drive systems in mosquitoes: rules of the road. Trends Parasitol. 2005 Feb;21(2):64-7.
3. Marrelli MT, Li C, Rasgon JL, Jacobs-Lorena M. Transgenic malaria-resistant mosquitoes have a fitness advantage when feeding on Plasmodium-infected blood. Proc Natl Acad Sci U S A. 2007 Mar 27;104(13):5580-3.
4. Marshall JM, Taylor CE. Malaria control with transgenic mosquitoes. PLoS Med. 2009 Feb 10;6(2):e20.
5. Ho MW and Cummins J. Stop release of GM insects! i-sis news9/10, July 2001.
6. Moreira LA, Iturbe-Ormaetxe I, Jeffery JA, Lu G, Pyke AT, Hedges LM, Rocha BC, Hall-Mendelin S, Day A, Riegler M, Hugo LE, Johnson KN, Kay BH, McGraw EA, van den Hurk AF, Ryan PA, O'Neill SL A Wolbachia symbiont in Aedes aegypti limits infection with dengue, Chikungunya, and Plasmodium. Cell. 2009, 139(7), 1268-78.
7. Jin C, Ren X, Rasgon JL The virulent Wolbachia strain wMelPop efficiently establishes somatic infections in the malaria vector Anopheles gambiae. Appl Environ Microbiol. 2009, 75(10), 3373-6
8. Favia G, Ricci I, Marzorati M, Negri I, Alma A, Sacchi L, Bandi C, Daffonchio D Bacteria of the genus Asaia: a potential paratransgenic weapon against malaria. Adv Exp Med Biol. 2008;627:49-59
9. Marshall JM. The effect of gene drive on containment of transgenic mosquitoes. J Theor Biol. 2009 May 21;258(2):250-6
10. Rasgon JL. Multi-locus assortment (MLA) for transgene dispersal and elimination in mosquito populations. PLoS One. 2009, 4(6), e5833.