sábado, junio 09, 2012

GM mosquitoes: hatch and release


By Camilo Rodríguez-Beltrán

In 2010 I read for the first time about the initial field experiments of genetically engineered mosquitoes that had taken place a year earlier in the Cayman Islands. This news came as a surprise to me, as I considered myself part of the independent scientific community continuously monitoring modern biotechnology advances and applications. Although British biotech company Oxitec's venture in developing GE mosquitoes was known, the astonishment came from the sudden jump to field release. I soon realized that I was not the only one missing a year of surveillance on this exercise: the release remained a de facto confidential test for a year.

It was difficult to understand the silence, intentional or not, on the experimental release of these mosquitoes, in particular because there were not hidden military or obscure purposes underlying the technology. In fact, the intended use was described as a tool to tackle dengue fever, one of the major public health issues in many developing countries. With over 50 million infections every year, the fight against this disease is one of the most important priorities for societies not only in the developing world but also in some regions of the developed world. Strategies range from vector management to early and accurate diagnosis, and while the research on vaccines and viral drugs is under development, no commercial vaccine is available for the moment.

Aedes aegypti is the principal, but not only, species of mosquito capable of transmitting the virus through bites from the female to humans. For this specific case, the technological strategy revolves around the release of mainly male engineered A. aegypti mosquitoes. This technology is called RIDL—Release with a Dominant Lethal—where the insects carry a specific genetic switch that under certain conditions causes death at the larval stage of their offspring. This application aims to reduce the incidence of dengue fever by suppressing the mosquito population.

At the molecular level, these GE mosquitoes have been designed with two transgenes. The first one (DsRed2) produces a red fluorescency in the organism under white light. This is a useful marker for selection and also for monitoring. But the most interesting, and also complex, piece of the system comes from the second transgene, the RIDL regulation system.

Imagine your office door slightly open on a windy day: little by little the door opens more and more as the wind pushes through. You can stand up to try to close it but the wind is so strong that it will reopen it again, and at one stage the door will be so wide open that the wind will be strong enough to create a chaos (flying pages, knocking over the coffee cup etc.). But suddenly you find the key to that door, and by closing the door you have reduced the flow necessary to create the chaos. Well, that is the RIDL system, a positive genetic feedback loop that produces a protein (tTAV) that is able to guide more production of itself (by acting positively on its own genetic promoter). This results in an over expression of tTAV, at a concentration that becomes lethal to mosquitoes' larvae. However there is one antidote, a chemical called tetracycline, which if present will bind the tTAV protein, reducing its presence in a free form to activate its promoter. tTAV will still be produced, but at a lower concentration with no toxic effect for the larvae. Just like the absence of a key allowed wind to knock over the coffee cup inside the office, absence of tetracycline will produce a lethal effect at the larval stage of the mosquitoes.

From a biosafety standpoint, risks related to these organisms follow some general issues:

(1) On modified mosquitoes: What will be the consequences in the ecological network of mosquitoes? What will be the effect on preys and predators? What will be the influence in other species of disease carrying mosquitoes? Could they benefit from a reduction in competition? Can the virus adapt better to other vectors because of this selection pressure?

(2) On GE organisms: What is the effect of the exposure to the DSRed2 and the tTAV proteins? What is the likelihood of instability of the genetically added trait? Could it evolve resistance to the lethal mechanism?

There are other specific issues related to the ability of flying and the difficulty of monitoring the distribution of the mosquitoes (in particular during transboundary movements), as well as issues related to the associated technology (for example, the need to act under absence of tetracycline).

Some of these uncertainties regarding the implications on ecosystems and health have apparently been accepted by some risk assessors, who have given approvals for the field release of the GM mosquitoes not only in the Cayman Islands but also in Malaysia and Brazil, with further approvals pending in the United States. Some have highly criticized the scientific approach used on these regulatory processes, and another article in this issue of GeneWatch addresses the regulatory gaps in these experiences.

I believe that the issues related to the associated technology are of particular interest. It has been acknowledged by Oxitec that in the absence of tetracycline, the survival rate of the GM mosquito larvae is about 3% under laboratory conditions (the specific reasons for this percentage of survival are unknown). It is interesting that some of the strongest discussions with the promoter of these technologies are about the numbers of surviving mosquitoes: Does it matter? Is it significant? Is it negligible? Debates are currently ongoing and will continue, but the fact is that the potential of having survivors is a reality, and some of these will be females—and female mosquitoes, genetically engineered or not, bite humans.

Another interesting factor is that the survival rate of GE mosquitoes can be underestimated in real conditions—not only because of the possibility of building a genetic resistance, but in particular because the antidote, tetracycline, is one of the major antibiotics used both for human health and agricultural practices. The major concentration of tetracycline in urban areas is likely to be in sewage systems, and recent literature has shown that A. aegypti does breed in dirty water; therefore the scenario of breeding and development in potentially tetracycline-contaminated aquatic environments, with the risk of suppressing the lethal system, should now be considered. One could argue that the concentrations in these environments will not be enough to trigger survival, but in order to know this a meticulous surveillance system of tetracycline concentration over time will be needed in the regions intended for release. For the moment I am not aware of any such initiatives, and I believe these will be very expensive and hard to put in place.

Aside from these questions, what has not been covered is a thorough analysis of the appropriateness of this strategy. It seems that the context is not ready for the technology. The RIDL system was not developed to tackle dengue; before mosquitoes, the technology was designed for cotton bollworms, and it seems that other agricultural pests will be targeted in the future. In other words, rather than developing a technology for the purpose of reducing the incidence of dengue fever, Oxitec developed the technology first and then looked for situations where it could be put into use. In this particular case, the use of tetracycline as an antidote makes things out in the environment a little bit more complicated. If the technological solution had started from the real challenge or opportunity then it seems very unlikely that it would rely on an antidote that is currently available exactly where you don't want it: in the waters where mosquito larvae grow.

I advocate for challenging solutions that rely solely on technology and forget to start from a context-centered approach. I put the weight on the challenge not really to the private companies, but on the governments and public research initiatives that should be deciding the best for all. Before asking "Does it work?" we need to ask: "Is it appropriate?"

Camilo Rodríguez-Beltrán, MSc, is co-founder of the Taleo Initiative and was awarded the TEDGlobal2010 Fellowship.

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