ISIS Report 07/03/12
Transgenic Mosquitoes Not a Solution
All attempts at controlling the spread of diseases by
transgenic mosquitoes are ineffective, inefficient, costly, and hazardous to
varying degrees Dr. Mae-Wan
Considerable controversy has been aroused by Oxitec, a UK
company based in Oxford, over its releases of transgenic mosquitoes designed to
control the spread of dengue fever, which at the same time, highlighted the
gross inadequacy of the regulation of transgenic insects releases worldwide
of Transgenic Insects Highly Inadequate and Unsafe, SiS 54).
More important than the adequacy
of regulation is whether transgenic insects are a solution to the problem of
Population reduction by
introducing dominant lethal gene
Transgenic mosquitoes such as
those created by Oxitec are designed to reduce or eradicate natural populations
of disease vectors, whereas newer approaches are designed instead to replace
natural populations with the minimum of ecological disturbance.
For far too
long, the extermination of the insect vector has been the preferred option to
controlling the spread of infectious disease, starting with DDT and other toxic
insecticides. Not surprisingly, the first efforts in creating transgenic
mosquitoes followed the same path, and Oxitec was no exception. It consisted in
the ‘release of insects with a dominant lethal’ (RIDL) intended to reduce
natural populations  (see Terminator
Mosquitoes to Control Dengue? SiS 39). Oxitec’s transgenic
mosquitoes were created with the transposon (jumping gene) piggyBac, the
dominant lethal gene incorporated is tTA, coding for the
tetracycline-repressible transcription activator protein, which when expressed
at high levels kills the developing embryo, for reasons still unknown ([2, 3] Can GM
Mosquitoes Eradicate Dengue Fever, SiS 50). When expressed, the tTA protein
binds to the tetO operator upstream of the tTA transgene and
drives more synthesis of tTA in a positive feedback loop. In the presence of
tetracycline, however, tetracycline binds to the tTA protein, and prevents it
from binding to tetO, thereby turning off the synthesis of more tTA,
allowing the insect embryo to survive.
transgenic mosquitoes can be grown in the presence of tetracycline. The adult
male mosquitoes (which do not bite humans) are sorted from the females that do
bite and transmit disease. The sorted males are then released into the field
to mate with wild-type female mosquitoes; whereupon the progeny will be killed
in the absence of tetracycline, and the wild population is decimated. That is
the theory, as far as it goes.
Oxitec transgenic mosquitoes
inefficient and unsafe
Oxitec claims that results from
Cayman trials showed a reduction in Aedes aegypti populations by 80 %
, while the journal Nature reported on its News Blog  that “the
controlled release of male mosquitoes genetically engineered to be sterile has
successfully wiped out dengue fever in a town of around 3, 000 people, in Grand
Cayman”. Both reports are inaccurate , if not outright false.
The mosquitoes are not sterile and Oxitec never
successfully eradicated dengue fever from any population; furthermore, dengue
is not endemic in the Cayman Islands.
The mosquitoes, engineered to be dependent on
tetracycline, are not completely killed in its absence; as some 3 to 4 % of the
mosquitoes embryos survive to adulthood in the laboratory . Tetracycline is
a very common antibiotic in the environment, particularly in sewers, septic
tanks and water treatment plants, common breeding grounds for A. aegypti.
Oxitec admitted that survival rates could be as high as 15 % in the presence of
contaminating levels of tetracycline . Moreover, Oxitec’s system of sorting
males from females is not perfect and up to 0.5 % of the released mosquitoes
could be female .
Much of that was confirmed in publications that appeared
in the biotech-friendly
Nature Biotechnology since. The journal’s news report of the experiment presented
a positive spin, if you only read the headline : “Results from the first
open-field trial of transgenic mosquitoes bode well for large-scale release to
fight infectious disease.” Only in the final paragraphs are the problems
revealed. The transgenic males were only half as successful as wild-type males
in mating with the wild-type females, but still, they were better than the
sterile male Mediterranean fruit flies used worldwide to protect fruit and
“The results are promising, but whether they portend
successful sterile insect release against A. aegypti is questionable”
, the commentators stated, for three reasons. First, they questioned the
reliability of the field data; too few traps were set, and only a small number
of larvae were scored for paternity, to determine whether they came from the
transgenic males or wild-type males. Second, implementation of RIDL-based
dengue control will require large-scale systems for producing the insects, transporting
and releasing them. Not only is basic research for optimal procedures missing,
there are not even established standards for assessing insect quality along the
entire rearing to release pathway; for example, the sorting of males from
females based on size of the pupae nevertheless left 0.5 % females in the
released population. And most importantly, the progeny of crosses between
transgenic males and wild type females in the laboratory survived at the
“disturbingly high rate of 3.5%.” This certainly does not sound like a ringing
was nothing in Oxitec’s own report in the journal  that contradicted the assessment
of the commentators.
immediate concern  is that decline in the natural populations of A.
aegypti could leave an ecological niche to be filled by other, possibly
more harmful pests. For example, the Asian Tiger mosquito, A. albopictus
is considered one of the most invasive species and carries many diseases
including dengue fever and the West Nile virus. Another possibility is that the
dengue virus could also evolve and become more virulent.
potential health hazards are associated with high levels of the transgene
product, the tTA protein, which could easily be injected into the bloodstream
of humans by biting females, not to mention the propensity of the piggyBac
vector integrated transgene -sequences to remobilize and transfer horizontally
to human cells (see ).
problems associated with Oxitec’s transgenic mosquitoes are to varying extent
generic to transgenic mosquitoes and other insects.
generic to transgene insect technologies
mentioned in relation to Oxitec’s transgenic mosquitoes, the technology
requires continual releases of lab-reared transgenic males in order to keep the
natural population down . We shall see that even with strategies involving
population replacement, continued releases are also required.
published in 2012 states : “Of particular concern for practical
transgenesis applications are the difficulties in defining and standardizing
the long-term effectiveness of transgenic manipulations.”
Apart from its relative inefficiency, transgene
expression is variable and can even be lost from established transgenic lines.
For example, the activity of a gene that suppresses dengue virus in a
transgenic strain of A. aegypti was lost after 17 generations .
Loss of transgene
activity could be due to gene silencing or actual loss of transgene through
remobilization of integrated transgenes. Transgenic lines created with
transposon- vectors – applying to most of them including the Oxitec transgenic
mosquitoes - are subject to instability due to remobilization of the
transposon-derived sequences. Remobilization can result in horizontal gene
transfer to unrelated species. The first generation of transposon vectors are
probably the worst offenders as I have commented  (Terminator insects give wings
to genome invaders, ISIS report): “These artificial transposons are already
aggressive genome invaders, and putting them into insects is to give them
wings, as well as sharp mouthparts for efficient delivery to all plants and
animals and their viruses.”
transgenic options still inefficient
are now more up-to-date, safer vectors for creating transgenic mosquitoes and
other insects that can target the transgenes more precisely into the genome
and increase their stability of expression, as well as decrease their tendency
to remobilize .
One of the best researched examples is the ‘selfish gene’
Medea model , which aims to replace the natural population with one that
confers resistance to disease or parasite, rather than decimate it. The goal is
to create the minimum ecological disturbance, and if the genetic modification
is precise and well designed, it also causes minimum physiological and
biochemical changes to the mosquito vector.
were first identified in the flour beetle Tribolium castaneum through
crosses between geographically isolated strains. They are located at a fixed
position in the genome, and when present in females, only progeny that inherit
the element-bearing chromosome from either the maternal and/or paternal genome
survive. In contrast, Medea-bearing males give rise to wild-type and
Medea-bearing progeny with equal frequency when mated to wild-type females. One
Tribolium Medea gene, MedeaM1 has been mapped,
and is associated with a composite Tc1 transposon that includes a number of
genes. Genetic analysis suggests a model in which Medea consists of two tightly
linked loci: one that encodes a toxin passed on to all progeny via the egg, and
a second that encodes an antidote active in the zygote.
A synthetic Medea
element Medeamyd88 was created, in which a modified version
of maternal-specific bicoid promoter was used to drive the expression of
a transcript encoding two synthetic microRNAs designed to silence the
expression of myd88, a maternally expressed protein required for dorso-ventral
pattern formation (by binding to complementary base sequences on the
transcript, thereby preventing its translation into protein). This kills the
embryo if left un-neutralized. The linked antidote gene encodes a
microRNA-insensitive version of the myd88 transcript lacking the target
sites present in the maternal transcript, and is placed under the control of a
transient early zygote-specific promoter. Expression of the antidote occurs
early enough that the rescued embryos develop normally. In a lab experiment,
introducing Medeamyd88 into a wild-type population with a 1:1
homozygous Medea male/wild-type male ratio resulted in the entire
population carrying at least one copy of Medea after 10-12 generations.
designed so that the maternally expressed microRNAs were located in an intron
of the antidote also showed Medea-like behaviour. This configuration
prevents recombination from creating Medea elements that lack the Medea
effector or the antidote-only elements, each of which can lead to the
appearance of wild-type individuals. Other implementations are possible.
Thus, no foreign
proteins are introduced into the natural population; there is only a shift of a
gene from mother to the zygote. The use of miRNAs to generate a pre-toxic state
provides an important degree of redundancy because multiple microRNAs each
processed and functioning as an independent unit can be linked into a
However, as the
researchers are aware, transgene silencing can still occur. The best that can
be done is to try and wall off the transgenes from the effects of repressive
chromatin that would silence the expression of genes. This can be achieved, at
least to some extent by flanking the Medea construct with sequences that confer
Still, as the
researchers point out, populations subject to replacement will always need
maintenance and modification over time. In particular, it is likely that first
generation transgenes will mutate to inactivity, become silenced and/or lose
effectiveness as the pathogens adapt and become resistant. Further rounds of
population replacement with novel toxin-antidote combinations are necessary.
mosquitoes as a strategy to controlling natural infectious disease vectors are
ineffective, inefficient, costly to implement, and hazardous to varying
degrees. More than a decade of dedicated efforts have not resulted in major advances.
It is against such a background that the latest non-transgenic approach appears
to be such a perfect solution (see  Non-transgenic
Mosquitoes to Combat Dengue, SiS
MW. Regulation of transgenic insects highly inadequate. Science in
Society 54 (to appear) 2012.
J and Ho MW. Terminator mosquitoes to control dengue? Science in Society 39, 33-35,
3. Cummins J.
Can GM mosquitoes eradicate dengue fever? Science in Society 50,
4. March 2011 Newsletter.” Oxitec, Mar. 2011. Web. <http://www.oxitec.com/our-news/newsletters/march-2011-newsletter
5. “GM Mosquitoes Wipe out Dengue Fever in Trial.” Nature
News Blog. Nature, 11 Nov. 2011. Web.
Genetically engineered mosquitoes in the U.S. Issue Brief,
Friends of the Earth, Washington DC, http://www.biosafety-info.net/file_dir/21277023124f348b11ef3c4.pdf
“Road test for genetically modified mosquitoes”, Todd Shelly
& Don McInnis, (News and Views), Nature Biotechnology 2011, 29, 984-5.
statement in response to NGO allegations. Press release, 12 January, http://www.oxitec.com/2012/01/press-release-oxitec-statement-in-response-to-ngo-allegations/#more-3170
9. Harris, AF, Nimmo D, McKenny AR, Kelly N, Scaife S Donnelly CA,
Beech C, Petrie WD and Alphey L. Field performance of engineered male
mosquitoes. Nature biotechnology 2011, 29, 1034-9.
Fraser Jr MJ. Insect transgenesis: current applications and future
prospects. Ann Rev Entomol 2012, 57, 267-89.
Franz AW, Sanchez-Vargas I, Piper J, et al. Stability
and loss of a virus resistance phenotype over time in transgenic mosquitoes
harbouring an antiviral effector gene. Insect Mol. Biol. 2009,
Ho MW Terminator insects give wings to genome invaders. ISIS Report, 19
March 2001, http://www.i-sis.org.uk/terminsects-pr.php
Ho MW. Non-transgenic mosquitoes to combat dengue. Science
in Society 54 (to appear) 2012.
There are 3 comments on this article so far. Add your comment
|Todd Millions Comment left 11th March 2012 09:09:09|
I can't restrain from asking-As well as the transpond leaks,has oxitec/epa quango completed their survey showing no background levels of tetracycline ozze from the swamp water bacterial culture that the drug was origianaly developed from?I await such biblical works of fiction with-quivering anticipation.
|henrymark101 Comment left 19th April 2013 09:09:05|
Nice title of the post as well as content on Transgenic Mosquitoes.
I like your post and the content that i want to get.
Thanks for the sharing.
You just describe the topics are wonderful and more useful topic is the insect technologies.
We hope that we will get more from you...!!!
|Alice Br Comment left 31st January 2016 07:07:33|
You do realize that this article explains why all the babies with missing frontal lobes are being born is area where these mosquitoes have been released. Its not the Zika virus its the tTA protein gene expression kill switch in the mosquito. The author of this article was right.