Genetically Modified Mosquitoes as a Means to Control Malaria

stubbornnessuglyBiotechnologie

12 déc. 2012 (il y a 8 années et 7 mois)

355 vue(s)

Justin Balido

Dr. D. Scott Smith

Humbio 153

9 March 2012

Genetically Modified Mosquitoes as a Means to Control Malaria


Malaria is one of the world’s most burdensome diseases. Approximately half of the
world population lives in areas at
-
risk for malaria
infection.

1,2

It is the leading cause of
death in many developing countries, with women and young children being the most
effected.
1
-
2

And the burden of disease is underestimated and understated. 1.2 million
deaths could be attributed to malaria in 2010.
2

Approximately 500 million people are
infected by malaria annually.
1

The infection results in economic loss, due to lost days of
work and absences from school.
1

Its burden is widespread and its impact is deep.

There are some interventions that have bee
n proven effective at reducing malarial
incidence. Indoor residual spraying of insecticides coats the interior walls of a home with
residual insecticide, thus
killing endophilic mosquitoes.
1

Bed nets impregnated with
insecticide offer both a physical bar
rier to mosquitoes, as well as insecticide to kill vectors
that land on the net, effectively killing mosquitoes before they can bite a human host and
spread infection.
1

Larval control involves the elimination of mosquito breeding grounds to
reduce vector
populations.
1


Current interventions
are effective, but still, ob
stacles exist.

Indoor residual
spraying must be reapplied after several months, and needs to be applied to at least 70% of
an affected area to be effective.
1

And mosquitoes have developed
resistance to the
insecticide used in both residual spraying and impregnated bed nets.
1

Larval control
requires constant surveillance to find pools of stagnant water in which mosquitoes can
breed.
1


Alternative interventions should be explored, to create
a sustainable solution to
reducing malaria prevalence and incidence globally.


One potential intervention is biological control of the vector, by releasing
engineered mosquitoes.
3,4

First, biologists breed mosquitoes, to express certain traits,
which will

be discussed later. Then, these mosquitoes are released into the wild, which the
target mosquito population inhabits. With some luck, the scientist
-
bred mosquitoes are fit
and competitive enough to mate with wild mosquitoes. Thus, the trait is passed o
n to the
next generation.
3,4


There are two goals to this type of intervention.
4

The first is population reduction.
By releasing sterile insects into the wild population, females that mate with the sterile
males will produce eggs that will ultimately not

produce mosquitoes.
5

Alternatively,
engineered males will mate with wild females, which will result in larvae with lethal genes,
thus killing the larvae before they can become a malaria
-
spreading mosquito.
5

By reducing
the mosquito population, you reduc
e the vector, and thus reduce the incidence of malaria.
The second goal is population replacement.
3,4

Instead of wiping out populations of
mosquitoes, the goal is to engineer mosquitoes
that are resistant to malaria. Thus, instead
of getting rid of the v
ector, we simply reduce the infectivity of the vector, and, again, reduce
malaria incidence.


Population reduction is a well
-
documented field, and the technique is known as the
Sterile Insect Technique (SIT). In the past, it has been done by irrad
iation a
nd
chemosterilization, and cytoplasmic incompatibility. Cytoplasmic incompatibility involves
the infection of mosquitoes with the symbiont Wolbachia, which will kill wild mosquito
eggs that are not naturally infected with the bacteria.
5

However, in movin
g forward,
transgenic steriliz
ation has gained more

interest, due to its potential efficacy, as well as its
reversibility should it result in undesirable effects.
4,5


Sterile Insect Technique ha
s been studied since the 196
0s as a means to control
pests,
including vectors of disease
, such as mosquitoes
. Mosquito releases were often
research
-
based in nature, and not intended to suppress or control populations. Releases
against
culex

mosquitoes were the fi
rst to experience success, in Myanmar with
cytoplas
mic incompatibility, and in Fr
ance with genetic modification.
5



The first trial of sterile insect technique application in a malaria vector that
experienced success was the elimination of the species
anopheles albimanus

in an isolated
area of El Salvador

in 1972. In this trial, vector sterilization was done chemically. Over 22
weeks, 4.4 million male mosquitoes were released into the area surrounding Lake
Apastepeque. By the end of the trial, the mosquito population was eliminated, and
researchers gain
ed an increased understanding of sterile insect technique.
5


However,

when an attempted a larger
-
scale project went underway in El Salvador,
there was inability to match results. In order to achieve population suppression, the
researchers had to reduce th
e target area by 20 square kilometers. Likewise, in India, the
World Health Organization (WHO) and the Indian Council of Medical Research (ICMR)
worked together to reduce mosquito populations, including the indigenous vector for
malaria.
Despite the larg
e amounts of mosquitoes released, there was minimal effect
observed on the overall populations of mosquitoes.

This was attributed to mosquito
migration from outside the target population. This both increased the population, and
introduced fertile males t
o mate with the females, reducing or eliminating any effect the
sterile insects may have had.
5


The other option to the biological control of the malaria vector is
population
replacement.
3,4


In pursuing this option, p
opulations of anopheline mosquitoes ar
e replaced
by
mosquito populations that express resistance to malaria. Research is currently
underway to determine the best genetic approach that would allow this to occur.
3
,4


While the ways to confer resistance to malaria in mosquitoes are many, there i
s one
model that has been proven successful in rodent malaria. Researchers at Johns Hopkins
have identified the receptor sites in the midgut of the mosquito that sporozoites require to
pass through the wall of the midgut, as well as the receptors on the o
utside of the salivary
glands that are required for the parasite to enter. Without these sites, sporozoites cannot
reach the salivary glands, and thus cannot infect another rodent. A protein has been found
that occupies the same receptor site as the sporo
zoite. By introducing large amounts of the
protein into the mosquito, the receptor sites are inaccessible to
the parasite, and therefore
the sporozoite will stay in the midgut, unable to pass on to a human host.
3


Another successful malaria resistance stu
dy has been conducted in chickens. In this
study, an antibody was engineered from a surface protein of the plasmodium parasite. A
virus was used to insert the gene into the mosquito DNA, and have the mosquito express
the gene. The antibodies were able t
o reduce the number of sporozoites in the salivary
glands of infected mosquitoes by 99.9%.
6



To induce resistance in humans, a number of factors must be identified in order to
make such an intervention feasible, let alone successful. For a protein to be

expressed in
the mosquito, an endogenous promoter must first be found


a gene already in the
mosquito that can both activate and control the production
of the exogenous gene product.
By finding an endogenous promoter, there would have to be less genetic

engineerin
g for an
outside gene to work. An engineered gene must be attached and placed in the DNA so that
the promoter will act upon it.
3,4,
7


One study accomplished this using bloodmeal
-
triggered gene activation. This study
has the distinct advantage
of using anti
-
malarial genes already present in the anopheles
mosquitoes. There has been extensive research done examining any immunity that
anopheles mosquitoes may already contain in their genome. Drawing upon this research,
this study genetically engi
neered mosquitoes to over
-
express these plasmodium
-
resistant
genes, and attached these genes to promoters activated upon bloodmeals. This has a
number of advantages. One of them is that fitness is less likely to reduced, because the
genes are already pre
sent in the genome. Another is that by attaching the malaria
resistance genes to promoters induced by bloodmeal, the immune response acts upon the
plasmodium soon after it enters the mosquito, before it has had a chance to penetrate the
wall of the midgut
.
7

This study highlights the advantages of endogenous promoters


because both the genes and the promoters were inherent in the mosquito’s genome, it
offers greater potential for success.


Another aspect of the use of genetically modified mosquitoes as an

intervention is
that needs further research i
s drive systems. Drive systems are constru
cts that help a
introduced gene spread quickly and effectively throughout a population.
3,4

In order for a
population replacement intervention to be successful, the ex
ogenous gene must be
introduced into the entire population. Thus, a mechanism is needed to ensure its spread.


Transposable elements were one of the fir
st drive systems to become popular in
research. The nature of tr
ansposable elements is such that they
can replicate themselves in
the host’s genome. By doing so, the probability of the exogenous gene being passed on to
offspring is raised. There have be
en a number of successful projects in which
Aedes aegypti

mosquitoes have been transformed. However, t
here have been a number of hurdles in
using transposable elements in mosquito engineering. One of these barriers is the fact that
the transposable elements tend to suppress their own activity in order to preserve the host
genome. So, even if a transposab
le element tied to a refractory gene were to sp
read
throughout the population, if it does not allow the exogenous gene to produce its anti
-
malarial protein, there is no point to population replacement. Furthermore, transposable
elements mutate over time, t
hreatening the sustainability of an intervention using genetic
modifications

based on transposable elements. Another barrier of transposable elements
is the reduction of activity with increasing size. Genes inducing malarial resistance are
large in size,

and thus adding them would greatly reduce the activity of the transposable
element, and thus the anti
-
malarial effect would be reduced.
3,4


Another drive system currently being researched

is Medea. Medea is a gene foun
d
naturally in the flour beetle. Th
is gene expresses both a toxin and an antidote to the toxin.
If an offspring is born heterozygous

for the Medea gene, it will experience only the toxin
and not the antidote, and die. Therefore, if you attach an anti
-
malarial gene to Medea, any
offspring
not carrying the malaria refractory gene will also die. Although it cannot be
ensured that every mosquito will inherit the refractory gene, Medea helps skew the
offspring ratio toward malaria resistance.
3


Another approach to using
genetic engineering to
reduce the incidence of malaria is
the infection of mos
quitoes with transgenic fungi. In one study, researchers modified the
fungus
Metarhizium anisopliae
to induce plasmodium resistance in mosquitoes
, which

has
the advantage of infected the mosquito dire
ctly through

its cuticle to the hemolymph. The
fungus was given a variety of genes to achieve this goal. One gene functioned similarly to
the rodent studies, and blocked receptors that plasmodium sporozoites need to bind to in
order to reach the salivary
glands. Another gene introduced promoted production of an
antibody, which causes

plasmodium to agglutinate within the mosqui
to, rendering it
unable to infect a human host. Infection of mosquitoes with fungi possessing these
properties would make the infe
cted mosquitoes resistant to malaria, and thus reduce
incidence in endemic areas.
8


One advantage of transgenic fungus

is that there is reduced


but not eliminated
-
concern for developed resistance. Fungal infection of mosquitoes only kills mosquitoes
th
at are older. Thus, mosquitoes
that

are resistant to fungal infection are not selected for,
since the mosquitoes dying from the infection are the same mosquitoes
that were going to
die anyway. Additionally, these fungal spores can survive on treated surf
aces for months


so, where insecticide resistance has developed, the same interventions can be used, except
with fungus treatment instead of insecticide.
8


The concern about
natural
selection can also be applied to transgenic mosquitoes.
In order for a
transgenic mosquito intervention to be successful, the genes must be selected
for, or at the very least, not selected against. Transgenic mosquitoes must be both fit
enough to survive, and competitive enough to mate.
3
-
5

One study points out that
mosquitoe
s have not developed complete resistance to malaria parasites, and hypothesizes
that defense mechanisms protecting the mosquito against malaria are costly, and confer
lesser fitness. Using statistical modeling, the study examined a
rodent
plasmodium
resist
ant population, a plasmodium susceptible population, and a wild control population.
In nearly all fronts, both the control and plasmodium susceptible populations
outperformed the plasmodium resistant ones.
The mean population growth rate was lower
in the

plasmodium resistant population than in both the plasmodium susceptible and
control. The plasmodium resistant only outperformed the plasmodium susceptible in
pupation success, and only outperformed the control in egg production. However, the most
life c
ycle stages that have the greatest affect on lifetime fitness are the larval stage and the
gravid female stage
-

stages in which the plasmodium resistant mosquito saw less success
than the control and susceptible groups.
9


Despite these obstacles, there ar
e advantages to pursuing interventions involving
ge
netically modified mosquitoes. One of the greatest advantages to using biological vector
control is the reduced dependence on human compliance. As mentioned earlier, indoor
residual spraying requires at
least 70% population compliance to be effective in reducing
the incidence of malaria in the target area. Likewise, despite the availability of insecticide
treated bed nets,
people do not always use them.
1

Even the transgenic fungus intervention
discussed

earlier in this paper has some reliance on human compliance.
8

In interventions
using

transgenic mosquitoes, the residents of the target area do not need to do anything.

The mere release of these mosquitoes in the environment is the extent of the interven
tion
that occurs in the target area.


Another advantage, and perhaps the most pro
mising one, is sustainability. If an
anti
-
malarial gene that is introduced into a population persists throughout generations,
then no further intervention may be needed. This
, indeed, is a lofty goal, but this is a
distinct advantage to this sort of intervention. While other prior interventions have
focused on affecting the population of the vector, no other intervention has focused on
changing the nature of the vector. If t
he nature of the vector is altered, then the nature of
the disease changes. While this goal is not feasible with current technologies, it is a
potentially sustainable solution to this humanitarian crisis.
3
-
4


However, many disadvantages exist to pursuing
such a treatment.
One of the
primary concerns is ethics. In
order to pursue this research, researchers must do field tests
to determine effectiveness. However, in doing field research, informed consent is required
of
the participating communiti
es.

It i
s questionable whether we have the resources to
educate entire communities about the
intervention.

Furthermore, if we were able to reach
to every member in the community
, we would have to ensure that they fully understand
the

nature of the technology,
and

more importantly, the risks involved with
intervention.

Also, given the migratory nature of mosquitoes, there is the issue of the genetically
modified mosquitoes going outside the target area, and thus subjecting the intervention on
a community that has
not given consent.
The

problem that arises, then, is whether or not it
is appropriate
to proceed
with this intervention without properly obtained informed
consent of the population.
10

Furthermore, in a previous Indian trial, media sensationalism
resulted

in backlash to the research. The researchers were accused of carrying out
biological warfare, which, while untrue, proved problematic in obtaining consent of the
community.
5


Another ethical concern is the conflict genetic modification has with religious issues.
Some
religions do not allow harm to other living beings


including insects. Thus, with a
intervention

like sterile insect technique, we must consider the desires of
the population.
For instance, Jainism is one such religion that does not allow for harm to other living
creatures. It is a religion based in In
dia, where malaria
is endemic.
10

This in itself is a
problem to treating the target area, but it also presents
a problem to neighboring areas.
Mosquito migration was problematic in the El Salvador trials, and poses a considerable
barrier to intervention success.
5


Biosafe
ty is of course another concern. The
Cartagena

Protocol on Biosafety to the
Convention on Bio
logical diversity is an international agreement regarding the usage of
transgenic organisms. One of the highlights of this protocol is the Precautionary Principle


it states that an action that may potentially cause risk to the environment must be weighte
d
against the ability for scientists to ameliorate potential damage. Furthermore, the protocol
states that we should pursue the least harmful alternative


that is, use genetically modified
mosquitoes only as a last resort.
11


Lastly, the issue of sustain
ability is also a question. For widespread application, we
must find a gene that has a high probability of effectiveness. However, a chance still
remains that the gene is not properly dr
iven into the wild population. Thus, a population
may be replaced b
riefly by a plasmodium resistant population, but the may not be stable
within the wild population.
9

Natural selection and fitness could cause the gene to
disappear.
8

Furthermore, population replacement cannot be managed with a single release
of mosquitoe
s. Millions of mosquitoes could be necessary to obtain an observable effect in
the population, and it could require regular releases of mosquitoes to ensure the refractory
gene remains in the population.
5


Given this information, there is no clear directi
on moving forward. Each technology
has its own merits, and has its own drawbacks. While some believe that genetically
modified mosquitoes should first be used to eliminate mosquito populations, there are
others who believe that the ecological ramificatio
ns do not outweigh the benefits.
11,12

Furthermore, while some believe that population replacement using genetically modified
mosquitoes should be a last resort, while others place it higher on their list of
interventions.
11,12

What is clear is that furth
er research should be done to properly assess
the risks, benefits, and effectiveness of such a technique. There is no clear direction to
move from this point


simply, forward.




Works Cited

1.

Smit
h, D. Scott Talk presented at: Global Health Education Cons
ortium; October
2009; Stanford, CA. Accessed March 8, 2012.

2.

Murray C, Rosenfeld L
,
Lim S
,
et al.
Global malaria mortality between 1980 and
2010: a systematic analy
sis.
The Lancet
. 2012;379(9814): 413
-
431.
http://www.thelancet.com/journals/lancet/article/PI
IS0140
-
6736(12)60034
-
8/fulltext. Accessed March 8, 2012.

3.

Marshall JM, Taylor CE (2009) Malaria Control with Transgenic Mosquitoes.
PLoS
.
2009;

6(2): e1000020.

Accessed March 8, 2012.


4.

Christophides, G. Transgenic mosquitoes and malaria transmission.
Cellular
Microbiology
. 2005;7(3):325
-
333. Accessed March 8, 2012.

5.

Benedict M, Robinson A. The first releases of transgenic mosquitoes: an argument
for the sterile insect technique.
TRENDS in Parasitology
. 2003;19(8): 349
-
355.
Accessed March 8, 2012.

6.

James
AA. Engineeering mosquito resistance to malaria parasites: the avian malaria
model.
Insect Biochemistry and Molecular Biology
. 2002; 32(10): 1317
-
1323.

Accessed March 8, 2012.

7.

Dong Y, Das S, Cirimotich C, Souza
-
Neto JA, McLean KJ, et al
.
Engineered

Anophe
les

Immunity to

Plasmodium

Infection.
PLoS

Pathog
. 2011;7(12): e1002458.

Accessed
March 8, 2012.

8.

Fang W, et al. Development of Transgenic Fungi That Kill Human Malaria Parasites
in Mosquitoes
.
Science
. 2011;331(1074):1074
-
1077. Accessed March 8, 2012.

9.

Voordouw M, Anholt B, Taylor P, Hurd H. Rodent malaria
-
resistant strains of the
mosquito,
Anopheles gambiae
, have slower population growth than susceptible
strains.
BMC Evolutionary Biology
. 2009;9(76). Accessed March 8, 2012.

10.

Lavery J, Harrington L, Scott

T.
Perspective: Ethical, Social, and Cultural
Considerations for Site Selection for Research with Genetically Modified Mosquitoes.
Am. J. Trop. Med. Hyg.

2008;79(3): 312
-
318. Accessed March 8, 2012.

11.


Ostera G, Gostin L. Biosafety Concerns Involving Geneti
cally Modified Mosquitoes to
Combat Malaria and Dengue in Developing Countries.
JAMA
. 2011;305(9): 930
-
931.
Accessed March 8, 2012.

12.

Benedict, M. Safety of Genetically Modified Mosquitoes.
JAMA
. 2011;305(20): 2069
-
2070. Accessed March 8, 2012.