Population Modification of Anopheline Species to Control Malaria Transmission

! " # $ $%%&&&' ( ') #% %*$ $" # () (+ $ $ ) ) # # , )-. /012+ *+'!# 3 ) ) !"#$% & '&(& &($ &$& & &&)# * '+ , ) -./" 012221 123 3 4 ) ! "#$% &' ()* & +' $''"! ,-'$. / ! $! $ - 0 & " ' .! 1 $'! 2 3!' ' .! 2 34 &'5 '' -!!&,4 $ $ %'..'$ .'$ % $ ' 333'$ % $! $. & '. $ 1 $'!#$% &' $61 $'!4 7" PATHOGENS AND GLOBAL HEALTH, 2018 https://doi.org/10.1080/20477724.2018.1427192 SPECIAL ISSUE ARTICLE OPEN ACCESS Population modification of Anopheline species to control malaria transmission Rebeca Carballar-Lejarazúa and Anthony A. Jamesa,b aDepartment of Microbiology & Molecular Genetics, University of California, Irvine, CA, USA; bDepartment of Molecular Biology & Biochemistry, University of California, Irvine, CA, USA ABSTRACT KEYWORDS Vector control strategies based on population modification of Anopheline mosquitoes may Population replacement; have a significant role in the malaria eradication agenda. They could consolidate elimination population alteration; gains by providing barriers to the reintroduction of parasites and competent vectors, and genetically-engineered allow resources to be allocated to new control sites while maintaining treated areas free of mosquitoes; gene drive; anti- parasite eﬀector genes malaria. Synthetic biological approaches are being used to generate transgenic mosquitoes for population modification. Proofs-of-principle exist for mosquito transgenesis, the construction of anti-parasite effector genes and gene-drive systems for rapidly introgressing beneficial genes into wild populations. Key challenges now are to develop field-ready strains of mosquitoes that incorporate features that maximize safety and efficacy, and specify pathways from discovery to development. We propose three pathways and a framework for target product profiles that maximize safety and efficacy while meeting the demands of the complexity of malaria transmission, and the regulatory and social diversity of potential end-users and stakeholders. Introduction Recent modeling provides additional support for the potential impact of the approaches [6,7]. We comple- Human disease and deaths resulting from malaria have ment these reports by reviewing briefly the concept of decreased significantly over the last two decades fueling population modification and posing questions about the the enthusiasm for pursuing the goal of malaria eradi- challenges of the next steps. A critical task is to specify cation [1]. These reductions have been achieved mostly pathways for further development of the technologies through the increased availability of effective therapeu- and to define the specific products to be used in popu- tic drug combinations and insecticide-treated bed nets lation modification control programs. and indoor spraying [2]. However, there are still over 400 thousand deaths every year, and modeling predicts that the downward trend will reverse with the failure of exist- What is population modification? ing technologies and the absence of new disease-control Core concepts of population modification for vec- tools [3]. tor-borne disease control were first aggregated in a sin- Vector biologists have proposed a variety of new gle paragraph by Christopher Curtis in which he stated: approaches to contribute to reducing parasite trans- “Mutant genes can be imagined the presence of which mission and some are actively pursuing genetic strate- in a population would be favorable to man, without gies that could impact mosquito populations [3]. These being very deleterious to the insect. As an example, approaches seek either to eliminate or reduce vector genes will be considered of a type already known, mosquito populations below thresholds needed for which make mosquitoes non-infectible by pathogens …. At fixation, the population would be harmless to stable parasite transmission (population suppression), man …. This type of procedure may be preferable to or make them incapable of transmitting pathogens methods which eradicate a pest species in an area, (population replacement/modification) 4[ ]. Proofs-of- leaving its ecological niche vacant for reinfestation by principle for both approaches have been demonstrated immigrants.” [8] and efforts are underway to move the advances from The publication was addressing the theoretical use the laboratory to the field. A case was made recently for of chromosome translocations to introgress ‘mutant the value of population modification in the eradication genes’ into wild mosquito populations, so while defin- agenda and this was based on the demonstrated need ing anti-parasite effector genes, Curtis was at the same for technologies that could provide long-term, cost-ef- time proposing a mechanism to achieve gene drive. We fective and sustainable regional malaria elimination [5]. recognize the translocation system as a variant of what CONTACT Anthony A. James [email protected] © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2 R. CARBALLAR-LEJARAZÚ AND A. A. JAMES is now designated ‘underdominance’. Curtis’ proposal As the program proceeds, population modification is was refined later into a set of operational hypotheses predicted in models to take on a larger role in the pre- to the effect that (1) the introduction into a population vention-of-reintroduction phase [5,6]. Parasite-resistant of vector insects of a gene that confers resistance to a mosquitoes would facilitate consolidation of elimination pathogen should lead to a decrease in transmission of and allow resources to be allocated to another target that pathogen and (2) implicit in this hypothesis is that region with the confidence that the malaria-free area will less transmission will result in less morbidity and mor- remain so. Furthermore, as Curtis proposed, unlike popu- tality [9]. lation suppression, population modification is expected Curtis originally called the approach ‘population to provide a level of environmental safety because it replacement’ but this was interpreted by some to mean would not result in an ‘empty’ ecological niche that could that mosquitoes developed in the laboratory would be be occupied by an opportunistic invasive species that switched, one-for-one, for those existing in the wild. may have an expanded vector competence and vectorial ‘Population modification’ was thought to convey more capacity. (Vector competence is defined as the intrinsic accurately the goals of the strategies and has been used ability of the insect to transmit a pathogen and includes extensively. More recently, opinions were expressed genetic components [13]. Vectorial capacity is an expres- that calling it ‘modification’ brought with it the negative sion of the efficiency of parasite transmission, and recent connotations of the broader use of the phrase ‘genet- formulations include vector competence as a parame- ically-modified organisms’ (GMOs), and ‘population ter [14]). In addition, modeling supports the conclusion alteration’ was adopted as a less charged description that modification approaches should be stable in low [10]. We use modification in this contribution, but it is mosquito population densities [6]. This is important in important to emphasize that population replacement, regions where mosquito population sizes fluctuate annu- modification and alteration all refer to the same cate- ally during wet and dry seasons. Similarly, population gory of technologies. World Health Organization (WHO) modification schemes are robust to locally-isolated wild documents adhere to the original descriptor, population populations since the modified population surrounding replacement [11]. a pocket of unmodified mosquitoes will persist until the modified mosquitoes eventually gain access to it. In con- trast, population suppression schemes can bypass such What are the benefits of population modification? disease-bearing populations and then vanish rapidly Population modification programs alone are not likely from the local environment resulting in repopulation of to eradicate malaria; this will be achieved using the full the region with the original unmodified, disease-com- array of tools that eliminate parasites in humans (prophy- petent mosquitoes. lactic and therapeutic drugs, vaccines [when available]) and those targeting the vector (source reduction, insec- What is needed to achieve population ticides, spatial repellants, and others). However, popula- modification and what has been done so far? tion modification is expected to provide sustainability to the WHO-defined control, pre-elimination, elimination Research challenges were defined as molecular biol- and prevention-of-reintroduction phases of local malaria ogists and geneticists sought to bring Curtis’ theory elimination [5,6]. This sustainability feature is important of population modification to a practical application because it is the basis for claims that population mod- [9,15,16]. It was important first to show that it would ification is expected to provide a cost-effective means be possible to use molecular genetic tools to engineer of preserving local elimination. Population modification genes that would result in parasite-resistance phe- strains can serve as barriers to the reinvasion of wild, par- notypes in previously

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