Feasible introgression of an anti-pathogen transgene into an urban mosquito population without using gene-drive

Spatial structure undermines parasite suppression by gene drive cargo

Gene drives may be used in two ways to curtail vectored diseases. Both involve engineering the drive to spread in the vector population. One approach uses the drive to directly depress vector numbers, possibly to extinction. The other approach leaves intact the vector population but suppresses the disease agent during its interaction with the vector. This second application may use a drive engineered to carry a genetic cargo that blocks the disease agent. An advantage of the second application is that it is far less likely to select vector resistance to block the drive, but the disease agent may instead evolve resistance to the inhibitory cargo. However, some gene drives are expected to spread so fast and attain such high coverage in the vector population that, if the disease agent can evolve resistance only gradually, disease eradication may be feasible. Here we use simple models to show that spatial structure in the vector population can greatly facilitate persistence and evolution of resistance by the disease agent. We suggest simple approaches to avoid some types of spatial structure, but others may be intrinsic to the populations being challenged and difficult to overcome.

Engineered Reciprocal Chromosome Translocations Drive High Threshold, Reversible Population Replacement in Drosophila

Replacement of wild insect populations with transgene-bearing individuals unable to transmit disease or survive under specific environmental conditions using gene drive provides a self-perpetuating method of disease prevention. Mechanisms that require the gene drive element and linked cargo to exceed a high threshold frequency in order for spread to occur are attractive because they offer several points of control: they bring about local, but not global population replacement; and transgenes can be eliminated by reintroducing wildtypes into the population so as to drive the frequency of transgenes below the threshold frequency required for drive. Reciprocal chromosome translocations were proposed as a tool for bringing about high threshold population replacement in 1940 and 1968. However, translocations able to achieve this goal have only been reported once, in the spider mite Tetranychus urticae, a haplo-diploid species in which there is strong selection in haploid males for fit homozygotes. We report the creation of engineered translocation-bearing strains of Drosophila melanogaster, generated through targeted chromosomal breakage and homologous recombination. These strains drive high threshold population replacement in laboratory populations. While it remains to be shown that engineered translocations can bring about population replacement in wild populations, these observations suggest that further exploration of engineered translocations as a tool for controlled population replacement is warranted.

Maintaining Quality of Candidate Strains of Transgenic Mosquitoes for Studies in Containment Facilities in Disease Endemic Countries

Transgenic mosquitoes are being developed as novel components of area-wide approaches to vector-borne disease control. Best practice is to develop these in phases, beginning with laboratory studies, before moving to field testing and inclusion in control programs, to ensure safety and prevent costly field testing of unsuitable strains. The process of identifying and developing good candidate strains requires maintenance of transgenic colonies over many generations in containment facilities. By working in disease endemic countries with target vector populations, laboratory strains may be developed and selected for properties that will enhance intended control efficacy in the next phase, while avoiding traits that introduce unnecessary risks. Candidate strains aiming toward field use must consistently achieve established performance criteria, throughout the process of scaling up from small study colonies to production of sufficient numbers for field testing and possible open release. Maintenance of a consistent quality can be demonstrated by a set of insect quality and insectary operating indicators, measured over time at predetermined intervals. These indicators: inform comparability of studies using various candidate strains at different times and locations; provide evidence of conformity relevant to compliance with terms of approval for regulated use; and can be used to validate some assumptions related to risk assessments covering the contained phase and for release into the environment.

Evaluating strategies for reversing CRISPR-Cas9 gene drives

A gene drive biases inheritance of a gene so that it increases in frequency within a population even when the gene confers no fitness benefit. There has been renewed interest in environmental releases of engineered gene drives due to recent proof of principle experiments with the CRISPR-Cas9 system as a drive mechanism. Release of modified organisms, however, is controversial, especially when the drive mechanism could theoretically alter all individuals of a species. Thus, it is desirable to have countermeasures to reverse a drive if a problem arises. Several genetic mechanisms for limiting or eliminating gene drives have been proposed and/or developed, including synthetic resistance, reversal drives, and immunizing reversal drives. While predictions about efficacy of these mechanisms have been optimistic, we lack detailed analyses of their expected dynamics. We develop a discrete time model for population genetics of a drive and proposed genetic countermeasures. Efficacy of drive reversal varies between countermeasures. For some parameter values, the model predicts unexpected behavior including polymorphic equilibria and oscillatory dynamics. The timing and number of released individuals containing a genetic countermeasure can substantially impact outcomes. The choice among countermeasures by researchers and regulators will depend on specific goals and population parameters of target populations.

Comparison of Model Predictions and Laboratory Observations of Transgene Frequencies in Continuously-Breeding Mosquito Populations

The persistence of transgenes in the environment is a consideration in risk assessments of transgenic organisms. Combining mathematical models that predict the frequency of transgenes and experimental demonstrations can validate the model predictions, or can detect significant biological deviations that were neither apparent nor included as model parameters. In order to assess the correlation between predictions and observations, models were constructed to estimate the frequency of a transgene causing male sexual sterility in simulated populations of a malaria mosquito Anopheles gambiae that were seeded with transgenic females at various proportions. Concurrently, overlapping-generation laboratory populations similar to those being modeled were initialized with various starting transgene proportions, and the subsequent proportions of transgenic individuals in populations were determined weekly until the transgene disappeared. The specific transgene being tested contained a homing endonuclease gene expressed in testes, I-PpoI, that cleaves the ribosomal DNA and results in complete male sexual sterility with no effect on female fertility. The transgene was observed to disappear more rapidly than the model predicted in all cases. The period before ovipositions that contained no transgenic progeny ranged from as little as three weeks after cage initiation to as long as 11 weeks.

Integrating Transgenic Vector Manipulation with Clinical Interventions to Manage Vector-Borne Diseases

Many vector-borne diseases lack effective vaccines and medications, and the limitations of traditional vector control have inspired novel approaches based on using genetic engineering to manipulate vector populations and thereby reduce transmission. Yet both the short- and long-term epidemiological effects of these transgenic strategies are highly uncertain. If neither vaccines, medications, nor transgenic strategies can by themselves suffice for managing vector-borne diseases, integrating these approaches becomes key. Here we develop a framework to evaluate how clinical interventions (i.e., vaccination and medication) can be integrated with transgenic vector manipulation strategies to prevent disease invasion and reduce disease incidence. We show that the ability of clinical interventions to accelerate disease suppression can depend on the nature of the transgenic manipulation deployed (e.g., whether vector population reduction or replacement is attempted). We find that making a specific, individual strategy highly effective may not be necessary for attaining public-health objectives, provided suitable combinations can be adopted. However, we show how combining only partially effective antimicrobial drugs or vaccination with transgenic vector manipulations that merely temporarily lower vector competence can amplify disease resurgence following transient suppression. Thus, transgenic vector manipulation that cannot be sustained can have adverse consequences—consequences which ineffective clinical interventions can at best only mitigate, and at worst temporarily exacerbate. This result, which arises from differences between the time scale on which the interventions affect disease dynamics and the time scale of host population dynamics, highlights the importance of accounting for the potential delay in the effects of deploying public health strategies on long-term disease incidence. We find that for systems at the disease-endemic equilibrium, even modest perturbations induced by weak interventions can exhibit strong, albeit transient, epidemiological effects. This, together with our finding that under some conditions combining strategies could have transient adverse epidemiological effects suggests that a relatively long time horizon may be necessary to discern the efficacy of alternative intervention strategies.

Despite decades of attempted vector control, several vector-borne diseases remain endemic. Recent high-profile studies suggest that candidate vaccines, particularly for dengue, may be less than completely effective as public health interventions. Nevertheless, the epidemiological consequences of using other novel approaches (e.g., transgenic strategies to reduce or replace vector populations) remain highly uncertain. Faced with unclear prospects of any one strategy succeeding in isolation, there is increasing interest in designing a comprehensive public health response to manage vector-borne diseases. Here we use a relatively simple model to study how combining vaccines, transgenic vector manipulation and antimicrobial medications can facilitate disease management. We explain why the epidemiological consequences for combining strategies are not expected to merely sum their effects. Contrary to the prevailing assumption that comprehensive disease management always yields public health benefits, we find integrating transgenic vector manipulation with clinical interventions can, in some cases, temporarily exacerbate the adverse consequences of any one strategy failing. These results highlight the need for system-specific modeling efforts aimed at assessing whether our conclusions apply to specific vector-borne diseases. We outline the implications for proceeding with public health responses integrating currently available products, as well as assessing their efficacy.

A critical assessment of vector control for dengue prevention

Recently, the Vaccines to Vaccinate (v2V) initiative was reconfigured into the Partnership for Dengue Control (PDC), a multi-sponsored and independent initiative. This redirection is consistent with the growing consensus among the dengue-prevention community that no single intervention will be sufficient to control dengue disease. The PDC's expectation is that when an effective dengue virus (DENV) vaccine is commercially available, the public health community will continue to rely on vector control because the two strategies complement and enhance one another. Although the concept of integrated intervention for dengue prevention is gaining increasingly broader acceptance, to date, no consensus has been reached regarding the details of how and what combination of approaches can be most effectively implemented to manage disease. To fill that gap, the PDC proposed a three step process: (1) a critical assessment of current vector control tools and those under development, (2) outlining a research agenda for determining, in a definitive way, what existing tools work best, and (3) determining how to combine the best vector control options, which have systematically been defined in this process, with DENV vaccines. To address the first step, the PDC convened a meeting of international experts during November 2013 in Washington, DC, to critically assess existing vector control interventions and tools under development. This report summarizes those deliberations.

Advances in genetically modified Aedes aegypti to control transmission of dengue viruses

ABSTRACT 

Dengue viruses (DENV) are mosquito-borne viruses that infect millions of humans each year. DENVs are endemic in tropical regions of the world and maintained in a transmission cycle between mosquito vectors (Aedes aegypti) and humans. DEN disease control relies on vector control approaches that have had limited success and are difficult to sustain. Genetically modified mosquitoes (GMM) may be an alternative control strategy to limit DENV transmission. GMM-based control strategies include: conditional expression of a dominant lethal gene (RIDL) to reduce vector populations; and introgression of antipathogen (AP) genes into wild-type vectors for population replacement. In this review, we describe novel GMM-based strategies to limit DENV transmission and discuss potential hurdles to their successful implementation in the field.

Disruption of dengue virus transmission by mosquitoes

Current control efforts for mosquito-borne arboviruses focus on mosquito control involving insecticide applications, which are becoming increasingly ineffective and unsustainable in urban areas. Mosquito population replacement is an alternative arbovirus control concept aiming at replacing virus-competent vector populations with laboratory-engineered incompetent vectors. A prerequisite for this strategy is the design of robust anti-pathogen effectors that can ultimately be genetically driven through a wild-type population. Several anti-pathogen effector concepts have been developed that target the RNA genomes of arboviruses such as dengue virus in a highly sequence-specific manner. Design principles are based on long inverted-repeat RNA triggered RNA interference, catalytic hammerhead ribozymes, and trans-splicing Group I Introns that are able to induce apoptosis in virus-infected cells following splicing with target viral RNA.

The role of RNA interference (RNAi) in arbovirus-vector interactions

RNA interference (RNAi) was shown over 18 years ago to be a mechanism by which arbovirus replication and transmission could be controlled in arthropod vectors. During the intervening period, research on RNAi has defined many of the components and mechanisms of this antiviral pathway in arthropods, yet a number of unexplored questions remain. RNAi refers to RNA-mediated regulation of gene expression. Originally, the term described silencing of endogenous genes by introduction of exogenous double-stranded (ds)RNA with the same sequence as the gene to be silenced. Further research has shown that RNAi comprises three gene regulation pathways that are mediated by small RNAs: the small interfering (si)RNA, micro (mi)RNA, and Piwi-interacting (pi)RNA pathways. The exogenous (exo-)siRNA pathway is now recognized as a major antiviral innate immune response of arthropods. More recent studies suggest that the piRNA and miRNA pathways might also have important roles in arbovirus-vector interactions. This review will focus on current knowledge of the role of the exo-siRNA pathway as an arthropod vector antiviral response and on emerging research into vector piRNA and miRNA pathway modulation of arbovirus-vector interactions. Although it is assumed that arboviruses must evade the vector’s antiviral RNAi response in order to maintain their natural transmission cycles, the strategies by which this is accomplished are not well defined. RNAi is also an important tool for arthropod gene knock-down in functional genomics studies and in development of arbovirus-resistant mosquito populations. Possible arbovirus strategies for evasion of RNAi and applications of RNAi in functional genomics analysis and arbovirus transmission control will also be reviewed.

Antipathogen genes and the replacement of disease-vectoring mosquito populations: a model-based evaluation

Recently, genetic strategies aimed at controlling populations of disease-vectoring mosquitoes have received considerable attention as alternatives to traditional measures. Theoretical studies have shown that female-killing (FK), antipathogen (AP), and reduce and replace (R&R) strategies can each decrease the number competent vectors. In this study, we utilize a mathematical model to evaluate impacts on competent Aedes aegypti populations of FK, AP, and R&R releases as well as hybrid strategies that result from combinations of these three approaches. We show that while the ordering of efficacy of these strategies depends upon population life history parameters, sex ratio of releases, and switch time in combination strategies, AP-only and R&R/AP releases typically lead to the greatest long-term reduction in competent vectors. R&R-only releases are often less effective at long-term reduction of competent vectors than AP-only releases or R&R/AP releases. Furthermore, the reduction in competent vectors caused by AP-only releases is easier to maintain than that caused by FK-only or R&R-only releases even when the AP gene confers a fitness cost. We discuss the roles that density dependence and inclusion of females play in the order of efficacy of the strategies. We anticipate that our results will provide added impetus to continue developing AP strategies.

Mosquito immune responses to arbovirus infections

The principal mosquito innate immune response to virus infections, RNA interference (RNAi), differs substantially from the immune response to bacterial and fungal infections. The exo-siRNA pathway constitutes the major anti-arboviral RNAi response and its essential genetic components have been identified. Recent research has also implicated the Piwi-interacting RNA pathway in mosquito anti-arboviral immunity, but Piwi gene-family components involved are not well-defined. Arboviruses must evade or suppress RNAi without causing pathogenesis in the vector to maintain their transmission cycle, but little is known about mechanisms of arbovirus modulation of RNAi. Genetic manipulation of mosquitoes to enhance their RNAi response can limit arbovirus infection and replication and could be used in novel strategies for interruption of arbovirus transmission and greatly reduce disease.