Combating mosquito-borne diseases using genetic control technologies

Abstract

Mosquito-borne diseases, such as dengue and malaria, pose significant global health burdens. Unfortunately, current control methods based on insecticides and environmental maintenance have fallen short of eliminating the disease burden. Scalable, deployable, genetic-based solutions are sought to reduce the transmission risk of these diseases. Pathogen-blocking Wolbachia bacteria, or genome engineering-based mosquito control strategies including gene drives have been developed to address these problems, both requiring the release of modified mosquitoes into the environment. Here, we review the latest developments, notable similarities, and critical distinctions between these promising technologies and discuss their future applications for mosquito-borne disease control.

Fig. 1. Wolbachia and transgene-based approaches for mosquito population suppression and population modification.

A Wolbachia and transgene-based approaches for population suppression. Wolbachia-infected males can suppress mosquito populations through CI effects in the early embryo. To prevent fertile Wolbachia-infected females from escaping the sex-sorting step, an irradiation step is included to render them sterile. Using transgene-based approaches, mosquitoes can be engineered to induce lethality in the immature or adult stage of the life cycle. In suppression approaches, reducing the number of mosquitoes will result in reduced pathogen transmission. B Wolbachia and transgene-based approaches for population modification. Several studies have demonstrated the pathogen-blocking capabilities of Wolbachia. This feature can be used to modify mosquito populations for pathogen resistance. As Wolbachia-infected females also have reproductive manipulation advantages (due to CI), pathogen blocking can spread throughout wild mosquito populations. In transgene-based approaches, strategies can be designed to inhibit replication of a specific pathogen through the desired mechanism (RNAi, over-expression of innate immune pathways, etc.). When linked to a gene drive, these strategies could possibly spread throughout mosquito populations. Both Wolbachia and transgene-based approaches seek to maintain the mosquito population. Arrows represent mosquito releases. The multiple arrows in the Wolbachia IIT approach indicate that multiple releases are needed to achieve suppression. For simplicity, the SIT, pgSIT, RIDL, and fsRDIL approaches are mentioned as examples in panel A due to their requirement of multiple releases. These approaches do not utilize Wolbachia, despite being under this category in the figure. MAYV mayaro virus, CHIKV chikungunya virus, DENV dengue virus, WNV West Nile virus, ZIKV Zika virus.

Fig. 2. Examples of novel suppression and modification approaches in transgenic mosquitoes.

Illustrations of recently developed population suppression approaches that utilize unique components to achieve mosquito suppression. A Gene drive (GD) suppression approach for Anopheles mosquitoes, which takes advantage of the sex determination pathway to produce fertile males and sterile females. B Sex-distorter GD programmed to home into dsx and express an endonuclease that shreds the X-chromosome. High sex-bias ratios towards males enable a population crash after sufficient generations. C RIDL, a self-limiting approach, consists of a dominant lethal gene that utilizes modified components of the Tet-OFF operon system^, . In the absence of tetracycline, transactivator (TtaV, green) binds to the operon sequence (orange) to induce toxic product expression in a tissue- and temporal-specific manner. High concentrations of toxic products will lead to lethality. D fsRIDL, a similar approach to RIDL, with added sex-specificity. A sex-specific intron ensures that TtaV protein will express only in flight muscles of females to prevent them from flying^, . E Potential application of pgSIT in mosquitoes. Transgenic mosquitoes carrying components encoding Cas9 and several guide RNAs (gRNAs) targeting sex-determination genes will enable the production of sterile male offspring. F Self-limiting split drive. Separating both Cas9 and gRNA/GD element components enables a safe, noninvasive, self-limiting system. G Recoded GD prevents fitness load associated with disrupting two copies of kh gene. H Non-autonomous GD designed to have minimal components is used to produce an antimicrobial peptide in mosquito midgut to inhibit Plasmodium in these tissues. I Multistage effector transgenes with the capacity to target several life stages of Plasmodium. Transgene containing five antimicrobial peptides is expressed after a blood meal. In another configuration, a single-chain antibody linked to an antimicrobial peptide was effective. J Transgenes produce microRNAs to induce the RNAi pathway of mosquitoes to target and inhibit dengue virus serotype 3 (DENV-3) and chikungunya virus (CHIKV) replication and transmission. K Anti-DENV transgene expresses an engineered single-chain antibody to confer resistance to four DENV serotypes. L Anti-Zika virus (ZIKV) transgene uses eight synthetic small RNAs to induce the RNAi pathway against ZIKV.