Gene drives to fight malaria: current state and future directions

Abstract

Self-propagating gene drive technologies have a number of desirable characteristics that warrant their development for the control of insect pest and vector populations, such as the malaria-transmitting mosquitoes. Theoretically easy to deploy and self-sustaining, these tools may be used to generate cost-effective interventions that benefit society without obvious bias related to wealth, age or education. Their species-specific design offers the potential to reduce environmental risks and aim to be compatible and complementary with other control strategies, potentially expediting the elimination and eradication of malaria. A number of strategies have been proposed for gene-drive based control of the malaria mosquito and recent demonstrations have shown proof-of-principle in the laboratory. Though several technical, ethical and regulatory challenges remain, none appear insurmountable if research continues in a step-wise and open manner.

Keywords: CRISPR; Gene drive; genetic vector control; genome editing; malaria; mosquito.

Figure 1.

Mendelian inheritance vs. gene drive inheritance. In mosquitoes, as for any other sexually reproducing organism, genetic elements in heterozygosis have a 50 percent chance of being inherited by the progeny and therefore its frequency remains constant in the population, or more likely, it is gradually lost if the transgene carries a cost (upper panel). A gene drive results in most or all progeny of heterozygotes receiving the driving genetic element, this allows the modification to spread rapidly throughout the population over a few generations (lower panel).

Figure 2.

Population suppression versus population replacement. In both cases the modified insects are released at a low initial frequency and spread in the population over generations. The modification can be designed to interfere with the mosquito reproduction or viability, aiming to the eliminate or suppress the population to levels that do not support disease transmission (left panel). Alternatively, the modification can be engineered to replace the vector population with insects unable to effectively transmit disease (right panel).

Figure 3.

Meiotic Y-drive. An endonuclease (blue block), placed onto the mosquito Y-chromosome, is expressed during male meiosis to cut a multicopy target sequence on the X-chromosome. The shredding of the X-chromosome favors the unaffected Y-carrying sperm and results in the production of a male-biased progeny.

Figure 4.

Homing gene-drive. The HEG (or TALEN, ZFN, CRISPR) is inserted within the target gene and expressed under a germline-specific promoter. The endonucleases can be designed to disrupt essential mosquito genes, genes required for reproduction or those involved with parasite development within the mosquito (grey block). Alternatively, the HEG can be linked to an antiparasitic effector. Homology-directed repair (HDR) of the double-strand break generated by the endonuclease leads to copying and ‘homing’ of the HEG⁺ allele.

Figure 5.

Gene drive system targeting female reproduction. The confinement of homing to the germline leads to super-Mendelian inheritance of the homing construct (in blue) that reduce the number of fertile females by targeting a haplosufficient, somatic female-fertility genes (in gray).

Figure 6.

Strategies to limit target site resistance. Schematic representation of some of the alternatives proposed to reduce the chance of resistance to homing-gene drives. (a) Alternative nucleases such as FokI-dCas9 fusion proteins retain the ability to cleave a sequence of DNA proximal to the target sites. Cleavage-induced mutations or pre-existing polymorphisms at the cut site, and outside the target, do not prevent further cleavage and homing [78]. (b) Target sites can be selected based on nucleotide sequence conservation or the likelihood that micro-homology mediated end-joining will generate null alleles. Mutations that may prevent cleavage are mostly null and get removed from population. (c) Multiple gRNA-expression cassettes or nuclease able to process its own CRISPR-RNA (crRNA), such as Cpf1, can be used to target multiple sites within the target gene. Mutations that prevent cleavage must simultaneously occur at all target sites to inhibit homing. Mutation occurring in only some of the targets can be removed during the homing-process stimulated by the cleavage of the intact sites.