Requirements for effective malaria control with homing endonuclease genes

Abstract

Malaria continues to impose a substantial burden on human health. We have previously proposed that biological approaches to control the mosquito vector of disease could be developed using homing endonuclease genes (HEGs), a class of selfish or parasitic gene that exists naturally in many microbes. Recent lab studies have demonstrated that HEGs can function in mosquitoes. We constructed and analyzed a model of mosquito population genetics and malaria epidemiology to determine how well HEGs need to function in order to have a significant effect on the burden of disease. Our model, combined with currently available data, indicates that populations of Anopheles gambiae could be eliminated by releasing 2-3 HEGs targeting female fertility genes, or a driving-Y chromosome that is transmitted to 75-96% of progeny. Combinations of fertility-targeting HEGs and Y drive may also be effective. It is possible to eliminate the disease without eliminating the vector, but the parameter space producing this outcome appears to be small. HEGs causing a quantitative reduction in adult survival can be more effective than those targeting female fertility, but the selection coefficients that need to be imposed are still large, unless many HEGs are to be released. Simulations show that HEG-based strategies can be effective over socially relevant time frames. Important limiting assumptions of the models are that there is only a single vector species, and we model a homogeneous population, not a landscape. Nevertheless, we conclude that HEG-based approaches could have a transformational effect on malaria control efforts.

Fig. 1.

Number of HEGs required to eliminate a population as a function of the homing rate when homing occurs in both sexes and the knockouts are recessive female sterile, for different values of R_m.

Fig. 2.

(A) Proportional reduction in the basic rate of increase of the parasite as a function of the reduction in the intrinsic rate of increase of the vector when mortality is imposed before (dashed line) or after (solid line) the density-dependent larval stage for different values of R_m. HEGs that affect female fecundity will follow the dashed lines, and HEGs that alter the sex ratio will follow the solid lines. (B) Synergistic effect of releasing multiple HEGs on the proportional reduction in R₀ for HEGs that target genes essential for female fecundity (red), prelarval survival (dashed black), postlarval survival (solid black), or causing quantitative reductions in adult survival (dark, medium, and light gray for s = 0.75, 0.5, and 0.25, respectively). (C) HEG load required to eliminate the disease when the load is imposed before (B, dashed lines) or after (A, solid lines) density dependence for R₀ = 5 (red), 50 (orange), or 500 (blue). For comparison the load needed to eliminate the vector is also shown (black line). (d) Contour plots showing combinations of R_m and R₀ (log scale) for which the disease is eliminated using the specified number of HEGs (n), for HEGs targeting genes essential for postlarval survival (black) or female fecundity (red), with a homing rate of e = 0.6. Malaria is eliminated in populations below and to the left of each line. All plots derived from Eq. S5 (SI Appendix).

Fig. 3.

Fate of the mosquito and the disease as a function of the homing rate and selection coefficient against homozygous knockouts for HEGs causing quantitative reductions in (A) adult survival and (B) female fecundity. Red lines show combinations of parameters in which the mosquito is eliminated, and black lines show combinations of parameters in which the disease is eliminated, for different initial values of R₀.

Fig. 4.

Example time courses after a single release of heterozygous HEG-bearing mosquitoes into a population at initial frequencies of 1% (arrows). R_m = 6; R₀ = 162. (A) The introduced mosquitoes carry two independent HEGs each of which targets a gene essential for female fertility (homing rate e = 0.6). (B) The introduced mosquitoes carry two independent HEGs each of which targets a gene involved in adult survival, with homozygous knockouts having s = 0.5 (homing rate e = 0.6). (C) The introduced mosquitoes carry on their Y chromosome a HEG that targets a repeated sequence on the X chromosome, resulting in transmission of the Y to m = 91% of sperm. The black curves refer to the left Y axis, whereas the red curves can be read on the right Y axis. In A and B, the abundance of the HEG among adult females is calculated as the number of homozygotes plus half the number of heterozygotes, whereas in C the abundance of the HEG is calculated as the number of adult males carrying it.

Fig. P1.

Two methods by which HEGs could be used to affect mosquito populations. (A) The canonical homing reaction, in which a HEG-encoded sequence-specific nuclease cleaves chromosomes not containing the HEG (H). The HEG gets copied to the cut chromosome during the repair process, thereby converting a heterozygous cell into a homozygous cell. The HEG is inserted in the middle of its own recognition sequence, protecting the chromosome it is on from being cut. If the HEG is in the middle of a host gene, it can disrupt the ability of the gene to function, thereby causing a population-wide gene knockout as the HEG spreads through the population. (B) A HEG on a Y chromosome that specifically cleaves a repeated sequence on the X chromosome during male meiosis could disrupt the transmission of that X chromosome, leading the Y chromosome (which carries the HEG) to be transmitted to a majority of progeny.