Dual effector population modification gene-drive strains of the African malaria mosquitoes, Anopheles gambiae and Anopheles coluzzii

Abstract

Proposed genetic approaches for reducing human malaria include population modification, which introduces genes into vector mosquitoes to reduce or prevent parasite transmission. We demonstrate the potential of Cas9/guide RNA (gRNA)–based gene-drive systems linked to dual antiparasite effector genes to spread rapidly through mosquito populations. Two strains have an autonomous gene-drive system coupled to dual anti- Plasmodium falciparum effector genes comprising single-chain variable fragment monoclonal antibodies targeting parasite ookinetes and sporozoites in the African malaria mosquitoes Anopheles gambiae (AgTP13) and Anopheles coluzzii (AcTP13). The gene-drive systems achieved full introduction within 3 to 6 mo after release in small cage trials. Life-table analyses revealed no fitness loads affecting AcTP13 gene-drive dynamics but AgTP13 males were less competitive than wild types. The effector molecules reduced significantly both parasite prevalence and infection intensities. These data supported transmission modeling of conceptual field releases in an island setting that shows meaningful epidemiological impacts at different sporozoite threshold levels (2.5 to 10 k) for human infection by reducing malaria incidence in optimal simulations by 50 to 90% within as few as 1 to 2 mo after a series of releases, and by ≥90% within 3 mo. Modeling outcomes for low sporozoite thresholds are sensitive to gene-drive system fitness loads, gametocytemia infection intensities during parasite challenges, and the formation of potentially drive-resistant genome target sites, extending the predicted times to achieve reduced incidence. TP13-based strains could be effective for malaria control strategies following validation of sporozoite transmission threshold numbers and testing field-derived parasite strains. These or similar strains are viable candidates for future field trials in a malaria-endemic region.

MGDrivE 3: A decoupled vector-human framework for epidemiological simulation of mosquito genetic control tools and their surveillance

“…The full gene drive system resembles one engineered in An. coluzzii [2], which includes dual linked effector genes targeting the malaria pathogen, and is one of the most promising population replacement systems in a mosquito vector to date. While we model this system in a setting chosen largely for its isolation [30], we note that regulatory and biosafety issues must be considered seriously for self-propagating systems with the potential to spread beyond their release site [31].…”

“…The inheritance dynamics of this system were fitted to laboratory cage data and are provided in Carballar-Lejarazú et al . [2] with model parameters summarized in Table S1. Notably, we considered a 10% fitness cost associated with mosquitoes carrying either one or two copies of the intact drive allele, as there were no major fitness loads inferred in the An.…”

“…Fitted reproductive fitness parameters for the TP13 construct in An. coluzzii [2] are loaded, and a 10% fitness load on male mating competitiveness and female fecundity is implemented, as described earlier.…”

“…For malaria vectors, recent constructs include a suppression gene drive targeting the doublesex gene in Anopheles gambiae [1], a replacement gene drive linked to dual antimalarial effector genes in both An. gambiae and Anopheles coluzzii [2], and a genetic version of the sterile insect technique engineered in An. gambiae [3].…”

“…In incorporating certain genetic vector control tools - e.g., gene drive systems intended to spread disease-refractory genes into mosquito populations [2, 23] - an important addition to the epidemiological framework is transmission parameters that are mosquito genotype-specific. In the ICL malaria model, the force of infection on humans, λ H ( a, t ), is dependent on both age, a , and time, t , and is a product of the EIR, ε ( a, t ), and the transmission probability, b , i.e.…”