Components of the Medea gene drive system in a mosquito Aedes aegypti

Mosquito-borne viral diseases, such as dengue, chikungunya, yellow fever, and Zika continue to be a major international public health problem. Despite control efforts, the numbers of infections are on the increase, which indicates that the existing control strategies, relying on suppression of mosquito vector populations, are not effective. The mosquito Aedes aegypti, responsible for the great majority of infections with these viruses, is difficult to control with the conventional tools. It breeds in various, often cryptic, man-made containers, and their females bite during the day, making their targeting with insecticides very challenging. Moreover, increasing resistance to insecticides in the Ae. aegypti populations further exacerbates the situation. To reduce disease burden, new genetic control strategies have been proposed, aiming at suppression of mosquito populations or at mosquito population modification by releasing engineered mosquitoes incapable of virus transmission. The latter strategies, to be effective, require a mechanism that would allow a rapid spread of the anti-pathogen synthetic constructs through the entire mosquito populations. A gene drive mechanism, called Medea, constructed thus far only in a fruit fly Drosophila melanogaster, has been demonstrated to possess the required invasive properties. Efforts to develop functional Medea element in mosquitoes have not yet met with success, because information about the relevant aspects of mosquito developmental biology is still lacking. In this study, we will focus on elucidation of the key developmental processes occurring in ovaries during egg production and in early Ae. aegypti embryos to identify the most optimal genetic constituents of the Medea element functional in this mosquito species. The identified sequences will be individually validated in transgenic mosquito lines and, subsequently used for the creation of a proof-of-principle functional Medea gene drive in Aedes.

A dramatic and constant rise of mosquito-borne arboviral infections calls for an urgent implementation of disease control measures that would be more effective than the currently used methods. Genetic approaches aiming at introduction of anti-pathogen constructs that make mosquitoes incapable of disease transmission are an attractive alternative to traditional disease control relying on suppression of mosquito numbers. Such genetic approaches require a gene drive mechanism to allow super-Mendelian inheritance of the anti-pathogen genes, and thus, their rapid spread through the natural mosquito populations. A synthetic element called Medea, created thus far only in Drosophila, has been demonstrated to possess the required invasive gene drive capabilities. Medea consists of two transcription units: (i) a microRNA-based toxin, whose expression, controlled by an ovarian promoter, leads to a knock-down of a target gene vital for embryo development, and (ii) an early zygotic promoter-driven variant of a target gene that is insensitive to the microRNA toxin. Despite efforts, functional Medea element has not yet been developed in mosquitoes. These unsuccessful attempts relied on a direct assembly of the element from DNA fragments orthologous (where possible) to the Medea components used in Drosophila. However, mosquitoes and fruit flies are phylogenetically distant, and the expression patterns of the key components may be different in these two taxa. Therefore, here we propose a different approach, based on an extensive analysis of transcription and open chromatin features in the Ae. aegypti ovaries (during egg development) and early embryos (during activation of a zygotic genome). We will conduct functional analysis of the relevant promoters and toxin-target gene pairs to provide a distilled list of the most optimal individual components prior to a subsequent construction of a proof-of-concept Medea element active in Aedes.

This study aims to explore developmental regulation of transcription in Aedes aegypti to create a proof-of-concept Medea gene drive that could be used to spread beneficial alleles, such as anti-viral constructs, through the natural mosquito populations. Ae. aegypti is the most important vector of human arboviral diseases, responsible for transmission of dengue, chikungunya, Zika, and yellow fever in the majority of cases. Thus, our study will provide data and resources that will have a direct influence on translational research on control of these diseases. The identification of key components of functional Medea gene drive in Ae. aegypti will greatly facilitate creation of the Medea element in other mosquito species. From this perspective, our study will have a major impact on control of mosquito-borne pathogens, in general. In result, in the long term, it will lead to an improved human health and wellbeing. Nearly half of the world's population is now at risk of dengue alone. Recent large outbreaks of chikungunya and Zika further underscore vulnerability of human populations to mosquito-borne infections. Therefore, the ultimate beneficiaries of the study will be hundreds of millions of people who, because of the harm inflicted by mosquitoes, suffer from recurrent vector-borne diseases, which are often an underlying cause of poverty and hunger in the low income countries.
Aside from the scientific community, our results will be of interest to both general public and to the policy makers, especially of the international bodies, such as the World Health Organization, the Food and Agriculture Organisation of the United Nations (FAO) and the International Atomic Energy Agency (IAEA). FAO and IAEA play a leading role in the development and implementation of the genetic control of mosquitoes and insect pests. The outputs of this study are expected to directly lead to the development of commercially exploitable products. Therefore, our results will also be of interest to industrial partners. The results of our research will be disseminated to the broad audience using various means. Important breakthroughs will be channelled through media: local, national and international, where appropriate, by the communications team at the Pirbright Institute. In addition, significant achievements will be publicised on the Institute's website and social media accounts. Furthermore, the web page of the PI, that is accessible to the general public, will be regularly updated to reflect progress of work. At an appropriate stage, we will contact IAEA or an industrial partner to facilitate translation of our findings into mosquito-borne disease control technology. These activities will be regulated by formal collaboration agreements prepared with the assistance of a legal team at the Institute to protect intellectual property of the outputs of our study.