Engineered mosquitoes may express a polycistronic cluster of synthetic RNA designed to be resistant to the Zika virus genome, according to a study published in the Proceedings of the National Academy of Sciences of the United States of America.

Zika virus is a mosquito-borne flavivirus that has spread rapidly throughout the Americas, causing hundreds of thousands of infections. Most cases of Zika infection are asymptomatic; however, if infected while pregnant, the viral genome has been associated with several congenital abnormalities and pregnancy loss. These symptoms provoked the World Health Organization to declare Zika virus a public health emergency of international concern in 2016. However, there are no currently available vaccines for Zika and no effective treatment options for those who are infected, which highlights the necessity for the development of novel vector control strategies to combat arboviral transmission. No previous anti-Zika virus refractory cargo genes have been developed in any mosquito. The current methods of vector control include removal of standing water and the use of insecticides, which have not been entirely effective against the spread of Aedes aegypti mosquitoes. Therefore, to urgently combat the spread of Zika and other Ae aegypti-borne diseases worldwide, this study attempted to engineer Ae aegypti mosquitoes to be resistant to Zika virus transmission.

To generate a cargo gene that conferred resistance to Zika, a synthetic small RNA-based approach was implemented into Ae aegypti mosquitoes. To achieve this, researchers engineered a piggyBac vector that comprised a polycistronic cluster of 8 Zika virus-targeting, miRNA-like, synthetic RNAs under the control of a promoter to drive the expression of the synthetic small RNAs in female midguts following a blood meal. Expression and processing of the Zika virus-targeting RNAs was confirmed through deep-sequencing small RNA populations that were dissected from midgut tissues isolated from both blood-fed and non-blood-fed female mosquitoes. Further testing was completed to characterize the functional significance of the Zika virus-targeting synthetic RNA expression and processing on vector competence, viral dissemination, and whether the RNAs were broadly inhibitory for Zika virus. Finally, the anti-Zika virus transgene was tested on a mouse model to see whether it inhibited Zika transmission to mice.

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In total, results demonstrated that Ae aegypti mosquitoes harboring this anti-Zika virus transgene expressed and fully processed the Zika virus-targeting synthetic RNAs in the midgut and had significantly reduced viral infection, dissemination rates, and transmission rates of Zika. Targeting the conserved genes in the Zika virus genome by expression of engineered polycistronic cluster of synthetic small RNAs resulted in complete refractoriness to multiple strains of Zika infection, dissemination, and transmission. However, it is uncertain how many synthetic RNAs are needed to ensure robust disease refractoriness and evolutionary stability in the wild population, as only 5 of 8 small RNAs were detected as expressed and processed. This also may suggest that the small RNA processing machinery may be overloaded or the promoter was not strong enough to ensure robust expression of all 8 small RNAs. However, this strategy may provide a suitable cargo gene for practical use to reduce or eliminate vector competence in mosquito populations.

Overall, the study authors concluded that, “Given the increasing incidence of these viral infections worldwide, such transgenes (coupled with gene drive systems) can provide an effective, sustainable, and comprehensive strategy for reducing the impact of arboviral mosquito-borne diseases.”

Reference

Buchman A, Gamez S, Li M, et al. Engineered resistance to Zika virus in transgenic Aedes aegypti expressing a polycistronic cluster of synthetic small RNAs [published online February 5, 2019]. Proc Natl Acad Sci U S A. doi: 10.1073/pnas.1810771116