Trends in Parasitology
ReviewMosquito transgenesis for malaria control
Section snippets
Malaria epidemics and vector control
Malaria, caused by Plasmodium parasites and transmitted by female Anopheles mosquitoes, remains one of the most devastating of human diseases, accounting for over 200 million cases and approximately half a million deaths each year [1]. The global age-standardized DALYs (see Glossary) rate of malaria in 2019 is 433 per 100 000 inhabitants [2], posing a great impact on both human mortality and economics. Malaria parasites have evolved a complex life cycle within their mosquito vectors (Figure 1).
Development of mosquito transgenesis
To generate a transgenic mosquito, a genetically engineered cassette (GEC) needs to be introduced into the mosquito genome. A GEC is either randomly integrated into the mosquito genome using transposable element (TE)-based transformation or precisely inserted into a DNA sequence through homology-directed repair (HDR) (Figure 2A). Four TEs have been demonstrated to be functional in mosquitoes, piggyBac [14., 15., 16., 17., 18., 19., 20.], mariner Mos1 [21,22], mariner Minos [23], and Hermes [24,
Transgenic expression of antiparasitic effector genes
The generation of fully parasite-refractory GE mosquitoes relies on the identification of efficient antiparasitic effector genes. As discussed previously, the transgene should provide maximum protection while imposing minimum fitness cost. Based on the working models for anti-Plasmodium effectors, GE mosquitoes can be categorized into three groups:
- (i)
Transgenic expression of exogenous antipathogen effectors to kill or suppress parasites. A variety of exogenous antimicrobial peptides (AMPs), such
Transgenic inactivation of host factor genes
Pathogen agonists, or host factors, are genes/proteins required for the development and replication of Plasmodium inside its vector mosquito. Depletion of these genes through genetic engineering of Anopheles mosquitoes aims at interrupting host–parasite interactions that are necessary for the development of the malaria parasite within the various cells and tissues of the mosquito [78]. Therefore, targeting host factors via GE Anopheles mosquitoes constitutes a promising strategy to halt
Transgenic manipulation of miRNAs and lncRNAs
In addition to expressing antiparasite effector genes and knocking out host factor genes, noncoding RNAs, (ncRNAs) such as microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), have also been used to generate GE mosquitoes or have been shown to have potential for such use. Unlike mRNAs, ncRNAs are not involved in gene translation; instead, they provide a vast array of possibilities for post-transcriptional regulation of gene expression.
miRNAs are small endogenous noncoding RNAs that regulate
Intervention of gene drives
Gene drives are considered an essential tool to make GE mosquitoes affordable for controlling malaria and other vector-borne diseases in the field, and this technique has advanced rapidly in recent years with the development of CRISPR/Cas9-based gene-editing technologies [103]. Gene drives spread a GEC through a population at higher-than-normal Mendelian rates of inheritance. The technology relies on homing endonuclease genes (HEGs), which copy themselves and a GEC to a defined target site
Concluding remarks
Tremendous progress has recently been achieved in mosquito transgenesis and gene-drive technology, and the prospect of using parasite-refractory GE mosquitoes for malaria control is rapidly approaching a reality. Here we have mainly discussed the population replacement strategy, a self-sustainable approach that requires minimal follow-up effort, once the gene-drive refractory transgenic mosquitoes are in place. However, these advances have also led to a polarizing debate on the benefits and
Acknowledgments
This work was supported by the National Institute of Allergy and Infectious Diseases, National Institutes of Health Grants R01AI122743 and R21AI131574, the University of California Irvine Malaria Initiative, and the Bloomberg Philanthropies. We thank Dr Deborah McClellan for editorial assistance.
Declaration of interests
There are no interests to declare.
Glossary
- Antiparasitic effectors
- peptides or genes with the ability to inhibit malaria parasite infection in their vertebrate hosts or mosquito vectors, including exogenous antiparasite peptides and endogenous mosquito immune genes such as antimicrobial peptide genes.
- CRISPR/Cas9
- clustered regularly interspaced short palindromic repeats/CRISPR-associated sequence 9, a genome-editing tool comprising a nuclease (Cas9) and a guide RNA that directs the nuclease to bind to specific genomic loci and introduce
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