Synthetic Selfish DNA as Genetic Technologies

 

Derric Nimmo, Ph.D., Product Development Manager, Oxitec Ltd.

Derric Nimmo, Ph.D., Product Development Manager, Oxitec Ltd.

Selfish genes are genes that can spread themselves rapidly within genomes and populations.   Think – transposable elements, B chromosomes, homing endonucleases, selfish sex chromosomes, genomic exclusion to name a few.

Some are promising candidates for the genetic engineering of wild populations. They offer the potential to spread or replace genes in wild populations with the release of only a few individuals.

There are many potential applications for gene-drive technology but it has been considered most extensively in mosquitoes as a means to  spread resistance genes to pathogens such as dengue virus, chikungunya virus, Plasmodium spp.  Gene drive systems are also being considered for suppressing or eliminating mosquito populations.  The goal of these approaches is to reduce the density of mosquitoes below the disease transmission threshold.

A Homing Endonuclease increasing in frequency with a genome.

A Homing Endonuclease increasing in frequency within a genome.

One group of selfish genes, known as Homing Endonuclease Genes (HEGs), have shown promise in recent years for disease control in mosquitoes. Austin Burt’s group at Imperial College have recently demonstrated in the laboratory that HEGs could be used to rapidly invade caged Anopheles gambiae populations (Windbichler et al. 2011). However, HEGs have been relatively hard to re-engineer to target-specific sequences in genomes of interest without affecting the function of the HEGs.  This has limited their adaptability and versatility.

Simoni and collaborators (2014) have proposed that using the basic functional principles of HEGs, the homing (site specific) insertion and the endonuclease activity, they could design Synthetic Selfish Elements (SSEs) as a model system. They used Zinc Finger Nucleases (ZFNs) and Transcription Activator-Like Effector Nuclease (TALENs) as highly site-specific nucleases, a germline-specific promoter for nuclease expression and fluorescent and phenotypic markers to discriminate different insertion events. The advantage of ZFN and especially TALEN nucleases is the ability to target a much wider array of sequences in the genome.

Simoni et al (2014) tested these SSEs in Drosophila melanogaster, looking at the efficiency and specificity of insertion, fitness effects and the ability to invade target populations.

Segregation Distorter's (SD) effects on spermatogenesis.  Only SD-containing sperm develop.  Left - normal spermatids. Right - SD caused sperm-killing. Credit: Reenan lab/Brown University

Segregation Distorter’s (SD) effects on spermatogenesis. SD is a gene drive system in Drosophila.  Only SD-containing sperm develop. Left – normal spermatids. Right – SD-caused sperm-killing.
Credit: Reenan lab/Brown University

The results showed that the design of SSEs used in this study were able to target the specific insertion sites highly efficiently with low toxicity, but unfortunately they were limited in their ability to invade host populations. The TALEN based SSEs produced too many dysfunctional homing products, limiting their spread enough that they were unable to invade a target population. ZFN based SSEs showed better population invasion but were still limited by their relatively lower homing rates.

Nonetheless the results are exciting and more research is required to answer some of the current issues with the technology.   This study clearly shows the promise of SSEs as ‘designer’ selfish genes and helps to understand better how HEGs function. SSEs potentially offer the flexibility to target almost any genomic region with little or no fitness effects and facilitate the spread of disease resistance genes into insect populations.

A selfish gene (blue) spreading through a mosquito population

A selfish gene (blue) spreading through a mosquito population

Simoni A, Siniscalchi C, Chan Y-S, Huen DS, Russell S, Windbichler N, Crisanti A (2014) Development of synthetic selfish elements based on modular nucleases in Drosophila melanogaster. Nucleic Acids Res 42: 7461-7472 doi: 10.1093/nar/gku387.

Other Reference Cited:  Windbichler N, Menichelli M, Papathanos PA, Thyme SB, Li H, Ulge UY, Hovde BT, Baker D, Monnat RJ Jr, Burt A, Crisanti A. (2011) A synthetic homing endonuclease-based gene drive system in the human malaria mosquito.  Nature. 473(7346):212-5. doi: 10.1038/nature09937.

Related Posts in Technology Topics:

The Road Ahead for Using Gene Drive Systems

Controlling Malaria Mosquitoes With Genetics

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