Researchers at the University of California, San Diego, have developed two new genetic systems that halt or eliminate gene drives after release. These systems help address concerns about gene drive releases into the wild.
Gene drives
Gene drives harness genetic engineering technology to propagate particular genes throughout populations. They are able to alter the probability of specific allele transmission to offspring from the natural 50% probability. Researchers hope to utilise gene drives to suppress devastating diseases such as malaria, dengue, Zika and yellow fever. Developments in CRISPR-Cas9 gene editing tools have allowed researchers to create several highly efficient gene drive systems in insects, yeast and bacteria. Nevertheless, there are several concerns regarding the safety of releasing such systems into the wild. This has raised questions about whether researchers should recall such systems should they spread beyond their intended zone of application.
Now, in a paper published in Molecular Cell, researchers describe two self-copying guide RNA-only genetic elements that either inactivate or delete the gene drive developed in fruit flies.
Professor Ethan Bier, senior author, stated:
“One way to mitigate the perceived risks of gene drives is to develop approaches to halt their spread or to delete them if necessary.
There’s been a lot of concern that there are so many unknowns associated with gene drives. Now we have saturated the possibilities, both at the genetic and molecular levels, and developed mitigating elements.”
The genetic systems
The two systems:
- e-CHACR (erasing construct hitchhiking on the autocatalytic chain reaction): This system can be inserted into the genome at any desired location. It encodes two or more gRNAs. One gRNA cuts at the genomic site of e-CHACR insertion. This allows self-copying in the presence of a trans-acting source of Cas9. The other gRNA cleaves the Cas9 transgene component of the gene drive; subsequently, inactivating the gene drive.
- ERACR (element reversing the autocatalytic chain reaction): This system inserts at the same genomic site as a gene drive. It encodes two gRNAs that combine with the Cas9 produced by the drive to cut either side of the drive element to delete and replace it. This system eliminates the gene drive altogether.
The team rigorously tested and analysed the activities of these systems in fruit flies. They found that all e-CHACRs were able to efficiently mutate and inactive Cas9. Similarly, they found that ERACRs copied and deleted gene drive elements as intended. However, they did find that ERACRs can damage target chromosomes and generate various rare recombinant outcomes.
Caution
The researchers note that these results provide optimism for ways to attenuate gene drive systems if needed; however, they still caution the use of such neutralising systems. They also believe that the decision to implement gene drives into the wild should not be determined by construction of neutralising elements and that they should only be developed for precautionary purposes.
Emily Bulger, University of California, San Diego, stated:
“Because ERACRs and e-CHACRs do not possess their own source of Cas9, they will only spread as far as the gene drive itself and will not edit the wild type population.
These technologies are not perfect, but we now have a much more comprehensive understanding of why and how unintended outcomes influence their function and we believe they have the potential to be powerful gene drive control mechanisms should the need arise.”
Image credit: Yana Tikhonova – canva.com
