Prowse et al. (2017) present their modelling work in the Proceedings of The Royal Society B on various different CRISPR/Cas9-based gene drives aimed at suppressing populations of exotic vertebrate species on islands (focusing on mice (Mus musculus)). The issues addressed here are of interest to insect biologists looking at gene drive systems for similar reasons.
There has been great excitement in recent years regarding advances in the use of CRISPR/Cas9 technology for the development of gene drive systems. However, notes of caution have been sounded due to the formation/spread of resistance alleles. Two of the main concerns here are due to standing genetic polymorphism and the potential for CRISPR/Cas9 systems to generate their own resistance alleles via non-homologous-end-joining (NHEJ).
Prowse et al. (2017) describe four distinct CRISPR/Cas9-based suppression drive designs. These are discussed in detail in the paper but briefly they are as follows.
- Strategy 1: Heterozygotic XX sterility – drives an XX male sex-reversing transgene to create a deficiency of females.
- Strategy 2: Heterozygotic XX sex reversal – similar to the above but with XX males able to transmit the gene drive.
- Strategy 3: Homozygotic embryonic non-viability – generates a loss-of-function mutation causing recessive embryonic lethality.
- Strategy 4: Homozygotic XX sterility – similar to the above but with infertility of homozygous females rather than recessive embryonic lethality.
To address the issue of resistance, the authors model the use of multiplexed guide RNAs. This is often discussed as a potential solution since a resistance phenotype would only be observed once an individual is simultaneously resistant at all gRNA target sites.
Results presented in this paper demonstrate that, for a single release of gene drive carrying mice, strategies 2, 3 and 4 are capable of eradicating an island mouse population; however each requires some number of multiplexed gRNAs to do so. Heterozygotic XX sterility (Strategy 1) fails to give eradication or suppression of a population under these conditions; however, it can successfully eradicate a population if gene drive carriers are continually released into the population.
The probability of a population being successfully eradicated when considering different numbers of gRNAs and initial mouse population sizes is then explored. This shows that the inclusion of extra gRNAs increases the probability that these strategies would be capable of eradicating a given island mouse population.
This is an interesting study that discusses CRISPR/Cas9-based gene drives in the important context of island populations. Such locations are likely to be considered for the first tests of any gene drive technologies since they are spatially isolated, thus limiting the risk of transgenes spreading beyond the target area.
The general principles outlined in this paper are not only of interest in the context of exotic vertebrates but will also have important implications for the future design of CRISPR/Cas9-based gene drives in insects.
Thomas A.A. Prowse, Phillip Cassey, Joshua V. Ross, Chandran Pfitzner, Talia A. Wittmann, Paul Thomas (2017) Dodging silver bullets: good CRISPR gene-drive design is critical for eradicating exotic vertebrates. Proc. R. Soc. B 284: 20170799. http://dx.doi.org/10.1098/rspb.2017.0799