The CRISPR/Cas9 technology emerged only recently. Still it is already obvious, that it makes the generation of genetically modified organisms more efficient than with conventional techniques. Moreover, species that were recalcitrant to those manipulations in the past are now amenable to genome editing.
What we initially saw in the rodent, especially the mouse research community, was a rush into the technique, where many scientists wanted to employ the new technology (me too). While there were a number of reports on off-target effects, it is now clear that off-target genome alterations are rarely found in an in vivo situation (Seruggia et al., 2015; Shen et al., 2014). Moreover modified nucleases are meanwhile available with no detectable off-target effects (Kleinstiver et al. 2016).
In the most widely used model organism in biomedical research, the mouse, the number of animals needed to generate a genetically modified line remains approximately the same with CRISPR/Cas9 as compared to conventional techniques. However, the ease at which new mutations are generated with the new system might lead to increased numbers of novel lines being produced (Williams et al., 2016). Moreover, numbers are also increasing for other species, especially zebrafish and rats (Auer and Del Bene, 2014; Hwang et al., 2013; Wang et al., 2015). As with all novel genetically modified lines, after their generation CRISPR/Cas9 generated animals need to be bred for experiments and are kept on the shelf for considerable time. Thereby, the increase in the number of lines generated will translate into larger numbers of additional animals bred.
At this time, conventional techniques still have their justification in the lab since precise modification of DNA via homologous recombination, especially with large constructs, is not as robust as necessary via CRISPR/Cas9. However, one has to take into account that this is a very recent technology, whilst work to improve the generation of genetically modified mice via homologous recombination in embryonic stem cells has been ongoing for more than 25 years. We therefore may see extensive improvements over the coming years once we gain a better understanding of the underlying mechanisms. With improvements in targeting efficiencies there could be opportunities for further reductions in the number of mice used for the generation of the animal model to be studied: In many cases mutations are still introduced into ES cells that are used for the generation of live mice. Even though culture conditions have been improved over the years, cell clones show a fairly high degree of aneuploidy so that multiple clones have to be injected to generate chimeric offspring that transmit the mutation to their progeny. Due to the non-chimeric nature of CRISPR/Cas9, mutated individuals will in most cases transmit. Therefore an improved ratio of mutant to wildtype offspring could directly reduce the number of animals produced. However, the degree of mosaicism could counteract this reduction. CRISPR/Cas9 technology has the potential that several mutations can be introduced on different alleles at the same time, even homozygously, once the mosaicism issue is solved. Alternatively, complex mutations can be generated by adding additional mutations in pronuclear stage embryos of mutant backgrounds. With a sufficient increase in efficiency, this promise does appear realistic (Williams et al., 2016). This would mean that instead of producing a plethora of unwanted mice with unwanted genotypes in the process of breeding compound mutants, one could proceed right to the desired combination of mutated alleles.
We expect to see an increase in the number of species that will undergo complex targeted genetic manipulation. Still, a significant increase in laboratory animal numbers is not expected. The research infrastructure has been developed and is still being developed for large scale mouse breeding. The breeding of rats takes up much more space and only a few centers have the infrastructure for larger animals. The applicability of the technology to non- human primates means they are also more often nowadays used for transgenic animal research, especially in countries where such research is not strictly regulated. This is another issue of public concern and will require an ethical evaluation process beyond the scope of the 3R approach. CRISPR/Cas9 technology is widely used to genetically manipulate zebrafish. An increase in zebrafish numbers similar to what is foreseen in the mouse can be expected. On the other hand there will be projects where the zebrafish is now amenable to the introduction of complex targeted genetic manipulation and can therefore replace the mouse as a model – a clear refinement in accordance with the 3Rs.
In summary, the ISTT’s 3Rs Committee acknowledges that the advent of the CRISPR/Cas9 technology has the potential to significantly increase the number of animals used and range of species genetically modified. However the committee believes that existing limits on space and other associated resources will inhibit the realization of this at the current time. In the long run, after the technology has been developed further and improved, the Committee is hopeful that opportunities for reducing the number of animals that are bred but not used will be realized.
Boris Jerchow, Chair
On behalf of the ISTT’s 3Rs Committee Berlin in March 2016
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Hwang, W.Y., Fu, Y., Reyon, D., Maeder, M.L., Tsai, S.Q., Sander, J.D., Peterson, R.T., Yeh, J.R., and Joung, J.K. (2013). Efficient genome editing in zebrafish using a CRISPR-Cas system. Nat Biotechnol 31, 227-229.
Kleinstiver, B.P., Pattanayak, V., Prew, M.S., Tsai, S.Q., Nguyen, N.T., Zheng, Z., and Joung, J.K. (2016). High-fidelity CRISPR–Cas9 nucleases with no detectable genome-wide off- target effects. Nature, 529, 490-495.
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Shen, B., Zhang, W., Zhang, J., Zhou, J., Wang, J., Chen, L., Wang, L., Hodgkins, A., Iyer, V., Huang, X., et al. (2014). Efficient genome modification by CRISPR-Cas9 nickase with minimal off-target effects. Nature methods 11, 399-402.
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