Zygote cryobanking applied to CRISPR/Cas9 microinjection in mice

Microinjection of CRISPR/Cas9 requires the availability of zygotes that implies animal breeding, superovulation schemes, and embryo collection. Vitrification of zygotes may allow having ready-to-use embryos and to temporally dissociate the workload of embryo production from microinjection. In this study, fresh (F group) or vitrified (V group) zygotes were microinjected with CRISPR/Cas9 system to test the hypothesis that vitrified zygotes could be a suitable source of embryos for microinjection. In Experiment 1 (in vitro evaluation), B6D2F1/J zygotes were microinjected and cultured until blastocyst stage. Embryo survival and cleavage rates after microinjection were similar between groups (~50% and ~80% respectively; P = NS). Development rate was significantly higher for F than V group (55.0% vs. 32.6%, respectively; P<0.05). Mutation rate did not show statistical differences among groups (P = NS). In Experiment 2 (in vivo evaluation), C57BL/6J zygotes were microinjected and transferred to recipient females. Embryo survival was significantly lower in fresh than in vitrified zygotes (49.2% vs. 62.7%, respectively; P<0.05). Cleavage rate did not show statistical differences (~70%; P = NS). Pregnancy rate (70.0% vs. 58.3%) and birth rate (11.9% vs. 11.2%) were not different between groups (F vs. V group; P = NS). Offspring mutation rate was higher for F than V group, in both heterodimer analysis (73.7% vs. 33.3%, respectively; P = 0.015) and Sanger sequencing (89.5% vs. 41.7%, respectively; P = 0.006). In conclusion, vitrified-warmed zygotes present a viable alternative source for CRISPR/Cas9 microinjection when the production of fresh embryos is impeded by limited technical support. The possibility of zygote cryobanking to perform microinjection sessions on demand seems to be a suitable alternative to avoid the breeding and maintenance of animals all over the year, enhancing the implementation of CRISPR technology.