Efficient genome editing of bovine in vitro derived zygotes via Cas9 RNP-electroporation using extended-stored bovine ovaries

Bovine embryos serve as a relevant model for human preimplantation development and are increasingly used in genome editing research. Although CRISPR-Cas9 electroporation in zygotes offers a promising approach, challenges with respect to editing efficiency and mosaicism remain a challenge. However, traditional in vitro fertilization (IVF) workflows enforce highly inconvenient electroporation times of zygotes usually around midnight. This study therefore aimed to investigate the feasibility of using bovine zygotes derived from extended-stored slaughterhouse derived ovaries (Phosphate buffered saline, 14-18 °C, 20-22 h) with particular attention to the strength of the voltage field during electroporation and the Cas9-RNP concentration with respect to embryonic development and genome editing efficiency. Oocytes obtained under these conditions demonstrated in vitro developmental competence similar to those derived from fresh ovaries, with comparable cleavage and blastocyst formation rates. Electroporation conducted 10 h post-insemination at voltages of 15V, 20V, and 25V revealed that 25V yielded the highest blastocyst editing rate (40.7 %) while maintaining acceptable developmental rates. Increasing voltage to 30V and 35V further enhanced blastocyst editing rate (66.7 % and 67.9 %, respectively), with a greater proportion of blastocyst showing a full edit. While higher voltages reduced developmental rates, the optimal voltage for electroporation was found to be 30V for bovine zygotes derived from extended stored ovaries. Using that setting, contrasting Cas9-RNP concentrations (6 μM vs. 3 μM) were tested. A concentration of 6 μM resulted in higher editing rates compared to 3 μM (81.5 % vs. 60.0 %), although the distribution of monoallelic, biallelic, and mosaic modifications did not differ significantly. Taken together, this study confirmed for the first time that zygotes from extend-stored bovine ovaries are suitable for genome editing via CRISPR Cas9-RNP electroporation. Mosaicism, however, remained prevalent, underscoring the need for strategies to improve the precision and consistency of embryo editing.