Analysis of Cytosine Base Editors in Bovine Zygotes: Efficiency and Editing Window Characterization Through Targeting the MYO7A Gene.

Cytosine base editors (CBEs) enable precise C-to-T (G-to-A) conversions in genomic DNA, offering significant potential for specific gene editing. This study compared the prototypical Base Editor 3 (BE3) and a modified variant, BE3-Y130F, which utilizes an hA3A deaminase with the Y130F mutation, focusing on their editing efficiency and editing window characteristics using bovine zygotes. Following in vitro fertilization (IVF), sgRNA and Cas9 mRNA were injected as a targeting efficiency control, which resulted in 100% editing with no wild-type sequence. Then, either BE3 or BE3-Y130F mRNA, synthesized via in vitro transcription, and an sgRNA targeting exon 4 of the MYO7A gene was injected into zygotes. Genomic DNA was extracted from both blastocysts and developmentally arrested embryos, and Sanger sequencing was performed to evaluate C-to-T conversion efficiency and editing window. Both BE3 and BE3-Y130F achieved 100% C-to-T conversion efficiency at the primary target cytosine. BE3 displayed a defined editing window, primarily affecting cytosines at positions 7 and 8, indicating a predictable profile. In contrast, BE3-Y130F maintained high efficiency but had a less clearly defined editing window, resulting in incomplete editing and a remaining cytosine on the target sequence.