Generation of germline-transmitting transgenic sheep by piggyBac-mediated transgenesis using pronuclear and cytoplasmic gene injection approaches.

Sheep represent an important large-animal model for biomedical research and biopharmaceutical production. Although the piggyBac transposon system offers efficient and stable genomic integration, the optimal gene delivery strategy for ovine embryos remains unclear. This study evaluated piggyBac-mediated transgenesis using pronuclear injection (PNI) in both in vivo- and in vitro-derived embryos and assessed cytoplasmic injection (CTI) as an alternative approach. In vivo-derived embryos were obtained from superovulated Kıvırcık ewes approximately 40 h after gonadotropin-releasing hormone administration, whereas in vitro-derived embryos were produced from slaughterhouse-derived oocytes. All embryos were injected with the hyperactive piggyBac transposase-based pmhyGENIE-3 construct (10 ng/µL). In vivo-derived embryos were transferred immediately after injection, whereas in vitro-derived embryos were cultured for 3 days and screened for EGFP expression prior to transfer. Among 65 in vitro-derived embryos injected by PNI, no transgenic offspring was obtained. In contrast, PNI of 19 in vivo-derived embryos resulted in one transgenic lamb (5.3%). CTI of 12 in vivo-derived embryos similarly produced one transgenic lamb (8.3%). Whole-genome sequencing of the healthy founder male produced by CTI identified a single detectable genomic integration locus on chromosome 10 within a non-coding RNA locus (LOC121820439). Germline transmission was confirmed by in vitro fertilization using sperm from the founder male, with EGFP expression detected in 10.1% (10/99) resulting embryos. These findings provide proof-of-concept evidence for piggyBac-mediated transgenesis in sheep and support the feasibility of cytoplasmic injection as an alternative gene-delivery approach under the conditions tested.