Early zygotic engineering promotes targeted large transgene integration and direct production of fully transgenic animals

Genetic engineering has become increasingly feasible with the advent of site-specific nucleases. This technology is highly effective for the generation of small mutations, however targeted insertion of large sequences to generate transgenic animals remains challenging, time consuming, and laborious. Following several failed attempts using the same reagents, we identified a protocol that achieved very high targeted integration of a 3.2 kb transgene into a single locus of a humanized mouse model of Down syndrome. Moreover, we demonstrate in multiple ways that this procedure directly generates numerous non-mosaic founder animals, which bred true to generate all transgenic progeny. In vitro fertilization of oocytes was followed by AAV-mediated delivery of donor sequence and electroporation of Cas9 ribonucleoprotein, all parameters designed to promote e arly z ygotic e ngineering (EZE). This strategy can obviate the need to breed mosaic animals for germline transmission and can enable engineering of difficult to breed animals. While efforts to improve genome engineering focus on reagents and delivery techniques, findings here suggest a narrow window of time in the early fertilized oocyte, likely before genome replication, is key to achieve both high integration efficiency and one-step generation of non-mosaic, engineered mice.