Efficient targeted transgenesis of large donor DNA into multiple mouse genetic backgrounds using bacteriophage Bxb1...

The development of mouse models of human disease and synthetic biology research by targeted transgenesis of large DNA constructs represent a significant genetic engineering hurdle. We developed an efficient, precise, single-copy integration of large transgenes directly into zygotes using multiple mouse genetic backgrounds. We used in vivo Bxb1 mediated recombinase-mediated cassette exchange (RMCE) with a transgene "landing pad" composed of dual heterologous Bxb1 attachment (att) sites in cis, within the Gt(ROSA)26Sor safe harbor locus. RMCE of donor was achieved by microinjection of vector DNA carrying cognate attachment sites flanking the donor transgene with Bxb1-integrase mRNA. This approach achieves perfect vector-free integration of donor constructs at efficiencies > 40% with up to ~ 43 kb transgenes. Coupled with a nanopore-based Cas9-targeted sequencing (nCATS), complete verification of precise insertion sequence was achieved. As a proof-of-concept we describe the development of C57BL/6J and NSG Krt18-ACE2 models for SARS-CoV2 research with verified heterozygous N1 animals within ~ 4 months. Additionally, we created a series of mice with diverse backgrounds carrying a single att site including FVB/NJ, PWK/PhJ, NOD/ShiLtJ, CAST/EiJ and DBA/2J allowing for rapid transgene insertion. Combined, this system enables predictable, rapid development with simplified characterization of precisely targeted transgenic animals across multiple genetic backgrounds.