Germline transgenesis is an important procedure for functional investigation of biological pathways, as well as for animal biotechnology. We have established a simple, nonviral protocol in three important biomedical model organisms frequently used in physiological studies. The protocol is based on the hyperactive Sleeping Beauty transposon system, SB100X, which reproducibly promoted generation of transgenic founders at frequencies of 50-64, 14-72, and 15% in mice, rats, and rabbits, respectively. The SB100X-mediated transgene integrations are less prone to genetic mosaicism and gene silencing as compared to either the classical pronuclear injection or to lentivirus-mediated transgenesis. The method was successfully applied to a variety of transgenes and animal models, and can be used to generate founders with single-copy integrations. The transposon vector also allows the generation of transgenic lines with tissue-specific expression patterns specified by promoter elements of choice, exemplified by a rat reporter strain useful for tracking serotonergic neurons. As a proof of principle, we rescued an inborn genetic defect in the fawn-hooded hypertensive rat by SB100X transgenesis. A side-by-side comparison of the SB100X- and piggyBac-based protocols revealed that the two systems are complementary, offering new opportunities in genome manipulation.

The CRISPR-Cas9 system is used for genome editing in mammalian cells by introducing double-strand breaks (DSBs) which are predominantly repaired via non-homologous end joining (NHEJ) or to lesser extent by homology-directed repair (HDR). To enhance HDR for improving the introduction of precise genetic modifications, we tested fusion proteins of Cas9 nuclease with HDR effectors to enforce their localization at DSBs. Using a traffic-light DSB repair reporter (TLR) system for the quantitative detection of HDR and NHEJ events in human HEK cells we found that Cas9 fusions with CtIP, Rad52, and Mre11, but not Rad51C promote HDR up to twofold in human cells and significantly reduce NHEJ events. We further compared, as an alternative to the direct fusion with Cas9, two components configurations that associate CtIP fusion proteins with a Cas9-SunTag fusion or with guide RNA that includes MS2 binding loops. We found that the Cas9-CtIP fusion and the MS2-CtIP system, but not the SunTag approach increase the ratio of HDR/NHEJ 4.5–6-fold. Optimal results are obtained by the combined use of Cas9-CtIP and MS2-CtIP, shifting the HDR/NHEJ ratio by a factor of 14.9. Thus, our findings provide a simple and effective tool to promote precise gene modifications in mammalian cells.

We describe a protocol for high-efficiency germline transgenesis and sustained transgene expression in two important biomedical models, the mouse and the rat, using the Sleeping Beauty transposon system. The procedure is based on co-injection of synthetic mRNA encoding the SB100X hyperactive transposase together with circular plasmid DNA carrying a transgene construct flanked by binding sites for the transposase, into the pronuclei of fertilized oocytes. Upon translation of the transposase mRNA, enzyme-mediated excision of the transgene cassettes from the injected plasmids followed by permanent genomic insertion produces stable transgenic animals.Generation of a germline-transgenic founder animal by using this protocol takes approximately three months. Transposon-mediated transgenesis compares favorably in terms of both efficiency and reliable transgene expression to classic pronuclear microinjection, and offers comparable efficacies to lentiviral approaches, without limitations on vector design, issues of transgene silencing, and the toxicity and biosafety concerns of working with viral vectors. Transpositionmediated gene delivery can easily be implemented by any laboratory, thereby providing an attractive method to genetically modify animals for biomedical and biotechnological purposes.

2The pig has emerged as an important large animal model in biomedical and pharmaceutical research.We describe a protocol for high-efficiency germline transgenesis and sustained transgene expression in pigs by using the Sleeping Beauty transposon system. The protocol is based on co-injection of a plasmid encoding the SB100X hyperactive transposase together with a second plasmid carrying a transgene flanked by binding sites for the transposase, into the cytoplasm of porcine zygotes. The transposase mediates excision of the transgene cassette from the plasmid vector and its permanent insertion into the genome to produce stable transgenic animals. This method compares favorably in terms of both efficiency and reliable transgene expression to classic pronuclear microinjection or somatic cell nuclear transfer, and offers comparable efficacies to lentiviral approaches, without limitations on vector design, issues of transgene silencing as well as the toxicity and biosafety concerns of working with viral vectors. Microinjection of the vectors into zygotes and transfer of the embryos to recipient animals can be performed in one day; generation of germline-transgenic lines by using this protocol takes approximately one year.