DNA Nucleases and their Use in Livestock Production
DNA nucleases, including zinc-finger nucleases (ZFN), transcription activator-like endonucleases (TALENS), and meganucleases, possess long recognition sites and cutting domains and are thus capable of cutting DNA in a very specific manner. These molecular scissors mediate targeted genetic alterations by enhancing the DNA mutation rate via induction of double-strand breaks at a predetermined genomic site. Compared to conventional homologous recombination-based gene targeting, DNA nucleases can increase the targeting rate up to 10,000-fold, and gene disruption via mutagenic DNA repair is stimulated at a similar frequency. The successful application of different DNA nucleases has been demonstrated in a multitude of organisms, including insects, amphibians, plants, nematodes, and mammals, including livestock animals. Recently, another novel class of molecular scissors was described that uses short RNA sequences to target a specific genomic site (Fig. 7.1). The CRISPR/CAS9 originates from a bacterial defense mechanism and can be programmed to target almost any site within a genome. The ease and low costs to create very specific genetic alterations by DNA nucleases have revolutionized the production of genetically modified livestock. Current results indicate that DNA nucleases can be successfully employed in a broad range of organisms which renders them useful for improving the understanding of complex physiological systems, producing genetically modified animals, including creating large animal models for human diseases and creating specific cell lines. Genetic modifications could also increase animal welfare by making dehorning and sexing obsolete or by making farm animals resistant/resilient against specific pathogens. Livestock with a desired phenotype or trait can now be produced with previously unknown precision and ease and within a very short time frame considered to be impossible before their advent. This chapter provides an update on DNA nucleases and their underlying mechanism and focuses on their use in livestock production. It has to be kept in mind that, at the time of writing this chapter, none of the genetically modified livestock has entered the food chain or had been used for the production of livestock-derived products.