Applications of genome editing in farm animals

Molecular scissors including zinc finger nucleases, transcription activator-like endonucleases, and clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) systems are capable of cutting the nuclear DNA precisely at a predetermined position. The CRISPR/Cas9 system is an RNA-based bacterial defense mechanism that recognizes the target sequence by specific base pair hybridization of an associated guide RNA, which has a complementary region to the 20 base targets. It mediates genetic alterations by enhancing the DNA mutation via induction of double-strand breaks at a predetermined genomic site. The design and construction of the CRISPR/Cas9 molecules is much simpler than that of the other molecular scissors, and allows multiplexed genomic modifications. Presently, CRISPR/Cas9 has been successfully employed in a broad range of organisms, which improves the understanding of complex physiological systems such as reproduction, immunology, disease resistance, and development. On top of this, the CRISPR/Cas9 system has dramatically improved the generation of transgenic and gene-edited animals, including large animal models for human diseases, and genetically modified livestock species for commercial disposition. This chapter provides an update on the use of CRISPR/Cas9 systems to modify the genome of farm animals, summarizes current knowledge on the underlying mechanism, and discusses new opportunities for generating genetically modified farm animals.