Application of genome-editing systems to enhance available pig resources for agriculture and biomedicine.

Traditionally, genetic engineering in the pig was a challenging task. Genetic engineering of somatic cells followed by somatic cell nuclear transfer (SCNT) could produce genetically engineered (GE) pigs carrying site-specific modifications. However, due to difficulties in engineering the genome of somatic cells and developmental defects associated with SCNT, a limited number of GE pig models were reported. Recent developments in genome-editing tools, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) 9 system, have markedly changed the effort and time required to produce GE pig models. The frequency of genetic engineering in somatic cells is now practical. In addition, SCNT is no longer essential in producing GE pigs carrying site-specific modifications, because direct injection of genome-editing systems into developing embryos introduces targeted modifications. To date, the CRISPR/Cas9 system is the most convenient, cost-effective, timely and commonly used genome-editing technology. Several applicable biomedical and agricultural pig models have been generated using the CRISPR/Cas9 system. Although the efficiency of genetic engineering has been markedly enhanced with the use of genome-editing systems, improvements are still needed to optimally use the emerging technology. Current and future advances in genome-editing strategies will have a monumental effect on pig models used in agriculture and biomedicine.