Recent advances in targeted genome engineering in mammalian systems.

Targeted genome engineering enables researchers to disrupt, insert, or replace a genomic sequence precisely at a predetermined locus. One well-established technology to edit a mammalian genome is known as gene targeting, which is based on the homologous recombination (HR) mechanism. However, the low HR frequency in mammalian cells (except for mice) prevents its wide application. To address this limitation, a custom-designed nuclease is used to introduce a site-specific DNA double-strand break (DSB) on the chromosome and the subsequent repair of the DSB by the HR mechanism or the non-homologous end joining mechanism results in efficient targeted genome modifications. Engineered homing endonucleases (also called meganucleases), zinc finger nucleases, and transcription activator-like effector nucleases represent the three major classes of custom-designed nucleases that have been successfully applied in many different organisms for targeted genome engineering. This article reviews the recent developments of these genome engineering tools and highlights a few representative applications in mammalian systems. Recent advances in gene delivery strategies of these custom-designed nucleases are also briefly discussed.