Genome editing systems in novel therapies.

Genome editing is the process in which DNA sequences at precise genomic locations are modified. In the past three decades, genome editing by homologous recombination has been successfully performed in mouse for generating genetic models. The low efficiency of this process in human cells, however, had prevented its clinical application until the recent advancements in designer endonuclease technologies. The significantly improved genome editing efficiencies aided by ZFN, TALEN, and CRISPR systems provide unprecedented opportunities not only for biomedical research, but also for developing novel therapies. Applications based on these genome editing tools to disrupt deleterious genes, correct genetic mutations, deliver functional transgenes more effectively or even modify the epigenetic landscape are being actively investigated for gene and cell therapy purposes. Encouraging results have been obtained in limited clinical trials in the past two years. While most of the applications are still in proof-of-principle or preclinical development stages, it is anticipated that the coming years will see increasing clinical success in novel therapies based on the modern genome editing technologies. It should be noted that critical issues still remain before the technologies can be translated into more reliable therapies. These key issues include off-target evaluation, establishing appropriate preclinical models and improving the currently low efficiency of homology-based precise gene replacement. In this review we discuss the preclinical and clinical studies aiming at translating the genome editing technologies as well as the issues that are important for more successful translation.