Strategies for In Vivo Genome Editing in Nondividing Cells

Programmable nucleases, including zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9), have enhanced our ability to edit genomes by the sequence-specific generation of double-strand breaks (DSBs) with subsequent homology-directed repair (HDR) of the DSB. However, the efficiency of the HDR pathway is limited in nondividing cells, which encompass most of the cells in the body. Therefore, the HDR-mediated genome-editing approach has limited in vivo applicability. Here, we discuss a mutation type-oriented viewpoint of strategies devised over the past few years to circumvent this problem, along with their possible applications and limitations. To bypass the problem of HDR inefficiency in nondividing cells, HDR-independent strategies are being developed to efficiently manipulate the genomes of these cells. These strategies can be categorised into two main groups based on whether a donor template is required. The type of mutation to be targeted dictates the choice of editing approach. Several novel approaches, including homology-independent targeted integration (HITI), obligate ligation-gated recombination (ObLiGaRe), precise integration into target chromosome (PITCH), recombinase Cas9 (RecCas9), homology-mediated end-joining (HMEJ), and base editing, have been described, some of which have been shown to be efficient both in vivo and in nondividing cells. However, the in vivo editing efficiency, possible off-targets, or creation of translocations, need to be further evaluated.