Abstract
On-target integration of large cassettes via homology-directed repair (HDR) has several applications. However, the HDR-mediated targeted knock-in suffered from low efficiency. In this study, we made several large plasmids (12.1-13.4 kb) which included the CRISPR/Cas9 system along with a puromycin transgene as part of the large DNA donor (5.3-7.1 kb insertion cassettes) and used them to evaluate their targeted integration efficiency into a transgenic murine embryonic fibroblast (MEF) cell line carrying a single copy of a Venus transgene. We established a detection assay by which HDR events could be discriminated from the error-prone non-homologous end-joining (NHEJ) events. Improving the plasmid quality could considerably leverage the cell toxicity impediment of large plasmids. The use of the TILD (targeted integration with linearized dsDNA) cassettes did not improve the HDR rate compared to the circular plasmids. However, the direct inclusion of nocodazole into the electroporation solution significantly improved the HDR rate. Also, simultaneous delivery of RNase HII and the donor plasmids into the electroporated cells considerably improved the HDR events. In conclusion, the results of this study showed that using cell synchronization reagents in the electroporation medium can efficiently induce HDR rate in the mammalian genome.