Abstract
Directly modulating the choice between homologous recombination (HR) and non-homologous end joining (NHEJ) - two independent pathways for repairing DNA double-strand breaks (DSBs) - has the potential to improve the efficiency of gene targeting by CRISPR/Cas9. Here, we have developed a rapid and easy-to-score screening approach for identifying small molecules that affect the choice between the two DSB repair pathways. Using this tool, we identified a small molecule, farrerol, that promotes HR but does not affect NHEJ. Further mechanistic studies indicate that farrerol functions through stimulating the recruitment of RAD51 to DSB sites. Importantly, we demonstrated that farrerol effectively promotes precise targeted integration in human cells, mouse cells and mouse embryos at multiple genomic loci. In addition, treating cells with farrerol did not have any obvious negative effect on genomic stability. Moreover, farrerol significantly improved the knock-in efficiency in blastocysts, and the subsequently generated knock-in mice retained the capacity for germline transmission.
Highlights
The ability to precisely edit genomes holds great promise for a wide range of applications in both biomedical research and the treatment of human genetic diseases (Su et al, 2016; Tebas et al, 2014)
Developed exogenous enzyme-based genome editing tools have greatly improved the efficiency of gene knock-out as the generated double-strand breaks (DSBs) can be repaired by the error-prone and cell cycleindependent non-homologous end joining (NHEJ) pathway, resulting in non-functional genes (Guo et al, 2018; Maeder and Gersbach, 2016)
Promoting homologous recombination (HR) directed repair or suppressing the NHEJ pathway have been explored as tools for improving the efficiency of SpCRISPR/Cas9-mediated gene targeting (Smirnikhina et al, 2019)
Summary
The ability to precisely edit genomes holds great promise for a wide range of applications in both biomedical research and the treatment of human genetic diseases (Su et al, 2016; Tebas et al, 2014). Suppressing NHEJ by blocking LIG4 activity with SCR7, or inhibiting DNA-PKcs kinase activity with NU7441 or NU7026, has been shown to improve the precise targeting efficiency of SpCRISPR/Cas (Chu et al, 2015; Robert et al, 2015; Zhang et al, 2017). Stimulating the HR factor, RAD51, with RS-1 improved SpCRISPR/Cas editing efficiency (Jayathilaka et al, 2008) Both inhibiting NHEJ and stimulating the activity of the recombinase involved in HR are potentially harmful to the maintenance of genome integrity (Chen et al, 2008; Vartak and Raghavan, 2015). Farrerol significantly improved the efficiency of SpCRISPR/ Cas9-mediated knock-in in blastocysts, and the generated knock-in founder mice retained the capability for germline transmission
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