Background Due to post-cleavage residence of the Cas9-sgRNA complex at its target, Cas9-induced DNA double-strand breaks (DSBs) have to be exposed to engage DSB repair pathways. Target interaction of Cas9-sgRNA determines its target binding affinity and modulates its post-cleavage target residence duration and exposure of Cas9-induced DSBs. This exposure, via different mechanisms, may initiate variable DNA damage responses, influencing DSB repair pathway choices and contributing to mutational heterogeneity in genome editing. However, this regulation of DSB repair pathway choices is poorly understood. Results In repair of Cas9-induced DSBs, repair pathway choices vary widely at different target sites and classical nonhomologous end joining (c-NHEJ) is not even engaged at some sites. In mouse embryonic stem cells, weakening the target interaction of Cas9-sgRNA promotes bias towards c-NHEJ and increases target dissociation and reduces target residence of Cas9-sgRNAs in vitro. As an important strategy for enhancing homology-directed repair, inactivation of c-NHEJ aggravates off-target activities of Cas9-sgRNA due to its weak interaction with off-target sites. By dislodging Cas9-sgRNA from its cleaved targets, DNA replication alters DSB end configurations and suppresses c-NHEJ in favor of other repair pathways, whereas transcription has little effect on c-NHEJ engagement. Dissociation of Cas9-sgRNA from its cleaved target by DNA replication may generate three-ended DSBs, resulting in palindromic fusion of sister chromatids, a potential source for CRISPR/Cas9-induced on-target chromosomal rearrangements. Conclusions Target residence of Cas9-sgRNA modulates DSB repair pathway choices likely through varying dissociation of Cas9-sgRNA from cleaved DNA, thus widening on-target and off-target mutational spectra in CRISPR/Cas9 genome editing.

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Background Due to post-cleavage residence of the Cas9-sgRNA complex at its target, Cas9-induced DNA double strand breaks (DSBs) have to be exposed in order to engage DSB repair pathways. Target interaction of Cas9-sgRNA determines its target-binding affinity and modulates its post-cleavage target-residence duration and exposure of Cas9-induced DSBs. This exposure by different mechanisms may initiate variable cellular DNA damage response, thus influencing DSB repair pathway choices and contributing to mutational heterogeneity in genome editing. However, this regulation of DSB repair pathway choices is poorly understood. Results In repair of Cas9-induced DSBs, repair pathway choices vary widely at different target sites and classical non-homologous end joining (c-NHEJ) is not even engaged at some sites. Weakening target interaction of Cas9-sgRNA promotes bias towards c-NHEJ, which is intrinsically accurate for Cas9-induced DSBs. As an important strategy for enhancing homology-directed repair, inactivation of c-NHEJ however aggravates off-target activities of Cas9-sgRNA due to its weak interaction with off-target sites. By dislodging Cas9-sgRNA from its cleaved targets, DNA replication alters DSB end configurations and suppresses c-NHEJ in favor of other repair pathways whereas transcription has little effect on DSB repair pathway choices. Dissociation of Cas9-sgRNA from its cleaved target by DNA replication may generate three-ended DSBs, resulting in palindromic fusion of sister chromatids, a potential source for CRISPR/Cas9-induced on-target chromosomal rearrangements. Conclusions Target interaction of Cas9-sgRNA modulates DSB repair pathway choices likely through varying dissociation of Cas9-sgRNA from cleaved DNA, thus widening on-target and off-target mutational spectra in CRISPR/Cas9 genome editing.

Given its different target-binding affinity and varying residence time after DNA cleavage, the Cas9-sgRNA complex that remains bound to cleaved DNA may influence DNA double strand break (DSB) repair pathway choices, contributing to highly heterogeneous mutations in genome editing. Here, we found that DSB repair pathway choices vary significantly at different sites for Cas9-induced DSBs. Reduced target-binding affinity of Cas9-sgRNA promotes a bias toward classical NHEJ (c-NHEJ), but inactivation of c-NHEJ aggravates off-target activities. By dislodging Cas9-sgRNA from the cleaved target, local DNA replication suppresses c-NHEJ in favor of other DSB repair pathways by altering DSB configuration and repair timing. It also results in sister chromatid fusion, which may promote chromosomal breakage-fusion-bridge cycles. This suggests that varying Cas9-sgRNA target-binding affinity or residence duration modulate DSB repair pathway choices for Cas9-induced DSBs and widen the mutational spectra in CRISPR/Cas9 genome editing.