Figure 1. DNA repair pathways involved in CRISPR-Cas nucleases mediated genome editing in mammalian cells.

CRISPR-Cas nuclease-induced DNA DSBs are repaired by either c-NHEJ or homology-based repair (HBR). The recruitment of 53BP1 inhibits end resection at the DSB site, promoting DSB repair via the c-NHEJ pathway. Without exogenous DNA donors, the two ends of the DSB are ligated together precisely or imprecisely with 1–4 bp small insertion or deletion (indel) through the c-NHEJ pathway. In the presence of a double-stranded DNA (dsDNA) donor without homology arms, the dsDNA donor can be inserted into the DSB site via the c-NHEJ pathway (HITI). In addition, the two ends of the DSB site may undergo initial phase end resection by RBBP8 (or named CtIP) and MRN, generating short 3′ overhang. Supplied with a DNA donor with homology arms, the 3′ overhang will prime DNA repair through the high-fidelity homology-based repair pathway. Supplied with a dsDNA donor with short homology arms (5–40 bp), the DSB site could be repaired via the MMEJ pathway. Furthermore, the short 3′ overhang may undergo second phase end resection, generating a longer 3′ overhang. In the presence of dsDNA donor with medium homology arms (>200 bp), the DSB site could be repaired via the SSA pathway. Supplied with a dsDNA donor with long homology arms (≥400 bp), the DSB site could be repaired via the HR pathway. In the presence of ssDNA donor with short homology arms (≥30 nt), the DSB site could be repaired via the SSTR pathway. Some major proteins involved in each pathway are shown in the figure. The direction of the arrow represents the 5′ to 3′ direction. Deletion, black cross. Inserted fragment, colored line. c-NHEJ, classical nonhomologous end joining. HITI, homology-independent targeted integration. MRN, MRE11-RAD50-NBS1 complex. HA, homology arm. SSTR, single-stranded templated repair. HR, homologous recombination. SSA, single-stranded annealing. MMEJ, microhomology-mediated end joining