Table 1 Modified Cas9 approaches to increasing the frequency of homology-dependent repair outcomes

Cas9-i53 (hypothetical)

Expression of i53 alone strongly suppressed the recruitment and function of 53BP1 at DSBs

Cas9-i53 fusion has not been reported, however expression of i53 in cell lines induces BRCA1 accumulation at DSB sites similar to 53BP1 knockout cells

Canny et al. [28]

Cas9-DN1S

DN1S inhibits the recruitment of key NHEJ proteins such as RIF1

two- to threefold increase in HDR and three- to fourfold reduction in NHEJ

Jayavaradhan et al. [24]

Cas9-yRAD52

yRAD52 can stimulate strand invasion and homologous recombination at a DSB site

Threefold HDR increase in human cells

Threefold HDR increase in chicken cells

Shao et al. [44], Wang et al. [45]

Cas9-HE (CtIP)

CtIP assists in end-resection and promotes HDR

Twofold HDR increase in human cell lines, human iPSCs, and rat zygotes

Charpentier et al. [49]

Cas9-UL12

UL12 recruits endogenous MRN complex which resects DSBs and promotes HDR

Twofold HDR increase in HEK293 cells

Reuven et al. [51]

miCas9 (cas9-Brex27)

Brex27 recruits RAD51, a pro-HDR protein that searches for homologous sequences and facilitates D-loop formation at DSB sites

Up to threefold HDR increase depending on donor size in human cell lines

Ma et al. [56]

REDIT

Exploits bacteriophage SSAPs to insert kilobases of sequence at DSB sites

Threefold increase in HDR in human AECs and iPSCs

Wang et al. [61]

Cas9-RecA

RecA is able to promote single-strand annealing pathway at DSBs in between repetitive sequences

2.5-fold increase in SSA in HEK293T cells

Lin et al. [66]

Cas9-hGem

Restricts Cas9 presence and activity to late S and G2 when HDR occurs most frequently

1.87-fold HDR increase in HEK293T cells

Gutschner et al. [79]

Cas9-PCV

Ensures proximity of donor DNA at DSB site through PCV-ssDNA covalent bonding

Up to 30-fold increase in HDR correcting frameshift mutations with short ssDNA donor

Aird et al. [84]