TABLE 1.
The characteristics of genome editing systems and their clinical application in cancer
| Genome editing systems | Composition | Target sequence | DNA repair way | Advantages | Disadvantages | Clinical application in cancer | ||
|---|---|---|---|---|---|---|---|---|
| Nucleases | Meganucleases | Each monomer can form αββαββα fold, with four‐stranded antiparallel β‐sheets | Intron/intein‐free sites | NHEJ/HDR | Fewer off‐targets | Difficult to construct | NA | |
| ZFNs | C2H2 zinc fingers and FokI C‐terminuse | Each Zinc finger recognizes three or four base pairs, generally 5′‐GNN‐3′ | GRm13Z40‐2 CTL modified by ZFN in recurrent malignant glioblastoma; ZFN‐603 and ZFN‐758 in human papillomavirus‐related malignant neoplasm | |||||
| TALENs | A non‐specific DNA‐cleavage domain of FokI and a DNA‐binding domain | Hypervariable residues NN, NI, HD and NG recognizing G, A, C, and T, respectively | T27, T512,TALEN‐HPV16 E6/E7, TALEN‐HPV18 E6/E7 in cervical intraepithelial neoplasia; UCART22, UCART123, UCARTCS1A in hematological malignancies | |||||
| CRISPR systems | CRISPR/Cas9 | Cas9 proteins,a specificity‐determining CRISPR RNA (crRNA), and an auxiliary trans‐activating RNA (tracrRNA) | 5′‐NGG‐3′ PAM | Easy to construct | Higher‐efficiency compared with TALEN and Cas12a | More off‐targets than Cas12a; p53 activation | CTX120 in multiple myeloma; anti‐mesothelin CAR‐T cells modified by CRISPR/Cas9 in solid tumors; CTX110 in B‐cell malignancies; UCART019 in CD19+ leukemia and lymphoma; PD‐1 knockout T cells modified by CRISPR/Cas9 in esophageal cancer | |
| CRISPR/Cas12a | Cas12a protein and crRNA | 5′‐TTN‐3′ PAM | Easy to construct; smaller molecular sizes; fewer off‐targets than CRISPR/Cas9 | Lower editing efficiency than Cas9 | NA | |||
| Novel gennome editing tools | Base editors | BE3: rat APOBEC1 and Cas9‐D10A nickase; ABE: tRNA specific adenosine deaminase and a Cas9 nickase | Different base editors need different PAM | Independent of NHEJ and HDR; not induce DSBs | Fewer off‐targets than HDR | – | Only four possible edits (C/T, G/A, A/G, and T/C) | NA |
| Prime editing | An engineered Cas9 (catalytically impaired Cas9 fused to a reverse transcriptase) and a pegRNA | 5′‐NGG‐3′ PAM | All 12 possible base‐to‐base conversions | Difficult to deliver due to large molecule size | ||||
| Transposon‐encoded CRISPR/Cas system | Tn7‐like transposase subunits and CRISPR effector | INTEGRATE: 5′‐CC‐3′ PAM; CAST: 5′‐GTN‐3′ PAM | Be able to apply for nonmitotic cells | Need more studies on human genome editing | ||||
Abbreviations: ZFNs, Zinc finger nucleases; TALEN, transcription activator‐like effector nuclease; PAM, protospacer adjacent motif; NHEJ, non‐homologous end joining; HDR, homology‐directed repair; CRISPR, clustered regularly interspaced short palindromic repeats; DSB, double strand breaks.