Traditional CRISPR-Cas9 nucleases induce blunt double-strand breaks (DSBs) that rely on uncontrolled cellular repair pathways (NHEJ/HDR), frequently causing large chromosomal deletions, retrotransposon insertions, and p53-mediated apoptosis. Base Editing and Prime Editing represent second-generation precision genomic correction technologies capable of installing targeted nucleotide transitions, transversions, insertions, and deletions without cutting both strands of the DNA double helix.
1. Base Editing: Cytosine & Adenine Deamination Chemistry
Developed by David Liu's laboratory, Base Editors fuse a catalytically impaired Cas9 nickase (nCas9-D10A) with an engineered nucleobase deaminase enzyme:
- Cytosine Base Editors (CBEs): Couple a cytidine deaminase (e.g., APOBEC1 or evoAPOBEC1) with a uracil glycosylase inhibitor (UGI). The enzyme deaminates cytidine (C) to uridine (U) within an editing window (~4–8 nt). Subsequent DNA replication or repair resolves the U:G intermediate into a permanent C•G to T•A transition.
- Adenine Base Editors (ABEs): Utilize an engineered laboratory-evolved deoxyadenosine deaminase (TadA-8e) to convert adenine (A) to inosine (I). Cellular polymerases read inosine as guanosine, installing precise A•T to G•C transitions.
2. Prime Editing: The Search-and-Replace Molecular Word Processor
While base editors are restricted to four transition mutations (C→T, T→C, A→G, G→A) and suffer from "bystander editing" when multiple target bases reside within the catalytic window, Prime Editing enables all 12 possible base-to-base conversions, as well as precise insertions (up to ~100 bp) and deletions without donor DNA templates.
A Prime Editor (such as PEmax or twinPE) consists of an engineered Cas9 nickase fused to an engineered Moloney Murine Leukemia Virus (M-MLV) Reverse Transcriptase, paired with a specialized **prime editing guide RNA (pegRNA)**:
- Spacer Sequence: Directs the Cas9 nickase to the target genomic locus.
- Primer Binding Site (PBS): Hybridizes to the single-stranded 3' DNA flap generated by the nicked non-target strand.
- Reverse Transcriptase (RT) Template: Encodes the desired genetic edit, which the fused reverse transcriptase directly writes into the genomic flap.
- Flap Resolution: Cellular 5' exonucleases degrade the unedited 5' flap, allowing the newly synthesized 3' edited flap to ligate into the chromosome.
| Feature | Base Editing (CBE / ABE) | Prime Editing (PE2 / PE3 / PEmax) |
|---|---|---|
| Permitted Mutations | Transitions only ($C ightarrow T$, $A ightarrow G$) | All 12 transitions, transversions, insertions, and deletions |
| Bystander Edits | Possible if multiple C/A bases are in editing window | Zero bystander edits (Single-nucleotide precision) |
| Editing Efficiency | High (typically 40% – 85%) | Moderate to High (20% – 70% with PEmax/twinPE) |
| Construct Size | ~4.5 – 5.2 kb (Compatible with dual-AAV) | ~6.3 kb (Requires dual split-intein AAV or LNP mRNA) |