Overview
Prime editing is a genome‑editing technology that enables precise insertion, deletion, or substitution of nucleotides in the DNA of living cells without creating double‑strand breaks (DSBs) or requiring donor DNA templates. First described in 2019 by researchers led by David Liu, the method expands on CRISPR/Cas systems by coupling a catalytically impaired Cas nuclease with an engineered reverse transcriptase and a specially designed prime editing guide RNA (pegRNA).
Mechanism of Action
- PegRNA design – The pegRNA contains three functional regions: a spacer sequence that directs the complex to a specific genomic locus, a primer‑binding site (PBS) that anneals to the nicked DNA strand, and a reverse‑transcriptase (RT) template encoding the desired edit.
- Targeting and nicking – The fusion protein consists of a Cas9 nickase (commonly an H840A mutant) that introduces a single‑strand break (“nick”) on the non‑target DNA strand adjacent to the editing site.
- Reverse transcription – The reverse transcriptase uses the RT template within the pegRNA as a template to synthesize a DNA flap containing the edited sequence, which is incorporated into the genome during cellular repair processes.
- Flap resolution and ligation – Cellular DNA‑repair pathways resolve the heteroduplex structure, sealing the edited strand and completing the modification without generating a DSB.
Core Components
- Prime editing guide RNA (pegRNA) – Provides target specificity and supplies the new genetic information.
- Cas9 nickase – A Cas9 variant that nicks only one DNA strand, preventing full double‑strand cleavage.
- Engineered reverse transcriptase – Fused to the Cas9 nickase; synthesizes DNA from the pegRNA template.
Variants and Extensions
- Cas12‑based prime editors employ a Cas12a (Cpf1) nuclease instead of Cas9, with analogous pegRNA designs adapted for the distinct PAM requirements of Cas12a.
- Accessory proteins such as small RNA‑binding proteins (e.g., La), MCP (MS2 coat protein) fusions, or engineered circular RNAs have been incorporated to improve editing efficiency and stability.
- Split prime editors separate the Cas9‑RT fusion into two polypeptides that reconstitute in cells, facilitating delivery of larger constructs via viral vectors.
Applications
Prime editing has been applied in a wide range of model systems, including mammalian cell lines, mouse embryos, and various plant species, to introduce precise point mutations, small insertions, or deletions. The technology holds promise for therapeutic genome correction because it can target up to ~90 % of known pathogenic human variants. In 2024, the prime‑editing–based gene therapy PM359 entered its first clinical trial in humans, marking the inaugural use of a prime editor in a clinical setting.
Advantages Over Earlier Methods
- No double‑strand breaks, reducing the risk of large chromosomal rearrangements and indels associated with conventional CRISPR/Cas9 editing.
- Template‑free: unlike homology‑directed repair, it does not require exogenous donor DNA.
- Versatility: capable of all twelve possible base substitutions, as well as small insertions and deletions, in a single platform.
Limitations and Challenges
While prime editing improves precision, its efficiency varies across cell types and target loci. Delivery of the relatively large Cas9‑RT‑pegRNA complex remains a hurdle for in vivo applications, prompting ongoing engineering of compact editors and optimized delivery vectors. Off‑target activity, though generally lower than that of nuclease‑based CRISPR, still requires thorough assessment before therapeutic use.
Historical Development
- 2019: Initial proof‑of‑concept published, demonstrating editing of multiple loci in human cells with efficiencies up to ~50 %.
- 2021–2022: Engineering of “PEmax” and other optimized architectures increased efficiency and broadened target scope.
- 2023–2024: Demonstrations in animal disease models and the launch of PM359 clinical trial highlighted translational potential.
References
Information synthesized from peer‑reviewed literature and reputable encyclopedia entries (e.g., Wikipedia article “Prime editing”, 2024).