Prime editing

01Overview and value chain

Markers: [EC: EU NGT Regulation & GMO Directive 2001/18/EC | OECD: Genomic medicine & biotechnology | Regulator: FDA (USA), EMA (EU), NMPA (China)]

Prime editing is the “search-and-replace” form of genome editing: a chimeric protein fusing a Cas9 nickase (nCas9) to a reverse transcriptase is guided by an extended prime-editing guide RNA (pegRNA) that both targets the locus and carries the edit as a template, so the new sequence is written directly into the gene. Unlike classic CRISPR it makes no double-strand break and needs no external donor DNA, and unlike base editors — which can only do C-to-T and A-to-G swaps — it can install all 12 base substitutions plus small insertions and deletions, in principle correcting the great majority of known disease-causing mutations. The latest editors (PE2, PE3, PE5, PE6 and the twin-prime systems) pair an engineered, thermostable reverse transcriptase with refined pegRNA scaffolds, and twin prime editing joins two pegRNAs to insert or delete thousands of base pairs. The field is now clinical: Prime Medicine’s PM-359 for chronic granulomatous disease is the flagship therapeutic programme, and the editing machinery is delivered as mRNA plus pegRNA inside lipid nanoparticles or recombinant AAV vectors. The open challenges are editing efficiency in vivo, immune responses to the editor, and pegRNA integrity during long-RNA synthesis.

The key directions of prime editing are:

1. All-12-substitution editing: writing any transition or transversion, and small indels, at a chosen locus without a double-strand break or donor template.
2. Engineered reverse transcriptases: thermostable, processive RT mutants (e.g. the 14-mutation M-MLV in PE6) that raise the efficiency of the new-strand synthesis inside the cell.
3. Twin prime editing: pairing two pegRNAs on opposite strands to insert or delete kilobase-scale fragments, extending reach beyond point edits.
4. In-vivo delivery: packaging the editor mRNA and pegRNA in lipid nanoparticles or AAV to reach liver, eye and the central nervous system in the body.

Sectoral value chain

[pegRNA design in silico] ──> [Long pegRNA synthesis & QC] ──> [Editor mRNA + LNP formulation]
                                             │
                                  (delivery + reverse transcription)
                                             │
                                             ▼
[Therapeutic dosing] <─── [Edit verification by sequencing] <─────┘

Value chain levels

Cross-cutting technologies of the sector:

• pegRNA scaffold engineering: the 3’ extension carrying the primer-binding site and RT template, plus 2’-OMe and phosphorothioate end-modifications that protect the long RNA from nucleases.
• Thermostable reverse transcriptases: engineered M-MLV RT variants (PE6 carries 14 mutations) that retain activity at higher temperature, raising the processivity and yield of the in-cell writing step.
• In-vivo delivery systems: lipid nanoparticles for liver and ex vivo editing, and recombinant AAV serotypes (developed at Zurich and elsewhere) for tissue-restricted delivery to the CNS and eye.

02US

The United States leads prime-editing commercialisation, anchored by the patent estate and the first clinical programme.

Prime Medicine, Broad Institute, FDA engagement

• Prime Medicine and PM-359: Prime Medicine (Cambridge, MA) is the prime-editing pure-play, with PM-359 for chronic granulomatous disease as its lead clinical programme and a pipeline across metabolic and ophthalmic diseases.
• Broad Institute IP and editor engineering: the Broad Institute of MIT and Harvard is the birthplace of prime editing and the PE2/PE3/PE5/twinPE system family, controlling the core patent portfolio that licensees build on.
• FDA engagement: the FDA works with developers on standards for pegRNA loop stability (hairpin risks) and reverse-transcriptase off-target activity as programmes approach the clinic.

03CN

China builds proprietary prime-editing platforms for agriculture and therapy, with state-backed academic groups and a maturing genome-editing biotech sector.

ePE platforms, EdiGene, NMPA orphan designations

• Enhanced prime editing (ePE): researchers at the Chinese Academy of Sciences developed ePE systems that raise reverse-transcription efficiency in plant and animal cells, applied to disease-resistant rice and other crops.
• EdiGene and the clinic: EdiGene runs genome-editing therapeutic programmes (including ET-01 for beta-thalassaemia) and is extending its platform toward prime-editing indications.
• NMPA pathway: the NMPA grants orphan designations to early Chinese prime-editing candidates, for example for Duchenne muscular dystrophy, and subsidises the trials.

04EU

European academic institutes lead on in-vivo delivery of prime editors into difficult tissues, under strict EMA oversight of long-RNA quality.

Zurich AAV delivery, EMA RNA-quality oversight

• University of Zurich delivery R&D: Swiss and neighbouring institutes pioneer AAV-based delivery of prime editors for in-vivo correction of inherited metabolic liver defects.
• EMA quality oversight: the EMA enforces strict sterility and identity standards for long synthetic RNA, requiring high-resolution chromatography to exclude truncated pegRNA isoforms.
• NGT context: EU gene-edited therapies are regulated under the GMO framework and the new genomic-techniques regulation, shaping the path for prime-editing clinical use in Europe.

05Leading companies and research institutes

06Tech stack and innovations

The prime-editing stack joins a molecular writer, a long-RNA supply chain and a delivery vehicle into a clinically dosable drug.

1. Prime-editor molecular stack:
   • The editor is a Cas9 nickase fused to an engineered reverse transcriptase; a pegRNA targets the locus and supplies the edit as a template, so the new sequence is reverse-transcribed directly into the nicked strand — no double-strand break, no donor DNA.
   • System generations PE2, PE3, PE5 and PE6 successively raise efficiency; PE6 pairs a 14-mutation thermostable M-MLV reverse transcriptase with optimised scaffolds, and twin prime editing joins two pegRNAs for kilobase-scale insertion or deletion.
2. Long modified-RNA synthesis and QC:
   • pegRNAs exceed 120 nucleotides and carry 2’-OMe and phosphorothioate end-modifications, synthesised on high-throughput solid-phase platforms and cleaved from the support with methylamine.
   • Two-stage purification — anion-exchange chromatography to resolve by length, then reverse-phase HPLC to remove isoforms — pushes purity above 95%, with mass confirmed by MALDI-TOF and integrity by capillary gel electrophoresis.
3. Delivery vehicles:
   • Lipid nanoparticles co-encapsulate the editor mRNA and pegRNA by microfluidic mixing, dosed to liver and used ex vivo.
   • Recombinant AAV serotypes (developed at the University of Zurich and peer institutes) enable tissue-restricted in-vivo delivery to the central nervous system and eye.

07Value chains and production pipelines

GMP pipeline for a therapeutic pegRNA + editor LNP drug product (GMP / sterile fill)

┌───────────────────────────┐      ┌───────────────────────────┐
│ 1. Solid-phase synthesis  │ ───> │ 2. Cleavage & deprotection│
│    of long pegRNA (>120nt)│      │    from the support        │
└───────────────────────────┘      └───────────────────────────┘
                                                  │
                                                  ▼
┌───────────────────────────┐      ┌───────────────────────────┐
│ 4. QC: MALDI-TOF mass &  │ <─── │ 3. Two-stage purification │
│    capillary electrophoresis│    │    (AEX + RP-HPLC)         │
└───────────────────────────┘      └───────────────────────────┘
              │
              ▼
┌───────────────────────────┐      ┌───────────────────────────┐
│ 5. Co-formulation with    │ ───> │ 6. Sterile filtration &   │
│    editor mRNA in LNPs    │      │    cryogenic fill-finish   │
└───────────────────────────┘      └───────────────────────────┘

Stage 1: Solid-phase synthesis of long pegRNA

The 120-150 nucleotide pegRNA, with 2’-OMe and phosphorothioate end-modifications, is built on a high-throughput solid-phase synthesiser base by base.

Stage 2: Cleavage and deprotection

The RNA is cleaved from the support and its protecting groups are removed with methylamine at elevated temperature, releasing the crude full-length pegRNA.

Stage 3: Two-stage purification

Anion-exchange chromatography resolves the product by length (removing truncated n and n-1 chains), and reverse-phase HPLC polishes away sequence isoforms, reaching above 95% purity.

Stage 4: QC by mass and electrophoresis

Molecular weight is confirmed by MALDI-TOF mass spectrometry and chain integrity and length-uniformity by capillary gel electrophoresis, releasing the GMP-grade pegRNA.

Stage 5: Co-formulation with editor mRNA in LNPs

The purified pegRNA and the editor-encoding mRNA are co-encapsulated into lipid nanoparticles by microfluidic mixing, forming the active drug substance.

Stage 6: Sterile filtration and cryogenic fill-finish

The LNP product is sterile-filtered, filled into vials and snap-frozen in liquid-nitrogen vapour to preserve the RNA–lipid complex, yielding the cryostored clinical lot ready for dosing.