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categories: ["genetics"]categories: ["genetics"]tags: ["crispr", "genome editing", "gene therapy", "reverse transcriptase", "biotechnology"]tags: ["crispr", "genome editing", "gene therapy", "reverse transcriptase", "biotechnology"]summary: "A genome editing method in which a nicking Cas9 fused to a reverse transcriptase writes a new DNA sequence specified by an extended guide RNA."summary: "A genome editing method in which a nicking Cas9 fused to a reverse transcriptase writes a new DNA sequence specified by an extended guide RNA."updated: "2026-08-23"updated: "2026-10-29"humanEvidence: "Human use is one ex vivo trial in chronic granulomatous disease, which reported restored immune-cell function in a treated patient in 2025; in vivo correction has been shown only in mice."humanEvidence: "Human use is one ex vivo trial in chronic granulomatous disease, which reported restored immune-cell function in a treated patient in 2025; in vivo correction has been shown only in mice."access: "Not obtainable as a therapy: no prime editing product is approved anywhere as of 2026 and human use is confined to early-phase trials; the reagents are sold for research."access: "Not obtainable as a therapy: no prime editing product is approved anywhere as of 2026 and human use is confined to early-phase trials; the reagents are sold for research."reversibility: "irreversible"reversibility: "irreversible"lines 18–24 → 18–243 unchanged lines not shown
{{ "caption": "Genome editing platform", "caption": "Genome editing platform", "rows": [ "rows": [ { "label": "Type", "value": "Nickase–reverse transcriptase fusion" }, { "label": "Type", "value": "Nickase-reverse transcriptase fusion" }, { "label": "First described", "value": "2019" }, { "label": "First described", "value": "2019" }, { "label": "Developed by", "value": "David Liu laboratory" }, { "label": "Developed by", "value": "David Liu laboratory" }, { "label": "Guide molecule", "value": "pegRNA" }, { "label": "Guide molecule", "value": "pegRNA" },lines 29–35 → 29–354 unchanged lines not shown
}}`````` **Prime editing** installs a chosen DNA sequence at a chosen site by nicking one strand of the double helix and then writing new bases onto the exposed end from an RNA template carried by the guide itself. Described in 2019, it removes two constraints at once: it needs no double-strand break, and unlike [[base-editing]] it is not restricted to particular chemical conversions.[^anzalone2019] Its practical difficulty is the reverse of that generality — the machine is large, the design space for each target is wide, and efficiency varies from excellent to negligible depending on the site.**Prime editing** installs a chosen DNA sequence at a chosen site by nicking one strand of the double helix and then writing new bases onto the exposed end from an RNA template carried by the guide itself. Described in 2019, it removes two constraints at once: it needs no double-strand break, and unlike [[base-editing]] it is not restricted to particular chemical conversions.[^anzalone2019] Its practical difficulty is the reverse of that generality: the machine is large, the design space for each target is wide, and efficiency varies from excellent to negligible depending on the site. ## How it works## How it works lines 50–56 → 50–5614 unchanged lines not shown
{ "year": "2019", "title": "Prime editing described", "text": "Anzalone and colleagues report search-and-replace editing without double-strand breaks or donor DNA, demonstrating all twelve base substitutions and small insertions and deletions in human cells." }, { "year": "2019", "title": "Prime editing described", "text": "Anzalone and colleagues report search-and-replace editing without double-strand breaks or donor DNA, demonstrating all twelve base substitutions and small insertions and deletions in human cells." }, { "year": "2021", "title": "Mismatch repair identified as the brake", "text": "Transient suppression of mismatch repair substantially raises editing efficiency, defining the PE4 and PE5 systems." }, { "year": "2021", "title": "Mismatch repair identified as the brake", "text": "Transient suppression of mismatch repair substantially raises editing efficiency, defining the PE4 and PE5 systems." }, { "year": "2022", "title": "Guide RNA stabilised", "text": "Structured RNA motifs added to the pegRNA 3' end protect it from degradation, and paired-pegRNA designs enable larger replacements and integrase-mediated insertions." }, { "year": "2022", "title": "Guide RNA stabilised", "text": "Structured RNA motifs added to the pegRNA 3' end protect it from degradation, and paired-pegRNA designs enable larger replacements and integrase-mediated insertions." }, { "year": "2023–2024", "title": "Compact and stabilised editors", "text": "Evolved reverse transcriptases shrink the construct, and fusion of an RNA-binding protein that recognises the pegRNA improves performance across cell types." }, { "year": "2023-2024", "title": "Compact and stabilised editors", "text": "Evolved reverse transcriptases shrink the construct, and fusion of an RNA-binding protein that recognises the pegRNA improves performance across cell types." }, { "year": "2025", "title": "First clinical data", "text": "Initial results are reported from the first prime editing trial, an ex vivo therapy for chronic granulomatous disease." } { "year": "2025", "title": "First clinical data", "text": "Initial results are reported from the first prime editing trial, an ex vivo therapy for chronic granulomatous disease." }]]``````lines 59–76 → 59–752 unchanged lines not shown
The original report defined a numbered progression that the field still uses. PE1 paired the nickase with an unmodified viral reverse transcriptase and worked poorly. PE2 used an engineered enzyme and worked well enough to be usable. PE3 added the second nick. Later systems addressed mismatch repair, pegRNA stability, and the size of the reverse transcriptase itself, with each generation typically improving efficiency several-fold in the cell types where it had previously been weakest.The original report defined a numbered progression that the field still uses. PE1 paired the nickase with an unmodified viral reverse transcriptase and worked poorly. PE2 used an engineered enzyme and worked well enough to be usable. PE3 added the second nick. Later systems addressed mismatch repair, pegRNA stability, and the size of the reverse transcriptase itself, with each generation typically improving efficiency several-fold in the cell types where it had previously been weakest. Two extensions widened the scope beyond small edits. Paired pegRNAs that write complementary flaps allow replacement of a whole segment rather than a few bases. Coupling prime editing to a serine integrase — using the editor to install a short recombinase landing site and the integrase to insert a payload into it — permits insertions of many kilobases, closing part of the gap between editing and gene addition.[^yarnall2023] That capability connects prime editing to the larger-scale ambitions described in [[synthetic-genomes]] and [[recoded-organisms]], where the goal is to rewrite rather than repair.Two extensions widened the scope beyond small edits. Paired pegRNAs that write complementary flaps allow replacement of a whole segment rather than a few bases. Coupling prime editing to a serine integrase, using the editor to install a short recombinase landing site and the integrase to insert a payload into it, permits insertions of many kilobases, closing part of the gap between editing and gene addition.[^yarnall2023] That capability connects prime editing to the larger-scale ambitions described in [[synthetic-genomes]] and [[recoded-organisms]], where the goal is to rewrite rather than repair. ## Current state## Current state Prime editing is a standard research tool for installing precise variants in cell lines, [[induced-pluripotent-stem-cells]] and [[organoids]], where the ability to write an exact patient mutation makes disease modelling far cleaner than nuclease-based approaches allow. In animals, prime editing has corrected the causal mutation in mouse models of metabolic liver disease and several other monogenic conditions, in each case in mice rather than in humans.[^bock2022]Prime editing is a standard research tool for installing precise variants in cell lines, [[induced-pluripotent-stem-cells]] and [[organoids]], where the ability to write an exact patient mutation makes disease modelling far cleaner than nuclease-based approaches allow. In animals, prime editing has corrected the causal mutation in mouse models of metabolic liver disease and several other monogenic conditions, in each case in mice rather than in humans.[^bock2022] Clinical translation began recently. The first prime editing therapy to reach patients treats chronic granulomatous disease by correcting a mutation in blood stem cells outside the body; initial results reported in 2025 indicated restoration of the missing immune-cell function in a treated patient.[^gori2026] The company behind it subsequently restructured and redirected effort toward liver targets, a reminder that in this field the binding constraint is often capital rather than biology. No prime editing product is approved anywhere as of 2026.Clinical translation began recently. The first prime editing therapy to reach patients treats chronic granulomatous disease by correcting a mutation in blood stem cells outside the body; initial results reported in 2025 indicated restoration of the missing immune-cell function in a treated patient.[^gori2026] The company behind it subsequently restructured and redirected effort toward liver targets, an illustration of how often funding rather than biology sets what gets pursued here. No prime editing product is approved anywhere as of 2026. > [!caution] How thin the human record is> [!caution] How thin the human record is> The entire clinical evidence for prime editing is one trial with two participants, whose blood stem cells were edited outside the body and returned after busulfan conditioning; restored NADPH oxidase activity was maintained to six and four months at the last reported follow-up.[^gori2026]> The entire clinical evidence for prime editing is one trial with two participants, whose blood stem cells were edited outside the body and returned after busulfan conditioning; restored NADPH oxidase activity was maintained to six and four months at the last reported follow-up.[^gori2026]> No prime editor has been delivered inside a human body — every in vivo correction is still in mice, in liver and a small number of other tissues.> No prime editor has been delivered inside a human body. Every in vivo correction is still in mice, in liver and a small number of other tissues. What would settle the platform's case is durability measured in years and a second indication, and as of 2026 neither exists.> What would settle the platform's case is durability measured in years and a second indication, and as of 2026 neither exists. > [!note] Terminology> [!note] Terminology> "Search and replace" is the phrase from the original paper and is apt for the mechanism, but it invites a word-processor analogy that overstates reliability. There is no undo, no confirmation step, and no guarantee the replacement is installed in every cell.> "Search and replace" is the phrase from the original paper and is apt for the mechanism, but it invites a word-processor analogy that overstates reliability. There is no undo, no confirmation step, and no guarantee the replacement is installed in every cell.lines 83–91 → 82–906 unchanged lines not shown
## Risks## Risks Because installing an edit requires three separate base-pairing events — spacer, primer binding site and template — prime editing is intrinsically more discriminating than a single-nick or single-cut system, and measured [[crispr-off-target-effects|off-target editing]] at guide-similar sites is generally lower than for [[crispr-cas9]]. The characteristic unintended products are different in kind: fragments of the pegRNA scaffold copied into the target site, tandem duplications, and small indels where the second nick was used.Because installing an edit requires three separate base-pairing events (spacer, primer binding site and template), prime editing is intrinsically more discriminating than a single-nick or single-cut system, and measured [[crispr-off-target-effects|off-target editing]] at guide-similar sites is generally lower than for [[crispr-cas9]]. The characteristic unintended products are different in kind: fragments of the pegRNA scaffold copied into the target site, tandem duplications, and small indels where the second nick was used. For [[somatic-gene-therapy]], the risk profile is favourable enough that the main open questions concern delivery vectors rather than the editor. For heritable applications, prime editing is sometimes advanced as the tool that would make [[germline-editing]] technically defensible. That argument runs into the same objections as its predecessors: efficiency in embryos is not the issue, mosaicism and verification are, and the international frameworks catalogued in [[governance-of-genome-editing]] are not written in terms of which enzyme is used — a posture that has not moved since the [[he-jiankui-affair]]. The practical alternative for most heritable disease risk remains embryo selection rather than editing.For [[somatic-gene-therapy]], the risk profile is favourable enough that the main open questions concern delivery vectors rather than the editor. For heritable applications, prime editing is sometimes advanced as the tool that would make [[germline-editing]] technically defensible. That argument runs into the same objections as its predecessors: efficiency in embryos is not the issue, mosaicism and verification are, and the international frameworks catalogued in [[governance-of-genome-editing]] are not written in terms of which enzyme is used, a posture that has not moved since the [[he-jiankui-affair]]. The practical alternative for most heritable disease risk remains embryo selection rather than editing. ## Outlook## Outlook removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
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