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categories: ["genetics", "society"]categories: ["genetics", "society"]tags: ["crispr", "genome editing", "safety", "gene therapy", "regulation", "mutagenesis"]tags: ["crispr", "genome editing", "safety", "gene therapy", "regulation", "mutagenesis"]summary: "Unintended DNA changes caused by genome editors, either at sites resembling the target or at the target itself, and the assays used to find them."summary: "Unintended DNA changes caused by genome editors, either at sites resembling the target or at the target itself, and the assays used to find them."updated: "2026-07-27"updated: "2026-10-29"issues: ["The 2024 FDA final guidance on genome editing products is described without a citation."]issues: ["The 2024 FDA final guidance on genome editing products is described without a citation."]------ lines 31–37 → 31–3717 unchanged lines not shown
Guide RNAs tolerate mismatches, particularly at positions distant from the PAM, so a twenty-nucleotide spacer typically has dozens to hundreds of partially matching sites in a mammalian genome. Whether any of them is actually cut depends on chromatin accessibility, the concentration and persistence of the editor, and the specific mismatch pattern. Cutting at such a site produces the same indels as an on-target cut, in a gene nobody chose.Guide RNAs tolerate mismatches, particularly at positions distant from the PAM, so a twenty-nucleotide spacer typically has dozens to hundreds of partially matching sites in a mammalian genome. Whether any of them is actually cut depends on chromatin accessibility, the concentration and persistence of the editor, and the specific mismatch pattern. Cutting at such a site produces the same indels as an on-target cut, in a gene nobody chose. Editors that do not cut have their own version of the problem. Base editors carry a deaminase that can act on transiently exposed single-stranded DNA anywhere in the genome, entirely independently of where the guide RNA directs the protein — a mechanism that leaves scattered single-base changes with no cut site to search near. Prime editors show lower guide-dependent off-target activity than nucleases, since three separate base-pairing events are required, but can insert fragments of their own guide RNA at the target. Editors that only bind DNA, as in [[epigenome-editing]], perturb transcription at sites they would never cut, because binding tolerates more mismatches than cleavage does.Editors that do not cut have their own version of the problem. Base editors carry a deaminase that can act on transiently exposed single-stranded DNA anywhere in the genome, entirely independently of where the guide RNA directs the protein. That mechanism leaves scattered single-base changes with no cut site to search near. Prime editors show lower guide-dependent off-target activity than nucleases, since three separate base-pairing events are required, but can insert fragments of their own guide RNA at the target. Editors that only bind DNA, as in [[epigenome-editing]], perturb transcription at sites they would never cut, because binding tolerates more mismatches than cleavage does. ## Detection## Detection lines 59–67 → 59–6721 unchanged lines not shown
The larger surprise of the past decade was that cutting the right place can still go wrong. Repair of a Cas9-induced break produces, at measurable frequency, deletions of many kilobases, inversions and complex rearrangements that standard short-amplicon sequencing does not detect because the primer sites themselves are lost.[^kosicki2018] More severe outcomes have been documented: whole-chromosome-arm loss, loss of heterozygosity extending far from the cut, and chromothripsis, in which a chromosome shatters and is stitched back together in scrambled order.[^leibowitz2021]The larger surprise of the past decade was that cutting the right place can still go wrong. Repair of a Cas9-induced break produces, at measurable frequency, deletions of many kilobases, inversions and complex rearrangements that standard short-amplicon sequencing does not detect because the primer sites themselves are lost.[^kosicki2018] More severe outcomes have been documented: whole-chromosome-arm loss, loss of heterozygosity extending far from the cut, and chromothripsis, in which a chromosome shatters and is stitched back together in scrambled order.[^leibowitz2021] Breaks also activate a p53-mediated damage response, which reduces editing efficiency in cells with intact p53 and therefore selects, weakly but systematically, for cells in which that pathway is impaired.[^ihry2018] The practical worry is not that editing causes cancer in any demonstrated case, but that the standard editing protocol applies a selection pressure whose direction is unfavourable.Breaks also activate a p53-mediated damage response, which reduces editing efficiency in cells with intact p53 and therefore selects, weakly but systematically, for cells in which that pathway is impaired.[^ihry2018] No demonstrated case links editing to cancer; the practical worry is that the standard editing protocol applies a selection pressure whose direction is unfavourable. Multiplexed editing compounds the problem. Making several cuts at once — routine in engineered cell therapies and in the heavily edited donor animals used for [[xenotransplantation]] — creates opportunities for translocation between the cut sites, and the frequency rises with the number of simultaneous breaks.Multiplexed editing compounds the problem. Making several cuts at once, which is routine in engineered cell therapies and in the heavily edited donor animals used for [[xenotransplantation]], creates opportunities for translocation between the cut sites, and the frequency rises with the number of simultaneous breaks. ## Editing in embryos## Editing in embryos lines 74–93 → 74–936 unchanged lines not shown
Several strategies reduce guide-dependent off-target activity, none to zero.Several strategies reduce guide-dependent off-target activity, none to zero. - **Engineered high-fidelity nucleases** with weakened non-specific DNA contacts, which lose activity faster on mismatched sites than on matched ones.[^kleinstiver2016]- **Engineered high-fidelity nucleases** with weakened non-specific DNA contacts, which lose activity faster on mismatched sites than on matched ones.[^kleinstiver2016]- **Short exposure**, by delivering preassembled protein–RNA complexes that degrade within hours rather than plasmids or viral vectors that express for weeks. This is one reason ex vivo editing is safer than in vivo editing with [[aav-vectors]], and why messenger RNA in [[lipid-nanoparticles]] is preferred to persistent expression.- **Short exposure**, by delivering preassembled protein-RNA complexes that degrade within hours rather than plasmids or viral vectors that express for weeks. This is one reason ex vivo editing is safer than in vivo editing with [[aav-vectors]], and why messenger RNA in [[lipid-nanoparticles]] is preferred to persistent expression.- **Paired nickases**, requiring two adjacent binding events to produce a break.- **Paired nickases**, requiring two adjacent binding events to produce a break.- **Guide selection** that avoids sites with close genomic relatives and avoids known polymorphic positions.- **Guide selection** that avoids sites with close genomic relatives and avoids known polymorphic positions.- **Switching editor class**, using [[base-editing]] or [[prime-editing]] where a double-strand break is not needed — which trades one error spectrum for another rather than eliminating error.- **Switching editor class**, using [[base-editing]] or [[prime-editing]] where a double-strand break is not needed. That trades one error spectrum for another rather than eliminating error. ## Regulatory practice## Regulatory practice The United States Food and Drug Administration issued final guidance on genome editing products in 2024 setting out expectations: orthogonal off-target nomination assays, confirmation in the clinical cell type, justification of any residual risk, and long-term follow-up of treated patients. The approval of [[casgevy]] in late 2023 was the first regulatory test of that framework, and the advisory discussion around it turned less on whether off-target edits had been found than on whether the assays used could have found them. That distinction — absence of evidence versus evidence of absence — remains the crux of every editing safety package.The United States Food and Drug Administration issued final guidance on genome editing products in 2024 setting out expectations: orthogonal off-target nomination assays, confirmation in the clinical cell type, justification of any residual risk, and long-term follow-up of treated patients. The approval of [[casgevy]] in late 2023 was the first regulatory test of that framework, and the advisory discussion around it turned less on whether off-target edits had been found than on whether the assays used could have found them. The distinction between absence of evidence and evidence of absence remains the crux of every editing safety package. For [[somatic-gene-therapy]] the residual uncertainty is managed the way oncology risk is managed: quantified where possible, monitored over years, and weighed against the disease. The frameworks surveyed in [[governance-of-genome-editing]] apply a different logic to heritable edits, and the [[he-jiankui-affair]] is the standing example of what happens when the safety analysis is performed by the person with an interest in the answer.For [[somatic-gene-therapy]] the residual uncertainty is managed the way oncology risk is managed: quantified where possible, monitored over years, and weighed against the disease. The frameworks surveyed in [[governance-of-genome-editing]] apply a different logic to heritable edits, and the [[he-jiankui-affair]] is the standing example of what happens when the safety analysis is performed by the person with an interest in the answer. ## Open problems## Open problems The field lacks a validated way to say how much off-target editing is too much, because no assay's sensitivity floor is well characterised in the tissue that matters. There is no established method for detecting rare structural rearrangements across a whole treated organ, no consensus on how to handle patient-specific variants at candidate sites, and little long-term human data — the treated cohorts are small and young. Editors deployed outside medicine, notably in [[gene-drive|gene drives]] released into wild populations, face the same detection limits with none of the follow-up infrastructure, which is one reason the [[precautionary-principle]] is invoked more forcefully there than in the clinic.The field lacks a validated way to say how much off-target editing is too much, because no assay's sensitivity floor is well characterised in the tissue that matters. There is no established method for detecting rare structural rearrangements across a whole treated organ, no consensus on how to handle patient-specific variants at candidate sites, and little long-term human data, since the treated cohorts are small and young. Editors deployed outside medicine, such as the [[gene-drive|gene drives]] released into wild populations, face the same detection limits with none of the follow-up infrastructure, which is one reason the [[precautionary-principle]] is invoked more forcefully there than in the clinic. ## See also## See also removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
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