{ "id": "ep49mdjwhy", "slug": "targeted-gene-repair", "title": "Targeted gene repair", "updated": "2026-10-15", "topic_path": [ { "id": "life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/topics/life" }, { "id": "life.biology", "label": "Biological foundations", "api_url": "https://www.edgepedia.org/api/v1/topics/life.biology" }, { "id": "life.biology.genetics", "label": "Genetics and genomic reference", "api_url": "https://www.edgepedia.org/api/v1/topics/life.biology.genetics" }, { "id": "life.biology.genetics.gene_editing_engineering", "label": "Genetic engineering, editing, and gene therapy", "api_url": "https://www.edgepedia.org/api/v1/topics/life.biology.genetics.gene_editing_engineering" } ], "geo": [ { "id": "geo.nongeo.t1946.life.biology.genetics", "label": "Non-geographic · 1946 to 2000: Genetics and genomic reference", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.life.biology.genetics", "path": [ { "id": "geo.nongeo", "label": 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"Targeted gene repair is a genome editing approach that corrects a specific disease-causing mutation at a defined genomic locus, using the cell's own repair machinery, from chimeraplasts to prime editors.", "snippet": "Targeted gene repair is a genome editing approach that corrects a specific disease-causing mutation at a defined genomic locus, using the cell's own repair machinery, from chimeraplasts to prime editors.", "node": "life.biology.genetics.gene_editing_engineering", "markdown": "# Targeted gene repair\n\nTargeted gene repair is a genome editing approach that corrects a specific disease-causing mutation at a defined genomic locus, relying on the cell's endogenous [DNA repair](https://www.edgepedia.org/dna-repair) mechanisms to convert the mutant sequence to the intended one.[1](https://www.benthamdirect.com/content/journals/cgt/10.2174/156652306777934847) It has been implemented with corrective molecules including chimeraplasts, modified single-stranded oligonucleotides, triplex-forming oligonucleotides, small DNA fragments, and AAV-based vectors[1](https://www.benthamdirect.com/content/journals/cgt/10.2174/156652306777934847) • [2](https://jbiomedsci.biomedcentral.com/articles/10.1186/1423-0127-18-10), and in its modern nuclease-era form with base editors and prime editors that write precise changes without donor DNA templates.[3](https://www.nature.com/articles/s41586-019-1711-4) The output is a corrected genomic allele, which in successful disease models produces a corrected transcript and a repaired cellular or organismal phenotype.\n\n| Key fact | Value |\n|---|---|\n| Classic repair molecule | Chimeric RNA/DNA oligonucleotide, 70–80 bases, double-hairpin configuration[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/) |\n| Early correction frequencies | 0.5–20%, varying even within the same laboratory[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/) |\n| PE3 prime editing | Typically 20–50% editing with 1–10% indels in HEK293T cells[3](https://www.nature.com/articles/s41586-019-1711-4) |\n| Optimized CFTR F508del correction | 58% in immortalized bronchial epithelial cells; 25% in patient-derived airway cells[5](http://nature.com/articles/s41551-024-01233-3.pdf) |\n| Prime editing of SCD HSPCs | 15–41% correction of HBB S to HBB A[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10195679/) |\n| Edit-to-indel ratio, PE3 vs HDR | 270-fold higher for PE3 at three test loci[3](https://www.nature.com/articles/s41586-019-1711-4) |\n| rAAV donor packaging limit | 4.5–4.7 kb including ITRs[7](https://link.springer.com/article/10.1007/s00335-024-10099-4) |\n\n## How it works\n\nThe oligonucleotide-directed approach delivers a synthetic molecule designed to align with the mutant locus and to contain the desired nucleotide change.[8](https://www.pnas.org/doi/abs/10.1073/pnas.93.5.2071) The chimeric RNA/DNA oligonucleotide is a single-stranded molecule, usually 70–80 bases long, whose sequence complementarity makes it fold into a double-hairpin configuration; its RNA bases stabilize the intermediate joint molecule while the DNA portion initiates the single-base exchange.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/) Mechanistically, each strand of the chimera can hybridize with double-stranded DNA to form a complement-stabilized D-loop, possibly via reciprocal four-strand exchange, a reaction not observed with oligonucleotides lacking 2'-O-methyl modifications.[9](https://pubs.acs.org/doi/abs/10.1021/bi9921891) The paired mismatch then recruits the cell's own repair machinery to complete the conversion.[1](https://www.benthamdirect.com/content/journals/cgt/10.2174/156652306777934847)\n\n[Prime editing](https://www.edgepedia.org/prime-editing) works differently: a Cas9 nickase with the HNH nuclease domain deactivated is fused to an engineered reverse transcriptase, and a prime editing guide RNA (pegRNA) both specifies the target site and carries the template for the desired edit, so prime editing ordinarily avoids an intentional double-strand break and a separate donor DNA molecule, using the pegRNA as an RNA template instead, although PE3, which adds a second nick, can produce double-strand-break-related outcomes.[10](https://link.springer.com/article/10.1186/s12967-024-05957-3) • [3](https://www.nature.com/articles/s41586-019-1711-4)\n\n## How it is done\n\nA practitioner first designs the repair molecule: a chimeraplast or ssODN matching the locus with the corrective base, or a pegRNA encoding the edit. Delivery is the main constraint. Non-viral delivery of plasmids by electroporation or lipid-mediated transfection achieves high editing efficiency in mammalian cell lines, and hydrodynamic tail vein injection delivers base and prime editors to mouse hepatocytes.[10](https://link.springer.com/article/10.1186/s12967-024-05957-3) Adenovirus vectors offer cargo capacity sufficient for full-length prime editors, at the cost of higher immunogenicity and integration risk.[11](https://doi.org/10.1016/j.ymthe.2026.04.033)\n\nFor donor-template approaches, recombinant AAV donors are limited to a DNA packaging capacity of 4.5–4.7 kb including ITRs, with homology arms requiring at minimum 2 × 0.3–0.4 kb[7](https://link.springer.com/article/10.1007/s00335-024-10099-4), and co-electroporation of CRISPR RNP with single-stranded DNA is only efficient for small mutations of a few hundred base pairs.[7](https://link.springer.com/article/10.1007/s00335-024-10099-4) HDR pathway activity peaks during the late S and G2 phases of the cell cycle, a dependence that two-cell embryo Cas9 RNP electroporation exploits.[7](https://link.springer.com/article/10.1007/s00335-024-10099-4) Verification matters: rAAV can integrate into the genome without HDR, without homology arms, at the Cas9 on-target cleavage site, and on-site concatemers occur, so alleles require careful validation[7](https://link.springer.com/article/10.1007/s00335-024-10099-4); clonal inheritance of corrected cells has been used to confirm stable repair.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/)\n\n## Origin\n\nTargeted gene repair is a process in which a synthetic molecule directs the exchange of a single DNA base at a specific site in an episome or chromosome.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/) Two 1996 papers established the approach: a chimeric RNA/DNA oligonucleotide directed correction of the sickle-cell mutation in the hemoglobin βS allele, producing a detectable level of gene conversion in lymphoblastoid cells homozygous for βS[12](https://www.science.org/doi/10.1126/science.273.5280.1386), and targeted correction of episomal DNA in mammalian cells was demonstrated in a model system.[8](https://www.pnas.org/doi/abs/10.1073/pnas.93.5.2071) DNA oligonucleotides could alter single bases in yeast.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/) The technique was then extended to liver cells, first in an HUH7 cell line and then in primary hepatocytes, and the Crigler-Najjar mutation was corrected in the Gunn rat model.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/)\n\nThe early reception was hostile. The reaction from the gene therapy and gene targeting community was generally disbelief, with many scientists considering the results most likely artifactual[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/), and controversy over the consistency and lack of reproducibility of early experiments, particularly for chimeraplasty, together with inefficient nuclear uptake and misleading assessment of repair frequencies, marked the field.[1](https://www.benthamdirect.com/content/journals/cgt/10.2174/156652306777934847)\n\n## Variants\n\nThe oligonucleotide-based approaches differ in the repair molecule used to induce local conversion: chimeraplasts (RNA/DNA chimeras, also called RDOs), modified single-stranded oligonucleotides (ssODNs/SSOs), small DNA fragments (SDFs, related to the SFHR approach), and triplex-forming oligonucleotides (TFOs). AAV-based viral donors are a donor-delivery method rather than an oligonucleotide, and zinc-finger nucleases (ZFNs) are engineered nucleases; both are distinct genome-editing approaches that can achieve targeted repair, as are the base editors and prime editors described below.[1](https://www.benthamdirect.com/content/journals/cgt/10.2174/156652306777934847) • [2](https://jbiomedsci.biomedcentral.com/articles/10.1186/1423-0127-18-10) • [13](https://mdpi-res.com/d_attachment/ijms/ijms-22-03348/article_deploy/ijms-22-03348.pdf?version=1616654595)\n\nThe nuclease-era variants avoid donor templates. Base editors chemically convert one base type within a window; a custom adenine base editor, ABE8e-NRCH, converted the SCD allele HBB S into the non-pathogenic Makassar β-globin variant HBB G, delivered ex vivo as mRNA.[14](https://pubmed.ncbi.nlm.nih.gov/34079130/) Prime editors performed more than 175 edits in human cells, including targeted insertions, deletions, and all 12 types of point mutations, without double-strand breaks or donor DNA.[3](https://www.nature.com/articles/s41586-019-1711-4)\n\n## Applications\n\n[Sickle cell disease](https://www.edgepedia.org/sickle-cell-disease) is the recurring test case across eras. Beyond the 1996 lymphoblastoid correction[12](https://www.science.org/doi/10.1126/science.273.5280.1386), prime editing corrected the SCD allele (HBB S) to wild type (HBB A) at 15–41% in HSPCs from SCD patients[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10195679/); seventeen weeks after transplantation into immunodeficient mice, an average of 42% of human erythroblasts and reticulocytes expressed HBB A, exceeding levels predicted for therapeutic benefit, and edited erythrocytes resisted hypoxia-induced sickling.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10195679/) [In vivo](https://www.edgepedia.org/in-vivo) base editing converted the sickle codon GTG to the benign G-Makassar codon GCG, with corrected disease hallmarks and no off-target editing at top-scored sites.[15](https://doi.org/10.1016/j.ymthe.2024.10.018)\n\nPrime editing corrected the genetic causes of sickle cell disease (HBB transversion) and [Tay–Sachs disease](https://www.edgepedia.org/tay-sachs-disease) (HEXA deletion) in human cells.[3](https://www.nature.com/articles/s41586-019-1711-4) For CFTR F508del, systematic optimization restored ion channel function to over 50% of wild-type levels in primary airway cells, similar to treatment with elexacaftor, tezacaftor, and ivacaftor.[5](http://nature.com/articles/s41551-024-01233-3.pdf)\n\n## Limitations and alternatives\n\nKnown failure modes include indels, bystander editing, wide editing windows, and off-target activity for base editors; both ABEs and CBEs can generate double-strand breaks, deletions, and translocations at target loci, though at lower frequency than nuclease Cas9.[10](https://link.springer.com/article/10.1186/s12967-024-05957-3) Prime editing efficiency varies greatly and remains low depending on the edit, target site, and cell type, and it does not entirely avoid target double-strand breaks.[10](https://link.springer.com/article/10.1186/s12967-024-05957-3) Reported correction efficiencies span orders of magnitude: early oligonucleotide work ranged from 0.5% to 20%, often within the same laboratory[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/), and one comparison of PEmax and PE2-NG systems reported F508del correction of 2.81% without normalization[16](http://protein.bio.msu.ru/biokhimiya/contents/v90/full/90060773.html), far below the 58% reported for the fully optimized system in bronchial epithelial cells[5](http://nature.com/articles/s41551-024-01233-3.pdf), a discrepancy that illustrates how strongly efficiency depends on editor version, target site, and cell type. Against alternatives: HDR knock-in with donor DNA reaches higher raw efficiency in reporter cells (21–24% vs PE2's 1.1–1.5%)[17](https://www.mdpi.com/2218-273X/13/5/870) but produces proportionally more indels, while NHEJ-based approaches are efficient but imprecise; donor-template methods are further constrained by AAV packaging limits and by rAAV integration at the cut site, which demands careful allele validation.[7](https://link.springer.com/article/10.1007/s00335-024-10099-4) Ex vivo cell therapy, as in the SCD HSPC work, adds engraftment as the key variable.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10195679/) Prime editing reached the clinic in the PM359 Phase 1/2 first-in-human trial, which began in October 2024, with the first patient treated in April 2025[11](https://doi.org/10.1016/j.ymthe.2026.04.033), and adenovirus-delivered PE5max corrected 43% of sickle hemoglobin alleles in a murine SCD model with phenotypic rescue.[11](https://doi.org/10.1016/j.ymthe.2026.04.033)\n\n## References\n\n1. [Targeted Gene Repair: The Ups and Downs of a Promising Gene Therapy Approach](https://www.benthamdirect.com/content/journals/cgt/10.2174/156652306777934847)\n2. [An update on targeted gene repair in mammalian cells: methods and mechanisms](https://jbiomedsci.biomedcentral.com/articles/10.1186/1423-0127-18-10)\n3. [Search-and-replace genome editing without double-strand breaks or donor DNA](https://www.nature.com/articles/s41586-019-1711-4)\n4. [Targeted gene repair – in the arena](https://pmc.ncbi.nlm.nih.gov/articles/PMC182220/)\n5. [Systematic optimization of prime editing to correct CFTR F508del](http://nature.com/articles/s41551-024-01233-3.pdf)\n6. [Ex vivo prime editing of patient haematopoietic stem cells rescues sickle-cell disease phenotypes after engraftment in mice](https://pmc.ncbi.nlm.nih.gov/articles/PMC10195679/)\n7. [Genome engineering with Cas9 and AAV repair templates, successes and pitfalls](https://link.springer.com/article/10.1007/s00335-024-10099-4)\n8. [Targeted gene correction of episomal DNA in mammalian cells mediated by a chimeric RNA.DNA oligonucleotide](https://www.pnas.org/doi/abs/10.1073/pnas.93.5.2071)\n9. [A Plausible Mechanism for Gene Correction by Chimeric Oligonucleotides](https://pubs.acs.org/doi/abs/10.1021/bi9921891)\n10. [From bench to bedside: cutting-edge applications of base editing and prime editing in precision medicine](https://link.springer.com/article/10.1186/s12967-024-05957-3)\n11. [A primer on prime: A prime editing update from advances to first-in-human trial (Molecular Therapy, 2026)](https://doi.org/10.1016/j.ymthe.2026.04.033)\n12. [Correction of the Mutation Responsible for Sickle Cell Anemia by an RNA-DNA Oligonucleotide](https://www.science.org/doi/10.1126/science.273.5280.1386)\n13. [On the Origins of Homology Directed Repair in Mammalian Cells](https://mdpi-res.com/d_attachment/ijms/ijms-22-03348/article_deploy/ijms-22-03348.pdf?version=1616654595)\n14. [Base editing of haematopoietic stem cells rescues sickle cell disease in mice (PubMed record)](https://pubmed.ncbi.nlm.nih.gov/34079130/)\n15. [Introducing a hemoglobin G-Makassar variant in HSCs by in vivo base editing treats sickle cell disease in mice (Molecular Therapy, 2024)](https://doi.org/10.1016/j.ymthe.2024.10.018)\n16. [Biochemistry (Moscow) article on prime editing for hereditary disorders](http://protein.bio.msu.ru/biokhimiya/contents/v90/full/90060773.html)\n17. [Comparison of In-Frame Deletion, Homology-Directed Repair, and Prime Editing-Based Correction of Duchenne Muscular Dystrophy Mutations](https://www.mdpi.com/2218-273X/13/5/870)\n\n---\n*Topic: Encyclopedia › Life and health › Biological foundations › Genetics and genomic reference › Genetic engineering, editing, and gene therapy*\n\n*Initially written Sep 29, 2026 · Reviewed: Sep 30, 2026 · Edited: Oct 15, 2026 · Last review: Sep 30, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/targeted-gene-repair", "markdown_url": "https://www.edgepedia.org/targeted-gene-repair.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Targeted gene repair\", Edgepedia (EdgeChat), https://www.edgepedia.org/targeted-gene-repair. Edgepedia Community License 1.0.", "credit_md": "\"[Targeted gene repair](https://www.edgepedia.org/targeted-gene-repair)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/targeted-gene-repair](https://www.edgepedia.org/targeted-gene-repair). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Targeted gene repair\", Edgepedia (EdgeChat), https://www.edgepedia.org/targeted-gene-repair. Edgepedia Community License 1.0.", "speakable": "Targeted gene repair is a genome editing approach that corrects a specific disease-causing mutation at a defined genomic locus, using the cell's own repair machinery, from chimeraplasts to prime editors." }