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Abstract
Gene targeting by single-stranded oligodeoxyribonucleotides (ssODNs) is emerging as a powerful tool for the introduction of subtle gene modifications in mouse embryonic stem (ES) cells and the generation of mutant mice. Here, we have studied the role of ssODN composition, transcription and replication of the target locus, and DNA repair pathways to gain more insight into the parameters governing ssODN-mediated gene targeting in mouse ES cells. We demonstrated that unmodified ssODNs of 35–40 nt were most efficient in correcting a chromosomally integrated mutant neomycin reporter gene. Addition of chemical modifications did not further enhance the efficacy of these ssODNs. The observed strand bias was not affected by transcriptional activity and may rather be caused by the different accessibility of the DNA strands during DNA replication. Consistently, targeting frequencies were enhanced when cells were treated with hydroxyurea to reduce the rate of replication fork progression. Transient down-regulation of various DNA repair genes by RNAi had no effect on the targeting frequency. Taken together, our data suggest that ssODN-mediated gene targeting occurs within the context of a replication fork. This implies that any given genomic sequence, irrespective of transcriptional status, should be amenable to ssODN-mediated gene targeting. The ability of ES cells to differentiate into various cell types after ssODN-mediated gene targeting may offer opportunities for future therapeutic applications.
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Affiliation(s)
- Marieke Aarts
- Division of Molecular Biology, The Netherlands Cancer Institute, Amsterdam, The Netherlands
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Wuepping M, Kenner O, Hegele H, Schwandt S, Kaufmann D. Higher efficiency of thymine-adenine clamp-modified single-stranded oligonucleotides in targeted nucleotide sequence correction is not correlated with lower intracellular degradation. Hum Gene Ther 2009; 20:283-7. [PMID: 19061415 DOI: 10.1089/hum.2008.138] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Specific single-stranded oligonucleotides can induce targeted nucleotide sequence correction in eukaryotic genes in vitro and in vivo. Our model for investigating the reasons for the low correction rates achieved by this method is the correction of a point mutation in the hypoxanthine-guanine phosphoribosyltransferase gene (hprt) in the cell line V79-151. Using single-stranded phosphorothioate-modified oligonucleotides, the correction rates of this hprt mutation were low but always reproducible. One reason for low exchange rates may be fast intracellular degradation of the oligonucleotides. Therefore we compared the exchange rates of different 3' and 5' end-modified oligonucleotides with their degradation rates. Thymine-adenine (TA) repeat (clamp)-modified oligonucleotides showed higher correction rates than those with a guanine-cytosine (GC) clamp and 5' clamps induced higher correction rates than clamps at the 3' end. Experiments on the stability of the most effective 5'-TA and 3'-TA clamp-modified oligonucleotide indicated rapid cleavage and the occurrence of shortened oligonucleotides in the presence of cytoplasmic and nuclear extracts. The phosphorothioate-modified oligonucleotides were more stable, but their correction rates were lower. We suggest that there is no direct correlation between the biological stability of the full-length oligonucleotides and the exchange rates achieved.
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Affiliation(s)
- M Wuepping
- Institute of Human Genetics, University of Ulm, Albert-Einstein-Allee 11, Ulm, Germany
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53
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Bonner M, Kmiec EB. DNA breakage associated with targeted gene alteration directed by DNA oligonucleotides. Mutat Res 2009; 669:85-94. [PMID: 19463835 DOI: 10.1016/j.mrfmmm.2009.05.004] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2008] [Revised: 05/01/2009] [Accepted: 05/07/2009] [Indexed: 10/20/2022]
Abstract
Understanding the mechanism by which single-stranded oligonucleotides (ODNs) elicit targeted nucleotide exchange (TNE) is imperative to achieving optimal correction efficiencies and medical applicability. It has been previously shown that introduction of an ODN into cells results in the activation of DNA damage response pathways, but there has been no evaluation of the damage created at the level of the DNA. The activation of H2AX, a hallmark protein of DNA breakage, suggests that a double-strand break (DSB) could be occurring during the targeted gene alteration (TGA) reaction. Using the human HCT116 cell line with a single integrated mutant eGFP gene as our model system, we demonstrate that the DNA strand breakage occurs when a specific ODN, designed to direct TGA, is transfected into the cells. Both single- and double-stranded DNA cleavage is observed dependent on the level of ODN added to the reaction. Possible mechanisms of ODN-dependent DSB formation, as a function of TGA, are discussed herein.
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Affiliation(s)
- Melissa Bonner
- Department of Biological Sciences, Delaware Biotechnology Institute, University of Delaware, Newark, DE 19716, USA
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54
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Papaioannou I, Disterer P, Owen JS. Use of internally nuclease-protected single-strand DNA oligonucleotides and silencing of the mismatch repair protein, MSH2, enhances the replication of corrected cells following gene editing. J Gene Med 2009; 11:267-74. [PMID: 19153972 DOI: 10.1002/jgm.1296] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
BACKGROUND Gene editing is potentially a powerful technology for introducing genetic changes by using short single-stranded DNA oligonucleotides (ssODNs). However, their efficiency is reduced by the mismatch repair system, especially MSH2, which may suppress gene editing, although findings vary depending on readout and type of oligonucleotide used. Additionally, successfully edited cells are reported to arrest at the S- or G2-phase. In the present study, we evaluate whether a novel ssODN design and down-regulation of MSH2 expression allows the isolation of replicating gene-edited cells. METHODS Cultured Chinese hamster ovary cells expressing mutated enhanced green fluorescent protein were targeted with ssODNs of varying design, all capable of restoring fluorescence, which allows the monitoring of correction events by flow cytometry. Converted cells were isolated by cell sorting and grown to determine colony formation efficiencies. MSH2 expression was suppressed with small interfering RNA and the cell cycle distribution of cells transfected with ssODN was quantified by flow cytometry, following propidium iodide or DRAQ5 staining. RESULTS Although efficiency was higher using ssODN end-protected with phosphorothioate, the potential of edited cells to form colonies was lower than those targeted with unmodified ssODN. We established that ssODN transfection itself perturbs the cell cycle and that MSH2 gene silencing increases correction efficiency. In both cases, however, the effect was dependent on the positioning of the protected nucleotides. Importantly, when internally protected ssODN was used in combination with MSH2 suppression, a higher proportion of G1-phase corrected cells was observed 48-64 h after transfection. CONCLUSIONS Use of internally protected ssODN and downregulating cellular MSH2 activity may facilitate isolation of viable, actively replicating gene-edited cells.
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Affiliation(s)
- Ioannis Papaioannou
- Department of Medicine, Royal Free and University College Medical School, Royal Free Campus, London, UK
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55
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Engstrom JU, Suzuki T, Kmiec EB. Regulation of targeted gene repair by intrinsic cellular processes. Bioessays 2009; 31:159-68. [PMID: 19204988 DOI: 10.1002/bies.200800119] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Targeted gene alteration (TGA) is a strategy for correcting single base mutations in the DNA of human cells that cause inherited disorders. TGA aims to reverse a phenotype by repairing the mutant base within the chromosome itself, avoiding the introduction of exogenous genes. The process of how to accurately repair a genetic mutation is elucidated through the use of single-stranded DNA oligonucleotides (ODNs) that can enter the cell and migrate to the nucleus. These specifically designed ODNs hybridize to the target sequence and act as a beacon for nucleotide exchange. The key to this reaction is the frequency with which the base is corrected; this will determine whether the approach becomes clinically relevant or not. Over the course of the last five years, workers have been uncovering the role played by the cells in regulating the gene repair process. In this essay, we discuss how the impact of the cell on TGA has evolved through the years and illustrate ways that inherent cellular pathways could be used to enhance TGA activity. We also describe the cost to cell metabolism and survival when certain processes are altered to achieve a higher frequency of repair.
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Affiliation(s)
- Julia U Engstrom
- University of Delaware, Department of Biological Sciences, Newark, DE 19716, USA
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56
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Cellular responses to targeted genomic sequence modification using single-stranded oligonucleotides and zinc-finger nucleases. DNA Repair (Amst) 2008; 8:298-308. [PMID: 19071233 DOI: 10.1016/j.dnarep.2008.11.011] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2008] [Revised: 10/23/2008] [Accepted: 11/05/2008] [Indexed: 01/27/2023]
Abstract
Single-stranded oligonucleotides (ssODNs) and zinc-finger nucleases (ZFNs) are two approaches that are being pursued to achieve sequence specific genome modification. ZFNs induce high rates of homologous recombination (HR) between the target sequence and a given donor by introducing site-specific genomic double-strand breaks (DSBs). The mode of action that is used by ssODNs remains largely unknown, but may involve genomic integration of the ssODNs. In this work, cellular responses following ssODN and ZFN mediated correction of a genomic reporter gene have been investigated in human cells. Comparison of the cell cycle distribution of corrected cells following ssODN or ZFN exposure, established that ssODN corrected cells were arrested in the late S and G2/M cell cycle phases, while ZFN corrected cells displayed normal cell cycle profiles. We demonstrate that after ssODN mediated gene correction, phosphorylation of the damage sensor protein H2AX could be observed in 5.8% and 29% of the corrected cells, using a single copy and a multi copy reporter, respectively. When using the ZFN strategy in a single copy reporter only 1.5% of the corrected cells were positive for gamma-H2AX staining. By direct detection of genomic DSBs we establish that the observed cell cycle arrest following ssODN mediated gene correction could be associated with the presence of unrepaired genomic DSBs. Lastly, we establish that although a mutant cellular mismatch repair (MMR) system as expected enhanced ssODN mediated gene correction, the capacity of the ssODN corrected cells to proliferate was not influenced by the MMR system. In conclusion gene correction by means of the ssODN strategy leads to activation of DNA damage signalling and cell cycle arrest due to formation of unrepaired genomic DSBs in a high proportion of the corrected cells. On the contrary, cells corrected using ZFNs displayed normal cell cycle distribution and lower rates of DNA damage.
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57
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Recent advances in gene therapy for severe congenital immunodeficiency diseases. Curr Opin Hematol 2008; 15:375-80. [PMID: 18536577 DOI: 10.1097/moh.0b013e328302c807] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
PURPOSE OF REVIEW To discuss new data on the safety and efficacy of the ongoing gene therapy trials for primary immune deficiencies, the first reports of new trials and the preclinical developments that are likely to be translated to the clinic in the near future. RECENT FINDINGS Both clinical successes and severe adverse events continue to be reported in trials of gammaretroviral gene therapy for severe combined immune deficiency-X1, adenosine deaminase-deficient forms of severe combined immune deficiency and chronic granulomatous disease. Insertion site analyses of recently reported trials on all of these diseases have discovered preferential insertion in the 5' ends of genes, including potentially dangerous ones such as proto-oncogenes and signal transduction and proliferation genes. Preclinical work on rodent and canine models has tested novel vectors, including lentiviruses and foamy viruses. SUMMARY Gene therapy for the most common forms of severe combined immune deficiency can lead to immune reconstitution in most patients, although a minority of patients has derived minimal clinical benefit and some have suffered severe adverse events including death. Ongoing preclinical work attempts to address the latter shortcoming. Meanwhile, in the presence of a careful risk-benefit assessment, gene therapy remains an appropriate subject of clinical investigation.
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58
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Parekh-Olmedo H, Kmiec EB. Progress and Prospects: targeted gene alteration (TGA). Gene Ther 2007; 14:1675-80. [DOI: 10.1038/sj.gt.3303053] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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Kow YW, Bao G, Reeves JW, Jinks-Robertson S, Crouse GF. Oligonucleotide transformation of yeast reveals mismatch repair complexes to be differentially active on DNA replication strands. Proc Natl Acad Sci U S A 2007; 104:11352-7. [PMID: 17592146 PMCID: PMC2040902 DOI: 10.1073/pnas.0704695104] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Transformation of both prokaryotes and eukaryotes with single-stranded oligonucleotides can transfer sequence information from the oligonucleotide to the chromosome. We have studied this process using oligonucleotides that correct a -1 frameshift mutation in the LYS2 gene of Saccharomyces cerevisiae. We demonstrate that transformation by oligonucleotides occurs preferentially on the lagging strand of replication and is strongly inhibited by the mismatch-repair system. These results are consistent with a mechanism in which oligonucleotides anneal to single-stranded regions of DNA at a replication fork and serve as primers for DNA synthesis. Because the mispairs the primers create are efficiently removed by the mismatch-repair system, single-stranded oligonucleotides can be used to probe mismatch-repair function in a chromosomal context. Removal of mispairs created by annealing of the single-stranded oligonucleotides to the chromosomal DNA is as expected, with 7-nt loops being recognized solely by MutS beta and 1-nt loops being recognized by both MutS alpha and MutS beta. We also find evidence for Mlh1-independent repair of 7-nt, but not 1-nt, loops. Unexpectedly, we find a strand asymmetry of mismatch-repair function; transformation is blocked more efficiently by MutS alpha on the lagging strand of replication, whereas MutS beta does not show a significant strand bias. These results suggest an inherent strand-related difference in how the yeast MutS alpha and MutS beta complexes access and/or repair mismatches that arise in the context of DNA replication.
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Affiliation(s)
| | | | | | | | - Gray F. Crouse
- Biology, Emory University, Atlanta, GA 30322
- To whom correspondence should be addressed. E-mail:
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60
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Ferrara L, Engstrom JU, Schwartz T, Parekh-Olmedo H, Kmiec EB. Recovery of cell cycle delay following targeted gene repair by oligonucleotides. DNA Repair (Amst) 2007; 6:1529-35. [PMID: 17560837 PMCID: PMC2083256 DOI: 10.1016/j.dnarep.2007.04.007] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2006] [Revised: 03/09/2007] [Indexed: 11/20/2022]
Abstract
We have previously shown that activation of the homologous recombinational repair pathway leads to a block of cell division in corrected cells, possibly through the activity of checkpoint proteins Chk1 and Chk2. In this study, we examine the long-term impact of this stalling on the growth of cells that have enabled gene repair events. Using a mutated eGFP gene as an episomal reporter, we show that corrected (eGFP-positive) cells contain only a few active replication templates 2 weeks after electroporation, yet do not display an apoptotic or senescent phenotype. By 6 weeks after electroporation, cells resume active replication with a cell cycle profile that is comparable to that of the non-corrected (eGFP-negative) population. These results indicate that the initial stalling is transient and eGFP-positive cells eventually resume a normal phenotypic growth pattern, allowing for passaging and expansion in vitro.
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Manipulation of cell cycle progression can counteract the apparent loss of correction frequency following oligonucleotide-directed gene repair. BMC Mol Biol 2007; 8:9. [PMID: 17284323 PMCID: PMC1797188 DOI: 10.1186/1471-2199-8-9] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2006] [Accepted: 02/06/2007] [Indexed: 11/10/2022] Open
Abstract
Background Single-stranded oligonucleotides (ssODN) are used routinely to direct specific base alterations within mammalian genomes that result in the restoration of a functional gene. Despite success with the technique, recent studies have revealed that following repair events, correction frequencies decrease as a function of time, possibly due to a sustained activation of damage response signals in corrected cells that lead to a selective stalling. In this study, we use thymidine to slow down the replication rate to enhance repair frequency and to maintain substantial levels of correction over time. Results First, we utilized thymidine to arrest cells in G1 and released the cells into S phase, at which point specific ssODNs direct the highest level of correction. Next, we devised a protocol in which cells are maintained in thymidine following the repair reaction, in which the replication is slowed in both corrected and non-corrected cells and the initial correction frequency is retained. We also present evidence that cells enter a senescence state upon prolonged treatment with thymidine but this passage can be avoided by removing thymidine at 48 hours. Conclusion Taken together, we believe that thymidine may be used in a therapeutic fashion to enable the maintenance of high levels of treated cells bearing repaired genes.
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Huen MSY, Li XT, Lu LY, Watt RM, Liu DP, Huang JD. The involvement of replication in single stranded oligonucleotide-mediated gene repair. Nucleic Acids Res 2006; 34:6183-94. [PMID: 17088285 PMCID: PMC1693898 DOI: 10.1093/nar/gkl852] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
Targeted gene repair mediated by single-stranded oligonucleotides (SSOs) has great potential for use in functional genomic studies and gene therapy. Genetic changes have been created using this approach in a number of prokaryotic and eukaryotic systems, including mouse embryonic stem cells. However, the underlying mechanisms remain to be fully established. In one of the current models, the ‘annealing-integration’ model, the SSO anneals to its target locus at the replication fork, serving as a primer for subsequent DNA synthesis mediated by the host replication machinery. Using a λ-Red recombination-based system in the bacterium Escherichia coli, we systematically examined several fundamental premises that form the mechanistic basis of this model. Our results provide direct evidence strongly suggesting that SSO-mediated gene repair is mechanistically linked to the process of DNA replication, and most likely involves a replication intermediate. These findings will help guide future experiments involving SSO-mediated gene repair in mammalian and prokaryotic cells, and suggest several mechanisms by which the efficiencies may be reliably and substantially increased.
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Affiliation(s)
- Michael S. Y. Huen
- Department of Biochemistry, The University of Hong Kong3/F Laboratory Block, Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong SAR, China
| | - Xin-tian Li
- Department of Biochemistry, The University of Hong Kong3/F Laboratory Block, Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong SAR, China
- National Laboratory of Medical Molecular Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences (CAMS) and Peking Union Medical College (PUMC)Beijing 100005, P.R. China
| | - Lin-Yu Lu
- Department of Biochemistry, The University of Hong Kong3/F Laboratory Block, Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong SAR, China
| | - Rory M. Watt
- Open Laboratory of Chemical Biology, The Institute of Molecular Technology for Drug Discovery and Synthesis, Department of Chemistry, The University of Hong Kong Pokfulam RoadHong Kong SAR, China
| | - De-Pei Liu
- National Laboratory of Medical Molecular Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences (CAMS) and Peking Union Medical College (PUMC)Beijing 100005, P.R. China
| | - Jian-Dong Huang
- Department of Biochemistry, The University of Hong Kong3/F Laboratory Block, Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong SAR, China
- To whom correspondence should be addressed. Tel: +852 2819 2810; Fax: +852 2855 1254;
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63
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Radecke F, Peter I, Radecke S, Gellhaus K, Schwarz K, Cathomen T. Targeted chromosomal gene modification in human cells by single-stranded oligodeoxynucleotides in the presence of a DNA double-strand break. Mol Ther 2006; 14:798-808. [PMID: 16904944 DOI: 10.1016/j.ymthe.2006.06.008] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2006] [Revised: 06/16/2006] [Accepted: 06/21/2006] [Indexed: 01/05/2023] Open
Abstract
A DNA double-strand break (DSB) cannot be tolerated by a cell and is dealt with by several pathways. Here, it was hypothesized that DSB induction close to a targeted mutation in the genome of a mammalian cell might attract oligodeoxynucleotide (ODN)-directed gene repair. A HEK-293-derived cell line had been engineered harboring a single target locus with open reading frames encoding the living-cell reporter proteins LacZ and EGFP, the latter translationally decoupled by a DNA spacer with a unique I-SceI recognition site for defined DSB induction. To enable expression of a fluorescent LacZ-EGFP fusion protein, single-stranded (ss) ODNs (80 or 96 nucleotides long) spanning the DSB were designed to fuse both reading frames by altering a few base-pair positions, deleting 59 bp or introducing a 10-bp fragment. The ssODNs alone generated few EGFP-positive cells. With I-SceI transiently expressed, more than 0.3% of cells revealed EGFP expression 7 days after transfection, with up to 96% of the loci faithfully corrected, depending on the ssODN used. During these correction events, the ssODN did not become physically incorporated into the chromosome, but served only as information template. Unwanted insertional mutagenesis also occurred. Both observations have important implications for gene therapy.
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Affiliation(s)
- Frank Radecke
- Department of Transfusion Medicine, University Hospital Ulm, and Institute for Clinical Transfusion Medicine and Immunogenetics Ulm, 89081 Ulm, Germany
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