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Braam SR, Denning C, Matsa E, Young LE, Passier R, Mummery CL. Feeder-free culture of human embryonic stem cells in conditioned medium for efficient genetic modification. Nat Protoc 2008; 3:1435-43. [PMID: 18772870 DOI: 10.1038/nprot.2008.140] [Citation(s) in RCA: 67] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Realizing the potential of human embryonic stem cells (hESCs) in research and commercial applications requires generic protocols for culture, expansion and genetic modification that function between multiple lines. Here we describe a feeder-free hESC culture protocol that was tested in 13 independent hESC lines derived in five different laboratories. The procedure is based on Matrigel adaptation in mouse embryonic fibroblast conditioned medium (CM) followed by monolayer culture of hESC. When combined, these techniques provide a robust hESC culture platform, suitable for high-efficiency genetic modification via plasmid transfection (using lipofection or electroporation), siRNA knockdown and viral transduction. In contrast to other available protocols, it does not require optimization for individual lines. hESC transiently expressing ectopic genes are obtained within 9 d and stable transgenic lines within 3 weeks.
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Affiliation(s)
- Stefan R Braam
- Hubrecht Institute, Developmental Biology and Stem Cell Research, Uppsalalaan 8, 3584 CT, Utrecht, The Netherlands
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102
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Rizvanov AA, Kiyasov AP, Gaziziov IM, Yilmaz TS, Kaligin MS, Andreeva DI, Shafigullina AK, Guseva DS, Kiselev SL, Matin K, Palotás A, Islamov RR. Human umbilical cord blood cells transfected with VEGF and L(1)CAM do not differentiate into neurons but transform into vascular endothelial cells and secrete neuro-trophic factors to support neuro-genesis-a novel approach in stem cell therapy. Neurochem Int 2008; 53:389-94. [PMID: 18948156 DOI: 10.1016/j.neuint.2008.09.011] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2008] [Accepted: 09/19/2008] [Indexed: 02/07/2023]
Abstract
Genetically modified mono-nuclear cell fraction from human umbilical cord blood (HUCB) expressing human vascular endothelial growth factor (VEGF) and mouse neural L(1) cell adhesion molecule (L(1)CAM) were used for gene-stem cell therapy of transgenic (G)93(A) mice adopted as an animal amyotrophic lateral sclerosis (ALS) model. We generated non-viral plasmid constructs, expressing human VEGF(165) (pcDNA-VEGF) and mouse neural L(1) cell adhesion molecule (pcDNA-mL(1)CAM). Mono-nuclear fraction of HUCB cells were transiently transfected by electro-poration with a mixture of expression plasmids (pcDNA-VEGF+pcDNA-mL(1)CAM). Sixteen transgenic female and male mice were randomly assigned to three groups: (1) transplantation of genetically modified HUCB cells expressing L(1) and VEGF (n=6), (2) transplantation of un-transfected HUCB cells (n=5), and (3) control group (n=5). In first two experimental groups 1x10(6) cells were injected retro-orbitally in pre-symptomatic 22-25-week-old (G)93(A) mice. Our results demonstrate that HUCB cells successfully grafted into nervous tissue of ALS mice and survived for over 3 months. Therefore, genetically modified HUCB cells migrate in the spinal cord parenchyma, proliferate, but instead of transforming into nerve cells, they differentiate into endothelial cells forming new blood vessels. We propose that: (A) expression of mouse neural L(1)CAM is responsible for increased homing and subsequent proliferation of transplanted cells at the site of neuro-degeneration, (B) expression of human VEGF directs HUCB cell differentiation into endothelial cells, and (C) neuro-protective effect may stem from the delivery of various neuro-trophic factors from newly formed blood vessels.
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Affiliation(s)
- Albert A Rizvanov
- Department of Histology, Kazan State Medical University, ul. Butlerova 49, R-420012 Kazan, Russia
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103
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Green JJ, Zhou BY, Mitalipova MM, Beard C, Langer R, Jaenisch R, Anderson DG. Nanoparticles for gene transfer to human embryonic stem cell colonies. NANO LETTERS 2008; 8:3126-30. [PMID: 18754690 PMCID: PMC3814161 DOI: 10.1021/nl8012665] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/07/2023]
Abstract
We develop biodegradable polymeric nanoparticles to facilitate nonviral gene transfer to human embryonic stem cells (hESCs). Small (approximately 200 nm), positively charged (approximately 10 mV) particles are formed by the self assembly of cationic, hydrolytically degradable poly(beta-amino esters) and plasmid DNA. By varying the end group of the polymer, we can tune the biophysical properties of the resulting nanoparticles and their gene-delivery efficacy. We created an OCT4-driven GFP hES cell line to allow the rapid identification of nanoparticles that facilitate gene transfer while maintaining an hESC undifferentiated state. Using this cell system, we synthesized nanoparticles that have gene delivery efficacy that is up to 4 times higher than that of the leading commercially available transfection agent, Lipofectamine 2000. Importantly, these materials have minimal toxicity and do not adversely affect hESC colony morphology or cause nonspecific differentiation.
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104
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Keravala A, Ormerod BK, Palmer TD, Calos MP. Long-term transgene expression in mouse neural progenitor cells modified with phiC31 integrase. J Neurosci Methods 2008; 173:299-305. [PMID: 18606184 PMCID: PMC2615000 DOI: 10.1016/j.jneumeth.2008.06.005] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2008] [Revised: 06/09/2008] [Accepted: 06/10/2008] [Indexed: 11/27/2022]
Abstract
Stem cells can potentially be utilized in combined gene/cell therapies for neural diseases. We examined the ability of the non-viral phiC31 integrase system to promote stable transgene expression in mouse neural progenitor cells (mNPCs). phiC31 integrase catalyzes the sequence-specific integration of attB-containing plasmids into pseudo attP sites in mammalian genomes, to produce long-term transgene expression. We achieved gene transfer by co-nucleofection of a plasmid carrying the luciferase marker gene and an attB site and a plasmid expressing integrase in mNPCs that had been generated in a neurosphere preparation. Luciferase expression was quantified in live cells for 8 weeks, revealing persistence of gene expression. Sequence-specific integration at a preferred pseudo attP site in the mouse genome was detected by using PCR. Furthermore, sustained transgene expression was demonstrated in genetically modified NPCs that were cultured in conditions that promoted either growth or differentiation into neurons and astrocytes. Our results demonstrate that the phiC31 integrase system produces stable transgene expression in adult mNPCs and their progeny and may be useful in strategies for combating neurodegenerative disorders.
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Affiliation(s)
- Annahita Keravala
- Department of Genetics, Stanford University School of Medicine, Stanford. CA 94305
| | - Brandi K. Ormerod
- Department of Neuroscience, Stanford University School of Medicine, Stanford. CA 94305
| | - Theo D. Palmer
- Department of Neuroscience, Stanford University School of Medicine, Stanford. CA 94305
| | - Michele P. Calos
- Department of Genetics, Stanford University School of Medicine, Stanford. CA 94305
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105
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Stewart R, Yang C, Anyfantis G, Przyborski S, Hole N, Strachan T, Stojkovic M, Keith WN, Armstrong L, Lako M. Silencing of the expression of pluripotent driven-reporter genes stably transfected into human pluripotent cells. Regen Med 2008; 3:505-22. [PMID: 18588473 DOI: 10.2217/17460751.3.4.505] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
AIMS & METHODS Marking of human embryonic stem (ES) and embryonal carcinoma (EC) cells with pluripotent promoter-driven reporter gene cassettes provides an important tool for studies related to maintenance of pluripotency, cell differentiation and cell selection. OCT4, TERF1 and telomerase reverse transcriptase component (TERT) are considered as pluripotent marker genes since they are expressed in both human ES and EC cells and significantly downregulated during the differentiation process. Our aim was to use core promoter regions from such pluripotent genes to drive expression of reporter genes that would be suitable for human ES cell selection amongst differentiated cells. RESULTS Human ES and EC cells were stably transfected with a number of TERT, OCT4 and TERF1 promoter-driven EGFP or NTR gene cassettes. Gradual loss of reporter gene expression was observed from 24 h post-transfection during transient transfection studies, while almost complete loss of reporter expression was observed upon stable transfections. The loss of reporter gene expression was partly reversed by addition of a histone deacetylase inhibitor and a demethylating agent, suggesting that in vitro methylation of these exogenous constructs and the epigenetic architecture around the site of integration are likely to play a major role in their transcriptional activity. Inclusion of gene-regulatory elements in addition to the core promoters has been shown to minimize such effects and should be considered as an important strategy in such studies. CONCLUSIONS Together our data suggest that human ES and EC cells are able to silence pluripotent promoter-driven reporter genes with high efficiency. Whether differentiated cells derived from human ES and EC cells retain this activity is unknown and need to be investigated before large-scale comparative reporter-based transfection studies can be used as a tool in human embryonic stem cell biology.
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Affiliation(s)
- Rebecca Stewart
- North East Institute for Stem Cell Research, Newcastle University International Centre for Life, Newcastle NE13BZ, UK
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106
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Kwok-Keung Chan K, Meiyun Wu S, Morin Nissom P, Oh SK, Choo AB. Generation of High-Level Stable Transgene Expressing Human Embryonic Stem Cell Lines Using Chinese Hamster Elongation Factor-1α Promoter System. Stem Cells Dev 2008; 17:825-36. [DOI: 10.1089/scd.2007.0233] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Affiliation(s)
- Ken Kwok-Keung Chan
- Stem Cell Group, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore
| | - Selena Meiyun Wu
- Stem Cell Group, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore
| | - Peter Morin Nissom
- Microarray Laboratory, Bioprocessing Technology Institute, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore
| | - Steve K.W. Oh
- Stem Cell Group, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore
| | - Andre B.H. Choo
- Stem Cell Group, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore
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107
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Fenno LE, Ptaszek LM, Cowan CA. Human embryonic stem cells: emerging technologies and practical applications. Curr Opin Genet Dev 2008; 18:324-9. [DOI: 10.1016/j.gde.2008.06.004] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2008] [Revised: 06/11/2008] [Accepted: 06/14/2008] [Indexed: 10/21/2022]
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108
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Lentiviral-Mediated HoxB4 Expression in Human Embryonic Stem Cells Initiates Early Hematopoiesis in a Dose-Dependent Manner but Does Not Promote Myeloid Differentiation. Stem Cells 2008; 26:2455-66. [DOI: 10.1634/stemcells.2007-0876] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
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109
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Wilson K, Yu J, Lee A, Wu JC. In vitro and in vivo bioluminescence reporter gene imaging of human embryonic stem cells. J Vis Exp 2008:740. [PMID: 19066577 DOI: 10.3791/740] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
The discovery of human embryonic stem cells (hESCs) has dramatically increased the tools available to medical scientists interested in regenerative medicine. However, direct injection of hESCs, and cells differentiated from hESCs, into living organisms has thus far been hampered by significant cell death, teratoma formation, and host immune rejection. Understanding the in vivo hESC behavior after transplantation requires novel imaging techniques to longitudinally monitor hESC localization, proliferation, and viability. Molecular imaging has given investigators a high-throughput, inexpensive, and sensitive means for tracking in vivo cell proliferation over days, weeks, and even months. This advancement has significantly increased the understanding of the spatio-temporal kinetics of hESC engraftment, proliferation, and teratoma-formation in living subjects. A major advance in molecular imaging has been the extension of noninvasive reporter gene assays from molecular and cellular biology into in vivo multi-modality imaging platforms. These reporter genes, under control of engineered promoters and enhancers that take advantage of the host cell s transcriptional machinery, are introduced into cells using a variety of vector and non-vector methods. Once in the cell, reporter genes can be transcribed either constitutively or only under specific biological or cellular conditions, depending on the type of promoter used. Transcription and translation of reporter genes into bioactive proteins is then detected with sensitive, noninvasive instrumentation (e.g., CCD cameras) using signal-generating probes such as D-luciferin. To avoid the need for excitatory light to track stem cells in vivo as is required for fluorescence imaging, bioluminescence reporter gene imaging systems require only an exogenously administered probe to induce light emission. Firefly luciferase, derived from the firefly Photinus pyralis, encodes an enzyme that catalyzes D-luciferin to the optically active metabolite, oxyluciferin. Optical activity can then be monitored with an external CCD camera. Stably transduced cells that carry the reporter construct within their chromosomal DNA will pass the reporter construct DNA to daughter cells, allowing for longitudinal monitoring of hESC survival and proliferation in vivo. Furthermore, because expression of the reporter gene product is required for signal generation, only viable parent and daughter cells will create bioluminescence signal; apoptotic or dead cells will not. In this video, the specific materials and methods needed for tracking stem cell proliferation and teratoma formation with bioluminescence imaging will be described.
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Affiliation(s)
- Kitchener Wilson
- Department of Radiology, Stanford University School of Medicine, USA
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110
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Braam SR, Denning C, van den Brink S, Kats P, Hochstenbach R, Passier R, Mummery CL. Improved genetic manipulation of human embryonic stem cells. Nat Methods 2008; 5:389-92. [PMID: 18391958 DOI: 10.1038/nmeth.1200] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2008] [Accepted: 03/13/2008] [Indexed: 02/08/2023]
Abstract
Low efficiency of transfection limits the ability to genetically manipulate human embryonic stem cells (hESCs), and differences in cell derivation and culture methods require optimization of transfection protocols. We transiently transferred multiple independent hESC lines with different growth requirements to standardized feeder-free culture, and optimized conditions for clonal growth and efficient gene transfer without loss of pluripotency. Stably transfected lines retained differentiation potential, and most lines displayed normal karyotypes.
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Affiliation(s)
- Stefan R Braam
- Hubrecht Institute, Developmental Biology and Stem Cell Research, Uppsalalaan 8, 3584 CT Utrecht, The Netherlands
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111
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PAX4 enhances beta-cell differentiation of human embryonic stem cells. PLoS One 2008; 3:e1783. [PMID: 18335054 PMCID: PMC2262135 DOI: 10.1371/journal.pone.0001783] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2007] [Accepted: 02/06/2008] [Indexed: 11/19/2022] Open
Abstract
Background Human embryonic stem cells (HESC) readily differentiate into an apparently haphazard array of cell types, corresponding to all three germ layers, when their culture conditions are altered, for example by growth in suspension as aggregates known as embryoid bodies (EBs). However, this diversity of differentiation means that the efficiency of producing any one particular cell type is inevitably low. Although pancreatic differentiation has been reported from HESC, practicable applications for the use of β-cells derived from HESC to treat diabetes will only be possible once techniques are developed to promote efficient differentiation along the pancreatic lineages. Methods and Findings Here, we have tested whether the transcription factor, Pax4 can be used to drive the differentiation of HESC to a β-cell fate in vitro. We constitutively over-expressed Pax4 in HESCs by stable transfection, and used Q-PCR analysis, immunocytochemistry, ELISA, Ca2+ microfluorimetry and cell imaging to assess the role of Pax4 in the differentiation and intracellular Ca2+ homeostasis of β-cells developing in embryoid bodies produced from such HESC. Cells expressing key β-cell markers were isolated by fluorescence-activated cell sorting after staining for high zinc content using the vital dye, Newport Green. Conclusion Constitutive expression of Pax4 in HESC substantially enhances their propensity to form putative β-cells. Our findings provide a novel foundation to study the mechanism of pancreatic β-cells differentiation during early human development and to help evaluate strategies for the generation of purified β-cells for future clinical applications.
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112
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Hohenstein KA, Pyle AD, Chern JY, Lock LF, Donovan PJ. Nucleofection mediates high-efficiency stable gene knockdown and transgene expression in human embryonic stem cells. Stem Cells 2008; 26:1436-43. [PMID: 18323409 DOI: 10.1634/stemcells.2007-0857] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
High-efficiency genetic modification of human embryonic stem (hES) cells would enable manipulation of gene activity, routine gene targeting, and development of new human disease models and treatments. Chemical transfection, nucleofection, and electroporation of hES cells result in low transfection efficiencies. Viral transduction is efficient but has significant drawbacks. Here we describe techniques to transiently and stably express transgenes in hES cells with high efficiency using a widely available vector system. The technique combines nucleofection of single hES cells with improved methods to select hES cells at clonal density. As validation, we reduced Oct4 and Nanog expression using siRNAs and shRNA vectors in hES cells. Furthermore, we derived many hES cell clones with either stably reduced alkaline phosphatase activity or stably overexpressed green fluorescent protein. These clones retained stem cell characteristics (normal karyotype, stem cell marker expression, self-renewal, and pluripotency). These studies will accelerate efforts to interrogate gene function and define the parameters that control growth and differentiation of hES cells. Disclosure of potential conflicts of interest is found at the end of this article.
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Affiliation(s)
- Kristi A Hohenstein
- Department of Biological Chemistry, Sue and Bill Gross Stem Cell Research Program, Center for Molecular and Mitochondrial Medicine and Genetics, University of California Irvine, Irvine, California 92697, USA
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113
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Hall VJ, Li JY, Brundin P. Restorative cell therapy for Parkinson's disease: A quest for the perfect cell. Semin Cell Dev Biol 2007; 18:859-69. [DOI: 10.1016/j.semcdb.2007.09.004] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2007] [Accepted: 09/05/2007] [Indexed: 12/09/2022]
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114
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Smith JR, Maguire S, Davis LA, Alexander M, Yang F, Chandran S, ffrench-Constant C, Pedersen RA. Robust, persistent transgene expression in human embryonic stem cells is achieved with AAVS1-targeted integration. Stem Cells 2007; 26:496-504. [PMID: 18024421 DOI: 10.1634/stemcells.2007-0039] [Citation(s) in RCA: 164] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Silencing and variegated transgene expression are poorly understood problems that can interfere with gene function studies in human embryonic stem cells (hESCs). We show that transgene expression (enhanced green fluorescent protein [EGFP]) from random integration sites in hESCs is affected by variegation and silencing, with only half of hESCs expressing the transgene, which is gradually lost after withdrawal of selection and differentiation. We tested the hypothesis that a transgene integrated into the adeno-associated virus type 2 (AAV2) target region on chromosome 19, known as the AAVS1 locus, would maintain transgene expression in hESCs. When we used AAV2 technology to target the AAVS1 locus, 4.16% of hESC clones achieved AAVS1-targeted integration. Targeted clones expressed Oct-4, stage-specific embryonic antigen-3 (SSEA3), and Tra-1-60 and differentiated into all three primary germ layers. EGFP expression from the AAVS1 locus showed significantly reduced variegated expression when in selection, with 90% +/- 4% of cells expressing EGFP compared with 57% +/- 32% for randomly integrated controls, and reduced tendency to undergo silencing, with 86% +/- 7% hESCs expressing EGFP 25 days after withdrawal of selection compared with 39% +/- 31% for randomly integrated clones. In addition, quantitative polymerase chain reaction analysis of hESCs also indicated significantly higher levels of EGFP mRNA in AAVS1-targeted clones as compared with randomly integrated clones. Transgene expression from the AAVS1 locus was shown to be stable during hESC differentiation, with more than 90% of cells expressing EGFP after 15 days of differentiation, as compared with approximately 30% for randomly integrated clones. These results demonstrate the utility of transgene integration at the AAVS1 locus in hESCs and its potential clinical application.
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Affiliation(s)
- Joseph R Smith
- Department of Surgery, Cambridge Institute for Medical Research, Wellcome Trust/MRC Building, Addenbrooke's Hospital, Hills Road, Cambridge CB2 OXY, United Kingdom.
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115
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Mummery C. Genetic Selection of Cardiomyocytes from Human Embryonic Stem Cells. Mol Ther 2007; 15:1908-9. [DOI: 10.1038/sj.mt.6300307] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
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116
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Thyagarajan B, Liu Y, Shin S, Lakshmipathy U, Scheyhing K, Xue H, Ellerström C, Strehl R, Hyllner J, Rao MS, Chesnut JD. Creation of engineered human embryonic stem cell lines using phiC31 integrase. Stem Cells 2007; 26:119-26. [PMID: 17962703 DOI: 10.1634/stemcells.2007-0283] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
It has previously been shown that the phage-derived phiC31 integrase can efficiently target native pseudo-attachment sites in the genome of various species in cultured cells, as well as in vivo. To demonstrate its utility in human embryonic stem cells (hESC), we have created hESC-derived clones containing expression constructs. Variant human embryonic stem cell lines BG01v and SA002 were used to derive lines expressing a green fluorescent protein (GFP) marker under control of either the human Oct4 promoter or the EF1alpha promoter. Stable clones were selected by antibiotic resistance and further characterized. The frequency of integration suggested candidate hot spots in the genome, which were mapped using a plasmid rescue strategy. The pseudo-attP profile in hESC differed from those reported earlier in differentiated cells. Clones derived using this method retained the ability to differentiate into all three germ layers, and fidelity of expression of GFP was verified in differentiation assays. GFP expression driven by the Oct4 promoter recapitulated endogenous Oct4 expression, whereas persistent stable expression of GFP expression driven by the EF1alpha promoter was seen. Our results demonstrate the utility of phiC31 integrase to target pseudo-attP sites in hESC and show that integrase-mediated site-specific integration can efficiently create stably expressing engineered human embryonic stem cell clones.
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