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Raab N, Zeh N, Kretz R, Weiß L, Stadermann A, Lindner B, Fischer S, Stoll D, Otte K. Nature as blueprint: Global phenotype engineering of CHO production cells based on a multi-omics comparison with plasma cells. Metab Eng 2024; 83:110-122. [PMID: 38561148 DOI: 10.1016/j.ymben.2024.03.007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2023] [Revised: 03/17/2024] [Accepted: 03/29/2024] [Indexed: 04/04/2024]
Abstract
Especially for the production of artificial, difficult to express molecules a further development of the CHO production cell line is required to keep pace with the continuously increasing demands. However, the identification of novel targets for cell line engineering to improve CHO cells is a time and cost intensive process. Since plasma cells are evolutionary optimized for a high antibody expression in mammals, we performed a comprehensive multi-omics comparison between CHO and plasma cells to exploit optimized cellular production traits. Comparing the transcriptome, proteome, miRNome, surfaceome and secretome of both cell lines identified key differences including 392 potential overexpression targets for CHO cell engineering categorized in 15 functional classes like transcription factors, protein processing or secretory pathway. In addition, 3 protein classes including 209 potential knock-down/out targets for CHO engineering were determined likely to affect aggregation or proteolysis. For production phenotype engineering, several of these novel targets were successfully applied to transient and transposase mediated overexpression or knock-down strategies to efficiently improve productivity of CHO cells. Thus, substantial improvement of CHO productivity was achieved by taking nature as a blueprint for cell line engineering.
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Affiliation(s)
- Nadja Raab
- Biberach University of Applied Sciences, Germany.
| | - Nikolas Zeh
- Biberach University of Applied Sciences, Germany; Cell Line Development, Bioprocess Development Biologicals, Boehringer Ingelheim Pharma GmbH & Co.KG, Biberach, Germany
| | - Robin Kretz
- Hochschule Albstadt Sigmaringen, Germany; NMI, Natural and Medical Sciences Institute at the University of Tuebingen, Reutlingen, Germany
| | - Linus Weiß
- Biberach University of Applied Sciences, Germany
| | - Anna Stadermann
- Cell Line Development, Bioprocess Development Biologicals, Boehringer Ingelheim Pharma GmbH & Co.KG, Biberach, Germany
| | - Benjamin Lindner
- Cell Line Development, Bioprocess Development Biologicals, Boehringer Ingelheim Pharma GmbH & Co.KG, Biberach, Germany
| | - Simon Fischer
- Cell Line Development, Bioprocess Development Biologicals, Boehringer Ingelheim Pharma GmbH & Co.KG, Biberach, Germany
| | - Dieter Stoll
- NMI, Natural and Medical Sciences Institute at the University of Tuebingen, Reutlingen, Germany
| | - Kerstin Otte
- Biberach University of Applied Sciences, Germany
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2
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Marx N, Eisenhut P, Weinguny M, Klanert G, Borth N. How to train your cell - Towards controlling phenotypes by harnessing the epigenome of Chinese hamster ovary production cell lines. Biotechnol Adv 2022; 56:107924. [PMID: 35149147 DOI: 10.1016/j.biotechadv.2022.107924] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2021] [Revised: 02/03/2022] [Accepted: 02/04/2022] [Indexed: 11/24/2022]
Abstract
Recent advances in omics technologies and the broad availability of big datasets have revolutionized our understanding of Chinese hamster ovary cells in their role as the most prevalent host for production of complex biopharmaceuticals. In consequence, our perception of this "workhorse of the biopharmaceutical industry" has successively shifted from that of a nicely working, but unknown recombinant protein producing black box to a biological system governed by multiple complex regulatory layers that might possibly be harnessed and manipulated at will. Despite the tremendous progress that has been made to characterize CHO cells on various omics levels, our understanding is still far from complete. The well-known inherent genetic plasticity of any immortalized and rapidly dividing cell line also characterizes CHO cells and can lead to problematic instability of recombinant protein production. While the high mutational frequency has been a focus of CHO cell research for decades, the impact of epigenetics and its role in differential gene expression has only recently been addressed. In this review we provide an overview about the current understanding of epigenetic regulation in CHO cells and discuss its significance for shaping the cell's phenotype. We also look into current state-of-the-art technology that can be applied to harness and manipulate the epigenetic network so as to nudge CHO cells towards a specific phenotype. Here, we revise current strategies on site-directed integration and random as well as targeted epigenome modifications. Finally, we address open questions that need to be investigated to exploit the full repertoire of fine-tuned control of multiplexed gene expression using epigenetic and systems biology tools.
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Affiliation(s)
- Nicolas Marx
- University of Natural Resources and Life Sciences, Vienna, Austria
| | - Peter Eisenhut
- Austrian Centre for Industrial Biotechnology GmbH, Vienna, Austria
| | - Marcus Weinguny
- University of Natural Resources and Life Sciences, Vienna, Austria; Austrian Centre for Industrial Biotechnology GmbH, Vienna, Austria
| | - Gerald Klanert
- Austrian Centre for Industrial Biotechnology GmbH, Vienna, Austria
| | - Nicole Borth
- University of Natural Resources and Life Sciences, Vienna, Austria; Austrian Centre for Industrial Biotechnology GmbH, Vienna, Austria.
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3
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Chang M, Kumar A, Kumar S, Huhn S, Timp W, Betenbaugh M, Du Z. Epigenetic Comparison of CHO Hosts and Clones Reveals Divergent Methylation and Transcription Patterns Across Lineages. Biotechnol Bioeng 2022; 119:1062-1076. [PMID: 35028935 DOI: 10.1002/bit.28036] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2021] [Revised: 12/08/2021] [Accepted: 12/26/2021] [Indexed: 11/11/2022]
Abstract
In this study, we examined DNA methylation and transcription profiles of recombinant clones derived from two different Chinese hamster ovary hosts. We found striking epigenetic differences between the clones, with global hypomethylation in the host 1 clones that produce bispecific antibody with higher productivity and complex assembly efficiency. Whereas the methylation patterns were found mostly inherited from the host, the host 1 clones exhibited continued demethylation reflected by the hypomethylation of newly emerged differential methylation regions (DMRs) even at the clone development stage. Several interconnected biological functions and pathways including cell adhesion, regulation of ion transport, and cholesterol biosynthesis were significantly altered between the clones at the RNA expression level and contained DMR in the promoter and/or gene-body of the transcripts, suggesting epigenetic regulation. Indeed, expression changes of epigenetic regulators were observed including writers (Dnmt1, Setdb1), readers (Mecp2), and erasers (Tet3, Kdm3a, Kdm1b/5c) involved in CpG methylation, histone methylation and heterochromatin maintenance. In addition, we identified putative transcription factors that may be readers or effectors of the epigenetic regulation in these clones. By combining transcriptomics with DNA methylation data, we identified potential processes and factors that may contribute to the variability in cell physiology between different production hosts. This article is protected by copyright. All rights reserved.
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Affiliation(s)
- Meiping Chang
- Process Cell Sciences, Biologics Process R&D, Merck & Co., Inc., Kenilworth, NJ, USA
| | - Amit Kumar
- Process Cell Sciences, Biologics Process R&D, Merck & Co., Inc., Kenilworth, NJ, USA
| | - Swetha Kumar
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University
| | - Steven Huhn
- Process Cell Sciences, Biologics Process R&D, Merck & Co., Inc., Kenilworth, NJ, USA
| | - Winston Timp
- Department of Biomedical Engineering, Johns Hopkins University
| | - Michael Betenbaugh
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University
| | - Zhimei Du
- Process Cell Sciences, Biologics Process R&D, Merck & Co., Inc., Kenilworth, NJ, USA
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4
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Electrospun Nanofibrous Membranes for Tissue Engineering and Cell Growth. APPLIED SCIENCES-BASEL 2021. [DOI: 10.3390/app11156929] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
In biotechnology, the field of cell cultivation is highly relevant. Cultivated cells can be used, for example, for the development of biopharmaceuticals and in tissue engineering. Commonly, mammalian cells are grown in bioreactors, T-flasks, well plates, etc., without a specific substrate. Nanofibrous mats, however, have been reported to promote cell growth, adhesion, and proliferation. Here, we give an overview of the different attempts at cultivating mammalian cells on electrospun nanofiber mats for biotechnological and biomedical purposes. Starting with a brief overview of the different electrospinning methods, resulting in random or defined fiber orientations in the nanofiber mats, we describe the typical materials used in cell growth applications in biotechnology and tissue engineering. The influence of using different surface morphologies and polymers or polymer blends on the possible application of such nanofiber mats for tissue engineering and other biotechnological applications is discussed. Polymer blends, in particular, can often be used to reach the required combination of mechanical and biological properties, making such nanofiber mats highly suitable for tissue engineering and other biotechnological or biomedical cell growth applications.
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Cid R, Bolívar J. Platforms for Production of Protein-Based Vaccines: From Classical to Next-Generation Strategies. Biomolecules 2021; 11:1072. [PMID: 34439738 PMCID: PMC8394948 DOI: 10.3390/biom11081072] [Citation(s) in RCA: 54] [Impact Index Per Article: 18.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2021] [Revised: 07/16/2021] [Accepted: 07/17/2021] [Indexed: 12/12/2022] Open
Abstract
To date, vaccination has become one of the most effective strategies to control and reduce infectious diseases, preventing millions of deaths worldwide. The earliest vaccines were developed as live-attenuated or inactivated pathogens, and, although they still represent the most extended human vaccine types, they also face some issues, such as the potential to revert to a pathogenic form of live-attenuated formulations or the weaker immune response associated with inactivated vaccines. Advances in genetic engineering have enabled improvements in vaccine design and strategies, such as recombinant subunit vaccines, have emerged, expanding the number of diseases that can be prevented. Moreover, antigen display systems such as VLPs or those designed by nanotechnology have improved the efficacy of subunit vaccines. Platforms for the production of recombinant vaccines have also evolved from the first hosts, Escherichia coli and Saccharomyces cerevisiae, to insect or mammalian cells. Traditional bacterial and yeast systems have been improved by engineering and new systems based on plants or insect larvae have emerged as alternative, low-cost platforms. Vaccine development is still time-consuming and costly, and alternative systems that can offer cost-effective and faster processes are demanding to address infectious diseases that still do not have a treatment and to face possible future pandemics.
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Affiliation(s)
- Raquel Cid
- ADL Bionatur Solutions S.A., Av. del Desarrollo Tecnológico 11, 11591 Jerez de la Frontera, Spain
| | - Jorge Bolívar
- Department of Biomedicine, Biotechnology and Public Health-Biochemistry and Molecular Biology, Campus Universitario de Puerto Real, University of Cadiz, 11510 Puerto Real, Spain
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Weinguny M, Eisenhut P, Klanert G, Virgolini N, Marx N, Jonsson A, Ivansson D, Lövgren A, Borth N. Random epigenetic modulation of CHO cells by repeated knockdown of DNA methyltransferases increases population diversity and enables sorting of cells with higher production capacities. Biotechnol Bioeng 2020; 117:3435-3447. [PMID: 32662873 PMCID: PMC7818401 DOI: 10.1002/bit.27493] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2020] [Revised: 05/25/2020] [Accepted: 07/12/2020] [Indexed: 12/15/2022]
Abstract
Chinese hamster ovary (CHO) cells produce a large share of today's biopharmaceuticals. Still, the generation of satisfactory producer cell lines is a tedious undertaking. Recently, it was found that CHO cells, when exposed to new environmental conditions, modify their epigenome, suggesting that cells adapt their gene expression pattern to handle new challenges. The major aim of the present study was to employ artificially induced, random changes in the DNA-methylation pattern of CHO cells to diversify cell populations and consequently increase the finding of cell lines with improved cellular characteristics. To achieve this, DNA methyltransferases and/or the ten-eleven translocation enzymes were downregulated by RNA interference over a time span of ∼16 days. Methylation analysis of the resulting cell pools revealed that the knockdown of DNA methyltransferases was highly effective in randomly demethylating the genome. The same approach, when applied to stable CHO producer cells resulted in (a) an increased productivity diversity in the cell population, and (b) a higher number of outliers within the population, which resulted in higher specific productivity and titer in the sorted cells. These findings suggest that epigenetics play a previously underestimated, but actually important role in defining the overall cellular behavior of production clones.
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Affiliation(s)
- Marcus Weinguny
- ACIB—Austrian Centre of Industrial BiotechnologyGrazAustria,Department of BiotechnologyUniversity of Natural Resources and Life SciencesViennaAustria
| | - Peter Eisenhut
- ACIB—Austrian Centre of Industrial BiotechnologyGrazAustria,Department of BiotechnologyUniversity of Natural Resources and Life SciencesViennaAustria
| | - Gerald Klanert
- ACIB—Austrian Centre of Industrial BiotechnologyGrazAustria
| | | | - Nicolas Marx
- ACIB—Austrian Centre of Industrial BiotechnologyGrazAustria,Department of BiotechnologyUniversity of Natural Resources and Life SciencesViennaAustria
| | | | | | | | - Nicole Borth
- ACIB—Austrian Centre of Industrial BiotechnologyGrazAustria,Department of BiotechnologyUniversity of Natural Resources and Life SciencesViennaAustria
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7
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Fluorescence-assisted sequential insertion of transgenes (FASIT): an approach for increasing specific productivity in mammalian cells. Sci Rep 2020; 10:12840. [PMID: 32732973 PMCID: PMC7392891 DOI: 10.1038/s41598-020-69709-1] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2020] [Accepted: 07/16/2020] [Indexed: 11/08/2022] Open
Abstract
Currently, the generation of cell lines for the production of recombinant proteins has the limitation of unstable gene expression due to the repeat-induced gene silencing or the loss of transgene copies resulting from recombination events. In this work, we developed a new strategy based on the sequential insertion of transgenes for generating stable clones producing high levels of a chimeric human follicle-stimulating hormone (hscFSH). Gene insertion was done by transducing HEK-293 cells with a lentiviral vector containing a bicistronic transcriptional unit for expressing hscFSH and GFP genes. Clone selection was performed by flow cytometry coupled to cell sorting, and the GFP gene was further removed by CRE-mediated site-specific recombination. High-producing clones of hscFSH were obtained after three rounds of lentiviral transduction. Expression levels increased in a step-wise manner from 7 to 23 pg/cell/day, with a relatively constant rate of 7 pg/cell/day in each round of transduction. The GFP gene was successfully removed from the cell genome without disturbing the hscFSH gene expression. Clones generated using this approach showed stable expression levels for more than two years. This is the first report describing the sequential insertion of transgenes as an alternative for increasing the expression levels of transformed cell lines. The methodology described here could notably impact on biotechnological industry by improving the capacity of mammalian cells to produce biopharmaceuticals.
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Verhagen N, Teleki A, Heinrich C, Schilling M, Unsöld A, Takors R. S-adenosylmethionine and methylthioadenosine boost cellular productivities of antibody forming Chinese hamster ovary cells. Biotechnol Bioeng 2020; 117:3239-3247. [PMID: 32644191 DOI: 10.1002/bit.27484] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2020] [Revised: 06/22/2020] [Accepted: 07/07/2020] [Indexed: 01/28/2023]
Abstract
The improvement of cell specific productivities for the formation of therapeutic proteins is an important step towards intensified production processes. Among others, the induction of the desired production phenotype via proper media additives is a feasible solution provided that said compounds adequately trigger metabolic and regulatory programs inside the cells. In this study, S-(5'-adenosyl)- l-methionine (SAM) and 5'-deoxy-5'-(methylthio)adenosine (MTA) were found to stimulate cell specific productivities up to approx. 50% while keeping viable cell densities transiently high and partially arresting the cell cycle in an anti-IL-8-producing CHO-DP12 cell line. Noteworthy, MTA turned out to be the chemical degradation product of the methyl group donor SAM and is consumed by the cells.
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Affiliation(s)
- Natascha Verhagen
- Institute of Biochemical Engineering, University of Stuttgart, Allmandring, Stuttgart, Germany
| | - Attila Teleki
- Institute of Biochemical Engineering, University of Stuttgart, Allmandring, Stuttgart, Germany
| | | | | | - Andreas Unsöld
- Boehringer Ingelheim Pharma GmbH & Co. KG, Biberach, Germany
| | - Ralf Takors
- Institute of Biochemical Engineering, University of Stuttgart, Allmandring, Stuttgart, Germany
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Luo X, Wang F, Wang G, Zhao Y. Identification of methylation states of DNA regions for Illumina methylation BeadChip. BMC Genomics 2020; 21:672. [PMID: 32138668 PMCID: PMC7057447 DOI: 10.1186/s12864-019-6019-0] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2019] [Accepted: 08/07/2019] [Indexed: 12/24/2022] Open
Abstract
Background Methylation of cytosine bases in DNA is a critical epigenetic mark in many eukaryotes and has also been implicated in the development and progression of normal and diseased cells. Therefore, profiling DNA methylation across the genome is vital to understanding the effects of epigenetic. In recent years the Illumina HumanMethylation450 (HM450K) and MethylationEPIC (EPIC) BeadChip have been widely used to profile DNA methylation in human samples. The methods to predict the methylation states of DNA regions based on microarray methylation datasets are critical to enable genome-wide analyses. Result We report a computational approach based on the two layers two-state hidden Markov model (HMM) to identify methylation states of single CpG site and DNA regions in HM450K and EPIC BeadChip. Using this mothed, all CpGs detected by HM450K and EPIC in H1-hESC and GM12878 cell lines are identified as un-methylated, middle-methylated and full-methylated states. A large number of DNA regions are segmented into three methylation states as well. Comparing the identified regions with the result from the whole genome bisulfite sequencing (WGBS) datasets segmented by MethySeekR, our method is verified. Genome-wide maps of chromatin states show that methylation state is inversely correlated with active histone marks. Genes regulated by un-methylated regions are expressed and regulated by full-methylated regions are repressed. Our method is illustrated to be useful and robust. Conclusion Our method is valuable for DNA methylation genome-wide analyses. It is focusing on identification of DNA methylation states on microarray methylation datasets. For the features of array datasets, using two layers two-state HMM to identify to methylation states on CpG sites and regions creatively, our method which takes into account the distribution of genome-wide methylation levels is more reasonable than segmentation with a fixed threshold. Electronic supplementary material The online version of this article (10.1186/s12864-019-6019-0) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- Ximei Luo
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin, China
| | - Fang Wang
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin, China
| | - Guohua Wang
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin, China.
| | - Yuming Zhao
- Information and Computer Engineering College, Northeast Forestry University, Harbin, China.
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Digital Twins and Their Role in Model-Assisted Design of Experiments. ADVANCES IN BIOCHEMICAL ENGINEERING/BIOTECHNOLOGY 2020; 177:29-61. [PMID: 32797268 DOI: 10.1007/10_2020_136] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Rising demands for biopharmaceuticals and the need to reduce manufacturing costs increase the pressure to develop productive and efficient bioprocesses. Among others, a major hurdle during process development and optimization studies is the huge experimental effort in conventional design of experiments (DoE) methods. As being an explorative approach, DoE requires extensive expert knowledge about the investigated factors and their boundary values and often leads to multiple rounds of time-consuming and costly experiments. The combination of DoE with a virtual representation of the bioprocess, called digital twin, in model-assisted DoE (mDoE) can be used as an alternative to decrease the number of experiments significantly. mDoE enables a knowledge-driven bioprocess development including the definition of a mathematical process model in the early development stages. In this chapter, digital twins and their role in mDoE are discussed. First, statistical DoE methods are introduced as the basis of mDoE. Second, the combination of a mathematical process model and DoE into mDoE is examined. This includes mathematical model structures and a selection scheme for the choice of DoE designs. Finally, the application of mDoE is discussed in a case study for the medium optimization in an antibody-producing Chinese hamster ovary cell culture process.
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Wehlage D, Blattner H, Mamun A, Kutzli I, Diestelhorst E, Rattenholl A, Gudermann F, Lütkemeyer D, Ehrmann A. Cell growth on electrospun nanofiber mats from polyacrylonitrile (PAN) blends. AIMS BIOENGINEERING 2020. [DOI: 10.3934/bioeng.2020004] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
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12
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Dhiman H, Gerstl MP, Ruckerbauer D, Hanscho M, Himmelbauer H, Clarke C, Barron N, Zanghellini J, Borth N. Genetic and Epigenetic Variation across Genes Involved in Energy Metabolism and Mitochondria of Chinese Hamster Ovary Cell Lines. Biotechnol J 2019; 14:e1800681. [DOI: 10.1002/biot.201800681] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2018] [Revised: 03/14/2019] [Indexed: 12/21/2022]
Affiliation(s)
- Heena Dhiman
- Department of BiotechnologyUniversity of Natural Resources and Life SciencesVienna Austria
- Austrian Centre of Industrial BiotechnologyMuthgasse 11 1190 Vienna Austria
| | - Matthias P. Gerstl
- Department of BiotechnologyUniversity of Natural Resources and Life SciencesVienna Austria
- Austrian Centre of Industrial BiotechnologyMuthgasse 11 1190 Vienna Austria
| | - David Ruckerbauer
- Austrian Centre of Industrial BiotechnologyMuthgasse 11 1190 Vienna Austria
| | - Michael Hanscho
- Austrian Centre of Industrial BiotechnologyMuthgasse 11 1190 Vienna Austria
| | - Heinz Himmelbauer
- Department of BiotechnologyUniversity of Natural Resources and Life SciencesVienna Austria
| | - Colin Clarke
- National Institute for Bioprocessing Research and TrainingBlackrock, Co Dublin Ireland
| | - Niall Barron
- National Institute for Bioprocessing Research and TrainingBlackrock, Co Dublin Ireland
- School of Chemical and Bioprocess EngineeringUniversity College DublinGlasnevin Whitehall Dublin Ireland
| | - Jürgen Zanghellini
- Austrian Centre of Industrial BiotechnologyMuthgasse 11 1190 Vienna Austria
- Austrian Biotech University of Applied SciencesKonrad Lorenz Strasse 10 3430 Tulln Austria
| | - Nicole Borth
- Department of BiotechnologyUniversity of Natural Resources and Life SciencesVienna Austria
- Austrian Centre of Industrial BiotechnologyMuthgasse 11 1190 Vienna Austria
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Möller J, Kuchemüller KB, Steinmetz T, Koopmann KS, Pörtner R. Model-assisted Design of Experiments as a concept for knowledge-based bioprocess development. Bioprocess Biosyst Eng 2019; 42:867-882. [DOI: 10.1007/s00449-019-02089-7] [Citation(s) in RCA: 46] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2018] [Accepted: 02/05/2019] [Indexed: 12/11/2022]
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14
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Hunter M, Yuan P, Vavilala D, Fox M. Optimization of Protein Expression in Mammalian Cells. ACTA ACUST UNITED AC 2018; 95:e77. [DOI: 10.1002/cpps.77] [Citation(s) in RCA: 64] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
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15
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Patel NA, Anderson CR, Terkildsen SE, Davis RC, Pack LD, Bhargava S, Clarke HR. Antibody expression stability in CHO clonally derived cell lines and their subclones: Role of methylation in phenotypic and epigenetic heterogeneity. Biotechnol Prog 2018; 34:635-649. [DOI: 10.1002/btpr.2655] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2018] [Revised: 04/24/2018] [Indexed: 11/11/2022]
Affiliation(s)
- Neha A. Patel
- Dept. of Bioprocess Development; Seattle Genetics; Bothell WA 98021
| | | | | | - Ray C. Davis
- Dept. of Bioprocess Development; Seattle Genetics; Bothell WA 98021
| | - Laura D. Pack
- Dept. of CMC Statistics; Seattle Genetics; Bothell WA 98021
| | - Swapnil Bhargava
- Dept. of Bioprocess Development; Seattle Genetics; Bothell WA 98021
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16
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Marx N, Grünwald-Gruber C, Bydlinski N, Dhiman H, Ngoc Nguyen L, Klanert G, Borth N. CRISPR-Based Targeted Epigenetic Editing Enables Gene Expression Modulation of the Silenced Beta-Galactoside Alpha-2,6-Sialyltransferase 1 in CHO Cells. Biotechnol J 2018; 13:e1700217. [DOI: 10.1002/biot.201700217] [Citation(s) in RCA: 37] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2018] [Revised: 05/01/2018] [Indexed: 12/20/2022]
Affiliation(s)
- Nicolas Marx
- Department of Biotechnology; BOKU University of Natural Resources and Life Sciences; Muthgasse 18 1190 Vienna Austria
- Austrian Centre of Industrial Biotechnology GmbH; Graz Austria
| | - Clemens Grünwald-Gruber
- Department of Biotechnology; BOKU University of Natural Resources and Life Sciences; Muthgasse 18 1190 Vienna Austria
| | - Nina Bydlinski
- Department of Biotechnology; BOKU University of Natural Resources and Life Sciences; Muthgasse 18 1190 Vienna Austria
| | - Heena Dhiman
- Department of Biotechnology; BOKU University of Natural Resources and Life Sciences; Muthgasse 18 1190 Vienna Austria
- Austrian Centre of Industrial Biotechnology GmbH; Graz Austria
| | - Ly Ngoc Nguyen
- Department of Biotechnology; BOKU University of Natural Resources and Life Sciences; Muthgasse 18 1190 Vienna Austria
- Austrian Centre of Industrial Biotechnology GmbH; Graz Austria
| | - Gerald Klanert
- Austrian Centre of Industrial Biotechnology GmbH; Graz Austria
| | - Nicole Borth
- Department of Biotechnology; BOKU University of Natural Resources and Life Sciences; Muthgasse 18 1190 Vienna Austria
- Austrian Centre of Industrial Biotechnology GmbH; Graz Austria
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17
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Stolfa G, Smonskey MT, Boniface R, Hachmann AB, Gulde P, Joshi AD, Pierce AP, Jacobia SJ, Campbell A. CHO-Omics Review: The Impact of Current and Emerging Technologies on Chinese Hamster Ovary Based Bioproduction. Biotechnol J 2017; 13:e1700227. [PMID: 29072373 DOI: 10.1002/biot.201700227] [Citation(s) in RCA: 59] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2017] [Revised: 10/12/2017] [Accepted: 10/16/2017] [Indexed: 01/07/2023]
Abstract
CHO cells are the most prevalent platform for modern bio-therapeutic production. Currently, there are several CHO cell lines used in bioproduction with distinct characteristics and unique genotypes and phenotypes. These differences limit advances in productivity and quality that can be achieved by the most common approaches to bioprocess optimization and cell line engineering. Incorporating omics-based approaches into current bioproduction processes will complement traditional methodologies to maximize gains from CHO engineering and bioprocess improvements. In order to highlight the utility of omics technologies in CHO bioproduction, the authors discuss current applications as well as limitations of genomics, transcriptomics, proteomics, metabolomics, lipidomics, fluxomics, glycomics, and multi-omics approaches and the potential they hold for the future of bioproduction. Multiple omics approaches are currently being used to improve CHO bioprocesses; however, the application of these technologies is still limited. As more CHO-omic datasets become available and integrated into systems models, the authors expect significant gains in product yield and quality. While individual omics technologies provide incremental improvements in bioproduction, the authors will likely see the most significant gains by applying multi-omics and systems biology approaches to individual CHO cell lines.
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Affiliation(s)
- Gino Stolfa
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
| | | | - Ryan Boniface
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
| | | | - Paul Gulde
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
| | - Atul D Joshi
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
| | - Anson P Pierce
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
| | - Scott J Jacobia
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
| | - Andrew Campbell
- Bioproduction R&D, Thermo Fisher Scientific, Grand Island, USA
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18
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Impact of different promoters, promoter mutation, and an enhancer on recombinant protein expression in CHO cells. Sci Rep 2017; 7:10416. [PMID: 28874794 PMCID: PMC5585415 DOI: 10.1038/s41598-017-10966-y] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2017] [Accepted: 08/17/2017] [Indexed: 11/09/2022] Open
Abstract
In the present study, six commonly used promoters, including cytomegalovirus major immediate-early (CMV), the CMV enhancer fused to the chicken beta-actin promoter (CAG), human elongation factor-1α (HEF-1α), mouse cytomegalovirus (mouse CMV), Chinese hamster elongation factor-1α (CHEF-1α), and phosphoglycerate kinase (PGK), a CMV promoter mutant and a CAG enhancer, were evaluated to determine their effects on transgene expression and stability in transfected CHO cells. The promoters and enhancer were cloned or synthesized, and mutation at C-404 in the CMV promoter was generated; then all elements were transfected into CHO cells. Stably transfected CHO cells were identified via screening under the selection pressure of G418. Flow cytometry, qPCR, and qRT-PCR were used to explore eGFP expression levels, gene copy number, and mRNA expression levels, respectively. Furthermore, the erythropoietin (EPO) gene was used to test the selected strong promoter. Of the six promoters, the CHEF-1α promoter yielded the highest transgene expression levels, whereas the CMV promoter maintained transgene expression more stably during long-term culture of cells. We conclude that CHEF-1α promoter conferred higher level of EPO expression in CHO cells, but the CMV promoter with its high levels of stability performs best in this vector system.
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19
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Wippermann A, Noll T. DNA methylation in CHO cells. J Biotechnol 2017; 258:206-210. [DOI: 10.1016/j.jbiotec.2017.08.005] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2017] [Revised: 08/06/2017] [Accepted: 08/07/2017] [Indexed: 01/22/2023]
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20
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Wang X, Xu Z, Tian Z, Zhang X, Xu D, Li Q, Zhang J, Wang T. The EF-1α promoter maintains high-level transgene expression from episomal vectors in transfected CHO-K1 cells. J Cell Mol Med 2017; 21:3044-3054. [PMID: 28557288 PMCID: PMC5661254 DOI: 10.1111/jcmm.13216] [Citation(s) in RCA: 52] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2017] [Accepted: 04/01/2017] [Indexed: 02/03/2023] Open
Abstract
In our previous study, we demonstrated that episomal vectors based on the characteristic sequence of matrix attachment regions (MARs) and containing the cytomegalovirus (CMV) promoter allow transgenes to be maintained episomally in Chinese hamster ovary (CHO) cells. However, the transgene expression was unstable and the number of copies was low. In this study, we focused on enhancers, various promoters and promoter variants that could improve the transgene expression stability, expression magnitude (level) and the copy number of a MAR‐based episomal vector in CHO‐K1 cells. In comparison with the CMV promoter, the eukaryotic translation elongation factor 1 α (EF‐1α, gene symbol EEF1A1) promoter increased the transfection efficiency, the transgene expression, the proportion of expression‐positive clones and the copy number of the episomal vector in long‐term culture. By contrast, no significant positive effects were observed with an enhancer, CMV promoter variants or CAG promoter in the episomal vector in long‐term culture. Moreover, the high‐expression clones harbouring the EF‐1α promoter tended to be more stable in long‐term culture, even in the absence of selection pressure. According to these findings, we concluded that the EF‐1α promoter is a potent regulatory sequence for episomal vectors because it maintains high transgene expression, transgene stability and copy number. These results provide valuable information on improvement of transgene stability and the copy number of episomal vectors.
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Affiliation(s)
- Xiaoyin Wang
- Department of Biochemistry and Molecular Biology, Xinxiang Medical University, Xinxiang, Henan, China.,Henan Collaborative Innovation Center of Molecular Diagnosis and Laboratory Medicine, Xinxiang, Henan, China
| | - Zhongjie Xu
- Life Science and Technology, Xinxiang Medical University, Xinxiang, Henan, China
| | - Zhengwei Tian
- Department of Biochemistry and Molecular Biology, Xinxiang Medical University, Xinxiang, Henan, China
| | - Xi Zhang
- Department of Biochemistry and Molecular Biology, Xinxiang Medical University, Xinxiang, Henan, China
| | - Danhua Xu
- Department of Biochemistry and Molecular Biology, Xinxiang Medical University, Xinxiang, Henan, China
| | - Qin Li
- Test Laboratory, Xinxiang Medical University, Xinxiang, Henan, China
| | - Junhe Zhang
- Department of Biochemistry and Molecular Biology, Xinxiang Medical University, Xinxiang, Henan, China
| | - Tianyun Wang
- Department of Biochemistry and Molecular Biology, Xinxiang Medical University, Xinxiang, Henan, China
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21
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Kelly PS, Clarke C, Costello A, Monger C, Meiller J, Dhiman H, Borth N, Betenbaugh MJ, Clynes M, Barron N. Ultra-deep next generation mitochondrial genome sequencing reveals widespread heteroplasmy in Chinese hamster ovary cells. Metab Eng 2017; 41:11-22. [DOI: 10.1016/j.ymben.2017.02.001] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2016] [Revised: 01/31/2017] [Accepted: 02/07/2017] [Indexed: 10/20/2022]
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22
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Alves CS, Dobrowsky TM. Strategies and Considerations for Improving Expression of "Difficult to Express" Proteins in CHO Cells. Methods Mol Biol 2017; 1603:1-23. [PMID: 28493120 DOI: 10.1007/978-1-4939-6972-2_1] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Despite substantial advances in the field of mammalian expression, there are still proteins that are characterized as difficult to express. Determining the expression bottleneck requires troubleshooting techniques specific for the given molecule and host. The complex array of intracellular processes involved in protein expression includes transcription, protein folding, post-translation processing, and secretion. Challenges in any of these steps could result in low protein expression, while the inherent properties of the molecule itself may limit its production via mechanisms such as cytotoxicity or inherent instability. Strategies to identify the rate-limiting step and subsequently improve expression and production are discussed here.
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23
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Dahodwala H, Sharfstein ST. The 'Omics Revolution in CHO Biology: Roadmap to Improved CHO Productivity. Methods Mol Biol 2017; 1603:153-168. [PMID: 28493129 DOI: 10.1007/978-1-4939-6972-2_10] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Increased understanding of Chinese hamster ovary (CHO) cell physiology has been ushered in upon availability of the parental CHO-K1 cell line genome. Free and openly accessible sequence information has complemented transcriptomic and proteomic studies. The previous decade has also seen an increase in sensitivity and accuracy of proteomic methods due to technology development. In this genomic era, high-throughput screening methods, sophisticated informatic tools, and models continually drive major innovations in cell line development and process engineering. This review describes the various achievements in 'omics techniques and their application to improve recombinant protein expression from CHO cell lines.
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Affiliation(s)
- Hussain Dahodwala
- Vaccine production program (VPP), VRC/NIAID/NIH, Gaithersburg, MD, 20878, USA
- SUNY Polytechnic Institute, 257 Fuller Road, Albany, NY, 12203, USA
| | - Susan T Sharfstein
- Vaccine production program (VPP), VRC/NIAID/NIH, Gaithersburg, MD, 20878, USA.
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24
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Wippermann A, Rupp O, Brinkrolf K, Hoffrogge R, Noll T. Integrative analysis of DNA methylation and gene expression in butyrate-treated CHO cells. J Biotechnol 2016; 257:150-161. [PMID: 27890772 DOI: 10.1016/j.jbiotec.2016.11.020] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2016] [Revised: 11/21/2016] [Accepted: 11/23/2016] [Indexed: 01/01/2023]
Abstract
The cellular mechanisms responsible for the versatile properties of CHO cells as the major production cell line for biopharmaceutical molecules are not entirely understood yet, although several 'omics' data facilitate the understanding of CHO cells and their reactions to environmental conditions. However, genome-wide studies of epigenetic processes such as DNA methylation are still limited. To prove the applicability and usefulness of integrating DNA methylation and gene expression data in a biotechnological context, we exemplarily analyzed the time course of cellular reactions upon butyrate addition in antibody-producing CHO cells by whole-genome bisulfite sequencing and CHO-specific cDNA microarrays. Gene expression and DNA methylation analyses showed that pathways known to be affected by butyrate, including cell cycle and apoptosis, as well as pathways potentially involved in butyrate-induced hyperproductivity such as central energy metabolism and protein biosynthesis were affected. Differentially methylated regions were furthermore found to contain binding-site motifs of specific transcription factors and were hypothesized to represent regulatory regions closely connected to the cellular response to butyrate. Generally, our experiment underlines the benefit of integrating DNA methylation and gene expression data, as it provided potential novel candidate genes for rational cell line development and allowed for new insights into the butyrate effect on CHO cells.
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Affiliation(s)
- Anna Wippermann
- Institute of Cell Culture Technology, Bielefeld University, Bielefeld, Germany; Center for Biotechnology, Bielefeld University, Bielefeld, Germany.
| | - Oliver Rupp
- Bioinformatics and Systems Biology, Justus-Liebig-University, Gießen, Germany
| | - Karina Brinkrolf
- Department of Biorescources, Fraunhofer Institute for Molecular Biology and Applied Ecology, Gießen, Germany
| | - Raimund Hoffrogge
- Institute of Cell Culture Technology, Bielefeld University, Bielefeld, Germany; Center for Biotechnology, Bielefeld University, Bielefeld, Germany
| | - Thomas Noll
- Institute of Cell Culture Technology, Bielefeld University, Bielefeld, Germany; Center for Biotechnology, Bielefeld University, Bielefeld, Germany
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25
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Feichtinger J, Hernández I, Fischer C, Hanscho M, Auer N, Hackl M, Jadhav V, Baumann M, Krempl PM, Schmidl C, Farlik M, Schuster M, Merkel A, Sommer A, Heath S, Rico D, Bock C, Thallinger GG, Borth N. Comprehensive genome and epigenome characterization of CHO cells in response to evolutionary pressures and over time. Biotechnol Bioeng 2016; 113:2241-53. [PMID: 27072894 PMCID: PMC5006888 DOI: 10.1002/bit.25990] [Citation(s) in RCA: 101] [Impact Index Per Article: 12.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2015] [Revised: 03/15/2016] [Accepted: 04/07/2016] [Indexed: 01/03/2023]
Abstract
The most striking characteristic of CHO cells is their adaptability, which enables efficient production of proteins as well as growth under a variety of culture conditions, but also results in genomic and phenotypic instability. To investigate the relative contribution of genomic and epigenetic modifications towards phenotype evolution, comprehensive genome and epigenome data are presented for six related CHO cell lines, both in response to perturbations (different culture conditions and media as well as selection of a specific phenotype with increased transient productivity) and in steady state (prolonged time in culture under constant conditions). Clear transitions were observed in DNA‐methylation patterns upon each perturbation, while few changes occurred over time under constant conditions. Only minor DNA‐methylation changes were observed between exponential and stationary growth phase; however, throughout a batch culture the histone modification pattern underwent continuous adaptation. Variation in genome sequence between the six cell lines on the level of SNPs, InDels, and structural variants is high, both upon perturbation and under constant conditions over time. The here presented comprehensive resource may open the door to improved control and manipulation of gene expression during industrial bioprocesses based on epigenetic mechanisms. Biotechnol. Bioeng. 2016;113: 2241–2253. © 2016 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.
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Affiliation(s)
- Julia Feichtinger
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria.,Graz University of Technology, Graz, Austria
| | - Inmaculada Hernández
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria
| | - Christoph Fischer
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria.,Graz University of Technology, Graz, Austria
| | - Michael Hanscho
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria
| | - Norbert Auer
- University of Natural Resources and Life Sciences Vienna, Muthgasse 18, Vienna, 1190, Austria
| | - Matthias Hackl
- University of Natural Resources and Life Sciences Vienna, Muthgasse 18, Vienna, 1190, Austria
| | - Vaibhav Jadhav
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria
| | - Martina Baumann
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria
| | - Peter M Krempl
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria.,Graz University of Technology, Graz, Austria
| | - Christian Schmidl
- CeMM Research Center for Molecular Medicine, Austrian Academy of Sciences, Vienna, Austria
| | - Matthias Farlik
- CeMM Research Center for Molecular Medicine, Austrian Academy of Sciences, Vienna, Austria
| | - Michael Schuster
- CeMM Research Center for Molecular Medicine, Austrian Academy of Sciences, Vienna, Austria
| | | | - Andreas Sommer
- Research Institute of Molecular Pathology, Vienna, Austria
| | - Simon Heath
- Center for Genomic Regulation (CRG), Barcelona, Spain
| | - Daniel Rico
- Spanish National Cancer Research Center (CNIO), Madrid, Spain
| | - Christoph Bock
- CeMM Research Center for Molecular Medicine, Austrian Academy of Sciences, Vienna, Austria
| | - Gerhard G Thallinger
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria.,Graz University of Technology, Graz, Austria
| | - Nicole Borth
- Austrian Center of Industrial Biotechnology, Muthgasse 11, Vienna, 1190, Austria. .,University of Natural Resources and Life Sciences Vienna, Muthgasse 18, Vienna, 1190, Austria.
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26
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Moritz B, Becker PB, Göpfert U. CMV promoter mutants with a reduced propensity to productivity loss in CHO cells. Sci Rep 2015; 5:16952. [PMID: 26581326 PMCID: PMC4652263 DOI: 10.1038/srep16952] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2015] [Accepted: 10/22/2015] [Indexed: 11/09/2022] Open
Abstract
The major immediate-early promoter and enhancer of the human cytomegalovirus (hCMV-MIE) is one of the most potent DNA elements driving recombinant gene expression in mammalian cells. Therefore, it is widely employed not only in research but also in large-scale industrial applications, e.g. for the production of therapeutic antibodies in Chinese hamster ovary cells (CHO). As we have reported previously, multi-site methylation of hCMV-MIE is linked to productivity loss in permanently transfected CHO cells lines. In particular, the cytosine located 179 bp upstream of the transcription start site (C-179) is frequently methylated. Therefore, our objective was to study whether mutation of C-179 and other cytosines within hCMV-MIE might lessen the instability of transgene expression. We discovered that the single mutation of C-179 to G can significantly stabilise the production of recombinant protein under control of hCMV-MIE in permanently transfected CHO cells.
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Affiliation(s)
- Benjamin Moritz
- Roche Pharmaceutical Research and Early Development, Large Molecule Research, Roche Innovation Center Penzberg, Germany
| | - Peter B Becker
- Biomedical Center and Center for Integrated Protein Science Munich, Ludwig Maximilian University, Munich, Germany
| | - Ulrich Göpfert
- Roche Pharmaceutical Research and Early Development, Large Molecule Research, Roche Innovation Center Penzberg, Germany
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27
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Monger C, Kelly PS, Gallagher C, Clynes M, Barron N, Clarke C. Towards next generation CHO cell biology: Bioinformatics methods for RNA-Seq-based expression profiling. Biotechnol J 2015; 10:950-66. [DOI: 10.1002/biot.201500107] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2015] [Revised: 04/08/2015] [Accepted: 05/07/2015] [Indexed: 12/24/2022]
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