A chimeric HS4 insulator-scaffold attachment region enhances transgene expression in transfected Chinese hamster ovary cells

Development of an Expression Vector Engineering Strategy Based on tDNA Insulator Element for the Stable Expression of Vascular Endothelial Growth Factor Receptor-Fc Fusion Protein

During the past decades, tremendous advances have occurred in manufacturing recombinant therapeutic proteins in Chinese hamster ovary (CHO) cells. Nevertheless, the production of stable high-producing cell lines has remained a major obstacle in the development process of the CHO cell line. It has been shown that genomic regulatory elements can promote cell line development efficiency by improving transgenes' productivity and stability. Such elements include insulators, ubiquitous chromatin opening elements, scaffold/matrix attachment regions, and antirepressors. In addition, tDNA elements are shown to act as insulators in mammalian cells. This study examines the effect of the tDNA insulator on stable expression of a vascular endothelial growth factor receptor-Fc fusion protein.

Stabilizing and Anti-Repressor Elements Effectively Increases Transgene Expression in Transfected CHO Cells

Chinese hamster ovary (CHO) cells are currently the most widely used host cells for recombinant therapeutic protein (RTP) production. Currently, the RTP yields need to increase further to meet the market needs and reduce costs. In this study, three stabilizing and anti-repressor (SAR) elements from the human genome were selected, including human SAR7, SAR40, and SAR44 elements. SAR elements were cloned upstream of the promoter in the eukaryotic vector, followed by transfection into CHO cells, and were screened under G418 pressure. Flow cytometry was used to detect enhanced green fluorescent protein (eGFP) expression levels. The gene copy numbers and mRNA expression levels were determined through quantitative real-time PCR. Furthermore, the effect of the stronger SAR elements on adalimumab was investigated. The results showed that transgene expression levels in the SAR-containing vectors were higher than that of the control vector, and SAR7 and SAR40 significantly increased and maintained the long-term expression of the transgene in CHO cells. In addition, the transgene expression level increase was related with gene copy numbers and mRNA expression levels. Collectively, SAR elements can enhance the transgene expression and maintain the long-term expression of a transgene in transfected CHO cells, which may be used to increase recombinant protein production in CHO cells.

Enhanced Transgene Expression by Optimization of Poly A in Transfected CHO Cells

The generation of the stable, high-level recombinant protein-producing cell lines remains a significant challenge in the biopharmaceutical industry. Expression vector optimization is an effective strategy to increase transgene expression levels and stability, and the choice of suitable poly A element is crucial for the expression of recombinant protein. In this study, we investigated the effects of different poly A elements on transgene expression in Chinese hamster ovary (CHO) cells. Five poly A elements, including bovine growth hormone (BGH), mutant BGH, herpes simplex virus type 1 thymidine kinase (HSV-TK), SV40, and a synthetic (Synt) poly A, were cloned into the expression vector and transfected into CHO cells. The results indicated the SV40 and Synt poly A sequences can significant improve eGFP transgene expression in stable transfected CHO cells and maintain long-term expression. However, qPCR results showed that the eGFP expression at protein level was not related to the gene copy number and mRNA level. Importantly, the SV40 and Synt poly A elements decreased the variation of eGFP transgene expression. Furthermore, it also showed that the SV40 and Synt poly A elements induced higher levels of adalimumab expression. In conclusion, SV40 poly A and Synt poly A are stronger elements that increase stable transgene expression and decrease the variation of expression, and the choice of suitable poly A element is helpful to improve the expression of recombinant protein.

Chromatin-modifying elements for recombinant protein production in mammalian cell systems

Mammalian cells are the preferred choice system for the production of complex molecules, such as recombinant therapeutic proteins. Although the technology for increasing the yield of proteins has improved rapidly, the process of selecting, identifying as well as maintaining high-yield cell clones is still troublesome, time-consuming and usually uncertain. Optimization of expression vectors is one of the most effective methods for enhancing protein expression levels. Several commonly used chromatin-modifying elements, including the matrix attachment region, ubiquitous chromatin opening elements, insulators, stabilizing anti-repressor elements can be used to increase the expression level and stability of recombinant proteins. In this review, these chromatin-modifying elements used for the expression vector optimization in mammalian cells are summarized, and future strategies for the utilization of expression cassettes are also discussed.

Fusion with matrix attachment regions enhances expression of recombinant protein in human HT-1080 cells

Like endogenous proteins, recombinant foreign proteins produced in human cell lines also need post-translational modifications. However, high and long-term expression of a gene of interest (GOI) presents significant challenges for recombinant protein production in human cells. In this work, the effect of human matrix attachment region elements (MARs), including the β-globin MAR (gMAR), chicken lysozyme MAR (cMAR), and a combination of these two, on the stable expression of GOI was assessed in human HT-1080 cells. After transfection with vectors containing the MAR elements and eGFP, stably HT-1080 cell pools were obtained under selective pressure. eGFP protein expression was analyzed by flow cytometry, while transgene copy number and eGFP mRNA expression levels were determined with qPCR and qRT-PCR technology. We found that MARs could not enhance transfection efficiency, but gMAR could significantly increase eGFP expression in stable HT-1080 cell pools by approximately 2.69-fold. Moreover, gMAR could also increase eGFP expression stability during long-term culture. Lastly, we showed that the effect of the MARs on transgenes was related to the gene copy number. In summary, this study found that MARs could both enhance the transgene expression and stability in HT-1080 cells.

Characterization of the cHS4 insulator in mouse embryonic stem cells

Synthetic biology circuits are often constructed with multiple gene expression units assembled in close proximity, and they can be used to perform complex functions in embryonic stem cells (ESCs). However, mutual interference between transcriptional units has not been well studied in mouse ESCs. To assess the efficiency of insulators at suppressing promoter interference in mouse ESCs, we used an evaluation scheme in which a tunable tetracycline response element promoter is connected to a constant Nanog promoter. The chicken hypersensitive site 4 (cHS4) insulator, widely used both for enhancer blocking and for barrier insulation in vitro and in vivo, was positioned between the two expression units for assessment. By inserting the cassette into various loci of the mouse ESC genome with PiggyBac transposon, we were able to quantitatively examine the protective effect of cHS4 by gradually increasing the transcriptional activity of the tetracycline response element promoter with doxycycline and then measuring the transcriptional activity of the Nanog promoter. Our results indicate that the cHS4 insulator has minimal insulating effects on promoter interference in mouse ESCs. Further studies show that the cHS4 insulation effect may be promoter specific and related to interaction with CCCTC‐binding factor‐mediated loop formation. In addition, we also compared DNA transposition and transgene expression with or without the cHS4 insulator using well‐established ESC reporters. The results indicate that cHS4 has no apparent effects on DNA transposition and transgene expression levels, but exerts modest protective effects on long‐term transgene silencing.

Effects of different 2A peptides on transgene expression mediated by tricistronic vectors in transfected CHO cells

Multicistronic vectors can increase transgene expression and decrease the imbalance of gene expression in the Chinese hamster ovary (CHO) cell expression system. Small, self-cleaving 2A peptides have a high cleavage efficiency and are essential for constructing high-expression multicistronic vectors. In this study, we investigated the effects of two different 2A peptides on transgene expression in CHO cells via their mediating action on tricistronic vectors. The enhanced green fluorescent protein (eGFP) and red fluorescent protein (RFP) genes were linked by the porcine teschovirus-1 (P2A) and Thosea asigna virus (T2A) peptides in a multicistronic vector. We transfected CHO cells with these vectors and screened for the presence of blasticidin-resistant colonies. Flow cytometry and real-time quantitative PCR (qPCR) were used to detect the expression levels of eGFP and RFP and the copy numbers of stably transfected cells. The results showed that P2A could enhance eGFP and RFP expression by 1.48- and 1.47-fold, respectively, compared to T2A. The expression levels of the genes were not proportional to their copy numbers. In conclusion, we found that P2A can effectively drive transgene expression in CHO cells and a potent 2A peptide can be used for recombinant protein production in the CHO cell system.

Novel short synthetic matrix attachment region for enhancing transgenic expression in recombinant Chinese hamster ovary cells

Matrix attachment regions (MARs) are DNA fragments with specific motifs that enhance transgenic expression; however, the characteristics and functions of these elements remain unclear. In this study, we designed and synthesized three short chimeric MARs, namely, SM4, SM5, and SM6, with different numbers and orders of motifs on the basis of the features and motifs of previously reported MARs, namely, SM1, SM2, and SM3, respectively. Expression vectors with six synthetic MARs flanking the down or upstream of the expression cassette for enhanced green fluorescence protein (EGFP) were constructed and introduced into Chinese hamster ovary (CHO) cells. Results indicated that the EGFP expression of the CHO cells with transfection bySM4, SM5, or SM6-containing vectors was higher than that of those containing SM1, SM2, or SM3 regardless of the MAR insertion position. The improving effect of SM5 was particularly pronounced. Transgenic expression was further enhanced with the increasing SM5 copy number. Bioinformatics analysis indicated that several arrangements of the DNA-binding motifs for CEBP, FAST, Hox, glutathione, and NMP4 may help increase transgenic expression levels and the average population of highly expressed cells. Our findings on novel synthetic MARs will help establish stable expression systems in mammalian cells.

Development of an improved lentiviral based vector system for the stable expression of monoclonal antibody in CHO cells

Therapeutic monoclonal antibodies (mAbs) have become the dominant products in biopharmaceutical industry. Mammalian cell expression systems including Chinese hamster ovary (CHO) cells are the most commonly used hosts for the production of complex recombinant proteins. However, development of stable, high producing CHO cell lines suffers from the low expression level and instability of the transgene. The increasing efforts in the development of novel therapeutic antibodies and the advent of biosimilars have revealed the necessity for the development of improved platforms for rapid production of products for initial characterization and testing. In line with this premise, vector design and engineering has been applied to improve the expression level and stability of the transgene. This study reports the application of an improved lentiviral vector system containing the human interferon-β scaffold attachment region (IFN-SAR) for the development of antibody producing stable CHO cells. mAb expressing clones producing 1100 µg/L of IgG1 monoclonal antibody were isolated without extensive screening of a large number of clones. Our results here indicate the positive effects of IFN-SAR on stable mAb expression using lentiviral based expression vectors. We also observed that although IFN-SAR can improve light chain (LC) and heavy chain (HC) gene copy numbers in stable cell pools, mAb expression in single cell clones was not affected by the transgene copy number.

Identification of consensus sequence from matrix attachment regions and functional analysis of its activity in stably transfected Chinese hamster ovary cells

Matrix attachment regions (MARs) can enhance transgene expression levels and maintain stability. However, the consensus sequence from MARs and its functional analysis remains to be examined. Here, we assessed a possible consensus sequence from MARs and assessed its activity in stably transfected Chinese hamster ovary (CHO) cells. First, we analyzed the effects of 10 MARs on transfected CHO cells and then analyzed the consensus motifs from these MARs using a bioinformatics method. The consensus sequence was synthesized and cloned upstream or downstream of the eukaryotic vector. The constructs were transfected into CHO cells and the expression levels and stability of enhanced green fluorescent protein were detected by flow cytometry. The results indicated that eight of the ten MARs increased transgene expression in transfected CHO cells. Three consensus motifs were found after bioinformatics analyses. The consensus sequence tandemly enhanced transgene expression when it was inserted into the eukaryotic expression vector; the effect of the addition upstream was stronger than that downstream. Thus, we found a MAR consensus sequence that may regulate the MAR-mediated increase in transgene expression.

Two human MARs effectively increase transgene expression in transfected CHO cells

Matrix attachment regions (MARs) can enhance the expression level of transgene in Chinese hamster ovaries (CHO) cell expression system. However, improvements in function and analyses of the mechanism remains unclear. In this study, we screened two new and more functional MAR elements from the human genome DNA. The human MAR-3 and MAR-7 element were cloned and inserted downstream of the polyA site in a eukaryotic vector. The constructs were transfected into CHO cells, and screened under G418 to produce the stably transfected cell pools. The expression levels and stability of enhanced green fluorescent protein (eGFP) were detected by flow cytometry. The transgene copy number and transgene expression at mRNA level were detected by quantitative real-time PCR. The results showed that the expression level of eGFP of cells transfected with MAR-containing vectors were all higher than those of the vectors without MARs under transient and stably transfection. The enhancing effect of MAR-7 was higher than that of MAR-3. Additionally, we found that MAR significantly increased eGFP copy numbers and eGFP gene mRNA expression level as compared with the vector without. In conclusion, MAR-3 and MAR-7 gene can promote the expression of transgene in transfected CHO cells, and its effect may be related to the increase of the number of copies.