Promoter from Chinese hamster elongation factor-1a gene and Epstein-Barr virus terminal repeats concatemer fragment maintain stable high-level expression of recombinant proteins.

Abstract

The Chinese hamster ovary (CHO) cell line is the main host for the high-titer production of therapeutic and diagnostic proteins in the biopharmaceutical industry. In most cases, plasmids for efficient protein expression in CHO cells are based on the cytomegalovirus (CMV) promoter. The autologous Chinese hamster eukaryotic translation elongation factor 1α (EEF1A1) promoter is a viable alternative to the CMV promoter in industrial applications. The EEF1A1 promoter and its surrounding DNA regions proved to be effective at maintaining high-level and stable expression of recombinant proteins in CHO cells. EEF1A1-based plasmids' large size can lead to low transfection efficiency and hamper target gene amplification. We hypothesized that an efficient EEF1A1-based expression vector with a long terminal repeat fragment from the Epstein-Barr virus (EBVTR) could be truncated without affecting promoter strength or the long-term stability of target gene expression. We made a series of deletions in the downstream flanking region of the EEF1A1 gene, and then in its upstream flanking region. The resulting plasmids, which coded for the enhanced green fluorescent protein (eGFP), were tested for the level of eGFP expression in the populations of stably transfected CHO DG44 cells and the stability of eGFP expression in the long-term culture in the absence of selection agents. It was shown that in the presence of the EBVTR fragment, the entire downstream flanking region of the EEF1A1 gene could be excluded from the plasmid vector. Shortening of the upstream flanking region of the EEF1A1 gene to a length of 2.5 kbp also had no significant effect on the level of eGFP expression or long-term stability. The EBVTR fragment significantly increased expression stability for both the CMV and EEF1A1 promoter-based plasmids, and the expression level drop during the two-month culture was more significant for both CMV promoter-based plasmids. Target protein expression stability for the truncated plasmid, based on the EEF1A1 gene and EBVTR fragment, is sufficient for common biopharmaceutical applications, making these plasmid vectors a viable alternative to conventional CMV promoter-based vectors.