Study of the mechanism for increased protein expression via transcription potency reduction of the selection marker

Weak promoters to drive selection marker expression: improvement of cell line development process for therapeutic protein production in CHO-K1 cells

Chinese Hamster Ovary cells have been widely used as host cells for production of recombinant therapeutic molecules. Cell line development is a decisive step, which must be carried out with an efficient process. In particular, degree of selection stringency is an important parameter for identification of rare, high-producing cell lines. In the CHOZN® CHO K1 platform, selection of top-producing clones is based on puromycin resistance, whose expression is driven by Simian Virus 40 Early (SV40E) promoter. In this study, novel promoters have been identified to drive expression of selection marker. Decrease of transcriptional activity compared to SV40E promoter was confirmed by RT-qPCR. Selection stringency was increased, as seen by decreased surviving rate of transfected mini-pools and longer recovery duration of transfected bulk pools. Several promoters led to a 1.5-fold increase of maximum titer and a 1.3-fold increase of mean specific productivity of the monoclonal antibody over the clone generation. Expression level was maintained stable over long term cultivation. Finally, productivity increase was confirmed on several monoclonal antibodies and fusion proteins. Lowering the strength of promoter for expression of selective pressure resistance is an efficient strategy to increase selection stringency, which can be applied on industrial CHO-based cell line development platforms.

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.