Currently, the titers of biopharmaceutical production from Chinese hamster ovary (CHO) cells have achieved gram per liter range and this can be attributed to advances in bioprocess development, media development and cell line development. To obtain high producing cell lines, extensive screening of producer clones during cell line development is often necessary. To improve the efficiency and efficacy of generating and isolating high producing clones, various expression vector engineering technologies can be applied, for example ubiquitous chromatin opening element (UCOE) , matrix attachment regions (MARs) (Mirkovitch et al. 1984), site specific recombination [3–9], to improve selection stringency [10–13], and to co-localize the GOI with the selection marker [14–16].
In our previous studies, we have similarly shown that specific productivities can be improved when we increased selection stringency by destabilizing the selection marker through the addition of AU-rich elements (ARE) to promote mRNA degradation and murine ornithine decarboxylase (MODC) PEST region to enhance protein degradation . While coexpression of GOI and selection marker using multiple promoters on the same vector may help in the co-localization, we have previously demonstrated that gene fragmentation can occurs at a high level of 14% during stable transfection of dual promoter dicistronic vector in CHO-DG44 cells . Subsequently, an attenuated IRES element was used together with the PEST region to allow for high recombinant protein titer using stably amplified cell pools .
In this study, we evaluated the use of tandem PEST sequence, further attenuation of the IRES element, and codon-deoptimization of the dhfr selection marker, to further optimizing the strength of selection marker expression in CHO cells for the production of recombinant human Alpha1-antitrypsin (rhA1AT), a serum protease inhibitor currently purified from human blood plasma as replacement therapy for patients who developed chronic obstructive pulmonary disease due to deficiency in the protein. Such vector combinations to attenuate translation initiation, protein elongation and protein stability for optimizing selection stringency have not been previously investigated. To our knowledge, there is also no report on high-titer production of rhA1AT in CHO cells, which is necessary for its manufacturability due to its high dosage requirement.