Increasing transgene expression and stability in recombinant CHO cells with DNA methyltransferase Dnmt3b gene knockout via CRISPR/Cas9

Chinese hamster ovary (CHO) cells are the preferred producers because of their capacity to perform proper protein folding, assembly, and post‐translational modifications, similar to human cells. When CHO cells are used to produce recombinant drug proteins, it is critical to maintain stable subculture of transgenic CHO cells and high expression levels of target genes when the recombinant expression seed cell bank is expanded to a large scale bioreactor. However, at present, the instability of recombinant protein expression during prolonged culture is one of the most common bottlenecks in mammalian protein production restricting the development of CHO cell expression system. It is believed that the instability of transgene expression in CHO cells is closely related to DNA methylation. DNA methylation was mainly catalyzed by the de novo methyltransferase Dnmt3a, Dnmt3b and DNA methylsustaining‐enzyme Dnmt1. DNA methyltransferase Dnmt3a and 3b play an important role in gene silencing induced by epigenetic modifications such as DNA methylation. It has been found that epigenetic modification mediated by Dnmt3b is more closely related to gene expression regulation. Therefore, in this paper, Dnmt3b gene‐deficient CHO cells were established through knock out of the Dnmt3b gene by CRISPR/Cas9 genome editing technology. Furthermore, the stability and efficiency of the expression system based on the Dnmt3b KO CHO cells were evaluated by determining the expression levels of the recombinant proteins. The results indicated that the long‐term stability of the green fluorescent reporter gene EGFP in the transfected Dnmt3b KO CHO cells over 30 passages increased regardless of the presence or absence of G418, compared with the normal transfected CHO cells. We also found that the expression of recombinant human‐vitronectin(VN) and monoclonal antibodies (mAbs) in the stable cell pools of the transgenic Dnmt3b KO CHO cells were at least 1.6 folds higher than those of the normal CHO cells. Under the condition of not increasing the cost of culture, it is beneficial to the screening of high yielding cells and industrial production. The findings indicated that the Dnmt3b‐deficient CHO cells could significantly improve the stability of target gene expression, which provide theoretical significance and application value for developing novel effective and stable CHO cell expression system.