Novel CRISPR/Cas9-mediated knockout of LIG4 increases efficiency of site-specific integration in Chinese hamster ovary cell line

Abstract

Aim: To investigate the impact of deficiency of LIG4 gene on site-specific integration in CHO cells.

Results: CHO cells are considered the most valuable mammalian cells in the manufacture of biological medicines, and genetic engineering of CHO cells can improve product yield and stability. The traditional method of inserting foreign genes by random integration (RI) requires multiple rounds of screening and selection, which may lead to location effects and gene silencing, making it difficult to obtain stable, high-yielding cell lines. Although site-specific integration (SSI) techniques may overcome the challenges with RI, its feasibility is limited by the very low efficiency of the technique. Recently, SSI efficiency has been enhanced in other mammalian cell types by inhibiting DNA ligase IV (Lig4) activity, which is indispensable in DNA double-strand break repair by NHEJ. However, this approach has not been evaluated in CHO cells. In this study, the LIG4 gene was knocked out of CHO cells using CRISPR/Cas9-mediated genome editing. Efficiency of gene targeting in LIG4^-/--CHO cell lines was estimated by a green fluorescence protein promoterless reporter system. Notably, the RI efficiency, most likely mediated by NHEJ in CHO, was inhibited by LIG4 knockout, whereas SSI efficiency strongly increased 9.2-fold under the precise control of the promoter in the ROSA26 site in LIG4^-/--CHO cells. Moreover, deletion of LIG4 had no obvious side effects on CHO cell proliferation.

Conclusions: Deficiency of LIG4 represents a feasible strategy to improve SSI efficiency and suggests it can be applied to develop and engineer CHO cell lines in the future.

Keywords: CRISPR/Cas9; Chinese hamster ovary (CHO) cells; DNA ligase IV; Nonhomologous end joining (NHEJ); Site-specific integration (SSI).