Increased expression of therapeutic proteins by identification of 3'-UTRs from high expressing genes in CHO cells

Therapeutic proteins, a.k.a. biopharmaceuticals, are most commonly produced in expression systems derived from Chinese Hamstery Ovary (CHO) cells, thanks to great capacity of post-translational modifications like secretation, folding and glycosylation. The engineering of cells for regulation of protein expression has many options including knock-in and knock-out of genes, epigenetic studies or improvement of the expression casette of the protein of interest by e.g. promotor variants or modifications of the 5’ and 3’ untranslated region (UTR). The 3’-UTR is therefore a good optimization candidate for attempting to achieve increased expression of therapeutic proteins. The final aim of this study was to identify and design 3’-UTRs for improved expression of therapeutic proteins in HyClone™ CHO cells from GE Healthcare Bio-Sciences AB (GEHC). The impact goal is to increase the efficiency and lower the costs for pharmaceutical companies when producing biopharmaceuticals in the HyClone™ CHO cell line, leading to increased accessibility of monoclonal antibodies (mAbs) on the pharmaceutical market. The study was initiated with bioinformatic analysis of the CHO cell transcriptome from a set of RNA-seq data of HyClone™ CHO to find high expressing, context independent genes. The 3’-UTRs from the best candidate genes were used for construction of plasmids for expression of a Fc-eGFP fusion protein. Nine selected 3’-UTRs were designed, synthesized and cloned into a parent plasmid (pGE0520) creating nine plasmid variants (pGE0523-531). The constructed plasmids were used for evaluation with site directed integration (SDI) into the HyClone™ CHO cell line and expression analysis were performed by flow cytometry and antibody titer measurements from cells with successfully integrated plasmid sorted by fluorescence-activated cell sorting (FACS). Result show a significant effect on protein expression when using different variants of 3’-UTRs. Two variants, pGE0524 and pGE0526, competing with the parent plasmid in expression levels and integration efficiency from SDI, making them candidates for further investigations against the parent plasmid. Results also show good correlation between flow cytometry data from pre- and post-sorting, which can make research for further 3’-UTRs more efficient by evaluations and prediction of expression levels before cell sorting