Eukaryotic genes are organised into chromatin domains, which exist in either a ‘closed’ state in which the genes are tightly condensed with proteins and are transcriptionally silent, or in an ‘open’ de-condensed state: a prerequisite for efficient gene expression.
When transfected genes integrate into mammalian chromosomes, the structure of the chromatin at the site of integration has a profound effect on expression of the transgene. Consequently, only relatively rare transfected clones where the expression vector has integrated into open chromatin show efficient expression of the transgene.
Ubiquitous Chromatin Opening Elements (UCOEs) have been isolated which ensure efficient expression in a wide range of cell types including CHO-K1, CHO-S, HeLa, and NS0 cells. UCOE containing vectors have been shown to markedly enhance a wide variety of intracellular, secreted and membrane-bound proteins.
Inclusion of a UCOE (4-8 kb in size) in a eukaryotic expression vector permits efficient expression in the vast majority of stable clones, whereas with conventional vectors, only a minor proportion of transfectants show high-level expression (Figure 1).
Figure 1: CHO cells stably transfected with a human CMV-EGFP reporter gene combination with, or without, an upstream 8kb UCOE. Median fluorescence indicates the level of expression of the reporter gene achieved in individual clonally derived transfectants.
Figure 2: High IgG1 yields can be obtained with Feed optimisation.
There is therefore no need for amplification and expression has been demonstrated to be stable over 130 generations. The combination of UCOE and CHO-S allows the generation of high yields of recombinant proteins (Figure 2).