Background: Gene therapy in mammalian cells requires vectors exhibiting long-term stability
and high expression. Episomal gene expression vectors offer a safe and attractive alternative to those
that integrate into the host cell genome.
Materials & Methods: In the present study, we developed a new episomal vector based on the insulator,
chicken hypersensitive site 4 (cHS4). The cHS4 element was artificially synthesized, cloned into the
pEGFP-C1 vector, and used to transfect Chinese hamster ovary (CHO) and human Chang liver cells.
The stably transfected cell colonies were further cultured in either the presence or absence of G418 selection.
Fluorescence in situ hybridization (FISH) analysis and vector rescue experiments demonstrated
that the vector replicated episomally in both CHO and human Chang liver cells. Compared with episomal
vectors mediated by matrix attachment region sequences, the cHS4 element-containing vector
yielded increased transgene expression levels, transfection efficiency, and stability during long-term
culture. The vector was present at a very low copy number in the cells and was stably maintained over
more than 100 generations without selection pressure.
Conclusion: In conclusion, apart from a few free vector forms, the cHS4-containing vector mainly replicates
episomally in mammalian cells and out- performs comparable systems in terms of yielding both
higher expression levels and stability levels.