Factors affecting amplification of BK virus episomal vectors in human cells. Brief report

Episomal vectors for gene expression in mammalian cells

An important reason for preferring mammalian cells for heterologous gene expression is their ability to make authentic proteins containing post-translational modifications similar to those of the native protein. The development of expression systems for mammalian cells has been ongoing for several years, resulting in a wide variety of effective expression vectors. The aim of this review is to highlight episomal expression vectors. Such episomal plasmids are usually based on sequences from DNA viruses, such as BK virus, bovine papilloma virus 1 and Epstein-Barr virus. In this review we will mainly focus on the improvements made towards the usefulness of these systems for gene expression studies and gene therapy.

Molecular integrity and usefulness of episomal expression vectors derived from BK and Epstein-Barr virus

High-level and stable production of a protein of interest is one of the most important parameters when considering the development of an efficient vector system for heterologous gene expression. In order to achieve this goal, we have used episomal vector elements derived from Epstein-Barr virus (EBV) or BK virus (BKV) in combination with the strictly regulated interferon-inducible Mx promoter. Here we demonstrate that EBV-derived vectors replicate efficiently in all cell lines tested (i.e. HEK293, HeLaH21 and Vero), yielding stable transfectants with a high, inducible expression level and almost no background. In contrast, BKV-derived vectors are much more restricted to particular cell types and hampered by DNA rearrangements, which is a serious drawback for use over a longer timespan.

New BK virus episomal vector for complementary DNA expression in human cells

The properties of pRP-c, a new vector for complementary DNA (cDNA) expression, are described. The vector contains the early region and replication origin of BK virus (BKV), a human papovavirus. Due to the presence of these BKV sequences, pRP-c replicates in human cells allowing amplification of inserted cDNAs. The promoter, intron and polyadenylation region for cDNA expression are separated by unique restriction sites and can therefore be individually excised and substituted with different transcription signals. Coding sequences of the bacterial genes for chloramphenicol-acetyl transferase (CAT) or neomycin phosphotransferase (neo) were inserted into the cDNA cloning site of pRP-c and expressed in human cells in transient assays or stable clones. In both cases expression of the inserted sequences was significantly more efficient than by using the integration vectors pSV2CAT and pSV2neo, demonstrating the advantages of episomal expression vectors in human cells. Possible uses of pRP-c to express viral and cellular cDNAs in human cells are discussed.