Simian virus 40 (SV40) large T antigen-dependent amplification of an Epstein-Barr virus-SV40 hybrid shuttle vector integrated into the human HeLa cell genome

Abstract

We analysed the DNA rearrangements that occurred during the integration and amplification of an Epstein-Barr virus (EBV)-simian virus 40 (SV40) hybrid shuttle vector in human cells. The human HeLa cell line was episomally transformed with the EBV-SV40 p205-GTI plasmid. After a 2 month culture in a selective medium, a HeLa cell-derived population (H-G1 cells) was obtained in which the p205-GTI vector was integrated as a single intact copy deleted in the EBV latent origin of replication (OriP). Sequencing data showed that the endpoints of the plasmid sequences, at the plasmid-cell DNA junctions, are located within the two essential elements of EBV OriP, which may form several secondary structures. This result suggests that a specific DNA sequence (OriP) or palindromic structures could play a role in this integration process. This represents the first fully characterized site of integration of an EBV vector in human cells. The transient expression of the SV40 large T antigen in H-G1 cells leads to the appearance of episomal molecules with an extremely heterogeneous size pattern. Individual analysis of these episomes after rescue in bacteria indicated that they retained sequences of both the p205-GTI plasmid and cellular DNA. Comparison of the structure of these circular DNAs with those of the integrated p205-GTI copy indicated that large T antigen expression in human cells leads to the amplification of the integrated shuttle vector according to the 'onion skin' model developed for transformed rodent cells. Indeed, amplified sequences were colinear with the integrated p205-GTI copy and its surrounding cellular sequences, distributed almost equally around the SV40 replication origin, and circularized by illegitimate recombination which did not involve specific nucleotide sequences. This system is of interest in that it enables easy recovery of individual recombined molecules in host bacteria. Each isolated clone contains a unique recombination junction which is easily and rapidly characterized and sequenced.