Studies on cellular tandemization of herpes simplex virus thymidine kinase DNA

Chromosomal integration of transduced recombinant baculovirus DNA in mammalian cells

Our group and others have recently demonstrated the ability of recombinant baculoviruses to transduce mammalian cells at high frequency. To further characterize the use of baculovirus as a mammalian gene delivery system, we examined the status of transduced DNA stably maintained in Chinese hamster ovary (CHO) cells. Four independent clones carrying two introduced markers, the genes for neomycin resistance (Neo) and green fluorescent protein (GFP), were selected. PCR analysis, Southern blotting, and DNA sequencing showed that discrete portions of the 148-kb baculovirus DNA were present as single-copy fragments ranging in size from 5 to 18 kb. Integration into the CHO cell genome was confirmed by fluorescent in situ hybridization (FISH) analysis. For one clone, the left and right viral/chromosomal junctions were determined by DNA sequencing of inverse PCR products. Similarly, for a different clone, the left viral/chromosomal junction was determined; however, the right junction sequence revealed the joining to another viral fragment by a short homology (microhomology), a hallmark of illegitimate recombination. The random viral breakpoints and the lack of homology between the virus and flanking chromosomal sequences are also suggestive of an illegitimate integration mechanism. To examine the long-term stability of reporter gene expression, all four clones were grown continuously for 36 passages in either the presence or absence of selection for Neo. Periodic assays over a 5-month period showed no loss of GFP expression for at least two of the clones. This report represents the first detailed analysis of baculovirus integrants within mammalian cells. The potential advantages of the baculovirus system for the stable integration of genetic material into mammalian genomes are discussed.

Recombination of selectable marker DNA in Nicotiana tabacum

A chimeric neomycin phosphotransferase II (NPT II) gene, which normally provides kanamycin resistance to transformed plant cells, was inactivated by in vitro deletions. Repair plasmids not containing plant-specific transcription signals but containing only the NPT II coding region (or parts of it) were used in co-transformation experiments involving direct DNA uptake into protoplasts isolated from Nicotiana tabacum. Recombination, or gene conversion mediated by homologous sequences produced active NPT II genes in about 1% of transformants, rendering these cells resistant to kanamycin. Analysis of the size of the active enzyme indicated that recombination had occurred producing an NPT II gene indistinguishable from the wild-type gene. Southern blot analysis revealed that the bulk of co-transformed donor plasmid DNA had suffered structural modifications; however, kanamycin resistance was inherited in a Mendelian fashion, indicating that at least one functional and structurally intact copy of the regenerated NPT II gene is integrated into the host genome.

Studies of the structure and functional organization of foreign DNA integrated into the genome of Nicotiana tabacum

In transgenic plants obtained either by Agrobacterium tumefaciens-mediated transformation or by direct DNA transfer, the structure of integrated chimeric donor DNA remains stable during vegetative proliferation, during sexual transmission, and under various selection conditions. We correlate the level of expression of the introduced gene in independent transformants and their offspring with the particular arrangement and modification of their integrated DNAs. Genetic analysis of transgenic plants shows that the chimeric gene is transmitted in a Mendelian fashion to the F1 and F2 progeny as a single dominant trait. Deviations from the expected segregation pattern are discussed with respect to different levels of gene activity. We compare the gene activity in heterozygotes versus homozygotes, and show variation in activity between plants regenerated independently from the same transformed callus. Cotransformation studies with two physically unlinked and partly homologous plasmids carrying two different marker genes indicate that they are physically linked after integration into the host genome.

Fate of selectable marker DNA integrated into the genome of Nicotiana tabacum

To compare the effects of different transformation methods on the integration behavior and structural stability of integrated foreign genes in plant cells, tobacco protoplasts were transformed with Escherichia coli plasmid pLGV2103neo DNA using the Ca phosphate DNA coprecipitation technique. Parallel transformations were done by cocultivation with Agrobacterium tumefaciens harboring the Ti plasmid derivatives pGV3850::2103neo or pGV3850::1103neo. A comparison of the fine structure of the integrated donor DNA obtained by direct gene transfer and by cocultivation indicates that the donor DNA in cells transformed by the former technique undergoes structural changes and concatemerizations, while the DNA integrated by the latter procedure is often unaltered. The cotransformed nopaline synthase gene, which is present in the donor Ti plasmid DNA, was inactivated in two out of nine cases. Once integrated, the arrays of selectable marker DNA appear to be structurally stable under different cell culture and selection conditions, as well as after genetic transmission.