Arrangement of integrated viral DNA sequences in cells transformed by adenovirus types 2 and 5

Inclusion of Moloney murine leukemia virus elements upstream of the transgene cassette in an E1-deleted adenovirus leads to an unusual genomic integration in epithelial cells

Classically, the 5' and 3' long terminal repeats (LTRs) are considered necessary but not sufficient for retroviral integration. Recently, we reported that inclusion of these and additional elements from Moloney murine leukemia virus (MoMLV) facilitated transgene integration, without retroviral integrase, when placed in an adenoviral context (AdLTR-luc vector) (Nat. Biotech. 18 (2000), 176; Biochem. Biophys. Res. Commun. 300 (2003), 115). To help understand this nonhomologous DNA recombination event, we constructed another vector, AdELP-luc, with 2.7 kb of MoMLV elements identically placed into an E1-deleted adenovirus type 5 backbone upstream of a luciferase cDNA reporter gene. Unlike AdLTR-luc, no MoMLV elements were placed downstream of the expression cassette. AdELP-luc readily infected epithelial cells in vitro. Southern hybridizations with DNA from cloned cells showed that disruption of the MoMLV sequences occurred. One cell clone, grown in vitro without any special selection medium for 9 months, exhibited stable vector integration and luciferase activity. Importantly, both Southern hybridization and FISH analyses showed that in addition to the MoMLV elements and expression cassette, substantial adenoviral sequence downstream of the luciferase cDNA was genomically integrated. These results suggest that the 2.7 kb of MoMLV sequence included in AdELP-luc have cis-acting functions and mediates an unusual integration event.

Chromosomal integration pattern of a helper-dependent minimal adenovirus vector with a selectable marker inserted into a 27.4-kilobase genomic stuffer

Helper-dependent minimal adenovirus vectors are promising tools for gene transfer and therapy because of their high capacity and the absence of immunostimulatory or cytotoxic viral genes. In order to characterize this new vector system with respect to its integrative properties, the integration pattern of a minimal adenovirus vector with a neo(r) gene inserted centrally into a noncoding 27.4-kb genomic stuffer element derived from the human X chromosome after infection of a sex chromosome aneuploid (X0) human glioblastoma cell line was studied. Our results indicate that even extensive homologies and abundant chromosomal repeat elements present in the vector did not lead to integration of the vector via homologous or homology-mediated mechanisms. Instead, integration occurred primarily by insertion of a monomer with no or little loss of sequences at the vector ends, apparently at random sites, which is very similar to E1 deletion adenovirus vectors. It is therefore unlikely that the incorporation of stuffer elements derived from human genomic DNA, which were shown to allow long-term transgene expression in vivo in a number of studies, leads to an enhanced risk of insertional mutagenesis. Furthermore, our findings indicate that the potential of minimal adenovirus vectors as tools for targeted insertion and gene targeting is limited despite the possibility of incorporating long stretches of homologous sequences. However, we found an enhanced efficiency of stable neo(r) transduction of the minimal adenovirus vector compared to an E1 deletion adenovirus vector, possibly caused by the absence of potential growth-inhibitory viral genes. Complete integration of the vector and tolerance of the integrated vector sequences by the cell might indicate a potential use of these vectors as tools for stable transfer of (large) genes.

Transgenesis by adenovirus-mediated gene transfer into mouse zona-free eggs

Zona-free mouse eggs at the pronucleus stage were infected with a replication-defective adenovirus vector containing a nuclear-targeted lacZ gene. Exogenous beta-galactosidase activity was detected in almost all eggs at the two-cell stage. Of 27 mice that developed from infected eggs, three carried the integrated exogenous gene mediated by the adenovirus. Two of the three expressed the lacZ gene, and all three mice transmitted the adenovirus-mediated transgene to F1 progeny Southern blot analysis was consistent with single copy integration. This finding should accelerate the development of new strategies for transgenesis and assist studies on the function of cloned genes in vivo.

Transduction of the CHO aprt gene into mouse L cells using an adeno-5/APRT recombinant virus

An adenovirus-5 recombinant virus Adapt1 carrying the Chinese hamster ovary (CHO) adenine phosphoribosyltransferase (aprt) gene was constructed by insertion of a 2.5-kb fragment containing the complete CHO aprt structural gene linked to a Moloney murine sarcoma virus (MSV) promoter into the E3 region of adenovirus-5. The CHO aprt gene was in the opposite orientation to the adenovirus E3 promoter. Mouse Lapt- tk- (LAT) cells expressed the CHO aprt gene when infected with the virus, even at low MOI (O.1). APRT activity was detectable from approximately 20 h postinfection. At a low frequency, LAT cells were transformed to aprt+, and four stable transductants were selected in adenine, azaserine (AA) medium. Such cells expressed APRT at approximately 50% wild-type activity and the enzyme was shown to be CHO APRT by starch gel electrophoresis. DNA was isolated from the transductants and probed with CHO aprt-specific DNA and with viral DNA probes. The results indicated that the CHO aprt gene was integrated into the LAT cells at a site other than mouse aprt. Although neighboring viral sequences were integrated and maintained in the transductants, viral sequences further upstream and downstream of the aprt gene were absent.

Infection of eucaryotic cells by helper-independent recombinant adenoviruses: early region 1 is not obligatory for integration of viral DNA

Recombinant viral genomes carrying a selectable drug resistance marker have been constructed by insertion of a hybrid gene for neomycin resistance into the helper-independent adenovirus vector, delta E1/X. The hybrid gene consists of sequences coding for the aminoglycoside 3'-phosphotransferase II from Tn5, under the control of the simian virus 40 early promoter, and renders mammalian cells resistant to the neomycin analog, G-418. Most of adenovirus early region 1 is deleted from delta E1/X (nucleotides 455 to 3330), and recombinant viral genomes carry the hybrid gene in its place. The large and small XbaI fragments of delta E1/X were ligated to the hybrid gene, and the mixture was transfected into 293 cells. Single plaques were isolated and subsequently passaged in 293 cells to produce virus stocks. The recombinant viruses efficiently rendered cultured rat (Rat2) and simian (CV1) cells resistant to G-418. Cloned cell lines selected for resistance to G-418 contained viral DNA integrated into the host cell genome, demonstrating that early region 1 is not essential for integration of the viral genome. Southern transfer experiments revealed that (i) the sites of integration in the host genome were not unique; (ii) in general, transformed CV1 cell lines contained single-copy, full-length viral genomes, colinear with the infecting virus; (iii) transformed Rat2 cell lines generally contained one to several copies of full-length viral genomes integrated colinearly with the infecting viral DNA; and (iv) three of these five lines of transformed Rat2 cell lines contained tandemly repeated viral DNA sequences in which the right and left ends of the viral genome were joined to each other.

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of integrated viral DNA sequences outside the transforming region in eight adenovirus-transformed cell lines

The expression of early and intermediate-early viral regions in eight adenovirus type 5 transformed cell lines was analyzed by radioimmuno-inhibition and RNA-DNA hybridization techniques. Details on the arrangement of the integrated viral DNA sequences in these cell lines have already been published (Visser, L., Wassenaar, A.D.C., Van Maarschalkerweerd, M.W. and Rozijn, T.H. (1981) J. Virol, 39, 684-693). In all cell lines tested, proteins encoded by the transforming region E1 are present. Dependent on the viral DNA content, additional early regions are expressed in most cell lines. In two of the cell lines polypeptides related to the adenoviral terminal protein, encoded by the recently described region E2b, could be detected. The viral DNA sequence encoding the body of the terminal protein mRNA is probably integrated intact, but the promoter region and at least some of the leaders are lacking in these cell lines.

Covalently closed circles of adenovirus 5 DNA

The genome of adenoviruses is a double-stranded linear DNA molecule with inverted terminal repeats about 100 base pairs (bp) in length and a terminal protein covalently linked to the 5' nucleotide of each strand. Both of these features permit the formation of DNA circles, the inverted repeats allowing the circularization of single-stranded DNA and the terminal protein the joining of one or more molecules to yield double-stranded circles or concatemers. However, although the existence of covalently closed circles has been postulated, double-stranded viral DNA purified from virions or infected cells by conventional methods (that is, using proteases and phenol or chloroform) has always been obtained in a linear form. Here, we present evidence for the existence in adenovirus 5 (Ad5) infected cells of novel structures resulting from covalent head-to-tail joining of viral DNA molecules and show that these structures are due at least in part to the formation of covalently closed circles.

Nucleotide sequence analysis of the linked left and right hand terminal regions of adenovirus type 5 DNA present in the transformed rat cell line 5RK20

A peculiar phenomenon is observed in several adenovirus type 2 or 5 (Ad2 or Ad5) transformed cell lines: the right hand and left hand terminal regions of the viral genome present in the viral DNA insertions of these cell lines are found to be linked together. A large part of the viral DNA insertion present in the Ad5 transformed rat cell line 5RK20 has been cloned in the lambda vector Charon21A, including the segment containing the linked terminal regions. Sequence analysis of the linkage region showed a perfect homology with the Ad5 DNA sequence and a direct linkage of basepair (bp) 63 of the left hand end of the viral genome to bp 108 of the right hand end. No cellular or rearranged viral sequences were present. Our findings suggest that the joining of viral sequences into the cellular genome.