Studies of adenovirus-SV40 hybrid viruses. V. Evidence for linkage between adenovirus and SV40 genetic materials

DNA cloning: a personal view after 40 years

In November 1973, my colleagues A. C. Y. Chang, H. W. Boyer, R. B. Helling, and I reported in PNAS that individual genes can be cloned and isolated by enzymatically cleaving DNA molecules into fragments, linking the fragments to an autonomously replicating plasmid, and introducing the resulting recombinant DNA molecules into bacteria. A few months later, Chang and I reported that genes from unrelated bacterial species can be combined and propagated using the same approach and that interspecies recombinant DNA molecules can produce a biologically functional protein in a foreign host. Soon afterward, Boyer's laboratory and mine published our collaborative discovery that even genes from animal cells can be cloned in bacteria. These three PNAS papers quickly led to the use of DNA cloning methods in multiple areas of the biological and chemical sciences. They also resulted in a highly public controversy about the potential hazards of laboratory manipulation of genetic material, a decision by Stanford University and the University of California to seek patents on the technology that Boyer and I had invented, and the application of DNA cloning methods for commercial purposes. In the 40 years that have passed since publication of our findings, use of DNA cloning has produced insights about the workings of genes and cells in health and disease and has altered the nature of the biotechnology and biopharmaceutical industries. Here, I provide a personal perspective of the events that led to, and followed, our report of DNA cloning.

Control of simian virus 40 gene expression in adenovirus-simian virus 40 hybrid viruses. Synthesis of hybrid adenovirus 2-simian virus 40 RNA molecules in cells infected with a nondefective adenovirus 2-simian virus 40 hybrid virus

The effect of interferon on simian virus 40 (SV40) and adenovirus 2 (Ad2) T antigen synthesis has been examined in cells infected with SV40, with Ad2, and with a nondefective Ad2-SV40 hybrid virus, Ad2(+)ND(4). The induction of SV40 T antigen by SV40 was highly sensitive to interferon, whereas the induction of Ad2 T-antigen by Ad2 was resistant. This difference in interferon sensitivity was also noted in cells simultaneously infected with both viruses. However, the induction of SV40 T antigen by Ad2(+)ND(4), which contains covalently linked SV40 and Ad2 DNAs, was as resistant to interferon as the induction of Ad2 T antigen. This change in the interferon sensitivity of SV40 T antigen synthesis suggests that the expression of at least this portion of the SV40 genetic information in Ad2(+)ND(4) is under Ad2 genetic control. When RNA extracted from Ad2(+)ND(4)-infected cells was examined by means of sequential hybridization with Ad2 DNA, elution, and rehybridization with SV40 DNA, 27% of the SV40-specific RNA was found to be linked to Ad2 RNA. No such linkage was detected in control mixtures of Ad2 and SV40 RNAs. The presence of Ad2 and SV40 nucleotide sequences in the same RNA molecule implies that, in Ad2(+)ND(4) infection, transcription is initiated in the DNA of one virus (Ad2 or SV40) and continues without interruption across the point of junction into the DNA of the other virus. Furthermore, the interferon resistance of Ad2(+)ND(4)-induced SV40 T antigen synthesis suggests that transcription of the genetic information for SV40 T antigen is initiated in a region of Ad2 DNA.

Use of nondefective adenovirus-simian virus 40 hybrids for mapping the simian virus 40 genome

A series of viable recombinants between adenovirus 2 (Ad2) and simian virus 40 (SV40) (nondefective Ad2-SV40 hybrids) have been isolated. The members of this series (designated Ad2(+)ND(1) through Ad2(+)ND(5)) differ from one another in the early SV40-specific antigens and the SV40-specific RNA species which they induce in infected cells. They also contain different amounts of SV40 DNA as shown by RNA-DNA hybridization techniques. We have examined the structure of the DNA molecules from these hybrids, using electron microscope heteroduplex mapping techniques. Each hybrid was found to contain a single segment of SV40 DNA of characteristic size covalently inserted at a unique location in the adenovirus 2 DNA molecule. The SV40 segments of the various hybrids formed an overlapping series with a common end point. When the results of the electron microscopic study were combined with data on antigen induction, it was found that a self-consistent map could be constructed which related specific regions of the SV40 genome to the induction of specific antigens. The order of these early SV40 antigen inducing regions in the SV40 DNA segments contained in the nondefective hybrids is: U antigen, tumor specific transplantation antigen, and T antigen with the U antigen region being nearest the common end point.

Studies of nondefective adenovirus 2-simian virus 40 hybrid viruses. IV. Characterization of the simian virus 40 ribonucleic acid species induced by wild-type simian virus 40 and by the nondefective hybrid virus, Ad2 + ND 1

Ad2(+)ND(1), a nondefective adenovirus 2 (Ad2)-simian virus 40 (SV40) hybrid virus, has been previously shown to contain a small segment of the SV40 genome covalently linked to Ad2 deoxyribonucleic acid (DNA). The SV40 portion of this hybrid virus has been characterized by relating the SV40-specific ribonucleic acid (RNA) sequences transcribed from the Ad2(+)ND(1) DNA to those transcribed from the DNA of SV40 itself. RNA-DNA hybridization-competition studies indicate that the SV40 component of Ad2(+)ND(1) consists of some, but not all, of that part of the SV40 genome which is transcribed early, i.e., prior to viral DNA replication, in SV40 lytic infection.

Simian virus 40 integration site in an adenovirus 7-simian virus 40 hybrid DNA molecule

The E46(+) strain of Adenovirus 7 is a mixed-virus population containing defective Adenoviurs 7-SV40 hybrid particles and helper, nonhybrid Adenovirus 7 particles. We have applied electron microscopic mapping techniques to obtain a physical map of the genome of the hybrid particles present in E46(+)PL1, a substrain of E46(+) derived from a single two-hit plaque. DNA molecules extracted from purified E46(+)Pl1 virions were found to be linear duplexes, with a mean lenght of 10.9 mum. When these molecules were denatured and renatured, a unique heteroduplex was formed that presumably derived one of its strands from an Adenovirus 7-SV40 hybrid molecule and the other from a nonhybrid Adenovirus 7 molecule. This heteroduplex was double-stranded, except for a short region near one end where the two strands were not paired. On the basis of measurements of the lengths of the single-and double-stranded regions in the heteroduplex, the structure of the Adenovirus 7-SV40 hybrid genome can be reconstructed as follows: The hybrid genome contains 16% less Adenovirus 7 DNA than the nonhybrid Adenovirus 7 genome. This deletion consists of the segment of DNA that maps between 0.05 and 0.21 molecular lenghts in the nonhybrid Adenovirus 7 DNA molecule. The deleted DNA has been partially replaced by an amount of heterologous DNA equivalent to 75% of the complete SV40 genome. A model for the generation of the hybrid genome is presented.

Studies on nondefective adenovirus-simian virus 40 hybrid viruses. I. A newly characterized simian virus 40 antigen induced by the Ad2+ND 1 virus

The nondefective adenovirus 2 (Ad2)-simian virus 40 (SV40) hybrid virus, Ad2(+)ND(1), does not induce heat-labile SV40 T antigen but does induce a previously uncharacterized heat-stable SV40 antigen-the SV40 "U" antigen. This antigen is detectable by both immunofluorescence and complement fixation by using sera from hamsters with SV40 tumors. Sera from hamsters bearing SV40 tumors can be divided into two groups, those that react with both SV40 T and U antigens (T(+)U(+) sera) and those that react with SV40 T antigen only (T(+)U(-) sera). SV40 U-specific sera from monkeys immunized with Ad2(+)ND(1)-infected cells do not react with SV40 T antigen by immunofluorescence but do react with an antigen in the nucleus of SV40-transformed cells and with an early, cytosine arabinoside-resistant antigen present in the nucleus of SV40-infected cells. A heat-stable SV40 antigen detectable by complement fixation with T(+)U(+) hamster sera is present in extracts of SV40-induced hamster tumors and in cell packs of SV40-infected or -transformed cells. SV40 U-antigen synthesis by Ad2(+)ND(1) virus is partially sensitive to inhibitors of deoxyribonucleic acid synthesis, whereas U-antigen synthesis by SV40 virus is an early cytosine arabinoside-resistant event. As an early SV40 antigen differing from SV40 T antigen, U antigen may play a role in malignant transformation mediated by SV40.

Equilibrium density gradient studies on simian virus 40-yielding variants of the adenovirus type 2-simian virus 40 hybrid population

The simian virus 40 (SV40)-yielding variants of the adenovirus type 2 (Ad.2)-SV40 hybrid (Ad.2(++)) population were studied by means of fixed-angle equilibrium density gradient centrifugation in cesium chloride. The hybrid virions of the Ad.2(++) high-efficiency yielder population banded at densities of 0.004 g/cm(3) lighter than the nonhybrid Ad.2 virions. The degree of separation of the hybrid particles was sufficient to permit greater than 100-fold relative purification by two cycles of centrifugation. Hybrid particles that produce adenovirus plaques in African green monkey kidney cells by two-hit kinetics (one-hit kinetics when assayed on lawns of nonhybrid adenovirus) were not separable from the particles that yield SV40 virus. The hybrid particle in the Ad.2(++) low-efficiency yielder population was not separable from the nonhybrid Ad.2 virions.

Density Differences Between Hybrid and Nonhybrid Particles in Two Adenovirus-Simian Virus 40 Hybrid Populations

The adenovirus 7-simian virus 40 hybrid virus population E46 + was subjected to fixed-angle equilibrium density gradient centrifugation in CsCl. A difference in buoyant density between the hybrid virion and its nonhybrid adenovirus 7 counterpart was noted, the hybrid virion possessing the lower buoyant density. This difference in buoyant density appeared to be accentuated in a population of adenovirus 2 +t7 , a derivative of E46 + in which the adenovirus 7-simian virus 40 genome had been transferred to an adenovirus 2 capsid.

In vitro transformation by the adenovirus-simian virus 40 hybrid viruses. V. Virus-specific ribonucleic acid in cell lines transformed by the adenovirus 2-simian virus 40 and adenovirus 12-simian virus 40 transcapsidant hybrid viruses

The ribonucleic acid-deoxyribonucleic acid hybridization technique was utilized to determine the presence of adenovirus (ad) and SV40 genetic information and to determine which ad genomes were present in clones of hamster cells transformed with the ad 2-SV40 and ad 12-SV40 transcapsidant hybrid virus populations. The results were correlated with the morphology of the transformed cells and colonies. It was found that cells transformed by either transcapsidant virus which had an SV40 morphology contained the ad 7 and SV40 genomes, whereas cells with a typical ad morphology contained only ad genetic information. Cells and colonies with morphological features of both ad- and SV40-transformed cells contained either the ad 2, or ad 12 genomes, depending on the transcapsidant used, together with the ad 7 and SV40 genomes. The results indicate the following: at least three different events occurred during transformation of hamster cells by the transcapsidant virus populations; the morphology of the resulting clones is determined by the viral genome(s) present; the linkage of the ad 7-SV40 genomes is confirmed since the ad 7- SV40 genomes were never found to be dissociated; the defective ad 7-SV40 genomes are capable of causing transformation; and the transcapsidant particle is probably composed of only ad 7 and SV40 genetic information.

In vitro transformation by adenovirus-simiam virus 40 hybrid viruses. IV. Properties of clones isolated from cell lines transformed by adenovirus 2-simiam virus 40 and adenovirus 12-simiam virus 40 transcapsidant hybird viruses

Clones were isolated from hamster cells transformed by the adenovirus 2-SV40 and adenovirus 12-SV40 transcapsidant hybrid viruses. The clones were characterized with respect to their cytomorphology, virus and antigen content, and the histomorphology of tumors induced by transplantation of the clonal sublines to hamsters. Three different cellular and colonial morphologies were observed. Clones with an SV40 morphology gave rise to tumors predominantly with an SV40 histology, whereas clones with an adenovirus morphology produced typical adenovirus tumors upon transplantation of the transformed cells. Clones which had features of both SV40 and adenovirus transformed cells gave rise to "intermediate" and adenovirus tumors. The results indicate that multiple events occur during transformation and tumorigenesis by the transcapsidant virus populations and provide an explanation for the multiplicity of findings which have been reported with these virus populations.

A nondefective (competent) adenovirus-SV40 hybrid isolated from the AD.2-SV40 hybrid population

A new nondefective hybrid virus has been plaque-isolated from the Ad.2-SV40 hybrid population. This virus replicates efficiently with one-hit kinetics in both human embryonic kidney and African green monkey kidney cells, induces an SV40 specific antigen which is detectable by immunofluorescence and complement-fixation using sera from SV40 tumor-bearing hamsters, and produces SV40-specific RNA detectable by DNA-RNA hybridization. The SV40-specific antigen induced by this virus is heat-stable, sensitive to inhibitors of DNA synthesis, serologically different from SV40 T and viral antigens, and is an unrecognized SV40 antigen.

In vitro transformation by the adenovirus-SV40 hybrid viruses. 3. Morphology of tumors induced with transformed cells

Tumors induced with hamster kidney cells transformed by the adeno 2-, adeno 3-, adeno 7-, and adeno 12-SV40 hybrid viruses, and by adenovirus type 12, were examined histologically. The tumors induced with adeno 2-, adeno 3-, and adeno 7-SV40-transformed cells were similar to tumors induced with SV40-transformed hamster kidney cells but contained cells intermediate in morphology between SV40 and adenovirus tumor cells and occasionally contained nests of adenovirus-like cells. Cells transformed by the adeno 12-SV40 hybrid and by adenovirus type 12 gave rise to morphologically similar tumors. The results suggest that both viral genomes are operative in hybrid-transformed cells but that one genome is apparently responsible for the predominant morphology of the tumor. Evidence that the morphology of a single transformed target cell is determined by the transforming genome was discussed.

In vitro transformation by the adenovirus-SV40 hybrid viruses. II. Characteristics of the transformation of hamster cells by the adeno 2-, adeno 3-, and adeno 12-SV40 viruses

Primary weanling hamster kidney cultures were transformed with the adeno 2-SV40, adeno 3-SV40, and adeno 12-SV40 hybrid viruses and with adenovirus type 12. The transformed cell lines which were established were characterized with respect to morphology, virus and antigen content, and chromosome aberrations. The adeno 2 and adeno 3-SV40 hybrid transformed cells had the morphology and T antigen content characteristic of SV40 transformations; cells transformed by the former hybrid had cytogenetic changes typical of SV40-transformed cells as well. The adeno 12-SV40 transformed cells were similar morphologically to adeno 12-transformed cells, contained both the SV40 and adeno T antigens and demonstrated the karyotypic instability of SV40-transformed cells, indicating that both viral genomes are operative in these cells. Although the results indicate that the SV40 genome in hybrids derived from the moderately or nononcogenic adenoviruses supplies the determinants for most of the characteristics investigated, and perhaps for oncogenesis, evidence was presented which suggests that a portion of a nononcogenic adenovirus genome may be integrated in adeno 2-SV40 transformed cells and directs the synthesis of adenovirus T antigens.