Increased infectivity of extracellular adenovirus type 12

Human Adenovirus Type 12

When viruses cross species barriers, they often change their biological and pathogenetic properties. In the author's laboratory the nonproductive interaction of Syrian hamster cells with human adenovirus type 12 (Ad12) has been studied. Ad12 induces undifferentiated tumors in newborn hamsters (Mesocricetus auratus) at high frequency. Ad12 inefficiently enters hamster (BHK21) cells, and only small amounts of viral DNA reach the nucleus. Viral DNA replication and late transcription are blocked. In Ad12-induced tumor cells, multiple copies of viral DNA are chromosomally integrated. The integrated viral DNA becomes de novo methylated. Cellular DNA methylation and transcription patterns in Ad12-transformed cells and in Ad12- induced tumor cells are altered. These changes may be related to the oncogenic potential of Ad12 in hamsters. In this chapter, concepts and techniques for the study of the Ad12-hamster cell system are summarized.

A new concept in (adenoviral) oncogenesis: integration of foreign DNA and its consequences

A new concept for viral oncogenesis is presented which is based on experimental work on the chromosomal integration of adenovirus DNA into mammalian genomes. The mechanism of adenovirus DNA integration is akin to non-sequence-specific insertional recombination in which patch homologies between the recombination partners are frequently observed. This reaction has been imitated in a cell-free system by using nuclear extracts from hamster cells and partly purified fractions derived from them. As a consequence of foreign DNA insertion into the mammalian genome, the foreign DNA is extensively de novo methylated in specific patterns, presumably as part of a mammalian host cell defense mechanism against inserted foreign DNA which can be permanently silenced in this way. A further corollary of foreign (adenovirus or bacteriophage lambda) DNA integration is seen in extensive changes in cellular DNA methylation patterns at sites far remote from the locus of insertional recombination. Repetitive cellular, retrotransposon-like sequences are particularly, but not exclusively, prone to these increases in DNA methylation. It is conceivable that these changes in DNA methylation are a reflection of a profound overall reorganization process in the affected genomes. Could these alterations significantly contribute to the transformation events during viral or other types of oncogenesis? These sequelae of foreign DNA integration into established mammalian genomes will have to be critically considered when interpreting results obtained with transgenic, knock-out, and knock-in animals and when devising schemes for human somatic gene therapy. The interpretation of de novo methylation as a cellular defense mechanism has prompted investigations on the fate of food-ingested foreign DNA. The gastrointestinal (GI) tract provides a large surface for the entry of foreign DNA into any organism. As a tracer molecule, bacteriophage M13 DNA has been fed to mice. Fragments of this DNA can be found in small amounts (about 1% of the administered DNA) in all parts of the intestinal tract and in the feces. Furthermore, M13 DNA can be traced in the columnar epithelia of the intestine, in Peyer's plaque leukocytes, in peripheral white blood cells, in spleen, and liver. Authentic M13 DNA has been recloned from total spleen DNA. If integrated, this DNA might elicit some of the described consequences of foreign DNA insertion into the mammalian genome. Food-ingested DNA will likely infiltrate the organism more frequently than viral DNA.

Sequence analysis of putative pVIII, E3 and fibre regions of porcine adenovirus type 3

Sequence analysis of a region of the genome of porcine adenovirus type 3 from map unit 79.5 to map unit 92 was performed. Homology studies revealed genes coding for the hexon-associated protein pVIII on the left and for the fibre protein on the right of the sequenced region. By analogy with the genomic organization of other adenoviruses, the 1179 bp sequence between the pVIII and fibre open reading frames, extending from map unit 81.3 to map unit 84.7, was identified as the equivalent of the E3 region of human adenoviruses. The deduced amino acid sequence of one of the three open reading frames of the putative E3 region showed homology with the 13.3K E3 protein of canine adenovirus type 2. The primary structure of the putative fibre protein was similar to that described for human adenovirus types 2 and 5, with a 14 pseudorepeat motif in the shaft region of the fibre. A 742 bp tandem repeat starting in the middle of the fibre gene and extending beyond the termination codon of this gene was observed.

A block in release of progeny virus and a high particle-to-infectious unit ratio contribute to poor growth of enteric adenovirus types 40 and 41 in cell culture

The fastidious enteric adenovirus (FEAd) types 40 (Ad40) and 41 (Ad41) are found in stool specimens of infants and young children in association with gastroenteritis. Although they can be isolated routinely from clinical specimens by using 293 cells, they are propagated with variable success in cell lines which support the replication of other adenovirus serotypes. HeLa cells are generally considered to be nonpermissive for the replication of FEAds, but in this study, Ad40 and Ad41 grew to comparable titers in individual 293 and HeLa cells. However, virus was not efficiently released from infected HeLa cells and thus did not undergo multiple cycles of infection in HeLa cell cultures. The block in virus release was not overcome in KB18 cells which, like 293 cells, constitutively express proteins encoded by the E1B region of a subgroup C adenovirus (in this case Ad2). Moreover, it was apparent from these studies that Ad40 and Ad41 have particle-to-infectious unit ratios several orders of magnitude greater than that for Ad5, even in 293 cells which express the E1A and E1B proteins of Ad5 and are considered to be permissive for replication of the FEAds. Neither the block in release of progeny virus nor the high particle-to-infectious unit ratio is explained solely by the defect in expression of the E1B 55K protein identified by Mautner et al. (V. Mautner, N. MacKay, and V. Steinthorsdottir, Virology 171:619-622, 1989; V. Mautner, N. MacKay, and K. Morris, Virology 179:129-138, 1990).

Chromosomal localization of integrated adenovirus DNA in productively infected and in transformed mammalian cells

The in situ hybridization technique has been used to localize adenovirus type 12 (Ad12) genomes on specific chromosomes of Ad12-transformed hamster cells as well as on specific chromosomes of human cells productively infected with Ad12. Hamster cell lines T637 and A2497-2 contain 20 to 22 and 17 copies of Ad12 DNA, respectively, in an integrated form. The results of in situ hybridization experiments demonstrate that the Ad12 DNA molecules are predominantly located on chromosomes 2, 7, 11 to 13, and 15 in cell line T637, and on chromosomes 7, 20, and perhaps 16 to 19 in cell line A2497-2. These data further document that viral DNA is chromosomally integrated and does not persist in a huge extrachromosomal, episomal structure. There is evidence from previous work that adenovirus DNA can also be covalently linked to human cellular DNA after productive adenovirus infection of human cells. In keeping with these earlier findings, we have now been able to show by in situ hybridization experiments that early in productive infection, i.e., 2 or 6 hr after infection, Ad12 genomes are predominantly associated with a limited number of human chromosomes. It appears biologically significant that these chromosomes, which belong to groups A and CII, have been the same ones in several independent infection experiments. Other chromosomes may also carry smaller amounts of viral DNA. In situ hybridization of Ad12 DNA to chromosomes of uninfected human cells yields no significant chromosomal association of viral DNA. The signal to noise ratio of grain counts over chromosomes from Ad12-infected cells to areas devoid of chromosomes is 5.6. The results presented raise the question of whether Ad12 DNA can integrate at selective sites in human chromosomes and whether this insertion event plays an essential role in early steps of viral transcription or replication. There is no direct evidence to suggest a biologic function for viral DNA integration in the productive infection cycle.

Adenovirus types 2 and 5 functions elicit replication and late expression of adenovirus type 12 DNA in hamster cells

Human adenovirus type 12 (Ad12) cannot replicate in hamster cells. There is a complete block of viral DNA replication and of the expression of late viral genes. Early viral functions are expressed. In contrast, hamster cells are permissive for human adenovirus type 2 (Ad2). Some of the Ad12-specific functions are insufficient to support viral replication in hamster cells, or else cellular functions are missing or inhibitory for Ad12 replication. It was shown that the block in the replication and late expression of the Ad12 genome in hamster cells could, at least in part, be complemented by Ad2 and adenovirus type 5 (Ad5) functions. When hamster cells were coinfected with Ad2 (or Ad5) and Ad12, both Ad2 (Ad5) and Ad12 DNA replicated. Ad2 (Ad5) virions were produced in double-infected hamster cells. The assembly of intact Ad12 virions was not detectable by the techniques used here. The analysis was further refined by Ad12 superinfecting Ad2- or Ad5-transformed cells which carried in an integrated form defined fragments of the Ad2 or Ad5 genome. Persistence and continued expression of the left terminus of the Ad2 or Ad5 DNA in these cells has been documented and helped to support replication and late expression of Ad12 DNA. It remains to be determined which of the E1 functions of Ad2 or Ad5 were responsible for the helper effect. Investigations on the biochemical mechanism of this complementation will entail studies on very complex viral and possibly cellular functions involved in the control of viral gene expression.