Cellular and early viral factors in the interaction of adenovirus type 12 with hamster cells: the abortive response

[Is there a risk of zoonotic disease due to adenoviruses?]

Every year brings another round of zoonotic viral infections. Usually they fall under the radar, but the occasional lethal epidemic brings another scare to the public and new urgency to the medical community. The types of these viruses (DNA vs. RNA genomes, enveloped vs. proteinaceous) as well as the preceding host(s) vary. Over the last 20 years, bats have been identified as an enigmatic carrier for several pathogens that have jumped the species barrier and infected humans. Factors that favour the emergence of zoonotic pathogens include the increasing overlap of the human and animal habitats, cultural activities, and the host reservoir. In this context, we asked whether bat and/or nonhuman primate adenoviruses are a risk for human health.

Epigenetic mechanisms in human adenovirus type 12 oncogenesis

For the past 30 years, my laboratory has concentrated its work on demonstrating that the epigenetic consequences of foreign DNA insertion into established mammalian genomes – de novo DNA methylation of the integrate and alterations of methylation patterns across the recipient genome – are essential elements in setting the stage towards oncogenic transformation. We have primarily studied human adenovirus type 12 (Ad12) which induces undifferentiated tumors in Syrian hamsters (Mesocricetus auratus) either at the site of subcutaneous Ad12 injection or intraperitoneally upon intramuscular injection. Up to 90% of the hamsters injected with Ad12 develop tumors within 3–6 weeks. Integration of foreign DNA, its de novo methylation, and the consequences of insertion on the cellular methylation and transcription profiles have been studied in detail. While viral infections are a frequent source of foreign genomes entering mammalian and other hosts and often their genomes, we have also pursued the fate of food-ingested foreign DNA in the mouse organism. The persistence of this DNA in the animals is transient and there is no evidence for the expression or germ line fixation of foreign DNA. Nevertheless, the occasional cell that carries integrated genomes from that foreign source deserves the oncologist's sustained interest.

Effect of 6-azacytidine on the course of experimental adenoviral infection in newborn Syrian hamsters

Adenoviral infection is a serious human pathology leading to respiratory, gastrointestinal and ocular disorders and epidemic outbreaks, especially in children's groups. Here we present the results from an investigation of anti- adenoviral effect of 6-azacytidine (6-AC) both in vitro and in vivo. The selectivity index of 6-AC for adenovirus type 5 in HEp-2 cells was 374, the 50% effective concentration was 0.5 mg/ml. For in vivo investigations we developed a model of disseminated adenoviral infection in newborn Syrian hamsters. The infectious virus was recovered from the liver, kidney, lungs and heart. Application of 6-AC led to a reduced period of the virus presence (7 days in the liver and 4 days in the kidney and heart) and lowered virus titers on day 3 post-inoculation (p.i.) (liver - 2.7 and 4.1, heart - 0 and 3.2, kidney - 0 and 2.4 log(10 )CPD(50)/mg tissue weight, in the presence and absence of 6-AC, respectively). Application of 6-AC to newborn Syrian hamsters led to partial destruction of their splenocytes. The results obtained suggest that 6-AC or 6-ACbased drugs with lower toxicity or applied topically may be suitable for therapy and prevention of adenoviral infection in humans.

Evasion of the immune system by adenoviruses

Human adenoviruses (Ads) have the ability to transform primary cells, and certain Ads, the subgenus A adenoviruses such as Ad12, induce tumours in immunocompetent rodents. The oncogenic phenotype of the subgenus A adenoviruses is determined by the viral E1A oncogene. In order to generate tumours, Ad12-transformed cells must evade the cellular immune system of the host. Ad12 E1A gene products mediate transcriptional repression of several genes in the major histocompatibility complex (MHC) involved in antigen processing and presentation, resulting in evasion of cytotoxic T lymphocyte (CTL) killing of transformed cells. In this review, the molecular mechanisms of E1A-mediated transcriptional repression of MHC gene expression are described. In addition, evasion of natural killer (NK) cell killing by Ad-transformed cells is also considered.

Tumorigenesis by adenovirus type 12 in newborn Syrian hamsters

Ad12 oncogenesis in hamsters has been studied in detail to provide the following new data in this tumor model. Cells in the Ad12-induced tumors, often thought to be of neuronal origin, actually exhibit mesenchymal and neuronal characteristics and are probably of an undifferentiated derivation. Their intraperitoneal spread upon intramuscular injection of Ad12 adds another important new aspect. Differences in the integration patterns among the tumors suggest clonal origins from individual transformation events. Ad12 gene expression in the tumors is determined, at least in part, by the patterns of DNA methylation imprinted de novo upon the integrated Ad12 genomes. Differential Ad12 gene expression patterns, which have previously not been described in tumors, are an important parameter in Ad12 oncogenesis. The availability of cellular DNA arrays has opened up unprecedented possibilities to document changes in cellular transcription patterns, particularly of cancer-specific genes. These patterns exhibit differences and similarities among the different Ad12-induced tumors. Among the cellular genes, which are expressed in the Ad12-induced tumors, many are cancer-specific. We pursue the hypothesis that these alterations in cellular transcription patterns as a consequence of viral DNA integration and expression play an essential role in Ad12 oncogenesis.

The abortive infection of Syrian hamster cells with human adenovirus type 12

Human adenovirus type 12 (Ad12) induces undifferentiated tumors in newborn Syrian hamsters, and this tumor model has been investigated in detail in our laboratory. One of the characteristics of the Ad12-hamster cell system is a strictly abortive infection cycle. In this chapter, we summarize previous and more recent results of studies on the interaction of Ad12 with the nonpermissive BHK21 hamster cell line. The block of Ad12 replication lies before viral DNA replication and late gene transcription which cannot be detected with the most sensitive techniques. Ad12 adsorption, cellular uptake and transport of the viral DNA to the nucleus are less efficient in the nonpermissive hamster cells than in permissive human cells. However, most of the early functions of the Ad12 genome are expressed in BHK21 cells, though at a low level. In the downstream region, the first exon, of the major late promoter (MLP) of Ad12 DNA, a mitigator element of 33 nucleotide pairs in length has been identified which contributes to the inactivity of the MLP in hamster cells and its markedly decreased activity in human cells. The E1 functions of Ad2 or Ad5 are capable of partly complementing the Ad12 deficiencies in hamster cells in that Ad12 viral DNA replication and late gene transcription can proceed, e.g. in a BHK hamster cell line, BHK297-C131,which carries in an integrated form and constitutively expresses the E1 region of Ad5 DNA. Nevertheless, the late Ad12 mRNAs, which are synthesized in this system with the authentic nucleotide sequence, fail to be translated to structural viral proteins. Hence, infectious virions are not produced in the partly complementing system. Probably there is also a translational block for late Ad12 mRNAs in hamster cells. We have recently shown that the overexpression of the Ad12 preterminal protein (pTP) gene or of the E1A gene facilitates the synthesis of full-length, authentic Ad12 DNA in BHK21 cells infected with Ad12. Apparently the pTP has a hitherto unknown function in eliciting full cycles of Ad12 DNA replication even in nonpermissive BHK21 cells when sufficient levels of Ad12 pTP are produced. We pursue the possibility that the completely abortive infection cycle of Ad12 in hamster cells ensures the survival of Ad12-induced hamster tumor cells which all carry, integrated in their genomes, multiple copies of Ad12 DNA. In this way, the viral genomes are immortalized and expanded in a huge number of tumor cells.

Biology of ovine adenovirus infection of nonpermissive cells

Nonhuman adenoviruses, including those of the genus Atadenovirus, have the potential to serve as vectors for vaccine and gene therapy applications in humans, since they are resistant to preexisting immunity induced by human adenoviruses in the majority of the population. In this study, we elucidate the outcome of infection by ovine adenovirus type 7 isolate 287 (OAdV) of several nonovine cell types. We show here that OAdV infects a wide range of nonovine cells but is unable to complete its replication cycle in any of them. In nonovine, nonfibroblast cells, viral replication is blocked at an early stage before the onset of, or early in, DNA replication. Some fibroblasts, on the other hand, allow viral DNA replication but block virus production at a later stage during or after the translation of late viral proteins. Late viral proteins are expressed in cells where viral DNA replication takes place, albeit at a reduced level. Significantly, late proteins are not properly processed, and their cellular distribution differs from that observed in infected ovine cells. Thus, our results clearly show that OAdV infection of all nonovine cells tested is abortive even if significant viral DNA replication occurs. These findings have significant positive implications with respect to the safety of the vector system and its future use in humans.

Overexpression of the adenovirus type 12 (Ad12) pTP or E1A gene facilitates Ad12 DNA replication in nonpermissive BHK21 hamster cells

In the adenovirus type 12 (Ad12) hamster cell system, abortive virus infection is one of the factors associated with the highly efficient oncogenesis in newborn Syrian hamsters. We have shown earlier that the replication and efficient late transcription of the Ad12 genome are blocked in Syrian hamster cells. Some of the early Ad12 functions are transcribed in these cells, although at a minimal rate. In the present study, we demonstrate that low expression levels of the E1A and precursor to terminal protein (pTP) genes of Ad12 seem to be responsible for the lack of Ad12 DNA replication in hamster cells. The Ad12 genes for the E1A functions or for pTP were tethered to the strong early promoter of the human cytomegalovirus and transfected into BHK21 cells. Subsequently, these cells were infected with Ad12 virions. In Ad12-infected BHK21 cells, which overexpressed pTP or E1A, full-length Ad12 DNA was de novo synthesized, as documented by metabolic labeling with [3H]thymidine and by zone velocity sedimentation in alkaline sucrose gradients followed by gel electrophoresis of the 3H-labeled DNA and Southern blot hybridization to 32P-labeled Ad12 DNA. Transfection of the cloned E1A region of Ad2 yielded similar results. The newly synthesized Ad12 DNA was covalently linked to pTP. The Ad12 DNA binding protein (DBP) and DNA polymerase (pol) genes were transcribed at levels similar to those in merely Ad12-infected cells. In pTP or E1A gene-transfected and Ad12-infected BHK21 cells, marginal levels of late Ad12 mRNA were detectable. Late Ad12 proteins were, however, not synthesized. Apparently, Ad12 DNA replication in hamster cells is rendered impossible due to insufficient threshold levels of the viral E1A and/or pTP.