Dissociation of carcinogen-induced SV40 DNA-amplification and amplification of AAV DNA in a Chinese hamster cell line

Spontaneous mobilization of integrated recombinant adenoassociated virus in a cell culture model of virus latency

A cell line containing integrated recombinant adenoassociated virus (AAV) was investigated for spontaneous mobilization of vector sequence. Detection of these rare events was facilitated by using a vector design that allowed the circular rescue product (cAAV) to be individually scored by bacterial transformation. Restriction and sequence analysis of captured clones revealed five highly ordered classes of cAAV, each of which contained a defined segment of the integrated vector locus. A common feature of all cAAV classes was the presence of a modified inverted terminal repeat that joined the ends of the liberated sequence. Assembly of extrachromosomal vector genomes was accompanied by deletions in the integration locus that could be mapped to one of the five cAAV classes, suggesting an excision-type mechanism. We propose that the spontaneous deletion and mobilization of vector sequence from the recombinant adenoassociated virus (rAAV) integration locus is mediated by a recombination event between the inverted terminal repeats that define the boundaries of the individual genome subunits.

Parvovirus-mediated antineoplastic activity exploits genome instability

The generation and accumulation of genetic mutations have been associated with carcinogenesis. Epidemiological and experimental evidence implicate parvoviruses in growth inhibition (oncosuppression) and selective destruction (oncolysis) of tumor cells. It is proposed here that parvoviruses can preferentially target genetically unstable tumor cells, which are deficient in DNA repair mechanisms. This selective strategy may serve as a virus-based therapeutic approach against cancer.

Carcinogen-induced accumulation of adeno-associated parvovirus DNA is transient as a result of two antagonistic activities that both require de novo protein synthesis

Helper-independent synthesis of adeno-associated parvovirus type 5 (AAV-5) DNA in SV40-transformed Syrian hamster cells is induced by chemical carcinogens, UV light and other stress treatments and has previously been shown to be transient. To gain some insight into the underlying mechanism, the influence of inhibitors of protein synthesis on AAV DNA accumulation was investigated. It is shown that transience is the result of 2 antagonistic activities--synthesis and decomposition. Both depend on de novo protein synthesis and are in part overlapping. The 2 activities are influenced by the amount of AAV uptake and/or by cell density. In less dense cultures, calculated initial amount of AAV DNA and relative accumulation per cell are higher. The effect may be due to the activity of transiently synthesized AAV-encoded regulatory proteins that increase replication and have inhibitory effects on DNA decomposition, or to the influence on cellular adhesion.

Influence of adeno-associated virus on adherence and growth properties of normal cells

It has been shown previously that infection of newly established cell cultures from malignant human tumors with adeno-associated parvovirus type 2 or type 5 results in growth arrest and cell death. Here we report that the additionally observed antiproliferative effect on diploid human fibroblasts is transient and is connected to a reduced number of cells in S phase. Progression through the cell cycle is disturbed either in G0/G1 or at the G1/S boundary, but an additional arrest in G2 cannot be excluded. DNA synthesis and cell proliferation are resumed when cells are recultured after loosening of cell-matrix adhesions by trypsin treatment. In contrast, they are not resumed by solely providing growth factors via higher amounts of fetal calf serum. The results suggest that cell adherence is altered in adeno-associated parvovirus-infected human embryo fibroblasts.

Infection with adeno-associated parvovirus leads to increased sensitivity of mammalian cells to stress

Infection of the SV40-transformed Syrian hamster cells Elona with adeno-associated parvovirus type 5 (AAV-5) affects the cellular stress response. Stress-induced growth delay becomes permanent and this promotes cell death. AAV DNA is replicated in the absence of helper virus. Induction of an incomplete stress response by cycloheximide treatment still results in replication of viral DNA but not in virus-mediated growth arrest. Thus, the reactions that induce AAV DNA replication and that direct growth delay are different and replication of AAV DNA is not sufficient to induce growth arrest and cell death. Additional steps that lead to effects resembling inhibition of protein synthesis are required. The possibility that they may have their origin in the action of AAV rep proteins is discussed.

Parvovirus replication

The members of the family Parvoviridae are among the smallest of the DNA viruses, with a linear single-stranded genome of about 5 kilobases. Currently the family is divided into three genera, two of which contain viruses of vertebrates and a third containing insect viruses. This review concentrates on the vertebrate viruses, with emphasis on recent advances in our insights into the molecular biology of viral replication. Traditionally the vertebrate viruses have been distinguished by the presence or absence of a requirement for a coinfection with a helper virus before productive infection can occur, hence the notion that the dependoviruses (adeno-associated viruses [AAV]) are defective. Recent data would suggest that not only is there a great deal of structural and genetic organizational similarity between the two types of vertebrate viruses, but also there is significant similarity in the molecular biology of productive replication. What differs is the physiological condition of the host cell that renders it permissive. Healthy dividing cells are permissive for productive replication by autonomous parvoviruses; such cells result in latent infection by dependoviruses. For a cell to become permissive for productive AAV replication, it must have been exposed to toxic conditions which activate a latent AAV genome. Such conditions can be caused by helper-virus infection or exposure to physical (UV light) or chemical (some carcinogens) agents. In this paper the molecular biology of replication is reviewed, with special emphasis on the role of the host and the consequences of viral infection for the host.

Adeno-associated parvoviruses inhibit growth of cells derived from malignant human tumors

Infection of freshly established malignant human tumor cells with the adeno-associated viruses AAV-2 or AAV-5 inhibits cell proliferation. Normal human fibroblasts are also affected, but appear to partially escape the inhibition. In cell cultures of permanent lines of human and rodent origin, cell number reduction could not be observed. The AAV-mediated influence on cell growth is not due to helper-independent replication of the virus, since neither of the cell cultures supported AAV propagation in the absence of a viral helper.