DNA damage stimulates human cell transformation by integrative but not episomal Epstein-Barr virus-derived plasmid

DNA Oncogenic Virus-Induced Oxidative Stress, Genomic Damage, and Aberrant Epigenetic Alterations

Approximately 20% of human cancers is attributable to DNA oncogenic viruses such as human papillomavirus (HPV), hepatitis B virus (HBV), and Epstein-Barr virus (EBV). Unrepaired DNA damage is the most common and overlapping feature of these DNA oncogenic viruses and a source of genomic instability and tumour development. Sustained DNA damage results from unceasing production of reactive oxygen species and activation of inflammasome cascades that trigger genomic changes and increased propensity of epigenetic alterations. Accumulation of epigenetic alterations may interfere with genome-wide cellular signalling machineries and promote malignant transformation leading to cancer development. Untangling and understanding the underlying mechanisms that promote these detrimental effects remain the major objectives for ongoing research and hope for effective virus-induced cancer therapy. Here, we review current literature with an emphasis on how DNA damage influences HPV, HVB, and EBV replication and epigenetic alterations that are associated with carcinogenesis.

Defective DNA repair in cells with human T-cell leukemia/bovine leukemia viruses: role of tax gene

BACKGROUND

Human T-cell leukemia virus (HTLV)/bovine leukemia virus (BLV) group retroviruses, which cause hematopoietic cancers, encode a unique protein, Tax, involved in the transformation of infected cells. Our purpose was to determine whether the mechanism by which Tax protein induces transformation in HTLV- or BLV-infected cells involves DNA damage.

METHODS

We used a micronucleus assay to measure chromosomal damage and alkali denaturation analysis to test host-cell DNA integrity in cells infected with HTLV, BLV, or simian T-lymphotropic virus or in cells transfected with the tax gene of HTLV or BLV. Controls included uninfected cells and cells infected with other oncogenic retroviruses or oncogenic DNA viruses. We used a plasmid reactivation assay to examine whether the damage might be due to the inhibition of DNA repair. To ascertain which of several repair pathways might be inhibited, chemical methods were used to selectively introduce lesions repaired by specific pathways into the reporter plasmid.

RESULTS

The presence of Tax was associated with DNA damage. HTLV- or BLV-infected or tax-transfected cells showed normal ability to repair damage induced by deoxyribonuclease I or psoralen but markedly decreased ability to repair damage induced by UV light, quercetin, or hydrogen peroxide.

CONCLUSIONS

These data suggest that the DNA repair pathway most inhibited by Tax is base-excision repair of oxidative damage. To our knowledge, this is the first report demonstrating inhibition of DNA repair by any retrovirus and suggests that this inhibition of DNA repair may contribute to the mechanism of cell transformation by the HTLV/BLV group of viruses.

A hybrid herpesvirus infectious vector based on Epstein-Barr virus and herpes simplex virus type 1 for gene transfer into human cells in vitro and in vivo

We have developed a miniviral vector, pH300, based on the human herpesviruses 1 and 4, herpes simplex virus type 1 (HSV-1), and Epstein-Barr virus (EBV), carrying EBV sequences for plasmid episomal maintenance and HSV-1 sequences for amplification and packaging in multimeric form into HSV-1 capsids in the presence of a helper virus and helper cell line. A reporter gene, the bacterial lacZ gene, which expressed beta-galactosidase, was inserted into the multiple cloning site of pH300 to make pH300-lac. The packaged pH300-lac DNA was very efficient in infecting human cells in tissue culture. The pH300-lac miniviral stock was used to infect in vitro various human cell types derived from breast cancer, lung cancer, and liver cancer. Up to 95% of cells were infected and expressed beta-galactosidase activity after exposure to viral stock at a multiplicity of infection of 3. There was essentially no apparent cytotoxicity after infection of cultured cells in vitro. To test in vivo gene delivery, human liver tumor cells preimplanted subcutaneously in nude mice and injected in situ with pH300-lac showed high efficiency of ectopic gene expression. The pH300 miniviral vector is a simple and effective gene transfer system which shows potential for gene therapy of cancer and inherited diseases.

Mammalian assay for site-specific DNA damage processing using the human H-ras proto-oncogene

The human genomic H-ras proto-oncogene was inserted into an Epstein-Barr virus (EBV) vector (p220.2) that replicates synchronously with the cell cycle. Unique restriction enzyme sites, 30 bp apart, were created on either side of codon 12 to enable the construction of gapped heteroduplex (GHD) DNA. Depending upon experimental protocol, the gap could be located either on the coding (non-transcribed) strand or the non-coding (transcribed) strand. GHD DNA was created using a 1.8 kb segment of H-ras DNA containing exon 1, into which a synthetic 30 nucleotide oligomer containing a strand- and site-specific mismatched nucleotide was annealed. The 1.8 kb segment of H-ras DNA containing a codon 12; middle G:T, A:C or T:C mismatch has been religated with high efficiency into the EBV vector and transfected into NIH 3T3 cells using a mild liposome-mediated protocol. Subsequent hygromycin resistant NIH 3T3 colonies have been PCR amplified and sequenced. In this study, codon 12; middle nucleotide mismatch correction rates to wild-type G:C during replication in NIH 3T3 cells were 96.4% of G:T mismatches, 87.5% of A:C mismatches and 67% of T:C mismatches.

Mutagen-induced recombination in mammalian cells in vitro

It is now clear from in vitro studies that mutagens induce recombination in the cell, both homologous and nonhomologous exchanges. The recombination events induced are extrachromosomal events, exchanges between extrachromosomal DNA and chromosomes, and inter- as well as intrachromosomal exchanges. However, not all types of DNA damage can induce recombination. The mechanisms involved in the induction process are not known but may involve activation of DNA repair systems. In addition, stimulation of mRNA transcription by mutagens, different recombination pathways and how the assay system is constructed may affect the frequency and characteristics of the observed recombination events.

Significance of the detection of Epstein-Barr virus DNA in lymph nodes in patients with Hodgkin's disease

Epstein-Barr virus (EBV) DNA is frequently identified in benign and malignant lymphoproliferative conditions. As shown by in situ hybridization studies viral DNA is localized within malignant cells as well as benign lymphocytes. Clonal and nonclonal EBV genomes are present in Hodgkin's disease (HD), lymphomas of the immunocompromised host and reactive lymph node hyperplasia. Lytic infection with formation of linear genomes is observed in the same conditions but appears to be infrequent in HD as shown by quantitation of mRNA coding for viral capsid antigen. Expression of the oncogene LMP (latent membrane protein) is seen in Sternberg-Reed (SR) cells and immunoblasts of AIDS-related lymphoma and infectious mononucleosis (IM). In HD, the region of the BNLF1 oncogene coding for the amino terminal and transmembrane domains (associated with oncogenic function) of LMP appears to be homogeneous whereas the region coding for the intracytoplasmic (carboxy terminal) domain of LMP is heterogeneous. Cytological similarities between SR cells and immunoblasts of IM and AIDS-related lymphomas are consistent with the hypothesis that the BNLF1 oncogene is one possible inducer of morphological features of SR cells. Whether chromosomal integration of EBV DNA is an important factor in activation of such a transforming activity remains to be elucidated. EBV DNA positive and negative HD cases with numerous SR cells lack significant mRNA expression of the two recombinase activating genes (RAG-1 and RAG-2). Therefore the SR cells appear to be derived from lymphocytes beyond the pre-B-cell or common thymocyte stage which may or may not subsequently become infected by EBV.