Integration of oncogenic viruses in mammalian cells

Genetic alterations in cancer as a result of breakage at fragile sites

The organization and replication of DNA render fragile sites (FSs) prone to breakage, recombination as well as becoming preferential targets for mutagens-carcinogens and integration of oncogenic viruses. For many years, attempts to link FSs and cancer generated mostly circumstantial evidence. The discoveries that chromosome translocations, amplification of proto-oncogenes, deletion of tumor suppressor genes, and integration of oncogenic viruses all result from the specific breakage of genomic DNA at FSs, however, have provided compelling support for such a link, further suggesting a causative role for FSs in cancer.

Chromosome-mediated alterations of the MYC gene in human cancer

The step-wise accumulation of genetic and epigenetic alterations in cancer development includes chromosome rearrangements and viral integration-mediated genetic alterations that frequently involve proto-oncogenes. Proto-oncogenes deregulation lead to unlimited, self-sufficient cell growth and ultimately generates invasive and destructive tumors. C-MYC gene, the cellular homologue of the avian myelocitic leukemia virus, is implicated in a large number of human solid tumors, leukemias and lymphomas as well as in a variety of animal neoplasias. Deregulated MYC expression is a common denominator in cancer. Chromosomal rearrangements and integration of oncogenic viruses frequently target MYC locus, causing structural or functional alterations of the gene. In this article, we illustrate how genomic rearrangements and viruses integration affect MYC locus in certain human lymphomas and solid tumors.

The sensitivities of SV40-transformed human fibroblasts to monofunctional and DNA-crosslinking alkylating agents

4 repair-deficient (Mer-) and 2 repair-proficient (Mer+) lines of SV40-transformed human fibroblasts were assayed for colony-forming ability after treatment with MNNG, methyl methanesulfonate (MMS), 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), and 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea (HECNU). The sensitivities to MMS, BCNU and HECNU of these SV40-transformed lines were similar to those of comparably treated human tumor cells observed previously. However, unlike human tumor lines, whose post-MNNG survival is strongly dependent upon Mer phenotype, SV40-transformed lines showed a lack of dependence of post-MNNG colony-forming ability on Mer phenotype. No differences in glutathione levels that might explain these differences were detected. The amounts of SV40-specific DNA and RNA among the lines were found to vary widely, but no correlation with Mer phenotype was found.

Viral integration, fragile sites, and proto-oncogenes in human neoplasia

To evaluate the trend of viral integration in the human genome, chromosomal localization of five DNA-containing viruses compiled from literature data was compared to the location of fragile sites and proto-oncogenes. A total of 35 regionally mapped viral integration sites from tumors and transformed cells were distributed over 19 chromosomes. Of the 35 integration sites 23 (66%) were at the bands of fragile sites, and 7 were one band away (20%). This statistically defines the correlation as highly significant (P = 0.0000183, Fisher's F-test). Five integration sites did not correspond to the location of a fragile site. Thirteen integration sites and proto-oncogenes mapped at the same bands (37%), 6 (17%) were one band apart, and at 16 integration sites (46%) no proto-oncogenes were localized (P = 0.00491). Eighteen viral integration sites, fragile sites, and proto-oncogenes (51%) were localized at the same bands or one band distant. This clustering of viral integration sites, fragile sites, and proto-oncogenes is statistically highly significant (P = 0.0000118), and indicates nonrandom viral integration in the human genome.

Preferential sites for viral integration on mammalian genome

Chromosomal localization of human papillomavirus (HPV) 16 and 18 on human cervical carcinomas and epithelial cell lines obtained after HPV transfection has uncovered a nonrandom association of viral integration and specific genome sites. Fragile sites appear to be preferential targets for viral integration because of their structural and functional characteristics through which chromosomal anomalies, alterations in protooncogene activity, and gene amplification can occur. Individually or in association, such changes lead to the acquisition of an unlimited cell growth potential but not tumorigenicity. Genetic instability and uncontrolled cell division resulting from HPV integration increase the cell's susceptibility to other exogenous carcinogenic factors that may complete the process of neoplastic development.

Human papillomavirus type 18 DNA is integrated at a single chromosome site in cervical carcinoma cell line SW756

SW756, a cervical carcinoma cell line, has multiple copies of human papillomavirus type 18 DNA sequences. The integration site of human papillomavirus type 18 DNA was localized by in situ hybridization to chromosome 12 at band q13. This single integration site corresponds to a heritable fragile site, which may have facilitated the integration of the viral DNA.

Chromosomal sites of integration of simian virus 40 DNA sequences mapped by in situ hybridization in two transformed hybrid cell lines

Simian virus 40 DNA sequences, integrated on human chromosome 7 in two transformed human-mouse hybrid somatic cell lines, were mapped to a specific chromosomal locus by in situ hybridization. To detect the integrated viral DNA by in situ hybridization, we increased the sensitivity of the technique by using as a probe 125I-labeled simian virus 40 cRNA (2 X 10(9) to 3 X 10(9) dpm/micrograms) prepared by in vitro transcription of simian virus 40 DNA with high-specific-radioactivity [125I]CTP. Although the viral nucleic acid was shown by blot hybridization in the two cell lines to be integrated in different restriction fragments, it was shown by in situ hybridization in the two cell lines to map to the same position, q31, on the long arm of human chromosome 7. The viral DNA integration sites were localized with a precision of +/- 1 silver grain diameter, equivalent to about 6.2 X 10(7) nucleotide pairs in the human genome. The procedures we describe may be adapted for localization of any gene on diploid chromosomes that can be cloned in a recombinant DNA vector.