The oncogenic DNA viruses: a review of in vitro transformation studies

Functional dissection of breast cancer risk-associated TERT promoter variants

The multi-cancer susceptibility locus at 5p15.33 includes TERT, encoding the telomerase catalytic subunit. Genome-wide association studies (GWAS) have identified six single nucleotide polymorphisms (SNPs) in the TERT promoter associated with decreased breast cancer risk, although the precise causal variants and their mechanisms of action have remained elusive. Luciferase reporter assays indicated that the protective haplotype reduced TERT promoter activity in human mammary epithelial and cancer cells in an estrogen-independent manner. Using single variant constructs, we identified rs3215401 and rs2853669 as likely functional variants. Silencing of MYC decreased TERT promoter activity but neither MYC nor ETS2 silencing conferred allele-specificity. In chromatin immunoprecipitation experiments, the ETS protein GABPA, but not ETS2 or ELF1, bound rs2853669 in an allele-specific manner in mammary epithelial cells. Investigation of open chromatin in mammoplasty samples suggested involvement of three additional variants, though not rs3215401 or rs2853669. Chromosome conformation capture revealed no interaction of the TERT promoter with regulatory elements in the locus, indicating limited local impact of candidate variants on the TERT promoter. Collectively, our functional studies of the TERT-CLPTM1L breast cancer susceptibility locus describe rs2853669 as a functional variant of this association signal among three other potentially causal variants and demonstrate the versatile mechanisms by which TERT promoter variants may affect breast cancer risk.

Papillomaviruses: a systematic review

In the last decades, a group of viruses has received great attention due to its relationship with cancer development and its wide distribution throughout the vertebrates: the papillomaviruses. In this article, we aim to review some of the most relevant reports concerning the use of bovines as an experimental model for studies related to papillomaviruses. Moreover, the obtained data contributes to the development of strategies against the clinical consequences of bovine papillomaviruses (BPV) that have led to drastic hazards to the herds. To overcome the problem, the vaccines that we have been developing involve recombinant DNA technology, aiming at prophylactic and therapeutic procedures. It is important to point out that these strategies can be used as models for innovative procedures against HPV, as this virus is the main causal agent of cervical cancer, the second most fatal cancer in women.

Long story short: p53 mediates innate immunity

The story of p53 and how we came to understand it is punctuated by fundamental insights into the essence of cancer. In the decades since its discovery, p53 has been shown to be centrally involved in most, if not all, of the cellular processes that maintain tissue homeostasis. Extensive functional analyses of p53 and its tumor-associated mutants have illuminated many of the common defects shared by most cancer cells. As the central character in a tale that continues to unfold, p53 has become increasingly familiar and yet remains surprisingly inscrutable. New relationships periodically come to light, and surprising, novel activities continue to emerge, thereby revealing new dimensions and aspects of its function. What lies at the very core of this complex protagonist? What is its prime motivation? As every avid reader knows, the elements of character are profoundly shaped by adversity--originating from within and without. And so it is with p53. This review will briefly recap the coordinated responses of p53 to viral infection, and outline a hypothetical model that would explain how an abundance of seemingly unrelated phenotypic attributes may in the end reflect a singular function. All stories eventually draw to a conclusion. This epic tale may eventually leave us with the realization that p53, most simply described, is a protein that evolved to mediate immune surveillance.

Ultrastructural and biological characterization of human choroid cell cultures transformed by Simian Virus 40

A human diploid cell line of choroid origin was isolated from the retrouveal portion of an enucleated eye and designated HC. After 10 passages, when the proliferative capacity of HC cells decreased, they were infected and transformed by Simian Virus 40 (SV40). A proliferating long-term cultured cell line designated HC/SV40 was established and it has been maintained as monolayer for more than 100 passages so far. The two cell lines, HC and HC/SV40, were compared for growth characteristics, capacity to form colonies in soft agar, presence of nuclear T-antigen, and ultrastructure. Cytogenetic analysis was also performed to determine the presence of chromosomal aberrations due to the permanent viral transformation of the cell line. The results indicate that HC/SV40 should be considered the transformed counterparts of HC cells because they are morphologically similar to the latter but can grow in soft agar, possess T-antigen, and show a pattern of karyotypic changes similar to that induced by SV40 in human fibroblasts. The choroid origin of HC and HC/SV40 cell lines was confirmed by the presence, in their cytoplasm, of typical electron dense granules. Their neural origin will make these cell lines very useful for neuropharmacological and differentiation studies.

Action spectrum for the in vitro induction of simian virus 40 by ultraviolet radiation

A line of simian virus 40-transformed hamster kidney cells was exposed to ultraviolet radiation at eleven different wavelengths in the region 238-302 nm. An action spectrum derived from the resulting exposure-response curves for the induction of simian virus 40 from these cells exhibits a broad peak in the region 260-270 nm suggesting DNA as the major chromophore for this response. This conclusion is consistent with results obtained by other investigators who have noted viral induction by a number of DNA-damaging agents.

Characterization of herpes simplex virus persistence in a human T lymphoblastoid cell line

Persistent, dynamic-state infection with herpes simplex virus (HSV) type 1 has been maintained in human T lymphoblastoid (CEM) cells for many months after initial infection with the wild-type virus (HSV0) (input virus/cell multiplicity of 1.0). Persistently infected cells grew as well as uninfected cells, except during occasional periods of crisis (increased viral replication and cytopathic effect). Cells could survive the crisis when they were maintained for twice the usual time interval (8 to 10 rather than 4 to 5 days) before subculture. Interferon was not detectable in the cultures. HSV0 was compared with HSVp1, a small plaque-forming isolate from persistently infected CEM cells. Primary infection of CEM cells with HSV0 at a low input multiplicity (0.01) led to abortive replication, whereas infection with HSVp1 at the same multiplicity resulted in either rapidly lytic or persistent infection depending upon the time interval of subculture. Approximately 55% of plaque-purified clones of HSVp1, as compared with only 5% of HSV0 clones, displayed temperature-sensitive growth in Vero cells. Defective interfering virus was not detectable in uncloned HSVp1 by interference assay. Persistently infected cultures "cured" by treatment with HSV antiserum or incubation at 39 degrees C were resistant to reinfection with HSV but permissive for vesicular stomatitis virus replication, suggesting that these treatments modulated a shift from the dynamic-state of the static-state, latent infection. These studies provide a model for characterization of HSV persistence and latency in a highly differentiated human cell line.

Isolation of a SV40-like Papovavirus from a human glioblastoma

A human glioblastoma multiforme (M27) tested in early cell cultures by indirect immunofluorescence staining showed SV40-related tumor (T)-antigen, 95% of the cells being positive. SV40-related viral capsid (V)-antigen was absent in all cells tested. Experiments to rescue this virus were performed by fusing M27 cells with CV-I monkey cells, which were permissive for SV40, using polyethylene glycol (PEG) as fusion factor. We succeeded in isolating virus particles SV40-GBM which electron microscopy showed to correspond in size and morphology to papovaviruses. Serological tests (hemagglutination, neutralization, fluorescent antibody) revealed that the virus is indistinguishable from SV40. Despite this apparent antigenic identity SV40-GBM differs slightly from SV40 wild type. This virus can propagate and produce CPE in both CV-I cells and primary fetal human kidney cells. Furthermore digestion of SV40-GBM DNA with the HindII/III restriction endonucleases revealed minor differences compared with the SV40 DNA. Therefore the virus SV40-GBM obtained from glioblastoma cells seems to be closely related to the SV40-PML viruses described earlier.

Biological properties of two strains of Simian virus 40 isolated from patients with progressive multifocal leukoencephalopathy

Biological properties of two strains of simian virus 40 (SV40) from brains of two patients with progressive multifocal leukoencephalopathy (PML) have been compared to those of a standard laboratory strain of SV40. Infectivity of both SV40-PML viruses was resistant to treatment with chloroform, low pH, and 50 C for 120 min. African green monkey kidney and BSC-1 cells were the most sensitive for viral replication, and cytopathology in these cultures was indistinguishable from that caused by SV40. Both viruses formed plaques in these cells. but, in African green monkey kidney cells, strain 1 virus produced plaques measuring 2 mm in diameter whereas strain 2 virus produced pleomorphic plaques varying from 1 to 10 mm in diameter. Hamster cells were not permissive for viral replication, and infection resulted only in viral transformation. Inoculation of human fetal glial cells resulted in a permissive lytic infection of one cell type and a persistent infection with only partial expression of the viral genome in the other. No morphological evidence of transformation was evident in the latter cells. Both strains of SV40-PML viruses were neutralized by commercial anti-SV40 serum, but in reciprocal kinetic neutralization tests differences in K values were noted when each was compared to SV40. Both viruses showed oncogenicity for hamsters, producing undifferentiated sarcomas when injected subcutaneously and choroid plexus papillomas after intracerebral inoculation. All hamster tumor cells contained intranuclear immunofluorescent tumor antigen. This was indistinguishable from SV40 T antigen in reciprocal staining reactions using hamster anti-T antibody induced by the two SV40-PML agents and SV40. These two human agents appear therefore to be new variants of simian virus 40.

Simian virus 40-host cell interactions. I. Temperature-sensitive regulation of SV40 T antigen in 3T3 mouse cells transformed by the ts*101 temperature-sensitive early mutant of SV40

BALB/3T3 and Swiss/3T3 mouse cells transformed at permissive temperature (33 C) by the early temperature-sensitive mutant of simian virus 40 (SV40), ts(*)101, exhibited a temperature-dependent modulation of SV40 tumor (T) antigen as assayed by immunofluorescence. The percentage of T antigen-positive nuclei in ts(*)101 transformed cells was reduced at restrictive temperature (39 C) when compared to 33 C and to wild-type SV40 transformed cells at either 33 C or 39 C. The percentage of T antigen-positive nuclei in ts(*)101 transformed cells returned to the 33 C control level when the cells were shifted from 39 to 33 C. The ts(*)101 transformed cells could be superinfected with wild-type, but not ts(*)101, virions at 39 C as assayed by an increase in T antigen-positive nuclei.

Cell surface antigen induced by influenza virus

By immunofluorescence staining, a specific surface antigen induced by influenza virus was detected on clone 1-5C-4 cells. The antigen was neither related to hemagglutinin nor dependent on the production of infectious virus. Formation of the virus-specific cell surface antigen was dependent on protein synthesis but independent of viral ribonucleic acid replication.

Demonstration of infectious deoxyribonucleic acid in transformed cells. I. Recovery of simian virus 40 from yielder and nonyielder transformed cells

Deoxyribonucleic acid (DNA) was extracted from virus-free simian virus 40 (SV40)-transformed hamster, mouse, and monkey cells and was inoculated into simian cells in the presence of diethylaminoethyl (DEAE)-dextran; infectious SV40 was recovered by using DNA from cell lines which fail to yield virus by the fusion technique as well as from cell lines which readily yield virus by fusion. The rescued virus was identified as SV40 by three methods: (i) neutralization of plaque formation by specific antiserum; (ii) induction of synthesis of viral-specific antigens detected by immunofluorescence; and (iii) presence of papovavirus particles seen by the electron microscope. Treatment of the transformed cell DNA with deoxyribonuclease or omission of the DEAE-dextran prevented the rescue of virus. Large amounts of transformed cell DNA were required (>10 mug/culture of 10(6) cells) to effect rescue of SV40 by passage through monkey cells. A linear response was obtained between the input of DNA with inocula between 10 and 45 mug of DNA/culture and the yield of SV40 recovered. Biological activity was demonstrable irregularly when the transformed cell DNA was assayed directly in the presence of DEAE-dextran. The DNA induced plaque formation in about 50% of the trials as well as the synthesis of SV40 tumor and viral antigens in rare simian cells. The infectious DNA appeared to be associated with cellular DNA. The infectivity was found in the pellet of precipitated DNA obtained by the Hirt technique and was inactivated by boiling for 15 min. These properties are characteristic of linear cellular DNA and not of free, circular SV40 DNA.

Activation of the viral genome in Simian Virus 40-transformed nonpermissive cells by permissive cell extracts

Simian virus 40-transformed nonpermissive cells in which neither infectious viral DNA nor virions had previously been detected reproducibly yield low levels of infectious simian virus-40 DNA after treatment with extracts of normal permissive cells. Virions were obtained in one out of 37 experiments. Activation of the viral genome is detected only with normal permissive cell extracts and only in simian virus 40-transformed cell lines that are rescuable by heterokaryon formation or by treatment with chemicals. The activating factor(s) is insensitive to deoxyribonuclease and ribonuclease, but is sensitive to heat at 56 degrees and to proteolytic enzymes.

Tumor induction by Agrobacterium tumefaciens: specific transfer of bacterial deoxyribonucleic acid to plant tissue

When Agrobacterium tumefaciens cells grown in the presence of tritiated thymidine to label specifically the bacterial deoxyribonucleic acid (DNA) are incubated with carrot root tissue for short periods of time, an appreciable fraction of the label becomes firmly associated with the root tissue. Such association is not observed in identical experiments when A. tumefaciens cell ribonucleic acid or protein are labeled. The extent of the retention of thymidine-derived label from bacterial cells by the root tissue in experiments with A. radiobacter and poorly tumorigenic strains of A. tumefaciens is significantly less than that afforded by tumorigenic strains of A. tumefaciens but greater than the level afforded by Escherichia coli. Transfer of DNA-specific label from A. tumefaciens to carrot root discs is not enhanced by treatments designed to provoke lysis of the bacterial cells, nor is it decreased by addition of deoxyribonuclease or excess unlabeled thymidine to the incubation medium. Bacterial cell-to-plant cell contact is necessary for transfer. Unlabeled A. radiobacter cells decrease in a competitive manner transfer of label when mixed with labeled A. tumefaciens cells. These findings suggest that transfer of DNA from A. tumefaciens to plant tissue after binding of the bacterial cells to specific plant tissue site(s) is a necessary feature of the mechanism by which A. tumefaciens provokes tumors in plants and provides an experimental technique of potentially great value in study of the early steps in the process of tumor induction by A. tumefaciens.