Genetics of SV40 T-antigen expression: studies of twins, heritable syndromes and cancer families

Depressed interferon synthesis in skin fibroblasts from lung cancer patients

Skin fibroblast cell cultures, derived from male adult lung cancer patients, an adult control population, and a newborn population were examined for their susceptibility to transformation with Kirsten murine sarcoma virus and their ability to respond to an interferon inducer (poly I.poly C). An association between sensitivity to viral transformation and induction of interferon was observed. Cultures derived from lung cancer patients demonstrated an increased sensitivity to virus transformation and a decreased ability to respond to interferon induction as compared with age-matched controls and newborns.

Simian virus 40 DNA replication correlates with expression of a particular subclass of T antigen in a human glial cell line

Immunocytochemistry and in situ hybridization were used to identify simian virus 40 (SV40) large T-antigen expression and viral DNA replication in individual cells of infected semipermissive human cell lines. SV40 infection aborts before T-antigen expression in many cells of each of the human cell lines examined. In all but one of the human cell lines, most of the T-antigen-producing cells replicated viral DNA. However, in the A172 line of human glial cells only a small percentage of the T-antigen-expressing cells replicated viral DNA. Since different structural and functional classes of T antigen can be recognized with anti-T monoclonal antibodies, we examined infected A172 cells with a panel of 10 anti-T monoclonal antibodies to determine whether viral DNA replication might correlate with the expression of a particular epitope of T antigen. One anti-T monoclonal antibody, PAb 100, did specifically recognize that subset of A172 cells which replicated SV40 DNA. The percentage of PAb 100-reactive A172 cells was dramatically increased by the DNA synthesis inhibitors hydroxyurea and aphidicolin. Removal of the hydroxyurea was followed by an increase in the percentage of cells replicating viral DNA corresponding to the increased percentage reactive with PAb 100. The pattern of SV40 infection in A172 cells was not altered by infection with viable viral mutants containing lesions in the small t protein, the agnoprotein, or the enhancer region. Finally, in situ hybridization was used to show that the percentage of human cells expressing T antigen was similar to the percentage transcribing early SV40 mRNA. Thus, the block to T-antigen expression in human cells is at a stage prior to transcription of early SV40 mRNA.

Immortalization of human fibroblasts transformed by origin-defective simian virus 40

Simian virus 40 (SV40)-mediated transformation of human diploid fibroblasts has provided an effective experimental system for studies of both "senescence" in cell culture and carcinogenesis. Previous interpretations may have been complicated, however, by the semipermissive virus-cell interaction. In earlier studies, we previously demonstrated that the human diploid fibroblast line HS74 can be efficiently transformed by DNA from replication-defective mutants of SV40 containing a deletion in the viral origin for DNA synthesis (SVori-). In the current study, we found that such SVori- transformants show a significantly increased life span in culture, as compared with either HS74 or an independent transformant containing an intact viral genome, but they nonetheless undergo senescence. We have clonally isolated six immortalized derivatives of one such transformant (SV/HF-5). Growth studies indicate that the immortalized cell lines do not invariably grow better than SV/HF-5 or HS74. Genetic studies involving karyotypic analysis and Southern analysis of integrated viral sequences demonstrated both random and nonrandom alterations. All immortalized derivatives conserved one of the two copies of SV40 sequences which expressed a truncated T antigen. These cloned SV40-transformed cell lines, pre- and postimmortalization, should be useful in defining molecular changes associated with immortalization.

Immortalization of human fibroblasts transformed by origin-defective simian virus 40

Simian virus 40 (SV40)-mediated transformation of human diploid fibroblasts has provided an effective experimental system for studies of both "senescence" in cell culture and carcinogenesis. Previous interpretations may have been complicated, however, by the semipermissive virus-cell interaction. In earlier studies, we previously demonstrated that the human diploid fibroblast line HS74 can be efficiently transformed by DNA from replication-defective mutants of SV40 containing a deletion in the viral origin for DNA synthesis (SVori-). In the current study, we found that such SVori- transformants show a significantly increased life span in culture, as compared with either HS74 or an independent transformant containing an intact viral genome, but they nonetheless undergo senescence. We have clonally isolated six immortalized derivatives of one such transformant (SV/HF-5). Growth studies indicate that the immortalized cell lines do not invariably grow better than SV/HF-5 or HS74. Genetic studies involving karyotypic analysis and Southern analysis of integrated viral sequences demonstrated both random and nonrandom alterations. All immortalized derivatives conserved one of the two copies of SV40 sequences which expressed a truncated T antigen. These cloned SV40-transformed cell lines, pre- and postimmortalization, should be useful in defining molecular changes associated with immortalization.

Depressed interferon synthesis in skin fibroblasts from lung cancer patients

Skin fibroblast cell cultures, derived from male adult lung cancer patients, an adult control population, and a newborn population were examined for their susceptibility to transformation with Kirsten murine sarcoma virus and their ability to respond to an interferon inducer (poly I X poly C). An association between sensitivity to viral transformation and induction of interferon was observed. Cultures derived from lung cancer patients demonstrated an increased sensitivity to virus transformation and a decreased ability to respond to interferon induction as compared with age-matched controls and newborns.

Increased expression of SV40 T antigen and cell division in skin fibroblast cell lines derived from a family at high risk of carcinoma (family G of Warthin)

T-antigen expression in 24 of 31 skin fibroblast cell lines from members of Family G was found to be significantly elevated compared with a healthy control population. However, the pattern of elevation did not appear to be associated with cancer risk. In addition, T-antigen values were independent of the age and sex, as well as the branch of family and generation of the cell donor. Cell lines from Family G tended to divide more frequently than cell lines from control donors. This tendency was negatively associated with elevated T-antigen expression in Family-G cell lines, while control cell lines showed no such correlation. These results demonstrate that caution must be taken in evaluating potential markers of cancer risk. Superficial analysis of the data would indicate the utility of the T-antigen assay as a marker for cancer family syndrome. However, consideration of additional factors, including pedigree relationships and cell biology in vitro, indicate that SV40 T-antigen expression is an ambiguous marker of this type of cancer risk, even when large groups are studied.

Replication of simian virus 40 DNA in normal human fibroblasts and in fibroblasts from xeroderma pigmentosum

Simian virus 40 infection of semipermissive human diploid fibroblasts (HF), at early passage in cell culture, was compared with that of permissive established monkey cell lines. Viral DNA can be readily detected at 24 to 48 h postinfection at 37 degrees C with a high multiplicity of infection, approaching 10% of that of monkey cells (TC7). The length of time necessary for replication of an average molecule of viral DNA was found to be indistinguishable in HF and TC7 cells. Strand elongation plus termination were assessed by following the accumulation of DNA I at 40 degrees C from replicative intermediates of tsA30 prelabeled at 33 degrees C, obviating isotope pool problems. Combined initiation and elongation of wild-type viral DNA was measured by density shift experiments involving a 5-bromodeoxyuridine chase of prelabeled [3H]thymidine-labeled viral DNA. Determination of accumulation of viral T and V antigens supports the conclusion that the most likely basis for the reduced virus yield in HF cells results from the inefficiency of an early stage in virus infection, before or during uncoating. Similar results were obtained in fibroblasts derived from patients with xeroderma pigmentosum, suggesting that enzymes of UV repair are not required in unirradiated simian virus 40 DNA synthesis.

Relationship of SV40 T-antigen expression in vitro to disorders of bone marrow function

Skin fibroblasts from patients with a variety of hematologic disorders were infected with SV40 virus in vitro in attempts to discover the reason for increased susceptibility of Fanconi anemia cells to this transforming virus. The proportion of skin fibroblasts expressing SV40 T-antigen by immunofluorescent methods was elevated in 12 patients with Fanconi anemia and in seven of nine obligate heterozygous relatives. Elevated expression was also observed in three patients with other hematological disorders at high risk of acute non-lymphocytic leukemia, but was not apparent in seven sporadic aplastic anemia patients or four of their relatives. T-antigen expression was elevated in about one-half of patients with thrombocytopenia-absent radius syndrome and related conditions, with familial aplastic anemia, and in their normal relatives. In the conditions under study, elevated T-antigen expression seemed clearly correlated with predisposition to leukemia, which may be genetically determined, but it was not associated with cytogenetic or anemic manifestations.

Human skin fibroblasts from individuals genetically predisposed to cancer are sensitive to an SV40-induced T antigen display and transformation

In the present study we have used a viral probe to determine the genetic susceptibility of fibroblastic cell strains derived from individuals with hereditary adenomatosis of the colon and rectum (ACR), an autosomal dominant trait. This report shows an increased sensitivity of apparently karyologically-normal diploid skin fibroblasts from ACR individuals to an SV40-induced T antigen display and transformation. None of the SV40-transformed cells grew as tumors in athymic mice and they all appeared to have a finite life span. The results suggest that the induction of T antigen positive cells by SV40 may be used as a marker of cancer risk in this cell system.

SV40 T-antigen expression in acute non-lymphocytic leukaemia

Susceptibility to SV40 virus infection has been suggested as an in vitro marker of predisposition to leukaemia and possibly other cancers in man. To evaluate this relationship, sporadic as well as familial cases of acute non-lymphocytic leukaemia were tested. Among skin fibroblast lines from 22 patients without family history of leukaemia, eight had values above the 95% confidence limit for the normal control population. In four leukaemia-prone families, elevated T-antigen expression was found in all four patients tested and in three-quarters of 36 blood relatives. In addition, elevated values were found in two of three cases of acute myelogenous leukaemia associated with constitutional cytogenetic anomalies, and in all three cases with preleukaemic haematologic disorders. Since SV40 T-antigen expression was elevated in most persons prone to acute non-lymphocytic leukaemia, as well as over one-third of sporadic cases, heritable risk factors may be involved in both groups.