Viral carcinogenesis: human cell transformation

W.C. Hahn MD PhD

Department of Adult Oncology, Dana-Farber Cancer Institute, Dana 710C, 44 Binney Street, Boston, MA 02115 USA. tel:617 632-2641, fax:617 632-2375,

The cancer cell results from the accumulation of genetic mutations that permit uncontrolled proliferation and independence from normal homeostatic regulation. The goal of our work is to create models of human cancer that will permit us to understand the molecular interactions that lead to the phenotypes associated with malignant transformation. The study of both transforming retroviruses and DNA tumor viruses has led to the identification and elucidation of many critical pathways involved in programming cancer cell growth. We have exploited several of these viral oncoproteins to create human cell models of epithelial cancers. The phenotype of immortality is essential for cell transformation, and several lines of evidence now implicate telomeres as a major molecular mechanism that regulates replicative lifespan. Ectopic expression of the telomerase catalytic subunit, hTERT, extends the lifespan of both pre-senescent and post-senescent, pre-crisis cells. These observations establish that repression of telomerase plays an important role in regulating both senescence and crisis. Corroborating these studies, disruption of telomere maintenance by expression of a dominantly-acting, catalytically inert mutant of hTERT in human cancer cell lines led to inhibition of telomerase activity, telomeric shortening, and widespread apoptosis. These observations support the notion that the up-regulation of hTERT observed in human tumors directly allows cancer cells to divide beyond the replicative capacity of normal cells. To investigate the role of hTERT in the transformation of human cells, we co-expressed combinations of hTERT, the simian virus 40 early region (ER) and an oncogenic allele of H-ras in normal human embryonic kidney epithelial, mammary epithelial, small airway epithelial cells and human foreskin fibroblasts. Only cells co-expressing all three genetic elements were immortal, maintained stable telomere length, exhibited anchorage-independent growth, and formed tumors in animal hosts. These results demonstrate that telomerase cooperates in the direct tumorigenic conversion of human cells. Using SV40 large T antigen mutants, we have begun to dissect the intracellular pathways that cooperate with telomerase and oncogenic ras to convert normal cells to tumorigenicity. The SV40 ER contributes to tumorigenic transformation by perturbing three intracellular pathways through the actions of the SV40 large T antigen (LT) and the SV40 small t antigen (ST). LT simultaneously disables the retinoblastoma (pRB) and p53 tumor suppressor pathways; however, complete transformation requires the additional perturbation of protein phosphatase 2A by ST. Taken together, these studies identify a limited set of genetic alterations that cooperate to program the conversion of normal human cells to the tumorigenic state.

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Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Molecular Basis of Oncogenesis.