Gene changes can affect cancer cells in many ways, but changes that increase disease severity-by allowing cells to proliferate when they should be quiescent, by enhancing their rate of growth under growth permissive conditions, or by increasing the risk that they will accumulate additional carcinogenic alterations-must be identified so that strategies to counter their effects can be developed. We describe a novel in vivo assay system based on hepatocyte transplantation that permits us to accomplish this objective for genetically modified hepatocytes. We find that the oncogenes c-myc and transforming growth factor a, but not simian virus 40 T-antigen, increase the rate of hepatocyte growth under growth permissive conditions. However, no single oncogene can induce hepatocyte growth in quiescent liver. In contrast, at least one oncogene combination, transforming growth factor a/ T-antigen, was sufficient to direct cell autonomous growth even in this nonpermissive environment. Furthermore, we could quantify risk for progression to neoplasia associated with oncogene expression; increased transformation frequency was the principal carcinogenic effect of T-antigen. Conclusion: This system identifies biological mechanistic role(s) in carcinogenesis for candidate genetic changes implicated in development of human liver cancer. The quantitative and comparative evaluation of gene effects on liver cancer allows us to prioritize targets for therapeutic intervention. (HEPATOLOGY 2010;52:634-643)

Identification of biomarkers that indicate an increased risk of breast cancer or that can be used as surrogates for evaluating treatment efficacy is paramount to successful disease prevention and intervention. An ideal biomarker would be identifiable before lesion development. To test the hypothesis that changes in cell turnover precede mammary carcinogenesis, we evaluated epithelial cell proliferation and apoptosis in mammary glands from transgenic mice engineered to develop mammary cancer due to expression in mammary epithelia of transforming growth factor ␣ (TGF-␣) or c-myc. In transgenic glands, before lesion development, epithelial cell turnover was enhanced overall compared with nontransgenic glands, indicating that aberrant cell turnover in normal epithelia may contribute to tumorigenesis. In addition, in tumor-containing glands, proliferation in normal epithelia was higher than in tumor-free transgenic glands, suggesting these cell populations influence one another. Finally, although c-myc glands displayed a uniformly high epithelial cell turnover regardless of age, cell turnover was reduced with aging in nontransgenic and TGF-␣ mice, indicating that some growth and death regulatory mechanisms remain intact in TGF-␣ epithelia. These observations support the evaluation of cell turnover as a biomarker of cancer risk and indicator of prevention/treatment efficacy in preclinical models and warrant validation in human breast cancer.