Reports describing transplantation of in vitro transduced fetal hepatoblasts21 or injection of oncogene-expressing transposon plasmids22 into mouse liver demonstrated feasibility of restricting oncogene expression to clones of hepatocytes, from which neoplasms arose, but were not quantitative. The comparative hepatocyte growth assay (CHeGA) represents a complement to other experimental in vivo models of liver carcinogenesis. Unlike previous systems, which assess oncogene carcinogenicity, CHeGA allows us to separate and quantify the effects of gene alterations on cellular growth in growth stimulatory and quiescent environments. Furthermore, we can determine whether there is posttransplantation

development GSI-IX of hepatocyte focus growth outliers. The presence of outliers implies that some transplanted cells possessed stable changes (genetic or epigenetic) in addition to oncogene expression at the time of transplantation, or developed these Bafilomycin A1 mouse changes shortly after transplantation. Because outlier growth continues in quiescent liver, these underlying changes must create the potential in affected cells for cell-autonomous

(environment-independent) growth, and in this way progenitors of outliers meet one criterion for preneoplastic cells. Using our growth assay, we can quantify, for any potential oncogene or oncogene combination, the associated risk of developing extreme outliers (EOs) with preneoplastic behavior. In fact, EO frequency is the best predictor of oncogene carcinogenicity in transgenic mice (see below), as expected if outliers are RANTES preneoplastic. This finding and our observation that EO microscopic anatomy is abnormal are consistent with suggestions by Laconi and colleagues20 that altered growth pattern is a principal marker of altered/nodular hepatocytes, although, in our system, these foci were identified by their ability to continue growth in a quiescent liver. Our findings, together with published data regarding oncogene effects in transgenic

mice, provide insight into the role of each oncogene in hepatocarcinogenesis. The principal effects of TGFα in transgenic mice are to stably increase hepatocyte number (liver mass increases up to twofold in transgenic mice),5, 8 and to increase the rate of hepatocyte replication after two-thirds partial hepatectomy and in 4-week-old but not 7-week-old mouse liver.7, 8 Consistent with these findings, TGFα quantifiably increases the rate at which hepatocytes can replicate under growth permissive conditions in CHeGA, but it does not uncouple replication from environmental controls in quiescent liver nor does it increase posttransplantation EOs. This liver phenotype is associated with a low risk for neoplastic progression on a per hepatocyte basis, because MT-TGFα transgenic mice develop a low tumor multiplicity with 10-month to 12-month latency.