The p53 gene, which encodes a growth regulatory protein, is frequently inactivated in human cancer and is considered a tumor suppressor gene. In exploring potential molecular mechanisms of hepatocellular carcinogenesis, we discovered p53 gene inactivations in primary human hepatocellular carcinoma (PHC). The frequency of such mutations in these tumors remains to be determined, but in our preliminary studies, we found p53 abnormalities in 8 out of 9 PHC-derived cell lines. Our results strongly suggest that the inactivation of p53 gene is a significant genetic lesion in hepatocellular carcinoma (HCC). We are beginning studies to determine the relationship between p53 gene inactivation and other suspected co- factors in the process of hepatocellular carcinogenesis, particularly Hepatitis B Virus (HBV). We hypothesize that HBV could play a role in HCC by directly altering the structure or expression of p53 gene. Alternatively, inactivation of p53 function and chronic infection with HBV may have cumulative effects in the multi-step process of hepatocellular transformation. We will begin our studies by determining the frequency of p53 gene inactivation in PHCs. Tumors, as well as uninvolved liver samples from the same patients, will be analyzed at the protein level using immunochemical methods; studies of the p53 gene will include restriction enzyme analysis of genomic DNA and sequencing of conserved regions after polymerase chain reaction amplification. PHCs with demonstrated p53 gene abnormalities will be used for preparation of genomic libraries, from which p53 genes as well as HBV integration sites will be cloned and sequenced. To further investigate the relationship between p53, HBV, and hepatocellular carcinogenesis, we will conduct transfection studies on three PHC-derived cell lines: 'minimally deviant' Hep G2 cells, which appear to express 'wide-type' p53, Hep 3B cells, which have no endogenous p53, and Huh 7 cells, which express a mutant p53 protein. These cell lines will be transfected with plasmids expressing 'wild-type' or mutant p53 and/or the HBV genome. Transfected cells will be assessed for consequent phenotypic changes, such as altered growth capability and tumorigenicity. Through our demonstration that the p53 gene is inactivated in HCC, we have developed a new perspective in our studies of hepatocellular carcinogenesis.
We aim to determine whether HBV interacts directly with the tumor suppressor function of p53 or contributes independently to the transformation process. We believe that these studies will be instrumental in clarifying the pathogenesis of HCC.
