Target gene modulation in hepatocellular carcinomas by decreased DNA-binding of p53 mutations

The crystallographic structure of the p53 core domain showed that most of the p53 mutations found in human tumors are located in conserved regions of the p53 DNA-binding domain. The aim of our study was to investigate the effect on DNA-binding and transactivation of three p53 mutations frequently found in hepatocellular carcinomas (HCC). Two of these mutations are located near the DNA-binding surface and are induced by aflatoxin B1 (249ser) and oxiradicals (249met). In contrast, mutation 220cys is not associated with a specific carcinogen in HCCs and is located outside the DNA binding structures of p53. Cotransfection experiments in two HCC cell lines, with mutated or deleted P53 genes, showed that all three mutations did not enhance reporter gene activity (RGC-CAT), in contrast to wt p53. However, in hepatoma cell lines all three mutations did suppress the p53 wild-type (wt) transactivation in a dose-dependent fashion. DNA-binding was monitored by gel shift assays using the consensus-, Waf-, and RGC-p53 binding sites. All three p53 mutations did decrease DNA-binding versus all binding sites included. Interestingly although all mutations showed the same DNA-binding and transactivation properties, differences in the ectopic expression in different hepatoma cells were observed. Therefore our results indicate that p53 mutations in HCC found in the DNA-binding domain and outside the conserved DNA-binding structures modulate target gene expression by decreasing sequence specific DNA-binding in a dominant negative fashion. The cellular environment may contribute to an additional selection advantage of some mutations.