Mutations in the tumor suppressor gene p53 in human liver cancer induced by alpha-particles.

ABSTRACT

The p53 tumor suppressor gene is mutated in varying fractions of almost all tumor types studied. The rate of mutations and the mutational spectrum in some tumors are specific for environmental mutagens assumed to be involved in the carcinogenic process. Thus, hepatocellular carcinomas supposedly induced by aflatoxin exposure often contain a specific point mutation in codon 249, and in lung cancers of miners with heavy radon exposure, another specific point mutation in codon 249 suggestive of an alpha-particle-specific mutation has been shown. The interpretation of studies linking the mutational spectrum with specific environmental exposures is complicated by the multifactorial or unknown genesis of most tumors. However, people given injections of the X-ray contrast medium Thorotrast (Th) in the past have experienced an enormous risk of liver tumors, and virtually all of these are supposedly induced by alpha-particles from the decay of 232Th. The examination of these tumors may provide evidence as to whether specific p53 point mutations are relevant in alpha-particle carcinogenesis. Therefore, we collected paraffin-embedded, formalin-fixed archival tissues from 18 hepatocellular carcinomas, 9 cholangiocarcinomas, and 9 hepatic angiosarcomas from Thorotrast-exposed patients. The tissues were analyzed for p53 protein expression by immunohistochemical staining by using the mAb DO-7 and for mutations of exons 5-8 by PCR and constant denaturant gel electrophoresis. G --> T transversions of the third base of codon 249 of the p53 gene were specifically screened for by restriction enzymes. No high score for p53 protein expression (i.e., positive staining of >20% of examined cells) was observed; lower scores were seen in 5 of 18 (28%) hepatocellular carcinomas, 1 of 9 (11%) cholangiocarcinomas, and 0 of 8 (0%) hepatic angiosarcomas. Only one p53 mutation, a heterozygous T --> G transversion of the first base codon 176, occurred in a hepatocellular carcinoma. The rate of p53 point mutations in alpha-particle-induced liver tumors seems to be lower than in European hepatocellular carcinomas in general. The study does not exclude the possibility that alpha-particle carcinogenesis may involve inactivation of p53 by gross deletions of the gene, but it speaks against the proposed specificity of point mutations of codon 249 in cancer supposedly induced by alpha-particles from radon progeny.