1995 Deichmann Lecture—p53 tumor suppressor gene: At the crossroads of molecular carcinogenesis, molecular epidemiology and cancer risk assessment

Abstract

Carcinogenesis is a multistage process involving activation of protooncogenes, e.g., ras, and inactivation of tumor suppressor genes, e.g., p53 and p16 1NK4. p53 is a prototype tumor suppressor gene that is well suited for analysis of mutational spectrum in human cancers; it is the most common genetic lesion in human cancers, it is a reasonable size for a molecular target, and it may indicate selection of mutations with pathobiological significance. The p53 mutational spectrum differs among cancers of the colon, lung, esophagus, breast, liver, brain, reticuloendothelial tissues and hemopoietic tissues. Analysis of these mutations can provide clues to the etiology of these diverse tumors and to the function of specific regions of p53. Transitions predominate in colon, brain and lymphoid malignancies. Mutational hotspots at CpG dinucleotides in codons 175, 245, 248, 273 and 282 may reflect endogenous mutagenic mechanisms, e.g., deamination of 5-methylcytosine to thymidine. Oxy-radicals including nitric oxide may enhance the rate of deamination. G:C to T:A transversions are the most frequent substitutions observed in cancers of the lung, breast, esophagus and liver, and are more likely to be due to bulky carcinogenDNA adducts. G to T transversion is more common in lung cancers from smokers when compared to never smokers. The high frequency of p53 mutations in the nontranscribed DNA strand is a reflection of strand specific repair. p53 mutation and/or accumulation of p53 protein can be preinvasive events in bronchial or esophageal carcinogenesis. p53 mutations also generally indicate a poor prognosis. In geographic areas where hepatitis B virus (HBV) and aflatoxin B 1 are cancer risk factors, most mutations are at the third nucleotide pair of codon 249. In geographic areas where hepatitis B and C virsus — but not aflatoxin B 1 — are risk factors, the p53 mutations are distributed in numerous codons. HBV X protein complexes with the p53 protein and inhibits its sequence specific DNA binding, transactivating and apoptotic capacity. The mutation load of 249 ser mutant cells in nontumorous liver is positively correlated with dietary aflatoxin B 1 exposure. The induction of skin carcinoma by ultraviolet light is indicated by the occurrence of p53 mutations at dipyrimidine sites including CC to TT double base changes. In summary, these differences in mutational frequency and spectrum among human cancer types suggest the etiological contributions in both exogenous and endogenous factors to human carcinogenesis and have implications for human cancer risk assessment.