Mutational hotspot in the p53 gene in human hepatocellular carcinomas

A conformation hypothesis for the suppressor and promoter functions of p53 in cell growth control and in cancer

Cancer is a genetic disease caused by defective control of cell proliferation. As cancer cells divide, the genetic defect is inherited by each daughter cell, leading to tumour development with possible progression to malignancy. The identification of those genes linked with cancer is essential for our understanding of the regulation of cell proliferation and for the therapeutic management of cancer cell growth. Recent studies have revealed that p53 is the most commonly affected gene in human cancer. It is a single copy gene and functions in the regulation of cell proliferation. Mutation of p53 is linked with tumour development, and this may involve abnormal functioning of mutant p53 protein. A mutant allele of p53 is functionally temperature-sensitive and can promote or suppress cell proliferation. The tertiary structure of the mutant protein is also sensitive to temperature and adopts promoter and suppressor forms of p53. A conformation model for the functioning of p53 proposes that wild-type p53 is induced to change from suppressor to promoter form during the cell growth response. This model predicts that any mutation that deregulates the normal control of p53 conformation may lead to cancer.

p53 mutations in human cancers

Mutations in the evolutionarily conserved codons of the p53 tumor suppressor gene are common in diverse types of human cancer. 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, whereas G:C to T:A transversions are the most frequent substitutions observed in cancers of the lung and liver. Mutations at A:T base pairs are seen more frequently in esophageal carcinomas than in other solid tumors. Most transitions in colorectal carcinomas, brain tumors, leukemias, and lymphomas are at CpG dinucleotide mutational hot spots. G to T transversions in lung, breast, and esophageal carcinomas are dispersed among numerous codons. In liver tumors in persons from geographic areas in which both aflatoxin B1 and hepatitis B virus are cancer risk factors, most mutations are at one nucleotide pair of codon 249. These differences may reflect the etiological contributions of both exogenous and endogenous factors to human carcinogenesis.

Selective G to T mutations of p53 gene in hepatocellular carcinoma from southern Africa

Hepatocellular carcinoma (HCC) is a prevalent cancer in sub-Saharan Africa and eastern Asia. Hepatitis B virus and aflatoxins are risk factors for HCC, but the molecular mechanism of human hepatocellular carcinogenesis is largely unknown. Abnormalities in the structure and expression of the tumour-suppressor gene p53 are frequent in HCC cell lines, and allelic losses from chromosome 17p have been found in HCCs from China and Japan. Here we report on allelic deletions from chromosome 17p and mutations of the p53 gene found in 50% of primary HCCs from southern Africa. Four of five mutations detected were G----T substitutions, with clustering at codon 249. This mutation specificity could reflect exposure to a specific carcinogen, one candidate being aflatoxin B1 (ref. 7), a food contaminant in Africa, which is both a mutagen that induces G to T substitution and a liver-specific carcinogen.

Contribution of the glutathione S-transferases to the mechanisms of resistance to aflatoxin B1

The harmful effects of Aflatoxin B1 (AFB1) are a consequence of it being metabolized to AFB1-8,9-epoxide, a compound that serves as an alkylating agent and mutagen. The toxicity of AFB1 towards different cells varies substantially; sensitivity can change significantly during development, can be modulated by treatment with xenobiotics and is decreased markedly in preneoplastic lesions as well as in tumors. Three types of resistance, namely intrinsic, inducible and acquired, can be identified. The potential resistance mechanisms include low capacity to form AFB1-8,9-epoxide, high detoxification activity, increase in AFB1 efflux from cells and high DNA repair capacity. Circumstantial evidence exists that amongst these mechanisms the glutathione S-transferases, through their ability to detoxify AFB1-8,9-epoxide, play a major role in determining the sensitivity of cells to AFB1.