Geographic variation of p53 mutational profile in nonmalignant human liver

Allele-specific PCR analysis of p53 codon 249 AGT transversion in liver tissues from patients with viral hepatitis

AGG to AGT mutations in codon 249 of the p53 tumor-suppressor gene are frequently observed in hepatocellular carcinomas (HCC) from areas where exposure to aflatoxin B1 (AFB) occurs. We developed a sensitive allele-specific polymerase chain reaction (AS-PCR) assay to detect this point mutation in non-neoplastic human liver tissues. Three oligonucleotide primers, 1 specific for the mutant allele and 2 specific for the wild-type allele were used. The mutant allele primer differed from the wild-type allele due to a G-to-T transversion in its terminal 3' nucleotide. The first stage involved amplification of exon 7 of p53 followed by a selective amplification of mutant codon 249 sequences. This method allowed for the detection of a mutant codon 249 allele in the presence of as many as 105 copies of the wild-type allele and was 100-fold more sensitive than the restriction fragment length polymorphism-PCR technique. We have applied this AS-PCR protocol to examine codon 249 AGT transversion in tumor and matched non-tumor liver samples from North American patients with hepatitis and from Mozambiquan patients exposed to AFB. Mutations were detected in 5 of 6 samples of non-neoplastic liver from Mozambiquan patients, all of whom were HBsAg- or HBcAg-positive and AFB-exposed. In contrast, no mutations were detected in non-neoplastic liver from North American patients with either HBV- or HCV-derived hepatitis and cirrhosis. This procedure is a simple and powerful approach for screening p53 codon 249 AGT mutation in heterogeneous non-neoplastic hepatocyte populations.

Inhibition of hepatitis-B-virus core promoter by p53: implications for carcinogenesis in hepatocytes

The incidence of hepatocellular carcinoma (HCC) is particularly high in regions of Asia and sub-Saharan Africa where rates of infection with human hepatitis-B virus (HBV) and aflatoxin-B1 contamination of food are high. In HCC tumors occurring in inhabitants of these regions, a G-to-T mutation frequently occurs at position 249 of the tumor-suppressor gene p53. This suggests that HBV and p53 mutation may collaborate in the carcinogenic process in liver. We have examined the effect of the HBV protein HBX in HCC lines with exogenous wild-type p53 or mutated p53 on transactivation of 2 different reporter genes. Transfection of HCC lines with wild-type p53 and a reporter with the promoter from the p53-responsive gene WAF1/p21 resulted in a high level of expression, as expected. When cells were co-transfected with a reporter gene driven by the HBV core promoter and with the HBX gene, expression was enhanced in the Hep 3B, HLE, PLC/PRF/5 and HuH 7 lines, but not in the HuH 1 line. Co-transfection of the reporter with a plasmid containing wild-type p53 resulted in significant inhibition of the HBV core promoter in all of the lines, whereas the mutated p53 gene had no effect. Our results indicate that wild-type p53 can inhibit transcription from the HBV core promoter. In similar experiments, both HBX and p53 were co-transfected into HCC lines with the WAF1/p2l reporter gene. HBX inhibited p53-induced expression in 4 of the 6 lines (Hep 3B, HuH 1, HuH 7 and HLE), there was no effect in one line (HLF), and enhancement was evident in PLC/PRF/5. Our results indicate that inhibition of p53 transcriptional activity by HBX does occur in HCC, but is highly cell-context-dependent. Inhibition of transcription from the HBV core promoter by wild-type p53 appears to be more universal, and may represent a mechanism by which wild-type p53 can protect against the carcinogenic process in liver.

Molecular and genetic events in schistosomiasis-associated human bladder cancer: role of oncogenes and tumor suppressor genes

Carcinoma of the urinary bladder is the most common malignancy in many tropical and subtropical countries and is mainly due to endemic schistosomal infection. Schistosomiasis-associated bladder cancer defines a characteristic pathology and cellular and molecular biology that differs from urothelial carcinoma of non-schistosomal origin. N-Nitroso compounds are suspected etiologic agents in the process of bladder cancer induction during schistosomiasis. Elevated levels of DNA alkylation damage have been detected in schistosome-infected bladders and are accompanied by an inefficient capacity of DNA repair mechanisms. Consequently, high frequency of G --> A transition mutations were observed in the H-ras gene and at the CpG sequences of the p53 tumor suppressor gene. Genetic changes have also been detected in the c-erbB-1 and c-erbB-2 oncogenes and in the cdkn2 and Rb tumor suppressor genes. The potential application of these mutational patterns in providing a biological marker suitable for the biomonitoring and early detection of this neoplasm could indicate new avenues of approach that might alleviate the problem in the future. It can also assist in elucidating the mechanisms by which schistosomiasis augments human bladder cancers.

Somatic mutation detection in human biomonitoring

Somatic cell gene mutation arising in vivo may be considered to be a biomarker for genotoxicity. Assays detecting mutations of the haemoglobin and glycophorin A genes in red blood cells and of the hypoxanthine-guanine phosphoribosyltransferase and human leucocyte antigenes in T-lymphocytes are available in humans. This MiniReview describes these assays and their application to studies of individuals exposed to genotoxic agents. Moreover, with the implementation of techniques of molecular biology mutation spectra can now be defined in addition to the quantitation of in vivo mutant frequencies. We describe current screening methods for unknown mutations, including the denaturing gradient gel electrophoresis, single strand conformation polymorphism analysis, heteroduplex analysis, chemical modification techniques and enzymatic cleavage methods. The advantage of mutation detection as a biomarker is that it integrates exposure and sensitivity in one measurement. With the analysis of mutation spectra it may thus be possible to identify the causative genotoxic agent.

Dietary contribution to genotoxic risk and its control

Both exogenous and endogenous mutagens have the potential to contribute to events leading to carcinogenesis. While many dietary exogenous mutagens have been characterized, aflatoxins are the only exogenous mutagens in food which have been shown to directly increase the risk of liver cancer in humans. Little attention has been given so far to endogenous DNA damage, its potential to contribute to carcinogenesis, and the influence of genetic and environmental factors such as the diet on the process. This paper reviews the use of biomarkers which may assist in measuring these effects, their potential use in improving risk analysis for dietary mutagens, their application in molecular epidemiology, and their potential for studying dietary protective factors.

The 1995 Walter Hubert Lecture--molecular epidemiology of human cancer: insights from the mutational analysis of the p53 tumour-suppressor gene

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Aflatoxin exposure and cytogenetic alterations in individuals from the Gambia, West Africa

Aflatoxin-albumin adducts in peripheral blood provide a measure of exposure to aflatoxin over the previous 2-3 months. In the present study, the levels of these adducts were determined in a group of individuals from The Gambia, West Africa and were compared in a cross-sectional study to the levels of various cytogenetic alterations (chromosomal aberrations, micronuclei, sister chromatid exchanges) in the same individuals to test whether an increase in genetic damage is associated with an increased exposure in this population. Of 35 subjects tested for aflatoxin-albumin adducts, all but 3 were positive. There were no correlations between the adduct level and the number of cytogenetic abnormalities at the individual level. A comparison of the cytogenetic alterations was made between Gambian individuals and a group of 22 healthy people from Italy where aflatoxin exposure is expected to be low. The levels of structural chromosomal aberrations, sister chromatid exchanges and micronuclei were all higher in the former group. Overall, these data are indicative of a higher exposure to genotoxins in Gambian subjects, one of which are aflatoxins, but suggest that more specific genetic markers of aflatoxin exposure are required to further examine the link between aflatoxin exposure and genetic alterations.

Etiology of cancer in humans and animals

Cancer is a multistep disease with a multifactorial aetiology. Among the avoidable causes of human cancer are exposure to environmental carcinogens, ionizing and non-ionizing radiation, chronic inflammatory states (viruses and parasites) and life style factors, in particular diet. There is increasing evidence that these aetiologic factors may interact with each other resulting in a more than additive cancer risk. This has been demonstrated for example with hepatitis B virus and aflatoxins in hepatocellular carcinoma and alcohol and tobacco in cancer of the esophagus. The integration of molecular markers of exposure, biological effect and individual susceptibility into epidemiological studies can contribute to strengthening the causal link between exposure and disease and thus help assess the relative contribution of multiple risk factors to the aetiology of a specific cancer. For some genotoxic carcinogens the sequence of events leading to tumour formation is well understood from exposure, to metabolism and ultimately to specific mutations in transformation-associated genes. The mechanisms of action of carcinogens which do not interact directly with DNA but exert adverse effects through receptor-mediated modulation of intercellular signal pathways is far less well understood and an example of this is the interaction between diet and hormones. Furthermore, there is increasing awareness that individual response to environmental agents may depend to a significant extent on the genetic background of the individual or population. This has long been known from animal experiments but human cancer susceptibility is a complex genetic trait involving genes responsible for carcinogen metabolism, DNA repair and as yet unidentified cell specific susceptibility genes. Phenotypic changes observed during tumour progression reflect the sequential acquisition of genetic alterations. To assess the contribution of mutations in the various genes involved in the carcinogenic process may require their expression in transgenic animals or knock out mice. This has again placed animal experimentation into the forefront of mainstream cancer research.

Somatic point mutations in the p53 gene of human tumors and cell lines: updated compilation

In 1994 we described a list of approximately 2500 point mutations in the p53 gene of human tumors and cell lines which we had compiled from the published literature and made available electronically through the file server at the EMBL Data Library. This database, updated twice a year, now contains records on 4496 published mutations (July 1995 release) and can be obtained from the EMBL Outstation-the European Bioinformatics Institute (EBI) through the network or on CD-ROM. This report describes the criteria for inclusion of data in this database, a description of the current format and a brief discussion of the current relevance of p53 mutation analysis to clinical and biological questions.

The p53 tumour suppressor gene: a model for molecular epidemiology of human cancer

The gene encoding the tumour suppressor protein p53 is one of the most commonly mutated genes in human cancers. Analysis of the mutational events that target the p53 gene has revealed evidence for both exogenous and endogenous mutational mechanisms. For example, the p53 mutational spectrum reveals evidence for a direct causal effect of ultraviolet radiation in skin cancer, of aflatoxin B1 in liver cancer and of tobacco smoke in lung cancer. This novel field, molecular epidemiology of human cancer risk, has added a new dimension to classical associative epidemiology by providing a direct link between human cancer and carcinogen exposure.

Molecular epidemiology and human risk monitoring

Molecular epidemiologic research involves the identification of relations between previous exposure to some putative causative agent and subsequent biological effects in a cluster of individuals in populations. There is intensive current research in the field of molecular epidemiology, and this research has a direct impact on risk assessment processes. Many of the challenges facing risk assessors today can be addressed by research focused on developing a better understanding of (a) exposure characteristics or assessment, (b) the relationship between exposure and dose, and (c) the ultimate exposure/dose effect response relationship. Results from this research can be used to design and implement preventive interventions in at risk populations. Thus, the application of research in exposure assessment and molecular epidemiology to risk assessment and preventive interventions makes this a core program for public health.

Alpha-fetoprotein and p53 autoantibodies in patients with chronic hepatitis C

Hepatitis C virus infection is a common cause of chronic liver disease and hepatocellular carcinoma. Recently, mutations in the p53 tumor suppressor gene with generation of circulating autoantibodies to p53 protein have been detected in a significant proportion of patients with different malignancies. Using ELISA methods we assessed alpha-fetoprotein and anti-p53 as serological screening parameters for hepatocellular carcinoma in 147 consecutive patients with chronic hepatitis C. Liver cirrhosis was histologically diagnosed in 58 patients (39.5%) and a hepatocellular carcinoma confirmed in seven patients (4.8%). Serum alpha-fetoprotein was raised above 20 ng/ml in 26/147 patients and above 100 ng/ml in 5/147 patients. In 6/7 patients with hepatocellular carcinoma, alpha-fetoprotein was raised above 20 ng/ml, but only in 3/7 cases above 100 ng/ml, resulting in a sensitivity and specificity of 85.7% and 85.7% (alpha-fetoprotein > 20 ng/ml) and 42.9% and 98.6% (alpha-fetoprotein > 100 ng/ml) for the detection of hepatocellular carcinoma, respectively. Autoantibodies to p53 were detected in 3/7 patients with hepatocellular carcinoma, but in 0/140 patients without malignancy (sensitivity 42.9%, specificity 100%). Screening for hepatocellular carcinoma was improved by combining alpha-fetoprotein measurement (level > 100 ng/ml) with detection for anti-p53 (sensitivity 71.4%, specificity 98.6%). In conclusion, the presence of anti-p53 was highly specific for malignancy and independent of alpha-fetoprotein status. Further studies including a larger number of patients with hepatitis C virus-related hepatocellular carcinoma are required to investigate whether serological testing for anti-p53 in combination with alpha-fetoprotein might improve the detection of hepatocellular carcinoma in high-risk patients with liver cirrhosis.

Low incidence of p53 mutations in European hepatocellular carcinomas with heterogeneous mutation as a rare event

BACKGROUND/AIMS

The aim of this study was to evaluate the role of p53 mutations in European hepatocarcinogenesis.

METHODS

DNA extracts from 20 microdissected tumor samples were investigated. Nucleotide sequence analysis of subcloned polymerase chain reaction-fragments of the conserved domain exons 5-8 was performed in order to detect heterogeneous distribution of p53 mutated cells within the tumors. In a screening procedure four clones of each exon 5-8 were analyzed. To confirm the observed mutations polymerase chain reaction and subcloning was repeated.

RESULTS

Sequence analysis confirmed a mutation in only two cases (10%). One at codon 220 (exon 6) was a homogeneous transition in nearly all clones from TAT to TGT. The other mutation was a transition from cGG to CAG at the known hot spot codon 248 (exon 7). It was found in 30% of the clones. We conclude that the other mutations from the first step were artefacts due to the infidelity of the taq-polymerase. All tumors had wild type sequence at the reported hot spot codon 249. The minor importance of p53 gene alterations in European hepatocarcinogenesis was further confirmed at the protein level by immunohistochemistry. Only the tumors with the heterogeneous p53 mutation at codon 248 showed a p53 overexpression in nearly 30% of the nuclei. None of the other tumors showed higher levels of p53 expression.

CONCLUSIONS

We therefore conclude that the incidence of p53 mutations in European hepatocellular carcinomas is very low. Generally there may be no heterogeneous distribution of p53 mutated cells within a tumor. The contribution of this genetic alteration to hepatocarcinogenesis in Europe seems of little importance.

The Aspergillus parasiticus polyketide synthase gene pksA, a homolog of Aspergillus nidulans wA, is required for aflatoxin B1 biosynthesis

Aflatoxins comprise a group of polyketide-derived carcinogenic mycotoxins produced by Aspergillus parasiticus and Aspergillus flavus. By transformation with a disruption construct, pXX, we disrupted the aflatoxin pathway in A. parasiticus SRRC 2043, resulting in the inability of this strain to produce aflatoxin intermediates as well as a major yellow pigment in the transformants. The disruption was attributed to a single-crossover, homologous integration event between pXX and the recipient A. parasiticus genome at a specific locus, designated pksA. Sequence analysis suggest that pksA is a homolog of the Aspergillus nidulans wA gene, a polyketide synthase gene involved in conidial wall pigment biosynthesis. The conserved beta-ketoacyl synthase, acyltransferase and acyl carrier-protein domains were present in the deduced amino acid sequence of the pksA product. No beta-ketoacyl reductase and enoyl reductase domains were found, suggesting that pksA does not encode catalytic activities for processing beta-carbon similar to those required for long chain fatty acid synthesis. The pksA gene is located in the aflatoxin pathway gene cluster and is linked to the nor-1 gene, an aflatoxin pathway gene required for converting norsolorinic acid to averantin. These two genes are divergently transcribed from a 1.5 kb intergenic region. We propose that pksA is a polyketide synthase gene required for the early steps of aflatoxin biosynthesis.

Enhanced levels of DNA adducts in the liver of woodchucks infected with hepatitis virus

Liver DNA specimens from woodchucks kept in captivity, 10 naturally infected with hepatitis virus (WHV) and five WHV-free, were examined for the presence of carcinogen-DNA adducts by 32P-postlabeling. The number of adducts was significantly higher in WHV carriers than in uninfected animals, and the total amounts of adducts per 10(9) nucleotides were also considerably enhanced by WHV infection, when using both butanol extraction (22.2 +/- 7.1 vs. 12.6 +/- 2.8, means +/- S.D.) and nuclease P1 enrichment (8.5 +/- 5.9 vs. 2.8 +/- 1.7). Two individual adducts were also significantly higher in WHV carriers. No significant variation occurred as related to age, sex or time length of captivity. These findings are consistent with our previous studies supporting an enhanced metabolism of chemical hepatocarcinogens in both human and woodchuck hepadnavirus infections. Several significant and remarkable correlations were pointed out by relating DNA adduct data to more than 30 virological, histopathological and metabolic parameters which had been previously evaluated in the same animals. For instance, numbers and/or levels of adducts were positively related to the amounts of virus present in hepatocytes, to cell damage (gamma-glutamyltranspeptidase activity), to the severity of the liver histopathological picture, and to monooxygenase activities, while they were inversely related to cellular glutathione concentrations and to detoxification of the direct-acting mutagen 4-nitroquinoline 1-oxide. The major adduct significantly correlated with the metabolic activation of the aromatic amine 2-aminofluorene and of the heterocyclic amines 3-amino-1-methyl-5H-pyrido(4,3)indole (Trp-P-2) and 2-amino-3,4-dimethylimidazo(4,5-f)quinoline (MeIQ), whereas another adduct significantly correlated with the metabolic activation of the mycotoxin aflatoxin B1. Thus, the enhanced metabolism of chemical hepatocarcinogens and the increased formation of carcinogen-DNA adducts in the liver of WHV carriers appear to represent one of the mechanisms contributing to the association between chronic hepadnavirus infection and development of primary hepatocellular carcinoma.

Codon 249 mutation of the p53 gene is a rare event in hepatocellular carcinomas from ethnic Chinese in Singapore

The present study characterised p53 mutations in 44 hepatocellular carcinomas (HCCs) from Chinese patients residing in a high-incidence area. Twelve point mutations (27%) were detected in tumour tissues using single-strand conformation polymorphism analysis followed by direct DNA sequencing. Remarkably, no mutations were observed at codon 249. This is in contrast to HCCs from other high HCC incidence areas with endemic aflatoxin exposures, in which codon 249 is a mutational hot spot. It is therefore suggested that risk factors other than dietary exposure to aflatoxin may contribute to the high HCC incidence in Singapore.

Characterization of a murine p53ser246 mutant equivalent to the human p53ser249 associated with hepatocellular carcinoma and aflatoxin exposure

A mutation in the tumor suppressor p53 gene resulting in an Arg-->Ser substitution in position 249 is found frequently in human hepatocellular carcinomas associated with hepatitis B infection and with aflatoxin exposure. To determine the significance of this mutation in an in vivo experimental model using transgenic mice, we introduced a two-nucleotide change in the mouse p53 gene at amino-acid position 246, which is equivalent to position 249 in human p53, by the recombinant polymerase chain reaction mismatched primer method. This p53 mutation resulted in the same change, an Arg-->Ser substitution, as in the human p53 gene at position 249. We now report that the protein product of this mutant mouse p53ser246 had properties similar to those of the wild-type protein when tested by binding to (i) monoclonal antibodies PAb246 and PAb240, ii) simian virus 40 large T antigen, and (iii) heat-shock protein. However, it had mutant-type transforming properties when tested for colony formation with an osteosarcoma cell line. It was not active, as is wild-type p53, in transcription activation of the muscle creatine kinase promoter. These properties are the same as those found in the p53trp248 product of the p53 mutation associated with the Li-Fraumeni syndrome. Although less is known about the human p53ser249 product associated with hepatocellular carcinoma, the mutant murine p53ser246 protein shares the known properties of the human gene product.

p53 expression in patients with cirrhosis with and without hepatocellular carcinoma

BACKGROUND

Mutated p53 acts as a dominant oncogene, whereas the wild type (wt) p53 gene product suppresses cell growth. Abnormalities in the p53 gene are reported in more than 50% of malignant tumors. Recently, an allelic loss of chromosome 17p, where the p53 gene is located, was found to be more frequent in hepatocellular carcinoma (HCC) cell lines and human tumors. In addition, in half of the cases of HCC from endemic areas for hepatitis B virus and aflatoxin, a hot spot point mutation at codon 249 was detected, as previously reported. Missense mutations in p53, mdm-2 complex formation, and other unknown mechanisms may lead to stabilization of the gene product, thus rendering it detectable by immunohistochemistry.

METHODS

To assess the relationship between p53 status at a premalignant stage and in HCC, the authors studied the immunohistologic expression of p53 in HCC and in the adjacent nontumorous resected liver tissue, using monoclonal antibody to wt and mutated p53.

RESULTS

Twelve of the 14 patients with liver tumors had HCC. Of the 12 patients with HCC and underlying cirrhosis, 8 (67%) had increased p53 expression in HCC cells. Eight of the 12 patients with p53-positive HCC cells had p53 overexpression in the nontumorous hepatocytes within regenerative nodules adjacent to HCC tissue. Three of 21 cirrhotic livers without a detectable tumor had increased p53 expression in the regenerative nodules. None of the 12 patients with chronic active hepatitis without cirrhosis or the 13 with a normal liver histology had increased p53 expression.

CONCLUSION

p53 overexpression in some cirrhotic livers and in nontumorous livers of patients with HCC may indicate a normal p53 gene response to cellular stress or, alternatively, to an abnormally or mutated p53 gene, and could occur before the development of HCC.

Mosaicism: the embryo as a target for induction of mutations leading to cancer and genetic disease

Mosaicism, the existence of "patches" of cells with a genetic constitution that differs from that of other cells of an organism, has been observed in both germinal and somatic tissues of several species, including humans. Mutational events occurring during early embryogenesis can give rise to an organism with a significant number of cells with the mutant genotype in one or more tissues. If this event occurs in a precursor of the germ cells, the mutation can be transferred to subsequent generations. In the F1 generation, this event will usually be perceived as a de novo germinal mutation rather than a transmitted variant allele, unless significant effort is directed toward detecting the mosaicism. Similarly, mutations in oncogenes and tumor-suppressor genes in proliferating somatic cells can generate populations of cells that are at increased risk of transforming into tumor cells. The number of potential preneoplastic cells is larger when the mutagenic event occurs in early development than if it occurs in the mature adult. Experimental data confirm that treatment of the developing embryo or fetus with carcinogenic and mutagenic agents increases the cancer incidence in these animals and the frequency of mutations in the offspring of the animals that were exposed in utero. The available data are conclusive that the developing organism is at risk from exposure to mutagenic and carcinogenic agents. However, the data are insufficient to estimate the level of risk associated with exposures in utero, relative to either the background (spontaneous) level of risk or risk associated with similar exposures to the adult organism.(ABSTRACT TRUNCATED AT 250 WORDS)