Aberrations of p53 gene in human hepatocellular carcinoma from China

Density Functional Theory Study on the Interaction between Aflatoxin B1/M1 and Gold Substrate

Aflatoxin M1 (AFM1) and its precursor, Aflatoxin B1 (AFB1), are highly pathogenic and mutagenic substances, making the detection and sensing of AFB1/M1 a long-standing focus of researchers. Among various detection techniques, surface-enhanced Raman spectroscopy (SERS) is considered an ideal method for AFB1/M1 detection due to its ability not only to enhance characteristic frequencies but also to detect shifts in these frequencies with high repeatability. Therefore, we employed density functional theory in conjunction with surface-enhanced Raman spectroscopy to investigate the interaction between AFB1/M1 and a Au substrate in the context of the SERS effect for the first time. To predict the potential binding sites of AFB1/M1 and Au within the SERS effect, we performed calculations on the molecular electrostatic potential of AFB1/M1. Considering the crucial role of the binding energy in molecular docking studies, we computed the binding energy between two molecules interacting with Au at different binding sites. The molecular frontier orbitals and related chemical parameters of AFB1/M1 and "molecular-Au" complexes were computed to elucidate the alterations in AFB1/M1 molecules under the SERS effect. Subsequently, the theoretical Raman spectra of AFB1/M1 and the complexes were compared and analyzed, enabling determination of the adsorption conformation of AFB1/M1 on the gold surface based on SERS surface selection rules. These findings not only provide a deeper understanding of the interaction mechanism between molecules and substrates in the SERS effect but also offer theoretical support for developing novel aflatoxin SERS sensors.

Research update on aflatoxins toxicity, metabolism, distribution, and detection: A concise overview

Serious health risks associated with the consumption of food products contaminated with aflatoxins (AFs) are worldwide recognized and depend predominantly on consumed AF concentration by diet. A low concentration of aflatoxins in cereals and related food commodities is unavoidable, especially in subtropic and tropic regions. Accordingly, risk assessment guidelines established by regulatory bodies in different countries help in the prevention of aflatoxin intoxication and the protection of public health. By assessing the maximal levels of aflatoxins in food products which are a potential risk to human health, it's possible to establish appropriate risk management strategies. Regarding, a few factors are crucial for making a rational risk management decision, such as toxicological profile, adequate information concerning the exposure duration, availability of routine and some novel analytical techniques, socioeconomic factors, food intake patterns, and maximal allowed levels of each aflatoxin in different food products which may be varied between countries.

Aflatoxin B1 and M1: Biological Properties and Their Involvement in Cancer Development

Aflatoxins are fungal metabolites found in feeds and foods. When the ruminants eat feedstuffs containing Aflatoxin B1 (AFB1), this toxin is metabolized and Aflatoxin M1 (AFM1) is excreted in milk. International Agency for Research on Cancer (IARC) classified AFB1 and AFM1 as human carcinogens belonging to Group 1 and Group 2B, respectively, with the formation of DNA adducts. In the last years, some epidemiological studies were conducted on cancer patients aimed to evaluate the effects of AFB1 and AFM1 exposure on cancer cells in order to verify the correlation between toxin exposure and cancer cell proliferation and invasion. In this review, we summarize the activation pathways of AFB1 and AFM1 and the data already reported in literature about their correlation with cancer development and progression. Moreover, considering that few data are still reported about what genes/proteins/miRNAs can be used as damage markers due to AFB1 and AFM1 exposure, we performed a bioinformatic analysis based on interaction network and miRNA predictions to identify a panel of genes/proteins/miRNAs that can be used as targets in further studies for evaluating the effects of the damages induced by AFB1 and AFM1 and their capacity to induce cancer initiation.

Mutational spectra of aflatoxin B1 in vivo establish biomarkers of exposure for human hepatocellular carcinoma

Aflatoxin B₁ (AFB₁) and/or hepatitis B and C viruses are risk factors for human hepatocellular carcinoma (HCC). Available evidence supports the interpretation that formation of AFB₁-DNA adducts in hepatocytes seeds a population of mutations, mainly G:C→T:A, and viral processes synergize to accelerate tumorigenesis, perhaps via inflammation. Responding to a need for early-onset evidence predicting disease development, highly accurate duplex sequencing was used to monitor acquisition of high-resolution mutational spectra (HRMS) during the process of hepatocarcinogenesis. Four-day-old male mice were treated with AFB₁ using a regimen that induced HCC within 72 wk. For analysis, livers were separated into tumor and adjacent cellular fractions. HRMS of cells surrounding the tumors revealed predominantly G:C→T:A mutations characteristic of AFB₁ exposure. Importantly, 25% of all mutations were G→T in one trinucleotide context (CGC; the underlined G is the position of the mutation), which is also a hotspot mutation in human liver tumors whose incidence correlates with AFB₁ exposure. The technology proved sufficiently sensitive that the same distinctive spectrum was detected as early as 10 wk after dosing, well before evidence of neoplasia. Additionally, analysis of tumor tissue revealed a more complex pattern than observed in surrounding hepatocytes; tumor HRMS were a composite of the 10-wk spectrum and a more heterogeneous set of mutations that emerged during tumor outgrowth. We propose that the 10-wk HRMS reflects a short-term mutational response to AFB₁, and, as such, is an early detection metric for AFB₁-induced liver cancer in this mouse model that will be a useful tool to reconstruct the molecular etiology of human hepatocarcinogenesis.

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

Molecular fingerprints of environmental carcinogens in human cancer

Identification of specific molecular changes (fingerprints) is important to identify cancer etiology. Exploitable biomarkers are related to DNA, epigenetics, and proteins. DNA adducts are the turning point between environmental exposures and biological damage. DNA mutational fingerprints are induced by carcinogens in tumor suppressor and oncogenes. In an epigenetic domain, methylation changes occurs in specific genes for arsenic, benzene, chromium, and cigarette smoke. Alteration of specific microRNA has been reported for environmental carcinogens. Benzo(a)pyrene, cadmium, coal, and wood dust hits specific heat-shock proteins and metalloproteases. The multiple analysis of these biomarkers provides information on the carcinogenic mechanisms activated by exposure to environmental carcinogens.

An integrated analysis of SOCS1 down-regulation in HBV infection-related hepatocellular carcinoma

Persistent inflammation together with genetic/epigenetic aberrations is strongly associated with chronic Hepatitis B virus (HBV) infection-related hepatocarcinogenesis. Here, we investigated the alterations of the suppressor of cytokine signalling (SOCS) family genes in HBV-related hepatocellular carcinoma (HCC). A total of 116 patients with HCC were enrolled in this study. The methylation statuses of SOCS1-7 and CISH genes were quantitatively measured and clinicopathological significance of SOCS1 methylation was statistically analysed. The gene copy number variation was assayed by aCGH. Luciferase reporter assay and Western blot were used to detect the involvement of SOCS1 in p53 signalling. We found high frequencies of SOCS1 gene hypermethylation in both tumour (56.03%) and adjacent nontumour tissues (54.31%), but tumour tissues exhibited increased methylation intensity (24.01% vs 13.11%, P < 0.0001), particularly in patients with larger tumour size or cirrhosis background (P < 0.0001). In addition, the frequency and intensity of SOCS1 hypermethylation in tumour tissues were both significantly higher than those in nontumour tissues in male gender patients and in patients ≥45 years old (P = 0.0214 and P < 0.0001, P = 0.0232 and P < 0.0001, respectively). SOCS1 gene deletion was found in 8 of 25 aCGH assayed tumour specimens, which was associated with lower SOCS1 mRNA expression (P = 0.0448). Furthermore, ectopic SOCS1 overexpression could activate the p53 signalling pathway in HCC cell lines. Hypermethylation of SOCS2-7 and CISH genes was seldom found in HCC. Our results suggested that the gene loss and epigenetic silencing of SOCS1 were strongly associated with HBV-related HCC.

Differential mutagenicity of aflatoxin B1 in the liver of neonatal and adult mice

Children are generally more sensitive to toxicants than adults, including an increased sensitivity to genotoxic carcinogens. We previously demonstrated that neonatal mice are also more sensitive to the mutagenic effects of the direct alkylating agents N-ethyl-N-nitrosoamine and the arylamine 4-aminobiphenyl than adult mice. In this study, we have evaluated the effect of age on the mutagenicity of the fungal toxin and liver carcinogen aflatoxin B(1) (AFB(1)). Neonatal Big Blue transgenic mice were treated with 6 mg/kg AFB(1), a treatment that produces liver tumors, while adult mice were treated with 6 and 60 mg/kg AFB(1), treatments that do not result in tumors. The cII liver mutant frequency (MF) in mice treated with AFB(1) as neonates was 22-fold higher than in control neonatal mice, whereas the treatment of adult mice with either dose of AFB(1) did not significantly increase the liver MF over the controls. In AFB(1)-treated neonatal mice, the frequency of G:C --> T:A transversion, a major type of mutation induced by AFB(1), was about 82-fold higher than for the control and 31-fold higher than for adult mice treated with 60 mg/kg AFB(1). Our mutagenicity findings parallel the relative carcinogenicity of AFB(1) in neonatal and adult mice, and are consistent with previous observations of the lower level of hepatic glutathione S-transferase and higher level of hepatic AFB(1)-DNA adduction in neonatal mice compared to adult mice.

p53 mutations as fingerprints for aristolochic acid: an environmental carcinogen in endemic (Balkan) nephropathy

The activation of protooncogenes and inactivation of tumor suppressor genes are considered to be the main molecular events in the multistep process of carcinogenesis. Mutations of the TP53 tumor suppressor gene have been found in nearly all tumor types and are estimated to contribute to more than 50% of all cancers. Most mutations lead to the synthesis of highly stable, inactive proteins that accumulate in the nucleus of cancer cells. Among the 393 codons of the human p53 gene, 222 are targets of 698 different types of mutations. Alterations of codons 175, 248, 273 and 282 correspond to 19% of all mutations and are considered general hot spot mutations. Dietary exposure to aristolochic acid (AA), an established nephrotoxin and human carcinogen found in all Aristolochia species was shown to be the causative agent of aristolochic acid nephropathy (previously called Chinese herbs nephropathy). This syndrome is characterized by proximal tubular damage, renal interstitial fibrosis, slow progression to the end stage renal disease and a high prevalence of upper urinary tract urothelial carcinoma (otherwise a highly unusual location). AA preferentially binds to purines in DNA and is associated with a high frequency of A-->T transversions in the p53 gene. Rats treated with AA develop A:T-->T:A mutations in codon 61. The pathological and clinical features of endemic (Balkan) nephropathy closely resemble those associated with aristolochic acid nephropathy except for the slower progression to end stage renal disease and longer cumulative period before the appearance of urothelial cancer. Recently, we reported the presence of AA-DNA adducts in renal cortex and A-->T p53 mutations in tumor tissue of patients from Croatia and Bosnia with endemic nephropathy. These data support the hypothesis that dietary exposure to AA is a major risk factor for endemic (Balkan) nephropathy.

The R246S hot-spot p53 mutant exerts dominant-negative effects in embryonic stem cells in vitro and in vivo

p53 is the most frequently mutated tumour-suppressor gene in human cancers. Mutant p53 is thought to contribute to carcinogenesis by the acquisition of gain-of-function properties or through the exertion of dominant-negative (DN) effects over the remaining wild-type protein. However, the context in which the DN effects are observed is not well understood. We have therefore generated `knock-in' mouse embryonic stem (ES) cells to investigate the effects of expressing a commonly found hot-spot p53 mutant, R246S – the mouse equivalent of human R249S, which is associated with hepatocellular carcinomas. We demonstrate here that R246S mutant p53 exhibits DN effects with respect to target gene expression, cell survival and cell cycle arrest both in cells that are in the undifferentiated state and upon differentiation. The knock-in cells contain higher levels of p53 that localizes to the nucleus even in the absence of genotoxic stress and yet remains non-functional, reminiscent of mutant p53 found in human tumours. In a model based on carbon-tetrachloride-induced liver injury, these cells were consistently highly tumorigenic in vivo, similar to p53–/– cells and in contrast to both p53+/+ and p53+/– ES cells. These data therefore indicate that the DN effects of mutant p53 are evident in the stem-cell context, in which its expression is relatively high compared with terminally differentiated cells.

TP53 mutations and hepatocellular carcinoma: insights into the etiology and pathogenesis of liver cancer

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide and the major risk factors include chronic infections with the hepatitis B (HBV) or C (HCV) virus, and exposure to dietary aflatoxin B(1) (AFB(1)) or alcohol consumption. Multiple genetic and epigenetic changes are involved in the molecular pathogenesis of HCC, for example, somatic mutations in the p53 tumor suppressor gene (TP53) and the activation of the WNT signal transduction pathway. AFB(1) frequently induces G:C to T:A transversions at the third base in codon 249 of TP53 and cooperates with HBV in causing p53 mutations in HCC. The detection of TP53 mutant DNA in plasma is a biomarker of both AFB(1) exposure and HCC risk. Chronic infection with HBV and HCV viruses, and oxyradical disorders including hemochromatosis, also generate reactive oxygen/nitrogen species that can both damage DNA and mutate cancer-related genes such as TP53. Certain mutant p53 proteins may exhibit a 'gain of oncogenic function'. The p53 biological network is a key responder to this oxidative and nitrosative stress. Depending on the extent of the DNA damage, p53 regulates the transcription of protective antioxidant genes and with extensive DNA damage, transactivates pro-oxidant genes that contribute to apoptosis. The X gene of HBV (HBx) is the most common open reading frame integrated into the host genome in HCC and the integrated HBx is frequently mutated. Mutant HBx proteins still retain their ability to bind to p53, and attenuate DNA repair and p53-mediated apoptosis. In summary, both viruses and chemicals are implicated in the etiology of TP53 mutations during the molecular pathogenesis of HCC.

Inactivation of PEMT2 in hepatocytes initiated by DENA in fasted/refed rats

Phosphatidylethanolamine N-methyltransferase (PEMT) is the enzyme that converts phosphatidylethanolamine (PE) into phosphatidylcholine. We have previously shown that PEMT suppressed hepatoma growth by triggering apoptosis. We investigate whether PEMT controlled cell death and cell proliferation triggered by fasting/refeeding and whether it is a marker of early preneoplastic lesions. The induction of programmed cell death and suppression of cell proliferation by fasting were associated with enhanced PEMT expression and activity, and with a decrease in CTP:phosphocholine cytidylyltransferase expression. Refeeding returned the liver growth and expression of CTP:phosphocholine cytidylyltransferase to control levels, while the expression of PEMT decreased to below control values. After DENA administration, PEMT protein, evaluated by Western blotting, slightly increased, but it remained below control levels. The treatment with 20 mg/kg DENA to refed rats induced the appearance of initiated hepatocytes that were negative for PEMT expression. Present findings indicate that PEMT is a novel tumour marker for early liver preneoplastic lesions.

KIAA0101 (OEACT-1), an expressionally down-regulated and growth-inhibitory gene in human hepatocellular carcinoma

BACKGROUND

Our previous cDNA array results indicated KIAA0101 as one of the differentially expressed genes in human hepatocellular carcinoma (HCC) as compared with non-cancerous liver. However, it is necessary to study its expression at protein level in HCC and its biological function for HCC cell growth.

METHOD

Western blot and tissue array were performed to compare KIAA0101 protein expression level in paired human HCC and non-cancerous liver tissues from the same patients. Investigation of its subcellular localization was done by using dual fluorescence image examination and enriched mitochondrial protein Western blot analysis. The in vitro cell growth curve was used for examining the effect of over-expression of KIAA0101 in HCC cells. FACS was used to analyze the cell cycle pattern in KIAA0101 expression positive (+) and negative (-) cell populations isolated by the pMACSKKII system after KIAA0101 cDNA transfection.

RESULTS

Western blot showed KIAA0101 protein expression was down-regulated in HCC tissues as compared with their counterpart non-cancerous liver tissues in 25 out of 30 cases. Tissue array also demonstrated the same pattern in 161 paired samples. KIAA0101 was predominantly localized in mitochondria and partially in nuclei. KIAA0101 cDNA transfection could inhibit the HCC cell growth in vitro. In cell cycle analysis, it could arrest cells at the G1 to S phase transition.

CONCLUSION

KIAA0101 protein expression was down-regulated in HCC. This gene could inhibit the HCC cell growth in vitro and presumably by its blocking effect on cell cycle.

Genetic alterations in cancer knowledge system: analysis of gene mutations in mouse and human liver and lung tumors

Mutational incidence and spectra for genes examined in both human and mouse lung and liver tumors were analyzed using the National Institute of Environmental Health Sciences (NIEHS) Genetic Alterations in Cancer (GAC) knowledge system. GAC is a publicly available, web-based system for evaluating data obtained from peer-reviewed studies of genetic changes in tumors associated with exposure to chemical, physical, or biological agents, as well as spontaneous tumors. In mice, mutations in Kras2 and Hras-1 were the most common events reported for lung and liver tumors, respectively, whether chemically induced or spontaneous. There was a significant difference in Kras2 mutation incidence for spontaneous versus induced mouse lung tumors and in Hras-1 mutation incidence and spectrum for spontaneous versus induced mouse liver tumors. The major gene changes reported for human lung and liver tumors were in KRAS2 (lung only) and TP53. The KRAS2 mutation incidence was similar for spontaneous and asbestos-induced human lung tumors, while the TP53 mutation incidence differed significantly. Aflatoxin B1, hepatitis B virus, hepatitis C virus, and vinyl chloride all caused TP53 mutations in human liver tumors, but the mutation spectrum for each agent differed. The incidence of KRAS2 mutations in human compared to mouse lung tumors differed significantly, as did the incidence of Hras and p53 gene mutations in human compared to mouse liver tumors. Differences observed in the mutation spectra for agent-induced compared to spontaneous tumors and similarities in spectra for structurally similar agents support the concept that mutation spectra can serve as a "fingerprint" of exposure based on chemical structure.

Molecular mechanisms in hepatocellular carcinoma development

Hepatocellular carcinoma (HCC) is one of the most common cancers in the world. It usually develops in patients with chronic viral hepatitis, aflatoxin exposure, or excessive alcohol use. Most patients with HCC present with advanced disease and have a poor prognosis. The implementation of antiviral drugs and the availability of a vaccine for hepatitis B should help reduce the incidence of HCC. Considerable effort has now focused on unraveling the molecular pathogenesis of HCC in order to design better treatments, or to prevent the disease altogether. However, so far, the pathogenesis of HCC appears to be quite heterogeneous among patients. In particular, several mechanisms of tumorigenesis seem to be involved, including loss of tumor suppressor gene function, oncogene activation, direct viral effects, DNA methylation, and angiogenesis. It is not clear which events are critical in tumor initiation versus tumor progression. RNA expression arrays and proteomics hold promise to provide further clues about this common and complex cancer.

Molecular cloning and characterization of LAPTM4B, a novel gene upregulated in hepatocellular carcinoma

Lysosomal-associated protein transmembrane-4 beta (LAPTM4B), a novel gene upregulated in hepatocellular carcinoma (HCC), was cloned using fluorescence differential display, RACE, and RT-PCR. It contains seven exons and encodes a 35-kDa protein with four putative transmembrane regions. Both the N- and C-termini of the protein are proline-rich, and may serve as potential ligands for the SH3 domain. Immunohistochemical analysis localized the protein predominantly to intracellular membranes. Northern blot showed that the LAPTM4B mRNAs were remarkably upregulated in HCC (87.3%) and correlated inversely with differentiation status. LAPTM4B was also overexpressed in many HCC-derived cell lines. It was also highly expressed in fetal livers and certain adult normal tissues including the heart, skeletal muscle, testis, and ovary. Promoter function assays showed a distinct difference in the gene's activities between BEL7402 and HLE cell lines, suggesting that the transcription factors responsible for regulation of the gene in the two cell lines are different, and that possible negative regulatory cis-elements may exist upstream of the promoter region. It was demonstrated that the N-terminus of LAPTM4B was essential for survival of the cells. Cells harboring the full-length LAPTM4B cDNA expression clone displayed a slightly increased efficiency in colony formation. These results suggest that LAPTM4B is a potential protooncogene, whose overexpression is involved in carcinogenesis and progression of HCC. In normal cells, it may also play important roles such as regulation of cell proliferation and survival.

Hepatocellular carcinoma: are we making progress?

Hepatocellular carcinoma is a very prevalent malignancy worldwide, with increasing incidence in the United States. Despite many available treatment options, the prognosis remains poor. Surgical resection or liver transplantation still represents the only potentially curative treatments for HCC. Until more effective systemic therapies are available, different localized treatment approaches will continue to be applied in the management of this disease. Although systemic chemotherapy has been disappointing, increased understanding of the tumor biology in HCC coupled with new drug development may lead to newer agents with novel mechanisms of action that are more efficacious. The poor treatment outcome and dismal prognosis make prevention of HCC an important strategy in controlling this aggressive type of malignancy. Vaccine programs for HBV are ongoing. Efforts are underway to develop a vaccine for HCV. Interferon therapy appears to decrease the risk of developing HCC in patients with hepatitis, especially those with HCV. A number of other approaches for decreasing risk in these patients as well as in those with alcoholic-related cirrhosis are currently being evaluated.

Dietary aflatoxin exposure and chemoprevention of cancer: a clinical review

Exposure to dietary aflatoxins is considered to be an important risk factor for the development of hepatocellular carcinoma in certain regions of the world. Significant advances have recently been made in understanding the clinical toxicology of aflatoxins. These include the development and validation of biomarkers of exposure and genotoxic effect. These biomarkers are currently being utilized to explore the potential that pharmaceutical interventions may have in modifying the toxicokinetics of dietary aflatoxin exposure. Preliminary results of clinical trials with the drug oltipraz suggest that it may modify the genotoxic effects of aflatoxin B1 by inhibiting bioactivation pathways and stimulating detoxification pathways. More recent results of a clinical trial with chlorophyllin suggest that this drug may have a role in preventing dietary exposure to aflatoxin B1 by reducing its oral bioavailability. The preliminary results of these chemoprevention studies may ultimately have implications for cancer prevention in high-risk populations in the future.

Allelic imbalance regions on chromosomes 8p, 17p and 19p related to metastasis of hepatocellular carcinoma: comparison between matched primary and metastatic lesions in 22 patients by genome-wide microsatellite analysis

To understand the molecular mechanisms of metastasis in hepatocellular carcinoma (HCC), it is necessary to identify the accumulating genetic alterations during its progression as well as those responsible for the acquisition of metastatic potential in cancer cells. In our previous study, using comparative genomic hybridization (CGH), we found that loss on chromosome 8p is more frequent in metastatic lesions than in matched primary tumors of HCC. Thus, 8p deletion might contribute to HCC metastasis. To narrow the location of metastasis-related alteration regions, we analyzed 22 primary and matched metastatic lesions of HCC by genome-wide microsatellite analysis. Common regions with high levels of allelic imbalance (AI) were identified on 17p, 8p11-cen, 8p21-23, 4q32-qter, 4q13-23, 16q, and 1p33. Regions with increased AI in metastatic lesions were 8p23.3, 8p11.2, 17p11.2-13.3, 4q21-22, 4q32-qter, 8q24.1, 9p11, 9q31, 11q23.1, 13q14.1-31, 13q32-qter, 16p13.3, 16q13, 16q22, and 19p13.1, and these were considered to be related to the metastasis phenotype. Among them, loss on 8p was again proved to be related to progression and metastasis of HCC, and 8p23.3 and 8p11.2 were two likely regions harboring metastasis-related genes. It was also shown for the first time in HCC that AI of 19p13.1 might also be related to metastatic potential. These results provide some candidate regions for further study to identify putative genes suppressing metastasis of HCC.

Codon 249 mutation in exon 7 of p53 gene in plasma DNA: maybe a new early diagnostic marker of hepatocellular carcinoma in Qidong risk area, China

AIM: One of the characteristics of hepatocellular carcinoma (HCC) in Qidong area is the selective mutation resulting in a serine substitution at codon 249 of the p53 gene (1, 20), and it has been identified as a “hotspot” mutation in heptocellular carcinomas occurring in populations exposed to aflatoxin and with high prevalence of hepatitis B virus carriers (2, 3, 9, 10, 16, 24). We evaluated in this paper whether this “hotspot” mutation could be detected in cell-free DNA circulating in plasma of patients with hepatocellular carcinoma and cirrhosis in Qidong, China, and tried to illustrate the significance of the detection of this molecular biomarker.

METHODS: We collected blood samples from 25 hepatocellular carcinoma patients, 20 cirrhotic patients and 30 healthy controls in Qidong area. DNA was extracted and purified from 200 µl of plasma from each sample. The 249^Ser p53 mutation was detected by restriction digestion analysis and direct sequencing of exon-7 PCR products.

RESULTS: We found in exon 7 of p53 gene G→T transversion at the third base of codon 249 resulting 249^Arg→249^Ser mutation in 10/25 (40%) hepatocellular carcinoma cases, 4/20 (20%) cirrhotics, and 2/30 (7%) healthy controls. The adjusted odds ratio for having the mutation was 22.1 (95%CI, 3.2~91.7) for HCC cases compared to controls.

CONCLUSION: These data show that the 249^Ser p53 mutation in plasma is strongly associated with hepatocellular carcinoma in Qidong patients. We found this mutation was also detected, although it was at a much lower frequency, in plasma DNA of Qidong cirrhotics and healthy controls; We consider that these findings, together with the usual method of HCC diagnosis, will give more information in early diagnosis of HCC, and 249^Ser p53 mutation should be developed to a new early diagnostic marker for HCC.

The ATF/CREB site is the key element for transcription of the human RNA methyltransferase like 1(RNMTL1) gene, a newly discovered 17p13.3 gene

The human RNA methyltransferase like 1 gene (RNMTL1) is one of thirteen newly discovered genes within a 116 Kb segment of the chromosome 17p13.3 that suffers from a high frequent loss of heterozygosity in human hepatocellular carcinoma in China[1-5]. To understand the molecular mechanisms underlying transcription control of the RNMTL1 gene in human cancers, we decline using of the conventional approach where the cis-elements bound by the known transcription factors are primary targets, and carried out the systematic analyses to dissect the promoter structure and identify/characterize the key cis-elements that are responsible for its strong expression in cell. The molecular approaches applied included 1, the primer extension for mapping of the transcription starts; 2, the transient transfection/reporter assays on a large number of deletion and site-specific mutants of the promoter segment for defining the minimal promoter and the crucial elements within; and 3, the electrophoresis mobility shift assay with specific antibodies for reconfirming the nature of the transcription factors and their cognate cis-elements. We have shown that the interaction of an ATF/CREB element (-38 to -31) and its cognate transcription factors play a predominant role in the promoter activity of the RNMTL1 gene. The secondary DNA structures of the ATF/CREB element play a more vital role in the protein-DNA interaction. Finally, we reported a novel mechanism underlying the YY1 mediated transcription repression, namely, the ATF/CREB dependent transcription-repression by YY1 is executed in absence of its own sequence-specific binding.

TP53 and liver carcinogenesis

Primary hepatocellular carcinoma (HCC) is one of the most common malignancies and has the fourth highest mortality rate worldwide. The major risk factors, including chronic infections with the hepatitis B or C virus, are exposure to dietary aflatoxin B1(AFB1), vinyl chloride, or alcohol consumption. Southern China and sub-Saharan Africa have the highest dietary AFB1 exposure, making it and hepatitis B virus (HBV) the major causes of cancer mortality in these geographic areas. Recent studies have discovered genetic and epigenetic changes involved in the molecular pathogenesis of HCC, including somatic mutations in the p53 tumor suppressor gene (TP53). AFB1 induces typical G:C to T:A transversions at the third base in codon 249 of p53. Chronic active hepatitis B and C (HCV) infection, and further inflammatory and oxyradical disorders including Wilson disease (WD) or hemochromatosis, generate reactive oxygen/nitrogen species that can damage DNA and mutate the p53 gene. The X gene of HBV (HBx) is the most common open reading frame integrated into the host genome in HCC. The integrated HBx is frequently mutated and has a diminished ability to function as a transcriptional cotransactivator and to activate the NF-kappa B pathway. However, the mutant HBx proteins still retain their ability to bind to and abrogate p53-mediated apoptosis. In summary, both viruses and chemicals are implicated in the etiology and molecular pathogenesis of HCC. The resultant molecular changes in the ras and Wnt signal-transduction pathways, and the p53 and Rb tumor suppressor pathways significantly contribute to liver carcinogenesis

Assessment of safety/risk vs. Public health concerns: Aflatoxins and hepatocarcinoma

Hepatocellular carcinoma, (HCC) is a serious health problem. It is prevalent in certain parts of the world where food contamination with aflatoxin is common. Aflatoxin, especially AFB(1), has been shown to induce HCC in many species of laboratory and wild animals, including subhuman primates. Carcinogenesis studies have demonstrated that AFB(1) is a potent genotoxic carcinogen. After bioactivation it may covalently bind with protein and with DNA. The former reaction is positively correlated with AFB(1) exposure, and the latter signifies initiation of the carcinogenesis process.With these biomarkers, epidemiological studies have amply demonstrated the etiological role of aflatoxin in HCC. However, hepatitis B virus also contributes to the development of HCC. Risks and VSD (virtual safe dose) have been estimated from animal and epidemiological studies. These estimates further confirm that AFB(1) is a potent carcinogen. Furthermore, the effects of AFB(1) exposure and hepatitis B are synergistic. Some preventive measures, such as lowering the contamination level of AFB(1) in food and appropriate vaccination programs, have been implemented in many parts of the world. Chemopreventive agents. which may abolish or reduce the effects of AFB(1) are being tested for their effectiveness.

Mutations in the factor IX gene (F9) during the past 150 years have relative rates similar to ancient mutations

Pollutants and dietary mutagens have been associated with somatic mutation and cancer, but the extent of their influence on germline mutation is not clear. Since deleterious germline mutations can be transmitted for thousands of years, any influence on germline mutation from the vast increase in man-made chemicals of the past 150 years would be an important public health issue. Observed disease causing mutations in the X-linked factor IX gene (F9) of hemophilia B patients originated predominantly in the past 150 years, since the half-life of these mutations in human populations had been about two generations before effective treatment became available about a generation ago. Recent changes in germline mutational processes may be detected by comparison of the observed hemophilia B causing mutation pattern in F9 with the pattern of neutral polymorphisms which occurred over a much longer period of time. By scanning a total of 1.5 megabases of deep intronic regions of F9 in the genomic DNA from 84 individuals, 42 neutral polymorphisms were found in 23 haplotypes that differed by at least 11 mutations from the ancestral primate haplotype. By sequencing F9 in seven non-human primates, 39 of these polymorphisms were characterized as ancient mutations relative to a unanimous ancestral primate allele. This ancient mutation pattern was compared to the recent pattern of hemophilia B causing mutations. Remarkably, no significant difference was found (P=0.5), suggesting that the vast increase in man-made chemicals during the past 150 years has not had a major impact on the pattern of human germline mutation. This result is consistent with the hypothesis that endogenous processes dominate germline mutation.

p53 as a mutagen test in breast cancer

The p53 gene is mutated in about half of all tumors. The p53 gene can be used as a "mutagen test," that is, the relative frequencies of the different types of mutation can be used as an epidemiological tool to explore the contribution of exogenous mutagens vs. endogenous processes in particular cancers. p53 has been used as a mutagen test in breast cancer. Surprisingly, the pattern of p53 mutations differs among 15 geographically and ethnically diverse populations. In contrast, mutation patterns in the human factor IX gene are similar in geographically and ethnically diverse populations. Diverse p53 mutation patterns in breast cancer are consistent with a significant contribution by a diversity of exogenous mutagens. Breast tissue may be uniquely sensitive to lipophilic mutagens because of its unique architecture, characterized by tiny islands of cancer-prone mammary epithelial cells surrounded by a sea of adipocytes. Mammary epithelial cells may be differentially susceptible to released lipophilic mutagens preferentially concentrated in adjacent adipocytes and originating in the diet. To test this hypothesis, we developed a method for measuring mutation load from ethanol-fixed, paraffin-embedded human tissues immunohistochemically stained with anti-p53 antibodies. Single cells staining positively for p53 overabundance are microdissected and the gene is sequenced. It is possible to identify individuals with a high mutation load in normal breast tissue and who are presumably at increased risk for breast cancer. In addition, analysis of the p53 gene with appropriate mutation detection methodology markedly improves the prediction of early recurrence, treatment failure, and death in breast cancer patients. Mutagen tests and mutation load measurements are useful tools to identify the role of mutagens in breast cancer.

Tumor suppressor genes: at the crossroads of molecular carcinogenesis, molecular epidemiology and human risk assessment

The p53 tumor suppressor gene is mutated in about half of all human cancer cases. The p53 protein modulates multiple cellular functions, such as gene transcription, DNA synthesis and repair, cell cycle arrest, senescence, and apoptosis. Mutations in the p53 gene can abrogate these functions and may lead to genetic instability and progress to cancer. The molecular archeology of the p53 mutation spectrum generates hypotheses concerning the etiology and molecular pathogenesis of cancer. The spectrum of somatic mutations in the p53 gene implicates environmental carcinogens and endogenous processes in the etiology of human cancer. The presence of a characteristic p53 mutation also can manifest a molecular link between exposure to a particular carcinogen and a specific type of human cancer, e.g. aflatoxin B1 (AFB1) exposure and codon 249ser mutations in hepatocellular carcinoma, ultraviolet (UV) exposure and CC to TT tandem mutations in skin cancer, and cigarette smoke and the prevalence of G to T transversions in lung cancer. Although several different exogenous carcinogens have been shown to selectively target p53, evidence supporting the endogenous insult of p53 from oxyradical and nitrogen-oxyradicals is accumulating. p53 mutations can be a biomarker of carcinogen effect. Determining the characteristic p53 mutation load in nontumorous tissue, with a highly sensitive mutation assay, can indicate a specific carcinogen exposure and also may help in identifying individuals at an increased risk of cancer.

Human germline mutation in the factor IX gene

The molecular epidemiology of factor IX germline mutations in patients with hemophilia B has been studied in detail because it is an advantageous model for analyzing recent germline mutations in humans. It is estimated that mutations have been defined in the majority of nucleotides that are the target for mutation. The likelihood that a factor IX missense mutation will cause disease correlates with the degree of evolutionary conservation of the amino acid. Mutation rates per base-pair have been estimated after careful consideration and correction for biases, predicting about 76 de novo mutations per generation per individual resulting in 0.3 deleterious changes. The male-to-female sex ratio of mutation varies with the type of mutation. There is evidence for a maternal age effect and an excess of non-CpG G:C to A:T transitions. The factor IX mutation pattern is similar among geographically, racially and ethnically diverse human populations. The data support primarily endogenous mechanisms of germline mutation in the factor IX gene. Mutations at splice junctions are compatible with simple rules for predicting disease causing mutations.

Computational analysis and prediction for exons of PAC579 genomic sequence

To isolate the novel genes related to human hepatocellular carcinoma (HCC), we sequenced P1-derived artificial chromosome PAC579 (D17S926 locus) mapped in the minimum LOH (loss of heterozygosity) deletion region of chromosome 17p13.3 in HCC. Four novel genes mapped in this genomic sequence area were isolated and cloned by wet-lab experiments, and the exons of these genes were located. 0-60 kb of this genomic sequence including the genes of interest was scanned with five different computational exon prediction programs as well as four splice site recognition programs. After analyzing and comparing the computationally predicted results with the wet-lab experiment results, some potential exons were predicted in the genomic sequence by using these programs.

Cloning and characterization of a novel gene (C17orf25) from the deletion region on chromosome 17p13.3 in hepatocelular carcinoma

Using a combination of hybridization of PAC to a cDNA library and RACE technique, we isolated a novel cDNA, designated as C17orf25 (Chromosome 17 open reading frame 25, previously named it HC71A), from the deletion region on chromosome 17p13.3. The cDNA encodes a protein of 313 amino acids with a calculated molecular mass of 34.8 kDa. C17orf25 is divided into 10 exons and 9 introns, spanning 23 kb of genomic DNA. Northern blot analysis showed that the mRNA expression of C17orf25 was decreased in hepatocellular carcinoma samples as compared to adjacent noncancerous liver tissues from the same patients. The transfection of C17orf25 into the hepatocellular carcinoma cell SMMC7721 and overexpression could inhibit the cell growth. The above results indicate that C17orf25 is a novel human gene, and the cloning and preliminary characterization of C17orf25 is a prerequisite for further functional analysis of this novel gene in human hepatocellular carcinoma.

p53 tumor suppressor gene: at the crossroads of molecular carcinogenesis, molecular epidemiology, and human risk assessment

The molecular archaeology of the mutation spectra of tumor suppressor genes generates hypotheses concerning the etiology and molecular pathogenesis of human cancer. The spectrum of somatic mutations in the p53 gene implicates environmental carcinogens and both endogenous agents and processes in the etiology of human cancer.

Genetic aberration in primary hepatocellular carcinoma: correlation between p53 gene mutation and loss-of-heterozygosity on chromosome 16q21-q23 and 9p21-p23

To elucidate the molecular pathology underlying the development of hepatocellular carcinoma (HCC), we used 41 highly polymorphic microsatellite markers to examine 55 HCC and corresponding non-tumor liver tissues on chromosome 9, 16 and 17. Loss-of-heterozygosity (LOH) is observed with high frequency on chromosomal region 17p13 (36/55, 65%), 9p21-p23 (28/55, 51%), 16q21-q23 (27/55, 49%) in tumors. Meanwhile, microsatellite instability is rarely found in these microsatellite loci. Direct sequencing was performed to detect the tentative mutation of tumor suppressor genes in these regions: p53, MTS1/p16, and CDH1/E-cadherin. Within exon 5-9 of p53 gene, 14 out of 55 HCC specimens (24%) have somatic mutations, and nucleotide deletion of this gene is reported in HCC for the first time. Mutation in MTS1/p16 is found only in one tumor case. We do not find mutations in CDH1/E-cadherin. Furthermore, a statistically significant correlation is present between p53 gene mutation and loss of chromosome region 16q21-q23 and 9p21-p23, which indicates that synergism between p53 inactivation and deletion of 16q21-q23 and 9p21-p23 may play a role in the pathogenesis of HCC.

Novel hotspot detector software reveals a non-CpG hotspot of germline mutation in the factor IX gene (F9) in Latin Americans

Two-base substitutions at each of two nucleotides in the factor IX gene (F9), but not part of CpG dinucleotides, were recently reported in a small population sample collected in Mexico, a significant observation of recurrent sites ("hotspots") of mutation (P=0.00005). When these new data were combined with previously collected mutation data into two progressively larger and inclusive Latin American samples, additional mutations were observed at one recurrent site, nucleotide 17747, and an additional recurrent nucleotide was observed such that the recurrent nucleotides in these larger samples were also significant (P=0.0003 and 0.0003). In contrast, in three non-Latin American control samples, there was at most only one nucleotide that recurred only once, most likely a chance recurrence (P>/=0.5). When the significance of substitutions was analyzed at each recurrent nucleotide individually, nucleotide 17747 was shown to be a significant recurrent nucleotide by itself in all the Latin American population samples (P</=0.02). Furthermore, a standard statistical comparison of mutation frequencies in the previously collected data alone confirmed that the frequency of mutation at nucleotide 17747 is significantly higher in Latin Americans than in all other populations combined (P=0.01). Thus, nucleotide 17747 is a germline mutation hotspot in F9 specific to Latin American populations. This may be the first evidence for population-specific effects on germline mutation that causes human genetic disease. The significance of the observed recurrent sites was analyzed using new software called Hotspot Detector which is capable of detecting significant recurrent sites in small samples, extending the sensitivity of F9 as a human germline mutagen test. Hotspot Detector uses a Monte-Carlo simulation method that was validated by comparing its results with those from an exact probability formula derived from statistical theory.

Genes and viruses in hepatobiliary neoplasia

Hepatobiliary neoplasms comprise a significant portion of the worldwide cancer burden. Advances in basic science research have led to rapid progress in our understanding of the molecular events responsible for these dreaded diseases. The genetic changes associated with hepatocellular carcinoma (HCC) have received the most attention. Aflatoxin B1 exposure leads to mutations in the p53 tumor suppressor gene, most commonly a transversion in codon 249 that leads to a substitution of serine for arginine in the p53 protein. Numerous other tumor suppressor genes, oncogenes, and tumor gene pathways are altered in HCC. Hepatitis B virus (HBV) infection is strongly associated with HCC. HBV may cause HCC either directly via the HBV X protein, or indirectly by causing liver inflammation and cirrhosis. Hepatitis C virus (HCV) infection is also associated with HCC. Recent evidence suggests that the HCV core protein may play a role in hepatocarcinogenesis. Several inherited metabolic diseases are associated with HCC. It is likely that these diseases cause HCC indirectly by causing cirrhosis. The molecular pathogenesis of cholangiocarcinoma and gallbladder cancer has not been well defined. However, multiple tumor suppressor genes and oncogenes, including p53 and K-ras, are altered in these tumors. Further molecular characterization of hepatobiliary tumors may lead to earlier diagnosis, better staging, improved treatment planning, and the development of more effective therapies.

Genetics of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the human cancers clearly linked to viral infections. Although the major viral and environmental risk factors for HCC development have been unravelled, the oncogenic pathways leading to malignant transformation of liver cells have long remained obscure. Recent outcomes have been provided by extensive allelotype studies which resulted in a comprehensive overview of the main genetic abnormalities in HCC, including DNA copy gains and losses. The differential involvement of the p53 tumor-suppressor gene in tumors associated with various risk factors has been largely clarified. Evidence for a crucial role of the reactivation of the Wnt/beta-catenin pathway, through mutations in the beta-catenin and axin genes in 30-40% of liver tumors, represents a major breakthrough. It has also been shown that the Rb pathway is frequently disrupted by methylation-dependent silencing of the p16INK4A gene and stimulation of Rb degradation by a proteosomal subunit. Presently, the identification of candidate oncogenes and tumor suppressors in the most frequently altered chromosomal regions is a major challenge. Great insights will come from integrating the signals from different pathways operating at preneoplastic and neoplastic stages. This search might, in time, permit an accurate evaluation of the major targets for therapeutic treatments.

Molecular epidemiology and carcinogenesis: endogenous and exogenous carcinogens

Mutations of the p53 tumor suppressor gene are found in about 50% of all human cancers. The p53 mutation spectra in these cancers are providing clues to the etiology and molecular pathogenesis of cancer. Recent studies indicate that the p53 protein is involved in several vital cellular functions, such as gene transcription, DNA synthesis and repair, cell cycle arrest, senescence and programmed cell death. Mutations in the p53 gene can abrogate these functions and may contribute to genomic instability and progression to cancer. Characteristic p53 mutation spectra have been associated with dietary aflatoxin B(1) (AFB(1)) exposure and hepatocellular carcinoma (HCC); sunlight exposure and skin cancer; and cigarette smoking and lung cancer. The mutation spectrum also reveals those p53 mutants that provide cells with a selective clonal expansion advantage during the multistep process of carcinogenesis. Although a number of different exogenous carcinogens have been shown to selectively target p53, pieces of evidence supporting the endogenous insult of p53 are accumulating. Furthermore, analysis of a characteristic p53 mutation load in nontumorous human tissue can indicate previous carcinogen exposure and may identify individuals at an increased cancer risk.

Apoptosis and cell proliferation are involved in the initiation of liver carcinogenesis by a subnecrogenic dose of diethylnitrosamine in refed rats

We investigated whether changes in apoptosis and cell proliferation induced by starvation and refeeding in rat liver may contribute to the initiation mechanism of liver cancer by 20 mg/kg of diethylnitrosamine (DENA). Rats were starved for 4 d, then refed and given 20 mg/kg of DENA after 1 d of refeeding. Rat livers were examined before and after DENA treatment to measure DNA loss and synthesis, the number of the placental form of glutathione S-transferase (P-GST) positive cells and their turnover. Four days of starvation depressed cell replication, as indicated by the labeling index (LI), and induced apoptosis, as shown by the decay of total DNA radioactivity and apoptotic index (AI, TUNEL technique). After 1 d of refeeding, AI significantly decreased and LI remained low, indicating that a high percentage of S phase cells was not required for the DNA damage due to 20 mg/kg of DENA. DENA induced apoptosis and the AI after 20 mg/kg of DENA was 3% in refed rats vs. 1% in fully-fed rats 5 d after DENA (P </= 0.05). Putative-initiated P-GST-positive hepatocytes appeared after administration of 20 mg/kg in refed rats, and they showed a higher LI (6%) than the surrounding P-GST-negative cells 3 d after DENA (LI = 2%; P </= 0.01), while very few P-GST-positive cells were found in fully-fed rats. These data indicate that starvation-induced cell loss and the subsequent refeeding trigger cell proliferation that gives a selective advantage to the cells initiated by 20 mg/kg of DENA to grow in the livers of refed rats.

Aflatoxin and liver cancer

This chapter reviews the data that have been accumulated implicating aflatoxin ingestion as an important risk factor in the aetiology of hepatocellular carcinoma (HCC). Numerous epidemiological studies have observed a correlation between areas of high aflatoxin exposure and a high incidence of HCC. The use of experimental models and specific biomarkers for aflatoxin exposure, such as urinary metabolites or aflatoxin adducts, have validated these findings. Ongoing clinical trials in Qidong, China, have indicated that oltipraz, a chemopreventive agent, can lower the biologically effective dose of aflatoxins by decreasing the metabolism of aflatoxin to its carcinogenic form and increasing the detoxification pathways of these metabolites. Intervention with chemicals such as these, alongside hepatitis B virus immunization programmes and improved storage conditions of staple foods, are prevention measures that can be undertaken to reduce the incidence of HCC in high-risk regions.

p53 mutation spectrum and load: the generation of hypotheses linking the exposure of endogenous or exogenous carcinogens to human cancer

The activation of protooncogenes and inactivation of tumor suppressor genes in affected cells are considered as the core events that provide a selective growth advantage and clonal expansion during the multistep process of carcinogenesis. Somatic mutations, induced by exogenous or endogenous mechanisms, were found to alter the normal functions of the p53 tumor suppressor gene. p53 is the most prominent example of tumor suppressor genes because it is mutated in about half of all human cancer. In contrast to other tumor suppressor genes (like APC and RB), about 80% of p53 mutations are missense mutations that lead to amino acid substitutions in proteins and can alter the protein conformation and increase the stability of p53. These changes can also alter the sequence-specific DNA binding and transcription factor activity of p53. These abnormalities can abrogate p53 dependent pathways involved in important cellular functions like cell-cycle control, DNA repair, differentiation, genomic plasticity and programmed cell death. A number of different carcinogens have been found to cause different characteristic mutations in the p53 gene. For example, exposure to ultraviolet light is correlated with transition mutations at dipyrimidine sites; aflatoxin B(1) exposure is correlated with a G:C to T:A transversion that leads to a serine substitution at residue 249 of p53 in hepatocellular carcinoma; and exposure to cigarette smoke is correlated with G:C to T:A transversions in lung carcinoma. Therefore, measuring the characteristic p53 mutation load or frequency of mutated alleles in nontumorous tissue (before the clonal expansion of mutated cells), can generate hypotheses, e.g., providing a molecular linkage between exposure to a particular carcinogen and cancer, and identifying individuals at increased cancer risk.

DNA damage by mycotoxins

Mycotoxins are toxic fungal metabolites which are structurally diverse, common contaminants of the ingredients of animal feed and human food. To date, mycotoxins with carcinogenic potency in experimental animal models include aflatoxins, sterigmatocystin, ochratoxin, fumonisins, zearalenone, and some Penicillium toxins. Most of these carcinogenic mycotoxins are genotoxic agents with the exception of fumonisins, which is currently believed to act by disrupting the signal transduction pathways of the target cells. Aflatoxin B1 (AFB1), a category I known human carcinogen and the most potent genotoxic agent, is mutagenic in many model systems and produces chromosomal aberrations, micronuclei, sister chromatid exchange, unscheduled DNA synthesis, and chromosomal strand breaks, as well as forms adducts in rodent and human cells. The predominant AFB1-DNA adduct was identified as 8, 9-dihydro-8-(N7-guanyl)-9-hydroxy-AFB1 (AFB1-N7-Gua), which derives from covalent bond formation between C8 of AFB1-8,9-epoxides and N7 of guanine bases in DNA. Initial AFB1-N7-guanine adduct can convert to a ring-opened formamidopyrimidine derivative, AFB1-FAPY. The formation of AFB1-N7-guanine adduct was linear over the low-dose range in all species examined, and liver, the primary target organ, had the highest level of the adduct. Formation of initial AFB1-N7-guanine adduct was correlated with the incidence of hepatic tumor in trout and rats. The AFB1-N7-guanine adduct was removed from DNA rapidly and was excreted exclusively in urine of exposed rats. Several human studies have validated the similar correlation between dietary exposure to AFB1 and excretion of AFB1-N7-guanine in urine. Replication of DNA containing AFB1-N7-guanine adduct-induced G-->T mutations in an experimental model. Activation of ras protooncogene has been found in AFB1-induced tumors in mouse, rat, and fish. More strikingly, the relationship between aflatoxin exposure and development of human hepatocellular carcinoma (HHC) was demonstrated by the studies on the p53 tumor suppressor gene. High frequency of p53 mutations (G-->T transversion at codon 249) was found to occur in HHC collected from populations exposed to high levels of dietary aflatoxin in China and Southern Africa. Furthermore, AFB1-induced DNA damage and hepatocarcinogenesis in experimental models can be modulated by a variety of factors including nutrients, chemopreventive agents, and other factors such as food restriction and viral infection, as well as genetic polymorphisms.

The human p53 gene mutated at position 249 per se is not sufficient to immortalize human liver cells

A particular point mutation of the tumor suppressor gene p53, namely a G-->T transversion at the third base of codon 249, is frequently detected in primary hepatocellular carcinomas from patients living in areas where the levels of dietary exposure to aflatoxin B1 and the rates of infection with the hepatitis B virus are very high. Very recently, a nontumorigenic liver epithelial cell line (HACL-1) with a finite life-span and expressing a number of hepatocyte-specific markers was established from a human hepatocellular adenoma in our laboratory. To analyze the role of mutated p53 in the immortalization of human liver cells, we transfected HACL-1 cells with an expression vector containing a human p53 complementary DNA mutated at the third base of codon 249 and analyzed the consequences of this gene transfer on the growth properties of this cell line. HACL-1 cells transfected with mutant p53 showed no increase in their life-span (when compared with HACL-1 cells transfected with the antibiotic resistance gene alone) and did not grow in soft agar, whereas transfection of wild-type p53 into HACL-1 cells led to a proliferation stop. Thus, these results strongly support the view that the mutation at codon 249 of the p53 gene may serve as a fingerprint for aflatoxin B1-induced hepatocellular carcinomas, but is not, by itself, sufficient to immortalize human liver cells.

Molecular epidemiology of human cancer

A challenging goal of molecular epidemiology is to identify an individual's risk of cancer. Molecular epidemiology integrates molecular biology, in vitro and in vivo laboratory models, biochemistry and epidemiology to infer individual cancer risk. Molecular dosimetry of carcinogen exposure is an important facet of molecular epidemiology and cancer risk assessment. Carcinogen macromolecular adduct levels, cytogenetic alterations and somatic cell mutations can be measured to determine the biologically effective doses of carcinogens. Molecular epidemiology also explores host cancer susceptibilities, such as carcinogen metabolism, DNA repair, and epigenetic and genetic alterations in tumor suppressor genes. p53 is a prototype tumor suppressor gene and is well suited for analysis of mutational spectrum in human cancer. The analyses of germ line and somatic mutation spectra of the p53 tumor suppressor gene provide important clues for cancer risk assessment in molecular epidemiology. For example, characteristic p53 mutation spectra have been associated with: dietary aflatoxin B1 exposure and hepatocellular carcinoma; sunlight exposure and skin carcinoma; and cigarette smoking and lung cancer. The mutation spectrum also reveals those p53 mutants that provide cells with a selective clonal expansion advantage during the multistep process of carcinogenesis. The p53 gene encodes a multifunctional protein involved in the cellular response to stress including DNA damage and hypoxia. Certain p53 mutants lose tumor suppressor activity and gain oncogenic activity, which is one explanation for the commonality of p53 mutations in human cancer. Molecular epidemiological results can be evaluated for causation by inference of the Bradford-Hill criteria, i.e., strength of association (consistency, specificity and temporality) and biological plausibility, which utilizes the "weight of the evidence principle."

Molecular and cellular pathology of hepatocellular carcinoma

Mutations of the p53 gene are common in hepatocellular carcinoma (HCC) and have been found in 13-33% of HCC in Asia and 23% of HCC in Hong Kong. In addition, p53 overexpression has been found to be associated with poorer cellular differentiation and larger tumour size and may be a late event in hepatocarcinogenesis. The p53 gene is important in controlling cell cycle, apoptosis and DNA repair. The cyclin-dependent kinase inhibitor p21^WAF1/CIP1 , which is downstream of p53, is regulated by both p53-dependent and p53-independent pathways. We found that HCC with p53 mutations had lower levels of p21 expression than those without p53 mutations. Moreover, p21 protein expression of the tumours was significantly higher in the tumours than in the corresponding non-tumorous livers. When the tumours were stratified into two groups, those with higher expression were found to have a significantly lower incidence of multiple tumour nodules and lower incidence of tumour microsatellite formation. p21 Expression was, however, not associated with p53 expression. Higher p21 expression is associated with solitary tumour nodules and fewer tumour microsatellites, but may not be enough to suppress tumour progression. Insulin-like growth factor II (IGF-II) gene has complex regulation of transcription resulting in multiple mRNA being produced and different mRNA occur in the adult and foetus. Using northern blot analysis, repression of normal adult promoter and re-expression of foetal promoters of IGF-II are common events in HCC, with repression of the normal adult promoter in 93% of the HCC transcripts and re-expression of the foetal transcripts (6 and 5 kb, respectively) in 40% of tumours. In addition, IGF-II expression was significantly more frequent in older patients. This may suggest that spontaneous expression of IGF-II late in life may promote the growth of tumours which have already arisen through other mechanisms, but foetal re-expression, itself, may not be enough to contribute to tumour progression.

p53 point mutation enhanced by hepatic regeneration in aflatoxin B1-induced rat liver tumors and preneoplastic lesions

Aflatoxin B1 (AFB1) is a well-known mutagen and carcinogen which induces human hepatocellular carcinoma (HCC). It has been found to be an important factor in inducing a high frequency of codon 249 mutation in the p53 gene. We characterized p53 mutations in specimens from preneoplastic lesions or tumors from the liver of rats induced by AFB1 with or without regeneration by partial hepatectomy treatment. PCR-SSCP and direct sequencing were used for screening and identification of p53 gene mutations in these samples. In rats treated with AFB1 with or without liver regeneration, 29% (5/17) of rats with hepatoma or neoplastic lesion had p53 mutation. No p53 mutations were found in the tumor samples from the rats without liver regeneration. However, in samples from the rats with liver regeneration, 38% (5/13) of the rats with hepatoma or neoplastic lesion were found to have p53 mutations. In one of these samples, we also observed a transversion mutation G --> T on codon 247, compared to codon 249 in humans. These findings suggest that the process of hepatocarcinogenesis induced by AFB1 does not necessarily involve p53 mutation, but mutation of the p53 gene can be enhanced by liver regeneration and that there is a possibility of a high mutability of the third base of codon 247, even though there is a small probability of detecting such a mutation in rats due to its silent mutation.

Aflatoxin sufferer and p53 gene mutation in hepatocellular carcinoma

AIM: To study the p53 gene mutation and its relationship to aflatoxin B1 exposure in hepatocellular carcinoma (HCC).

METHODS: Restriction fragment length polymorphism analysis method was used in 62 HCC samples, and DNA direct sequencing in another 45 HCC samples.

RESULTS: In HCC and AFB1 high and low-risk areas, 36/52 (69%) and 2/10 (20%) cases were found losing the HaeIII allele respectively, suggesting one of the base G mutation at the p53 gene codon 249. Similar results appeared in DNA direct sequencing, 20/35 (57%) and 1/10 (10%) respectively mutated at the codon 249 third base G to C transversion.

CONCLUSION: In HCC after AFB1 exposure, mutation of p53 gene is fixed at codon 249 third base and take the form of G to T transversion. This is a definite marker of mutation which is induced by AFB1 mutagen. It is applicable for molecular epidemiologic survey of the sufferers of AFB1 among HCC cases and for discovering more unknown natural AFB1 contaminated areas.

Expression of insulin-like growth factor II mRNA in hepatocellular carcinoma

Insulin-like growth factor II (IGF-II) is a mitogenic polypeptide closely related to insulin. Its gene has complex regulation of transcription, resulting in multiple mRNA initiated by different promoters. To study its role in hepatocarcinogenesis, we examined the expression of IGF-II mRNA in hepatocellular carcinomas (HCC) and correlated it with the pathological features of the tumours. Using northern blot analysis, transcription of the normal adult promoter was repressed in all but two of the 30 HCC. Instead, there was re-expression of two foetal transcripts (6 and 5 kb) in 12 tumours. In contrast, most (93.3%) of the non-tumorous livers showed expression of adult transcript only, and there were three livers demonstrating expression of foetal transcripts in addition to the adult one. There was a significant association of IGF-II expression with direct tumour invasion into the adjacent liver parenchyma but foetal expression did not influence other parameters directly related to tumour invasiveness, including venous permeation, formation of tumour microsatellites and positive resection margin. Besides, IGF-II expression was significantly more frequently seen in tumours from older patients. To conclude, repression of normal adult promoter and re-expression of foetal promoters of IGF-II are common events in HCC. The observation that foetal IGF-II expression was significantly more frequent in older patients suggests that spontaneous foetal expression of IGF-II late in life may promote the growth of tumours which have already arisen through other mechanisms, but foetal re-expression itself is not enough to contribute to tumour progression.

Mutations of the p53 tumor suppressor gene and ras oncogenes in aflatoxin hepatocarcinogenesis

Aflatoxin B1 (AFB1) is classified as a group I carcinogen in humans by IARC. However, the exact mechanisms of AFB1 hepatocarcinogenesis have not been fully elucidated. Recent studies have suggested that oncogenes are critical molecular targets for AFB1, and AFB1 causes characteristic genetic changes in the p53 tumor suppressor gene and ras protooncogenes. Up to date, more than 1500 human hepatocellular carcinoma (HCC) samples have been examined for p53 mutations with respect to different AFB1 exposure levels. The most significant finding is that more than 50% of HCC patients from high aflatoxin exposure areas such as southern Africa and Qidong, China harboured a codon 249 G to T transversion in the p53 tumor suppressor gene, which is found to be consistent with the mutagenic specificity of AFB1 observed in vitro. In contrast, this mutational pattern is not found in HCC samples from moderate or low aflatoxin exposure countries or regions. Therefore, this hot-spot mutation is believed to be a molecular fingerprint linking the initial event of AFB1-DNA adduct formation with the ultimate development and progress of human HCC. However, some important points still remain to be explicated. First, in many of these studies, the systematic evaluation of AFB1 exposure is rather limited and the classification of AFB1 exposure level is speculative and confusing, without the definite evidence for the actual aflatoxin exposure level. Second, the role of hepadnaviral infection has to be considered in the induction of this unique mutational spectrum. On the other hand, ras oncogene mutations are frequently found in AFB1-induced HCC samples in experimental animals, while the frequency of ras mutation in human HCC in contrast is much lower than that of p53. Recent studies have provided additional evidence that reactive oxygen species (ROS) and oxidative DNA damage may be involved in AFB1-induced p53 and ras mutations. In future, follow-up cohorts exposed to different levels of AFB1 combined with the determination of putative gene markers are much needed.

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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