Mutational hotspot in the p53 gene in human hepatocellular carcinomas

p53 mutations in primary human lung tumors and their metastases

In a total of 26 primary human lung tumors and 60 metastases derived from them, exons 5-8 of the p53 tumor suppressor gene were analyzed by single-strand conformation polymorphism and subsequent direct DNA sequencing of amplified DNA. Mutational inactivation of the p53 gene was identified in four of five squamous cell carcinomas, three of nine adenocarcinomas, and two of nine small-cell carcinomas, the overall incidence being 35%. Point mutations occurred at a similar incidence in exons 5-8, with a preference for G-->T transversions. In seven of nine cases (78%), mutations were identical in the primary tumor and all of its metastases, indicating that in lung tumors, p53 mutations usually precede metastasis and that hematogenic and lymphogenic dissemination of tumor cells to other tissues is not associated with a selection against p53 inactivation. In one case, a kidney metastasis had the same mutation as the primary squamous cell carcinoma, whereas a liver metastasis had no mutation, indicating heterogeneity of the primary lung neoplasm and selective metastasis of mutated and nonmutated tumor cells to kidney and liver, respectively. Only in one liver metastasis was a mutation identified that was neither present in the primary lung tumor nor in a kidney metastasis, suggesting that occasionally p53 mutations occur after metastatic spread.

p53 oncogene mutations in human prostate cancer specimens

Eighty-five prostate cancer specimens from prostate resections were analyzed for the presence of p53 gene mutations by immunohistochemical staining for P53 protein. DNA from thirty-four of these samples was also analyzed for mutations in exons 5-8 by single-strand chain polymorphism (SSCP) analysis. One sample had P53 staining by immunohistochemistry, one sample was positive for a p53 mutation by SSCP, and one sample was positive by both techniques. Mutations in the two samples that were positive by SSCP were confirmed by nucleotide sequencing. In a separate study of ten lymph nodes that contained prostate cancer metastases, one had detectable P53 protein by immunohistochemical staining. Therefore, p53 mutations appear to be low frequency events in primary prostate cancer.

Aflatoxin B1-adduct formation in rat and human small bowel enterocytes

BACKGROUND/AIMS

Hepatic CYP3A enzymes have been implicated in the bioactivation of aflatoxin B1 (AFB1) to DNA binding metabolites. CYP3A enzymes are also abundant in the small bowel, and we therefore examined the ability of this tissue to form intracellular AFB1 adducts.

METHODS

Immunohistochemistry using a antibody to the stable AFB1-DNA adduct was performed on small bowel sections obtained from rats orally gavaged with AFB1 and on human small bowel biopsy specimens maintained in explant culture. 3H-AFB1 was instilled into a loop of small bowel of untreated rats and rats pretreated with the CYP3A inducer dexamethasone during vivisection. DNA was isolated from the loop 2 hours later and assayed for specific activity.

RESULTS

In both rats and humans, AFB1-adducts were detected exclusively in mature enterocytes in a pattern similar to the distribution of CYP3A enzymes. Induction of enterocyte CYP3A in rats resulted in an increase in enterocyte immunoreactive AFB1 adducts and in a 1.8-fold increase in 3H-AFB1-nucleic acid adducts (P = 0.01).

CONCLUSIONS

Intracellular AFB1 adducts are formed in the small intestine, and this reflects, at least in part, the catalytic activity of CYP3A enzymes. Because these AFB1 adducts should ultimately pass in stool, enterocyte CYP3A may represent a regulatable barrier to dietary aflatoxins.

Screening for p53 mutations in C3H/He mouse liver tumors derived spontaneously or induced with diethylnitrosamine or phenobarbitone

Distinct differences have been described in the development of C3H/He mouse liver tumors induced by the genotoxic carcinogen diethylnitrosamine (DEN) and by the nongenotoxic agent phenobarbitone (PB) in terms of pathology and the frequency of mutation at codon 61 of the Ha-ras oncogene. To further define the mechanisms involved, we screened the tumor suppressor gene p53 for mutations in exons 5, 7, and 8 using polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) analysis. Nearly all the mutations so far described have been found within these three exons. In this study a total of six spontaneous tumors, eight tumors induced by PB, 14 tumors induced by DEN, and five samples of normal liver tissue were screened, and no mutations were found in any of the tumors examined. The positive control, the plasmid LTRp53cG (val), had a point mutation in exon 5 that was detected by PCR-SSCP. Since many of the tumors were late-stage hepatocellular carcinomas, we concluded that mutations in exons 5, 7, and 8 of the p53 gene do not play an important role in the development of chemically induced liver tumors in the C3H/He mouse.

UV and skin cancer: specific p53 gene mutation in normal skin as a biologically relevant exposure measurement

Many human skin tumors contain mutated p53 genes that probably result from UV exposure. To investigate the link between UV exposure and p53 gene mutation, we developed two methods to detect presumptive UV-specific p53 gene mutations in UV-exposed normal skin. The methods are based on mutant allele-specific PCRs and ligase chain reactions and designed to detect CC to TT mutations at codons 245 and 247/248, using 10 micrograms of DNA samples. These specific mutations in the p53 gene have been reported in skin tumors. CC to TT mutations in the p53 gene were detected in cultured human skin cells only after UV irradiation, and the mutation frequency increased with increasing UV dose. Seventeen of 23 samples of normal skin from sun-exposed sites (74%) on Australian skin cancer patients contained CC to TT mutations in one or both of codons 245 and 247/248 of the p53 gene, and only 1 of 20 samples from non-sun-exposed sites (5%) harbored the mutation. None of 15 biopsies of normal skin from non-sun-exposed or intermittently exposed sites on volunteers living in France carried such mutations. Our results suggest that specific p53 gene mutations associated with human skin cancer are induced in normal skin by solar UV radiation. Measurement of these mutations may be useful as a biologically relevant measure of UV exposure in humans and as a possible predictor of risk for skin cancer.

Duck hepatitis B virus infection, aflatoxin B1 and liver cancer in domestic Chinese ducks

The oncogenicity of Duck hepatitis B virus (DHBV) is unclear since hepatocellular carcinomas (HCCs) have been reported only in domestic ducks in Qidong, an area of China where hepatitis B virus (HBV) and aflatoxin B1 (AFB1) are risk factors for liver cancer in man. In order to better define the association between DHBV infection, AFB1 and HCC we analysed a series of 16 duck liver samples collected from local farms in Qidong. HCC was found in eight and cirrhosis in one of these samples. Furthermore bile duct proliferation, characteristic of AFB1 exposure in ducks and other animal species, was found in these ducks. Integration of DHBV DNA into cellular DNA was observed in only one out of four DHBV positive HCCs, indicating that viral integration is not prerequisite for tumour development. In four remaining HCCs the polymerase chain reaction (PCR) failed to show any DHBV DNA suggesting that liver tumours do occur in polymerase chain reaction (PCR) failed to show any DHBV DNA suggesting that liver tumours do occur in these ducks in the absence of DHBV infection. In addition, AFB1-DNA adducts were detected by hplc-immunoassay in one such DHBV-negative tumour. In summary we demonstrate that risk factors other than DHBV, including AFB1 exposure, may be important in duck liver carcinogenesis in Qidong.

Historical origins of current concepts of carcinogenesis

The first attempts to understand the causes of cancer were based on generalizations of what might now be termed a "holistic" nature, and hereditary influences were recognized at an early stage; these views survive principally through a supposed positive connection between psychological factors such as stress and diminished ability to combat the progressive development of tumors through some form of immunologically mediated rejection of potentially cancerous cells. While evidence for immunosurveillance is generally accepted, it is now widely regarded as almost wholly confined to instances where tumor viruses are involved as causative agents. The earliest theorists drew an analogy between the processes of carcinogenesis and of evolution; the cancer cells acquired the ability to outstrip their normal counterparts in their capacity for proliferation. This was even before evolution had been interpreted as involving a continuous succession of mutations. Evidence was already to hand before the end of the 18th century that exogenous agents, notably soot, a product of the "industrial revolution," could cause skin cancer. Somewhat over 100 years later, another industrial innovation, the manufacture of synthetic dyestuffs, implicated specific chemical compounds that could act systemically to cause bladder cancer. Meanwhile, the 19th century saw the establishment of the fundamentals of modern medical science; of particular relevance to cancer was the demonstration that it involved abnormalities in the process of cell division. The commencement of the 20th century was marked by a rediscovery of the concept of mutation; and it was proposed that cancer originated through uncontrolled division of somatically mutated cells. At around this time, two further important exogenous causative agents were discovered: X-rays and tumor viruses. In the late 1920s, x-radiation became the first established exogenous cause of mutagenesis. The discoverer of this phenomenon, H. J. Muller, suggested that while mutation in a single cell was the primary causative mechanism in carcinogenesis, its generally observed logarithmic increase in incidence with age reflected a "multihit" process, and that multiple successive mutations were required in the progeny of the original mutants. He also recognized that the rate of proliferation of potentially cancerous cells would markedly influence the probability of their subsequent mutation. These considerations are essentially the foundation of the generally accepted view of carcinogenesis that now seems unlikely to be superseded. However, this acceptance did not come about unopposed. The analogy between carcinogenesis and evolution was disliked by many biologists because it embodied the concept that cancer was an inevitable consequence of our evolutionary origins.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of mutant p53 protein in hepatocellular carcinoma

p53 mutations are a common genetic finding in hepatocellular carcinoma from areas of high aflatoxin exposure. Recent small studies have shown that p53 gene mutations may be less common in areas with a low prevalence of hepatocellular carcinoma such as Great Britain. The protein product of mutant p53 can be detected immunohistochemically because of its longer half life in comparison with native protein. This study used a novel monoclonal antibody DO-7, raised against recombinant p53 and effective in routinely processed biopsy specimen tissue, to detect the mutant protein in a series of 45 cases of hepatocellular carcinoma occurring in white subjects resident in the United Kingdom. Focal nuclear labelling was seen in four cases (9%); surrounding cirrhotic tissue in one of these was negative for p53 expression. This study shows that p53 mutations are a rare event in hepatocarcinogenesis in Great Britain, an area of low aflatoxin exposure, and supports the concept of geographical variations in the cause and pathogenesis of hepatocellular carcinoma.

Molecular mechanisms of radiation oncogenesis

Resolving the molecular mechanisms of radiation oncogenesis represents an important but daunting challenge in radiation research. This brief review outlines the principal oncogenic mechanisms that need to be considered in the context of radiation effects on the genome, how these might relate to specific gene and chromosomal changes relevant to neoplasia and the possible implications of such knowledge for the modelling of cancer risk. The long-term application of this mechanistic knowledge to the determination of tumour causality and for the assessment of individual cancer risk is also briefly discussed.

G-->A mutations in p53 and Ha-ras genes in esophageal papillomas induced by N-nitrosomethylbenzylamine in two strains of rats

In human esophageal cancers, no ras gene mutations but a relatively high prevalence of p53 gene mutations have been reported. We found a high prevalence of point mutations in Ha-ras and p53 genes in N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumors in two strains of rats (BD VI and F344). Our analysis showed the point mutation GGA-->GAA (expected from the known mechanisms of action of NMBA) at Ha-ras codon 12 in 22 of 46 (48%) and 22 of 38 (58%) papillomas from BD VI and F344 rats, respectively. There was no significant difference in the prevalence of ras mutations in tumors induced by high doses (5.0 mg/kg) and low doses (2.5 mg/kg) of NMBA. Eleven papillomas from each strain were analyzed for p53 mutations. The prevalent mutations found were G-->A and C-->T transitions. The frequency of p53 mutation was 36% (four of 11) for each strain. No apparent hot-spot codon or exon was found in the p53 gene, and two papillomas contained double mutations in this gene. The high prevalence of G-->A mutations in the rat Ha-ras gene contrasts with that in the human gene, in which no ras mutations have been found in primary tumors, and suggests either that the biology of esophageal carcinogenesis differs in humans and rats or that nitrosamines are not the major etiological risk factor for human esophageal cancers.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 6. Estimation of genetic risks of exposure to chemical mutagens: relevance of data on spontaneous mutations and of experience with ionizing radiation

This paper examines the impact of advances in knowledge on the molecular biology of human Mendelian diseases on the estimation of genetic risks of exposure to ionizing radiation and to chemical mutagens. More specifically, it addresses the question of whether and to what extent naturally occurring Mendelian diseases can be used as a baseline for efforts in this area. Data on the molecular nature and mechanisms of origin of spontaneous mutations underlying naturally occurring Mendelian diseases and on radiation-induced mutations in experimental systems suggest that for ionizing radiation, naturally occurring Mendelian diseases may not constitute an entirely adequate frame of reference and that current risk estimates for this class of diseases are conservative; these estimates however provide a margin of safety in formulating radiation protection guidelines. Currently available data on mechanisms and specificities of action of chemical mutagens, molecular dosimetry, repair of chemically induced adducts in the DNA, adduct-mutation relationships etc., permit the tentative conclusion that naturally occurring Mendelian diseases may provide a better baseline for genetic risk estimation for chemical mutagens than for ionizing radiation. With both ionizing radiation and chemical mutagens, the question of which Mendelian diseases are potentially inducible will become answerable in the near future when more molecular data on human genetic diseases become available. It is therefore essential that risk estimators keep abreast of advances in human genetics and integrate these into their conceptual framework. However, induced Mendelian diseases (especially the dominant ones which are of more immediate concern) are likely to represent a very small fraction of the adverse genetic effects of induced mutations. More attention therefore needs to be devoted to studies on the heterozygous effects of induced mutations.

Alterations of the p53 tumor-suppressor gene in transformed mouse liver cells

Mutational inactivation of p53, a potential tumor-suppressor gene, has been found in many tumors of humans as well as rodents. The p53 status in normal and transformed mouse liver cell lines has, however, not been investigated. We examined possible point mutations and compared mRNA and protein expression of the p53 gene in normal vs. transformed mouse liver cells. The transformed cells studied included lines spontaneously transformed by sub-culture, virally transformed by simian virus 40 (SV40), and chemically transformed by N-methyl-N-nitro-N-nitrosoguanidine (MNNG) or methylcholanthrene epoxide (MC). A heterozygous G-->A point mutation at codon 241, position 1, of p53 was detected in MNNG-transformed cells after screening of 5 evolutionarily conserved regions where mutation hot-spots are clustered. The mutation causes a gly-->arg substitution. No mutations were found in normal or other transformed cells. The steady-state levels of p53 mRNA were decreased in chemically transformed (both MNNG- and MC-transformed) cells. Elevated levels of p53 protein were found in spontaneously transformed and SV40-transformed cells, an observation that may reflect a longer half-life of the protein, as has been shown in other transformed lines. The low level of the p53 protein in MC-transformed cells may result from transcriptional depression of the p53 gene. We conclude from these data that abnormal p53 status, such as point mutation or altered expression, may play a role during the malignant transformation of mouse liver cells.

Comparative analysis of p53 and c-myc expression and cell proliferation in human hepatocellular carcinomas--an enhanced immunohistochemical approach

Expression of p53 and c-myc was investigated and compared with cell proliferative activity in a series of 40 hepatocellular carcinomas (HCC), by means of enhanced immunohistochemistry. p53 expression was demonstrated in 5 out of 40 HCC (12.5%) with the incidence increasing in 5 out of 40 HCC (12.5%) with the incidence increasing in proportion to the histological grading of malignancy: thus, 0% of well-differentiated, 6.9% of moderately differentiated and 33.3% of poorly differentiated lesions were positive. The proliferating-cell nuclear antigen (PCNA) labeling index also showed a statistically significant increase with this grading. Distribution patterns of PCNA-positive cell were divided into four types: scatter, marginal, mosaic and diffuse. Four HCC cases, predominantly of the poorly differentiated type, exhibited the diffuse pattern. Generally, p53 overexpression corresponded well with PCNA positivity. In contrast, there was no correlation between c-myc overexpression, found in 19 out of 40 HCC (47.5%), and histological grading of HCC or PCNA labeling index. The distribution pattern of c-myc-positive HCC cells was also different from that of PCNA and p53. Our results suggest that p53 overexpression closely relates to proliferation of HCC cells. Furthermore, there may be a consistent difference in regulatory mechanisms between p53 and c-myc expression in multistep hepatocarcinogenesis.

Current pathogenetic and molecular concepts in viral liver carcinogenesis

Hepatocellular carcinoma (HCC) is one of the most frequent malignancies in humans and in most cases a consequence of chronic infection of the liver by hepatotropic viruses (Hepatitis B Virus (HBV) and possibly Hepatitis C Virus (HCV)). Formation of HCC results from a stepwise process involving different preneoplastic lesions that reflect multiple genetic events, like protooncogene activation, tumor suppressor gene inactivation, and growth factor over- or reexpression. Recent investigations have gained new insights into how these factors are activated and may interact. In addition, improved knowledge of the molecular biology of HBV has led to better understanding of its pleiotropic effects on induction and progression in hepatocarcinogenesis.

Immunohistochemical detection of mutant P53 protein and human papillomavirus-related E6 protein in anal cancers

PURPOSE

The wild-type P53 protein, a product of the P53 gene, is a normal growth controlling protein. Mutation of the P53 gene generates a mutant P53 protein which promotes tumor formation through loss of growth suppression. Some of the agents responsible for P53 gene mutation are known, one of which may be tumorigenic human papillomavirus (HPV) infection. Anal cancers are demonstrating a changing trend in the affected population, from older females in the older reported series to younger males more recently. This may be a reflection of infection with tumorigenic HPV types 16 and 18. The E6 oncoprotein of these viruses inactivates the growth-controlling wild-type P53 protein. In this study, our purpose was to determine the incidence of mutant P53 and HPV-16 and 18-related E6 protein and their coexpression in anal cancers.

METHODS

We examined 29 anal cancers immunohistochemically for mutant P53 protein, HPV 16 and 18 E6 protein, and coexpression of the two.

RESULTS

Mutant P53 protein was present in 58.6 percent of anal cancers overall and in 85.7 percent of anal adenocarcinomas. E6 oncoprotein was present in five cases (17.2 percent), all of which were squamous-cell carcinomas. Coexpression of both mutant P53 and E6 proteins was seen in only three cases (10.3 percent).

CONCLUSION

Although tumorigenic HPV may be an important cause for P53 gene mutation in anal cancers, perhaps other mutagenic factors play a predominant role.

An atypical carcinoid tumor of the lung with mutations in the p53 gene and the retinoblastoma gene

Molecular analysis of a metastatic lesion of an atypical carcinoid tumor of the lung obtained from a 77-year-old man at autopsy revealed a point mutation in the p53 gene and a deletion in the retinoblastoma (Rb) mRNA. This case suggests that both these antioncogenes may be involved in the progression of atypical carcinoid tumor.

Identification of DNA adducts at specific locations by sequencing techniques

1. A simple procedure for the accurate identification of the positions of adduct formation when viral M13mp18 single stranded DNA is treated with chloroacetaldehyde (CAA) is presented. 2. The normal dideoxy sequencing reaction protocol was employed except no dideoxy nucleotides were used to cause chain termination. 3. Instead, the CAA adducted bases of the template DNA act as points of polymerase fall off causing the appearance of bands of DNA sequencing gel autoradiographs marking locations where adduct formation may have occurred.

p53 protein in aggressive and non-aggressive basal cell carcinoma

Basal cell carcinoma (BCC) is the most frequent cutaneous neoplasm, with a generally favorable clinical behavior. Sometimes, indeed, it recurs after therapy and/or metastasizes. As point mutations in the coding sequence of the p53 tumor suppressor gene have been implicated in the progression of many human tumors, we studied the expression of p53 protein on this neoplasia. We tested immunohistochemically the positivity for p53 protein (NCL-p53-CM1, YLEM) on 19 cases of morphologically "non aggressive" BCC (BCC1) and on 19 "aggressive" BCC (BCC2), all with one or more relapses and 3 with distant metastases also. Results were related to clinico-pathological and follow-up data. All but one BCC2 were found positive for p53 protein. Conversely, only 2 cases of BCC1 exhibited low immunoreactivity for p53 protein, with high statistical differences between the two groups. No correlation was found between the immunoreactivity, age of patients, and site of the lesions. The availability of immunohistochemistry and the relatively easy interpretation of the results make screening for p53 protein a possibly useful tool in the prognostic evaluation of BCC.

The mutation of codon 249 in the p53 gene is not specific in Japanese hepatocellular carcinoma

Hepatocellular carcinoma samples obtained from 59 patients at surgical resection were examined for mutations of the third base at codon 249 of the p53 gene, using the polymerase chain reaction and oligonucleotide hybridization techniques. This point mutation, which is frequently observed in HCC cases from Southern Africa and Quidong in China, was not recognized in either 60 hepatocellular carcinomas or 53 noncancerous liver tissue samples from Japan. Thirty-four of 45 patients (75.6%) were positive for the hepatitis C virus, which was a higher rate than that for hepatitis B virus infection (9 of 55; 16.4%). The exposure to aflatoxin B1 was not considered to be remarkable. These results suggest that the point mutation of the third base at codon 249 is not common in Japanese patients, and it is suggested that numerous other factors affect the mutation of the p53 gene and the development of hepatocellular carcinoma.

Infrequent involvement of p53 gene mutations in the tumourigenesis of Japanese prostate cancer

A study was made of the incidence of p53 mutations in Japanese males with prostate cancer or benign prostatic hyperplasia. Polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) was used as a primary screening technique with gene sequencing being carried out in positive cases. Two out of 21 prostate cancers (9.5%) were found to have p53 mutations. These were stage B2 and D2 prostate cancers. No abnormalities were found in the remaining cases or benign prostatic hyperplasia. Mutations of the p53 gene would thus appear infrequent in the tumourigenesis of primary prostate cancer.

Deletion of the E-cadherin gene in hepatitis B virus-positive Chinese hepatocellular carcinomas

Frequent allele loss from chromosome 16q was recently described for human tumors of the breast, prostate gland and liver, indicating the possible presence of a tumor-suppressor gene on that chromosome arm. In this study, the chromosome 16 allele status of 38 hepatocellular carcinomas in Chinese patients was determined with restriction-fragment-length polymorphism analysis. Tumor-specific allele loss was detected in 14 (74%) of 19 patients informative for 16p markers and in 22 (85%) of 26 patients informative for 16q markers. Quantitative densitometric analysis revealed reduction to hemizygosity of the E-cadherin cell adhesion gene (localized to 16q22.1) in 18 (64%) of the 28 patients for whom quantitative data were available. Reduced expression of E-cadherin has been associated with invasion and metastasis in several human cell lines and primary tumors, and our results suggest that one mechanism of reduced E-cadherin expression is the loss of one copy of the E-cadherin gene.

Infrequent involvement of p53 mutations and loss of heterozygosity of 17p in the tumorigenesis of renal cell carcinoma

Restriction fragment length polymorphism (RFLP) analysis and the polymerase chain reaction of the single-strand conformation polymorphism (PCR-SSCP) method were conducted to assess the loss of heterozygosity of chromosome 17p and mutations of the p53 gene in 30 surgical specimens of human renal cell carcinoma. Six of 29 tumors (20.6%) showed loss of heterozygosity on chromosome 17p in RFLP analysis, and in none of 21 tumors could a mutation be found on exons 5 to 8 of the p53 gene in PCR-SSCP analysis. We conclude that the p53 gene mutation does not play a role in the development of the majority of cases of renal cell carcinoma and that there may be another tumor suppressor gene on 17p.

Tumourigenesis associated with the p53 tumour suppressor gene

The p53 gene is contained within 16-20 kb of cellular DNA located on the short arm of human chromosome 17 at position 17p13.1. This gene encodes a 393-amino-acid nuclear phosphoprotein involved in the regulation of cell proliferation. Current evidence suggests that loss of normal p53 function is associated with cell transformation in vitro and development of neoplasms in vivo. More than 50% of human malignancies of epithelial, mesenchymal, haematopoietic, lymphoid, and central nervous system origin analysed thus far, were shown to contain an altered p53 gene. The oncoproteins derived from several tumour viruses, including the SV40 large T antigen, the adenovirus E1B protein and papillomavirus E6 protein, as well as specific cellular gene products, e.g. murine double minute-2 (MDM2), were found to bind to the wild-type p53 protein and presumably lead to inactivation of this gene product. Therefore, the inactivation of p53 tumour suppressor gene is currently regarded as an almost universal step in the development of human cancers. The current data on p53-associated tumourigenesis are briefly discussed in this minireview.

Aflatoxin B1 induces the transversion of G-->T in codon 249 of the p53 tumor suppressor gene in human hepatocytes

Approximately half of hepatocellular carcinoma (HCC) from regions in the world with high contamination of food with the mycotoxin aflatoxin B1 (AFB1) contain a mutation in codon 249 of the p53 tumor suppressor gene. The mutation almost exclusively consists of a G-->T transversion in the third position of this codon, resulting in the insertion of serine at position 249 in the mutant protein. To gain insight into the mechanism of formation of this striking mutational hot spot in hepatocarcinogenesis, we studied the mutagenesis of codons 247-250 of p53 by rat liver microsome-activated AFB1 in human HCC cells HepG2 by restriction fragment length polymorphism/polymerase chain reaction genotypic analysis. AFB1 preferentially induced the transversion of G-->T in the third position of codon 249. However, AFB1 also induced G-->T and C-->A transversions into adjacent codons, albeit at lower frequencies. Since the latter mutations are not observed in HCC it follows that both mutability on the DNA level and altered function of the mutant serine 249 p53 protein are responsible for the observed mutational hot spot in p53 in HCC from AFB1-contaminated areas. Our results are in agreement with an etiological role of AFB1 in hepatocarcinogenesis in regions of the world with AFB1-contaminated food.

Wild-type p53 differentially affects tumorigenic and metastatic potential of murine metastatic cell variants

The structure and the function of the p53 gene were studied in two metastatic cell variants derived from Lewis lung carcinoma. Single missense mutation at codon 334 was detected in the p53 gene of both cell variants. In spite of the identical mutation, the in vitro and in vivo growth rates of the two cell variants were differentially affected by the constitutive expression of exogenous wild-type (wt) p53 gene. In fact, only the more malignant cell line (C87) was severely affected by the wt-p53 gene introduction. However, the in vivo effects on this cell line were transient because during serial in vivo passages, cell populations lacking the wt-p53 gene were selected. Genetic mechanisms responsible for the resistance of the less metastatic cell variant (BC215) to the wt-p53 expression, were investigated. Intrinsic ability to mutate exogenous cDNA sequences was tested. We report that BC215 cells continued to express exogenous wt-p53 sequences after several in vitro passages. The expression of mdm2 gene was evaluated. The data demonstrated that BC215 cells constitutively express higher levels of mdm2 gene than C87 cells. We conclude that the overexpression of this gene might be responsible for the resistance of BC215 cells to exogenous wt-p53 gene expression.

Point mutations of the P53 gene, human hepatocellular carcinoma and aflatoxins

The tumor suppressor p53 exerts important protective functions towards DNA-damaging agents. Its inactivation by allelic deletions or point mutations within the P53 gene as well as complex formation of wildtype p53 with cellular or viral proteins is a common and crucial event in carcinogenesis. Mutations increase the half-life of the p53 protein allowing the immunohistochemical detection and anti-p53 antibody formation. Distinct G to T point mutations in codon 249 leading to a substitution of the basic amino acid arginine by the neutral amino acid serine are responsible for the altered functionality of the mutant gene product and were originally identified in 8 of 16 Chinese and 5 of 10 African HCC patients. Both groups are frequently exposed to mycotoxin contaminations of their food. Today an average P53 gene mutation rate of 25% is assumed for high-aflatoxin B1-exposure regions. This is double the rate observed in low-aflatoxin B1-exposure countries. Although many HCC patients displaying P53 mutations also suffer from HBV infection, which itself can lead to rearrangements of P53 coding regions or induce the synthesis of viral proteins possibly interacting with p53, the specific G to T transversion within codon 249 of the P53 gene seems to directly reflect the extent of aflatoxin B1 exposure.

Natural history of chronic hepatitis B virus infection: new light on an old story

Chronic hepatitis B virus (HBV) infection is one of the most common persistent virus infection in man. It causes significant morbidity and mortality, and therefore is important. Extensive studies on clinicopathologic studies and long-term follow up on hepatitis B surface antigen (HBsAg) carriers have largely disclosed the natural history of chronic HBV infection. The infection easily becomes chronic when contracted in early infancy. As high as 90% of babies born to HBV carrier mothers will also become HBsAg carriers. Once chronic infection is established, it is refractory, and HBsAg carriage usually persists for life. However, the chronic infection is not monotonous, it actually evolves from an HBV replicative phase to a non-replicative phase. The host responds differently and with more complexity in different phases. The virus-host interactions, divided into three phases, virus tolerance, virus clearance and residual HBV integrated phases, result in a heterogeneous variety of hepatic lesions. The first two phases occur when HBV is actively replicating, and the last corresponds to the non-replicative phase. The high HBV level (and hence HBV gene products) renders the host's immune system tolerant to the virus, and the infected host does not exert an effort to get rid of the virus. At this stage, the liver is nearly normal, and the host is asymptomatic. However, later in the replicative phase, the HBV replication begins to wane, and the immune tolerance is no longer maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

The molecular biology of carcinogenesis

Carcinogenesis may result from the action of any one or a combination of chemical, physical, biologic, and/or genetic insults to cells. The process of carcinogenesis may be divided into at least three stages: initiation, promotion, and progression. The first stage of carcinogenesis, initiation, results from an irreversible genetic alteration, most likely one or more simple mutations, transversions, transitions, and/or small deletions in DNA. The reversible stage of promotion does not involve changes in the structure of DNA but rather in the expression of the genome mediated through promoter-receptor interactions. The final irreversible stage of progression is characterized by karyotypic instability and malignant growth. Critical molecular targets during the stages of carcinogenesis include proto-oncogenes, cellular oncogenes, and tumor suppressor genes, alterations in both alleles of the latter being found only in the stage of progression. Although many of these critical target genes have been identified, the ultimate number and characteristics of molecular alterations that define neoplasia have not been elucidated.

Genetic changes and histopathological grades in human hepatocellular carcinomas

Loss of heterozygosity (LOH) on chromosomes 1p, 4q, 5q, 8p, 13q, 16q, 17p, and 22q, and mutation of the p53 gene were simultaneously analyzed in 63 hepatocellular carcinomas (HCCs) with distinct histopathological grades, 80% of the tumors being from patients who had been exposed to hepatitis B virus (HBV) or hepatitis C virus (HCV). The frequencies of LOH on 8 chromosomes were 0-25% in 10 well differentiated HCCs, LOH being observed on 4q, 5q and 17p, 21-53% in 26 moderately differentiated HCCs, LOH on 8p and 17p being high, and 29-75% in 27 poorly differentiated HCCs, LOH on 17p, 4q and 8p being the most frequent. p53 gene mutation was detected in moderately and poorly differentiated HCCs at 15% and 52%, respectively, but not at all in well differentiated HCCs. Of the mutations detected, 42% were transition mutation and only 5% were CpG transition, in contrast to the high frequencies of these types of mutations in colon tumors (78% and 54%, respectively). LOH on every chromosome and p53 mutation were more frequent in more advanced tumors, and accumulation of genetic changes increased with increase of the histopathological grade. Frequency of genetic changes in HCCs from HBV-positive patients was comparable to that from HCV-positive patients. The present results suggest that accumulation of genetic changes in multiple tumor suppressor genes, especially LOH on 17p, 4q and 8p, and mutation in p53 gene, are involved in the progression of liver cancer, and LOH on 17p and 4q precedes other genetic changes. Differences in the direction of p53 mutation between HCC and colon carcinoma suggest that liver carcinogens are distinct from colon carcinogens. Furthermore, mechanisms affecting the frequency of LOH in HCCs in HBV-infected patients may be similar to those in HCV-infected patients.

The molecular biology of carcinogenesis

Carcinogenesis may result from the action of any one or a combination of chemical, physical, biologic, and/or genetic insults to cells. The process of carcinogenesis may be divided into at least three stages: initiation, promotion, and progression. The first stage of carcinogenesis, initiation, results from an irreversible genetic alteration, most likely one or more simple mutations, transversions, transitions, and/or small deletions in DNA. The reversible stage of promotion does not involve changes in the structure of DNA but rather in the expression of the genome mediated through promoter-receptor interactions. The final irreversible stage of progression is characterized by karyotypic instability and malignant growth. Critical molecular targets during the stages of carcinogenesis include proto-oncogenes, cellular oncogenes, and tumor suppressor genes, alterations in both alleles of the latter being found only in the stage of progression. Although many of these critical target genes have been identified, the ultimate number and characteristics of molecular alterations that define neoplasia have not been elucidated.

Establishment of two distinct human hepatocellular carcinoma cell lines from a single nodule showing clonal dedifferentiation of cancer cells

Hepatocellular carcinomas often contain tumor cells of more than one histological grade. The clonal relationship and biological behavior of hepatocellular carcinoma cells in histologically heterogeneous areas have not been fully explored. We established two distinct human hepatocellular carcinoma cell lines (HAK-1A and 1B) from a single nodule showing a three-layered structure with a different histological grade in each layer. Morphologically, HAK-1A and 1B resembled well-differentiated hepatocellular carcinoma cells in the outer layer of the original tumor and poorly differentiated ones in the inner layer, respectively. HAK-1B appeared less differentiated morphologically and more aggressive biologically than HAK-1A; HAK-1B had a shorter doubling time, higher tumorigenicity and an aneuploid DNA index. Chromosome analysis revealed many different abnormalities in the two cell lines, in which, however, two identical structural abnormalities (2q+ and 17p+) were identified. Moreover, sequence analysis of the p53 gene showed identical mutations at codon 242 in both cell lines. These findings suggest that the two cell lines are of clonal origin and that hepatocellular carcinomas consisting of cancerous tissues of more than one histological grade may reflect clonal dedifferentiation in the tumor. Furthermore, we predict that a clonal, morphologically less differentiated subpopulation such as HAK-1B is more aggressive in proliferation and may be closely related to subsequent tumor progression in hepatocellular carcinoma.

Urinary aflatoxin levels, hepatitis-B virus infection and hepatocellular carcinoma in Taiwan

Using a urinary immunoassay to measure aflatoxin metabolites, we examined the associations between exposure to aflatoxin, chronic infection with the hepatitis-B virus (HBV) and background rates of hepatocellular carcinoma (HCC) mortality in a cross-sectional survey of 250 residents from 8 areas of Taiwan with a 4-fold variation in age-adjusted HCC mortality. Specimens of fasting blood and overnight urines were used to determine HBV carrier status and excretion of aflatoxin in the subjects surveyed. While the prevalence of hepatitis-B virus carriers showed moderate variability, there was a 500-fold range in urinary aflatoxin levels. Mean log-transformed levels of aflatoxin metabolites were similar in males and females and in HBV carriers and non-carriers. In the 8 townships, HCC mortality correlated positively with both area HBV carrier prevalence and mean aflatoxin levels. The primary analyses, however, were conducted at the individual level. Each subject's aflatoxin level was treated as the response variable in a multiple regression model, and the corresponding sex-specific area HCC rate was included as a predictor along with the individual's carrier status, age and sex; alcohol consumption and cigarette smoking were also considered. In these analyses, a significant association was again observed between the marker of aflatoxin exposure and the background rate of HCC mortality. In females, the slope of the regression line was somewhat steeper in HBV carriers, but this pattern was not seen in males and formal testing yielded no statistically significant evidence of an interaction. Our findings are consistent with the hypothesis that aflatoxin plays an independent role in hepatocellular carcinoma in Taiwan.

p53 gene mutations and 17p allelic deletions in hepatocellular carcinoma from Japan

BACKGROUND

p53 gene mutations at codon 249 have been reported in hepatocellular carcinoma (HCC) from China and South Africa, a phenomenon shown to be closely associated with food contamination by aflatoxin. There have been few reports, however, in regard to p53 gene mutations in HCC from other geographic areas.

METHODS

The authors analyzed 20 HCC from Japan for alteration of the p53 gene by restriction fragment length polymorphisms and for nucleic acid mutations by polymerase chain reaction with direct sequencing.

RESULTS

Alterations associated with the p53 gene were found in 6 of 20 HCC (30%). Allelic loss of chromosome 17p occurred in 5 of 14 informative (heterozygous) cases (36%). Mutations in the p53 gene were detected in three cases (15%), at codons 176 (exon 5), 236 (exon 7), and 294 (exon 8). These cases were different from the HCC cases from China and South Africa, where point mutations in the p53 gene were reported at the same codon 249 in half of the cases and where aflatoxin food contamination and hepatitis B virus infection are recognized risk factors of HCC. No p53 gene alterations were found in smaller HCC (< 3 cm) or at earlier stages.

CONCLUSIONS

In Japan, p53 gene alterations seem to be a late event in the progression of hepatocarcinogenesis, which is often associated with persistent infection by the hepatitis C or B virus, but not usually with exposure to aflatoxin.

p53 mutation in gastric cancer: a genetic model for carcinogenesis is common to gastric and colorectal cancer

Mutation of the p53 tumour-suppressor gene in exons 5 through 8 was examined in 118 cases of gastric cancer (59 early gastric cancers and 59 advanced gastric cancers) using PCR-SSCP (polymerase-chain-reaction-single-strand-conformation polymorphism) analysis and direct sequencing. In early gastric cancer, mutations were found in 15 of 41 (37%) cases of the cohesive type, i.e., papillary adenocarcinoma, well to moderately differentiated tubular adenocarcinoma, and poorly differentiated adenocarcinoma with solid nests or focal tubular structures, but were not detected in 18 cases of the non-cohesive type, i.e., signet-ring-cell carcinoma and poorly differentiated adenocarcinoma growing in a scattered manner. In advanced gastric cancer, 25 of 59 (42%) cases of the cohesive type had p53 mutation. No significant association was found between p53 mutation and other histopathological parameters such as macroscopic classification, lymph-node involvement and depth of tumour invasion. Fifteen of 25 (60%) mutations in the advanced gastric-cancer group were accompanied by allele loss at the p53 gene locus. Eighty-three percent of mutations in early gastric cancer and 52% of mutations in advanced gastric cancer showed G:C-to-A:T transition, almost exclusively at CpG dinucleotide mutational hot spots, indicating that the spectrum of p53 mutation was similar to that of colorectal cancer. These data suggest that the p53 mutation occurs selectively in gastric cancer of the cohesive type from the intramucosal cancer stage.

In vitro cytochrome P450 monooxygenase and prostaglandin H-synthase mediated aflatoxin B1 biotransformation in guinea pig tissues: effects of beta-naphthoflavone treatment

In the present study, we examined the effects of treating guinea pigs with beta-naphthoflavone (BNF) on aflatoxin B1 (AFB1) metabolism by microsomal cytochrome P450 monooxygenase (P450) and prostaglandin H synthase (PHS) in liver, lung and kidney tissues. After BNF treatment, microsomal 7-ethoxyresorufin O-deethylase activity was induced 13-, 25- and 11-fold in lung, kidney and liver, respectively, confirming that the BNF treatment protocol was effective at inducing monooxygenase activity. Treatment of guinea pigs with BNF did not change [3H]AFB1-DNA binding catalyzed by microsomal PHS or P450 in lung, kidney or liver. In contrast, AFM1 formation by P450 was significantly increased in microsomes from all three organs. The data indicate that BNF-inducible P450 isozymes of the P4501A class are responsible for the biotransformation of AFB1 to non-toxic metabolites. Guinea pig kidney microsomes could also catalyze NADPH-dependent formation of aflatoxicol (AFL), a metabolite usually produced by a cytosolic steroid dehydrogenase. Renal microsomal AFL formation was not altered by prior BNF treatment. The results in the present study suggest that BNF may alter the bioactivation of AFB1 in guinea pig tissues by inducing P450 activity, leading to the formation of less reactive metabolite.

Tumor-suppressor p53 gene in hepatitis C and B virus-associated human hepatocellular carcinoma

Abnormalities of the tumor-suppressor p53 gene have been discovered in human hepatocellular carcinoma (HCC). It is unclear, however, whether HCC related to chronic viral hepatitis is associated with p53 gene alterations. In this study, we have examined p53 abnormalities in HCC associated with hepatitis C and B virus (HCV and HBV) infections. Tissues from 18 HCC patients from several hospitals throughout the United States were collected (9 were HCV-infected, 5 were HBV-infected, 1 was HCV/HBV-infected, and 3 were non-virus-associated). Immunostaining with monoclonal pAb 1801 revealed expression of p53 protein in tumor-cell nuclei in one HCV-associated HCC, and in no case of HBV-associated HCC, while the nuclei of adjacent hepatocytes were negative. Using Hae III-digestion of chromosomal DNA, mutations at codon 249 were not found in any of 18 HCC tissues studied. Direct sequencing demonstrated a mutated codon 244 and a wild-type codon 249 in the conserved regions (exon 5-8) of p53 gene from the tumor tissue with nuclear p53 expression. By reverse-transcription-polymerase chain reaction (RT-PCR), the expression of p53 mRNA was demonstrated in tumor cells from 10 out of 16 HCC tissues. In conclusion, the specific mutation at codon 249 with G to T transversion was not observed in the HCCs associated with HCV or HBV infections. In HBV or non-virus-associated HCCs studied, no other p53 gene abnormalities were found. A point mutation at codon 244 with G to A transition of p53 gene was detected in only one of 10 HCV-associated HCCs, which suggests that p53 mutations may not play a significant role in HCV- or HBV-associated hepatocarcinogenesis.

The expression of p53 antigen in primary malignant epithelial tumors of the liver: an immunohistochemical study

We examined the expression of mutant p53 gene products in primary malignant epithelial tumors of the liver. Fourteen of 68 hepatocellular carcinomas, one of seven hepatoblastomas and one of nine intrahepatic cholangiocarcinomas showed nuclear staining for p53 proteins. None of the surrounding non-tumorous tissues expressed nuclear staining. The detection of p53 proteins in tumor cells was significantly higher in hepatocellular carcinomas of Oriental patients (31.6%) compared to non-Orientals (6.7%, p < 0.015). No significant differences were seen in p53 antigen expression between hepatitis B and non-hepatitis B associated hepatocellular carcinomas in Oriental patients. These results suggest a role for other environmental factors, such as aflatoxin, in the etiology of p53 mutation in hepatocellular carcinoma in Oriental patients.

Analysis of germline and in vivo somatic mutations in the human adenine phosphoribosyltransferase gene: mutational hot spots at the intron 4 splice donor site and at codon 87

We have characterized 18 germline and 10 in vivo somatic mutations in the human adenine phosphoribosyltransferase (APRT) gene. Both germline and in vivo somatic mutations were clustered at the intron 4 splice donor site and at codon 87. In vitro somatic mutations in human APRT do not appear to show this clustering. These findings suggest that the spectrum of germline mutations in APRT may be similar to that incurred by somatic cells in vivo, but different from that seen in cultured cells. Thus, in vivo, rather than in vitro, somatic mutations in this gene may be more representative of mutational events occurring in the germline.

Introduction to molecular endocrine pathology

The applications of molecular techniques to endocrine pathology have been rapidly expanding in the past few years. This article reviews some of the basic concepts and techniques in molecular biology and summarizes recent applications of these techniques in the study of endocrine diseases. The applications of clonal analysis, hybridization techniques, single strand conformation polymorphism analysis, and the polymerase chain reaction technique in the study of neoplasms including specific mutations of oncogenes and suppressor genes in various endocrine tissues are discussed. The rapidly developing field of homeobox genes and their role in development including studies of normal pituitary development and pituitary tumor expression of the Pit-1 transcription factor, a member of the POU family of homeobox genes, are also summarized.Endocr Pathol 4:64-72, 1993.

High prevalence of mutations at codon 249 of the p53 gene in hepatocellular carcinomas from Senegal

In hepatocellular carcinoma, mutation within the p53 gene occurs mainly at codon 249 and its frequency has been associated with exposure to aflatoxin. As Senegal is a country where liver cancer incidence is one of the highest in the world and where people are highly exposed to aflatoxin, we screened 15 liver cancer samples from this country for mutation at codon 249 of the p53 gene. Non-tumoral DNA from the patients showed a wild type genotype. Mutation at codon 249 of the p53 gene was detected in 10 of the 15 tumour tissues tested (67%). This frequency of mutation in codon 249 of the p53 gene is the highest described. These results confirmed that there is an association between countries of high aflatoxin intake and a high frequency of mutation in codon 249 of p53 gene, and that HBV alone does not contribute to these base changes.