Mutational hotspot in the p53 gene in human hepatocellular carcinomas

Functional inactivation but not structural mutation of p53 causes liver cancer

Structural mutations in the p53 gene are seen in virtually every form of human cancer. To determine whether such mutations are important for initiating tumorigenesis, we have been studying hepatocellular carcinoma, in which most cases are associated with chronic hepatitis B virus infections. Using a transgenic mouse model where expression of a single HBV gene product, the HBx protein, induces progressive changes in the liver, we show that tumour development correlates precisely with p53 binding to HBx in the cytoplasm and complete blockage of p53 entry into the nucleus. Analysis of tumour cell DNA shows no evidence for p53 mutation, except in advanced tumours where a small proportion of cells may have acquired specific base substitutions. Our results suggest that genetic changes in p53 are late events which may contribute to tumour progression.

Transactivation of the human p53 tumor suppressor gene by c-Myc/Max contributes to elevated mutant p53 expression in some tumors

Elevated levels of mutant forms of the p53 tumor suppressor are a hallmark of many transformed cells. Multiple mechanisms such as increased stability of the protein and increased transcription of the gene can account for elevated p53 expression. Recent findings indicate that c-Myc/Max heterodimers can bind to an essential CA(C/T)GTG-containing site in the p53 promoter and elevate its expression. We have addressed the possibility that elevated mutant p53 expression is due to deregulated c-Myc expression. Here we demonstrate that the human p53 promoter is transactivated by high c-Myc expression and repressed by high Max expression. In examining the relative levels of c-Myc and p53 in human Burkitt's lymphomas and other B-lymphoid lines, we found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression. In particular, cells that express very low levels of c-Myc protein also express low levels of p53 mRNA, while cells that express high levels of c-Myc tend to express high levels of p53 mRNA. To determine whether the p53 gene can be a target for c-Myc in vivo, we assayed the effects of antisense c-myc RNA on the levels of endogenous p53 mRNA. The results indicate that the presence of antisense c-myc RNA leads to a reduction in the levels of c-Myc protein, p53 mRNA, and expression from the p53 promoter. Taken together, our findings support a direct role for c-Myc in elevating expression of the mutant p53 gene in some tumors.

The genetics of human cancer: implications for ecotoxicology

The study of human cancers has provided evidence that malignant progression is associated with genetic change. It has been suggested that some genetic alterations in tumors may be the result of direct or indirect processes related to environmental chemical exposure. This hypothesis has been supported by genetic evidence in liver tumors which has associated aflatoxin B1 exposure with the detection of inactivating DNA mutations within the human p53 tumor suppressor gene. The detection of activating ras oncogene mutations at high frequency in liver tumors of feral fish suggest that the survey of mutations in genes, such as p53 or other genes, might provide a genetic signature for specific chemical exposure in tissues of aquatic animals derived from environmentally damaged sites.

Activation of ras oncogene in livers with cirrhosis

Activation of cellular oncogenes and inactivation of anti-oncogenes have been postulated as important mechanisms during hepatocarcinogenesis. This study was conducted to detect abnormal levels of several proto-oncogenes (c-jun, c-fos, c-H-ras) and of the p53 and the alpha-fetoprotein gene in the liver during cirrhosis, a pathological process which predisposes to the development of hepatocarcinoma. Liver tissue from 11 patients with cirrhosis of different etiologies, and seven histologically normal liver fragments taken at the periphery of benign liver tumors of metastases were studied. Transcripts of the various oncogenes and of the alpha-fetoprotein gene were detected by in situ hybridization, and the p53 protein was revealed by immunocytochemistry. No overexpression of any of the mRNA tested or of the p53 protein was found in histologically normal liver in contact with benign or metastatic tumors. In contrast, 10 of the 11 specimens with cirrhosis (90.9%) displayed abnormally high levels of c-H-ras transcripts. Five samples with cirrhosis revealed a moderate increase in the level of c-fos mRNA. Only one case and two cases, respectively, exhibited increased levels of c-jun and alpha-fetoprotein mRNA. No cases were positive for the p53 antigen. Liver-cell proliferation, as assessed by immunocytochemistry with the Ki 67 monoclonal antibody, was low in both the group with cirrhosis and the control groups (0.49% and 0.55% positive cells, respectively). These data demonstrate that activation of c-H-ras mRNA is an almost constant finding in hepatocytes of livers with cirrhosis. This gene overexpression is not linked to hepatocellular proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)

Double P53 point mutation in extramedullary blast crisis of chronic myelogenous leukemia

A patient with Philadelphia-positive chronic myelogenous leukemia (CML) evolved in extramedullary blast crisis, was studied for the presence of alterations of the P53 tumor suppressor gene in the different stages of disease progression. No P53 gene aberrations were detected during the chronic and accelerated phases. Two identical missense point mutations, involving codons 249 and 281 and leading to the amino acid substitutions Arg-Ser and Thy-Asp, were identified in cells of an extramedullary mass and then in peripheral blood blast crisis cells. The data indicate that the medullary and extramedullary blast cells belong to the same cellular clone. They also strongly suggest that in this case, the alteration of P53 gene is strictly related to the progression of the disease, although this mechanism is certainly neither the only nor the most frequent molecular event leading to the acute phase.

DNA recombination induced by aflatoxin B1 activated by cytochrome P450 1A enzymes

Mutations in tumor suppressor genes are intricately associated with the etiology of neoplasia. Often, such mutations are followed by the loss of the second, functional alleles of tumor suppressor genes, a phenomenon known as loss of heterozygosity. Loss of heterozygosity may occur by different molecular mechanisms, including mitotic recombination, and it is conceivable that these molecular events are influenced by endogenous as well as exogenous factors. To test whether mitotic recombination is induced by certain carcinogens, we genetically engineered a Saccharomyces cerevisiae tester strain so that it metabolizes two important classes of carcinogens, polycyclic aromatic hydrocarbons and heterocyclic arylamines. This was accomplished by expressing human cDNA's coding for the cytochrome P450 (CYP) enzymes CYP1A1 or CYP1A2 in combination with NADPH-CYP oxidoreductase in a strain heterozygous for two mutations in the trp5 gene. Microsomes isolated from the transformed yeast strains activated various xenobiotics to powerful mutagens that were detected in the Ames test. Of these, the mycotoxin aflatoxin B1, when activated intracellularly in the strains containing either human CYP enzyme, significantly induced mitotic recombination. These results are discussed in light of possible mechanisms that are involved in aflatoxin B1-mediated hepatocarcinogenesis. Similarly, benzo[a]pyrene-trans-7,8-dihydrodiol and 3-amino-1-methyl-5H-pyrido[4,3-b]indole were activated to recombinagenic products, whereas benzo[a]pyrene and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline were negative in this assay. Our results argue that the constructed yeast strains may be a valuable tool for the investigation of drug-induced mitotic recombination.

Prognostic significance of p53 and ras gene abnormalities in lung adenocarcinoma patients with stage I disease after curative resection

We investigated the prognostic significance of p53 gene abnormalities and ras gene mutations in patients with curatively resected stage I lung adenocarcinoma. Formalin-fixed and paraffin-embedded tissues were obtained from 30 patients who had undergone curative resection for stage I lung adenocarcinoma. Abnormalities of the p53 gene were detected using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis and immunohistochemistry and ras mutations were detected using PCR-restriction fragment length polymorphism (RFLP) analysis. Both univariate and multivariate analyses were performed to assess the relationship between the presence of abnormalities of these genes and the patients' disease-free survival. Eleven tumors (37%) had mutated p53 sequences and 11 (37%) showed p53 overexpression. A total of 15 tumors (50%) had p53 gene abnormalities and the concordance rate was 73%. Seven tumors (23%) showed mutated ras sequences. The univariate analysis revealed that the disease-free survival of patients with any p53 abnormality was shorter than that of those without abnormalities (P = 0.02, generalized Wilcoxon test), and survival of those with p53 protein overexpression was more significantly shorter (P = 0.003, generalized Wilcoxon test). Multivariate analysis using the Cox proportional hazards model indicated that the presence of p53 abnormalities was a significantly (P = 0.01) unfavorable prognostic factor. There was no significant correlation between the presence of ras mutation and survival. These results suggest that analysis of the p53 gene may be helpful for the selection of high-risk patients for clinical trials of adjuvant therapy for stage I lung adenocarcinoma.

Transactivation of the human p53 tumor suppressor gene by c-Myc/Max contributes to elevated mutant p53 expression in some tumors

Elevated levels of mutant forms of the p53 tumor suppressor are a hallmark of many transformed cells. Multiple mechanisms such as increased stability of the protein and increased transcription of the gene can account for elevated p53 expression. Recent findings indicate that c-Myc/Max heterodimers can bind to an essential CA(C/T)GTG-containing site in the p53 promoter and elevate its expression. We have addressed the possibility that elevated mutant p53 expression is due to deregulated c-Myc expression. Here we demonstrate that the human p53 promoter is transactivated by high c-Myc expression and repressed by high Max expression. In examining the relative levels of c-Myc and p53 in human Burkitt's lymphomas and other B-lymphoid lines, we found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression. In particular, cells that express very low levels of c-Myc protein also express low levels of p53 mRNA, while cells that express high levels of c-Myc tend to express high levels of p53 mRNA. To determine whether the p53 gene can be a target for c-Myc in vivo, we assayed the effects of antisense c-myc RNA on the levels of endogenous p53 mRNA. The results indicate that the presence of antisense c-myc RNA leads to a reduction in the levels of c-Myc protein, p53 mRNA, and expression from the p53 promoter. Taken together, our findings support a direct role for c-Myc in elevating expression of the mutant p53 gene in some tumors.

High frequency of p53 abnormality in laryngeal cancers of heavy smokers and its relation to human papillomavirus infection

A series of 41 laryngeal squamous cell carcinomas was examined for p53 abnormalities and human papillomavirus (HPV) infection by an immunohistochemical and/or molecular approach. Immunohistochemically, p53 over-expression was observed in about 60% of the cancers, of which 12 were revealed to contain point mutations of p53 by a combination of the single-strand conformational polymorphism technique and direct sequencing. The p53 point mutations ranged from codons 157 to 278 and most of these mutations lay in two "hot spots" (codon 157 in four cancers and codon 248 in three cancers). The majority of p53 mutations, both transversions (seven cancers) and transitions (five cancers), occurred at the G nucleotide of the codons. An analysis of the clinical information indicated that p53 point mutation was frequently observed in heavy smokers with an average Brinkman index score of more than 1000. On the other hand, HPV DNA, type 16 or 18, was detected in a quarter of the laryngeal cancers. Of eleven HPV-positive cases, nine were immunohistochemically positive for p53, of which four contained a p53 point mutation. These results suggested no inverse relation between p53 mutation and HPV infection in laryngeal cancers. Our study indicates that p53 abnormalities are related to smoking history and the correlation might be better for smoking and chemical mutagenesis than for HPV.

Spontaneous mutations in lacI-containing lambda lysogens derived from transgenic mice: the observed patterns differ in liver and spleen

The pattern of somatic mutation observed in tumor suppressor genes, such as the p53 gene, vary dramatically with tumor type. Some of the observed differences are due to tissue specific effects of mutagens, but it is also possible that some differences may reflect the tissue/cell type specificity of spontaneous mutation. Transgenic mouse models with recombinant shuttle vectors containing the lacI or lacZ target genes may shed light on the extent to which spontaneous mutation displays tissue specificity. Herein we utilize a recently described selectable system to obtain spontaneous mutants for analysis of the molecular lesions. Spontaneous mutations were isolated in the lacI gene recovered from five transgenic mice carrying a lambda shuttle vector. Seventy-three and 67 independent mutations derived from liver and spleen DNA, respectively, were defined in the amino terminal region of lacI. Although technical barriers preclude a direct assessment of the E. coli derived pattern of mutation in this system, five pieces of circumstantial evidence suggest that many of the mutations arose in mouse rather than in E. coli. In DNA from both liver and spleen, mutations at CpG dinucleotides predominate (58% and 51%, respectively). In spleen, most of the mutations at CpG are transitions, while in liver most are transversions. In addition, liver has a higher frequency of GC-->TA transversions at non-CpG dinucleotides while spleen had a higher frequency of deletions and insertions. The data provide evidence that the spontaneous pattern of mutation is tissue specific. In addition, the high frequency of transversions at CpG suggests the need to reevaluate the mechanisms by which mutations occur at this methylated dinucleotide.

TP53 gene mutations and p53 protein immunoreactivity in malignant and premalignant Barrett's esophagus

BACKGROUND/AIMS

Limited data are available regarding TP53 gene alterations in Barrett's esophagus. This study was undertaken to characterize TP53 mutations and p53 protein immunoreactivity in cancers and preinvasive lesions of Barrett's esophageal mucosa.

METHODS

Seventeen Barrett's adenocarcinomas were examined by polymerase chain reaction amplification, denaturant gradient gel electrophoresis, and sequencing for the presence of TP53 mutations in exons 5-8. In 9 cases, Barrett's epithelium adjacent to the cancer was investigated. p53 protein immunoreactivity was studied with PAb 1801.

RESULTS

Sixteen mutations were found in 15 adenocarcinomas, including 10 missense, 3 nonsense, 1 frameshift, and 2 mutations located within consensus splice donor and acceptor sequences. All nucleotide substitutions were transitions. Eight of the 12 transitions involving a GC base pair occurred within the context of a CpG dinucleotide. p53 immunostaining was present in all 10 cases with missense mutations and in 1 case without a detectable mutation. The surrounding Barrett's mucosa showed TP53 mutations identical to that observed in the carcinoma in only 3 of 5 specimens showing high-grade dysplasia.

CONCLUSIONS

TP53 gene mutations and p53 protein immunostaining are present in a majority of Barrett's adenocarcinomas. Our results suggest that these mutations are involved at an early stage during malignant transformation of Barrett's esophagus.

Activation of promutagens in a human bronchial epithelial cell line stably expressing human cytochrome P450 1A2

Normal human bronchial epithelial (NHBE) cells are the putative progenitor cells of all types of lung cancer. NHBE cells immortalized by SV40 T-antigen retain many characteristics of the primary cells and are a useful model for investigating the role of oncogenes, tumor suppressor genes, and certain chemical carcinogens in the molecular pathogenesis of lung cancer. In this study, SV40 T-antigen-positive cells (BEAS-2B) were characterized for their metabolic functions and were shown to continue to express epoxide hydrolase, glutathione S-transferase pi, glutathione peroxidase, and catalase. To increase their metabolic activity towards human procarcinogens, human cytochrome P450 1A2 (CYP1A2) was stably expressed by introducing CYP1A2 cDNA into BEAS-2B cells either by infection with a high-titer recombinant retrovirus (pXT-1A2) or by transfection with a CYP1A2 expression vector (pCMV1A2), which produced the cell lines B-1A2 and B-CMV1A2, respectively. Cell lines established with either expression system expressed enzymatically active CYP1A2 protein and were 50- to 400-fold more sensitive to the cytotoxic effect of the carcinogen aflatoxin B1 (AFB1) than the corresponding control cell lines. The cytotoxic effects of AFB1 were paralleled by increased metabolism of AFB1 and enhanced formation of the AFB1-N7 guanine adduct in B-CMV1A2 cells. Cytotoxicity and adduct formation correlated with a significantly higher protein expression of CYP1A2 by the cytomegalovirus promoter-driven plasmid. Since this human epithelial cell line is the precursor cell type of lung cancer, has normal phase II enzymes, and exhibits highly reproducible expression of phase I enzymes, this in vitro model should aid in the evaluation of putative human carcinogens and anticarcinogens.

p53 gene mutations in human prostate cancers in Japan: different mutation spectra between Japan and western countries

The involvement of p53 mutations in prostate cancers in Japan was investigated. To evaluate any possible clinicopathological significance, p53 mutations in 40 samples from 36 Japanese prostate cancers of different stages (five cases of latent tumors, three of stage A cancers, 10 of stage B, five of stage C and 13 of stage D), including four lymph node metastases of stage D cases, were examined by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and sequencing. Mutations were detected in five of 40 samples (12.5%); four were in primary cancers and the other in a lymph node metastasis from one of them. All mutation-positive cases were in stage D, and the mutation frequency in stage D cases was 31%. This result indicates that p53 mutations may play a role in the progression of a subgroup of prostate cancers in Japanese, as observed for Americans and Europeans. However, a difference was noted between Japanese and Americans in the p53 mutational spectrum (at CpG site), presumably arising from variation in the underlying etiologic factors.

The molecular pathogenesis of hepatocellular carcinoma

Some of the multiple factors involved in the molecular pathogenesis of hepatocellular carcinoma have been elucidated in recent years but no clear picture of how and in what sequence these factors interact at the molecular level has emerged yet. Transformation of hepatocytes to the malignant phenotype may occur irrespective of the aetiological agent through a pathway of chronic liver injury, regeneration and cirrhosis. The activation of cellular oncogenes, the inactivation of tumour suppressor genes and overexpression of certain growth factors contribute to the development of HCC. There is increasing evidence that the hepatitis B virus may play a direct role in the molecular pathogenesis of HCC. Aflatoxins have been shown to induce specific mutations of the p53 tumour suppressor gene thus providing a clue to how an environmental factor may contribute to tumour development at the molecular level.

Inactivation of the p53 gene by either mutation or HPV infection is extremely frequent in human oral squamous cell carcinoma cell lines

The state of p53 tumour suppressor and the frequency of high-risk human papillomavirus (HPV) infections were studied in nine human oral cancer cell lines. Three cancer cell lines (SCC-4, Tu-177 and FaDu) had similar amounts of p53 transcripts to normal cells, but contained significantly higher levels of p53 protein than the normal control cells. Sequencing highly conserved open reading frames of the p53 gene of these cancer cells showed point mutations in the SCC-4 and Tu-177 cell lines, a base transition from CCC to TCC occurred at codon 151; and in the line FaDu, a mutation of CGG to CTG occurred at codon 248. The HEp-2 and 1483 cancer lines contained significantly lower levels of p53 protein compared to the normal counterpart. Sequencing of p53 cDNA for HEp-2 and 1483 lines showed no mutations, but northern analysis revealed that these cell lines expressed HPV-18 E6/E7 messages. Four cell lines (SCC-9, SCC-15, SCC-25, and Tu-139) expressed negligible amounts of p53 transcripts compared to the normal counterpart and undetectable levels of p53 protein. These cell lines contained mutations in the highly conserved open reading frames of the p53 gene as follows: the SCC-9 had a deletion of 32 base pairs between codons 274 and 285; the line SCC-15 had an insertion of five base pairs between codons 224 and 225; the line SCC-25 had a deletion of two base pairs in codon 209; and the Tu-139 line had a deletion of 46 base pairs between codons 171 and 186.(ABSTRACT TRUNCATED AT 250 WORDS)

Overexpression of p53 protein and Ki67 proliferative index in hepatocellular carcinoma: an immunohistochemical study on 109 Italian patients

The overexpression of p53 protein and the Ki67 proliferative index was evaluated in 96 hepatocellular carcinomas (HCC), 67 in cirrhotic livers and 29 in non-cirrhotic ones, and in 13 non-carcinomatous lesions, all surgically resected from Italian patients. Overexpression of p53 was detected only in carcinomatous lesions, and was significantly related to the grade of HCC (P < 0.001). In fact, p53 was observed in 7/7 (100%) cases of grade IV, 13/43 (30.3%) of grade III, and 10/46 (21.7%) of grade II. The relationship between p53 and Ki67 scores was determined in serial sections from corresponding areas of both diffuse and patchy immunoreactivity. In the overall population, p53-positive tumors showed a significantly higher Ki67 score (15.9 +/- 5.5% vs 9.2 +/- 4.3% [P < 0.001]). This observation was evident in all grades of HCC.

Alterations of the RB tumour suppressor gene in hepatocellular carcinoma and hepatoblastoma cell lines in association with abnormal p53 expression

Alterations in the expression of the RB tumour suppressor gene were found by Western immunoblot in three of seven hepatocellular carcinoma and hepatoblastoma cell lines. Abnormalities were detected by single-strand conformation polymorphism (SSCP) within exons 17-21 in RB cDNA from two of these three cell lines and within exons 20-21 in the third cell line. In these three cell lines with abnormal RB expression, abnormal expression of the p53 tumour suppressor gene was also found, apparently the product of a mutant gene. Thus, mutations within the RB gene (or splice-site mutations with exon-skipping) and apparent mutations of the p53 gene together may have contributed to the development of three of these tumours or to the establishment of these cell lines.

Carcinogen daily dose-dependence of the biological features and development rate of hepatocellular carcinomas induced by 3'-methyl-4-dimethylaminoazobenzene in the rat

Differences in biological features and the rate development of hepatocellular carcinomas (HCCs) induced with various daily doses of 3'-methyl-4-dimethylaminoazobenzene were investigated. Male Donryu rats at 21 days old were fed the carcinogen at concentrations ranging from 50 to 600 ppm in the diet continuously, or 600 ppm for 3 weeks followed by a dietary promoting regimen of 500 ppm phenobarbital. Large (> or = 10 mm) HCCs were monitored and the histological features and alpha-fetoprotein (AFP) production analysed. High doses of the carcinogen predominantly induced HCCs of high grade malignancy with AFP production in a short latent period whereas lower doses and the initiation-promotion protocol were primarily associated with low grade HCCs lacking AFP production and developing after long latent periods. Thus the experimental results clearly document that biological features of neoplasms, viewed as a spectrum, may markedly differ according to the daily dose of the carcinogen applied.

Ultraviolet B light-induced mutagenesis of p53 hotspot codons 248 and 249 in human skin fibroblasts

Mutations in the p53 tumor suppressor gene are detected in approximately half of non-melanoma skin cancers. The type of base-pair changes observed strongly suggests solar radiation as the causative mutagen. Mutations are distributed nonrandomly and form moderate hotspots. We studied the capacity of ultraviolet B light (UVB, 280-320 nm) to induce base-pair changes into the p53 exon 7 sequence extending from nt 14067 to 14075 in human skin fibroblasts. This sequence contains hotspot codon 248. UVB induced mostly C-->A and G-->T transversions. The base-pair change with the highest relative abundance was C-->A in the first position of codon 250 (CCC-->ACC), followed by (in diminishing relative abundance) G-->T in the third position of codon 249 (AGG-->AGT), C-->A in the first position of codon 248 (CGG-->AGG), and C-->A in the third position of codon 247 (AAC-->AAA). The C-->T transition in the third position of codon 247 (AAC-->AAT) occurred with moderate efficiency. These base-pair changes are compatible with pyrimidine photodimers as premutagenic lesions, but they could also form opposite 8-hydroxyguanine, which is the major oxidation product of guanine. No evidence was obtained for the presence of tandem double CC-->TT transitions in the untranscribed strand at codons 247/248 and 250. The relative abundance of mutations induced by UVB in the p53 sequence extending from codon 247 to 250 in human fibroblasts does not correlate with mutations observed in the DNA from non-melanoma skin cancer. This lack of correlation suggests that the mutability of this p53 sequence at the DNA level plays only a minor role in the pathogenesis of non-melanoma skin cancer in humans.

Similar patterns of hypomethylation in the beta-hydroxy-beta-methylglutaryl coenzyme A reductase gene in hepatic nodules induced by different carcinogens

Our earlier studies demonstrated that the beta-hydroxy-beta-methylglutaryl coenzyme A (HMG CoA) reductase gene is hypomethylated and overexpressed in hepatic nodules initiated by 1,2-dimethylhydrazine (1,2-DMH). The study presented here examined whether the pattern of DNA methylation of the HMG CoA reductase gene in hepatic nodules reflected carcinogen-DNA interaction during initiation. Accordingly, hepatic nodules were generated in male Fischer 344 rats with either 1,2-DMH or aristolochic acid (AA), which interact predominantly with the guanine and adenine bases in DNA, respectively. DNA from individual nodules was restricted with HpaII, MspI, and HhaI, and the fragments obtained were hybridized to a cDNA probe for HMG CoA reductase. The results indicated that the hypomethylation pattern in the reductase gene in the nodules initiated with these two carcinogens was similar, although they interacted with different bases in the DNA. The question still remained whether the DNA fragments obtained by digesting the two sets of nodules with the restriction endonucleases were from the same domains in the genome of HMG CoA reductase. To examine this, probes for the different domains of the HMG CoA reductase gene were generated from the cDNA using the restriction enzyme Accl. Three probes were obtained: (i) a 5'-end fragment corresponding to the membrane-spanning region, (ii) a second fragment corresponding to the 3'-end of the protein, and (iii) a third fragment spanning the region between (i) and (ii). Each of these probes was radiolabeled and hybridized to the HpaII- and HhaI-generated fragments from the DNA of hepatic nodules initiated with 1,2-DMH and AA. Irrespective of the carcinogen used for initiation, hypomethylation occurred in all three domains of the gene. More important, the pattern of hypomethylation was similar in the nodules initiated by the two carcinogens. Furthermore, an overall similarity was seen in the hypomethylation patterns in the c-myc and c-Ha-ras genes in the DNA of nodules initiated with either 1,2-DMH or AA. These results raise the possibility that the pattern of hypomethylation established in the hepatic nodules may not directly reflect the interaction between the initiating carcinogen and DNA but may represent a unique phenotype of hepatic nodules.

mRNA and protein expression of p53 mutations in human bladder cancer cell lines

We investigated mRNA and protein expression in p53 gene mutations in four human bladder cancer cell lines using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) and Northern blot and Western blot analyses. The following mutations were identified in three of the four cell lines: a missense transversion at codon 110, a missense transition at codon 250 and a non-sense transversion at codon 126. These mutations were located outside previously identified hot spot codons and have rarely been reported in bladder cancer tissues or other neoplasms. Positive intranuclear p53 immunostaining in neoplastic cells in the two missense mutations and the premature stop codon in the non-sense mutation suggested the presence of structural and functional alterations in the p53 protein. Northern and Western blot analyses revealed either an intense or a weak p53 mRNA band together with an intense p53 protein band in the missense mutations, but no p53 mRNA or protein band in the non-sense mutation. A weak p53 mRNA band, but no distinct p53 protein band was observed in the cell line without a mutation and in normal control bladder cells. Our findings suggest that regulation of p53 expression in these cell lines differs at the post-transcriptional and/or post-translational level between the wildtype and the mutant p53 genes and also among different mutant p53 genes. The three cell lines with mutations were derived from high-grade carcinomas; the cell line without mutation was derived from a low-grade carcinoma.

Comparative analysis of mutations in the p53 and K-ras genes in pancreatic cancer

Mutations in codon 12 of K-ras occur in a high proportion of pancreatic cancer cases. Although there is evidence that p53 mutations also occur in this tumor, few studies have been reported to date and no comparison has been made of K-ras and p53 mutations in the same tissues. Single-strand conformation polymorphism and sequencing of the PCR products were used to determine mutations in p53 gene; to detect mutations in K-ras genes, the artificial restriction fragment length polymorphism (RFLP) approach was used. Eight out of 30 tissues from primary pancreas cancer and 3 of 4 samples from metastases showed p53 mutations. Fifteen out of 17 pancreatic cancer cell lines had p53 mutations. In 2 cases, the same p53 mutation was identified in the original tumor and in a tumor-derived cell line. The majority of p53 mutations were present in exons 5-9 of the gene. Mutations at codon 12 of the K-ras gene were identified in 23/32 pancreas cancer tissues and in 14/17 cell lines. There was no relationship between the types of mutation observed in the 2 genes. In conclusion, mutations in K-ras and p53 genes are common in pancreatic cancer. p53 mutations may occur more frequently in metastatic lesions than in primary tumors, although further work is necessary to investigate this point.

Histochemical and immunohistochemical similarities between hepatic tumors in two chimpanzees and man

A well-differentiated trabecular hepatocellular carcinoma (HCC) and a well-differentiated tumor resembling HCC from each of two chimpanzees were found to have histochemical and immunohistochemical staining characteristics similar to those in human HCCs. Transforming growth factor alpha was overexpressed in both tumors. Oval cells, thought to be liver stem cell progeny with a possible role in hepatocarcinogenesis, were observed among nontumorous hepatocytes, particularly near the tumors. Hepatic tumors are rare in chimpanzees but their similarities to human HCC provides a useful research model.

Overexpression and point mutations of p53 tumor suppressor gene in hepatocellular carcinomas in Hong Kong Chinese people

BACKGROUND

Gene deletion, point mutations, and abnormalities in expression of the tumor suppressor gene p53 in hepatocellular carcinoma have been reported to occur with varying frequency in different geographic regions.

METHODS

To assess the expression and point mutation of the p53 gene, 31 patients with hepatocellular carcinomas were examined using Northern blotting, immunohistochemical methods, and DNA sequencing. All patients were Chinese, and 90.3% were positive for hepatitis B surface antigen (HBsAg).

RESULTS

p53 transcript or protein was found in 14 (48.4%) of the 31 patients. Detectable p53 mRNA transcripts were found in 10 patients, and p53 protein was detected in 8 patients. In most cases of patients who had detectable p53 mRNA transcripts, the transcripts in the tumors were exhibited at a higher level than they were in the corresponding nontumorous livers. No p53 protein was detected in the nontumorous livers in all 31 patients. Six (23.1%) of the 26 tumors sequenced showed point mutation scattered in exons 5-9. Of these, only two were at codon-249, and the nature of these two mutations was G-to-T transversions. All but one of the six patients with point mutations had overexpression of the gene.

CONCLUSIONS

Our results show that scattered point mutations are not uncommon in hepatocellular carcinomas in patients from Hong Kong. The distribution pattern of the mutations seems to have no particular correlation with HBsAg status despite a high prevalence rate of HBsAg positivity in our patients. Consistent with a low aflatoxin exposure, aflatoxin-related specific mutation at codon-249 is much less related to the pathogenesis of hepatocellular carcinoma in Hong Kong Chinese people than in other regions with a high-aflatoxin exposure.

High proliferative rates demonstrated by bromodeoxyuridine labeling index in breast carcinomas with p53 overexpression

The protein p53 is a product of a suppressor oncogene. Mutations occurring in 13-15% of breast carcinomas are associated with p53 stainability within nuclei and progression of the tumor. We determined the extent to which p53 abnormality was associated with proliferation by measuring p53 immunohistochemically with a polyclonal antibody and monoclonal PAb1801 in invasive carcinomas of known S-phase fraction (SPF) assessed histologically by bromodeoxyuridine incorporation. Results with the two antibodies always agreed. One of 20 low, 2/18 midrange, and 9/17 high SPF carcinomas were positive for p53. P53 positivity was also related to other indicators of aggressiveness including size of primary tumor, nuclear and nucleolar size, and estrogen and progesterone receptor content, but relationships between p53 and vascular invasion and lymph node metastasis were not found. We conclude that nuclear p53 accumulation is more closely related to proliferation than to invasion and metastasis, and that it identifies some but not all breast carcinomas with high proliferative indices.

Immunohistochemical analysis of p53 protein overexpression in liver cell dysplasia and in hepatocellular carcinoma

We analysed p53 protein immunoreactivity in hepatocellular carcinomas (HCCs) and in liver cell dysplasia (LCD) of patients from an area in Northern China, using five anti-p53 protein antibodies recognizing different epitopes of the protein. In HCCs, the overall prevalence of p53 protein immunoreactivity was 78.3%. However, prevalence was strongly influenced by the type of antibody used, ranging from 67.5% for antibody PAb-1801 to only 10.8% for antibodies PAb-421 and DO-7. p53 protein immunoreactivity was not related to type or grade of HCC. In contrast to former reports, p53 protein staining was restricted to nuclei only when using the CM-1 antibody, whereas two other antibodies yielded both, nuclear and cytoplasmic or membrane staining, and no nuclear staining was observed with antibodies PAb-421 and DO-7, the latter two, however, demonstrating cytoplasmic and membrane staining. For LCD, three subtypes were morphologically and karyometrically defined. Nuclei of some LCD cells were p53 immunoreactive, but positivity was restricted to the small cell variant of LCD. Positivity was different for cirrhosis with or without associated HCC, amounting to 18.9% in the former and 39.4% in the latter. Interestingly, p53 protein immunoreactivity also occurred in a set of small hepatocytes not showing the typical feature of LCD and therefore classified as simple regenerating liver cells.

Frequent alterations of the tumor suppressor genes p53 and DCC in human pancreatic carcinoma

BACKGROUND/AIMS

The pathogenesis of pancreatic cancer is poorly understood. The multigenetic nature of carcinogenesis has been best documented in colon cancer. The relevance of this model was suggested for other epithelial tumors. Only advanced stages of pancreatic cancer are usually detected because of late diagnosis. Analysis of accumulated, diverse genetic changes could allow further understanding of putative mechanisms involved in tumor development. Activated c-Ki-ras oncogene has been shown to be a frequent event. However, additional alterations of tumor suppressor genes are expected. Therefore, concomitant genetic changes of p53 and deleted in colon carcinoma (DCC) in pancreatic carcinoma cell lines and primary tumors were analyzed.

METHODS

p53 protein and transcript expression were revealed by immunocytochemistry and immunohistochemistry, immunoassay, and Northern blot analysis. p53 mutations were identified by sequence analysis. DCC expression was investigated by reverse-transcription polymerase chain reaction.

RESULTS

p53 overexpression was observed in 9 of 12 cell lines. p53 point mutations were confirmed in seven cell lines overexpressing p53. The majority of cell lines showed concomitant p53 and DCC alterations. Four of 6 primary tumors overexpressing p53 also showed loss of DCC expression.

CONCLUSIONS

p53 and DCC genetic changes are associated with pancreatic cancer and the frequently activated c-Ki-ras oncogene. Therefore, the multihit model of carcinogenesis could prove relevant for pancreatic cancer.

p53 mutations in Barrett's adenocarcinoma and high-grade dysplasia

BACKGROUND/AIMS

Allelic losses of chromosome 17p and overexpression of p53 protein have been reported in Barrett's adenocarcinomas. This study aimed to determine the stage in which p53 mutations arise in neoplastic progression in Barrett's esophagus and their relationship to the clonal evolution of cancer.

METHODS

Fourteen patients with high-grade dysplasia, adenocarcinoma, or both arising in Barrett's esophagus were evaluated. Flow cytometric cell sorting was used to obtain purified populations of neoplastic cells for analysis of p53 mutations. DNA was extracted, and exons 5 through 9 of the p53 gene were amplified by polymerase chain reaction. Amplified DNA was sequenced and analyzed by automated sequencing.

RESULTS

Nine of the 14 patients had p53 mutations. Six of the 9 patients had regions of high-grade dysplasia that could be evaluated; all 6 had p53 mutations in high-grade dysplasia. In 3 patients, the same p53 mutations were found in both high-grade dysplasia and adenocarcinoma. All 14 patients had aneuploidy. In 4 patients, diploid cell populations could also be evaluated for p53 mutations; 3 of the 4 patients had p53 mutations in diploid cell populations. In 2 patients, the same p53 mutation was found in multiple aneuploid cell populations within a cancer.

CONCLUSIONS

p53 mutations occur frequently in Barrett's adenocarcinomas. They develop in diploid cell populations. The same p53 mutations are then found in aneuploid cell populations in high-grade dysplasia, in cancer, and in multiple aneuploid cell populations in cancer.

How precisely can data from transgenic mouse mutation-detection systems be extrapolated to humans?: lesions from the human factor IX gene

Transgenic mutation-detection systems have been pioneered in mice, but the approach is applicable to any species in which transgenic animals can be generated. The observed mutations seen in mutation-detection systems are influenced by the underlying pattern of mutation, i.e., the mutational pattern that occurs in wild-type organisms in endogenous segments of DNA that are not under selective pressure. Unfortunately, the biology of most genes and assays markedly skew the underlying pattern of mutation. Herein, we raise multiple issues that must be addressed in order to estimate the underlying pattern of spontaneous mutation from transgenic mouse mutation-detection systems. If these issues can be addressed, the underlying pattern of spontaneous mutation can then be deduced for multiple cell types and for transgenes integrated into different parts of the genome. Even though transgenic methodology cannot be applied directly to humans, it is likely that comparable data on the underlying pattern of spontaneous mutation will be available in humans. Such data are currently available for germline mutations in the factor IX gene. These data are reviewed because of their relevance to two of the multiple issues that must be addressed in transgenic mouse mutation-detection systems: (1) How can the underlying pattern of mutation be deduced from the observed pattern? and (2) How similar are the underlying patterns of mutation in humans and in mice? The analysis of recent germ-line mutation in the factor IX gene yield estimates of the mutation rates per base pair per generation. In brief, the mutation rates vary from 0.037 x 10(-10) for deletions (> 20 bp) to 360 x 10(-10) for transitions at the dinucleotide CpG. If these mutation rates are extrapolated to the entire genome, the aggregate mutation rate is estimated to be 36 x 10(-10). This implies that the diploid genome of each person contains about 21 de novo mutations. In the future, the underlying pattern of spontaneous mutation will be deduced for multiple human genes and these will serve as benchmarks to assess the similarity of the mutational process in humans and in mice.

Geographic variation of p53 mutational profile in nonmalignant human liver

Fifty-eight percent of hepatocellular carcinomas (HCCs) from Qidong, China, contain an AGG to AGT mutation at codon 249 of the p53 tumor suppressor gene, a mutation that is rarely seen in HCCs from Western countries. The population of Qidong is exposed to high levels of aflatoxin B1 (AFB1), a fungal toxin that has been shown to induce the same mutation in cultured human HCC cells. To investigate the role of AFB1 and of these p53 mutations in hepatocarcinogenesis, normal liver samples from the United States, Thailand, and Qidong (where AFB1 exposures are negligible, low and high, respectively) were examined for p53 mutations. The frequency of the AGG to AGT mutation at codon 249 paralleled the level of AFB1 exposure, which supports the hypothesis that this toxin has a causative--and probably early--role in hepatocarcinogenesis.

Absence of p53 mutations and various frequencies of Ki-ras exon 1 mutations in rat hepatic tumors induced by different carcinogens

Mutations of p53 and Ki-ras exon 1 were investigated in rat hepatic lesions induced by four kinds of hepatocarcinogenic protocols: continuous feeding of 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB), daily intraperitoneal injection of aflatoxin B1 (AFB1), and the Solt and Farber regimen (Nature 236:701-703, 1976), in which diethylnitrosamine (DEN) or nitrosomethylurea (NMU) was used as initiating agents. DNA from microdissected tissue sections was amplified by the polymerase chain reaction (PCR) directly using primers for p53 exons 5-8 and Ki-ras exon 1 and analyzed for mutations by denaturing gradient gel electrophoresis (DGGE) or constant denaturant gel electrophoresis (CDGE). One or both of the p53 PCR primers were located within introns to prevent amplifying the p53 processed pseudogenes. In a total of 59 hepatocellular carcinomas (HCCs), no p53 aberrations were detected, indicating that p53 mutations are not very important in rat hepatic carcinogenesis. On the other hand, Ki-ras codon 12 mutations were found at low frequency in HCCs, hyperplastic foci, and cholangiofibroses induced by 3'-Me-DAB and by AFB1 but not in the lesions induced by the Solt and Farber regimen. Although Ki-ras codon 12 mutations are generally infrequent in rat hepatic tumors, their incidence thus appears to vary depending on the carcinogen used for their induction.

p53 gene mutation in hepatocellular carcinoma induced by 2-amino-3-methylimidazo[4,5-f]quinoline in nonhuman primates

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) is one of several heterocyclic amines formed during the cooking of proteinaceous foods. IQ is a potent carcinogen in rodent bioassays and causes a high incidence of hepatocellular carcinomas in nonhuman primates. We examined 20 hepatocellular carcinomas (HCCs) from nonhuman primates for mutations of the p53 gene using polymerase chain reaction-single strand conformational polymorphism analysis. Mutations in the p53 gene were detected in 4 of 20 HCCs (20%) with 3 showing G-to-T transversions and one a G-to-A transition. Three of these mutations were observed in codons 175 and 248 that are known mutational hot spots in human cancers. These data indicate that part of the IQ-induced HCCs in nonhuman primates may involve inactivation of the p53 gene and suggest that IQ and possibly other heterocyclic amines may participate in human carcinogenesis by a similar mechanism.

Overexpression of the oncoprotein p53 in primary hepatic tumors of childhood does not correlate with gene mutations

High levels of expression of the p53 protein and gene mutations have been described in adult hepatocellular carcinomas. It has been postulated that specific mutations in exon 7 may be caused by aflatoxin exposure. To determine whether p53 mutations occur in childhood liver cancer unassociated with aflatoxin exposure or hepatitis B virus (HBV) infection, we have analyzed three histologically distinct tumor types. Two techniques were used to access p53 in the tumors: (1) expression studies of the p53 protein were performed using the polyclonal antibody CM1 and immunohistochemistry, and (2) DNA sequencing was performed. p53 Protein was detectable by immunohistochemistry in 10 of 15 hepatoblastomas, six of nine hepatocellular carcinomas, and one of one embryonal sarcomas. Solid phase single-stranded DNA sequencing across exons 5 through 9 showed normal sequence in all cases. These results indicate that p53 is overexpressed in a majority of childhood liver cancers, but this abnormal p53 expression does not seem to be caused by mutations in the p53 gene.

p53 gene mutations and steroid receptor status in breast cancer. Clinicopathologic correlations and prognostic assessment

BACKGROUND

There is increasing evidence linking development and progression of cancer to an accumulation of mutations at the genomic level. The most frequently mutated gene known to date in sporadic breast cancer appears to be the tumor suppressor gene p53. This study was designed to determine the frequency of p53 gene mutations in primary breast cancer, to correlate the presence of p53 mutations with established clinicopathologic parameters, including the estrogen receptor (ER) and progesterone receptor (PR) status, and to assess the prognostic significance of p53 mutations regarding patient survival.

METHODS

We examined the p53 gene in genomic DNA samples from 192 primary breast cancers. Using denaturant gradient gel electrophoresis, the authors analyzed exons 5-9 in all tumors for mutations and performed DNA sequencing in 20 tumors to identify the exact nature of the p53 mutations.

RESULTS

p53 gene alterations were identified in 43 of the 192 tumors (22%), the majority localized in exons 5 and 6. DNA sequencing showed mostly missense mutations resulting from G or C substitutions. p53 mutations were found more often in tumors of younger women (P = 0.002), Afro-American women (P = 0.05), and in tumors lacking ER (P = 0.03), PR (P = 0.04), or both (P = 0.06). There were no significant correlations with family history, tumor size, histologic grade or type, nodal status, or disease stage. The overall survival rates showed no significant difference between patients with mutant and wild-type p53 tumors. The same was true when the comparison was limited to node-negative patients or patients with ER-positive or ER-negative tumors. Finally, there was no significant difference in survival between patients with tumors harboring mutations in exons 5 and 6 versus exons 7-9.

CONCLUSIONS

The results of this and other studies demonstrate a consistent relationship between ER-positive tumors and wild-type p53 on one hand and ER-negative cancers and p53 mutations on the other. Our data do not support a significant prognostic role for p53 mutations in predicting survival.

A new mutational hot-spot in the p53 gene in human hepatocellular carcinoma

Mutations in the p53 gene are frequent genetic alterations in human hepatocellular carcinoma. We have examined 38 hepatocellular carcinoma cases from Taiwan for the presence of p53 alterations in exons 5-8 of the gene using the single-stranded conformational polymorphism method and direct sequencing of polymerase chain reaction products. Using the single-stranded conformational polymorphism method, we found mutations in 16 (42.1%) cases. Twelve mutations were found in exon 5, three in exon 7, and one in exon 8. No mutations were found in exon 6. Sequencing of polymerase chain reaction products showed that all mutations in exon 5 were clustered at codon 166 and were T/A transversions resulting in an amino acid change from serine to threonine, identifying a new hot-spot for point mutations in the p53 gene. The mutations in exon 7 were all at codon 249, and were G/T transversions leading to an amino acid change of arginine to serine. Finally, the mutation at exon 8 was a G-to-T transversion at codon 286 leading to a stop codon. These data indicate that mutations of the p53 gene may be important in the development of human hepatocellular carcinoma and that, in contrast to other tumors, the mutations of the p53 gene in hepatocellular carcinomas can be clustered in a specific codon of the gene.

Cytokine-induced modulation of tumor suppressor gene expression in ovarian cancer cells: up-regulation of p53 gene expression and induction of apoptosis by tumor necrosis factor-alpha

OBJECTIVE

Our purpose was to determine the effect of tumor necrosis factor-alpha on anti-oncogene expression and to investigate the relationship between the up-regulation of the p53 tumor suppressor gene and tumor necrosis factor-alpha-mediated apoptosis in epithelial ovarian cancer cell lines.

STUDY DESIGN

By means of Northern blot techniques p53 messenger ribonucleic acid expression was assayed in ovarian cancer cells. Tumor cells explanted from patients into Balb/c nude mice were exposed to supernatants from activated monocytes, activated T cells, or the recombinant cytokines interleukin-6 and tumor necrosis factor-alpha. Time- and dose-dependence of p53 up-regulation was measured. Induction of programmed cell death (apoptosis) by tumor necrosis factor-alpha was quantitated by means of a deoxyribonucleic acid fragmentation assay.

RESULTS

Detectable levels of messenger ribonucleic acid for p53 were seen in ovarian cancer cells. Tumor necrosis factor-alpha induced a significant up-regulation of p53 messenger ribonucleic acid levels in ovarian cancer cells grown in nude mice and in vitro, whereas interleukin-6 did not. The maximum level of induction was 8 hours, and the up-regulation of p53 was dose dependent. In addition, tumor necrosis factor-alpha induced a dose-dependent increase in deoxyribonucleic acid fragmentation.

CONCLUSION

Tumor necrosis factor-alpha induced up-regulation of p53 tumor suppressor gene expression in epithelial ovarian cancer cell lines, together with the induction of cell death by apoptosis.

Tumor suppressor genes, growth factor genes, and oncogenes in hepatitis B virus-associated hepatocellular carcinoma

A series of changes in the genes that control hepatocyte growth, or interference with the protein products of these genes, appears to have an important role in the etiology of hepatocellular carcinoma (HCC). Mutations of the p53 tumor suppressor gene have been identified in 30-50% of HCC patients in some geographic areas. Abnormalities of the RB tumor suppressor gene have been found in 20-25% of HCCs, including 80-86% of HCCs with p53 mutations. Overexpression of transforming growth factor alpha (TGF-alpha), insulin-like growth factor II (IGF-II), and the oncogenes N-ras, c-myc, and c-fos have been found in high percentages of HCC patients. The cumulative effect of these changes may be more important than the order in which they occur. Some of these changes may explain the mechanism(s) by which the hepatitis B virus participates in the development of HCC.

Hepatitis B virus X protein inhibits p53 sequence-specific DNA binding, transcriptional activity, and association with transcription factor ERCC3

Chronic active hepatitis caused by infection with hepatitis B virus, a DNA virus, is a major risk factor for human hepatocellular carcinoma. Since the oncogenicity of several DNA viruses is dependent on the interaction of their viral oncoproteins with cellular tumor-suppressor gene products, we investigated the interaction between hepatitis B virus X protein (HBX) and human wild-type p53 protein. HBX complexes with the wild-type p53 protein and inhibits its sequence-specific DNA binding in vitro. HBX expression also inhibits p53-mediated transcriptional activation in vivo and the in vitro association of p53 and ERCC3, a general transcription factor involved in nucleotide excision repair. Therefore, HBX may affect a wide range of p53 functions and contribute to the molecular pathogenesis of human hepatocellular carcinoma.

The 1993 Walter Hubert Lecture: the role of the p53 tumour-suppressor gene in tumorigenesis

The p53 tumour-suppressor gene is mutated in 60% of human tumours, and the product of the gene acts as a suppressor of cell division. It is thought that the growth-suppressive effects of p53 are mediated through the transcriptional transactivation activity of the protein. Overexpression of the p53 protein results either in arrest in the G1 phase of the cell cycle or in the induction of apoptosis. Both the level of the protein and its transcriptional transactivation activity increase following treatment of cells with agents that damage DNA, and it is thought that p53 acts to protect cells against the accumulation of mutations and subsequent conversion to a cancerous state. The induction of p53 levels in cells exposed to gamma-irradiation results in cell cycle arrest in some cells (fibroblasts) and apoptosis in others (thymocytes). Cells lacking p53 have lost this cell cycle control and presumably accumulate damage-induced mutations that result in tumorigenesis. Thus, the role of p53 in suppressing tumorigenesis may be to rescue the cell or organism from the mutagenic effects of DNA damage. Loss of p53 function accelerates the process of tumorigenesis and alters the response of cells to agents that damage DNA, indicating that successful strategies for radiation therapy may well need to take into account the tissue of origin and the status of p53 in the tumour.

Two-dimensional polyacrylamide gel electrophoresis in experimental hepatocarcinogenesis studies

High resolution two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) in combination with computer-assisted densitometry was used to analyze sequential changes in polypeptide expression during chemically (aflatoxin Bl; AFB), spontaneously, and oncogene (v-Ha-ras, v-raf, and v-raflv-myc)-induced experimental rat hepatocarcinogenesis. Two-dimensional mapping of [35S]methionine and [32P]orthophosphate-labeled whole cell lysate and nuclear polypeptides revealed subsets of polypeptides specific for each transformation modality in the in vitro rat liver epithelial (RLE) transformation model. Many of the observed changes in whole cell lysate preparations were localized to specific subcellular organelles. Significant alterations in the expression of the extracellular matrix protein, fibronectin, as well as tropomyosin- and intermediate filament-related polypeptides (vimentin, beta-tubulin, cytokeratins 8, 14, and 18, and actin) were observed among the various transformant cell lines. Whereas alterations in the tropomyosin isoforms appeared to be transformation specific, concomitant modulation of intermediate filament expression was related more to the differentiation state of the individual cell lines than to the transformed phenotype. To integrate protein and DNA information of polypeptides believed to be critically involved during cellular transformation, N-terminal amino acid microsequencing of selected nuclear polypeptides was performed. Preliminary results suggest that N-terminal blockage of rat liver epithelial nuclear proteins to be minor (approximately 20%) with sequencing sensitivity of one pmol. These studies extend our on-going efforts toward the establishment of computerized database of rat liver epithelial cellular proteins (Wirth et al., Electrophoresis, 1991, 12, 931-954) to aid in the delineation of polypeptides critically involved in cellular growth and differentiation as well as transformation.

Analysis of p53 gene mutations in low- and high-grade astrocytomas by polymerase chain reaction-assisted single-strand conformation polymorphism and immunohistochemistry

Using polymerase chain reaction-assisted single-strand conformation polymorphism (PCR-SSCP) and immunohistochemical analyses, mutations in the p53 tumor suppressor gene were examined in 19 low- and high-grade gliomas. By PCR-SSCP and nucleotide analyses, p53 gene mutation was seen in 7 gliomas. Out of the 7 mutations, 3 were located at the CpG site of the previously proposed hot-spot codons 248 and 273, 2 were at codons 171 and 214 and the other 2 were in intron 5, 1 at the splice acceptor site and the other in the vicinity of the splice donor site. The latter 4 mutations have not, or only rarely, been observed in gliomas or in other tumors. However, their effect on the structural and functional alteration of the p53 protein was suggested by positive intranuclear p53 immunostaining in neoplastic cells in 3 mutations including the 1 at the splice acceptor site. In connection with glioma grading, the p53 gene mutation was shown to have occurred in both low- and high-grade gliomas, often in most of the neoplastic cells, as suggested by lack of distinct normal bands and ladders in SSCP and direct sequencing, respectively. The absence of recurrence and malignant transformation over a considerably long postoperative time in our low-grade glioma cases suggested that the p53 gene mutation might not be sufficient for the progression from low- to high-grade gliomas. The frequency of detection of mutation was 7/19(37%) by PCR-SSCP, 8/19(42%) by immunohistochemistry and 10/19(53%) by both methods. The results of PCR-SSCP and immunohistochemistry were consistent in 14 cases (73.7%), but not in 5 cases(26.3%).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular events in the pathogenesis of hepadnavirus-associated hepatocellular carcinoma

Chronic hepadnavirus infection is associated with hepatocellular carcinoma (HCC) in natural hosts such as humans, woodchucks, and Beechey ground squirrels. Several possible oncogenic mechanisms have been identified, including a potential role of the hepadnavirus x (hbx) gene, which transactivates transcription regulated by certain cis-acting sequences, e.g. regulatory sequences of the hepatitis B virus (HBV) and heterologous regulatory sequences of other viruses and cellular genes. The oncogenic potential of hbx is suggested by the observation of HCCs in hbx transgenic mice, the oncogenic transformation of cells expressing hbx in culture, and the transactivation of oncogenes c-myc and c-jun by hbx. Cis-activation of cellular oncogenes N-myc and c-myc by viral promoter insertion has been a common finding in woodchuck hepatitis virus (WHV)-associated HCCs of woodchucks. No such cis-activation of any cellular gene has been shown in virus-associated HCCs of ground squirrels or humans. Amplification and overexpression of the c-myc gene has been a common finding in HCCs of ground squirrels, and is rare in woodchuck or human HCCs. Point mutations in the p53 gene and allelic deletion of p53 have been common findings in human HCCs, but have not been found in HCCs in woodchucks and have been found rarely in ground squirrels. How each of these genetic changes in the different hosts contributes to HCC remains to be determined, but apparently different changes in different HCCs of hepadnavirus-infected hosts suggest that several separate genetic events may contribute to the development of HCC. These events may differ in each host, and some may not result from a direct virus-specific mechanism. Chronic hepadnavirus infection is often associated with chronic necroinflammatory liver disease and cirrhosis, a pathologic process common to several other risk factors for HCC. This suggests that this pathologic process (necroinflammatory disease) may be hepatocarcinogenic regardless of the inciting agent. Thus hepadnavirus infection may play an important role in the development of HCC by causing chronic hepatitis and HCC with the same mechanisms by which other risk factors for HCC cause chronic necroinflammatory liver disease and HCC.