p53: at the crossroads of molecular carcinogenesis and risk assessment

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

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

Heterogeneity, biodiversity and bioperversity in solid neoplasms

Heterogeneity of biological structure and function is an impediment to the analysis and treatment of human solid tumours. Its importance is frequently underestimated in clinico-pathological research. This article reviews the many facets of heterogeneity in tumour systems, and its importance to the interpretation of tumour biology.

Lack of mutations in K-ras codons 12 and 13 in human atherosclerotic lesions

In the framework of a project investigating the possible involvement of cancer biomarkers in human atherogenesis, we evaluated the occurrence of K-ras mutations in the DNA extracted from smooth muscle cells of abdominal aorta atherosclerotic lesions. The molecular analysis of the DNA from 32 surgical specimens, using PCR-based denaturing gradient gel electrophoresis (DGGE), did not reveal any variant in K-ras codons 12 and 13, which are the most frequently involved codons among the ras genes mutated in various types of human tumors. Analysis of the DNA extracted from four cell lines carrying known K-ras mutational alleles showed typically positive DGGE patterns. Thus, on the whole, the conclusions of this study and of previous studies using the same biological material are consistent with the occurrence of DNA adducts in human atherosclerotic lesions but in the absence of p53 involvement or of K-ras mutations in codons 12 and 13. The search for candidate genes which may possibly be involved in the atherogenetic process warrants further studies.

The single-stranded DNA end binding site of p53 coincides with the C-terminal regulatory region

p53 is a transcription factor that binds double-stranded (ds) DNA in a sequence-specific manner. In addition, p53 can bind the ends of single-stranded (ss) DNA. We previously demonstrated that ssDNA oligonucleotides interact with the C-terminal domain of p53 and stimulate binding to internal segments of long ssDNA by the p53 core domain. Here we show that the p53 C-terminal domain can recognize staggered ss ends of dsDNA. We have mapped the binding site for ssDNA ends to residues 361-382 in human p53 using a p53 deletion mutant (p53-delta 30) lacking the 30 C-terminal amino acid residues and a series of 22mer peptides. The binding site for DNA ends coincides with a region previously implicated in regulation of sequence-specific DNA binding by the core domain. The interaction of the C-terminal regulatory domain with the ends of ssDNA or with the protruding ends of dsDNA stimulates both sequence-specific and non-specific DNA binding via the core domain. Electron microscopy demonstrated the simultaneous binding of p53 to dsDNA and a ssDNA end. These results suggest a model in which interaction of the p53 C-terminal tail with DNA ends generated after DNA damage causes activation of sequence-specific p53 DNA binding in vivo and may thus provide a molecular link between DNA damage and p53-mediated growth arrest and apoptosis.

Absence of germline mutations in exons 5-9 of the p53 gene in patients with Li-Fraumeni-like (SBLA) and familial adenomatous polyposis heritable cancer syndromes

Although acquired mutations in the human p53 gene occur in many tumor types, germline mutations are rare. An exception is the occurrence of germline p53 mutations in a fraction of families afflicted with the Li-Fraumeni syndrome (LFS). Previous studies from our laboratory demonstrated increased levels of wild type p53 protein in skin fibroblasts (SF) of patients from heritable cancer syndrome, including familial adenomatous polyposis (FAP), neurofibromatosis type 1 (NF1), and bilateral retinoblastoma (bRB) (Kopelovich and DeLeo, 1984,1986). Here, we further address the association between germline p53 alterations and genetic predisposition to cancer in the SBLA syndrome and in FAP. DNA sequencing and single-stranded conformational polymorphism analysis (SSCP) were utilized to screen for the presence of mutations within exons 5-9 of the p53 gene in SF and in benign tumors. Thus we observed no germline mutations in exons 5-9 of the p53 gene in SF from SBLA or FAP patients, including the Gardner variant. In addition, we observed no acquired mutations in exons 5-9 of the p53 gene in benign tumors from FAP patients. In conclusion, we found no association between germline p53 mutations and SBLA or FAP. How mechanisms that involve nonmutational activation of the p53 protein might affect genetic predisposition to cancer remains to be established.

Future approaches to genetic toxicology risk assessment

Short-term genetic toxicology tests were developed for the purpose of identifying chemical carcinogens in the environment. After two decades of development and validation, the tests are well-established in routine testing schemes, but our views of their utility for safety evaluation have undergone re-assessment. The correlation between identified mutagens and identified carcinogens has turned out to be significantly less than one. Processes or mechanisms that are not directly genotoxic appear to play a role in carcinogenesis. While short term test data are still components of the assessment of carcinogenic risk, genetic damage also has been recognized as important in its own right, in relation to heritable genetic risk and other health-related effects, such as aging, reproductive failure and developmental toxicity. The revolution in molecular biology and genetic analysis occurring over the past 20 years has contributed to the wealth of new information on the complexities of cell regulation, differentiation, and the carcinogenic process. These technologies have provided new experimental approaches to genetic toxicology assessments, including transgenic cell and animal models, human monitoring, and analysis of macromolecular interactions at environmentally relevant exposures. The potential exists for the development of more efficient and more relevant genetic toxicology testing schemes for use assessing human safety. A delineation of contemporary needs, a modern view of the elements of cancer induction, and an examination of new assays and technologies may provide a framework for integrating new approaches into current schemes for evaluating the potential genetic and carcinogenic risk of environmental chemicals.

Clonal heterogeneity in human esophageal squamous cell carcinomas on DNA analysis

Cancers are thought to arise through multistep accumulation of somatic mutations in the progeny of a single cell. Multiple mutations may induce molecular intratumor heterogeneity. Therefore, we examined molecular clonal heterogeneity in esophageal squamous cell carcinomas. Twenty-four esophageal squamous cell carcinomas and associated lymph node metastases were examined for microsatellite alterations, and abnormalities of the p53 and transforming growth factor-beta type II receptor (TGF-beta RII) genes. There were eight cases (33%) showing different patterns of loss of heterozygosity in primary tumors and metastatic lymph nodes with microsatellite markers. On the other hand, the abnormalities of p53 were identical in all these cases. No mutation was detected in the simple repeated sequences of the TGF-beta RII gene. These results indicate that molecular clonal heterogeneity exists in esophageal squamous cell carcinomas. Therefore, care is necessary in preoperative genetic diagnosis using biopsy samples.

Laboratory probing of oncogenes from human liquid and solid specimens as markers of exposure to toxicants

Recent discoveries regarding the mechanistic role of oncogenes and tumor suppressor genes in cancer development have opened a new era of molecular diagnosis. It has been observed repeatedly that genetic lesions serve as tumor markers in a broad variety of human cancers. The ras gene family, consisting of three related genes, H-ras, K-ras, and N-ras, acquires transforming activity through amplification or mutation in many tissues. If not all, then most types of human malignancies have been found to contain an altered ras gene. Because the ras oncogenes actively participate in both early and intermediate stages of cancer, several highly specific and sensitive approaches have been introduced to detect these genetic alterations as biomarkers of exposure to carcinogens. There is also mounting evidence that implicate chemical-specific alterations of the p53 tumor suppressor gene detected in most human tumors. Therefore, it seems a reliable laboratory approach to identify both altered p53 and ras genes as biomarkers of human chronic or intermittent exposure to toxicants in a variety of occupational settings.

The Use of Isolated Hepatocytes from Rats with Enzyme-altered Foci: Studies of p53 Expression

This paper describes a new version of the widely used technique of isolated rat hepatocyte primary culture. Hepatocytes are isolated from carcinogen-treated rats, and the cultures contain two populations of cells. Data indicate that one cell type derives from enzyme-altered foci (EAF) and one from unaltered tissue. EAF are regarded as precarcinogenic lesions, and the model might be useful in establishing mechanistic and test data for the risk assessment of carcinogens. The two populations differ in their responses to many xenobiotics and toxins. They also differ in their responses to endogenous cell growth control factors. Data are presented on p53 expression in cultured hepatocytes in response to exposure to methyl iodide or diethyl nitrosamine. The responses of the two cell types differed. For example, 0.2mM methyl iodide induced p53 expression in 2–4% of the non-EAF cells, but in less than 1% of the EAF cells. 2% of non–EAF cells and 8% of EAF cells incorporated thymidine. It is concluded that many EAF cells have an attenuated expression of p53 in response to DNA-damaging agents. EAF hepatocytes with damaged DNA may replicate DNA, and this effect could be critical for tumour development.

Assignment of the TP53 orthologue to a new linkage group (LG XIV) in fish of the genus Xiphophorus (Teleostei: Poeciliidae)

Using a p53 encoding cDNA fragment of rainbow trout (Oncorhynchus mykiss) as probe, a lambda clone from a platyfish (Xiphophorus maculatus) genomic library was isolated. DNA sequencing of the insert from this clone revealed that it contained the highly conserved domains IV and V of the p53 polypeptide. To map the Xiphophorus p53 gene, joint segregation analysis of the inheritance of a PstI-generated DNA restriction fragment length polymorphism (RFLP) and the inheritance of 36 polymorphic protein and DNA markers was performed in backcross hybrids of X. clemenciae x (X. clemenciae x X. milleri) and X. helleri x X. (helleri x X. maculatus Jp 163 B) using Oncorhynchus cDNA and Xiphophorus genomic p53 probes, respectively. The p53-hybridizing sequence (TP53) was linked to the ACO1 (cytosolic aconitase) locus in both crosses, and defines a new Xiphophorus linkage group, designated LG XIV. This is the first mapping assignment of a known human tumor suppressor gene in fish. Since ACO1 is not linked with melanoma severity in X. helleri x X. maculatus Jp 163 A backcross hybrids, these data indicate that homozygosity for the X. helleri TP53 genotype in backcross hybrids of the cross type is not associated with genetically regulated malignant melanoma formation in the Gordon-Kosswig hybrid melanoma model.

Clinical utility of molecular probes in lymphoproliferative disorders

The use of molecular probes to diagnose and evaluate human malignancies has been most prominent in the disease category of malignant lymphoma. While this technology is daunting, it has gained rapid entry into the laboratories of both academic and community hospitals. This is because the molecular analysis of genetic changes in human disease, especially the human lymphomas, contributes to many facets of care including aiding the diagnosis, providing prognosis and helping to monitor the disease process. This review aims to highlight both the current routinely available technology and its practical applications in human lymphoma.

p53 gene and cell cycling in uveal melanoma

PURPOSE

To determine whether alterations of p53, a tumor suppressor gene, were present in uveal melanoma, and to characterize further the nature of those changes.

METHODS

Immunohistochemical analysis with a monoclonal antibody was used to determine whether alterations of p53 were present in 35 enucleated archival uveal melanomas. Further characterization was done by comparing the p53 gene and cell cycling status by using bromodeoxyuridine staining. The alterations in p53 were characterized using polymerase chain reaction single-strand conformational polymorphism analysis and sequencing.

RESULTS

Four of 35 uveal melanomas showed low levels (0.5% to 5.0%) of positive immunostaining for altered p53 in tumor cell nuclei using monoclonal antibody DO-7. These four tumors had the three highest and the 14th highest bromodeoxyuridine labeling indices, ranging from 1.3% to 7.0%. Polymerase chain reaction single-strand conformational polymorphism analysis of p53 exons 5 to 8 was performed on three p53-positive and six p53-negative tumors, and no altered motility shifts were detected. Sequencing of one of the positive staining specimens confirmed no mutations in exons 5 through 8 in the p53 gene. Double immunohistochemical labeling for both bromodeoxyuridine and p53 in one tumor showed that most of p53-positive cells were in S phase.

CONCLUSIONS

Mutation of p53 is an uncommon event in uveal melanomas. Nuclear accumulation of p53 protein was found in three of the four tumors with the highest levels of cell cycling.

The role of individual susceptibility in cancer burden related to environmental exposure

Individual susceptibility to cancer may result from host factors including differences n metabolism, DNA repair, altered expression of protooncogenes and tumor suppressor genes, and nutritional status. Since most carcinogens require metabolic activation before binding to DNA, variations in an individual's metabolic phenotype that have detected in enzymes involved in activation and detoxification should play an essential role in the development of environmental cancer. This phenotypic metabolic variation has now been related to genetic polymorphisms, and many genes encoding carcinogen-metabolizing enzymes have been identified and cloned. Consequently, allelic variants or genetic defects that give rise to the observed variation and new polymorphisms have been recognized. Development of simple polymerase chain reaction (PCR)-based assays has enabled identification of an individual's genotype for a variety of metabolic polymorphisms. Thus, recent knowledge of the genetic basis for individual metabolic variation has opened new possibilities of studies focusing on increased individual susceptibility to environmentally induced cancer, which are reviewed with special reference to smoking-induced lung cancer. Cancer susceptibility due to chemical exposure is likely to be determined by an individual's phenotype for a number of enzymes (both activating and detoxifying) relevant to that of a single carcinogen or mixtures of carcinogens. Given the number and variability in expression of carcinogen-metabolizing enzymes and the complexity of chemical exposures, assessment of a single polymorphic enzyme (genotype) may not be sufficient. Mutations in the p53 gene are among the most common genetic changes in human cancer. The frequency and type p53 mutations can act as a fingerprint of carcinogen exposure and may therefore provide information about external etiological agents, intensity of exposure, and host factors affecting the tumorigenesis process. In human lung cancer, p53 mutations (both the mutation pattern and frequency) have been linked with tobacco smoking; the type of mutation most frequently observed is G:C to T:A transversion, a mutation preferentially induced by benzo[a]pyrene diol epoxide. An association between the presence of this transversion and the genotype deficient in glutathione S-transferase M1-mediated detoxification has been observed in lung cancer. Taken together, these findings suggest that determination of metabolic at risk genotypes in combination with levels of DNA adducts in target (surrogate) tissues and the p53 mutation pattern should allow the identification of susceptible individuals and subgroups in carcinogen-exposed populations.

p53 overexpression in head and neck squamous cell carcinoma: review of the literature

As a tumour suppressor gene, the inactivation of p53 induces the development of numerous human cancers. Mutations of p53 have been implicated in the pathogenesis of head and neck squamous cell carcinoma (HN-SCC) at a high incidence. In premalignant lesions and in situ carcinomas, p53 overexpression is not exclusively restricted to neoplastic cells, but frequently affects the normal appearing keratinocytes adjacent to p53 positive neoplasms or present in dysplastic areas. These results suggest that as contributors to the early phases of HN-SCC development, p53 alterations may be excellent biomarkers that indicate the predisposition of a particular oral cavity premalignant lesion toward malignancy. In most cases, the p53 overexpression status of a tumour metastasis is identical to that of a primary tumour, indicating that a p53 mutation precedes metastatic spread. In patients with multiple primary tumours, multiple foci of p53 overexpression are observed in epithelia distant from the tumour. So the expression of p53 in normal epithelium would indicate an increased risk for transformation to second or third primary cancers. Distinct p53 mutations in different primary tumours of the same patient indicate that these cancers arise as independent events; these results support the existence of multifocal polyclonal processes. Regardless of the aforementioned results that support p53 as a valid tumour biomarker, most studies have shown no relationship between the expression of p53 and clinical and histopathological parameters. The role played by p53 mutations in the progression and vital prognosis of HN-SCC has not yet been demonstrated.

Molecular genetic differentiation between primary lung cancers and lung metastases of other tumors

When solitary pulmonary tumors are observed in patients with a history of cancer, differentiation between metastasis and primary lung cancer is crucial for appropriate therapy. Assuming that p53 mutations are conserved in metastases, mutation analysis of the p53 gene would be a valuable tool in differentiating metastases from primary carcinomas of the lung. In nine of 267 resected lung tumors, the origin of the lung tumor could not be defined histologically. Five patients had a history of colorectal carcinoma, one had a history of breast carcinoma, one had a history of soft-tissue carcinoma, and one had a history of head and neck carcinoma. One patient with a clear cell carcinoma of the lung had been surgically treated for both renal and thyroid cancer. Material from one patient with adenocarcinoma of the lung, histologically defined regional lymph nodes, and distant brain metastasis served as a control. We extracted deoxyribonucleic acid from the snap-frozen tissue of the unclassified lung tumors, from paraffin-embedded tissue of the previously removed primary cancers, and also from peripheral blood of the patients. Exons 2 to 11 of the p53 gene were amplified in separated polymerase chain reactions and directly sequenced. In all cases, the presence of germline mutations was excluded by analysis of peripheral blood deoxyribonucleic acid. The p53 mutation detected in the deoxyribonucleic acid of the lung tumor of the control patient proved to be conserved in the lymph nodes as well as in the brain metastasis. In two cases, the lung tumors exhibited a p53 mutation not present in the previously removed primary tumor and were therefore classified as new primary lung cancers. In five cases, the lung tumors proved to be metastases of the first tumor, exhibiting the identical p53 mutation. One of these lung tumor samples could be identified as a metastasis from the renal cancer, but the corresponding thyroid cancer material was different. For two cases, molecular analysis remained inconclusive. In one case, no p53 mutation could be found in the compared samples; in the other, no deoxyribonucleic acid could be extracted. Analysis of p53 mutations allowed exact classification in tumors for which standard methods failed to distinguish between metastasis or primary tumor. More than two thirds of lung tumors in patients with previous gastrointestinal carcinoma were revealed to be metastases, but second primary lung cancer could also be diagnosed. This diagnosis allowed correct surgical and adjuvant treatment of these patients.

Characterization of p53 gene mutations in basal-cell carcinomas: comparison between sun-exposed and less-exposed skin areas

Mutations in the p53 gene in 32 basal-cell carcinomas (BCC) developed in Japanese patients were identified by the polymerase chain reaction and single-strand-conformation polymorphism analysis, followed by sequencing the DNA. Among 16 BCC developed in continuously sun-exposed areas, 6 tumors showed 7 base substitutions, most of which were G:C to A:T transitions, mainly at the dipyrimidine sites. Seven out of 16 BCC developed in less-exposed areas showed 8 base substitutions, but the majority (75.0%) of them were transversions. These results suggest that the mutation in the p53 gene plays a significant role in the tumorigenesis of BCC developed in less-exposed areas as well as those in sun-exposed areas in Japanese patients. There must be therefore causative factors other than UV irradiation for BCC in less-exposed areas.

Transformation in recurrent ovarian granulosa cell tumors: Ki67 (MIB-1) and p53 immunohistochemistry demonstrates a possible molecular basis for the poor histopathologic prediction of clinical behavior

Ovarian granulosa cell tumors (GCTs) behave unpredictably. Stage I patients suffer recurrences many years after treatment, and histopathologic evaluation of the primary GCT offers only a few clues. Grading, in particular, is largely ineffective. Ki67 (MIB-1) and p53 monoclonal antibodies (active on paraffin embedded tissues) provide insight into nuclear proliferation and control, respectively. In this study, the authors hypothesized that these molecular markers will help predict the clinical behavior of GCTs. Paraffin sections from 68 GCTs (arising in 56 patients: 53 primary and 15 recurrent) including 34 typical and 27 diffuse adult, and seven juvenile types were immunostained for Ki67 (MIB-1 clone; Immunotech, Westbrook, ME) and p53 (DO7 clone; Novacastra Laboratories, UK). The Ki67 proliferation index (Ki67PI = percentage immunoreactive on a count of at least 400 nuclei) ranges from 1 to 50% (mean, = 12.2%; median, 9.3%). Nineteen percent of GCTs exhibited focal p53 immunoreactivity; the number of GCTs and proportion of nuclei decorated were as follows: four, <1%; seven, 1% to 10%; and one, 20%. Ki67PI was higher in recurrent tumors (P<.001) and correlated with mitotic rate (r = .75; P<.0001). p53 staining was associated with juvenile type GCTs (P<.001) and higher Ki67PI (P<.005). Other histopathologic features exhibited no association with p53 staining or Ki67PI. Follow-up was available for 54 of 56 patients: 18 suffered recurrences after 16 to 229 months (mean, 72.1 months; median 59 months), and 36 were disease free 16 to 369 months (mean, 78.2 months; median, 70 months) after diagnosis. Curiously, high Ki67PI and mitotic rates of primary GCT correlated weakly with a disease-free course (P=.03 and .07 respectively). Disease recurrence was associated with stage >I (P<.0005), vessel invasion in the capsule (P<.001), ruptured tumors (P<.005), and older patients (P<.02). p53 staining and size or subtype of GCT exhibited no prognostic value. For 12 patients, paired primary and first recurrence of GCT showed a striking increase in Ki67PI (P<.00005) and mitotic activity (P<.02) in the recurrence. p53 expression also appeared (de novo) in two recurrent GCTs. The interpretation of focal p53 staining (>10% nuclei decorated in only one GCT) is controversial. Some investigators suggest that this represents overexpression of wild type p53 rather than p53 gene mutation. Primary GCTs exhibit a wide spectrum of proliferative activity, and the seven juvenile GCTs (the most proliferative type) demonstrated no recurrences in this study. Recurrent GCTs displayed a transformation of molecular markers to increased proliferative activity and overexpression of p53, fundamentally, by these markers, a different GCT than the primary one. These findings suggest a molecular basis for the lack of histopathologic predictors for recurrence. Factors other than proliferation of the primary GCT (which relates most closely to grading) either extrinsic to the neoplasm (host dependent) or as yet undetectable must determine malignant behavior.

A possible role for human papillomaviruses in head and neck cancer

Human papillomaviruses (HPVs) cause benign tumors in the respiratory tract. Mounting evidence suggests that they also play a role in the etiology of a subset of head and neck cancers. Carcinomas in patients with a history of recurrent respiratory papillomatosis clearly are caused by persisting HPV interacting with one of more carcinogenic agents. Verrucous carcinomas of the oral cavity, tonsillar and tongue carcinomas are strongly linked with HPVs, based on molecular epidemiologic data. Tonsillar cancer have been shown to express HPV RNA, presumed necessary to induce and maintain a carcinoma, supporting a viral etiology. This paper reviews the molecular and cellular basis for considering HPVs as causative agents of cancer, and reviews the literature that considers the possible role of HPVs in head and neck cancer.

p16/CDKN2 gene and p53 gene alterations in Japanese non-smoking female lung adenocarcinoma

Primary lung adenocarcinomas in non-smoking females are increasing in the USA and Japan. Environmental factors such as passive smoking, asbestos, domestic radon, and hormonal effects have been implicated, but the etiology is still uncertain. We therefore analyzed point mutations of p16 gene, a newly characterized tumor suppressor gene, and compared the results with alterations of p53 gene in 28 primary lung adenocarcinomas in non-smoking Japanese females. There were no cases with somatic point mutation of p16 gene, except for one case with two germline mutations (silent mutations). In contrast, six out of 16 informative cases showed loss of heterozygosity of p53 gene using a TP53 microsatellite marker and 19 out of 28 cases showed expression of oncoprotein using DO-7 immunohistochemistry. These findings suggest that p16 gene alteration is a rare event in primary lung adenocarcinomas in Japanese non-smoking females, compared with alterations of the p53 gene.

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

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Compilation of somatic mutations of the CDKN2 gene in human cancers: non-random distribution of base substitutions

The CDKN2 gene, encoding the cyclin-dependent kinase inhibitor p16, is a tumour suppressor gene that maps to chromosome band 9p21-p22. The most common mechanism of inactivation of this gene in human cancers is through homozygous deletion; however, in a smaller proportion of tumours and tumour cell lines intragenic mutations occur. In this study we have compiled a database of over 120 published point mutations in the CDKN2 gene from a wide variety of tumour types. A further 50 deletions, insertions, and splice mutations in CDKN2 have also been compiled. Furthermore, we have standardised the numbering of all mutations according to the full-length 156 amino acid form of p16. From this study we are able to define several hot spots, some of which occur at conserved residues within the ankyrin domains of p16. While many of the hotspots are shared by a number of cancers, the relative importance of each position varies, possibly reflecting the role of different carcinogens in the development of certain tumours. As reported previously, the mutational spectrum of CDKN2 in melanomas differs from that of internal malignancies and supports the involvement of UV in melanoma tumorigenesis. Notably, 52% of all substitutions in melanoma-derived samples occurred at just six nucleotide positions. Nonsense mutations comprise a comparatively high proportion of mutations present in the CDKN2 gene, and possible explanations for this are discussed.

Alteration of the p53 gene of lung carcinomas with sarcomatous transformation (spindle cell carcinoma): analysis of four cases

Lung carcinoma with sarcomatous transformation (LCST) is highly aggressive and characterized by local invasion and/or distant metastasis, which leads to a shorter survival than ordinary lung carcinomas. Therefore, to elucidate whether the malignant potential of the spindle cell element in LCST is associated with the alteration of the p53 gene, four cases were examined by analyses of overexpression of the p53 oncoprotein, mutation of the p53 gene and loss-of-heterozygosity (LOH) at chromosome 17p. In two cases overexpression of the p53 oncoprotein of the spindle cell component showed a higher degree of staining than that of the carcinoma component; LOH was identified in both carcinoma and sarcomatous components in one case, while in contrast, another case showed LOH in the sarcomatous component only. Mutations were clearly detected in two cases; one showed a CTT to CGT transversion in codon 194 of exon 6 in both components, whereas the other showed a CTG to CAG transversion in codon 265 of exon 8 in the sarcomatous component only. On the basis of these observations, it suggested that the sarcomatous component shows a higher frequency of p53 gene abnormalities in comparison to the carcinoma component. These results also suggested that the acquisition of malignant potential in the sarcomatous component, or the morphological alteration of carcinoma cells, is correlated with abnormalities associated with the p53 gene.

Molecular analysis of the TP53 gene in Barrett's adenocarcinoma

The TP53 gene is the most frequently mutated gene in human cancers. Barrett's esophagus provides an excellent model by which to understand the genetic events that lead from dysplasia to cancer. We screened for mutations in the TP53 gene by a combination of denaturing gradient gel electrophoresis and DNA sequencing in ten cases of adenocarcinoma arising in Barrett's mucosa. We have identified missense mutations in five of the ten samples, three transitions (R282W, G245S, R248W) and two transversions (E286Q and C176F). In one case we have analyzed biopsy specimens taken from the same site, one year before the patient developed an intra mucosal carcinoma. The mutation that was identified in this high grade dysplastic area was identical to that detected in the cancer. This would suggest TP53 mutations occur as an early genetic event in the development of Barrett's adenocarcinoma.

A unique spectrum of p53 mutations in B-cell chronic lymphocytic leukemia distinct from that of other lymphoid malignancies

The spectrum and pattern of p53 mutations detected in 42 cases of B-cell chronic lymphocytic leukemia (B-CLL) were analyzed, and several interesting features were noted. Codon 209 in the p53 gene may be a new hot-spot for p53 mutation in B-CLL disease. Four of the 42 (10%) reported B-CLL p53 mutations occurred at codon 209 versus none in 214 cases of other lymphoid malignancies screened for p53 mutations (P = 0.0006). Transversion mutations predominated at codon 273 rather than the transition mutations that are known to occur at this CpG site. Four of six (67%) B-CLL cases had transversions at codon 273 compared with two of 17 (12%) of all other lymphoid tumors examined (P = 0.02). In addition, over 65% of the p53 mutations detected in B-CLL showed a strand bias for p53 mutations on the untranscribed DNA strand. This feature of DNA strand bias is notable in cancers of the lung, esophagus, and head and neck, which may result from high exposure to carcinogens. This spectrum of p53 mutations in B-CLL together with the high frequency of transversion mutations and DNA strand bias may implicate environmental carcinogens associated with p53 gene damage in some B-CLL patients.

p53 protein expression in colorectal adenomas: an immunohistochemical study using an antigen retrieval system

The immunohistochemical expression of the p53 gene product was examined in 91 colorectal adenomas from patients without (group 1, 50 cases) or with (group 2, 41 cases) concurrent sporadic colorectal carcinoma, and in 15 additional cases of randomly selected carcinomas from group 2 patients. Immunohistochemical reactions were performed with the DO-7 monoclonal and the CM1 polyclonal antibodies, following microwave irradiation of the tissues in an antigen retrieval solution, and the proportion of the immunoreactive cells was semiquantitatively assessed. p53 protein immunoreactivity was present in 46.1% (42, of 91, i.e., 20 out of 50 of group 1 and 22 out of 41 of group 2) and 33% (30 of 91, i.e. 14 out of 50 of group 1 and 16 out of 41 of group 2) of the adenomas using DO-7 and CM1 antibody, respectively. High p53 expression (i.e. immunolabelling of more than 30% of the tumour cell nuclei) was found in 13.2% of the adenomas (12 of 91, i.e. three out of 50 of group 1 and nine out of 41 of group 2; P = 0.025 using the chi 2 test) using the DO-7 antibody, and in 6.6% of the cases (six of 91, i.e. two out of 50 of group 1 and four out of 41 of group 2) using the CM1 antibody. In carcinomas, 80% of the cases (i.e. 12 of 15) were found to express p53 protein with both antibodies. p53 immunoreactivity in colorectal adenomas increased with the degree of dysplasia: only five (17.8%) of the 28 adenomas with mild dysplasia were found to be DO-7 positive, while all of them remained CM1 negative. From the 50 adenomas exhibiting moderate dysplasia, 28 (56%) were DO-7 positive, and 22 (44%) were CM1 positive. Finally, from the 13 adenomas with severe dysplasia, nine (69.2%) and eight (61.5%) were found to be positive with the DO-7 and the CM1 antibody, respectively. Our results indicate that an increased number of group 2 adenomas express p53 protein, when compared with group 1 adenomas, and suggest that a strong correlation exists between p53 protein expression and the degree of dysplasia in colorectal neoplasms.

Dose-response relationships for carcinogens

Biotransformation of chemical carcinogens involves both metabolic activation and detoxication. The molecular dose present on DNA as adducts represents a balance between these two pathways (formation) and DNA repair. All of these are enzymatic processes subject to saturation. When none of the pathways is saturated, linear molecular dosimetry is expected, whereas if metabolic activation is saturated, a supralinear response occurs. If detoxication or DNA repair is saturated, a sublinear response occurs. With chronic exposure, steady-state concentrations of DNA adducts develop and these follow the same patterns. With several alkylating agents, multiple adducts are formed. The extent of formation is chemically defined, but different DNA repair pathways can be involved for different adducts. By understanding the molecular dose and biology of each adduct and comparing these to the dose-response for tumor induction, it may be possible to identify the most appropriate biomarkers for risk assessment. Recently, endogenous DNA adducts identical to those induced by known human carcinogens have been identified. These endogenously formed adducts may play an important role in human carcinogenesis.

Molecular epidemiology and human risk monitoring

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

Integrin-mediated entry into S phase of human gastric adenocarcinoma cells

The integrins are a family of integral membrane receptors that participate in binding to various extracellular and cell surface proteins during adhesion, migration, and homing of normal and neoplastic cells. In this study, we characterized the involvement of integrins in mediating the growth of an adhesion-dependent gastric adenocarcinoma line, ST2. This line was distinguished and selected for study based on its inability to grow when suspended in soft agar or plated on poly(2-hydroxyethyl methacrylate)-coated dishes. ST2 cells arrested in G0/G1 of the cell cycle when deprived of adhesion to substrate. Using purified matrix components, collagen was found to be highly active in promoting beta 1 integrin-mediated cell attachment and spreading. Subsequent to spreading on collagen, the cells were released from G0/G1 block and progressed into S phase. Monoclonal antibodies to alpha 2 or beta 1 integrin blocked the reinduction of both cell spreading and entry into S phase. These studies suggest that during the metastatic process, integrin receptor interaction with the insoluble matrix may be an important step leading to proliferation of some tumors.

Molecular biology of colon polyps and colon cancer

From a histologic and endoscopic standpoint, colon and rectal cancer (CRC) begins as a small neoplastic polyp which progressively enlarges and transforms through a dysplasia stage into invasive cancer. Recently, molecular abnormalities underlying the adenomacarcinoma progression have been defined. The adenomatous polyposis coli (APC) gene and mismatch repair genes are found to be dysfunctional early in the neoplastic process; either as inherited or somatic mutations. Subsequently, polyps progress to cancer along one of two paths depending on which gene is abnormal. When the APC gene is the initial mutation tumor development follows the "loss of heterozygocity" (LOH) pathway. If mismatch repair genes are altered, the "replication error" (RER) pathway is followed. Somatic mutations of the K-ras oncogene and the MCC, DCC, and p53 tumor suppressor genes accumulate in the LOH pathway and mark the progression through polyp stages. Microsatellite instability is a characteristic of the RER pathway but the precise genes involved in this pathway currently are not known. Defining these pathways has led to a new classification scheme for CRC with resultant changes in our clinical approach to screening, surveillance, and treatment.

DNA mismatch repair and cancer

The genetic basis of cancer involves certain classes of genes, particularly oncogenes, tumor-suppressor genes, and DNA mismatch repair genes. Originally identified in bacteria and yeast, the human homologues of DNA mismatch repair genes have been implicated in the pathogenesis of the hereditary nonpolyposis colorectal cancer syndromes, as well as a variety of different sporadic cancers. An appreciation of their role in cancer is predicated on an understanding of their function in the processes of DNA repair. This article reviews the recent developments and advances in the biology of the human DNA mismatch repair genes and their involvement in the pathogenesis of cancer.

Frameshift mutation in codon 176 of the p53 gene in rat esophageal epithelial cells transformed by benzo[a]pyrene dihydrodiol

Mutations in the p53 tumor suppressor gene have been associated with exposure to environmental chemical carcinogens. Cultured rat esophageal epithelial cells were transformed in vitro by treatment with benzo[a]pyrene dihydrodiol (BP-DHD). A BP-DHD-transformed cell line and control cell lines were analyzed for mutations in the p53 gene and in the Ha-ras gene by single-strand conformation polymorphism analysis of polymerase chain reaction-amplified products and direct DNA sequencing. The deletion of one cytosine in codons 174-176 (TGCCCCCAC-->TGCCCCAC) of the p53 gene was found only in the BP-DHD-transformed cell line. The BP-DHD-transformed cells were highly invasive and tumorigenic when transplanted into syngeneic rats, whereas control lines either were nontumorigenic or formed epithelial cysts. BP-DHD-transformed cells and control lines were negative for mutations in the Ha-ras gene. Our results suggest that the tumorigenic potential of the BP-DHD-transformed cell line is associated with a frameshift mutation in codon 176 of the p53 gene but not with mutations in the Ha-ras gene. The G/C-rich codons 174-176 in the rat p53 gene may be specific targets for BP-DHD.

Cooperation of simian virus 40 large and small T antigens in metabolic stabilization of tumor suppressor p53 during cellular transformation

Metabolic stabilization of the tumor suppressor p53 is a key event in cellular transformation by simian virus 40 (SV40). Expression of the SV40 large tumor antigen (large T) is necessary but not sufficient for this process, as metabolic stabilization of p53 complexed to large T in abortively SV40-infected cells strictly depends on the cellular systems analyzed (F. Tiemann and W. Deppert, J. Virol. 68:2869-2878, 1994). Comparative analyses of various cells differing in metabolic stabilization of p53 upon abortive infection with SV40 revealed that metabolic stabilization of p53 closely correlated with expression of the SV40 small t antigen (small t) in these cells: 3T3 cells do not express small t and do not stabilize p53 upon infection with wild-type SV40. However, ectopic expression of small t in 3T3 cells provided these cells with the capacity to stabilize p53 upon SV40 infection. Conversely, precrisis mouse embryo cells express small t and mediate metabolic stabilization of p53 upon infection with wild-type SV40. Infection of these cells with an SV40 small-t deletion mutant did not lead to metabolic stabilization of p53. Small-t expression and metabolic stabilization of p53 correlated with an enhanced transformation efficiency by SV40, supporting the conclusion that at least part of the documented helper effect of small t in SV40 transformation is its ability to promote metabolic stabilization of p53 complexed to large T.

A splice donor site mutation results in the insertion of five extra amino acids into P53 from SEWA mouse sarcoma cells

The status of the p53 gene in SEWA-AS12-ADH (S-ADH) cells, a subline of the mouse sarcoma cell line SEWA, was examined. Immunoprecipitation with wild-type (wt) or mutant P53-specific monoclonal antibodies (mAb) showed that both wt and mutant P53 were produced. Sequence analysis of the p53 cDNA and genomic DNA revealed a single nucleotide (nt) substitution at a splice donor site at the beginning of intron 7. As a result of this mutation, an alternative splice site 15 nt further 3' in intron 7 is used. The P53 protein translated from this aberrantly spliced mRNA carries an Arg258-->Ser substitution, followed by an insertion of 5 extra amino acids. This is the first example of a splice-site mutation in the mouse p53 gene.

High prevalence of mutations in the p53 gene in poorly differentiated squamous cell carcinomas in xeroderma pigmentosum patients

Mutations in the p53 gene were analyzed in 23 squamous cell carcinomas (SCCs) and five basal cell carcinomas from 10 xeroderma pigmentosum patients in Tunisia. Fourteen mutations were detected. Most occurred at the dipyrimidine sequences of DNA, suggesting that they were caused by ultraviolet light. A strong correlation was noted between the presence of the p53 mutations and clinical characteristics such as histology and growth of SCC. In well-differentiated grade 1 SCCs, three (27.3%) of 11 tumors had the p53 gene mutations, whereas in grade 2 and grade 3 SCCs, six (85.7%) of seven tumors had the p53 gene mutations (p < 0.05). Tumors less than 8.0 mm in diameter showed a relatively low frequency of mutation (two of 10 tumors, 20.0%), whereas most of the tumors larger than 8.1 mm (seven of eight tumors, 87.5%) had mutations of the p53 gene (p < 0.025). Multiple tumors in the same xeroderma pigmentosum patients also showed this relation. These results suggest that mutations in the p53 gene lead to the invasive and rapid-growing character of SCC.

Risk assessment and biological mechanisms: lessons learned, future opportunities

During the past decade, toxicological research has been dominated by two themes; investigations to elucidate the mechanisms of action of toxicants and studies to provide information to support improved assessments of human health risks. The conduct of mechanistic investigations was given an early impetus by advances in biochemistry and cell biology and, more recently, by related advances in molecular biology. Research to provide information for improved human health risk assessments was stimulated by the 1983 NAS/NRC report that provided a codified structure for conducting risk assessments. At first glance, it would appear that the two themes are closely related and, indeed, should represent parts of a joined theme. However, examination of the toxicology/risk assessment literature of the past decade indicates that this has not been the case. Reports of mechanistic studies infrequently indicate how the information can be used to provide improved estimates of human risk from exposure to toxicants. If reference is made, it is usually qualitative in nature. Neither is examination of the risk assessment literature reassuring. Mechanistic studies may be cited; however, the final step of the process, risk characterization, is usually dominated by the use of default options grounded in conservative interpretations of generic scientific knowledge. Two examples are reviewed that stand out as illustrations of how mechanistic information can be used to make a difference in risk assessments: (1) consideration of the alpha 2u-globulin-mediated mechanism for evaluating male rat data for relevance in assessing human risks of renal cancer and (2) the use of DNA-protein cross-links as an internal dose metric in cross-species extrapolation of nasal cancer risks from inhaled formaldehyde. This paper reviews past experience on these topics and suggests a strategy for increasing the use of mechanistic information in risk assessments. A key component of the strategy is to use the risk assessment process to identify research needs/opportunities that, if addressed, will reduce the use of default options, thereby reducing the uncertainties in risk assessments. Another component of the strategy is to identify a few chemicals anticipated to exert their effect via different mechanisms and whose mechanisms of dosimetry and disease pathogenesis can be investigated in-depth within a risk assessment framework; this identification will create prototype approaches as alternatives to the use of default options that have major impact on the outcome of the risk assessment process.

Allelic frequency of p53 gene codon 72 polymorphism in urologic cancers

Alterations in the p53 tumor suppressor gene appear to be important in the development of many human tumors. The wild-type p53 gene has a polymorphism at codon 72 that presents the arginine (CGC) or proline (CCC) genotype, which recently has been reported to be associated with genetically determined susceptibility to smoking-related lung cancers. To determine whether this p53 genotype influences individual risk of urologic cancer and/or its progression, we used polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis to assay the allelic frequencies of this polymorphism in 85 renal cell carcinoma patients, 151 urothelial cancer patients, 33 testicular cancer patients, 28 prostatic cancer patients and 56 patients without neoplastic disease. The allelic distributions of the three genotypes (Arg/Arg, Arg/Pro, Pro/Pro) in patients with renal cell carcinoma (29.4%, 55.3%, 15.3%), urothelial cancers (45.7%, 39.7%, 14.6%), testicular cancer (45.4%, 48.5%, 6.1%) or prostate cancer (42.9%, 50.0%, 7.1%) did not differ significantly from those in the normal controls. However, Pro/Pro genotype in renal cell carcinoma and urothelial cancer (smoking-related cancers) was more frequent than that in prostate cancer and testicular cancer (smoking-unrelated cancers) with borderline significance (P = 0.0881). There was no particular correlation between frequency of the three genotypes and grade or stage of each type of tumor. The association of genetic predisposition to urologic cancers with p53 gene codon 72 polymorphism is not so clear as the previous study of Japanese lung cancer patients, but this polymorphism may play some role in urothelial cancers and renal cell carcinoma, in which smoking is an epidemiological risk factor.

Augmented DNA-binding activity of p53 protein encoded by a carboxyl-terminal alternatively spliced mRNA is blocked by p53 protein encoded by the regularly spliced form

DNA-binding activity of the wild-type p53 is central to its function in vivo. However, recombinant or in vitro translated wild-type p53 proteins, unless modified, are poor DNA binders. The fact that the in vitro produced protein gains DNA-binding activity upon modification at the C terminus raises the possibility that similar mechanisms may exist in the cell. Data presented here show that a C-terminal alternatively spliced wild-type p53 (ASp53) mRNA expressed by bacteria or transcribed in vitro codes for a p53 protein that efficiently binds DNA. Our results support the conclusion that the augmented DNA binding activity of an ASp53 protein is probably due to attenuation of the negative effect residing at the C terminus of the wild-type p53 protein encoded by the regularly spliced mRNA (RSp53) rather than acquisition of additional functionality by the alternatively spliced C' terminus. In addition, we found that ASp53 forms a complex with the non-DNA-binding RSp53, which in turn blocks the DNA-binding activity of ASp53. Interaction between these two wild-type p53 proteins may underline a mechanism that controls the activity of the wild-type p53 protein in the cell.

Constitutional p53 mutations associated with brain tumors in young adults

Identification of patients at risk for developing brain tumors is important for the development of preventative strategies. Because individuals with germline p53 mutations may be at increased risk, we examined DNA from brain tumor-derived cell lines and malignant and normal nervous system tissue for p53 gene mutations using the single strand conformation polymorphism assay and direct sequencing of polymerase chain reaction-amplified DNA. We found mutations in the p53 gene in eight of 22 adult glioma tissue specimens and germline mutations in two of these eight patients. In contrast, mutation of the p53 gene was not detectable in either 16 glial tumors occurring in children, glial tumor tissue from three unrelated glioblastoma multiforme patients with a familial history of cancer, or in benign meningiomas. One constitutional p53 mutation was a G to T transversion at codon 154, and the second was a C to T transition at codon 256. Both patients with germline mutations developed glioblastoma multiforme before the age of 31, although the median age for glioma patients is above 50. These findings suggest that p53 germline mutations may identify a subset of young adults predisposed to the development of high-grade astrocytic tumors.

TP53 mutations in prostatic cancer. Analysis of pre- and post-treatment archival formalin-fixed tumour tissue

The TP53 gene mutation pattern in prostatic cancer was examined in relation to progression and survival, using archival formalin-fixed pre- and post-treatment tumour specimens from 84 prostatic cancer patients. Thirty-four had hormone-sensitive tumours and 50 were hormone-resistant. Six of the 34 (18 per cent) therapy-responding tumours and 19 of the 50 (38 per cent) hormone-resistant tumours showed p53 protein accumulation in the post-treatment specimen. Both pre- and post-treatment specimens from these 25 patients were analysed for mutation of the conserved regions of the TP53 gene (exons 5-8), using constant denaturant gel electrophoresis (CDGE) followed by DNA sequencing. In the post-treatment samples, mutations were detected in three of the six patients with hormone-responsive tumours and in 11 of the 19 patients with hormone-resistant tumours. The three (100 per cent) patients with therapy-responsive tumours with mutations and nine of the 11 (82 per cent) patients with therapy-resistant tumours with mutations died of the disease. Thirteen of the 14 mutations in the post-treatment specimens were transitions, 11 occurring at CpG dinucleotides in which codon 273 was involved in ten. A significantly higher proportion of tumours with mutations were poorly differentiated compared with tumours without mutation (P < 0.04). Our findings indicate that TP53 mutation is a late event in tumour development of the prostate gland and that codon 273 might be a 'hotspot' for mutation in the progression of the disease.

p53 mutation and MDM2 amplification are rare even in human papillomavirus-negative cervical carcinomas

BACKGROUND

Mutation of the p53 tumor suppressor gene is the most commonly found genetic alteration in human cancer. The E6 gene product of human papillomavirus (HPV) 16 and 18 can inactivate the p53 protein by promoting its degradation. Because most HPV-positive cervical carcinoma cell lines contain wild-type p53 whereas HPV-negative cell lines have point mutations in the p53 gene, a major role in the development of HPV-negative cervical cancer has been attributed to p53. Recent studies, however, have observed no consistent presence of p53 mutation in HPV-negative primary cervical carcinomas. The MDM2 oncogene, which forms an autoregulatory loop with the wild-type p53 protein, has been found amplified in a high percentage of human sarcomas, thus abolishing the antiproliferative function of p53.

METHODS

Forty-three primary cervical carcinomas and 10 autopsy-derived distant metastases from one patient were examined for p53 mutation and MDM2 amplification. These tumors had been selected from 238 cervical cancers that had been HPV-typed by Southern blot hybridization and polymerase chain reaction as a representative sample for their HPV status and their clinicopathologic characteristics. Seventeen of the cases had a remarkably good or poor clinical outcome. Human papillomavirus DNA sequences had been detected in 30 of these 43 primary tumors and 13 were negative for HPV by both methods. p53 mutation in the highly conserved exons 5-8 was studied by single-strand conformation polymorphism analysis and direct sequencing. MDM2 amplification was analyzed by Southern blot hybridization.

RESULTS

Only two missense point mutations and one nucleotide sequence polymorphism were detected: a TAC-->TGC transition in codon 234 in exon 7, resulting in a Tyr-->Lys substitution, a CGT-->TGT transition in codon 273 in exon 8, resulting in an Arg-->Cys substitution and a polymorphism (CGA-->CGG) in codon 213 in exon 6. Both tumors revealing the point mutations were HPV-negative carcinomas. Amplification of the MDM2 gene was observed in 1 of the 53 specimens tested.

CONCLUSIONS

In contrast to data derived from cultured cervical carcinoma cell lines and primary sarcomas, these results indicate that p53 mutation and amplification of the MDM2 oncogene are rare even in HPV-negative primary cervical carcinomas. However, to the authors; knowledge, this is the first observation of MDM2 amplification in humans outside sarcomas and neuroepithelial tumors.