p53: at the crossroads of molecular carcinogenesis and risk assessment

Clinical applications of the p53 tumor suppressor gene

The interest on the p53 gene has grown enormously in the last 4 years. It is now known that p53 is directly involved in important cellular functions including regulation of the cell cycle, and that its alteration may be one of the important steps in the initiation of cancer. In this review I will cover briefly basic and clinical aspects related to the p53 gene and protein and explore ways of using the accumulated knowledge for patient diagnosis and monitoring. The literature suggests that it is now appropriate to start assessing the p53 gene status of breast tumors for prognosis. Therapeutic options are at an infancy stage. A new diagnostic approach based on the immune response of cancer patients against mutant proteins is discussed, by using the p53 mutant protein as a model. Although the cancer patient has not as yet benefited directly from the enormous number of investigations on the p53 gene and protein, there is hope that in the long-term these studies will promote the understanding of cancer initiation and progression at the molecular level with a practical return at a later phase.

Apoptosis, cancer and the p53 tumour suppressor gene

One of the most commonly detected abnormalities in human cancer is mutation of the p53 tumour suppressor gene. Intrinsic to the function of p53 is its ability to induce apoptotic cell death and to cause cell cycle arrest. Moreover, p53 plays an important role in controlling the cellular response to DNA damaging agents such as ionizing radiation and cancer chemotherapeutic drugs. Loss of p53 function causes increased resistance to radiation and chemotherapeutic agents, and there is increasing evidence that p53 mutational status is an important determinant of clinical outcome in cancer. This review will focus on recent data describing the biochemistry of p53 function, its role in mediating apoptosis and cell cycle arrest and in the control of tumour growth and death.

Tumor suppressor genes and clonal evolution in B-CLL

This review highlights the genetic alterations that have been detailed in the malignant B-cell clones of patients with B-chronic lymphocytic leukemia (CLL). In particular, the alterations seen in p53 and the retinoblastoma (Rb) genes are reviewed. In addition, the multiplicity of cytogenetic alterations observed at baseline and on sequential analysis are summarized. The cytogenetic and molecular biologic analysis of B-CLL clones has revealed that there is a dynamic array of genetic events which occur within a B-cell clone. This latter data strongly suggests that clonal evolution may occur in B-CLL patients. However the relationship of the clonal instability to the patient's clinical course is still unclear. The relatively frequent detection of multiple tumor suppressor gene alterations in the B-CLL clones offer several interesting clues regarding the transformation event within B-CLL. A model is proposed which attempts to explain the potential contribution and interaction of p53 and Rb gene alterations in a malignant B-cell transformation.

Potential genotoxicity of chronically elevated nitric oxide: a review

Several human cancers are associated with chronic bacterial, viral and parasitic infections. Nitric oxide, which is a short-lived free radical produced by many types of cells for a number of important physiological functions, is elevated in these infections. Long-term exposure to elevated NO. in cells could have potential genotoxic effects on hosts. There are at least three mechanisms by which intracellular elevated NO. could exert genotoxic affects after reacting with O2. These include formation of carcinogenic N-nitroso compounds, direct deamination of DNA bases, and oxidation of DNA after formation of peroxynitrite and/or hydroxyl radicals. One or more of these mechanisms could, theoretically, explain why chronic infection increases the risk of certain cancers.

Cell proliferation markers in predicting metastases in malignant melanoma

It is often difficult to predict the outcome of melanoma in patients with Clark level III-IV disease. We sought to identify markers of cell proliferation which may be useful in predicting prognosis. Patients with Clark's level III-IV malignant melanoma who had no local recurrences or metastases were matched with patients of comparable level and thickness who did experience recurrences of metastases. Cell proliferation markers p53, proliferating cell nuclear antigen (PCNA), and Ki-67 were assessed by immunohistochemistry. DNA ploidy was determined by flow cytometry. There was no difference in the expression of p53, PCNA, and Ki-67 between patients with metastases and patients without metastases. However, patients with metastases were more likely to have an aneuploid tumor cell population than were patients without metastases (p < 0.03). Expression of cell proliferation markers do not appear to help predict prognosis in advanced level melanoma; however, aneuploidy may be associated with a greater probability of metastasis.

N-methylnitrosourea-induced Ki-ras codon 12 mutations: early events in mouse thymic lymphomas

N-Methylnitrosourea (NMU)-induced codon 12 Ki-ras mutations were analyzed in premalignant thymic lymphomas from C57BL/6J mice by using a selective polymerase chain reaction amplification strategy. The frequency of codon 12 Ki-ras mutations was 67% (16 of 24) in NMU-treated animals with premalignant stage I disease. Previously, animals with different stages of disease had been analyzed for cytogenetic changes and for mutations in the p53 tumor suppressor gene. The genetic changes observed were early-activating codon 12 G35-->A transition mutations of the Ki-ras gene, followed closely by trisomy 15 and infrequent mutation of the p53 gene late in tumor development. The consistent and early detection of Ki-ras mutations in NMU-treated animals but not in untreated controls suggests that the mutations result from direct carcinogen exposure. Alternate pathways of NMU-induced thymic lymphomagenesis were implicated. One pathway involved putative NMU-induced mutations in other, non-ras oncogenes that cooperate with trisomy 15 to produce similar T-cell tumors. The frequency of p53 gene mutations in human and murine T-cell tumors is similar but low.

Technological trends in clinical laboratory science

OBJECTIVE

This article will review the advancements and new developments being made in (1) advanced computers, (2) microtechnology, (3) advanced immunodiagnostics, (4) neural networks, and (5) molecular biology. The influence of these technologies and their products on clinical laboratories is also discussed.

CONCLUSION

Significant evolutionary and revolutionary technological changes are occurring in a number of scientific and engineering disciplines that impact the medical profession. As a consequence of this era of rapid change, the discipline of Clinical Laboratory Science is undergoing a "technological explosion" which is having a significant and profound effect on how clinical laboratories of today and tomorrow are and will be staffed, equipped, and operated.

p53 oncoprotein overexpression correlates with mutagen-induced chromosome fragility in head and neck cancer patients with multiple malignancies

In this study, we analysed immunocytochemically p53 expression in first primary and second primary cancers from 25 head and neck cancer patients (HNCPs) with multiple malignancies in comparison with oncoprotein expression in tumour tissues from 25 historical HNCP controls with single cancer in a match-paired analysis. Moreover, we investigated bleomycin-induced chromosome fragility in both groups of HNCPs and in 21 additional healthy controls. Thirty-nine out of 75 tumour specimens analysed (52%) showed positive p53 immunostaining. Eleven out of 25 (44%) from single cancer patients and 28 out of 50 (56%) tumours from HNCPs with multiple malignancies were p53 positive. In the group of multiple primary cancers, nine patients (36%) showed positive staining of both first and second primaries, whereas six (24%) had positive labelling of first primary cancer but not of the subsequent second primary, four (16%) patient showed p53 expression only in the second primary cancer and six (24%) patients showed no p53 immunoreactivity in both tumours. Chromosomal analysis demonstrated a higher sensitivity to clastogens of HNCPs with multiple tumours than of HNCPs with a single cancer (P < 0.01), and a significant correlation between chromosome fragility and p53 overexpression (P < 0.01) only in HNCPs with multiple malignancies more than in those with single head and neck cancer (P = 0.11). Moreover, we found that patients with p53-positive staining of both first and second primaries showed a statistically significant higher mutagen sensitivity than those with a single p53 immunoreactive tumour or those in whom both cancers were p53 negative (P < 0.01). Our data suggest that subjects with increased susceptibility to carcingogens after exposure to tobacco or alcohol are at higher risk for multiple cancers in which one of the most common genetic events is aberrant p53 expression.

How much of the human disease burden is attributable to environmental chemicals?

According to the evaluations made by IARC, 66 agents or exposures have been recognised as human carcinogens. About 60% of all cancer cases occur in people over 65 years of age, which is indeed the most important risk factor for cancer, including both the duration of exposure to the variety of carcinogenic agents and allowing expression of genetically determined disorders. Diet as a source of mutagens as well as other environmental mutagens may affect blood pressure either directly or by favouring the development of arteriosclerosis: mutagens have been shown to accelerate arteriosclerotic plaque development. Some concern has also been expressed on the possible adverse effect that environmental chemicals may have on reproductive ability, on the basis of the decline in semen quality reported over the past decades. Better defining the role of aetiological agents would result in a more precise definition of attributable risks. Of particular interest to the latter goal are the studies of inter-individual variability in the susceptibility to carcinogens, which will hopefully contribute to define the role of low-level exposure to carcinogens.

Backbone dynamics of the oligomerization domain of p53 determined from 15N NMR relaxation measurements

The backbone dynamics of the tetrameric p53 oligomerization domain (residues 319-360) have been investigated by two-dimensional inverse detected heteronuclear 1H-15N NMR spectroscopy at 500 and 600 MHz. 15N T1, T2, and heteronuclear NOEs were measured for 39 of 40 non-proline backbone NH vectors at both field strengths. The overall correlation time for the tetramer, calculated from the T1/T2 ratios, was found to be 14.8 ns at 35 degrees C. The correlation times and amplitudes of the internal motions were extracted from the relaxation data using the model-free formalism (Lipari G, Szabo A, 1982, J Am Chem Soc 104:4546-4559). The internal dynamics of the structural core of the p53 oligomerization domain are uniform and fairly rigid, with residues 327-354 exhibiting an average generalized order parameter (S2) of 0.88 +/- 0.08. The N- and C-termini exhibit substantial mobility and are unstructured in the solution structure of p53. Residues located at the N- and C-termini, in the beta-sheet, in the turn between the alpha-helix and beta-sheet, and at the C-terminal end of the alpha-helix display two distinct internal motions that are faster than the overall correlation time. Fast internal motions (< or = 20 ps) are within the extreme narrowing limit and are of uniform amplitude. The slower motions (0.6-2.2 ns) are outside the extreme narrowing limit and vary in amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)

P53 gene mutations in lymphoid diseases and their possible relevance to drug resistance

Mutations in the p53 tumor suppressor gene occur with a frequency of 12.5% in lymphoid malignancies. The viral-associated diseases, Adult T-cell Leukemia (ATL) and Burkitt's lymphoma, showed higher p53 mutation frequencies of 24% and 41%, respectively. Mutations occurred in the highly conserved regions of the p53 gene. Two new hot spots for mutation were noted in exon 7 at codons 239 and 245. The spectrum of p53 mutations differs among different cancers. Transition mutations occurring in colon and brain tumors also predominated in the majority of the lymphoid malignancies. However, B-cell chronic lymphocytic leukemia (B-CLL) and non-Hodgkin's lymphoma (NHL) had an unusually high frequency of G to T transversions. Among carcinomas of the lung, liver, breast and esophagus there is also a high frequency of G to T transversions. The differences in mutation spectra between different lymphoid diseases may be due to differences in mutagenic factors or differences in the biological properties of the p53 protein in different lymphoid compartments. Mutation of the p53 gene is associated with advanced stage of lymphoid disease and poor prognosis. For B-CLL disease, p53 mutations are associated with drug resistance. Overexpression of the bcl-2 protein is also associated with a block in apoptosis. Resistance to apoptosis could be a general mechanism for drug resistance in B-CLL and other lymphoid diseases.

Refined solution structure of the oligomerization domain of the tumour suppressor p53

The NMR solution structure of the oligomerization domain of the tumour suppressor p53 (residues 319-360) has been refined. The structure comprises a dimer of dimers, oriented in an approximately orthogonal manner. The present structure determination is based on 4,472 experimental NMR restraints which represents a three and half fold increase over our previous work in the number of NOE restraints at the tetramerization interface. A comparison with the recently solved 1.7 A resolution X-ray structure shows that the structures are very similar and that the average angular root-mean-square difference in the interhelical angles is about 1 degree. The results of recent extensive mutagenesis data and the possible effects of mutations which have been identified in human cancers are discussed in the light of the present structure.

p53 mutations, protein expression and cell proliferation in squamous cell carcinomas of the head and neck

Thirty-three patients with squamous cell carcinoma of the head and neck region were studied concerning p53 protein expression and mutations in exons 4-9 of the p53 gene using immunohistochemistry, polymerase chain reaction (PCR)-single strand conformation polymorphism analysis and DNA sequencing. Immunoreactivity was found in 64% and p53 gene mutations in 39% of the tumours. Thirty-three per cent of the immunopositive and 50% of the immunonegative tumours were mutated within exons 5-8. In one immunopositive tumour three variants of deletions were observed. Sequencing of the p53 mutated, immunonegative tumours revealed four cases with deletions, one case with a transversion resulting in a stop codon and one case with a splice site mutation which could result in omission of the following exon at splicing. All mutations in the immunonegative tumours resulted in a truncated p53 protein. No association between p53 gene status and expression of proliferating cell nuclear antigen (PCNA) or cell proliferation as judged by in vivo incorporation of the thymidine analogue iododeoxyuridine (IdUrd) was found.

Accumulation of wild-type p53 in meningiomas

The p53 tumour suppressor gene contributes to the regulation of DNA repair and the initiation of apoptosis. Mutations of this gene and nuclear accumulation of its protein product are features of a large variety of tumours. A histological spectrum of meningiomas, including anaplastic examples and a meningosarcoma, was analysed for accumulation of the p53 protein and mutations in exons 4-9 of the p53 gene. No mutations were found, but 9/34 (26%) tumours showed accumulation of the p53 protein. The p53-positive meningiomas came from across the histological spectrum and were not restricted to the anaplastic group. The accumulation of wild-type p53 in a proportion of meningiomas may reflect this gene's role in DNA repair, or its enhanced stability when bound to cellular proteins such as the mdm-2 gene product.

Family history of cancer, body weight, and p53 nuclear overexpression in Duke's C colorectal cancer

To examine the hypothesis that colorectal carcinomas with and without TP53 mutations may be characterised by aetiological heterogeneity, we analysed a group of 107 patients with primary Dukes' C colorectal cancer seen at the Memorial Sloan-Kettering Cancer Center (MSKCC) from 1986 to 1990. We assessed p53 overexpression using the monoclonal antibody PAb 1801, and identified 42 (39%) patients displaying p53-positive phenotype, defined as > or = 25% of positive cells. Patients with two or more first-degree relatives with cancer had an odds ratio (OR) of 2.9 (95% CI 1.0-8.3) for p53 overexpression in comparison with those without a family history of cancer (trend test, P = 0.11). A possible association between body weight and p53 overexpression was observed. The ORs were 1.9 for the second quartile, 1.9 for the third quartile and 3.4 for the highest quartile in comparison with the lowest quartile (trend test, P = 0.06). No association between occupational physical activity, smoking, drinking, parity and p53 overexpression was identified. The results suggest that p53 overexpression may be related to genetic predisposition to colorectal cancer, and p53-positive and p53-negative colorectal cancers may be controlled by different aetiological pathways.

Regulation of the sequence-specific DNA binding function of p53 by protein kinase C and protein phosphatases

The p53 tumor suppressor protein is a transcription factor with sequence-specific DNA binding activity that is thought to be important for the growth-inhibitory function of p53. DNA binding appears to require activation of a cryptic form of p53 by allosteric mechanisms involving a negative regulatory domain at the carboxyl terminus of p53. The latent form of p53, reactive to the carboxyl-terminal antibody PAb421, is produced in a variety of eukaryotic cells, suggesting that activation of p53 is an important rate-limiting step in vivo. In this report we provide evidence that phosphorylation of serine 378 within the carboxyl-terminal negative regulatory domain of the human p53 protein by protein kinase C correlates with loss of PAb421 reactivity and a concomitant activation of sequence-specific DNA binding. These effects are reversed by subsequent dephosphorylation of the protein kinase C-reactive site by protein phosphatases 1 (PP1) and 2A (PP2A), which restore the reactivity of p53 to PAb421 and regenerate the latent form of p53 lacking significant DNA binding activity. Thus, p53 is subject to both positive and negative regulation by reversible enzymatic modifications affecting the latent or active state of the protein, suggesting a possible mechanism for the regulation of its tumor suppressor function.

P53, cell cycle control and apoptosis: implications for cancer

Cellular proliferation depends on the rates of both cell division and cell death. Tumors frequently have decreased cell death as a primary mode of increased cell proliferation. Genetic changes resulting in loss of programmed cell death (apoptosis) are likely to be critical components of tumorigenesis. Many of the gene products which appear to control apoptotic tendencies are regulators of cell cycle progression; thus, cell cycle control and cell death appear to be tightly linked processes. P53 protein is an example of a gene product which affects both cell cycle progression and apoptosis. The ability of p53 overexpression to induce apoptosis may be a major reason why tumor cells frequently disable p53 during the transformation process. Unfortunately, the same genetic changes which cause loss of apoptosis during tumor development, may also result in tumor cell resistance to anti-neoplastic therapies which kill tumor cells by apoptosis. Elucidation of the genetic and biochemical controls of these cellular responses may provide insights into ways to induce cell death and thus hopefully suggest new targets for improving therapeutic index in the treatment of malignancies.

The effects of hydrogen peroxide on DNA repair activities

Oxygen free radicals generated by H2O2 are involved in the multistage carcinogenic process; mechanisms include carcinogen activation, oxidative DNA damage, and tumor promotion. In this study, we have evaluated another potential mechanism of H2O2 in carcinogenesis--modulation of DNA repair activities. Preexposure of human peripheral mononuclear leukocytes to H2O2 significantly inhibited DNA repair activities in response to damage induced by N-methyl-N'-nitro-N-nitrosoguanidine, measured as unscheduled DNA synthesis. The responses to H2O2 were compared in four healthy human subjects with two sample preparations on different days. Results from multivariate general linear models showed that H2O2 significantly inhibited DNA repair in a dose-dependent manner after adjustment for between- and within-subject variabilities. There was an estimate of 5.0 units (dpm/5 x 10(5) cells) decrease in induced unscheduled DNA synthesis per unit (microM) increase of H2O2 treatment. Furthermore, there was substantial variability in DNA repair activities for the same individual sampled on different days regardless of H2O2 dose level. Results from this study suggest that H2O2 not only can induce DNA damage, but also have suppressive effects on DNA repair.

Heterogeneity in Li-Fraumeni families: p53 mutation analysis and immunohistochemical staining

We have screened two families for constitutional TP53 mutations, one family with Li-Fraumeni syndrome and the other with features of this syndrome. We report a germline mutation in exon 7 of the TP53 gene in the family with "Li-Fraumeni-like" syndrome. The mutation occurred at codon 245 and causes a Gly-Ser amino acid change. It was inherited by both affected and unaffected subjects. Malignant tumours from all members of this family showed strong positive nuclear immunohistochemical staining with antibodies CM-1 and DO1, directed against TP53. In contrast, no constitutional TP53 mutations were found in a "classic" Li-Fraumeni family. In this family positive staining was seen in both malignant and normal tissues. These results support previous findings that variants of the Li-Fraumeni syndrome exist since not all LFS families carry TP53 germline mutations. Secondly, immunohistochemical positivity is not synonymous with an underlying mutation and is therefore inadequate as an exclusive diagnostic marker.

p53 binds single-stranded DNA ends through the C-terminal domain and internal DNA segments via the middle domain

We have previously reported that wild-type p53 can bind single-stranded (ss) DNA ends and catalyze renaturation of ss complementary DNA molecules. Here we demonstrate that p53 can also bind to internal segments of ss DNA molecules via a binding site (internal DNA site) distinct from the binding site for DNA ends (DNA end site). Using p53 deletion mutants, the internal DNA site was mapped to the central region (residues 99-307), while the DNA end site was mapped to the C-terminal domain (residues 320-393) of the p53 protein. The internal DNA site can be activated by the binding of ss DNA ends to the DNA end site. The C-terminal domain alone was sufficient to catalyze DNA renaturation, although the central domain was also involved in promotion of renaturation by the full-length protein. Our results suggest that the interaction of the C-terminal tail of p53 with ss DNA ends generated by DNA damage in vivo may lead to activation of non-specific ss DNA binding by the central domain of p53.

Stable and unstable genes: distribution among species and range of mutations

An expansion of the 'stable' gene hypothesis proposed previously is the hypothesis that there is a range of genes, some proto-oncogenes and some not, varying from those with multiple copies in the genome, wide distribution among species, and limited mutability because of a specific protective stabilizing mechanism of such genes, to those that are in single copies in the genome, unprotected, unique to one species, and highly mutable along their entire length.

p53 transcriptional activation mediated by coactivators TAFII40 and TAFII60

The tumor suppressor protein p53 is a transcriptional regulator that enhances the expression of proteins that control cellular proliferation. The multisubunit transcription factor IID (TFIID) is thought to be a primary target for site-specific activators of transcription. Here, a direct interaction between the activation domain of p53 and two subunits of the TFIID complex, TAFII40 and TAFII60, is reported. A double point mutation in the activation domain of p53 impaired the ability of this domain to activate transcription and, simultaneously, its ability to interact with both TAFII40 and TAFII60. Furthermore, a partial TFIID complex containing Drosophila TATA binding protein (dTBP), human TAFII250, dTAFII60, and dTAFII40 supported activation by a Gal4-p53 fusion protein in vitro, whereas TBP or a subcomplex lacking TAFII40 and TAFII60 did not. Together, these results suggest that TAFII40 and TAFII60 are important targets for transmitting activation signals between p53 and the initiation complex.

Spontaneous in vitro immortalization of breast epithelial cells from a patient with Li-Fraumeni syndrome

Individuals with germ line mutations in the p53 gene, such as Li-Fraumeni syndrome (LFS), have an increased occurrence of many types of cancer, including an unusually high incidence of breast cancer. This report documents that normal breast epithelial cells obtained from a patient with LFS (with a mutation at codon 133 of the p53 gene) spontaneously immortalized in cell culture while the breast stromal fibroblasts from this same patient did not. Spontaneous immortalization of human cells in vitro is an extremely rare event. This is the first documented case of the spontaneous immortalization of breast epithelial cells from a patient with LFS in culture. LFS patient breast stromal fibroblasts infected with a retroviral vector containing human papillomavirus type 16 E7 alone were able to immortalize, whereas stromal cells obtained from patients with wild-type p53, similarly infected with human papillomavirus type 16 E7, did not. The present results indicate a protective role of normal pRb-like functions in breast stromal fibroblasts but not in breast epithelial cells and reinforces an important role of wild-type p53 in the regulation of the normal growth and development of breast epithelial tissue.

Impact of the human genome project on medical practice

BACKGROUND

The Human Genome Project is a coordinated effort to define the human genetic blueprint. The goals include construction of a variety of maps of the human genome, including the identification and localization of all genes. The discovery of genes responsible for human diseases has had a significant impact on the practice of medicine.

METHODS

Methods for defining the human genome include cytogenetic, physical, and genetic mapping techniques. A variety of strategies have been used to identify human genes, especially those genes that are responsible for disease. Once a disease gene has been identified, this information can be used to develop new diagnostic and therapeutic procedures.

RESULTS

A number of disease genes have already been identified, leading to improved diagnosis and novel approaches to therapy. A new type of mutation, trinucleotide repeat expansion, has been found to be responsible for at least seven diseases with an unusual inheritance pattern.

CONCLUSIONS

Materials and technology generated by the Human Genome Project and related research have provided important tools for the diagnosis and treatment of patients afflicted with genetic diseases.

Structures of protein complexes by multidimensional heteronuclear magnetic resonance spectroscopy

With the advent of multidimensional heteronuclear-edited and -filtered NMR experiments, the field of three-dimensional structure determination by NMR has again increased in scope, making it possible to move the technology beyond the approximately 10 kDa limit inherent to conventional two-dimensional NMR to systems up to potentially 35 to 40 kDa. This article outlines the basic strategies for solving three-dimensional structures of larger systems, in particular, protein complexes and multimeric proteins using three- and four-dimensional NMR spectroscopy, summarizes the key experiments, and illustrates the power of these methods using several examples of protein-DNA, protein-peptide complexes, and oligomeric proteins from the authors' laboratories.

Mutagenic potency of exocyclic DNA adducts: marked differences between Escherichia coli and simian kidney cells

A single-stranded shuttle vector containing a single 3,N4-etheno-2'-deoxycytidine (epsilon dC) or 1,N2-(1,3-propano)-2'- deoxyguanosine (PdG) DNA adduct was used to investigate translesional DNA synthesis in Escherichia coli and simian kidney (COS) cells. The presence of either exocyclic adduct was associated with reduced numbers of transformants. In E. coli, this inhibitory effect could be overcome partially by irradiating cells with UV light before transformation. Translesional synthesis past both exocyclic lesions was accompanied by targeted mutations. For PdG, the primary mutagenic events observed in both hosts were PdG-->T transversions; in preirradiated E. coli, PdG-->A transitions were also observed. The targeted mutation frequency for single-stranded DNA that contained PdG was 100% in nonirradiated E. coli, 68% in preirradiated cells, and 8% in COS cells. In contrast, the targeted mutation frequency for single-stranded DNA that contained epsilon dC was 2% in nonirradiated E. coli, 32% in preirradiated cells, and 81% in COS cells. The primary mutations generated by epsilon dC in both E. coli and COS cells were epsilon dC-->A and epsilon dC-->T base substitutions. These observations appear to reflect the variable specificity of DNA replication complexes in incorporating bases opposite certain adducts. We conclude that DNA synthesis past the same DNA adduct can have strikingly different consequences in bacteria and mammalian cells, underscoring the importance of establishing the intrinsic mutagenic potential of DNA adducts in mammalian cells.

Atypical alveolar hyperplasia: relationship with pulmonary adenocarcinoma, p53, and c-erbB-2 expression

Atypical alveolar hyperplasia (AAH) has recently been described in human lungs in association with primary lung cancer, particularly adenocarcinoma. Unlike proximal bronchogenic carcinoma, peripheral (parenchymal) adenocarcinoma of the lung does not have a well-recognized progenitor lesion. Epidemiological morphometric, and cytofluorometric data in the literature suggest that AAH is a candidate premalignant entity. In this study, 97 AAH lesions were found in lungs resected from 29 patients (1-13 lesions per case, mean 3.5) being treated for presumed carcinoma (25/29 had adenocarcinoma). From a study case-load of 285 adenocarcinoma-bearing lungs, the AAH incidence was 8.8 per cent. Sections of 67 AAH lesions from 19 patients were stained using monoclonal antibodies against Ki67 (MIB1), p53 (DO7), and c-erbB-2 (NCL-CB11). Ki67 was expressed in up to 10 per cent of AAH nuclei. Thirty-nine lesions (58 per cent) showed stainable p53 protein, while five (7 per cent) expressed membrane c-erbB-2 oncoprotein. These latter five lesions were all strongly positive for p53, and both p53 and c-erbB staining was associated with increased cellular crowding and pleomorphism in AAH. These data demonstrate that AAH exhibits some genetic changes associated with malignancy and thereby support the hypothesis that AAH is premalignant.

Cdk inhibitors: on the threshold of checkpoints and development

The regulation of cyclin-dependent kinases is at the heart of cell cycle control and, by inference, the control of cell proliferation. Recent advances in regulation of these kinases have uncovered a group of small proteins that bind to and inhibit them, thus preventing cell cycle progression. Linking these proteins to tumor suppressor functions has provided a much sought after connection between cancer and cell cycle control.

Detection and classification of mutagens: a set of base-specific Salmonella tester strains

A detection and classification system for mutagens has been developed that identifies the six possible base-pair substitution mutations. A set of six Salmonella typhimurium (TA7001 to TA7006) strains has been constructed, each of which carries a unique missense mutation in the histidine biosynthetic operon. In addition to the his mutation, these strains carry different auxiliary features that enhance the mutability of the target his mutation. These include the R factor pKM101, which has the SOS-inducible mucAB system; a deletion of the uvrB component of excision repair; and rfa mutations to increase the accessibility of bulky chemicals to the bacteria. Another set of strains (TA7041 to TA7046) contain a wild-type rfa gene. Reversion via the base substitution unique to each strain was verified by sequence analyses of > 800 revertants obtained from different types of mutagens. The strains have considerably lower spontaneous reversion frequencies and detect a variety of mutagens at a sensitivity comparable to the Salmonella tester strains TA100, TA102, and TA104. The low spontaneous frequency of reversion of a mixture of the six tester strains (approximately 10 revertants per plate) enables a single mutation assay with the mixture that is followed by classification of the type of mutation with the individual strains.

Changes in p53 and cyclin D1 protein levels and cell proliferation in different stages of human esophageal and gastric-cardia carcinogenesis

The objective of this study was to quantify the changes in p53 and cyclin D1 protein levels in different stages of human esophageal and gastric cardia carcinogenesis in a high-risk population in Henan, China. Immunoreactivity of p53, cyclin D1 and proliferating-cell nuclear antigen (PCNA) was observed in the cell nuclei of esophageal and gastric cardia biopsies. The number of p53-immunostaining-positive cells was low in normal epithelia, slightly increased in basal-cell hyperplasia (BCH), markedly increased in dysplasia (DYS) (10-fold), and further increased in squamous-cell carcinoma (SCC) (40-fold). This pattern of change was similar to that of cell proliferation as indicated by PCNA immunostaining. On the other hand, the number of cyclin D1-immunostaining-positive cells did not increase from BCH to DYS, although a slight increase from DYS to SCC was noted. In the gastric cardia, again, the pattern of change of p53-positive cells in different stages of lesions paralleled the pattern of cell proliferation. The number of p53-positive cells was very low, much lower than that of PCNA-positive cells, in normal, chronic superficial gastritis (CSG) and chronic atrophic gastritis (CAG); therefore, the increase of p53-positive cells from CAG to DYS was more dramatic (100-fold). From DYS to adenocarcinoma (AC), the p53-positive and the PCNA-positive cells increased 4-fold. On the other hand, the number of cyclin D1-positive cells did not increase in pre-cancerous lesions, but increased slightly in AC. This study demonstrates that p53 protein accumulation increased with the progression of pre-cancerous lesions, especially in the genesis of dysplasia, both in the esophagus and in the gastric cardia. Our approach of quantitative immunohistochemistry sheds light on the mechanisms of genesis of esophageal and gastric-cardia cancers, which frequently occur together in many high-incidence areas.

Flow cytometric quantitation of C-myc and P53 proteins in bovine papillomavirus type 1-transformed primary mouse fibroblasts

Bovine papillomavirus type 1 (BPV-1)-transformed mouse fibroblast cell lines were analyzed via flow cytometry (FCM) for expression of p53 and c-myc proteins along with their DNA content. In comparison to the nontransformed control cell line, significantly elevated levels of both the p53 and the c-myc protein were present in some but not all of the transformed cell lines. Quantitation of p53 and c-myc proteins in cell lines containing BPV-1 DNA revealed that the tumorigenic cell lines expressed higher levels of both the p53 (P = 0.0034; Mann-Whitney U test) and the c-myc protein (P = 0.0039; Mann-Whitney U test) as compared to the nontumorigenic cell lines. On average, at least 9,000-10,000 p53 or c-myc protein molecules per cell were detected in the transformed tumorigenic cell lines. These results show that quantitative FCM can be reliably used to detect very low levels (3,000 molecules per cell) of specific protein, and FCM is a useful tool to study the virus-induced changes in the levels of nuclear proteins within a cell population and in tumorigenesis.

Gene regulation by temperature-sensitive p53 mutants: identification of p53 response genes

The ability of the p53 protein to act as a sequence-specific transcriptional activator suggests that genes induced by p53 may encode critical mediators of p53 tumor suppression. Using a tetracycline-regulated p53 expression system and cDNA library subtraction procedure, we identified several p53-induced gene transcripts in human Saos-2 osteosarcoma cells that are novel on the basis of their size, regulation, and low abundance. Wild-type p53-dependent induction of these transcripts was observed in cells that are growth arrested by p53, as well as in cells that undergo apoptosis upon expression of an inducible wild-type p53 transgene. These results show that p53 activates the expression of numerous response genes and suggest that multiple effectors may play a role in mediating cellular functions of p53.

The molecular biology of esophageal and gastric cancer and their precursors: oncogenes, tumor suppressor genes, and growth factors

The evolution of sequential histological changes from normal cells through invasive cancer affords the cancer biologist the opportunity to identify separate molecular steps involved in cancer progression. As one studies the development of human carcinoma, it becomes apparent that multiple genetic alterations affecting both cellular proto-oncogenes and tumor suppressor genes are involved during the development and progression of both esophageal and gastric cancers. The different histological forms of both esophageal and gastric carcinomas as well as their differing etiologies result in the possibility that a spectrum of genetic changes is involved in different tumor types. p53 abnormalities occur frequently in tumors arising in both organs, and in both sites p53 abnormalities can be observed in precancerous lesions as well as in overt cancer. Subsequent abnormalities affecting other genes (eg, epithelial growth factor receptors [EGFRs]) potentially enhance the growth potential of tumors. This review focuses on abnormalities of oncogenes, tumor suppressor genes, and growth factors commonly found in cancers of the esophagus and stomach.

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.

Analysis of the p16 gene (CDKN2) as a candidate for the chromosome 9p melanoma susceptibility locus

A locus for familial melanoma, MLM, has been mapped within the same interval on chromosome 9p21 as the gene for a putative cell cycle regulator, p16INK4 (CDKN2) MTS1. This gene is homozygously deleted from many tumour cell lines including melanomas, suggesting that CDKN2 is a good candidate for MLM. We have analysed CDKN2 coding sequences in pedigrees segregating 9p melanoma susceptibility and 38 other melanoma-prone families. In only two families were potential predisposing mutations identified. No evidence was found for heterozygous deletions of CDKN2 in the germline of melanoma-prone individuals. The low frequency of potential predisposing mutations detected suggests that either the majority of mutations fall outside the CDKN2 coding sequence or that CDKN2 is not MLM.

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.

Toward the development of an equitable cancer prevention

The most reasonable and socially acceptable development of cancer prevention should be the blending of the population approach, that is the shifting of the distribution of risk factors across an entire population in a favourable direction, with the high risk approach aimed at the identification, surveillance and possibly early interventions on individuals with particularly high values of predisposition to cancer. Interventions aimed at reducing or eliminating genetically determined weaknesses with regard to interactions with the environment, will not make, therefore, in any way obsolete or redundant, interventions aimed at eliminating or reducing exposure to environmental carcinogens.

Indirect mechanisms of HIV pathogenesis: how does HIV kill T cells?

Although twelve years have passed since the identification of HIV as the cause of AIDS, we do not yet know how HIV kills its target, the CD4+ T cell, nor how this killing cripples the immune system. Prominent theories include direct killing of infected CD4+ T cells by the action or accumulation of cytopathic viral DNA, transcripts or proteins, or by virus-specific cytotoxic T lymphocytes, and indirect killing of uninfected CD4+ T cells (and other immune cells) by autoimmune mechanisms, cytokines, superantigens, or apoptosis. In the past year, studies have provided tantalizing clues as to why infected cells may not die and how these infected cells kill innocent bystander cells.

Crystal structure of a p53 tumor suppressor-DNA complex: understanding tumorigenic mutations

Mutations in the p53 tumor suppressor are the most frequently observed genetic alterations in human cancer. The majority of the mutations occur in the core domain which contains the sequence-specific DNA binding activity of the p53 protein (residues 102-292), and they result in loss of DNA binding. The crystal structure of a complex containing the core domain of human p53 and a DNA binding site has been determined at 2.2 angstroms resolution and refined to a crystallographic R factor of 20.5 percent. The core domain structure consists of a beta sandwich that serves as a scaffold for two large loops and a loop-sheet-helix motif. The two loops, which are held together in part by a tetrahedrally coordinated zinc atom, and the loop-sheet-helix motif form the DNA binding surface of p53. Residues from the loop-sheet-helix motif interact in the major groove of the DNA, while an arginine from one of the two large loops interacts in the minor groove. The loops and the loop-sheet-helix motif consist of the conserved regions of the core domain and contain the majority of the p53 mutations identified in tumors. The structure supports the hypothesis that DNA binding is critical for the biological activity of p53, and provides a framework for understanding how mutations inactivate it.

High-resolution structure of the oligomerization domain of p53 by multidimensional NMR

The three-dimensional structure of the oligomerization domain (residues 319 to 360) of the tumor suppressor p53 has been solved by multidimensional heteronuclear magnetic resonance (NMR) spectroscopy. The domain forms a 20-kilodalton symmetric tetramer with a topology made up from a dimer of dimers. The two primary dimers each comprise two antiparallel helices linked by an antiparallel beta sheet. One beta strand and one helix are contributed from each monomer. The interface between the two dimers forming the tetramer is mediated solely by helix-helix contacts. The overall result is a symmetric, four-helix bundle with adjacent helices oriented antiparallel to each other and with the two antiparallel beta sheets located on opposing faces of the molecule. The tetramer is stabilized not only by hydrophobic interactions within the protein core but also by a number of electrostatic interactions. The implications of the structure of the tetramer for the biological function of p53 are discussed.

Identification of p53 mutations in endometrial adenocarcinoma by polymerase chain reaction-single-strand conformation polymorphism

OBJECTIVE

We determined whether mutations in p53 exons 5-6-7-8, as detected in the polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) test, might be correlated with stage or grade in endometrial adenocarcinoma.

METHODS

We amplified sequences containing exons 5, 6, 7, or 8 in DNA from tumors and controls. Mutation within the amplified sequences was indicated by changes in electrophoretic mobility (band shifts) in the SSCP test. The results were analyzed statistically and compared with the results of other, similar studies.

RESULTS

We identified 15 band shifts among 47 endometrial tumors studied (band shifts in two different exons in two cases) and none among 42 controls. Band shifts in exons 5 and 8 were associated uniformly with grade 2 or grade 3 histology. In other studies p53 mutations were correlated with advanced-stage malignancy.

CONCLUSION

Further evaluation of particular p53 mutations and their relation to disease course in endometrial adenocarcinoma seems warranted. The PCR-SSCP test seems well-suited to this purpose.