Amplification of a gene encoding a p53-associated protein in human sarcomas

Accumulation of p53 in a mutant cell line defective in the ubiquitin pathway

The wild-type p53 gene product plays an important role in the control of cell proliferation, differentiation, and survival. Altered function is frequently associated with changes in p53 stability. We have studied the role of the ubiquitination pathway in the degradation of p53, utilizing a temperature-sensitive mutant, ts20, derived from the mouse cell line BALB/c 3T3. We found that wild-type p53 accumulates markedly because of decreased breakdown when cells are shifted to the restrictive temperature. Introduction of sequences encoding the human ubiquitin-activating enzyme E1 corrects the temperature sensitivity defect in ts20 and prevents accumulation of p53. The data therefore strongly indicate that wild-type p53 is degraded intracellularly by the ubiquitin-mediated proteolytic pathway.

Assignment of two human epidermal squamous cell carcinomas cell lines to more than one complementation group for the immortal phenotype

Two human cell lines derived from squamous cell carcinomas (SCCs) of the epidermis, SCC-12 clone F and SCC-13 clone Y, were made to be independent of the Swiss 3T3 feeder layer to perform somatic-cell genetic experiments. We fused these SCC lines with normal human fibroblasts, and all resulting hybrids senesced after completing 12-17 population doublings, suggesting that in part, immortalization of the keratinocyte during SCC development results from the loss of gene function. We also tested whether these two SCC lines mapped to known complementation groups for immortality by fusing them with representatives of groups A (GM847), B (HeLa), and C (143B), but most of these hybrids were indistinguishable from those derived from homotypic crosses set up as immortal hybrid controls. As reported by others, fusions of cell lines from different complementation groups-143B (group C) x HeLa (group B) or GM847 (group A) x Hela (group B)--resulted in predominantly senescent hybrids. Our results confirmed and extended previous observations by others that the phenomenon of senescence is dominant to that of immortality, but they did not allow us to assign either of the SCC lines we studied to a complementation group for immortality.

A benign cultured colon adenoma bears three genetically altered colon cancer oncogenes, but progresses to tumorigenicity and transforming growth factor-beta independence without inactivating the p53 tumor suppressor gene

We describe the spontaneous progression of a colon adenoma cell line to tumorigenicity and growth factor independence. This system allows direct comparison of biologic stages of malignant progression with alterations of colon cancer suppressor genes and oncogenes. VACO-235, a human colon adenoma cell line, is at early passages nontumorigenic in the nude mouse, unable to grow in soft agar, growth stimulated by serum and EGF, and growth inhibited by TGF-beta. VACO-235 daughter passages 93 and higher have in culture spontaneously progressed to being weakly tumorigenic, but retain all other growth characteristics of VACO-235 early passages. A mouse xenograft from late passage VACO-235 was reestablished in culture as the granddaughter cell line, VACO-411. VACO-411 is highly tumorigenic, clones in soft agar, and is unresponsive to serum, EGF, and TGF-beta. Early passage VACO-235 bears a mutant K-ras allele, bears only mutant APC alleles, expresses no DCC transcripts, and expresses only wild type p53 transcripts. VACO-411 retains the identical genotype, still expressing only wild type p53. Colonic cells after ras mutation, APC mutation, and DCC inactivation remain nontumorigenic and growth factor dependent. Malignant progression involves at least two additional steps, and in VACO-411 can proceed by a novel pathway not requiring p53 inactivation.

Physical and functional interaction between wild-type p53 and mdm2 proteins

The mdm2 oncogene, which is often amplified in mammalian tumors, produces a number of transcripts that encode distinct protein forms. Previous studies demonstrating that overexpression of the mdm2 gene can activate its transforming potential, and can inhibit the transcriptional activation function of p53, prompted us to begin to explore possible functional differences among the various mdm2 products. Utilizing a transient transfection assay, we have evaluated four naturally occurring murine mdm2 forms for their ability to inhibit p53-mediated transcriptional activation of reporter genes regulated by p53 response elements. Three of these mdm2 forms were found to physically associate with the wild-type p53 protein and to possess the ability to inhibit its transactivation function. A fourth form failed to exhibit either of these functions. This last mdm2 form lacks the N-terminal protein domain that is present in the other three splice forms examined, pointing to this region as one that is critical for complex formation with the p53 protein. Identifying such differences among mdm2 proteins provides important clues for dissecting their functional domains, and emphasizes that defining the individual properties of these products will be critical in elucidating the overall growth control function of the mdm2 gene.

Physical and functional interaction between wild-type p53 and mdm2 proteins

The mdm2 oncogene, which is often amplified in mammalian tumors, produces a number of transcripts that encode distinct protein forms. Previous studies demonstrating that overexpression of the mdm2 gene can activate its transforming potential, and can inhibit the transcriptional activation function of p53, prompted us to begin to explore possible functional differences among the various mdm2 products. Utilizing a transient transfection assay, we have evaluated four naturally occurring murine mdm2 forms for their ability to inhibit p53-mediated transcriptional activation of reporter genes regulated by p53 response elements. Three of these mdm2 forms were found to physically associate with the wild-type p53 protein and to possess the ability to inhibit its transactivation function. A fourth form failed to exhibit either of these functions. This last mdm2 form lacks the N-terminal protein domain that is present in the other three splice forms examined, pointing to this region as one that is critical for complex formation with the p53 protein. Identifying such differences among mdm2 proteins provides important clues for dissecting their functional domains, and emphasizes that defining the individual properties of these products will be critical in elucidating the overall growth control function of the mdm2 gene.

Cancer genetics

The accumulation of multiple genetic changes underlies the process of tumorigenesis, and both dominantly acting oncogenes and inactivated tumor suppressor genes co-exist in the same tumor. Individual mutations are thought to independently contribute to the kaleidoscopic transformed phenotype. Several examples have now been found of mutations in genes that, through different mechanisms, act on central control points either to ensure genome stability or to regulate the common pathways that signal cell proliferation, survival and differentiation. Mutations at these loci may have multiple, and apparently unrelated, phenotypic consequences.

Cytogenetic studies of four hemangiopericytomas

We report cytogenetic studies on four hemangiopericytomas using in situ culture and robotic harvesting. Simple clonal chromosome abnormalities were observed in each of the three successful tumors. A cerebral tumor had a nonconstitutional 46,XX,t(7;12)(p15;q15) karyotype. A recurrent cerebral tumor had two cell lines: one with a 46,XY,t(2;12)(p13;q24.1) karyotype and one with a 46,XY,t(5;7)(q13;p15) karyotype. Each of the metaphases from a lung tumor had a 46,Y,inv(X)(p22.3q13) karyotype. We conclude that simple nonconstitutional balanced chromosome anomalies of 7p15 and 12q are common in cerebral hemangiopericytomas. It is possible that balanced anomalies of these chromosome regions are related to the pathogenesis of this tumor type.

The molecular basis of genetic dominance

Studies of mutagenesis in many organisms indicate that the majority (over 90%) of mutations are recessive to wild type. If recessiveness represents the 'default' state, what are the distinguishing features that make a minority of mutations give rise to dominant or semidominant characters? This review draws on the rapid expansion in knowledge of molecular and cellular biology to classify the molecular mechanisms of dominant mutation. The categories discussed include (1) reduced gene dosage, expression, or protein activity (haploinsufficiency); (2) increased gene dosage; (3) ectopic or temporally altered mRNA expression; (4) increased or constitutive protein activity; (5) dominant negative effects; (6) altered structural proteins; (7) toxic protein alterations; and (8) new protein functions. This provides a framework for understanding the basis of dominant genetic phenomena in humans and other organisms.

Lack of MDM2 amplification in human leukaemia

While deletion or mutation of the p53 gene is one of the most common molecular alterations detected in a wide variety of tumours, it has been shown to occur in only a relatively small percentage of the leukaemia cases examined. However, it may be that other components of the p53 pathway are involved. Amplification of the MDM2 gene has recently been demonstrated in human sarcomas resulting in an increase in MDM2 protein levels. This protein can bind to p53 preventing the transactivation of p53 responsive genes, thus mimicking mutation or deletion of p53. We have investigated the prevalence of MDM2 amplification in human leukaemias. 101 leukaemia or lymphoma samples and nine cell lines were studied using Southern blotting. In no case was MDM2 amplification present. We conclude that MDM2 amplification is not a common event in human leukaemias.

Detection of MDM2-proto-oncogene in paraffin embedded human bronchial epithelium

Recently a new proto-oncogene, the murine double-minute 2 (MDM2), has been described. MDM2 becomes oncogenic due to amplification and overexpression. Among other proto-oncogenes MDM2 becomes interesting since MDM2 protein can associate with both mutant and wild type p53 tumor suppressor gene products and thus inhibit p53-mediated transactivation of other genes. Loss of p53 tumor suppressor function is the most frequently observed alteration in human tumors. Immunohistochemical studies investigating the quantity of MDM2 protein in human sarcomas revealed an overexpression in 30% of the specimens. Here we describe the successful use of a monoclonal antibody (IF2) for the detection of MDM2 protein in paraffin-embedded tissue from human lung biopsies. 18 out of 44 specimens (41%), predominantly mucosal epithelial and glandular epithelial cells, stained positive for MDM2. No significant difference was observed between non-cancerogenic cells adjacent to tumor cells and those specimens without any tumor cells but altered by inflammatory processes. In general, the staining pattern was restricted not to the nuclei, but to selected subnuclear compartments, probably representing the golgi apparatus or the endoplasmatic reticulum. Our data support the hypothesis that in addition to its nuclear function of forming a complex with p53, MDM2 may also be secreted and thus have a transcellular effect.

TP53 allele loss, mutations and expression in malignant melanoma

p53 alterations at the DNA, mRNA and protein levels were studied in tumour metastases sampled from 30 patients with malignant melanoma. Paraffin-embedded sections from these and an additional 12 patients were examined for the presence of p53 protein. TP53 gene aberrations were found in 7 of 30 (23%) of the patients, six of which showed loss of heterozygosity (LOH). Point mutations were detected in only two cases, one of which had LOH whereas the other was non-informative. Increased levels of p53 mRNA were present in only one tumour with, but in six cases without, detectable DNA abnormalities. Four of the latter and six tumours with normal transcript levels had immunohistochemically detectable levels of p53 protein. In 25 cases in which corresponding primary and metastatic lesions could be compared, closely similar immunoreactivity patterns were observed. Increased expression of the MDM2 gene was found in only one tumour in parallel with overexpression of p53. Altogether, the data indicate that inactivation of the p53 regulatory pathway is not of major significance in the tumorigenesis of malignant melanoma. However, a significant association was found between p53 immunoreactivity and the relapse-free period in patients with superficial spreading melanoma. That increased protein expression was predominantly found in tumours without DNA alterations might suggest a role for the wild-type p53 protein in restricting malignant cell proliferation in these cases.

Pattern of mutant p53 expression in human astrocytomas suggests the existence of alternate pathways of tumorigenesis

BACKGROUND

Clinical observations suggest that malignant astrocytomas may arise from well-differentiated, low-grade tumors that have undergone anaplastic progression or may develop de novo. Mutations that alter the function of the p53 gene product are thought to play a critical role in astrocytoma tumorigenesis. The authors studied the pattern of mutant p53 expression in astrocytomas to define its role in the formation of malignant tumors by these different pathways.

METHODS

Tissues from 44 astrocytomas corresponding to Grades I-IV of the World Health Organization (WHO) classification were analyzed for the presence of mutations in exons 5, 7, and 8 of the p53 gene using single strand conformation polymorphism (SSCP) and sequence analysis of DNA amplified by the polymerase chain reaction. Immunostaining for mutant p53 proteins was performed on tissues fixed in formaldehyde solution and embedded in paraffin; the tissues were from these 44 astrocytomas and another 103 astrocytomas obtained from archival material.

RESULTS

Tumors with mutant p53 genes were reliably identified by immunostaining for mutant p53 proteins. A higher percentage of astrocytomas of histologic Grades II-IV stained positively for p53 than were identified by mutational analysis. The average ages of patients with Grade III/IV astrocytomas with prominent (> 10%) p53 staining and those with sparse (< 10%) or no p53 staining were 44.5, 64.3, and 67.9 years, respectively (P < 0.0001).

CONCLUSIONS

The pattern of mutant p53 expression is consistent with a role in driving the progression of low-grade astrocytomas to more malignant tumors. These results provide a genetic basis for the clinical observation that malignant astrocytomas resulting from anaplastic progression occur in a younger patient population than do malignant astrocytomas arising de novo.

p53 tumor suppressor oncogene expression in squamous cell carcinoma of the hypopharynx

BACKGROUND

Although abnormalities of the p53 tumor suppressor oncogene system are being detected in many human cancers, the frequency and prognostic significance of such events in squamous cell cancer of the head and neck remain unknown.

METHODS

Immunohistochemical studies were performed on archival tumor tissue taken from 43 patients with squamous cell carcinoma of the hypopharynx. Statistical analyses examining potential associations between p53 oncoprotein accumulation and various clinicopathologic parameters (including survival) were conducted.

RESULTS

Sixteen (37%) tumors demonstrated strong specific staining for p53. Patients whose tumors stained strongly for p53 were significantly younger, presented at a more advanced clinical disease stage, and tended to have increased expression of epidermal growth factor receptor (P = 0.056). There was no correlation between p53 expression and the incidence of multiple primary cancers, tumor grade, DNA ploidy, or percent of S-phase. Median survival times were 11.6 months and 18.0 months for the p53-positive and p53-negative groups (NS). A Cox regression analysis confirmed the lack of significance of p53 in overall survival.

CONCLUSIONS

These data suggest that although abnormalities of the p53 oncoprotein are common in this cancer, this finding is of little clinical significance.

An evaluation of six antibodies for immunohistochemistry of mutant p53 gene product in archival colorectal neoplasms

Immunohistochemical detection of intranuclear p53 gene product may indicate mutation of the p53 suppressor gene on chromosome 17p. We used six commercially available antibodies for p53 immunohistochemistry on 19 archival colorectal neoplasms and compared the results with the mutation status of the p53 gene and 17p allelic deletion status. By Friedman's ranking analysis, use of mouse monoclonal antibody DO7 with Target Unmasking Fluid (TUF) for antigen retrieval was the most sensitive and specific procedure (P < 0.0001). Six of 7 cases with high expression (p53 Labeling Index > 30 per cent using a CAS 200 image analyser) had p53 mutation. Of seven tumours without expression (LI < 1 per cent), six had no mutation and one had a truncating mutation which prohibited nuclear localization of gene product. The low expression group (1 per cent < LI < 30 per cent, n = 5) consisted of three tumours without and two tumours with mutation. The sensitivity of high expression with the DO7-TUF method for p53 gene mutation was 67 per cent with specificity of 90 per cent, predictive value of a positive of 86 per cent, predictive value of a negative of 75 per cent, and efficiency of 79 per cent. This study suggests that immunohistochemistry is valuable for assessing p53 gene mutations in colorectal neoplasms, but further study is needed to elucidate the precise link between immunohistochemistry and molecular genetic alterations.

Application of molecular genetics and cytogenetics to breast cancer and soft tissue sarcomas

BACKGROUND

Over the last decade there have been considerable advances in understanding of the molecular events involved in the initiation and progression of a wide range of solid tumours. In many instances, cytogenetic abnormalities have been the first indication that there is a mutated gene at a particular locus. In colonic polyposis, for example, the loss of 5q material in a man with the disease indicated the importance of the region and led to the subsequent cloning of the gene. Interestingly, loss of this tumour suppressor gene is also seen in approximately 20%-50% of sporadic colorectal carcinomas, which suggests that it may be important to study these inherited syndromes to identify genes that may be more widely involved in carcinogenesis. It is also important to realize that the order in which different molecular events occur is unlikely to fit into predictable chronological patterns, and that there will be great variation in the molecular defects of tumours that otherwise appear morphologically similar and have a common pattern of clinical presentation and progression. This is because established tumours are probably composed of a number of subclones at the molecular level, and clonal selection will only take place if a particular mutation provides a selective growth advantage or it is necessary for a stage in progression, such as invasion or metastasis. Multiple sites of loss of heterozygosity (LOH) have been identified in a range of tumours, many of which may be sites of as yet unidentified tumour suppressor genes.(ABSTRACT TRUNCATED AT 250 WORDS)

MDM2 gene amplification in human breast cancer

The MDM2 gene is a gene whose product binds to p53 and regulates its functions. The amplification of the MDM2 gene has been found in one third of human sarcomas, and a differential expression of MDM2 gene in relation with oestrogen receptor status was recently found in human breast cancer cell lines. We analysed 60 breast cancers for MDM2 gene amplification by Southern blot. This event was observed in 1 case with high levels of oestrogen receptor (ER). Thus, MDM2 gene amplification seems to be a rare event in breast cancer. Further studies are needed to define precisely the relationship between MDM2 amplification and ER status.

Problems with p53 immunohistochemical staining: the effect of fixation and variation in the methods of evaluation

The availability of antibodies which recognise p53 protein in paraffin-embedded tissue has created the opportunity to use immunohistochemistry to study the expression of p53 in a wide variety of clinical material. In this paper we have investigated the relationship between the type of fixative and the pattern of p53 staining in mammary carcinoma. Optimal results were obtained from breast tissue fixed in phenol formol saline, methacarn or cold formol saline with positive staining for stabilised p53 protein occurring in 69/95 (73%) cases studied. Care must be taken in the interpretation of these results since positive staining for p53 protein is not always indicative of mutation of the p53 gene. Furthermore, a range of staining patterns is seen in mammary carcinomas, making interpretation difficult. Assessment of staining needs to be standardised in order that different studies can be compared. However, in breast carcinoma, p53 immunohistochemistry appears to give information relating to tumour grade and, independently, to prognosis.

Absence of MDM-2 gene amplification in experimentally induced tumors regardless of p53 status

To assess the generality of the hypothesis that murine double-minute-2 (MDM-2) gene amplification complements the absence of p53 mutation during tumor development, we analyzed 143 murine tumors induced by a variety of carcinogenic agents in two different mouse strains. Only three of 143 tumors showed p53 genetic alterations and none showed MDM-2 amplification, indicating the existence of alternative pathways that permit tumor cells to bypass p53-MDM-2 control.

Analysis of the p53 gene, its expression and protein stabilization in non-Hodgkin's lymphomas

We have investigated the p53 gene, expression of its mRNA, and stabilization of its protein in a series of non-Hodgkin's lymphomas (NHLs). Immunohistochemistry revealed positive staining of the p53 protein in node biopsies from 6/36 NHL patients, all of whom had high-grade disease. The remaining NHL samples, together with 3 reactive nodes, showed either negative staining or the staining of only occasional cells. In one case that exhibited intense nuclear staining in 90% of the cells, a mutation in the p53 gene was also observed. There was no evidence of rearrangements of the p53 gene in any of the NHL samples. Although p53 mRNA could not be detected in nonmalignant tissue, it was apparently overexpressed in 16/38 NHL, but this did not correlate with positive staining of the p53 protein. These data suggest that p53 dysfunction might play an important role in the evolution of some cases of NHL, and that mechanisms other than mutation of the p53 gene may be involved in stabilizing the p53 protein in these neoplasms.

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

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

Activation of the human immunodeficiency virus type 1 long terminal repeat by transforming mutants of human p53

We have studied the effects of human wild-type and mutant p53s on the long terminal repeat (LTR) promoter of human immunodeficiency virus type 1 (HIV). HeLa cells were cotransfected with a wild-type or mutant p53 expression plasmid and a plasmid containing a chloramphenicol acetyltransferase reporter gene under HIV LTR promoter control. As expected, expression of wild-type p53 inhibited promoter function. Expression of a p53 mutated at any one of the four amino acid positions 175, 248, 273, and 281 correlated with a significant increase of the HIV promoter activity. The HIV LTR was also significantly activated in Saos-2 cells that do not express endogenous p53. This finding suggests a gain-of-transactivation function by mutation of the p53 gene. Cotransfection of wild-type and mutant p53-281G expression plasmids indicated that either the wild type or the mutant was dominant in inhibiting or enhancing promoter activity, respectively, when transfected in excess of the other. Transfection experiments showed transactivation even when the Sp1, NF-kappa B, and TATA sites in the LTR were individually mutated. Synthetic minimal promoter constructs containing two Sp1 sites or two NF-kappa B sites or an ATF site are also significantly activated by the mutant p53-281G. Thus, the mutant protein may activate transcription through interaction with either a general transcription factor or a common factor that bridges the basal transcription machinery and the transcription factors Sp1, NF-kappa B, and ATF.

Infrequency of ras, p53, WT1, or RB gene alterations in Wilms tumors

BACKGROUND

Alteration of the ras family of oncogenes and of the tumor suppressor genes p53 and RB are the most common genetic events in human tumors. Although there have been no reports of the prevalence of these alterations in Wilms tumors, overexpression of the N-myc and insulin-like growth factor-II (IGF-II) genes have been observed, and alteration of another tumor suppressor gene (WT1) has been demonstrated.

METHODS

Forty-four Wilms tumor specimens were tested for the presence of N-, K-, and H-ras mutations in codons 12, 13, and 61 by single-strand conformation polymorphism (SSCP) analysis and direct DNA sequence analysis. Sixteen tumors were tested for abnormalities of WT1 by Southern and northern blot analysis and reverse transcriptase polymerase chain reaction (RT-PCR). N-myc, c-myc, WT1, and IGF-II mRNA expression was measured in 16 tumors by Northern blot analysis. Thirty-eight tumors were screened for p53 mutations by SSCP analysis and direct DNA sequence analysis. Nine tumors were analyzed for loss of heterozygosity (LOH) of RB.

RESULTS

Although the authors confirmed that N-myc and IGF-II are overexpressed in Wilms tumors, no mutations of ras family, p53, or RB genes were identified, and no gross alterations of WT1 were detected by Southern or Northern blot analysis.

CONCLUSIONS

These findings suggest that H-ras, K-ras, N-ras, p53, and RB are not involved in the pathogenesis of Wilms tumor.

p53 expression and its relationship to DNA alterations in bone and soft tissue sarcomas

The p53 gene is one of the best studied tumour suppressor genes. Recently we performed mutation analysis on the p53 gene in a large number of bone and soft tissue sarcomas, and found that approximately one-third of the sarcomas have some type of DNA alteration at the p53 locus (Toguchida et al., 1992). However, the expression of the p53 protein resulting from these alterations still remains to be clarified. In this study, p53 expression in the sarcoma tissues was analysed immunohistochemically using antibody PAb421 (Oncogene Science) and its relationship to DNA alterations was examined. Of 113 tumours, 29 (25.7%) showed positive staining for the p53 protein. These included 19 of 67 osteosarcomas, five of 20 chondrosarcomas, four of 11 malignant fibrous histiocytomas (MFHs) and one Ewing's sarcoma. In chondrosarcomas, most of the p53-positive tumours belonged to highly malignant and atypical tumour types (dedifferentiated or mesenchymal type), suggesting a role for p53 mutation in the progression of cartilaginous tumours. All the cases with a missense mutation showed strongly positive staining, while no immunoreactivity was observed in the remaining three-quarters with DNA alterations including gross rearrangement, frame-shift mutation, nonsense mutation or mutation at splicing site except in one case. These results demonstrated the dominance of the p53 mutations with null protein expression in bone and soft tissue sarcomas, showing a unique characteristic of these types of tumours compared with other malignancies such as colon carcinomas.

The tumor suppressor p53 and the oncoprotein simian virus 40 T antigen bind to overlapping domains on the MDM2 protein

The oncogene mdm2 has been found to be amplified in human sarcomas, and the gene product binds to the tumor suppressor p53. In this report, we describe the dissection of the MDM2-binding domain on p53 as well as the p53-binding domain on MDM2. We also demonstrate that the oncoprotein simian virus 40 T antigen binds to the product of cellular oncogene mdm2. We have constructed several N- and C-terminal deletion mutants of p53 and MDM2, expressed them in vitro, and assayed their in vitro association capability. The N-terminal boundary of the p53-binding domain on MDM2 is between amino acids 1 and 58, while the C-terminal boundary is between amino acids 221 and 155. T antigen binds to an overlapping domain on the MDM2 protein. On the other hand, the MDM2-binding domain of p53 is defined by amino acids 1 and 159 at the N terminus. At the C terminus, binding is progressively reduced as amino acids 327 to 145 are deleted. We determined the effect of human MDM2 on the transactivation ability of wild-type human p53 in the Saos-2 osteosarcoma cell line, which does not have any endogenous p53. Human MDM2 inhibited the ability of human p53 to transactivate the promoter with p53-binding sites. Thus, human MDM2 protein, like the murine protein, can inactivate the transactivation ability of human p53. Interestingly, both the transactivation domain and the MDM2-binding domain of p53 are situated near the N terminus. We further show that deletion of the N-terminal 58 amino acids of MDM2, which eliminates p53 binding, also abolishes the capability of inactivating p53-mediated transactivation. This finding suggests a correlation of in vitro p53-MDM2 binding with MDM2's ability in vivo to interfere with p53-mediated transactivation.

In vivo cloning of PCR products in E. coli

This report describes an efficient method to clone PCR products exploiting endogenous Escherichia coli enzymatic activities. PCR products are engineered to contain terminal sequences identical to sequences at the two ends of a linearized vector. PCR products and vector DNA are then simply co-transfected into E. coli strain JC8679, obviating the requirement for enzymatic treatment of the PCR product or in vitro ligation. The high rate of homologous recombination in this strain results in efficient incorporation of the insert into the vector, a process we refer to as in vivo cloning (IVC).

The tumor suppressor p53 and the oncoprotein simian virus 40 T antigen bind to overlapping domains on the MDM2 protein

The oncogene mdm2 has been found to be amplified in human sarcomas, and the gene product binds to the tumor suppressor p53. In this report, we describe the dissection of the MDM2-binding domain on p53 as well as the p53-binding domain on MDM2. We also demonstrate that the oncoprotein simian virus 40 T antigen binds to the product of cellular oncogene mdm2. We have constructed several N- and C-terminal deletion mutants of p53 and MDM2, expressed them in vitro, and assayed their in vitro association capability. The N-terminal boundary of the p53-binding domain on MDM2 is between amino acids 1 and 58, while the C-terminal boundary is between amino acids 221 and 155. T antigen binds to an overlapping domain on the MDM2 protein. On the other hand, the MDM2-binding domain of p53 is defined by amino acids 1 and 159 at the N terminus. At the C terminus, binding is progressively reduced as amino acids 327 to 145 are deleted. We determined the effect of human MDM2 on the transactivation ability of wild-type human p53 in the Saos-2 osteosarcoma cell line, which does not have any endogenous p53. Human MDM2 inhibited the ability of human p53 to transactivate the promoter with p53-binding sites. Thus, human MDM2 protein, like the murine protein, can inactivate the transactivation ability of human p53. Interestingly, both the transactivation domain and the MDM2-binding domain of p53 are situated near the N terminus. We further show that deletion of the N-terminal 58 amino acids of MDM2, which eliminates p53 binding, also abolishes the capability of inactivating p53-mediated transactivation. This finding suggests a correlation of in vitro p53-MDM2 binding with MDM2's ability in vivo to interfere with p53-mediated transactivation.

Discerning the function of p53 by examining its molecular interactions

Of the many genes mutated on the road to tumor formation, few have received as much attention as p53. The gene has come to occupy center stage for the simple reason that it is more frequently altered in human tumors than any other known gene, undergoing mutation at a significant rate in almost every tumor type in which it has been studied. This association between p53 mutation and tumorigenesis has spurred a flurry of research attempting to delineate the normal function of p53 and, by extension, the role of p53 mutation in tumor formation. At the cellular level, p53 has been shown to suppress growth. Recent efforts to further discern the function of p53 have centered on the underlying molecular basis for this growth suppression. In particular, research has focused on the identification of cellular molecules (specifically DNA and proteins) with which the p53 protein associates. p53 has now been shown to bind DNA in a sequence-specific manner, and mounting evidence suggests that p53 acts as a transcription factor, perhaps regulating the expression levels of genes involved in the inhibition of cell growth. The logical next step in understanding p53 function involves the resolution of two questions: (1) what are the physiological transcriptional targets of p53, and (2) what cellular proteins regulate or mediate the ability of p53 to modulate transcription? Some initial clues to these puzzles are now emerging, and these form the subject of this review.

Putative dehydrogenase tms1 suppresses growth arrest induced by a p53 tumour mutant in fission yeast

Expression of a human tumour-derived p53 His 273 cDNA induced growth arrest in fission yeast Schizosaccharomyces pombe. Based on the p53-induced growth arrest, we cloned an extragenic suppressor, termed tms1, by complementation. The open reading frame of the tms1 gene corresponded to a protein of 347 amino acids with a calculated mass of 37380 Da. The transcriptional start site of the tms1 gene was mapped and, in addition, the corresponding cDNA was isolated and expressed in Escherichia coli. Recombinant tms1 protein served as an antigen to produce specific polyclonal antibodies to aid identification of the tms1-gene-product in total yeast lysates. Comparison of the deduced amino acid sequence of tms1 with available databases revealed significant similarity to dehydrogenases, suggesting that the tms1 protein itself might possess dehydrogenase activity.

Relationship of tobacco/alcohol use to p53 expression in patients with lingual squamous cell carcinomas

This study examined p53 expression immunocytochemically in 40 lingual squamous cell carcinomas from Dutch patients with known histories of smoking and/or drinking alcohol. 30% of neoplasms showed positive p53 reactivity, suggesting increased levels of p53 protein. No alcohol or tobacco risk factors were evident in 33.3% (4/12) of p53-positive neoplasms whereas only 7.1% (2/28) of p53-negative neoplasms showed an absence of these risk factors. 25% (3/12) of p53-positive neoplasms and 71.4% (20/28) of p53-negative neoplasms were found in patients who had been exposed to both alcohol and tobacco. A similar negative association with p53 reactivity was also found when either tobacco or alcohol were used in isolation. The results contrast with previous observations in head/neck and oral carcinomas and indicate that the association of alcohol/tobacco and p53 expression remains open to question.

Transactivation of the P2 promoter of parathyroid hormone-related protein by human T-cell lymphotropic virus type I Tax1: evidence for the involvement of transcription factor Ets1

Expression of the parathyroid hormone-related protein (PTHrP), a protein that plays a primary role in the development of the humoral hypercalcemia of malignancy, is regulated by two distinct promoters, P1 and P2. PTHrP is overexpressed in lymphocytes from adult T-cell leukemia patients. We now demonstrate that in the human T-cell lymphotropic virus type I-transformed cell line MT-2, RNA synthesis is initiated primarily at the P2 promoter. Furthermore, in cotransfection experiments, Tax1 transactivates the P2 promoter 10- to 12-fold. By using deletion and site-specific point mutations, we have identified a promoter-proximal sequence (positions -72 to -40) which is important for Tax1 transactivation. The PTHrP promoter-proximal element contains two potential overlapping Ets1 binding sites, EBS I and EBS II. Gel shift analysis demonstrated that Ets1 binds specifically to both EBS I and EBS II. Mutation of the consensus GGAA core motif in EBS I abolished binding and Tax1 transactivation in Jurkat T lymphocytes. In Ets1-deficient cells, cotransfection of Tax1 and Ets1 expression plasmids stimulates PTHrP promoter activity. In the absence of Ets1, minimal transactivation of the PTHrP promoter is observed. These data suggest that Ets1 binds to EBS I and cooperates with Tax1 to transactivate the PTHrP P2 promoter.

Tumourigenesis associated with the p53 tumour suppressor gene

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

p53-dependent apoptosis modulates the cytotoxicity of anticancer agents

Although the primary cellular targets of many anticancer agents have been identified, less is known about the processes leading to the selective cell death of cancer cells or the molecular basis of drug resistance. p53-deficient mouse embryonic fibroblasts were used to examine systematically the requirement for p53 in cellular sensitivity and resistance to a diverse group of anticancer agents. These results demonstrate that an oncogene, specifically the adenovirus E1A gene, can sensitize fibroblasts to apoptosis induced by ionizing radiation, 5-fluorouracil, etoposide, and adriamycin. Furthermore, the p53 tumor suppressor is required for efficient execution of the death program. These data reinforce the notion that the cytotoxic action of many anticancer agents involves processes subsequent to the interaction between drug and cellular target and indicate that divergent stimuli can activate a common cell death program. Consequently, the involvement of p53 in the apoptotic response suggests a mechanism whereby tumor cells can acquire cross-resistance to anticancer agents.

Functional domains of wild-type and mutant p53 proteins involved in transcriptional regulation, transdominant inhibition, and transformation suppression

The wild-type (wt) p53 protein has transcriptional activation functions which may be linked to its tumor suppressor activity. Many mutant p53 proteins expressed in cancers have lost the ability to function as transcriptional activators and furthermore may inhibit wt p53 function. To study the mechanisms by which mutant forms of p53 have lost their transactivation function and can act in a dominant negative manner, a structure-function analysis of both mutant and engineered truncated forms of p53 was carried out. We show that different mutant p53 proteins found in cancers vary in the ability to inhibit the transcriptional transactivation and specific DNA binding activities of wt human p53. This transdominant effect was mediated through the carboxy-terminal oligomerization region. The role of the transactivation activity in transformation suppression by wt p53 was also examined by constructing an N-terminal deletion mutant lacking the transactivation domain. This mutant was unable to transactivate but could bind specifically to DNA. Although it was impaired in its ability to suppress transformation of primary rat embryo fibroblasts by adenovirus E1A plus activated ras, the N-terminal deletion mutant still had some suppression activity, suggesting that additional functions of p53 may contribute to transformation suppression.

Prognostic significance of TP53 alterations in breast carcinoma

Constant denaturant gel electrophoresis (CDGE) was used to screen 179 breast carcinomas for mutations in the conserved regions of the TP53 gene (exons 5 through 8). Mutations were found in 35 of 163 primary tumours (21%) and in 5 of 16 metastases (31%) and resided predominantly in exon 7. The majority of the mutations were G:C-->A:T transitions. Immunohistochemistry demonstrated nuclear accumulation of p53 protein in 35 of 162 primary tumours (22%) and in four of 15 metastases (27%). TP53 mutation was strongly associated with nuclear accumulation of p53 protein. In total 42 of 163 primary tumours (26%) and 5 of 16 metastases (31%) were demonstrated to contain TP53 alterations (mutation and/or nuclear protein accumulation). TP53 alteration in primary tumour was significantly associated with the following parameters: positive node status, T status > 1, negative oestrogen receptor status, negative progesterone receptor status, presence of ERBB2 gene amplification, and invasive ductal histology. Furthermore, there were statistically significant associations, independent of other prognostic factors, between TP53 alterations in primary tumour and disease-free and overall survival.

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

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

Functional domains of wild-type and mutant p53 proteins involved in transcriptional regulation, transdominant inhibition, and transformation suppression

The wild-type (wt) p53 protein has transcriptional activation functions which may be linked to its tumor suppressor activity. Many mutant p53 proteins expressed in cancers have lost the ability to function as transcriptional activators and furthermore may inhibit wt p53 function. To study the mechanisms by which mutant forms of p53 have lost their transactivation function and can act in a dominant negative manner, a structure-function analysis of both mutant and engineered truncated forms of p53 was carried out. We show that different mutant p53 proteins found in cancers vary in the ability to inhibit the transcriptional transactivation and specific DNA binding activities of wt human p53. This transdominant effect was mediated through the carboxy-terminal oligomerization region. The role of the transactivation activity in transformation suppression by wt p53 was also examined by constructing an N-terminal deletion mutant lacking the transactivation domain. This mutant was unable to transactivate but could bind specifically to DNA. Although it was impaired in its ability to suppress transformation of primary rat embryo fibroblasts by adenovirus E1A plus activated ras, the N-terminal deletion mutant still had some suppression activity, suggesting that additional functions of p53 may contribute to transformation suppression.

Cytogenetic findings in 33 osteosarcomas

Thirty-three osteosarcomas (OS) were analyzed cytogenetically. Clonal chromosome changes were detected in 17 cases. Six tumors had chromosome numbers in the diploid range, 6 in the triploid range, 1 in the tetraploid range and 1 in the pentaploid range, while 3 tumors had multiple clones with different ploidy levels. Including the present 17 tumors, a total of 27 OS with clonal aberrations have been reported. The recognizable structural rearrangements in these 27 tumors clustered to chromosome arms 1p, 1q, 3p, 3q, 7q, 11p, 17p and 22q. Chromosome bands 1q11, 1q21, 1q42 and 7q11 were the most frequently rearranged, and the most common numerical rearrangements were -3, -10, -13 and -15. Supernumerary ring chromosomes, in 2 tumors as the sole change, were found in all 3 parosteal OS, which is in agreement with the findings in 1 previously reported parosteal OS. The association between ring formation and parosteal morphology represents the first cytogenetic-morphologic entity among OS.

Lessons from our children

While the incidence of cancer is increasing among both children and adults, mortality rates have decreased for children, while they have increased for adults. Of children diagnosed with cancer today, 80% are predicted to be long-term survivors. Although there are differences between children and adults with respect to the tumor types, biology, and outcome, there are common lessons which we can learn from our children regarding the genetics of cancer, its management and treatment, and the importance of longitudinal studies of the survivors. Specific pediatric cancers, such as retinoblastoma, have led to the recognition of tumor suppressor genes, now also observed among adult tumors including sarcomas, breast, lung, and bladder cancer. The presence of the tumor suppressor gene provides an understanding for the incidence of second malignant tumors among patients with heritable diseases. Furthermore, cancer prone families, such as those with the Li-Fraumeni syndrome, also carry the p 53 tumor suppressor gene; the presence of which greatly increases the risk of developing invasive cancer. Childhood cancer is rare; it represents only 1% of the total US cancer problem. However, 53% of all children with cancer, but only 2% of all adults, are studied via the NCI cooperative group mechanism. For some specific childhood tumors such as rhabdomyosarcoma and Wilms' tumor, as many as 70-85% of all cases are managed via NCI sponsored trials. Essentially all pediatric cancer is treated by interdigitating radiation with surgical resection and systemic chemotherapy. This approach has contributed to high cure rates. Finally, our understanding of the late effects of being a cancer survivor have come from longitudinal studies of children. The most severe long-term effects related to radiation in childhood pertain to growth and development, infertility, and second malignant tumor induction. Here the children treated for Hodgkin's disease have taught us the dose and volume effects on axial skeletal and soft tissue growth. Infertility issues are also treatment-related and may often be obviated by using gonadal shielding. The risk of secondary leukemia is related to dose and class of specific chemotherapeutic agents administered; it is 5.5% among children receiving 6 cycles of MOPP. There is a 22-fold risk at 30 years of age of solid tumor induction following radiotherapy for children with Hodgkin's disease. These serious concerns have been offset by current therapeutic approaches of using lower doses and smaller volumes of radiation with fewer cycles of less toxic chemotherapeutic agents. Childhood cancer ranks high among number of person-years of potential life saved annually.(ABSTRACT TRUNCATED AT 400 WORDS)

Absence of human papillomavirus type 16 E6 transcripts in HPV 16-infected, cytologically normal cervical scrapings

By a combination of reversed transcription and subsequent polymerase chain reaction (RNA-PCR), 23 cytologically normal cervical scrapings, positive for the presence of human papillomavirus type 16 (HPV 16) DNA, were analyzed for the presence of transcripts originating from the E6 region of the viral genome. This region is thought to be involved in transformational, tumorigenic events. No mRNAs of the E6 region were detectable using the most sensitive PCR-mediated procedure currently available. Since it was previously shown that in cytological abnormal cervical scrapings about one-half of the samples positive for HPV 16 DNA express mRNAs of the E6 region, a difference between normal and abnormal cervical scrapings, when the HPV 16 is present, exists. The observed difference between cytologically normal and abnormal, HPV-DNA-positive cervical scrapes may eventually be used as a prognostic marker for screening of women at risk for the development of cervical carcinoma. However, firm establishment of the putative correlation between tumor progression and the presence of E6 transcripts requires extensive follow-up analysis of HPV-positive patients.