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

Wild-type p53 protein undergoes cytoplasmic sequestration in undifferentiated neuroblastomas but not in differentiated tumors

Neuroblastoma (NB), a tumor arising from the sympathetic nervous system, is one of the most common malignancies in childhood. Several recent reports on the p53 genotype found virtually exclusive wild-type status in primary tumors, and it was postulated that p53 plays no role in the development of NB. Here, however, we report that the vast majority of undifferentiated NBs exhibit abnormal cytoplasmic sequestration of wild-type p53. This inability of p53 to translocate to the nucleus presumably prevents the protein from functioning as a suppressor. Thirty of 31 cases (96%) of undifferentiated NB showed elevated levels of wild-type p53 in the cytoplasm of all tumor cells concomittant with a lack of nuclear staining. p53 immunoprecipitation from tumor tissues showed a 4.5- to 8-fold increase over normal protein levels. All of 10 tumors analyzed harbored wild-type p53 by direct sequencing of full-length cDNA and Southern blot. In addition, no MDM-2 gene amplification was seen in all 11 tumors analyzed. In contrast, no p53 abnormality was detected in 14 differentiated ganglioneuroblastomas and 1 benign ganglioneuroma. We conclude that loss of p53 function seems to play a major role in the tumorigenesis of undifferentiated NB. This tumor might abrogate the transactivating function of p53 by inhibiting its access to the nucleus, rather than by gene mutation. Importantly, our results suggest that (i) this could be a general mechanism for p53 inactivation not limited to breast cancer (where we first described it) and that (ii) it is found in a tumor previously not thought to be affected by p53 alteration.

Towards understanding the molecular basis of thyroid cancer

Cancer is a multistep phenomenon and multiple genetic lesions are involved in the emergence of the cancerous lesion. This has best been demonstrated in colonic cancer. The authors review their work and that of others highlighting what is known about thyroid cancer. They implicate ras mutations predominantly in follicular carcinoma, rearrangement of the ret proto-oncogene in papillary carcinoma and the tumor suppressor genes p53 and retinoblastoma gene product in all stages of thyroid carcinoma. They find a low rate of ret proto-oncogene rearrangement in the Saudi population (>5%) as compared to elsewhere in the world (20%). They find TSH receptor message abundance to be predictive of prognosis in thyroid cancer patients. Lastly, they examine whether the abundance of the anti-metastatic gene nm23 message abundance negatively correlated with the tendency of thyroid tumors to metastasize and find that not to be the case in thyroid carcinoma. The study of oncogenes and tumor suppressor genes in the pathogenesis of thyroid cancer is in its infancy; however, rapid progress is being made in identifying genes participating in malignant thyroid cell transformation.

Biologic indicators of exposure: are markers associated with oncogenesis useful as biologic markers of effect?

Radiation-induced molecular and cellular alterations play an important role in the transformation of a normal cell into a cancer cell. However, the basic molecular and cellular alterations upon exposure to ionizing irradiation are still poorly understood. Identification of such alterations would be of importance for the assessment of exposure dose, as well as for the assessment of an exposed individual's risk of developing cancer. Extensive studies of the mechanisms of oncogenesis have led to the identification of altered genes, such as proto-oncogenes and tumor suppressor genes as well as other genes intimately involved in cellular proliferation and differentiation, that are more or less frequently associated with a variety of human malignancies. It can be assumed that at least some of these mechanisms are associated with radiation-induced oncogenesis. The longevity of stem cells, particularly those of the hemopoietic system, makes them the prime target cell population to accumulate genetic alterations due to exposure to a variety of agents. Improvements in purification strategies for hemopoietic stem cells, as well as the availability of sensitive techniques such as the polymerase chain reaction (PCR) and flow cytometry analysis, should allow in-depth studies at the molecular and cellular level after exposure to physical and chemical agents.

Amplification of the MDM2 gene in human breast cancer and its association with MDM2 and p53 protein status

The present study reports on the frequency of MDM2 gene amplification and MDM2 protein expression in a series of 100 breast carcinomas and its association with accumulation of the p53 protein. Of the 100 cases, frozen samples for 82 cases were available for Southern blotting. Three of the 82 (4%) demonstrated MDM2 gene amplification of up to 6-fold. Immunohistochemical analysis of the formalin-fixed, paraffin-embedded tumours demonstrated that 7/97 (7%) had nuclear expression for MDM2 in 10-50% of the tumour cells (type 2 staining) and were denoted MDM2+. Two of the MDM2-amplified samples were MDM2+ with one of the two tumours also displaying type 2 p53 nuclear staining. Finally at the protein level, MDM2+ tumours were significantly associated with tumours having low levels of p53 staining (0-10% cells positive) (P = 0.03). We conclude that MDM2 gene amplification occurs at a lower frequency in breast cancer than in non-epithelial tumours. Alterations in MDM2 and p53 may represent alternative pathways in tumorigenesis, but they are not mutually exclusive in all cases.

p53 protein accumulation in European hepatocellular carcinoma is not always dependent on p53 gene mutation

BACKGROUND/AIMS

Immunohistochemical reactivity for p53 protein is common in various human malignancies and often related to p53 gene mutation. However, in some tumor types, accumulation of wild-type p53 has been shown. Previously, we analyzed 96 European hepatocellular carcinomas using immunohistochemistry and found that 31% of these tumors overexpressed p53 in the cell nucleus. The aim of the present study was to establish whether p53 positivity correlates with the presence of structural p53 gene abnormalities in European hepatocellular carcinoma.

METHODS

DNA from 20 tumors, 10 with strong immunostaining and 10 with undetectable staining for p53, was extracted from frozen sections, and the entire coding portion of the p53 gene was sequenced.

RESULTS

Five of the 10 tumors containing high levels of p53 protein showed missense point mutations. The remaining 5 tumors with high p53 levels showed the wild-type coding sequence. One of the 10 tumors containing undetectable levels of p53 protein had a 1-base pair deletion in the splice acceptor site of intron 4.

CONCLUSIONS

The results strongly suggest that, in European hepatocellular carcinomas, stabilization of the p53 protein depends on factors other than p53 gene mutation, such as binding to other molecules of cellular or viral origin.

Germline p53 mutations are frequently detected in young children with rhabdomyosarcoma

We investigated the possibility that a proportion of children with sporadic rhabdomyosarcoma (RMS) carry constitutional mutations of the p53 tumor suppressor gene. 33 patients with sporadic RMS at two large outpatient pediatric oncology clinics submitted blood samples. Genomic DNA was extracted from peripheral blood leukocytes and PCR was used to amplify exons 2-11 of the p53 gene. Amplified genomic DNA was screened for the presence of germline p53 mutations using single-strand conformation polymorphism (SSCP) analysis. The DNA sequence of those samples that showed aberrant migration of bands on SSCP analysis was determined to identify the precise nature of the gene mutations. Patient records were reviewed to assess clinical correlates of the mutant p53 carrier state. Heterozygous constitutional mutations were detected in 3/33 patient samples screened. Two of these missense mutations are located in exon 7 and one in exon 8 of the p53 gene. The presence of mutations was not correlated with tumor histology, stage, or site. However, an association between young age at diagnosis and presence of a constitutional p53 mutation was noted: 3/13 children under the age of 3 yr at diagnosis carried mutations, whereas none of 20 children over 3 yr of age at diagnosis harbored a detectable constitutional mutation. These results in children with RMS corroborates previous findings in other clinical settings suggesting that the mutant p53 carrier state may predispose individuals to malignancy at an early age. Although this study did not assess whether the mutations were preexisting or new germline alterations, assessment of close relatives of RMS patients for cancer risk and predictive genetic testing may be indicated.

p53 status in spontaneous and dimethylnitrosamine-induced renal cell tumors from rats

Rats carrying the Eker tumor-susceptibility mutation (Eker rats) are predisposed to developing renal cell carcinoma. Rats heterozygous for the Eker mutation develop spontaneous multiple bilateral renal cell tumors by the age of 1 yr. In a previous study, Eker-mutation carrier and noncarrier rats were exposed to the renal carcinogen dimethylnitrosamine (DMN), and male rats carrying the Eker mutation exhibited a 70-fold increase in the induction of renal adenomas and carcinomas when compared with noncarrier rats. In this study, spontaneous and DMN-induced rat renal cell tumors (adenomas and carcinomas) were analyzed for mutations of the p53 gene by direct sequencing of cDNA polymerase chain reaction products. There were no mutations in p53 cDNA derived from renal tumors from six untreated rats. Mutations were found in one of 15 of the DMN-induced tumors: a transition at codon 140, CCT-->CTT, in a renal adenoma. Additionally, seven cell lines derived from spontaneous renal cell tumors did not contain mutations in p53. The low frequency of p53 mutations (one of 21 renal cell tumors and none of seven cell lines derived from renal cell tumors) indicates that the development of both spontaneous and carcinogen-induced renal tumors involved a non-p53-dependent pathway. As p53 is infrequently mutated in human renal cell carcinomas and in rat renal mesenchymal tumors, it is likely that a tumor suppressor gene or genes other than p53 are involved in the development of renal cancer.

Phosphorylation studies on rat p53 using the baculovirus expression system. Manipulation of the phosphorylation state with okadaic acid and influence on DNA binding

To elucidate the role of phosphorylation of p53 we used the baculovirus expression system to obtain high yields of protein eventually in distinct phosphorylation states. Initially, we obtained only marginal phosphorylation, despite high levels of expression. Two-dimensional phosphopeptide maps exhibited the same pattern as known from rat cells although some sites were underrepresented. Coexpression of simian virus 40 (SV40) large T antigen or cyclin-dependent kinases, cdc2 or cdk2, had only marginal effects on the phosphorylation state of p53. However, when we employed the phosphatase inhibitor okadaic acid, overall phosphorylation of p53 was drastically enhanced in a dose-dependent manner and resembled that of p53 from SV40-transformed rat cells. This hyperphosphorylation resulted in enhanced binding of a consensus oligonucleotide as revealed by electrophoretic mobility shift assays. To assess the role of individual phosphorylation sites, we generated a set of mutants at putative or identified sites. All mutants retained the ability to bind wild-type conformation-specific antibody Pab1620, to complex with SV40 large T antigen, and to bind to the consensus oligonucleotide. Moreover, most mutants exhibited enhanced DNA binding upon okadaic acid treatment, except for a mutant at the cdk site which failed to do so. These data show that: (a) insect cells contain all the protein kinases necessary for phosphorylation of a mammalian protein, p53; (b) in insect cells the ratio of kinase/phosphatase activities differs from that in mammalian cells so that underphosphorylation of recombinant proteins in this system may result from high phosphatase activities rather than saturation of kinases with recombinant substrate; (c) the system can be manipulated to obtain subpopulations of recombinant protein in a desired phosphorylation state, and (d) phosphorylation may regulate the DNA-binding activity of p53.

Presence of human papillomavirus sequences in tumour-derived human oral keratinocytes expressing mutant p53

A series of eight oral epithelial cell lines derived from untreated human oral squamous cell carcinomas, which had arisen in patients with different tobacco histories, were examined for the presence of human papillomavirus (HPV) DNA, expression of stable p53 protein and p53 point mutation. Polymerase chain reaction (PCR)-based screening, but not Southern blot analysis, showed HPV-16 early region sequences to be present at low copy number (< 1 copy per cell) in two cell lines at early passage (3-5) in vitro (H400, T45), implying that only subpopulations of cells harboured viral DNA. HPV sequences were undetectable in cells at later passage (12-15), suggesting that viral sequences had been lost during growth in vitro, or that negative selection of HPV-containing cells had occurred. High levels of p53 were detected in the two HPV-positive cell lines and in three others (H103, H314, H357) by Western blotting, suggesting expression of mutant (stable) p53 molecules. A sixth cell line (H157) expressed a truncated p53. Sequence analysis of exons 2-11 of the p53 gene revealed missense mutations in six cell lines, one of which (H413) did not result in high levels of protein, and nonsense mutations in the remaining two cell lines (H157, H376). The results suggest that p53 mutation is a frequent genetic event in oral cancer. In addition, the expression of mutant p53 in oral cancer cells does not preclude a papillomaviral aetiology for these tumours. Analysis of p53 expression alone may result in underestimation of the frequency of p53 mutations in human cancers.(ABSTRACT TRUNCATED AT 250 WORDS)

The p53-mediated G1 checkpoint is retained in tumorigenic rat embryo fibroblast clones transformed by the human papillomavirus type 16 E7 gene and EJ-ras

Rat embryo fibroblast clones transformed with the human papillomavirus type 16 E7 gene and the H-ras oncogene (ER clones) fall into two groups on the basis of endogenous p53 genotype, wild type or mutant. We have compared these clones with the aim of indentifying physiological differences that could be attributed to p53 protein function. We show that all ER clones, regardless of p53 gene status, are tumorigenic and metastatic in severe combined immunodeficiency mice. We demonstrate that only the wild-type p53 protein expressed in ER clones is functional on the basis of its site-specific double-stranded DNA-binding activity and its ability to confer a G1 delay on cells following treatment with ionizing radiation. These data indicate that disruption of the p53 growth-regulatory pathway is not a prerequisite for the malignant conversion of rat embryo fibroblasts expressing the E7 gene and mutant ras. Differences in phenotype that were correlated with loss of p53 protein function included the following: serum-independent growth of ER clones in culture, decreased tumor doubling time in vivo, and increased radioresistance. In addition, we demonstrate the p53-dependent G1 checkpoint alone does not determine radiosensitivity.

p53 expression in non-Hodgkin's lymphomas: a marker of p53 inactivation?

The p53 gene located in the short arm of chromosome 17 at position 17p13, is involved in the negative regulation of cellular growth. p53 mutation seems to be the most frequent genetic alteration found in human cancer. Mutant conformation of the p53 gene is associated with cell proliferation and tumour progression, and in most cases implies p53 stabilization, which renders the p53 protein detectable through the use of immunohistochemical techniques. p53 expression is a frequent finding in high grade lymphomas of either B or T cell lineage, having been detected in 30% of cases in our series. The focal presence of p53+ cells was seen in a wide range of low and high grade lymphomas, including lymphadenitis and reactive tonsils. In 37.5% of cases this increased expression of p53 was secondary to mutation in highly conserved regions (exons 5-8). Unlike findings reported in other tumours, in lymphomas, p53 expression seems to be secondary to genetic alterations other than p53 mutation. Initial data suggest that the MDM2 protein could be involved in inactivating p53 protein in most of these cases. Finally, p53 expression has been found to be a poor prognostic marker in high grade B-cell lymphomas in a large series of cases. High p53 expression was associated with a short survival, this relation being stronger in cases with simultaneous bcl2 expression.

The p53 and mdm-2 genes in human testicular germ-cell tumors

Mutations in the p53 gene are common in many cancers. They have been documented to occur in about 55% of all cancers of 51 different cell and tissue types. These mutations are accompanied by overexpression of the p53 protein in the nucleus of the cell, and this protein has lost its tumor suppressor function. In this study, 25 testicular germ-cell (TGC) tumors were tested for p53 mutations and the level of p53 protein expression. While 67% of the tumors overproduced the p53 protein in the nucleus of 10-60% of their cells, in all cases the DNA sequence of exons 4-9 of the p53 gene was wild type. In this tumor type, there was apparently no selection pressure for p53 mutations. The mdm-2 gene residues on chromosome 12 (12q13-q14), a chromosome often altered in TGC tumors. mdm-2 gene amplification (2.5- to 10-fold) was detected in three (12%) of these TGC tumors. These three tumors, and eight additional TGC tumors, overexpressed mdm-2 mRNA. There was a good correlation between overexpression of p53 protein and overexpression of mdm-2 mRNA (P = 0.01). This may well result from the fact that the level of mdm-2 mRNA is regulated by the p53 level. These studies demonstrate that TGC tumors fail to be selected for p53 mutations but nonetheless frequently expressed high levels of wild-type p53 protein in the cell nucleus. Perhaps this produces the excellent response to radiation and chemotherapy of these tumors, which generally have a good prognosis. Wild-type p53 may mediate apoptosis in these cells in response to the DNA damage.

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.

Multiple patterns of MDM-2 deregulation in human leukemias: implications in leukemogenesis and prognosis

The human analogue of the mouse double minute-2 (MDM-2) protein binds to p53 protein and abrogates its tumor-suppressing activity. MDM-2 overexpression may represent an alternative mechanism to p53 mutation for escaping the p53-mediated growth control. Interestingly, multiple MDM-2 protein isoforms have been described and the possibility of functional differences between various isoforms has been raised. Previously, we demonstrated significant MDM-2 mRNA overexpression in human leukemias and suggested that MDM-2 overexpression may be a marker of aggressiveness of the disease. Polyclonal antibodies (Ab) have been generated to detect various isoforms of the MDM-2 protein. Using these Abs, we confirmed MDM-2 protein overexpression in leukemias. Furthermore, we observed heterogeneity in the isoforms expressed in various types of leukemias. In addition, we demonstrated that analysis by flow cytometry could be used as a diagnostic tool for detecting altered MDM-2 protein expression in leukemias. Here we review and expand our initial observations and confirm MDM-2 mRNA and protein overexpression by reverse transcription-polymerase chain reaction (RT-PCR), flow cytometry, and western blot analyses. Understanding the possible role of MDM-2 oncogene expression in leukemias may establish the scientific basis for new therapeutic approaches.

Single cell monitoring of growth arrest and morphological changes induced by transfer of wild-type p53 alleles to glioblastoma cells

Mutation of the p53 tumor suppressor gene is one of the earliest identified genetic lesions during malignant progression of human astrocytomas. To assess the functional significance of these mutations, wild-type (WT) p53 genes were introduced into glioblastoma cell lines having mutant, WT, or null endogenous p53 alleles. Populations of cells with mutant or null endogenous p53 alleles and exogenous WT p53 were spontaneously selected in culture for cells expressing only mutant p53 or no p53, which then displayed a growth and tumorigenic phenotype identical to the parental cells. To determine the phenotypic consequences of WT p53 expression before the occurrence of mutations, we developed a single cell assay to monitor WT p53-dependent transcription activity. Transfer and expression of exogenous WT p53 genes to cells with endogenous mutant or deleted, but not WT, p53 alleles caused growth arrest and morphological changes, including increased cell size and acquisition of multiple nuclei. This supports the hypothesis that genetic lesions of the p53 gene play an important role in the genesis of astrocytomas. Furthermore, the high sensitivity of the episomal single cell reporter strategy developed here has potential clinical applications in the rapid screening of patients for germ-line mutations of the p53 gene or any other gene with known targets for transcriptional transactivation.

The role of p53 inactivation in human cervical cell carcinoma development

We investigated the association between human papillomavirus (HPV) infection and p53 gene mutation in 47 primary uterine cervical cancers. HPV DNA sequences were present in 43 cancers (91.5%), and one of these cancers contained a p53 gene mutation. In addition, one of the remaining four HPV-negative cancers also contained a p53 gene mutation. As a result, p53 inactivation corresponded to the development of 44 of the primary uterine cervical cancers studied (93.6%). We obtained both primary and recurrent tumours from four cases. In two of these cases, the HPV genomes that were present in an episomal state in the primary tumours were observed to have disappeared in the recurrent tumours. One of these recurrent tumours also contained a p53 gene mutation, which suggested the possibility that p53 inactivation was required in order to maintain the aggressive behaviour in this cancer either by an HPV infection or by a p53 gene mutation. No MDM2 gene amplification was observed in the tumours that carried neither HPV DNAs nor p53 gene mutations.

MDM2 and CDK4 gene amplification in Ewing's sarcoma

Amplification of the MDM2 gene, which maps to chromosome band 12q13 and encodes a p53-binding protein, may result in functional inactivation of p53 and has been observed in various bone and soft tissue sarcomas. Published studies have included few cases of Ewing's sarcoma (ES) or peripheral neuroectodermal tumour (PNET), a tumour group in which alterations of the p53 pathway have so far not been extensively studied. We examined two ES cell lines, RD-ES and SK-ES-1, and 30 specimens from 27 patients (24 ES, 6 PNET; 19 primary, 4 local recurrence, 7 metastasis) for MDM2 gene amplification by Southern blot analysis. All 30 clinical specimens had been confirmed to contain sufficient ES/PNET DNA by the demonstration of a rearrangement of the t(11;22)-associated EWS gene using an EWS cDNA probe on the same blots. MDM2 gene amplification was detected in 3 of 30 specimens (10 per cent), including two ES and one PNET, but in neither of the cell lines. The three cases with amplification were morphologically typical primary tumours. Two of the three cases also showed co-amplification of the CDK4 gene, which encodes a cyclin-dependent kinase and also maps to band 12q13. Clinically, all three cases had metastatic disease at diagnosis, compared with only 1 of 15 MDM2-negative cases where the primary tumour was studied. The difference was statistically significant (P = 0.005), suggesting an association of MDM2 amplification with advanced stage.(ABSTRACT TRUNCATED AT 250 WORDS)

PCR-mismatch analysis of p53 gene mutation in Hodgkin's disease

Expression of the p53 protein can be detected by immunohistochemistry in Reed-Sternberg (RS) cells, the presumed neoplastic component of Hodgkin's disease (HD) lesions. At present, there is no clear molecular evidence that p53 positive immunostaining in HD correlates with the presence of mutations or other structural alterations of the p53 gene. To address this question, 49 cases of HD have been investigated for p53 expression by immunohistochemistry, using the DO1 monoclonal antibody on paraffin sections. Thirty-seven out of 49 cases (75 per cent) exhibited positive immunostaining restricted to RS cells and variants. Among these 37 positive cases, ten cases were selected on the basis of a rich content of RS cells showing virtually 100 per cent DO1 positivity. A PCR-mismatch strategy was chosen for the detection of p53 mutations. The threshold level of sensitivity was assessed on positive cell-line controls. A proportion of 10-15 per cent p53 mutated cells mixed in a normal population could be identified. Total genomic DNA was extracted from the ten selected HD cases and PCR amplification of exons 5-8 of the p53 gene was performed. Heteroduplex mismatch analysis revealed no structural alterations of the p53 gene in any case. In view of these findings, it appears unlikely that the sensitivity of the method by itself can fully explain the negative results, although this possibility cannot be completely ruled out. Thus, it is conceivable that p53 positive immunostaining in HD may not necessarily imply genomic alterations in the classic 'hot spot' regions and may be related to another mechanism of p53 protein stabilization.

On the regulation of the p53 tumour suppressor, and its role in the cellular response to DNA damage

The p53 gene is required for the normal apoptotic response of mammalian cells to DNA damage caused by ionizing radiation and DNA damaging drugs. DNA damage results in the accumulation of biologically active p53. This response is potentially lethal and is therefore highly regulated. By using both biochemical and cell biological approaches a number of discrete control pathways have been identified. These include analysis of cellular and viral proteins that bind to p53 to inactivate its function, the discovery of cells with defects in the p53 activation pathway and the analysis of an allosteric regulation of p53 function controlled by phosphorylation.

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.

Lack of a correlation between p53 protein expression and radiation response in human tumor primary cultures

We investigated the possible relationship between immunohistochemically detected p53 expression and in vitro response to gamma-irradiation in 24 primary cultures of human ovarian cancers and cutaneous melanomas. The frequency of p53-positive tumors was around 60% within each tumor histotype. The range of the surviving fraction at 2 Gy (SF2) was similar in p53-positive (0.10-0.76) and p53-negative (0.23-0.65) tumors, with median values of 0.36 and 0.33, respectively. No differences were observed in the accumulation of DNA-double strand breaks, assessed by neutral filter elution after exposure to 50 Gy, between p53-positive and p53-negative tumors. As regards DNA lesion repair, after 2 h of recovery the percentage of rejoined DNA-double strand breaks ranged from 19% to 99% in the different cultures, but again the distribution of values was similar for p53-positive and p53-negative tumors. Specifically, the median percentage of repaired DNA-double strand breaks was 70% and 74% in the two groups. On the whole, our data do not support the hypothesis that p53 overexpression is a major determinant of in vitro radiation response.

Genetic alterations associated with metastatic dissemination and chemoresistance in neuroblastoma

Knowledge about genetic alterations specific to the metastatic process and chemoresistance in neuroblastoma is progressing steadily. Low or no CD44 expression, increased NM23 expression and specific mutations of the 5' coding regions of NM23 are distinct features of aggressive, metastatic neuroblastoma. MYCN down-regulates Class I HLA antigen expression in many neuroblastoma cell lines and, in turn, may be regulated by a suppressor gene. The MYCN amplified human neuroblastoma cell line, IGR-N-91, established in vitro, metastasises in the nude mouse and has exhibited co-activation of MYCN and PGY1, resulting from direct activation of the oncoprotein on the PGY1 promoter. In this model, the MYCN product activates angiogenesis, the dissemination process and chemoresistance via specific genes (PGY1 and GST3). MYCN, like the BCL-2 and TP53 products, may also play a key role in apoptosis. The implication of these genes in the potential for metastasis and chemoresistance in neuroblastoma is discussed.

Amplification of the E2F1 transcription factor gene in the HEL erythroleukemia cell line

The E2F transcription factor plays an important regulatory role in cell proliferation, mediating the expression of genes whose products are essential for inducing resting cells to enter the cell cycle and synthesize DNA. To investigate the possible involvement of E2F in hematopoietic malignancies, we isolated genomic clones encompassing the human E2F1 gene. We then used fluorescence in situ hybridization to localize E2F1 to human chromosome 20q11, telomeric to the p107 locus, a gene whose product is related to the retinoblastoma gene product (pRb). This finding contrasts with the 1p36 and 6q22 chromosomal locations previously assigned E2F2 and E2F3, two additional members of the E2F family. Although deletions or structural rearrangements of E2F1 were not detected in 14 primary acute leukemia or myelodysplasia samples with structural abnormalities of chromosome 20q11, the gene was amplified and overexpressed in HEL erythroleukemia cells and translocated to other chromosomes in several established human leukemia cell lines. This study provides the first evidence of gene amplification involving a member of the E2F family of transcription factors. We propose that E2F1 overexpression in erythroid progenitors may stimulate abnormal cell proliferation by overriding negative regulatory signals mediated by tumor suppressor proteins such as pRb.

Cytogenetic evolution of MYCN and MDM2 amplification in the neuroblastoma LS tumour and its cell line

Amplification of the MYCN gene is frequently seen either in extrachromosomal double minutes (DMs) or in homogeneously staining regions (HSRs) of aggressively growing neuroblastomas. Total genomic DNA from cell line LS, from early passages of the same line and from original tumour material was biotinylated and hybridised to metaphase chromosomes of normal human lymphocytes. The reverse genomic hybridisation revealed the amplified DNA to be derived both from chromosome 2p23-24, which is the position of MYCN, and from chromosome 12 band q13-14. The MDM2 gene, located at 12q13-14, was found amplified both in early and late passages of LS, in addition to amplified MYCN. Amplification units of MYCN and MDM2 appear first to develop within DMs, which then integrate into different chromosomes to develop to HSRs.

mdm2 gene alterations and mdm2 protein expression in breast carcinomas

This study investigates the mdm2 gene status and expression in 66 surgically resected human breast carcinomas, with correlations with clinico-pathological and biological data (histological type, grading, steroid receptor status, p53 expression, proliferative activity). Four (7.7 per cent) out of 52 informative cases bear mdm2 gene amplification (four-to ten-fold) and 8 (15.4 per cent) of 52 cases showed borderline amplification (three-fold). Nine (13.6 per cent) out of 66 cases showed strong mdm2 nuclear immunoreactivity. Twenty-seven (40.9 per cent) cases showed isolated mdm2 reactive nuclei. All cases with clear amplification showed a high percentage of mdm2 immunoreactive nuclei. The relationship between gene amplification and mdm2 protein expression is highly significant (P < 0.0001). No association was observed between mdm2 gene amplification and any of the considered clinico-pathological and biological parameters, while mdm2 immunoreactivity showed a significant association only with oestrogen receptor immunoreactivity (P = 0.009). p53 expression was associated neither with mdm2 gene amplification nor with mdm2 immunoreactivity. It could be tempting to hypothesize that the evaluation of the combined mdm2/p53 immunohistochemical phenotype in human breast carcinoma could give us better prognostic information than the evaluation of the expression of the p53 protein alone.

Prognostic implications of chromosome 17p deletions in human medulloblastomas

DNA derived from medulloblastoma biopsies was analyzed to determine if deletions of the 17p region, mutations of the TP53 gene, or amplification of the c-myc, N-myc, EGFR (epidermal growth factor receptor), or MDM2 (murine double-minute-2) genes was indicative of a poor prognosis. Loss of heterozygosity for 17p, observed in 8/28 (29%) paired samples, was associated with a shortened survival period (p = 0.045 by the logrank test). TP53 mutations occurred in 2/46 (4.3%) tumor samples. c-myc Amplification was seen in 3/43 (6.9%) cases, while none of the tumors contained amplified N-myc, EGFR, or MDM2 genes. These results demonstrate that, while only rare medulloblastomas contain TP53 gene mutations or amplification of the c-myc gene, loss of heterozygosity on chromosome 17p is indicative of a significantly worse prognosis among patients with these tumors. Further, these results provide a strong impetus for a prospective analysis of loss of heterozygosity in a cooperative group setting, which would include tumor staging, a selection of treatment modalities, and multivariate analyses.

Expression of mdm-2 and p53 protein in transitional cell carcinoma

Amplification of the mdm-2 gene and overexpression of the mdm-2 protein might inactivate p53 function, and may have prognostic relevance. The present paper investigated the immunohistochemical overexpression of the mdm-2 and p53 proteins in 25 biopsy specimens of transitional cell bladder carcinomas (10 pT1 and 15 pT2 or higher stages). Five cases (20%) showed strong mdm-2 protein immunoreactivity in more than 5% of the tumor cells; 14 cases (56%) showed p53 immunoreactivity in more than 20% of the cells, and were considered as overexpressing p53 protein. Four of the five cases with strong mdm-2 immunoreactivity did not show p53 overexpression, and 13 of the 14 cases with p53 overexpression did not show mdm-2 immunoreactivity. Our data are consistent with the hypothesis that p53 overaccumulation (and hence possible p53 gene mutation) or mdm-2 overexpression (and hence possible mdm-2 gene amplification) may mirror two different and possibly complementary gene alterations, which might finally interfere with the control of cell proliferation and apoptosis. In this perspective, evaluation of the combined mdm-2/p53 protein phenotype in human bladder carcinomas could have prognostic relevance and give us better prognostic information than evaluation of the p53 protein alone.

Molecular biological aspects of soft tissue tumors

In the preceding, the reader has hopefully developed an appreciation of the major malignant tumors to be encountered in somatic soft tissues in children, adolescents, and young adults. In aggregate, this group of tumors accounts for about 20% of cancer in this age group. Importantly, they are curable tumors when nonmetastatic at presentation, but therapy appropriate to prognosis and tumor responsiveness is highly dependent on precise diagnosis. The historical morphologic methods alone will not suffice for this purpose, but the anticipated rapid advent of molecular genetic diagnostic and prognostic methods should. Useful, practical, and rapid genetic tests, available in the same time frame as the routine histopathologic evaluation of these tumors, are likely to forever change the diagnosis and management of these tumors, individually and as a group.

Characteristic chromosome abnormalities and karyotype profiles in soft tissue tumors

Characteristic chromosome abnormalities and karyotype profiles are emerging for the soft tissue tumors. The notable findings are summarized in the Table 1. Within the broad range of solid tumors, it is certainly the soft tissue tumors in which the most spectacular success has occurred with regard to neoplasia-associated chromosome abnormalities. Cytogenetic studies of soft tissue tumors have been encouraged by the early and growing supporting interest of pathologists and clinicians concerned with soft tissue tumors. However, when one considers the variety of types and subtypes of benign and malignant soft tissue tumors, the number that has been so far characterized by a specific chromosome change is still very small. But, as we attempt to demonstrate in this report, these data should be viewed as paradigms for the importance of cytogenetic investigations in solid tumors. Cytogenetic studies of solid tumors are of more than clinical interest. Cytogenetic studies allow molecular investigations of the chromosomal breakpoints. They allow the search to proceed for genes involved in the chromosomal changes, providing a better knowledge of the malignant transformation process. In addition, the fruits of the combined efforts in cytogenetic and molecular technologies, from which has come "molecular cytogenetics," will let us recognize more conveniently, more quickly and, hopefully, less expensively the well-characterized diagnostic chromosome markers in tumor cells. Thus, we may be able to reach the goal of incorporating cytogenetics into standard diagnostic procedures for solid tumors, as has been achieved with hematological malignancies. Molecular cytogenetics including fluorescent in situ hybridization (FISH) technology promises to bring soft tissue tumor cytogenetics into regular diagnostic armamentaria and concurrently speed research into the basis of soft tissue tumors.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

Low Grade Amplification of MDM2 Gene in a Subset of Human Breast Cancers without p53 Alterations

MDM2 protein is thought to bind to p53 tumor suppressor protein leading to inhibition of p53-mediated transactivation. Amplification of the MDM2 gene has been frequently observed in human sarcoma, and relevant overexpression of the MDM2 protein is assumed to contribute to tumorigenesis through inactivation of the p53 function. In order to determine whether MDM2 amplification plays a role in the development of human breast cancer without genetic alteration of p53, we analyzed, MDM2 gene amplification by quantitative hybridization and genetic alteration of p53, in 32 primary tumors and 26 metastatic lymph nodes. Low grade amplification of the MDM2 gene (2-6 fold) was observed in four cases, none of which showed even subtle genetic alterations of p53 or loss of alleles on 17p. Moreover, in three of the four cases with MDM2 gene amplification, the level of gene amplification in the metastatic lymph nodes was slightly higher than that in the primary tumors. These results, taken together with previous findings, suggest that a subset of breast cancers without genetic alteration of p53 may also arise by inactivation of the p53 function through interaction with the overexpressed MDM2 protein induced by gene amplification.

Cell cycle control and cancer

Multiple genetic changes occur during the evolution of normal cells into cancer cells. This evolution is facilitated in cancer cells by loss of fidelity in the processes that replicate, repair, and segregate the genome. Recent advances in our understanding of the cell cycle reveal how fidelity is normally achieved by the coordinated activity of cyclin-dependent kinases, checkpoint controls, and repair pathways and how this fidelity can be abrogated by specific genetic changes. These insights suggest molecular mechanisms for cellular transformation and may help to identify potential targets for improved cancer therapies.

The protooncogene CHOP/GADD153, involved in growth arrest and DNA damage response, is amplified in a subset of human sarcomas

The C/EBP-homologous transcription factor CHOP (GADD153) is inducible by growth inhibition or DNA damage, and has been shown to be oncogenically activated by the specific (12;16) translocation in human myxoid liposarcoma. We have now found CHOP amplification in two sarcoma cell lines with previously reported amplification of the nearby GLI gene. Among 98 other human sarcomas of various types, CHOP was amplified in a hemangiopericytoma, a liposarcoma, and two osteosarcoma. High constitutive expression levels of CHOP were observed in tumors with gene amplification, but also in some other samples. The nearby MDM2 gene, which codes for a protein that may inactivate wild-type p53, has previously been reported to be frequently amplified in sarcoma. In our sarcoma panel, MDM2 was amplified in 9 cases. MDM2 and CHOP were co-amplified in two of these, whereas the two osteosarcomas had amplified CHOP but not MDM2. CHOP was amplified in both cell lines with GLI amplification, and MDM2 only in one. No mutations in the TP53 gene have been found in samples with amplification of MDM2. In contrast, the cell line in which CHOP but not MDM2 was amplified had mutated TP53, suggesting that selection of this amplicon was not mediated through p53 inactivation.

Linkage studies in a Li-Fraumeni family with increased expression of p53 protein but no germline mutation in p53

We report a family with the Li-Fraumeni syndrome (LFS) in whom we have been unable to detect a mutation in the coding sequence of the p53 gene. Analysis of linkage to three polymorphic markers within p53 enabled direct involvement of p53 to be excluded. This is the first example of a LFS family in whom exclusion of p53 has been possible. Four affected members of the family with sarcoma or premenopausal breast cancer showed increased expression of p53 protein in their normal tissues as detected by immunohistochemistry. It therefore appears that the LFS phenotype has been conferred by an aberrant gene, showing a dominant pattern of inheritance, which may be acting to compromise normal p53 function rather than by a mutation in p53 itself. In order to try to determine the chromosomal location of this putative gene, we have carried out studies of linkage to candidate loci. By these means we have excluded involvement of Rb1 and BRCA1 on chromosomes 13q and 17q respectively. The MDM2 oncogene on chromosome 12q was considered to be the prime candidate as MDM2 is amplified in sarcomas and the MDM2 product binds to p53. Furthermore, p53 mutation and amplification of MDM2 have been shown to be mutually exclusive events in tumour development. Linkage analysis to two polymorphic markers within MDM2 yielded a three-point LOD score of -5.4 at a recombination fraction theta equal to zero. Therefore MDM2 could be excluded. It is possible that the gene which is responsible for cancer susceptibility in this family, possibly via interaction with p53, will be important in the histogenesis of breast cancer in general. We are now carrying out further studies to locate and identify this gene.

p53 gene mutations in pituitary adenomas: rare events

OBJECTIVE

Occult pituitary adenomas are said to occur in up to 20% of random autopsy examinations, yet the only oncogene known to be associated with pituitary adenomas, gsp, is found in only 40% of somatotrophinomas, a subtype that accounts for a minority of pituitary tumours. Mutations of the p53 tumour suppressor gene are thought to be involved in the pathogenesis of as many as 50% of all human cancers, including tumours of the central nervous system. The objective of this study was to determine whether p53 gene mutations are associated with pituitary adenomas.

DESIGN AND PATIENTS

Fragments of pituitary adenoma tissue from 29 patients undergoing routine hypophysectomy for pituitary tumour were coated in cryostat embedding medium and frozen at -80 degrees C within 24 hours of resection. They consisted of 9 somatotroph, 4 corticotroph, 1 mammotroph and 15 endocrinologically inactive adenomas, all of the non-invasive clinical phenotype. Sequential frozen sections were subjected to in situ hybridization analysis for anterior pituitary hormone transcripts and examined histologically to ensure that the frozen sections used to generate DNA templates for polymerase chain reaction amplification were not contaminated with non-tumour tissue.

MEASUREMENTS

p53 exons 7 and 8, within which 98% of substitution mutations are thought to occur, and exons 4-6 in tumours immunopositive for p53, were amplified by polymerase chain reaction and ligated into the vector pCR2. DNA from small-scale plasmid preparations of pCR2 containing cloned p53 exons from human pituitary adenomas was sequenced using an automated fluorescence-based system (DuPont Genesis 2000) and compared with wild-type sequence. Apparent mutations were confirmed or refuted by sequencing a further 2-4 clones isolated from the same template.

RESULTS

Although immunocytochemical staining patterns for wild-type p53 varied markedly between different tumours, no mutations were identified in any of the exonic sequences examined.

CONCLUSIONS

p53 mutations, at least within the high mutation domains of p53, occur infrequently in human pituitary adenomas. Increased steady-state levels of p53 protein identified immunocytochemically may be a consequence of binding to other cellular proteins in these tumours.

The ribosomal L5 protein is associated with mdm-2 and mdm-2-p53 complexes

Throughout the purification of the mdm-2 or mdm-2-p53 protein complexes, a protein with a molecular weight of 34,000 was observed to copurify with these proteins. Several monoclonal antibodies directed against distinct epitopes in the mdm-2 or p53 protein coimmunoprecipitated this 34,000-molecular-weight protein, which did not react to p53 or mdm-2 polyclonal antisera in a Western immunoblot. The N-terminal amino acid sequence of this 34,000-molecular-weight protein demonstrated that the first 40 amino acids were identical to the ribosomal L5 protein, found in the large rRNA subunit and bound to 5S RNA. Partial peptide maps of the authentic L5 protein and the 34,000-molecular-weight protein were identical. mdm-2-L5 and mdm-2-L5-p53 complexes were shown to bind 5S RNA specifically, presumably through the known specificity of L5 protein for 5S RNA. In 5S RNA-L5-mdm-2-p53 ribonucleoprotein complexes, it was also possible to detect the 5.8S RNA which has been suggested to be covalently linked to a percentage of the p53 protein in a cell. These experiments have identified a unique ribonucleoprotein complex composed of 5S RNA, L5 protein, mdm-2 proteins, p53 protein, and possibly the 5.8S RNA. While the function of such a ribonucleoprotein complex is not yet clear, the identity of its component parts suggests a role for these proteins and RNA species in ribosomal biogenesis, ribosomal transport from the nucleus to the cytoplasm, or translational regulation in the cell.

Elevated levels of MDM-2 and p53 expression are associated with high grade non-Hodgkin's lymphomas

The role of p53 in the evolution of non-Hodgkin's lymphomas (NHL) is unclear. Mutations of the p53 gene appear to be relatively uncommon but stabilized p53 protein, as detected by immunohistochemistry, has indicated a more frequent involvement of p53. As dysfunction of p53 protein has also been suggested to occur after overexpression of the mdm-2 protein, we have therefore investigated a series of non-malignant hyperplastic reactive lymphoid tissues and NHL to examine whether the levels of expression of MDM-2 correlated to positivity of p53 protein staining. Northern blot analysis of MDM-2 expression was compared to glucose-6-phosphate dehydrogenase (G6PD) expression by densitometry to quantify the relative levels of MDM-2 expression. Consistent low levels of MDM-2 expression were observed in non-malignant lymphoid tissue and in low grade NHL, however, 13/15 high grade NHL exhibited a 2-15-fold increase in MDM-2 expression. Interestingly similar elevations in p53 mRNA expression were also observed in 6/15 high grade NHL. Positive staining of the p53 protein did not, however, correlate with elevated mRNA levels of either MDM-2 or p53. The significance of these observations is discussed.

Accumulation of nuclear p53 and tumor progression in bladder cancer

BACKGROUND

We have previously demonstrated a strong association between nuclear accumulation of p53 protein, as determined by immunohistochemical analysis, and mutations in the p53 gene. The purpose of this study was to determine the relation between nuclear accumulation of p53 and tumor progression in transitional-cell carcinoma of the bladder.

METHODS

Histologic specimens of transitional-cell carcinoma of the bladder (stages Pa, noninvasive disease, to P4, disease with direct extension into adjacent organs or structures) from 243 patients who were treated by radical cystectomy were examined for the immunohistochemical detection of p53 protein. Nuclear p53 reactivity was then analyzed in relation to time to recurrence and overall survival.

RESULTS

The detection of nuclear p53 was significantly associated with an increased risk of recurrence of bladder cancer (P < 0.001) and with decreased overall survival (P < 0.001). In patients with cancer confined to the bladder, the rates of recurrence for stage P1, P2, and P3a tumors that had no detectable nuclear p53 reactivity at five years were 7, 12, and 11 percent, respectively, as compared with 62, 56, and 80 percent, respectively, for tumors that had p53 immunoreactivity. Similar results were obtained when the presence or absence of p53 in the nuclei of the tumor cells was studied in relation to overall survival. In a multivariable analysis stratified according to grade, pathological stage, and lymph-node status, nuclear p53 status was an independent predictor (and in cancer confined to the bladder, the only independent predictor) of recurrence and overall survival (P < 0.001).

CONCLUSIONS

In patients with transitional-cell carcinoma confined to the bladder, an accumulation of p53 in the tumor-cell nuclei detected by immunohistochemical methods predicts a significantly increased risk of recurrence and death, independently of tumor grade, stage, and lymph-node status. Patients with transitional-cell carcinoma confirmed to the bladder that demonstrates nuclear p53 reactivity should be considered for protocols of adjuvant treatment.

Deregulated transcription factor E2F-1 expression leads to S-phase entry and p53-mediated apoptosis

E2F-1 is a transcription factor suspected of activating genes required for S phase and a known target for the action of RB, the retinoblastoma gene product. Its induction in quiescent fibroblasts led to S-phase entry followed by apoptosis. E2F-1-mediated apoptosis was suppressed by coexpression of wild-type RB or a transdominant negative mutant species of p53. In contrast, coexpression of a naturally occurring loss-of-function RB mutant or wild-type p53 did not suppress the induction of apoptosis under these conditions. Thus, deregulated E2F-1 activity gives rise to proliferative and apoptotic signals. p53 appears to participate in the execution of the latter.

The ribosomal L5 protein is associated with mdm-2 and mdm-2-p53 complexes

Throughout the purification of the mdm-2 or mdm-2-p53 protein complexes, a protein with a molecular weight of 34,000 was observed to copurify with these proteins. Several monoclonal antibodies directed against distinct epitopes in the mdm-2 or p53 protein coimmunoprecipitated this 34,000-molecular-weight protein, which did not react to p53 or mdm-2 polyclonal antisera in a Western immunoblot. The N-terminal amino acid sequence of this 34,000-molecular-weight protein demonstrated that the first 40 amino acids were identical to the ribosomal L5 protein, found in the large rRNA subunit and bound to 5S RNA. Partial peptide maps of the authentic L5 protein and the 34,000-molecular-weight protein were identical. mdm-2-L5 and mdm-2-L5-p53 complexes were shown to bind 5S RNA specifically, presumably through the known specificity of L5 protein for 5S RNA. In 5S RNA-L5-mdm-2-p53 ribonucleoprotein complexes, it was also possible to detect the 5.8S RNA which has been suggested to be covalently linked to a percentage of the p53 protein in a cell. These experiments have identified a unique ribonucleoprotein complex composed of 5S RNA, L5 protein, mdm-2 proteins, p53 protein, and possibly the 5.8S RNA. While the function of such a ribonucleoprotein complex is not yet clear, the identity of its component parts suggests a role for these proteins and RNA species in ribosomal biogenesis, ribosomal transport from the nucleus to the cytoplasm, or translational regulation in the cell.

Radiation-induced genomic instability

Quantitative assessment of the heritable somatic effects of ionizing radiation exposures has relied upon the assumption that radiation-induced lesions were 'fixed' in the DNA prior to the first postirradiation mitosis. Lesion conversion was thought to occur during the initial round of DNA replication or as a consequence of error-prone enzymatic processing of lesions. The standard experimental protocols for the assessment of a variety of radiation-induced endpoints (cell death, specific locus mutations, neoplastic transformation and chromosome aberrations) evaluate these various endpoints at a single snapshot in time. In contrast with the aforementioned approaches, some studies have specifically assessed radiation effects as a function of time following exposure. Evidence has accumulated in support of the hypothesis that radiation exposure induces a persistent destabilization of the genome. This instability has been observed as a delayed expression of lethal mutations, as an enhanced rate of accumulation of non-lethal heritable alterations, and as a progressive intraclonal chromosomal heterogeneity. The genetic controls and biochemical mechanisms underlying radiation-induced genomic instability have not yet been delineated. The aim is to integrate the accumulated evidence that suggests that radiation exposure has a persistent effect on the stability of the mammalian genome.

Prognostic implications of p53 overexpression in supratentorial astrocytic tumors

The wild-type p53 gene is thought to play a critical role in tumor suppression and has been shown to reverse the transformed phenotype of tumor cells in vitro. Mutational inactivation of this aspect of p53 activity occurs frequently in many human neoplasms, including astrocytomas, and is thought to represent a critical step in tumor progression. We have found previously that the presence of p53 immunoreactivity was significantly associated with malignant astrocytomas arising in younger patients, although occurring infrequently in tumors in older patients. Given that young age is the most consistent clinical factor predictive of longer survival in patients with astrocytomas, this suggested that p53 protein accumulation might be a molecular predictor of enhanced survival. To test this hypothesis, we retrospectively studied the association of p53 overexpression with survival in 149 patients with astrocytomas, using univariate and multivariate analysis to determine its value in predicting survival. Although our analysis reaffirmed the strong association between young age and increased survival, we were unable to demonstrate any difference in survival between patients with Grade III and IV tumors with p53 immunoreactivity compared with those without. Presumably, once a tumor has progressed to high grade, the relative importance of p53 status as a predictor of survival is low, probably because of the large number of accumulated genetic alterations associated with malignant tumors. In contrast, the presence of p53 overexpression in Grade II astrocytomas seemed from survival curves to indicate shorter survival compared with patients who had no p53 immunoreactivity. However, this variable did not quite reach statistical significance (P = 0.08) as an independent predictive variable in multivariate analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

TP53 tumor suppressor gene and skin carcinogenesis

The tumor suppressor gene TP53 encodes for a nuclear phosphoprotein involved in the control of cell proliferation, particularly in stressed cells. TP53 gene mutations are the most frequent genetic event found in human cancers. Most mutations locate in the highly conserved domains of the gene. Their localizations vary according to the tissue and tumor type, but define some hot spot regions that may have a certain degree of tissue specificity. In certain cases, the type of nucleotide substitutions observed can help to find the carcinogenic agent. In recent years, TP53 gene mutations have been frequently observed in human skin tumors. In epithelial carcinomas, they involve mainly exons 5, 7, and 8. Interestingly, many are C to T transitions at dipyrimidine sites; particularly, one can find CC to TT double-base changes that are known to be specific to ultraviolet radiation. These data confirm at the molecular level the role of ultraviolet radiation as an important etiologic factor in the genesis of these lesions. The high incidence of TP53 mutations suggest that they play a role in keratinocyte transformation. Nevertheless, this event has not yet been defined as an early or late event. In melanomas, most studies have shown the detection of the p53 protein by immunohistochemistry, suggestive of the presence of a mutation in the gene prolonging the protein half-life. Anti-p53 reactivity is frequent in these tumors and seems to correlate with tumor aggressiveness. Confirmation and characterization of TP53 gene mutation at the DNA level would help to precisely define the role of this gene in the development of these tumors.

Cytogenetic findings in 19 malignant bone tumors

BACKGROUND

The majority of karyotypes observed in osteosarcomas (OS) and chondrosarcomas (CS) are complex. Specific chromosomal abnormalities have not yet been characterized in either tumor except for a ring chromosome in parosteal OS. The purpose of this study was to determine recurrent chromosomal abnormalities and establish a possible correlation between the cytogenetic changes and the pathologic findings.

METHODS

Ten OS and nine CS were cytogenetically analyzed. Tumor samples were obtained from patients having a resection or incisional biopsy. Cytogenetic study of short term cell cultures included harvesting and G-banding, which were performed by routine methodologies.

RESULTS

Clonal abnormalities were observed in six OS and six CS. Modal chromosome numbers ranged from near diploid to near tetraploid in both types of tumors. The structural rearrangements observed in OS involved mostly chromosomes 1, 2, 6, 12, and 17. Nonreciprocal translocations were the most frequent event. Two OS had a single clonal abnormality involving 11p15 and 14q32, respectively. Double minute chromosomes were observed in three cases. In CS, the most frequent structural abnormalities were nonreciprocal translocations and deletions involving numerous chromosomes. Rearrangements of 1p together with other abnormalities were observed in four CS.

CONCLUSIONS

The karyotypes were usually complex consisting of numerical and structural changes, particularly in high grade tumors. Rearrangements of 11p15 and 14q32 in OS and possibly 1p in CS were found as primary cytogenetic aberrations. Cytogenetic analysis in more cases of OS and CS together with molecular studies are necessary to characterize further the consistent genetic changes in these tumors.