The dominating effect of mutant p53

Inactivation of p53 in Human Keratinocytes Leads to Squamous Differentiation and Shedding via Replication Stress and Mitotic Slippage

Tumor suppressor p53 is a major cellular guardian of genome integrity, and its inactivation is the most frequent genetic alteration in cancer, rising up to 80% in squamous cell carcinoma (SCC). By adapting the small hairpin RNA (shRNA) technology, we inactivated endogenous p53 in primary epithelial cells from the epidermis of human skin. We show that either loss of endogenous p53 or overexpression of a temperature-sensitive dominant-negative conformation triggers a self-protective differentiation response, resulting in cell stratification and expulsion. These effects follow DNA damage and exit from mitosis without cell division. p53 preserves the proliferative potential of the stem cell compartment and limits the power of proto-oncogene MYC to drive cell cycle stress and differentiation. The results provide insight into the role of p53 in self-renewal homeostasis and help explain why p53 mutations do not initiate skin cancer but increase the likelihood that cancer cells will appear.

SW480, a p53 Double-mutant Cell Line Retains Proficiency for Some p53 Functions

During certain types of cellular stress, the p53 tumor suppressor protein binds to DNA and transactivates a variety of genes that regulate critical responses including apoptosis, cell cycle checkpoints, differentiation, and angiogenesis. In addition, functional p53 is known to be required for efficient nucleotide excision repair (NER) of bulky DNA adducts generated through exposure to environmental mutagens such as UV light. Nonetheless, we previously showed that the model p53-mutated human adenocarcinoma strain SW480 is proficient in the removal of UV-induced cyclobutane pyrimidine dimers (CPD) via NER. We undertook the present study to begin probing the molecular basis for this unexpected repair phenotype. Cytogenetic analysis indicated that SW480 is stable at the chromosomal level, i.e. manifests a karyotypic profile very similar to that revealed for this line as far back as 14 years ago. After fluorescence in situ hybridization (FISH), using a probe complementary to the p53 gene, we found that 98% of the SW480 interphase nuclei contains three copies of the gene, later revealed to be localized on intact short arms of three chromosomes 17. DNA sequence analysis further showed that all three p53 copies in SW480 carry two point mutations (R273H and P309S), and levels of the corresponding mutated p53 protein are about 20-fold higher than in the closely related p53 wild-type strain LoVo. Using an electrophoretic mobility shift assay (EMSA), we demonstrated that R273H/P309S p53 is able to bind with wild-type affinity to its consensus DNA sequence in vitro. Analysis of p21(Cip1/WAF1) expression and in vivo footprinting by ligation-mediated PCR (LMPCR) showed that, in wild-type LoVo cells, an exposure to cellular stress (e.g. UV or ionizing radiation) is necessary for p53 activation of the p21(Cip1/WAF1) promoter. In contrast, the R273H/P309S-mutated p53 protein in SW480 constitutively activates p21(Cip1/WAF1) in the absence of stress through an unknown mechanism. A similar phenomenon whereby mutated p53 in SW480 is able to induce NER-related proteins might explain the normal DNA repair phenotype previously observed in this strain. For now we conclude that, in general, results obtained using SW480 as a p53-deficient cell line should be interpreted very cautiously.

Transgenic mouse with human mutant p53 expression in the prostate epithelium

BACKGROUND

Apoptosis is disrupted in prostate tumor cells, conferring a survival advantage. p53 is a nuclear protein believed to regulate cancer progression, in part by inducing apoptosis. To test this possibility in future studies, the objective of the present study was to generate a transgenic mouse model expressing mutant p53 in the prostate (PR).

METHODS

Transgene incorporation was tested using Southern analysis. Expression of mutant p53 protein was examined using immunofluorescence microscopy. Apoptosis in the PR was evaluated using the Tunnel method.

RESULTS

A construct, consisting of the rat probasin promoter and a mutant human p53 fragment, was prepared and used to generate transgenic mice. rPB-mutant p53 transgene incorporation, as well as nuclear accumulation of mutant human p53 protein, was demonstrated. Prostatic intraepithelial neoplasia (PIN) III and IV were found in PR of 52-week old transgenic mice, whereas no pathological changes were found in the other organs examined. PR ability to undergo apoptosis following castration was reduced in rPB-mutant p53 mice as compared to non transgenic littermates.

CONCLUSIONS

Transgenic rPB-mutant p53 mice accumulate mutant p53 protein in PR, resulting in neoplastic lesions and reduced apoptotic potential in the PR. Breeding rPB-mutant p53 mice with mice expressing an oncogene in their PR will be useful in examining interactions of multiple genes that result in progression of slow growing prostate tumors expressing oncogenes alone to metastatic cancer.

Structure and functionality of a designed p53 dimer

P53 is a homotetrameric tumor suppressor protein involved in transcriptional control of genes that regulate cell proliferation and death. In order to probe the role that oligomerization plays in this capacity, we have previously designed and characterized a series of p53 proteins with altered oligomeric states through hydrophilc substitution of residues Met340 or Leu344 in the normally tetrameric oligomerization domain. Although such mutations have little effect on the overall secondary structural content of the oligomerization domain, both solubility and the resistance to thermal denaturation are substantially reduced relative to that of the wild-type domain. Here, we report the design and characterization of a double-mutant p53 with alterations of residues at positions Met340 and Leu344. The double-mutations Met340Glu/Leu344Lys and Met340Gln/Leu344Arg resulted in distinct dimeric forms of the protein. Furthermore, we have verified by NMR structure determination that the double-mutant Met340Gln/Leu344Arg is essentially a "half-tetramer". Analysis of the in vivo activities of full-length p53 oligomeric mutants reveals that while cell-cycle arrest requires tetrameric p53, transcriptional transactivation activity of monomers and dimers retain roughly background and half of the wild-type activity, respectively.

Spectral karyotyping (SKY) refinement of a complex karyotype with t(20;21) in a Ph-positive CML patient submitted to peripheral blood stem cell transplantation

A patient with a Ph-positive chronic myeloid leukaemia (CML) was submitted to allogeneic peripheral blood stem cell transplantation from an HLA-haploidentical related donor 7 years after the diagnosis. Six months later, he showed a disease relapse while cytogenetic analysis displayed a complex karyotype. To characterise the chromosomal rearrangements spectral karyotype (SKY) analysis was used. This redefined all chromosome rearrangements and revealed a t(20;21)(q11;q22). FISH analysis with a specific probe for the AML1 gene disclosed disruption of this gene which was partially translocated on to the long arm of chromosome 20. It is likely that this rearrangement, unusual for CML, was implicated in the disease evolution towards blastic crisis (BC).

Intracellular antibody-caspase-mediated cell killing: an approach for application in cancer therapy

Antibodies have been expressed inside cells in an attempt to ablate the function of oncogene products. To make intracellular antibodies more generally applicable and effective in cancer therapy, we have devised a method in which programmed cell death or apoptosis can be triggered by specific antibody-antigen interaction. When intracellular antibodies are linked to caspase 3, the "executioner" in the apoptosis pathway, and bind to the target antigen, the caspase 3 moieties are self-activated and thereby induce cell killing. We have used this strategy in a model system with two pairs of intracellular antibodies and antigens. In vivo coexpression of an antibody-caspase 3 fusion with its antigenic target induced apoptosis that was specific for antibody, antigen, and active caspase 3. Moreover, the antibody-caspase 3 fusion protein was not toxic to cells in the absence of antigen. Therefore, intracellular antibody-mediated apoptosis should be useful as a specific therapeutic approach for the treatment of cancers, a situation where target cell killing is required.

Adenoviral vector-mediated gene transfer: timing of wild-type p53 gene expression in vivo and effect of tumor transduction on survival in a rat glioma brachytherapy model

OBJECTIVE

This study sought to investigate modification of the radiation response in a rat 9L brain tumor model in vivo by the wild-type p53 gene (wtp53). Determination of the timing and dose of radiation therapy required the assessment of the duration of the effect of wtp53 expression on 9L tumors after in vivo transfection.

METHODS

Anesthetized male F-344 rats each were stereotactically inoculated with 4 x 10(4) 9L gliosarcoma cells through a skull screw into the cerebrum in the right frontal region. Twelve-day-old tumors were inoculated through the screw with recombinant adenoviral vectors under isoflurane anaesthesia: control rats with Ad5/RSV/GL2 (carrying the luciferase gene), and study rats with Ad5CMV-p53 (carrying the wtp53 gene). Brain tumors removed at specific times after transfection were measured, homogenized, and lysed and wtp53 expression determined by Western blot analysis. Four groups of nine rats were, subsequently, implanted with iodine-125 seeds 15 days post-tumor inoculation to give a minimum tumor dose of 40 or 60 Gy.

RESULTS

We demonstrated transfer of wtp53 into rat 9L tumors in vivo using the Ad5CMV-p53 vector. The expression of wtp53 was demonstrated to be maximum between days 1 and 3 post-vector inoculation. Tumors expressing wtp53 were smaller than controls transfected with Ad5/RSV/GL2 but this difference was not statistically significant. Radiation made a significant difference to the survival of tumor-bearing rats. Moreover, wtp53 expression conferred a significant additional survival advantage.

CONCLUSION

The expression of wtp53 significantly improves the survival of irradiated tumor-bearing rats in our model.

Induction of wild-type p53 activity in human cancer cells by ribozymes that repair mutant p53 transcripts

Several groups have attempted to develop gene therapy strategies to treat cancer via introduction of the wild-type (wt) p53 cDNA into cancer cells. Unfortunately, these approaches do not result in regulated expression of the p53 gene and do not reduce expression of the mutant p53 that is overexpressed in cancerous cells. These shortcomings may greatly limit the utility of this gene replacement approach. We describe an alternative strategy with trans-splicing ribozymes that can simultaneously reduce mutant p53 expression and restore wt p53 activity in various human cancers. The ribozyme accomplished such conversion by repairing defective p53 mRNAs with high fidelity and specificity. The corrected transcripts were translated to produce functional p53 that can transactivate p53-responsive promoters and down-modulate expression of the multidrug resistance (MDR1) gene promoter. The level of wt p53 activity generated was significant, resulting in a 23-fold induction of a p53-responsive promoter and a 3-fold reduction in MDR1 promoter expression in transfected cancer cells. Once efficient delivery systems are developed, this strategy should prove useful for making human cancers more responsive to p53 activity and more sensitive to chemotherapeutic agents.

Aberrations of the K-ras, p53, and APC genes in extrahepatic bile duct cancer

BACKGROUND AND OBJECTIVES

The genetic alterations involved in extrahepatic bile duct (EHBD) cancer are poorly understood. Our aim was to identify aberrations of the K-ras, p53, and APC genes in EHBD cancer.

METHODS

We investigated aberrations of these genes in 52 EHBD cancers using polymerase chain reaction (PCR) single-strand conformation polymorphism analysis, followed by direct sequence determination and a PCR restriction fragment length polymorphism assay.

RESULTS

The K-ras, p53, and APC genes were mutated in 9.6%, 32.7%, and 0% of EHBD cancers, respectively. Loss of heterozygosity at the p53 and APC gene loci was identified in 15.6% and 38.5% of EHBD cancers, respectively.

CONCLUSIONS

Our results suggest that an unknown suppressor gene on 5q other than the APC gene may be responsible for EHBD cancer.

5-methylcytosine at HpaII sites in p53 is not hypermutable after UVC irradiation

Using a yeast based p53 functional assay we previously demonstrated that the UVC-induced p53 mutation spectrum appears to be indistinguishable from the one observed in Non Melanoma Skin Cancer (NMSC). However, position 742 (codon 248, CpG site) represented the major hot spot in NMSC but was not found mutated in the yeast system. In order to determine whether UVC-induced mutagenic events may be facilitated at methylated cytosine (5mC), a yeast expression vector harbouring a human wild-type p53 cDNA (pLS76) was methylated in vitro by HpaII methylase. Methylation induced 98% protection to HpaII endonuclease. Unmethylated and methylated pLS76 vectors were then UVC irradiated (lambda(max): 254 nm) and transfected into a yeast strain containing the ADE2 gene regulated by a p53-responsive promoter. The results revealed that: (i) 5mC at HpaII sites did not cause any difference in the UVC-induced survival and/or mutagenicity; (ii) none of the 20 mutants derived from methylated pLS76 showed p53 mutations targeted at HpaII sites; (iii) the UVC-induced p53 mutation spectra derived from methylated and unmethylated pLS76 were indistinguishable not only when classes of mutations and hot spots were concerned, but also when compared through a rigorous statistical test to estimate their relatedness (P = 0.85); (iv) the presence of 5mC did not increase the formation of photo-lesions at codon 248, as determined by using a stop polymerase assay. Although based on a limited number of mutants, these results suggest that the mere presence of 5mC at position 742 does not cause a dramatic increase of its mutability after UVC irradiation. We propose that position 742 is a hot spot in NMSC either because of mutagenic events at 5mC caused by other UV components of solarlight and/or because not all the NMSC are directly correlated with UV mutagenesis but may have a "spontaneous" origin.

Overexpression of bax associated with mutations in the loop-sheet-helix motif of p53

Recent investigations have revealed that mutations of the loop-sheet-helix motif of p53 is a significant factor for a poor prognosis in patients with non-small-cell lung cancer (NSCLC). To clarify this mechanism, bcl-2 and bax expression were evaluated in relation to mutations of p53. Tumor tissues of 203 patients with NSCLC were analyzed. Immunohistochemistry was performed to evaluate bcl-2 and bax expression, and polymerase chain reaction single-strand conformation polymorphism following direct sequencing was performed to investigate p53 status. A total of 79 carcinomas were bcl-2 positive, 146 carcinomas were bax positive, and 72 carcinomas had missense mutations of p53. There was no difference in bcl-2 expression in relation to p53 status. On the other hand, tumors with structural mutations of p53 had significantly lower expression of bax than those with wild-type p53 (P = 0.0026). In contrast, tumors with mutations of the loop-sheet-helix motif of p53 had significantly higher expression of bax than those with wild-type p53 (P = 0.0236). The frequency of a bcl-2/bax ratio of >/=1 was significantly lower in tumors with mutations of the loop-sheet-helix motif than that in tumors with wild-type p53 (P = 0.0240). The bcl-2/bax ratio status was a significant factor for a prognosis in patients with NSCLC (P = 0.0083). Mutations of the loop-sheet-helix motif of p53 were correlated with overexpression of bax, while other mutations of p53 were correlated with low levels of bax expression. This variation in pattern of bax expression in relation to mutant p53 might reflect the biological behavior of tumors in patients with bcl-2-positive NSCLC.

Isolation of differentially expressed cDNAs from p53-dependent apoptotic cells: activation of the human homologue of the Drosophila peroxidasin gene

Inactivation of the p53 tumor suppressor protein has been observed in a large number of human cancers. Overexpression of p53 induces either growth arrest or programmed cell death (apoptosis). The growth arrest function of p53 is mediated by induction of p21 (WAF1/CIP1), but the mechanisms underlying p53-dependent apoptosis are still largely unknown. To investigate these mechanisms, we have identified six differentially expressed transcripts in a human colon cancer cell line undergoing p53-dependent apoptosis. One of the p53-responsive genes showed significant homology to Drosophila peroxidasin, an extracellular matrix-associated peroxidase, and is likely to be its human homologue. Our results suggest a possible connection between p53-dependent apoptosis and the production of reactive oxygen species.

A novel tumor-specific gene therapy for bladder cancer

Gene therapy has been successfully used to treat genetic diseases. Currently, much investigation involves the role of gene therapy in malignant tumors. One problem associated with the retroviral system used for gene therapy is its non-specificity. Herein a vector delivery system is described, using human telomerase reverse transcriptase (hTRT) promotor, which can specifically affect telomerase-positive tumor cells while sparing nearby telomerase-negative cells. By combining a self-containing Cre/loxP site-specific recombination system into the design, this vector will destroy telomerase-positive, p53-negative tumor cells, while sparing normal cells which are telomerase-positive with wild type p53 (such as activated lymphocytes). This vector design appears especially suited to bladder transitional cell carcinoma, because of easy access transurethrally and high rate of local recurrence, and biologically secondary to high proportion of telomerase activity and p53 dysfunction.

Mutually compensatory mutations during evolution of the tetramerization domain of tumor suppressor p53 lead to impaired hetero-oligomerization

We have measured the stability and stoichiometry of variants of the human p53 tetramerization domain to assess the effects of mutation on homo- and hetero-oligomerization. The residues chosen for mutation were those in the hydrophobic core that we had previously found to be critical for its stability but are not conserved in human p73 or p51 or in p53-related proteins from invertebrates or vertebrates. The mutations introduced were either single natural mutations or combinations of mutations present in p53-like proteins from different species. Most of the mutations were substantially destabilizing when introduced singly. The introduction of multiple mutations led to two opposite effects: some combinations of mutations that have occurred during the evolution of the hydrophobic core of the domain in p53-like proteins had additive destabilizing effects, whereas other naturally occurring combinations of mutations had little or no net effect on the stability, there being mutually compensating effects of up to 9.5 kcal/mol of tetramer. The triple mutant L332V/F341L/L344I, whose hydrophobic core represents that of the chicken p53 domain, was nearly as stable as the human domain but had impaired hetero-oligomerization with it. Thus, engineering of a functional p53 variant with a reduced capacity to hetero-oligomerize with wild-type human p53 can be achieved without any impairment in the stability and subunit affinity of the engineered homo-oligomer.

p53-oriented cancer therapies: current progress

Nearly twenty years after the initial discovery of p53, we are now in an ideal position to exploit our vast knowledge of p53 biology in the creation of novel cancer therapies. Disruption of p53 function through mutation, or other means, occurs very frequently in human cancer. Loss of p53 function has been linked with unfavourable prognosis in a large number of tumour types, as indicated by more aggressive tumours, early metastasis and decreased survival rates. Many different avenues of research have converged upon p53 to highlight this protein as being one of the foremost cellular responders to stress, in particular to DNA damage. Huge advances have been made in understanding the complex role p53 plays in the regulation of apoptosis and cell cycle arrest. This review is not meant to be a comprehensive description of p53 biology, but rather serves to highlight current progress in the development of p53-oriented cancer therapies. These may be categorised into three basic strategies: gene replacement therapy using wild-type p53, restoration of p53 function by other means and, finally, targeting of the p53 dysfunction itself. Rapid progress is expected to be made regarding the identification of conventional pharmaceutical agents which either work in a p53-independent manner or act preferentially in p53 defective cells. Gene replacement therapy with wild-type p53 also holds considerable potential for obtaining clinically relevant results quickly. The other forms of cancer therapies based around p53 are much further behind in the developmental process, but may prove to more efficacious in the long run, especially in terms of specificity. As with many other fields, the innovation of successful p53-oriented cancer therapies is only limited by our understanding of p53 biology and the creative use of such knowledge.

p53 mutations in cutaneous lesions induced in the hairless mouse by a solar ultraviolet light simulator

We investigated skin lesions induced in hairless SKH:HR1 mice by chronic exposure to a solar ultraviolet light (UV) simulator for alterations of the p53 gene in conserved domains. Mutations of exons 5-8 of the p53 gene in skin lesions were screened in 31 benign skin lesions (hyperplasias), 25 precancerous skin lesions (keratoacanthomas), and 25 malignant skin lesions (squamous cell carcinomas; SCC) by polymerase chain reaction-single-strand conformation polymorphism analysis. Most of the mutations occurred at dipyrimidine sequences located on the nontranscribed strand; the most frequent modifications were C-->T transitions (77%) and CC-->TT tandem mutations (5%); the latter are considered the UV fingerprint. p53 mutations were detected in 3% of the hyperplasias, 12% of the keratoacanthomas, and 52% of the SCCs. Hence, the high frequency of p53 mutations in SCCs compared with keratoacanthomas induced by a solar UV simulator suggested that, in our study, p53 mutations probably occurred as a late event in the skin carcinogenesis progression of SCC. Interestingly, the level of CC-->TT tandem mutations in the SCCs (5%) was similar to that found in SCCs induced in hairless mice by UVB alone. p53 protein was also detected in the different types of skin lesions by immunohistochemical analysis. Thus, our data from hairless mouse skin tumors induced by a solar UV simulator confirmed the major role of UVB-induced DNA damage in skin carcinogenesis and suggested that UVA plays a minor role in bringing about p53 alterations.

Nine hydrophobic side chains are key determinants of the thermodynamic stability and oligomerization status of tumour suppressor p53 tetramerization domain

The contribution of almost each amino acid side chain to the thermodynamic stability of the tetramerization domain (residues 326-353) of human p53 has been quantitated using 25 mutants with single-residue truncations to alanine (or glycine). Truncation of either Leu344 or Leu348 buried at the tetramer interface, but not of any other residue, led to the formation of dimers of moderate stability (8-9 kcal/mol of dimer) instead of tetramers. One-third of the substitutions were moderately destabilizing (<3.9 kcal/mol of tetramer). Truncations of Arg333, Asn345 or Glu349 involved in intermonomer hydrogen bonds, Ala347 at the tetramer interface or Thr329 were more destabilizing (4.1-5.7 kcal/mol). Strongly destabilizing (8.8- 11.7 kcal/mol) substitutions included those of Met340 at the tetramer interface and Phe328, Arg337 and Phe338 involved peripherally in the hydrophobic core. Truncation of any of the three residues involved centrally in the hydrophobic core of each primary dimer either prevented folding (Ile332) or allowed folding only at high protein concentration or low temperature (Leu330 and Phe341). Nine hydrophobic residues per monomer constitute critical determinants for the stability and oligomerization status of this p53 domain.

An oncogenic form of p53 confers a dominant, gain-of-function phenotype that disrupts spindle checkpoint control

Although it is well-established that p53 functions as a tumor suppressor gene, certain mutations exhibit gain-of-function activities that increase oncogenic transformation. We have found a common class of p53 missense mutation that exhibits a dominant, gain-of-function activity that generates genomic instability. Fibroblasts from Li-Fraumeni syndrome heterozygotes with such mutations generate polyploid cells when exposed to spindle depolymerizing agents. Expression of such mutant alleles in normal fibroblasts yields the same phenotype. This class of dominant, gain-of-function p53 mutation (p53(RSC), relaxed spindle checkpoint allele) does not require the transcriptional activation function of p53 for this behavior. Thus p53 mutations can contribute to progression of a cancer cell not only by absence of p53 tumor suppressor activity but also by the presence of an activity that promotes genetic instability.

Genetic selection of intragenic suppressor mutations that reverse the effect of common p53 cancer mutations

Several lines of evidence suggest that the presence of the wild-type tumor suppressor gene p53 in human cancers correlates well with successful anti-cancer therapy. Restoration of wild-type p53 function to cancer cells that have lost it might therefore improve treatment outcomes. Using a systematic yeast genetic approach, we selected second-site suppressor mutations that can overcome the deleterious effects of common p53 cancer mutations in human cells. We identified several suppressor mutations for the V143A, G245S and R249S cancer mutations. The beneficial effects of these suppressor mutations were demonstrated using mammalian reporter gene and apoptosis assays. Further experiments showed that these suppressor mutations could override additional p53 cancer mutations. The mechanisms of such suppressor mutations can be elucidated by structural studies, ultimately leading to a framework for the discovery of small molecules able to stabilize p53 mutants.

Expression of wild-type p53 is required for efficient global genomic nucleotide excision repair in UV-irradiated human fibroblasts

We have shown previously that Li-Fraumeni syndrome fibroblasts homozygous for p53 mutations are deficient in the removal of UV-induced cyclobutane pyrimidine dimers from genomic DNA, but still proficient in the transcription-coupled repair pathway (Ford, J. M., and Hanawalt, P. C. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 8876-8880). We have now utilized monoclonal antibodies specific for cyclobutane pyrimidine dimers or 6-4 photoproducts, respectively, to measure their repair in UV-irradiated human fibroblasts. Cells homozygous for p53 mutations were deficient in the repair of both photoproducts, whereas cells heterozygous for mutant p53 exhibited normal repair of 6-4 photoproducts, but decreased initial rates of removal of cyclobutane pyrimidine dimers, compared with normal cells. The specificity of the effect of wild-type p53 on nucleotide excision repair was demonstrated in a p53 homozygous mutant cell line containing a tetracycline-regulated wild-type p53 gene. Wild-type p53 expression and activity were suppressed in the presence of tetracycline, whereas withdrawal of tetracycline resulted in the induction of p53 expression, cell cycle checkpoint activation, and DNA damage-induced apoptosis. The regulated expression of wild-type p53 resulted in the recovery of normal levels of repair of both cyclobutane pyrimidine dimers and 6-4 photoproducts in genomic DNA, but did not alter the transcription-coupled repair of cyclobutane pyrimidine dimers. Therefore, the wild-type p53 gene product is an important determinant of nucleotide excision repair activity in human cells.

p53: functions, mutations and sarcomas

The p53 gene is the most commonly altered gene in a multitude of human cancers. The alterations can be acquired somatically or transmitted through the germ-line. Bone and soft tissue sarcomas are frequently found to have acquired abnormalities in the p53 and mdm-2 genes. In soft tissue sarcoma, the amplification of the mdm-2 gene and the binding of its oncogene product to wild-type p53 protein functionally inactivates normal p53-regulated growth. Inherited mutations of the p53 gene are associated with the rare Li-Fraumeni familial cancer syndrome. Various tumor types arise in these families, with sarcomas of the bone and soft tissues and carcinoma of the breast being the most frequently observed. Transgenic mice with highly expressed mutated p53 have a higher incidence of tumors, including predominantly osteosarcomas and soft tissue sarcomas. In close similarity with the Li-Fraumeni syndrome, homozygously p53-null mice (transgenic mice carrying two non-functional p53 allele) are developmentally normal however they are susceptible to spontaneous tumor formation. This article reviews briefly the structure, function, and dysfunction of the p53 tumor-suppressor gene with particular focus on its role in the development of bone and soft tissue sarcoma.

The tumor suppressor p53 modifies mutational processes in a human lymphoblastoid cell line

Abnormalities in the p53 gene play an important role in genomic instability and tumorigenesis. Our previous work showed that p53 status is correlated with differential mutability in two closely related human lymphoblastoid cell lines, TK6 and WTK1. WTK1 cells, which contain a mutation in p53 (p53Ile237) show a remarkably increased mutability, larger genetic alterations at the thymidine kinase locus (tk), an increased ability to catalyze recombination, and a delay in the onset of apoptosis after X-irradiation, compared to TK6 (p53 +/+). In the present study, we demonstrate that after transfection and subsequent overexpression of the known dominant negative mutant p53 Ala143 allele (mp53Ala143) in TK6, there were significantly enhanced spontaneous and X-ray-induced mutant frequencies at the tk locus, and delayed onset of X-ray-induced apoptosis, to a similar extent as in WTK1. In addition, high protein expression of mp53Ala143 in transfectants was correlated with both increased mutation frequency and altered apoptosis kinetics. Similar results were obtained with p53 Ile237 transfection into TK6. Our observations indicate that the product of the p53 gene affects mutational processes. We hypothesize that p53 dysfunction can lead to increased mutagenicity at the endogenous tk gene in human lymphoblastoid cell lines either through delayed apoptosis in response to DNA damage or by mediating increased recombination.

The p53 gene as a modifier of intrinsic radiosensitivity: implications for radiotherapy

BACKGROUND AND PURPOSE

Experimental studies have implicated the normal or "wild type' p53 protein (i.e. WTp53) in the cellular response to ionizing radiation and other DNA damaging agents. Whether altered WTp53 protein function can lead to changes in cellular radiosensitivity and/or clinical radiocurability remains an area of ongoing study. In this review, we describe the potential implications of altered WTp53 protein function in normal and tumour cells as it relates to clinical radiotherapy, and describe novel treatment strategies designed to re-institute WTp53 protein function as a means of sensitizing cells to ionizing radiation.

METHODS AND MATERIALS

A number of experimental and clinical studies are critically reviewed with respect to the role of the p53 protein as a determinant of cellular oncogenesis, genomic stability, apoptosis, DNA repair and radioresponse in normal and transformed mammalian cells.

RESULTS

In normal fibroblasts, exposure to ionizing radiation leads to a G1 cell cycle delay (i.e. a "G1 checkpoint') as a result of WTp53 mediated inhibition of G1-cyclin-kinase and retinoblastoma (pRb) protein function. The G1 checkpoint response is absent in tumour cells which express a mutant form of the p53 protein (i.e. MTp53), leading to acquired radioresistance in vitro. Depending on the cell type studied, this increase in cellular radiation survival can be mediated through decreased radiation-induced apoptosis, or altered kinetics of the radiation-induced G1 checkpoint. Recent biochemical studies support an indirect role for the p53 protein in both nucleotide excision and recombinational DNA repair pathways. However, based on clinicopathologic data, it remains unclear as to whether WTp53 protein function can predict for human tumour radiocurability and normal tissue radioresponse.

CONCLUSIONS

Alterations in cell cycle control secondary to aberrant WTp53 protein function may be clinically significant if they lead to the acquisition of mutant cellular phenotypes, including the radioresistant phenotype. Pre-clinical studies suggest that these phenotypes may be reversed using adenovirus-mediated gene therapy or pharmacologic strategies designed to re-institute WTp53 protein function. Our analysis of the published data strongly argues for the use of functional assays for the determination of WTp53 protein function in studies which attempt to correlate normal and tumour tissue radioresponse with p53 genotype, or p53 protein expression.

Dominant-negative p53 mutations selected in yeast hit cancer hot spots

Clinically important mutant p53 proteins may be tumorigenic through a dominant-negative mechanism or due to a gain-of-function. Examples for both hypotheses have been described; however, it remains unclear to what extent they apply to TP53 mutations in general. Here it is shown that the mutational spectrum of dominant-negative p53 mutants selected in a novel yeast assay correlates tightly with p53 mutations in cancer. Two classes of dominant-negative mutations are described; the more dominant one affects codons that are essential for the stabilization of the DNA-binding surface of the p53 core domain and for the direct interaction of p53 with its DNA binding sites. These results predict that the vast majority of TP53 mutations leading to cancer do so in a dominant-negative fashion.

P53, apoptosis, and breast cancer

Wild-type p53 is a tumor suppressor gene that plays a central role in maintaining the genetic integrity of the cell by preventing cells with damaged DNA from further proliferation. Mutation and deletion of p53 are the most common genetic defects seen in clinical cancer. About 40% of breast carcinomas show high levels of stabilized, often mutant, p53 protein in their cells as detected by immunohistochemistry. p53-related defects in tumor cells correlate with a poor prognosis and may also indicate a poor response to chemotherapy. In experimental systems, the p53 status of cells is important in determining their sensitivity to radiation and chemotherapeutic drugs. Cells with functional p53 die by apoptosis, whilst similar cells lacking p53 function continue to proliferate, perpetuating potentially oncogenic mutations. Not only may p53 status be a marker of the biological aggressiveness of individual tumors and of their likely response to therapy, but restoration of normal p53 function is itself already a goal of cancer therapy.

No association between p53 status and alpha-particle-induced chromosomal instability in human lymphoblastoid cells

Previous work has demonstrated that alpha-particle irradiation of primary human bone marrow cells leads to the transmission of chromosomal instability in the descendants of the irradiated cell, although there is some interindividual variation. We have extended these studies to human EBV-transformed lymphoblastoid cell lines in order to establish an in vitro model system. The five cell lines analyzed, including one from a Fanconi anaemia patient, exhibited high levels of persistent chromatid aberrations up to approximately 40 cell generations after alpha-irradiation. The p53 status of the cell lines was defined according to whether cellular p53 levels were induced by irradiation, translocated to the nucleus and were able to bind a p53 DNA consensus recognition sequence in vitro. Together with the primary bone marrow cell studies, we conclude that alpha-particle induced chromosomal instability is independent of the p53 status of the cell as defined in these studies.

Clinical implications of the p53 gene

The capacity for malignant growth is acquired by the stepwise accumulation of defects in specific genes regulating cell growth and tissue homeostasis. Although several hundred genes are known to control growth, molecular genetic studies in cancer show that few of these are consistently involved in the natural history of human cancer, and those typically in only certain types of malignancy. Prospects for development of molecular-based diagnostic and therapeutic strategies with widespread application did not look promising, until it was realized that the p53 tumor suppressor gene was defective in approximately half of all malignancies. This discovery generated research efforts of unparalleled intensity to determine how p53 functions at the molecular level, and how to apply this knowledge to clinical ends.

Alterations of p53 and expression of WAF1/p21 in human thyroid tumors

We have investigated p53 expression in 178 thyroid tumors from 162 patients by immunohistochemistry using two antibodies, DO1 and CM1. In addition, 35 tumors were analyzed for expression of WAF1/p21, one of the downstream mediators of p53. Only 15 tumors (8.4%) had greater than 10% of tumor cell nuclei positively stained for p53. Six of 14 Hürthle tumors and three of 34 papillary thyroid carcinomas showed staining of p53 in the cytoplasm. A total of 40 tumors, including all p53 positive tumors, and all anaplastic and poorly differentiated tumors were screened for mutations in exons 5-8 of the TP53 gene by constant denaturing gel electrophoresis and subsequent sequence analysis. A mutation was detected in five tumors only: one anaplastic carcinoma, one poorly differentiated follicular carcinoma (negative by p53 immunohistochemistry), one atypical follicular adenoma, and two papillary thyroid carcinoma metastases, of which the primary tumors had no detectable mutation. We conclude that p53 immunohistochemistry cannot be used for diagnostic and prognostic purposes in thyroid tumors. The tumors with TP53 mutation showed a markedly reduced WAF1/p21 expression. Three anaplastic carcinomas with highly expressed p53, but with no detectable mutation, also showed high expression of WAF1/p21. This may be explained by overexpression of wild-type p53, possibly due to the patients' preoperative treatment, including external radiation of the neck region. The results indicate that WAF1/p21 immunohistochemistry contributes to the information of the functional status of p53, and may facilitate the interpretation of results from p53 immunohistochemistry in these tumors.