Direct growth stimulation of normal human epithelial cells by mutant p53

Oral Progenitor Cell Line-Derived Small Extracellular Vesicles as a Treatment for Preferential Wound Healing Outcome

Scar formation during wound repair can be devastating for affected individuals. Our group previously documented the therapeutic potential of novel progenitor cell populations from the non-scarring buccal mucosa. These Oral Mucosa Lamina Propria-Progenitor Cells (OMLP-PCs) are multipotent, immunosuppressive, and antibacterial. Small extracellular vesicles (sEVs) may play important roles in stem cell-mediated repair in varied settings; hence, we investigated sEVs from this source for wound repair. We created an hTERT immortalized OMLP-PC line (OMLP-PCL) and confirmed retention of morphology, lineage plasticity, surface markers, and functional properties. sEVs isolated from OMLP-PCL were analyzed by nanoparticle tracking analysis, Cryo-EM and flow cytometry. Compared to bone marrow-derived mesenchymal stromal cells (BM-MSC) sEVs, OMLP-PCL sEVs were more potent at driving wound healing functions, including cell proliferation and wound repopulation and downregulated myofibroblast formation. A reduced scarring potential was further demonstrated in a preclinical in vivo model. Manipulation of OMLP-PCL sEVs may provide novel options for non-scarring wound healing in clinical settings.

iTRAQ-coupled 2D LC/MS-MS analysis of CXCR7-transfected papillary thyroid carcinoma cells: A new insight into CXCR7 regulation of papillary thyroid carcinoma progression and identification of potential biomarkers

Previous studies have demonstrated that C-X-C chemokine receptor type 7 (CXCR7) regulates papillary thyroid carcinoma (PTC) growth and metastasis; however, the molecular mechanisms underlying this regulation remain unclear. In the present study, the protein expression profiles of the PTC cell line GLAG-66 and GLAG-66 cells stably transfected with CXCR7 cDNA were analyzed and compared using isobaric tag for relative and absolute quantification-coupled two-dimensional liquid chromatography-tandem mass spectrometry. In total, 2,983 proteins were quantified and 130 proteins were identified to be differentially expressed, of which 87 were significantly upregulated and 43 were significantly downregulated. Gene Ontology enrichment analysis revealed that the differentially expressed proteins were primarily enriched in a number of biological processes, including metabolism-related processes, cellular component organization, transport, cellular development process and the immune response. The differentially expressed proteins identified included fibronectin 1, basigin, periplakin and serpin family B member 5, all of which are associated with cellular junctions and cancer progression. In addition, transgelin-2 and AHNAK nucleoprotein 2 were identified as potential novel biomarkers for the prognosis and treatment of PTC.

Family of microRNA-146 Regulates RARβ in Papillary Thyroid Carcinoma

Retinoic acid is a promising tool in adjuvant cancer therapies, including refractory thyroid cancer, and its biological role is mediated by the retinoic acid receptor beta (RARβ). However, expression of RARβ is lowered in papillary thyroid carcinoma (PTC), contributing to promotion of tumor growth and inefficiency of retinoic acid and radioactive iodine treatment. The causes of aberrant RARB expression are largely unknown. We hypothesized that the culpable mechanisms include the action of microRNAs from the miR-146 family, previously identified as significantly upregulated in PTC tumors. To test this hypothesis, we assessed the expression of RARB as well as miR-146a-5p and miR-146b-5p in 48 PTC tumor/normal tissue pairs by Taqman assay to reveal that the expression of RARB was 3.28-fold decreased, and miR-146b-5p was 28.9-fold increased in PTC tumors. Direct interaction between miRs and RARB was determined in the luciferase assay and further confirmed in cell lines, where overexpression of miR-146a-5p and miR-146b-5p caused a 31% and 33% decrease in endogenous RARB mRNA levels. Inhibition of miR-146a and miR-146b resulted in 62.5% and 45.4% increase of RARB, respectively, and a concomitant decrease in proliferation rates of thyroid cancer cell lines, analyzed in xCELLigence system.We showed that two microRNAs of the miR-146 family directly regulate RARB. Inhibition of miRs resulted in restoration of RARB expression and decreased rates of proliferation of thyroid cancer cells. By restoring RARB levels, microRNA inhibitors may become part of an adjuvant therapy in thyroid cancer patients.

Use of p38 MAPK Inhibitors for the Treatment of Werner Syndrome

Werner syndrome provides a convincing model for aspects of the normal ageing phenotype and may provide a suitable model for therapeutic interventions designed to combat the ageing process. Cultured primary fibroblast cells from Werner syndrome patients provide a powerful model system to study the link between replicative senescence in vitro and in vivo pathophysiology. Genome instability, together with an increased pro-oxidant state, and frequent replication fork stalling, all provide plausible triggers for intracellular stress in Werner syndrome cells, and implicates p38 MAPK signaling in their shortened replicative lifespan. A number of different p38 MAPK inhibitor chemotypes have been prepared rapidly and efficiently using microwave heating techniques for biological study in Werner syndrome cells, including SB203580, VX-745, RO3201195, UR-13756 and BIRB 796, and their selectivity and potency evaluated in this cellular context. Werner syndrome fibroblasts treated with a p38 MAPK inhibitor reveal an unexpected reversal of the accelerated ageing phenotype. Thus the study of p38 inhibition and its effect upon Werner pathophysiology is likely to provide new revelations into the biological mechanisms operating in cellular senescence and human ageing in the future.

Persistent infection of a carcinoma thyroid cell line with coxsackievirus B

BACKGROUND

Viral infections are described as environmental factors that are implicated in various thyroid diseases. The role of enteroviruses (EV) in the pathogenesis of thyroid diseases has been suspected. Recently, we found that EV RNA could be detected in postoperative thyroid specimens. We decided to investigate the infection of a human thyroid cell line with coxsackievirus B4 (CVB4).

METHODS

The wild-type human thyroid carcinoma cells K1 were inoculated with CVB4E2 at 2.1 x 10(7) TCID50/mL. The titer of the virus progeny was determined every 3 days on HEp-2 cells. CVB replication at the molecular level was monitored by searching for intracellular viral genomes using reverse transcription (RT)-polymerase chain reaction (PCR). EV VP1 capsid protein was detected by indirect immunocytofluorescence staining. Cell viability was determined by 3-(4,5)-dimethylthiahiazo(-z-y1)-3,5-di-phenytetrazoliumromide (MTT) absorbance, and the nuclear morphology was assessed by Hoechst Dye staining.

RESULTS

Infectivity assays with CVB4E2 revealed an increase in viral titers. Virus production decreased thereafter, but was not stopped by serial subculture for 24 days after infection. Detection of intracellular positive and negative viral RNA strands by RT-PCR was positive between days 1 and 14 postinfection (p.i.), and by semi-nested RT-PCR up to 24 days. K1 cell cultures infected with CVB4 were stained positively for EV VP1: the number of VP1-positive cells decreased rapidly within 6 days and remained low up to the end of culture. Compared with mock-infected cultures, viability in CVB4-infected cultures was around 100% up to 24 days. Cells with strongly fluorescent nuclei and/or fragmented nuclei were observed.

CONCLUSION

We demonstrate for the first time that CVB4 could replicate in thyroid cells and could persist, with predominance of viral negative RNA strands for up to 24 days p.i. without obvious cytopathic effect. Our results suggest that CVB4 may lead to thyroid cell apoptosis. Further studies are needed to determine whether CVB could play a role in thyroid pathologies.

Mutant p53 can delay growth arrest and loss of CDK2 activity in senescing human fibroblasts without reducing p21(WAF1) expression

Functional wild-type p53 is required for human diploid fibroblasts (HDF) to enter an irreversible growth arrest known as replicative senescence. Experimentally, abrogation of p53 function by expression of human papillomavirus type 16 E6 or disruption of a key downstream effector p21 by homologous recombination both extended HDF life span. However, although sufficient to extend life span, p21 down-regulation is not necessary, because expression of a dominant-negative mutant p53 (143(ala)) extends life span without apparently decreasing p21 expression. Given the importance of p53 in cellular senescence and the general assumption that p21 may be the sole mediator of its action in this process, we have investigated how abrogation of p53 function can overcome senescence without lowering expression of p21. We have found up-regulated levels of the cyclin-dependent kinase 2 (cdk2) protein in HDF expressing 143(ala) mutant p53 as compared to senescent controls, together with an increase in p21-free cdk2 which, in conjunction with cyclin E, is able to form an active kinase which can phosphorylate the retinoblastoma protein. However, forced overexpression of cdk2 in near-senescent HDF failed to restore cdk2-associated kinase activity. Our data suggest that p53-mediated senescence depends on factor(s) other than p21 which modulate formation of cyclin E-cdk2 complexes.

Telomere-based proliferative lifespan barriers in Werner-syndrome fibroblasts involve both p53-dependent and p53-independent mechanisms

Werner-syndrome fibroblasts have a reduced in vitro life span before entering replicative senescence. Although this has been thought to be causal in the accelerated ageing of this disease, controversy remains as to whether Werner syndrome is showing the acceleration of a normal cellular ageing mechanism or the occurrence of a novel Werner-syndrome-specific process. Here, we analyse the signalling pathways responsible for senescence in Werner-syndrome fibroblasts. Cultured Werner-syndrome (AG05229) fibroblasts senesced after approximately 20 population doublings with most of the cells having a 2N content of DNA. This was associated with hypophosphorylated pRb and high levels of p16(Ink4a) and p21(Waf1). Senescent AG05229 cells re-entered the cell cycle following microinjection of a p53-neutralizing antibody. Similarly, production of the human papilloma virus 16 E6 oncoprotein in presenescent AG05229 cells resulted in senescence being bypassed and extended cellular life span. Werner-syndrome fibroblasts expressing E6 did not proliferate indefinitely but reached a second proliferative lifespan barrier, termed M(int), that could be bypassed by forced production of telomerase in post-M1 E6-producing cells. The conclusions from these studies are that: (1) replicative senescence in Werner-syndrome fibroblasts is a telomere-induced p53-dependent event; and (2) the intermediate lifespan barrier M(int) is also a telomere-induced event, although it appears to be independent of p53. Werner-syndrome fibroblasts resemble normal human fibroblasts for both these proliferative lifespan barriers, with the strong similarity between the signalling pathway linking telomeres to cell-cycle arrest in Werner-syndrome and normal fibroblasts providing further support for the defect in Werner syndrome causing the acceleration of a normal ageing mechanism.

Spontaneous and induced aneuploidy, considerations which may influence chromosome malsegregation

Aneuploidy plays a major role in the production of human birth defects and is becoming increasingly recognised as a critical event in the etiology of a wide range of human cancers. Thus, the detection of aneuploidy and the characterisation of the mechanisms which lead to chromosome malsegregation is an important area of genotoxicological research. As an aid to aneuploidy research, methods have been developed to analyse the mechanisms of chromosome malsegregation and to investigate the role of aneuploidy in tumour progression. The presence of aneuploid cells is a common characteristic of many of tumour cell types as illustrated by the wide range of chromosome number changes detected in post-menopausal breast tumours. To investigate the time of occurrence of aneuploidy during tumour progression, we have studied the chromosome number status of Syrian hamster dermal (SHD) cells cultures progressing to morphological transformation. The production of both polyploid and aneuploid cells is a common feature of progressing cells in this model. The elevation of both progression to morphological transformation and aneuploid frequencies can be produced by exposure to a diverse range of carcinogens and tumour promoters. Analysis of the genotoxic activity of the hormone 17-beta oestradiol demonstrated its ability to induce both chromosome loss and non-disjunction in human lymphoblastoid cells implicating aneugenic activity in hormone related cancers. Mutations in the p53 tumour suppressor gene introduced into human fibroblasts produced modifications in chromosome separation at mitosis which may lead to the production of both aneuploidy and polyploid cells. Our studies indicate that the production of aneuploid cells can be influenced by both endogenous and exogenous factors and occur throughout the progression of normal cells to a malignant phenotype.

p53-Dependent growth arrest and altered p53-immunoreactivity following metabolic labelling with 32P ortho-phosphate in human fibroblasts

The tumour suppressor gene p53 plays a major role in the cellular response to DNA damage, mediating growth arrest and/or apoptosis. Phosphorylation of the protein occurs at numerous sites in vivo and is likely to be a major mechanism for modulation of its activity as a transcriptional transactivator. Not surprisingly, therefore, p53 has been intensively studied by 32P metabolic labelling. Here we show however, using normal human fibroblasts, that typical labelling conditions induce (i) a p53-dependent inhibition of DNA synthesis and (ii) an increase in the cellular content of p53 protein detectable by the phosphorylation-sensitive antibody DO-1 but not by antibody DO-12. These data demonstrate for the first time that 32P labelling is sufficient to induce a biologically-significant, p53-mediated cellular response and strongly suggest that it perturbs the phosphorylation state of p53 which it is being used to measure. This highlights the need to re-evaluate earlier data by non-radioactive approaches using phospho-specific antibodies.

Control of replicative life span in human cells: barriers to clonal expansion intermediate between M1 senescence and M2 crisis

The accumulation of genetic abnormalities in a developing tumor is driven, at least in part, by the need to overcome inherent restraints on the replicative life span of human cells, two of which-senescence (M1) and crisis (M2)-have been well characterized. Here we describe additional barriers to clonal expansion (Mint) intermediate between M1 and M2, revealed by abrogation of tumor-suppressor gene (TSG) pathways by individual human papillomavirus type 16 (HPV16) proteins. In human fibroblasts, abrogation of p53 function by HPVE6 allowed escape from M1, followed up to 20 population doublings (PD) later by a second viable proliferation arrest state, MintE6, closely resembling M1. This occurred despite abrogation of p21(WAF1) induction but was associated with and potentially mediated by a further approximately 3-fold increase in p16(INK4a) expression compared to its level at M1. Expression of HPVE7, which targets pRb (and p21(WAF1)), also permitted clonal expansion, but this was limited predominantly by increasing cell death, resulting in a MintE7 phenotype similar to M2 but occurring after fewer PD. This was associated with, and at least partly due to, an increase in nuclear p53 content and activity, not seen in younger cells expressing E7. In a different cell type, thyroid epithelium, E7 also allowed clonal expansion terminating in a similar state to MintE7 in fibroblasts. In contrast, however, there was no evidence for a p53-regulated pathway; E6 was without effect, and the increases in p21(WAF1) expression at M1 and MintE7 were p53 independent. These data provide a model for clonal evolution by successive TSG inactivation and suggest that cell type diversity in life span regulation may determine the pattern of gene mutation in the corresponding tumors.

Human thyroid cancer cells as a source of iso-genic, iso-phenotypic cell lines with or without functional p53

Differentiated thyroid carcinomas (in contrast to the rarer anaplastic form) are unusual among human cancers in displaying a remarkably low frequency of p53 mutation and appear to retain wild-type (wt) p53 function as assessed by the response of derived cell lines to DNA damage. Using one such cell line, K1, we have tested the effect of experimental abrogation of p53 function by generating matched sub-clones stably expressing either a neo control gene, a dominant-negative mutant p53 (143ala) or human papilloma virus protein HPV16 E6. Loss of p53 function in the latter two groups was confirmed by abolition of p53-dependent 'stress' responses including induction of the cyclin/CDK inhibitor p21WAF1 and G1/S arrest following DNA-damage. In contrast, no change was detected in the phenotype of 'unstressed' clones, with respect to any of the following parameters: proliferation rate in monolayer, serum-dependence for proliferation or survival, tumorigenicity, cellular morphology, or tissue-specific differentiation markers. The K1 line therefore represents a 'neutral' background with respect to p53 function, permitting the derivation of functionally p53 + or - clones which are not only iso-genic but also iso-phenotypic. Such a panel should be an ideal tool with which to test the p53-dependence of cellular stress responses, particularly the sensitivity to potential therapeutic agents, free from the confounding additional phenotypic differences which usually accompany loss of p53 function. The results also further support the hypothesis that p53 mutation alone is not sufficient to drive progression of thyroid cancer to the aggressive anaplastic form.

Cellular senescence and cancer

The proliferative lifespan of normal mammalian cells is limited by intrinsic controls, which desensitize the cell-cycle machinery to extrinsic stimulation after a given number of cell divisions. One underlying clock driving this process of 'replicative senescence' is the progressive erosion of chromosome telomeres, which occurs with each round of DNA replication. This appears to trigger growth inhibition via activation of the tumour suppressor gene (TSG) product, p53, and the consequent up-regulation of the cell-cycle inhibitor p21WAF1. Other inhibitory pathways are also activated (possibly by additional clocks), including the TSG p16INK4a and the less well-defined complementation group genes. Loss of one pathway can be compensated, after a limited extension of lifespan, by further up-regulation of the others, so that to escape mortality a developing tumour must overcome multiple 'proliferative lifespan barriers' (PLBs) by successive genetic events, each conferring a new wave of clonal expansion. This provides one explanation for the existence of multiple genetic abnormalities in human cancers; furthermore, the diversity in the nature and timing of these PLBs between different cell types may explain the variation in the spectrum of abnormalities observed between the corresponding cancers. Even if all senescence pathways are inactivated, immortalization can only be achieved if erosion of telomeres is halted, before their end-protecting function is lost. This usually requires either activation of telomerase during tumour development, if the cell of origin is telomerase-negative, or up-regulation if the normal cell already has some activity, but not enough to prevent erosion. In either case, cancers often maintain near-critical telomere lengths; hence pharmacological inhibition of telomerase remains an attractive approach to the selective killing of tumour cells.

The use of molecular technology in the differentiation of pancreatic cancer and chronic pancreatitis

CONCLUSION

It is concluded that currently there are limitations in the use of some of the proposed tests, whereas in the future, further progress in our understanding of the molecular biology of pancreatic disease and the development and application of existing techniques should have a greater impact on clinical practice.

BACKGROUND

Fifteen to 20% of patients with pancreatic cancer present with a resectable mass in the head of the pancreas, but there is a subgroup of patients for whom it is difficult to reach the correct diagnosis.

METHOD

This article addresses how molecular technology can be used to aid in the diagnosis of this group of patients. The clinical and scientific literature is reviewed by accessing papers through the Medline database.

RESULTS

This article reviews the limitations of conventional imaging techniques and the limitations of fine needle aspiration cytology and cytological examination of pancreatic duct secretions. The molecular biology of both pancreatic cancer and chronic pancreatitis is then reviewed with emphasis on the common molecular defects seen in these diseases. The current use of molecular techniques in the examination of cytological and histological specimens, stool, blood, and pancreatic duct secretions and how this helps discriminate between benign and malignant lesions of the pancreas is addressed. Finally, the use of novel serum screening tests in groups at high risk of pancreatic cancer is discussed.

An amphotropic retroviral vector expressing a mutant gsp oncogene: effects on human thyroid cells in vitro

Point mutations of the gsp protooncogene (encoding the alpha-subunit of the Gs protein) that constitutively activate the cAMP signaling pathway are a common feature of and a plausible causative mechanism for thyroid hyperfunctioning adenomas (hot nodules). To investigate the extent to which mutant gsp acting alone can induce proliferation of thyroid follicular cells, we generated an amphotropic retroviral vector (based on the pBABE-neo plasmid and psi-CRIP packaging line) to permit stable introduction of a hemagglutinin-tagged Gln227-->Leu mutant gsp gene into normal human thyrocytes in vitro. The biological activity of the vector was confirmed by detection of HA-tagged Gsp protein expression and induction of cAMP synthesis in selected target cells. Normal human thyroid follicular cells in primary monolayer culture were infected with the gsp retroviral vector or with corresponding vectors expressing mutant H-ras or neo only as positive and negative controls, respectively. Although, as before, mutant ras generated 10-20 well differentiated epithelial colonies/dish of 10(5) infected cells, with an average lifespan of 15-20 population doublings, only small groups of no more than 15-50 differentiated thyrocytes were observed with the gsp vector. In addition to standard conditions (10% FCS), infections were performed in reduced serum (1% FCS, TSH, and insulin), in the presence of isobutylylmethylxanthine, or in the presence of agents capable of closing gap junctions, with no significant difference in outcome. Although little or no proliferative response was observed regardless of the conditions, there was clear evidence of morphological response (rearrangement of the actin cytoskeleton and increased cell size). The results suggest that gsp mutation may not be a sufficient proliferogenic stimulus by itself to account for hot nodule formation.

Proliferative lifespan checkpoints: cell-type specificity and influence on tumour biology

Lifespan checkpoints are viewed here as intrinsic mechanisms which desensitise cells to external growth signals as a programmed response to proliferative age, as distinct from externally-triggered differentiation. This review focuses on the role of tumour suppressor gene products as essential mediators of cell cycle arrest at lifespan checkpoints, concentrating in particular on p53. Although drawing inevitably on fibroblast senescence and telomere erosion paradigms, other lifespan clocks and signal pathways are discussed. Particular emphasis is placed on cell-type diversity in the nature, number and timing of lifespan checkpoints and its importance for tumour biology. Breast and thyroid cancer are used to illustrate the concept that the "choice" of checkpoint(s) in a given normal cell may have a determining influence on the mutational spectrum and clinical behaviour of its tumours.

p53 mutation without allelic loss and absence of mdm-2 amplification in a transplantable hamster pancreatic ductal adenocarcinoma and derived cell lines but not primary ductal adenocarcinomas in hamsters

An investigation of p53 gene mutation by single-stranded conformation polymorphism analysis of polymerase chain reaction products followed by direct sequencing and of murine double minute 2 (mdm-2) gene amplification by Southern blot analysis was performed, using a series of hamster pancreatic duct adenocarcinomas: 18 primary adenocarcinomas induced by N-nitrosobis(2-oxopropyl)amine, a transplantable adenocarcinoma (HPD), and three cell lines derived from HPD (HPD1NR, HPD2NR, and HPD3NR). A mutation in the p53 gene was detected at codon 197, resulting in an amino acid change from leucine to phenylalanine, in both HPD and the three cell lines but in none of the 18 primary adenocarcinomas. In the three HPD cell lines, which were confirmed to contain only cancer cells, a normal p53 gene allele was retained. Immunohistochemical investigation of p53 expression using polyclonal antibody Ab-7 revealed positive nuclear staining in the HPD and two back-transplanted tumors derived from HPD1NR and HPD2NR but not in the 18 primary adenocarcinomas. mdm-2 gene amplification was not detected in 18 primary adenocarcinomas or any of the tumor cell lines. The results suggest that a p53 gene mutation without allelic loss, together with overexpression of p53 protein, may be a genetic alteration involved in the progression stage of multistep pancreatic carcinogenesis in hamsters and that mdm-2 gene amplification is not important for this process.

Abnormalities of the RB1 and DCC tumor suppressor genes: uncommon in human pancreatic adenocarcinoma

Evidence of RB1 allele loss was found in only 6% of pancreatic cancers, and we found no significant sequence abnormalities nor loss of RB protein expression in a panel of tumors and cell lines. Using reverse transcription-polymerase chain reaction and Southern blot analysis, we found no evidence for loss of DCC expression in pancreatic cancer cell lines, and allele loss only rarely in tumor biopsies. These findings suggest that abnormalities of RB1 and DCC are unlikely to play a major role in pancreatic carcinogenesis.

Antisense oligonucleotides directed against p53 have antiproliferative effects unrelated to effects on p53 expression

Antisense oligonucleotides targeting p53 have been hailed as a potentially new technique for treating patients with cancer, and there have been encouraging reports of good patient tolerance in vivo and of antiproliferative effects in vitro. However, evidence is lacking that these oligonucleotides are acting via an antisense interaction to modulate p53 expression. We examined a phosphorothioate antisense oligonucleotide, directed against exon 10 of the TP53 gene, and a chimaeric phosphorothioate-phosphodiester oligonucleotide directed against the p53 translation initiation codon. Both failed to specifically suppress p53 protein production in a cell-free assay system or to have any effect on mutant p53 expression by human pancreatic cancer cell lines. Antiproliferative effects were apparent, especially with the phosphorothioate antisense oligonucleotide, but this was independent of the p53 status of the cells (mutant, wild-type or absent) and also occurred with the control (sense and randomised) oligonucleotides. The most dramatic antiproliferative effects were seen with the 'control' phosphorothioate oligonucleotides. These findings suggest that the antiproliferative effects of some antisense oligonucleotides may be unrelated to expression of the gene they have been designed to target.

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.