Germ-line mutations of the p53 tumor suppressor gene in patients with high risk for cancer inactivate the p53 protein

A matched case-control study of the prognosis of early breast cancer in patients with Li-Fraumeni syndrome (BREAST TP53)

INTRODUCTION

Breast cancer (BC) is the most common type of cancer in premenopausal women with germline TP53 pathogenic variants (mTP53) (Li Fraumeni syndrome - LFS). However, little is known about the BC prognosis in these patients. This study analyzed the BC-related oncologic outcomes of patients with LFS.

METHODS

We evaluated a cohort of LFS patients with BC in comparison with a control cohort of BC patients with no pathogenic variant in a hereditary cancer panel. The primary endpoint was recurrence-free survival (RFS). Due to the risk of second malignancies in LFS, only locoregional and distant recurrences were considered events for RFS. Secondary endpoints included rates of contralateral BC, overall survival (OS), and breast cancer-specific survival (BCSS).

RESULTS

Forty-one patients were evaluated in the mTP53 group and 82 in the control group. Median age at BC diagnosis was 40 and 41 years, respectively. The mTP53 group received less adjuvant radiotherapy than the control group (63.4% vs 93.9%, P < 0.001). Other relevant baseline characteristics and treatment received were similar between groups. 5y-RFS rates were 79.4% in the mTP53 versus 93.6% in the control group (HR 2.43, 95%CI 0.74-8.01, P = 0.143); and were not impacted by the use of adjuvant radiotherapy. 5y-BCSS rates were 92.2% and 98.6%, respectively (HR 1.87, IC95% 0.25-13.48, P = 0.534).

CONCLUSIONS

Our results showed no statistically significant difference in BC-related RFS and BCSS between patients with mTP53 and a control group with no pathogenic variant. Larger multicentric studies are warranted to confirm these results.

Li-Fraumeni Syndrome: A Rare Genetic Disorder

Li-Fraumeni syndrome (LFS) is an inherited genetic condition that makes individuals predisposed to specific types of cancer. As a result, cancer risk can be passed down from generation to generation. TP53 is the genetic blueprint for a protein called p53 and most commonly causes this condition by mutations or alterations in that gene. Mutations prevent the gene from functioning properly. LFS is associated with TP53 gene mutations in approximately 70% of families. Most patients with LFS have one normal copy of TP53 and one mutated copy of TP53, usually inherited from a parent with the condition. This is a case report of a 40-year-old female who underwent genetic testing to determine her p53 mutation status. Her mother was diagnosed with breast cancer at a young age, despite the fact that her brothers and sisters' genetic tests came out normal. The genetic testing showed her as a carrier for the TP53 gene mutation. Despite the fact that she had no signs or symptoms of any linked tumors associated with the condition, she was diagnosed with LFS.

Clonal Hematopoiesis and Mosaicism Revealed by a Multi-Tissue Analysis of Constitutional TP53 Status

BACKGROUND

Though germline TP53 pathogenic/likely pathogenic variants (PV) are associated with Li-Fraumeni syndrome, many detected by multigene panels represent aberrant clonal expansion (ACE), most due to clonal hematopoiesis (CH). Discerning ACE/CH from germline variants and postzygotic mosaicism (PZM) is critically needed for risk assessment and management.

METHODS

Participants in the Li-Fraumeni &amp; TP53 Understanding &amp; Progress (LiFT UP) study with a TP53 PV were eligible. Demographics, personal/family cancer history, and clinical laboratory test reports were obtained. DNA from multiple tissues was analyzed using a custom QIAseq assay (ACE panel) that included TP53 and other CH-associated genes; the ACE panel and eyebrow follicles were assessed in a workflow to discern TP53 PV clinical categories.

RESULTS

Among 134 participants there was a significant difference for the age at diagnosis (P &lt; 0.001), component cancers (P = 0.007), and clinical testing criteria (P &lt; 0.001), comparing germline with PZM or ACE. ACE panel analysis of DNA from 55 sets of eyebrow follicles (mean 1.4 ug) and 36 formalin-fixed, paraffin imbedded tissues demonstrated low variance (SE, 3%; P = 0.993) for TP53 variant allele fraction, with no significant difference (P = 0.965) between tissue types, and detected CH gene PVs. Of 55 multi-tissue cases, germline status was confirmed for 20, PZM in seven, ACE for 25, and three were indeterminate. Additional CH variants were detected in six ACE and two germline cases.

CONCLUSIONS

We demonstrated an effective approach and tools for discerning germline TP53 status.

IMPACT

Discernment of PZM and TP53-driven CH increases diagnostic accuracy and enables risk-appropriate care.

Aberrantly Activated APOBEC3B Is Associated With Mutant p53-Driven Refractory/Relapsed Diffuse Large B-Cell Lymphoma

Tumor protein 53 (TP53) mutation predicts an unfavorable prognosis in diffuse large B-cell lymphoma (DLBCL), but the molecular basis for this association remains unclear. In several malignancies, the cytidine deaminase apolipoprotein B mRNA editing enzyme catalytic subunit 3B (APOBEC3B) has been reported to be associated with the TP53 G/C-to-A/T mutation. Here, we show that the frequency of this mutation was significantly higher in relapsed/refractory (R/R) than in non-R/R DLBCL, which was positively associated with the APOBEC3B expression level. APOBEC3B overexpression induced the TP53 G/C-to-A/T mutation in vitro, resulting in a phenotype similar to that of DLBCL specimens. Additionally, APOBEC3B-induced p53 mutants promoted the growth of DLBCL cells and enhanced drug resistance. These results suggest that APOBEC3B is a critical factor in mutant p53-driven R/R DLBCL and is therefore a potential therapeutic target.

Partial p53 reactivation is sufficient to induce cancer regression

Background

Impaired p53 function is one of the central molecular features of a tumor cell and even a partial reduction in p53 activity can increase the cancer risk in mice and men. From a therapeutic perspective it is noteworthy that tumor cells often become addicted to the absence of p53 providing a rationale for developing p53 reactivating compounds to treat cancer patients. Unfortunately, many of the compounds that are currently undergoing preclinical and clinical testing fail to fully reactivate mutant p53 proteins, raising the crucial question: how much p53 activity is needed to elicit a therapeutic effect?

Methods

We have genetically modelled partial p53 reactivation using knock-in mice with inducible expression of the p53 variant E177R. This variant has a reduced ability to bind and transactivate target genes and consequently causes moderate cancer susceptibility. We have generated different syngeneically transplanted and autochthonous mouse models of p53-deficient acute myeloid leukemia and B or T cell lymphoma. After cancer manifestation we have activated E177R expression and analyzed the in vivo therapy response by bioluminescence or magnetic resonance imaging. The molecular response was further characterized in vitro by assays for gene expression, proliferation, senescence, differentiation, apoptosis and clonogenic growth.

Results

We report the conceptually intriguing observation that the p53 variant E177R, which promotes de novo leukemia and lymphoma formation, inhibits proliferation and viability, induces immune cell infiltration and triggers cancer regression in vivo when introduced into p53-deficient leukemia and lymphomas. p53-deficient cancer cells proved to be so addicted to the absence of p53 that even the low-level activity of E177R is detrimental to cancer growth.

Conclusions

The observation that a partial loss-of-function p53 variant promotes tumorigenesis in one setting and induces regression in another, underlines the highly context-specific effects of individual p53 mutants. It further highlights the exquisite sensitivity of cancer cells to even small changes in p53 activity and reveals that changes in activity level are more important than the absolute level. As such, the study encourages ongoing research efforts into mutant p53 reactivating drugs by providing genetic proof-of-principle evidence that incomplete p53 reactivation may suffice to elicit a therapeutic response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02269-6.

Multiple meningiomas arising within the same hemisphere associated with Li-Fraumeni syndrome

Background:

While meningiomas are some of the most common intracranial tumors, the presence of multiple ones at the time of presentation is rare and can most commonly be observed in patients with well-described syndromes (i.e., neurofibromatosis type 2) or those with prior cranial radiation history. In others, however, the pathophysiology remains unclear.

Case Description:

A 49-year-old female with no significant personal or familial oncologic medical history presented with a generalized seizure and was found to have ten meningiomas arising within the right hemisphere. She underwent a two-staged resection of all tumors, with pathology revealing the World Health Organization Grade I meningioma. Whole-exome sequencing revealed somatic NF2 mutations and heterozygous deletion of chromosome 22 overlapping with NF2, and analysis of the germline uncovered mutations of TP53, rendering a diagnosis of Li-Fraumeni Syndrome.

Conclusions:

This case represents a novel presentation of multiple meningiomas in a patient with newly diagnosed Li-Fraumeni syndrome, suggesting meningioma may be considered as part of this tumor-predisposed patient population.

TP53 mutations in head and neck cancer cells determine the Warburg phenotypic switch creating metabolic vulnerabilities and therapeutic opportunities for stratified therapies

Patients with mutated TP53 have been identified as having comparatively poor outcomes compared to those retaining wild-type p53 in many cancers, including squamous cell carcinomas of the head and neck (SCCHN). We have examined the role of p53 in regulation of metabolism in SCCHN cells and find that loss of p53 function determines the Warburg effect in these cells. Moreover, this metabolic adaptation to loss of p53 function creates an Achilles' heel for tumour cells that can be exploited for potential therapeutic benefit. Specifically, cells lacking normal wild-type p53 function, whether through mutation or RNAi-mediated downregulation, display a lack of metabolic flexibility, becoming more dependent on glycolysis and losing the ability to increase energy production from oxidative phosphorylation. Thus, cells that have compromised p53 function can be sensitised to ionizing radiation by pre-treatment with a glycolytic inhibitor. These results demonstrate the deterministic role of p53 in regulating energy metabolism and provide proof of principle evidence for an opportunity for patient stratification based on p53 status that can be exploited therapeutically using current standard of care treatment with ionising radiation.

Residual apoptotic activity of a tumorigenic p53 mutant improves cancer therapy responses

Engineered p53 mutant mice are valuable tools for delineating p53 functions in tumor suppression and cancer therapy. Here, we have introduced the R178E mutation into the Trp53 gene of mice to specifically ablate the cooperative nature of p53 DNA binding. Trp53^R178E mice show no detectable target gene regulation and, at first sight, are largely indistinguishable from Trp53^−/− mice. Surprisingly, stabilization of p53^R178E in Mdm2^−/− mice nevertheless triggers extensive apoptosis, indicative of residual wild‐type activities. Although this apoptotic activity suffices to trigger lethality of Trp53^R178E;Mdm2^−/− embryos, it proves insufficient for suppression of spontaneous and oncogene‐driven tumorigenesis. Trp53^R178E mice develop tumors indistinguishably from Trp53^−/− mice and tumors retain and even stabilize the p53^R178E protein, further attesting to the lack of significant tumor suppressor activity. However, Trp53^R178E tumors exhibit remarkably better chemotherapy responses than Trp53^−/− ones, resulting in enhanced eradication of p53‐mutated tumor cells. Together, this provides genetic proof‐of‐principle evidence that a p53 mutant can be highly tumorigenic and yet retain apoptotic activity which provides a survival benefit in the context of cancer therapy.

Li-Fraumeni syndrome presenting as mucosal melanoma: Case report and treatment considerations

BACKGROUND

Li-Fraumeni syndrome (LFS) is a familial cancer predisposition associated with a germline mutation in TP53. Patients with LFS are at risk of developing malignancies and require comprehensive screening. We describe an index case of LFS presenting with mucosal melanoma.

METHODS

A 21-year-old woman presented with a left maxillary mucosal lesion and a left neck mass. Biopsies revealed metastatic mucosal melanoma, which is a pathology previously unreported in LFS families. Genetic testing revealed LFS, with a germline TP53 mutation, and pedigree analysis identified 9 first-degree and second-degree relatives with hematologic malignancies.

RESULTS

The patient underwent a maxillectomy and left neck dissection, followed by adjuvant radiotherapy. At 30-month follow-up, there was no evidence of local, regional, or distant failure, nor did she develop a second primary tumor.

CONCLUSION

This represents the first reported case of LFS associated with mucosal melanoma. Treatment considerations, specifically the risks of adjuvant therapy in LFS, are discussed. © 2016 Wiley Periodicals, Inc. Head Neck 39: E20-E22, 2017.

Establishment and characterization of the first pediatric adrenocortical carcinoma xenograft model identifies topotecan as a potential chemotherapeutic agent

PURPOSE

Pediatric adrenocortical carcinoma (ACC) is a rare and highly aggressive malignancy. Conventional chemotherapeutic agents have shown limited utility and are largely ineffective in treating children with advanced ACC. The lack of cell lines and animal models of pediatric ACC has hampered the development of new therapies. Here we report the establishment of the first pediatric ACC xenograft model and the characterization of its sensitivity to selected chemotherapeutic agents.

EXPERIMENTAL DESIGN

A tumor from an 11-year-old boy with previously untreated ACC was established as a subcutaneous xenograft in immunocompromised CB17 scid(-/-) mice. The patient harbored a germline TP53 G245C mutation, and the primary tumor showed loss of heterozygosity with retention of the mutated TP53 allele. Histopathology, DNA fingerprinting, gene expression profiling, and biochemical analyses of the xenograft were conducted and compared with the primary tumor and normal adrenal cortex. The second endpoint was to assess the preliminary antitumor activity of selected chemotherapeutic agents.

RESULTS

The xenograft maintained the histopathologic and molecular features of the primary tumor. Screening the xenograft for drug responsiveness showed that cisplatin had a potent antitumor effect. However, etoposide, doxorubicin, and a panel of other common cancer drugs had little or no antitumor activity, with the exception of topotecan, which was found to significantly inhibit tumor growth. Consistent with these preclinical findings, topotecan as a single agent in a child with relapsed ACC resulted in disease stabilization.

CONCLUSION

Our study established a novel TP53-associated pediatric ACC xenograft and identified topotecan as a potentially effective agent for treating children with this disease.

Li-fraumeni syndrome

Li-Fraumeni syndrome (LFS) is a classic cancer predisposition disorder that is commonly associated with germline mutations of the p53 tumor suppressor gene. Examination of the wide spectrum of adult-onset and childhood cancers and the distribution of p53 mutations has led to a greater understanding of cancer genotype-phenotype correlations. However, the complex LFS phenotype is not readily explained by the simple identification of germline p53 mutations in affected individuals. Recent work has identified genetic events that modify the LFS phenotype. These include intragenic polymorphisms, mutations/polymorphisms of genes in the p53 regulatory pathway, as well as more global events such as aberrant copy number variation and telomere attrition. These genetic events may, in part, explain the breadth of tumor histiotypes within and across LFS families, the apparent accelerated age of onset within families, and the range of clinical outcomes among affected family members. This review will examine the clinical and genetic definitions of LFS and offer insight into how lessons learned from the study of this rare disorder may inform similar questions in other familial cancer syndromes.

Modeling human osteosarcoma in the mouse: From bedside to bench

Osteosarcoma (OS) is the most common primary tumour of bone, occurring predominantly in the second decade of life. High-dose cytotoxic chemotherapy and surgical resection have improved prognosis, with long-term survival for patients with localized (non-metastatic) disease approaching 70%. At presentation approximately 20% of patients have metastases and almost all patients with recurrent OS have metastatic disease and cure rates for patients with metastatic or recurrent disease remain poor (<20% survival). Over the past 20 years, considerable progress has been made in the understanding of OS pathogenesis, yet these insights have not translated into substantial therapeutic advances and clinical outcomes. Further progress is essential in order to develop molecularly based therapies that target both primary lesions as well as metastatic disease. The increasing sophistication with which gene expression can be modulated in the mouse, both positively and negatively in addition to temporally, has allowed for the recent generation of more faithful OS models than have previously been available. These murine OS models can recapitulate all aspects of the disease process, from initiation and establishment to invasion and dissemination to distant sites. The development and utilisation of murine models that faithfully recapitulate human osteosarcoma, complementing existing approaches using human and canine disease, holds significant promise in furthering our understanding of the genetic basis of the disease and, more critically, in advancing pre-clinical studies aimed at the rational development and trialing of new therapeutic approaches.

The role of mutant p53 in human cancer

Mutations in the TP53 (p53) gene are present in a large fraction of human tumours, which frequently express mutant p53 proteins at high but heterogeneous levels. The clinical significance of this protein accumulation remains clouded. Mouse models bearing knock-in mutations of p53 have established that the mutant p53 proteins can drive tumour formation, invasion and metastasis through dominant negative inhibition of wild-type p53 as well as through gain of function or 'neomorphic' activities that can inhibit or activate the function of other proteins. These models have also shown that mutation alone does not confer stability, so the variable staining of mutant proteins seen in human cancers reflects tumour-specific activation of p53-stabilizing pathways. Blocking the accumulation and activity of mutant p53 proteins may thus provide novel cancer therapeutic and diagnostic targets, but their induction by chemotherapy may paradoxically limit the effectiveness of these treatments.

Life or death: p53-induced apoptosis requires DNA binding cooperativity

The tumor suppressor p53 provides exquisite protection from cancer by balancing cell survival and death in response to stress. Sustained stress or irreparable damage trigger p53's killer functions to permanently eliminate genetically-altered cells as a potential source of cancer. To prevent the unnecessary loss of cells that could cause premature aging as a result of stem cell attrition, the killer functions of p53 are tightly regulated and balanced against protector functions that promote damage repair and support survival in response to low stress or mild damage. In molecular terms these p53-based cell fate decisions involve protein interactions with cofactors and modifying enzymes, which modulate the activation of distinct sets of p53 target genes. In addition, we demonstrate that part of this regulation occurs at the level of DNA binding. We show that the killer function of p53 requires the four DNA binding domains within the p53 tetramer to interact with one another. These intermolecular interactions enable cooperative binding of p53 to less perfect response elements in the genome, which are present in many target genes essential for apoptosis. Modulating p53 interactions within the tetramer could therefore present a novel promising strategy to fine-tune p53-based cell fate decisions.

MDM2 SNP309 G allele increases risk but the T allele is associated with earlier onset age of sporadic breast cancers in the Chinese population

Sporadic breast cancer in women <40 years is uncommon in Caucasians, in contrast to a much earlier onset in Chinese Asians. However, the molecular determinants for this earlier onset are unclear. It has been reported that SNP309 in the promoter of MDM2, the negative regulator of p53, affects the onset age of cancers in females. Essentially, the G allele, rather than the T allele, has been suggested to accelerate the age of cancer onset. Hence, we examined if MDM2 and p53 polymorphisms would be determinants of the early onset phenomenon in Chinese women. Our results indicate that the MDM2 SNP309 G allele is more prevalent in the Chinese population compared with reported frequencies in Caucasians, and increases breast cancer risk of both sporadic cases and those with family history. However, it was the T/T genotype that was associated with earlier onset age of sporadic breast cancers in contrast to the G allele that was associated with the familial cases. Though p53 codon 72 single-nucleotide polymorphism (SNP) did not affect general cancer risk or age of onset, arginine homozygozity, in contrast to proline homozygozity, was found to decrease breast cancer risk in the later onset sporadic cases. Both SNP309 and codon 72 polymorphisms did not affect the stage of cancer. Together, the data suggest that though the MDM2 SNP309 G allele is a risk factor for breast cancer, it does not accelerate, but delays the onset of the sporadic disease in Chinese women, highlighting that differences in ethnicity and family history may influence the role of MDM2 SNP309 in cancer susceptibility.

HCV NS2 protein inhibits cell proliferation and induces cell cycle arrest in the S-phase in mammalian cells through down-regulation of cyclin A expression

Chronic hepatitis C virus (HCV) infection often leads to liver cancer. NS2 protein is a HCV hydrophobic transmembrane protein that associates with several cellular proteins in mammalian cells. In this report, we investigated the functions of NS2 protein by examining its effects on cell growth and cell cycle progression. Stable NS2-expressing HeLa and Vero cell lines were established by transfection of the cells with pcDNA3.1(-)-NS2 followed by selection of the transfected cells in the presence of G418. We found that the proliferation rates of both NS2-expressing cell lines were inhibited by 40-50% compared with the control cells that were transfected with pcDNA3.1(-) control vector. Cell cycle analysis of these NS2-expressing cell lines shows that the proportion of cells in the S-phase increased significantly compared to that of control cells that do not express NS2 protein, suggesting NS2 protein induces cell cycle arrest in the S-phase. Further studies showed that the induction of cell cycle arrest in the S-phase by NS2 protein is associated with the decrease of cyclin A level. In contrast, the expression of NS2 protein does not affect the levels of cyclin-dependent kinase CDK2, CDK4, cyclin D1, or cyclin E. Our results suggest that HCV NS2 protein inhibits cell growth and induces the cell cycle arrest in the S-phase through down-regulation of cyclin A expression, which may be beneficial to HCV viral replication. Our findings not only provide information in the understanding mechanism of HCV infection, but also provide guidance for the future development of potential therapeutics for the prevention and treatment of the viral infection.

Functional analysis of p53 tumor suppressor in yeast

The p53 tumor suppressor protein is a transcription factor that mediates the cell's response to various kinds of stress by preventing cell division and/or inducing apoptosis. p53 gene mutations have been detected in nearly 50% of human cancers. These gene aberrations are mostly missense point mutations located predominantly in the central DNA-binding domain. In addition to the classical inactivating mutations, there are also dominant-negative, gain-of-function, temperature-sensitive, and cold-sensitive, discriminating, superactive p53 mutations, and some mutations that do not inactivate p53 activity. Several approaches have been developed for detection and analyses of p53 mutations: first, immunochemical methods have been developed to detect p53 protein levels; second, molecular analyses targeting changes in DNA structure are utilized; and third, functional assays are used to explore the biological properties of the p53 protein. Functional analysis of separated alleles in yeast targets the transactivation capability of the p53 protein expressed in yeast cells. This method uses p53 mRNA isolated from cells and tissues to produce a p53 product by RT-PCR. This method has undergone continuous improvement and now serves as a powerful tool for distinguishing various functional types of p53 mutations. Understanding the exact impact of p53 mutation on its function is an important prerequisite for establishment of efficient anti-cancer therapies.

PDBSite: a database of the 3D structure of protein functional sites

The PDBSite database provides comprehensive structural and functional information on various protein sites (post-translational modification, catalytic active, organic and inorganic ligand binding, protein-protein, protein-DNA and protein-RNA interactions) in the Protein Data Bank (PDB). The PDBSite is available online at http://wwwmgs.bionet.nsc.ru/mgs/gnw/pdbsite/. It consists of functional sites extracted from PDB using the SITE records and of an additional set containing the protein interaction sites inferred from the contact residues in heterocomplexes. The PDBSite was set up by automated processing of the PDB. The PDBSite database can be queried through the functional description and the structural characteristics of the site and its environment. The PDBSite is integrated with the PDBSiteScan tool allowing structural comparisons of a protein against the functional sites. The PDBSite enables the recognition of functional sites in protein tertiary structures, providing annotation of function through structure. The PDBSite is updated after each new PDB release.

Using germ-line genetic variation to investigate and treat cancer

For many years, there has been spirited debate as to the relative importance of environmental and genetic factors in the pathogenesis of cancer. Current efforts to annotate the human genome for germ-line genetic variants should establish the foundation for dissecting the contribution of genetics to the risk for cancer susceptibility. Population-based studies should be conducted to determine the influence of germline genetic variation on cancer outcomes, including the efficacy of anti-cancer drugs and the risk for life-threatening toxicities. Although we are early in the investigation of the influence of germline genetics on cancer outcomes, it is likely that, in the future, it will be possible to individualize therapeutic interventions. In turn, knowledge of genetic risk factors could afford opportunities for prevention, early intervention and minimization of deleterious toxicities associated with cancer therapy.

The Li-Fraumeni syndrome

Li-Fraumeni syndrome (LFS) has been the most common terminology used for the syndrome. It is a rare familial dominantly inherited cancer syndrome characterized by a wide spectrum of neoplasms occurring in children and young adults. The canonical definition of LFS includes a proband diagnosed with sarcoma before 45 years of age, a first-degree relative with cancer before this same age and another first- or second-degree relative in the lineage with any cancer before this age or sarcoma at any age. Multiple studies have reported p53 germline mutations in LFS families in various parts of the world. As in sporadic tumors, loss of heterozygosity leading to the inactivation of the wild-type allele by deletion or mutation is observed in LFS tumors. Cancer-risk in mutation carriers has been estimated to be 73% in males and nearly 100% in females, the difference almost entirely explained by breast cancer. The identification of germline p53 mutations in rare cancer-prone families has given rise to the medical, counseling, psychological and ethical problems.

Absence of germline and somatic p53 alterations in children with sporadic brain tumors

Cancers of the central nervous system are the most common solid tumors of childhood. Although somatic alterations of the p53 tumor suppressor gene have been implicated in brain tumorigenesis, the role of germline p53 mutations in the development of childhood brain tumors has not been well defined. As a component of an ongoing extensive study of the epidemiology of childhood brain tumors, we prospectively examined the germline and tumor p53 gene status in 85 children without a family history of cancer who were diagnosed with a sporadic malignant central nervous system tumor. Using PCR/single-strand conformational polymorphism analysis and direct DNA sequencing, 85 children were screened for the presence of constitutional p53 sequence alterations in exons 2 and 4 through 11. No mutations were identified. Commonly reported sequence polymorphisms were observed at codon 72, as well as in 2 other previously described nucleotide residues. Forty-four brain tumor samples were available for analysis and of these 40 were paired with peripheral blood. Once again, no p53 mutations were found. Of the 5 germline samples with the 2 common polymorphisms, only one had a paired tumor sample for comparison and the tumor contained the same alteration as the germline. Of note, one tumor, a PNET of the cerebellum (medulloblastoma), showed loss of heterozygosity at codon 72. We can conclude that the frequency of germline and somatic p53 mutations in sporadic childhood brain tumors is very low, probably less than 1%, and there is no need to screen these patients routinely for their germline p53 status. However, the potential significance of LOH at codon 72 remains to be elucidated.

p53 compound heterozygosity in a severely affected child with Li-Fraumeni syndrome

The Li-Fraumeni Syndrome (LFS) is a rare, dominantly inherited syndrome that features high risk of cancers in childhood and early adulthood. Affected families tend to develop bone and soft tissue sarcomas, breast cancers, brain tumors, leukemias, and adrenocortical carcinomas. In some kindreds, the genetic abnormality associated with this cancer phenotype is a heterozygous germline mutation in the p53 tumor suppressor gene. Recently, we identified one patient who presented in early childhood with multiple primary cancers and who harbored three germline p53 alterations (R156H and R267Q on the maternal allele and R290H on the paternal allele). To classify the biologic effects of these alterations, functional properties of each of the p53 mutants were examined using in vitro assays of cellular growth suppression and transcriptional activation. Each amino acid substitution conferred partial or complete loss of wild-type p53 function, but the child completed normal embryonic development. This observation has not been previously reported in a human, but is consistent with observations of normal embryogenesis in p53-deficient mice.

Continuous human cell lines inducibly expressing hepatitis C virus structural and nonstructural proteins

Investigation of the hepatitis C virus (HCV) life cycle and the evaluation of novel antiviral strategies are limited by the lack of an efficient cell culture system. Therefore, continuous human cell lines inducibly expressing the entire HCV open reading frame were generated with use of a tetracycline-regulated gene expression system. HCV transgenes were found to be chromosomally integrated in a head-to-tail configuration. Northern blot analyses revealed a tightly regulated unspliced transcript of approximately 9 kilobases (kb). HCV structural and nonstructural proteins were faithfully processed, indicating that the cellular and viral proteolytic machineries and posttranslational modification pathways are fully functional in these cell lines. Steady state expression levels could be regulated over a broad range by the concentration of tetracycline present in the culture medium. Kinetic analyses revealed a half-life of less than 1 hour for the HCV RNA whereas a half-life of approximately 9.5, 12, 11, and 10 hours was found for core, NS3, NS4A, and NS5A proteins, respectively. Viral proteins were found to colocalize in the cytoplasm in a pattern characteristic of the endoplasmic reticulum. High-level expression of HCV proteins in the fully induced state was toxic to the cells. These cell lines provide a unique in vitro system to analyze structural and functional properties of HCV proteins, their interactions with cellular proteins and pathways, and the requirements for HCV morphogenesis. In addition, they should prove useful for the evaluation of novel antiviral strategies against hepatitis C in a well-defined and reproducible cellular context.

Li-Fraumeni syndrome and the role of the p53 tumor suppressor gene in cancer susceptibility

Mutation of the tumor suppressor gene p53 is a molecular genetic event frequently observed in human cancer, and inactivating missense mutations usually are accompanied by the resultant overexpression of mutant p53 protein. In gynecologic cancers, p53 is also often altered; the frequency varies depending on types of cancers and where they develop. Further, human papillomavirus oncoproteins that inactivate p53 and Rb proteins play important roles in the development of several gynecologic cancers. Individuals who are heterozygous for germline mutations of the p53 gene are strongly predisposed to a variety of cancers. The identification of these individuals may have profound value in the future when therapies or chemopreventive agents specific for the p53 alteration are available. The role of p53 tumor suppressor gene in gynecologic cancers and heritable cancer susceptibility syndromes including Li-Fraumeni and Lynch II syndromes is an active and important area of study.

Genetic predisposition to cancer and familial cancer syndromes

Approximately 10% to 15% of childhood cancers are hereditary or familial in nature. For several genetic disorders, the development of cancer is a secondary manifestation of the clinical phenotype, whereas cancer predisposition syndromes are generally recognized by the manifestation of characteristic malignancies. The study of pediatric cancer and rare hereditary cancer syndromes and associations has led to the identification of numerous cancer genes that are known to play critical roles in both normal and abnormal cellular growth, differentiation, and proliferation. The potential to identify such genetic markers of cancer predisposition poses difficult social, legal, and ethical questions in their application to clinical practice.

Human BRCA1 inhibits growth in yeast: potential use in diagnostic testing

Germline-inactivating mutations of BRCA1 result in a hereditary predisposition to breast and ovarian cancer. Truncating mutations of BRCA1 predispose to cancer and can be ascertained by protein truncation testing or sequencing. However, cancer-predisposing missense mutations of BRCA1 are difficult to distinguish from polymorphisms by genetic testing methods currently used. Here we show that expression of BRCA1 or BRCA1 fused to a GAL4 activation domain in Saccharomyces cerevesiae inhibits growth, resulting in small colonies easily distinguishable from vector-transformed controls. The growth inhibitory effect can be localized to sequences encoding the recently described BRCA1 C-terminal domains. Growth suppression by a BRCA1 fusion protein is not influenced by introduction of neutral polymorphisms but is diminished or abolished by frameshift, nonsense, or disease-associated missense mutations located in the C-terminal 305 amino acids of BRCA1. These observations may permit the functional significance of many BRCA1 sequence changes to be assessed in yeast. Additionally, the correlation of growth suppression with wild-type forms of BRCA1 suggests that the assay may be capable of detecting functionally conserved interactions between the evolutionarily conserved BRCA1 C-terminal domains and cellular elements found in both human and yeast cells.

Oligomerization is not essential for growth suppression by p53 in p53-deficient osteosarcoma Saos-2 cells

The carboxy-terminal portion of the p53 protein contains the tetramerization domain, and the introduction of multiple missense mutations in this domain disrupts the formation of p53 tetramers, resulting in the production of dimeric or monomeric forms of p53. It has recently been shown that a single missense or nonsense mutation in this domain affects the functional properties of p53 both in yeast and in mammalian cells. In this study, we tested the oligomerization of p53 with mutations in the oligomerization domain, when expressed in a human osteosarcoma cell line, Saos-2, in vivo. We found that single point mutations, including two missense and two nonsense mutations, in the alpha-helix of the oligomerization domain disrupted the oligomerization of p53, but that p53 still retained its ability to inhibit colony formation of cells to some degree. These results suggest that oligomerization and the carboxy-terminal basic domain are not prerequisite for p53-dependent tumor suppression, and this may explain why few of the tumor-derived p53 mutations that have been examined so far are carboxy-terminal mutations.

Clinical and pathological associations with p53 tumour-suppressor gene mutations and expression of p21WAF1/Cip1 in colorectal carcinoma

Inactivation of the p53 tumour-suppressor gene is common in a wide variety of human neoplasms. In the majority of cases, single point mutations in the protein-encoding sequence of p53 lead to positive immunohistochemistry (IHC) for the p53 protein, and are accompanied by loss of the wild-type allele. Recently, the WAF1/Cip1 gene was identified as one of the genes induced by wild-type p53, and increased expression of p21WAF1/Cip1 has been found to reflect the status of the p53 tumour-suppressor pathway. We investigated the inactivation of p53 in a relatively small, but well-characterised, group of 46 colorectal carcinomas that were previously studied for allelic alterations, ras oncogene mutations and DNA aneuploidy. Alterations in p53 were identified by IHC, loss of 17p and DNA sequence analysis of exons 5-8, whereas p21WAF1/Cip1 protein expression was determined by IHC. p53 mutations were identified in 19 of the 46 tumours (41%), whereas positive IHC for p53 was found in 21 of the 46 tumours (46%). Positive IHC for p21WAF1/Cip1 was detected in 16 of 42 cases (38%). We found no relationship between p21WAF1/Cip1 staining and p53 protein expression or p53 mutational status. Inactivating mutations in the p53 gene correlated with LOH at 17p but not with LOH at 5q or 18q, Dukes' stage, tumour grade or DNA ploidy. There was a higher survival rate independent of Dukes' stage in the group with no alterations in p53 compared with those with evidence of dysfunction of p53, but the difference was not statistically significant. We conclude that inactivation of p53 and altered expression of p21WAF1/Cip1 are common in colorectal carcinoma but do not correlate with each other or with the clinical or pathological parameters investigated.

Transcriptional repression of p53 by human T-cell leukemia virus type I Tax protein

The human T-cell leukemia virus type I oncoprotein Tax transcriptionally deregulates a wide variety of viral and cellular genes. Tax deregulation of gene expression is mediated through interaction with a variety of structurally unrelated cellular transcription factors, as Tax does not bind DNA in a sequence-specific manner. Although most of these cellular transcription factors have been shown to mediate activation by Tax, we have recently demonstrated that members of the basic helix-loop-helix (bHLH) family of transcription factors, which play a critical role in progression through the cell cycle, mediate repression by Tax. In this report, we examined whether Tax might repress transcription of the tumor suppressor p53, as the p53 gene has recently been demonstrated to be regulated by the bHLH protein c-Myc. Furthermore, loss or inactivation of the p53 gene has been shown to be causally associated with oncogenic transformation. We show that Tax represses transcription of the p53 gene and that this repression is dependent upon the bHLH recognition element in the p53 promoter. Together, these results suggest that Tax may promote malignant transformation through repression of p53 transcription.

A novel polymorphism in intron 6 of the human p53 gene: a possible association with cancer predisposition and susceptibility

We present a novel polymorphic 8-bp sequence in intron 6 of the p53 gene that maps between bp 55 and 62 of the 3' end of exon 6. Of normal blood samples, 32% were heterozygotic for this polymorphism and display a NN' genotype, whereas 68% of the population is homozygotic for the N genotype. The rare homozygotic genotype N' was detected only in four blood samples of cancer patients. Peripheral blood of gastrointestinal (GI) and breast tumor patients demonstrated a higher incidence of heterozygosity (50%) than that of normal individuals. Analysis of the distribution of this polymorphism in tumor samples showed loss of heterozygosity (LOH). This LOH during tumor progression could exhibit preference to each one of the polymorphic alleles. The rare presentation of one allele and the increased incidence of heterozygosity in carcinoma patients may suggest an association between this polymorphism with cancer predisposition and susceptibility. The fact that genetic alterations occurring in noncoding regions may play a role in tumor development only further increases the extent of involvement of p53 in carcinogenesis.

Immunofluorometric analysis of p53 protein and prostate-specific antigen in breast tumours and their association with other prognostic indicators

Mutation and overexpression of p53 occurs in 20-40% of breast cancers and has been shown to be an independent prognostic indicator. Recently we have demonstrated prostate-specific antigen (PSA) expression in breast tumours to be suggestive of favourable prognosis, but quantitative relationships between PSA and p53, and between these and other prognostic factors in breast cancer, have not been investigated. Time-resolved immunofluorometric procedures were used to quantify both p53 protein and PSA in 200 breast tumour extracts, which were also assayed for oestrogen (ER) and progesterone receptors (PGR), epidermal growth factor receptors (EGFR), cathepsin D and HER-2/neu, and characterised for S-phase fraction and DNA ploidy. Weak Spearman correlations were found between p53 and ER (r = - 0.18, P = 0.010), PGR (r = - 0.15, P = 0.0385) and S-phase fraction (r = 0.17, P = 0.016), while PSA was correlated only with PGR (r = 0.16, P = 0.025). Wilcoxon rank sum analysis revealed that levels of ER (P = 0.0001), PGR (P = 0.0001), S-phase fraction (P = 0.0001) and EGFR (P = 0.0014) differed significantly between the two groups categorised as p53 negative or p53 positive. Tumours classified as PSA negative or PSA positive were found to differ with respect to PGR (P = 0.0091) and S-phase fraction (P = 0.011) in a similar analysis. Contingency tables indicated significant negative associations between the status of p53 and that of ER (P = 0.003) and PGR (P = 0.001) and between PSA and S-phase fraction (P = 0.012), and positive associations between p53 and EGFR (P = 0.017), HER-2/neu (P = 0.008), S-phase fraction (P = 0.001) and aneuploidy (P = 0.007), and between PSA and both ER (P = 0.061) and PGR (P = 0.010). No significant associations were found between p53 and PSA. Our results demonstrate that the presence of p53 in breast tumours relates to several other variables which are suspected to predict aggressive tumour phenotypes and that the presence of PSA relates to variables associated with good prognosis.

Intracranial germ cell tumors: detection of p53 gene mutations by single-strand conformation polymorphism analysis

Using polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) analysis, p53 gene mutation was examined in 12 intracranial germ cell tumors (5 yolk sac carcinomas and 7 germinomas), many of which were derived from young patients in the first to the second decade. A total of 10 mutations were detected in 4 of the 12 cases and, in 3 of them, the mutations were multiple or tandem. Among the 10 mutations, 7 were missense, 1 was splicing and 2 were silent. The 7 missense mutations were located at previously proposed hot spot codons or in their vicinity or, when outside the hot spots, at a codon encoding an amino acid conserved in most vertebrates. These findings suggested that all 7 missense mutations may actually give rise to functional alteration of the p53 protein. The splicing mutation was considered to be a germ-line mutation, though its biological effect was equivocal, since the neoplastic tissue contained an additional mutation. The pattern of the mutations was predominancy of G:C-A:T transition with frequent involvement of the CpG site. These mutations were more frequently detected in yolk sac carcinomas (60%; 3/5 cases) than in germinomas (14%; 1/7 cases), suggesting that the contribution of the p53 mutation to carcinogenesis differed with the histological type of the intracranial germ cell tumor.

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.

Germ-line p53 mutations in 15 families with Li-Fraumeni syndrome

Germ-line mutations of the tumor-suppressor gene p53 have been observed in some families with the Li-Fraumeni syndrome (LFS), a familial cancer syndrome in which affected relatives develop a diverse set of early-onset malignancies including breast carcinoma, sarcomas, and brain tumors. The analysis of the p53 gene in LFS families has been limited, in most studies to date, to the region between exon 5 and exon 9. In order to determine the frequency and distribution of germ-line p53 mutations in LFS, we sequenced the 10 coding exons of the p53 gene in lymphocytes and fibroblast cell lines derived from 15 families with the syndrome. Germ-line mutations were observed in eight families. Six mutations were missense mutations located between exons 5 and 8. One mutation was a nonsense mutation in exon 6, and one mutation was a splicing mutation in intron 4, generating aberrant shorter p53 RNA(s). In three families, a mutation of the p53 gene was observed in the fibroblast cell line derived from the proband. However, the mutation was not found in affected relatives in two families and in the blood from the one individual, indicating that the mutation probably occurred during cell culture in vitro. In four families, no mutation was observed. This study indicates that germ-line p53 mutations in LFS are mostly located between exons 5 and 8 and that approximately 50% of patients with LFS have no germ-line mutations in the coding region of the p53 gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Wild type p53 stimulates expression from the human multidrug resistance promoter in a p53-negative cell line

The effect of human wild type and mutant p53 proteins on the human multidrug resistance (MDR1) promoter was studied in a p53-negative human cell line. Transient expression of MDR1 promoter-chloramphenicol acetyltransferase reporter gene constructs (MDRCAT) cotransfected with p53 expression vectors was analyzed in H358 lung carcinoma cells. Cotransfection with a wild type p53 expression vector stimulated MDRCAT activity, while cotransfection with mutant p53 expression vectors altered at amino acid positions 181, 252, 258, or 273 failed to stimulate expression. Wild type p53 stimulation of MDRCAT activity was time dependent with maximal expression occurring 24-30 h following transfection and correlating with high p53 protein levels. MDR1 promoter deletion analysis suggested that the sequences involved in wild type p53 stimulation of MDRCAT activity were contained within the region from -39 to +53 relative to the start of transcription at +1. This region contains no TATA or p53 consensus binding sequence but does contain an initiator sequence. Wild type p53 stimulation of MDRCAT expression also occurred in parental and doxorubicin-resistant SW620 colon and parental 2780 ovarian cancer cell lines, indicating that wild type p53-mediated simulation of the MDR1 promoter is not restricted to a single cell line.

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.

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.

Genetics of primary brain tumors: a review

In this review we provide evidence for the existence of genes associated with primary malignant brain tumors. We summarize the current knowledge from studies of familial cancer aggregation, hereditary syndromes, and molecular and cytogenetic studies. The epidemiologic evidence is suggestive but inconclusive for an association between brain tumors and cancers in other family members, including cancers of the breast, lung and colon. Central nervous system (CNS) tumors have been associated with several hereditary syndromes including the Li-Fraumeni cancer family syndrome, neurofibromatosis (types 1 and 2), tuberous sclerosis, nevoid basal cell carcinoma syndrome, familial polyposis, and von Hippel-Lindau disease. Significant studies leading to the recognition of molecular and cytogenetic abnormalities in malignant gliomas are described in detail. The genetic studies conducted thus far suggest a role for inherited susceptibility in some CNS tumors.

Screening patients for heterozygous p53 mutations using a functional assay in yeast

Inherited mutations of the p53 gene significantly increase the risk of developing diverse malignancies, and germline p53 mutations can be detected by assaying the transcriptional activity of the p53 protein in mammalian cells. Here we describe a method starting with lymphocytes that allows detection of germline p53 mutations by 'functional' analysis of p53 protein expressed in Saccharomyces cerevisiae. The p53 PCR products are directly cloned into yeast expression vectors in vivo and subsequently tested for transcriptional activity in a simple growth assay. This technique, functional analysis of separated alleles in yeast (FASAY), requires only a few steps, can be automated readily and should permit screening for germline or somatic heterozygous mutations in any gene whose function can be monitored in yeast.

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

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

Functional studies of a germ-line polymorphism at codon 47 within the p53 gene

A rare germ-line polymorphism in codon 47 of the p53 gene replaces the wild-type proline (CCG) with a serine (TCG). Restriction analysis of 101 human samples revealed the frequency of the rare allele to be 0% (n = 69) in Caucasians and 4.7% (3/64, n = 32) among African-Americans. To investigate the consequence of this amino acid substitution, a cDNA construct (p53 mut47ser) containing the mutation was introduced into a lung adenocarcinoma cell line (Calu-6) that does not express p53. A growth suppression similar to that obtained after introduction of a wild-type p53 cDNA construct was observed, in contrast to the result obtained by introduction of p53 mut143ala. Furthermore, expression of neither p53 mut47ser nor wild-type p53 was tolerated by growing cells. In transient expression assays, both mut47ser and wild-type p53 activated the expression of a reporter gene linked to a p53 binding sequence (PG13-CAT) and inhibited the expression of the luciferase gene under the control of the Rous sarcoma virus promoter (RSVluc). In the same assay, mut143ala did not activate the expression of PG13-CAT and produced only a slight inhibitory effect on RSVluc. These findings indicate that the p53 variant with a serine at codon 47 should be considered as a rare germ-line polymorphism that does not alter the growth-suppression activity of p53.

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

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

Phosphorylation at Ser-15 and Ser-392 in mutant p53 molecules from human tumors is altered compared to wild-type p53

The product of the p53 gene suppresses cell growth and plays a critical role in suppressing development of human tumors. p53 protein binds DNA, activates transcription, and can be phosphorylated at N- and C-terminal sites. Previously, wild-type p53 was shown to be hyperphosphorylated compared to mutant p53 during p53-mediated growth arrest in vivo. Here we show that Ser-15 and Ser-9 in the N-terminal transactivation domain of wild-type human p53 are phosphorylated in vivo in cells derived from the human glioblastoma line T98G. In [Ile237]p53 and [Ala143]p53, two natural p53 mutants from human tumors that are defective for activation of transcription, phosphorylation at Ser-15 was reduced and phosphorylation at Ser-392 was increased compared to wild-type p53. No change was observed at Ser-9. [His273]p53, a third mutant, had a phosphorylation state similar to that of wild-type p53. We suggest that phosphorylation of Ser-15 may depend on the ability of p53 to adopt a wild-type conformation and may contribute to p53's ability to block cell growth.

Analysis of a protein-binding domain of p53

The tumor suppressor protein p53 was first isolated as a simian virus 40 large T antigen-associated protein and subsequently was found to function in cell proliferation control. Tumor-derived mutations in p53 occur predominantly in four evolutionarily conserved regions spanning approximately 50% of the polypeptide. Previously, three of these regions were identified as essential for T-antigen binding. We have examined the interaction between p53 and T antigen by using Escherichia coli-expressed human p53. By a combination of deletion analysis and antibody inhibition studies, a region of p53 that is both necessary and sufficient for binding to T antigen has been localized. This function is contained within residues 94 to 293, which include the four conserved regions affected by mutation in tumors. Residues 94 to 293 of p53 were expressed in both wild-type and mutant forms. T-antigen binding was unaffected by tumor-derived mutations which have been associated with the wild-type conformation of p53 but was greatly reduced by mutations which were previously shown to alter p53 conformation. Our results show that, like T-antigen binding to the Rb tumor suppressor protein, T antigen appears to interact with the domain of p53 that is commonly mutated in human tumors.