p53 mutants have selective dominant-negative effects on apoptosis but not growth arrest in human cancer cell lines

P53 Gene Therapy with ZIF-8 Metal-Organic Framework: A Platform in Cancer Gene Therapy

Gene therapy holds great promise as a therapeutic approach for combating cancer, with the choice of gene delivery vector being a critical factor in its success. In recent years, metal-organic frameworks (MOFs) have emerged as valuable tools for intracellular plasmid delivery in this field. This study aimed to encapsulate plasmid DNA encoding the TP53 tumor suppressor gene (pEGFP-N1-TP53) within zeolitic imidazolate framework-8 (ZIF-8) MOFs and ZIF-8-PEI. Subsequently, the transfection efficiency and ability to induce cell death were assessed in MDA-MB-231, MCF-7, and HeLa cancer cells. A comparative analysis was conducted to evaluate the induction of cell death by pEGFP-N1-TP53@ZIF-8-PEI, pEGFP-N1-TP53-ZIF-8 nanoparticles, and Lipofectamine in the aforementioned cell lines. Additionally, an optimal condition for loading the plasmid into ZIF-8 was proposed. The findings from cell transfection assays, MTT assay, and flow cytometry revealed that both pEGFP-N1-TP53@ZIF-8-PEI and pEGFP-N1-TP53-ZIF-8 effectively delivered the plasmid to the cells. Notably, pEGFP-N1-TP53@ZIF-8-PEI exhibited significant results, inducing 77% cell death in the HeLa cell line and 73% in the MDA-MB-231 cell line. Our observations indicated that MDA-MB-231 and HeLa cells exhibited heightened responsiveness to TP53 gene therapy when delivered through ZIF-8-PEI and ZIF-8. Based on these findings, further investigation of pEGFP-N1-TP53@ZIF-8-PEI as a potential cancer therapeutic platform in other cancer cell types is warranted.

Clinical impact of TP53 functional mutations in patients with metastatic colorectal cancer treated with bevacizumab and chemotherapy

BACKGROUND

Clinical and experimental studies indicate that the tumor protein p53 (TP53) gene loss of function due to missense mutations (MMs) may confer sensitivity to anti-angiogenics. This effect seems to be linked to cross-talk mechanisms among TP53, vascular endothelial growth factor (VEGF), and VEGF receptors. We investigated whether specific TP53 MMs are associated with clinical outcomes of patients with metastatic colorectal cancer (mCRC) treated with first-line chemotherapy plus Bevacizumab. The study focused on KRAS-mutated, liver-only mCRC cases as a homogeneous subgroup that may represent a relevant setting for exploring this association.

MATERIALS AND METHODS

MMs were identified on primary tumors. MMs were classified by mutant-specific residual transcriptional activity scores (TP53RTAS) as transcriptionally inactive (TP53inactive = TP53RTAS 0%) or active (TP53active = TP53RTAS ≥ 1%) and used for stratifying patients in progression-free survival (PFS), response rate, and overall survival (OS) analyses.

RESULTS

The study population consisted of 62 patients. MMs were found in 39 cases (62%) with 16 having TP53inactive and 23 TP53active MMs. Patients with TP53inactive MMs showed better PFS in comparison with the remaining groups (wild-type and TP53active). This effect was retained in the multivariate model. A similar clinical impact was observed in the OS analysis. There was a significant difference in the overall response rate and rate of post-treatment resection of liver metastases between the TP53inactive and the wild-type or TP53active MMs cases.

CONCLUSIONS

Specific TP53 MMs may identify sub-groups of patients who benefit from Bevacizumab-based systemic therapy and these findings could lead to novel tailored treatment strategies in this setting.

Mutant p53 oncogenicity: dominant-negative or gain-of-function?

The p53 protein is mutated in about 50% of human cancers. Aside from losing its tumor-suppressive activities, mutant p53 may acquire pro-oncogenic activity, which is facilitated by two underlying mechanisms. The first mechanism is the inhibition of co-expressed wild-type p53 (WTp53) activity, dubbed the dominant-negative effect (DNE). The second mechanism is a neomorphic pro-oncogenic activity that does not involve the inhibition of WTp53, termed gain-of-function (GOF). Throughout the years, both mechanisms were demonstrated in a plethora of in vitro and in vivo models. However, whether both account for protumorigenic activities of mutant p53 and in which contexts is still a matter of ongoing debate. Here, we discuss evidence for both DNE and GOF in a variety of models. These models suggest that both GOF and DNE can be relevant, but are highly dependent on the specific mutation type, genetic and cellular context and even the phenotype that is being assessed. In addition, we discuss how mutant and WTp53 might not exist as two separate entities, but rather as a continuum that may involve a balance between the two forms in the same cells, which could be tilted by various factors and drugs. Further elucidation of the factors that dictate the balance between the WT and mutant p53 states, as well as the factors that govern the impact of DNE and GOF in different cancer types, may lead to the development of more effective treatment regimens for cancer patients.

p53 tetramerization: at the center of the dominant-negative effect of mutant p53

In this review, Gencel-Augusto and Lozano summarize the data on p53 mutants with a functional tetramerization domain that form mixed tetramers and in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. They conclude that the DNE is mostly observed after DNA damage but fails in other contexts.

The p53 tumor suppressor functions as a tetrameric transcription factor to regulate hundreds of genes—many in a tissue-specific manner. Missense mutations in cancers in the p53 DNA-binding and tetramerization domains cement the importance of these domains in tumor suppression. p53 mutants with a functional tetramerization domain form mixed tetramers, which in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. DNA damage appears necessary but not sufficient for DNE, indicating that upstream signals impact DNE. Posttranslational modifications and protein–protein interactions alter p53 tetramerization affecting transcription, stability, and localization. These regulatory components limit the dominant-negative effects of mutant p53 on wild-type p53 activity. A deeper understanding of the molecular basis for DNE may drive development of drugs that release WT p53 and allow tumor suppression.

Functional characterization of novel germline TP53 variants in Swedish families

Pathogenic germline TP53 variants predispose to a wide range of early onset cancers, often recognized as the Li-Fraumeni syndrome (LFS). They are also identified in 1% of families with hereditary breast cancer (HrBC) that do not fulfill the criteria for LFS. In this study, we present a total of 24 different TP53 variants identified in 31 Swedish families with LFS or HrBC. Ten of these variants, nine exonic and one splice, have previously not been described as germline pathogenic variants. The nine exonic variants were functionally characterized and demonstrated partial transactivation activity compared to wild-type p53. Some show nuclear localization similar to wild-type p53 while others possess cytoplasmic or perinuclear localization. The four frameshift variants (W91Gfs*32, L111 Wfs*12, S227 Lfs*20 and S240Kfs*25) had negligible, while F134 L and T231del had low level of p53 activity. The L111 Wfs*12 and T231del variants are also deficient for induction of apoptosis. The missense variant R110C retain p53 effects and the nonsense E349* shows at least partial transcription factor activity but has reduced ability to trigger apoptosis. This is the first functional characterization of novel germline TP53 pathogenic or likely pathogenic variants in the Swedish cohort as an attempt to understand its association with LFS and HrBC, respectively.

Genetic Effects on Dispersion in Urinary Albumin and Creatinine in Three House Mouse (Mus musculus) Cohorts

Conventionally, quantitative genetics concerns the heredity of trait means, but there is growing evidence for the existence of architectures in which certain alleles cause random variance in phenotype, termed ‘phenotypic dispersion’ (PD) or ‘variance QTL’ (vQTL), including in physiological traits like disease signs. However, the structure of this phenomenon is still poorly known. PD for urinary albumin (PD_UAlb) and creatinine (PD_UCrea) was mapped using curated data from two nearly genetically identical F₂ mouse (Mus musculus) cohorts (383 male F₂ C57BL/6J×A/J (97 SNP) and 207 male F₂ C57BL/6J×A/J ApoE knockout mice (144 SNP)) and a related mapping cohort (340 male F₂ DBA/2J×C57BL/6J (83 SNP, 8 microsatellites)). PD_UAlb was associated with markers in regions of Chr 1 (5-64 megabases (MB); 141-158 MB), 3 (∼113 MB), 8 (37-68 MB), 14 (92-117 MB) and 17 (14-24 MB) with several positions and quantitative architectures in common between the two C57BL/6J×A/J cohorts, most of which had a negative dominant construction. One locus for PD_UCrea was detected on Chr 19 (57 MB) in the C57BL/6J×A/J ApoE^−/− cohort. The large number of negative dominant loci for albuminuria dispersion relative to conventional quantitative trait loci suggests that the development of albuminuria may be largely genetically dynamic and that randomization in this development is detrimental.

O6-methylguanine-induced transcriptional mutagenesis reduces p53 tumor-suppressor function

The impact of DNA lesions on replication and mutagenesis is of high relevance for human health; however, the role of lesion-induced transcriptional mutagenesis (TM) in disease development is unknown. Here, the impact of O⁶-methylguanine–induced TM on p53 function as a tumor suppressor was investigated in human cells. Results showed that TM in 15% of the transcripts resulted in a reduced ability of p53 protein to transactivate genes that regulate cell-cycle arrest and induction of apoptosis. This resulted in the loss of functional cell-cycle checkpoints and in impaired activation of apoptosis, both canonical p53 tumor-suppressor functions. This work provides evidence that TM can induce phenotypic changes in mammalian cells that have important implications for its role in tumorigenesis.

Altered protein function due to mutagenesis plays an important role in disease development. This is perhaps most evident in tumorigenesis and the associated loss or gain of function of tumor-suppressor genes and oncogenes. The extent to which lesion-induced transcriptional mutagenesis (TM) influences protein function and its contribution to the development of disease is not well understood. In this study, the impact of O⁶-methylguanine on the transcription fidelity of p53 and the subsequent effects on the protein’s function as a regulator of cell death and cell-cycle arrest were examined in human cells. Levels of TM were determined by RNA-sequencing. In cells with active DNA repair, misincorporation of uridine opposite the lesion occurred in 0.14% of the transcripts and increased to 14.7% when repair by alkylguanine–DNA alkyltransferase was compromised. Expression of the dominant-negative p53 R248W mutant due to TM significantly reduced the transactivation of several established p53 target genes that mediate the tumor-suppressor function, including CDKN1A (p21) and BBC3 (PUMA). This resulted in deregulated signaling through the retinoblastoma protein and loss of G1/S cell-cycle checkpoint function. In addition, we observed impaired activation of apoptosis coupled to the reduction of the tumor-suppressor functions of p53. Taking these findings together, this work provides evidence that TM can induce phenotypic changes in mammalian cells that have important implications for the role of TM in tumorigenesis.

Class I HDAC Inhibitors Display Different Antitumor Mechanism in Leukemia and Prostatic Cancer Cells Depending on Their p53 Status

Previously, we designed and synthesized a series of o-aminobenzamide-based histone deacetylase (HDAC) inhibitors, among which the representative compound 11a exhibited potent inhibitory activity against class I HDACs. In this study, we report the development of more potent hydrazide-based class I selective HDAC inhibitors using 11a as a lead. Representative compound 13b showed a mixed, slow, and tight binding inhibition mechanism for HDAC1, 2, and 3. The most potent compound 13e exhibited low nanomolar IC50s toward HDAC1, 2, and 3 and could down-regulate HDAC6 in acute myeloid leukemia MV4-11 cells. The EC50 of 13e against MV4-11 cells was 34.7 nM, which is 26 times lower than its parent compound 11a. In vitro responses to 13e vary significantly and interestingly based on cell type: in p53 wild-type MV4-11 cells, 13e induced cell death via apoptosis and G1/S cell cycle arrest, which is likely mediated by a p53-dependent pathway, while in p53-null PC-3 cells, 13e caused G2/M arrest and inhibited cell proliferation without inducing caspase-3-dependent apoptosis.

A comprehensive analysis and functional characterization of naturally occurring non-synonymous variants of nuclear receptor PXR

Pregnane & Xenobiotic Receptor (PXR) acts as a xenosensing transcriptional regulator of many drug metabolizing enzymes and transporters of the 'detoxification machinery' that coordinate in elimination of xenobiotics and endobiotics from the cellular milieu. It is an accepted view that some individuals or specific populations display considerable differences in their ability to metabolize different drugs, dietary constituents, herbals etc. In this context we speculated that polymorphisms in PXR gene might contribute to variability in cytochrome P450 (CYP450) metabolizing enzymes of phase I, drug metabolizing components of phase II and efflux components of the detoxification machinery. Therefore, in this study, we have undertaken a comprehensive functional analysis of seventeen naturally occurring non-synonymous variants of human PXR. When compared, we observed that some of the PXR SNP variants exhibit distinct functional and dynamic responses on parameters which included transcriptional function, sub-cellular localization, mitotic chromatin binding, DNA-binding properties and other molecular interactions. One of the unique SNP located within the DNA-binding domain of PXR was found to be functionally null and distinct on other parameters. Similarly, some of the non-synonymous SNPs in PXR imparted reduced transactivation function as compared to wild type PXR. Interestingly, PXR is reported to be a mitotic chromatin binding protein and such an association has been correlated to an emerging concept of 'transcription memory' and altered transcription output. In view of the observations made herein our data suggest that some of the natural PXR variants may have adverse physiological consequences owing to its influence on the expression levels and functional output of drug-metabolizing enzymes and transporters. The present study is expected to explain not only the observed inter-individual responses to different drugs but may also highlight the mechanistic details and importance of PXR in drug clearance, drug-drug interactions and diverse metabolic disorders. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Analytical Validation and Application of a Targeted Next-Generation Sequencing Mutation-Detection Assay for Use in Treatment Assignment in the NCI-MPACT Trial

Robust and analytically validated assays are essential for clinical studies. We outline an analytical validation study of a targeted next-generation sequencing mutation-detection assay used for patient selection in the National Cancer Institute Molecular Profiling-Based Assignment of Cancer Therapy (NCI-MPACT) trial (NCT01827384). Using DNA samples from normal or tumor cell lines and xenografts with known variants, we assessed the sensitivity, specificity, and reproducibility of the NCI-MPACT assay in five variant types: single-nucleotide variants (SNVs), SNVs at homopolymeric (HP) regions (≥3 identical bases), small insertions/deletions (indels), large indels (gap ≥4 bp), and indels at HP regions. The assay achieved sensitivities of 100% for 64 SNVs, nine SNVs at HP regions, and 11 large indels, 83.33% for six indels, and 93.33% for 15 indels at HP regions. Zero false positives (100% specificity) were found in 380 actionable mutation loci in 96 runs of haplotype map cells. Reproducibility analysis showed 96.3% to 100% intraoperator and 98.1% to 100% interoperator mean concordance in detected variants and 100% reproducibility in treatment selection. To date, 38 tumors have been screened, 34 passed preanalytical quality control, and 18 had actionable mutations for treatment assignment. The NCI-MPACT assay is well suited for its intended investigational use and can serve as a template for developing next-generation sequencing assays for other cancer clinical trial applications.

Progeny from irradiated colorectal cancer cells acquire an EMT-like phenotype and activate Wnt/β-catenin pathway

Radiotherapy remains a major approach to adjuvant therapy for patients with advanced colorectal cancer, however, the fractionation schedules frequently allow for the repopulation of surviving tumors cells, neoplastic progression, and subsequent metastasis. The aim of the present study was to analyze the transgenerational effects induced by radiation and evaluate whether it could increase the malignant features on the progeny derived from irradiated parental colorectal cancer cells, Caco-2, HT-29, and HCT-116. The progeny of these cells displayed a differential radioresistance as seen by clonogenic and caspase activation assay and had a direct correlation with survivin expression as observed by immunoblotting. Immunofluorescence showed that the most radioresistant progenies had an aberrant morphology, disturbance of the cell-cell adhesion contacts, disorganization of the actin cytoskeleton, and vimentin filaments. Only the progeny derived from intermediary radioresistant cells, HT-29, reduced the E-cadherin expression and overexpressed β-catenin and vimentin with increased cell migration, invasion, and metalloprotease activation as seen by immunoblotting, wound healing, invasion, and metalloprotease activity assay. We also observed that this most aggressive progeny increased the Wnt/β-catenin-dependent TCF/LEF activity and underwent an upregulation of mesenchymal markers and downregulation of E-cadherin, as determined by qRT-PCR. Our results showed that the intermediate radioresistant cells can generate more aggressive cellular progeny with the EMT-like phenotype. The Wnt/β-catenin pathway may constitute an important target for new adjuvant treatment schedules with radiotherapy, with the goal of reducing the migratory and invasive potential of the remaining cells after treatment.

Long noncoding RNA lincRNA-p21 is the major mediator of UVB-induced and p53-dependent apoptosis in keratinocytes

LincRNA-p21 is a long noncoding RNA and a transcriptional target of p53 and HIF-1α. LincRNA-p21 regulates gene expression in cis and trans, mRNA translation, protein stability, the Warburg effect, and p53-dependent apoptosis and cell cycle arrest in doxorubicin-treated mouse embryo fibroblasts. p53 plays a key role in the response of skin keratinocytes to UVB-induced DNA damage by inducing cell cycle arrest and apoptosis. In skin cancer development, UVB-induced mutation of p53 allows keratinocytes upon successive UVB exposures to evade apoptosis and cell cycle arrest. We hypothesized that lincRNA-p21 has a key functional role in UVB-induced apoptosis and/or cell cycle arrest in keratinocytes and loss of lincRNA-p21 function results in the evasion of apoptosis and/or cell cycle arrest. We observed that lincRNA-p21 transcripts are highly inducible by UVB in mouse and human keratinocytes in culture and in mouse skin in vivo. LincRNA-p21 is regulated at the transcriptional level in response to UVB, and the UVB induction of lincRNA-p21 in keratinocytes and in vivo in mouse epidermis is primarily through a p53-dependent pathway. Knockdown of lincRNA-p21 blocked UVB-induced apoptosis in mouse and human keratinocytes, and lincRNA-p21 was responsible for the majority of UVB-induced and p53-mediated apoptosis in keratinocytes. Knockdown of lincRNA-p21 had no effect on cell proliferation in untreated or UVB-treated keratinocytes. An early event in skin cancer is the mutation of a single p53 allele. We observed that a mutant p53^+/R172H allele expressed in mouse epidermis (K5Cre^+/tg;LSLp53^+/R172H) showed a significant dominant-negative inhibitory effect on UVB-induced lincRNA-p21 transcription and apoptosis in epidermis. We conclude lincRNA-p21 is highly inducible by UVB and has a key role in triggering UVB-induced apoptotic death. We propose that the mutation of a single p53 allele provides a pro-oncogenic function early in skin cancer development through a dominant inhibitory effect on UVB-induced lincRNA-p21 expression and the subsequent evasion of UVB-induced apoptosis.

IKK-β mediates chemoresistance by sequestering FOXO3; a critical factor for cell survival and death

Chemotherapeutic drugs proved only 50% successful in breast cancer because of cell type-dependent resistance mechanisms. FOXO3 is known to be involved in the regulation of several cell death-related genes; however, the extent of FOXO3 regulation in chemoresistance is still not fully understood. Here, we show that FOXO3 critically mediates cisplatin chemosensitivity of MCF-7 breast cancer cells which express higher levels of FOXO3 compared to resistant MDA-MB-231 cells. Administration of cisplatin induces apoptosis in MCF-7 cells in a FOXO3-dependent manner as indicated by RNA interference. On the other hand, IKK-β (IκB kinase) appears to inhibit FOXO3 action after cisplatin treatment and promotes chemoresistance in MDA-MB-231 cells. IKK-β directly interacts and sequesters FOXO3 in the cytosol preventing its nuclear localization. Moreover, cisplatin treatment induces autophagosome formation through LC-3 conversion while inhibiting the cleavage of caspase 9 and caspase 3 in MDA-MB-231 cells manipulated to overexpress FOXO3. In brief, our findings demonstrate that in addition to cellular level of active FOXO3, cisplatin chemoresistance is also regulated by IKK-β sequestration of FOXO3 in cytosol.

p53 directly suppresses BNIP3 expression to protect against hypoxia-induced cell death

Hypoxia stabilizes the tumour suppressor p53, allowing it to function primarily as a transrepressor; however, the function of p53 during hypoxia remains unclear. In this study, we showed that p53 suppressed BNIP3 expression by directly binding to the p53-response element motif and recruiting corepressor mSin3a to the BNIP3 promoter. The DNA-binding site of p53 must remain intact for the protein to suppress the BNIP3 promoter. In addition, taking advantage of zebrafish as an in vivo model, we confirmed that zebrafish nip3a, a homologous gene of mammalian BNIP3, was indeed induced by hypoxia and p53 mutation/knockdown enhanced nip3a expression under hypoxia resulted in cell death enhancement in p53 mutant embryos. Furthermore, p53 protected against hypoxia-induced cell death mediated by p53 suppression of BNIP3 as illustrated by p53 knockdown/loss assays in both human cell lines and zebrafish model, which is in contrast to the traditional pro-apoptotic role of p53. Our results suggest a novel function of p53 in hypoxia-induced cell death, leading to the development of new treatments for ischaemic heart disease and cerebral stroke.

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.

Restoration of DNA-binding and growth-suppressive activity of mutant forms of p53 via a PCAF-mediated acetylation pathway

Tumor-derived mutant forms of p53 compromise its DNA binding, transcriptional, and growth regulatory activity in a manner that is dependent upon the cell-type and the type of mutation. Given the high frequency of p53 mutations in human tumors, reactivation of the p53 pathway has been widely proposed as beneficial for cancer therapy. In support of this possibility p53 mutants possess a certain degree of conformational flexibility that allows for re-induction of function by a number of structurally different artificial compounds or by short peptides. This raises the question of whether physiological pathways for p53 mutant reactivation also exist and can be exploited therapeutically. The activity of wild-type p53 is modulated by various acetyl-transferases and deacetylases, but whether acetylation influences signaling by p53 mutant is still unknown. Here, we show that the PCAF acetyl-transferase is down-regulated in tumors harboring p53 mutants, where its re-expression leads to p53 acetylation and to cell death. Furthermore, acetylation restores the DNA-binding ability of p53 mutants in vitro and expression of PCAF, or treatment with deacetylase inhibitors, promotes their binding to p53-regulated promoters and transcriptional activity in vivo. These data suggest that PCAF-mediated acetylation rescues activity of at least a set of p53 mutations. Therefore, we propose that dis-regulation of PCAF activity is a pre-requisite for p53 mutant loss of function and for the oncogenic potential acquired by neoplastic cells expressing these proteins. Our findings offer a new rationale for therapeutic targeting of PCAF activity in tumors harboring oncogenic versions of p53.

Genetic modeling of Li-Fraumeni syndrome in zebrafish

Li-Fraumeni syndrome (LFS) is a highly penetrant, autosomal dominant, human familial cancer predisposition. Although a key role for the tumor suppressor p53 has been implicated in LFS, the genetic and cellular mechanisms underpinning this disease remain unknown. Therefore, modeling LFS in a vertebrate system that is accessible to both large-scale genetic screens and in vivo cell biological studies will facilitate the in vivo dissection of disease mechanisms, help identify candidate genes, and spur the discovery of therapeutic compounds. Here, we describe a forward genetic screen in zebrafish embryos that was used to identify LFS candidate genes, which yielded a p53 mutant (p53(I166T)) that as an adult develops tumors, predominantly sarcomas, with 100% penetrance. As in humans with LFS, tumors arise in heterozygotes and display loss of heterozygosity (LOH). This report of LOH indicates that Knudson's two-hit hypothesis, a hallmark of human autosomal dominant cancer syndromes, can be modeled in zebrafish. Furthermore, as with some LFS mutations, the zebrafish p53(I166T) allele is a loss-of-function allele with dominant-negative activity in vivo. Additionally, we demonstrate that the p53 regulatory pathway, including Mdm2 regulation, is evolutionarily conserved in zebrafish, providing a bona fide biological context in which to systematically uncover novel modifier genes and therapeutic agents for human LFS.

The expanding universe of p53 targets

The p53 tumour suppressor is modified through mutation or changes in expression in most cancers, leading to the altered regulation of hundreds of genes that are directly influenced by this sequence-specific transcription factor. Central to the p53 master regulatory network are the target response element (RE) sequences. The extent of p53 transactivation and transcriptional repression is influenced by many factors, including p53 levels, cofactors and the specific RE sequences, all of which contribute to the role that p53 has in the aetiology of cancer. This Review describes the identification and functionality of REs and highlights the inclusion of non-canonical REs that expand the universe of genes and regulation mechanisms in the p53 tumour suppressor network.

Blockage of NF-kappaB by IKKbeta- or RelA-siRNA rather than the NF-kappaB super-suppressor IkappaBalpha mutant potentiates adriamycin-induced cytotoxicity in lung cancer cells

Ambiguous roles of genotoxic anticancer therapeutic-induced NF-kappaB activation in regulating gene expression (activation or suppression) and apoptosis (anti- or pro-apoptosis) have recently been suggested. In order to clarify this controversy and determine the usefulness of NF-kappaB blockage for sensitizing anticancer therapy, we have systematically investigated the effect of distinct NF-kappaB-blocking approaches on lung cancer cells' responses to Adriamycin-induced cytotoxicity. The results show that Adriamycin-induced NF-kappaB activation functions as a transcriptional activator triggering the expression of anti-apoptotic genes. Blocking NF-kappaB with IKKbeta- or RelA siRNA substantially sensitized Adriamycin-induced cytotoxicity, suggesting that the NF-kappaB pathway could be a target for sensitizing lung cancer cells to Adriamycin's anticancer effect. Surprisingly, although it effectively blocks NF-kappaB activation, the IkappaBalpha super-suppressor (IkappaBalphaAA) antagonized Adriamycin-induced cell death. Additionally, the induction of death receptor 5 (DR5), which contributes to Adriamycin-induced cytotoxicity, was not affected by NF-kappaB blockage. Thus, our results suggest that Adriamycin-induced NF-kappaB is a transcriptional activator that protects lung cancer cells against apoptosis, and IKKbeta- or RelA siRNA rather than IkappaBalphaAA is an appropriate NF-kappaB blocking approach for sensitizing lung cancer cells to Adriamycin-induced cytotoxicity.

The R246S hot-spot p53 mutant exerts dominant-negative effects in embryonic stem cells in vitro and in vivo

p53 is the most frequently mutated tumour-suppressor gene in human cancers. Mutant p53 is thought to contribute to carcinogenesis by the acquisition of gain-of-function properties or through the exertion of dominant-negative (DN) effects over the remaining wild-type protein. However, the context in which the DN effects are observed is not well understood. We have therefore generated `knock-in' mouse embryonic stem (ES) cells to investigate the effects of expressing a commonly found hot-spot p53 mutant, R246S – the mouse equivalent of human R249S, which is associated with hepatocellular carcinomas. We demonstrate here that R246S mutant p53 exhibits DN effects with respect to target gene expression, cell survival and cell cycle arrest both in cells that are in the undifferentiated state and upon differentiation. The knock-in cells contain higher levels of p53 that localizes to the nucleus even in the absence of genotoxic stress and yet remains non-functional, reminiscent of mutant p53 found in human tumours. In a model based on carbon-tetrachloride-induced liver injury, these cells were consistently highly tumorigenic in vivo, similar to p53–/– cells and in contrast to both p53+/+ and p53+/– ES cells. These data therefore indicate that the DN effects of mutant p53 are evident in the stem-cell context, in which its expression is relatively high compared with terminally differentiated cells.

A role for the deubiquitinating enzyme USP28 in control of the DNA-damage response

The Chk2-p53-PUMA pathway is a major regulator of DNA-damage-induced apoptosis in response to double-strand breaks in vivo. Through analysis of 53BP1 complexes we have discovered a new ubiquitin protease, USP28, which regulates this pathway. Using a human cell line that faithfully recapitulated the Chk2-p53-PUMA pathway, we show that USP28 is required to stabilize Chk2 and 53BP1 in response to DNA damage. In this cell line, both USP28 and Chk2 are required for DNA-damage-induced apoptosis, and they accomplish this in part through regulation of the p53 induction of proapoptotic genes like PUMA. Our studies implicate DNA-damage-induced ubiquitination and deubiquitination as a major regulator of the DNA-damage response for Chk2, 53BP1, and a number of other proteins in the DNA-damage checkpoint pathway, including several mediators, such as Mdc1, Claspin, and TopBP1.

Inactive full-length p53 mutants lacking dominant wild-type p53 inhibition highlight loss of heterozygosity as an important aspect of p53 status in human cancers

Over 1000 different mutants of the tumor suppressor protein p53 with one amino acid change in the core domain have been reported in human cancers. In mouse knock-in models, two frequent mutants displayed loss of wild-type (wt) p53 function, inhibition of wt p53 and wt p53-independent gain of function. The remaining mutants have been systematically characterized for loss of wt p53 function, but not other phenotypes. We report the concomitant assessment of loss of function and interference with wt p53 using URA3-based p53 yeast and confirmatory mammalian assays. We studied 76 mutants representing 54% of over 15 000 reported missense core domain mutations. The majority showed the expected complete loss of wt p53 function and dominant p53 inhibition. A few infrequent p53 mutants had wt p53-like activity. Remarkably, one-third showed no interference with wt p53 despite loss of wt p53 function at 37 degrees C. Half of this group consisted of temperature-sensitive p53 mutants, but the other half was surprisingly made up of mutants with complete loss of wt p53 function. Our findings illustrate the diverse behavior of p53 mutants and mechanisms of malignant transformation by p53 mutants. The identification of full-length p53 mutants without dominant inhibition of wt p53 highlights the importance of determining the status of the wt p53 allele in human cancers, in particular in the context of clinical studies. In the case of p53 mutants with no or weak dominant p53 inhibition, presence of the wt allele may indicate a good prognosis cancer, whereas loss of heterozygosity may spell an aggressive, therapy-resistant cancer.

Up-regulation of NFκB-responsive gene expression by ΔNp73α in p53 null cells

Transactivation domain (TAD)-truncated p73, DeltaNp73, associates with p53, resulting in suppression of p53's functions. Using p53 null cell lines, we examined whether or not DeltaNp73 can regulate gene expression in a p53-independent manner. When DeltaNp73alpha was co-transfected with a luciferase reporter plasmid with various enhancer elements, NFkappaB-responsive luciferase gene expression was selectively up-regulated by DeltaNp73alpha, but not by other p73-isoforms with TAD and DeltaNp73beta. Deletion of the TAD endowed p73alpha with the ability to enhance the responsive gene's expression, but deletion of the N-terminal proline-rich domain (PRD) rendered the TAD-deleted p73alpha inactive. Considering the inability of DeltaNp73beta, which is the C-terminus-truncated form of DeltaNp73alpha, to function, these results indicate that both the PRD and C-terminus are necessary for DeltaNp73alpha to can activate NFkappaB-responsive luciferase expression. Over-expression of p53 suppressed the TAD-truncated p73alpha-mediated luciferase expression, suggesting that p53 interferes with the TAD-truncated p73alpha-mediated activation of NFkappaB. Inhibitors for NFkappaB activation reduced the TAD-truncated p73alpha-dependent NFkappaB-responsive gene expression, indicating that TAD-truncated p73alpha activates NFkappaB as does TNFalpha. In addition to the results obtained in the reporter gene assay, TAD-truncated p73alpha stimulated the translocation of NFkappaB to the nucleus and the expression of an endogenous NFkappaB-responsive gene, Bcl-XL. Taken together, these results demonstrate that TAD-truncated p73alpha can activate NFkappaB.

Predictive value of p53 and 14-3-3sigma for the effect of chemoradiation therapy on esophageal squamous cell carcinoma

BACKGROUND AND OBJECTIVES

The p53 family regulates cell-cycle arrest, triggers apoptosis, repairs DNA damage caused by various genotoxic stresses, and protects cells from death upon irradiation. The purpose of the present study was to examine the expressions of p53 and one of the p53 family proteins, 14-3-3sigma, in biopsy specimens and to predict the clinical and histological responses to chemoradiation therapy (CRT) in patients with esophageal squamous cell carcinoma (ESCC).

METHODS

We investigated with the relationship between p53 and 14-3-3sigma expressions in biopsy specimens obtained from 62 patients with ESCC and analyzed these patients' clinical and histological responses to CRT. Chemoradiation therapy consisted of 5-fluorouracil plus cisplatin and 40 Gy of radiation.

RESULTS

Following CRT, 71.0% of patients showed a positive clinical response and 52.8% showed a positive histological response. The rate of positive expression was 43.5% for p53 and 58.1% for 14-3-3sigma. Statistically significant correlations were found between p53 expression and clinical response to CRT (P = 0.001) and histological response to CRT (P = 0.041), and between 14-3-3sigma expression and histological response to CRT (P = 0.01). Furthermore, in p53-positive tumors, CRT was more effective in tumors with 14-3-3sigma-positive expressions than those with 14-3-3sigma-negative expressions (P = 0.037). The survival rate of the patients with 14-3-3sigma-positive tumors was better than those with 14-3-3sigma-negative tumors in patients with p53-positive tumors (P = 0.047).

CONCLUSIONS

We demonstrated that p53-negative or 14-3-3sigma-positive expressions were closely related to the response to CRT. It is clinically useful to examine the expression of these genes in biopsy specimens for predicting the CRT outcomes in patients with ESCC.

A Novel Human p53 Isoform Is an Essential Element of the ATR-Intra-S Phase Checkpoint

The archetypal human tumor suppressor p53 is considered to have unique transactivation properties. The assumption is based on the fact that additionally identified human p53 isoforms lack transcriptional activity. However, we provide evidence for the existence of an alternatively spliced p53 isoform (Deltap53) that exerts its transcriptional activity independent from p53. In contrast to p53, Deltap53 transactivates the endogenous p21 and 14-3-3sigma but not the mdm2, bax, and PIG3 promoter. Cell cycle studies showed that Deltap53 displays its differential transcriptional activity only in damaged S phase cells. Upon activation of the ATR-intra-S phase checkpoint, Deltap53, but not p53, transactivates the Cdk inhibitor p21. Induction of p21 results in downregulation of cyclin A-Cdk activity and accordingly attenuation of S phase progression. Data demonstrate that the Deltap53-p21-cyclin A-Cdk pathway is crucial to facilitate uncoupling of repair and replication events, indicating that Deltap53 is an essential element of the ATR-intra-S phase checkpoint.

Mutant p53 gain of function in two mouse models of Li-Fraumeni syndrome

The p53 tumor suppressor gene is commonly altered in human tumors, predominantly through missense mutations that result in accumulation of mutant p53 protein. These mutations may confer dominant-negative or gain-of-function properties to p53. To ascertain the physiological effects of p53 point mutation, the structural mutant p53R172H and the contact mutant p53R270H (codons 175 and 273 in humans) were engineered into the endogenous p53 locus in mice. p53R270H/+ and p53R172H/+ mice are models of Li-Fraumeni Syndrome; they developed allele-specific tumor spectra distinct from p53+/- mice. In addition, p53R270H/- and p53R172H/- mice developed novel tumors compared to p53-/- mice, including a variety of carcinomas and more frequent endothelial tumors. Dominant effects that varied by allele and function were observed in primary cells derived from p53R270H/+ and p53R172H/+ mice. These results demonstrate that point mutant p53 alleles expressed under physiological control have enhanced oncogenic potential beyond the simple loss of p53 function.

Caspase-1 activator Ipaf is a p53-inducible gene involved in apoptosis

The tumor suppressor protein p53 regulates transcription of many genes that mediate cell cycle arrest, apoptosis, DNA repair and other cellular responses. Here we show that Ipaf, a human CED-4 homologue and an activator of caspase-1, is induced by p53. Overexpression of p53 by transfection in U2OS and A549 cells increased Ipaf mRNA levels. Treatment of p53-positive cell lines U2OS and MCF-7 with the DNA damaging drug, doxorubicin, which increases p53 protein level, induced expression of Ipaf mRNA but similar treatment of MCF-7-mp53 (a clone of MCF-7 cells expressing mutant p53) and p53-negative K562 cells showed much less induction of Ipaf gene expression. Expression analysis for Ipaf mRNA in doxorubicin-treated human tumor cell lines suggests that p53-dependent as well as p53-independent mechanisms are involved in the regulation of Ipaf gene expression in a cell-type-specific manner. The Ipaf promoter was activated by normal p53 but not by His(273) mutant of p53. A functional p53-binding site was identified in the Ipaf promoter. A dominant-negative mutant of Ipaf inhibited p53-induced and doxorubicin-induced apoptosis by about 50%. Ipaf-directed small hairpin RNA downregulated p53-induced Ipaf gene expression and also reduced p53-induced apoptosis. Doxorubicin-induced apoptosis was also inhibited by Ipaf-directed small hairpin RNA. Our results show that p53 can directly induce Ipaf gene transcription, which contributes to p53-dependent apoptosis in at least some human cells.

Effects of febrile temperature on adenoviral infection and replication: implications for viral therapy of cancer

We previously conducted a phase I/II study using arterial infusions of ONYX-015 (dl1520), a replication-selective adenoviral vector, with E1b deleted, for patients with metastatic colorectal cancer. No dose-limiting toxicities occurred, but >90% of the patients experienced fever. The effects of temperature on the replication of dl1520 in normal and transformed cells had not been studied. Therefore, replication and cell viability assays were performed with a panel of nontransformed and transformed cell lines cultured at 37 and 39.5 degrees C and treated with adenovirus type 5 (Ad5) or dl1520. Ad5-mediated cytolytic effects were inhibited and production of infectious particles decreased by >1,000-fold in the nontransformed cells at 39.5 degrees C. Seven of nine of the tumor cell lines retained significant cell-killing effects when treated with Ad5 at 39.5 degrees C. When dl1520 was used, no cytolytic effects were observed at 39.5 degrees C in the nontransformed cell lines; however, cytolytic effects occurred in six of nine tumor cell lines at 39.5 degrees C. Notably, a subset of the tumor cell lines demonstrated increased dl1520-mediated cytolytic effect and replication at 39.5 degrees C. Suppression of Ad5 and dl1520 replication at 39.5 degrees C was not related to p53 status or HSP70 expression. Also, at 39.5 degrees C, E1a expression was inhibited in nontransformed cells but was still abundant in the transformed cells, indicating that a novel early block in viral replication occurred in the nontransformed cells. Fever may therefore augment the therapeutic index of oncolytic viruses by inhibiting replication in normal cells while permitting or enhancing viral replication in some tumor cells.

Bnip3L is induced by p53 under hypoxia, and its knockdown promotes tumor growth

p53-dependent apoptosis is a major determinant of its tumor suppressor activity and can be triggered by hypoxia. No p53 target is known to be induced by p53 or to mediate p53-dependent apoptosis during hypoxia. We report that p53 can directly upregulate expression of Bnip3L, a cell death inducer. During hypoxia, Bnip3L is highly induced in wild-type p53-expressing cells, in part due to increased recruitment of p53 and CBP to Bnip3L. Apoptosis is reduced in hypoxia-exposed cells with functional p53 following Bnip3L knockdown. In vivo, Bnip3L knockdown promotes tumorigenicity of wild-type versus mutant p53-expressing tumors. Thus, Bnip3L, capable of attenuating tumorigenicity, mediates p53-dependent apoptosis under hypoxia, which provides a novel understanding of p53 in tumor suppression.

How many mutant p53 molecules are needed to inactivate a tetramer?

The tumor suppressor p53 is transcription factor composed of four identical subunits. The majority of the mutations in p53 are missense mutations that impair DNA binding. On the other hand, the p53-related p63 and p73 genes are rarely mutated, but many cell types express natural variants lacking the N-terminal transactivation domain (NDelta). Compelling evidence indicates that both the DNA binding-defective and NDelta mutants can impair the function of wild-type p53 in a dominant-negative manner. Interestingly, it is uncertain how many mutant subunit(s) a p53 tetramer can tolerate. In this study, we first made theoretical predictions based on the number of mutant p53 monomers needed to inactivate a tetramer and then tested how well the experimental data fit the predicted values. Surprisingly, these experiments reveal that DNA binding-defective p53 mutants (R249S and R273H) are very ineffective in impairing the transcriptional activity of p53: at least three mutants are required to inactivate a tetramer. In marked contrast, p53NDelta is a very potent inhibitor of p53: one NDelta subunit per tetramer is sufficient to abolish the transcriptional activity. DNA binding is not necessary for the NDelta proteins to inactivate p53. Similarly, NDelta variants of p63 and p73 are also powerful inhibitors of members of the p53 family. These results have important implications for our thinking about the mechanism of tumorigenesis involving missense p53 mutants or the N-terminally truncated isoforms.

Mutant p53 exerts a dominant negative effect by preventing wild-type p53 from binding to the promoter of its target genes

Mutation of the p53 tumor suppressor gene is the most common genetic alteration in human cancer. A majority of these mutations are missense mutations in the DNA-binding domain. As a result, the mutated p53 gene encodes a full-length protein incapable of transactivating its target genes. In addition to this loss of function, mutant p53 can have a dominant negative effect over wild-type p53 and/or gain of function activity independently of the wild-type protein. To better understand the nature of the tumorigenic activity of mutant p53, we have investigated the mechanism by which mutant p53 can exert a dominant negative effect. We have established several stable cell lines capable of inducibly expressing a p53 mutant alone, wild-type p53 alone, or both proteins concurrently. In this context, we have used chromatin immunoprecipitation to determine the ability of wild-type p53 to bind to its endogenous target genes in the presence of various p53 mutants. We have found that p53 missense mutants markedly reduce the binding of wild-type p53 to the p53 responsive element in the target genes of p21, MDM2, and PIG3. These findings correlate with the reduced ability of wild-type p53 in inducing these and other endogenous target genes and growth suppression in the presence of mutant p53. We also showed that mutant p53 suppresses the ability of wild-type p53 in inducing cell cycle arrest. This highlights the sensitivity and utility of the dual inducible expression system because in previous studies, p53-mediated cell cycle arrest is not affected by transiently overexpressed p53 mutants. Together, our data showed that mutant p53 exerts its dominant negative activity by abrogating the DNA binding, and subsequently the growth suppression, functions of wild-type p53.

Control of apoptosis by p53

The p53 tumor suppressor acts to integrate multiple stress signals into a series of diverse antiproliferative responses. One of the most important p53 functions is its ability to activate apoptosis, and disruption of this process can promote tumor progression and chemoresistance. p53 apparently promotes apoptosis through transcription-dependent and -independent mechanisms that act in concert to ensure that the cell death program proceeds efficiently. Moreover, the apoptotic activity of p53 is tightly controlled, and is influenced by a series of quantitative and qualitative events that influence the outcome of p53 activation. Interestingly, other p53 family members can also promote apoptosis, either in parallel or in concert with p53. Although incomplete, our current understanding of p53 illustrates how apoptosis can be integrated into a larger tumor suppressor network controlled by different signals, environmental factors, and cell type. Understanding this network in more detail will provide insights into cancer and other diseases, and will identify strategies to improve their therapeutic treatment.

Characterization of p53-mediated up-regulation of CD95 gene expression upon genotoxic treatment in human breast tumor cells

Death receptor CD95 gene expression is frequently low in human breast tumors and is up-regulated by genotoxic treatments in a p53-dependent manner. We have evaluated the relative contribution of promoter and intronic p53 consensus sites to the regulation of the human CD95 gene in breast tumor cells following doxorubicin treatment. Deletion constructs of the promoter region and site-directed mutagenesis of p53 consensus sites in a fragment spanning 1448 bp of the 5'-promoter demonstrate that these sites are not involved in the observed up-regulation of the CD95 gene upon doxorubicin treatment. In contrast, a p53 consensus site located within the first intron of CD95 gene is absolutely required for the inducible expression of CD95 upon genotoxic treatment in breast tumor cells. Analysis of the transcriptional activity of the two most common p53 mutants found in human breast tumors that are associated with resistance to doxorubicin reveals that these mutations completely eliminate the ability of p53 protein to transactivate CD95 gene expression. On the other hand, Bcl-2 overexpression albeit preventing doxorubicin-induced apoptosis, has no effect on p53-mediated CD95 up-regulation in breast tumor cells. Altogether, these results indicate the lack of involvement of p53 consensus sites of the CD95 promoter region and the pivotal role of intronic p53-responsive element in the regulation of human CD95 gene expression in breast tumor cells. Our results also suggest that in breast cancer patients with certain mutations in the p53 gene, expression of death receptor CD95 in response to genotoxic treatments could be severely compromised.

ARF tumor suppressor induces mitochondria-dependent apoptosis by modulation of mitochondrial Bcl-2 family proteins

A tumor suppressor gene product, ARF, sensitizes cells to apoptosis in the presence of appropriate collateral signals. In this study, we analyzed the mechanism of ARF-dependent apoptosis and demonstrated that ARF induces mitochondria-dependent apoptosis in p53 wild-type, ARF/p16-null cells. We also found that ARF evokes cytochrome c release from mitochondria, decreases mitochondrial membrane potential, and activates pro-caspase-9 to induce apoptosis. Our findings suggest that this apoptotic cellular modulation is brought about by up-regulation of the proapoptotic Bcl-2 family proteins Bax and Bim and down-regulation of antiapoptotic Bcl-2 in mitochondrial fractions. Additionally, ARF seems to down-regulate Bcl-2 in a p53-dependent manner while up-regulating Bax/Bim via a p53-independent pathway.

Structure, function, and aggregation of the zinc-free form of the p53 DNA binding domain

The p53 DNA binding domain (DBD) contains a single bound zinc ion that is essential for activity. Zinc remains bound to wild-type DBD at temperatures below 30 degrees C; however, it rapidly dissociates at physiological temperature. The resulting zinc-free protein (apoDBD) is folded and stable. NMR spectra reveal that the DNA binding surface is altered in the absence of Zn(2+). Fluorescence anisotropy studies show that Zn(2+) removal abolishes site-specific DNA binding activity, although full nonspecific DNA binding affinity is retained. Surprisingly, the majority of tumorigenic mutations that destabilize DBD do not appreciably destabilize apoDBD. The R175H mutation instead substantially accelerates the rate of Zn(2+) loss. A considerable fraction of cellular p53 may therefore exist in the folded zinc-free form, especially when tumorigenic mutations are present. ApoDBD appears to promote aggregation of zinc-bound DBD via a nucleation-growth process. These data provide an explanation for the dominant negative phenotype exhibited by many mutations. Through a combination of induced p53 aggregation and diminished site-specific DNA binding activity, Zn(2+) loss may represent a significant inactivation pathway for p53 in the cell.

TP53 mutations in familial breast cancer: functional aspects

Mutation in p53 (TP53) remains one of the most commonly described genetic events in human neoplasia. The occurrence of mutations is somewhat less common in sporadic breast carcinomas than in other cancers, with an overall frequency of about 20%. There is, however, evidence that p53 is mutated at a significantly higher frequency in breast carcinomas arising in carriers of germ-line BRCA1 and BRCA2 mutations. Some of the p53 mutants identified in BRCA1 and BRCA2 mutation carriers are either previously undescribed or infrequently reported in sporadic human cancers. Functional characterization of such mutants in various systems has revealed that they frequently possess properties not commonly associated with those occurring in sporadic cases: they retain apoptosis-inducing, transactivating, and growth-inhibitory activities similar to the wild-type protein, yet are compromised for transformation suppression and also possess an independent transforming phenotype. The occurrence of such mutants in familial breast cancer implies the operation of distinct selective pressures during tumorigenesis in BRCA-associated breast cancers.

A nuclear protein tyrosine phosphatase activates p53 and induces caspase-1-dependent apoptosis

PTP-S2/TC45 is a nuclear protein tyrosine phosphatase, which induces p53-dependent apoptosis. Here we show that the p53 protein level increased in MCF-7 cells in response to PTP-S2 overexpression. PTP-S2-induced p53 protein was transcriptionally active and it could activate caspase-1 gene expression from endogenous as well as ectopic promoter. Coexpression of an active site mutant of procaspase-1 strongly inhibited PTP-S2-induced apoptosis. Mutant procaspase-1 also inhibited apoptosis induced by p53 overexpression or doxorubicin treatment, which induce caspase-1 gene expression. In contrast, apoptosis induced by staurosporine or cycloheximide, which do not increase caspase-1 gene expression, was not affected by mutant procaspase-1. These results suggest that caspase-1 may be one of the mediators of p53-dependent apoptosis in human cells.

A novel p53 mutational hotspot in skin tumors from UV-irradiated Xpc mutant mice alters transactivation functions

A mutation in codon 122 of the mouse p53 gene resulting in a T to L amino acid substitution (T122-->L) is frequently associated with skin cancer in UV-irradiated mice that are both homozygous mutant for the nucleotide excision repair (NER) gene Xpc (Xpc(-/-)) and hemizygous mutant for the p53 gene. We investigated the functional consequences of the mouse T122-->L mutation when expressed either in mammalian cells or in the yeast Saccharomyces cerevisiae. Similar to a non-functional allele, high expression of the T122-->L allele in p53(-/-) mouse embryo fibroblasts and human Saos-2 cells failed to suppress growth. However, the T122-->L mutant p53 showed wild-type transactivation levels with Bax and MDM2 promoters when expressed in either cell type and retained transactivation of the p21 and the c-Fos promoters in one cell line. Using a recently developed rheostatable p53 induction system in yeast we assessed the T122-->L transactivation capacity at low levels of protein expression using 12 different p53 response elements (REs). Compared to wild-type p53 the T122-->L protein manifested an unusual transactivation pattern comprising reduced and enhanced activity with specific REs. The high incidence of the T122-->L mutant allele in the Xpc(-/-) background suggests that both genetic and epigenetic conditions may facilitate the emergence of particular functional p53 mutations. Furthermore, the approach that we have taken also provides for the dissection of functions that may be retained in many p53 tumor alleles.

Single nucleotide polymorphism seeking long term association with complex disease

Successful investigation of common diseases requires advances in our understanding of the organization of the genome. Linkage disequilibrium provides a theoretical basis for performing candidate gene or whole-genome association studies to analyze complex disease. However, to constructively interrogate SNPs for these studies, technologies with sufficient throughput and sensitivity are required. A plethora of suitable and reliable methods have been developed, each of which has its own unique advantage. The characteristics of the most promising genotyping and polymorphism scanning technologies are presented. These technologies are examined both in the context of complex disease investigation and in their capacity to face the unique physical and molecular challenges (allele amplification, loss of heterozygosity and stromal contamination) of solid tumor research.

Restoration of p53 tumor suppressor pathway in human cervical carcinoma cells by sodium arsenite

In most cervical cancer cells, p53 and Rb are disrupted by human papillomaviruses (HPVs) E6 and E7, respectively. Restoration of p53 or Rb function by blocking E6/p53 or E7/Rb pathway might be a potential therapeutic purpose for these cancer cells. Treatment with sodium arsenite (SA) resulted in significant repression of E6 and E7 mRNA levels in SiHa cells. After E6 and E7 repression, p53 was dramatically induced and accumulated in cellular nuclei and Rb was also induced. Two p53-responsive genes, p21(waf1/cip1) and mdm2, were induced after SA treatment. Furthermore, SA also reduced the expressions of Cdc25A and cyclin B, blocked cell cycle progression at G2/M phase, and induced apoptosis in SiHa cells. SA-induced apoptosis was greatly reduced by expression of a dominant-negative mutated p53. In this study, we have first demonstrated that SA did repress E6 and E7 oncogenes, restore the p53 tumor suppressor pathway and induce apoptosis in SiHa cells. Therefore, it would be a potential strategy to promote SA as therapeutic purpose for HPV-positive cancer cells.

Biogenesis of p53 involves cotranslational dimerization of monomers and posttranslational dimerization of dimers. Implications on the dominant negative effect

Precisely how mutant p53 exerts a dominant negative effect over wild type p53 has been an enigma. To understand how wild type and mutant p53 form hetero-oligomers, we studied p53 biogenesis in vitro. We show here that p53 dimers are formed cotranslationally (on the polysome), whereas tetramers are formed posttranslationally (by the dimerization of dimers in solution). Coexpression of wild type and mutant p53 therefore results in 50% of the p53 generated being heterotetramers comprised of a single species: wild type dimer/mutant dimer. Using hot spot mutants of p53 and a variety of natural target sites, we show that all wild type/mutant heterotetramers manifest impaired DNA binding activity. This impairment is not due to the mutant dimeric subunit inhibiting association of the complex with DNA but rather due to the lack of significant contribution (positive cooperativity) from the mutant partner. For all heterotetramers, bias in binding is particularly pronounced against those sequences in genes responsible for apoptosis rather than cell growth arrest. These results explain the molecular basis of p53 dominant negative effect and suggest a functional role in the regulation of p53 tetramerization.

Dissecting p53 tumor suppressor functions in vivo

Although the p53 tumor suppressor acts in a plethora of processes that influence cellular proliferation and survival, it remains unclear which p53 functions are essential for tumor suppression and, as a consequence, are selected against during tumor development. Using a mouse model harboring primary, genetically modified myc-driven lymphomas, we show that disruption of apoptosis downstream of p53 by Bcl2 or a dominant-negative caspase 9 confers-like p53 loss-a selective advantage, and completely alleviates pressure to inactivate p53 during lymphomagenesis. Despite their p53-null-like aggressive phenotype, apoptosis-defective lymphomas that retain intact p53 genes do not display the checkpoint defects and gross aneuploidy that are characteristic of p53 mutant tumors. Therefore, apoptosis is the only p53 function selected against during lymphoma development, whereas defective cell-cycle checkpoints and aneuploidy are mere byproducts of p53 loss.

An endonuclease/ligase based mutation scanning method especially suited for analysis of neoplastic tissue

Knowledge of inherited and sporadic mutations in known and candidate cancer genes may influence clinical decisions. We have developed a mutation scanning method that combines thermostable EndonucleaseV (Endo V) and DNA ligase. Variant and wild-type PCR amplicons are generated using fluorescently labeled primers, and heteroduplexed. Thermotoga maritima (Tma) EndoV recognizes and primarily cleaves heteroduplex DNA one base 3' to the mismatch, as well as nicking matched DNA at low levels. Thermus species (Tsp.) AK16D DNA ligase reseals the background nicks to create a highly sensitive and specific assay. The fragment mobility on a DNA sequencing gel reveals the approximate position of the mutation. This method identified 31/35 and 8/8 unique point mutations and insertions/deletions, respectively, in the p53, VHL, K-ras, APC, BRCA1, and BRCA2 genes. The method has the sensitivity to detect K-ras mutations diluted 1 : 20 with wild-type DNA, a p53 mutation in a 1.7 kb amplicon, and unknown p53 mutations in pooled DNA samples. EndoV/Ligase mutation scanning combined with PCR/LDR/Universal array proved superior to automated DNA sequencing for detecting p53 mutations in colon tumors. This technique is well suited for scanning low-frequency mutations in pooled samples and for analysing tumor DNA containing a minority of the unknown mutation.

Tumour p53 mutations exhibit promoter selective dominance over wild type p53

The tumour suppressor gene p53 is frequently mutated in human cancer. Tumour derived p53 mutants are usually transcriptionally inactive, but some mutants retain the ability to transactivate a subset of p53 target genes. In addition to simple loss of function, some p53 mutants may be carcinogenic through a dominant negative mechanism. Aiming at a more general classification of p53 mutants into predictive functional categories it is important to determine (i) which p53 mutants are dominant, (ii) what features characterize dominant mutants and (iii) whether dominance is target gene specific. The ability of 71 p53 mutants to inhibit wild type p53 was determined using a simple yeast transcriptional assay. Approximately 30% of the mutants were dominant. They preferentially affect highly conserved amino acids (P<0.005), which are frequently mutated in tumours (P<0.005), and usually located near the DNA binding surface of the protein (P<0.001). Different tumour-derived amino acid substitutions at the same codon usually have the same dominance phenotype. To determine whether the ability of p53 mutants to inhibit wild type p53 is target gene specific, the dominance towards p21, bax, and PIG3 binding sites was examined. Approximately 40% of the 45 mutants examined were dominant for the p21 (17/45) or PIG3 (20/45) responsive elements and 71% (32/45) were dominant for the bax responsive element. These differences are statistically significant (p21 vs bax, P<0.003; bax vs PIG3, P<0.02, Fisher's exact test) and defined a hierarchy of dominance. Finally, we extended the analysis to a group of mutants isolated in BRCA-associated tumours, some of which retained wild type level of transcription in yeast as well as in human cells, but show gain of function in transformation assays. Since transformation assays require transdominant inhibition of the endogenous wild type allele, one possible explanation for the behaviour of the BRCA-associated mutants is that they adopt conformations able to bind DNA alone but not in mixed tetramers with wild type p53. The yeast data do not support this explanation, because all BRCA-associated mutants that behaved as wild type in transcription assay were recessive in dominance assays.

Selective cytotoxicity of intracellular amyloid beta peptide1-42 through p53 and Bax in cultured primary human neurons

Extracellular amyloid beta peptides (Abetas) have long been thought to be a primary cause of Alzheimer's disease (AD). Now, detection of intracellular neuronal Abeta1--42 accumulation before extracellular Abeta deposits questions the relevance of intracellular peptides in AD. In the present study, we directly address whether intracellular Abeta is toxic to human neurons. Microinjections of Abeta1--42 peptide or a cDNA-expressing cytosolic Abeta1--42 rapidly induces cell death of primary human neurons. In contrast, Abeta1--40, Abeta40--1, or Abeta42--1 peptides, and cDNAs expressing cytosolic Abeta1--40 or secreted Abeta1--42 and Abeta1--40, are not toxic. As little as a 1-pM concentration or 1500 molecules/cell of Abeta1--42 peptides is neurotoxic. The nonfibrillized and fibrillized Abeta1--42 peptides are equally toxic. In contrast, Abeta1--42 peptides are not toxic to human primary astrocytes, neuronal, and nonneuronal cell lines. Inhibition of de novo protein synthesis protects against Abeta1--42 toxicity, indicating that programmed cell death is involved. Bcl-2, Bax-neutralizing antibodies, cDNA expression of a p53R273H dominant negative mutant, and caspase inhibitors prevent Abeta1--42-mediated human neuronal cell death. Taken together, our data directly demonstrate that intracellular Abeta1--42 is selectively cytotoxic to human neurons through the p53--Bax cell death pathway.