Aflatoxin B1-induced TP53 mutational pattern in normal human cells using the FASAY (Functional Analysis of Separated Alleles in Yeast)

Mutations in the TP53 gene are the most common alterations in human tumours. In hepatocellular carcinoma (HCC) related to exposure to aflatoxin B1, a specific G>T transversion in codon 249 is classically described as a hot spot. However, AFB1 is suspected to be a potent carcinogen in tissues other than the liver. By using the FASAY functional assay in yeast, the present study aimed at depicting the mutational pattern of TP53 in normal human fibroblasts after in vitro exposure to AFB1. Molecular analysis of mutants revealed that codon 245 was the main hot spot, whereas no mutations were found in codon 249. The locations of mutations within GG and GC/CG sequences are well in accordance with AFB1-adduct location data. In our assay with normal human fibroblasts, AFB1 mainly induced G>A transitions, followed by G>T and A>G mutations. This suggests that G>T transversions at codon 249 were likely the result of a selection bias in human HCC rather than a true fingerprint of AFB1 adducts. Indeed, a comparison of the mutation pattern with that found in human HCC excluding codon 249 reveals that the two spectra are quite similar. Furthermore, the similarity between our in vitro spectrum with that identified in AFB1-induced lung tumours in mice suggests that AFB1 may be a potent lung carcinogen in humans.     

Mutations in the p53 tumor suppressor gene in tree shrew hepatocellular carcinoma associated with hepatitis B virus infection and intake of aflatoxin B1

Infection with hepadnaviruses and exposure to aflatoxin B1 (AFB1) are considered to be major risk factors in the development of hepatocellular carcinoma (HCC) in humans. A high rate of p53 mutations at codon 249 has been reported in these tumors. The tree shrew (Tupaia belangeri chinensis) is a useful animal model for the development of HCC after human hepatitis B virus (HBV) infection or AFB1 treatment. Therefore, it was of particular interest to determine whether the p53 gene in tree shrew HCCs associated with HBV infection and/or with exposure to AFB1 is affected in the same manner as in human HCCs. We determined the tree shrew p53 wild-type nucleotide sequences by RT-PCR and automatic DNA-sequencing. Tree shrew wild-type p53 sequence showed 91.7 and 93.4% homologies with human p53 nucleotide and amino acids sequences, respectively, while it showed 77.2 and 73.7% homologies in mice. One HCC and normal liver tissue from AFB1 treated and one HCC from AFB1- and HBV-treated tree shrew showed no change in p53 sequences, while three HCCs from AFB1- and HBV-treated tree shrews showed point mutations in p53 sequences. One HCC showed point mutations at codon 275, which is on the DNA-binding domain of p53 gene, which might be a cause of gain-of-function during the development of HCC. As a result, our finding indicates that tree shrews exposed to AFB1 and/or HBV had neither codon 249 mutations nor significant levels of other mutations in the p53 gene, as is the case with humans.     

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

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

Hepatocellular carcinoma and liver cirrhosis TP53 mutation analysis reflects a moderate dietary exposure to aflatoxins in Espírito Santo State, Brazil

The close relationship between aflatoxins and 249^ser TP53 gene mutation (AGG to AGT, Arg to Ser) in hepatocellular carcinoma (HCC) makes this mutation an indirect indicator of dietary contamination with this toxin. We have examined the prevalence of codon 249 TP53 mutation in 41 HCC and 74 liver cirrhosis (without HCC) cases diagnosed at the HUCAM University Hospital in Vitoria, Espírito Santo State, Brazil. DNA was extracted from paraffin sections and from plasma. The mutation was detected by DNA amplification, followed by restriction endonuclease digestion and confirmed by direct sequencing. DNA restriction showed 249^ser mutation in 16 HCC and 13 liver cirrhosis, but sequencing confirmed mutations in only 6 HCC and 1 liver cirrhosis. In addition, sequencing revealed 4 patients with mutations at codon 250 (250^ser and 250^leu) in HCC cases. The prevalence of TP53 mutation was 10/41 (24.3 %) in HCC and 1/74 (1.4 %) in liver cirrhosis. No relationship between the presence of mutations and the etiology of HCC was observed. TP53 exon 7 mutations, which are related to aflatoxins exposure, were found at 14.6 % (249^ser), 7.3 % (250^leu) and 2.4 % (250^ser) in 41 cases of HCC and 1.4 % in 74 liver cirrhosis (without HCC) cases, suggesting a moderate dietary exposure to aflatoxins in the Espírito Santo State, Brazil.     

Retroviral expression of the hepatitis B virus x gene promotes liver cell susceptibility to carcinogen-induced site specific mutagenesis

Mutational inactivation of the tumor suppressor gene p53 is common in hepatocellular carcinomas (HCC). AGG to AGT transversion in codon 249 of exon 7 of the p53 gene occurs in over 50% of HCC from endemic regions, where both chronic infection with the hepatitis B virus (HBV) and exposure to carcinogens such as aflatoxin B1 (AFB1) prevail. In this study, we report the effect of the HBV x protein (HBx) on carcinogen-induced cytotoxicity and AGG to AGT mutation in codon 249 of the p53 gene in the human liver cell line CCL13. Expression of HBx, as revealed by its transactivation function, results in enhanced cell susceptibility to cytotoxicity induced by the AFB1 active metabolite, AFB1-8,9-epoxide, and benzo(a)pyrene diol-epoxide. Under similar conditions, expression of HBx promotes apoptosis in a subset of cell population. Exposure to AFB1-8, 9-epoxide alone induces a low frequency of AGG to AGT mutation in codon 249 of the p53 gene, as determined by an allele-specific polymerase chain reaction (AS-PCR) assay. However, expression of HBx enhances the frequency of AFB1-epoxide-induced AGG to AGT mutation compared to control cells. In summary, this study demonstrates that expression of HBx enhances liver cell susceptibility to carcinogen-induced mutagenesis, possibly through alteration of the balance between DNA repair and apoptosis, two cellular defense mechanisms against genotoxic stress.     

Structures of p53 cancer mutants and mechanism of rescue by second-site suppressor mutations

We have solved the crystal structures of three oncogenic mutants of the core domain of the human tumor suppressor p53. The mutations were introduced into a stabilized variant. The cancer hot spot mutation R273H simply removes an arginine involved in DNA binding without causing structural distortions in neighboring residues. In contrast, the "structural" oncogenic mutations H168R and R249S induce substantial structural perturbation around the mutation site in the L2 and L3 loops, respectively. H168R is a specific intragenic suppressor mutation for R249S. When both cancer mutations are combined in the same molecule, Arg(168) mimics the role of Arg(249) in wild type, and the wild type conformation is largely restored in both loops. Our structural and biophysical data provide compelling evidence for the mechanism of rescue of mutant p53 by intragenic suppressor mutations and reveal features by which proteins can adapt to deleterious mutations.     

Mutations in the Trp53 gene of UV-irradiated Xpc mutant mice suggest a novel Xpc-dependent DNA repair process

Mutational hot spots in the human p53 gene are well established in tumors in the human population and are frequently negative prognosticators of the clinical outcome. We previously developed a mouse model of skin cancer with mutations in the xeroderma pigmentosum group C gene (Xpc). UVB radiation-induced skin cancer is significantly enhanced in these mice when they also carry a mutation in one copy of the Trp53 gene (Xpc-/-Trp53+/-). Skin tumors in these mice often contain inactivating mutations in the remaining Trp53 allele and we have previously reported a novel mutational hot spot at a non-dipyrimidine site (ACG) in codon 122 of the Trp53 gene in the tumors. Here we show that this mutation is not a hot spot in Xpa or Csa mutant mice. Furthermore, the mutation in codon T122 can be identified in mouse skin DNA from (Xpc-/-Trp53+/-) mice as early as 2 weeks after exposure to UVB radiation, well before histological evidence of dysplastic or neoplastic changes. Since this mutational hot spot is not at a dipyrimidine site and is apparently Xpc-specific, we suggest that some form of non-dipyrimidine base damage is normally repaired in a manner that is distinct from conventional nucleotide excision repair, but that requires XPC protein.     

Detection of clinically relevant point mutations by a novel piezoelectric biosensor

Piezoelectric sensing is here applied to point mutation detection in human DNA. The mutation investigated is in the TP53 gene, which results inactivated in most cancer types. TP53 gene maps on chromosome 17 (17p13.1). It contains 11 exons and codifies for the relative protein, involved in cell proliferation. The TP53 gene has a wide mutation spectrum that is related to different tumours. In particular, those occurring in the structurally important L2 and L3 zinc-binding domains, have been linked to patient prognosis and more strongly to radiotherapy and chemotherapy resistance in several major cancers. For this reason, the identification of these mutations represents an important clinical target and biosensors could represent good candidate for fast mutation screening. In this paper, a DNA-based piezoelectric biosensor for the detection of the TP53 gene mutation at codon 248 is reported. A biotinylated probe was immobilised on the sensor surface via dextran-streptavidin modified surfaces. The sensor was optimised using synthetic oligonucleotides. Finally, the sensor system was successfully applied to polymerase chain reaction (PCR)-amplified real samples of DNA extracted from two cell lines, one normal (wild-type) and one mutated, carrying the mutation at codon 248 of the TP53 gene. The results obtained demonstrate that the DNA-based piezoelectric biosensor is able to detect the point mutations in PCR-amplified samples showing the potentialities of this approach for routine analysis.     

Mutational spectrum and genotoxicity of the major lipid peroxidation product,trans-4-hydroxy-2-nonenal,induced DNA adducts in nucleotide excision repair-proficient and -deficient human cells

trans-4-Hydroxy-2-nonenal (4-HNE), a major product of lipid peroxidation, is able to interact with DNA to form 6-(1-hydroxyhexanyl)-8-hydroxy-1,N(2)-propano-2'-deoxyguanosine (4-HNE-dG) adducts, but its genotoxicity and mutagenicity remain elusive. It has been reported that 4-HNE treatment in human cells induces a high frequency of G.C to T.A mutations at the third base of codon 249 (AGG*) of the p53 gene, a mutational hot spot in human cancers, particularly in hepatocellular carcinoma. This G.C to T.A transversion at codon 249, however, has been thought to be caused by etheno-DNA adducts induced by the endogenous metabolite of 4-HNE, 2,3-epoxy-4-hydroxynonanal. We have recently found that 4-HNE preferentially forms 4-HNE-dG adducts at the GAGG*C/A sequence in the p53 gene including codon 249 (GAGG*C). Our finding supports the possibility that G.C to T.A mutations at codon 249 may be induced by 4-HNE-dG adducts. To investigate this possibility, we determined the mutational spectrum induced by 4-HNE-dG adducts in the supF gene of shuttle vector pSP189 replicated in human cells. We have found that 4-HNE-dG adducts are mutagenic and genotoxic in human cells, and that G.C to T.A transversions are the most prevalent mutations induced by 4-HNE-dG adducts. Furthermore, 4-HNE-dG adducts induce a significantly higher level of genotoxicity and mutagenicity in nucleotide excision repair (NER)-deficient human and Escherichia coli cells than in NER-proficient cells, indicating that NER is a major pathway for repairing 4-HNE-dG adducts in both human and E. coli cells. Together, these results suggest that 4-HNE-dG adducts may contribute greatly to the G.C to T.A mutation at codon 249 of the p53 gene, and may play an important role in carcinogenesis.     

Restoration of mutant TP53 to normal TP53 function by glycerol as a chemical chaperone

We have previously reported that heat stress induces expression of wild-type TP53 (formerly known as p53) activated factor 1 (CDKN1A, formerly known as WAF1) only when TP53 protein is wild-type using cells of a human glioblastoma cell line (A-172) and cells of its transformant (A-172/mp53/ 143) with a mutant TP53 (point mutation at codon 143 from Val to Ala) vector. Transfection of A-172 cells with the mutant TP53 vector abolished the heat-induced expression of CDKN1A, demonstrating the dominant negative nature of this TP53 mutant over the endogenous wild-type TP53. This kind of dominant negative TP53 mutant occurs frequently in various types of cancer. Overcoming this dominance or restoring the normal functions to these TP53 mutants is a new strategy for TP53-targeted cancer therapies. We examined whether glycerol can act as a chemical chaperone to correct the mutant TP53 conformation. No CDKN1A expression was induced after heating or treatment with glycerol at concentrations of 0.6 and 1.2 M in these transformants. In contrast, A-172/mp53/ 143 cells showed CDKN1A expression when they were heated in the presence of glycerol at 0.6 or 1.2 M, which was similar to the response of the parental and neo vector-transfected control cells. To test the generality of the effects of glycerol on mutant TP53, we used human osteosarcoma Saos-2 cells (lacking TP53) transfected with mutant TP53 and cells of two other human glioblastoma cell lines carrying mutant TP53. These cells showed similar CDKN1A expression when heated in the presence of glycerol at 0.6 or 1.2 M. These results suggest that glycerol is effective in restoring several TP53 mutants to normal TP53 function, leading to normal CDKN1A expression after heat stress. This observation provides a novel tool for correction of mutant TP53 conformation and may be applicable for TP53-targeted cancer therapy.     

Structural distortion of p53 by the mutation R249S and its rescue by a designed peptide: implications for "mutant conformation"

Missense mutations in the DNA-binding core domain of the tumour suppressor protein p53 are frequent in cancer. Many of them result in loss of native structure. The mutation R249S is one of the six most common cancer-associated p53 mutations ("hot-spots"). As it is highly frequent in hepatocellular carcinoma, its rescue is an important therapeutic target. We have used NMR techniques to study the structural effects of the R249S mutation. The overall fold of the core domain is retained in R249S, and it does not take up a denatured "mutant conformation". However, the beta-sandwich had increased flexibility and, according to changes in chemical shift, there was local distortion throughout the DNA-binding interface. It is likely that the R249S mutation resulted in an ensemble of native and native-like conformations in a dynamic equilibrium. The peptide FL-CDB3 that was designed to rescue mutants of p53 by binding specifically to its native structure was found to revert the chemical shifts of R249S back towards the wild-type values and so reverse the structural effects of mutation. We discuss the implications for a rescue strategy and also for the analysis of antibody-binding data.     

Transforming activity of mutant human p53 alleles

Mutant forms of the p53 gene have been shown to cooperate with an activated ras gene in transforming primary cells in culture. The aberrant proteins encoded by p53 mutants are thought to act in a dominant negative manner in these assays. In vivo data, however, reveal that where p53 has undergone genetic change in tumors, both alleles have been affected. We previously identified a case of human acute myelogenous leukemia (AML) in which both alleles of the p53 gene had undergone independent missense mutations (at codons 135 cys to ser and 246 met to val). In these blasts, p53 mutations appear to be acting recessively. We have assayed the transforming potential of these p53 mutations, as well as that of another mutation at codon 273, also identified in a human neoplasm. Both mutations from the AML blasts (codon 135 and codon 246) confer transforming ability on the mutant protein. While transformation assays may define functionally different subsets of p53 mutations, the overexpression phenotype of mutants in this assay may not accurately reflect the pathological effects of p53 mutations in vivo.     

Benzo[a]pyrene, aflatoxine B₁ and acetaldehyde mutational patterns in TP53 gene using a functional assay: relevance to human cancer aetiology

Mutations in the TP53 gene are the most common alterations in human tumours. TP53 mutational patterns have sometimes been linked to carcinogen exposure. In hepatocellular carcinoma, a specific G>T transversion on codon 249 is classically described as a fingerprint of aflatoxin B(1) exposure. Likewise G>T transversions in codons 157 and 158 have been related to tobacco exposure in human lung cancers. However, controversies remain about the interpretation of TP53 mutational pattern in tumours as the fingerprint of genotoxin exposure. By using a functional assay, the Functional Analysis of Separated Alleles in Yeast (FASAY), the present study depicts the mutational pattern of TP53 in normal human fibroblasts after in vitro exposure to well-known carcinogens: benzo[a]pyrene, aflatoxin B(1) and acetaldehyde. These in vitro patterns of mutations were then compared to those found in human tumours by using the IARC database of TP53 mutations. The results show that the TP53 mutational patterns found in human tumours can be only partly ascribed to genotoxin exposure. A complex interplay between the functional impact of the mutations on p53 phenotype and the cancer natural history may affect these patterns. However, our results strongly support that genotoxins exposure plays a major role in the aetiology of the considered cancers.     

p53 mutants without a functional tetramerisation domain are not oncogenic

p53 is altered in about 50 % of cancers. Most of the p53 mutants have lost the wild-type tumour suppressor activity but show oncogenic properties. The majority of the p53 alterations are missense mutations of residues located in its DNA binding domain (DBD). Only a few mutations concern residues in its tetramerisation domain (TD). However, the study of mutant proteins identified in tumors that do not form tetramers has shown that they have lost the wild-type activity like most of the p53 DBD mutants. Here, we show that two of such mutant proteins, Arg342Pro and Leu344Pro are not dominant negative and do not stimulate the expression of a reporter gene under the control of the multi-drug resistance gene-1 (MDR-1). This suggests that to be oncogenic, p53 mutants need to form tetramers. Accordingly, the dominant negative effect and the ability of a tetrameric mutant protein, Asp281Gly, to stimulate the MDR-1 promoter are abolished when its TD is rendered non-functional by the mutation of leucine 344 to a proline residue. These results suggest that mutations in the TD, are less selected in tumors than mutations in the DBD because they do not lead to oncogenic proteins.     

TP53: an oncogene in disguise

The standard classification used to define the various cancer genes confines tumor protein p53 (TP53) to the role of a tumor suppressor gene. However, it is now an indisputable fact that many p53 mutants act as oncogenic proteins. This statement is based on multiple arguments including the mutation signature of the TP53 gene in human cancer, the various gains-of-function (GOFs) of the different p53 mutants and the heterogeneous phenotypes developed by knock-in mouse strains modeling several human TP53 mutations. In this review, we will shatter the classical and traditional image of tumor protein p53 (TP53) as a tumor suppressor gene by emphasizing its multiple oncogenic properties that make it a potential therapeutic target that should not be underestimated. Analysis of the data generated by the various cancer genome projects highlights the high frequency of TP53 mutations and reveals that several p53 hotspot mutants are the most common oncoprotein variants expressed in several types of tumors. The use of Muller''s classical definition of mutations based on quantitative and qualitative consequences on the protein product, such as ‘amorph'', ‘hypomorph'', ‘hypermorph'' ‘neomorph'' or ‘antimorph'', allows a more meaningful assessment of the consequences of cancer gene modifications, their potential clinical significance, and clearly demonstrates that the TP53 gene is an atypical cancer gene.     

Co-localization of mutant p53 and amyloid-like protein aggregates in breast tumors

P53 is one of the most important tumor suppressor proteins in human cancers. Mutations in the TP53 gene are common features of malignant tumors and normally correlate to a more aggressive disease. In breast cancer, these gene alterations are present in approximately 20% of cases and are characteristically of missense type. In the present work we describe TP53 mutations in breast cancer biopsies and investigate whether wild and mutant p53 participate in protein aggregates formation in these breast cancer cases. We analyzed 88 biopsies from patients residing in the metropolitan area of Rio de Janeiro, and performed TP53 mutation screening using direct sequencing of exons 5-10. Seventeen mutations were detected, 12 of them were of missense type, 2 nonsenses, 2 deletions and 1 insertion. The presence of TP53 mutation was highly statistically associated to tumor aggressiveness of IDC cases, indicated here by Elston Grade III (p<0.0001). Paraffin embedded breast cancer tissues were analyzed for the presence of p53 aggregates through immunofluorescence co-localization assay, using anti-aggregate primary antibody A11, and anti-p53. Our results show that mutant p53 co-localizes with amyloid-like protein aggregates, depending on mutation type, suggesting that mutant p53 may form aggregates in breast cancer cells, in vivo.     

A Novel p53 Mutant Found in Iatrogenic Urothelial Cancers Is Dysfunctional and Can Be Rescued by a Second-site Global Suppressor Mutation

Exposure to herbal remedies containing the carcinogen aristolochic acid (AA) has been widespread in some regions of the world. Rare A→T TP53 mutations were recently discovered in AA-associated urothelial cancers. The near absence of these mutations among all other sequenced human tumors suggests that they could be biologically silent. There are no cell banks with established lines derived from human tumors with which to explore the influence of the novel mutants on p53 function and cellular behavior. To investigate their impact, we generated isogenic mutant clones by integrase-mediated cassette exchange at the p53 locus of platform (null) murine embryonic fibroblasts and kidney epithelial cells. Common tumor mutants (R248W, R273C) were compared with the AA-associated mutants N131Y, R249W, and Q104L. Assays of cell proliferation, migration, growth in soft agar, apoptosis, senescence, and gene expression revealed contrasting outcomes on cellular behavior following introduction of N131Y or Q104L. The N131Y mutant demonstrated a phenotype akin to common tumor mutants, whereas Q104L clone behavior resembled that of cells with wild-type p53. Wild-type p53 responses were restored in double-mutant cells harboring N131Y and N239Y, a second-site rescue mutation, suggesting that pharmaceutical reactivation of p53 function in tumors expressing N131Y could have therapeutic benefit. N131Y is likely to contribute directly to tumor phenotype and is a promising candidate biomarker of AA exposure and disease. Rare mutations thus do not necessarily point to sites where amino acid exchanges are phenotypically neutral. Encounter with mutagenic insults targeting cryptic sites can reveal specific signature hotspots.