Change of conformation of the DNA-binding domain of p53 is the only key element for binding of and interference with p73

TP53: the unluckiest of genes?

The transcription factor p53 plays a key role in the cellular defense against cancer development. It is inactivated in virtually every tumor, and in every second tumor this inactivation is due to a mutation in the TP53 gene. In this perspective, we show that this diverse mutational spectrum is unique among all other cancer-associated proteins and discuss what drives the selection of TP53 mutations in cancer. We highlight that several factors conspire to make the p53 protein particularly vulnerable to inactivation by the mutations that constantly plague our genome. It appears that the TP53 gene has emerged as a victim of its own evolutionary past that shaped its structure and function towards a pluripotent tumor suppressor, but came with an increased structural fragility of its DNA-binding domain. TP53 loss of function - with associated dominant-negative effects - is the main mechanism that will impair TP53 tumor suppressive function, regardless of whether a neomorphic phenotype is associated with some of these variants.

Evolutionary history of the p53 family DNA-binding domain: insights from an Alvinella pompejana homolog

The extremophile Alvinella pompejana, an annelid worm living on the edge of hydrothermal vents in the Pacific Ocean, is an excellent model system for studying factors that govern protein stability. Low intrinsic stability is a crucial factor for the susceptibility of the transcription factor p53 to inactivating mutations in human cancer. Understanding its molecular basis may facilitate the design of novel therapeutic strategies targeting mutant p53. By analyzing expressed sequence tag (EST) data, we discovered a p53 family gene in A. pompejana. Protein crystallography and biophysical studies showed that it has a p53/p63-like DNA-binding domain (DBD) that is more thermostable than all vertebrate p53 DBDs tested so far, but not as stable as that of human p63. We also identified features associated with its increased thermostability. In addition, the A. pompejana homolog shares DNA-binding properties with human p53 family DBDs, despite its evolutionary distance, consistent with a potential role in maintaining genome integrity. Through extensive structural and phylogenetic analyses, we could further trace key evolutionary events that shaped the structure, stability, and function of the p53 family DBD over time, leading to a potent but vulnerable tumor suppressor in humans.

Transcriptional Regulation by Wild-Type and Cancer-Related Mutant Forms of p53

TP53 missense mutations produce a mutant p53 protein that cannot activate the p53 tumor suppressive transcriptional response, which is the primary selective pressure for TP53 mutation. Specific codons of TP53, termed hotspot mutants, are mutated at elevated frequency. Hotspot forms of mutant p53 possess oncogenic properties in addition to being deficient in tumor suppression. Such p53 mutants accumulate to high levels in the cells they inhabit, causing transcriptional alterations that produce pro-oncogenic activities, such as increased pro-growth signaling, invasiveness, and metastases. These forms of mutant p53 very likely use features of wild-type p53, such as interactions with the transcriptional machinery, to produce oncogenic effects. In this review, we discuss commonalities between wild-type and mutant p53 proteins with an emphasis on transcriptional processes.

Aggregation and Prion-Like Properties of Misfolded Tumor Suppressors: Is Cancer a Prion Disease?

Prion diseases are disorders that share several characteristics that are typical of many neurodegenerative diseases. Recently, several studies have extended the prion concept to pathological aggregation in malignant tumors involving misfolded p53, a tumor-suppressor protein. The aggregation of p53 and its coaggregation with p53 family members, p63 and p73, have been shown. Certain p53 mutants exert a dominant-negative regulatory effect on wild-type (WT) p53. The basis for this dominant-negative effect is that amyloid-like mutant p53 converts WT p53 into an aggregated species, leading to a gain-of-function (GoF) phenotype and the loss of its tumor-suppressor function. Recently, it was shown that p53 aggregates can be internalized by cells and can coaggregate with endogenous p53, corroborating the prion-like properties of p53 aggregates. The prion-like behavior of oncogenic p53 mutants provides an explanation for its dominant-negative and GoF properties, including the high metastatic potential of cancer cells carrying p53 mutations. The inhibition of p53 aggregation appears to represent a promising target for therapeutic intervention in patients with malignant tumors.

Oncogenic Intra-p53 Family Member Interactions in Human Cancers

The p53 gene family members p53, p73, and p63 display several isoforms derived from the presence of internal promoters and alternative splicing events. They are structural homologs but hold peculiar functional properties. p53, p73, and p63 are tumor suppressor genes that promote differentiation, senescence, and apoptosis. p53, unlike p73 and p63, is frequently mutated in cancer often displaying oncogenic “gain of function” activities correlated with the induction of proliferation, invasion, chemoresistance, and genomic instability in cancer cells. These oncogenic functions are promoted either by the aberrant transcriptional cooperation of mutant p53 (mutp53) with transcription cofactors (e.g., NF-Y, E2F1, Vitamin D Receptor, Ets-1, NF-kB and YAP) or by the interaction with the p53 family members, p73 and p63, determining their functional inactivation. The instauration of these aberrant transcriptional networks leads to increased cell growth, low activation of DNA damage response pathways (DNA damage response and DNA double-strand breaks response), enhanced invasion, and high chemoresistance to different conventional chemotherapeutic treatments. Several studies have clearly shown that different cancers harboring mutant p53 proteins exhibit a poor prognosis when compared to those carrying wild-type p53 (wt-p53) protein. The interference of mutantp53/p73 and/or mutantp53/p63 interactions, thereby restoring p53, p73, and p63 tumor suppression functions, could be among the potential therapeutic strategies for the treatment of mutant p53 human cancers.

Evolution of p53 transactivation specificity through the lens of a yeast-based functional assay

Co-evolution of transcription factors (TFs) with their respective cis-regulatory network enhances functional diversity in the course of evolution. We present a new approach to investigate transactivation capacity of sequence-specific TFs in evolutionary studies. Saccharomyces cerevisiae was used as an in vivo test tube and p53 proteins derived from human and five commonly used animal models were chosen as proof of concept. p53 is a highly conserved master regulator of environmental stress responses. Previous reports indicated conserved p53 DNA binding specificity in vitro, even for evolutionary distant species. We used isogenic yeast strains where p53-dependent transactivation was measured towards chromosomally integrated p53 response elements (REs). Ten REs were chosen to sample a wide range of DNA binding affinity and transactivation capacity for human p53 and proteins were expressed at two levels using an inducible expression system. We showed that the assay is amenable to study thermo-sensitivity of frog p53, and that chimeric constructs containing an ectopic transactivation domain could be rapidly developed to enhance the activity of proteins, such as fruit fly p53, that are poorly effective in engaging the yeast transcriptional machinery. Changes in the profile of relative transactivation towards the ten REs were measured for each p53 protein and compared to the profile obtained with human p53. These results, which are largely independent from relative p53 protein levels, revealed widespread evolutionary divergence of p53 transactivation specificity, even between human and mouse p53. Fruit fly and human p53 exhibited the largest discrimination among REs while zebrafish p53 was the least selective.

Mutant p53: one name, many proteins

There is now strong evidence that mutation not only abrogates p53 tumor-suppressive functions, but in some instances can also endow mutant proteins with novel activities. Such neomorphic p53 proteins are capable of dramatically altering tumor cell behavior, primarily through their interactions with other cellular proteins and regulation of cancer cell transcriptional programs. Different missense mutations in p53 may confer unique activities and thereby offer insight into the mutagenic events that drive tumor progression. Here we review mechanisms by which mutant p53 exerts its cellular effects, with a particular focus on the burgeoning mutant p53 transcriptome, and discuss the biological and clinical consequences of mutant p53 gain of function.

Correlation between low-level expression of the tumor suppressor gene TAp73 and the chemoresistance of human glioma stem cells

OBJECTIVE

Glioma stem cells (GSCs) are regarded as the root of glioma growth and recurrence. Chemoresistance is one of the characteristics of GSCs that increases the difficulties in eradicating the cells by anticancer drugs.

PURPOSE

The objective of this study is to investigate the correlation between expression of the tumor suppressor gene TAp73 and the chemoresistance of human GSCs.

METHODS

MTT and tumor sphere formation assays were used to analyze the chemoresistance phenotype of GSCs derived from primary human glioma specimens under cisplatin exposure. Reverse transcription real-time PCR was applied for assaying mRNA levels of TAp73. Protein levels of TAp73, p21, Bax, and cleared caspase 3 were assayed by western blot. Cell apoptosis was analyzed by flow cytometry after the annexin V fluorescence staining.

RESULTS

GSCs exhibited increased chemoresistance compared to differentiated glioma cells (DGCs) derived from the same tumor specimen. The expression of TAp73 was lower in GSCs and was not sensitive to cisplatin treatment as compared to DGCs. Overexpression of TAp73 by transfection increased the apoptosis of GSCs in the presence of cisplatin and reduced the chemoresistance of GSC. TAp73 knockdown by siRNA in DGCs reduced cisplatin-induced apoptosis and increased the resistance to cisplatin.

CONCLUSION

These findings indicate that TAp73 silencing is hallmark of GSC to maintain their chemoresistance phenotype. Thus, targeting TAp73 may provide a novel strategy to eradicating GSCs.

Targeting p73 in cancer

p73 is a member of the p53 family of tumor suppressors. Transactivating isoforms of p73 (TAp73) have p53-like, anti-proliferative and pro-apoptotic activities that are crucial for an efficient chemotherapy response. In line with this, genetic studies in mice have confirmed that TAp73 acts as a tumor suppressor. However, in contrast to p53, which is commonly inactivated in human cancer by point mutations, the TP73 gene is almost never mutated. Instead, the tumor suppressor activity of TAp73 is inhibited through a variety of mechanisms including epigenetic silencing and complex formation with inhibitory proteins. All these mechanisms have in common that they are in principle reversible and therefore amenable to therapeutic intervention. Here, we will review how tumor cells control the tumor suppressor activity of TAp73 and discuss possible strategies targeting p73 for reactivation.

Gain of function of mutant p53 by coaggregation with multiple tumor suppressors

Many p53 missense mutations possess dominant-negative activity and oncogenic gain of function. We report that for structurally destabilized p53 mutants, these effects result from mutant-induced coaggregation of wild-type p53 and its paralogs p63 and p73, thereby also inducing a heat-shock response. Aggregation of mutant p53 resulted from self-assembly of a conserved aggregation-nucleating sequence within the hydrophobic core of the DNA-binding domain, which becomes exposed after mutation. Suppressing the aggregation propensity of this sequence by mutagenesis abrogated gain of function and restored activity of wild-type p53 and its paralogs. In the p53 germline mutation database, tumors carrying aggregation-prone p53 mutations have a significantly lower frequency of wild-type allele loss as compared to tumors harboring nonaggregating mutations, suggesting a difference in clonal selection of aggregating mutants. Overall, our study reveals a novel disease mechanism for mutant p53 gain of function and suggests that, at least in some respects, cancer could be considered an aggregation-associated disease.

Translating p53 into the clinic

Mutations in the TP53 gene are a feature of 50% of all reported cancer cases. In the other 50% of cases, the TP53 gene itself is not mutated but the p53 pathway is often partially inactivated. Cancer therapies that target specific mutant genes are proving to be highly active and trials assessing agents that exploit the p53 system are ongoing. Many trials are aimed at stratifying patients on the basis of TP53 status. In another approach, TP53 is delivered as a gene therapy; this is the only currently approved p53-based treatment. The p53 protein is overexpressed in many cancers and p53-based vaccines are undergoing trials. Processed cell-surface p53 is being exploited as a target for protein-drug conjugates, and small-molecule drugs that inhibit the activity of MDM2, the E3 ligase that regulates p53 levels, have been developed by several companies. The first MDM2 inhibitors are being trialed in both hematologic and solid malignancies. Finally, the first agent found to restore the active function of mutant TP53 has just entered the clinic. Here we discuss the basis of these trials and the future of p53-based therapy.

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.

TAR1, a human anti-p53 single-chain antibody, restores tumor suppressor function to mutant p53 variants

The tumor suppressor gene p53 is mutated in more than half of human tumors. One important characteristic of p53 mutants is their accumulation in the nucleus of cancer cells. Thus, reactivation of mutant p53 proteins may trigger massive apoptosis in tumor cells. Pharmacologic methods are currently under development to induce mutant p53 proteins to resume their wild-type function. We have identified a human single-chain Fv fragment, designated as transcriptional transactivation and apoptosis restoring (TAR1), which specifically and with high affinity binds to mutant p53 and restores its wild-type active conformation. Binding of TAR1 to mutant p53 induced transcriptional transactivation of p53 target genes and down-regulation of mutant p53 transcriptional target genes. TAR1 treatment induced apoptosis in a variety of cell lines endogenously expressing p53 carrying different point mutations DNA contact or structural p53 mutants. Moreover, in an animal model of mice carrying human xenografts, TAR1 induced tumor regression with no apparent deleterious side effects. Thus, it may be considered as a potential candidate for anticancer treatment, targeting tumors with mutant p53.

Mutant p53 mediates survival of breast cancer cells

Background:

p53 is the most commonly mutated tumour-suppressor gene in human cancers. Unlike other tumour-suppressor genes, most p53 cancer mutations are missense mutations within the core domain, leading to the expression of a full-length mutant p53 protein. Accumulating evidence has indicated that p53 cancer mutants not only lose tumour suppression activity but also gain new oncogenic activities to promote tumourigenesis.

Methods:

The endogenous mutant p53 function in human breast cancer cells was studied using RNA interference (RNAi). Gene knockdown was confirmed by quantitative PCR and western blotting. Apoptosis was evaluated by morphological changes of cells, their PARP cleavage and annexin V staining.

Results:

We show that cancer-associated p53 missense mutants are required for the survival of breast cancer cells. Inhibition of endogenous mutant p53 by RNAi led to massive apoptosis in two mutant p53-expressing cell lines, T47D and MDA-MB-468, but not in the wild-type p53-expressing cells, MCF-7 and MCF-10A. Reconstitution of an RNAi-insensitive mutant p53 in MDA-MB-468 cells completely abolished the apoptotic effects after silencing of endogenous mutant p53, suggesting the specific survival effects of mutant p53. The apoptotic effect induced by mutant p53 ablation, however, is independent of p63 or p73 function.

Conclusion:

These findings provide clear evidence of a pro-survival ‘gain-of-function’ property of a subset of p53 cancer mutants in breast cancer cells.

Protein expression following gamma-irradiation relevant to growth arrest and apoptosis in colon cancer cells

PURPOSE

To study expression of proteins previously connected to radiotherapy response in rectal cancer patients, namely, p53, TAp73, DeltaNp73, survivin and PRL-3, after irradiation in colon cancer cells to gain standing ground for further studies of pathways and mechanisms.

METHODS

Three colon cancer cell lines (KM12C, KM12SM and KM12L4a) with one origin were radiated with gamma-radiation. Radiosensitivity was determined with cell cycle, survival fraction at 5 Gy (SF5) and apoptosis analysis and protein expression by Western blot.

RESULTS

Following irradiation, KM12C showed no cell cycle arrest, and low SF5 and apoptosis, whilst KM12L4a showed high SF5 and apoptosis. KM12SM had moderate radiosensitivity. After irradiation, the anti-apoptotic DeltaNp73 and mitosis-factor PRL-3 increased in KM12C and the radioresistance factor survivin increased in KM12L4a.

CONCLUSIONS

The cell lines seem to have evolved different protein patterns regarding the studied proteins and partly therefore developed different resistance mechanisms, less apoptosis for KM12C and continued proliferation for KM12L4a, after gamma-irradiation.

Restoration of p53 to limit tumor growth

PURPOSE OF REVIEW

p53 mutation occurs in over half of all human tumors. Among the remaining tumors, although they may process a wild-type p53, the pathways of p53-induced cell-cycle arrest and apoptosis are deficient. Therefore, p53 serves as a unique molecular target for cancer therapy. This review focuses on the current progress regarding restoration of p53 function in human tumors for molecularly targeted therapy.

RECENT FINDINGS

Targeting p53 for cancer therapy has been intensively pursued. CP-31398 was the first small molecule identified with the ability to restore the wild-type conformation to mutant p53. Subsequently, PRIMA-1 and ellipticine were found to be able to induce mutant p53-dependent cell death. Nutlin was developed to rescue wild-type p53 from degradation mediated by MDM2. More recently, p53 family members can be activated and therefore serve as substitutes of p53 in tumor cells and induce cell death.

SUMMARY

Loss of p53 function is a characteristic of almost all human tumors. Recent advances demonstrate that reconstitution of p53 function is possible and practical as a promising antitumor strategy.

Differential effects of novel tumour-derived p53 mutations on the transformation of NIH-3T3 cells

The p53 tumour suppressor gene is frequently mutated in human tumours and different tumour-derived mutations have varying effects on cells. The effect of a novel tumour-derived p53 mutation and two recently described mutations from South African breast cancer patients on the growth rate, colony formation, cell cycle arrest after irradiation and response to chemotherapeutic drugs was investigated. None of the p53 mutations had any significant effect on the inherent growth rate of the cells; however, contact inhibition of growth in two of the mutants was lost. These same two mutants formed colonies in soft agar, whereas the third mutant did not. All three of the mutants failed to show a G(1) cell cycle arrest after exposure to 7 Gy of [(60)Co] radiation, albeit to different degrees. Cells expressing the p53 mutants were either more sensitive to cisplatin and melphalan or more resistant than the untransfected cells, depending on the mutation. However, there was no difference in response to daunorubicin treatment. These results demonstrate that different p53 mutations exert varying biological effects on normal cells, with some altering checkpoint activation more effectively than others. The data also suggest that the nature of the p53 mutation influences the sensitivity to cytostatic drugs.

Versatile functions of p53 protein in multicellular organisms

The p53 tumor suppressor plays a pivotal role in multicellular organism by enforcing benefits of the organism over those of an individual cell. The task of p53 is to control the integrity and correctness of all processes in each individual cell and in the organism as a whole. Information about the state of ongoing events in the cell is gathered through multiple signaling pathways that convey signals modifying activities of p53. Changes in the activities depend on the character of damages or deviations from optimum in processes, and the activity of p53 changes depending on the degree of the aberration, which results in either stimulation of repair processes and protective mechanisms, or the cessation of further cell divisions and the induction of programmed cell death. The strategy of p53 ensures genetic identity of cells and prevents the selection of abnormal cells. By accomplishing these strategic tasks, p53 may use a wide spectrum of activities, such as its ability to function as a transcription factor, by inducing or repressing different genes, or as an enzyme, by acting as an exonuclease during DNA reparation, or as an adaptor or a regulatory protein, intervening into functions of numerous signaling pathways. Loss of function of the p53 gene occurs in virtually every case of cancer, and deficiency in p53 is an unavoidable prerequisite to the development of malignancies. The functions of p53 play substantial roles in many other pathologies as well as in the aging process. This review is focused on strategies of the p53 gene, demonstrating individual mechanisms underlying its functions.

Transcriptional regulation by p53: one protein, many possibilities

The p53 tumor suppressor protein is a DNA sequence-specific transcriptional regulator that, in response to various forms of cellular stress, controls the expression of numerous genes involved in cellular outcomes including among others, cell cycle arrest and cell death. Two key features of the p53 protein are required for its transcriptional activities: its ability to recognize and bind specific DNA sequences and to recruit both general and specialized transcriptional co-regulators. In fact, multiple interactions with co-activators and co-repressors as well as with the components of the general transcriptional machinery allow p53 to either promote or inhibit transcription of different target genes. This review focuses on some of the salient features of the interactions of p53 with DNA and with factors that regulate transcription. We discuss as well the complexities of the functional domains of p53 with respect to these interactions.

Mutant p53 proteins: between loss and gain of function

Cancer might result from both the aberrant activation of genes, whose physiological tuning is essential for the life of a normal cell, and the inactivation of tumor suppressor genes, whose main job is to preserve the integrity of cell genome. Among the latter, p53 is considered a key tumor suppressor gene that is inactivated mainly by missense mutations in half of human cancers. It is becoming increasingly clear that the resulting mutant p53 proteins gain oncogenic properties favoring the insurgence, the maintenance, and the spreading of malignant tumors. In this review, we mainly discuss the molecular mechanisms underlying gain of function of human tumor-derived p53 mutants, their impact on the chemoresistance and the prognosis of human tumors, with a special focus on head and neck cancers, and the perspectives of treating tumors through the manipulation of mutant p53 proteins.

Are interactions with p63 and p73 involved in mutant p53 gain of oncogenic function?

Although still controversial, the presence of mutant p53 in cancer cells may result in more aggressive tumors and correspondingly worse outcomes. The means by which mutant p53 exerts such pro-oncogenic activity are currently under extensive investigation and different models have been proposed. We focus here on a proposed mechanism by which a subset of tumor-derived p53 mutants physically interact with p53 family members, p63 and p73, and negatively regulate their proapoptotic function. Both cell-based assays and knock-in mice expressing mutant forms of p53 support this model. As more than half of human tumors harbor mutant forms of p53 protein, approaches aimed at disrupting the pathological interactions among p53 family members might be of clinical value.

Reactivation of mutant p53: molecular mechanisms and therapeutic potential

The p53 tumor suppressor gene is the most frequently mutated gene in cancer. Most p53 mutations are missense point mutations that cluster in the DNA-binding core domain. This results in distortion of core domain folding and disruption of DNA binding and transcriptional transactivation of p53 target genes. Structural studies have demonstrated that mutant p53 core domain unfolding is not irreversible. Mutant p53 is expressed at high levels in many tumors. Therefore, mutant p53 is a promising target for novel cancer therapy. Mutant p53 reactivation will restore p53-dependent apoptosis, resulting in efficient removal of tumor cells. A number of strategies for targeting mutant p53 have been designed, including peptides and small molecules that restore the active conformation and DNA binding to mutant p53 and induce p53-dependent suppression of tumor cell growth in vitro and in vivo. This opens possibilities for the clinical application of mutant p53 reactivation in the treatment of cancer.

The oncogenic roles of p53 mutants in mouse models

Tumors associated with p53 usually contain missense mutations in the p53 tumor suppressor gene rather than deletions of p53, suggesting a growth advantage for cells with missense mutations. The oncogenic roles of p53 mutants have been examined extensively in cell lines. Mouse models that inherit p53 mutations expressed at physiological levels have now been generated to examine the activities of mutant p53 upon tumorigenesis in vivo. Mice with p53 mutations develop tumor spectrums and metastatic phenotypes different from those of mice with a p53-null allele. Embryo fibroblasts with mutant p53 also show increased proliferative and transformation properties. One mechanism for this gain-of-function potential is the inhibition of function of the p53 family members p63 and p73.

The role of p73 in hematological malignancies

The P73 gene is a homologue of the P53 tumor suppressor. Owing to its structural similarity with p53, p73 was originally considered to have tumor suppressor function. However, the discovery of N-terminal truncated isoforms with oncogenic properties showed a 'two in one' structure of its product, p73 protein. The full-length variants are strong inducers of apoptosis, whereas the truncated isoforms inhibit proapoptotic activity of p53 and the full-length p73. Thus, p73 is involved in the regulation of cell cycle, cell death and development. Moreover, it plays a role in carcinogenesis and controls tumor sensitivity to treatment. p73 is commonly expressed in tumor cells in hematological malignancies. Overexpression of p73 protein and aberrant expression of its particular isoforms, with very low frequency of P73 hypermethylation or mutations, were found in malignant myeloproliferations, including acute myeloblastic leukemia. In contrast, hypermethylation and subsequent inactivation of the P73 gene are the most common findings in malignant lymphoproliferative disorders, especially acute lymphoblastic leukemia (ALL) and non-Hodgkin's lymphomas. Assessment of P73 methylation may provide important prognostic information, as was confirmed in patients with ALL. This review summarizes some aspects of p73 biology with particular reference to its possible pathogenetic role and prognostic significance in hematological malignancies.

p53 mutation heterogeneity in cancer

The p53 gene is inactivated in about 50% of human cancers and the p53 protein is an essential component of the cell response induced by genotoxic stresses such as those generated by radiotherapy or chemotherapy. It is therefore highly likely that these alterations are an important component in tumor resistance to therapy. The particular characteristics of these alterations, 80% of which are missense mutations leading to functionally heterogeneous proteins, make p53 a unique gene in the class of tumor suppressor genes. A considerable number of mutant p53 proteins probably have an oncogenic activity per se and therefore actively participate in cell transformation. The fact that the apoptotic and antiproliferative functions of p53 can be dissociated in certain mutants also suggests another level of complexity in the relationships between p53 inactivation and neoplasia.

Modulation of gene expression by tumor-derived p53 mutants

p53 mutants with a single amino acid substitution are overexpressed in a majority of human cancers containing a p53 mutation. Overexpression of the mutant protein suggests that there is a selection pressure on the cell indicative of an active functional role for mutant p53. Indeed, H1299 cells expressing mutant p53-R175H, p53-R273H or p53-D281G grow at a faster rate compared with a control cell line. Using p53-specific small interfering RNA, we show that the growth rate of mutant p53-expressing cells decreases as mutant p53 level decreases, demonstrating that the increased cellular growth is dependent on p53 expression. Increased growth rate is not observed for H1299 cell clones expressing mutant p53-D281G (L22Q/W23S), which has been shown to be defective in transactivation in transient transcriptional assays. This shows that the increased growth rate imparted by mutant p53 in H1299 cells requires the transactivation function of mutant p53. By performing microarray hybridization analyses, we show that constitutive expression of three common p53 mutants (p53-R175H, p53-R273H, and p53-D281G) in H1299 human lung carcinoma cells evokes regulation of a common set of genes, a significant number of which are involved in cell growth regulation. Predictably, H1299 cells expressing p53-D281G (L22Q/W23S) are defective in up-regulating a number of these genes. The differences in expression profiles induced by individual p53 mutants in the cells may be representative of the p53 mutants and how they can affect gene expression resulting in the observed "gain of function" phenotypes (i.e., increased growth rate, decreased sensitivity to chemotherapeutic agents, and so forth).

Tumor-derived p53 mutants induce oncogenesis by transactivating growth-promoting genes

We have studied the mechanism of mutant p53-mediated oncogenesis using several tumor-derived mutants. Using a colony formation assay, we found that the majority of the mutants increased the number of colonies formed compared to the vector. Expression of tumor-derived p53 mutants increases the rate of cell growth, suggesting that the p53 mutants have 'gain of function' properties. We have studied the gene expression profile of cells expressing tumor-derived p53-D281G to identify genes transactivated by mutant p53. We report the transactivation of two genes, asparagine synthetase and human telomerase reverse transcriptase. Quantitative real-time PCR confirms this upregulation. Transient transfection promoter assays verify that tumor-derived p53 mutants transactivate these promoters significantly. An electrophoretic mobility shift assay shows that tumor-derived p53-mutants cannot bind to the wild-type p53 consensus sequence. The results presented here provide some evidence of a possible mechanism for mutant p53-mediated transactivation.

Reassessment of theTP53 mutation database in human disease by data mining with a library ofTP53 missense mutations

TP53 alteration is the most frequent genetic alteration found in human cancers. To date, more than 15,000 tumors with TP53 mutations have been published, leading to the description of more than 1,500 different TP53 mutants (http://p53.curie.fr). The frequency of these mutants is highly heterogeneous, with 11 hotspot mutants found more than 100 times, whereas 306 mutants have been reported only once. So far, little is known concerning the biological significance of these rare mutants, as the majority of biological studies have focused on classic hotspot mutants. In order to gain a deeper knowledge about the significance of all of these mutants, we have cross-checked each mutant of the TP53 mutation database for its activity, derived from a library of 2,314 TP53 mutants representing all possible amino acid substitutions caused by a point mutation. The transactivation activity of all of these mutant was analyzed with respect to eight transcription promoters [Kato S, et al., Proc Natl Acad Sci USA (2003)100:8424-8429]. Although the most frequent TP53 mutants sustain a clear loss of transactivation activity, more than 50% of the rare TP53 mutants display significant activity. Analysis in specific types of cancer or in normal skin patches demonstrates a similar distribution of TP53 loss of activity, with the exception of melanoma, in which the majority of TP53 mutants display significant activity. Our data indicate that TP53 mutants represent a highly heterogeneous population with a large diversity in terms of loss of transactivation activity that could account for the heterogeneous tumor phenotypes and the difficulty of clinical studies.

The European flounder (Platichthys flesus) TP53 functions as a temperature-sensitive transcription factor which inhibits cell growth in yeast

Numerous studies focus on biological roles of the TP53 tumor suppressor gene in mammals but little is known about the actual function of TP53 in lower vertebrates. In this study, we used an in vivo functional assay in yeast to address the transactivation capacity of the flounder TP53 protein. We showed that the flounder TP53 acts as a sequence-specific transcription factor which is able to transactivate various human promoters containing a p53-responsive element (RE). This transcriptional activity was completely abrogated in the Val147Glu TP53 mutant previously identified in two flounder hepatic hyperplasia. In addition, we showed that the wild-type (wt) flounder TP53 but not the Val147Glu mutant inhibits cell growth when expressed in yeast. We finally reported that transcription regulation and growth inhibition by the wild-type flounder TP53 is temperature-dependent. The flounder TP53 optimal temperature appeared lower than those reported for the Xenopus and human homologues.

Chemosensitivity linked to p73 function

Most chemotherapeutic agents induce DNA damage, leading to p53 accumulation and apoptosis. The factors that determine chemosensitivity in p53-defective tumor cells are poorly understood. We found that the p53 family member p73 is induced by a wide variety of chemotherapeutic drugs. Blocking p73 function with a dominant-negative mutant, siRNA, or homologous recombination led to chemoresistance of human tumor cells and engineered transformed cells, irrespective of p53 status. Mutant p53 can inactivate p73 and downregulation of mutant p53 enhanced chemosensitivity. These findings indicate that p73 is a determinant of chemotherapeutic efficacy in humans.

p53 mutations and resistance to chemotherapy: A stab in the back for p73

p73 is essential for apoptosis induced by many cytotoxic agents, but this function can be blocked by a particular category of p53 mutations that have consequently acquired a gain of function.