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

Mechanisms of differential activation of target gene promoters by p53 hinge domain mutants with impaired apoptotic function

Suppression of tumor cell growth by p53 results from the activation of both apoptosis and cell cycle arrest functions that have been shown to be separable activities of p53. We report here that some mutants in the p53 hinge domain, a short linker between the DNA binding and tetramerization domains, differentially activated the promoters of p53 target genes and possessed an impaired apoptotic function. Our results indicate that the hinge domain may play an important role in differentially regulating p53 cell cycle arrest and apoptotic functions. However, the mechanisms by which p53 hinge domain mutants differentially activate its target genes, e.g. p21(WAF1/CIP1) and Bax, remain unknown. To investigate the possible mechanisms, recombinant p21(WAF1/CIP1) and Bax promoters were constructed, resulting in rearrangement of the existing p53 binding sites within a given promoter or actually swapping p53 binding sites between the two promoters. Our results suggest that multiple mechanisms of differential transactivation occur, depending on the molecular nature of the relevant hinge domain mutant, such as the possibility that dual separate DNA binding sites in the p21(WAF1/CIP1) promoter are responsible for the selective transactivation activity of p53 hinge domain mutant del300-327, which has a large deletion in the hinge domain. Lack of ideal p53 binding sites in the Bax promoter results in less potent activation than that seen with the p21(WAF1/CIP1) promoter when it is transactivated by hinge domain point mutant mutR306P or short deletion mutant del300-308 proteins. How the single mutation or the short deletion affect the conformation of p53 and consequently the transactivation of the Bax promoter will require further investigation of the relevant p53 protein: DNA-binding domain by NMR and x-ray crystallographic techniques.

Novel human p53 mutations that are toxic to yeast can enhance transactivation of specific promoters and reactivate tumor p53 mutants

Since highly expressed human p53 can inhibit human and yeast cell growth, we predicted that p53 mutants could be generated with increased growth inhibition of the yeast Saccharomyces cerevisiae and that these would be useful for characterizing p53 functions and tumor p53 mutants. A random mutagenesis screen led to the isolation of mutations in the DNA binding domain that result in p53 being lethal even at moderate expression levels in yeast. Three independent mutants had an alanine change at the evolutionary invariant V122 in the L1 loop. The other toxic mutations affected codons 277 (C277R, C277W) and 279 (G279R). This latter amino acid change was also reported in tumors, while all the other mutations are novel. A recently developed rheostatable GALI promoter system that provides graded increases in expression of p53 was used to examine the transactivation function of the toxic mutations when expression was greatly reduced and cells were viable. At low expression levels the toxic mutants lacked transactivation from a 3xRGC responsive element (RE). Surprisingly some exhibited enhanced transactivation with p21 and bax REs. The V122A mutant was able to re-activate transactivation of various p53 tumor mutants and retained growth inhibition when co-expressed with dominant-negative tumor mutations. Upon expression in human Saos-2 cells the V122A p53 mutant caused growth suppression, was capable of transactivation and exhibited higher than wild type activity with the bax promoter in luciferase assays. A non-functional p53 tumor mutant was partially reactivated by V122A for both transactivation and growth suppression. Thus, the screen for toxic p53 mutants in yeast can identify novel p53 variants that may be useful in dissecting p53 regulated cellular responses and in developing p53-based cancer therapies.

p53 contains a DNA break-binding motif similar to the functional part of BRCT-related region of Rb

The BRCT regions are two repeating structures at BRCA1 carboxyl-terminus and are ubiquitous in some proteins involved in DNA repair and cell cycle checkpoints. It was shown that BRCTs of TopBP1, BRCA1, and BRCT-Ws of Rb bound DNA ends and breaks. We indicate here that the C-terminus of p53 tumor suppressor contains a DNA binding motif (residues 327-333 in human), whose features are similar to those of the part of BRCT-W in Rb with DNA binding activity. The short motif was required for the gel retardation activity of DNA fragments, since residues 311-333 showed the activity while residues 311-326 showed no activity. Significant numbers of total p53 and its fragments with the motif formed multimerizing complexes and associated with DNA ends and breaks. These results suggest the common presence of DNA binding motifs that can recognize DNA ends or damages in major tumor suppressors, Rb, BRCA1 and p53. The oncogenic activity of p53 C-terminus (residues 311-393) required both the DNA damage recognition motif and the repair enzyme-associating domain.

EGR2 mutations in inherited neuropathies dominant-negatively inhibit myelin gene expression

The identification of EGR2 mutations in patients with neuropathies and the phenotype Egr2/Krox20(-/-) have demonstrated that the Egr2 transcription factor is critical for peripheral nerve myelination. However, the mechanism by which these mutations cause disease remains unclear, as most patients present with disease in the heterozygous state, whereas Egr2(+/-) mice are phenotypically normal. To understand the effect of aberrant Egr2 activity on Schwann cell gene expression, we performed microarray expression profiling to identify genes regulated by Egr2 in Schwann cells. These include genes encoding myelin proteins and enzymes required for synthesis of normal myelin lipids. Using these newly identified targets, we have shown that neuropathy-associated EGR2 mutants dominant-negatively inhibit wild-type Egr2-mediated expression of essential myelin genes to levels sufficiently low to result in the abnormal myelination observed in these patients.

Direct transcriptional activation of human caspase-1 by tumor suppressor p53

The tumor suppressor protein p53 is a sequence-specific DNA-binding protein, and its biological responses are very often mediated by transcriptional activation of various target genes. Here we show that caspase-1 (interleukin-1beta converting enzyme), which plays a role in the production of proinflammatory cytokines and in apoptosis, is a transcriptional target of p53. Caspase-1 mRNA levels increased upon overexpression of p53 by transfection in MCF-7 cells. Human caspase-1 promoter showed a sequence homologous to the consensus p53-binding site. This sequence bound to p53 in gel shift assays. A caspase-1 promoter-reporter construct was activated 6-8-fold by cotransfection with normal p53 but not by mutant p53 (His(273)) in HeLa, as well as MCF-7, cells. Mutation of the p53-binding site in caspase-1 promoter abolished transactivation by p53. Treatment of p53-positive MCF-7 cells with the DNA-damaging drug, doxorubicin, which increases p53 levels, enhanced caspase-1 promoter activity 4-5-fold, but similar treatment of MCF-7-mp53 (a clone of MCF-7 cells expressing mutant p53) and p53-negative HeLa cells with doxorubicin did not increase caspase-1 promoter activity. Doxorubicin treatment increased caspase-1 mRNA levels in MCF-7 cells but not in MCF-7-mp53 or HeLa cells. These results show that endogenous p53 can regulate caspase-1 gene expression.

p53 mutants exhibiting enhanced transcriptional activation and altered promoter selectivity are revealed using a sensitive, yeast-based functional assay

Changes in promoter specificity and binding affinity that may be associated with p53 mutations or post-translational modifications are useful in understanding p53 structure/function relationships and categorizing tumor mutations. We have exploited variable expression of human p53 in yeast to identify mutants with novel phenotypes that would correspond to altered promoter selectivity and affinity. The p53 cDNA regions coding for the DNA binding and tetramerization domains were subjected to random PCR mutagenesis and were cloned directly by recombination in yeast into a vector with a GAL1 promoter whose level of expression could be easily varied. p53 variants exhibiting higher than wild type levels of transactivation (supertrans) for the RGC responsive element were identified at low level of p53 protein expression. All the p53 mutants obtained with this screen were located in the DNA binding domain. Two out of 17 supertrans mutants have been found in tumors. Six mutations were in the L1 loop region between amino acids 115 and 124. The transactivation potential of a panel of supertrans p53 mutants on different promoters was evaluated using the p53 responsive elements, RGC, PIG3, p21 and bax. Although all mutants retained some activity with all promoters, we found different patterns of induction based on strength and promoter specificity. In particular none of the mutants was supertrans for the p21 responsive element. Interestingly, further analysis in yeast showed that the transactivation function could be retained even in the presence of dominant-negative p53 tumor mutations that could inhibit wild type p53. Five mutants were also characterized in human cells in terms of growth suppression and transactivation of various promoters. These novel supertrans p53 mutants may be useful in studies aimed at dissecting p53 downstream pathways, understanding specific interactions between p53 and the DNA, and could replace wild type p53 in cancer gene therapy protocols. The approach may also prove useful in identifying p53 tumor mutations.

The gene encoding p202, an interferon-inducible negative regulator of the p53 tumor suppressor, is a target of p53-mediated transcriptional repression

The p53 tumor suppressor protein regulates the transcription of regulatory genes involved in cell cycle arrest and apoptosis. We reported previously that overexpression of p202, an interferon-inducible negative regulator of cell growth, negatively regulates the transcriptional activity of p53. Now we identify the gene encoding p202 as one whose mRNA and protein expression decrease in cells following the expression of wild-type, but not mutant, p53. Furthermore, the levels of p202 also decrease after exposure of cells to ultra violet light, which correlate with increase in the levels of p53. We report that the sequence-specific DNA binding of p53 to the 5'-regulatory region of the 202 gene contributes to the transcriptional repression of the 202 gene. Interestingly, overexpression of p202 in cells induced to undergo p53-dependent apoptosis significantly delays this process, indicating that the negative regulation of the 202 gene by wild-type p53 is important to potentiate apoptosis.