Mutation at p53 serine 389 does not rescue the embryonic lethality in mdm2 or mdm4 null mice

Regulating tumor suppressor genes: post-translational modifications

Tumor suppressor genes cooperate with each other in tumors. Three important tumor suppressor proteins, retinoblastoma (Rb), p53, phosphatase, and tensin homolog deleted on chromosome ten (PTEN) are functionally associated and they regulated by post-translational modification (PTMs) as well. PTMs include phosphorylation, SUMOylation, acetylation, and other novel modifications becoming growing appreciated. Because most of PTMs are reversible, normal cells use them as a switch to control the state of cells being the resting or proliferating, and PTMs also involve in cell survival and cell cycle, which may lead to abnormal proliferation and tumorigenesis. Although a lot of studies focus on the importance of each kind of PTM, further discoveries shows that tumor suppressor genes (TSGs) form a complex “network” by the interaction of modification. Recently, there are several promising strategies for TSGs for they change more frequently than carcinogenic genes in cancers. We here review the necessity, characteristics, and mechanisms of each kind of post-translational modification on Rb, p53, PTEN, and its influence on the precise and selective function. We also discuss the current antitumoral therapies of Rb, p53 and PTEN as predictive, prognostic, and therapeutic target in cancer.

Somatic Trp53 mutations differentially drive breast cancer and evolution of metastases

TP53 mutations are the most frequent genetic alterations in breast cancer and are associated with more aggressive disease and worse overall survival. We have created two conditional mutant Trp53 alleles in the mouse that allow expression of Trp53R172H or Trp53R245W missense mutations in single cells surrounded by a normal stroma and immune system. Mice with Trp53 mutations in a few breast epithelial cells develop breast cancers with high similarity to human breast cancer including triple negative. p53R245W tumors are the most aggressive and exhibit metastases to lung and liver. Development of p53R172H breast tumors with some metastases requires additional hits. Sequencing of primary tumors and metastases shows p53R245W drives a parallel evolutionary pattern of metastases. These in vivo models most closely simulate the genesis of human breast cancer and will thus be invaluable in testing novel therapeutic options.

Mutations in TP53 gene are very common in cancer development. Here the authors take advantage of murine models to show that somatic Trp53 mutations differentially drive breast cancer and evolution of metastases.

The gain of function of p53 mutant p53S in promoting tumorigenesis by cross-talking with H-RasV12

The loss of wild type p53 tumor suppressive function and oncogenic gain-of-function of p53 mutants have been showing important implications in tumorigenesis. The p53^N236S (p53^N239S in human, p53S) mutation has been shown to lose wild type p53 function by yeast assay. However, its gain of function is still not clear. By gel shift assay, we showed that mutant p53S had lost its DNA binding ability to its target promoters. Further real-time PCR data confirmed that p53S had lost the function of regulating the transcription of p21 ^Cip1/Waf1, cyclin G, PUMA, and Bax in response to 10Gy irradiation. These data confirmed the loss of function of p53S in mammalian cells. By xenograft assay, we showed that the p53S per se was not oncogenic enough to form tumor, however, cooperating with H-RasV12, p53S could dramatically promote tumorigenesis in p53 null MEFs. Further study showed that co-expression of p53S and H-RasV12 could increase the expression level of H-RasV12 and partially eliminate the elevation of stress response proteins such as Chk2, γ-H2AX, Hsp70, Rb, p16^Ink4a caused by either p53S or H-RasV12. These data suggested that p53S cross-talked with H-RasV12 and reduced the cellular stress response to oncogenic signals, which facilitated the cell growth and tumorigenesis. Together these data provided the molecular basis for the cooperation of p53S and H-RasV12 and revealed the gain of function of p53S in cross-talking with H-RasV12. This study revealed an important aspect of gain of function for p53 mutant, therefore might shed light on the clinical strategy in targeting p53 mutant.

Ubiquitination of human AP-endonuclease 1 (APE1) enhanced by T233E substitution and by CDK5

Apurinic/apyrimidinic endonuclease-1 (APE1) is a multifunctional DNA repair/gene regulatory protein in mammalian cells, and was recently reported to be phosphorylated at Thr233 by CDK5. We here report that ubiquitination of T233E APE1, a mimicry of phospho-T233 APE1, was markedly increased in multiple cell lines. Expression of CDK5 enhanced monoubiquitination of endogenous APE1. Polyubiquitinated APE1 was decreased when K48R ubiquitin was expressed, suggesting that polyubiquitination was mediated mainly through Lys48 of ubiquitin. The ubiquitination activity of MDM2, consistent in its role for APE1 ubiquitination, was increased for T233E APE1 compared to the wild-type APE1. In mouse embryonic fibroblasts lacking the MDM2 gene, ubiquitination of T233E APE1 was still observed probably because of the decreased degradation activity for monoubiquitinated APE1 and because of backup E3 ligases in the cells. Monoubiquitinated APE1 was present in the nucleus, and analyzing global gene expression profiles with or without induction of a ubiquitin-APE1 fusion gene suggested that monoubiquitination enhanced the gene suppression activity of APE1. These data reveal a delicate balance of ubiquitination and phosphorylation activities that alter the gene regulatory function of APE1.

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.

Acidic domain is indispensable for MDM2 to negatively regulate the acetylation of p53

MDM2 is the most important negative regulator of tumor suppressor p53. Both RING finger domain and acidic domain of MDM2 contribute to the ubiquitination of p53. The crosstalk between ubiquitination and acetylation of p53 prompts us to examine whether acidic domain is essential for MDM2 to regulate the acetylation of p53. We find that the acidic domain of MDM2 is necessary to inhibit p300-mediated acetylation of p53 as well as to mediate the deacetylation of p53. Our results indicate that acidic domain of MDM2 provides essential information for acetyltransferase p300 and deacetylase HDAC1 and is indispensable for MDM2 to negatively regulate the acetylation of p53.

The inherent instability of mutant p53 is alleviated by Mdm2 or p16INK4a loss

The p53 tumor suppressor is often disrupted in human cancers by the acquisition of missense mutations. We generated mice with a missense mutation at codon 172 that mimics the p53R175H hot spot mutation in human cancer. p53 homozygous mutant mice have unstable mutant p53 in normal cells and stabilize mutant p53 in some but not all tumors. To investigate the significance of these data, we examined the regulation of mutant p53 stability by Mdm2, an E3 ubiquitin ligase that targets p53 for degradation, and p16INK4a, a member of the Rb tumor suppressor pathway. Mice lacking Mdm2 or p16INK4a stabilized mutant p53, and revealed an earlier age of tumor onset than p53 mutant mice and a gain-of-function metastatic phenotype. Analysis of tumors from p53 homozygous mutant mice with stable p53 revealed defects in the Rb pathway. Additionally, ionizing radiation stabilizes wild-type and mutant p53. Thus, the stabilization of mutant p53 is not a given but it is a prerequisite for its gain-of-function phenotype. Since mutant p53 stability mimics that of wild-type p53, these data indicate that drugs aimed at activating wild-type p53 will also stabilize mutant p53 with dire consequences.

Mtbp haploinsufficiency in mice increases tumor metastasis

Mdm2 inhibits the function of the p53 tumor suppressor. Mdm2 is overexpressed in many tumors with wild-type p53 suggesting an alternate mechanism of loss of p53 activity in tumors. An Mdm2-binding protein (MTBP) was identified using a yeast two-hybrid screen. In tissue culture, MTBP inhibits Mdm2 self-ubiquitination, leading to stabilization of Mdm2 and increased degradation of p53. To address the role of MTBP in the regulation of the p53 pathway in vivo, we deleted the Mtbp gene in mice. Homozygous disruption of Mtbp resulted in early embryonic lethality, which was not rescued by loss of p53. Mtbp+/- mice were not tumor prone. When mice were sensitized for tumor development by p53 heterozygosity, we found that the Mtbp+/-p53+/- mice developed significantly more metastatic tumors (18.2%) as compared to p53+/- mice (2.6%). Results of in vitro migration and invasion assays support the in vivo findings. Downmodulation of Mtbp in osteosarcoma cells derived from p53+/- mice resulted in increased invasiveness, and overexpression of Mtbp in Mtbp+/-p53+/- osteosarcoma cells inhibited invasiveness. These results suggest that MTBP is a metastasis suppressor. These results advance our understanding of the cellular roles of MTBP and raise the possibility that MTBP is a novel therapeutic target for metastasis.

Mammary tumor modifiers in BALB/cJ mice heterozygous for p53

BALB/c mice are predisposed to developing spontaneous mammary tumors, which are further increased in a p53 heterozygous state. C57BL/6J mice are resistant to induced mammary tumors and develop less than 1% mammary tumors in both wild-type and p53+/- states. To map modifiers of mammary tumorigenesis, we have established F1 and F2 crosses and backcrosses to BALB/cJ (N2-BALB/cJ) and C57BL/6J (N2-C57BL/6J) strains. All cohorts developed mammary carcinomas in p53+/- females, suggesting that multiple loci dominantly and recessively contributed to mammary tumorigenesis. We mapped two modifiers of mammary tumorigenesis in the BALB/cJ strain. Mtsm1 (mammary tumor susceptibility modifier), a dominant-acting modifier, is located on chromosome 7. Mtsm1 is suggestive for linkage to mammary tumorigenesis (p = 0.001). We have analyzed the Mtsm1 region to locate candidate genes by comparing it to a rat modifier region, Mcs3, which shares syntenic conservation with Mtsm1. Expression data and SNPs were also taken into account. Five potential candidate genes within Mtsm1 are Aldh1a3, Chd2, Nipa2, Pcsk6, and Tubgcp5. The second modifier mapped is Mtsm2, a recessive-acting modifier. Mtsm2 is located on chromosome X and is significantly linked to mammary tumorigenesis (p = 1.03 x 10(-7)).

Crippling p53 activities via knock-in mutations in mouse models

The tumor suppressor p53 is the most frequently mutated gene in human cancer. In vivo models have been generated using knock-in alleles in which missense mutations are introduced that mimic the kinds of mutations found in human cancers, or that abolish specific p53 functions. Critically, these studies examine the in vivo and physiological functions of p53. Studies indicate that p53 missense mutations in the DNA-binding domain identical with those inherited in the Li-Fraumeni syndrome, have distinct properties. Studies in mice with mutants that separate cell-cycle arrest and apoptosis functions of p53 show delayed onset of tumor development, suggesting that both p53 functions are crucial for suppressing tumors. Mice with mutations at post-translational modification sites exhibit subtle effects on p53 activity and tumor development, indicating a fine-tuning mechanism of p53 activity in vivo. Importantly, each mutant mouse has a distinct phenotype, suggesting diverse and exquisite mechanisms of p53 regulation in different environments, different tissues and different genetic backgrounds. The generation of these mutant p53 knock-in mice has laid the groundwork for future studies to elucidate the in vivo physiological function of mutant p53 and to examine cooperating effects in combination with other alterations.

p21 delays tumor onset by preservation of chromosomal stability

The p53 protein suppresses tumorigenesis by initiating cellular functions such as cell cycle arrest and apoptosis in response to DNA damage. A p53 mutant, p53R172P, which is deficient for apoptosis but retains a partial cell cycle arrest function, delays tumor onset in mice. Remarkably, lymphomas arising in Trp53(515C/515C) mice (encoding p53R172P) retain stable genomes. Given the dominant role of p21 in p53 cell cycle control, we crossed Trp53(515C/515C) mice onto a p21-null background to determine whether p21 was required for maintaining chromosomal stability and delaying tumor onset. Loss of p21 completely abolished the cell cycle arrest function of p53R172P and accelerated tumor onset in Trp53(515C/515C) mice. Cytogenetic examination of Trp53(515C/515C) p21(-/-) sarcomas and lymphomas revealed aneuploidy and chromosomal aberrations that were absent in Trp53(515C/515C) malignancies. Thus, p21 coupled p53-dependent checkpoint control and preservation of chromosomal stability, and cooperated with apoptosis in suppressing tumor onset in mice.

Gain of function of a p53 hot spot mutation in a mouse model of Li-Fraumeni syndrome

Individuals with Li-Fraumeni syndrome carry inherited mutations in the p53 tumor suppressor gene and are predisposed to tumor development. To examine the mechanistic nature of these p53 missense mutations, we generated mice harboring a G-to-A substitution at nucleotide 515 of p53 (p53+/515A) corresponding to the p53R175H hot spot mutation in human cancers. Although p53+/515A mice display a similar tumor spectrum and survival curve as p53+/- mice, tumors from p53+/515A mice metastasized with high frequency. Correspondingly, the embryonic fibroblasts from the p53515A/515A mutant mice displayed enhanced cell proliferation, DNA synthesis, and transformation potential. The disruption of p63 and p73 in p53-/- cells increased transformation capacity and reinitiated DNA synthesis to levels observed in p53515A/515A cells. Additionally, p63 and p73 were functionally inactivated in p53515A cells. These results provide in vivo validation for the gain-of-function properties of certain p53 missense mutations and suggest a mechanistic basis for these phenotypes.

What have animal models taught us about the p53 pathway?

Mouse models have provided important insight into the in vivo significance of upstream and downstream signals that regulate the p53 tumour suppressor. One important lesson learned from these models is that negative regulators of p53 are critical in vivo modulators of p53 activity. Additionally, upstream regulators of p53 activity, such as p19(Arf) and Atm, are themselves critical tumour modifiers/suppressors. The presence of multiple positive regulators of p53 and numerous downstream targets indicates a redundancy that ensures activation of the p53 pathway. Importantly, p53 plays a prominent role as a tumour suppressor in vivo by virtue of its ability both to block cell cycle progression and to induce cell death. Finally, different p53 mutants have different properties in vivo. Three missense mutations have been generated at the p53 locus and all three exhibit unique differences in their ability to contribute to the tumour phenotype. Clearly, determining the levels of p53 inhibitors, and the typing of p53 mutations in human tumours should be performed to determine the best avenue for treatment.