Polymorphisms of the p53 gene in women with ovarian or endometrial carcinoma

The p53 gene is frequently mutated in various human tumors. Polymorphism is an additional genetic alteration observed in exons and introns of the p53 gene of normal tissues and tumors. Distributions of alleles of three common polymorphisms of the p53 gene; a 16 bp duplication in intron 3, codon 72 of exon 4 and a sequence in intron 6, were studied in peripheral white blood cells (WBC) of patients with ovarian or endometrial carcinomas. The analysis was performed by PCR and direct sequencing. The 100% linkage observed between the most common haplotypes of each polymorphism in healthy subjects was lower in the patients. A significant difference was observed between frequencies of genotype and haplotype combinations in patients with ovarian carcinoma and endometrial carcinoma. The incidence of heterozygosity was increased in ovarian carcinoma and decreased in endometrial carcinoma. Our results suggest that the p53 gene may be involved in susceptibility and predisposition to various cancers not only by mutations but also by preferential presentation of polymorphic alleles.

Immunity to p53 induced by an idiotypic network of anti-p53 antibodies: generation of sequence-specific anti-DNA antibodies and protection from tumor metastasis

The general overexpression of p53 by different types of tumor cells suggests that p53 immunity might be generally useful for tumor immunotherapy. We describe here the induction of immunity to p53 and resistance to tumor metastasis using an idiotypic network. Mice were immunized with domain-specific anti-p53 monoclonal antibodies (Ab1): PAb-248 directed to the N-terminus; PAb-246 directed to the specific DNA-binding region; or PAb-240 directed to a mutant p53 that does not bind specific DNA. Immunized mice responded by making anti-idiotypic antibodies (Ab2) specific for the Ab1 inducer. Ab1 PAb-246 induced Ab2 that, like p53 itself, could bind the specific DNA oligonucleotide sequence of the p53 responsive element. Mice immunized with Ab1 PAb-240 or PAb-246 spontaneously made Ab3 anti-p53 antibodies that reflected the specificity of their Ab1 inducers: Ab1 PAb-246 induced Ab3 specific for wild-type p53; PAb-240 induced Ab3 specific for mutant p53. Ab1 PAb-248 induced only Ab2. The spontaneously arising Ab3 were of T cell-dependent IgG isotypes. Peptides from the complementarity determining regions of the Ab1 antibodies PAb-240 and PAb-246 could also induce Ab3 anti-p53. Finally, mice that produced Ab3 anti-p53 acquired resistance to tumor metastases. Therefore, an anti-idiotypic network built around certain domains of p53 seems to be programmed within the immune system, specific Ab2 antibodies can mimic the DNA binding domain of p53, and Ab3 network immunity to p53 can be associated with resistance to tumor cells.

Mutant p53 protein expression interferes with p53-independent apoptotic pathways

Loss of normal p53 function was found frequently to interfere with response of cancer cells to conventional anticancer therapies. Since more than half of all human cancers possess p53 mutations, we decided to explore the involvement of mutant p53 in drug induced apoptosis. To further evaluate the relationship between the p53-dependent and p53-independent apoptotic pathways, and to elucidate the function of mutant p53 in modulating these processes, we investigated the role of a p53 temperature-sensitive (ts) mutant in a number of apoptotic pathways induced by chemotherapeutic drugs that are currently used in cancer therapy. To that end, we studied the M1/2, myeloid p53 non-producer cells, and M1/2-derived temperature-sensitive mutant p53 expressing clones. Apoptosis caused by DNA damage induced with gamma-irradiation, doxorubicin or cisplatin, was enhanced in cells expressing wild type p53 as compared to that seen in parental p53 non-producer cells; mutant p53 expressing clones were found to be more resistant to apoptosis induced by these factors. Actinomycin D, a potent inhibitor of transcription, as well as a DNA damaging agent, abrogated the restraint apoptosis mediated by mutant p53. These observations suggest that while loss of wild type p53 function clearly reduces the rate of apoptosis, p53 mutations may result in a gain of function which significantly interferes with chemotherapy induced apoptosis. Therefore, to achieve a successful cancer therapy, it is critical to consider the specific relationship between a given mutation in p53 and the chemotherapy selected.

Idiotypic immunization induces immunity to mutated p53 and tumor rejection

The p53 molecule might serve as a common tumor-associated antigen, as the tumor suppressor gene p53 is mutated and the p53 protein is often over-expressed in tumor cells. We report that effective immunity to p53 can be induced through an idiotypic network by immunization of mice with a monoclonal antibody (PAb-240) specific for mutated p53, or with a peptide derived from the complementarity determining region (CDR) 3 of the variable domain of the light chain (VL) of this antibody. The immunized mice produced IgG antibodies to p53 and mounted a cytotoxic reaction to a tumor line bearing mutated p53. The idiotypically immunized mice were resistant to challenge with the tumor cells. Thus antibodies to p53 might serve as immunogens for activating resistance to some tumors. At the basic level, these findings indicate that a network of p53 immunity may be organized naturally within the immune system.

P53 gene mutation in a T-acute lymphoblastic leukemia cell line (loucy) with t(16:20) and 5q- chromosomal aberrations

A human T-acute lymphoblastic leukemia (ALL) cell line (Loucy), derived from cells from a patient with resistant ALL with a t(16:20) and 5q- chromosomal aberrations was evaluated for p53 gene alterations and expression. Western blot analysis of p53 showed elevated levels of the protein. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis and direct sequencing identified a point mutation at codon 272 (GTG --> ATG) of the p53 gene. Possible molecular mechanisms underlying these alterations and their role in the establishment of this cell line and in leukemogenesis in general are discussed.

Alternatively spliced mRNA transcripts encoding the extracellular domain of the FSH receptor gene. Expression in the mouse ovary during the ovulatory cycle

OBJECTIVE

To evaluate regulation of follicle-stimulating hormone (FSH) receptor expression in the mouse ovary during different stages of an artificially induced ovulatory cycle.

STUDY DESIGN

Follicular maturation was achieved in pubertal female mice by pregnant mares' serum gonadotropin (PMSG). Ovulation was induced 48 hours later by human chorionic gonadotropin (hCG). Ovaries were harvested before treatment, at 24 and 48 hours after PMSG and at 3, 9 and 12 hours after hCG. RNA was extracted using a single-step isolation method and used for reverse transcription. The cDNA was amplified by polymerase chain reaction (PCR) using primers designed to amplify a 512-basepair product corresponding to the extracellular fragment of the FSH receptor.

RESULTS

PCR products, resolved by electrophoresis on agarose gels, showed four bands corresponding to four discrete, alternatively spliced forms of the FSH receptor. Expression of the various transcripts varied at different stages of the ovulatory cycle such that the larger transcripts increased up to 48 hours following PMSG and began to decrease thereafter, reaching a trough 12 hours following hCG administration. Conversely, a smaller transcript reached a peak 9 hours following hCG administration and decreased thereafter.

CONCLUSION

The various transcripts represent different FSH receptor mRNA splicing and may mediate changes in receptor function. Since these alternative spliced forms encode different portions of the extracellular domain, it is possible that they have altered hormone-binding affinity serving a regulatory purpose, such as decreasing hormone binding affinity.

DNA-binding activity of wild-type p53 protein is mediated by the central part of the molecule and controlled by its C terminus

The DNA binding activity of wild type p53 is central to its activity. The "central" part of the molecule, where most mutations appear in primary human tumors, is the actual DNA binding domain. The C-terminal part was shown to exert a negative effect on the DNA binding activity. In the present study we show that while anti-p53 antibodies recognizing the C terminus of the wild type p53 facilitate DNA binding activity, blocking of the wild type specific epitope by specific anti-p53 antibodies, inhibited the DNA binding activity of the wild type p53 protein. An alternatively spliced p53 protein exhibits an augmented DNA binding activity. The fact that most p53 mutants have lost the wild type p53 conformation specific epitope, coupled with the observation that blocking of this site by binding specific antibodies, prevents the interaction of wild type p53 with DNA, suggests that maintaining the correct structural conformation of this site is central for DNA binding activity. Still, the internal structure of the p53 target and particularly the length of the sequence between the two tandem inverted repeats, is critical for protein-DNA interaction behavior.

Molecular alterations in the TP53 gene of peripheral blood cells of patients with chronic myeloid leukemia

The TP53 gene has been extensively studied in patients with chronic myeloid leukemia (CML), both in chronic phase and in blast crisis. Mutations in the gene were found in up to 30% of the patients, especially among those in blast crisis. We report the results of an analysis of 29 blood samples from CML patients: 8 samples from chronic phase patients, 8 from patients in the accelerated phase, and 13 from patients in blast crisis. By using genomic DNA, we sequenced PCR products of the coding exons and most introns of the TP53 gene, finding genetic changes in 30% of the blast crisis samples and 12% in chronic phase. All mutations were found in introns and were previously unreported. Immunocytochemical studies revealed accumulation of TP53 in blood cells of samples both from chronic phase and blast crisis patients. Since these samples had no TP53 mutations, we believe that wild type TP53 accumulates in blood cells of CML patients. Our results, therefore, indicate that molecular changes in coding regions of the TP53 gene are rare. The significance of the abundance of intronic changes should be investigated further. Accumulation of wild type TP53 in CML cells may indicate an additional mechanism involving this gene in the pathogenesis of this disease.

The DNA binding regulatory domain of p53: see the C

The p53 tumor suppressor gene is a critical regulator of normal development involved in cell cycle control pathways, such as growth arrest, differentiation and apoptosis. The DNA binding activity of p53 is central to its function. In addition to the specific DNA binding activity that is confined to the "core" domain of the molecule, the C-terminus seems to play an important role in both controlling the specific as well as exhibiting a non-specific DNA binding activity, which is directly associated with sensing damaged DNA. The C-terminal DNA binding activity appears to be regulated by phosphorylation, glycosylation, splicing and binding of several factors. The C-terminus seems to recognize single and double stranded DNA breaks that occur during DNA replication and recombination, as well as following external DNA stress signals. Unless the cell manages to correct the DNA damage it has the tempting option to progress towards apoptosis. Imagine the C-terminus as a traffic light ensuring the safe "on going" through the cell cycle; in case damaged DNA could not be corrected, p53 dependent apoptosis or terminal differentiation "signs" are turned on!

Role of wild type p53 in the G2 phase: regulation of the gamma-irradiation-induced delay and DNA repair

Upregulation of the p53 protein was shown to induce cell cycle arrest at the G1/S border and in some cases at the G2/M border. Furthermore, it was suggested that p53 is associated with the induction of the various DNA repair pathways. Previously, we demonstrated that cells co-expressing endogenous wild type p53 protein, together with dominant negative mutant p53, exhibit deregulation of apoptosis, G1 arrest and delay in G2 following gamma-irradiation. In the present study, we investigated the role of p53 protein in the DNA damage response at the G2 phase. Using p53-null, wild type p53 and mutant p53-producer cell lines, we found that the two C-terminally spliced p53 forms could prevent gamma-irradiation induced mutagenesis prior to mitosis, at the G2/M checkpoint. We found that at the G2 phase, p53 may facilitate repair of DNA breaks giving rise to micronuclei, and regulate the exit from the G2 checkpoint. At the G1 phase, only the regularly spliced form of p53 caused growth arrest. In contrast, both the regularly and the alternatively spliced p53 forms directed postmitotic micronucleated cells towards apoptosis. These results provide a functional explanation for the cell cycle-independent expression of p53 in normal cycling cells, as well as in cells where p53 is up-regulated, following DNA damage.

Differential expression of the regularly spliced wild-type p53 and its COOH-terminal alternatively spliced form during epidermal differentiation

In the present study, we investigated the role of p53 in the differentiation of epidermal keratinocyte cells. The interrelationship between p53 expression and the various stages of epidermal differentiation and the role of the COOH terminus of the p53 molecule in this process were determined by comparing the expression of the regularly spliced p53 (RSp53) molecule and that of the COOH-terminal alternatively spliced (ASp53) form. p53 mRNA distribution was studied by in situ analysis of frozen skin sections and by reverse transcription-PCR analysis of the various wild-type p53 forms expressed in neonatal skin cell fractions separated by Percoll gradient. p53 protein levels were measured by fluorescence-activated cell sorting analysis and immunohistochemistry, using antibodies that recognize either the COOH terminus of RSp53 or ASp53. The results show that although less mature keratinocyte cells predominantly express the RSp53 form, the more mature cells preferentially express the ASp53 form. Therefore, it is possible that the two p53 forms are associated with different functions required at the various stages of keratinocyte differentiation. The results suggest that the COOH-terminal domain of the p53 molecule is important for its activity in the process of keratinocyte differentiation.

Expression of p53 in differentiation and apoptosis and its deregulation in tumor cell

The observation that wild type p53 may induce cells to undergo either apoptosis or differentiation raises the question of whether these two events share similar p53-dependent pathways. To evaluate the interrelationship between these two p53-dependent processes, our study focused on the human HL-60, a pro-myelocytic p53 non-producer cell line in which p53 expression was introduced and the induction of apoptosis and differentiation was followed under controlled conditions. p53 expression was induced in the HL-60 cell line by infection with the recombinant wild type p53 (p53WT) vaccinia virus. Viral infection gave rise to cells expressing various levels of wild type p53 protein. High levels of p53 protein induced cells to undergo rapid apoptosis, whereas lower levels of p53 protein induced cells to undergo cell differentiation at a more moderate rate of kinetics. These results suggest that p53 protein levels may determine whether a given cell should prefer one pathway over the other to exit the cell cycle.

The murine C'-terminally alternatively spliced form of p53 induces attenuated apoptosis in myeloid cells

The onset of p53-dependent apoptosis results from the accumulation of damaged DNA. Recently, it was shown that the C' terminus of the p53 protein plays a central role in sensing damaged DNA. In our present study, we examined the role of the C' terminus in the induction of apoptosis. A temperature-sensitive (ts) mutant of the alternatively spliced form of p53 (p53AS-ts) and the ts mutant of the regularly spliced form (p53RS-ts) were used to generate series of stable clones with increasing amounts of p53 protein. Apoptotic patterns induced by either the regularly spliced p53 product (p53RS) or a C'-terminally alternatively spliced p53 product (p53AS) were compared. We found that although both forms of p53 induced apoptosis following expression of the wild-type protein conformation, the kinetics were different. Apoptosis induced by the p53AS protein was attenuated compared to that induced by p53RS. The delay in the manifestation of the apoptotic features following p53AS expression was in agreement with a delay in the regulation of the expression of apoptosis-related genes. The observation that p53 with an altered C' terminus is still capable of inducing apoptosis suggests that the actual onset of the apoptotic process most probably involves structural domains other than the C' terminus of the p53 molecule. However, the fact that the apoptotic activity mediated by the p53AS product was slower than that mediated by the p53RS product suggests that the C' terminus indeed exerts a certain control on the apoptotic activity of the p53 molecule.

PACT: cloning and characterization of a cellular p53 binding protein that interacts with Rb

Cellular functions of tumor suppressor proteins can be mediated by protein-protein interactions. Using p53 as a probe to screen an expression library, a cDNA encoding a 250 kDa protein was isolated. Recombinant forms of this protein, designated PACT, bind to wild type p53 while two different mutations abolish this interaction. PACT protein can also interfere with p53 specific DNA binding. PACT contains a serine/arginine (SR) rich region and a C' terminal lysine rich domain. The 250 kDa PACT protein can be precipitated from cell lysates by a method specific for SR proteins. snRNPs can be co-immunoprecipitated from cells with anti-PACT antibodies. These antibodies stain cell nuclei in a speckled pattern reminiscent of the distribution of known splicing factors. Recently, RBQ1, a truncated human homologue of PACT was identified by virtue of Rb binding. We show that RBQ1 is truncated as a result of a possible mutational event. PACT can interact with both cellular Rb and p53.

A novel transcript encoded within the 10-kb first intron of the human p53 tumor suppressor gene (D17S2179E) is induced during differentiation of myeloid leukemia cells

Two promoters were previously shown to map to the 5'-end of the human p53 gene. p53p1 was located upstream of the first exon and is responsible for transcription of the major p53 mRNA species. p53p2 is a stronger promoter than p53p1 and was located within the 10, 738-bp first intron, approximately 1000 bp downstream of exon 1. mRNA transcripts that initiated from p53p2 were previously identified in HL-60 cells by primer extension analysis and were observed to increase in abundance during differentiation of HL-60 cells to granulocytes. By screening a cDNA library with a probe derived from sequences downstream of the p53p2 start site, we have cloned and characterized a cDNA that represents a mRNA that appears to have been initiated from the p53p2 promoter. We have designated the gene encoding this transcript Hp53int1 (the GDB designation is D17S2179E). The cDNA is 1125 bp and is polyadenylated downstream from a consensus poly(A) addition site. The entire 1125 bp is derived from intron 1 of the p53 gene, with no introns having been removed. The cDNA contains no major open reading frame although reading frame +1 contains a relatively low abundance of stop codons compared to the other two reading frames and could possibly encode a protein of 119 amino acids. Analysis of the +1 reading frame shows a region of high homology to a portion of the DNA-binding domain of NF-kappaB. These results indicate that a novel polyadenylated transcript is encoded by the first intron of the human p53 gene. Hp53int1 may be a pseudogene for a gene that may have encoded a DNA-binding protein. Alternatively, the transcript may have a function, since RNA transcripts of this gene are present in a number of human cells and their levels are induced during terminal differentiation of myeloid leukemia cells such as HL-60 and U937.

The role of p53 in the induction of polyploidity of myelomonocytic leukemic M1/2 cells

p53 was shown to play a central role in the maintenance of genomic integrity. The present experiments suggest that p53 is involved in the control of cell ploidity. Using a p53 non-producer cell line, M1/2, that was reconstituted to express either wild type or mutant p53 protein, by infection with the temperature sensitive (Ts) p53Val135 virus, it was found that both loss of wild type p53 or overexpression of mutant p53, may be associated with the generation of cell polyploidity. Overexpression of mutant p53 protein enhanced the appearance of giant cells that further accumulated following gamma-irradiation. Expression of wild type p53 reduced the level of giant cells which accumulated in the parental M1/2 p53 non-producer cells following gamma-irradiation. This activity of the wild type p53 seems to be mediated by either the reduction in the rate of giant cell generation, as observed in M1/2 derived cell lines expressing low levels of wild type p53 protein or by facilitating their apoptosis, as observed in wild type p53 high-producer cells. The latter conclusion is further supported by the observation that isolated giant cells are directly induced to undergo apoptosis following wild type p53 expression.

Induction of apoptosis and p53 expression in immature thymocytes by direct interaction with thymic epithelial cells

Apoptosis of normal thymocytes was shown to be triggered by several mechanisms (e.g. glucocorticoids, gamma-irradiation). In the present study the authors report on thymocyte apoptosis that is induced by thymic epithelial cells. The thymocytes undergo a massive apoptotic death within 24 h of cocultivation with thymic epithelial cell monolayers derived from primary cultures (PTEC) or from a thymic epithelial cell line (TEC). Non-thymic monolayers were inactive. Apoptosis induction in this experimental model requires direct contact between the thymocytes and the thymic epithelial monolayer and can be blocked by anti-CD2 and anti-LFA-1 antibodies. The immature CD3-/+dull CD4+CD8+ thymocytes were the cells which undergo apoptosis. The fact that the authors are dealing with a massive apoptotic process of immature cells in the absence of exogenous antigen suggests that it involves the nonselected thymocytes. The apoptotic pathway selected by thymocytes following their culturing on TEC involves p53 expression. Indeed it was found that TEC-induced apoptosis, led to the accumulation of p53 protein that preceded the step of DNA fragmentation in freshly isolated thymocytes as well as in a glucocorticoid resistant thymoma cell line. Since glucocorticoid-induced thymocyte apoptosis is p53-independent, glucocorticoids are conceivably not involved in TEC-induced thymocyte death. The in vitro experimental model presented here may reflect the physiological sequence of events leading to thymocyte death in the thymus.

p53 plays a regulatory role in differentiation and apoptosis of central nervous system-associated cells

This study demonstrated the involvement of the tumor suppressor protein p53 in differentiation and programmed cell death of neurons and oligodendrocytes, two cell types that leave the mitotic cycle early in development and undergo massive-scale cell death as the nervous system matures. We found that primary cultures of rat oligodendrocytes and neurons, as well as of the neuronal PC12 pheochromocytoma cell line, constitutively express the p53 protein. At critical points in the maturation of these cells in vitro, the subcellular localization of p53 changes: during differentiation it appears mainly in the nucleus, whereas in mature differentiated cells it is present mainly in the cytoplasm. These subcellular changes were correlated with changes in levels of immunoprecipitated p53. Infection of cells with a recombinant retrovirus encoding a C-terminal p53 miniprotein (p53 DD), previously shown to act as a dominant negative inhibitor of endogenous wild-type p53 activity, inhibited the differentiation of oligodendrocytes and of PC12 cells and protected neurons from spontaneous apoptotic death. These findings suggest that p53, upon receiving appropriate signals, is recruited into the nucleus, where it plays a regulatory role in directing primary neurons', oligodendrocytes, and PC12 cells toward either differentiation or apoptosis in vitro.

Cooperation between p53-dependent and p53-independent apoptotic pathways in myeloid cells

Apoptosis may involve p53-dependent and -independent pathways. Results presented here suggest a possible cooperation between these two types of pathways. M1/2 is a p53-nonproducer subclone that may undergo either a p53-independent apoptosis following growth factor deprivation or a p53-dependent apoptosis following reconstitution of wild-type p53 expression. The p53-independent apoptosis in these cells is a slow process occurring after a G0-G1 arrest. In contrast, the p53-dependent apoptosis is much more rapid and is characterized by early and late apoptotic phases, taking place in cell arrested at G0-G1 and S phase. The transition from early to late apoptosis correlated with the levels of the p53 protein. Concomitant induction of both apoptotic pathways accelerated cell death and facilitated the transition from early to late apoptotic phase. The interaction between these pathways is further supported by the finding that mutant p53 interferes with p53-independent apoptosis. Thus, although apoptosis can occur via either p53-dependent or -independent pathways, under certain conditions the two pathways may interact with each other.

Induction of HL-60 cells to undergo apoptosis is determined by high levels of wild-type p53 protein whereas differentiation of the cells is mediated by lower p53 levels

The observation that wild-type p53 may induce cells to undergo either apoptosis or differentiation raises the question of whether these two events share similar p53-dependent pathways. To evaluate the interrelationship between these two p53-dependent processes, our study focused on the human HL-60, a promyelocytic p53 nonproducer cell line in which p53 expression was introduced and the induction of apoptosis and differentiation was followed under controlled conditions. p53 expression was induced in the HL-60 cell line by infection with the recombinant wild-type p53 (p53WT) vaccinia virus. Viral infection gave rise to cells expressing various levels of wild-type p53 protein. High levels of p53 protein induced cells to undergo rapid apoptosis, whereas lower levels of p53 protein induced cells to undergo cell differentiation at a more moderate rate of kinetics. These results suggest that p53 protein levels may determine whether a given cell should prefer one pathway over the other to exit the cell cycle. Accordingly, we propose that the p53 vaccinia virus may be used as a potential vector for cell therapy leading toward the exit of p53 null human primary hematopoetic tumors from the malignant state in vivo via the apoptotic or cell differentiation pathways.

A novel polymorphism in intron 6 of the human p53 gene: a possible association with cancer predisposition and susceptibility

We present a novel polymorphic 8-bp sequence in intron 6 of the p53 gene that maps between bp 55 and 62 of the 3' end of exon 6. Of normal blood samples, 32% were heterozygotic for this polymorphism and display a NN' genotype, whereas 68% of the population is homozygotic for the N genotype. The rare homozygotic genotype N' was detected only in four blood samples of cancer patients. Peripheral blood of gastrointestinal (GI) and breast tumor patients demonstrated a higher incidence of heterozygosity (50%) than that of normal individuals. Analysis of the distribution of this polymorphism in tumor samples showed loss of heterozygosity (LOH). This LOH during tumor progression could exhibit preference to each one of the polymorphic alleles. The rare presentation of one allele and the increased incidence of heterozygosity in carcinoma patients may suggest an association between this polymorphism with cancer predisposition and susceptibility. The fact that genetic alterations occurring in noncoding regions may play a role in tumor development only further increases the extent of involvement of p53 in carcinogenesis.