P53 mutation in acute T cell lymphoblastic leukemia is of somatic origin and is stable during establishment of T cell acute lymphoblastic leukemia cell lines

TP53 lacks tetramerization and N-terminal domains due to novel inactivating mutations detected in leukemia patients

Background

TP53 is a highly conserved tumor suppressor gene present on chromosome 17 and comprised 11 exons and 12 introns. The TP53 protein maintained the genomic integrity of the cell by regulating different pathways. The association of TP53 with leukemia and the increasing prevalence of leukemia in Pakistan instigated us to initiate the current study.

Materials and Methods

The TP53 gene of acute myeloid leukemia patients (n = 23) and normal individuals (n = 30) was amplified through polymerase chain reaction (PCR). The PCR amplified products of 3 samples 1 normal (NC-30) and 2 cancerous (LK-6 and LK-19) were subjected to deoxyribonucleic acid (DNA) sequence analysis. Bioinformatics analysis of the obtained DNA sequences helped to identify nature, type, and functional impact of mutations, if any.

Results

Results revealed 2 novel mutations in Case No. 1 (c. G >A10987 and c. InsA13298_13299) and Case No. 2 (c. InsC13284_13285, c. T >A13365) which generate a premature codon (ocher) at position 239 and lead to truncated TP53 protein. In Case No. 3, 16 novel mutations were identified and c. delC11093 mutation created a premature codon (opal) at 59^th position. Hence, the resultant protein will lack its tetramerization and N-terminal domain required for its normal functioning. Moreover, some intronic mutations were noticed and found to have a negative impact on splicing related regulatory sequences.

Conclusion

Results suggest the role of TP53 inactivating mutations in pathogenesis of leukemia.

LEF1 Induces DHRS2 Gene Expression in Human Acute Leukemia Jurkat T-Cells

Objective

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive disease resulting from the accumulation of genetic changes that affect the development of T-cells. The precise role of lymphoid enhancer-binding factor 1 (LEF1) in T-ALL has been controversial since both overexpression and inactivating LEF1 mutations have been reported to date. Here, we investigate the potential gene targets of LEF1 in the Jurkat human T-cell leukemia cell line.

Materials and Methods

We used small interfering RNA (siRNA) technology to knock down LEF1 in Jurkat cells and then compared the gene expression levels in the LEF1 knockdown cells with non-targeting siRNA-transfected and non-transfected cells by employing microarray analysis.

Results

We identified DHRS2, a tumor suppressor gene, as the most significantly downregulated gene in LEF1 knockdown cells, and we further confirmed its downregulation by real-time quantitative polymerase chain reaction (qRT-PCR) in mRNA and at protein level by western blotting.

Conclusion

Our results revealed that DHRS2 is positively regulated by LEF1 in Jurkat cells, which indicates the capability of LEF1 as a tumor suppressor and, together with previous reports, suggests that LEF1 exhibits a regulatory role in T-ALL via not only its oncogenic targets but also tumor suppressor genes.

Abnormal expression of p27kip1 protein in levator ani muscle of aging women with pelvic floor disorders - a relationship to the cellular differentiation and degeneration

BACKGROUND: Pelvic floor disorders affect almost 50% of aging women. An important role in the pelvic floor support belongs to the levator ani muscle. The p27/kip1 (p27) protein, multifunctional cyclin-dependent kinase inhibitor, shows changing expression in differentiating skeletal muscle cells during development, and relatively high levels of p27 RNA were detected in the normal human skeletal muscles. METHODS: Biopsy samples of levator ani muscle were obtained from 22 symptomatic patients with stress urinary incontinence, pelvic organ prolapse, and overlaps (age range 38-74), and nine asymptomatic women (age 31-49). Cryostat sections were investigated for p27 protein expression and type I (slow twitch) and type II (fast twitch) fibers. RESULTS: All fibers exhibited strong plasma membrane (and nuclear) p27 protein expression. cytoplasmic p27 expression was virtually absent in asymptomatic women. In perimenopausal symptomatic patients (ages 38-55), muscle fibers showed hypertrophy and moderate cytoplasmic p27 staining accompanied by diminution of type II fibers. Older symptomatic patients (ages 57-74) showed cytoplasmic p27 overexpression accompanied by shrinking, cytoplasmic vacuolization and fragmentation of muscle cells. The plasma membrane and cytoplasmic p27 expression was not unique to the muscle cells. Under certain circumstances, it was also detected in other cell types (epithelium of ectocervix and luteal cells). CONCLUSIONS: This is the first report on the unusual (plasma membrane and cytoplasmic) expression of p27 protein in normal and abnormal human striated muscle cells in vivo. Our data indicate that pelvic floor disorders are in perimenopausal patients associated with an appearance of moderate cytoplasmic p27 expression, accompanying hypertrophy and transition of type II into type I fibers. The patients in advanced postmenopause show shrinking and fragmentation of muscle fibers associated with strong cytoplasmic p27 expression.

Relationship of p15 and p16 gene alterations to elevated dihydrofolate reductase in childhood acute lymphoblastic leukaemia

The downstream effects of p15 and p16 gene deletions and loss of transcripts on dihydrofolate reductase (DHFR) were examined in 63 B-precursor (BP) acute lymphoblastic leukaemia (ALL) samples. p15 and/or p16 gene deletions were seen in 6% and 8%, respectively, of BP-ALL samples; however, losses of p15 and/or p16 transcripts were seen in 26 out of 63 (41%) samples. Loss of p15 transcripts (36.5%) exceeded that for p16 (17.5%). For the 26 BP-ALLs that lacked p15 and/or p16 transcripts, only six (23%) exhibited low levels of DHFR by flow cytometry assay with Pt430, a fluorescent anti-folate. Conversely, 18 out of 37 (49%) BP-ALL samples with intact p15 and/or p16 genes and transcripts showed low levels of DHFR (P = 0.04). In p15- and p16-null K562 cells transfected with a tetracycline-inducible p15 cDNA construct, induction of p15 transcripts and protein was accompanied by decreased growth rates, decreased S-phase fraction, decreased retinoblastoma protein phosphorylation, and markedly reduced levels of DHFR transcripts and protein. Collectively, our results suggest that losses of p15 and/or p16 gene expression result in elevated levels of DHFR in BP-ALL in children. However, additional downstream factors undoubtedly also contribute to elevated levels of this enzyme target.

[Mutational analysis of p53 in T-cell lymphoblastic leukemia]

BACKGROUND

p53 mutations were analysed in a consecutive series of patients with T-ALL from one hospital over a period of 9 years.

PATIENTS AND METHOD

Thirty two samples from 27 patients with T-ALL were included in the study. Exons 5-9 were analysed using SSCP.

RESULTS

A mutation at codon 273 was identified in a child in third relapse. The mutated clone became dominant in a sample obtained one month following a course of salvage chemotherapy.

CONCLUSION

p53 mutations are not frequent in T-ALL even in patients at relapse.

p53AIP1, a potential mediator of p53-dependent apoptosis, and its regulation by Ser-46-phosphorylated p53

Through direct cloning of p53 binding sequences from human genomic DNA, we have isolated a novel gene, designated p53AIP1 (p53-regulated Apoptosis-Inducing Protein 1), whose expression is inducible by wild-type p53. Ectopically expressed p53AIP1, which is localized within mitochondria, leads to apoptotic cell death through dissipation of mitochondrial A(psi)m. We have found that upon severe DNA damage, Ser-46 on p53 is phosphorylated and apoptosis is induced. In addition, substitution of Ser-46 inhibits the ability of p53 to induce apoptosis and selectively blocks expression of p53AIP1. Our results suggest that p53AIP1 is likely to play an important role in mediating p53-dependent apoptosis, and phosphorylation of Ser-46 regulates the transcriptional activation of this apoptosis-inducing gene.

Pooled analysis of p53 mutations in hematological malignancies

A computerized database is described that contains information about 507 mutations in the p53 gene of hematologic tumors and corresponding cell lines. Analysis of these mutations indicated the following findings: First, mutational spectrum analysis in these tumors was found to be similar to the pattern found for other solid tumors. However, when the patterns of base substitutions were examined separately according to the types of hematologic malignancies, followed by subgroup analysis, notable differences (in some cases of statistical significance) emerged. Second, mutational pattern analysis indicates that about 48% of base substitutions in hematologic tumors are suspected to be associated with carcinogen exposure. Third, deletions and insertions are localized mainly to exons 5-8 and repeated DNA sequences. However, the unusual profile of variations in frequency within each type of tumor suggests that, in addition to endogenous damage to template DNA, there is the factor of exposure to environmental physical and chemical carcinogens/mutagens. Fourth, p53 protein alterations analysis indicate that most of the changes in the amino acids are "semiconservative," presumably in order to avoid disrupting the structure of the p53 monomer. Consistent with this notion, structural mutations are more conservative than the binding mutations. Finally, molecular mechanisms that lead to p53 mutations, etiological factors that play a role in their formation, and the pathophysiological significance of consequent p53 protein alterations are discussed.

p53 alterations in human leukemia-lymphoma cell lines: in vitroartifact or prerequisite for cell immortalization?

Alteration of the p53 gene is one of the most frequent events in human tumorigenesis. The inactivation of p53 tumor suppressor function can be caused by chromosome deletion, gene deletion, or mainly by point mutations. p53 mutations occur moderately often in hematopoietic malignancies. A significantly higher frequency of p53 alterations in cell lines vs primary samples has been observed for all types of malignant hematopoietic cell lines. It has been postulated that p53 gene abnormalities arise in cell lines during in vitro establishment of the culture or prolonged culture; but it is also conceivable that those cases that carry p53 mutations may be more suitable for in vitro establishment as permanent cell lines. We analyzed data on the p53 gene status in a panel of matched primary hematopoietic tumor cells and the respective cell lines derived from this original material. In 85% (53/62) of the pairs of matched primary cells and cell lines, the in vivo and in vitro data were identical (both with p53 wild-type or both with the same p53 mutation). In some instances, serial clinical samples (eg at presentation and relapse) and serial sister cell lines were available. These cases showed that a clinical sample at presentation often had a p53 wild-type configuration whereas the derived cell line and a relapse specimen carried an identical p53 point mutation. These findings suggest that a minor clone, at first undetectable by standard analysis, represents a reservoir for the outgrowth of resistant cells in vivo and also a pool of cells with a growth advantage in vitro, providing a significantly higher chance of immortalization in culture. This was further supported by studies employing mutant allele-specific gene amplifications, a technique which is significantly more sensitive (100- to 1000-fold) than the commonly applied SSCP assay with a sensitivity threshold of about 10% mutated cells within a pool of wild-type cells. Taken together, this analysis confirms the usefulness of human hematopoietic cell lines as in vitro model systems for the study of the biology of hematopoietic malignancies. It further underlines the notion that p53 gene alterations confer a survival advantage to, at least some, malignant cells in vitro and presumably also in vivo; however, it is highly unlikely that a p53 mutation alone would suffice for the immortalization of a cell line in vitro or tumor development in vivo. Leukemia (2000) 14, 198-206.

Molecular and cellular correlates of methotrexate response in childhood acute lymphoblastic leukemia

The improved outlook for children diagnosed today with acute lymphoblastic leukemia (ALL) over that 40 years ago is remarkable. With modern therapies and supportive care, complete remissions are achieved in up to 95% of patients and long-term disease-free survival rates approach 80%. Methotrexate is a key component in ALL consolidation and maintenance therapies and is administered intrathecally in the prophylaxis and treatment of central nervous system leukemia. Recent reports have significantly extended the results of preclinical studies of methotrexate response and resistance to patients with ALL. The application of new and sensitive molecular biology techniques makes it possible to study specific chromosomal and genetic alterations [t(12;21), hyperdiploidy, deletions or methylation of p15INK4B and p16INK4A] which potentially contribute to methotrexate response and resistance in childhood ALL. Studies of the relationships between genetic alterations and ALL progression, methotrexate pharmacology, and long term event-free-survivals may lead to the better identification of subgroups of patients who exhibit unique levels of sensitivity or resistance to chemotherapy including methotrexate. Further, by characterizing the roles of translocation-generated fusion genes (TEL-AML 1) and tumor suppressor genes (p15INK4B and p16INK4A) in treatment response, it may be possible to identify new and selective targets and/or treatment strategies for both children and adults with ALL who are refractory to current therapies.

Detecting single base substitutions, mismatches and bulges in DNA by temperature gradient gel electrophoresis and related methods

Temperature gradient gel electrophoresis (TGGE) and related methods can separate DNA fragments that differ by a single base pair or defect. This article describes the basic features of TGGE, and reviews the theoretical model of DNA unwinding and its ability to predict DNA mobility in a temperature gradient gel. Recent applications of TGGE and related methods that were directed at detecting point mutations, and evaluating the effects of single site defects are also reported.

Increased Frequency of Expression of Elevated Dihydrofolate Reductase in T-Cell Versus B-Precursor Acute Lymphoblastic Leukemia in Children

The relationships between dihydrofolate reductase (DHFR) levels or methotrexate membrane transport and acute lymphoblastic leukemia (ALL) immunophenotype were evaluated on 51 T-cell and 44 B-precursor ALL specimens from 90 pediatric ALL patients at diagnosis and relapse, using a fluorescent methotrexate analog (PT430) and flow cytometry assay (Matherly et al, Blood 85:500, 1995). For T-cell ALL, 35 of 45 (78%) of newly diagnosed patients' specimens exhibited elevated DHFR relative to DHFR levels in ALL blasts from methotrexate-responsive patients. For 30 of 45 diagnostic T-ALL specimens, DHFR expression was heterogeneous, with up to 3 separate subpopulations covering a 44-fold range; the DHFR-overproducing fractions comprised 10% to 88% of the total blasts. Elevated DHFR was less common in B-precursor ALL at diagnosis, being detected in only 17 of 36 specimens (47%); 11 of these samples exhibited DHFR heterogeneity. Median maximal DHFR levels were fourfold higher in T-cell than B-precursor ALL at diagnosis. Within a particular phenotypic group, there were no correlations between DHFR levels and patient prognostic features, including age, sex, chromosomal abnormalities, white blood cell counts (WBCs), and percentage of S-phase. However, for B-precursor ALL, there was a notably higher fraction of African-American than white patients with elevated DHFR. For patients (both phenotypes) with low WBCs (<50,000/μL), event-free survival times were significantly shorter for those expressing DHFR above a threshold level than for patients expressing DHFR below this level (P < .016); this relationship was not seen for patients with high WBCs (<50,000/μL). Elevated DHFR was detected in 11 of 14 relapse specimens (5 of 6 T-cell and 6 of 8 B-precursor). Two of five paired relapse specimens (both T-cell) from patients treated with methotrexate exhibited increased DHFR levels over those at diagnosis (2.5- to 5-fold); the fraction of DHFR-overproducing blasts was also increased in 4 of 5 paired relapse specimens (2 B-precursor and 2 T-cell). In contrast to the variations in DHFR, highly impaired methotrexate transport was detected in only 6 of 95 ALL specimens, including both diagnosis and relapse. There was no correlation between phenotype and impaired transport. These data provide further rationale for the use of mechanistically based prognostic factors to complement known biologic or disease-based prognostic indicators in the design of ALL therapy including methotrexate, particularly with patients presenting with low WBCs. The finding of a markedly increased frequency of DHFR overexpression in T-cell over B-precursor ALL suggests that this difference is associated with the poorer prognosis of patients with T-cell ALL treated with standard-dose antimetabolite therapy and implies that higher-dose methotrexate (≥1 g/m2) during consolidation therapy may be useful in the treatment of this disease.

Increased Frequency of Expression of Elevated Dihydrofolate Reductase in T-Cell Versus B-Precursor Acute Lymphoblastic Leukemia in Children

The relationships between dihydrofolate reductase (DHFR) levels or methotrexate membrane transport and acute lymphoblastic leukemia (ALL) immunophenotype were evaluated on 51 T-cell and 44 B-precursor ALL specimens from 90 pediatric ALL patients at diagnosis and relapse, using a fluorescent methotrexate analog (PT430) and flow cytometry assay (Matherly et al, Blood 85:500, 1995). For T-cell ALL, 35 of 45 (78%) of newly diagnosed patients' specimens exhibited elevated DHFR relative to DHFR levels in ALL blasts from methotrexate-responsive patients. For 30 of 45 diagnostic T-ALL specimens, DHFR expression was heterogeneous, with up to 3 separate subpopulations covering a 44-fold range; the DHFR-overproducing fractions comprised 10% to 88% of the total blasts. Elevated DHFR was less common in B-precursor ALL at diagnosis, being detected in only 17 of 36 specimens (47%); 11 of these samples exhibited DHFR heterogeneity. Median maximal DHFR levels were fourfold higher in T-cell than B-precursor ALL at diagnosis. Within a particular phenotypic group, there were no correlations between DHFR levels and patient prognostic features, including age, sex, chromosomal abnormalities, white blood cell counts (WBCs), and percentage of S-phase. However, for B-precursor ALL, there was a notably higher fraction of African-American than white patients with elevated DHFR. For patients (both phenotypes) with low WBCs (<50,000/μL), event-free survival times were significantly shorter for those expressing DHFR above a threshold level than for patients expressing DHFR below this level (P < .016); this relationship was not seen for patients with high WBCs (<50,000/μL). Elevated DHFR was detected in 11 of 14 relapse specimens (5 of 6 T-cell and 6 of 8 B-precursor). Two of five paired relapse specimens (both T-cell) from patients treated with methotrexate exhibited increased DHFR levels over those at diagnosis (2.5- to 5-fold); the fraction of DHFR-overproducing blasts was also increased in 4 of 5 paired relapse specimens (2 B-precursor and 2 T-cell). In contrast to the variations in DHFR, highly impaired methotrexate transport was detected in only 6 of 95 ALL specimens, including both diagnosis and relapse. There was no correlation between phenotype and impaired transport. These data provide further rationale for the use of mechanistically based prognostic factors to complement known biologic or disease-based prognostic indicators in the design of ALL therapy including methotrexate, particularly with patients presenting with low WBCs. The finding of a markedly increased frequency of DHFR overexpression in T-cell over B-precursor ALL suggests that this difference is associated with the poorer prognosis of patients with T-cell ALL treated with standard-dose antimetabolite therapy and implies that higher-dose methotrexate (≥1 g/m2) during consolidation therapy may be useful in the treatment of this disease.

Differential in vitro effects of physiological and atmospheric oxygen tension on normal human peripheral blood mononuclear cell proliferation, cytokine and immunoglobulin production

Since the early work of Mischell & Dutton (Science 153, 1004-1008, 1966), it has been recognized that certain lymphocyte cultures are exquisitely sensitive to the harsh effects of atmospheric oxygen tension. The influence of oxygen partial pressure (pO2) on normal human peripheral blood mononuclear cell (PBMC) phenotype, proliferative ability, cytokine, immunoglobulin production, and redox status was examined by culturing PBMC under ambient oxygen (high pO2) or a more physiological pO2 (5% O2; low pO2). Low pO2 conditions promoted a significant increase in overall viable PBMC number and enhanced Concanavalin A (Con A)- or pokeweed mitogen (PWM)-stimulated PBMC proliferation by approximately 30% and 50%, respectively. No differential pO2 effects were apparent on phytohemagglutinin (PHA)- or staphylococcal enterotoxin B (SEB)-induced proliferation. Both resting and Con A-stimulated lymphocytes incubated for 24 h under high pO2 had a greater baseline carboxy-2',7'-dichlorofluorescin (C-DCF) fluorescence, and were less able to quench the effect of H2O2 treatment compared to lymphocytes cultured under low pO2 conditions. Supernatant gamma-IFN, IL-2, and IL-4 concentrations were elevated 50-65% when PBMC were stimulated with Con A for 24 h under low pO2; however, lipopolysaccharide (LPS)-stimulated IL-1 beta production was reduced by over 75%. PWM-stimulated IgM production by PBMC was significantly reduced in day 7 cultures incubated under low pO2, whereas IgG and IgA production remained relatively unaltered. Immunophenotyping analyses did not reveal any significant alterations in cell subset or marker distribution at the time points examined; however, an interesting trend of increased CD69 expression was observed for Con A-stimulated PBMC incubated under low pO2. These results demonstrate that O2 is a critical parameter for the in vitro culture of lymphocytes, and suggests that varying pO2 may differentially alter PBMC functionality.

p53-dependent G1 arrest and p53-independent apoptosis influence the radiobiologic response of glioblastoma

PURPOSE

Loss of the p53 tumor suppressor gene has been associated with tumor progression, disease relapse, poor response to antineoplastic therapy, and poor prognosis in many malignancies. We have investigated the contribution of p53-mediated radiation-induced apoptosis and G1 arrest to the well described radiation resistance of glioblastoma multiforme (GM) cells.

METHODS AND MATERIALS

Radiation survival in vitro was quantitated using linear quadratic and repair-saturation mathematical models. Isogenic derivatives of glioblastoma cells differing only in their p53 status were generated using a retroviral vector expressing a dominant negative mutant of p53. Radiation-induced apoptosis was assayed by Fluorescence-activated cell sorter (FACS) analysis, terminal deoxynucleotide transferase labeling technique, and chromatin morphology. Cells were synchronized in early G1 phase and mitotic and labeling indices were measured.

RESULTS

Radiation-induced apoptosis of GM cells was independent of functional wild-type p53 (wt p53). Decreased susceptibility to radiation-induced apoptosis was associated with lower alpha values characterizing the shoulder of the clonogenic radiation survival curve. Using isogenic GM cells differing only in their p53 activity, we found that a p53-mediated function, radiation-induced G1 arrest, could also influence the value of alpha and clonogenic radiation resistance. Inactivation of wt p53 function by a dominant negative mutant of p53 resulted in a significantly diminished alpha value with no alteration in cellular susceptibility to radiation-induced apoptosis. The clonal derivative U87-LUX.8 expressing a functional wt p53 had an alpha (Gy-1) value of 0.609, whereas the isogenic clonal derivative U87-175.4 lacking wt p53 function had an alpha (Gy-1) value of 0.175.

CONCLUSION

We conclude that two distinct cellular responses to radiation, p53-independent apoptosis and p53-dependent G1-arrest, influence radiobiological parameters that characterize the radiation response of glioblastoma cells. Further understanding of the molecular basis of GM radiation resistance will lead to improvement in existing therapeutic modalities and to the development of novel treatment approaches.

p53 mediated tumor cell response to chemotherapeutic DNA damage: a preliminary study in matched pairs of breast cancer biopsies

Wild type p53 plays a crucial role in maintaining genomic stability in both normal and tumor cells in vitro. When DNA damage occurs, p53 acts as a cell cycle checkpoint and induces a cellular response that aims at restoring genomic integrity. p53 may either allow the repair of damaged DNA by inducing a transient G1 arrest or may eliminate the damaged cells by triggering apoptosis. Mutant p53 fails to mediate any of these effects. From this, a p53 status-dependent response to therapy might be expected when tumors are treated with DNA-damaging genotoxic agents: Although wild type p53-harboring tumors have an intact checkpoint that might allow them to restore genomic integrity back to a pre-exposure level, mutant p53 tumors have a corrupted checkpoint that could lead to an accelerated loss of genomic stability. Until now, no studies have been described that examine such a p53-mediated effect in vivo. The authors tested this response model in vivo comparing 32 matched biopsy pairs from patients with breast cancer before and after rigorously standardized polychemotherapy. Four of the five drugs specifically induce a wild type p53-mediated checkpoint response. Tumor tissue from matched pairs of untreated and treated biopsies of the same patient were analyzed for treatment-associated changes of p53 protein expression by immunocytochemistry and, in a few available specimens, of p53 genotype changes by polymerase chain reaction-based DNA analysis. Treatment-associated changes of the p53 immunophenotype, which the authors speculate to reflect clonal selection, occurred in 39% (12 of 31) of the specimens. One specimen was not informative. Most tumors undergoing clonal selection originally harbored mutant p53 (nine of 12), and only three of 12 tumors were wild type. This study shows that exposure to genotoxic agents is commonly associated with a change in p53 immunophenotype. Although the limited material in this cohort prevented direct analysis of genetic instability, these results suggest that tumors with altered p53 may be genomically less stable and, therefore, may be more likely to undergo treatment-induced clonal changes than wild type tumors. This study also shows that the rigorous matched sample approach, although difficult to obtain, is an important tool that allows the in vivo assessment of the tumor response to genotoxic therapy in a controlled fashion.

Absence of p53 mutation in Japanese patients with malignant thyroid lymphoma

Abnormalities of the gene encoding the sequence-specific DNA-binding protein, nuclear phosphorprotein p53, are among the most common genetic alterations observed in human cancers. A mutation of this tumor suppressor gene has been reported with a low prevalence in differentiated thyroid carcinomas, while the prevalence was high in undifferentiated thyroid carcinomas. We used denaturing gradient gel electrophoresis (DGGE) to probe for mutations of p53 gene in order to determine its role in the genesis of malignant thyroid lymphoma. Involvement of 27 samples had been proven by histopathologic examination of specimen obtained by open biopsy of the thyroid gland or from cervical lymph nodes. DNA was extracted from tissues embedded in paraffin blocks and exons 5-8 of p53 gene were examined for the presence of mutations by DGGE following amplification by PCR using GC-clamped primers. To confirm accuracy of the method, samples with known p53 mutations were included in the study. No mutations were detected in any of the amplified exons of malignant thyroid lymphoma samples. These results suggest that p53 mutations are not present or are uncommon in Japanese patients with malignant thyroid lymphomas. The role of p53 in this form of carcinogenesis cannot be fully excluded since we have not examined the occurrence of mutations in regions upstream of exon 5.

Gain-of-function mutations of the p53 gene induce lymphohematopoietic metastatic potential and tissue invasiveness

Leukemia cell infiltration and the induction of lethal hematopoietic disease in immune-deficient SCID mice transplanted with human T cell acute lymphoblastic T leukemia (T-ALL) cells occurred only when the cells possessed mutant p53 genes and lacked a wild-type allele or when T-ALL cells lacking p53 protein were infected with specific mutant p53 genes. A series of six mutant p53 genes were cloned from relapse T-ALL-derived cell lines and were constructed into defective retroviral expression vectors. Viruses encoding mutant p53 proteins were used to infect relapse T-ALL cells in a study designed to compare their pathogenic potency. The mutant p53 genes possessed a distinct hierarchy in vivo and in vitro: mutants inducing the greatest increase in proliferation of different T-ALL lines in vitro and colony formation in methylcellulose cultures also induced tissue invasiveness of infected T-ALL cells in vivo. Mutant p53 gene transfer to a cell line lacking p53 protein showed that the more potent p53 mutants possessed a distinctive dominant oncogenic activity in vitro and in vivo. The dominant oncogenic activity of these mutant p53 proteins was not dependent on the presence of and on complex formation with wild-type p53 protein. These "hot" p53 mutations thus represent bona fide gain-of-function mutations. Infection of p53-negative T-ALL cells with viruses encoding gain-of-function mutant p53 genes resulted in the acquisition of metastatic potential and tissue invasiveness. Taken together, our results suggest that specific mutant p53 genes play a role in the generation of lymphohematopoietic metastatic potential and tissue invasiveness as assayed in SCID mice, whereas the expression of wild-type p53 is capable of keeping this metastatic potential in check.

p53-dependent apoptosis modulates the cytotoxicity of anticancer agents

Although the primary cellular targets of many anticancer agents have been identified, less is known about the processes leading to the selective cell death of cancer cells or the molecular basis of drug resistance. p53-deficient mouse embryonic fibroblasts were used to examine systematically the requirement for p53 in cellular sensitivity and resistance to a diverse group of anticancer agents. These results demonstrate that an oncogene, specifically the adenovirus E1A gene, can sensitize fibroblasts to apoptosis induced by ionizing radiation, 5-fluorouracil, etoposide, and adriamycin. Furthermore, the p53 tumor suppressor is required for efficient execution of the death program. These data reinforce the notion that the cytotoxic action of many anticancer agents involves processes subsequent to the interaction between drug and cellular target and indicate that divergent stimuli can activate a common cell death program. Consequently, the involvement of p53 in the apoptotic response suggests a mechanism whereby tumor cells can acquire cross-resistance to anticancer agents.