p53 in chronic myelogenous leukemia. Study of mechanisms of differential expression

Wild-type p53 induces apoptosis of myeloid leukaemic cells that is inhibited by interleukin-6

Wild-type p53 protein has many properties consistent with its being the product of a tumour suppressor gene. Although the normal roles of tumour suppressor genes are still largely unknown, it seems that they could be involved in promoting cell differentiation as well as in mediating growth arrest by growth-inhibitory cytokines. Hence, the abrogation of wild-type p53 expression, which is a common feature of many tumours, could eliminate these activities. We have now tested this notion by restoring the expression of p53 in a murine myeloid leukaemic cell line that normally lacks p53. The use of a temperature-sensitive p53 mutant allowed us to analyse cells in which the introduced p53 had either wild-type or mutant properties. Although there seemed to be no effect on differentiation, the introduction of wild-type p53 resulted in rapid loss of cell viability in a way characteristic of apoptosis (programmed cell death). The effect of wild-type p53 was counteracted by interleukin-6. Thus products of tumour suppressor genes could be involved in restricting precursor cell populations by mediating apoptosis.

p53 in chronic myelogenous leukemia in acute phase

All patients with chronic myelogenous leukemia (CML) undergo clinical transition from chronic to acute phase. This transition is often associated with deletion of the short arm of chromosome 17 in the form of the i(17q) aberration. Since the p53 gene is a suppressor gene and is located on 17p13, we examined the possibility that it is inactivated during progression of CML. Therefore, we studied the structure and expression of p53 in the leukemic cells of a large number of CML patients in acute phase. We found that although the gene is rarely rearranged, one p53 allele is completely deleted in patients with the i(17q) aberration as well as in some patients who do not show karyotypic changes. In all of these patients the remaining allele is inactivated through loss of expression, rearrangement, or point mutation. Detailed analysis of some patients who carry both p53 alleles indicated neither loss of expression nor structural alterations. It appears that p53 loss of function is associated with progression of around 25% of CML patients.

Expression of wild-type p53 is not compatible with continued growth of p53-negative tumor cells

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced murine erythroleukemia cell lines and may represent a necessary step in the progression of this disease. As well, frequent loss or mutation of p53 alleles in diverse human tumors is consistent with the view of p53 as a tumor suppressor gene. To examine the significance of p53 gene inactivation in tumorigenesis, we have attempted to express transfected wild-type p53 in three p53-negative tumor cell lines: murine DP16-1 Friend erythroleukemia cells, human K562 cells, and SKOV-3 cells. We found that aberrant p53 proteins, which differ from wild-type p53 by a single amino acid substitution, were expressed stably in these cells, whereas wild-type p53 expression was not tolerated. The inability of p53-negative tumor cell lines to support long-term expression of wild-type p53 protein is consistent with the view that p53 is a tumor suppressor gene.

Expression of wild-type p53 is not compatible with continued growth of p53-negative tumor cells

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced murine erythroleukemia cell lines and may represent a necessary step in the progression of this disease. As well, frequent loss or mutation of p53 alleles in diverse human tumors is consistent with the view of p53 as a tumor suppressor gene. To examine the significance of p53 gene inactivation in tumorigenesis, we have attempted to express transfected wild-type p53 in three p53-negative tumor cell lines: murine DP16-1 Friend erythroleukemia cells, human K562 cells, and SKOV-3 cells. We found that aberrant p53 proteins, which differ from wild-type p53 by a single amino acid substitution, were expressed stably in these cells, whereas wild-type p53 expression was not tolerated. The inability of p53-negative tumor cell lines to support long-term expression of wild-type p53 protein is consistent with the view that p53 is a tumor suppressor gene.

Frequent mutations in the p53 tumor suppressor gene in human leukemia T-cell lines

Human T-cell leukemia and T-cell acute lymphoblastic leukemia cell lines were studied for alterations in the p53 tumor suppressor gene. Southern blot analysis of 10 leukemic T-cell lines revealed no gross genomic deletions or rearrangements. Reverse transcription-polymerase chain reaction analysis of p53 mRNA indicated that all 10 lines produced p53 mRNA of normal size. By direct sequencing of polymerase chain reaction-amplified cDNA, we detected 11 missense and nonsense point mutations in 5 of the 10 leukemic T-cell lines studied. The mutations are primarily located in the evolutionarily highly conserved regions of the p53 gene. One of the five cell lines in which a mutation was detected possesses a homozygous point mutation in both p53 alleles, while the other four cell lines harbor from two to four different point mutations. An allelic study of two of the lines (CEM, A3/Kawa) shows that the two missense mutations found in each line are located on separate alleles, thus both alleles of the p53 gene may have been functionally inactivated by two different point mutations. Since cultured leukemic T-cell lines represent a late, fully tumorigenic stage of leukemic T cells, mutation of both (or more) alleles of the p53 gene may reflect the selection of cells possessing an increasingly tumorigenic phenotype, whether the selection took place in vivo or in vitro. Previously, we have shown that the HSB-2 T-cell acute lymphoblastic leukemia cell line had lost both alleles of the retinoblastoma tumor suppressor gene. Taken together, our data show that at least 6 of 10 leukemic T-cell lines examined may have lost the normal function of a known tumor suppressor gene, suggesting that this class of genes serves a critical role in the generation of fully tumorigenic leukemic T cells.

Frequent mutations in the p53 tumor suppressor gene in human leukemia T-cell lines

Human T-cell leukemia and T-cell acute lymphoblastic leukemia cell lines were studied for alterations in the p53 tumor suppressor gene. Southern blot analysis of 10 leukemic T-cell lines revealed no gross genomic deletions or rearrangements. Reverse transcription-polymerase chain reaction analysis of p53 mRNA indicated that all 10 lines produced p53 mRNA of normal size. By direct sequencing of polymerase chain reaction-amplified cDNA, we detected 11 missense and nonsense point mutations in 5 of the 10 leukemic T-cell lines studied. The mutations are primarily located in the evolutionarily highly conserved regions of the p53 gene. One of the five cell lines in which a mutation was detected possesses a homozygous point mutation in both p53 alleles, while the other four cell lines harbor from two to four different point mutations. An allelic study of two of the lines (CEM, A3/Kawa) shows that the two missense mutations found in each line are located on separate alleles, thus both alleles of the p53 gene may have been functionally inactivated by two different point mutations. Since cultured leukemic T-cell lines represent a late, fully tumorigenic stage of leukemic T cells, mutation of both (or more) alleles of the p53 gene may reflect the selection of cells possessing an increasingly tumorigenic phenotype, whether the selection took place in vivo or in vitro. Previously, we have shown that the HSB-2 T-cell acute lymphoblastic leukemia cell line had lost both alleles of the retinoblastoma tumor suppressor gene. Taken together, our data show that at least 6 of 10 leukemic T-cell lines examined may have lost the normal function of a known tumor suppressor gene, suggesting that this class of genes serves a critical role in the generation of fully tumorigenic leukemic T cells.

p53: oncogene or anti-oncogene?

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Wild-type p53 can inhibit oncogene-mediated focus formation

Mutant forms of the p53 cellular tumor antigen elicit neoplastic transformation in vitro. Recent evidence indicated that loss of normal p53 expression is a frequent event in certain types of tumors, raising the possibility that such loss provides transformed cells with a selective growth advantage. Thus, it was conceivable that the mutants might contribute to transformation by abrogating normal p53 function. We therefore studied the effect of plasmids encoding wild-type (wt) p53 on the ability of primary rat embryo fibroblasts to be transformed by a combination of mutant p53 and ras. It was found that wt p53 plasmids indeed caused a marked reduction in the number of transformed foci. Furthermore, wt p53 plasmids also suppressed the induction of transformed foci by combinations of bona fide oncogenes, such as myc plus ras or adenovirus E1A plus ras. On the other hand, plasmids carrying mutations in the p53 coding region totally failed to inhibit oncogene-mediated focus induction and often even slightly stimulated it. Hence, such mutations completely abolished the activity of wt p53 that is responsible for the "suppressor" effect. The latter fact is of special interest, since similar mutations in p53 are often observed in human and rodent tumors. The inhibitory effect of p53 was most pronounced when early-passage cells were used as targets, whereas established cell lines were less sensitive. These data support the notions that wt p53 expression may be restrictive to neoplastic progression and that p53 inactivation may play a crucial role in tumorigenesis.

High incidence of lung, bone, and lymphoid tumors in transgenic mice overexpressing mutant alleles of the p53 oncogene

We have investigated the role of the p53 gene in oncogenesis in vivo by generating transgenic mice carrying murine p53 genomic fragments isolated from a mouse Friend erythroleukemia cell line or BALB/c mouse liver DNA. Elevated levels of p53 mRNA were detected in several tissues of two transgenic lines tested. Increased levels of p53 protein were also detected in most of the tissues analyzed by Western blotting (immunoblotting). Because both transgenes encoded p53 proteins that were antigenically distinct from wild-type p53, it was possible to demonstrate that overexpression of the p53 protein was mostly, if not entirely, due to the expression of the transgenes. Neoplasms developed in 20% of the transgenic mice, with a high incidence of lung adenocarcinomas, osteosarcomas, and lymphomas. Tissues such as ovaries that expressed the transgene at high levels were not at higher risk of malignant transformation than tissues expressing p53 protein at much lower levels. The long latent period and low penetrance suggest that overexpression of p53 alone is not sufficient to induce malignancies and that additional events are required. These observations provide direct evidence that mutant alleles of the p53 oncogene have oncogenic potential in vivo and that different cell types show intrinsic differences in susceptibility to malignant transformation by p53. Since recent data suggest that p53 may be a recessive oncogene, it is possible that the elevated tumor incidence results from functional inactivation of endogenous p53 by overexpression of the mutant transgene. The high incidence of lung and bone tumors suggests that p53 transgenic mice may provide a useful model to investigate the molecular events that underlie these malignancies in humans.

High incidence of lung, bone, and lymphoid tumors in transgenic mice overexpressing mutant alleles of the p53 oncogene

We have investigated the role of the p53 gene in oncogenesis in vivo by generating transgenic mice carrying murine p53 genomic fragments isolated from a mouse Friend erythroleukemia cell line or BALB/c mouse liver DNA. Elevated levels of p53 mRNA were detected in several tissues of two transgenic lines tested. Increased levels of p53 protein were also detected in most of the tissues analyzed by Western blotting (immunoblotting). Because both transgenes encoded p53 proteins that were antigenically distinct from wild-type p53, it was possible to demonstrate that overexpression of the p53 protein was mostly, if not entirely, due to the expression of the transgenes. Neoplasms developed in 20% of the transgenic mice, with a high incidence of lung adenocarcinomas, osteosarcomas, and lymphomas. Tissues such as ovaries that expressed the transgene at high levels were not at higher risk of malignant transformation than tissues expressing p53 protein at much lower levels. The long latent period and low penetrance suggest that overexpression of p53 alone is not sufficient to induce malignancies and that additional events are required. These observations provide direct evidence that mutant alleles of the p53 oncogene have oncogenic potential in vivo and that different cell types show intrinsic differences in susceptibility to malignant transformation by p53. Since recent data suggest that p53 may be a recessive oncogene, it is possible that the elevated tumor incidence results from functional inactivation of endogenous p53 by overexpression of the mutant transgene. The high incidence of lung and bone tumors suggests that p53 transgenic mice may provide a useful model to investigate the molecular events that underlie these malignancies in humans.

Alternative forms of the BCR-ABL oncogene have quantitatively different potencies for stimulation of immature lymphoid cells

The Philadelphia chromosome (t9:22;q34:q11) is found in more than 90% of patients with chronic myelogenous leukemia, in 10 to 20% of patients with acute lymphocytic leukemia, and in 1 to 2% of patients with acute myelogenous leukemia. Alternative chimeric oncogenes are formed by splicing different sets of BCR gene exons on chromosome 22 across the translocation breakpoint to a common set of ABL oncogene sequences on chromosome 9. This results in an 8.7-kilobase mRNA that encodes the P210 BCR-ABL gene product commonly found in patients with chronic myelogenous leukemia or a 7.0-kilobase mRNA that produces the P185 BCR-ABL gene product found in most Philadelphia chromosome-positive patients with acute lymphocytic leukemia. To compare the efficiency of growth stimulation by these two proteins, we derived cDNA clones for each with identical 5' and 3' untranslated regions and expressed them from retrovirus vectors. Matched stocks were compared for potency to transform immature B-lymphoid lineage precursors. The growth-stimulating effects of P185 for this cell type were found to be significantly greater than those of P210. Structural changes in BCR may regulate the effectiveness of the ABL tyrosine kinase function, as monitored by lymphocyte growth response. Changes in mitogenic potency may help to explain the more acute leukemic presentation usually associated with expression of the P185 BCR-ABL oncogene.

Alternative forms of the BCR-ABL oncogene have quantitatively different potencies for stimulation of immature lymphoid cells

The Philadelphia chromosome (t9:22;q34:q11) is found in more than 90% of patients with chronic myelogenous leukemia, in 10 to 20% of patients with acute lymphocytic leukemia, and in 1 to 2% of patients with acute myelogenous leukemia. Alternative chimeric oncogenes are formed by splicing different sets of BCR gene exons on chromosome 22 across the translocation breakpoint to a common set of ABL oncogene sequences on chromosome 9. This results in an 8.7-kilobase mRNA that encodes the P210 BCR-ABL gene product commonly found in patients with chronic myelogenous leukemia or a 7.0-kilobase mRNA that produces the P185 BCR-ABL gene product found in most Philadelphia chromosome-positive patients with acute lymphocytic leukemia. To compare the efficiency of growth stimulation by these two proteins, we derived cDNA clones for each with identical 5' and 3' untranslated regions and expressed them from retrovirus vectors. Matched stocks were compared for potency to transform immature B-lymphoid lineage precursors. The growth-stimulating effects of P185 for this cell type were found to be significantly greater than those of P210. Structural changes in BCR may regulate the effectiveness of the ABL tyrosine kinase function, as monitored by lymphocyte growth response. Changes in mitogenic potency may help to explain the more acute leukemic presentation usually associated with expression of the P185 BCR-ABL oncogene.