P53-mediated radioresistance does not correlate with metastatic potential in tumorigenic rat embryo cell lines following oncogene transfection

PURPOSE

Changes in wild-type p53 protein function occur in the majority of human tumors, and may alter genomic stability and the cellular response to ionizing radiation. Whether oncoproteins can render tumor cells both radioresistant and metastatic, may have implications for clinical strategies designed to improve local tumor control. In the studies reported here, we tested the hypothesis that acquired radioresistance correlates with metastatic potential within a large panel of transformed rat embryo cell (REF) lines following transfection with activated H-ras, mutant p53, and HPV16-E7 alleles.

METHODS AND MATERIALS

Rat embryo cells (REF cells) were transfected using the calcium-phosphate technique with an activated H-ras gene alone, or in combination with human papillomavirus HPV16-E7 and/or human or murine mutant p53 sequences. Other rat embryo cell clones expressing transfected HPV-E7 and activated ras sequences subsequently acquired endogenous p53 gene mutations during culture in vitro. The relative expression of p21ras and p53 protein for each REF transformant was determined by Western blot analysis following transfection. REF clones were phenotypically characterized at early passage (i.e., passages 5-7) and late passage (i.e., passages 10-20) for their: (a) relative tumor growth rate, and (b) their ability to undergo spontaneous metastasis following intramuscular injection into the hind legs of SCID mice. In vivo phenotypic end points were then compared to previously measured parameters of in vitro radiosensitivity for each cell line. Additionally, the expression of the cellular protease, plasminogen activator, was determined for a number of metastatic and nonmetastatic cell lines.

RESULTS

We found no evidence that selected oncogene-transfected REF transformants that were radioresistant in culture had a greater spontaneous metastatic potential than nonradioresistant REF transformants. Neither the level of expression of the p21ras protein nor that of the p53 protein was correlated with the spontaneous metastatic phenotype when tested at early passage. The metastatic phenotype appeared to be independent of p53 genotype. The majority of metastatic REF clones tested (7 out of 9 clones) expressed plasminogen activator following oncogene transfection, in contrast to nonmetastatic REF transformed cell lines.

CONCLUSIONS

Our results suggest that (a) intrinsic radioresistance does not correlate with spontaneous metastatic potential in oncogene-expressing REF transformant cell lines, and (b), novel clinical strategies designed to overcome oncogene-mediated radioresistance could potentially impact on overall survival, as gains in local tumor control may not be offset by a greater risk of distant metastasis.

From telomere loss to p53 induction and activation of a DNA-damage pathway at senescence: the telomere loss/DNA damage model of cell aging

In the cold winter of 1966 Aleksay Olovnikov, a theoretical biologist at the Academy of Sciences in Moscow, was waiting in the subway station where he was hit by the idea that the ends of linear chromosomes can't be replicated fully during each round of replication. In a theoretical paper (Olovnikov, 1971) he proposed that in somatic cells the ends of the chromosomes are not fully replicated during DNA synthesis, resulting in the shortening of linear DNA molecules with each cell division, and that this may be the cause of cell cycle arrest in senescent cells. Almost two decades after this proposal, Calvin Harley and co-workers found that telomeres, the physical ends of human chromosomes, shorten as a function of age in human cells in vitro and in vivo. The telomere hypothesis proposes that critically short telomeres may act as a mitotic clock to signal the cell cycle arrest at senescence (Harley, 1991). Here, we extend the telomere hypothesis and propose a model that incorporates recent advances in tumor suppressors and cell cycle control with several areas of cell aging. We propose that telomere shortening per se is not the direct signal for cell cycle arrest. It is the consequence of telomere loss, which may lead to generation of ds or ss DNA breaks. These breaks activate a p53 dependent or independent DNA-damage pathway that leads to the induction of a family of inhibitors of cyclin dependent kinases (including p21 and p16) and the eventual G1 block of senescence. In agreement with this hypothesis, we demonstrate that the level of p53 protein increases in near senescent cultures of MDFs. This increase may be responsible for induction of p21 (Noda, 1993) and IGF-Bp3 (Goldstein, 1991).

Cytokines inhibit p53-mediated apoptosis but not p53-mediated G1 arrest

Murine erythroleukemia cells that lack endogenous p53 expression were transfected with a temperature-sensitive p53 allele. The temperature-sensitive p53 protein behaves as a mutant polypeptide at 37 degrees C and as a wild-type polypeptide at 32 degrees C. Three independent clones expressing the temperature-sensitive p53 protein were characterized with respect to p53-mediated G1 cell cycle arrest, apoptosis, and differentiation. Clone ts5.203 responded to p53 activation at 32 degrees C by undergoing G1 arrest, apoptosis, and differentiation. Apoptosis was seen in cells representative of all phases of the cell cycle and was not restricted to cells arrested in G1. The addition of a cytokine (erythropoietin, c-kit ligand, or interleukin-3) to the culture medium of ts5.203 cells blocked p53-mediated apoptosis and differentiation but not p53-mediated G1 arrest. These observations indicate that apoptosis and G1 arrest can be effectively uncoupled through the action of cytokines acting as survival factors and are consistent with the idea that apoptosis and G1 arrest represent separate functions of p53. Clones ts15.15 and tsCB3.4 responded to p53 activation at 32 degrees C by undergoing G1 arrest but not apoptosis. We demonstrate that tsCB3.4 secretes a factor with erythropoietin-like activity and that ts15.15 secretes a factor with interleukin-3 activity and suggest that autocrine secretion of these cytokines blocks p53-mediated apoptosis. These data provide a framework in which to understand the variable responses of cells to p53 overexpression.

p53 mutations, c-myc and bcl-2 rearrangements in human non-Hodgkin's lymphoma cell lines

Fourteen Non-Hodgkin's lymphoma cell lines were generated and assessed for the presence of structural p53, c-myc and bcl-2 gene changes. Single or multiple changes were observed in 11 of the lines. Alterations of the p53 gene were most frequent and documented for 10 lines by immunoprecipitation using the antibodies PAb 240 and PAb 1801, sequencing studies and Southern blot analysis. A detailed study was performed in one of the cell lines (OCI-Ly 4) for which material of the original tumor sample was available. Two point mutations identified by sequencing cDNA derived from the cell line were also present in the original tumor specimen. In contrast, DNA prepared from fibroblasts of the same patient did not show the mutations. Six of the 14 lines demonstrated c-myc rearrangements, while bcl-2 changes were observed in 4. The presence of c-myc was associated with shorter survival of this group of patients with aggressive disease. None of the other changes present as single or composite alterations were correlated with clinical outcome measures.

The p53-mediated G1 checkpoint is retained in tumorigenic rat embryo fibroblast clones transformed by the human papillomavirus type 16 E7 gene and EJ-ras

Rat embryo fibroblast clones transformed with the human papillomavirus type 16 E7 gene and the H-ras oncogene (ER clones) fall into two groups on the basis of endogenous p53 genotype, wild type or mutant. We have compared these clones with the aim of indentifying physiological differences that could be attributed to p53 protein function. We show that all ER clones, regardless of p53 gene status, are tumorigenic and metastatic in severe combined immunodeficiency mice. We demonstrate that only the wild-type p53 protein expressed in ER clones is functional on the basis of its site-specific double-stranded DNA-binding activity and its ability to confer a G1 delay on cells following treatment with ionizing radiation. These data indicate that disruption of the p53 growth-regulatory pathway is not a prerequisite for the malignant conversion of rat embryo fibroblasts expressing the E7 gene and mutant ras. Differences in phenotype that were correlated with loss of p53 protein function included the following: serum-independent growth of ER clones in culture, decreased tumor doubling time in vivo, and increased radioresistance. In addition, we demonstrate the p53-dependent G1 checkpoint alone does not determine radiosensitivity.

The role of p53 in tumor progression

p53 mutation is a common event in tumors. The evidence that such mutation is an important step in the sequence of genetic changes which underly tumor progression is outlined. Such evidence includes the study of human tumors, animal tumor models and cell culture. In several instances, p53 mutation seems to be a late event. This is by no means a universal finding, and important reservations are stated concerning the role of p53 mutation in tumor progression.

Mutant p53 increases radioresistance in rat embryo fibroblasts simultaneously transfected with HPV16-E7 and/or activated H-ras

Recently, it has been suggested that abrogation of the wild type p53 protein function may alter the cellular response to DNA damaging agents, including ionizing radiation. This study was designed to compre the relative radiosensitivity and tumorigenicity of rat embryo fibroblast (REF) cell lines transfected with a mutant form of the p53 gene (plasmid MTp53pro193), alone, or in combination, with the H-ras (plasmid pEJ6.6) and HPV16-E7 (plasmid pJ4 omega 16.E7) oncogenes. Transfection of the mutant p53pro193 gene alone resulted in selected clones having increased radioresistance in culture which correlated with increased mutant p53 expression in these clones. However, the co-transfection of mutant p53 and H-ras genes or triple transfection of mutant p53, H-ras and E7 genes resulted in clones with high mutant p53 expression, significantly increased radioresistance and uniform tumorigenicity. There was no correlation between intrinsic radioresistance and spontaneous metastasis in the tumorigenic REF transfectant clones. Stepwise acquisition of radioresistance and an aggressive tumor cell phenotype is observed when the mutant p53 gene and HPV E7 co-operate with the ras oncogene in transfection assays, and can be correlated to increases in mutant p53 expression.

Mutation of the endogenous p53 gene in cells transformed by HPV-16 E7 and EJ c-ras confers a growth advantage involving an autocrine mechanism

Rat embryo fibroblasts transformed with the HPV-16 E7 gene and the activated c-H-ras gene fall into two distinct phenotypic classes. At high cell density, clones of one class form colonies in methylcellulose supplemented with low serum; at low cell density, these cells display responsiveness to mitogenic factors present in serum-free conditioned medium from rat embryo fibroblasts. In contrast, clones of the second class exhibit an absolute dependency on growth factors present in serum at all cell densities in the methylcellulose colony assay and fail to respond to conditioned medium. We find that the status of the endogenous p53 gene is tightly correlated with these two classes of clones. Clones of the first class contain missense mutations in the p53 gene and have lost the wild-type allele. Clones of the second class express wild-type p53 protein. The importance of mutant p53 expression in reducing the growth factor dependency of transformed clones was confirmed in a separate series of experiments in which rat embryo fibroblasts were transformed with three genes, E7 + ras + mutant p53. The growth behaviour of these triply transfected clones was similar to that of the E7 + ras clones expressing endogenous mutant p53. We demonstrate that the enhanced proliferation of E7 + ras clones expressing mutant p53 protein involves an autocrine mechanism.

Alterations of p53 and c-myc in the clonal evolution of malignant lymphoma

We derived the lymphoma cell lines OCI-Ly 13.1 and OCI-Ly 13.2 from a patient with non-Hodgkin's lymphoma at the time of presentation and during chemotherapy-resistant relapse. These lines were of T-cell phenotype and contained the identical T-cell receptor beta-chain rearrangement, indicating that both lines were members of the same malignant clone. The lines differed in their growth characteristics; OCI-Ly 13.1 grew slowly and required growth factors for colony formation, whereas OCI-Ly 13.2 grew rapidly and formed colonies without addition of growth factors. To test whether or not these biologic differences were associated with specific genetic changes, we evaluated the status of the c-myc and p53 genes of both cell lines. The p53 and c-myc genes of OCI-Ly 13.1 were in germline configuration and produced normal-sized transcripts. The p53 protein expressed in OCI-Ly 13.1 was recognized by the anti-p53 monoclonal antibody, PAb240, indicating a conformation typical of p53 proteins expressed by p53 alleles containing a missense mutation. However, sequencing studies of the entire p53 coding region did not reveal any point mutations. In contrast, the cell line OCI-Ly 13.2 contained structural abnormalities of both the c-myc and p53 genes. In addition, one of the p53 alleles was lost as determined by a cDNA probe for the p53 gene (17p 13.1) and the YNZ22.1 probe (17p 13.3). These changes resulted in the absence of p53 protein and mRNA in OCI-Ly 13.2 as detected by immunoprecipitation and Northern blot analysis, respectively. They may be a reflection of disease progression and may be associated with the altered behavior of the malignant cell population within the patient and in vitro.

The core promoter region of the P-glycoprotein gene is sufficient to confer differential responsiveness to wild-type and mutant p53

The overexpression of P-glycoprotein is thought to be responsible for resistance to chemotherapy in some non-responsive cancers. The mechanism by which P-glycoprotein is overexpressed in human tumors is poorly understood. However, several lines of evidence suggest that the major regulatory mechanism of P-glycoprotein overexpression in human tumors is at the transcriptional level. During tumor progression one of the most commonly observed alterations is mutation of the p53 tumor-suppressor gene. It has been shown that the p53 protein plays a role in transcriptional regulation. To gain insight into the effect p53 protein may have on P-glycoprotein promoter activity, we transiently co-transfected plasmids containing the hamster pgp1 or human mdr1 promoter linked to the chloramphenicol acetyltransferase (CAT) reporter gene with plasmids encoding either wild-type or mutant p53 protein into Chinese hamster ovary (CHO) cells. In this report, we show that wild-type p53 protein represses P-glycoprotein promoter activity, while mutant forms of p53 protein enhance P-glycoprotein promoter activity. Furthermore, we present data which indicate that the transcriptional regulatory effects of p53 are mediated through interactions with pgp1/mdr1 core promoter sequences. These findings have implications for our understanding of the molecular mechanism(s) by which p53 protein functions as a transcriptional regulator of gene expression. In addition, our results suggest a mechanism by which P-glycoprotein may be overexpressed in human cancers that also express mutant forms of p53 protein.

The transforming and suppressor functions of p53 alleles: effects of mutations that disrupt phosphorylation, oligomerization and nuclear translocation

Mutant p53 alleles that have a recessive phenotype in human tumors can, in cooperation with an activated H-ras gene, transform rat embryo fibroblasts (REFs). Mutant p53 proteins differ from wild type, and from each other in conformation, localization and transforming potential. Missense mutations in codons 143, 175 and 275 confer strong transforming potential. A serine 135 p53 mutant has an intermediate transforming potential, while the histidine codon 273 allele transforms weakly, if at all. In contrast to the wild type p53 gene, mutant p53 alleles with strong transforming ability cannot suppress the transformation of REFs by other oncogenes. The His273 allele retains partial suppressor function in this assay. The relevance of p53 oligomerization, phosphorylation and nuclear translocation to the transforming potential of mutant p53 and to wild type p53 suppressor function were examined. The inability of mutant p53 polypeptides to form homodimers correlates with loss of transforming function. Monomeric variants of wild type p53 protein, however, retain the ability to suppress focus formation. Phosphorylation of serine residues 315 and 392 is not required for the transforming function of mutant p53, nor is serine 315 required for suppressor function when these alleles are constitutively expressed in REF assays. Nuclear translocation-defective mutant and wild type p53 proteins retain transforming and suppressor function in REF assays.

Growth suppression of Friend virus-transformed erythroleukemia cells by p53 protein is accompanied by hemoglobin production and is sensitive to erythropoietin

The murine allele temperature-sensitive (ts) p53Val-135 encodes a ts p53 protein that behaves as a mutant polypeptide at 37 degrees C and as a wild-type polypeptide at 32 degrees C. This ts allele was introduced into the p53 nonproducer Friend erythroleukemia cell line DP16-1. The DP16-1 cell line was derived from the spleen cells of a mouse infected with the polycythemia strain of Friend virus, and like other erythroleukemia cell lines transformed by this virus, it grows independently of erythropoietin, likely because of expression of the viral gp55 protein which binds to and activates the erythropoietin receptor. When incubated at 32 degrees C, DP16-1 cells expressing ts p53Val-135 protein, arrested in the G0/G1 phase of the cell cycle, rapidly lost viability and expressed hemoglobin, a marker of erythroid differentiation. Erythropoietin had a striking effect on p53Val-135-expressing cells at 32 degrees C by prolonging their survival and diminishing the extent of hemoglobin production. This response to erythropoietin was not accompanied by down-regulation of viral gp55 protein.

Identification of a human cancer related organ-specific neoantigen

Human cancers express organ-specific neoantigens (OSNs) which elicit specific cellular immune responses in the cancer patient, as demonstrated by leukocyte adherence inhibition (LAI), an in vitro immune response assay. A purified protein of MW 40,000 (p40) exhibiting OSN (colon specific) activity was cleaved into specific peptide fragments and their partial amino acid sequences determined. This information was used in the polymerase chain reaction (PCR) to obtain a 992 bp cDNA clone (PCR-992) from a human colon adenocarcinoma cell line (LS-180). By comparison of the predicted amino acid sequence of PCR-992 with the known sequence of p40 peptides, PCR-992 was shown to correspond to almost the entire coding region of p40. Nucleotide sequence analysis suggested that the protein was mycoplasmal in origin due to its high A+T content (76%) and the presence of five in frame TGA termination codons; at least two of the latter are actually read as tryptophan, a known feature of mycoplasma translation. We have confirmed this origin by direct isolation of a contaminating mycoplasma species from the LS-180 cell line and demonstration that it could be hybridized with the PCR-992 probe. Northern and PCR analysis of RNA preparations from the contaminated LS-180 cell line showed that p40 was part of the high affinity transport system operon of Mycoplasma hyorhinis (Dudler et al, EMBO J., 7: 3963-3970, 1988). Total protein lysates of Mycoplasma hyorhinis cultivated without animal cells could elicit positive LAI responses when incubated with cancer patient leukocytes but not with normal patient leukocytes. The organ-specific nature of the response was, however, not observed indicating that host cell-mycoplasmal interactions may play a role in determining the organ-specific nature of p40 seen with the LAI. The significance of these findings will be discussed in the context of previous thinking regarding the origin of OSNs.

Transforming activity of mutant human p53 alleles

Mutant forms of the p53 gene have been shown to cooperate with an activated ras gene in transforming primary cells in culture. The aberrant proteins encoded by p53 mutants are thought to act in a dominant negative manner in these assays. In vivo data, however, reveal that where p53 has undergone genetic change in tumors, both alleles have been affected. We previously identified a case of human acute myelogenous leukemia (AML) in which both alleles of the p53 gene had undergone independent missense mutations (at codons 135 cys to ser and 246 met to val). In these blasts, p53 mutations appear to be acting recessively. We have assayed the transforming potential of these p53 mutations, as well as that of another mutation at codon 273, also identified in a human neoplasm. Both mutations from the AML blasts (codon 135 and codon 246) confer transforming ability on the mutant protein. While transformation assays may define functionally different subsets of p53 mutations, the overexpression phenotype of mutants in this assay may not accurately reflect the pathological effects of p53 mutations in vivo.

Mutation of the p53 gene in human acute myelogenous leukemia

Heterogeneity of p53 protein expression is seen in blast cells of patients with acute myelogenous leukemia (AML). p53 protein is detected in the blasts of certain AML patients but not in others. We have identified p53 protein variants with abnormal mobility on gel electrophoresis and/or prolonged half-life (t 1/2). We have sequenced the p53 coding sequence from primary blast cells of five AML patients and from the AML cell line (OCIM2). In OCIM2, a point mutation in codon 274 was identified that changes a valine residue to aspartic acid. A wild type p53 allele was not detected in these cells. Two point mutations (codon 135, cysteine to serine; codon 246, methionine to valine) were identified in cDNA from blasts of one AML patient. Both mutations were present in blast colonies grown from single blast progenitor cells, indicating that individual leukemia cells had sustained mutation of both p53 alleles. The cDNAs sequenced from blast samples of four other patients, including one with prolonged p53 protein t 1/2 and one with no detectable p53 protein, were fully wild type. Thus, the heterogeneity of p53 expression cannot be explained in all cases by genetic change in the p53 coding sequence. The prolonged t 1/2 of p53 protein seen in some AML blasts may therefore reflect changes not inherent to p53. A model is proposed in which mutational inactivation of p53, although not required for the evolution of neoplasia, would confer a selective advantage, favoring clonal outgrowth during disease progression.

Synergism between pairs of immortalizing genes in transformation assays of rat embryo fibroblasts

A number of cellular and viral genes encode proteins that play a role in the establishment of normal cells in culture. In addition, these genes cooperate with activated ras genes to induce cellular transformation. We show that ras-dependent transformation of rat embryo fibroblasts is more efficient when two establishment genes are used together compared with one alone. Both quantitative and qualitative differences in the efficiency of transformation were detected. The number of transformed foci generated was greater than the sum of the foci obtained with ras and each of the establishment genes used separately. In addition, the foci had a distinct morphology. Synergism was seen between the HPV-16 E7 gene and certain mutant alleles of the cellular p53 gene as well as between E7 and c-myc.

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced erythroleukemia: relationship of inactivation to dominant transforming alleles

The Friend erythroleukemia virus complex contains no cell-derived oncogene. Transformation by this virus may therefore involve mutations affecting cellular gene expression. We provide evidence that inactivating mutations of the cellular p53 gene are a common feature in Friend virus-induced malignancy, consistent with an antioncogene role for p53 in this disease. We have shown that frequent rearrangements of the p53 gene cause loss of expression or synthesis of truncated proteins, whereas overexpression of p53 protein is seen in other Friend cell lines. We now demonstrate that p53 expression in the latter cells is also abnormal, as a result of missense mutations in regions encoding highly conserved amino acids. Three of these aberrant alleles obtained from cells from different mice were cloned and found to function as dominant oncogenes in gene transfer assays, supporting the view that certain naturally occurring missense mutations in p53 confer a dominant negative phenotype on the encoded protein.

p53: oncogene or anti-oncogene?

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Alterations in the p53 gene and the clonal evolution of the blast crisis of chronic myelocytic leukemia

Molecular mechanisms responsible for the clinical progression of chronic myelocytic leukemia to its accelerated phase or to blast crisis have not been defined. We found alterations of the p53 gene (p53 is a 53-kDa nuclear protein) including deletions and rearrangements in 8 of 34 patients in blast crisis and 1 of 4 patients in the accelerated phase, but in only 1 of 38 patients in the chronic phase of chronic myelocytic leukemia. Only two other examples of p53 gene alterations were found among 203 patients with hematologic malignancies and solid tumors. Transcripts of the p53 gene were uniformly found in chronic-phase cells, but gene expression was variable in blast crisis, and transcripts were reduced or undetectable in 10 of 16 patients. Heterogeneous alterations in the structure and expression of the p53 gene appear to be relatively frequent in blast crisis and may be involved in the evolution of disease.

Carcinoembryonic antigen, a human tumor marker, functions as an intercellular adhesion molecule

Carcinoembryonic antigen (CEA) is a member of a family of cell surface glycoproteins that are produced in excess in essentially all human colon carcinomas and in a high proportion of carcinomas at many other sites. The function of this widely used tumor marker and its relevance to malignant transformation is therefore of considerable interest. We demonstrate here that CEA mediates Ca2+-independent, homotypic aggregation of cultured human colon adenocarcinoma cells (LS-180) and rodent cells transfected with functional CEA cDNA. Furthermore, CEA can effect the homotypic sorting of cells in heterogeneous populations of aggregating cells. CEA can thus be considered a new addition to the family of intercellular adhesion molecules. We also show that, whereas CEA is localized mainly to epithelial cell membranes facing the lumen in normal adult intestine, it is found on adjacent cell membranes in both embryonic intestine and colonic tumors. A model for the role of CEA in the tissue architecture of adult, embryonic, and aberrant tumor intestinal epithelium is presented.

Inactivation of the p53 oncogene by internal deletion or retroviral integration in erythroleukemic cell lines induced by Friend leukemia virus

The p53 gene is rearranged in a high proportion of erythroleukemic cell lines derived from the spleens of mice infected with Friend leukemia virus. These rearrangements result in either the synthesis of a truncated protein or the inactivation of the p53 gene. Here we have molecularly characterized the rearrangements in two murine erythroleukemic cell lines induced by Friend leukemia virus, DP20-1 and CB3, that contain a rearranged p53 gene and fail to express p53 protein. The rearrangement in the DP20-1 cell line is due to the insertion of Friend spleen focus-forming provirus (SFFV) in the 3' end of the p53 gene in intron sequences between exons 9 and 10. Transfection of molecular clones of this SFFV provirus into NIH3T3 cells results in the generation of infectious virus as determined by its ability, in the presence of helper virus, to induce rapid splenomegaly and polycythemia when injected into adult DBA/2J mice. Insertion of SFFV in DP20-1 cells resulted in the expression of an aberrant 2.9 kb RNA species. Analysis of a molecular clone of the rearranged p53 gene in a second cell line, CB3, revealed that the p53 gene in this clone has sustained a large deletion within the p53 gene resulting in the loss of coding sequences between exons 4 and 8. The 5' end of the deletion originates within exon 4 and extends 3' to within the eighth intron. The significance of these findings with regard to the multi-stage nature of Friend virus induced erythroleukemia is discussed.