ras mutations in human leukemia and related disorders

ALOX5 exhibits anti-tumor and drug-sensitizing effects in MLL-rearranged leukemia

MLL-rearranged acute myeloid leukemia (AML) remains a fatal disease with a high rate of relapse and therapeutic failure due to chemotherapy resistance. In analysis of our Affymetrix microarray profiling and chromatin immunoprecipitation (ChIP) assays, we found that ALOX5 is especially down-regulated in MLL-rearranged AML, via transcription repression mediated by Polycomb repressive complex 2 (PRC2). Colony forming/replating and bone marrow transplantation (BMT) assays showed that Alox5 exhibited a moderate anti-tumor effect both in vitro and in vivo. Strikingly, leukemic cells with Alox5 overexpression showed a significantly higher sensitivity to the standard chemotherapeutic agents, i.e., doxorubicin (DOX) and cytarabine (Ara-C). The drug-sensitizing role of Alox5 was further confirmed in human and murine MLL-rearranged AML cell models in vitro, as well as in the in vivo MLL-rearranged AML BMT model coupled with treatment of “5 + 3” (i.e. DOX plus Ara-C) regimen. Stat and K-Ras signaling pathways were negatively correlated with Alox5 overexpression in MLL-AF9-leukemic blast cells; inhibition of the above signaling pathways mimicked the drug-sensitizing effect of ALOX5 in AML cells. Collectively, our work shows that ALOX5 plays a moderate anti-tumor role and functions as a drug sensitizer, with a therapeutic potential, in MLL-rearranged AML.

Blastic leukaemias (AML): a biologist's view

Acute myeloblastic leukaemia is characterised by the extreme clonal proliferation of haematopoietic precursor cells with abnormal or arrested differentiation. Chemotherapy of acute leukaemia is channelled towards the reduction and eradication of leukaemic cells. However, relapse is generally assumed to occur in residual host cells, which are refractory to or elude therapy. The cancer stem cell hypothesis has gained considerable importance in recent years and could interpret this behaviour. This persuasive theory states that cells within a tumour are organised in a hierarchy similar to that of normal tissues and are maintained by a small subset of cells responsible for tumour dormancy. These cells, defined as 'tumour initiating cells' (TICs), possess several properties of normal tissue stem cells. Recently, the TICs associated with AML have been shown to comprise distinct, hierarchically arranged classes similar to those observed for haematopoietic stem cells. We know now that the growth and survival of blasts in AML are driven by the same growth factors that stimulate normal cells. Furthermore, direct evidence of the role of membrane stem cell factor and its receptor c-Kit in cell-cell interactions and cell survival in primary AML blasts have been provided, defining the importance of juxtacrine stimulation. Inhibition of c-Kit signalling induces combinations of cell death: autophagy (compensatory mechanism towards survival) and apoptosis. While recent work confirmed that c-Kit inhibitors reduce cancer cell proliferation, it also demonstrated that future inappropriate prescriptions could cause normal tissue deterioration. The purpose of this paper was to review some of the salient features of leukaemic blasts in support of the proposal that research into neoplasia be increased. Rather than presenting the details of various studies, I have attempted to indicate general areas in which work has been done or is in progress. It is hoped that this survey of the subject will demonstrate a variety of opportunities for additional research in human neoplasia.

Not the presence but the amount of clonal DNA detectable in remission of acute myeloid leukemia is predictive for relapse

OBJECTIVES

The persistence of clonal cells after chemotherapy, or a re-emerging of clonal cells in remission (CR) or at relapse in patients with acute myeloid leukemia (AML) was studied to assess the prognostic significance of the amount of clonal DNA in predicting the clinical outcome.

METHODS

Clonal rearrangements in the gene sequences of retinoic acid receptor (RAR) alpha, major breakpoint cluster region (M-bcr), immunoglobulin (Ig)-JH, T-cell receptor (TcR) beta, myeloid lymphoid leukemia or cytokines (GM-CSF, G-CSF, IL-3) detected in bone marrow samples from 37 patients with primary AML (pAML) or secondary AML (sAML) were investigated. A relative increase or decrease of clonal DNA in the course of AML was evaluated by comparing the optical densities of DNA bands of the rearranged genes and the total amount of DNA.

RESULTS

High amounts of clonal DNA were detectable at diagnosis, during persisting disease and at relapse (Ø 39%, 35%, or 38% of total DNA, respectively), compared to 20% in complete remission (CR). Amounts of clonal DNA (except for Ig-JH gene rearrangements) were of prognostic significance at diagnosis, patients with less than 33% clonal DNA were characterized by significantly longer relapse-free survival times (all cases: p = 0.01; pAML: p = 0.002). Patients in CR exhibiting less than 5% (all cases) or 15% (pAML) clonal DNA showed longer relapse-free survival times (p = 0.08 or p = 0.03, respectively). Vice versa, significantly higher amounts of clonal DNA (all cases 51% vs. pAML 54%) could be detected in cases studied at diagnosis who relapsed in the following 5 months (all cases p = 0.01) or 14 months (pAML p = 0.007). Significantly higher amounts of clonal DNA (33%) could be detected in cases studied in CR who relapsed in the following 4 months (all cases p = 0.002 or pAML p = 0.006, respectively). Moreover, we could prove disease progression on a cellular level months before the clinical onset of sAML after a period of MDS.

CONCLUSIONS

Clonal, gene-rearranged DNA is regularly detectable at diagnosis and during persisting AML, in CR and at relapse. However, the presence, rather than the amount of clonal DNA detectable in CR is predictive for relapse. These data might indicate the significance of gene rearrangement analyses in the course of AML to identify cases with a high risk of relapse, independently from the karyotype.

N-ras mutations in radiation-induced murine leukemic cells

N-ras mutations were examined in DNA samples extracted from the spleen of CBA/Ca mice that developed myeloid leukemia (ML) following exposure to radiations of different qualities. A total of 17 ML cases, i.e. 5 cases of neutron-induced and 12 cases of photon- (3 gamma-ray and 9 x-ray) induced ML were included in the study along with 12 DNA samples from the bone marrow cells of control mice. Polymerase chain reaction-single strand conformational polymorphisms (PCR-SSCP) and the direct sequencing of PCR products were used to analyze three regions of the N-ras gene: (i) a 120 base-pair (bp) long portion of exon I (codons 2-37); (ii) a 103 bp long portion of exon II (codons 48-82); and (iii) a 107 bp long portion of exon III (codons 118-150). PCR-SSCP mobility shifts indicated mutations within only exon II of the N-ras gene. Such mutations were more prevalent in samples from mice exposed to fast neutrons. The exact type and location of these mutations were then determined by direct DNA sequencing. Silent point mutations, i.e. base transitions at the third base of codons 57 (GAC-->GAT), 62 (CAA-->CAC), or 70 (CAG-->CAA) were present only in mice that developed ML after exposure to fast neutrons. A base transversion at the third base of codon 61 (CAA-->CAC) was also observed in some ML cases. DNA sequencing demonstrated that ML samples contained normal as well as mutated DNA sequences. The higher frequency of N-ras mutations in neutron-induced ML suggested that fast neutrons are more effective in inducing genomic instability at the N-ras region of the genome. More importantly, N-ras mutations are not the initiating event in radiation leukemogenesis. This conclusion was supported by the finding that N-ras mutations were detected only in mice with an overt leukemic phenotype but not in mice with minimal tissue infiltration of leukemic cells, suggesting that the disease may be present prior to the presence of N-ras mutations. Alternatively, N-ras may be present in these mice but a large number of normal spleen cells in these mice interferes with the detection of mutation in a small population of leukemic cells.

Analysis of mutations of neurofibromatosis type 1 gene and N-ras gene in acute myelogenous leukemia

Neurofibromatosis type 1 (NF1) gene is a tumor suppressor gene, and the NF1 gene product, neurofibromin, can downregulate the N-ras gene. Because the N-ras gene is often mutated in acute myelogenous leukemia (AML), we wondered if the NF1 gene might be mutated in those AML samples not having N-ras mutations. We investigated the mutational status of the N-ras gene and the FLR exon of codons 1371-1423 of the open reading frame of the full-length NF1 cDNA, which has a strong homology with the mammalian ras GTPase-activating protein (GAP), especially for a stretch of three consecutive amino acids (F, L, R), by single-strand conformation polymorphism analysis and direct sequencing in samples from patients with AML. Of 48 AML patients, 10 (21%) had point (missense) mutations of the N-ras gene involving codons 12, 13 and 61. However, mutations in the FLR exon of the NF1 gene were not detected in any of the AML samples. We also examined the difference of clinical response to induction therapy between AML patients with and without N-ras mutation. A significantly lower rate of complete remission was noted in individuals with N-ras gene mutations. These results suggest that mutation of the NF1 gene, at least in the FLR exon, is very rare in AML and the NF1 gene probably is not a functional complement of the N-ras gene mutation. The presence of N-ras gene mutation may be associated with a lower clinical response to antileukemic therapy.

DNA sequence analysis of mutations induced by melphalan in the CHO aprt locus

In previous work, we established that treatment with melphalan (L-phenylalanine mustard) produced a predominance of A.T-->T.A transversions in the Simian virus 40 (SV40)-based shuttle vector pZ189 during replication in human 293 cells. Mutations were induced with varying doses (4-12 microM) melphalan in the aprt gene of the hemizygous Chinese hamster ovary (CHO) cell line D422 to determine whether a similar mutation spectrum would be observed in an endogenous gene. DNA sequence alterations were determined for 39 spontaneous and 41 melphalan-induced independent mutant clones. Other than a predominance of transversions in both systems, the spectrum of melphalan-induced aprt mutations bears little resemblance to the spectrum observed in the supF gene of the shuttle plasmid pZ189. In aprt, mutations at G.C base pairs (bp) predominated (29 of 41 base substitutions). Significantly enhanced mutagenesis was observed at 5' G-G-C 3' and 5' G-G-C-C 3' sites in the aprt gene. Almost half of the melphalan-induced base substitutions occurred at 5' G-N-C 3' sequences, which are believed to be potential interstrand crosslink sites.

The v-abl, c-fms, or v-myc oncogene induces gamma radiation resistance of hematopoietic progenitor cell line 32d cl 3 at clinical low dose rate

A variety of viral and cellular oncogenes have been described with differing mechanisms of action but with the common property of inducing morphologic alteration of cells in culture. Subclonal lines of oncogene expressing cells have been shown to produce tumors in vivo. Expression of the N-ras oncogene in embryo fibroblast NIH/3T3 cells has been demonstrated to increase radioresistance in vitro, and these results have been confirmed and extended to human cell lines expressing the c-raf oncogene. In the present report, we have examined the effects of expression of the c-fms, v-abl, or v-myc oncogene in a clonal hematopoietic progenitor cell line 32D cl 3. The 32D cell line is nonmalignant in vivo and is dependent upon a source of Interleukin-3 (IL-3) for growth in vitro. The radiation survival of 32D cl 3 cells transfected and expressed in the c-fms oncogene showed significant increase in the radioresistance at both 5 cGy/min and 116 cGy/min. A clone of 32D cl 3 transfected and expressing the v-myc oncogene demonstrated increased radioresistance at both dose rates. Results of split dose experiments suggested significant repair of sublethal irradiation damage of 32D-v-abl cells. Results were compared with expression of the same v-abl oncogene in the NIH/3T3 embryo fibroblast cell line. The data demonstrate that gamma irradiation resistance is significantly increased by each oncogene expressed in 32D cl 3 cells. The data on cell line 32D cl 3 may correlate with the radioresistance of v-abl expressing human hematopoietic cell malignancies treated by irradiation therapy.

The hematopoietic microenvironment

Hematopoietic microenvironment is comprised of an admixture of several adherent cell types including fibroblasts, reticular adventitial cells, and marcophages. The biologic interaction of these cells with the most primitive hematopoietic progenitor cells capable of reconstituting all hematopoietic lineages within an irradiated host, as well as differentiated progenitor cells and cells of each committed lineage, has been the subject of intense investigation. Transplantation of the hematopoietic microenvironment has recently been demonstrated and this technique has been used to partially correct the microenvironmental defect in the Sl/Sld mouse. The molecular mechanism of cell surface interaction between stromal and hematopoietic stem cells is being elucidated by molecular transfection techniques in which genes for specific receptors are introduced into hematopoietic stem cell lines and then demonstrated to adhere and proliferate in contact with stromal cells expressing transfected recombinant ligands. This model has been demonstrated with the EGF receptor bearing 32D cl 3 stem cells bound to TGF alpha-producing stromal cells. Extracellular matrix components of the adherent cell layer, the binding of hematopoietic growth factor interaction with matrix components, as well as the positive and negative feedback regulatory role of hematopoietic stem cells bound to the microenvironment, represents the focus of current investigation.