BCR first exon sequences specifically activate the BCR/ABL tyrosine kinase oncogene of Philadelphia chromosome-positive human leukemias.

The c-abl proto-oncogene encodes a cytoplasmic tyrosine kinase which is homologous to the src gene product in its kinase domain and in the upstream kinase regulatory domains SH2 (src homology region 2) and SH3 (src homology region 3). The murine v-abl oncogene product has lost the SH3 domain as a consequence of N-terminal fusion of gag sequences. Deletion of the SH3 domain is sufficient to render the murine c-abl proto-oncogene product transforming when myristylated N-terminal membrane localization sequences are also present. In contrast, the human BCR/ABL oncogene of the Philadelphia chromosome translocation has an intact SH3 domain and its product is not myristylated at the N terminus. To analyze the contribution of BCR-encoded sequences to BCR/ABL-mediated transformation, the effects of a series of deletions and substitutions were assessed in fibroblast and hematopoietic-cell transformation assays. BCR first-exon sequences specifically potentiate transformation and tyrosine kinase activation when they are fused to the second exon of otherwise intact c-ABL. This suggests that BCR-encoded sequences specifically interfere with negative regulation of the ABL-encoded tyrosine kinase, which would represent a novel mechanism for the activation of nonreceptor tyrosine kinase-encoding proto-oncogenes.     

PTPROt inactivates the oncogenic fusion protein BCR/ABL and suppresses transformation of K562 cells

Chronic myelogenous leukemia is typified by constitutive activation of the c-abl kinase as a result of its fusion to the breakpoint cluster region (BCR). Because the truncated isoform of protein-tyrosine phosphatase receptor-type O (PTPROt) is specifically expressed in hematopoietic cells, we tested the possibility that it could potentially dephosphorylate and inactivate the fusion protein bcr/abl. Ectopic expression of PTPROt in the chronic myelogenous leukemia cell line K562 indeed resulted in hypophosphorylation of bcr/abl and reduced phosphorylation of its downstream targets CrkL and Stat5, confirming that PTPROt could inactivate the function of bcr/abl. Furthermore, the expression of catalytically active PTPROt in K562 cells caused reduced proliferation, delayed transition from G0/G1 to S phase, loss of anchorage independent growth, inhibition of ex vivo tumor growth, and increased their susceptibility to apoptosis, affirming that this tyrosine phosphatase can revert the transformation potential of bcr/abl. Additionally, the catalytically inactive PTPROt acted as a trapping mutant that was also able to inhibit anchorage independence and facilitate apoptosis of K562 cells. The inhibitory action of PTPROt on bcr/abl was also confirmed in a murine myeloid cell line overexpressing bcr/abl. PTPROt expression was suppressed in K562 cells and was relieved upon treatment of the cells with 5-azacytidine, an inhibitor of DNA methyltransferase, with concomitant hypomethylation of the PTPRO CpG island. These data demonstrate that suppression of PTPROt by promoter methylation could contribute to the augmented phosphorylation and constitutive activity of its substrate bcr/abl and provide a potentially significant molecular therapeutic target for bcr/abl-positive leukemia.     

IGF-IR determines the fates of BCR/ABL leukemia

Background

The tyrosine kinase receptor insulin-like growth factor 1 receptor (IGF-IR) contributes to the initiation and progression of many types of malignancies. We previously showed that IGF-2, which binds IGF-IR, is an extrinsic factor that supports the ex vivo expansion of hematopoietic stem cells (HSCs). We also demonstrated that IGF-IR is not required for HSC activity in vivo.

Methods and results

Here we investigated the role of IGF-IR in chronic myeloid leukemia (CML) using the retroviral BCR/ABL transplantation mouse model. Existing antibodies against IGF-IR are not suitable for flow cytometry; therefore, we generated a fusion of the human IgG Fc fragment with mutant IGF-2 that can bind to IGF-IR. We used this fusion protein to evaluate mouse primary hematopoietic populations. Through transplantation assays with IGF-IR⁺ and IGF-IR⁻ cells, we demonstrated that IGF-IR is expressed on all mouse HSCs. The expression of IGF-IR is much higher on CML cells than on acute lymphoblastic leukemia (ALL) cells. The depletion of IGF-IR expression in BCR/ABL⁺ cells led to the development of ALL (mostly T cell ALL) but not CML. Lack of IGF-IR resulted in decreased self-renewal of the BCR/ABL⁺ CML cells in the serial replating assay.

Conclusion

IGF-IR regulates the cell fate determination of BCR/ABL⁺ leukemia cells and supports the self-renewal of CML cells.     

Structural alterations of the BCR and ABL genes in Ph1 positive acute leukemias with rearrangements in the BCR gene first intron: further evidence implicating Alu sequences in the chromosome translocation.

In the Philadelphia positive bcr negative acute leukemias (Ph1+bcr- AL), the chromosomal breakpoints on chromosome 22 have been shown clustered within 10.8kb (bcr2) and 5kb (bcr3) fragments of the first intron of the BCR gene. We previously reported that the breakpoints were localized in Alu repeats on chromosomes 9 and 22 in a Ph1+bcr- acute lymphoblastic leukemia with a rearrangement involving bcr2. Molecular data of two other Ph1 translocations, one a Ph1+bcr- acute myeloblastic leukemia in the bcr2 region, and the other an acute lymphoblastic leukemia in the bcr3 region are presented. In the former, the breakpoints on chromosomes 9 and 22 are localized in Alu repeats, in regions with two inverted Alu sequences, as in our previously reported case. In the second leukemia, the breakpoints are not located in Alu sequences, but such repeats are found in their vicinity. The implications of these findings are discussed.     

Global effects of BCR/ABL and TEL/PDGFRbeta expression on the proteome and phosphoproteome: identification of the Rho pathway as a target of BCR/ABL

Many leukemic oncogenes form as a consequence of gene fusions or mutation that result in the activation or overexpression of a tyrosine kinase. To identify commonalities and differences in the action of two such kinases, breakpoint cluster region (BCR)/ABL and TEL/PDGFRbeta, two-dimensional gel electrophoresis was employed to characterize their effects on the proteome. While both oncogenes affected expression of specific proteins, few common effects were observed. A number of proteins whose expression is altered by BCR/ABL, including gelsolin and stathmin, are related to cytoskeletal function whereas no such changes were seen in TEL/PDGFRbeta-transfected cells. Treatment of cells with the kinase inhibitor STI571 for 4-h reversed changes in expression of some of these cytoskeletal proteins. Correspondingly, BCR/ABL-transfected cells were less responsive to chemotactic and chemokinetic stimuli than non-transfected cells and TEL/PDGFRbeta-transfected Ba/F3 cells. Decreased motile response was reversed by a 16-h treatment with STI571. A phosphoprotein-specific gel stain was used to identify TEL/PDGFRbeta and BCR/ABL-mediated changes in the phosphoproteome. These included changes on Crkl, Ras-GAP-binding protein 1, and for BCR/ABL, cytoskeletal proteins such as tubulin, and Nedd5. Decreased phosphorylation of Rho-GTPase dissociation inhibitor (Rho GDI) was also observed in BCR/ABL-transfected cells. This results in the activation of the Rho pathway, and treatment of cells with Y27632, an inhibitor of Rho kinase, inhibited DNA synthesis in BCR/ABL-transfected Ba/F3 cells but not TEL/PDGFRbeta-expressing cells. Expression of a dominant-negative RhoA inhibited both DNA synthesis and transwell migration, demonstrating the significance of this pathway in BCR/ABL-mediated transformation.     

The Functional Interplay Between the t(9;22)-Associated Fusion Proteins BCR/ABL and ABL/BCR in Philadelphia Chromosome-Positive Acute Lymphatic Leukemia

The hallmark of Philadelphia chromosome positive (Ph⁺) leukemia is the BCR/ABL kinase, which is successfully targeted by selective ATP competitors. However, inhibition of BCR/ABL alone is unable to eradicate Ph⁺ leukemia. The t(9;22) is a reciprocal translocation which encodes not only for the der22 (Philadelphia chromosome) related BCR/ABL, but also for der9 related ABL/BCR fusion proteins, which can be detected in 65% of patients with chronic myeloid leukemia (CML) and 100% of patients with Ph⁺ acute lymphatic leukemia (ALL). ABL/BCRs are oncogenes able to influence the lineage commitment of hematopoietic progenitors. Aim of this study was to further disclose the role of p96^ABL/BCR for the pathogenesis of Ph⁺ ALL. The co-expression of p96^ABL/BCR enhanced the kinase activity and as a consequence, the transformation potential of p185^BCR/ABL. Targeting p96^ABL/BCR by RNAi inhibited growth of Ph⁺ ALL cell lines and Ph⁺ ALL patient-derived long-term cultures (PD-LTCs). Our in vitro and in vivo stem cell studies further revealed a functional hierarchy of p96^ABL/BCR and p185^BCR/ABL in hematopoietic stem cells. Co-expression of p96^ABL/BCR abolished the capacity of p185^BCR/ABL to induce a CML-like disease and led to the induction of ALL. Taken together our here presented data reveal an important role of p96^ABL/BCR for the pathogenesis of Ph⁺ ALL.     

Comparison of baculovirus-expressed c-Abl and BCR/ABL protein tyrosine kinases

Mouse c-Abl type IV and human BCR/ABL proteins have been expressed in insect cells using the baculovirus system. The proteins were expressed as full-length polypeptides as judged by electrophoresis in denaturing gels. They were identified by immunoprecipitation and immunoblotting with antibodies against ABL peptides and, for BCR/ABL, against a BCR peptide. In these immunoprecipitates both proteins gave autophosphorylation principally on tyrosine. Both proteins were active tyrosine kinases, phosphorylating a variety of tyrosine-containing substrates. In fresh extracts both proteins contained phosphotyrosine as shown by Western blots with antiphosphotyrosine antibodies. Partial purification could be achieved readily using ion exchange columns, and the BCR/ABL protein, p210^BCR/ABL, could be further purified to near-homogeneity using an antiphosphotyrosine column. Both enzymes required a divalent metal ion for activity. At low concentrations of ATP (2 μM) and with angiotensin II as substrate both enzymes were activated by Mn²⁺ or by Mg²⁺. No major differences in catalytic properties were found between the two isolated enzymes in solution. The oncogenic properties of p210^BCR/ABL may be due to its different subcellular location, or to the presence of an intracellular inhibitor of c-Abl that does not inhibit BCR/ABL, or to altered substrate-specificity such that it can phosphorylate a unique substrate which is not recognised by c-Abl.     

Effect of Ras Inhibition in Hematopoiesis and BCR/ABL Leukemogenesis

Introduction

Breast cancer is the most common female cancer and the second most common cause of female cancer-related deaths in the United States. World-wide, more than one million women will be diagnosed with breast cancer annually. In 2007, more than 175,000 women were diagnosed with breast cancer in the United States. However, deaths due to breast cancer have decreased in the recent years in part because of improved screening techniques, surgical interventions, understanding of the pathogenesis of the disease, and utilization of traditional chemotherapies in a more efficacious manner. One of the more exciting areas of improvement in the treatment of breast cancer is the entrance of novel therapies now available to oncologists. In the field of cancer therapeutics, the area of targeted and biologic therapies has been progressing at a rapid rate, particularly in the treatment of breast cancer. Since the advent of imatinib for the successful treatment of chronic myelogenous leukemia in the 2001, clinicians have been searching for comparable therapies that could be as efficacious and as tolerable. In order for targeted therapies to be effective, the agent must be able to inhibit critical regulatory pathways which promote tumor cell growth and proliferation. The targets must be identifiable, quantifiable and capable of being interrupted. In the field of breast cancer, two advances in targeted therapy have led to great strides in the understanding and treatment of breast cancer, namely hormonal therapy for estrogen positive receptor breast cancer and antibodies directed towards the inhibition of human epidermal growth factor receptor (HER)2. These advances have revolutionized the understanding and the treatment strategies for breast cancer. Building upon these successes, a host of novel agents are currently being investigated and used in clinical trials that will hopefully prove to be as fruitful. This review will focus on novel therapies in the field of breast cancer with a focus on metastatic breast cancer (MBC) and updates from the recent annual ASCO meeting and contains a summary of the results.     

Curcumin potentiates the anti-leukemia effects of imatinib by downregulation of the AKT/mTOR pathway and BCR/ABL gene expression in Ph+ acute lymphoblastic leukemia

Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL) is triggered by BCR/ABL and SRC family tyrosine kinases. They interact with each other and subsequently activate downstream growth-signaling pathways, including Raf/MEK/ERK, Akt/mTOR, and STAT5 pathways. Although imatinib is the standard treatment for Ph+ leukemia, response rate of Ph+ ALL to imatinib is low, relapse is frequent and quick. Studies have documented the potential anti-tumor activities of curcumin. However, whether curcumin can be used in the therapy for Ph+ ALL remains obscure. Here, we reported that curcumin induced apoptosis by inhibition of AKT/mTOR and ABL/STAT5 signaling, down-regulation of BCR/ABL expression, and induction of the BCL2/BAX imbalance. Curcumin exerted synergetic anti-leukemia effects with imatinib by inhibition of the imatinib-mediated overactivation of AKT/mTOR signaling and down-regulation of BCR/ABL gene expression. In primary samples from Ph+ ALL patients, curcumin inhibited cellular proliferation and down-regulated constitutive activation of growth-signaling pathways not only in newly diagnosed patients but also in imatinib-resistant patients. In Ph+ ALL mouse models, curcumin exhibited synergetic anti-leukemia effects with imatinib. These results demonstrated that curcumin might be a promising agent for Ph+ ALL patients.     

Deletion of ABL/BCR on der(9) associated with severe basophilia

Chronic basophilic leukemia is a rare form in chronic myeloid leukemia patients. Only limited number of reports are available. Herein, we describe a patient who presented with fatigue, weight loss, leucocytosis, prominent basophilia, and mild eosinophilia. On biopsy, bone marrow was hypercellular with marked basophils. The immunophenotype showed abnormal expression of CD7, which is suggestive of basophilic maturation. Chromosomal analysis from GTG-banded metaphases revealed Ph positivity, and fluorescence in situ hybridization (FISH) with BCR/ABL dual color, dual fusion probe showed single fusion on the der(22) chromosome and ABL/BCR fusion was deleted on the der(9) chromosome. The deletion (ABL/BCR) on der(9) may be associated with basophilia which may be also indicative of the transformation of CML to acute myeloid leukemia.     

The two major imatinib resistance mutations E255K and T315I enhance the activity of BCR/ABL fusion kinase

The resistance to the tyrosine kinase inhibitor imatinib in BCR/ABL-positive leukemias is mostly associated with mutations in the kinase domain of BCR/ABL, which include the most prevalent mutations E255K and T315I. Intriguingly, these mutations have also been identified in some patients before imatinib treatment. Here we examined the effects of these mutations on the kinase activity of a BCR/ABL kinase domain construct that also contained the SH3 and SH2 domains. When expressed in COS7 cells, the BCR/ABL construct with either E255K or T315I exhibited not only the resistance to imatinib but also the increase in activity to induce autophosphorylation as well as tyrosine phosphorylation of various cellular proteins, which included STAT5. The mutant kinases also showed increased activities in in vitro kinase assays. These results raise a possibility that the major imatinib resistance mutations E255K and T315I may confer the growth advantage on leukemic cells to expand in the absence of selective pressure from imatinib treatment.     

BCR/ABL Recruits p53 Tumor Suppressor Protein to Induce Drug Resistance

Tumors expressing the ABL oncoproteins (BCR/ABL, TEL/ABL, v-ABL) can avoidapoptosis triggered by DNA damaging agents. The tumor suppressor protein p53 is animportant activator of apoptosis in normal cells; conversely its functional loss may causedrug resistance. The ABL oncoprotein - p53 paradigm represents the relationship between anoncogenic tyrosine kinase and a tumor suppressor gene. Here we show that BCR/ABLoncoproteins employ p53 to induce resistance to DNA damage in myeloid leukemia cells.Cells transformed by the ABL oncoproteins displayed accumulation of p53 upon DNAdamage. In contrast, only a modest increase of p53 expression followed by activation ofcaspase-3 were detected in normal cells expressing endogenous c-ABL. Phosphatidylinositol-3 kinase-like protein kinases (ATR and also ATM) -dependent phosphorylation of p53-Ser15residue was associated with the accumulation of p53, and stimulation of p21Waf-1 andGADD45, resulting in G2/M delay in BCR/ABL cells after genotoxic treatment. Inhibition ofp53 by siRNA or by the temperature-sensitive mutation reduced G2/M accumulation anddrug resistance of BCR/ABL cells. In conclusion, accumulation of the p53 proteincontributed to prolonged G2/M checkpoint activation and drug resistance in myeloid cellsexpressing the BCR/ABL oncoproteins.     

BCR/ABL modifies the kinetics and fidelity of DNA double-strand breaks repair in hematopoietic cells

The oncogenic BCR/ABL tyrosine kinase facilitates the repair of DNA double-strand breaks (DSBs). We find that after gamma-irradiation BCR/ABL-positive leukemia cells accumulate more DSBs in comparison to normal cells. These lesions are efficiently repaired in a time-dependent fashion by BCR/ABL-stimulated non-homologous end-joining (NHEJ) followed by homologous recombination repair (HRR) mechanisms. However, mutations and large deletions were detected in HRR and NHEJ products, respectively, in BCR/ABL-positive leukemia cells. We propose that unfaithful repair of DSBs may contribute to genomic instability in the Philadelphia chromosome-positive leukemias.     

Assessment of downstream effectors of BCR/ABL protein tyrosine kinase using combined proteomic approaches

Leukaemic transformation is frequently associated with the aberrant activity of a protein tyrosine kinase (PTK). As such it is of clinical relevance to be able to map the effects of these leukaemogenic PTKs on haemopoietic cells at the level of phosphorylation modulation. In this paradigm study we have employed a range of proteomic approaches to analyse the effects of one such PTK, BCR/ABL. We have employed phosphoproteome enrichment techniques allied to peptide and protein quantification to identify proteins and pathways involved in cellular transformation. Amongst the proteins shown to be regulated at the post‐translational level were cofilin, an actin‐severing protein thus linked to altered motility and Cbl an E3 ubiquitin ligase integrally linked to the control of tyrosine kinase signalling (regulated by 5 and 6 PTKs respectively). The major class of proteins identified however were molecular chaperones. We also showed that HSP90 phosphorylation is altered by BCR/ABL action and that HSP90 plays a crucial role in oncogene stability. Further investigation with another six leukaemogenic PTKs demonstrates that this HSP90 role in oncogene stability appears to be a common phenomenon in a range of leukaemias. This opens up the potential opportunity to treat different leukaemias with HSP90 inhibitors.