Overlapping cDNA clones define the complete coding region for the P210c-abl gene product associated with chronic myelogenous leukemia cells containing the Philadelphia chromosome

Studying the enhancement of programmed cell death by combined AG1024 and paclitaxel in a model of chronic myelogenous leukemia

AIMS

Chronic myelogenous leukemia is a clonal malignancy of the pluripotent hematopoietic stem cells that is characterized by the uncontrolled proliferation and expansion of myeloid progenitors. Myeloid progenitors express the fusion oncogene BCR-ABL, which has uncontrollable activity in malignant cells and prevents the cell apoptosis caused by some antineoplastic agents, such as paclitaxel. Targeting these abnormalities by blocking the tyrosine kinase enzymes of BCR-ABL is a promising approach for chronic myelogenous leukemia therapy.

MAIN METHODS

Conventional Liu's staining is an auxiliary technique used in microscopy to enhance the contrast in microscopic images, aiding the observation of cell morphology. The MTT assay, flow cytometry of the sub-G1 analysis and the TUNEL assay were applied to estimate the apoptosis levels. RT-PCR and western blot methods were used to evaluate the key molecules conferring anti-cell-death properties.

KEY FINDINGS

The effects of the tyrosine kinase inhibitor AG1024 were evaluated with regard to the regulation of BCR-ABL expression, inhibition of cell proliferation, and enhanced paclitaxel-induced apoptosis in BCR-ABL-expressing K562 cell lines. AG1024 downregulated the expression of BCR-ABL and anti-apoptosis factors, such as Bcl-2 and Bcl-xL, which were present in K562 cells. Moreover, the combination of AG1024 with paclitaxel inhibited cell proliferation and enhanced paclitaxel-induced apoptosis within 24h.

SIGNIFICANCE

In summary, the present study shows that the combination of AG1024 with paclitaxel inhibited model cancer cell proliferation, suggesting a new use of paclitaxel-based chemotherapy for cancer control.

Functional mechanisms and roles of adaptor proteins in abl-regulated cytoskeletal actin dynamics

Abl is a nonreceptor tyrosine kinase and plays an essential role in the modeling and remodeling of F-actin by transducing extracellular signals. Abl and its paralog, Arg, are unique among the tyrosine kinase family in that they contain an unusual extended C-terminal half consisting of multiple functional domains. This structural characteristic may underlie the role of Abl as a mediator of upstream signals to downstream signaling machineries involved in actin dynamics. Indeed, a group of SH3-containing accessory proteins, or adaptor proteins, have been identified that bind to a proline-rich domain of the C-terminal portion of Abl and modulate its kinase activity, substrate recognition, and intracellular localization. Moreover, the existence of signaling cascade and biological outcomes unique to each adaptor protein has been demonstrated. In this paper, we summarize functional roles and mechanisms of adaptor proteins in Abl-regulated actin dynamics, mainly focusing on a family of adaptor proteins, Abi. The mechanism of Abl's activation and downstream signaling mediated by Abi is described in comparison with those by another adaptor protein, Crk.

Translation of the Philadelphia chromosome into therapy for CML

Throughout its history, chronic myeloid leukemia (CML) has set precedents for cancer research and therapy. These range from the identification of the first specific chromosomal abnormality associated with cancer to the development of imatinib as a specific, targeted therapy for the disease. The successful development of imatinib as a therapeutic agent for CML can be attributed directly to decades of scientific discoveries. These discoveries determined that the BCR-ABL tyrosine kinase is the critical pathogenetic event in CML and an ideal target for therapy. This was confirmed in clinical trials of imatinib, with imatinib significantly improving the long-term survival of patients with CML. Continuing in this tradition of scientific discoveries leading to improved therapies, the understanding of resistance to imatinib has rapidly led to strategies to circumvent resistance. Continued studies of hematologic malignancies will allow this paradigm of targeting molecular pathogenetic events to be applied to many additional hematologic cancers.

Ph+/VE-cadherin+ identifies a stem cell like population of acute lymphoblastic leukemia sustained by bone marrow niche cells

Although leukemic stem cells (LSCs) show a symbiotic relationship with bone marrow microenvironmental niches, the mechanism by which the marrow microenvironment contributes to self-renewal and proliferation of LSCs remains elusive. In the present study, we identified a unique subpopulation of Philadelphia chromosome-positive (Ph(+)) acute lymphoblastic leukemia (ALL) cells coexpressing markers of endothelial cells (including VE-cadherin, PECAM-1, and Flk-1) and committed B-lineage progenitors. After long-term coculture with bone marrow stromal cells, tumor cells formed hematopoietic colonies and cords, expressed early stem- cell markers, and showed endothelial sprouting. Gene expression profiles of LSCs were altered in the presence of stromal cell contact. Stromal cell contact promoted leukemic cell VE-cadherin expression, stabilized beta-catenin, and up-regulated Bcr-abl fusion gene expression. Our study indicates that these specific tumor cells are uniquely positioned to respond to microenvironment-derived self-renewing and proliferative cues. Ph(+)/VE-cadherin(+) tumor subpopulation circumvents the requirement of exogenous Wnt signaling for self-renewal through stromal cell support of leukemic cell VE-cadherin expression and up-regulated Bcr-abl tyrosine kinase activity. These data suggest that strategies targeting signals in the marrow microenvironment that amplify the Bcr-abl/VE-cadherin/beta-catenin axis may have utility in sensitizing drug-resistant leukemic stem cells.

Biphosphinic palladacycle complex mediates lysosomal-membrane permeabilization and cell death in K562 leukaemia cells

The cell death mechanism of cytotoxicity induced by the Biphosphinic Palladacycle Complex (BPC) was studied using a K562 leukaemia cell line. The IC50 values obtained for K562 cells post-72 h of BPC were less than 5.0 microM by using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and trypan blue assays. Using the Acridine Orange vital staining combining fluorescence microscopy it was observed that the complex triggers apoptosis in K562 cells, inducing DNA fragmentation, as analysed through electrophoresis. Lysosomal-membrane permeabilization was also observed in K562 cells post-5 h of BPC, which suggests intralysosomal accumulation by proton-trapping, since its pKa value ranged from 5.1 to 6.5. Caspase-3, and -6 activity induced by BPC in K562 cells was prevented by the cathepsin-B inhibitor [N-(L-3-trans-propylcarbamoyl-oxirane-2-carbonyl)-L-isoleucyl-L-proline] (CA074). These events occurred in the presence of endogenous bcl-2 and bax expression. Acute toxicological studies demonstrated that BPC produces no lesions for liver and kidney fourteen-days after drug administration (100 mg/kg--i.p.). White and red blood cells of BPC-treated mice presented normal morphological characteristics. Taken together, these data suggest a novel lysosomal pathway for BPC-induced apoptosis, in which lysosomes are the primary target and cathepsin B acts as death mediator.

The BCR-ABL story: bench to bedside and back

The twenty-first century is beginning with a sharp turn in the field of cancer therapy. Molecular targeted therapies against specific oncogenic events are now possible. The BCR-ABL story represents a notable example of how research from the fields of cytogenetics, retroviral oncology, protein phosphorylation, and small molecule chemical inhibitors can lead to the development of a successful molecular targeted therapy. Imatinib mesylate (Gleevec, STI571, or CP57148B) is a direct inhibitor of ABL (ABL1), ARG (ABL2), KIT, and PDGFR tyrosine kinases. This drug has had a major impact on the treatment of chronic myelogenous leukemia (CML) as well as other blood neoplasias and solid tumors with etiologies based on activation of these tyrosine kinases. Analysis of CML patients resistant to BCR-ABL suppression by Imatinib mesylate coupled with the crystallographic structure of ABL complexed to this inhibitor have shown how structural mutations in ABL can circumvent an otherwise potent anticancer drug. The successes and limitations of Imatinib mesylate hold general lessons for the development of alternative molecular targeted therapies in oncology.

Imatinib as a Paradigm of Targeted Therapies

Imatinib (Gleevec) exemplifies the successful development of a rationally designed, molecularly targeted therapy for the treatment of a specific cancer. This article reviews the identification of the BCR-ABL tyrosine kinase as a therapeutic target in chronic myeloid leukemia and the steps in the development of an agent to specifically inactivate this abnormality. The clinical trials results are reviewed along with a description of resistance mechanisms. As imatinib also inhibits the tyrosine kinase activity of KIT and the platelet-derived growth factor receptors, the extension of imatinib to malignancies driven by these kinases will be described. Issues related to clinical trials of molecularly targeted agents are discussed, including patient and dose selection. Last, the translation of this paradigm to other malignancies is explored.

Inhibition of Bcr-Abl kinase activity by PD180970 blocks constitutive activation of Stat5 and growth of CML cells

Chronic myelogenous leukemia (CML) is a myeloproliferative disease characterized by the BCR-ABL genetic translocation and constitutive activation of the Abl tyrosine kinase. Among members of the Signal Transducers and Activators of Transcription (STAT) family of transcription factors, Stat5 is activated by the Bcr-Abl kinase and is implicated in the pathogenesis of CML. We recently identified PD180970 as a new and highly potent inhibitor of Bcr-Abl kinase. In this study, we show that blocking Bcr-Abl kinase activity using PD180970 in the human K562 CML cell line resulted in inhibition of Stat5 DNA-binding activity with an IC(50) of 5 nM. Furthermore, abrogation of Abl kinase-mediated Stat5 activation suppressed cell proliferation and induced apoptosis in K562 cells, but not in the Bcr-Abl-negative myeloid cell lines, HEL 92.1.7 and HL-60. Dominant-negative Stat5 protein expressed from a vaccinia virus vector also induced apoptosis of K562 cells, consistent with earlier studies that demonstrated an essential role of Stat5 signaling in growth and survival of CML cells. RNA and protein analyses revealed several candidate target genes of Stat5, including Bcl-x, Mcl-1, c-Myc and cyclin D2, which were down-regulated after treatment with PD180970. In addition, PD180970 inhibited Stat5 DNA-binding activity in cultured primary leukemic cells derived from CML patients. To detect activated Stat5 in CML patient specimens, we developed an immunocytochemical assay that can be used as a molecular end-point assay to monitor inhibition of Bcr-Abl signaling. Moreover, PD180970 blocked Stat5 signaling and induced apoptosis of STI-571 (Gleevec, Imatinib)-resistant Bcr-Abl-positive cells. Together, these results suggest that the mechanism of action of PD180970 involves inhibition of Bcr-Abl-mediated Stat5 signaling and provide further evidence that compounds in this structural class may represent potential therapeutic agents for CML.

Modelling haematopoietic malignancies in the mouse and therapeutical implications

Modelling human disease in the mouse has become an essential activity in biomedical research in order to unravel molecular mechanisms underlying pathological conditions as well as to determine in vivo the consequences of aberrant gene function. The mouse is by far the most accessible mammalian system physiologically similar to humans. Furthermore, the development of novel techniques for manipulating the murine genome, which allow the in vivo modification of virtually any genomic region in a time and/or tissue specific manner, renders the mouse an ideal model system to study human pathological conditions. Modelling human diseases in mice has reached an even greater relevance in the field of haematological malignancies, due to the already advanced characterization of the molecular basis of many haematological disorders. In this review, we describe the most important technological developments that made it possible to reproduce in the mouse the genetic lesions that characterize human haematological malignancies, thus often generating faithful mouse models of the human condition. We provide specific examples of the advantages and limitations of the various genetic approaches utilized to model leukaemia and lymphoma in the mouse. Finally, we discuss the power of mouse modelling in developing and testing novel therapeutic modalities in pre-clinical studies.

Fusion of the BCR and the fibroblast growth factor receptor-1 (FGFR1) genes as a result of t(8;22)(p11;q11) in a myeloproliferative disorder: the first fusion gene involving BCR but not ABL

Constitutive activation of tyrosine kinases as a consequence of chromosomal translocations, forming fusion genes, plays an important role in the development of hematologic malignancies, in particular, myeloproliferative syndromes (MPSs). In this respect, the t(9;22)(q34;q11) that results in the BCR/ABL fusion gene in chronic myeloid leukemia is one of the best-studied examples. The fibroblast growth factor receptor 1 (FGFR1) gene at 8p11 encodes a transmembrane receptor tyrosine kinase and is similarly activated by chromosomal translocations, in which three alternative genes-ZNF198 at 13q12, CEP110 at 9q34, and FOP at 6q27-become fused to the tyrosine kinase domain of FGFR1. These 8p11-translocations are associated with characteristic morphologic and clinical features, referred to as "8p11 MPS." In this study, we report the isolation and characterization of a novel fusion gene in a hematologic malignancy with a t(8;22)(p11;q11) and features suggestive of 8p11 MPS. We show that the breakpoints in the t(8;22) occur within introns 4 and 8 of the BCR and FGFR1 genes, respectively. On the mRNA level, the t(8;22) results in the fusion of BCR exons 1-4 in-frame with the tyrosine kinase domain of FGFR1 as well as in the expression of a reciprocal FGFR1/BCR chimeric transcript. By analogy with data obtained from previously characterized fusion genes involving FGFR1 and BCR/ABL, it is likely that the oligomerization domain contributed by BCR is critical and that its dimerizing properties lead to aberrant FGFR1 signaling and neoplastic transformation.

NF-kappaB/RelA transactivation is required for atypical protein kinase C iota-mediated cell survival

In chronic myelogenous leukemia (CML), the oncogene bcr-abl encodes a dysregulated tyrosine kinase that inhibits apoptosis. We showed previously that human erythroleukemia K562 cells are resistant to antineoplastic drug (taxol)-induced apoptosis through the atypical protein kinase C iota isozyme (PKC iota), a kinase downstream of Bcr-Abl. The mechanism(s) by which PKC iota mediates cell survival to taxol is unknown. Here we demonstrate that PKC iota requires the transcription factor nuclear factor-kappaB (NF-kappaB) to confer cell survival. At apoptosis-inducing concentrations, taxol weakly induces IkappaB(alpha) proteolysis and NF-kappaB translocation in K562 cells, but potently induces its transcriptional activity. Inhibition of NF-kappaB activity (by blocking IkappaB(alpha) degradation) significantly sensitizes cells to taxol-induced apoptosis. Likewise, K562 cells expressing antisense PKC iota mRNA or kinase dead PKC iota (PKC iota-KD) are sensitized to taxol; these cells are rescued from apoptosis by NF-kappaB overexpression. Expression of constitutively active PKC iota (PKC iota-CA) upregulates NF-kappaB transactivation and rescues cells from apoptosis in the absence of Bcr-Abl tyrosine kinase activity. Using a chimeric GAL4-RelA transactivator, we find that taxol potently activates GAL4-RelA-dependent transcription. This activation was further upregulated by expression of PKC iota-CA and inhibited by expression of PKC iota-KD. Our results indicate that RelA transactivation is an important downstream target of the PKC iota-mediated Bcr-Abl signaling pathway and is required for resistance to taxol-induced apoptosis.

Philadelphia-positive B-cell acute lymphoblastic leukemia is initiated in an uncommitted progenitor cell

BCR-ABL is a chimeric oncogene generated by translocation of sequences from the c-ABLgene on chromosome 9 into the BCR gene on chromosome 22. Alternative chimeric proteins, BCR-ABLp190 and BCR-ABLp210, are produced that are characteristic of chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (Ph1-ALL) respectively. In CML, it is evident that the transformation occurs at the level of pluripotent stem cells. However, Ph1-ALL has been thought to affect progenitor cells with lymphoid differentiation. Recently, it has been demonstrated that normal primitive cells, rather than committed progenitor cells, are the target for leukemic transformation in Ph1-ALL. In this review, we discuss what is known about the relationship between the specific BCR-ABLp190 oncogene, the target cell and the characteristics of the subsequent disease process it causes. We also discuss how this information may be applied to the establishment of new directions in therapy.

Multiple signaling interactions of Abl and Arg kinases with the EphB2 receptor

The Eph family of receptor tyrosine kinases and the Abl family of non-receptor tyrosine kinases have both been implicated in tissue morphogenesis. They regulate the organization of the actin cytoskeleton in the developing nervous system and participate in signaling pathways involved in axon growth. Both Eph receptors and Abl are localized in the neuronal growth cone, suggesting that they play a role in axon pathfinding. Two-hybrid screens identified regions of Abl and Arg that bind to the EphB2 and EphA4 receptors, suggesting a novel signaling connection involving the two kinase families. The association of full-length Abl and Arg with EphB2 was confirmed by co-immunoprecipitation and found to involve several distinct protein interactions. The SH2 domains of Abl and Arg bind to tyrosine-phosphorylated motifs in the juxtamembrane region of EphB2. A second, phosphorylation-independent interaction with EphB2 involves non-conserved sequences in the C-terminal tails of Abl and Arg. A third interaction between Abl and EphB2 is probably mediated by an intermediary protein because it requires tyrosine phosphorylation of EphB2, but not the binding sites for the Abl SH2 domain. The connection between EphB2 and Abl/Arg appears to be reciprocal. Activated EphB2 causes tyrosine phosphorylation of Abl and Arg, and vice versa. Interestingly, treatment of COS cells and B35 neuronal-like cells with ephrin-B1 to activate endogenous EphB2 decreased the kinase activity of endogenous Abl. These data are consistent with the opposite effects that Eph receptors and Abl have on neurite ougrowth and suggest that Eph receptors and Abl family kinases have shared signaling activities.

Regulation of Cbl phosphorylation by the Abl tyrosine kinase and the Nck SH2/SH3 adaptor

The Cbl proto-oncogene product is tyrosine phosphorylated in response to a wide variety of stimuli. Cbl and the Abl nonreceptor tyrosine kinase both bind to SH3 domains from the SH2/SH3 adaptor Nck, and are candidate effectors for Nck function. Numerous additional SH2- and SH3-domain-mediated interactions are also possible between Cbl, Abl, and Nck. We find that these three signaling proteins associate when overexpressed in mammalian cells and can regulate each other's activity. Co-expression of wt Cbl together with c-Abl, the activity of which is normally repressed in vivo, led to extensive Abl-dependent phosphorylation of Cbl. The major proline-rich region of Cbl was required for its phosphorylation by c-Abl, but not by a constitutively activated Abl mutant, suggesting Cbl activates c-Abl by engaging its SH3 domain. Efficient phosphorylation of Cbl and its stable association with Abl required the SH2 domain of Abl, suggesting that SH2-phosphotyrosine interactions prevent dissociation of active Abl from Cbl. We also show that overexpression of Nck could repress the phosphorylation of Cbl by Abl in vivo. Studies with Nck mutants suggested that the Nck SH2 domain is responsible for inhibiting the activity of Abl toward both Cbl and Nck itself, most likely by competing with the Abl SH2 for tyrosine-phosphorylated binding sites.

Molecular abnormalities in chronic myeloid leukemia: deregulation of cell growth and apoptosis

Chronic myeloid leukemia (CML) is a disease of the hematopoietic system, characterized by the presence of the Bcr-Abl oncoprotein. The main characteristics of this disease include adhesion independence, growth factor independence, and resistance to apoptosis. Loss or mutation of the tumor suppressor gene, p53, is one of the most frequent secondary mutations in CML blast crisis. The transition between chronic phase and blast crisis is associated with increased resistance to apoptosis correlating with poor prognosis. This review focuses on the involvement of these two oncoproteins in the development and progression of the apoptotic-resistant phenotype in CML.

An in vivo and in vitro comparison of the effects of b2-a2 and b3-a2 p210BCR-ABL splice variants on murine 32D cells

The Philadelphia (Ph) chromosome, a characteristic cytogenetic marker of chronic myeloid leukaemia (CML), is caused by a reciprocal translocation juxtaposing the 3' region of the ABL gene onto the 5' region of the BCR gene. Due to conservation of the reading frame, but depending on the site of the breakpoint in the BCR gene, two alternatively spliced variants of the p210BCR-ABL mRNA (known as b2-a2 and b3-a2) are produced. To investigate whether there are any biological differences between these splice variants we have transfected the b3-a2 or b2-a2 cDNA into a murine myeloid cell line, 32D. We have also included the previously prepared 32Dp210 cell line (which expresses the b3-a2 transcript) in all of our comparisons. RT-PCR analysis indicated that transcription levels were comparable between the variants. Morphological examination of the cells expressing either of the BCR-ABL transcripts indicated that these cells were more mature with increased cytoplasm:nuclear ratios compared to the 32D parental and 32Dneo vector control cells. However, the 32Dp210 cells had a very different appearance from the other panel members and flow karyotyping indicated a clonal evolution and cytogenetic instability in these cells alone. At 10(6) and 10(7) cell doses all 32D cells expressing BCR-ABL caused ill health and tissue infiltration in SCID mice with such rapidity that statistical analysis was not informative. However, at the 10(5) and 10(4) dosage levels there were similar survival rates between mice injected with 32Db2-a2 or 32Db3-a2 while mice injected with 32Dp210 had a significantly shorter survival time. The study of this 32D cell line panel indicated that there were no overt differences in the biological properties conferred by the b3-a2 or b2-a2 transcripts to the 32D cells although these transcripts were able to confer in vitro and in vivo biological effects. This panel of BCR-ABL expressing 32D cells provides a useful model for CML disease progression studies.

A primitive hematopoietic cell is the target for the leukemic transformation in human Philadelphia-positive acute lymphoblastic leukemia

BCR-ABL is a chimeric oncogene generated by translocation of sequences from the chromosomal counterpart (c-ABLgene) on chromosome 9 into the BCR gene on chromosome 22. Alternative chimeric proteins, BCR-ABLp190 and BCR-ABLp210, are produced that are characteristic of chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph1-ALL). In CML, the transformation occurs at the level of pluripotent stem cells. However, Ph1-ALL is thought to affect progenitor cells with lymphoid differentiation. Here we demonstrate that the cell capable of initiating human Ph1-ALL in non-obese diabetic mice with severe combined immunodeficiency disease (NOD/SCID), termed SCID leukemia–initiating cell (SL-IC), possesses the differentiative and proliferative capacities and the potential for self-renewal expected of a leukemic stem cell. The SL-ICs from all Ph1-ALL analyzed, regardless of the heterogeneity in maturation characteristics of the leukemic blasts, were exclusively CD34+CD38−, which is similar to the cell-surface phenotype of normal SCID-repopulating cells. This indicates that normal primitive cells, rather than committed progenitor cells, are the target for leukemic transformation in Ph1-ALL.

In vivo inhibition by a site-specific catalytic RNA subunit of RNase P designed against the BCR-ABL oncogenic products: a novel approach for cancer treatment

One major obstacle to the effective treatment of cancer is to distinguish between tumor cells and normal cells. The chimeric molecules created by cancer-associated chromosomal abnormalities are ideal therapeutic targets because they are unique to the disease. We describe the use of a novel approach based on the catalytic RNA subunit of RNase P to destroy specifically the tumor-specific fusion genes created as a result of chromosome abnormalities. Using as a target model the abnormal BCR-ABL p190 and p210 products, we constructed M1-RNA with guide sequences that recognized the oncogenic messengers at the fusion point (M1-p190-GS and M1-p210-GS). To test the effectiveness and the specificity of M1-p190-GS and M1-p210-GS, we studied in vitro and in vivo effects of these RNA enzymes againstBCR-ABLp190 andBCR-ABLp210, bearing in mind that both fusion genes share the ABL sequence but differ in the sequence coming from the BCR gene. We showed that M1-p190-GS and M1-p210-GS can act as sequence-specific endonucleases and can exclusively cleave target RNA that forms a base pair with the guide sequence (GS). We also demonstrated that when M1-p190-GS and M1-p210-GS were expressed in proper mammalian cell models, they abolished the effect of BCR-ABL by specifically decreasing the amount of the target BCR-ABL mRNA and preventing the function of theBCR-ABL oncogenes. These data clearly demonstrate the usefulness of the catalytic activity of M1-GS RNA to cleave specifically the chimeric molecules created by chromosomal abnormalities in human cancer and to represent a novel approach to cancer treatment.

Activation of the Abl tyrosine kinase in vivo by Src homology 3 domains from the Src homology 2/Src homology 3 adaptor Nck

The nonreceptor tyrosine kinase c-Abl is tightly regulated in vivo, but the mechanisms that normally repress its activity are not well understood. We find that a construct encoding the first two Src homology 3 (SH3) domains of the Src homology 2/SH3 adaptor protein Nck can activate c-Abl in human 293T cells. A myristoylated Nck SH3 domain construct, which is expected to localize to membranes, potently activated Abl when expressed at low levels. An unmyristoylated Nck SH3 domain construct, which localizes to the cytosol and nucleus, also activated Abl but only at high levels of expression. Activation by both myristoylated and unmyristoylated Nck constructs required the C terminus of Abl; a C-terminally truncated form of Abl was not activated, although this construct could still be activated by deletion of its SH3 domain. Activation did not require the major binding sites in the Abl C terminus for Nck SH3 domains, however, suggesting that the mechanism of activation does not require direct binding to the C terminus. Activation of c-Abl by Nck SH3 domains provides a robust experimental system for analyzing the mechanisms that normally repress Abl activity and how that normal regulation can be perturbed.

In vivo analysis of the molecular pathogenesis of acute promyelocytic leukemia in the mouse and its therapeutic implications

Acute promyelocytic leukemia (APL) is characterized by the expansion of malignant myeloid cells blocked at the promyelocytic stage of hemopoietic development, and is associated with reciprocal chromosomal translocations always involving the retinoic acid receptor alpha (RARalpha) gene on chromosome 17. As a consequence of the translocation RARalpha variably fuses to the PML, PLZF, NPM and NUMA genes (X genes), leading to the generation of RARalpha-X and X-RARalpha fusion genes. The aberrant chimeric proteins encoded by these genes may exert a crucial role in leukemogenesis. Retinoic acid (RA), a metabolite of vitamin A, can overcome the block of maturation at the promyelocytic stage and induce the malignant cells to terminally mature into granulocytes resulting in complete albeit transient disease remission. APL has become, for this reason, the paradigm for 'cancer differentiation therapy'. Furthermore, APL associated with translocation between the RARalpha and the PLZF genes (PLZF-RARalpha) shows a distinctly worse prognosis with poor response to chemotherapy and little or no response to treatment with RA, thus defining a new APL syndrome. Here we will focus our attention on the recent progresses made in defining the molecular mechanisms underlying the pathogenesis of this paradigmatic disease in vivo in the mouse. We will review the critical contribution of mouse modeling in unraveling the transcriptional basis for the differential response to RA in APL. We will also discuss how this new understanding has allowed to propose, develop and test in these murine leukemia models as well as in human APL patients novel therapeutic strategies.

Proteasome inhibitors as potential novel anticancer agents

Ubiquitin/proteasome-dependent proteolysis plays an essential role in degrading regulatory proteins and thereby controlling processes of cell proliferation and cell death (apoptosis). Most recent experiments using cell cultures and mouse models have demonstrated that proteasome inhibitors induce cancer cell apoptosis and therefore inhibit tumor growth. The proteasome inhibitors have the following unique features: (i) greater apoptosis-inducing potency when tested in various human tumor cell lines than current anticancer drugs; (ii) ability to selectively target transformed and tumor, but not normal, human cells; and (iii) ability to overcome tumor cell resistance to cytotoxic therapies. We suggest that proteasome inhibitors have potential use as novel anticancer agents. Copyright 1999 Harcourt Publishers Ltd.

Mammalian Srb/Mediator complex is targeted by adenovirus E1A protein

Adenovirus E1A proteins prepare the host cell for viral replication, stimulating cell cycling and viral transcription through interactions with critical cellular regulatory proteins such as RB and CBP. Here we show that the E1A zinc-finger domain that is required to activate transcription of viral early genes binds to a host-cell multiprotein complex containing homologues of yeast Srb/Mediator proteins. This occurs through a stable interaction with the human homologue of Caenorhabditis elegans SUR-2, a protein required for many developmental processes in the nematode. This human Srb/Mediator complex stimulates transcription in vitro in response to both the E1A zinc-finger and the herpes simplex virus VP16 activation domains. Interaction with human Sur-2 is also required for transcription to be activated by the activation domain of a transcription factor of the ETS-family in response to activated mitogen-activated protein (MAP) kinase.

Bcr-abl protein detection in peripheral blood mononuclear cells for follow-up of chronic myelogenous leukaemia patients

We have assessed the value of p210 protein detection in peripheral blood cells for follow-up of chronic myelogenous leukaemia (CML) patients. Quantification was achieved by comparing the relative intensities of the p210 bands with those of the normal abl protein (p145). Serial dilution of Ph-positive K562 cells with Ph-negative HL60 or KG1 cells revealed a linear correlation between the p210/p145 ratio and the number of Ph-positive cells (r = 0.998; p < 0.001) with a sensitivity of detecting less than 1% Ph-positive cells in 5 x 10(6) cells. Ninety-six consecutive patients were enrolled in the study and a total of 155 Western blot analyses have been performed and compared to chromosomal analyses of bone marrow. Parallel RT-PCR analyses have been performed on 99 occasions. All patients with positive cytogenetic findings also probed positive for p210. In 23 instances p210 was detectable despite negative chromosomal analysis. In 16 samples these results were confirmed by RT-PCR. In patients with partial cytogenetic remission (n = 26) the results of the p210 assay correlated significantly with the percentage of Ph-positive metaphases (r = 0.69; p < 0.001). In conclusion, monitoring of CML patients by quantification of the bcr-abl protein is a feasible and sensitive alternative to chromosomal analysis of bone marrow.

Nucleophosmin-anaplastic lymphoma kinase of large-cell anaplastic lymphoma is a constitutively active tyrosine kinase that utilizes phospholipase C-gamma to mediate its mitogenicity

Large-cell anaplastic lymphoma is a subtype of non-Hodgkin's lymphoma characterized by the expression of CD30. More than half of these lymphomas have a chromosomal translocation, t(2;5), that leads to the expression of a hybrid protein comprised of the nucleolar phosphoprotein nucleophosmin (NPM) and the anaplastic lymphoma kinase (ALK). Here we show that transfection of the constitutively active tyrosine kinase NPM-ALK into Ba/F3 and Rat-1 cells leads to a transformed phenotype. Oncogenic tyrosine kinases transform cells by activating the mitogenic signal transduction pathways, e.g., by binding and activating SH2-containing signaling molecules. We found that NPM-ALK binds most specifically to the SH2 domains of phospholipase C-gamma (PLC-gamma) in vitro. Furthermore, we showed complex formation of NPM-ALK and PLC-gamma in vivo by coimmunoprecipitation experiments in large-cell anaplastic lymphoma cells. This complex formation leads to the tyrosine phosphorylation and activation of PLC-gamma, which can be corroborated by enhanced production of inositol phosphates (IPs) in NPM-ALK-expressing cells. By phosphopeptide competition experiments, we were able to identify the tyrosine residue on NPM-ALK responsible for interaction with PLC-gamma as Y664. Using site-directed mutagenesis, we constructed a comprehensive panel of tyrosine-to-phenylalanine NPM-ALK mutants, including NPM-ALK(Y664F). NPM-ALK(Y664F), when transfected into Ba/F3 cells, no longer forms complexes with PLC-gamma or leads to PLC-gamma phosphorylation and activation, as confirmed by low IP levels in these cells. Most interestingly, Ba/F3 and Rat-1 cells expressing NPM-ALK(Y664F) also show a biological phenotype in that they are not stably transformed. Overexpression of PLC-gamma can partially rescue the proliferative response of Ba/F3 cells to the NPM-ALK(Y664F) mutant. Thus, PLC-gamma is an important downstream target of NPM-ALK that contributes to its mitogenic activity and is likely to be important in the molecular pathogenesis of large-cell anaplastic lymphomas.

A BCR-ABLp190 Fusion Gene Made by Homologous Recombination Causes B-Cell Acute Lymphoblastic Leukemias in Chimeric Mice With Independence of the Endogenous bcr Product

BCR-ABLp190 oncogene is the result of a reciprocal translocation between chromosomes 9 and 22 and is associated with B-cell acute lymphoblastic leukemia (B-ALL) in humans. Current models expressing the BCR-ABLp190 chimeric gene fail to consistently reproduce the phenotype with which the fusion gene is associated in human pathology, mainly due to the difficulty of being expressed in the appropriate cell type in vivo. We have used here homologous recombination in ES cells to create an in-frame fusion of BCR-ABLp190 that mimics the consequences of the human chromosomal translocation by fusion of BCR-ABL coding sequences into the bcr endogenous gene. The chimeric mice generated with the mutant embryonic stem cells systematically develop B-ALL. Using these chimeric mice, we further show that BCR-ABL oncogene does not require the endogenous bcr product in leukemogenesis. Our results show that BCR-ABLp190 chimeric mice are a new model to study the biology of the BCR-ABL oncogene and indicate the efficacy of this strategy for studying the role of specific chromosome abnormalities in tumor development.

A BCR-ABLp190 Fusion Gene Made by Homologous Recombination Causes B-Cell Acute Lymphoblastic Leukemias in Chimeric Mice With Independence of the Endogenous bcr Product

BCR-ABLp190 oncogene is the result of a reciprocal translocation between chromosomes 9 and 22 and is associated with B-cell acute lymphoblastic leukemia (B-ALL) in humans. Current models expressing the BCR-ABLp190 chimeric gene fail to consistently reproduce the phenotype with which the fusion gene is associated in human pathology, mainly due to the difficulty of being expressed in the appropriate cell type in vivo. We have used here homologous recombination in ES cells to create an in-frame fusion of BCR-ABLp190 that mimics the consequences of the human chromosomal translocation by fusion of BCR-ABL coding sequences into the bcr endogenous gene. The chimeric mice generated with the mutant embryonic stem cells systematically develop B-ALL. Using these chimeric mice, we further show that BCR-ABL oncogene does not require the endogenous bcr product in leukemogenesis. Our results show that BCR-ABLp190 chimeric mice are a new model to study the biology of the BCR-ABL oncogene and indicate the efficacy of this strategy for studying the role of specific chromosome abnormalities in tumor development.

Intramolecular interactions of the regulatory domains of the Bcr-Abl kinase reveal a novel control mechanism

BACKGROUND

The Abl nonreceptor tyrosine kinase is implicated in a range of cellular processes and its transforming variants are involved in human leukemias. The N-terminal regulatory region of the Abl protein contains Src homology domains SH2 and SH3 which have been shown to be important for the regulation of its activity in vivo. These domains are often found together in the same protein and biochemical data suggest that the functions of one domain can be influenced by the other.

RESULTS

We have determined the crystal structure of the Abl regulatory region containing the SH3 and SH2 domains. In general, the individual domains are very similar to those of previously solved structures, although the Abl SH2 domain contains a loop which is extended so that one side of the resulting phosphotyrosine-binding pocket is open. In our structure the protein exists as a monomer with no intermolecular contacts to which a biological function may be attributed. However, there is a significant intramolecular contact between a loop of the SH3 domain and the extended loop of the SH2 domain. This contact surface includes the SH2 loop segment that is responsible for binding the phosphate moiety of phosphotyrosine-containing proteins and is therefore critical for orienting peptide interactions.

CONCLUSIONS

The crystal structure of the composite Abl SH3-SH2 domain provides the first indication of how SH2 and SH3 domains communicate with each other within the same molecule and why the presence of one directly influences the activity of the other. This is the first clear evidence that these two domains are in contact with each other. The results suggest that this direct interaction between the two domains may affect the ligand binding properties of the SH2 domain, thus providing an explanation for biochemical and functional data concerning the Bcr-Abl kinase.

Retroviral vector design for gene therapy of cancer: specific inhibition and tagging of BCR-ABLp190 cells

BACKGROUND

The main difficulty in providing effective treatment of patients with cancer is distinguishing between tumor and normal cells. The chimeric molecules created by cancer-associated chromosomal abnormalities (such as the BCR-ABL fusion protein) represent ideal therapeutic targets since they are unique to the disease state. A major challenge, however, is how to deliver the specific anti-tumor agent into every tumor cell.

MATERIAL AND METHODS

In this report we describe the use of a novel strategy to introduce specific anti-tumor reagents into every tumor cell. It uses retroviral vectors encoding both antisense transcripts specific for the BCR-ABLp190 fusion junction (the specific anti-tumor drug) and a truncated human CD5 cDNA, which allows selection of the infected cells. In order to coexpress the antisense molecule with the truncated human CD5 gene, the picornavirus internal ribosome-entry site was incorporated in the constructs.

RESULTS

When the antisense transcripts in the CD5-retroviral vector were introduced into Ba/F3+p190 cells rendered interleukin 3 (IL-3) independent by expression of the BCR-ABL sequences, the cells died upon IL-3 withdrawal, as measured by the absence of CD5-positive cells. Control Ba/F3+p210 cells infected with the same virus did not die in the absence of IL-3.

CONCLUSIONS

These data suggest a novel strategy for cancer treatment which incorporates the use of a retrovirus coexpressing both a selectable surface marker and a tumor-specific agent.

Evidence that SH2 domains promote processive phosphorylation by protein-tyrosine kinases

BACKGROUND

Non-receptor protein-tyrosine kinases often contain at least one Src homology 2 (SH2) domain, a protein module that binds with high affinity to tyrosine-phosphorylated peptides. Because SH2 domains would be predicted to bind with high affinity to proteins phosphorylated by the kinase, but not to the unphosphorylated substrate, their presence in tyrosine kinases has been puzzling. An important role for the SH2 domain of the Abl tyrosine kinase was suggested by work showing that Abl requires an intact SH2 domain in order to malignantly transform cells, and that replacement of the Abl SH2 domain with heterologous SH2 domains alters the spectrum of proteins phosphorylated detectably by Abl in vivo.

RESULTS

We have used purified wild-type and mutant Abl kinases to examine the roles of the Abl's SH2 and catalytic domains in phosphorylation of p130CAS, a model substrate that has multiple potential phosphorylation sites. We find that an SH2 domain is required for efficient hyperphosphorylation of p130 in vitro. We use chimeric mutants with heterologous SH2 domains to demonstrate that the SH2 domain of the oncogenically transforming adaptor protein Crk, which is the SH2 domain predicted to bind with highest affinity (of those tested) to potential phosphorylation sites in p130, is best able to facilitate hyperphosphorylation. This is the case whether the catalytic domain of the kinase is derived from Abl or from its distant relative, Src. These studies also reveal a role for binding of Crk to Abl in mediating phosphorylation by the kinase. Using purified proteins, we demonstrate that association with Crk strikingly enhances the ability of Abl to hyperphosphorylate p130. There is an excellent correlation between the ability of mutant Crk proteins to promote hyperphosphorylation of p130 by Abl and their ability to transform rodent fibroblasts.

CONCLUSION

Our data suggest that, ultimately, the substrate specificity of a non-receptor tyrosine kinase is dependent on the binding specificity of its associated SH2 domain. The SH2 domain binds tightly to a subset of proteins phosphorylated by the catalytic domain, leading to processive phosphorylation of those proteins. Substrate specificity can be broadened by an association between the kinase and proteins, such as Crk, that contain additional SH2 domains; this may play a role in malignant transformation by Crk.

Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

We have used in vitro mutagenesis to examine in detail the roles of two modular protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As previously shown, the SH3 domain suppresses an intrinsic transforming activity of the normally nontransforming c-Abl product in vivo. We show here that this inhibitory activity is extremely position sensitive, because mutants in which the position of the SH3 domain within the protein is subtly altered are fully transforming. In contrast to the case in vivo, the SH3 domain has no effect on the in vitro kinase activity of the purified protein. These results are consistent with a model in which the SH3 domain binds a cellular inhibitory factor, which in turn must physically interact with other parts of the kinase. Unlike the SH3 domain, the SH2 domain is required for transforming activity of activated Abl alleles. We demonstrate that SH2 domains from other proteins (Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit, and Crk) can complement the absence of the Abl SH2 domain and that mutants with heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated proteins in vivo. The positive function of the SH2 domain is relatively position independent, and the effect of multiple SH2 domains appears to be additive. These results suggest a novel mechanism for regulation of tyrosine kinases in which the SH2 domain binds to, and thereby enhances the phosphorylation of, a subset of proteins phosphorylated by the catalytic domain. Our data also suggest that the roles of the SH2 and SH3 domains in the regulation of Abl are different in several respects from the roles proposed for these domains in the closely related Src family of tyrosine kinases.

Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

We have used in vitro mutagenesis to examine in detail the roles of two modular protein domains, SH2 and SH3, in the regulation of the Abl tyrosine kinase. As previously shown, the SH3 domain suppresses an intrinsic transforming activity of the normally nontransforming c-Abl product in vivo. We show here that this inhibitory activity is extremely position sensitive, because mutants in which the position of the SH3 domain within the protein is subtly altered are fully transforming. In contrast to the case in vivo, the SH3 domain has no effect on the in vitro kinase activity of the purified protein. These results are consistent with a model in which the SH3 domain binds a cellular inhibitory factor, which in turn must physically interact with other parts of the kinase. Unlike the SH3 domain, the SH2 domain is required for transforming activity of activated Abl alleles. We demonstrate that SH2 domains from other proteins (Ras-GTPase-activating protein, Src, p85 phosphatidylinositol 3-kinase subunit, and Crk) can complement the absence of the Abl SH2 domain and that mutants with heterologous SH2 domains induce altered patterns of tyrosine-phosphorylated proteins in vivo. The positive function of the SH2 domain is relatively position independent, and the effect of multiple SH2 domains appears to be additive. These results suggest a novel mechanism for regulation of tyrosine kinases in which the SH2 domain binds to, and thereby enhances the phosphorylation of, a subset of proteins phosphorylated by the catalytic domain. Our data also suggest that the roles of the SH2 and SH3 domains in the regulation of Abl are different in several respects from the roles proposed for these domains in the closely related Src family of tyrosine kinases.

Human transcription factor IIIC box B binding subunit

Transcription factor IIIC (TFIIIC) is a multisubunit basic TF for RNA polymerase III. It initiates transcription complex assembly on tRNA and related genes by binding to the internal box B promoter element and is also required for transcription of 5S rRNA and other stable nuclear and cytoplasmic RNAs transcribed by polymerase III. In mammalian cells, regulation of TFIIIC activity controls overall polymerase III transcription in response to growth factors and viral infection. Here, we report the cloning and sequencing of a full-length cDNA (and genomic DNA from the transcription initiation region) encoding the box B binding subunit of human TFIIIC, the 243-kDa alpha subunit. Specific antisera raised against the encoded protein super shifts a TFIIIC-box B DNA complex during an electrophoretic mobility shift assay and immunodepletes TFIIIC transcriptional activity from a partially purified TFIIIC fraction, proving that the cDNA encodes a component of TFIIIC. The human protein shows surprisingly little similarity to the box B binding subunit of yeast TFIIIC.

A coiled-coil oligomerization domain of Bcr is essential for the transforming function of Bcr-Abl oncoproteins

In Philadelphia chromosome-positive human leukemias, the c-abl proto-oncogene on chromosome 9 becomes fused to the bcr gene on chromosome 22, and chimeric Bcr-Abl proteins are produced. The fused Bcr sequences activate the tyrosine kinase, actin-binding, and transforming functions of Abl. Activation of the Abl transforming function has been shown to require two distinct domains of Bcr: domain 1 (Bcr amino acids 1 to 63) and domain 2 (Bcr amino acids 176 to 242). The amino acid sequence of domain 1 indicates that it may be a coiled-coil oligomerization domain. We show here that domain 1 of Bcr forms a homotetramer. Tetramerization of Bcr-Abl through Bcr domain 1 correlates with activation of the tyrosine kinase and F-actin-binding functions of Abl. Disruption of the coiled coil by insertional mutagenesis inactivates the oligomerization function as well as the ability of Bcr-Abl to transform Rat-1 fibroblasts or to abrogate interleukin-3 dependence in lymphoid cells. These results strongly suggest that Bcr-Abl oligomers are the active entities in transformation.

A coiled-coil oligomerization domain of Bcr is essential for the transforming function of Bcr-Abl oncoproteins

In Philadelphia chromosome-positive human leukemias, the c-abl proto-oncogene on chromosome 9 becomes fused to the bcr gene on chromosome 22, and chimeric Bcr-Abl proteins are produced. The fused Bcr sequences activate the tyrosine kinase, actin-binding, and transforming functions of Abl. Activation of the Abl transforming function has been shown to require two distinct domains of Bcr: domain 1 (Bcr amino acids 1 to 63) and domain 2 (Bcr amino acids 176 to 242). The amino acid sequence of domain 1 indicates that it may be a coiled-coil oligomerization domain. We show here that domain 1 of Bcr forms a homotetramer. Tetramerization of Bcr-Abl through Bcr domain 1 correlates with activation of the tyrosine kinase and F-actin-binding functions of Abl. Disruption of the coiled coil by insertional mutagenesis inactivates the oligomerization function as well as the ability of Bcr-Abl to transform Rat-1 fibroblasts or to abrogate interleukin-3 dependence in lymphoid cells. These results strongly suggest that Bcr-Abl oligomers are the active entities in transformation.

BCR-ABL and v-abl oncogenes induce distinct patterns of thymic lymphoma involving different lymphocyte subsets

The human BCR-ABL oncogenes encoded by the Philadelphia chromosome (Ph) affect the pathogenesis of diverse types of leukemia and yet are rarely associated with T-lymphoid leukemia. To determine whether BCR-ABL kinases are inefficient in transforming T lymphocytes, BCR-ABL-expressing retroviruses were injected intrathymically into mice. Thymomas that expressed BCR-ABL kinase developed after a relatively long latent period. In most thymomas, deletion of 3' proviral sequences resulted in loss of tk-neo and occasionally caused expression of kinase-active carboxy-terminally truncated BCR-ABL oncoprotein. In contrast, deletion of 3' proviral sequences was not observed in thymomas induced with Abelson murine leukemia virus (A-MuLV). BCR-ABL viruses induced distinct patterns of disease and involved different thymocyte subsets than A-MuLV and Moloney murine leukemia virus (Mo-MuLV). While Mo-MuLV only induced Thy-1+ thymomas, v-abl- and BCR-ABL-induced thymomas often contained mixed populations of B220+ and Thy-1+ lymphocytes in the same tumor. In most v-abl and BCR-ABL tumors, Thy-1+ lymphoid cells expressed CD8 and a continuum of CD4 ranging from negative to positive. Conversely, Mo-MuLV thymomas contained distinct populations of CD4+ cells that were either CD8+ or CD8-. A-MuLV-transformed T-lymphoid cells did not express the CD3/T-cell receptor complex, while BCR-ABL tumors were CD3+. Thus, BCR-ABL viruses preferentially induce somewhat more differentiated T lymphocytes than are transformed by A-MuLV. Furthermore, rare B220+ lymphocytes may represent preferred v-abl and BCR-ABL transformation targets in the thymus.

Isolation of genetic suppressor elements, inducing resistance to topoisomerase II-interactive cytotoxic drugs, from human topoisomerase II cDNA

Many cytotoxic anticancer drugs act at topoisomerase II (topo II) by stabilizing cleavable complexes with DNA formed by this enzyme. Several cell lines, selected for resistance to topo II-interactive drugs, show decreased expression or activity of topo II, suggesting that such a decrease may be responsible for drug resistance. In the present study, etoposide resistance was used as the selection strategy to isolate genetic suppressor elements (GSEs) from a retroviral library expressing random fragments of human topo II (alpha form) cDNA. Twelve GSEs were isolated, encoding either peptides corresponding to short segments of the topo II alpha molecule (2.4-6.5% of the protein) or 163- to 220-bp-long antisense RNA sequences. Expression of a GSE encoding antisense RNA led to decreased cellular expression of the topo II alpha protein. Both types of GSE induced resistance to several topo II poisons but not to drugs that do not act at topo II. These results provide direct evidence that inhibition of topo II results in resistance to topo II-interactive drugs, indicate structural domains of topo II capable of independent functional interactions, and demonstrate that expression selection of random fragments constitutes an efficient approach to the generation of GSEs in mammalian cells.

Structural organization of BCR-ABL gene in chronic phase and blast transformation in chronic myeloid leukemia patients

We studied 36 DNA samples of 18 patients affected with chronic myeloid leukemia (CML) for the presence of mutations in the first exon of the BCR gene was divided into four regions amplified by polymerase chain reaction (PCR). By single strand conformation polymorphism analysis (SSCP) and direct sequencing of amplified fragments, we found different banding profiles in 9 out of 18 patients in the PCR fragment spanning nucleotide 506-826. In one patient, sequence analysis revealed the presence of a point mutation at nucleotide 669 (A-T; Gln-Leu). No difference was found between DNA samples collected during the chronic phase and the blastic transformation. No different mobility shifts of single stranded PCR products were found in the other amplified fragments. The activation of BCR-ABL involves direct interaction between BCR first exon sequences and the tyrosine kinase regulatory domains of ABL. In the first BCR exon, and around the mutated sequences two SH-2-binding sites, are retained. These domains are essential for BCR-ABL-mediated transformation. Our results demonstrate the presence of point mutation in this regulatory region, which may suggest a role for the altered BCR sequence in activation of the BCR-ABL oncogene.

Characterization and genetic mapping of a short, highly repeated, interspersed DNA sequence from rice (Oryza sativa L.)

A short, highly repeated, interspersed DNA sequence from rice was characterized using a combination of techniques and genetically mapped to rice chromosomes by restriction fragment length polymorphism (RFLP) analysis. A consensus sequence (GGC)n, where n varies from 13-16, for the repeated sequence family was deduced from sequence analysis. Southern blot analysis, restriction mapping of repeat element-containing genomic clones, and DNA sequence analysis indicated that the repeated sequence is interspersed in the rice genome, and is heterogeneous and divergent. About 200,000 copies are present in the rice genome. Single copy sequences flanking the repeat element were used as RFLP markers to map individual repeat elements. Eleven such repeat elements were mapped to seven different chromosomes. The strategy for characterization of highly dispersed repeated DNA and its uses in genetic mapping, DNA fingerprinting, and evolutionary studies are discussed.

Differences in oncogenic potency but not target cell specificity distinguish the two forms of the BCR/ABL oncogene

Two forms of activated BCR/ABL proteins, P210 and P185, that differ in BCR-derived sequences, are associated with Philadelphia chromosome-positive leukemias. One of these diseases is chronic myelogenous leukemia, an indolent disease arising in hematopoietic stem cells that is almost always associated with the P210 form of BCR/ABL. Acute lymphocytic leukemia, a more aggressive malignancy, can be associated with both forms of BCR/ABL. While it is virtually certain that BCR/ABL plays a central role in both of these diseases, the features that determine the association of a particular form with a given disease have not been elucidated. We have used the bone marrow reconstitution leukemogenesis model to test the hypothesis that BCR sequences influence the ability of activated ABL to transform different types of hematopoietic cells. Our studies reveal that both P185 and P210 induce a similar spectrum of hematological diseases, including granulocytic, myelomonocytic, and lymphocytic leukemias. Despite the similarity of the disease patterns, animals given P185-infected marrow developed a more aggressive disease after a shorter latent period than those given P210-infected marrow. These data demonstrate that the structure of the BCR/ABL oncoprotein does not affect the type of disease induced by each form of the oncogene but does control the potency of the oncogenic signal.

Differences in oncogenic potency but not target cell specificity distinguish the two forms of the BCR/ABL oncogene

Two forms of activated BCR/ABL proteins, P210 and P185, that differ in BCR-derived sequences, are associated with Philadelphia chromosome-positive leukemias. One of these diseases is chronic myelogenous leukemia, an indolent disease arising in hematopoietic stem cells that is almost always associated with the P210 form of BCR/ABL. Acute lymphocytic leukemia, a more aggressive malignancy, can be associated with both forms of BCR/ABL. While it is virtually certain that BCR/ABL plays a central role in both of these diseases, the features that determine the association of a particular form with a given disease have not been elucidated. We have used the bone marrow reconstitution leukemogenesis model to test the hypothesis that BCR sequences influence the ability of activated ABL to transform different types of hematopoietic cells. Our studies reveal that both P185 and P210 induce a similar spectrum of hematological diseases, including granulocytic, myelomonocytic, and lymphocytic leukemias. Despite the similarity of the disease patterns, animals given P185-infected marrow developed a more aggressive disease after a shorter latent period than those given P210-infected marrow. These data demonstrate that the structure of the BCR/ABL oncoprotein does not affect the type of disease induced by each form of the oncogene but does control the potency of the oncogenic signal.

Evidence for regulation of the human ABL tyrosine kinase by a cellular inhibitor

Phosphotyrosine cannot be detected on normal human ABL protein-tyrosine kinases, but activated oncogenic forms of the human ABL protein are phosphorylated on tyrosine in vivo. Activation of ABL can occur by substitution of the ABL first exon with breakpoint cluster region (BCR) sequences or by deletion of the noncatalytic SH3 (src homology region 3) domain. An alternative mode for the activation of the ABL kinases is hyperexpression at greater than 500-fold over endogenous levels. This is not a consequence of transphosphorylation of the hyperexpressed ABL molecules. ABL proteins translated in vitro lack phosphotyrosine, but tyrosine kinase activity is uncovered after immunoprecipitation and removal of lysate components. The rates of dephosphorylation of ABL and BCR-ABL fusion protein by phosphotyrosine-specific phosphatases are approximately the same. These combined results indicate that inhibition of ABL activity is reversible and suggest that a cellular component interacts noncovalently with ABL to inhibit its autophosphorylation.

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.