The molecular biology of chronic myeloid leukemia

ERG activates a stem-like proliferation-differentiation program in prostate epithelial cells with mixed basal-luminal identity

To gain insight into how ERG translocations cause prostate cancer, we performed single cell transcriptional profiling of an autochthonous mouse model at an early stage of disease initiation. Despite broad expression of ERG in all prostate epithelial cells, proliferation was enriched in a small, stem-like population with mixed-luminal basal identity (called intermediate cells). Through a series of lineage tracing and primary prostate tissue transplantation experiments, we find that tumor initiating activity resides in a subpopulation of basal cells that co-express the luminal genes Tmprss2 and Nkx3.1 (called BasalLum) but not in the larger population of classical Krt8+ luminal cells. Upon ERG activation, BasalLum cells give rise to the highly proliferative intermediate state, which subsequently transitions to the larger population of Krt8+ luminal cells characteristic of ERG-positive human cancers. Furthermore, this proliferative population is characterized by an ERG-specific chromatin state enriched for NFkB, AP-1, STAT and NFAT binding, with implications for TF cooperativity. The fact that the proliferative potential of ERG is enriched in a small stem-like population implicates the chromatin context of these cells as a critical variable for unmasking its oncogenic activity.

Allosteric regulation of kinase activity in living cells

The dysregulation of protein kinases is associated with multiple diseases due to the kinases' involvement in a variety of cell signaling pathways. Manipulating protein kinase function, by controlling the active site, is a promising therapeutic and investigative strategy to mitigate and study diseases. Kinase active sites share structural similarities, making it difficult to specifically target one kinase, and allosteric control allows specific regulation and study of kinase function without directly targeting the active site. Allosteric sites are distal to the active site but coupled via a dynamic network of inter-atomic interactions between residues in the protein. Establishing an allosteric control over a kinase requires understanding the allosteric wiring of the protein. Computational techniques offer effective and inexpensive mapping of the allosteric sites on a protein. Here, we discuss the methods to map and regulate allosteric communications in proteins, and strategies to establish control over kinase functions in live cells and organisms. Protein molecules, or 'sensors,' are engineered to function as tools to control allosteric activity of the protein as these sensors have high spatiotemporal resolution and help in understanding cell phenotypes after immediate activation or inactivation of a kinase. Traditional methods used to study protein functions, such as knockout, knockdown, or mutation, cannot offer a sufficiently high spatiotemporal resolution. We discuss the modern repertoire of tools to regulate protein kinases as we enter a new era in deciphering cellular signaling and developing novel approaches to treat diseases associated with signal dysregulation.

Allosteric regulation of kinase activity in living cells

The dysregulation of protein kinases is associated with multiple diseases due to the kinases' involvement in a variety of cell signaling pathways. Manipulating protein kinase function, by controlling the active site, is a promising therapeutic and investigative strategy to mitigate and study diseases. Kinase active sites share structural similarities making it difficult to specifically target one kinase, allosteric control allows specific regulation and study of kinase function without directly targeting the active site. Allosteric sites are distal to the active site but coupled via a dynamic network of inter-atomic interactions between residues in the protein. Establishing an allosteric control over a kinase requires understanding the allosteric wiring of the protein. Computational techniques offer effective and inexpensive mapping of the allosteric sites on a protein. Here, we discuss methods to map and regulate allosteric communications in proteins, and strategies to establish control over kinase functions in live cells and organisms. Protein molecules, or "sensors" are engineered to function as tools to control allosteric activity of the protein as these sensors have high spatiotemporal resolution and help in understanding cell phenotypes after immediate activation or inactivation of a kinase. Traditional methods used to study protein functions, such as knockout, knockdown, or mutation, cannot offer a sufficiently high spatiotemporal resolution. We discuss the modern repertoire of tools to regulate protein kinases as we enter a new era in deciphering cellular signaling and developing novel approaches to treat diseases associated with signal dysregulation.

Fluid retention-associated adverse events in patients treated with BCR::ABL1 inhibitors based on FDA Adverse Event Reporting System (FAERS): a retrospective pharmacovigilance study

OBJECTIVES

This study aimed to conduct a thorough analysis of fluid retention-associated adverse events (AEs) associated with BCR::ABL inhibitors.

DESIGN

A retrospective pharmacovigilance study.

SETTING

Food and Drug Administration Adverse Event Reporting System (FAERS) database for BCR::ABL inhibitors was searched from 1 January 2004 to 30 September 2021.

MAIN OUTCOME MEASURES

Reporting OR (ROR) and 95% CI were used to detect the signals. ROR was calculated by dividing the odds of fluid retention event reporting for the target drug by the odds of fluid retention event reporting for all other drugs. The signal was considered positive if the lower limit of 95% CI of ROR was >1. The analysis was run only considering coupled fluid retention events/BCR::ABL inhibitors with at least three cases.

RESULTS

A total of 97 823 reports were identified in FAERS. Imatinib had the most fluid retention signals, followed by dasatinib and nilotinib, while bosutinib and ponatinib had fewer signals. Periorbital oedema (ROR=24.931, 95% CI 22.404 to 27.743), chylothorax (ROR=161.427, 95% CI 125.835 to 207.085), nipple swelling (ROR=48.796, 95% CI 26.270 to 90.636), chylothorax (ROR=35.798, 95% CI 14.791 to 86.642) and gallbladder oedema (ROR=77.996, 95% CI 38.286 to 158.893) were the strongest signals detected for imatinib, dasatinib, nilotinib, bosutinib and ponatinib, respectively. Pleural effusion, pericardial effusion and pulmonary oedema were detected for all BCR::ABL inhibitors, with dasatinib having the highest RORs for pleural effusion (ROR=37.424, 95% CI 35.715 to 39.216), pericardial effusion (ROR=14.146, 95% CI 12.649 to 15.819) and pulmonary oedema (ROR=11.217, 95% CI 10.303 to 12.213). Patients aged ≥65 years using dasatinib, imatinib, nilotinib or bosutinib had higher RORs for pleural effusion, pericardial effusion and pulmonary oedema. Patients aged ≥65 years and females using imatinib had higher RORs for periorbital oedema, generalised oedema and face oedema.

CONCLUSIONS

This pharmacovigilance study serves as a clinical reminder to physicians to be more vigilant for fluid retention-associated AEs with BCR::ABL inhibitors.

Studies on the inhibitory effect of isavuconazole on flumatinib metabolism in vitro and in vivo

As the validated agent for the treatment of chronic myelogenous leukemia (CML), flumatinib is a novel oral tyrosine kinase inhibitor (TKI) with higher potency and selectivity for BCR-ABL1 kinase compared to imatinib. Many patients experience aspergillosis infection and they may start using isavuconazole, which is an inhibitor of CYP3A4. However, there is no study on their interaction in vitro and in vivo. In the present study, the concentrations of flumatinib and its major metabolite M1 were rapidly determined using an stable ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method. The half-maximal inhibitory concentration (IC₅₀) was 6.66 μM in human liver microsomes (HLM), while 0.62 μM in rat liver microsomes (RLM) and 2.90 μM in recombinant human CYP3A4 (rCYP3A4). Furthermore, the mechanisms of inhibition of flumatinib in human liver microsomes, rat liver microsomes and rCYP3A4 by isavuconazole were mixed. Moreover, ketoconazole, posaconazole, and isavuconazole showed more potent inhibitory effects than itraconazole, fluconazole, and voriconazole on HLM-mediated flumatinib metabolism. In pharmacokinetic experiments of rats, it was observed that isavuconazole could greatly change the pharmacokinetic parameters of flumatinib, including AUC_(0-t), AUC_(0-∞), C_max and CLz/F, but had no effect on the metabolism of M1. According to the results of in vitro and in vivo studies, the metabolism of flumatinib was inhibited by isavuconazole, suggesting that isavuconazole may raise the plasma concentration of flumatinib. Thus, it is important to take special care of the interactions between flumatinib and isavuconazole in clinical applications.

Imatinib disassembles the regulatory core of Abelson kinase by binding to its ATP site and not by binding to its myristoyl pocket

It was recently reported (Xie et al., 2022) that the Abelson tyrosine kinase (Abl) ATP-site inhibitor imatinib also binds to Abl's myristoyl binding pocket, which is the target of allosteric Abl inhibitors. This was based on a crystal structure of a truncated Abl kinase domain construct in complex with imatinib bound to the allosteric site as well as further isothermal titration calorimetry (ITC), NMR, and kinase activity data. Although imatinib's affinity for the allosteric site is significantly weaker (10 µ M) than for the ATP site (10 nM), imatinib binding to the allosteric site may disassemble the regulatory core of Abl, thereby stimulating kinase activity, in particular for Abl mutants with reduced imatinib ATP-site affinity. It was argued that the previously observed imatinib-induced opening of the Abl regulatory core (Skora et al., 2013; Sonti et al., 2018) may be caused by the binding of imatinib to the allosteric site and not to the ATP site. We show here that this is not the case but that indeed imatinib binding to the ATP site induces the opening of the regulatory core at nanomolar concentrations. This agrees with findings that other type-II ATP-site inhibitors (nilotinib, ponatinib) disassemble the regulatory core despite demonstrated negligible binding to the allosteric site.

Characteristics of BCR-ABL gene variants in patients of chronic myeloid leukemia

Background

Depending on breakpoints of rearrangement different types of BCR–ABL fusion protein can be generated in patients of chronic myeloid leukemia (CML). The aim of this study is to observe frequencies of major transcripts in CML patients by reverse transcriptase polymerase chain reaction (RT-PCR) and their hematological features at the time of presentation.

Materials and methods

This cross sectional study was performed at Molecular Lab of Riphah International University, Islamabad from January to June 2019. Consecutive peripheral blood samples of 70 newly diagnosed CML patients in chronic phase were analyzed by RT-PCR to detect different BCR–ABL transcripts. Routine blood cell counts were assessed by an automated hematology analyzer.

Results

All samples expressed typical BCR–ABL rearrangement. Expression of either e14a2 or e13a2 transcript was detected in 38 (54%) and 30 (43%) patients, respectively. Coexpression of e13a2 + e14a2 was found in 2 (3%) patients. The mean total leukocyte count was higher in group expressing e13a2 (P = 0.01). Higher mean platelet count was noted in patients with e14a2 transcript, but this difference was statistically insignificant (P = 0.1). The association of male gender was observed with the group exhibiting e14a2 (P = 0.01). There was no statistically significant association between transcript type and different ranges of age, hemoglobin levels, and platelet and total leukocyte counts (P > 0.05).

Conclusion

e14a2 transcript was most common transcript in CML patients. Patients exhibiting e13a2 subgroup presented with significantly higher mean white blood cell count at the time of presentation. Significantly higher proportion of male patients was found to express e14a2 transcript over e13a2.

Eye: the door to undiagnosed chronic myeloid leukaemia

Chronic myeloid leukaemia (CML) is a myeloproliferative neoplasm that can present in varied ways from incidental finding on haemogram to symptomatic presentation such as splenomegaly. We report an interesting case of a 22-year-old man who presented with loss of vision in right eye for 1 month. There were no pre-existing ocular or systemic diseases. On detailed ocular examination, a diagnosis of right eye rubeosis iridis, hyphaema, cataract and vitreous haemorrhage with left eye suspected leukaemic retinopathy was made. Routine haemogram revealed high leucocytosis. Systemic evaluation with investigations confirmed the diagnosis of CML and the patient was started on appropriate therapy.

Laboratory Monitoring of Chronic Myeloid Leukemia in Patients on Tyrosine Kinase Inhibitors

Chronic Myeloid Leukemia (CML) is a myeloproliferative neoplasm characterized by translocation of genetic material from chromosome 9 to chromosome 22 to form a fusion gene (BCR-ABL1) that is responsible for abnormal tyrosine kinase activity and alteration of various downstream signaling pathways. In addition to morphological diagnosis of CML phase, it is essential to detect BCR-ABL1 fusion by either metaphase cytogenetics or reverse transcriptase polymerase chain reaction that also determines type of mRNA transcript. Once treatment begins, monitoring the response to Tyrosine Kinase Inhibitor (TKI) using standardized techniques and guidelines is important to check for failure of response and thus, plan timely intervention by increasing the dose of TKI or opting for second line TKIs. The goal is to stop evolution of CML to accelerated phase or blast crisis that has poor response to treatment. Also, it is desirable to achieve good outcomes and even treatment free remission in patients of CML on TKI. Thus, molecular monitoring by reverse transcriptase quantitative PCR (RT-qPCR) is done at regular intervals. There are international recommendations and quality control measures to standardize the reporting of fusion gene transcript levels by quantitative PCR (RT-qPCR) in CML to achieve and maintain sensitivity in molecular detection of CML disease burden. Various state-of-the-art molecular techniques have emerged to accurately determine the number of fusion-gene transcript levels. This review highlights various methodologies and their practical implications in management of CML patients on TKI.

New alternative splicing BCR/ABL-OOF shows an oncogenic role by lack of inhibition of BCR GTPase activity and an increased of persistence of Rac activation in chronic myeloid leukemia

In Chronic Myeloid Leukemia 80% of patients present alternative splice variants involving BCR exons 1, 13 or 14 and ABL exon 4, with a consequent impairment in the reading frame of the ABL gene. Therefore BCR/ABL fusion proteins (BCR/ABL-OOF) are characterized by an in-frame BCR portion followed by an amino acids sequence arising from the out of frame (OOF) reading of the ABL gene. The product of this new transcript contains the characteristic BCR domains while lacking the COOH-terminal Rho GTPase GAP domain. The present work aims to characterize the protein functionality in terms of cytoskeleton (re-)modelling, adhesion and activation of canonical oncogenic signalling pathways. Here, we show that BCR/ABL-OOF has a peculiar endosomal localization which affects EGF receptor activation and turnover. Moreover, we demonstrate that BCR/ABL-OOF expression leads to aberrant cellular adhesion due to the activation of Rac GTPase, increase in cellular proliferation, migration and survival. When overexpressed in a BCR/ABL positive cell line, BCR/ABL-OOF induces hyperactivation of Rac signaling axis offering a therapeutic window for Rac-targeted therapy. Our data support a critical role of BCR/ABL-OOF in leukemogenesis and identify a subset of patients that may benefit from Rac-targeted therapies.

BCR-ABL1 promotes leukemia by converting p27 into a cytoplasmic oncoprotein

Recent studies have revealed that p27, a nuclear cyclin-dependent kinase (Cdk) inhibitor and tumor suppressor, can acquire oncogenic activities upon mislocalization to the cytoplasm. To understand how these antagonistic activities influence oncogenesis, we dissected the nuclear and cytoplasmic functions of p27 in chronic myeloid leukemia (CML), a well-characterized malignancy caused by the BCR-ABL1 tyrosine kinase. p27 is predominantly cytoplasmic in CML and nuclear in normal cells. BCR-ABL1 regulates nuclear and cytoplasmic p27 abundance by kinase-dependent and -independent mechanisms, respectively. p27 knockdown in CML cell lines with predominantly cytoplasmic p27 induces apoptosis, consistent with a leukemogenic role of cytoplasmic p27. Accordingly, a p27 mutant (p27(CK-)) devoid of Cdk inhibitory nuclear functions enhances leukemogenesis in a murine CML model compared with complete absence of p27. In contrast, p27 mutations that enhance its stability (p27(T187A)) or nuclear retention (p27(S10A)) attenuate leukemogenesis over wild-type p27, validating the tumor-suppressor function of nuclear p27 in CML. We conclude that BCR-ABL1 kinase-dependent and -independent mechanisms convert p27 from a nuclear tumor suppressor to a cytoplasmic oncogene. These findings suggest that cytoplasmic mislocalization of p27 despite BCR-ABL1 inhibition by tyrosine kinase inhibitors may contribute to drug resistance, and effective therapeutic strategies to stabilize nuclear p27 must also prevent cytoplasmic mislocalization.

Physiologic hypoxia promotes maintenance of CML stem cells despite effective BCR-ABL1 inhibition

Hypoxia mediates TKI resistance. Hypoxia enhances CML stem cell maintenance.

CaMKII γ, a critical regulator of CML stem/progenitor cells, is a target of the natural product berbamine

Bcr-Abl tyrosine kinase inhibitors (TKIs) have been a remarkable success for the treatment of Ph(+) chronic myeloid leukemia (CML). However, a significant proportion of patients treated with TKIs develop resistance because of leukemia stem cells (LSCs) and T315I mutant Bcr-Abl. Here we describe the unknown activity of the natural product berbamine that efficiently eradicates LSCs and T315I mutant Bcr-Abl clones. Unexpectedly, we identify CaMKII γ as a specific and critical target of berbamine for its antileukemia activity. Berbamine specifically binds to the ATP-binding pocket of CaMKII γ, inhibits its phosphorylation and triggers apoptosis of leukemia cells. More importantly, CaMKII γ is highly activated in LSCs but not in normal hematopoietic stem cells and coactivates LSC-related β-catenin and Stat3 signaling networks. The identification of CaMKII γ as a specific target of berbamine and as a critical molecular switch regulating multiple LSC-related signaling pathways can explain the unique antileukemia activity of berbamine. These findings also suggest that berbamine may be the first ATP-competitive inhibitor of CaMKII γ, and potentially, can serve as a new type of molecular targeted agent through inhibition of the CaMKII γ activity for treatment of leukemia.

BMI1 collaborates with BCR-ABL in leukemic transformation of human CD34+ cells

The major limitation for the development of curative cancer therapies has been an incomplete understanding of the molecular mechanisms driving cancer progression. Human models to study the development and progression of chronic myeloid leukemia (CML) have not been established. Here, we show that BMI1 collaborates with BCR-ABL in inducing a fatal leukemia in nonobese diabetic/severe combined immunodeficiency mice transplanted with transduced human CD34+ cells within 4-5 months. The leukemias were transplantable into secondary recipients with a shortened latency of 8-12 weeks. Clonal analysis revealed that similar clones initiated leukemia in primary and secondary mice. In vivo, transformation was biased toward a lymphoid blast crisis, and in vitro, myeloid as well as lymphoid long-term, self-renewing cultures could be established. Retroviral introduction of BMI1 in primary chronic-phase CD34+ cells from CML patients elevated their proliferative capacity and self-renewal properties. Thus, our data identify BMI1 as a potential therapeutic target in CML.

The new tyrosine-kinase inhibitor and anticancer drug dasatinib reversibly affects platelet activation in vitro and in vivo

Dasatinib is an oral potent adenosine triphosphate (ATP)-competitive inhibitor of BCR-ABL, cKIT, platelet-derived growth factor receptor, and SRC family kinases (SFKs), which has demonstrated high efficiency in patients with imatinib-resistant chronic myelogenous leukemia. Here, we show that dasatinib weakly affects platelet activation by thrombin or adenosine diphosphate but is a potent inhibitor of platelet signaling and functions initiated by collagen or FcgammaRIIA cross-linking, which require immunoreceptor tyrosine-based activation motif phosphorylation by SFKs. Accordingly, dasatinib treatment rapidly decreases the volume of thrombi formed under arterial flow conditions in whole blood from patients or mice perfused over a matrix of collagen. Moreover, treatment of mice with dasatinib increases the tail bleeding time in a dose-dependent manner. Interestingly, these effects are rapidly reversible after interruption of the treatment. Our data clearly demonstrate that, in contrast to imatinib, dasatinib affects platelet functions in vitro and in vivo, which has important implications in clinic and could explain increased risks of bleeding observed in patients. Moreover, dasatinib efficiently prevents platelet activation mediated by FcgammaRIIA cross-linking and by sera from patients with heparin-induced thrombocytopenia, suggesting that reversible antiplatelet agents acting as ATP-competitive inhibitors of SFKs may be of therapeutic interest in the treatment of this pathology.

Down-regulation of hsa-miR-10a in chronic myeloid leukemia CD34+ cells increases USF2-mediated cell growth

MicroRNAs (miRNA) are small noncoding, single-stranded RNAs that inhibit gene expression at a posttranscriptional level, whose abnormal expression has been described in different tumors. The aim of our study was to identify miRNAs potentially implicated in chronic myeloid leukemia (CML). We detected an abnormal miRNA expression profile in mononuclear and CD34(+) cells from patients with CML compared with healthy controls. Of 157 miRNAs tested, hsa-miR-10a, hsa-miR-150, and hsa-miR-151 were down-regulated, whereas hsa-miR-96 was up-regulated in CML cells. Down-regulation of hsa-miR-10a was not dependent on BCR-ABL1 activity and contributed to the increased cell growth of CML cells. We identified the upstream stimulatory factor 2 (USF2) as a potential target of hsa-miR-10a and showed that overexpression of USF2 also increases cell growth. The clinical relevance of these findings was shown in a group of 85 newly diagnosed patients with CML in which expression of hsa-miR-10a was down-regulated in 71% of the patients, whereas expression of USF2 was up-regulated in 60% of the CML patients, with overexpression of USF2 being significantly associated with decreased expression of hsa-miR-10a (P = 0.004). Our results indicate that down-regulation of hsa-miR-10a may increase USF2 and contribute to the increase in cell proliferation of CML implicating a miRNA in the abnormal behavior of CML.

BMS-214662 potently induces apoptosis of chronic myeloid leukemia stem and progenitor cells and synergizes with tyrosine kinase inhibitors

Chronic myeloid leukemia (CML), a hematopoietic stem-cell disorder, cannot be eradicated by conventional chemotherapy or the tyrosine kinase inhibitor imatinib mesylate (IM). To target CML stem/progenitor cells, we investigated BMS-214662, a cytotoxic farnesyltransferase inhibitor, previously reported to kill nonproliferating tumor cells. IM or dasatinib alone reversibly arrested proliferation of CML stem/progenitor cells without inducing apoptosis. In contrast, BMS-214662, alone or in combination with IM or dasatinib, potently induced apoptosis of both proliferating and quiescent CML stem/progenitor cells with less than 1% recovery of Philadelphia-positive long-term culture-initiating cells. Normal stem/progenitor cells were relatively spared by BMS-214662, suggesting selectivity for leukemic stem/progenitor cells. The ability to induce selective apoptosis of leukemic stem/progenitor cells was unique to BMS-214662 and not seen with a structurally similar agent BMS-225975. BMS-214662 was cytotoxic against CML blast crisis stem/progenitor cells, particularly in combination with a tyrosine kinase inhibitor and equally effective in cell lines harboring wild-type vs mutant BCR-ABL, including the T315I mutation. This is the first report of an agent with activity in resistant and blast crisis CML that selectively kills CML stem/progenitor cells through apoptosis and offers potential for eradication of chronic phase CML.

Ubp43 regulates BCR-ABL leukemogenesis via the type 1 interferon receptor signaling

Interferon (IFN) signaling induces the expression of interferon-responsive genes and leads to the activation of pathways that are involved in the innate immune response. Ubp43 is an ISG15-specific isopeptidase, the expression of which is activated by IFN. Ubp43 knock-out mice are hypersensitive to IFN-alpha/beta and have enhanced resistance to lethal viral and bacterial infections. Here we show that in addition to protection against foreign pathogens, Ubp43 deficiency increases the resistance to oncogenic transformation by BCR-ABL. BCR-ABL viral transduction/transplantation of wild-type bone marrow cells results in the rapid development of a chronic myeloid leukemia (CML)-like myeloproliferative disease; in contrast, a significantly increased latency of disease development is observed following BCR-ABL viral transduction/transplantation of Ubp43-deficient bone marrow cells. This resistance to leukemic development is dependent on type 1 IFN (IFN-alpha/beta) signaling in Ubp43-deficient cells. Increased levels of type 1 IFN are also detected in the serum of CML mice. These results suggest that inhibition of Ubp43-negative effect on IFN signaling can potentiate the response to increased endogenous IFN levels in innate immune responses against cancer development, indicating that pharmacological inhibition of Ubp43 may be of benefit in cancers and others diseases in which interferon is currently prescribed.

MEK1/2 inhibitors sensitize Bcr/Abl+ human leukemia cells to the dual Abl/Src inhibitor BMS-354/825

Interactions between MEK1/2 inhibitors and the dual Abl/Src kinase inhibitor dasatinib (BMS-354825) were examined in chronic myeloid leukemia (CML) cell lines and primary specimens. Cotreatment of K562 or LAMA cells with subtoxic or marginally toxic concentrations of PD184352 (or U0126) and dasatinib synergistically potentiated mitochondrial damage, caspase activation, and apoptosis. Similar interactions were observed in CD34(+) cells from one CML patient-derived but not in a normal human CD34(+) bone marrow cell specimen. These interactions were associated with multiple perturbations in survival signaling pathways, including inactivation of Bcr/Abl, STAT5, and ERK1/2; down-regulation of Bcl-x(L) and Mcl-1; and dephosphorylation/activation of Bim. They were also associated with BAX/BAK conformational change, mitochondrial dysfunction, and caspase activation. Bim knockdown by shRNA suppressed BAX and BAK conformational change and protected cells from dasatinib/PD184352 lethality. Conversely, K562 cells ectopically expressing Mcl-1 or Bcl-x(L) were significantly less susceptible to dasatinib/PD184352 toxicity. Notably, the dasatinib/PD184352 regimen was active against leukemic cells exhibiting various forms of imatinib mesylate resistance, including Bcr/Abl overexpression, Lyn activation, and several Bcr/Abl kinase domain mutations (eg, E255K, M351T), but not T315I. Together, these findings suggest that strategies combining dasatanib with MEK1/2 inhibitors warrant further investigation in Bcr/Abl(+) malignancies, particularly in the setting of imatinib mesylate-resistant disease.

Antileukemic activity of lysophosphatidic acid acyltransferase-beta inhibitor CT32228 in chronic myelogenous leukemia sensitive and resistant to imatinib

PURPOSE

Lysophosphatidic acid acyltransferase (LPAAT)-beta catalyzes the conversion of lysophosphatidic acid to phosphatidic acid, an essential component of several signaling pathways, including the Ras/mitogen-activated protein kinase pathway. Inhibition of LPAAT-beta induces growth arrest and apoptosis in cancer cell lines, implicating LPAAT-beta as a potential drug target in neoplasia.

EXPERIMENTAL DESIGN

In this study, we investigated the effects of CT32228, a specific LPAAT-beta inhibitor, on BCR-ABL-transformed cell lines and primary cells from patients with chronic myelogenous leukemia.

RESULTS

CT32228 had antiproliferative activity against BCR-ABL-positive cell lines in the nanomolar dose range, evidenced by cell cycle arrest in G2-M and induction of apoptosis. Treatment of K562 cells with CT32228 led to inhibition of extracellular signal-regulated kinase 1/2 phosphorylation, consistent with inhibition of mitogen-activated protein kinase signaling. Importantly, CT32228 was highly active in cell lines resistant to the Bcr-Abl kinase inhibitor imatinib. Combination of CT32228 with imatinib produced additive inhibition of proliferation in cell lines with residual sensitivity toward imatinib. In short-term cultures in the absence of growth factors, CT32228 preferentially inhibited the growth of granulocyte-macrophage colony-forming units from chronic myelogenous leukemia patients compared with healthy controls.

CONCLUSION

These data establish LPAAT-beta as a potential drug target for the treatment of BCR-ABL-positive leukemias.

Bcr-Abl signaling through the PI-3/S6 kinase pathway inhibits nuclear translocation of the transcription factor Bach2, which represses the antiapoptotic factor heme oxygenase-1

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the Bcr-Abl oncoprotein. We have previously reported that expression of the Bach2 transcription factor, which induces apoptosis in response to oxidative stress, is greatly reduced in CML cells. Because these cells are resistant to apoptosis, we tested whether Bach2 could also be regulated through posttranslational mechanisms that promote inhibition of the apoptotic response to mutagenic stimuli in CML. We found that Bach2 is phosphorylated on S521 via the phosphatidylinositol-3/S6 kinase pathway, and substitution of this site to alanine leads to nuclear accumulation of the protein, indicating that this phosphorylation is important for its subcellular localization. Ectopic expression of the S521 mutant imparts greater impairment to CML cell growth than the wild-type factor. Furthermore, we showed that Bach2 transcriptionally represses heme oxygenase-1, an antiapoptotic factor up-regulated in CML. Because CML cells are known to produce high levels of intracellular reactive oxygen species, overexpression of heme oxygenase-1 resulting from inhibition of Bach2 activity may contribute to their genomic instability and leukemic phenotype.

KLF4 suppresses transformation of pre-B cells by ABL oncogenes

Genes that are strongly repressed after B-cell activation are candidates for being inactivated, mutated, or repressed in B-cell malignancies. Krüppel-like factor 4 (Klf4), a gene down-regulated in activated murine B cells, is expressed at low levels in several types of human B-cell lineage lymphomas and leukemias. The human KLF4 gene has been identified as a tumor suppressor gene in colon and gastric cancer; in concordance with this, overexpression of KLF4 can suppress proliferation in several epithelial cell types. Here we investigate the effects of KLF4 on pro/pre-B-cell transformation by v-Abl and BCR-ABL, oncogenes that cause leukemia in mice and humans. We show that overexpression of KLF4 induces arrest and apoptosis in the G1 phase of the cell cycle. KLF4-mediated death, but not cell-cycle arrest, can be rescued by Bcl-XL overexpression. Transformed pro/pre-B cells expressing KLF4 display increased expression of p21CIP and decreased expression of c-Myc and cyclin D2. Tetracycline-inducible expression of KLF4 in B-cell progenitors of transgenic mice blocks transformation by BCR-ABL and depletes leukemic pre-B cells in vivo. Collectively, our work identifies KLF4 as a putative tumor suppressor in B-cell malignancies.

Synergistic interactions between DMAG and mitogen-activated protein kinase kinase 1/2 inhibitors in Bcr/abl+ leukemia cells sensitive and resistant to imatinib mesylate

PURPOSE

To characterize interactions between the heat shock protein 90 antagonist 17-dimethylaminoethylamino-17-demethoxygeldanamycin (DMAG) and the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase 1/2 inhibitor PD184352 in Bcr/abl(+) leukemia cells sensitive and resistant to imatinib mesylate.

EXPERIMENTAL DESIGN

K562 and LAMA 84 cells were exposed to varying concentrations of DMAG and PD184352 for 48 hours; after which, mitochondrial integrity, caspase activation, and apoptosis were monitored. Parallel studies were done in imatinib mesylate-resistant cells, including BaF3 cells transfected with plasmids encoding clinically relevant Bcr/abl mutations conferring imatinib mesylate resistance (e.g., E255K, M351T, and T315I) and primary CD34(+) bone marrow cells from patients refractory to imatinib mesylate.

RESULTS

Cotreatment of Bcr/abl(+) cells with minimally toxic concentrations of DMAG and PD184352 resulted in synergistic induction of mitochondrial injury (cytochrome c release and Bax conformational change), events associated with the pronounced and sustained inactivation of ERK1/2 accompanied by down-regulation of Bcl-x(L). Conversely, cells ectopically expressing Bcl-x(L) displayed significant protection against PD184352/DMAG-mediated lethality. This regimen effectively induced apoptosis in K562 cells overexpressing Bcr/abl, in BaF3 cells expressing various clinically relevant Bcr/abl mutations, and in primary CD34(+) cells from patients resistant to imatinib mesylate, but was relatively sparing of normal CD34(+) bone marrow cells.

CONCLUSIONS

A regimen combining the heat shock protein 90 antagonist DMAG and the mitogen-activated protein kinase/ERK kinase 1/2 inhibitor potently induces apoptosis in Bcr/abl(+) cells, including those resistant to imatinib mesylate through various mechanisms including Bcr/abl kinase mutations, through a process that may involve sustained ERK1/2 inactivation and Bcl-x(L) down-regulation. This strategy warrants further attention in Bcr/abl(+) hematopoietic malignancies, particularly those resistant to Bcr/abl kinase inhibitors.

Two Single-Nucleotide Polymorphisms with Linkage Disequilibrium in the Human Programmed Cell Death 5 Gene 5′ Regulatory Region Affect Promoter Activity and the Susceptibility of Chronic Myelogenous Leukemia in Chinese Population

PURPOSE

Chronic myelogenous leukemia (CML) is a disease characterized cytogenetically by the presence of the Philadelphia chromosome. Recent studies suggested that altered PDCD5 expression may have significant implications in CML progression. The aim of this study was to identify single-nucleotide polymorphisms (SNP) within the programmed cell death 5 (PDCD5) promoter region and show their functional relevance to PDCD5 expression as well as their genetic susceptibility to CML.

EXPERIMENTAL DESIGN

One hundred twenty-nine CML subjects and 211 healthy controls were recruited for identification of SNPs and subsequent genetic analysis. Luciferase reporter assays were carried out to show the functional significance of the SNPs located in the promoter region to PDCD5 expression. Real-time quantitative PCR and Western blot analysis were done to determine the expression differences of PDCD5 in CML patients with different genotypes.

RESULTS

Two SNPs were identified within the PDCD5 promoter. They are -27A>G and -11G>A (transcription start site as position 1), respectively. The complete linkage disequilibrium was found between these two polymorphisms. The frequencies of -27G+/-11A+ genotype and -27G/-11A allele were significantly higher in CML patients than in healthy controls (genotype: 26.36% versus 11.85%, chi2=11.75, P<0.01; allele: 13.57% versus 6.40%, chi2=9.48, P<0.01). Luciferase reporter assays revealed that the promoter with -27G/-11A had significantly lower transcriptional activity and could not be up-regulated after apoptotic stimulations compared with the promoter with -27A/-11G. PDCD5 expression analysis in mononuclear cells derived from CML patients and cell lines with different -27/-11 genotypes showed consistent results with the reporter assays.

CONCLUSIONS

These data suggest that -27G/-11A is associated with reduced PDCD5 promoter activity and increased susceptibility to CML.

Bcr-Abl expression levels determine the rate of development of resistance to imatinib mesylate in chronic myeloid leukemia

Chronic myeloid leukemia (CML) starts with the acquisition of a BCR-ABL fusion gene in a single hematopoietic stem cell, but the time to progression is unpredictable. Although the tyrosine kinase inhibitor imatinib mesylate is highly effective in the treatment of CML, its continuous administration is associated with development of resistance, particularly in advanced phase or blast crisis. We investigate here whether a feature of disease progression (i.e., elevated expression of Bcr-Abl in CD34+ progenitor cells from CML patients in blast crisis) has any bearing on the kinetics of resistance to imatinib. By studying cell lines that exogenously express Bcr-Abl over the range found from chronic phase to blast crisis of CML, we show that cells expressing high amounts of Bcr-Abl, as in blast crisis, are much less sensitive to imatinib and, more significantly, take a substantially shorter time for yielding a mutant subclone resistant to the inhibitor than cells with low expression levels, as in chronic phase. Our data suggest that the differential levels of the Bcr-Abl oncoprotein expressed by CD34+ CML cells may reflect the extent and duration of their response to imatinib; the relatively high levels of oncoprotein in advanced-phase disease may underlie the observed rapid development of resistance.

Dose-dependent effects of Bcr-Abl in cell line models of different stages of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is caused by Bcr-Abl, an activated tyrosine kinase. The amounts of Bcr-Abl mRNA and protein in cells from patients in blast crisis (BC) are higher than in those chronic phase (CP), indicating that their expression rises with disease progression. In order to study this phenomenon on cells with the same genetic background, we transfected the 32D cell line with the BCR-ABL transgene and selected clones with graded expression of Bcr-Abl within the range found in cells from CP to BC. In vitro, we found that Bcr-Abl exerted dose-dependent effects upon growth factor dependence, clonogenicity and migration. However, the relationship between Bcr-Abl expression and cellular adhesion to fibronectin was more complex: rather than a direct positive correlation, clones that expressed low, but not high, levels of Bcr-Abl were less adhesive than growth factor stimulated, parental BCR-ABL-negative 32D cells. This finding parallels the situation with normal and CML-CP progenitors where the latter exhibit defective adhesion to fibronectin. Treatment with the tyrosine kinase inhibitor imatinib mesylate reversed the adhesion deficient phenotype of clones expressing low levels of Bcr-Abl. When injected subcutaneously into syngeneic mice, cell lines expressing high levels of Bcr-Abl rapidly induced tumors, whereas low-expressing clones led to tumor formation only after a prolonged latency. These findings suggest that the level of Bcr-Abl may be essential in determining the phenotype of the leukemic clone at different stages of the disease.

Distinct molecular phenotype of malignant CD34(+) hematopoietic stem and progenitor cells in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a malignant disorder of the hematopoietic stem cell characterized by the BCR-ABL oncogene. We examined gene expression profiles of highly enriched CD34(+) hematopoietic stem and progenitor cells from patients with CML in chronic phase using cDNA arrays covering 1.185 genes. Comparing CML CD34(+) cells with normal CD34(+) cells, we found 158 genes which were significantly differentially expressed. Gene expression patterns reflected BCR-ABL-induced functional alterations such as increased cell-cycle and proteasome activity. Detoxification enzymes and DNA repair proteins were downregulated in CML CD34(+) cells, which might contribute to genetic instability. Decreased expression of junction plakoglobulin and CXC chemokine receptor 4 (CXCR-4) might facilitate the release of immature precursors from bone marrow in CML. GATA-2 was upregulated in CML CD34(+) cells, suggesting an increased self-renewal in comparison with normal CD34(+) cells. Moreover, we found upregulation of the proto-oncogene SKI and of receptors for neuromediators such as opioid mu1 receptor, GABA B receptor, adenosine A1 receptor, orexin 1 and 2 receptors and corticotropine-releasing hormone receptor. Treatment of CML progenitor cells with the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) resulted in a dose-dependent significant inhibition of clonogenic growth by 40% at a concentration of 10(-5) M, which could be reversed by the equimolar addition of the receptor agonist 2-chloro-N6-cyclopentyladenosine (P<0.05). The incubation of normal progenitor cells with DPCPX resulted in an inhibition of clonogenic growth to a significantly lesser extent in comparison with CML cells (P<0.05), suggesting that the adenosine A1 receptor is of functional relevance in CML hematopoietic progenitor cells.

BCR-ABL induces the expression of Skp2 through the PI3K pathway to promote p27Kip1 degradation and proliferation of chronic myelogenous leukemia cells

Chronic myelogenous leukemia (CML) is characterized by the expression of the BCR-ABL tyrosine kinase, which results in increased cell proliferation and inhibition of apoptosis. In this study, we show in both BCR-ABL cells (Mo7e-p210 and BaF/3-p210) and primary CML CD34+ cells that STI571 inhibition of BCR-ABL tyrosine kinase activity results in a G(1) cell cycle arrest mediated by the PI3K pathway. This arrest is associated with a nuclear accumulation of p27(Kip1) and down-regulation of cyclins D and E. As a result, there is a reduction of the cyclin E/Cdk2 kinase activity and of the retinoblastoma protein phosphorylation. By quantitative reverse transcription-PCR we show that BCR-ABL/PI3K regulates the expression of p27(Kip1) at the level of transcription. We further show that BCR-ABL also regulates p27(Kip1) protein levels by increasing its degradation by the proteasome. This degradation depends on the ubiquitinylation of p27(Kip1) by Skp2-containing SFC complexes: silencing the expression of Skp2 with a small interfering RNA results in the accumulation of p27(Kip1). We also demonstrate that BCR-ABL cells show transcriptional up-regulation of Skp2. Finally, expression of a p27(Kip1) mutant unable of being recognized by Skp2 results in inhibition of proliferation of BCR-ABL cells, indicating that the degradation of p27(Kip1) contributes to the pathogenesis of CML. In conclusion, these results suggest that BCR-ABL regulates cell cycle in CML cells at least in part by inducing proteasome-mediated degradation of the cell cycle inhibitor p27(Kip1) and provide a rationale for the use of inhibitors of the proteasome in patients with BCR-ABL leukemias.

Identification of heme oxygenase-1 as a novel BCR/ABL-dependent survival factor in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a stem cell disease in which BCR/ABL promotes the survival of leukemic cells. Heme oxygenase-1 (HO-1) is an inducible stress protein that catalyzes the degradation of heme and has recently been implicated in the regulation of growth and survival of various neoplastic cells. In the present study, we analyzed the expression and role of HO-1 in CML cells. As assessed by Northern and Western blot analysis as well as immunostaining, primary CML cells were found to express HO-1 mRNA and the HO-1 protein in a constitutive manner. Exposure of these cells to the BCR/ABL tyrosine kinase inhibitor STI571 resulted in decreased expression of HO-1 mRNA and protein. In addition, BCR/ABL was found to up-regulate HO-1 promoter activity, mRNA levels, and protein levels in Ba/F3 cells. To investigate the role of HO-1 for survival of primary CML cells, the HO-1 inducer hemin was used. Hemin-induced expression of HO-1 was found to protect CML cells from STI571-induced cell death. In addition, inhibition of HO-1 by zinc-(II)-deuteroporphyrin-IX-2,4-bisethyleneglycol resulted in a substantial decrease of cell viability. Furthermore, overexpression of HO-1 in the CML-derived cell line K562 was found to counteract STI571-induced apoptosis. Together, our data identify HO-1 as a novel BCR/ABL-driven survival molecule and potential target in leukemic cells in patients with CML. The pathogenetic and clinical implications of this observation remain to be elucidated.

Translational regulation by the p210 BCR/ABL oncoprotein

The ability of oncogenic proteins to regulate the rate of translation of specific mRNA subsets may be a rapid and efficient mechanism to modulate the levels and, in many cases, the activity of the corresponding proteins. In the past few years, we have identified several RNA binding proteins with translation regulatory activity whose expression is markedly activated in the blast crisis of chronic myelogenous leukemia, which represents the most malignant disease stage. Perturbation of the activity of some RNA binding proteins suppresses the leukemogenic potential of BCR/ABL-expressing cells. Most importantly, we have identified some of the targets of these RNA binding proteins. Two of these targets, c/ebp alpha and mdm2 mRNAs, are directly relevant for the altered differentiation and survival of leukemic cells. The identification of mRNA targets translationally regulated by RNA binding proteins overexpressed in tumor cells may lead to the development of therapeutic strategies aimed at modulating the translation rate of specific mRNAs.

MEK kinase 1 mediates the antiapoptotic effect of the Bcr-Abl oncogene through NF-κB activation

Bcr-Abl tyrosine kinase, a chimeric oncoprotein responsible for chronic myelogenous leukemia, constitutively activates several signal transduction pathways that stimulate cell proliferation and prevent apoptosis in hematopoietic cells. The antiapoptotic function of Bcr-Abl is necessary for hematopoietic transformation, and also contributes to leukemogenesis. Herein, we show for the first time that cell transformation induced by Bcr-Abl leads to increased expression and kinase activity of MEK kinase 1 (MEKK1), which acts upstream of the c-Jun N-terminal kinase (JNK), extracellular signal regulated kinase (ERK) and NF-kappaB signaling pathways. Inhibition of MEKK1 activity using a dominant-negative MEKK1 mutant (MEKK1km) diminished the ability of Bcr-Abl to protect cells from genotoxin-induced apoptosis, but had no effect on the proliferation of Bcr-Abl-transformed cells. Expression of MEKK1km also reduced NF-kappaB activation, and inhibited antiapoptotic c-IAP1 and c-IAP2 mRNA expression in response to the genotoxin. By contrast, neither JNK nor ERK activation was affected. These results indicate that MEKK1 is a downstream target of Bcr-Abl, and that the antiapoptotic effect of Bcr-Abl in chronic myelogenous leukemia cells is mediated via the MEKK1-NF-kappaB pathway.

Dendritic cells from CML patients have altered actin organization, reduced antigen processing, and impaired migration

Chronic myeloid leukemia (CML) is characterized by expression of the BCR-ABL fusion gene that encodes a 210-kDa protein, which is a constitutively active tyrosine kinase. At least 70% of the oncoprotein is localized to the cytoskeleton, and several of the most prominent tyrosine kinase substrates for p210(BCR-ABL) are cytoskeletal proteins. Dendritic cells (DCs) are bone marrow-derived antigen-presenting cells responsible for the initiation of immune responses. In CML patients, up to 98% of myeloid DCs generated from peripheral blood mononuclear cells are BCR-ABL positive. In this study we have compared the morphology and behavior of myeloid DCs derived from CML patients with control DCs from healthy individuals. We show that the actin cytoskeleton and shape of CML-DCs of myeloid origin adherent to fibronectin differ significantly from those of normal DCs. CML-DCs are also defective in processing and presentation of exogenous antigens such as tetanus toxoid. The antigen-processing defect may be a consequence of the reduced capacity of CML-DCs to capture antigen via macropinocytosis or via mannose receptors when compared with DCs generated from healthy individuals. Furthermore, chemokine-induced migration of CML-DCs in vitro was significantly reduced. These observations cannot be explained by a difference in the maturation status of CML and normal DCs, because phenotypic analysis by flow cytometry showed a similar surface expression of maturation makers. Taken together, these results suggest that the defects in antigen processing and migration we have observed in CML-DCs may be related to underlying cytoskeletal changes induced by the p210(BCR-ABL) fusion protein.

Emergence of clonal cytogenetic abnormalities in Ph- cells in some CML patients in cytogenetic remission to imatinib but restoration of polyclonal hematopoiesis in the majority

Chronic myelogenous leukemia (CML) is characterized by the presence of a Bcr-Abl fusion protein with deregulated tyrosine kinase activity that is required for maintaining the malignant phenotype. Imatinib, a selective inhibitor of Bcr-Abl, induces major cytogenetic remission (MCR) or complete cytogenetic remission (CCR) in the majority of patients with CML in first chronic phase. However, thorough re-evaluation of cytogenetics in a cohort of patients in MCR or CCR demonstrated clonal karyotypic abnormalities in more than 10% of cases, some of which were clinically associated with a myelodysplastic syndrome (MDS). Further analysis identified previous exposure to cytarabine and idarubicin as significant risk factors for the subsequent occurrence of abnormalities in Philadelphia chromosome-negative (Ph-) cells. To investigate if cytogenetically normal but clonal hematopoiesis might be present in other patients in cytogenetic remission, we studied X-chromosome inactivation as a marker of clonality by polymerase chain reaction analysis of the human androgen receptor (HUMARA). We find that imatinib restores a polyclonal pattern in most patients in CCR and MCR. Nonetheless, our results are consistent with the notion that targeted therapy of CML with imatinib favors the manifestation of Ph- clonal disorders in some patients. They indicate that patients on imatinib should be followed with conventional cytogenetics, even after induction of CCR.

Specific inhibition of bcr-abl gene expression by small interfering RNA

Small interfering RNAs (siRNAs) were designed to target the bcr-abl oncogene, which causes chronic myeloid leukemia (CML) and bcr-abl-positive acute lymphoblastic leukemia (ALL). Chemically synthesized anti-bcr-abl siRNAs were selected using reporter gene constructs and were found to reduce bcr-abl mRNA up to 87% in bcr-abl-positive cell lines and in primary cells from CML patients. This mRNA reduction was specific for bcr-abl because c-abl and c-bcr mRNA levels remained unaffected. Furthermore, protein expression of BCR-ABL and of laminA/C was reduced by specific siRNAs up to 80% in bcr-abl-positive and normal CD34(+) cells, respectively. Finally, anti-bcr-abl siRNA inhibited BCR-ABL-dependent, but not cytokine-dependent, proliferation in a bcr-abl-positive cell line. These data demonstrate that siRNA can specifically and efficiently interfere with the expression of an oncogenic fusion gene in hematopoietic cells.

The molecular mechanism of chronic myelogenous leukemia and its therapeutic implications: studies in a murine model

Chronic myelogenous leukemia (CML) is a malignant disease resulting from the neoplastic transformation of a hematopoietic stem cell. Generation of the BCR-ABL fusion gene plays an essential role in causing the vast majority of CML. Clinical and laboratory studies have indicated that development of CML involves both the effects of BCR-ABL within its correct target cells and interactions of BCR-ABL target cells with the rest of the in vivo environment, and that the progression of the disease to blast crisis involves multiple genetic alterations. An efficient mouse bone marrow transduction and transplantation model for CML has recently been developed. This review summarizes the analysis of the roles of functional domains and downstream signaling pathways of BCR-ABL, of altered cytokine production, of interferon signaling pathways and of oncogene cooperation in the pathogenesis of CML using this murine model. The in vivo studies of leukemogenesis will help to advance mechanism-based therapies for CML, as well as to understand fundamental rules of leukemogenesis and hematopoiesis.

BCR/ABL genes and leukemic phenotype: from molecular mechanisms to clinical correlations

The Philadelphia chromosome (Ph), a minute chromosome that derives from the balanced translocation between chromosomes 9 and 22, was first described in 1960 and was for a long time the only genetic lesion consistently associated with human cancer. This chromosomal translocation results in the fusion between the 5' part of BCR gene, normally located on chromosome 22, and the 3' part of the ABL gene on chromosome 9 giving origin to a BCR/ABL fusion gene which is transcribed and then translated into a hybrid protein. Three main variants of the BCR/ABL gene have been described, that, depending on the length of the sequence of the BCR gene included, encode for the p190(BCR/ABL), P210(BCR/ABL), and P230(BCR/ABL) proteins. These three main variants are associated with distinct clinical types of human leukemias. Herein we review the data on the correlations between the type of BCR/ABL gene and the corresponding leukemic clinical features. Lastly, drawing on experimental data, we provide insight into the different transforming power of the three hybrid BCR/ABL proteins.

Novel targeted therapies for Bcr-Abl positive acute leukemias: beyond STI571

In the pathophysiology of CML, the constitutive activity of the Bcr-Abl tyrosine kinase (TK) is, most likely, the sole molecular abnormality of the chronic phase. It also remains a critical molecular determinant of malignant behavior of the leukemic progenitors in the accelerated and blastic phase of CML. Therefore, downregulation of the levels and activity of Bcr-Abl is clearly the lynchpin of a rational therapeutic strategy against all phases of CML. Support for this has only been strengthened by the observations that resistance to imatinib mesylate (imatinib) commonly involves a breakthrough and the persistent activity of Bcr-Abl TK. This is due to either mutations that inhibit imatinib action on Bcr-Abl TK or amplification of the bcr-abl gene. Recent studies have demonstrated that other small molecule tyrosine kinase inhibitors that also inhibit Bcr-Abl TK may be highly active in inducing differentiation and apoptosis of CML progenitors, regardless of their sensitivity to imatinib. Small molecule inhibitors that downregulate the levels of Bcr-Abl by inhibiting its translation, e.g., arsenic trioxide, or promoting its proteasomal degradation, e.g., geldanamycin analogues, have also been identified. Finally the identification of other potent survival and antiapoptotic signaling pathways in imatinib-resistant CML progenitors indicates that inhibitors of these pathways will eventually be treatment strategies for advanced phases of CML.

Jak2 is involved in c-Myc induction by Bcr-Abl

We have previously shown that the Jak2 tyrosine kinase is activated in Bcr-Abl positive cell lines and blood cells from CML blast crisis patients by tyrosine phosphorylation. We are searching for downstream targets of Jak2 in Bcr-Abl positive cells. It is known that c-Myc expression is required for the oncogenic effects of Bcr-Abl, and that over-expression of c-Myc complements the transformation defect of the Bcr-Abl SH2 deletion mutant. Moreover, the Bcr-Abl SH2 deletion mutant and an Abl C-terminal deletion mutant are deficient in activating c-Myc expression. Since the Jak2 binds to the C-terminal domain of Bcr-Abl and optimal Jak2 activation requires the SH2 domain, we tested whether Jak2 was involved in c-Myc protein induction by Bcr-Abl. We treated the 32Dp210 Bcr-Abl cells with the Jak2 specific tyrosine kinase inhibitor, AG490, and found that this drug, like the Abl tyrosine kinase inhibitor STI-571, inhibited c-Myc protein induction by Bcr-Abl. Treatment of 32Dp210 Bcr-Abl cells with AG490 also inhibited c-MYC RNA expression. It is also known that c-Myc protein is a labile protein that is increased in amounts in response to various growth factors by a mechanism not involving new Myc protein formation. Treatment of 32Dp210 Bcr-Abl cells with both the proteasome inhibitor MG132 and AG490 blocked the reduction of the c-Myc protein observed by AG490 alone. An adaptor protein SH2-Bbeta is involved in the enhancement of the tyrosine kinase activity of Jak2 following ligand/receptor interaction. In this regard we showed that the Jak2/Bcr-Abl complex contains SH2-Bbeta. Expression of the SH2-Bbeta R555E mutant in 32Dp210 Bcr-Abl cells reduced c-Myc expression about 40% compared to a vector control. Interestingly, we found the reduction of the c-Myc protein in several clones of dominant-negative (DN) Jak2 expressing K562 cells correlated very well with the reduction of tumor growth of these cells in nude mice as compared to vector transfected K562 cells. Both STI-571 and AG490 also induced apoptosis in 32Dp210 cells. Of interest, IL-3 containing medium reversed the STI-571 induced apoptosis of 32Dp210 cells but did not reverse the induction of apoptosis by AG490, which strongly supports the specificity of the inhibitory effects of AG490 on the Jak2 tyrosine kinase. In summary, our findings indicate that Jak2 mediates the increase in c-Myc expression that is induced by Bcr-Abl. Our results indicate that activated Jak2 not only mediates an increase of c-MYC RNA expression but also interferes with proteasome-dependent degradation of c-Myc protein.

Cyclosporine inhibition of P-glycoprotein in chronic myeloid leukemia blast phase

Chronic myeloid leukemia blast phase (CML-BP) cells commonly express the multidrug transporter, P-glycoprotein (Pgp). To determine whether Pgp inhibition improves treatment outcome in CML-BP, the Southwest Oncology Group performed a randomized, controlled trial testing the benefit of the Pgp modulator, cyclosporin A (CsA). Seventy-three eligible patients were assigned to treatment with cytarabine and infusional daunorubicin with or without intravenous CsA. Treatment with CsA yielded no improvement in treatment outcome as measured by the frequency of induction resistance (68% vs 53%), rate of complete remission or restored chronic phase (CR/CP, 8% vs 30%), and survival (3 vs 5 months). Blast expression of Pgp (63%) and LRP (71%) was common, whereas only Pgp adversely impacted the rate of CR/CP (P = .025). We conclude that Pgp has prognostic relevance in CML-BP but that the modulation of Pgp function with CsA as applied in this trial is ineffective.

BCR/ABL P190 transgenic mice develop leukemia in the absence of Crkl

The Bcr/Abl fusion protein directly causes chronic myelogenous leukemia and Philadelphia-chromosome positive acute lymphoblastic leukemia. Multiple independent studies have implicated Crkl, a small adapter protein, in transduction of oncogenic signals of Bcr/Abl and Crkl tyrosine-phosphorylation is used as a diagnostic tool for Philadelphia-positive leukemia. To evaluate the contribution of Crkl to this type of leukemia, we generated mutant mice that lack Crkl expression. We found that the overall survival of P190 BCR/ABL crkl-/- mice was comparable to that of genetically matched P190 BCR/ABL crkl +/+ mice. Both genotypes developed lymphoid lineage leukemia/lymphoma. Western blot analysis of -/- and +/+ lymphomas showed that the related Crk protein was tyrosine phosphorylated and could be found complexed with Bcr-Abl P190. These data indicate that possible therapeutic approaches that target Crkl may be complicated by the presence of pathways that compensate for lack of Crkl function.

Modeling Philadelphia chromosome positive leukemias

The Ph chromosome has been genetically linked to CML and ALL. Its chimeric fusion gene product, BCR-ABL, can generate leukemia in mice. This review will discuss selected model systems developed to study BCR-ABL induced leukemia and focuses on what we have learned about the human disease from these models. Five main experimental approaches will be discussed including: (i) Reconstitution of mice with bone marrow cells retrovirally transduced with BCR-ABL; (ii) Transgenic mice expressing BCR-ABL; (iii) Knock-in mice with BCR-ABL expression driven from the endogenous bcr locus; (iv) Development of CML-like disease in mice with loss of function mutations in heterologous genes; and (v) ES in vitro hematopoietic differentiation coupled with regulated BCR-ABL expression.