A randomised phase II trial of hydroxychloroquine and imatinib versus imatinib alone for patients with chronic myeloid leukaemia in major cytogenetic response with residual disease

In chronic-phase chronic myeloid leukaemia (CP-CML), residual BCR-ABL1+ leukaemia stem cells are responsible for disease persistence despite TKI. Based on in vitro data, CHOICES (CHlorOquine and Imatinib Combination to Eliminate Stem cells) was an international, randomised phase II trial designed to study the safety and efficacy of imatinib (IM) and hydroxychloroquine (HCQ) compared with IM alone in CP-CML patients in major cytogenetic remission with residual disease detectable by qPCR. Sixty-two patients were randomly assigned to either arm. Treatment 'successes' was the primary end point, defined as ≥0.5 log reduction in 12-month qPCR level from trial entry. Selected secondary study end points were 24-month treatment 'successes', molecular response and progression at 12 and 24 months, comparison of IM levels, and achievement of blood HCQ levels >2000 ng/ml. At 12 months, there was no difference in 'success' rate (p = 0.58); MMR was achieved in 80% (IM) vs 92% (IM/HCQ) (p = 0.21). At 24 months, the 'success' rate was 20.8% higher with IM/HCQ (p = 0.059). No patients progressed. Seventeen serious adverse events, including four serious adverse reactions, were reported; diarrhoea occurred more frequently with combination. IM/HCQ is tolerable in CP-CML, with modest improvement in qPCR levels at 12 and 24 months, suggesting autophagy inhibition maybe of clinical value in CP-CML.

Correction: CD93 is expressed on chronic myeloid leukemia stem cells and identifies a quiescent population which persists after tyrosine kinase inhibitor therapy

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

CD93 is expressed on chronic myeloid leukemia stem cells and identifies a quiescent population which persists after tyrosine kinase inhibitor therapy

The introduction of BCR-ABL tyrosine kinase inhibitors has revolutionized the treatment of chronic myeloid leukemia (CML). A major clinical aim remains the identification and elimination of low-level disease persistence, termed "minimal residual disease". The phenomenon of disease persistence suggests that despite targeted therapeutic approaches, BCR-ABL-independent mechanisms exist which sustain the survival of leukemic stem cells (LSCs). Although other markers of a primitive CML LSC population have been identified in the preclinical setting, only CD26 appears to offer clinical utility. Here we demonstrate consistent and selective expression of CD93 on a lin^-CD34⁺CD38^-CD90⁺ CML LSC population and show in vitro and in vivo data to suggest increased stem cell characteristics, as well as robust engraftment in patient-derived xenograft models in comparison with a CD93^- CML stem/progenitor cell population, which fails to engraft. Through bulk and single-cell analyses of selected stem cell and cell survival-specific genes, we confirmed the quiescent character and demonstrate their persistence in a population of CML patient samples who demonstrate molecular relapse on TKI withdrawal. Taken together, our results identify that CD93 is consistently and selectively expressed on a lin^-CD34⁺CD38^-CD90⁺ CML LSC population with stem cell characteristics and may be an important indicator in determining poor TKI responders.

Targeting quiescent leukemic stem cells using second generation autophagy inhibitors

In chronic myeloid leukemia (CML), tyrosine kinase inhibitor (TKI) treatment induces autophagy that promotes survival and TKI-resistance in leukemic stem cells (LSCs). In clinical studies hydroxychloroquine (HCQ), the only clinically approved autophagy inhibitor, does not consistently inhibit autophagy in cancer patients, so more potent autophagy inhibitors are needed. We generated a murine model of CML in which autophagic flux can be measured in bone marrow-located LSCs. In parallel, we use cell division tracing, phenotyping of primary CML cells, and a robust xenotransplantation model of human CML, to investigate the effect of Lys05, a highly potent lysosomotropic agent, and PIK-III, a selective inhibitor of VPS34, on the survival and function of LSCs. We demonstrate that long-term haematopoietic stem cells (LT-HSCs: Lin-Sca-1+c-kit+CD48-CD150+) isolated from leukemic mice have higher basal autophagy levels compared with non-leukemic LT-HSCs and more mature leukemic cells. Additionally, we present that while HCQ is ineffective, Lys05-mediated autophagy inhibition reduces LSCs quiescence and drives myeloid cell expansion. Furthermore, Lys05 and PIK-III reduced the number of primary CML LSCs and target xenografted LSCs when used in combination with TKI treatment, providing a strong rationale for clinical use of second generation autophagy inhibitors as a novel treatment for CML patients with LSC persistence.

Targeting BCR-ABL-Independent TKI Resistance in Chronic Myeloid Leukemia by mTOR and Autophagy Inhibition

Background

Imatinib and second-generation tyrosine kinase inhibitors (TKIs) nilotinib and dasatinib have statistically significantly improved the life expectancy of chronic myeloid leukemia (CML) patients; however, resistance to TKIs remains a major clinical challenge. Although ponatinib, a third-generation TKI, improves outcomes for patients with BCR-ABL-dependent mechanisms of resistance, including the T315I mutation, a proportion of patients may have or develop BCR-ABL-independent resistance and fail ponatinib treatment. By modeling ponatinib resistance and testing samples from these CML patients, it is hoped that an alternative drug target can be identified and inhibited with a novel compound.

Methods

Two CML cell lines with acquired BCR-ABL-independent resistance were generated following culture in ponatinib. RNA sequencing and gene ontology (GO) enrichment were used to detect aberrant transcriptional response in ponatinib-resistant cells. A validated oncogene drug library was used to identify US Food and Drug Administration-approved drugs with activity against TKI-resistant cells. Validation was performed using bone marrow (BM)-derived cells from TKI-resistant patients (n = 4) and a human xenograft mouse model (n = 4-6 mice per group). All statistical tests were two-sided.

Results

We show that ponatinib-resistant CML cells can acquire BCR-ABL-independent resistance mediated through alternative activation of mTOR. Following transcriptomic analysis and drug screening, we highlight mTOR inhibition as an alternative therapeutic approach in TKI-resistant CML cells. Additionally, we show that catalytic mTOR inhibitors induce autophagy and demonstrate that genetic or pharmacological inhibition of autophagy sensitizes ponatinib-resistant CML cells to death induced by mTOR inhibition in vitro (% number of colonies of control[SD], NVP-BEZ235 vs NVP-BEZ235+HCQ: 45.0[17.9]% vs 24.0[8.4]%, P = .002) and in vivo (median survival of NVP-BEZ235- vs NVP-BEZ235+HCQ-treated mice: 38.5 days vs 47.0 days, P = .04).

Conclusion

Combined mTOR and autophagy inhibition may provide an attractive approach to target BCR-ABL-independent mechanism of resistance.

hsa-mir183/EGR1-mediated regulation of E2F1 is required for CML stem/progenitor cell survival

Chronic myeloid leukemia (CML) stem/progenitor cells (SPCs) express a transcriptional program characteristic of proliferation, yet can achieve and maintain quiescence. Understanding the mechanisms by which leukemic SPCs maintain quiescence will help to clarify how they persist during long-term targeted treatment. We have identified a novel BCR-ABL1 protein kinase-dependent pathway mediated by the upregulation of hsa-mir183, the downregulation of its direct target early growth response 1 (EGR1), and, as a consequence, upregulation of E2F1. We show here that inhibition of hsa-mir183 reduced proliferation and impaired colony formation of CML SPCs. Downstream of this, inhibition of E2F1 also reduced proliferation of CML SPCs, leading to p53-mediated apoptosis. In addition, we demonstrate that E2F1 plays a pivotal role in regulating CML SPC proliferation status. Thus, for the first time, we highlight the mechanism of hsa-mir183/EGR1-mediated E2F1 regulation and demonstrate this axis as a novel, critical factor for CML SPC survival, offering new insights into leukemic stem cell eradication.

Bone marrow niche trafficking of miR-126 controls the self-renewal of leukemia stem cells in chronic myelogenous leukemia

Leukemia stem cells (LSCs) in individuals with chronic myelogenous leukemia (CML) (hereafter referred to as CML LSCs) are responsible for initiating and maintaining clonal hematopoiesis. These cells persist in the bone marrow (BM) despite effective inhibition of BCR-ABL kinase activity by tyrosine kinase inhibitors (TKIs). Here we show that although the microRNA (miRNA) miR-126 supported the quiescence, self-renewal and engraftment capacity of CML LSCs, miR-126 levels were lower in CML LSCs than in long-term hematopoietic stem cells (LT-HSCs) from healthy individuals. Downregulation of miR-126 levels in CML LSCs was due to phosphorylation of Sprouty-related EVH1-domain-containing 1 (SPRED1) by BCR-ABL, which led to inhibition of the RAN-exportin-5-RCC1 complex that mediates miRNA maturation. Endothelial cells (ECs) in the BM supply miR-126 to CML LSCs to support quiescence and leukemia growth, as shown using mouse models of CML in which Mir126a (encoding miR-126) was conditionally knocked out in ECs and/or LSCs. Inhibition of BCR-ABL by TKI treatment caused an undesired increase in endogenous miR-126 levels, which enhanced LSC quiescence and persistence. Mir126a knockout in LSCs and/or ECs, or treatment with a miR-126 inhibitor that targets miR-126 expression in both LSCs and ECs, enhanced the in vivo anti-leukemic effects of TKI treatment and strongly diminished LSC leukemia-initiating capacity, providing a new strategy for the elimination of LSCs in individuals with CML.

Cooperation of imipramine blue and tyrosine kinase blockade demonstrates activity against chronic myeloid leukemia

The use of tyrosine kinase inhibitors (TKI), including nilotinib, has revolutionized the treatment of chronic myeloid leukemia (CML). However current unmet clinical needs include combating activation of additional survival signaling pathways in persistent leukemia stem cells after long-term TKI therapy. A ubiquitous signaling alteration in cancer, including CML, is activation of reactive oxygen species (ROS) signaling, which may potentiate stem cell activity and mediate resistance to both conventional chemotherapy and targeted inhibitors. We have developed a novel nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, imipramine blue (IB) that targets ROS generation. ROS levels are known to be elevated in CML with respect to normal hematopoietic stem/progenitor cells and not corrected by TKI. We demonstrate that IB has additive benefit with nilotinib in inhibiting proliferation, viability, and clonogenic function of TKI-insensitive quiescent CD34⁺ CML chronic phase (CP) cells while normal CD34⁺ cells retained their clonogenic capacity in response to this combination therapy in vitro. Mechanistically, the pro-apoptotic activity of IB likely resides in part through its dual ability to block NF-κB and re-activate the tumor suppressor protein phosphatase 2A (PP2A). Combining BCR-ABL1 kinase inhibition with NADPH oxidase blockade may be beneficial in eradication of CML and worthy of further investigation.

Targeting mitochondrial oxidative phosphorylation eradicates therapy-resistant chronic myeloid leukemia stem cells

Treatment of chronic myeloid leukemia (CML) with imatinib mesylate and other second- and/or third-generation c-Abl-specific tyrosine kinase inhibitors (TKIs) has substantially extended patient survival. However, TKIs primarily target differentiated cells and do not eliminate leukemic stem cells (LSCs). Therefore, targeting minimal residual disease to prevent acquired resistance and/or disease relapse requires identification of new LSC-selective target(s) that can be exploited therapeutically. Considering that malignant transformation involves cellular metabolic changes, which may in turn render the transformed cells susceptible to specific assaults in a selective manner, we searched for such vulnerabilities in CML LSCs. We performed metabolic analyses on both stem cell-enriched (CD34+ and CD34+CD38-) and differentiated (CD34-) cells derived from individuals with CML, and we compared the signature of these cells with that of their normal counterparts. Through combination of stable isotope-assisted metabolomics with functional assays, we demonstrate that primitive CML cells rely on upregulated oxidative metabolism for their survival. We also show that combination treatment with imatinib and tigecycline, an antibiotic that inhibits mitochondrial protein translation, selectively eradicates CML LSCs both in vitro and in a xenotransplantation model of human CML. Our findings provide a strong rationale for investigation of the use of TKIs in combination with tigecycline to treat patients with CML with minimal residual disease.

Stem Cell Guardians - Old and New Perspectives in LSC Biology

The introduction of tyrosine kinase inhibitors in chronic myeloid leukaemia (CML) has revolutionised disease outcome. However, despite this, progression to blast phase disease is high in those that do not achieve complete cytogenetic and major molecular response on standard therapy. As well as BCR-ABL-dependent mechanisms, disease persistence has been shown to play a key role. Disease persistence suggests that, despite a targeted therapeutic approach, BCR-ABL-independent mechanisms are being exploited to sustain the survival of a small population of cells termed leukaemic stem cells (LSCs). Increasing evidence highlights the importance of self-renewal and survival pathways in this process. This review will focus on the role of stem-cell restricted self-renewal pathways, namely Hedgehog, Notch, and Bone Morphogenic Pathway (BMP). Wingless-Int/β-Catenin (Wnt/β-Catenin) signalling will be discussed within a further review in this series in view of its regulatory role in GSK3β. Further to this, we will highlight the role of key transcriptional regulators, namely p53 and c- MYC, in targeting wider deregulated networks.

A new monoclonal antibody detects downregulation of protein tyrosine phosphatase receptor type γ in chronic myeloid leukemia patients

BACKGROUND

Protein tyrosine phosphatase receptor gamma (PTPRG) is a ubiquitously expressed member of the protein tyrosine phosphatase family known to act as a tumor suppressor gene in many different neoplasms with mechanisms of inactivation including mutations and methylation of CpG islands in the promoter region. Although a critical role in human hematopoiesis and an oncosuppressor role in chronic myeloid leukemia (CML) have been reported, only one polyclonal antibody (named chPTPRG) has been described as capable of recognizing the native antigen of this phosphatase by flow cytometry. Protein biomarkers of CML have not yet found applications in the clinic, and in this study, we have analyzed a group of newly diagnosed CML patients before and after treatment. The aim of this work was to characterize and exploit a newly developed murine monoclonal antibody specific for the PTPRG extracellular domain (named TPγ B9-2) to better define PTPRG protein downregulation in CML patients.

METHODS

TPγ B9-2 specifically recognizes PTPRG (both human and murine) by flow cytometry, western blotting, immunoprecipitation, and immunohistochemistry.

RESULTS

Co-localization experiments performed with both anti-PTPRG antibodies identified the presence of isoforms and confirmed protein downregulation at diagnosis in the Philadelphia-positive myeloid lineage (including CD34⁺/CD38^bright/dim cells). After effective tyrosine kinase inhibitor (TKI) treatment, its expression recovered in tandem with the return of Philadelphia-negative hematopoiesis. Of note, PTPRG mRNA levels remain unchanged in tyrosine kinase inhibitors (TKI) non-responder patients, confirming that downregulation selectively occurs in primary CML cells.

CONCLUSIONS

The availability of this unique antibody permits its evaluation for clinical application including the support for diagnosis and follow-up of these disorders. Evaluation of PTPRG as a potential therapeutic target is also facilitated by the availability of a specific reagent capable to specifically detect its target in various experimental conditions.

Preclinical approaches in chronic myeloid leukemia: from cells to systems

Advances in the design of targeted therapies for the treatment of chronic myeloid leukemia (CML) have transformed the prognosis for patients diagnosed with this disease. However, leukemic stem cell persistence, drug intolerance, drug resistance, and advanced-phase disease represent unmet clinical needs demanding the attention of CML investigators worldwide. The availability of appropriate preclinical models is essential to efficiently translate findings from the bench to the clinic. Here we review the current approaches taken to preclinical work in the CML field, including examples of commonly used in vivo models and recent successes from systems biology-based methodologies.

Axl Blockade by BGB324 Inhibits BCR-ABL Tyrosine Kinase Inhibitor-Sensitive and -Resistant Chronic Myeloid Leukemia

Purpose: BCR-ABL kinase inhibitors are employed successfully for chronic myeloid leukemia (CML) treatment. However, resistant disease and persistence of BCR-ABL1-independent leukemia stem and progenitor cells (LSPC) remain clinical challenges. The receptor tyrosine kinase Axl can mediate survival and therapy resistance of different cancer cells. We investigated the therapeutic potential of Axl inhibition in CML.Experimental Design: We used primary cells from patients with CML and TKI-sensitive and -resistant BCR-ABL1⁺ CML cell lines and a novel ponatinib-resistant cell line KCL-22 PonR. We analyzed the effects of genetic and pharmacologic Axl blockade by the small-molecule Axl inhibitor BGB324 in vitro and in vivo In BCR-ABL1-unmutated cells, we also investigated BGB324 in combination with imatinib.Results: We demonstrate overexpression of Axl receptor tyrosine kinase in primary cells of patients with CML compared with healthy individuals and a further increase of Axl expression in BCR-ABL TKI-resistant patients. We show that Axl blockage decreased growth of BCR-ABL TKI-sensitive CML cells including CD34⁺ cells and exerts additive effects with imatinib via inhibition of Stat5 activation. BGB324 also inhibits BCR-ABL TKI-resistant cells, including T315I-mutated and ponatinib-resistant primary cells. BGB324 exerted therapeutic effects in BCR-ABL1 T315I-mutated and ponatinib-resistant preclinical mouse models. Notably, BGB324 does not inhibit BCR-ABL1 and consequently inhibits CML independent of BCR-ABL1 mutational status.Conclusions: Our data show that Axl inhibition has therapeutic potential in BCR-ABL TKI-sensitive as well as -resistant CML and support the need for clinical trials. Clin Cancer Res; 23(9); 2289-300. ©2016 AACR.

Lifting the Differentiation Embargo

Effective differentiation therapy for acute myeloid leukemia (AML) has been restricted to a small subset of patients with one defined genetic abnormality. Using an unbiased small molecule screen, Sykes et al. now identify a mechanism of de-repression of differentiation in several models of AML driven by distinct genetic drivers.

Epigenetic Reprogramming Sensitizes CML Stem Cells to Combined EZH2 and Tyrosine Kinase Inhibition

A major obstacle to curing chronic myeloid leukemia (CML) is residual disease maintained by tyrosine kinase inhibitor (TKI)-persistent leukemic stem cells (LSC). These are BCR-ABL1 kinase independent, refractory to apoptosis, and serve as a reservoir to drive relapse or TKI resistance. We demonstrate that Polycomb Repressive Complex 2 is misregulated in chronic phase CML LSCs. This is associated with extensive reprogramming of H3K27me3 targets in LSCs, thus sensitizing them to apoptosis upon treatment with an EZH2-specific inhibitor (EZH2i). EZH2i does not impair normal hematopoietic stem cell survival. Strikingly, treatment of primary CML cells with either EZH2i or TKI alone caused significant upregulation of H3K27me3 targets, and combined treatment further potentiated these effects and resulted in significant loss of LSCs compared to TKI alone, in vitro, and in long-term bone marrow murine xenografts. Our findings point to a promising epigenetic-based therapeutic strategy to more effectively target LSCs in patients with CML receiving TKIs.

SIGNIFICANCE

In CML, TKI-persistent LSCs remain an obstacle to cure, and approaches to eradicate them remain a significant unmet clinical need. We demonstrate that EZH2 and H3K27me3 reprogramming is important for LSC survival, but renders LSCs sensitive to the combined effects of EZH2i and TKI. This represents a novel approach to more effectively target LSCs in patients receiving TKI treatment. Cancer Discov; 6(11); 1248-57. ©2016 AACR.See related article by Xie et al., p. 1237This article is highlighted in the In This Issue feature, p. 1197.

CXCR2 and CXCL4 regulate survival and self-renewal of hematopoietic stem/progenitor cells

The regulation of hematopoietic stem cell (HSC) survival and self-renewal within the bone marrow (BM) niche is not well understood. We therefore investigated global transcriptomic profiling of normal human HSC/hematopoietic progenitor cells [HPCs], revealing that several chemokine ligands (CXCL1-4, CXCL6, CXCL10, CXCL11, and CXCL13) were upregulated in human quiescent CD34(+)Hoescht(-)Pyronin Y(-) and primitive CD34(+)38(-), as compared with proliferating CD34(+)Hoechst(+)Pyronin Y(+) and CD34(+)38(+) stem/progenitor cells. This suggested that chemokines might play an important role in the homeostasis of HSCs. In human CD34(+) hematopoietic cells, knockdown of CXCL4 or pharmacologic inhibition of the chemokine receptor CXCR2, significantly decreased cell viability and colony forming cell (CFC) potential. Studies on Cxcr2(-/-) mice demonstrated enhanced BM and spleen cellularity, with significantly increased numbers of HSCs, hematopoietic progenitor cell-1 (HPC-1), HPC-2, and Lin(-)Sca-1(+)c-Kit(+) subpopulations. Cxcr2(-/-) stem/progenitor cells showed reduced self-renewal capacity as measured in serial transplantation assays. Parallel studies on Cxcl4 demonstrated reduced numbers of CFC in primary and secondary assays following knockdown in murine c-Kit(+) cells, and Cxcl4(-/-) mice showed a decrease in HSC and reduced self-renewal capacity after secondary transplantation. These data demonstrate that the CXCR2 network and CXCL4 play a role in the maintenance of normal HSC/HPC cell fates, including survival and self-renewal.

The Scotland Leukaemia Registry Audit of Incidence, Diagnosis and Clinical Management of New Patients with Chronic Myeloid Leukaemia in 1999 and 2000

The management of CML has recently become increasingly complex. The Scotland Leukaemia Registry (SLR) sent questionnaires to all 26 Scottish haematology units, of which 18 (69%) responded. From January 1999 to December 2000, 64 new cases of CML were identified by the audit (incidence 0.64/100,00/yr), of which 46 were registered with the SLR. At diagnosis, all 18 units combined bone marrow examination with cytogenetics/FISH, but only 13 performed RT-PCR. Of four units that calculated the Hasford Score, only two used it to inform clinical decisions. 52% of patients entered clinical trials, 57% involving imatinib mesylate (IM). Of the 23 patients who were tissue typed, suitable donors were found for 18, 11 sibling, and 7 unrelated, representing 28% of the total patient population. Only 13/64 patients (20%) did not have a BMT donor identified or enter a clinical trial. Although 38% of units would consider reduced intensity allografting in patients > 60 years, no centres currently routinely tissue-type such patients. For first line therapy 56% of patients received hydroxyurea +/- interferon. Of the newer agents, 83% of units believed imatinib mesylate should be reserved for clinical trials, 83% would consider using oral ara-C and 89% pegylated-interferon.

Adult hematopoietic stem cells lacking Hif-1α self-renew normally

The hematopoietic stem cell (HSC) pool is maintained under hypoxic conditions within the bone marrow microenvironment. Cellular responses to hypoxia are largely mediated by the hypoxia-inducible factors, Hif-1 and Hif-2. The oxygen-regulated α subunits of Hif-1 and Hif-2 (namely, Hif-1α and Hif-2α) form dimers with their stably expressed β subunits and control the transcription of downstream hypoxia-responsive genes to facilitate adaptation to low oxygen tension. An initial study concluded that Hif-1α is essential for HSC maintenance, whereby Hif-1α-deficient HSCs lost their ability to self-renew in serial transplantation assays. In another study, we demonstrated that Hif-2α is dispensable for cell-autonomous HSC maintenance, both under steady-state conditions and following transplantation. Given these unexpected findings, we set out to revisit the role of Hif-1α in cell-autonomous HSC functions. Here we demonstrate that inducible acute deletion of Hif-1α has no impact on HSC survival. Notably, unstressed HSCs lacking Hif-1α efficiently self-renew and sustain long-term multilineage hematopoiesis upon serial transplantation. Finally, Hif-1α-deficient HSCs recover normally after hematopoietic injury induced by serial administration of 5-fluorouracil. We therefore conclude that despite the hypoxic nature of the bone marrow microenvironment, Hif-1α is dispensable for cell-autonomous HSC maintenance.

ATG7 regulates energy metabolism, differentiation and survival of Philadelphia-chromosome-positive cells

A major drawback of tyrosine kinase inhibitor (TKI) treatment in chronic myeloid leukemia (CML) is that primitive CML cells are able to survive TKI-mediated BCR-ABL inhibition, leading to disease persistence in patients. Investigation of strategies aiming to inhibit alternative survival pathways in CML is therefore critical. We have previously shown that a nonspecific pharmacological inhibition of autophagy potentiates TKI-induced death in Philadelphia chromosome-positive cells. Here we provide further understanding of how specific and pharmacological autophagy inhibition affects nonmitochondrial and mitochondrial energy metabolism and reactive oxygen species (ROS)-mediated differentiation of CML cells and highlight ATG7 (a critical component of the LC3 conjugation system) as a potential specific therapeutic target. By combining extra- and intracellular steady state metabolite measurements by liquid chromatography-mass spectrometry with metabolic flux assays using labeled glucose and functional assays, we demonstrate that knockdown of ATG7 results in decreased glycolysis and increased flux of labeled carbons through the mitochondrial tricarboxylic acid cycle. This leads to increased oxidative phosphorylation and mitochondrial ROS accumulation. Furthermore, following ROS accumulation, CML cells, including primary CML CD34(+) progenitor cells, differentiate toward the erythroid lineage. Finally, ATG7 knockdown sensitizes CML progenitor cells to TKI-induced death, without affecting survival of normal cells, suggesting that specific inhibitors of ATG7 in combination with TKI would provide a novel therapeutic approach for CML patients exhibiting persistent disease.

Hif-1α and Hif-2α synergize to suppress AML development but are dispensable for disease maintenance

Leukemogenesis occurs under hypoxic conditions within the bone marrow (BM). Knockdown of key mediators of cellular responses to hypoxia with shRNA, namely hypoxia-inducible factor-1α (HIF-1α) or HIF-2α, in human acute myeloid leukemia (AML) samples results in their apoptosis and inability to engraft, implicating HIF-1α or HIF-2α as therapeutic targets. However, genetic deletion of Hif-1α has no effect on mouse AML maintenance and may accelerate disease development. Here, we report the impact of conditional genetic deletion of Hif-2α or both Hif-1α and Hif-2α at different stages of leukemogenesis in mice. Deletion of Hif-2α accelerates development of leukemic stem cells (LSCs) and shortens AML latency initiated by Mll-AF9 and its downstream effectors Meis1 and Hoxa9. Notably, the accelerated initiation of AML caused by Hif-2α deletion is further potentiated by Hif-1α codeletion. However, established LSCs lacking Hif-2α or both Hif-1α and Hif-2α propagate AML with the same latency as wild-type LSCs. Furthermore, pharmacological inhibition of the HIF pathway or HIF-2α knockout using the lentiviral CRISPR-Cas9 system in human established leukemic cells with MLL-AF9 translocation have no impact on their functions. We therefore conclude that although Hif-1α and Hif-2α synergize to suppress the development of AML, they are not required for LSC maintenance.

The antiproliferative activity of kinase inhibitors in chronic myeloid leukemia cells is mediated by FOXO transcription factors

Chronic myeloid leukemia (CML) is initiated and maintained by the tyrosine kinase BCR-ABL which activates a number of signal transduction pathways, including PI3K/AKT signaling and consequently inactivates FOXO transcription factors. ABL-specific tyrosine kinase inhibitors (TKIs) induce minimal apoptosis in CML progenitor cells, yet exert potent antiproliferative effects, through as yet poorly understood mechanisms. Here, we demonstrate that in CD34+ CML cells, FOXO1 and 3a are inactivated and relocalized to the cytoplasm by BCR-ABL activity. TKIs caused a decrease in phosphorylation of FOXOs, leading to their relocalization from cytoplasm (inactive) to nucleus (active), where they modulated the expression of key FOXO target genes, such as Cyclin D1, ATM, CDKN1C, and BCL6 and induced G1 arrest. Activation of FOXO1 and 3a and a decreased expression of their target gene Cyclin D1 were also observed after 6 days of in vivo treatment with dasatinib in a CML transgenic mouse model. The over-expression of FOXO3a in CML cells combined with TKIs to reduce proliferation, with similar results seen for inhibitors of PI3K/AKT/mTOR signaling. While stable expression of an active FOXO3a mutant induced a similar level of quiescence to TKIs alone, shRNA-mediated knockdown of FOXO3a drove CML cells into cell cycle and potentiated TKI-induced apoptosis. These data demonstrate that TKI-induced G1 arrest in CML cells is mediated through inhibition of the PI3K/AKT pathway and reactivation of FOXOs. This enhanced understanding of TKI activity and induced progenitor cell quiescence suggests that new therapeutic strategies for CML should focus on manipulation of this signaling network. Stem Cells2014;32:2324–2337

Identification of CD25 as STAT5-Dependent Growth Regulator of Leukemic Stem Cells in Ph+ CML

PURPOSE

In chronic myelogenous leukemia (CML), leukemic stem cells (LSC) represent a critical target of therapy. However, little is known about markers and targets expressed by LSCs. The aim of this project was to identify novel relevant markers of CML LSCs.

EXPERIMENTAL DESIGN

CML LSCs were examined by flow cytometry, qPCR, and various bioassays. In addition, we examined the multipotent CD25(+)CML cell line KU812.

RESULTS

In contrast to normal hematopoietic stem cells, CD34(+)/CD38(-)CML LSCs expressed the IL-2 receptor alpha chain, IL-2RA (CD25). STAT5 was found to induce expression of CD25 in Lin(-)/Sca-1(+)/Kit(+)stem cells in C57Bl/6 mice. Correspondingly, shRNA-induced STAT5 depletion resulted in decreased CD25 expression in KU812 cells. Moreover, the BCR/ABL1 inhibitors nilotinib and ponatinib were found to decrease STAT5 activity and CD25 expression in KU812 cells and primary CML LSCs. A CD25-targeting shRNA was found to augment proliferation of KU812 cellsin vitroand their engraftmentin vivoin NOD/SCID-IL-2Rγ(-/-)mice. In drug-screening experiments, the PI3K/mTOR blocker BEZ235 promoted the expression of STAT5 and CD25 in CML cells. Finally, we found that BEZ235 produces synergistic antineoplastic effects on CML cells when applied in combination with nilotinib or ponatinib.

CONCLUSIONS

CD25 is a novel STAT5-dependent marker of CML LSCs and may be useful for LSC detection and LSC isolation in clinical practice and basic science. Moreover, CD25 serves as a growth regulator of CML LSCs, which may have biologic and clinical implications and may pave the way for the development of new more effective LSC-eradicating treatment strategies in CML.

Do we need more drugs for chronic myeloid leukemia?

The introduction of protein tyrosine kinase inhibitors (TKIs) in 1998 transformed the management of chronic myeloid leukemia (CML), leading to significantly reduced mortality and improved 5 year survival rates. However, the CML community is faced with several clinical issues that need to be addressed. Ten to 15% of CML patients are diagnosed in advanced phase, and small numbers of chronic phase (CP) cases experience disease progression each year during treatment. For these patients, TKIs induce only transient responses and alternative treatment strategies are urgently required. Depending on choice of first line TKI, approximately 30% of CML CP cases show suboptimal responses, due to a combination of poor compliance, drug intolerance, and drug resistance, with approximately 50% of TKI-resistance caused by kinase domain mutations and the remainder due to unknown mechanisms. Finally, the chance of successful treatment discontinuation is on the order of only 10-20% related to disease persistence. Disease persistence is a poorly understood phenomenon; all CML patients have functional Philadelphia positive (Ph+) stem and progenitor cells in their bone marrows and continue to express BCR-ABL1 by DNA PCR, even when in very deep remission and following treatment discontinuation. What controls the maintenance of these persisting cells, whether it is necessary to fully eradicate the malignant clone to achieve cure, and how that might be approached therapeutically are open questions.

Concise review: cancer cells escape from oncogene addiction: understanding the mechanisms behind treatment failure for more effective targeting

Oncogene addiction describes the dependence of some cancers on one or a few genes for their survival. Inhibition of the corresponding oncoproteins can lead to dramatic responses. However, in some cases, such as chronic myeloid leukemia (CML), a disease characterized by the presence of the abnormal fusion tyrosine kinase BCR-ABL, cancer stem cells may never acquire addiction to the oncogene that drives disease development. The suggested mechanism(s) for treatment failure include a quiescent stem cell population capable of reinstating disease, high levels of oncoprotein expression, or acquired mutations in the oncogene. In this review, we discuss the evidence for oncogene addiction in several solid tumors and their potential escape mechanism(s) with a particular focus on CML stem cells.

Dual glutathione-S-transferase-θ1 and -μ1 gene deletions determine imatinib failure in chronic myeloid leukemia

Approximately 40% of patients with chronic myeloid leukemia (CML) receiving imatinib fail treatment. There is an increased risk of CML in subjects with (i) deletions of genes encoding glutathione-S-transferase (GST)-θ1 (GSTT1) and -μ1, (GSTM1) and (ii) the GST-π1 (GSTP1) single-nucleotide polymorphism (SNP) Ile105Val (GSTP1*B; rs1695); however, their effects on imatinib treatment outcome are not known. Here, we assess the role of these GSTs in relation to imatinib treatment outcome in 193 CML patients. Deletion of GSTT1 alone, or in combination with deletion of the GSTM1 gene, significantly increased the likelihood of imatinib failure (P = 0.021 and P < 0.001, respectively). The GSTP1*B SNP was not associated with time to imatinib failure. Losses of the GSTT1 and GSTM1 genes are therefore important determinants of imatinib failure in CML. Screening for GSTT1 and GSTM1 gene deletions during diagnosis may identify patients who may be better treated using an alternative therapy.

Arachidonate 15-lipoxygenase is required for chronic myeloid leukemia stem cell survival

Cancer stem cells (CSCs) are responsible for the initiation and maintenance of some types of cancer, suggesting that inhibition of these cells may limit disease progression and relapse. Unfortunately, few CSC-specific genes have been identified. Here, we determined that the gene encoding arachidonate 15-lipoxygenase (Alox15/15-LO) is essential for the survival of leukemia stem cells (LSCs) in a murine model of BCR-ABL-induced chronic myeloid leukemia (CML). In the absence of Alox15, BCR-ABL was unable to induce CML in mice. Furthermore, Alox15 deletion impaired LSC function by affecting cell division and apoptosis, leading to an eventual depletion of LSCs. Moreover, chemical inhibition of 15-LO function impaired LSC function and attenuated CML in mice. The defective CML phenotype in Alox15-deficient animals was rescued by depleting the gene encoding P-selectin, which is upregulated in Alox15-deficient animals. Both deletion and overexpression of P-selectin affected the survival of LSCs. In human CML cell lines and CD34+ cells, knockdown of Alox15 or inhibition of 15-LO dramatically reduced survival. Loss of Alox15 altered expression of PTEN, PI3K/AKT, and the transcription factor ICSBP, which are known mediators of cancer pathogenesis. These results suggest that ALOX15 has potential as a therapeutic target for eradicating LSCs in CML.

Autophagy in blood cancers: biological role and therapeutic implications

Autophagy is a cell recycling process the molecular apparatus of which has been identified over the past decade. Autophagy allows cells to survive starvation and inhospitable conditions and plays a key role in numerous physiological functions, including hematopoiesis and immune responses. In hematologic malignancies, autophagy can either act as a chemo-resistance mechanism or have tumor suppressive functions, depending on the context. In addition, autophagy is involved in other important aspects of blood cancers as it promotes immune competence and anti-cancer immunity, and may even help enhance patient tolerance to standard treatments. Approaches exploiting autophagy, either to activate or inhibit it, could find broad application in hematologic malignancies and contribute to improved clinical outcomes. These aspects are discussed here together with a brief introduction to the molecular machinery of autophagy and to its role in blood cell physiology.

Autophagy in chronic myeloid leukaemia: stem cell survival and implication in therapy

The insensitivity of Chronic Myeloid Leukaemia (CML) stem cells to Tyrosine Kinase Inhibitor (TKI) treatment is now believed to be the main reason for disease persistence experienced in patients. It has been shown that autophagy, an evolutionarily conserved catabolic process that involves degradation of unnecessary or harmful cellular components via lysosomes, is induced following TKI treatment in CML cells. Of clinical importance, autophagy inhibition, using the anti-malarial drug hydroxychloroquine (HCQ), sensitised CML cells, including primitive CML stem cells, to TKI treatment. In this review we discuss the role of autophagy in the maintenance and survival of stem cells in more detail, with a focus on its role in survival of CML stem cells and the possibility to inhibit this pathway as a way to eliminate persistent CML stem cells in vitro and in patients.

Targeting survival pathways in chronic myeloid leukaemia stem cells

Chronic myeloid leukaemia (CML) is a clonal myeloproliferative disorder characterized by the presence of a fusion oncogene BCR-ABL, which encodes a protein with constitutive TK activity. The implementation of tyrosine kinase inhibitors (TKIs) marked a major advance in CML therapy; however, there are problems with current treatment. For example, relapse occurs when these drugs are discontinued in the majority of patients who have achieved a complete molecular response on TKI and these agents are less effective in patients with mutations in the BCR-ABL kinase domain. Importantly, TKI can effectively target proliferating mature cells, but do not eradicate quiescent leukaemic stem cells (LSCs), therefore allowing disease persistence despite treatment. It is essential that alternative strategies are used to target the LSC population. BCR-ABL activation is responsible for the modulation of different signalling pathways, which allows the LSC fraction to evade cell death. Several pathways have been shown to be modulated by BCR-ABL, including PI3K/AKT/mTOR, JAK-STAT and autophagy signalling pathways. Targeting components of these survival pathways, alone or in combination with TKI, therefore represents an attractive potential therapeutic approach for targeting the LSC. However, many pathways are also active in normal stem cells. Therefore, potential targets must be validated to effectively eradicate CML stem cells while sparing normal counterparts. This review summarizes the main pathways modulated in CML stem cells, the recent developments and the use of novel drugs to target components in these pathways which may be used to target the LSC population.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Redirecting traffic using the XPO1 police

In this issue of Blood, Walker et al investigate the preclinical potential of KPT-330, an exportin-1 (XPO1, also known as chromosome maintenance protein 1 [CRM1]) inhibitor, against both accelerated phase (AP) and blast crisis chronic myeloid leukemia (CML-BC) and against Philadelphia chromosome-positive (Ph1) acute lymphoblastic leukemia (ALL), all of which are diseases of significant unmet clinical need.1 The authors provide encouraging data from both a leukemic mouse model and a single CML-AP patient, corroborating mechanistic studies suggesting that KPT-330 efficacy relies on targeting abundantly expressed XPO1, followed by the reactivation of protein phosphatase 2A (PP2A).

Hif-2α is not essential for cell-autonomous hematopoietic stem cell maintenance

Local hypoxia in hematopoietic stem cell (HSC) niches is thought to regulate HSC functions. Hypoxia-inducible factor-1 (Hif-1) and Hif-2 are key mediators of cellular responses to hypoxia. Although oxygen-regulated α-subunits of Hifs, namely Hif-1α and Hif-2α, are closely related, they play overlapping and also distinct functions in nonhematopoietic tissues. Although Hif-1α-deficient HSCs lose their activity on serial transplantation, the role for Hif-2α in cell-autonomous HSC maintenance remains unknown. Here, we demonstrate that constitutive or inducible hematopoiesis-specific Hif-2α deletion does not affect HSC numbers and steady-state hematopoiesis. Furthermore, using serial transplantations and 5-fluorouracil treatment, we demonstrate that HSCs do not require Hif-2α to self-renew and recover after hematopoietic injury. Finally, we show that Hif-1α deletion has no major impact on steady-state maintenance of Hif-2α-deficient HSCs and their ability to repopulate primary recipients, indicating that Hif-1α expression does not account for normal behavior of Hif-2α-deficient HSCs.

PP2A-activating drugs selectively eradicate TKI-resistant chronic myeloid leukemic stem cells

The success of tyrosine kinase inhibitors (TKIs) in treating chronic myeloid leukemia (CML) depends on the requirement for BCR-ABL1 kinase activity in CML progenitors. However, CML quiescent HSCs are TKI resistant and represent a BCR-ABL1 kinase-independent disease reservoir. Here we have shown that persistence of leukemic HSCs in BM requires inhibition of the tumor suppressor protein phosphatase 2A (PP2A) and expression--but not activity--of the BCR-ABL1 oncogene. Examination of HSCs from CML patients and healthy individuals revealed that PP2A activity was suppressed in CML compared with normal HSCs. TKI-resistant CML quiescent HSCs showed increased levels of BCR-ABL1, but very low kinase activity. BCR-ABL1 expression, but not kinase function, was required for recruitment of JAK2, activation of a JAK2/β-catenin survival/self-renewal pathway, and inhibition of PP2A. PP2A-activating drugs (PADs) markedly reduced survival and self-renewal of CML quiescent HSCs, but not normal quiescent HSCs, through BCR-ABL1 kinase-independent and PP2A-mediated inhibition of JAK2 and β-catenin. This led to suppression of human leukemic, but not normal, HSC/progenitor survival in BM xenografts and interference with long-term maintenance of BCR-ABL1-positive HSCs in serial transplantation assays. Targeting the JAK2/PP2A/β-catenin network in quiescent HSCs with PADs (e.g., FTY720) has the potential to treat TKI-refractory CML and relieve lifelong patient dependence on TKIs.

A specific PTPRC/CD45 phosphorylation event governed by stem cell chemokine CXCL12 regulates primitive hematopoietic cell motility

CXCL12 governs cellular motility, a process deregulated by hematopoietic stem cell oncogenes such as p210-BCR-ABL. A phosphoproteomics approach to the analysis of a hematopoietic progenitor cell line treated with CXCL12 and the Rac 1 and 2 inhibitor NSC23766 has been employed to objectively discover novel mechanisms for regulation of stem cells in normal and malignant hematopoiesis. The proteomic data sets identified new aspects of CXCL12-mediated signaling and novel features of stem cell regulation. We also identified a novel phosphorylation event in hematopoietic progenitor cells that correlated with motile response and governed by the chemotactic factor CXCL12. The novel phosphorylation site on PTPRC/CD45; a protein tyrosine phosphatase, was validated by raising an antibody to the site and also using a mass spectrometry absolute quantification strategy. Site directed mutagenesis and inhibitor studies demonstrated that this single phosphorylation site governs hematopoietic progenitor cell and lymphoid cell motility, lies downstream from Rac proteins and potentiates Src signaling. We have also demonstrated that PTPRC/CD45 is down-regulated in leukemogenic tyrosine kinase expressing cells. The use of discovery proteomics has enabled further understanding of the regulation of PTPRC/CD45 and its important role in cellular motility in progenitor cells.

Quantitative proteomics analysis of BMS-214662 effects on CD34 positive cells from chronic myeloid leukaemia patients

Chronic myeloid leukaemia (CML) arises in a haemopoietic stem cell and is driven by the Bcr-Abl oncoprotein. Abl kinase inhibitors (protein tyrosine kinase inhibitors) represent standard treatment for CML and induce remission in the majority of patients with early disease, however these drugs do not target leukaemic stem cells (LSCs) effectively, thus preventing cure. Previously, we identified the farnesyl transferase inhibitor BMS-214662 as a selective inducer of apoptosis in LSCs of CML patients relative to normal controls; however, the mechanism underlying LSC-specific apoptosis remains unclear. To identify pathways involved in the favourable effects of BMS-214662 in CML, we employed a proteomic approach (based on iTRAQ) to analyse changes in protein expression in response to drug treatment in the nuclear and cytoplasmic fractions of CD34(+) CML cells. The study identified 88 proteins as altered after drug treatment, which included proteins known to be involved in nucleic acid metabolism, oncogenesis, developmental processes and intracellular protein trafficking. We found that expression of Ebp1, a negative regulator of proliferation, was upregulated in the nucleus of BMS-214662-treated cells. Furthermore, proteins showing altered levels in the cytosol, such as histones, were predominantly derived from the nucleus and BMS-214662 affected expression levels of nuclear pore complex proteins. Validation of key facets of these observations suggests that drug-induced alterations in protein localisation, potentially via loss of nuclear membrane integrity, contributes to the LSC specificity of BMS-214662, possibly via Ran proteins as targets.

Megakaryocytes assemble podosomes that degrade matrix and protrude through basement membrane

Megakaryocytes give rise to platelets via extension of proplatelet arms, which are released through the vascular sinusoids into the bloodstream. Megakaryocytes and their precursors undergo varying interactions with the extracellular environment in the bone marrow during their maturation and positioning in the vascular niche. We demonstrate that podosomes are abundant in primary murine megakaryocytes adherent on multiple extracellular matrix substrates, including native basement membrane. Megakaryocyte podosome lifetime and density, but not podosome size, are dependent on the type of matrix, with podosome lifetime dramatically increased on collagen fibers compared with fibrinogen. Podosome stability and dynamics depend on actin cytoskeletal dynamics but not matrix metalloproteases. However, podosomes degrade matrix and appear to be important for megakaryocytes to extend protrusions across a native basement membrane. We thus demonstrate for the first time a fundamental requirement for podosomes in megakaryocyte process extension across a basement membrane, and our results suggest that podosomes may have a role in proplatelet arm extension or penetration of basement membrane.

Targeting primitive chronic myeloid leukemia cells by effective inhibition of a new AHI-1-BCR-ABL-JAK2 complex

Background

Imatinib mesylate (IM) induces clinical remission of chronic myeloid leukemia (CML). The Abelson helper integration site 1 (AHI-1) oncoprotein interacts with BCR-ABL and Janus kinase 2 (JAK2) to mediate IM response of primitive CML cells, but the effect of the interaction complex on the response to ABL and JAK2 inhibitors is unknown.

Methods

The AHI-1–BCR-ABL–JAK2 interaction complex was analyzed by mutational analysis and coimmunoprecipitation. Roles of the complex in regulation of response or resistance to ABL and JAK2 inhibitors were investigated in BCR-ABL⁺ cells and primary CML stem/progenitor cells and in immunodeficient NSG mice. All statistical tests were two-sided.

Results

The WD40-repeat domain of AHI-1 interacts with BCR-ABL, whereas the N-terminal region interacts with JAK2; loss of these interactions statistically significantly increased the IM sensitivity of CML cells. Disrupting this complex with a combination of IM and an orally bioavailable selective JAK2 inhibitor (TG101209 [TG]) statistically significantly induced death of AHI-1–overexpressing and IM-resistant cells in vitro and enhanced survival of leukemic mice, compared with single agents (combination vs TG alone: 63 vs 53 days, ratio = 0.84, 95% confidence interval [CI] = 0.6 to 1.1, P = .004; vs IM: 57 days, ratio = 0.9, 95% CI = 0.61 to 1.2, P = .003). Combination treatment also statistically significantly enhanced apoptosis of CD34⁺ leukemic stem/progenitor cells and eliminated their long-term leukemia-initiating activity in NSG mice. Importantly, this approach was effective against treatment-naive CML stem cells from patients who subsequently proved to be resistant to IM therapy.

Conclusions

Simultaneously targeting BCR-ABL and JAK2 activities in CML stem/progenitor cells may improve outcomes in patients destined to develop IM resistance.

Role of autophagy in cancer prevention, development and therapy

Autophagy is a process that takes place in all mammalian cells and ensures homoeostasis and quality control. The term autophagy [self (auto)-eating (phagy)] was first introduced in 1963 by Christian de Duve, who discovered the involvement of lysosomes in the autophagy process. Since then, substantial progress has been made in understanding the molecular mechanism and signalling regulation of autophagy and several reviews have been published that comprehensively summarize these findings. The role of autophagy in cancer has received a lot of attention in the last few years and autophagy modulators are now being tested in several clinical trials. In the present chapter we aim to give a brief overview of recent findings regarding the mechanism and key regulators of autophagy and discuss the important physiological role of mammalian autophagy in health and disease. Particular focus is given to the role of autophagy in cancer prevention, development and in response to anticancer therapy. In this regard, we also give an updated list and discuss current clinical trials that aim to modulate autophagy, alone or in combination with radio-, chemo- or targeted therapy, for enhanced anticancer intervention.

Lineage tracing of Pf4-Cre marks hematopoietic stem cells and their progeny

The development of a megakaryocyte lineage specific Cre deleter, using the Pf4 (CXCL4) promoter (Pf4-Cre), was a significant step forward in the specific analysis of platelet and megakaryocyte cell biology. However, in the present study we have employed a sensitive reporter-based approach to demonstrate that Pf4-Cre also recombines in a significant proportion of both fetal liver and bone marrow hematopoietic stem cells (HSCs), including the most primitive fraction containing the long-term repopulating HSCs. Consequently, we demonstrate that Pf4-Cre activity is not megakaryocyte lineage-specific but extends to other myeloid and lymphoid lineages at significant levels between 15-60%. Finally, we show for the first time that Pf4 transcripts are present in adult HSCs and primitive hematopoietic progenitor cells. These results have fundamental implications for the use of the Pf4-Cre mouse model and for our understanding of a possible role for Pf4 in the development of the hematopoietic lineage.

Mechanisms and novel approaches in overriding tyrosine kinase inhibitor resistance in chronic myeloid leukemia

Chronic myeloid leukemia is a stem cell-initiated but progenitor-driven disease induced by the BCR-ABL oncogene. Tyrosine kinase inhibitors (TKIs) were introduced in the late 1990s and have revolutionized the management of chronic myeloid leukemia in chronic phase. The majority of patients can now expect to live a normal life as long as they continue to comply with TKI treatment. However, in a significant proportion of cases TKI resistance develops over time, requiring a switch of therapy. The most frequent mechanism for drug resistance is the development of kinase domain mutations that reduce or completely ablate drug efficacy. Fortunately, the last 10 years have seen an impressive array of new drugs, some modeled on the mechanism of action of imatinib, others employing more novel approaches, for these patients.

Expression of p89(c-Mybex9b), an alternatively spliced form of c-Myb, is required for proliferation and survival of p210BCR/ABL-expressing cells

The c-Myb gene encodes the p75^c-Myb isoform and less-abundant proteins generated by alternatively spliced transcripts. Among these, the best known is p^c-Mybex9b, which contains 121 additional amino acids between exon 9 and 10, in a domain involved in protein–protein interactions and negative regulation. In hematopoietic cells, expression of p^c-Mybex9b accounts for 10–15% of total c-Myb; these levels may be biologically relevant because modest changes in c-Myb expression affects proliferation and survival of leukemic cells and lineage choice and frequency of normal hematopoietic progenitors. In this study, we assessed biochemical activities of p^c-Mybex9b and the consequences of perturbing its expression in K562 and primary chronic myeloid leukemia (CML) progenitor cells. Compared with p75^c-Myb, p^c-Mybex9b is more stable and more effective in transactivating Myb-regulated promoters. Ectopic expression of p^c-Mybex9b enhanced proliferation and colony formation and reduced imatinib (IM) sensitivity of K562 cells; conversely, specific downregulation of p^c-Mybex9b reduced proliferation and colony formation, enhanced IM sensitivity of K562 cells and markedly suppressed colony formation of CML CD34⁺ cells, without affecting the levels of p75^c-Myb. Together, these studies indicate that expression of the low-abundance p^c-Mybex9b isoform has an important role for the overall biological effects of c-Myb in BCR/ABL-transformed cells.

Rac2-MRC-cIII-generated ROS cause genomic instability in chronic myeloid leukemia stem cells and primitive progenitors

Chronic myeloid leukemia in chronic phase (CML-CP) is induced by BCR-ABL1 oncogenic tyrosine kinase. Tyrosine kinase inhibitors eliminate the bulk of CML-CP cells, but fail to eradicate leukemia stem cells (LSCs) and leukemia progenitor cells (LPCs) displaying innate and acquired resistance, respectively. These cells may accumulate genomic instability, leading to disease relapse and/or malignant progression to a fatal blast phase. In the present study, we show that Rac2 GTPase alters mitochondrial membrane potential and electron flow through the mitochondrial respiratory chain complex III (MRC-cIII), thereby generating high levels of reactive oxygen species (ROS) in CML-CP LSCs and primitive LPCs. MRC-cIII-generated ROS promote oxidative DNA damage to trigger genomic instability, resulting in an accumulation of chromosomal aberrations and tyrosine kinase inhibitor-resistant BCR-ABL1 mutants. JAK2(V617F) and FLT3(ITD)-positive polycythemia vera cells and acute myeloid leukemia cells also produce ROS via MRC-cIII. In the present study, inhibition of Rac2 by genetic deletion or a small-molecule inhibitor and down-regulation of mitochondrial ROS by disruption of MRC-cIII, expression of mitochondria-targeted catalase, or addition of ROS-scavenging mitochondria-targeted peptide aptamer reduced genomic instability. We postulate that the Rac2-MRC-cIII pathway triggers ROS-mediated genomic instability in LSCs and primitive LPCs, which could be targeted to prevent the relapse and malignant progression of CML.

Altered microenvironmental regulation of leukemic and normal stem cells in chronic myelogenous leukemia

We characterized leukemia stem cells (LSC) in chronic phase chronic myelogenous leukemia (CML) using a transgenic mouse model. LSC were restricted to cells with long-term hematopoietic stem cell (LTHSC) phenotype. CML LTHSC demonstrated reduced homing and retention in the bone marrow (BM), related to decreased CXCL12 expression in CML BM, resulting from increased G-CSF production by leukemia cells. Altered cytokine expression in CML BM was associated with selective impairment of normal LTHSC growth and a growth advantage to CML LTHSC. Imatinib (IM) treatment partially corrected abnormalities in cytokine levels and LTHSC growth. These results were validated using human CML samples and provide improved understanding of microenvironmental regulation of normal and leukemic LTHSC and their response to IM in CML.