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Malignant stem cells have long been considered a key therapeutic target in leukemia. Therapeutic strategies designed to target the fundamental biology of leukemia stem cells while sparing normal hematopoietic cells may provide better outcomes for leukemia patients. One process in leukemia stem cell biology that has intriguing therapeutic potential is energy metabolism. In this article we discuss the metabolic properties of leukemia stem cells and how targeting energy metabolism may provide more effective therapeutic regimens for leukemia patients. In addition, we highlight the similarities and differences in energy metabolism between leukemia stem cells and malignant stem cells from solid tumors.

Background Patients with relapsed and refractory (R/R) acute myeloid leukemia (AML) have limited treatment options. Genomically-defined personalized therapies are only applicable for a minority of patients. Therapies without identifiable targets can be effective but patient selection is challenging. The sequential combination of azacitidine with high-dose lenalidomide has shown activity; we aimed to determine the efficacy of this genomically-agnostic regimen in patients with R/R AML, with the intention of applying sophisticated methods to predict responders. Methods Thirty-seven R/R AML/myelodysplastic syndrome patients were enrolled in a phase 2 study of azacitidine with lenalidomide. The primary endpoint was complete remission (CR) and CR with incomplete blood count recovery (CRi) rate. A computational biological modeling (CBM) approach was applied retrospectively to predict outcomes based on the understood mechanisms of azacitidine and lenalidomide in the setting of each patients’ disease. Findings Four of 37 patients (11%) had a CR/CRi; the study failed to meet the alternative hypothesis. Significant toxicity was observed in some cases, with three treatment-related deaths and a 30-day mortality rate of 14%. However, the CBM method predicted responses in 83% of evaluable patients, with a positive and negative predictive value of 80% and 89%, respectively. Interpretation Sequential azacitidine and high-dose lenalidomide is effective in a minority of R/R AML patients; it may be possible to predict responders at the time of diagnosis using a CBM approach. More efforts to predict responses in non-targeted therapies should be made, to spare toxicity in patients unlikely to respond and maximize treatments for those with limited options.

Acute myeloid leukemia (AML) is a cancer of myeloid-lineage cells with limited therapeutic options. We previously combined ex vivo drug sensitivity with genomic, transcriptomic, and clinical annotations for a large cohort of AML patients, which facilitated discovery of functional genomic correlates. Here, we present a dataset that has been harmonized with our initial report to yield a cumulative cohort of 805 patients (942 specimens). We show strong cross-cohort concordance and identify features of drug response. Further, deconvoluting transcriptomic data shows that drug sensitivity is governed broadly by AML cell differentiation state, sometimes conditionally affecting other correlates of response. Finally, modeling of clinical outcome reveals a single gene, PEAR1, to be among the strongest predictors of patient survival, especially for young patients. Collectively, this report expands a large functional genomic resource, offers avenues for mechanistic exploration and drug development, and reveals tools for predicting outcome in AML.

Melampomagnolide B (MMB, 3) is a parthenolide (PTL, 1) based sesquiterpene lactone that has been used as a template for the synthesis of a plethora of lead anticancer agents owing to its reactive C-10 primary hydroxyl group. Such compounds have been shown to inhibit the IKKβ subunit, preventing phosphorylation of the cytoplasmic IκB inhibitory complex. The present study focuses on the synthesis and in vitro antitumor properties of novel benzyl and phenethyl carbamates of MMB (7a-7k). Screening of these MMB carbamates identified analogs with potent growth inhibition properties against a panel of 60 human cancer cell lines (71% of the molecules screened had GI(50) values

A series of succinamide derivatives of melampomagnolide B have been synthesized by coupling MMB monosuccinate (2) with various heterocyclic amines to afford compounds 3a–3l. MMB monosuccinate was also reacted with terminal diaminoalkanes to afford dimeric succinamido analogs of MMB (4a–4h). These succinamide analogs of MMB were evaluated for their anti-cancer activity against a panel of sixty human cancer cell lines. Analogs 3d–3i and dimers 4f–4g exhibited promising anti-cancer activity with GI50 values ranging from 0.28-33.5 μM against most of the cell lines in the panel. The dimeric analogs 4f and 4g were identified as lead compounds with GI50 values in the nanomolar range (GI50 = 280–980 nM) against several cell lines in the panel; i.e. leukemia cell lines CCRF-CEM, HL-60(TB), K-562, MOLT-4, RPMI-8226 and SR; and solid tumor cell lines NCI-H522 (non-small cell lung cancer), SW-620 and HCT-116 (colon cancer), LOX IMVI (melanoma), RXF 393 (renal cancer), and MCF7, BT-549 and MDA-MB-468 (breast cancer). Succinamide analogs 3a, 3c–3l and 4b–4h were also evaluated for their apoptotic activity against M9-ENL1 acute myelogenous leukemia cells; compounds 3h–3j and 4g were equipotent with parthenolide, exhibiting LC50 values in the range 4.1–8.1 μM. Molecular docking studies indicate that these molecules interact covalently with the highly conserved Cys-46 residue of the N-terminal lobe (1–109) of human IKKβ to inhibit the NFκB transcription factor complex, resulting in down-regulation of anti-apoptotic genes under NFκB control.

Although multidrug approaches to cancer therapy are common, few strategies are based on rigorous scientific principles. Rather, drug combinations are largely dictated by empirical or clinical parameters. In the present study we developed a strategy for rational design of a regimen that selectively targets human acute myelogenous leukemia (AML) stem cells. As a starting point, we used parthenolide, an agent shown to target critical mechanisms of redox balance in primary AML cells. Next, using proteomic, genomic, and metabolomic methods, we determined that treatment with parthenolide leads to induction of compensatory mechanisms that include up-regulated NADPH production via the pentose phosphate pathway as well as activation of the Nrf2-mediated oxidative stress response pathway. Using this knowledge we identified 2-deoxyglucose and temsirolimus as agents that can be added to a parthenolide regimen as a means to inhibit such compensatory events and thereby further enhance eradication of AML cells. We demonstrate that the parthenolide, 2-deoxyglucose, temsirolimus (termed PDT) regimen is a potent means of targeting AML stem cells but has little to no effect on normal stem cells. Taken together our findings illustrate a comprehensive approach to designing combination anticancer drug regimens.

Parthenolide is a naturally occurring terpene with promising anticancer properties, in particular in the context of acute myeloid leukemia (AML). Optimization of this natural product has been challenged by limited opportunities for the late-stage functionalization of this molecule without affecting the pharmacologically important α-methylene-γ-lactone moiety. Here, we report the further development and application of a chemoenzymatic strategy to afford a series of new analogs of parthenolide functionalized at the aliphatic positions C9 and C14. Several of these compounds were determined to be able to kill leukemia cells and patient-derived primary AML specimens with improved activity compared to parthenolide, exhibiting LC50 values in the low micromolar range. These studies demonstrate that different O−H functionalization chemistries can be applied to elaborate the parthenolide scaffold and that modifications at the C9 or C14 position can effectively enhance the antileukemic properties of this natural product. The C9-functionalized analogs 22a and 25b were identified as the most interesting compounds in terms of antileukemic potency and selectivity toward AML versus healthy blood cells.

Adipose tissue (AT) has previously been identified as an extra-medullary reservoir for normal hematopoietic stem cells (HSCs) and may promote tumor development. Here, we show that a subpopulation of leukemic stem cells (LSCs) can utilize gonadal adipose tissue (GAT) as a niche to support their metabolism and evade chemotherapy. In a mouse model of blast crisis chronic myeloid leukemia (CML), adipose-resident LSCs exhibit a pro-inflammatory phenotype and induce lipolysis in GAT. GAT lipolysis fuels fatty acid oxidation in LSCs, especially within a subpopulation expressing the fatty acid transporter CD36. CD36(+) LSCs have unique metabolic properties, are strikingly enriched in AT, and are protected from chemotherapy by the GAT microenvironment. CD36 also marks a fraction of human blast crisis CML and acute myeloid leukemia (AML) cells with similar biological properties. These findings suggest striking interplay between leukemic cells and AT to create a unique microenvironment that supports the metabolic demands and survival of a distinct LSC subpopulation.

Hematopoietic stem cell (HSC) quiescence supports lifelong blood regeneration and guards against pre-leukemic clonal expansion. Acute exposure to the pro-inflammatory cytokine interleukin (IL)-1 drives myeloid cell production and HSC cell cycle entry. However, HSC return to a quiescent state suggesting the presence of a ‘braking’ mechanism that limits HSC proliferative capacity during chronic inflammation. To identify mechanism(s) regulating HSC cell cycle activity, we injected mice with IL-1 for 20 days modeling chronic inflammation in vivo. RNA-seq analysis of HSC following IL-1 exposure revealed repression of cell cycle and protein synthesis genes, suggesting the activation of a ‘growth arrest’ gene program. This gene program coincided with increased PU.1 expression, and ChIP-seq analysis identified PU.1 binding on nearly all repressed genes, suggesting PU.1 enforces HSC quiescence during chronic inflammation. Strikingly, HSC from IL-1 treated PU.1-deficient mice exhibited loss of quiescence associated with aberrant myeloid expansion in the bone marrow and spleen. Together, our results suggest PU.1 induction maintains HSC quiescence under chronic inflammatory stress. They also suggest IL-1 may confer a competitive advantage to HSC with impaired PU.1 function, triggering aberrant proliferation and myeloid expansion. Several oncogenic mutations found in acute myelogenous leukemia (AML) impair the expression and/or function of PU.1. As AML pre-dominantly affects elderly individuals and leukemogenesis is often associated with chronic inflammation, our data supports a model where chronic inflammation triggers the selective expansion of HSC harboring oncogenic mutations, leading to a pre-leukemic state. Thus, blockade of inflammation may be a tractable approach to delay and/or prevent leukemia.

Myelodysplastic syndrome (MDS) is a group of heterogeneous disorders caused by ineffective hematopoiesis and characterized by bone marrow dysplasia and cytopenia. Current treatment options for MDS are limited to supportive care, hypomethylating agents, and stem cell transplant. Most patients eventually succumb to the disease or progress to leukemia. Previously, we found that CD123 can be used to delineate MDS stem cells in patients at high risk for MDS and that the CD123-positive population is biologically distinct from normal hematopoietic stem cells. Furthermore, selective targeting of MDS stem cells can dramatically reduce tumor burden in preclinical models. On the basis of these findings, we propose CD123 as a candidate target for chimeric antigen receptor (CAR) T-cell therapy in high-risk MDS patients. To test this concept, we employed a CAR lentiviral vector containing a CD123-specific single-chain variable fragment in combination with the CD28 costimulatory domain, CD3ζ signaling domain, and truncated estimated glomerular filtration rate. Utilizing this system, we illustrate that CD123 CAR can be expressed on both healthy donor and MDS patient-derived T lymphocytes with high efficiency, leading to the successful elimination of MDS stem cells both in vitro and in patient-derived xenografts. These results provide the concept for the use of CD123-targeted CAR T cells as a therapeutic option for patients with MDS.

SUMMARY The NADPH-dependent oxidase NOX2 is an important effector of immune cell function, and its activity has been linked to oncogenic signaling. Here, we describe a role for NOX2 in leukemia-initiating stem cell populations (LSCs). In a murine model of leukemia, suppression of NOX2 impaired core metabolism, attenuated disease development, and depleted functionally defined LSCs. Transcriptional analysis of purified LSCs revealed that deficiency of NOX2 collapses the self-renewal program and activates inflammatory and myeloid-differentiation-associated programs. Downstream of NOX2, we identified the forkhead transcription factor FOXC1 as a mediator of the phenotype. Notably, suppression of NOX2 or FOXC1 led to marked differentiation of leukemic blasts. In xenotransplantation models of primary human myeloid leukemia, suppression of either NOX2 or FOXC1 significantly attenuated disease development. Collectively, these findings position NOX2 as a critical regulator of malignant hematopoiesis and highlight the clinical potential of inhibiting NOX2 as a means to target LSCs., Graphical Abstract, In Brief The NADPH-dependent oxidase NOX2 is important for normal myeloid cell function. Adane et al. show that NOX2 is expressed in leukemic stem cells, where it regulates the balance of myeloid differentiation and self-renewal. Deficiency of NOX2 altered core metabolism, exacerbated inflammatory signaling, and limited in vivo disease development.

A phase I study utilizing decitabine (DAC) followed by the mammalian target of rapamycin (mTOR) inhibitor, rapamycin, in patients with relapsed/refractory adult AML was undertaken to assess safety and feasibility. Patients received DAC 20mg/m(2) intravenously daily for 5 days followed by rapamycin from day 6 to day 25 at doses of 2 mg, 4 mg, and 6 mg/day in a standard 3+3 dose escalation design. Twelve patients completed treatment for safety evaluation. Maximum tolerated dose (MTD) was not reached, and except for grade 3 mucositis in 4 patients, no other significant unexpected non-hematologic toxicities have occurred indicating safety of this regimen. This trial is registered at clinical trials.gov as NCT00861874.

The development of strategies to eradicate primary human acute myelogenous leukemia (AML) cells is a major challenge to the leukemia research field. In particular, primitive leukemia cells, often termed leukemia stem cells, are typically refractory to many forms of therapy. To investigate improved strategies for targeting of human AML cells we compared the molecular mechanisms regulating oxidative state in primitive (CD34(+)) leukemic versus normal specimens. Our data indicate that CD34(+) AML cells have elevated expression of multiple glutathione pathway regulatory proteins, presumably as a mechanism to compensate for increased oxidative stress in leukemic cells. Consistent with this observation, CD34(+) AML cells have lower levels of reduced glutathione and increased levels of oxidized glutathione compared with normal CD34(+) cells. These findings led us to hypothesize that AML cells will be hypersensitive to inhibition of glutathione metabolism. To test this premise, we identified compounds such as parthenolide (PTL) or piperlongumine that induce almost complete glutathione depletion and severe cell death in CD34(+) AML cells. Importantly, these compounds only induce limited and transient glutathione depletion as well as significantly less toxicity in normal CD34(+) cells. We further determined that PTL perturbs glutathione homeostasis by a multifactorial mechanism, which includes inhibiting key glutathione metabolic enzymes (GCLC and GPX1), as well as direct depletion of glutathione. These findings demonstrate that primitive leukemia cells are uniquely sensitive to agents that target aberrant glutathione metabolism, an intrinsic property of primary human AML cells.

SummaryWe used an in vivo small hairpin RNA (shRNA) screening approach to identify genes that are essential for MLL-AF9 acute myeloid leukemia (AML). We found that Integrin Beta 3 (Itgb3) is essential for murine leukemia cells in vivo and for human leukemia cells in xenotransplantation studies. In leukemia cells, Itgb3 knockdown impaired homing, downregulated LSC transcriptional programs, and induced differentiation via the intracellular kinase Syk. In contrast, loss of Itgb3 in normal hematopoietic stem and progenitor cells did not affect engraftment, reconstitution, or differentiation. Finally, using an Itgb3 knockout mouse model, we confirmed that Itgb3 is dispensable for normal hematopoiesis but is required for leukemogenesis. Our results establish the significance of the Itgb3 signaling pathway as a potential therapeutic target in AML.

There are a number of approaches for selective targeting of leukemic stem cells (LSCs). These include targeting stem-cell properties, such as self-renewal, inducing cycling of quiescent LSCs to sensitize them to conventional agents, employing or inducing immune-based mechanisms, and targeting tumor-specific physiology. Agents such as parthenolide inhibit the ability of leukemic stem cells to respond to oxidative stress and make leukemic stem cells and bulk leukemic cells susceptible to cell death, while normal stem cells remain relatively unharmed by these agents. The major mechanism of action of these small molecules appears to revolve around the aberrant glutathione metabolism pathway found in leukemic cells.

SummaryLeukemia stem cells (LSCs) represent a biologically distinct subpopulation of myeloid leukemias, with reduced cell cycle activity and increased resistance to therapeutic challenge. To better characterize key properties of LSCs, we employed a strategy based on identification of genes synergistically dysregulated by cooperating oncogenes. We hypothesized that such genes, termed “cooperation response genes” (CRGs), would represent regulators of LSC growth and survival. Using both a primary mouse model and human leukemia specimens, we show that CRGs comprise genes previously undescribed in leukemia pathogenesis in which multiple pathways modulate the biology of LSCs. In addition, our findings demonstrate that the CRG expression profile can be used as a drug discovery tool for identification of compounds that selectively target the LSC population. We conclude that CRG-based analyses provide a powerful means to characterize the basic biology of LSCs as well as to identify improved methods for therapeutic targeting.

Psoriasis is a common, immune-mediated genetic disorder of the skin and is associated with arthritis in approximately 30% of cases. Previously, we localized PSORS2 (psoriasis susceptibility locus 2) to chromosomal region 17q25.3-qter after a genome-wide linkage scan in a family of European ancestry with multiple cases of psoriasis and psoriatic arthritis. Linkage to PSORS2 was also observed in a Taiwanese family with multiple psoriasis-affected members. In caspase recruitment domain family, member 14 (CARD14), we identified unique gain-of-function mutations that segregated with psoriasis by using genomic capture and DNA sequencing. The mutations c.349G>A (p.Gly117Ser) (in the family of European descent) and c.349+5G>A (in the Taiwanese family) altered splicing between CARD14 exons 3 and 4. A de novo CARD14 mutation, c.413A>C (p.Glu138Ala), was detected in a child with sporadic, early-onset, generalized pustular psoriasis. CARD14 activates nuclear factor kappa B (NF-kB), and compared with wild-type CARD14, the p.Gly117Ser and p.Glu138Ala substitutions were shown to lead to enhanced NF-kB activation and upregulation of a subset of psoriasis-associated genes in keratinocytes. These genes included chemokine (C-C motif) ligand 20 (CCL20) and interleukin 8 (IL8). CARD14 is localized mainly in the basal and suprabasal layers of healthy skin epidermis, whereas in lesional psoriatic skin, it is reduced in the basal layer and more diffusely upregulated in the suprabasal layers of the epidermis. We propose that, after a triggering event that can include epidermal injury, rare gain-of-function mutations in CARD14 initiate a process that includes inflammatory cell recruitment by keratinocytes. This perpetuates a vicious cycle of epidermal inflammation and regeneration, a cycle which is the hallmark of psoriasis.

Myeloid leukemias are clonal disorders originating in a primitive multipotential hematopoietic cell and characterized by aberrant proliferation, differentiation and maturation of leukemic progenitors and precursor cells. These diseases are the result of multiple genetic and epigenetic events, although the nature and number of events vary widely among patients. For over four decades, studies have identified sub-populations of leukemic cells possessing different functional capabilities. Investigators have struggled to understand the origin and significance of this heterogeneity. The stem cell model for myeloid malignancies has offered one potential explanation. Since 1994, experimental data supporting the presence of leukemia stem cells has been reported and validated in numerous studies. We will review the history of the model, data from the past decade supporting the stem cell model for myeloid malignancies, more recent data regarding patient specific variability in leukemic stem cell surface antigen phenotype and the impact the stem cell model has on the care of patients with myeloid malignancies.

Melampomagnolide B has been identified as a new antileukemic sesquiterpene. A biotin-conjugated derivative of melampomagnolide B was designed and synthesized in order to elucidate its mechanism of action. A study of the biochemical interactions of the biotin probe suggests that melampomagnolide B derives its remarkable selectivity for leukemic cells over normal hematopoietic cells from its unique ability to exploit biochemical differences between the two cell types.

Summary From an organismal perspective, cancer cell populations can be considered analogous to parasites that compete with the host for essential systemic resources such as glucose. Here, we employed leukemia models and human leukemia samples to document a form of adaptive homeostasis, where malignant cells alter systemic physiology through impairment of both host insulin sensitivity and insulin secretion to provide tumors with increased glucose. Mechanistically, tumor cells induce high-level production of IGFBP1 from adipose tissue to mediate insulin sensitivity. Further, leukemia-induced gut dysbiosis, serotonin loss, and incretin inactivation combine to suppress insulin secretion. Importantly, attenuated disease progression and prolonged survival are achieved through disruption of the leukemia-induced adaptive homeostasis. Our studies provide a paradigm for systemic management of leukemic disease.

A series of aminoparthenolide analogs (6-37) were synthesized and evaluated for their anti-leukemic activity. Eight compounds exhibited good anti-leukemic activity with LD(50)'s in the low microM range (1.5-3.0microM). Compounds 16, 24 and 30 were the most potent compounds in the series, causing greater than 90% cell death at 10microM concentration against primary AML cells in culture, with LD(50) values of 1.7, 1.8 and 1.6microM.

Two key issues must be addressed in the discussion of targeting leukemic stem cells: (1) can the leukemic stem cell be targeted in vivo, and (2) how to assess whether the leukemic stem cell is actually being targeted. Currently several small molecule and antibody- or ligand-based agents have shown activity in selectively targeting the leukemic stem cell. However, there is debate about how to use these targeted agents and how to identify and quantitate the leukemic stem cell to determine whether or not it is being targeted. Parameters are suggested here to help identify and quantitate leukemic stem cells in the clinical context.

Background: Given the significant activity and tolerability of gemcitabine in patients with relapsed Hodgkin's lymphoma (HL), the critical role that nuclear factor kappa B (NF-κB) appears to play in the pathogenesis of this tumor, the ability of bortezomib to inhibit NF-κB activity, and laboratory studies suggesting synergistic antitumor effects of gemcitabine and bortezomib, we hypothesized that this combination would be efficacious in patients with relapsed or refractory HL. Patients and methods: A total of 18 patients participated. Patients received 3-week cycles of bortezomib 1 mg/m2 on days 1, 4, 8, and 11 plus gemcitabine 800 mg/m2 on days 1 and 8. Results: The overall response rate for all patients was 22% (95% confidence interval 3% to 42%). Three patients developed grade III transaminase elevation: one was removed from the study and two had doses of gemcitabine held. Almost all patients exhibited inhibition of proteasome activity with treatment. Conclusions: The combination of gemcitabine and bortezomib is a less active and more toxic regimen in relapsed HL than other currently available treatments. It poses a risk of severe liver toxicity and should be pursued with caution in other types of cancer.

We previously demonstrated that activation of the Parathyroid Hormone Receptor (PTH1R) in osteoblastic cells increases the Notch ligand Jagged1 and expands hematopoietic stem cells (HSC) through Notch signaling. However, regulation of Jagged1 by PTH in osteoblasts is poorly understood. The present study demonstrates that PTH treatment increases Jagged1 levels in a subpopulation of osteoblastic cells in vivo and in UMR106 osteoblastic cells in vitro. Since PTH(1-34) activates both Adenylate Cyclase/Protein Kinase A (AC/PKA) and Protein Kinase C (PKC) downstream of the PTH1R in osteoblastic cells, we independently determined the effect of either pathway on Jagged1. Activation of AC with Forskolin or PKA with PTH(1-31) or cell-permeable cAMP analogues increased osteoblastic Jagged1. This PTH-dependent Jagged1 increase was blocked by H89 and PKI, specific PKA inhibitors. In contrast, PKC activation with phorbol ester (PMA) or PTH(13-34) did not stimulate Jagged1 expression, and PTH-dependent Jagged1 stimulation was not blocked by Gö6976, a conventional PKC inhibitor. Therefore, PTH treatment stimulates osteoblastic Jagged1 mainly through the AC/PKA signaling pathway downstream of the PTH1R. Since Jagged1/Notch signaling has been implicated not only in stromal-HSC interactions but also in osteoblastic differentiation, Jagged1 may play a critical role in mediating the PTH-dependent expansion of HSC, as well as the anabolic effect of PTH in bone.

The biology of stem cells and their intrinsic properties are now recognized as integral to tumor pathogenesis in several types of cancer. This observation has broad ramifications in the cancer research field and is likely to impact our understanding of the basic mechanisms of tumor formation and the strategies we use to treat cancers. A role for stem cells has been demonstrated for cancers of the hematopoietic system, breast and brain. Going forward it is likely that stem cells will also be implicated in other malignancies. Hence, a detailed understanding of stem cells and how they mediate tumor pathogenesis will be critical in developing more effective cancer therapies.

JunB is important in maintaining normal hematopoietic stem cell functions, but the mechanisms underlying its activity are not well understood. In the current issue of Cancer Cell , a study by Santaguida et al. provides new insights into JunB's function and the genesis of myeloid disease.

While a broad range of expertise has recently come to bear on the intriguing topic of “cancer stem cells,” the overall relevance of stem cells as they relate to cancer remains in dispute. In this commentary, underlying points of contention are described with the aim of defining focal points for discussion and future consideration.

Significant advances in the use of genetic and molecular biology strategies have recently begun to identify genes that have a major impact on the determination, commitment and developmental potential of hematopoietic stem cells. Using a variety of experimental strategies, genes such as SCL, GATA-2, HoxB4, Flk-2, c-mpl, dlk, and others have been implicated as important regulators of stem cell growth. In addition, genetic mapping has identified several loci that correlate strongly with stem cell numbers and proliferation.

Defining the characteristics of leukemia stem cells is critical in order to better understand both the genesis of leukemic disease and strategies by which such cells may be eradicated. In this issue of Cancer Cell, Somervaille and Cleary describe studies in which the properties of malignant stem cells are elucidated in a mouse model of leukemia induced by expression of the MLL-AF9 translocation. Biological features of leukemia stem cells in this system challenge previous thinking in several ways and suggest an unexpected degree of heterogeneity among stem cells in various forms of leukemia.

Chronic myelogenous leukemia (CML) can progress from an indolent chronic phase to an aggressive blast crisis phase but the molecular basis of this transition remains poorly understood. Here we have used mouse models of CML to show that disease progression is regulated by the Musashi-Numb signaling axis. Specifically we find that chronic phase is marked by high and blast crisis phase by low levels of Numb expression, and that ectopic expression of Numb promotes differentiation and impairs advanced phase disease in vivo. As a possible explanation for the decreased levels of Numb in blast crisis phase, we show that NUP98-HOXA9, an oncogene associated with blast crisis CML, can trigger expression of Musashi 2 (Msi2) an RNA binding protein which in turn represses Numb. Importantly, loss of Msi2 significantly impairs the development and propagation of blast crisis CML in vitro and in vivo. Finally, we show that Msi2 expression is not only highly upregulated during human CML progression but is also an early indicator of poorer prognosis. These data show that the Musashi-Numb pathway can control the differentiation of CML cells, and raise the possibility that targeting this pathway may provide a new strategy for therapy of aggressive leukemias.