The IL-7R antagonist Lusvertikimab reduces leukemic burden in xenograft-ALL via antibody-dependent cellular phagocytosis.

Acute lymphoblastic leukemia (ALL) arises from the uncontrolled proliferation of precursor B or T cells (BCP- or T-ALL). Current treatment protocols obtain high cure rates in children but are based on toxic polychemotherapy. Novel therapies are urgently needed, especially in relapsed/refractory (r/r) disease, high-risk leukemias and T-ALL, where immunotherapy approaches remain scarce. While the Interleukin-7 receptor (IL-7R) plays a pivotal role in ALL development, no IL-7R-targeting immunotherapy has yet reached clinical application in ALL. The IL-7Rα chain (CD127)-targeting IgG4 antibody Lusvertikimab (formerly OSE-127) is a full antagonist of the IL-7R pathway showing a good safety profile in healthy volunteers. Here, we show that ~85% of ALL cases express surface CD127. We demonstrate significant in vivo efficacy of Lusvertikimab immunotherapy in a heterogeneous cohort of BCP- and T-ALL patient-derived xenografts (PDX) in minimal residual disease (MRD) and overt leukemia models, including r/r and high-risk leukemias. Importantly, Lusvertikimab was particularly effective when combined with polychemotherapy in a phase 2-like PDX study with CD127high samples leading to MRD-negativity in >50% of mice treated with combination therapy. Mechanistically, Lusvertikimab targeted ALL cells via a dual mode of action comprising direct IL-7R antagonistic activity and induction of macrophage-mediated antibody-dependent cellular phagocytosis (ADCP). Lusvertikimab-mediated in vitro ADCP levels significantly correlated with CD127 expression levels and the reduction of leukemia burden upon treatment of PDX animals in vivo. Altogether, through its dual mode of action and good safety profile, Lusvertikimab may represent a novel immunotherapy option for any CD127-positive ALL, particularly in combination with standard-of-care polychemotherapy.

CLEC-1 Restrains Acute Inflammatory Response and Recruitment of Neutrophils following Tissue Injury.

The inflammatory response is a key mechanism for the elimination of injurious agents but must be tightly controlled to prevent additional tissue damage and progression to persistent inflammation. C-type lectin receptors expressed mostly by myeloid cells play a crucial role in the regulation of inflammation by recognizing molecular patterns released by injured tissues. We recently showed that the C-type lectin receptor CLEC-1 is able to recognize necrotic cells. However, its role in the acute inflammatory response following tissue damage had not yet been investigated. We show in this study, in a mouse model of liver injury induced by acetaminophen intoxication, that Clec1a deficiency enhances the acute immune response with increased expression of Il1b, Tnfa, and Cxcl2 and higher infiltration of activated neutrophils into the injured organ. Furthermore, we demonstrate that Clec1a deficiency exacerbates tissue damage via CXCL2-dependent neutrophil infiltration. In contrast, we observed that the lack of CLEC-1 limits CCL2 expression and the accumulation, beyond the peak of injury, of monocyte-derived macrophages. Mechanistically, we found that Clec1a-deficient dendritic cells increase the expression of Il1b, Tnfa, and Cxcl2 in response to necrotic cells, but decrease the expression of Ccl2. Interestingly, treatment with an anti-human CLEC-1 antagonist mAb recapitulates the exacerbation of acute immunopathology observed by genetic loss of Clec1a in a preclinical humanized mouse model. To conclude, our results demonstrate that CLEC-1 is a death receptor limiting the acute inflammatory response following injury and represents a therapeutic target to modulate immunity.

First-in-Human Study in Healthy Subjects with the Noncytotoxic Monoclonal Antibody OSE-127, a Strict Antagonist of IL-7Rα.

OSE-127 is a humanized mAb targeting the IL-7Rα-chain (CD127), under development for inflammatory and autoimmune disease treatment. It is a strict antagonist of the IL-7R pathway, is not internalized by target cells, and is noncytotoxic. In this work, a first-in-human, phase I, randomized, double-blind, placebo-controlled, single-center study was carried out to determine the safety, pharmacokinetics, pharmacodynamics, and immunogenicity of OSE-127 administration. Sixty-three healthy subjects were randomly assigned to nine groups: six single ascending dose groups with i.v. administration (0.002-10 mg/kg), a single s.c. treatment group (1 mg/kg), and two double i.v. injection groups (6 or 10 mg/kg). Subjects were followed during <146 d. OSE-127's pharmacokinetic half-life after a single dose increased from 4.6 (1 mg/kg) to 11.7 d (10 mg/kg) and, after a second dose, from 12.5 (6 mg/kg) to 16.25 d (10 mg/kg). Receptor occupancy was ≥95% at doses ≥0.02 mg/kg, and this saturation level was maintained >100 d after two i.v. infusions at 10 mg/kg. IL-7 consumption was inhibited by OSE-127 administration, as demonstrated by a decreased IL-7 pathway gene signature in peripheral blood cells and by ex vivo T lymphocyte restimulation experiments. OSE-127 was well tolerated, with no evidence of cytokine-release syndrome and no significant alteration of blood lymphocyte counts or subset populations. Altogether, the observed lack of significant lymphopenia or serious adverse events, concomitant with the dose-dependent inhibition of IL-7 consumption by target cells, highlights that OSE-127 may show clinical activity in IL-7R pathway-involved diseases.

CLEC-1 is a death sensor that limits antigen cross-presentation by dendritic cells and represents a target for cancer immunotherapy.

Tumors exploit numerous immune checkpoints, including those deployed by myeloid cells to curtail antitumor immunity. Here, we show that the C-type lectin receptor CLEC-1 expressed by myeloid cells senses dead cells killed by programmed necrosis. Moreover, we identified Tripartite Motif Containing 21 (TRIM21) as an endogenous ligand overexpressed in various cancers. We observed that the combination of CLEC-1 blockade with chemotherapy prolonged mouse survival in tumor models. Loss of CLEC-1 reduced the accumulation of immunosuppressive myeloid cells in tumors and invigorated the activation state of dendritic cells (DCs), thereby increasing T cell responses. Mechanistically, we found that the absence of CLEC-1 increased the cross-presentation of dead cell-associated antigens by conventional type-1 DCs. We identified antihuman CLEC-1 antagonist antibodies able to enhance antitumor immunity in CLEC-1 humanized mice. Together, our results demonstrate that CLEC-1 acts as an immune checkpoint in myeloid cells and support CLEC-1 as a novel target for cancer immunotherapy.

Vesicular trafficking is a key determinant of the statin response in acute myeloid leukemia.

Cholesterol homeostasis has been proposed as one mechanism contributing to chemoresistance in AML and hence, inclusion of statins in therapeutic regimens as part of clinical trials in AML has shown encouraging results. Chemical screening of primary human AML specimens by our group led to the identification of lipophilic statins as potent inhibitors of AMLs from a wide range of cytogenetic groups. Genetic screening to identify modulators of the statin response uncovered the role of protein geranylgeranylation and of RAB proteins, coordinating various aspect of vesicular trafficking, in mediating the effects of statins on AML cell viability. We further show that statins can inhibit vesicle-mediated transport in primary human specimens, and that statins sensitive samples show expression signatures reminiscent of enhanced vesicular trafficking. Overall, this study sheds light into the mechanism of action of statins in AML and identifies a novel vulnerability for cytogenetically diverse AML.

Mubritinib Targets the Electron Transport Chain Complex I and Reveals the Landscape of OXPHOS Dependency in Acute Myeloid Leukemia.

To identify therapeutic targets in acute myeloid leukemia (AML), we chemically interrogated 200 sequenced primary specimens. Mubritinib, a known ERBB2 inhibitor, elicited strong anti-leukemic effects in vitro and in vivo. In the context of AML, mubritinib functions through ubiquinone-dependent inhibition of electron transport chain (ETC) complex I activity. Resistance to mubritinib characterized normal CD34+ hematopoietic cells and chemotherapy-sensitive AMLs, which displayed transcriptomic hallmarks of hypoxia. Conversely, sensitivity correlated with mitochondrial function-related gene expression levels and characterized a large subset of chemotherapy-resistant AMLs with oxidative phosphorylation (OXPHOS) hyperactivity. Altogether, our work thus identifies an ETC complex I inhibitor and reveals the genetic landscape of OXPHOS dependency in AML.

Chemogenomic Landscape of RUNX1-mutated AML Reveals Importance of RUNX1 Allele Dosage in Genetics and Glucocorticoid Sensitivity.

Purpose:RUNX1-mutated (RUNX1mut) acute myeloid leukemia (AML) is associated with adverse outcome, highlighting the urgent need for a better genetic characterization of this AML subgroup and for the design of efficient therapeutic strategies for this disease. Toward this goal, we further dissected the mutational spectrum and gene expression profile of RUNX1mut AML and correlated these results to drug sensitivity to identify novel compounds targeting this AML subgroup.Experimental Design: RNA-sequencing of 47 RUNX1mut primary AML specimens was performed and sequencing results were compared to those of RUNX1 wild-type samples. Chemical screens were also conducted using RUNX1mut specimens to identify compounds selectively affecting the viability of RUNX1mut AML.Results: We show that samples with no remaining RUNX1 wild-type allele are clinically and genetically distinct and display a more homogeneous gene expression profile. Chemical screening revealed that most RUNX1mut specimens are sensitive to glucocorticoids (GCs) and we confirmed that GCs inhibit AML cell proliferation through their interaction with the glucocorticoid receptor (GR). We observed that specimens harboring RUNX1 mutations expected to result in low residual RUNX1 activity are most sensitive to GCs, and that coassociating mutations as well as GR levels contribute to GC sensitivity. Accordingly, acquired glucocorticoid sensitivity was achieved by negatively regulating RUNX1 expression in human AML cells.Conclusions: Our findings show the profound impact of RUNX1 allele dosage on gene expression profile and glucocorticoid sensitivity in AML, thereby opening opportunities for preclinical testing which may lead to drug repurposing and improved disease characterization. Clin Cancer Res; 23(22); 6969-81. ©2017 AACR.

The transcriptomic landscape and directed chemical interrogation of MLL-rearranged acute myeloid leukemias.

Using next-generation sequencing of primary acute myeloid leukemia (AML) specimens, we identified to our knowledge the first unifying genetic network common to the two subgroups of KMT2A (MLL)-rearranged leukemia, namely having MLL fusions or partial tandem duplications. Within this network, we experimentally confirmed upregulation of the gene with the most subtype-specific increase in expression, LOC100289656, and identified cryptic MLL fusions, including a new MLL-ENAH fusion. We also identified a subset of MLL fusion specimens carrying mutations in SPI1 accompanied by inactivation of its transcriptional network, as well as frequent RAS pathway mutations, which sensitized the leukemias to synthetic lethal interactions between MEK and receptor tyrosine kinase inhibitors. This transcriptomics-based characterization and chemical interrogation of human MLL-rearranged AML was a valuable approach for identifying complementary features that define this disease.

Suppression of early hematogenous dissemination of human breast cancer cells to bone marrow by retinoic Acid-induced 2.

UNLABELLED: Regulatory pathways that drive early hematogenous dissemination of tumor cells are insufficiently defined. Here, we used the presence of disseminated tumor cells (DTC) in the bone marrow to define patients with early disseminated breast cancer and identified low retinoic acid-induced 2 (RAI2) expression to be significantly associated with DTC status. Low RAI2 expression was also shown to be an independent poor prognostic factor in 10 different cancer datasets. Depletion of RAI2 protein in luminal breast cancer cell lines resulted in dedifferentiation marked by downregulation of ERα, FOXA1, and GATA3, together with increased invasiveness and activation of AKT signaling. Functional analysis of the previously uncharacterized RAI2 protein revealed molecular interaction with CtBP transcriptional regulators and an overlapping function in controlling the expression of a number of key target genes involved in breast cancer. These results suggest that RAI2 is a new metastasis-associated protein that sustains differentiation of luminal breast epithelial cells. SIGNIFICANCE: We identified downregulation of RAI2 as a novel metastasis-associated genetic alteration especially associated with early occurring bone metastasis in ERα-positive breast tumors. We specified the role of the RAI2 protein to function as a transcriptional regulator that controls the expression of several key regulators of breast epithelial integrity and cancer.

Defined conditions for the isolation and expansion of basal prostate progenitor cells of mouse and human origin.

Methods to isolate and culture primary prostate epithelial stem/progenitor cells (PESCs) have proven difficult and ineffective. Here, we present a method to grow and expand both murine and human basal PESCs long term in serum- and feeder-free conditions. The method enriches for adherent mouse basal PESCs with a Lin(-)SCA-1(+)CD49f(+)TROP2(high) phenotype. Progesterone and sodium selenite are additionally required for the growth of human Lin(-)CD49f(+)TROP2(high) PESCs. The gene-expression profiles of expanded basal PESCs show similarities to ESCs, and NF-kB function is critical for epithelial differentiation of sphere-cultured PESCs. When transplanted in combination with urogenital sinus mesenchyme, expanded mouse and human PESCs generate ectopic prostatic tubules, demonstrating their stem cell activity in vivo. This novel method will facilitate the molecular, genomic, and functional characterization of normal and pathologic prostate glands of mouse and human origin.

Co-expression of MET and CD47 is a novel prognosticator for survival of luminal breast cancer patients.

Although luminal-type primary breast cancer can be efficiently treated, development of metastatic disease remains a significant clinical problem. We have previously shown that luminal-type circulating tumor cells (CTCs) co-expressing the tyrosine-kinase MET and CD47, a ligand involved in cancer cell evasion from macrophage scavenging, are able to initiate metastasis in xenografts. Here, we investigated the clinical relevance of MET-CD47 co-expression in 255 hormone receptor positive breast tumors by immunohistochemistry and found a 10.3- year mean overall-survival difference between MET-CD47 double-positive and double-negative patients (p<0.001) MET-CD47 co-expression defined a novel independent prognosticator for overall-survival by multivariate analysis (Cox proportional hazards model: HR: 4.1, p<0.002) and CD47 expression alone or in combination with MET was strongly associated with lymph node metastasis. Furthermore, flow cytometric analysis of metastatic patient blood revealed consistent presence of MET+CD47+ CTCs (range 0.8 - 33.3% of CTCs) and their frequency was associated with increased metastatic spread. Finally, primary uncultured CTCs with high MET+CD47+ content showed an enhanced capacity to initiate metastasis in mice. Detection and targeting of MET and CD47 may thus provide a rational basis for risk stratification and treatment of patients with luminal-type breast cancer.

Serial enumeration of circulating tumor cells predicts treatment response and prognosis in metastatic breast cancer: a prospective study in 393 patients.

BACKGROUND: To prospectively assess circulating tumor cell (CTC) status at baseline (CTCBL) and after one cycle of a new line of systemic therapy (CTC1C), and changes from CTCBL to CTC1C (CTC kinetics, CTCKIN) for their utility in predicting response, progression-free (PFS) and overall survival (OS) in metastatic breast cancer (MBC). METHODS: CTCBL and CTC1C status was determined as negative (-) or positive (+) for < 5 or ≥ 5 CTCs/7.5 ml blood using CellSearch™ (Veridex). CTCKIN was categorized as favorable (CTC1C-) or unfavorable (CTC1C+). Tumor response was to be assessed every 2-3 months using the Response Evaluation Criteria in Solid Tumors (RECIST) criteria. Statistical analysis focused on the relation between CTC status and CTCKIN, and response, PFS, and OS. RESULTS: 133/393 (34%) patients enrolled were CTCBL+. CTC1C status after one cycle and radiological tumor response were assessed after median (range) periods of 1.2 (0.5-3.2) and 2.9 (0.5-4.8) months, respectively. 57/201 (28%) were CTC1C+. Median [95% confidence interval] PFS and OS (months) were significantly reduced in CTCBL+ vs. CTCBL- patients (PFS 4.7 [3.7-6.1] vs. 7.8 [6.4-9.2]; OS 10.4 [7.9-15.0] vs. 27.2 [22.3-29.9]), and for CTC1C+ vs. CTC1C- patients (PFS 4.3 [3.6-6.0] vs. 8.5 [6.6-10.4]; OS 7.7 [6.4-13.9] vs. 30.6 [22.6-not available]). Unfavorable CTCKIN was significantly associated with progressive disease. Multivariate Cox regression analysis revealed prognostic factors for shorter PFS (CTCBL+, persistent CTCs after one cycle, ≥ 3rd-line therapy, and triple-negative receptor status) and shorter OS (CTCBL+, persistent CTCs after one cycle, bone-and-visceral/local metastases, ≥ 3rd-line therapy, and triple-negative receptor status). CONCLUSIONS: CTCBL, CTC1C, and CTCKIN are predictive of outcome in MBC. Serial CTC enumeration is useful in tailoring systemic treatment of MBC. TRIAL REGISTRATION: Not applicable.

Expression and prognostic significance of cancer stem cell markers CD24 and CD44 in urothelial bladder cancer xenografts and patients undergoing radical cystectomy.

OBJECTIVES: To evaluate CD24/CD44/CD47 cancer stem cell marker expressions in bladder cancer (BCa) and provide data on their prognostic significance for clinical outcome in patients undergoing radical cystectomy (RC). MATERIAL AND METHODS: Primary BCa tissue was used for xenograft studies. A tissue microarray was prepared using specimens from a cohort of 132 patients. All patients underwent RC for urothelial BCa between 2001 and 2010. Expression of CD24, CD44, and CD47 was examined in primary samples and xenografts by fluorescence-activated cell sorting. Populations of CD24(low)- and CD24(high)-expressing cells were sorted and evaluated for tumorigenicity in vivo. Tissue microarray was analyzed for CD24/CD44 staining intensity and tumor-specific vs. stromal cell staining. Associations with BCa survival, BCa stage, and lymph node status were evaluated by univariate and multivariate analyses. RESULTS: CD24 and CD44/CD47 expressions mark distinct cell populations within the normal urothelium as well as in BCa. CD24(high/low) expression was not sufficient to characterize CD24 as a BCa-initiating marker in in vivo primary xenotransplants. CD24 and CD44 expressions correlated with lower cancer-specific survival in patients. However, multivariate analyses of CD24 or CD44 did not demonstrate significantly increased hazards for cancer-specific death if analyzed together with stage, grade, and nodal status of patients. CONCLUSIONS: Cancer stem cell markers CD24/CD44/CD47 are differentially expressed in cells of urothelial BCa in patients undergoing RC and influence cancer-specific survival of patients. Further evaluation of CD24/CD44/CD47 protein expression could be of high therapeutic value in BCa. However, both CD24 and CD44 expressions cannot be regarded as independent prognostic parameters for patients undergoing RC.

Pten loss in the bone marrow leads to G-CSF-mediated HSC mobilization.

The phosphatase and tumor suppressor PTEN inhibits the phosphoinositol-3-kinase (PI3K) signaling pathway and plays a key role in cell growth, proliferation, survival, and migration. Pten conditional deletion using MxCre or Scl-CreER(T) leads to splenomegaly and leukemia formation, which occurs after the relocation of normal hematopoietic stem cells (HSCs) from the bone marrow to the spleen. Unexpectedly, dormant HSCs in the bone marrow do not enter the cell cycle upon Pten loss, they do not lose self-renewal activity, and they are not exhausted. Instead, Pten deficiency causes an up-regulation of the PI3K pathway in myeloid cells, but not in HSCs. Strikingly, myeloid cells secrete high levels of G-CSF upon Pten loss, leading to the mobilization of HSCs from the bone marrow and accumulation in the spleen. After deletion of Pten in mice lacking G-CSF, the splenomegaly, myeloproliferative disease, and splenic HSC accumulation are rescued. Our data show that although PTEN has little if any role in normal HSCs, it is essential to prevent overt G-CSF production by myeloid and stromal cells which otherwise causes HSCs to relocate to the spleen followed by lethal leukemia initiation.

Effects of essential amino acid deficiency: down-regulation of KCC2 and the GABAA receptor; disinhibition in the anterior piriform cortex.

The anterior piriform cortex (APC) is activated by, and is the brain area most sensitive to, essential (indispensable) amino acid (IAA) deficiency. The APC is required for the rapid (20 min) behavioral rejection of IAA deficient diets and increased foraging, both crucial adaptive functions supporting IAA homeostasis in omnivores. The biochemical mechanisms signaling IAA deficiency in the APC block initiation of translation in protein synthesis via uncharged tRNA and the general amino acid control kinase, general control nonderepressing kinase 2. Yet, how inhibition of protein synthesis activates the APC is unknown. The neuronal K(+) Cl(-) cotransporter, neural potassium chloride co-transporter (KCC2), and GABAA receptors are essential inhibitory elements in the APC with short plasmalemmal half-lives that maintain control in this highly excitable circuitry. After a single IAA deficient meal both proteins were reduced (vs. basal diet controls) in western blots of APC (but not neocortex or cerebellum) and in immunohistochemistry of APC. Furthermore, electrophysiological analyses support loss of inhibitory elements such as the GABAA receptor in this model. As the crucial inhibitory function of the GABAA receptor depends on KCC2 and the Cl(-) transmembrane gradient it establishes, these results suggest that loss of such inhibitory elements contributes to disinhibition of the APC in IAA deficiency. The circuitry of the anterior piriform cortex (APC) is finely balanced between excitatory (glutamate, +) and inhibitory (GABA, -) transmission. GABAA receptors use Cl(-), requiring the neural potassium chloride co-transporter (KCC2). Both are rapidly turning-over proteins, dependent on protein synthesis for repletion. In IAA (indispensable amino acid) deficiency, within 20 min, blockade of protein synthesis prevents restoration of these inhibitors; they are diminished; disinhibition ensues. GCN2 = general control non-derepressing kinase 2, eIF2α = α-subunit of the eukaryotic initiation factor 2.

Identification of a population of blood circulating tumor cells from breast cancer patients that initiates metastasis in a xenograft assay.

It has been hypothesized that carcinoma metastasis is initiated by a subpopulation of circulating tumor cells (CTCs) found in the blood of patients. However, although the presence of CTCs is an indicator of poor prognosis in several carcinoma entities, the existence and phenotype of metastasis-initiating cells (MICs) among CTCs has not been experimentally demonstrated. Here we developed a xenograft assay and used it to show that primary human luminal breast cancer CTCs contain MICs that give rise to bone, lung and liver metastases in mice. These MIC-containing CTC populations expressed EPCAM, CD44, CD47 and MET. In a small cohort of patients with metastases, the number of EPCAM(+)CD44(+)CD47(+)MET(+) CTCs, but not of bulk EPCAM(+) CTCs, correlated with lower overall survival and increased number of metastasic sites. These data describe functional circulating MICs and associated markers, which may aid the design of better tools to diagnose and treat metastatic breast cancer.

The prognostic impact of circulating tumor cells in subtypes of metastatic breast cancer.

The detection of circulating tumor cells (CTCs) in the peripheral blood of metastatic breast cancer (MBC) patients is an independent marker of prognosis. This large prospective multicenter study aimed to assess the impact of CTCs on overall survival (OS) and progression free survival (PFS) in patients with predefined molecular subgroups of MBC. To this end, 468 MBC patients were divided into three subgroups based on immunohistochemical staining of the primary tumor: (1) hormone receptor-positive/HER2-negative (HorR+/HER2-), (2) HER2-positive (HER2+), and (3) HorR-negative/HER2-negative (HorR-/HER2-) patients. CTC status (<5 CTCs/7.5 ml blood (CTC-negative) vs. ≥5 CTCs/7.5 ml blood (CTC-positive)) was determined using the CellCearch(®) system before patients started a new line of therapy. At baseline, 205 (42 %) patients were CTC-positive. On multivariate analysis, CTC-positivity was an independent prognostic factor for shorter PFS and OS. In HorR+/HER2- patients, median PFS [95 % CI] of CTC-negative versus CTC-positive patients was 8.60 [5.93-11.27] versus 4.33 [3.29-5.38] months (p < 0.001), in HER2+ patients 7.60 [5.40-9.79] versus 6.60 [4.20-9.00] months (p = 0.477) and in HorR-/HER2- patients 5.83 [5.09-6.78] versus 3.05 [1.81-4.29] months (p < 0.001), respectively. Median OS [95 % CI] of CTC-negative versus CTC-positive patients was as follows: not reached by either in the HorR+/HER2- subgroup (p < 0.001), not reached versus 18.07 [11.10-25.05] months (p = 0.001) in the HER2+ subgroup, and not reached versus 8.57 [4.07-13.07] months in the HorR-/HER2- subgroup (p = 0.001). In conclusion, our results strongly confirm the independent prognostic value of CTC enumeration in MBC patients. In contrast to recent reports, there was no association between primary tumor-based molecular subgroups and the impact of CTC status on OS. Hence, CTC status may help to identify patients who require aggressive therapy, especially among those with triple-negative MBC.

The evolving concept of cancer and metastasis stem cells.

The cancer stem cell (CSC) concept, which arose more than a decade ago, proposed that tumor growth is sustained by a subpopulation of highly malignant cancerous cells. These cells, termed CSCs, comprise the top of the tumor cell hierarchy and have been isolated from many leukemias and solid tumors. Recent work has discovered that this hierarchy is embedded within a genetically heterogeneous tumor, in which various related but distinct subclones compete within the tumor mass. Thus, genetically distinct CSCs exist on top of each subclone, revealing a highly complex cellular composition of tumors. The CSC concept has therefore evolved to better model the complex and highly dynamic processes of tumorigenesis, tumor relapse, and metastasis.