Gene expression and mutation-guided synthetic lethality eradicates proliferating and quiescent leukemia cells

Quiescent and proliferating leukemia cells accumulate highly lethal DNA double-strand breaks that are repaired by 2 major mechanisms: BRCA-dependent homologous recombination and DNA-dependent protein kinase-mediated (DNA-PK-mediated) nonhomologous end-joining, whereas DNA repair pathways mediated by poly(ADP)ribose polymerase 1 (PARP1) serve as backups. Here we have designed a personalized medicine approach called gene expression and mutation analysis (GEMA) to identify BRCA- and DNA-PK-deficient leukemias either directly, using reverse transcription-quantitative PCR, microarrays, and flow cytometry, or indirectly, by the presence of oncogenes such as BCR-ABL1. DNA-PK-deficient quiescent leukemia cells and BRCA/DNA-PK-deficient proliferating leukemia cells were sensitive to PARP1 inhibitors that were administered alone or in combination with current antileukemic drugs. In conclusion, GEMA-guided targeting of PARP1 resulted in dual cellular synthetic lethality in quiescent and proliferating immature leukemia cells, and is thus a potential approach to eradicate leukemia stem and progenitor cells that are responsible for initiation and manifestation of the disease. Further, an analysis of The Cancer Genome Atlas database indicated that this personalized medicine approach could also be applied to treat numerous solid tumors from individual patients.

Deregulation of DUX4 and ERG in acute lymphoblastic leukemia

Chromosomal rearrangements deregulating hematopoietic transcription factors are common in acute lymphoblastic leukemia (ALL). Here we show that deregulation of the homeobox transcription factor gene DUX4 and the ETS transcription factor gene ERG is a hallmark of a subtype of B-progenitor ALL that comprises up to 7% of B-ALL. DUX4 rearrangement and overexpression was present in all cases and was accompanied by transcriptional deregulation of ERG, expression of a novel ERG isoform, ERGalt, and frequent ERG deletion. ERGalt uses a non-canonical first exon whose transcription was initiated by DUX4 binding. ERGalt retains the DNA-binding and transactivation domains of ERG, but it inhibits wild-type ERG transcriptional activity and is transforming. These results illustrate a unique paradigm of transcription factor deregulation in leukemia in which DUX4 deregulation results in loss of function of ERG, either by deletion or induced expression of an isoform that is a dominant-negative inhibitor of wild-type ERG function.

A renewed model of pancreatic cancer evolution based on genomic rearrangement patterns

Pancreatic cancer, a highly aggressive tumour type with uniformly poor prognosis, exemplifies the classically held view of stepwise cancer development. The current model of tumorigenesis, based on analyses of precursor lesions, termed pancreatic intraepithelial neoplasm (PanINs) lesions, makes two predictions: first, that pancreatic cancer develops through a particular sequence of genetic alterations (KRAS, followed by CDKN2A, then TP53 and SMAD4); and second, that the evolutionary trajectory of pancreatic cancer progression is gradual because each alteration is acquired independently. A shortcoming of this model is that clonally expanded precursor lesions do not always belong to the tumour lineage, indicating that the evolutionary trajectory of the tumour lineage and precursor lesions can be divergent. This prevailing model of tumorigenesis has contributed to the clinical notion that pancreatic cancer evolves slowly and presents at a late stage. However, the propensity for this disease to rapidly metastasize and the inability to improve patient outcomes, despite efforts aimed at early detection, suggest that pancreatic cancer progression is not gradual. Here, using newly developed informatics tools, we tracked changes in DNA copy number and their associated rearrangements in tumour-enriched genomes and found that pancreatic cancer tumorigenesis is neither gradual nor follows the accepted mutation order. Two-thirds of tumours harbour complex rearrangement patterns associated with mitotic errors, consistent with punctuated equilibrium as the principal evolutionary trajectory. In a subset of cases, the consequence of such errors is the simultaneous, rather than sequential, knockout of canonical preneoplastic genetic drivers that are likely to set-off invasive cancer growth. These findings challenge the current progression model of pancreatic cancer and provide insights into the mutational processes that give rise to these aggressive tumours.

Global proteomics dataset of miR-126 overexpression in acute myeloid leukemia

A deep proteomics analysis was conducted on a primary acute myeloid leukemia culture system to identify potential protein targets regulated by miR-126. Leukemia cells were transduced either with an empty control lentivirus or one containing the sequence for miR-126, and resulting cells were analyzed using ultra-high performance liquid chromatography (UHPLC) coupled with high resolution mass spectrometry. The mass spectrometry data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PRIDE: PXD001994. The proteomics data and statistical analysis described in this article is associated with a research article, “miR-126 regulates distinct self-renewal outcomes in normal and malignant hematopoietic stem cells” (Lechman et al., 2016) [1], and serves as a resource for researchers working in the field of microRNAs and their regulation of protein levels.

Abstract LB-341: Evolving functional heterogeneity in B-acute lymphoblastic leukemia

Current cancer therapies are directed at molecular markers or dominant pathways present in the bulk of neoplastic cells. However, recent studies have identified many genetically distinct subclones co-existing within a single neoplasm. In over 50% of patients with relapsed acute lymphoblastic leukemia (ALL), the genetic clones present at relapse are not the dominant clone present at diagnosis, but have evolved from a minor or ancestral clone (Mullighan et al., Science, 2008). Previous work has shown that this subclonal diversity in B-ALL exists at the level of the leukemia-initiating cells (L-IC) capable of generating patient derived xenografts (Notta et al., Nature, 2011). In order to investigate the functional consequences of genetic clonal evolution during disease progression, we performed in-depth genomic and functional analysis of 14 paired diagnosis/relapse samples from adult and pediatric B-ALL patients of varying cytogenetics. Patient samples were subjected to whole exome sequencing (WES), SNP analysis and RNA sequencing. Diagnosis-specific, relapse-specific, and shared variants at both clonal and subclonal frequencies were identified. Limiting dilution analysis by transplantation of CD19+ leukemic blasts into 870 immune-deficient mice (xenografts) identified no significant trend in enrichment in L-IC frequency between paired patient samples with a median frequency of 1 in 2691. Despite similar frequencies of L-IC, functional differences within identically sourced patient xenografts were observed, including increased leukemic dissemination of relapse cells to distal sites such as the central nervous system (CNS), differences in engraftment levels and differences in immunophenotypes. Targeted-sequencing and copy number analysis of the xenografts, in comparison to the patient sample from which they were derived, has uncovered clonal variation and the unequivocal identification of minor subclones ancestral to the relapse in xenografts transplanted with the diagnostic sample from 8 patients. Some of these subclones are rare and were not captured through standard WES analysis of the patient samples, highlighting the value of xenografting to functionally identify and viably isolate subclones for further study. Interrogation of the therapeutic responses of the ‘relapse-like’ diagnosis subclones in secondary xenografts displayed differential resistance to standard chemotherapeutic agents (vincristine and L-asparaginase) pre-existing in the patient diagnosis samples prior to treatment. Furthermore, investigation of different sites of leukemic infiltration in the xenografts provided evidence of distinct clonal selection in the CNS, a known site of disease relapse, in comparison to the bone marrow. Using this data we can begin to draw the evolutionary paths to relapse. We have shown evidence that minor subclones at diagnosis, ancestral to the relapsing clone, possess functional advantages over other diagnostic clones. Overall, this work provides a substantial advance in connecting genetic diversity to functional consequences, thereby furthering our understanding of the heterogeneity identified in B-ALL and its contributions to therapy failure and disease recurrence.

Citation Format: Stephanie M. Dobson, Robert Vanner, Esmé Waanders, Olga I. Gan, Jessica McLeod, Ildiko Grandal, Debbie Payne-Turner, Michael Edmonson, Zhaohui Gu, Xioatu Ma, Yiping Fan, Pankaj Gupta, Sagi Abelson, Michael Rusch, Ying Shao, Scott Olsen, Geoffrey Neale, John Easton, Cynthia J. Guidos, Jayne S. Danska, Jinghui Zhang, Mark D. Minden, Charles G. Mullighan, John E. Dick. Evolving functional heterogeneity in B-acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-341.

Ectopic miR-125a Expression Induces Long-Term Repopulating Stem Cell Capacity in Mouse and Human Hematopoietic Progenitors

Umbilical cord blood (CB) is a convenient and broadly used source of hematopoietic stem cells (HSCs) for allogeneic stem cell transplantation. However, limiting numbers of HSCs remain a major constraint for its clinical application. Although one feasible option would be to expand HSCs to improve therapeutic outcome, available protocols and the molecular mechanisms governing the self-renewal of HSCs are unclear. Here, we show that ectopic expression of a single microRNA (miRNA), miR-125a, in purified murine and human multipotent progenitors (MPPs) resulted in increased self-renewal and robust long-term multi-lineage repopulation in transplanted recipient mice. Using quantitative proteomics and western blot analysis, we identified a restricted set of miR-125a targets involved in conferring long-term repopulating capacity to MPPs in humans and mice. Our findings offer the innovative potential to use MPPs with enhanced self-renewal activity to augment limited sources of HSCs to improve clinical protocols.

miRNA-126 Orchestrates an Oncogenic Program in B Cell Precursor Acute Lymphoblastic Leukemia

MicroRNA (miRNA)-126 is a known regulator of hematopoietic stem cell quiescence. We engineered murine hematopoiesis to express miRNA-126 across all differentiation stages. Thirty percent of mice developed monoclonal B cell leukemia, which was prevented or regressed when a tetracycline-repressible miRNA-126 cassette was switched off. Regression was accompanied by upregulation of cell-cycle regulators and B cell differentiation genes, and downregulation of oncogenic signaling pathways. Expression of dominant-negative p53 delayed blast clearance upon miRNA-126 switch-off, highlighting the relevance of p53 inhibition in miRNA-126 addiction. Forced miRNA-126 expression in mouse and human progenitors reduced p53 transcriptional activity through regulation of multiple p53-related targets. miRNA-126 is highly expressed in a subset of human B-ALL, and antagonizing miRNA-126 in ALL xenograft models triggered apoptosis and reduced disease burden.

Molecular landscapes of human hematopoietic stem cells in health and leukemia

Blood cells are organized as a hierarchy with hematopoietic stem cells (HSCs) at the root. The advent of genomic technologies has opened the way for global characterization of the molecular landscape of HSCs and their progeny, both in mouse and human models, at the genetic, transcriptomic, epigenetic, and proteomics levels. Here, we outline our current understanding of the molecular programs that govern human HSCs and how dynamic changes occurring during HSC differentiation are necessary for well-regulated blood formation under homeostasis and upon injury. A large body of evidence is accumulating on how the programs of normal hematopoiesis are modified in acute myeloid leukemia, an aggressive adult malignancy driven by leukemic stem cells. We summarize these findings and their clinical implications.

Truncating Erythropoietin Receptor Rearrangements in Acute Lymphoblastic Leukemia

Chromosomal rearrangements are a hallmark of acute lymphoblastic leukemia (ALL) and are important ALL initiating events. We describe four different rearrangements of the erythropoietin receptor gene EPOR in Philadelphia chromosome-like (Ph-like) ALL. All of these rearrangements result in truncation of the cytoplasmic tail of EPOR at residues similar to those mutated in primary familial congenital polycythemia, with preservation of the proximal tyrosine essential for receptor activation and loss of distal regulatory residues. This resulted in deregulated EPOR expression, hypersensitivity to erythropoietin stimulation, and heightened JAK-STAT activation. Expression of truncated EPOR in mouse B cell progenitors induced ALL in vivo. Human leukemic cells with EPOR rearrangements were sensitive to JAK-STAT inhibition, suggesting a therapeutic option in high-risk ALL.

miR-126 Regulates Distinct Self-Renewal Outcomes in Normal and Malignant Hematopoietic Stem Cells

To investigate miRNA function in human acute myeloid leukemia (AML) stem cells (LSC), we generated a prognostic LSC-associated miRNA signature derived from functionally validated subpopulations of AML samples. For one signature miRNA, miR-126, high bioactivity aggregated all in vivo patient sample LSC activity into a single sorted population, tightly coupling miR-126 expression to LSC function. Through functional studies, miR-126 was found to restrain cell cycle progression, prevent differentiation, and increase self-renewal of primary LSC in vivo. Compared with prior results showing miR-126 regulation of normal hematopoietic stem cell (HSC) cycling, these functional stem effects are opposite between LSC and HSC. Combined transcriptome and proteome analysis demonstrates that miR-126 targets the PI3K/AKT/MTOR signaling pathway, preserving LSC quiescence and promoting chemotherapy resistance.

Abstract IA10: The role of stem cells in the origin of relapse in acute leukemia

The cellular and molecular basis for intra-tumoral heterogeneity is poorly understood. Tumor cells can be genetically diverse due to mutations and clonal evolution resulting in intra-tumoral functional heterogeneity. Often proposed as mutually exclusive, cancer stem cell (CSC) models postulate that tumors are cellular hierarchies created due to epigenetic programs that are sustained by CSC. I will focus on three lines of evidence showing these models are highly integrated. Gene signatures specific to either AML LSC or normal HSC are highly similar and define a common stemness program. Compared to non-stem cell transcriptional programs, only stem cell signatures were significant independent predictors of patient survival in 4 large clinical databases of >1000 samples. Thus, determinants of stemness influence clinical outcome of AML across a spectrum of mutations indicating that many genetic abnormalities coalesce around stem cell properties. Secondly, we have carried out a series of combined genetic and functional studies of the LSC from either B-ALL or AML that point to commonalities between clonal evolution and CSC models of cancer. LSC from diagnostic patient samples were genetically diverse and reconstruction of their genetic ancestry showed that multiple LSC subclones were related through a complex branching evolutionary process and specific genetic events influence L-IC frequency. Also study of paired diagnostic (Dx) and relapse (Rx) samples are revealing that individual subclones possess distinct functional growth properties and that rare Dx subclones are chemotherapy resistant and become enriched at Rx. Thus the clonal evolution models are highly relevant in cancer but need to be extended to adopt the concept that CSC are subject to clonal evolutionary forces. Finally, the combined genetic and functional analysis of AML is revealing fundamental insights into the cell of origin, nature and biological consequences of initiating lesions and order of subsequent mutations; concepts that demonstrate how highly integrated the CSC and genetic evolution models must be. Highly purified hematopoietic stem cells (HSC), progenitor and mature cell fractions from the blood of AML patients were found to contain recurrent DNMT3a mutations (DNMT3amut) at high allele frequency, but without coincident NPM1 mutations (NPM1c) present in AML blasts. DNMT3amut-bearing HSC exhibited multilineage repopulation advantage over non-mutated HSC in xenografts, establishing their identity as pre-leukemic-HSC (preL-HSC). preL-HSC were found in remission samples indicating that they survive chemotherapy. Thus DNMT3amut arises early in AML evolution, likely in HSC, leading to a clonally expanded pool of preL-HSC from which AML evolves. For therapy to be more effective, our findings indicate that each genetic subclones must be targeted and that any cells that possess stemness properties, whether they are LSC or ancestral preL-HSC, must also be eradicated.

Citation Format: John E. Dick. The role of stem cells in the origin of relapse in acute leukemia. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr IA10.

Abstract A25: Evolving functional heterogeneity in B-acute lymphoblastic leukemia

Despite high survival rates for children with acute lymphoblastic leukemia (ALL), only 40% of adult patients will achieve long-term disease-free survival, and relapses in both pediatric and adult ALL are often fatal. B-ALL leukemic blasts exhibit considerable subclonal genomic diversity. In 50% of patients, the clones present at relapse are not the dominant clone at diagnosis, but have evolved from a minor or ancestral clone. B-ALL subclonal diversity also exists in leukemia-initiating cells (L-IC) that are functionally capable of initiating xenografts. In order to investigate the clonal evolution during disease progression and link genetic diversity with functional characteristics, including differentiation, migratory properties, L-IC frequency and therapy resistance, we performed in depth genomic and functional analysis of 14 paired diagnosis/relapse samples from adult and pediatric B-ALL patients of varying cytogenetics. Time to relapse ranged from 6 to 97 months. Patient samples were subjected to whole exome/genome sequencing, SNP analysis and RNA sequencing. Diagnosis-specific, relapse-specific and shared variants at both clonal and subclonal frequencies were identified. Limiting dilution analysis by transplantation of CD19+ leukemic blasts into immune-deficient mice (xenografts) identified no significant trend in enrichment in L-IC frequency between paired patient samples with a median frequency of 1 in 2691. Despite similar frequencies of L-IC, functional differences within identically sourced patient xenografts were observed, including increased leukemic dissemination of relapse cells to the spleen and/or central nervous system, differences in engraftment levels and differences in immunophenotypes. Copy number analysis and ongoing variant sequencing of the xenografts, in comparison to the patient sample from which they were derived, has uncovered clonal variation and the outgrowth of ‘relapse-like’ subclones in xenografts transplanted with the diagnostic sample. Interrogation of the therapeutic response of these subclones in secondary xenografts displayed evidence of resistance to standard chemotherapeutic agents (vincristine and L-asparaginase). Therefore we have shown evidence that relapse subclones/ ancestral clones present at diagnosis possess functional advantages over other diagnostic clones. Overall, this work will provide further understanding of the heterogeneity identified in B-ALL and how it contributes to lymphoid leukemogenesis, therapy failure, and disease recurrence.

Citation Format: Stephanie M. Dobson, Robert Vanner, Esmé Waanders, Jessica McLeod, Olga I. Gan, Zhaohui Gu, Debbie Payne-Turner, Xiaotu Ma, Yiping Fan, Pankaj Gupta, Michael Rusch, John Easton, Cynthia J. Guidos, Jayne S. Danska, Jinghui Zhang, Mark D. Minden, Charles G. Mullighan, John E. Dick. Evolving functional heterogeneity in B-acute lymphoblastic leukemia. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr A25.

An Integrated Analysis of Heterogeneous Drug Responses in Acute Myeloid Leukemia That Enables the Discovery of Predictive Biomarkers

Many promising new cancer drugs proceed through preclinical testing and early-phase trials only to fail in late-stage clinical testing. Thus, improved models that better predict survival outcomes and enable the development of biomarkers are needed to identify patients most likely to respond to and benefit from therapy. Here, we describe a comprehensive approach in which we incorporated biobanking, xenografting, and multiplexed phospho-flow (PF) cytometric profiling to study drug response and identify predictive biomarkers in acute myeloid leukemia (AML) patients. To test the efficacy of our approach, we evaluated the investigational JAK2 inhibitor fedratinib (FED) in 64 patient samples. FED robustly reduced leukemia in mouse xenograft models in 59% of cases and was also effective in limiting the protumorigenic activity of leukemia stem cells as shown by serial transplantation assays. In parallel, PF profiling identified FED-mediated reduction in phospho-STAT5 (pSTAT5) levels as a predictive biomarker of in vivo drug response with high specificity (92%) and strong positive predictive value (93%). Unexpectedly, another JAK inhibitor, ruxolitinib (RUX), was ineffective in 8 of 10 FED-responsive samples. Notably, this outcome could be predicted by the status of pSTAT5 signaling, which was unaffected by RUX treatment. Consistent with this observed discrepancy, PF analysis revealed that FED exerted its effects through multiple JAK2-independent mechanisms. Collectively, this work establishes an integrated approach for testing novel anticancer agents that captures the inherent variability of response caused by disease heterogeneity and in parallel, facilitates the identification of predictive biomarkers that can help stratify patients into appropriate clinical trials.

Distinct routes of lineage development reshape the human blood hierarchy across ontogeny

In a classical view of hematopoiesis, the various blood cell lineages arise via a hierarchical scheme starting with multipotent stem cells that become increasingly restricted in their differentiation potential through oligopotent and then unipotent progenitors. We developed a cell-sorting scheme to resolve myeloid (My), erythroid (Er), and megakaryocytic (Mk) fates from single CD34(+) cells and then mapped the progenitor hierarchy across human development. Fetal liver contained large numbers of distinct oligopotent progenitors with intermingled My, Er, and Mk fates. However, few oligopotent progenitor intermediates were present in the adult bone marrow. Instead, only two progenitor classes predominate, multipotent and unipotent, with Er-Mk lineages emerging from multipotent cells. The developmental shift to an adult "two-tier" hierarchy challenges current dogma and provides a revised framework to understand normal and disease states of human hematopoiesis.

Abstract A176: Efficacy and safety of allogeneic double negative T cell as anti-AML therapy and its underlying mechanism

Acute myeloid leukemia (AML) is the most common form of adult acute leukemia that is associated with a low long-term survival rate. While chemotherapy achieves remission in the majority of AML patients, many relapse due to residual chemotherapy-resistant AML populations. Allogeneic hematopoetic stem cell transplantation is a potential curative treatment for AML that demonstrates the efficacy of a cell-mediated treatment for chemotherapy-resistant disease. However, its wide application is limited by suitable donor availability and associated toxicity such as graft-versus-host disease (GvHD). Hence, there is a need for a new treatment approach that targets chemotherapy-resistant AML blasts with minimal side effects. The goals of this study were to characterize allogeneic CD3+CD4-CD8- double negative T (DNT) cells as a potential new target of treatment for AML patients, including ones that are resistant to chemotherapy, and to dissect its underlying mechanisms. Using two-hour flow cytometry-based in vitro killing assay, we demonstrated that the allogeneic DNT cells expanded from healthy volunteers were cytotoxic against 23/29 primary AML patient blasts in a dose-dependent manner. Of those, 13 blasts were obtained from chemotherapy refractory or relapsing AML patients and nine of them were susceptible to DNT cells. The anti-leukemia activity of DNT cells was further validated in a AML-NSG xenograft model: a single infusion of DNT cells into mice pre-engrafted with primary AML blasts from chemoresponsive, chemorefractory, and relapsed patients significantly reduced the leukemia burden. Although residual blasts were observed from DNT cell-treated group, they did not develop resistance to DNT cells, as they remained susceptible to DNT cells in ex vivo killing assay. Further, we demonstrated the safety of allogeneic DNT cells, as DNT cells did not target allogeneic peripheral blood mononuclear cells (PBMC) and hematopoietic stem/progenitor cells (HSPC) in vitro. In addition, administration of allogeneic DNT cells into NSG mice, engrafted with human HSPC, had no effect on the engraftment level of human hematopoetic cells and their differentiation into different lineages. Further, infusion of human DNT cells did not cause xenogeneic GvHD in mice, collectively demonstrating the safety of allogeneic DNT cells. Using blocking assays, we showed the roles of HLA-class I, NKG2D, and DNAM-1 in DNT cell-mediated cytotoxicity against AML, whereas HLA-class II and T cell receptor were not involved. While IFNγ release correlated with cytotoxicity of DNT cells, IFNγ treatment alone did not induce AML cell death. Neutralizing IFNγ reduced susceptibility to DNT cell-mediated cytotoxicity while pretreating AML with recombinant IFNγ increased their susceptibility by inducing higher expression of NKG2D and DNAM-1 ligands, where NKG2D- and DNAM-1-blocking antibodies abrogated the effect of IFNγ pretreatment. Cytolytic activity of DNT cells was mediated in perforin-granzyme-dependent fashion, which subsequently activated caspase-8 and caspase-9 pathways in AML cells to induce AML cell death. Collectively, these studies demonstrated the safety and efficacy of allogeneic DNT cell therapy as a potential treatment for AML patients, including those with chemotherapy-resistant leukemia, and revealed important molecules for the anti-leukemia activity of DNT cells.

Citation Format: Jong Bok Lee, Claire Weihsu Chen, Mark D. Minden, John E. Dick, Li Zhang. Efficacy and safety of allogeneic double negative T cell as anti-AML therapy and its underlying mechanism. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A176.

Abstract A04: Differential dependence on sphingolipid metabolism in the normal and leukemic human hematopoietic hierarchy

Metabolic alterations are a cancer hallmark that are typically evaluated in bulk tissues. However most normal tissues and cancers are hierarchically organized and the metabolic requirements of both normal and cancer stem cells are poorly understood, particularly beyond energy metabolism. By analyzing a comprehensive transcriptional roadmap of human hematopoiesis, and comparing this to a leukemia stem cell (LSC) signature developed from 84 human acute myeloid leukemia (AML) samples, we found that: (1) several metabolic pathways, specifically in bioactive lipids, distinguish normal hematopoietic stem cells (HSC) from progenitors; and (2) while LSC are similar to HSC, specific metabolic pathways are more comparable to those of normal progenitors. We defined a lipid stem signature of 24 lipid genes, including sphingolipid genes, whose expression is higher in HSC than progenitors. Interestingly, sphingosine-1-phosphate (S1P) is known to play a role in HSC egress, and ceramide vs S1P levels serve as a rheostat to regulate cell growth and survival. To determine if sphingolipids play a functional role in the primitive hematopoietic compartment, we altered sphingolipid signaling by plating sorted populations of HSC or granulocyte-myeloid progenitors (GMP) in methylcellulose containing myriocin, which inhibits the first step of de novo sphingolipid synthesis, or FTY720, a S1P mimetic. Myriocin decreased GMP colony output but did not affect CFC derived from HSC. By contrast FTY720 affected HSC-derived CFC but not those from GMP, suggesting differential sensitivity to sphingolipid pathway inhibition between stem and progenitor cells. In vitro treatment of lineage depleted cord blood (Lin- CB) with myriocin for 8 days limited only myeloid differentiation compared to control treated cells. In contrast, FTY720 treatment reduced levels of immunophenotypic stem cells, erythroid and myeloid cells, as would be expected with inhibition of S1P proliferative signaling. However, in vitro FTY720-treated Lin- CB cells exhibit 16-week engraftment capacity comparable to that of controlled-treated cells, suggesting that any effects on HSC function are reversible after drug withdrawal. Furthermore, in vivo treatment with FTY720 in mice with established CB grafts did not decrease engraftment.

Our lipid stem signature is enriched in LSC gene expression profiles from our 84 AML cohort by GSEA analysis, suggesting that differences in lipid metabolism may also exist in LSC vs. non-LSC. To determine if sphingolipids play a functional role in AML biology, we transplanted mice with peripheral blood cells from 13 AML patients, including those with therapy resistant and relapsed disease, and treated engrafted mice with myriocin or FTY720. We observed heterogeneous responses in our cohort, with reduction of leukemic burden in 3 and 5 samples following treatment with FTY720 and myriocin, respectively. Remarkably, serial transplantation of FTY720 responders into untreated secondary mice at limiting dilution demonstrated decreased LSC frequency in FTY720-treated primary mice compared to vehicle-treated controls, whereas myriocin responders showed no alteration of LSC frequency. These results suggest that FTY720 but not myriocin treatment affects LSC number and/or function. To stratify responders from nonresponders, we performed transcriptional analysis of untreated patient samples and compared these data to transcriptional signatures generated from the normal and AML hierarchy. Bioinformatic analysis demonstrated that FTY720 responders had an enriched LSC signature compared to nonresponders. In contrast, myriocin responders exhibited a strong GMP signature compared to nonresponders. Thus, normal human hematopoietic stem and progenitor cells display a variable dependence on sphingolipid biology that is also distinct between LSC and HSC, pointing to targeting of bioactive sphingolipids as a novel therapeutic strategy in AML to eradicate LSC while sparing HSC.

Citation Format: Stephanie Z. Xie, Elisa Laurenti, Robin Ferrari, John E. Dick. Differential dependence on sphingolipid metabolism in the normal and leukemic human hematopoietic hierarchy. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr A04.

MLL5 Orchestrates a Cancer Self-Renewal State by Repressing the Histone Variant H3.3 and Globally Reorganizing Chromatin

Mutations in the histone 3 variant H3.3 have been identified in one-third of pediatric glioblastomas (GBMs), but not in adult tumors. Here we show that H3.3 is a dynamic determinant of functional properties in adult GBM. H3.3 is repressed by mixed lineage leukemia 5 (MLL5) in self-renewing GBM cells. MLL5 is a global epigenetic repressor that orchestrates reorganization of chromatin structure by punctuating chromosomes with foci of compacted chromatin, favoring tumorigenic and self-renewing properties. Conversely, H3.3 antagonizes self-renewal and promotes differentiation. We exploited these epigenetic states to rationally identify two small molecules that effectively curb cancer stem cell properties in a preclinical model. Our work uncovers a role for MLL5 and H3.3 in maintaining self-renewal hierarchies in adult GBM.

Efficacy of Retinoids in IKZF1-Mutated BCR-ABL1 Acute Lymphoblastic Leukemia

Alterations of IKZF1, encoding the lymphoid transcription factor IKAROS, are a hallmark of high-risk acute lymphoblastic leukemia (ALL), however the role of IKZF1 alterations in ALL pathogenesis is poorly understood. Here, we show that in mouse models of BCR-ABL1 leukemia, Ikzf1 and Arf alterations synergistically promote the development of an aggressive lymphoid leukemia. Ikzf1 alterations result in acquisition of stem cell-like features, including self-renewal and increased bone marrow stromal adhesion. Retinoid receptor agonists reversed this phenotype, partly by inducing expression of IKZF1, resulting in abrogation of adhesion and self-renewal, cell cycle arrest, and attenuation of proliferation without direct cytotoxicity. Retinoids potentiated the activity of dasatinib in mouse and human BCR-ABL1 ALL, providing an additional therapeutic option in IKZF1-mutated ALL.

AGS67E, an Anti-CD37 Monomethyl Auristatin E Antibody-Drug Conjugate as a Potential Therapeutic for B/T-Cell Malignancies and AML: A New Role for CD37 in AML

CD37 is a tetraspanin expressed on malignant B cells. Recently, CD37 has gained interest as a therapeutic target. We developed AGS67E, an antibody-drug conjugate that targets CD37 for the potential treatment of B/T-cell malignancies. It is a fully human monoclonal IgG2 antibody (AGS67C) conjugated, via a protease-cleavable linker, to the microtubule-disrupting agent monomethyl auristatin E (MMAE). AGS67E induces potent cytotoxicity, apoptosis, and cell-cycle alterations in many non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL) cell lines and patient-derived samples in vitro. It also shows potent antitumor activity in NHL and CLL xenografts, including Rituxan-refractory models. During profiling studies to confirm the reported expression of CD37 in normal tissues and B-cell malignancies, we made the novel discovery that the CD37 protein was expressed in T-cell lymphomas and in AML. AGS67E bound to >80% of NHL and T-cell lymphomas, 100% of CLL and 100% of AML patient-derived samples, including CD34(+)CD38(-) leukemic stem cells. It also induced cytotoxicity, apoptosis, and cell-cycle alterations in AML cell lines and antitumor efficacy in orthotopic AML xenografts. Taken together, this study shows not only that AGS67E may serve as a potential therapeutic for B/T-cell malignancies, but it also demonstrates, for the first time, that CD37 is well expressed and a potential drug target in AML.

CDK6 levels regulate quiescence exit in human hematopoietic stem cells

Regulated blood production is achieved through the hierarchical organization of dormant hematopoietic stem cell (HSC) subsets that differ in self-renewal potential and division frequency, with long-term (LT)-HSCs dividing the least. The molecular mechanisms underlying this variability in HSC division kinetics are unknown. We report here that quiescence exit kinetics are differentially regulated within human HSC subsets through the expression level of CDK6. LT-HSCs lack CDK6 protein. Short-term (ST)-HSCs are also quiescent but contain high CDK6 protein levels that permit rapid cell cycle entry upon mitogenic stimulation. Enforced CDK6 expression in LT-HSCs shortens quiescence exit and confers competitive advantage without impacting function. Computational modeling suggests that this independent control of quiescence exit kinetics inherently limits LT-HSC divisions and preserves the HSC pool to ensure lifelong hematopoiesis. Thus, differential expression of CDK6 underlies heterogeneity in stem cell quiescence states that functionally regulates this highly regenerative system.

Evolution of the cancer stem cell model

Genetic analyses have shaped much of our understanding of cancer. However, it is becoming increasingly clear that cancer cells display features of normal tissue organization, where cancer stem cells (CSCs) can drive tumor growth. Although often considered as mutually exclusive models to describe tumor heterogeneity, we propose that the genetic and CSC models of cancer can be harmonized by considering the role of genetic diversity and nongenetic influences in contributing to tumor heterogeneity. We offer an approach to integrating CSCs and cancer genetic data that will guide the field in interpreting past observations and designing future studies.

Abstract 2914: Phosphoproteomic and transcriptional biomarkers predict response to SAR302503, a JAK2 inhibitor, in human acute myeloid leukemia preclinical models

Research to develop new anti-cancer treatments has recently shifted focus to identifying and targeting molecules and pathways essential for cancer stem cell survival. However, preclinical models that rely on cell lines for drug testing do not capture the heterogeneity of response typically seen in the clinic that likely reflects the underlying genetic and functional heterogeneity of the tumors. Moreover, the use of cell lines makes it difficult to develop companion biomarker tools for patient stratification. We have taken a novel approach that combines drug testing of a large cohort of patient samples in xenograft assays, with multiplexed phosphoflow cytometric and RNA-Seq analysis of each patient sample to develop biomarkers that predict drug response. We applied this approach to study the efficacy of SAR302503 (Sanofi), a small molecule inhibitor of JAK2, against leukemia stem cells (LSCs) in acute myeloid leukemia (AML). JAK2 inhibitors have demonstrated efficacy in clinical trials for treatment of myeloproliferative disorders. Activated JAK2 signaling has been reported in AML, however it is not clear whether JAK2 inhibitors are effective in this disease, particularly against the disease-sustaining LSCs. SAR302503 treatment reduced leukemic engraftment in 22 of 34 (65%) AML patient samples of multiple subtypes with heterogeneous cytogenetic and molecular abnormalities. Phosphoflow analysis showed that AML samples that were sensitive to JAK2 inhibition in xenotransplantation assays exhibited high basal levels of pSTAT5 that were rapidly decreased by SAR302503 treatment in vitro, whereas non-responding samples showed low levels of pSTAT5, indicating that pSTAT5 is a useful drug response biomarker. This biomarker has now been validated in an independent cohort of AML patient samples. Phosphoflow cytometric profiling also enabled the rational design and testing of a novel drug combination regimen (SAR302503+Dasatinib) with efficacy against LSCs. Additionally, RNA-Seq analysis of paired vehicle- and drug-treated patient samples revealed that samples that were responsive to JAK2 inhibition in vivo had distinct transcriptional profiles compared to those that were not, suggesting that a molecular signature predictive of response to JAK2 inhibition can be identified. Overall, our approach, involving large-scale analysis of patient samples using state-of-the-art xenograft assays, captures the heterogeneity of response typically seen in the clinic that likely reflects the underlying genetic and functional heterogeneity of the tumors and offers a new paradigm for development of both novel agents that effectively target LSCs and biomarker tools to identify the patients most likely to benefit from targeted treatment.

Citation Format: Weihsu Claire Chen, Julie S. Yuan, Nathan Mbong, Andreea C. Popescu, Yan Xing, Gitte Gerhard, Wei Zhang, Yussanne Ma, Richard Moore, Marco Marra, Mark D. Minden, Donna E. Hogge, Cynthia Guidos, John E. Dick, Jean C.Y. Wang. Phosphoproteomic and transcriptional biomarkers predict response to SAR302503, a JAK2 inhibitor, in human acute myeloid leukemia preclinical models. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2914. doi:10.1158/1538-7445.AM2014-2914

Abstract 4792: Elucidating stem cell-specific metabolic pathways in normal and malignant hematopoiesis to target human acute myeloid leukemia stem cells

Metabolic alterations are a cancer hallmark but the metabolic requirements of hematopoietic stem cells (HSC) in general, and leukemic stem cells (LSCs) specifically are poorly understood, particularly beyond energy metabolism. By analyzing a comprehensive transcriptional roadmap of human hematopoiesis, and comparing this to a LSC signature developed from 84 primary human acute myeloid leukemia (AML) samples, we uncovered two unexpected findings: (1) several metabolic pathways, specifically in bioactive lipids, distinguish HSC from progenitors and are essential for their function; and LSC are more similar to HSC overall, but also possess specific metabolic pathways that are more comparable to those of normal progenitors. We compiled a comprehensive list of 67 genes involved in fatty acid metabolism based on literature and database information and found that 36 of these were differentially expressed between HSC and progenitors, with 24 of them higher in HSC. In particular, sphingolipid enzymes are differentially regulated throughout the hematopoietic hierarchy. Interesting, sphingosine-1-phosphate (S1P) plays a role in HSC egress and ceramide vs S1P levels serve as a rheostat to regulate growth and cell survival. We defined these 24 genes as a lipid stem signature, and find that it is enriched in the LSC gene expression profiles from our 84 primary AML cohort by GSEA analysis indicating that fatty acid metabolism is differentially regulated in LSCs and non-LSCs, as it is for HSCs vs. normal progenitors. Our signature was found to be prognostic for patient survival in a Dutch cohort of 181 cytogenetically normal AMLs. To determine if lipid metabolism plays a functional role in AML biology, we screened a fatty acid compound library using a novel AML cell line (8227) that retains hierarchical organization and assessed cell viability, phenotypic LSC content, and differentiation. We identified myriocin, which targets serine palmitoyltransferase (1st step of sphingolipid synthesis) in this screen. Myriocin decreased 8227 viability and altered differentiation in vitro and reduced leukemia burden in vivo following transplantation into NOD/SCID mice transgenic for human cytokines. Treatment of mice bearing primary AML xenografts, including those from therapy resistant and relapsed patients, with myriocin or its derivative FTY720, a S1P mimetic, resulted in reduction of leukemic engraftment. Moreover, serial transplantation of AML samples by limiting dilution showed decreased LSC frequency of FTY720- and myriocin-treated cells in secondary mice compared to vehicle-treated cells. Importantly, myriocin or FTY720 treatment do not disrupt engraftment in mice bearing normal hematopoietic grafts. Thus, sphingolipid biology in LSC is different from that of HSC, and we present a novel AML therapeutic strategy targeting bioactive sphingolipids as a means to eradicate LSC while sparing HSC.

Citation Format: Stephanie Z. Xie, Elisa Laurenti, John E. Dick. Elucidating stem cell-specific metabolic pathways in normal and malignant hematopoiesis to target human acute myeloid leukemia stem cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4792. doi:10.1158/1538-7445.AM2014-4792

Abstract SY13-01: Genetic and non-genetic mechanisms contribute to long term clonal growth dynamics and therapy resistance

The cellular and molecular basis for intra-tumoral heterogeneity is poorly understood. Tumor cells can be genetically diverse due to mutations and clonal evolution resulting in intra-tumoral functional heterogeneity. Often proposed as mutually exclusive, cancer stem cell (CSC) models postulate that tumors are cellular hierarchies sustained by CSC heterogeneity due to epigenetic differences (i.e. long term tumor propagation only derives from CSC). The presentation will focus on three lines of evidence showing these models are highly integrated. Gene signatures specific to either AML LSC or normal HSC are highly similar and define a common stemness program. Compared to non-stem cell transcriptional programs, only stem cell signatures were significant independent predictors of patient survival in 4 large clinical databases of >1000 samples. Thus, determinants of stemness influence clinical outcome of AML across a spectrum of mutations indicating that many genetic abnormalities coalesce around stem cell properties. These studies provide strong support that the CSC model is clinically relevant and not an artifact of the xenografts model. Secondly, we have carried out a series of combined genetic and functional studies of the LSC from either B-ALL or AML that point to commonalities between clonal evolution and CSC models of cancer. LSC from diagnostic patient samples were genetically diverse and reconstruction of their genetic ancestry showed that multiple LSC subclones were related through a complex branching evolutionary process and specific genetic events influence L-IC frequency. Also study of paired diagnostic (Dx) and relapse (Rx) samples are revealing that individual subclones possess distinct functional growth properties and that rare Dx subclones are chemotherapy resistant and become enriched at Rx. Thus the clonal evolution models are highly relevant in cancer but need to be extended to adopt the concept that CSC are subject to clonal evolutionary forces. Finally, the combined genetic and functional analysis of AML is revealing fundamental insights into the cell of origin, nature and biological consequences of initiating lesions and order of subsequent mutations; concepts that demonstrate how highly integrated the CSC and genetic evolution models must be. Highly purified hematopoietic stem cells (HSC), progenitor and mature cell fractions from the blood of AML patients were found to contain recurrent DNMT3a mutations (DNMT3amut) at high allele frequency, but without coincident NPM1 mutations (NPM1c) present in AML blasts. DNMT3amut-bearing HSC exhibited multilineage repopulation advantage over non-mutated HSC in xenografts, establishing their identity as pre-leukemic-HSC (preL-HSC). preL-HSC were found in remission samples indicating that they survive chemotherapy. Thus DNMT3amut arises early in AML evolution, likely in HSC, leading to a clonally expanded pool of preL-HSC from which AML evolves. Our findings provide a paradigm for the detection and treatment of pre-leukemic clones before the acquisition of additional genetic lesions engenders greater therapeutic resistance. Collectively, these three lines of evidence show that the CSC and evolution models can be unified. Moreover, these studies point to the need to develop therapies that effective target, all the genetic subclones present in a tumor, the ancestral cell types from which recurrences can arise and ultimately all leukemia and pre-leukemic cells possessing stemness properties, to ensure further evolution and recurrence are prevented.

Citation Format: John E. Dick. Genetic and non-genetic mechanisms contribute to long term clonal growth dynamics and therapy resistance. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr SY13-01. doi:10.1158/1538-7445.AM2014-SY13-01

Reduced lymphoid lineage priming promotes human hematopoietic stem cell expansion

The hematopoietic system sustains regeneration throughout life by balancing self-renewal and differentiation. To stay poised for mature blood production, hematopoietic stem cells (HSCs) maintain low-level expression of lineage-associated genes, a process termed lineage priming. Here, we modulated expression levels of Inhibitor of DNA binding (ID) proteins to ask whether lineage priming affects self-renewal of human HSCs. We found that lentiviral overexpression of ID proteins in cord blood HSCs biases myeloerythroid commitment at the expense of lymphoid differentiation. Conversely, reducing ID2 expression levels increases lymphoid potential. Mechanistically, ID2 inhibits the transcription factor E47 to attenuate B-lymphoid priming in HSCs and progenitors. Strikingly, ID2 overexpression also results in a 10-fold expansion of HSCs in serial limiting dilution assays, indicating that early lymphoid transcription factors antagonize human HSC self-renewal. The relationship between lineage priming and self-renewal can be exploited to increase expansion of transplantable human HSCs and points to broader implications for other stem cell populations.

Intercellular network structure and regulatory motifs in the human hematopoietic system

The hematopoietic system is a distributed tissue that consists of functionally distinct cell types continuously produced through hematopoietic stem cell (HSC) differentiation. Combining genomic and phenotypic data with high-content experiments, we have built a directional cell-cell communication network between 12 cell types isolated from human umbilical cord blood. Network structure analysis revealed that ligand production is cell type dependent, whereas ligand binding is promiscuous. Consequently, additional control strategies such as cell frequency modulation and compartmentalization were needed to achieve specificity in HSC fate regulation. Incorporating the in vitro effects (quiescence, self-renewal, proliferation, or differentiation) of 27 HSC binding ligands into the topology of the cell-cell communication network allowed coding of cell type-dependent feedback regulation of HSC fate. Pathway enrichment analysis identified intracellular regulatory motifs enriched in these cell type- and ligand-coupled responses. This study uncovers cellular mechanisms of hematopoietic cell feedback in HSC fate regulation, provides insight into the design principles of the human hematopoietic system, and serves as a foundation for the analysis of intercellular regulation in multicellular systems.

Identification of pre-leukaemic haematopoietic stem cells in acute leukaemia

In acute myeloid leukemia (AML), the cell of origin, nature and biological consequences of initiating lesions and order of subsequent mutations remain poorly understood, as AML is typically diagnosed without observation of a pre-leukemic phase. Here, highly purified hematopoietic stem cells (HSC), progenitor and mature cell fractions from the blood of AML patients were found to contain recurrent DNMT3a mutations (DNMT3a^mut) at high allele frequency, but without coincident NPM1 mutations (NPM1c) present in AML blasts. DNMT3a^mut-bearing HSC exhibited multilineage repopulation advantage over non-mutated HSC in xenografts, establishing their identity as pre-leukemic-HSC (preL-HSC). preL-HSC were found in remission samples indicating that they survive chemotherapy. Thus DNMT3a^mut arises early in AML evolution, likely in HSC, leading to a clonally expanded pool of preL-HSC from which AML evolves. Our findings provide a paradigm for the detection and treatment of pre-leukemic clones before the acquisition of additional genetic lesions engenders greater therapeutic resistance.

Abstract BS01-1: Genetic and non-genetic mechanisms contribute to longterm clonal growth dynamics and therapy resistance

The cellular and molecular basis for intra-tumoral heterogeneity is poorly understood. Tumor cells can be genetically diverse due to mutations and clonal evolution resulting in intra-tumoral functional heterogeneity. Often proposed as mutually exclusive, cancer stem cell (CSC) models postulate that tumors are cellular hierarchies sustained by CSC heterogeneity due to epigenetic differences (i.e. long term tumor propagation only derives from CSC). Two lines of evidence support the CSC model in leukemia. We have recently developed gene signatures specific to either AML LSC or normal HSC and found they share a set of genes that define a common stemness program. Only these stem cell related gene signatures were significant independent predictors of patient survival in large clinical databases. Thus, determinants of stemness influence clinical outcome of AML across a spectrum of mutations indicating that many genetic abnormalities coalesce around stem cell properties. Second, we have carried out a series of combined genetic and functional studies of the LSC from either B-ALL or AML that point to commonalities between clonal evolution and CSC models of cancer. LSC from diagnostic patient samples were genetically diverse and reconstruction of their genetic ancestry showed that multiple LSC subclones were related through a complex branching evolutionary process. The discovery that specific genetic events influence L-IC frequency and that genetically distinct L-IC evolve through a complex evolutionary process indicates that genetic and functional heterogeneity are highly inter-related. Finally, we have also begun to study paired diagnostic (Dx) and relapse (Rx) samples and found that rare Dx subclones are chemotherapy resistant and become enriched at Rx. Collectively, our study points to the need to develop therapies that effective target all genetic subclones present in a tumor and also ensure that cells possessing stemness properties are eliminated to prevent further evolution and recurrence.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr BS01-1.

Self-renewal as a therapeutic target in human colorectal cancer

Tumor recurrence following treatment remains a major clinical challenge. Evidence from xenograft models and human trials indicates selective enrichment of cancer-initiating cells (CICs) in tumors that survive therapy. Together with recent reports showing that CIC gene signatures influence patient survival, these studies predict that targeting self-renewal, the key 'stemness' property unique to CICs, may represent a new paradigm in cancer therapy. Here we demonstrate that tumor formation and, more specifically, human colorectal CIC function are dependent on the canonical self-renewal regulator BMI-1. Downregulation of BMI-1 inhibits the ability of colorectal CICs to self-renew, resulting in the abrogation of their tumorigenic potential. Treatment of primary colorectal cancer xenografts with a small-molecule BMI-1 inhibitor resulted in colorectal CIC loss with long-term and irreversible impairment of tumor growth. Targeting the BMI-1-related self-renewal machinery provides the basis for a new therapeutic approach in the treatment of colorectal cancer.

Identification of genes expressed by immune cells of the colon that are regulated by colorectal cancer-associated variants

A locus on human chromosome 11q23 tagged by marker rs3802842 was associated with colorectal cancer (CRC) in a genome-wide association study; this finding has been replicated in case–control studies worldwide. In order to identify biologic factors at this locus that are related to the etiopathology of CRC, we used microarray-based target selection methods, coupled to next-generation sequencing, to study 103 kb at the 11q23 locus. We genotyped 369 putative variants from 1,030 patients with CRC (cases) and 1,061 individuals without CRC (controls) from the Ontario Familial Colorectal Cancer Registry. Two previously uncharacterized genes, COLCA1 and COLCA2, were found to be co-regulated genes that are transcribed from opposite strands. Expression levels of COLCA1 and COLCA2 transcripts correlate with rs3802842 genotypes. In colon tissues, COLCA1 co-localizes with crystalloid granules of eosinophils and granular organelles of mast cells, neutrophils, macrophages, dendritic cells and differentiated myeloid-derived cell lines. COLCA2 is present in the cytoplasm of normal epithelial, immune and other cell lineages, as well as tumor cells. Tissue microarray analysis demonstrates the association of rs3802842 with lymphocyte density in the lamina propria (p = 0.014) and levels of COLCA1 in the lamina propria (p = 0.00016) and COLCA2 (tumor cells, p = 0.0041 and lamina propria, p = 6 × 10^–5). In conclusion, genetic, expression and immunohistochemical data implicate COLCA1 and COLCA2 in the pathogenesis of colon cancer. Histologic analyses indicate the involvement of immune pathways.

Catalytic site remodelling of the DOT1L methyltransferase by selective inhibitors

Selective inhibition of protein methyltransferases is a promising new approach to drug discovery. An attractive strategy towards this goal is the development of compounds that selectively inhibit binding of the cofactor, S-adenosylmethionine, within specific protein methyltransferases. Here we report the three-dimensional structure of the protein methyltransferase DOT1L bound to EPZ004777, the first S-adenosylmethionine-competitive inhibitor of a protein methyltransferase with in vivo efficacy. This structure and those of four new analogues reveal remodelling of the catalytic site. EPZ004777 and a brominated analogue, SGC0946, inhibit DOT1L in vitro and selectively kill mixed lineage leukaemia cells, in which DOT1L is aberrantly localized via interaction with an oncogenic MLL fusion protein. These data provide important new insight into mechanisms of cell-active S-adenosylmethionine-competitive protein methyltransferase inhibitors, and establish a foundation for the further development of drug-like inhibitors of DOT1L for cancer therapy.

Abstract 2292: Enforced expression of miR-125b promotes the in vivo expansion of human Lin- CB multi-lymphoid progenitors (MLP) and AML leukemia stem cells

We recently demonstrated stem cell gene signatures predict clinical outcome in acute myeloid leukemia (AML) (Eppert et. al., Nature Medicine, 2011). Concomitant to this work, miRNA signatures for hematopoietic stem cells (HSC) and leukemia stem cells (LSC) were also generated. miRNA are small non-coding RNAs that regulate the translation and mRNA stability of protein coding genes with significant roles in the maintenance of human HSC (Lechman et. al., Cell Stem Cell, in press). To understand the functional role of miRNA in normal human blood development, we undertook an in vivo over-expression screen of 10 miRNA candidates over-represented in HSC and LSC. Lineage depleted human umbilical cord blood cells (Lin- CB) were transduced with lentivirus expressing either a candidate miRNA or control vector and xeno-transplanted into NSG mice. Three miRNA displayed a competitive growth advantage while 4 miRNA induced a growth disadvantage along with skewing of lineage output. A top LSC array candidate, miR-125b, showed the most pronounced phenotype with overt expansion of marked cells, enlarged spleens and increased lymphoid and erythroid output. Detailed analysis of miR-125b grafts revealed a greatly expanded MLP population, in comparison to HSC and MPP. Furthermore, upon enforced in vivo expression of miR-125b in 3 AML patient samples, we observed large increases in the CD34+CD117+ populations for all three AML samples, suggesting increased LSC numbers. Secondary LDA experiments revealed up to a 34 fold increase in LSC activity in comparison to control vector transduced AML cells. These data suggest that miR-125b normally functions in the limited self-renewal of lymphoid committed early progenitors and this function may be usurped during leukemogenesis to enhance LSC self-renewal.

Citation Format: Eric R. Lechman, Karin G. Hermans, Stephanie Dobson, Kolja Eppert, Mark Minden, John E. Dick. Enforced expression of miR-125b promotes the in vivo expansion of human Lin- CB multi-lymphoid progenitors (MLP) and AML leukemia stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2292. doi:10.1158/1538-7445.AM2013-2292

Abstract SY05-01: Genetic and non-genetic mechanisms contribute to long-term clonal growth dynamics and therapy resistance

Individual cells of a tumor vary in many hallmarks of cancer and this heterogeneity can contribute to therapy failure and disease recurrence. The cellular and molecular basis for intra-tumoral heterogeneity is poorly understood. Individual tumor cells can exhibit genetic diversity due to mutations and clonal evolution resulting in intra-tumoral functional heterogeneity. Thus, a single tumor can be composed of genetically distinct subclones, each with distinct functional properties. However, non-genetic determinants also can drive heterogeneity. Often proposed as mutually exclusive to genetic models, the cancer stem cell (CSC) model postulates that tumors are cellular hierarchies, where subpopulations of self-renewing cancer stem cells (CSC) sustain long-term clonal maintenance (i.e. long term tumor propagation only derives from CSC). CSC and non-CSC are considered to be genetically identical (although evidence is very limited), therefore developmental pathways and epigenetic modifications govern the stemness state and drive this functional heterogeneity. There is strong evidence for the CSC model in AML and some solid tumors. By fractionating primary AML into functionally defined leukemia stem cells (LSC) and non-LSC populations and subjecting these to mRNA and miRNA profiling, we have identified an LSC specific gene signature. The LSC signature is highly similar to normal HSC-specific signatures revealing a common and shared stemness program that governs the stem cell state. Critically, the LSC signature and the common stemness signature were shown to be strong independent predictors of patient survival in large clinical databases; bulk blasts and non-LSC fractions lacked prognostic power (Eppert Nature Med 2011). Thus, determinants of stemness influence clinical outcome of AML establishing the clinical relevance of LSC are and not artifacts of xenotransplantation. This work and other recent reports of stem cell prognostic power establishes clinical relevance and helps to place the entire CSC concept onto a stronger footing.

In an effort to determine if the clonal evolution genetic model and the CSC model of cancer can be unified, we have carried out three different studies examining CSC from both the functional and the genetic level. To conclusively establish that human acute leukemia possess functionally distinct genetic subclones we focused on Ph+ B-ALL. Genome sequencing is clearly showing that genetic diversity exists in leukemia and solid tumors, however an in silico demonstration of diversity cannot establish how the cellular properties of the tumor cells are influenced by specific gene abnormalities. We found that diagnostic patient samples possessed extensive subclonal genetic diversity and through the use of xenotransplant assays, we found that this diversity originated from within the leukemia initiating cells (L-IC). Individual mice derived from the same primary sample were repopulated with genetically distinct subclones and the growth properties, L-IC frequency, etc were distinct for each subclone. Reconstruction of their genetic ancestry showed that multiple L-IC subclones were related through a complex branching evolutionary process indicating that genetic and functional heterogeneity are closely connected (Notta Nature 2011). Direct clonal evidence that an individual L-IC can evolve came from our prior study of human B-ALL initiated in an experimental leukemogenesis model where human progenitors were transformed with an MLL-ENL vector (Barabe Science 2007). Serial xenografts of B-ALL showed evolution at the Ig locus, yet the viral insertion confirmed the disease was initiated with a single L-IC. Thus the evolution and CSC models need not be mutually exclusive: CSC are not static and they are subject to acquisition of new mutations and subsequent evolution during disease progression. New additional evidence on changes in LSC during disease progression will be presented where we have examined the genetic diversity of paired diagnostic and relapse sample of AML, combined with parallel xenografting and LSC studies. Collectively our study points to the need to develop effective therapies to eradicate all genetic subclones to prevent further evolution and recurrence.

Our third functional/genetic study involved colorectal cancer (CRC) and directly tested functional repopulation behaviour of single cells. In parallel the genetic diversity of the single clones that propagated in xenogafts was also ascertained enabling an evaluation of the relative contribution of genetic and non-genetic determinants to variable growth properties. Our lab developed a highly reliable xenograft assay for primary CRC. By combining DNA copy number alteration (CNA) profiling, targeted and exome sequencing and lentiviral lineage tracing, we followed the repopulation dynamics of many single lentivirus-marked lineages from colorectal cancers through serial xenograft passages. The xenografts were genetically stable on serial transplantation. Despite this genetic stability, the proliferation, persistence, and chemotherapy tolerance of lentivirally marked lineages was variable within each clone. Not all tumor-initiating cells (T-IC) cells are contributing to tumor growth continuously: some are held in reserve, while others have the ability to oscillate between periods of dormancy and activity. This distinct intraclonal behavior affected response to conventional chemotherapy, as actively proliferating progeny were preferentially eliminated, while the relatively dormant CRC cells became dominant during tumor re-initiation following chemotherapy. The intraclonal diversity in genetically identical, single cell functional behavior of primary human CRC cells in vivo has the net effect of contributing to tumor growth during both homeostasis and therapy response (Kreso Science 2013). Our findings reveal another layer of complexity, beyond genetic diversity, that drives intratumoral heterogeneity of CRC. The prospect of understanding how genetic and non-genetic determinants interact to influence the functional diversity and therapy response for other cancers should drive future cancer research.

Citation Format: John E. Dick. Genetic and non-genetic mechanisms contribute to long-term clonal growth dynamics and therapy resistance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr SY05-01. doi:10.1158/1538-7445.AM2013-SY05-01

Abstract 907: Efficacy of SAR302503, a JAK2 inhibitor, in primary human acute myeloid leukemia xenografts

Small molecule inhibitors of Janus kinase 2 (JAK2) such as Ruxolitinib and SAR302503 have demonstrated efficacy in clinical trials for treatment of myeloproliferative disorders. Activated JAK2 signaling has been reported in some acute myeloid leukemia (AML) samples even though JAK2 mutations are relatively rare in AML. Whether JAK2 inhibitors are effective in AML, particularly against the disease-sustaining leukemia stem cells (LSC), is not clear. We report that SAR302503 (Sanofi, Cambridge MA), an orally administered small molecule JAK2 inhibitor, shows efficacy in a xenograft model of human AML established by intrafemoral injection of primary AML cells into sublethally irradiated NOD.SCID mice. The AML samples tested were of multiple subtypes with heterogeneous cytogenetic and molecular abnormalities. Starting 2 weeks post transplantation to permit establishment of an AML graft, mice received twice daily oral gavage with 60 mg/kg SAR302503 or vehicle alone (0.5% methylcellulose) for 14 consecutive days. In 17 of 34 AML samples, SAR302503 treatment reduced leukemic engraftment in the injected femur (56-94% relative reduction, RR, compared to controls; p<0.05) as well as non-injected bones (30-95% RR; p<0.05). 5 additional samples exhibited a partial response (<50% RR in injected femur and 31-64% RR in non-injected bones, p<0.05). In preliminary serial transplantation studies, AML cells harvested from SAR302503-treated primary mice showed reduced ability to generate a leukemic graft in untreated secondary mice compared to controls, suggesting that JAK2 inhibition reduces LSC function and/or survival. Given the heterogeneous response to SAR302503 treatment observed in vivo, we carried out phosphoflow cytometric analysis of patient samples to identify biomarkers that could predict drug response. AML samples that were sensitive to JAK2 inhibition in xenotransplantation assays exhibited high basal levels of pSTAT5, often in only a small subset of cells, that were rapidly decreased by SAR302503 treatment in vitro, whereas non-responding samples showed low levels of pSTAT5, suggesting that pSTAT5 is a useful drug response biomarker. Our results demonstrate the potential of SAR302503 to target AML cells including LSCs in a broad cross section of AML patients, and warrant further studies to identify responders and non-responders and better characterize proteomic biomarkers of drug response. The approach we have taken, which focuses on large-scale analysis of primary samples using state-of-the-art xenograft assays, offers a new paradigm for preclinical drug development to identify both novel agents that effectively target LSCs and the patients most likely to benefit from targeted treatment.

Citation Format: Weihsu C. Chen, Andreea C. Popescu, Yan Xing, Gitte Gerhard, Julie Yuan, Mark Minden, Cynthia Guidos, Donna E. Hogge, John E. Dick, Jean CY Wang. Efficacy of SAR302503, a JAK2 inhibitor, in primary human acute myeloid leukemia xenografts. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 907. doi:10.1158/1538-7445.AM2013-907

Variable clonal repopulation dynamics influence chemotherapy response in colorectal cancer

Intratumoral heterogeneity arises through the evolution of genetically diverse subclones during tumor progression. However, it remains unknown whether cells within single genetic clones are functionally equivalent. By combining DNA copy number alteration (CNA) profiling, sequencing, and lentiviral lineage tracking, we followed the repopulation dynamics of 150 single lentivirus-marked lineages from 10 human colorectal cancers through serial xenograft passages in mice. CNA and mutational analysis distinguished individual clones and showed that clones remained stable upon serial transplantation. Despite this stability, the proliferation, persistence, and chemotherapy tolerance of lentivirally marked lineages were variable within each clone. Chemotherapy promoted the dominance of previously minor or dormant lineages. Thus, apart from genetic diversity, tumor cells display inherent functional variability in tumor propagation potential, which contributes to both cancer growth and therapy tolerance.

Lysosomal disruption preferentially targets acute myeloid leukemia cells and progenitors

Despite efforts to understand and treat acute myeloid leukemia (AML), there remains a need for more comprehensive therapies to prevent AML-associated relapses. To identify new therapeutic strategies for AML, we screened a library of on- and off-patent drugs and identified the antimalarial agent mefloquine as a compound that selectively kills AML cells and AML stem cells in a panel of leukemia cell lines and in mice. Using a yeast genome-wide functional screen for mefloquine sensitizers, we identified genes associated with the yeast vacuole, the homolog of the mammalian lysosome. Consistent with this, we determined that mefloquine disrupts lysosomes, directly permeabilizes the lysosome membrane, and releases cathepsins into the cytosol. Knockdown of the lysosomal membrane proteins LAMP1 and LAMP2 resulted in decreased cell viability, as did treatment of AML cells with known lysosome disrupters. Highlighting a potential therapeutic rationale for this strategy, leukemic cells had significantly larger lysosomes compared with normal cells, and leukemia-initiating cells overexpressed lysosomal biogenesis genes. These results demonstrate that lysosomal disruption preferentially targets AML cells and AML progenitor cells, providing a rationale for testing lysosomal disruption as a novel therapeutic strategy for AML.

Attenuation of miR-126 activity expands HSC in vivo without exhaustion

Lifelong blood cell production is governed through the poorly understood integration of cell-intrinsic and -extrinsic control of hematopoietic stem cell (HSC) quiescence and activation. MicroRNAs (miRNAs) coordinately regulate multiple targets within signaling networks, making them attractive candidate HSC regulators. We report that miR-126, a miRNA expressed in HSC and early progenitors, plays a pivotal role in restraining cell-cycle progression of HSC in vitro and in vivo. miR-126 knockdown by using lentiviral sponges increased HSC proliferation without inducing exhaustion, resulting in expansion of mouse and human long-term repopulating HSC. Conversely, enforced miR-126 expression impaired cell-cycle entry, leading to progressively reduced hematopoietic contribution. In HSC/early progenitors, miR-126 regulates multiple targets within the PI3K/AKT/GSK3β pathway, attenuating signal transduction in response to extrinsic signals. These data establish that miR-126 sets a threshold for HSC activation and thus governs HSC pool size, demonstrating the importance of miRNA in the control of HSC function.

Molecular and functional characterization of early human hematopoiesis

Through improvements in xenograft assay methods and in the identification of novel cell surface markers, significant progress has been made in our understanding of the human hematopoietic stem and progenitor hierarchy. The isolation of clonally pure populations of stem cells and early progenitors opens the way to carry out gene expression profiling studies to uncover the molecular regulators of each developmental step and to gain insight into the process of lineage commitment in human hematopoiesis.

Disruption of SIRPα signaling in macrophages eliminates human acute myeloid leukemia stem cells in xenografts

Inhibition of macrophage SIRPα–CD47 interactions mediates phagocytosis and clearance of acute myeloid leukemia stem cells.

Although tumor surveillance by T and B lymphocytes is well studied, the role of innate immune cells, in particular macrophages, is less clear. Moreover, the existence of subclonal genetic and functional diversity in some human cancers such as leukemia underscores the importance of defining tumor surveillance mechanisms that effectively target the disease-sustaining cancer stem cells in addition to bulk cells. In this study, we report that leukemia stem cell function in xenotransplant models of acute myeloid leukemia (AML) depends on SIRPα-mediated inhibition of macrophages through engagement with its ligand CD47. We generated mice expressing SIRPα variants with differential ability to bind human CD47 and demonstrated that macrophage-mediated phagocytosis and clearance of AML stem cells depend on absent SIRPα signaling. We obtained independent confirmation of the genetic restriction observed in our mouse models by using SIRPα-Fc fusion protein to disrupt SIRPα–CD47 engagement. Treatment with SIRPα-Fc enhanced phagocytosis of AML cells by both mouse and human macrophages and impaired leukemic engraftment in mice. Importantly, SIRPα-Fc treatment did not significantly enhance phagocytosis of normal hematopoietic targets. These findings support the development of therapeutics that antagonize SIRPα signaling to enhance macrophage-mediated elimination of AML.

Human solid tumor xenografts in immunodeficient mice are vulnerable to lymphomagenesis associated with Epstein-Barr virus

Xenografting primary human solid tumor tissue into immunodeficient mice is a widely used tool in studies of human cancer biology; however, care must be taken to prove that the tumors obtained recapitulate parent tissue. We xenografted primary human hepatocellular carcinoma (HCC) tumor fragments or bulk tumor cell suspensions into immunodeficient mice. We unexpectedly observed that 11 of 21 xenografts generated from 16 independent patient samples resembled lymphoid neoplasms rather than HCC. Immunohistochemistry and flow cytometry analyses revealed that the lymphoid neoplasms were comprised of cells expressing human CD45 and CD19/20, consistent with human B lymphocytes. In situ hybridization was strongly positive for Epstein-Barr virus (EBV) encoded RNA. Genomic analysis revealed unique monoclonal or oligoclonal immunoglobulin heavy chain gene rearrangements in each B-cell neoplasm. These data demonstrate that the lymphoid neoplasms were EBV-associated human B-cell lymphomas. Analogous to EBV-associated lymphoproliferative disorders in immunocompromised humans, the human lymphomas in these HCC xenografts likely developed from reactivation of latent EBV in intratumoral passenger B lymphocytes following their xenotransplantation into immunodeficient recipient mice. Given the high prevalence of latent EBV infection in humans and the universal presence of B lymphocytes in solid tumors, this potentially confounding process represents an important pitfall of human solid tumor xenografting. This phenomenon can be recognized and avoided by routine phenotyping of primary tumors and xenografts with human leukocyte markers, and provides a compelling biological rationale for exclusion of these cells from human solid tumor xenotransplantation assays.

ID1 and ID3 regulate the self-renewal capacity of human colon cancer-initiating cells through p21

There is increasing evidence that some cancers are hierarchically organized, sustained by a relatively rare population of cancer-initiating cells (C-ICs). Although the capacity to initiate tumors upon serial transplantation is a hallmark of all C-ICs, little is known about the genes that control this process. Here, we establish that ID1 and ID3 function together to govern colon cancer-initiating cell (CC-IC) self-renewal through cell-cycle restriction driven by the cell-cycle inhibitor p21. Regulation of p21 by ID1 and ID3 is a central mechanism preventing the accumulation of excess DNA damage and subsequent functional exhaustion of CC-ICs. Additionally, silencing of ID1 and ID3 increases sensitivity of CC-ICs to the chemotherapeutic agent oxaliplatin, linking tumor initiation function with chemotherapy resistance.

Hematopoiesis: a human perspective

Despite its complexity, blood is probably the best understood developmental system, largely due to seminal experimentation in the mouse. Clinically, hematopoietic stem cell (HSC) transplantation represents the most widely deployed regenerative therapy, but human HSCs have only been characterized relatively recently. The discovery that immune-deficient mice could be engrafted with human cells provided a powerful approach for studying HSCs. We highlight 2 decades of studies focusing on isolation and molecular regulation of human HSCs, therapeutic applications, and early lineage commitment steps, and compare mouse and humanized models to identify both conserved and species-specific mechanisms that will aid future preclinical research.

Abstract 1013: Enriched miR-126 bioactivity marks the primitive compartment in human AML and regulates leukemia stem cell numbers

Previous work has shown miRNAs are dysregulated in acute myeloid leukemia (AML), however, there is little known regarding miRNA expression and function in human leukemia stem cells (LSC). In order to elucidate the role of miRNA in LSC, we performed miRNA profiling on fractionated subpopulations of primary AML patient samples. Supervised analysis guided by the in vivo SCID leukemia initiating cell (SL-IC) capacity of each sub-population generated a unique miRNA signature associated with LSC enriched fractions. The biological activity of our top candidate, miR-126, was confirmed at single cell resolution by using a novel bidirectional lentivirus miRNA reporter system in vitro and within primary AML patient samples xenografted into immune-deficient NSG mice. These data suggest that primitive AML cells may express high levels of bioactive miR-126 relative to more “differentiated” blast populations. To test this hypothesis, we FACS sorted miR-126 genetic reporter vector transduced primary AML patient samples and transplanted these populations into immune-compromised secondary mouse recipients. The results of these proof-of-concept experiments demonstrates our ability to prospectively isolate LSC enriched fractions in all 4 AML patient samples tested using only a single biomarker, miR-126. Finally, to understand the functional relevance of miR-126 expression within primitive human AML cells, stable enforced expression and knockdown of miR-126 was achieved using lentiviral vectors. Enforced expression in four primary AML xenografts resulted in a several fold increase of CD34+CD117+ lentivirus marked leukemia cells after 12 weeks. In addition, the miR-126/OE cells showed reduced differentiation marker expression (CD14, CD15) with no significant differences in AML graft size. To determine if the expanded population had SL-IC activity or was a downstream leukemic progenitor, limiting dilution assays were performed by transplantation of FACS sorted lentivirus marked cells into secondary recipient mice for 12 weeks. A 3-20 fold increase in LSC activity was observed with miR-126 forced expression compared to control cells. These data suggest that high levels of miR-126 bioactivity support self-renewal/maintenance of primitive AML cells at the cost of aberrant differentiation. We performed microarray analysis of a primitive human AML cells after miR-126/OE and miR-126/KD. The principal signalling pathway(s) under direct control of miR-126 in primitive AML cells were revealed by subjecting our array data to Gene Set Enrichment Analysis (GSEA) in combination with several published miRNA target prediction algorithms. In summary, this work demonstrates that miR-126 is more abundant and biologically active within the leukemia stem/progenitor cell compartment of the AML functional hierarchy and serves to regulate AML stem cell numbers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1013. doi:1538-7445.AM2012-1013

Abstract 3496: Self-renewal as a therapeutic target in human colorectal cancer

Tumor recurrence following treatment remains a major clinical challenge. Evidence from xenograft models and human trials indicates selective enrichment of cancer-initiating cells (C-ICs) in tumors that survive therapy. Together with recent reports showing that C-IC gene signatures influence patient survival, these studies predict that targeting self-renewal, the key stemness property unique to C-ICs, may represent a new paradigm in cancer therapy. Here we demonstrate that tumor formation, and more specifically human colorectal C-IC function are dependent on the canonical self-renewal regulator BMI-1. Down-regulation of BMI-1 inhibits the ability of colorectal C-ICs to self-renew resulting in the abrogation of their tumorigenic potential. Treatment of primary colorectal cancer xenografts with small molecule BMI-1 inhibitors resulted in colorectal C-IC loss with long-term and irreversible impairment of tumor growth. Targeting the Bmi-1 related self-renewal machinery provides the basis for a new therapeutic approach in the treatment of colorectal cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3496. doi:1538-7445.AM2012-3496