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Tumor heterogeneity is a major challenge for oncology drug discovery and development. Understanding of the spatial tumor landscape is key to identifying new targets and impactful model systems. Here, we test the utility of spatial transcriptomics (ST) for oncology discovery by profiling 40 tissue sections and 80,024 capture spots across a diverse set of tissue types, sample formats, and RNA capture chemistries. We verify the accuracy and fidelity of ST by leveraging matched pathology analysis, which provides a ground truth for tissue section composition. We then use spatial data to demonstrate the capture of key tumor depth features, identifying hypoxia, necrosis, vasculature, and extracellular matrix variation. We also leverage spatial context to identify relative cell-type locations showing the anti-correlation of tumor and immune cells in syngeneic cancer models. Lastly, we demonstrate target identification approaches in clinical pancreatic adenocarcinoma samples, highlighting tumor intrinsic biomarkers and paracrine signaling.

Tumors put in a vulnerable position Cancer cells often display alterations in metabolism that help fuel their growth. Such metabolic “rewiring” may also work against the cancer cells, however, by creating new vulnerabilities that can be exploited therapeutically. A variety of human tumors show changes in methionine metabolism caused by loss of the gene coding for 5-methylthioadenosine phosphorylase (MTAP). Mavrakis et al. and Kryukov et al. found that the loss of MTAP renders cancer cell lines sensitive to growth inhibition by compounds that suppress the activity of a specific arginine methyltransferase called PRMT5. Conceivably, drugs that inhibit PRMT5 activity could be developed into a tailored therapy for MTAP-deficient tumors. Science , this issue pp. 1208 and 1214

To address limitations of current high-throughput methods for studying cell-cell communication and determining the cell-of-origin of proteins in multicellular environments, we have developed a technique that selectively and continuously labels the proteome of individual cell types in co-culture. Through transgenic expression of exogenous amino acid biosynthesis enzymes, vertebrate cells overcome their dependence on essential amino acids and can be selectively labeled through metabolic incorporation of amino acids produced from heavy isotope-labeled precursors. We have named this method Type specific labeling with Amino acid Precursors (CTAP). Testing CTAP in several human and mouse cell lines, we were able to differentially label the proteome of distinct cell populations in co-culture and determine the relative expression of proteins by quantitative mass spectrometry. In addition, CTAP successfully identified the cell-of-origin of extracellular proteins in co-culture, highlighting its potential use in biomarker discovery for linking secreted factors to their cellular source.

We have developed a screening platform for the isolation of genetic entities involved in metastatic reactivation. Retroviral libraries of cDNAs from fully metastatic breast-cancer cells or pooled microRNAs were transduced into breast-cancer cells that become dormant upon infiltrating the lung. Upon inoculation in the tail vein of mice, the cells that had acquired the ability to undergo reactivation generated metastatic lesions. Integrated retroviral vectors were recovered from these lesions, sequenced, and subjected to a second round of validation. By using this strategy, we isolated canonical genes and microRNAs that mediate metastatic reactivation in the lung. To identify genes that oppose reactivation, we screened an expression library encoding shRNAs, and we identified target genes that encode potential enforcers of dormancy. Our screening strategy enables the identification and rapid biological validation of single genetic entities that are necessary to maintain dormancy or to induce reactivation. This technology should facilitate the elucidation of the molecular underpinnings of these processes.

The translational control of oncoprotein expression is implicated in many cancers. Here we report an eIF4A RNA helicase-dependent mechanism of translational control that contributes to oncogenesis and underlies the anticancer effects of silvestrol and related compounds. For example, eIF4A promotes T-cell acute lymphoblastic leukaemia development in vivo and is required for leukaemia maintenance. Accordingly, inhibition of eIF4A with silvestrol has powerful therapeutic effects against murine and human leukaemic cells in vitro and in vivo. We use transcriptome-scale ribosome footprinting to identify the hallmarks of eIF4A-dependent transcripts. These include 5′ untranslated region (UTR) sequences such as the 12-nucleotide guanine quartet (CGG)4 motif that can form RNA G-quadruplex structures. Notably, among the most eIF4A-dependent and silvestrol-sensitive transcripts are a number of oncogenes, superenhancer-associated transcription factors, and epigenetic regulators. Hence, the 5′ UTRs of select cancer genes harbour a targetable requirement for the eIF4A RNA helicase. The translation of many messenger RNAs that encode important oncogenes and transcription factors depends on the eIF4A RNA helicase to resolve G-quadruplex structures, implying eIF4A inhibition as an effective cancer therapy. The expression of some oncoproteins is regulated at the translational level. Hans-Guido Wendel and colleagues show that a subset of oncoprotein- and transcription factor-encoding mRNAs that are dependent on the translation initiation factor eIF4A contain a G-quadruplex-forming structure in their 5′ untranslated regions. These findings explain why silvestrol, a plant-derived anticancer agent that targets eIF4A-dependent translation, is not generally toxic but can be well tolerated except in cancer cells which are dependent on the activities of these proteins. In a separate study in this issue, Stephan Vagner and colleagues show that inhibition of eIF4F cooperates with BRAF inhibitors in reducing the growth of melanomas linked to BRAF mutations.

The importance of individual microRNAs (miRNAs) has been established in specific cancers. However, a comprehensive analysis of the contribution of miRNAs to the pathogenesis of any specific cancer is lacking. Here we show that in T-cell acute lymphoblastic leukemia (T-ALL), a small set of miRNAs is responsible for the cooperative suppression of several tumor suppressor genes. Cross-comparison of miRNA expression profiles in human T-ALL with the results of an unbiased miRNA library screen allowed us to identify five miRNAs (miR-19b, miR-20a, miR-26a, miR-92 and miR-223) that are capable of promoting T-ALL development in a mouse model and which account for the majority of miRNA expression in human T-ALL. Moreover, these miRNAs produce overlapping and cooperative effects on tumor suppressor genes implicated in the pathogenesis of T-ALL, including IKAROS (also known as IKZF1), PTEN, BIM, PHF6, NF1 and FBXW7. Thus, a comprehensive and unbiased analysis of miRNA action in T-ALL reveals a striking pattern of miRNA-tumor suppressor gene interactions in this cancer.

Genetically and pathologically accurate mouse models of leukemia and lymphoma have been developed in recent years. Adoptive transfer of genetically modified hematopoietic progenitor cells enables rapid and highly controlled gain- and loss-of-function studies for these types of cancer. In this Commentary, we discuss how these highly versatile experimental approaches can be used as biological filters to pinpoint transformation-relevant activities from complex cancer genome data. We anticipate that the functional identification of genetic ‘drivers’ using mouse models of leukemia and lymphoma will facilitate the development of molecular diagnostics and mechanism-based therapies for patients that suffer from these diseases.

Our recent findings on Rheb and eIF4E address key questions of translational control in cancer and have implications for tumor therapy.(1) Briefly, we find that Rheb a proximal activator of mTORC1 and protein translation can cooperate with c-Myc in tumorigenesis in vivo in a manner resembling Akt or the oncogenic eIF4E translation initiation factor. Rheb is highly expressed in some human lymphomas as well as other cancers and likely contributes to malignancies in different tissues.(2) The cancer-relevant activities emanating from increased Rheb depend on activation of mTORC1 and are sensitive to rapamycin. Moreover, farnesyltransferase inhibitors (FTIs) can directly block Rheb activity and this is responsible for the therapeutic effect of these drugs in certain tumors. We will discuss here how translational control mechanisms contribute to oncogenesis and speculate on the potential and limitations of targeting these co-operating oncogenic events for therapy.

Transforming growth factor-beta (TGF-beta) signaling is controlled by a variety of regulators that target either signaling receptors or activated Smad complexes. Among the negative regulators, Smad7 antagonizes TGF-beta signaling mainly through targeting the signaling receptors, whereas SnoN and c-Ski repress signaling at the transcriptional level through inactivation of Smad complexes. We previously found that Arkadia is a positive regulator of TGF-beta signaling that induces ubiquitin-dependent degradation of Smad7 through its C-terminal RING domain. We report here that Arkadia induces degradation of SnoN and c-Ski in addition to Smad7. Arkadia interacts with SnoN and c-Ski in their free forms as well as in the forms bound to Smad proteins, and constitutively down-regulates levels of their expression. Arkadia thus appears to effectively enhance TGF-beta signaling through simultaneous down-regulation of two distinct types of negative regulators, Smad7 and SnoN/c-Ski, and may play an important role in determining the intensity of TGF-beta family signaling in target cells.

In this paper, we describe the characterization of DEF6, a novel PH-DH-like protein related to SWAP-70 that functions as an upstream activator of Rho GTPases. In NIH 3T3 cells, stimulation of the PI 3-kinase signaling pathway with either H 2 O 2 or platelet-derived growth factor (PDGF) resulted in the translocation of an overexpressed DEF6–GFP fusion protein to the cell membrane and induced the formation of filopodia and lamellipodia. In contrast to full-length DEF6, expression of the DH-like (DHL) domain as a GFP fusion protein potently induced actin polymerization, including stress fiber formation in COS-7 cells, in the absence of PI 3-kinase signaling, indicating that it was constitutively active. The GTP-loading of Cdc42 was strongly enhanced in NIH 3T3 cells expressing the DH domain while filopodia formation, membrane ruffling, and stress fiber formation could be inhibited by the co-expression of the DH domain with dominant negative mutants of either N17Rac1, N17Cdc42, or N19RhoA, respectively. This indicated that DEF6 acts upstream of the Rho GTPases resulting in the activation of the Cdc42, Rac1, and RhoA signaling pathways. In vitro, DEF6 specifically interacted with Rac1, Rac2, Cdc42, and RhoA, suggesting a direct role for DEF6 in the activation of Rho GTPases. The ability of DEF6 to both stimulate actin polymerization and bind to filamentous actin suggests a role for DEF6 in regulating cell shape, polarity, and movement.

Elucidation of the mutational landscape of human cancer has progressed rapidly and been accompanied by the development of therapeutics targeting mutant oncogenes. However, a comprehensive mapping of cancer dependencies has lagged behind and the discovery of therapeutic targets for counteracting tumor suppressor gene loss is needed. To identify vulnerabilities relevant to specific cancer subtypes, we conducted a large-scale RNAi screen in which viability effects of mRNA knockdown were assessed for 7,837 genes using an average of 20 shRNAs per gene in 398 cancer cell lines. We describe findings of this screen, outlining the classes of cancer dependency genes and their relationships to genetic, expression, and lineage features. In addition, we describe robust gene-interaction networks recapitulating both protein complexes and functional cooperation among complexes and pathways. This dataset along with a web portal is provided to the community to assist in the discovery and translation of new therapeutic approaches for cancer.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive form of leukemia that is mainly diagnosed in children and shows a skewed gender distribution toward males. In this study, we report somatic loss-of-function mutations in the X-linked histone H3K27me3 demethylase ubiquitously transcribed X (UTX) chromosome, in human T-ALL. Interestingly, UTX mutations were exclusively present in male T-ALL patients and allelic expression analysis revealed that UTX escapes X-inactivation in female T-ALL lymphoblasts and normal T cells. Notably, we demonstrate in vitro and in vivo that the H3K27me3 demethylase UTX functions as a bona fide tumor suppressor in T-ALL. Moreover, T-ALL driven by UTX inactivation exhibits collateral sensitivity to pharmacologic H3K27me3 inhibition. All together, our results show how a gender-specific and therapeutically relevant defect in balancing H3K27 methylation contributes to T-cell leukemogenesis.

Metabolic genes are increasingly recognized as targets of somatic genetic alteration in human cancer often leading to profound changes in intracellular metabolite concentrations. 5-Methylthioadenosine Phosphorylase (MTAP) is a key enzyme in the methionine salvage pathway that metabolizes methylthioadenosine (MTA) to adenine and methionine. Its chromosomal position proximal to CDKN2A results in frequent collateral homozygous deletion in a wide range of human cancers. By interrogating data from a large scale deep-coverage pooled shRNA screen across 390 cancer cell line models we found that the viability of MTAP null cancer cells is strongly impaired upon shRNA-mediated depletion of the protein arginine methyltransferase PRMT5. In MTAP deleted cells there is marked accumulation of the substrate MTA and surprisingly, we find that MTA is a specific inhibitor of the catalytic activity of PRMT5. In keeping with these data, knockout of MTAP in an MTAP-proficient cell line led to increased MTA levels and rendered them sensitive to PRMT5 depletion. Moreover, reconstitution of MTAP in an MTAP-deficient cell line fully rescued PRMT5 dependence. Collectively, these findings indicate that the collateral loss of MTAP in CDNK2A deleted cancers leads to accumulation of MTA that thereby creates a hypomorphic PRMT5 state that is selectively sensitized towards further PRMT5 inhibition. Citation Format: Konstantinos Mavrakis, E Robert McDonald III, Michael R. Schlabach, Eric Billy, Gregory R. Hoffman, Antoine deWeck, David A. Ruddy, Kavitha Venkatesan, Greg McAllister, Rosalie deBeaumont, Samuel Ho, Yue Liu, Yan Yan-Neale, Guizhi Yang, Fallon Lin, Hong Yin, Hui Gao, David Randal Kipp, Songping Zhao, Joshua T. McNamara, Elizabeth R. Sprague, Young Shin Cho, Justin Gu, Ken Crawford, Vladimir Capka, Kristen Hurov, Jeffrey A. Porter, John Tallarico, Craig Mickanin, Emma Lees, Raymond Pagliarini, Nicholas Keen, Tobias Schmelzle, Francesco Hofmann, Frank Stegmeier, William R. Sellers. Disordered methionine metabolism in MTAP/CDKN2A-deleted cancers leads to marked dependence on PRMT5. [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-017.

Individual microRNAs (miRNAs) have been implicated as oncogenes in experimental cancer models and their expression may affect clinical outcomes. To gain a more comprehensive view of miRNA action in leukemia, we analyzed miRNA expression patters in T-cell leukemia ALL (T-ALL) and cross-referenced the results with an unbiased genetic screen and computational analyses.1 We found that multiple microRNAs contribute to leukmogenesis and act as multi-targeted regulators of several tumor suppressor genes. The oncomirs form a network of overlapping and partially redundant interactions that stabilize the malignant phenotype though coordinate repression of cellular failsafe programs. The emerging network pattern of oncomir action is distinct from the notion of single oncogenic 'driver' mutation. We will discuss experimental, diagnostic and therapeutic implications of this concept of miRNA action in cancer.

Insights into cancer genetics can lead to therapeutic opportunities. By cross-referencing chromosomal changes with an unbiased genetic screen we identify the ephrin receptor A7 (EPHA7) as a tumor suppressor in follicular lymphoma (FL). EPHA7 is a target of 6q deletions and inactivated in 72% of FLs. Knockdown of EPHA7 drives lymphoma development in a murine FL model. In analogy to its physiological function in brain development, a soluble splice variant of EPHA7 (EPHA7(TR)) interferes with another Eph-receptor and blocks oncogenic signals in lymphoma cells. Consistent with this drug-like activity, administration of the purified EPHA7(TR) protein produces antitumor effects against xenografted human lymphomas. Further, by fusing EPHA7(TR) to the anti-CD20 antibody (rituximab) we can directly target this tumor suppressor to lymphomas in vivo. Our study attests to the power of combining descriptive tumor genomics with functional screens and reveals EPHA7(TR) as tumor suppressor with immediate therapeutic potential.

We recently identified miR-19 as the critical activity for leukemogenesis within the oncogenic 17~92 cluster of microRNAs. ( 1) This finding prompted us to test an unbiased method for pinpointing those miR-19 targets may be key to its oncogenic action. Specifically, we used a large-scale short hairpin RNA screen to identify those miR-19 target genes, whose knockdown could reproduce miR-19's effects on lymphocyte transformation. In this way, we found that miR-19 produces a coordinate clampdown on multiple negative regulators of PI3K-related survival signals. These findings have implications for the therapy of miR-19 expressing tumors. They also validate a new strategy for the unbiased identification of functionally important microRNA target genes. Using the example of miR-19 in leukemia, we will discuss some possibilities and limitations of this new approach.

MicroRNAs (miRNAs) have emerged as novel cancer genes. In particular, the miR-17-92 cluster, containing six individual miRNAs, is highly expressed in haematopoietic cancers and promotes lymphomagenesis in vivo. Clinical use of these findings hinges on isolating the oncogenic activity within the 17-92 cluster and defining its relevant target genes. Here we show that miR-19 is sufficient to promote leukaemogenesis in Notch1-induced T-cell acute lymphoblastic leukaemia (T-ALL) in vivo. In concord with the pathogenic importance of this interaction in T-ALL, we report a novel translocation that targets the 17-92 cluster and coincides with a second rearrangement that activates Notch1. To identify the miR-19 targets responsible for its oncogenic action, we conducted a large-scale short hairpin RNA screen for genes whose knockdown can phenocopy miR-19. Strikingly, the results of this screen were enriched for miR-19 target genes, and include Bim (Bcl2L11), AMP-activated kinase (Prkaa1) and the phosphatases Pten and PP2A (Ppp2r5e). Hence, an unbiased, functional genomics approach reveals a coordinate clampdown on several regulators of phosphatidylinositol-3-OH kinase-related survival signals by the leukaemogenic miR-19.

The Transforming Growth Factor (TGF) beta signalling family includes morphogens, such as Nodal and Activin, with important functions in vertebrate development. The concentration of the morphogen is critical for fate decisions in the responding cells. Smad2 and Smad3 are effectors of the Nodal/Activin branch of TGFbeta signalling: they are activated by receptors, enter the nucleus and directly transcribe target genes. However, there have been no studies correlating levels of Smad2/3 activation with expression patterns of endogenous target genes in a developmental context over time. We used mouse Embryonic Stem (ES) cells to create a system whereby levels of activated Smad2/3 can be manipulated by an inducible constitutively active receptor (Alk4*) and an inhibitor (SB-431542) that blocks specifically Smad2/3 activation. The transcriptional responses were analysed by microarrays at different time points during activation and repression. We identified several genes that follow faithfully and reproducibly the Smad2/3 activation profile. Twenty-seven of these were novel and expressed in the early embryo downstream of Smad2/3 signalling. As they responded to Smad2/3 activation in the absence of protein synthesis, they were considered direct. These immediate responsive genes included negative intracellular feedback factors, like SnoN and I-Smad7, which inhibit the transcriptional activity of Smad2/3. However, their activation did not lead to subsequent repression of target genes over time, suggesting that this type of feedback is inefficient in ES cells or it is counteracted by mechanisms such as ubiquitin-mediated degradation by Arkadia. Here we present an ES cell system along with a database containing the expression profile of thousands of genes downstream of Smad2/3 activation patterns, in the presence or absence of protein synthesis. Furthermore, we identify primary target genes that follow proportionately and with high sensitivity changes in Smad2/3 levels over 15-30 hours. The above system and resource provide tools to study morphogen function in development.

The AKT–mTOR pathway harbors several known and putative oncogenes and tumor suppressors. In a phenotypic screen for lymphomagenesis, we tested candidate genes acting upstream of and downstream from mTOR in vivo. We find that Rheb, a proximal activator of mTORC1, can produce rapid development of aggressive and drug-resistant lymphomas. Rheb causes mTORC1-dependent effects on apoptosis, senescence, and treatment responses that resemble those of Akt. Moreover, Rheb activity toward mTORC1 requires farnesylation and is readily blocked by a pharmacological inhibitor of farnesyltransferase (FTI). In Pten-deficient tumor cells, inhibition of Rheb by FTI is responsible for the drug’s anti-tumor effects, such that a farnesylation-independent mutant of Rheb renders these tumors resistant to FTI therapy. Notably, RHEB is highly expressed in some human lymphomas, resulting in mTORC1 activation and increased sensitivity to rapamycin and FTI. Downstream from mTOR, we examined translation initiation factors that have been implicated in transformation in vitro. Of these, only eIF4E was able to enhance lymphomagenesis in vivo. In summary, the Rheb GTPase is an oncogenic activity upstream of mTORC1 and eIF4E and a direct therapeutic target of farnesyltransferase inhibitors in cancer.

Regulation of transforming growth factor-β (TGF-β) signaling is critical in vertebrate development, as several members of the TGF-β family have been shown to act as morphogens, controlling a variety of cell fate decisions depending on concentration. Little is known about the role of intracellular regulation of the TGF-β pathway in development. E3 ubiquitin ligases target specific protein substrates for proteasome-mediated degradation, and several are implicated in signaling. We have shown that Arkadia, a nuclear RING-domain E3 ubiquitin ligase, is essential for a subset of Nodal functions in the embryo, but the molecular mechanism of its action in embryonic cells had not been addressed. Here, we find that Arkadia facilitates Nodal signaling broadly in the embryo, and that it is indispensable for cell fates that depend on maximum signaling. Loss of Arkadia in embryonic cells causes nuclear accumulation of phospho-Smad2/3 (P-Smad2/3), the effectors of Nodal signaling; however, these must be repressed or hypoactive as the expression of their direct target genes is reduced or lost. Molecular and functional analysis shows that Arkadia interacts with and ubiquitinates P-Smad2/3 causing their degradation, and that this is via the same domains required for enhancing their activity. Consistent with this dual function, introduction of Arkadia in homozygous null (−/−) embryonic stem cells activates the accumulated and hypoactive P-Smad2/3 at the expense of their abundance. Arkadia−/− cells, like Smad2−/− cells, cannot form foregut and prechordal plate in chimeras, confirming this functional interaction in vivo. As Arkadia overexpression never represses, and in some cells enhances signaling, the degradation of P-Smad2/3 by Arkadia cannot occur prior to their activation in the nucleus. Therefore, Arkadia provides a mechanism for signaling termination at the end of the cascade by coupling degradation of P-Smad2/3 with the activation of target gene transcription. This mechanism can account for achieving efficient and maximum Nodal signaling during embryogenesis and for rapid resetting of target gene promoters allowing cells to respond to dynamic changes in extracellular signals., Author Summary In development, cells respond to secreted signals (called morphogens) by turning on or off sets of target genes. How does gene activity adjust quickly in response to rapidly changing extracellular signals? This should require efficient removal of old/used signaling effectors (signal-activated transcription factors) from the promoters of target genes to allow new ones to assume control. We previously discovered Arkadia, an E3 ubiquitin ligase, and showed that it is an essential factor for normal development. (Ubiquitin ligases trigger the addition of ubiquitin residues to proteins, typically marking them for degradation.) Here, we show that Arkadia is required for high activity of the major signaling pathway, TGF-β/Nodal. Arkadia has a dual role to degrade Smads, the TGF-β signaling effectors, and enhance their transcriptional activity. This coupling of degradation with activation provides a mechanism to ensure that only effectors “in use” are degraded, allowing the new ones to proceed. It is possible that very similar mechanisms operate in other pathways to establish dynamic regulation and efficient signaling, while their failure may be associated with developmental abnormalities and disease, including cancer., Arkadia enhances TGF-β family activity by degrading its inhibitory Smads but also stimulating transcription of phospho-Smads.

Large scale cancer genome sequencing efforts, such as TCGA, revealed the landscape of genomic alterations across many tumor types, but the functional relevance of these alterations and genetic interdependencies cannot be fully assessed from these datasets. Thus, functional genomic screens, such as pooled shRNA 9pshRNA) screens, hold great promise to systematically identify cancer dependencies. However, RNAi discovery screens have been plagued by off target effects causing false positive results and possibly masking on-target effects. In order to address the previous problems with RNAi screens for cancer target discovery, we conducted a large scale pshRNA screening campaign targeting 7500 genes at a depth of 20 shRNAs per gene, across more than 300 cell lines of the cancer cell line encyclopedia (CCLE). The substantial shRNA and cell line depth in this screen significantly increased the robustness of results relative to previous published screens, and allowed the more robust identification of cancer dependencies. Moreover, integration of those growth phenotypes with known features of the cancer cell line encyclopedia (CCLE) enabled the identification of biomarkers (genetic, epigenetic, or proteomic) that correlate with sensitivity, and thus permitted discovery of synthetic lethal relationships. The increased depth provides a robust data set that can be used to find and validate new drug targets as well as infer pathway membership of novel genes by simple phenotypic similarity. The findings of this screen will be presented here. Citation Format: Michael Schlabach, Eric Billy, Konstantinos Mavrakis, Greg Hoffman, Tobias Schmelzle, Francesco Hofmann, Nicholas Keen, Frank Stegmeier, William Sellers. Systematic discovery of cancer dependencies through deep coverage pshRNA screens. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C151.

Abstract 2553 The Philadelphia (Ph) chromosome (t(9;22)(q34;q11)) is the most common recurrent cytogenetic abnormality in adult B-cell acute lymphoblastic leukemia (B-ALL). Resulting expression of the BCR-ABL fusion gene product, a constitutively active tyrosine kinase, leads to an extremely poor prognosis when the disease is treated with chemotherapy alone. In recent years targeted therapy with tyrosine kinase inhibitors (TKIs) in combination with chemotherapy has improved outcomes. Therapeutic resistance remains a major clinical problem, however, with less than half of patients surviving three years from initial therapy in most series even with up-front TKI-chemotherapy regimens. TKI resistance in B-ALL may result from acquired mutations of the BRC-ABL protein or from BCR-ABL-independent causes, including feedback activation of BCL6 or activation of other oncogenic signaling pathways through poorly understood mechanisms. In this study, we screened a whole-genome library of microRNAs for ability to produce resistance to TKI therapy in murine pro-B cells transformed with BCR-ABL. Several initial candidates from the screen were individually confirmed to confer TKI resistance, including miR 148/152 and 19, known oncomirs in T-ALL. Computational analysis of the gene targets of validated miRs shows significant overlap at several tumor-suppressor pathways, including down-regulation of PTEN, a known mechanism of TKI resistance in Ph+ B-ALL cell lines. In sum, we have identified a novel mechanism of TKI resistance in Ph+ B-ALL mediated by miRs and leading to down-regulation of specific tumor suppressive intermediates. These findings will allow development of new therapeutic combinations to improve outcomes for B-ALL patients. Disclosures: No relevant conflicts of interest to declare.

Corrigendum: A cooperative microRNA-tumor suppressor gene network in acute T-cell lymphoblastic leukemia (T-ALL)