NOTCH1-driven T-cell Acute Lymphoblastic Leukemia Research Articles

Integrating rapamycin with novel PI3K/Akt/mTOR inhibitor microRNAs on NOTCH1-driven T-cell acute lymphoblastic leukemia (T-ALL).

The PI3K/AKT/mTOR signaling pathway plays a significant role in the development of T-cell acute lymphoblastic leukemia (T-ALL). Rapamycin is a potential therapeutic strategy for hematological malignancies due to its ability to suppress mTOR activity. Additionally, microRNAs (miRNAs) have emerged as key regulators in T-ALL pathophysiology and treatment. This study aimed to investigate the combined effects of rapamycin and miRNAs in inhibiting the PI3K/AKT/mTOR pathway in T-ALL cells. Bioinformatic algorithms were used to find miRNAs that inhibit the PI3K/AKT/mTOR pathway. Twenty-five bone marrow samples were collected from T-ALL patients, alongside five control bone marrow samples from non-leukemia patients. The Jurkat cell line was chosen as a representative model for T-ALL. Gene and miRNA expression levels were assessed using quantitative real-time PCR (qRT-PCR). Two miRNAs exhibiting down-regulation in both clinical samples and Jurkat cells were transfected to the Jurkat cell line to investigate their impact on target gene expression. Furthermore, in order to evaluate the potential of combination therapy involving miRNAs and rapamycin, apoptosis and cell cycle assays were carried out. Six miRNAs (miR-3143, miR-3182, miR-99a/100, miR-155, miR-576-5p, and miR-501- 3p) were predicted as inhibitors of PI3K/AKT/mTOR pathway. The expression analysis of both clinical samples and the Jurkat cell line revealed a simultaneous downregulation of miR-3143 and miR-3182. Transfection investigation demonstrated that the exogenous overexpression of miR-3143 and miR-3182 can effectively inhibit PI3K/AKT/mTOR signaling in the Jurkat cell line. Moreover, when used as a dual inhibitor along with rapamycin, miR-3143 and miR-3182 significantly increased apoptosis and caused cell cycle arrest in the Jurkat cell line. These preliminary results highlight the potential for improving T-ALL treatment through multi-targeted therapeutic strategies involving rapamycin and miR-3143/miR-3182.

KDM6B protects T-ALL cells from NOTCH1-induced oncogenic stress.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematopoietic neoplasm resulting from the malignant transformation of T-cell progenitors. While activating NOTCH1 mutations are the dominant genetic drivers of T-ALL, epigenetic dysfunction plays a central role in the pathology of T-ALL and can provide alternative mechanisms to oncogenesis in lieu of or in combination with genetic mutations. The histone demethylase enzyme KDM6A (UTX) is also recurrently mutated in T-ALL patients and functions as a tumor suppressor. However, its gene paralog, KDM6B (JMJD3), is never mutated and can be significantly overexpressed, suggesting it may be necessary for sustaining the disease. Here, we used mouse and human T-ALL models to show that KDM6B is required for T-ALL development and maintenance. Using NOTCH1 gain-of-function retroviral models, mouse cells genetically deficient for Kdm6b were unable to propagate T-ALL. Inactivating KDM6B in human T-ALL patient cells by CRISPR/Cas9 showed KDM6B-targeted cells were significantly outcompeted over time. The dependence of T-ALL cells on KDM6B was proportional to the oncogenic strength of NOTCH1 mutation, with KDM6B required to prevent stress-induced apoptosis from strong NOTCH1 signaling. These studies identify a crucial role for KDM6B in sustaining NOTCH1-driven T-ALL and implicate KDM6B as a novel therapeutic target in these patients.

The Histone Demethylase KDM6B Is a Genetic Dependency of NOTCH1-Driven T-ALL

T-cell acute lymphoblastic leukemia (T-ALL) arises from the accumulation of genomic abnormalities and the malignant proliferation of immature T-cells. Despite recent advancements in understanding the genetic alterations driving T cell leukemogenesis, patients still suffer from recurrent relapses and treatment-related toxicities. Genome sequencing has revealed significant heterogeneity and important insights into the genetic landscape of T-ALL. Mutations in epigenetic modifiers are frequently observed and serve as an attractive target for novel therapeutic approaches.Histone demethylase enzymes play a critical role in the regulation of gene expression programs in T-ALL. KDM6A (UTX) is known to behave as a tumor suppressor in most T-ALL subtypes. However, it's gene paralog, KDM6B (JMJD3), is never mutated and can be significantly overexpressed in patients. Here, we show that KDM6B is required for T-ALL initiation. Using genetic mouse models, Sca-1 enriched WBM from Vav-Cre: Kdm6b+/+, Vav-Cre: Kdm6bfl/+, and Vav-Cre: Kdm6bfl/fl adult mice was transduced with a retrovirus expressing Notch1 Intracellular Domain (NICD). NOTCH1 gain-of-function mutations are the most frequent driver events in adult T-ALL, and this model recapitulates many of the human pathologies. Transduced cells were transplanted into irradiated mice. While there was robust engraftment in all groups at four weeks post-transplant, T-ALL cells were not sustained in the genetic absence of Kdm6b. Mice receiving control NICD-GFP+ cells succumbed to T-ALL with median survival of 79 days, whereas the only mice receiving Kdm6b-null NICD-GFP+ cells that developed disease were found to retain one copy of the Kdm6b floxed allele (Fig 1). To investigate the translational potential, we targeted KDM6B for genetic inactivation by CRISPR/Cas9 in primary T-ALL patient cells, followed by xenograft into NSG mice. The effect of KDM6B targeting was quantified over time by monitoring the variant allele fraction (VAF) of the transplanted cells. In most patient samples, KDM6B-targeted cells were significantly outcompeted over time, thus further highlighting the requirement of KDM6B in sustaining T-ALL tumorigenesis.To examine the mechanism by which KDM6B sustain T-ALL cells, gene expression profiling was performed by RNA-seq on mouse T-ALL cells of genetic backgrounds Control, Kdm6b-Het, and Kdm6b-KO. Three gene sets were significantly downregulated in Kdm6b-deficient T-ALL cells compared to the Control group, all of which are involved in cell cycle processes. Additional validation of these findings with cell cycle functional studies is currently ongoing. Additionally, while Kdm6b has been described for its H3K27me3 histone demethylase function, recent studies have shown its involvement in regulating various gene expression programs through demethylase-independent mechanisms. To determine if the role of Kdm6b in T-ALL oncogenesis was related to it demethylase activity, we performed the same NICD retroviral transduction experiment with inclusion of a Kdm6b +/H1388A mouse genotype, a point mutation which renders Kdm6b catalytically dead. Our data shows that Kdm6b +/H1388A phenocopies T-ALL with Kdm6b homozygous loss-of-function, showing no evidence of disease in the blood beyond 8-weeks post-transplant. We conclude from this that the leukemogenic role of Kdm6b requires it's H3K27me3 demethylase function. In summary, these data reveal Kdm6b as an oncogenic dependency in Notch1-driven T-ALL in both mouse and human systems, and present Kdm6b as a high value therapeutic target in adult T-ALL patients. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Downregulation of Glutamine Synthetase, not glutaminolysis, is responsible for glutamine addiction in Notch1-driven acute lymphoblastic leukemia.

The cellular receptor Notch1 is a central regulator of T-cell development, and as a consequence, Notch1 pathway appears upregulated in >65% of the cases of T-cell acute lymphoblastic leukemia (T-ALL). However, strategies targeting Notch1 signaling render only modest results in the clinic due to treatment resistance and severe side effects. While many investigations reported the different aspects of tumor cell growth and leukemia progression controlled by Notch1, less is known regarding the modifications of cellular metabolism induced by Notch1 upregulation in T-ALL. Previously, glutaminolysis inhibition has been proposed to synergize with anti-Notch therapies in T-ALL models. In this work, we report that Notch1 upregulation in T-ALL induced a change in the metabolism of the important amino acid glutamine, preventing glutamine synthesis through the downregulation of glutamine synthetase (GS). Downregulation of GS was responsible for glutamine addiction in Notch1-driven T-ALL both invitro and invivo. Our results also confirmed an increase in glutaminolysis mediated by Notch1. Increased glutaminolysis resulted in the activation of the mammalian target of rapamycin complex 1 (mTORC1) pathway, a central controller of cell growth. However, glutaminolysis did not play any role in Notch1-induced glutamine addiction. Finally, the combined treatment targeting mTORC1 and limiting glutamine availability had a synergistic effect to induce apoptosis and to prevent Notch1-driven leukemia progression. Our results placed glutamine limitation and mTORC1 inhibition as a potential therapy against Notch1-driven leukemia.

Overcoming NOTCH1-Driven Chemoresistance in T-Cell Acute Lymphoblastic Leukemia Via Metabolic Intervention with Oxphos Inhibitor

Overcoming NOTCH1-Driven Chemoresistance in T-Cell Acute Lymphoblastic Leukemia Via Metabolic Intervention with Oxphos Inhibitor

Abstract 3434: A tumor suppressor enhancer of PTEN in T-cell development and transformation

Abstract Recent studies have dissected the mutational landscape of T-cell acute lymphoblastic leukemia (T-ALL), identifying several oncogenes and tumor suppressors implicated in T-cell transformation. However, most genetic lesions in cancer are located in intergenic regions, whose role remains poorly understood. In this context, 20% of T-ALL patients show loss of expression of the tumor suppressor PTEN, but not all can be explained by mutations/deletions of its coding region, defective splicing or promoter hypermethylation. Thus, we hypothesized that alterations of yet unidentified enhancers might play a critical role in the regulation of PTEN expression in T-ALL. To test this, we first performed 4C-seq of the PTEN promoter in human T-ALL cells. Integration of our results with epigenetic profiling revealed a distal non-coding region (named PE for PTEN enhancer) that shows concomitant interaction with the PTEN promoter and bona fide enhancer marks. Moreover, reciprocal 4C-seq assays using PE as viewpoint confirmed its interaction with the PTEN promoter. Next, multispecies DNA sequence alignment revealed notable conservation of PE in mammals and detailed analysis of H3K27ac across 64 cell lines and tissues revealed that PE is specifically active in T-cells and other hematopoietic cells, but is largely repressed in non-hematopoietic cells. Moreover, luciferase assays showed strong, orientation-independent activation of reporter constructs containing either the mouse or human PE sequence in Jurkat cells, and CRISPR/Cas9-induced deletion of PE in human T-ALL cells led to reduced PTEN levels, increased phospho-AKT and enhanced cellular proliferation. To formally test the functional relevance of PE in T-cell development and transformation, we generated PE knockout and conditional knockout mice. PE-null animals are viable and show a marked increase in thymus size and cellularity as their only developmental alteration. Mechanistically, this phenotype is associated with a marked reduction in Pten expression. Next, we analyzed the role of PE in the induction of NOTCH1-driven T-ALLs by transplanting mice with PE wild-type or knockout hematopoietic progenitors infected with retroviruses expressing a mutant constitutively active form of NOTCH1. In this context, mice transplanted with NOTCH-infected PE-null cells showed 100% penetrance and developed T-ALL ~50 days after transplant. However, mice transplanted with NOTCH-infected PE wild-type cells showed delayed T-ALL kinetics and reduced penetrance. In addition, secondary loss of PE in already established NOTCH1-induced T-ALLs from PE conditional knockout mice led to accelerated T-ALL progression and a gene expression signature driven by Pten loss. Finally, analyses of human T-ALL patient samples uncovered recurrent deletions encompassing PE without concomitant deletion of the PTEN coding sequence. Altogether, our results identify PE as the first long-range tumor suppressor enhancer directly implicated in the pathogenesis of human leukemia. Citation Format: Luca Tottone, Olga Lancho, Maria Victoria da Silva-Diz, Shirley Luo, Shreeta Chakraborty, Pedro P. Rocha, Daniel Herranz. A tumor suppressor enhancer of PTEN in T-cell development and transformation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3434.

The Nucleotide Kinase Nadk Is Required for ROS Detoxification and Constitutes a Metabolic Vulnerability of NOTCH1-Driven T-ALL

Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer with a cumulative risk of ~1 in 2,000 children by the age of 15 years and an increasing incidence over the last 30 years.Whilst advances in the use of combination chemotherapy have significantly improved outcomes, current treatments remain toxic and can have long-term health consequences. T-cell acute lymphoblastic leukemia (T-ALL) represents between 15 to 25% of all ALL and affects both children and adults, and carries a worse prognosis. Activating mutations in the NOTCH1gene are found in more than 60% of cases and are being targeted therapeutically with compounds such as γ-secretase inhibitors (GSIs) and Notch inhibiting antibodies (mAbs). However, none of these approaches has entered mainstream therapy as yet.To address this, we performed a genome wide CRISPR-Cas9 screen in T-ALL cell lines driven by NOTCH1 overexpression (Jurkat, CEM-CCRF), or mutation (PEER) and in a line not dependent on NOTCH1 (Loucy) (Figure 1A). To discover NOTCH1-specific genetic vulnerabilities, we compared essential genes for NOTCH1-dependent T-ALL cell lines with those for Loucy and for 6 human myeloid leukemia cells (MV4;11, MOLM13, OCI-AML2, OCI-AML3, K562 and HL60)1. This identified 69 NOTCH-specific essential genes, including well known players in NOTCH signaling such as NOTCH1, RBPJ, BCL11B, GATA3 and CTPB1(Figure 1B). In addition to these genes, we also identified a separate group of genes associated to cellular pathways involved in the reduction of reactive oxygen species (ROS). One of these was NADK, the gene for nicotinamide adenine dinucleotide kinase, which we investigate further. NADK drives the conversion of NAD+to NADP+, which in its reduced form (NADPH) is used in the reduction of ROS levels through the production of reduced Glutathione. Exposure of Loucy (NOTCH1-WT) to the oxidant H2O2showed that it was not able to reduce intracellular ROS levels as efficiently as NOTCH1-driven lines, suggesting increased NADK activity in the latter. In keeping with this, intracellular NADP+/NADPH levels that were significantly higher in NOTCH1-driven cell lines than in Loucy (Figure 1C). In addition, pharmaceutical or genetic inhibition of NADK led to a significant increase in ROS level in NOTCH1-driven Jurkat cells, while Loucy cells were not affected (Figure 1D). This increase in ROS levels was associated with a significant decrease in cell proliferation (Figure 1E). It was previously reported that, in both human and mouse T-ALL cells, the intracellular domain of Notch1 (NiCD) suppresses production of ROS levels and imparts a higher resistance to H2O2,in so doing facilitating the proliferation of these cells2. We hypothesize that NADK is the mediator of this effect of NOTCH1 and this makes it a therapeutic vulnerability in NOTCH1-driven T-ALL and potentially in other NOTCH1-driven cancers. We currently testing this hypothesis in vivo using patient-derived T-ALL xenografts into immunocompromised mice and investigating the molecular events involved in the NOTCH1-NADK interaction.Our findings propose NADK as a novel therapeutic vulnerability in NOTCH1-driven T-ALL and demonstrate the power genome-wide CRISPR screens in finding novel genetic and therapeutic targets for this disease.1. Tzelepis, K.et al.A CRISPR Dropout Screen Identifies Genetic Vulnerabilities and Therapeutic Targets in Acute Myeloid Leukemia. Cell Rep17, 1193-1205 (2016).2. Giambra, V.et al.NOTCH1 promotes T cell leukemia-initiating activity by RUNX-mediated regulation of PKC-theta and reactive oxygen species. Nat Med18, 1693-8 (2012). [Display omitted] DisclosuresVassiliou:KYMAB: Consultancy, Equity Ownership; Celgene: Research Funding.

Mitochondrial Complex I Inhibitor Iacs-010759 Reverses the NOTCH1-Driven Metabolic Reprogramming in T-ALL Via Blockade of Oxidative Phosphorylation: Synergy with Chemotherapy and Glutaminase Inhibition

Adult T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy characterized by limited therapeutic options and a high rate of treatment failure due to chemoresistance. T-ALL is largely driven by activating NOTCH1 mutations, where oncogenic NOTCH1 facilitates glutamine oxidation, induces metabolic stress, and facilitates reliance on oxidative phosphorylation (OXPHOS)1. In other malignancies, the shift toward OXPHOS-dependent high-energy status is associated with acquired chemoresistance.In this study, we found that the novel inhibitor of mitochondrial complex I (OXPHOSi) IACS-0107592 has preclinical activity in NOTCH1-mutated T-ALL; we also characterize the cellular and metabolic responses to OXPHOS inhibition and propose that an OXPHOSi be incorporated into standard-of-care therapy to improve outcomes in patients harboring NOTCH1-mutated T-ALL.Exposure to IACS-010759 (0-370 nM) in vitro drastically reduced T-ALL viability, with EC50 ranging from 0.1-10 nM for cell lines (n=7) and from 13-60 nM for patient-derived xenograft (PDX)-derived and primary T-ALL cells (n=10) (Fig.1). Oral administration of IACS-010759 (7.5 mg/kg/day) significantly reduced leukemia burden and extended overall survival (p<0.0001) in two aggressive NOTCH1-mutated T-ALL PDX models and in a murine NOTCH1-driven T-ALL model (Fig.4). Addition of OXPHOS inhibitor to dexamethasone (X), vincristine (V), asparaginase (L), or a combination (VXL) led to additive/synergistic inhibition of cell proliferation in vitro and to doubling of overall survival in vivo (p<0.0001) (Fig.4). Metabolic characterization confirmed that IACS-010759 caused striking dose-dependent decreases in basal and maximal oxygen consumption rates (OCR) and ATP and NADH production in T-ALL cell lines and primary T-ALL samples (p<0.001; Fig.2). Further, pretreatment with V, X, or L shifted T-ALL cell metabolism toward OXPHOS, increasing significantly the OCR that was effectively inhibited by IACS-010759. Pharmacological inhibition of complex I with IACS-010759, similar to knockout of complex I subunit NDUFS4 using CRISPR-CAS9, induced catastrophic changes in mitochondria, with induction of mitochondrial reactive oxygen species (ROS), DNA damage, and activation of the compensatory mTOR pathway. OXPHOS inhibition altered cellular energy homeostasis through reduction of TCA cycle intermediates; decreased glutathione level (by UPLC-MS/MS; p<0.0001) with ROS induction (Fig.3); and depleted the pool of intracellular nucleotides, affecting DNA and RNA synthesis (Fig.2C). Stable isotope-resolved metabolomics (SIRM) flux analysis showed that IACS-010759 (30 nM at 24 h) significantly decreased the flux of glucose through the TCA cycle and redirected it toward lactate production and increased utilization of glutamine for fueling the TCA cycle, in particular through reductive metabolism, uncovering reliance on glutaminolysis as an additional therapeutic target. Consistent with this was the finding that combined OXPHOSi with glutaminase inhibitor CB-839 caused additive reduction of viability of T-ALL cells lines and primary T-ALL cells in vitro (Fig.2), decreased tumor burden (p<0.02), and increased survival in a T-ALL PDX model (p<0.01). This was supported by IACS-induced reduction of tumor burden in a NOTCH1-mutated GLS fl/fl murine model upon tamoxifen-induced GLS knockout (p<0.01).In summary, our findings indicate that OXPHOSi, alone and particularly in combination with standard chemotherapy and GLS inhibition, constitutes a novel therapeutic modality that targets a unique metabolic vulnerability of NOTCH1-mutated T-ALL cells.

MBD2 Ablation Impairs Lymphopoiesis and Impedes Progression and Maintenance of T-ALL.

Aberrant DNA methylation patterns in leukemia might be exploited for therapeutic targeting. In this study, we employed a genetically deficient mouse model to explore the role of the methylated DNA binding protein MBD2 in normal and malignant hematopoiesis. MBD2 ablation led to diminished lymphocytes. Functional defects of the lymphoid compartment were also observed after in vivo reconstitution of MBD2-deficient hematopoietic stem cells (HSC). In an established model of Notch1-driven T-cell acute lymphoblastic leukemia (T-ALL), MBD2 ablation impeded malignant progression and maintenance by attenuating the Wnt signaling pathway. In clinical specimens of human T-ALL, Wnt signaling pathway signatures were significantly enhanced and positively correlated with the expression and function of MBD2. Furthermore, a number of typical Wnt signaling inhibitory genes were abnormally hypermethylated in primary human T-ALL. Abnormal activation of Wnt signaling in T-ALL was switched off by MBD2 deletion, partially by reactivating epigenetically silenced Wnt signaling inhibitors. Taken together, our results define essential roles for MBD2 in lymphopoiesis and T-ALL and suggest MBD2 as a candidate therapeutic target in T-ALL.Significance: This study highlights a methylated DNA binding protein as a candidate therapeutic target to improve the treatment of T-cell acute lymphoblastic leukemias, as a new starting point for developing epigenetic therapy in this and other lymphoid malignancies. Cancer Res; 78(7); 1632-42. ©2018 AACR.

Synergistic Drug Combinations with a CDK4/6 Inhibitor in T-Cell Acute Lymphoblastic Leukemia

While significant progress has been made in the treatment of T-cell acute lymphoblastic leukemia (T-ALL), approximately 10-20% of newly diagnosed patients will experience either induction failure or relapse. Additionally, fewer than 50% of T-ALL patients who experience a relapse are long-term survivors. New targeted therapies are needed for the treatment of this disease.Multiple lines of evidence point to Cyclin D3/CDK4/6 as a potential therapeutic target in T-ALL. Cyclin D3 (CCND3), a direct target of activated NOTCH1, is upregulated in T-ALL, and CCND3 null animals are refractory to NOTCH1 driven T-ALL. CCND3 binds and activates CDK4/6, and the CCND3-CDK complex phosphorylates the tumor suppressor RB leading to cell cycle progression. Previous studies have demonstrated that CDK4/6 small-molecule inhibition is an effective therapeutic strategy for the treatment of NOTCH1-driven T-ALL mouse models.Using the publicly available Genomics of Drug Sensitivity in Cancer data set, we identified NOTCH1 mutations as a biomarker of response and RB mutations as a biomarker of resistance to the CDK4/6 inhibitor palbociclib. We validated that RB null status predicts resistance to the Novartis CDK4/6 inhibitor LEE011 in a panel of T-ALL cell lines. Interestingly, we identified both NOTCH1 mutant, as well as NOTCH1 wildtype, T-ALL cell lines that were sensitive to LEE011 treatment. Mining of publicly available data revealed that CDK6 is consistently marked by a super-enhancer in T-ALL cell lines, both NOTCH1 mutant and wildtype, suggesting another potential reason for sensitivity to CDK4/6 inhibition in this lineage. Treatment with LEE011 also led to a dose-dependent cell cycle arrest and cell death in T-ALL cells, including MOLT4 (NOTCH1 mutant) and MOLT16 (NOTCH1 wildtype).Combinations of drugs with CDK4/6 inhibitors will likely be critical for the successful translation of this drug class because they generally do not induce cell death. Combinations with cytotoxic chemotherapy are predicted to be antagonistic, however, as most of these drugs rely on rapidly proliferating cells, and CDK4/6 inhibition induces cell cycle arrest. To discover effective, and immediately translatable combination therapies with LEE011 in T-ALL, we performed combination studies of LEE011 with agents standardly used for T-ALL treatment, including corticosteroids, methotrexate, mercaptopurine, asparaginase, vincristine and doxorubicin. Combinations of LEE011 with methotrexate, mercaptopurine, vincristine or asparaginase were antagonistic in T-ALL cell lines while the combination with doxorubicin was additive. Combination treatment of LEE011 with corticosteroids had a synergistic effect on cell viability in MOLT4 and MOLT16 cell lines as measured by excess over Bliss additive and isobologram analyses. This combination also decreased phospho RB signaling, increased cell cycle arrest and induced cell death to a greater degree than either drug alone. LEE011 treatment increased CCND3 protein levels, an effect mitigated by glucocorticoid treatment, one possible mechanism contributing to the observed synergy. Additionally, the combination of LEE011 with everolimus, an mTOR inhibitor, was synergistic in these cell lines.We next extended testing to in vivo models of T-ALL. In a MOLT16 orthotopic mouse model, the combination of LEE011 and everolimus significantly prolonged mouse survival compared to treatment with each individual drug alone. The combination of LEE011 with dexamethasone did not extend survival over treatment with LEE011 alone and dexamethasone was inactive in vivo. Both LEE011 and everolimus had on-target activity in the treated mice as measured by inhibition of peripheral blood phospho-RB and phospho-4EBP1. We then tested the combination of LEE011 with dexamethasone in a second mouse model, a MOLT4 orthotopic model. Here, the combination of LEE011 with dexamethasone was more effective in prolonging survival compared to each treatment alone, supporting a heterogeneous response to the combination of LEE011 with dexamethasone in vivo.We conclude that LEE011 is active in T-ALL, and that combination therapy with corticosteroids and/or mTOR inhibitors warrants further investigation in the clinical setting. DisclosuresKim:Novartis Pharmaceuticals: Employment. Stegmaier:Novartis Pharmaceuticals: Consultancy.

Rictor/mammalian target of rapamycin 2 regulates the development of notch1 induced murine T-cell acute lymphoblastic leukemia via forkhead box O3

Mammalian target of rapamycin (mTOR) is composed of two distinct biochemical complexes, mTORC1 and mTORC2. In response to nutrients and growth factors, mTORC1 is known to control cellular growth by regulating the translational regulators S6 kinase 1 and 4E binding protein 1, whereas mTORC2 mediates cell proliferation and survival by activating Akt through phosphorylation at Ser473. Studies have shown that the deregulation of mTORC2 leads to the development of myeloproliferative disorder and leukemia in the phosphatase and tensin homolog deleted on chromosome ten (PTEN)-deleted mouse model. However, the mechanism by which mTORC2 specifically affects leukemogenesis is still not fully understood. Here, we investigated the role of mTORC2 in NOTCH1-driven T-cell acute lymphoblastic leukemia (T-ALL) in a Rictor-deficient mouse model. We found that, by deleting Rictor, an essential component of mTORC2, leukemia progression was significantly suppressed by arresting a greater proportion of Rictor(△/△) leukemic cells at the G0 phase of the cell cycle. Furthermore, the absence of Rictor led to the overexpression of chemotaxis-related genes, such as CCR2, CCR4 and CXCR4, which contributed to the homing and migration of Rictor-deficient T-ALL cells to the spleen but not the bone marrow. In addition, we demonstrated that inactivation of mTORC2 caused the overexpression of forkhead box O3 and its downstream effectors and eased the progression of leukemia in T-ALL mice. Our study thus indicates that forkhead box O3 could be a potential drug target for the treatment of T-ALL leukemia.