NOTCH1 In T-cell Acute Lymphoblastic Leukemia Research Articles

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

Synergistic Targeting of Protein Translation and Inhibition of NOTCH Signaling in T-ALL

Oncogenic NOTCH signaling is a major driver of T-cell transformation in T-cell acute lymphoblastic leukemia (T-ALL). However, clinical studies testing the efficacy of NOTCH1 inactivation with γ-secretase inhibitors (GSIs) have shown limited antileukemic activity for these drugs as single agents. Here we used an expression-based virtual screening approach and network perturbation analyses to identify and functionally characterize new highly active antileukemic drugs synergistic with NOTCH1 inhibition in T-ALL. Gene expression profiling studies have shown a prominent gene expression signature dominated by genes involved in growth and metabolism downstream of NOTCH1 in T-ALL. Notably, loss of the PTEN tumor suppressor gene confers resistance to GSI therapy and effectively rescues the gene expression signature induced by NOTCH1 inhibition in T-ALL. We hypothesized that drugs inducing transcriptional programs overlapping with those driven by NOTCH1 inhibition and antagonizing those resulting from PTEN loss could have synergistic antileukemic effects with GSIs in PTEN wild type and PTEN null leukemia cells. To address this question we generated gene expression signatures from Pten conditional-inducible knockout NOTCH1-driven leukemias in basal condition, upon NOTCH1 inhibition by GSI treatment and upon deletion of Pten. Connectivity Map (cMAP) analysis in this series identified 17 high scoring compounds as candidate antileukemic drugs (p<0.01). Reassuringly these included two inhibitors of the mTOR/PI3K/AKT pathway (rapamycin, wortmannin), but also histone deacetylase inhibitors (vorinostat, trichostatin A and valproic acid), phenothiazine antipsychotic drugs (trifluoperazine and thioridazine), antimalarial agents (astemizole, mefloquine) and compounds with less characterized activities such as withaferin A, parthenolide and pyrvinium pamoate. Transcriptional profiling followed by pairwise gene set enrichment analysis of these compounds identified groups of drugs with highly interconnected transcriptional programs suggestive of an overlapping mechanism of action (e.g. mTOR/PI3K inhibitors, HDAC inhibitors and phenothiazines), as well as compounds with more unique expression signatures suggestive of a more distinct mode of action (e.g. withaferin A, astemizole and mefloquine). Detailed characterization of the antileukemic effects of these 17 cMAP hits alone and in combination with the GSI DBZ in a broad panel of human NOTCH1-mutated T-ALL cell lines, identified withaferin A, rapamycin, wortmannin, parthenolide and vorinostat as the most active (lethal dose 50 <0.5 µM) and GSI-synergistic (combination index <0.4) drugs in this series. Among these, withaferin A, stood out as the most cytotoxic and GSI-synergistic compound against both PTEN positive and PTEN null T-ALL cell lines. Moreover, withaferin A treatment of primary mouse NOTCH1-induced T-ALLs and primary human T-ALL xenografts demonstrated strong and GSI-synergistic antileukemic activity in vivo.To address the mechanisms mediating the antileukemic effects of withaferin A we performed a detailed analysis of the gene expression signatures induced by this drug in T-ALL lymphoblasts. These studies revealed a strong enrichment of downregulated genes involved in translation regulation in T-ALL cells upon treatment with withaferin A (p<0.001). Mechanistically, transcriptional network perturbation analysis identified the eIF2A translation initiation complex as a potential effector of the antileukemic effects of withaferin A, and withaferin A treatment induced strong dose dependent phosphorylation of eIF2S1 in position S51, a modification responsible for blocking the activity of the eIF2A complex. Consistently, polysome profiling and nascent-protein assays revealed decreased translation in T-ALL cells treated with withaferin A. In this context, expression a phosphomimetic mutant form of eIF2S1 (S51D) impaired leukemia cell viability. Moreover, expression of a non-phosphorylatable form of eIF2S1 (eIF2S1 S51A) in T-ALL cells abrogated the antileukemic effects of withaferin A.These results support a direct role of eIF2S1 phosphorylation and the inhibition of eIF2A-dependent translation as a critical mediators of the antileukemic effects of withaferin A in T-ALL and a role for the combination of GSIs and inhibitors of protein translation for the treatment of high risk T-ALL. DisclosuresCalifano:Therasis Inc: Employment; Cancer Genetics Inc: Consultancy; Ipsen pharmaceuticals: Consultancy; Thermo Fischer Scientific: Consultancy.

PI3K/AKT signaling inhibits NOTCH1 lysosome-mediated degradation.

The pathways of NOTCH and PI3K/AKT are dysregulated in about 60% and 48% of T-cell acute lymphoblastic leukemia (T-ALL) patients, respectively. In this context, they interact and cooperate in controlling tumor cell biology. Here, we propose a novel mechanism by which the PI3K/AKT pathway regulates NOTCH1 in T-ALL, starting from the evidence that the inhibition of PI3K/AKT signaling induced by treatment with LY294002 or transient transfection with a dominant negative AKT mutant downregulates NOTCH1 protein levels and activity, without affecting NOTCH1 transcription. We showed that the withdrawal of PI3K/AKT signaling was associated to NOTCH1 phosphorylation in tyrosine residues and monoubiquitination of NOTCH1 detected by Ubiquitin capture assay. Co-immunoprecipitation assay and colocalization analysis further showed that the E3 ubiquitin ligase c-Cbl interacts and monoubiquitinates NOTCH1, activating its lysosomal degradation. These results suggest that the degradation of NOTCH1 could represent a mechanism of control by which NOTCH1 receptors are actively removed from the cell surface. This mechanism is finely regulated by the PI3K/AKT pathway in physiological conditions. In pathological conditions characterized by PI3K/AKT hyperactivation, such as T-ALL, the excessive AKT signaling could lead to NOTCH1 signaling dysregulation. Therefore, a therapeutic strategy directed to PI3K/AKT in T-ALL could contemporaneously inhibit the dysregulated NOTCH1 signaling. © 2015 Wiley Periodicals, Inc.

­­­­­­N-Me, a Long Range T-Cell Specific Oncogenic Enhancer in T-ALL

Over the last years, numerous studies have dissected the mutational landscape of T-cell acute lymphoblastic leukemia (T-ALL) resulting in the identification of numerous oncogenes and tumor suppressors implicated in T-cell transformation. However, most genetic abnormalities found in cancer are located in intergenic regions, whose role in cancer development, if any, remains poorly understood. Here we hypothesized that recurrent cancer-associated intergenic mutations, amplifications and deletions may implicate strong transcriptional regulatory sequences responsible for the activation of key oncogenic factors in the pathogenesis of T-ALL. To address this question, we first used ChIPseq analysis to map the genomic landscape of enhancer sequences controlled by NOTCH1, a critical T-ALL oncogene activated by mutations in over 60% of human T-ALLs. In addition, we performed high resolution aCGH analysis of somatic chromosomal amplifications and deletions in a comprehensive series of 160 T-ALL samples. These analyses revealed recurrent focal duplications at chromosome 8q24 in 8/160 (5%) T-ALL cases in an area devoid of protein-coding genes containing a prominent 1 kb NOTCH1-binding peak. Strikingly, this putative oncogenic element is located +1,427 kb 3’ from the MYC locus and chromatin configuration 3C analysis demonstrated its direct association with the MYC proximal promoter. Multispecies DNA sequence alignment revealed remarkable conservation of this region in mammals, birds and reptiles and local ChIP analysis revealed bona fide active enhancer features including P300 occupancy and high levels of H3K4me1 with low levels of H3K4me3. In addition, detailed analysis of epigenetic marks across 64 hematopoietic and non-hematopoietic cell lines and tissues revealed that his regulatory element is located within a major superenhancer specifically active in T-cells. Based on these results, we proposed that this regulatory sequence, hereby named N-Me for NOTCH-bound MYCenhancer, could function as an important regulatory element driving the activation of MYC downstream of NOTCH1 in T-ALL. Consistently, luciferase reporter assays showed strong, orientation-independent and NOTCH-dependent activation of reporter constructs containing N-Me in association with a -2.5 kb MYC proximal promoter in JURKAT T-ALL cells.Next and to formally test the functional relevance of this enhancer in T-cell development and transformation we generated and characterized N-Me knockout and conditional knockout mice. N-Me null animals were viable and showed a marked and selective reduction in thymus size and cellularity as their only developmental alteration. Detailed immunophenotypic analysis of N-Me knockout thymocytes demonstrated accumulation of double negative 3 (DN3) T cells and marked reductions in double positive and CD4+ and CD8+ single positive cells. Mechanistically, this phenotype was associated with a marked reduction in Myc expression in DN3, DN4 and ISP cells. Moreover, transplantation of N-Me knockout bone marrow hematopoietic progenitors demonstrated that this phenotype is cell autonomous, and can be rescued upon retroviral expression of MYC. Next, we analyzed the role of N-Me in the induction of NOTCH-driven leukemias by transplanting mice with N-Me wild-type and knockout hematopoietic progenitors infected with retroviruses expressing a mutant constitutively active form of NOTCH1 (ΔE-NOTCH1). In this context, mice transplanted with ΔE-NOTCH1-infected N-Me wild type cells developed T-ALL with 100% penetrance 6 weeks after transplant. In contrast, mice transplanted with ΔE-NOTCH1-N-Me knockout cells showed complete resistance to NOTCH1-induced transformation remaining 100% leukemia-free at 15 weeks post-transplant. In addition, secondary deletion of N-Me in established NOTCH1 induced leukemias from tamoxifen-inducible N-Me conditional knockout mice (Rosa26TMCre N-Meflox/flox) induced profound antileukemic effects with extended survival and almost complete suppression of leukemia initiating cell activity. Altogether, these results identify N-Me as the first long range oncogenic enhancer directly implicated in the pathogenesis of human leukemia. DisclosuresNo relevant conflicts of interest to declare.

Suppression of notch1-dependent T-cell leukemia by β-catenin inhibition

T-cell acute lymphoblastic leukemia (T-ALL) is frequently associated with activating mutations of Notch1, which however are not sufficient to recapitulate the disease. Here, we investigated the role of β-catenin as a co-operating factor for Notch1 in T-ALL, prompted by the observations that β-catenin and Notch1 share the regulation of functions associated with cancer and stemness. Utilizing a small molecule (PKF115-584), which inhibits the transcriptional activity of β-catenin, we show that Notch and Wnt pathways are synergistically required for T-ALL cell growth.

Alteration ofNOTCH1in T-cell Acute Lymphoblastic Leukemia and Development of Target Therapeutic Agent

T-cell acute lymphoblastic leukemia (T-ALL) accounts for approximately 10-15% of entire ALL in children. The outcome of T-ALL has been improved through the intensified therapeutic strategy, however, it is still a more aggressive disease. In T-ALL a couple of transcription factor oncogenes are known to be relocated to the juxtaposition of T-cell receptor genes, potent promoter, by chromosome translocation. However the incidence of each chimeric gene formation in T-ALL is less than 5% and their clinical significance as a prognostic marker is lacking. A decade ago it was identified that activating mutations in NOTCH1 in about 60% of T-ALL. After then, activating NOTCH1 mutations present in T-ALL have been extensively investigated with regard to understanding its molecular pathogenesis, its prognostic significance, and developing molecularly tailored novel agents. Small molecule γ-secretase inhibitor, blocking a proteolytic step required for creation of a fragment of NOTCH intracellular domain which actually act as a controller of its target gene expression, was tried as a target therapeutic drug for T-ALL. Although outcome of this drug was not satisfactory, challenges have been launched to develop new drugs which specifically act on the aberrant behavior of mutated NOTCH1 in T-ALL. pISSN 2233-5250 / eISSN 2233-4580 http://dx.doi.org/10.15264/cpho.2014.21.1.1 Clin Pediatr Hematol Oncol 2014;21:1∼8

Friend or foe: can activating mutations in NOTCH1 contribute to a favorable treatment outcome in patients with T-ALL?

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of thymocytes. Despite significant improvement in the treatment of T-ALL, approximately 20% of children and most adults succumb to resistant or relapsed disease. Transformation events occur during crucial steps of thymocyte development and have been related to the expression of certain oncogenes such as TAL2, TLX1, LYL1, LMO1, and NOTCH1. Mutations that lead to constitutive activation of NOTCH1 are most commonly found in human patients with T-ALL. Moreover, overexpression of the intracellular portion of NOTCH1 can lead to the initiation of T-ALL in mouse models. These findings suggest that NOTCH1 may promote tumorigenesis through the regulation of differentiation of leukemic cells, and, potentially, of leukemia-initiating cell identity and function. Multiple studies and clinical trials aimed at targeting NOTCH1 in T-ALL or using NOTCH1 mutations as a prognostic tool are currently underway. Recent studies unexpectedly found that activating mutations in NOTCH1 are correlated with better treatment outcome. Here we review these studies and discuss possible explanations for these findings.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature

Characterization of transgenic mice expressing cancer‐associated variants of human NOTCH1

The Notch1 receptor plays a critical role in cell fate decisions during development. Activation of Notch signaling has been implicated in several types of cancer, particularly T-cell acute lymphoblastic leukemia (T-ALL). Consequently, several transgenic mouse strains have been made to study the role of Notch1 in T-ALL. However, the existing Notch1 transgenic lines mimic a translocation event found in only ∼1% of T-ALL cases. Here we describe three novel NOTCH1 transgenic mouse strains that have Cre-inducible expression of the entire human NOTCH1 locus, each possessing a common mutation found in T-ALL. Unlike existing Notch1 transgenic strains, these NOTCH1 transgenic strains express full-length receptors from an endogenous human promoter that should be susceptible to a number of Notch antagonists that have recently been developed. These strains will allow researchers to modulate Notch signaling to study both normal development and cancer biology.

Intersecting High-Throughput Screens Identifies SERCA As a Target for Modulating NOTCH1 In Hematopoietic Malignancies

Abstract 555While long-term survival rates for children with acute lymphoblastic leukemia (ALL) are approximately 80%, we have reached a plateau over the past decade with combination chemotherapy. ALL remains a leading cause of cancer-related death in children, and even for those children cured, morbidity is significant. Moreover, prognosis for adults with ALL is poor. Alternative approaches to treating this disease are needed. NOTCH1 is a well-validated target in hematopoietic malignancy, with NOTCH1 activating mutations identified in over 50% of T-cell acute lymphoblastic leukemia (T-ALL) and in 12% of chronic lymphocytic leukemias. NOTCH1 is a heterodimeric protein, normally activated by a series of proteolytic cleavages upon ligand stimulation, which forms a transcription factor complex altering gene expression. We intersected the results of two high-throughput screens focused on modulating NOTCH1 to identify new NOTCH1 inhibitors. First, we applied an unbiased, cell-based gene expression signature screening approach to identify new NOTCH1 inhibitors (Gene Expression-based High-throughput Screening (GE-HTS)) in which a gene signature serves as a surrogate for inactive NOTCH1. Among top-scoring compounds inducing the NOTCH1 inactivation signature, we identified inhibitors of SERCA (Sarco/endoplasmic Reticulum Ca++ATPase) channels, such as the natural products thapsigargin and cyclopiazonic acid. We integrated these results with a cDNA library screen for NOTCH1 activation in which U2OS cells expressing a leukemic NOTCH1 allele, L1601PδP, were scored for NOTCH1-sensitive RBPJ luciferase reporter activity. This screen also identified the SERCA genes ATP2A2 and ATP2A3 as top candidates, whose expression activates NOTCH1, complementing our original observation of SERCA inhibition as an approach to inhibit NOTCH1.SERCA proteins reside on the endoplasmic reticulum and are responsible for the transfer of calcium from the cytosol of the cell to the lumen of the endoplasmic reticulum. We focused our attention on the more potent molecule, thapsigargin, and confirmed that it induces the NOTCH1 off signatures across multiple T-ALL cell lines and targets NOTCH1 signaling by demonstrating its inhibitory effects on the RBPJ luciferase reporter in heterologous U2OS cells expressing the L1601PδP mutation. The anti-leukemic phenotype induced by thapsigargin was consistent with that previously reported with inhibition of gamma secretase, the enzyme involved in the final activation step of NOTCH1. Decreased viability and cell size and a G1/G0 arrest were observed upon low nanomolar thapsigargin treatment in multiple T-ALL cell lines. These effects were on-target for NOTCH1 inhibition. Overexpression of the activated form of NOTCH1, (ICN1) restored the expression of the NOTCH1 target genes c-MYC and DTX1 upon thapsigargin treatment and rescued the effects of thapsigargin on T-ALL viability.As in gamma secretase inhibitor (GSI) treatment, we observed that thapsigargin altered the level of active ICN1 protein in T-ALL cell lines. However, in contrast to treatment with GSI, thapsigargin also decreased levels of the furin-processed transmembrane forms of NOTCH1, while the full length NOTCH1 precursor accumulated as demonstrated by western blot and immunofluorescence studies. To further confirm the role of SERCA activity in NOTCH1 maturation, we inhibited Ca++ATPase activity with SERCA-directed shRNA in T-ALL cell lines and demonstrated that genetically-driven inhibition of ATP2A2 results in loss of ICN1 and trans-membrane NOTCH1 and the expected phenotypic consequences: reduced viability and G1/G0 arrest.To confirm that both chemical and genetic inhibition of SERCA leads to NOTCH inactivation in vivo, we adopted a Drosophila intestinal cancer stem cell model in which the inhibition of NOTCH1 prevents differentiation of stem cell daughter cells resulting in an expansion of the stem cell population. Treating Drosophila with thapsigargin or cyclopiazonic acid resulted in an expansion of the stem cell pool consistent with NOTCH inhibition. Indeed, a transgenic RNAi hairpin directed against Ca-P60A (the Drosophila SERCA orthologue) recapitulated the NOTCH phenotype observed with chemical treatment. In summary, these pre-clinical studies suggest a role for SERCA modulation of NOTCH1 maturation and identify SERCA as a potentially druggable target to inhibit NOTCH1 in T-ALL. Disclosures:No relevant conflicts of interest to declare.

Therapy of Philadelphia Chromosome-Negative Acute Lymphoblastic Leukemia in Adults: New Paradigms

Although the outcomes for adults with acute lymphoblastic leukemia (ALL) lag behind the stunningly successful results seen in children, new paradigms and new discoveries bring hope that this disparity will steadily lessen. The adoption of the use of pediatric intensity-type regimens in adolescents and young adults show promise in improving outcomes in this population. Recent donor-versus-no-donor comparisons in the allogeneic transplant setting highlight a potent graft-versus-leukemia effect in ALL, and the application of reduced intensity conditioning transplants may exploit this effect while reducing nonrelapse mortality. New therapeutic targets, such as CD22 in precusor B-cell ALL and mutations in NOTCH1 in T-cell ALL, are being exploited in clinical trials. Finally, use of molecular techniques and flow cytometry to quantitate minimal residual disease will allow further stratifications of patients by risk, identification of new therapeutic targets and will lessen drug toxicity through the use of pharmacogenomics.

Targeting the Notch1 and mTOR pathways in a mouse T-ALL model

AbstractMutations in NOTCH1 are frequently detected in patients with T-cell acute lymphoblastic leukemia (T-ALL) and in mouse T-ALL models. Treatment of mouse or human T-ALL cell lines in vitro with γ-secretase inhibitors (GSIs) results in growth arrest and/or apoptosis. These studies suggest GSIs as potential therapeutic agents in the treatment of T-ALL. To determine whether GSIs have antileukemic activity in vivo, we treated near-end-stage Tal1/Ink4a/Arf+/− leukemic mice with vehicle or with a GSI developed by Merck (MRK-003). We found that GSI treatment significantly extended the survival of leukemic mice compared with vehicle-treated mice. Notch1 target gene expression was repressed and increased numbers of apoptotic cells were observed in the GSI-treated mice, demonstrating that Notch1 inhibition in vivo induces apoptosis. T-ALL cell lines also exhibit PI3K/mTOR pathway activation, indicating that rapamycin may also have therapeutic benefit. When GSIs are administered in combination with rapamycin, mTOR kinase activity is ablated and apoptosis induced. Moreover, GSI and rapamycin treatment inhibits human T-ALL growth and extends survival in a mouse xenograft model. This work supports the idea of targeting NOTCH1 in T-ALL and suggests that inhibition of the mTOR and NOTCH1 pathways may have added efficacy.

Evolving paradigms in the therapy of Philadelphia chromosome–negative acute lymphoblastic leukemia in adults

Important studies challenging previous approaches to the treatment of adults with Philadelphia chromosome-negative acute lymphoblastic leukemia (ALL) have emerged in the past decade. Donor versus no donor comparisons of allogeneic transplant highlight a potent graft-versus-leukemia effect in ALL, and the application of reduced-intensity conditioning transplants may exploit this effect while reducing non-relapse mortality. The adoption of the use of pediatric intensity-type regimens in adolescents and young adults shows promise to improve outcomes in this population. New therapeutic targets such as mutations in NOTCH1 in T-cell ALL and CD22 in pre-B ALL are being exploited in clinical trials. The application of molecular techniques and flow cytometry to quantitate minimal residual disease will allow further stratification of patients by risk. Although the outcomes of adults with ALL lag behind the stunningly successful results seen in children, new paradigms and new discoveries bring hope that this disparity will steadily lessen.

Chemical Genomic Screen Identifies Ionophores as Modulators of Notch1 in T-ALL

Although in recent years intensive chemotherapy regimens have improved outcome for patients with T-cell acute lymphoblastic leukemia (T-ALL), side effects and relapse-rates remain high. An alternative therapeutic approach would be to target constitutively active mutated Notch1. Notch1, a trans-membrane receptor that acts as a transcription factor following proteolytic activation by the γ-secretase complex, harbors gain of function mutations in over 50% of T-ALL. Early clinical trials attempting to modulate Notch1 with γ-secretase inhibitors (GSI) were closed secondary to gastrointestinal toxicity, suggesting that alternative approaches to inhibiting Notch1 signaling are needed. Transcription factor complexes are difficult to target pharmacologically with conventional high-throughput screening approaches. We therefore sought to target Notch1 with a new gene expressionbased approach to small molecule library screening. Signatures for the Notch1 on versus off states were defined using microarray expression profiling of 7 different Notch1 mutant T-ALL cell lines treated with vehicle (Notch1 on) versus a GSI (Notch1 off). We adapted this 32-gene signature to our gene expression-based high-throughput screening (GE-HTS) assay, which uses ligation-mediated amplification (LMA) and a Luminex bead-based detection system. As proof of principle, we confirmed that an shRNA against Notch1 induced the Notch1 off signature in DND41 cells (a T-ALL cell line bearing a mutated Notch1 allele). In order to facilitate future clinical translation, we screened a collection of 4,500 bioactive compounds highly enriched for FDA-approved drugs in DND41 cells. Twenty-eight compounds that induced the Notch1 off signature were identified as top hits. These compounds were tested in a secondary screen in a 2-fold dilution series in two GSI sensitive cell lines, DND41 and KOPTK1, and one GSI resistant cell line, PF382. Ion channel modulators emerged as the top class of compounds confirmed to induce the Notch1 off signature across all cell lines. We next focused on two representative compounds, the ionophores salinomycin and ionomycin. Similar to GSI, both of these molecules induced cell cycle arrest and markedly decreased levels of the activated intracellular portion of Notch (ICN). Both salinomycin and ionomycin, as well as numerous other ionophores that scored as inhibitors in the screen, are known to increase cytosolic calcium. These studies are consistent with a role for calcium in the activation of oncogenic forms of Notch1 and suggest a potential role for ion channel modulators in the treatment of T-ALL.

Gain-of-Function NOTCH1 Mutations Occur Frequently in Human T Cell Acute Lymphoblastic Leukemia.

NOTCH1 was discovered originally through its involvement in a rare (7;9) translocation found in human T cell acute lymphoblastic leukemia (T-ALL). Here, we report that >50% of human T-ALLs have activating NOTCH1 mutations, occurring as amino acid substitutions in an extracellular heterodimerization (HD) domain and/or as frameshift and stop codon mutations that result in the deletion of a C-terminal PEST destruction box. Normal pro-NOTCH1 is processed into a heterodimer consisting of an extracellular subunit and a transmembrane subunit, which associate non-covalently through the HD domain. NOTCH1 activation is triggered by binding of Serrate or Delta-like ligands to the extracellular subunit, which induces successive proteolytic cleavages in the transmembrane subunit that are dependent on i) metalloproteases and ii) gamma-secretase. The γ-secretase cleavage releases intracellular NOTCH1 (ICN1), which translocates to the nucleus and forms a transcriptional activation complex with the transcription factor CSL and co-activators of the Mastermind family. Normal turnover of ICN1 is regulated by the C-terminal PEST sequence. Data pointing to the existence of frequent abnormalities of NOTCH1 in T-ALL stemmed from a functional screen of 30 T-ALL cell lines. This identified five T-ALL cell lines that underwent growth arrest in response to i) treatment with an inhibitor γ-secretase, and ii) retroviral transduction of dominant negative Mastermind-like-1. Sequencing of of cDNAs from 4 of these 5 cell lines demonstrated both a missense mutation in the HD domain and a frameshift mutation in the PEST domain lying in cis in the same NOTCH1 allele. Subsequent sequencing of genomic DNA obtained from bone marrow lymphoblasts of 96 children and adolescents with T-ALL demonstrated identical or similar mutations in NOTCH1 in 53 samples (55.2%). Mutations in the HD domain alone were observed in 26 cases (27.1%), in the PEST domain alone in 11 cases (11.4%), and in both the HD and PEST domains in 16 cases (16.7%). Mutations were observed in tumors associated with expression of HOX11 (2/3), HOX11L2 (10/13; 77%), TAL1 (12/31; 39%), LYL1 (9/14; 64%), MLL-ENL (1/3) or CALM-AF10 (1/2), which span the major molecular T-ALL subtypes. In contrast, NOTCH1 mutations were not observed in genomic DNAs samples obtained from B-ALL lymphoblasts (N=89), or from T-ALL patients with NOTCH1-associated disease at the time of clinical remission (N=4). Reporter gene assays conducted with plasmids expressing normal and mutated forms of NOTCH1 showed that a PEST deletion or various HD mutations alone caused ~1.5-fold and 3–9-fold stimulations of reporter gene activity, respectively, whereas normal NOTCH1 lacked intrinsic signaling activity. More strikingly, the combination of various HD mutations and a PEST deletion in cis caused synergistic 20–40-fold stimulations of reporter gene activity that were completely abrogated by a γ-secretase inhibitor, indicating that signaling depends on proteolysis. These results suggest a model in which HD domain mutations promote ICN1 production, and PEST domain mutations enhance ICN1 stability. Our findings greatly expand the role of NOTCH1 in the pathogenesis of human T-ALL, and provide a rationale for targeted therapies that interfere with NOTCH signaling.