Activating Notch1 mutations in mouse models of T-ALL

Oncogenic Kras and Notch-1 cooperate in T-cell acute lymphoblastic leukemia/lymphoma

Mutations of the Ras family are one of the most common somatic events found in all human cancers, although they are relatively rare in T-cell acute lymphoblastic leukemia (T-ALL). In mice, conditional expression of oncogenic Kras(G12D) from its endogenous promoter causes a fatal myeloproliferative disorder, and only rarely a T-ALL-like disease. In the article being evaluated, the authors demonstrate that primary mice expressing oncogenic Kras have a block in T-cell differentiation at the double-negative 1 stage. Interestingly, most secondarily transplanted mice develop a fatal T-ALL-like disease. Sequencing of NOTCH-1 showed that 50% of these mice harbored truncating mutations in the PEST domain that would be predicted to activate Notch signaling. Cell lines established from some of the mice demonstrated sensitivity to γ-secretase inhibition, suggesting that even when NOTCH-1 mutations occur as secondary collaborating events, tumors retain a dependency on this pathway that might be exploitable clinically.

Promotion and maintenance of leukemia by ERG

The Ets-related gene (ERG) located on human chromosome 21 encodes a transcription factor and is thought to be causally related to Down syndrome-associated acute megakaryocytic leukemia in childhood. In clinical adult leukemia, however, increased expression of ERG is indicative of poor prognosis in T-cell acute lymphoblastic leukemia and cytogenetically normal acute myeloid leukemia, although the involvement of ERG in the development of adult leukemia remains elusive. Here, we show that forced expression of ERG in adult BM cells alters differentiation and induces expansion of T and erythroid cells and increases frequencies of myeloid progenitors in mouse BM transplantation models. The expanded T cells then develop T-cell acute lymphoblastic leukemia after acquisition of mutations in the Notch1 gene. Targeted expression of ERG into B cells also altered differentiation and promoted growth of precursor B cells. Overall, these findings suggest a general role of ERG in promoting growth of adult hematopoietic cells in various lineages. In line with this, shRNA-mediated silencing of ERG expression attenuated growth of human leukemia cell lines of various lineages. Thus, ERG is capable of promoting the development of leukemia and is crucial for its maintenance.

Notch Signaling in T-Cell Development and T-ALL

The Notch signaling pathway is an evolutionarily conserved cell signaling system present in most multicellular organisms, as it controls cell fate specification by regulating cell proliferation, differentiation, apoptosis, and survival. Regulation of the Notch signaling pathway can be achieved at multiple levels. Notch proteins are involved in lineage fate decisions in a variety of tissues in various species. Notch is essential for T lineage cell differentiation including T versus B and αβ versus γδ lineage specification. In this paper, we discuss Notch signaling in normal T-cell maturation and differentiation as well as in T-cell acute lymphoblastic lymphoma/leukemia.

Notch3 activation promotes invasive glioma formation in a tissue site-specific manner

Although Notch signaling has been widely implicated in neoplastic growth, direct evidence for in vivo initiation of neoplasia by the pathway in murine models has been limited to tumors of lymphoid, breast, and choroid plexus cells. To examine tumorigenic potential in the eye and brain, we injected retroviruses encoding activated forms of Notch1, Notch2, or Notch3 into embryonic mice. Interestingly, the majority of animals infected with active Notch3 developed proliferative lesions comprised of pigmented ocular choroid cells, retinal and optic nerve glia, and lens epithelium. Notch3-induced lesions in the choroid, retina, and optic nerve were capable of invading adjacent tissues, suggesting that they were malignant tumors. Although Notch3 activation induced choroidal tumors in up to 67% of eyes, Notch1 or Notch2 activation never resulted in such tumors. Active forms of Notch1 and Notch2 did generate a few small proliferative glial nodules in the retina and optic nerve, whereas Notch3 was 10-fold more efficient at generating growths, many of which were large invasive gliomas. Expression of active Notch1/Notch3 chimeric receptors implicated the RBPjk-association molecule and transactivation domains of Notch3 in generating choroidal and glial tumors, respectively. In contrast to our findings in the optic nerve and retina, introduction of active Notch receptors, including Notch3, into the brain never caused glial tumors. Our results highlight the differential ability of Notch receptor paralogs to initiate malignant tumor formation, and suggest that glial precursors of the optic nerve, but not the brain, are susceptible to transformation by Notch3.

Notch signalling in T-cell lymphoblastic leukaemia/lymphoma and other haematological malignancies

Notch receptors participate in a highly conserved signalling pathway that regulates normal development and tissue homeostasis in a context- and dose-dependent manner. Deregulated Notch signalling has been implicated in many diseases, but the clearest example of a pathogenic role is found in T-cell lymphoblastic leukaemia/lymphoma (T-LL), in which the majority of human and murine tumours have acquired mutations that lead to aberrant increases in Notch1 signalling. Remarkably, it appears that the selective pressure for Notch mutations is virtually unique among cancers to T-LL, presumably reflecting a special context-dependent role for Notch in normal T-cell progenitors. Nevertheless, there are some recent reports suggesting that Notch signalling has subtle, yet important roles in other forms of haematological malignancy as well. Here, we review the role of Notch signalling in various blood cancers, focusing on T-LL with an eye towards targeted therapeutics.

Cellular context-dependent effects of H2ax and p53 deletion on the development of thymic lymphoma

H2AX and Artemis each cooperate with p53 to suppress lymphoma. Germline H2ax(-/-)p53(-/-) mice die of T-cell receptor-β(-) (TCR-β(-)) thymic lymphomas with translocations and other lesions characteristic of human T-cell acute lymphoblastic leukemia. Here, we demonstrate that mice with inactivation of H2ax and p53 in thymocytes die at later ages to TCR-β(-) or TCR-β(+) thymic lymphomas containing a similar pattern of translocations as H2ax(-/-)p53(-/-) tumors. Germline Artemis(-/-) p53(-/-) mice die of lymphomas with antigen receptor locus translocations, whereas Artemis(-/-)H2ax(-/-)p53(-/-) mice die at earlier ages from multiple malignancies. We show here that Artemis(-/-) mice with p53 deletion in thymocytes die of TCR-β(-) tumors containing Tcrα/δ translocations, other clonal translocations, or aneuploidy, as well as Notch1 mutations. Strikingly, Artemis(-/-) mice with H2ax and p53 deletion in thymocytes exhibited a lower rate of mortality from TCR-β(-) tumors, which harbored significantly elevated levels of genomic instability. Our data reveal that the cellular origin of H2ax and p53 loss impacts the rate of mortality from and developmental stage of thymic lymphomas, and suggest that conditional deletion of tumor suppressor genes may provide more physiologic models for human lymphoid malignancies than germline inactivation.

NOTCH1 and FBXW7 mutations have a favorable impact on early response to treatment, but not on outcome, in children with T-cell acute lymphoblastic leukemia (T-ALL) treated on EORTC trials 58881 and 58951

Risk-adjusted treatment stratification in T-cell acute lymphoblastic leukemias (T-ALLs) is currently based only on early response to chemotherapy. We investigated the prognostic implication of hyperactivation of NOTCH pathway resulting from mutations of NOTCH1 or FBXW7 in children with T-ALL enrolled in EORTC-CLG trials. Overall, 80 out of 134 (60%) patients were NOTCH+ (NOTCH1 and/or FBXW7 mutated). Although clinical presentations were not significantly associated with NOTCH status, NOTCH+ patients showed a better early response to chemotherapy as compared with NOTCH- patients, according to the rate of poor pre-phase 'responders' (25% versus 44%; P=0.02) and the incidence of high minimal residual disease (MRD) levels (11% (7/62) versus 32% (10/31); P=0.01) at completion of induction. However, the outcome of NOTCH+ patients was similar to that of NOTCH- patients, with a 5-year event-free survival (EFS) of 73% and 70% (P=0.82), and 5-year overall survival of 82% and 79% (P=0.62), respectively. In patients with high MRD levels, the 5-year EFS rate was 0% (NOTCH+) versus 42% (NOTCH-), whereas in those with low MRD levels, the outcome was similar: 76% (NOTCH+) versus 78% (NOTCH-). The incidence of isolated central nervous system (CNS) relapses was relatively high in NOTCH1+ patients (8.3%), which could be related to a higher propensity of NOTCH+ leukemic blasts to target the CNS.

The molecular basis of Lmo2-induced T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is commonly caused by the overexpression of oncogenic transcription factors in developing T cells. In a mouse model of one such oncogene, LMO2, the cellular effect is to induce self-renewal of committed T cells in the thymus, which persist long-term while acquiring additional mutations and eventually giving rise to leukemia. These precancerous stem cells (pre-CSC) are intrinsically resistant to radiotherapy, implying that they may be refractory to conventional cancer therapies. However, they depend on an aberrantly expressed stem cell-like self-renewal program for their maintenance, in addition to a specialized thymic microenvironmental niche. Here, we discuss potential approaches for targeting pre-CSCs in T-ALL by using therapies directed at oncogenic transcription factors themselves, downstream self-renewal pathways, and the supportive cell niche.

Deletion-based mechanisms of Notch1 activation in T-ALL: key roles for RAG recombinase and a conserved internal translational start site in Notch1

Point mutations that trigger ligand-independent proteolysis of the Notch1 ectodomain occur frequently in human T-cell acute lymphoblastic leukemia (T-ALL) but are rare in murine T-ALL, suggesting that other mechanisms account for Notch1 activation in murine tumors. Here we show that most murine T-ALLs harbor Notch1 deletions that fall into 2 types, both leading to ligand-independent Notch1 activation. Type 1 deletions remove exon 1 and the proximal promoter, appear to be RAG-mediated, and are associated with mRNA transcripts that initiate from 3' regions of Notch1. In line with the RAG dependency of these rearrangements, RAG2 binds to the 5' end of Notch1 in normal thymocytes near the deletion breakpoints. Type 2 deletions remove sequences between exon 1 and exons 26 to 28 of Notch1, appear to be RAG-independent, and are associated with transcripts in which exon 1 is spliced out of frame to 3' Notch1 exons. Translation of both types of transcripts initiates at a conserved methionine residue, M1727, which lies within the Notch1 transmembrane domain. Polypeptides initiating at M1727 insert into membranes and are subject to constitutive cleavage by γ-secretase. Thus, like human T-ALL, murine T-ALL is often associated with acquired mutations that cause ligand-independent Notch1 activation.

The role of Notch signaling pathway in epithelial-mesenchymal transition (EMT) during development and tumor aggressiveness

The Notch signaling pathway maintains a balance between cell proliferation and apoptosis, and thus it is believed that Notch signaling pathways may play an important role in the development and progression of several malignancies. However, the functions of Notch signaling in EMT are largely unknown. This mini review describes the role of Notch signaling pathway in EMT, and cataloging how its deregulation is involved in EMT and tumor aggressiveness. Further attempts have been made to summarize the role of several chemopreventive agents that could be useful for targeted inactivation of Notch signaling, and thus it may cause reversal of EMT, which could become a novel approach for cancer prevention and treatment.

Deletion of the RNA-binding proteins ZFP36L1 and ZFP36L2 leads to perturbed thymic development and T lymphoblastic leukemia

ZFP36L1 and ZFP36L2 are RNA-binding proteins (RBPs) that interact with AU-rich elements in the 3' untranslated region of mRNA, which leads to mRNA degradation and translational repression. Here we show that mice that lacked ZFP36L1 and ZFP36L2 during thymopoiesis developed a T cell acute lymphoblastic leukemia (T-ALL) dependent on the oncogenic transcription factor Notch1. Before the onset of T-ALL, thymic development was perturbed, with accumulation of cells that had passed through the beta-selection checkpoint without first expressing the T cell antigen receptor beta-chain (TCRbeta). Notch1 expression was higher in untransformed thymocytes in the absence of ZFP36L1 and ZFP36L2. Both RBPs interacted with evolutionarily conserved AU-rich elements in the 3' untranslated region of Notch1 and suppressed its expression. Our data establish a role for ZFP36L1 and ZFP36L2 during thymocyte development and in the prevention of malignant transformation.

ATM-deficient thymic lymphoma is associated with aberrant tcrd rearrangement and gene amplification

Ataxia telangiectasia mutated (ATM) deficiency predisposes humans and mice to T lineage lymphomas with recurrent chromosome 14 translocations involving the T cell receptor alpha/delta (Tcra/d) locus. Such translocations have been thought to result from aberrant repair of DNA double-strand breaks (DSBs) during Tcra locus V(D)J recombination, and to require the Tcra enhancer (Ealpha) for Tcra rearrangement or expression of the translocated oncogene. We now show that, in addition to the known chromosome 14 translocation, ATM-deficient mouse thymic lymphomas routinely contain a centromeric fragment of chromosome 14 that spans up to the 5' boundary of the Tcra/d locus, at which position a 500-kb or larger region centromeric to Tcra/d is routinely amplified. In addition, they routinely contain a large deletion of the telomeric end of one copy of chromosome 12. In contrast to prior expectations, the recurrent translocations and amplifications involve V(D)J recombination-initiated breaks in the Tcrd locus, as opposed to the Tcra locus, and arise independently of the Ealpha. Overall, our studies reveal previously unexpected mechanisms that contribute to the oncogenic transformation of ATM-deficient T lineage cells.

Modeling T-cell acute lymphoblastic leukemia induced by the SCL and LMO1 oncogenes

Deciphering molecular events required for full transformation of normal cells into cancer cells remains a challenge. In T-cell acute lymphoblastic leukemia (T-ALL), the genes encoding the TAL1/SCL and LMO1/2 transcription factors are recurring targets of chromosomal translocations, whereas NOTCH1 is activated in >50% of samples. Here we show that the SCL and LMO1 oncogenes collaborate to expand primitive thymocyte progenitors and inhibit later stages of differentiation. Together with pre-T-cell antigen receptor (pre-TCR) signaling, these oncogenes provide a favorable context for the acquisition of activating Notch1 mutations and the emergence of self-renewing leukemia-initiating cells in T-ALL. All tumor cells harness identical and specific Notch1 mutations and Tcrbeta clonal signature, indicative of clonal dominance and concurring with the observation that Notch1 gain of function confers a selective advantage to SCL-LMO1 transgenic thymocytes. Accordingly, a hyperactive Notch1 allele accelerates leukemia onset induced by SCL-LMO1 and bypasses the requirement for pre-TCR signaling. Finally, the time to leukemia induced by the three transgenes corresponds to the time required for clonal expansion from a single leukemic stem cell, suggesting that SCL, LMO1, and Notch1 gain of function, together with an active pre-TCR, might represent the minimum set of complementing events for the transformation of susceptible thymocytes.

Notch signaling: emerging molecular targets for cancer therapy

The Notch signaling pathway is a highly conserved developmental pathway, which plays a critical role in cell-fate decision, tissue patterning and morphogenesis. There is increasing evidence that this pathway is dysregulated in a variety of malignancies, and can behave as either an oncogene or a tumor suppressor depending upon cell context. This review highlights the current evidence for aberration of the Notch signaling pathway in a wide range of tumors from hematological cancers, such as leukemia and lymphoma through to skin, breast, lung, pancreas, colon and brain tumors. It proposes that the Notch signaling pathway may represent novel therapeutic targets and will be a welcome asset to the cancer therapeutic arena.

Lymphomagenesis in SCID-X1 mice following lentivirus-mediated phenotype correction independent of insertional mutagenesis and gammac overexpression

The development of leukemia as a consequence of vector-mediated genotoxicity in gene therapy trials for X-linked severe combined immunodeficiency (SCID-X1) has prompted substantial research effort into the design and safety testing of integrating vectors. An important element of vector design is the selection and evaluation of promoter-enhancer elements with sufficient strength to drive reliable immune reconstitution, but minimal propensity for enhancer-mediated insertional mutagenesis. In this study, we set out to explore the effect of promoter-enhancer selection on the efficacy and safety of human immunodeficiency virus-1-derived lentiviral vectors in gammac-deficient mice. We observed incomplete or absent T- and B-cell development in mice transplanted with progenitors expressing gammac from the phosphoglycerate kinase (PGK) and Wiscott-Aldrich syndrome (WAS) promoters, respectively. In contrast, functional T- and B-cell compartments were restored in mice receiving an equivalent vector containing the elongation factor-1-alpha (EF1alpha) promoter; however, 4 of 14 mice reconstituted with this vector subsequently developed lymphoma. Extensive analyses failed to implicate insertional mutagenesis or gammac overexpression as the underlying mechanism. These findings highlight the need for detailed mechanistic analysis of tumor readouts in preclinical animal models assessing vector safety, and suggest the existence of other ill-defined risk factors for oncogenesis, including replicative stress, in gene therapy protocols targeting the hematopoietic compartment.

Notch promotes radioresistance of glioma stem cells

Radiotherapy represents the most effective nonsurgical treatments for gliomas. However, gliomas are highly radioresistant and recurrence is nearly universal. Results from our laboratory and other groups suggest that cancer stem cells contribute to radioresistance in gliomas and breast cancers. The Notch pathway is critically implicated in stem cell fate determination and cancer. In this study, we show that inhibition of Notch pathway with gamma-secretase inhibitors (GSIs) renders the glioma stem cells more sensitive to radiation at clinically relevant doses. GSIs enhance radiation-induced cell death and impair clonogenic survival of glioma stem cells but not non-stem glioma cells. Expression of the constitutively active intracellular domains of Notch1 or Notch2 protect glioma stem cells against radiation. Notch inhibition with GSIs does not alter the DNA damage response of glioma stem cells after radiation but rather reduces Akt activity and Mcl-1 levels. Finally, knockdown of Notch1 or Notch2 sensitizes glioma stem cells to radiation and impairs xenograft tumor formation. Taken together, our results suggest a critical role of Notch signaling to regulate radioresistance of glioma stem cells. Inhibition of Notch signaling holds promise to improve the efficiency of current radiotherapy in glioma treatment.

Rap signaling in normal lymphocyte development and leukemia genesis

Although Rap GTPases of the Ras family remained enigmatic for years, extensive studies in this decade have revealed diverse functions of Rap signaling in the control of cell proliferation, differentiation, survival, adhesion, and movement. With the use of gene-engineered mice, we have uncovered essential roles of endogenous Rap signaling in normal lymphocyte development of both T- and B-lineage cells. Deregulation of Rap signaling, on the other hand, results in the development of characteristic leukemia in manners highly dependent on the contexts of cell lineages. These results highlight crucial roles of Rap signaling in the physiology and pathology of lymphocyte development.

Early T cell differentiation lessons from T-cell acute lymphoblastic leukemia

T cells develop from bone marrow-derived self-renewing hematopoietic stem cells (HSC). Upon entering the thymus, these cells undergo progressive commitment and differentiation driven by the thymic stroma and the pre-T cell receptor (pre-TCR). These processes are disrupted in T-cell acute lymphoblastic leukemia (T-ALL). More than 70% of recurring chromosomal rearrangements in T-ALL activate the expression of oncogenic transcription factors, belonging mostly to three families, basic helix-loop-helix (bHLH), homeobox (HOX), and c-MYB. This prevalence is indicative of their importance in the T lineage, and their dominant mechanisms of transformation. For example, bHLH oncoproteins inhibit E2A and HEB, revealing their tumor suppressor function in the thymus. The induction of T-ALL, nonetheless, requires collaboration with constitutive NOTCH1 signaling and the pre-TCR, as well as loss-of-function mutations for CDKN2A and PTEN. Significantly, NOTCH1, the pre-TCR pathway, and E2A/HEB proteins control critical checkpoints and branchpoints in early thymocyte development whereas several oncogenic transcription factors, HOXA9, c-MYB, SCL, and LYL-1 control HSC self-renewal. Together, these genetic lesions alter key regulatory processes in the cell, favoring self-renewal and subvert the normal control of thymocyte homeostasis.

Stem cells in normal development and cancer

In this chapter we provide an overview of stem cells in normal tissues as well as in many different types of cancers. All tissues in the body are derived from organ-specific stem cells that retain the ability to self-renew and differentiate into specific cell types. The cancer stem cell hypothesis suggests that tumors arise from cell populations with dysregulated self-renewal. This may be tissue stem cells or more differentiated cells that acquire self-renewal capabilities. In addition, we outline some useful assays for purification and isolation of cancer stem cells including the dye exclusion side population assay, flow cytometry sorting techniques for identification of putative cancer stem cell markers, tumorspheres assay, aldehyde dehydrogenase activity assay, PKH, and other membrane staining used to label the cancer stem cells, as well as in vivo xenograft transplantation assays. We also examine some of the cell signaling pathways that regulate stem cell self-renewal including the Notch, Hedgehog, HER2/PI3K/Akt/PTEN, and p53 pathways. We also review information demonstrating the involvement of the microenvironment or stem cell niche and its effects on the growth and maintenance of cancer stem cells. Finally, we highlight the therapeutic implications of targeting stem cells by inhibiting these pathways for the treatment and prevention of cancer.

Acute T-cell leukemias remain dependent on Notch signaling despite PTEN and INK4A/ARF loss

NOTCH1 is activated by mutation in more than 50% of human T-cell acute lymphoblastic leukemias (T-ALLs) and inhibition of Notch signaling causes cell-cycle/growth arrest, providing rationale for NOTCH1 as a therapeutic target. The tumor suppressor phosphatase and tensin homolog (PTEN) is also mutated or lost in up to 20% of cases. It was recently observed among human T-ALL cell lines that PTEN loss correlated with resistance to Notch inhibition, raising concern that patients with PTEN-negative disease may fail Notch inhibitor therapy. As these studies were limited to established cell lines, we addressed this issue using a genetically defined mouse retroviral transduction/bone marrow transplantation model and observed primary murine leukemias to remain dependent on NOTCH1 signaling despite Pten loss, with or without additional deletion of p16(Ink4a)/p19(Arf). We also examined 13 primary human T-ALL samples obtained at diagnosis and found no correlation between PTEN status and resistance to Notch inhibition. Furthermore, we noted in the mouse model that Pten loss accelerated disease onset and produced multiclonal tumors, suggesting NOTCH1 activation and Pten loss may collaborate in leukemia induction. Thus, in contrast to previous findings with established cell lines, these results indicate PTEN loss does not relieve primary T-ALL cells of their "addiction" to Notch signaling.

Dual antitumor mechanisms of Notch signaling inhibitor in a T-cell acute lymphoblastic leukemia xenograft model

Constitutive activation of Notch signaling is required for the proliferation of a subgroup of human T-cell acute lymphoblastic leukemias (T-ALL). Previous in vitro studies have demonstrated the therapeutic potential of Notch signaling inhibitors for treating T-ALL. To further examine this possibility, we applied a gamma-secretase inhibitor (GSI) to T-ALL xenograft models. Treatment of established subcutaneous tumors with GSI resulted in partial or complete regression of tumors arising from four T-ALL cell lines that were also sensitive to GSI in vitro. To elucidate the mechanism of action, we transduced DND-41 cells with the active form of Notch1 (aN1), which conferred resistance to in vitro GSI treatment. Nevertheless, in vivo treatment with GSI induced a partial but significant regression of subcutaneous tumors that developed from aN1-transduced DND-41 cells, whereas it induced complete regression of tumors that developed from mock-transduced DND-41 cells. These findings indicate that the remarkable efficacy of GSI might be attributable to dual mechanisms, directly via apoptosis of DND-41 cells through the inhibition of cell-autonomous Notch signaling, and indirectly via disturbance of tumor angiogenesis through the inhibition of non-cell-autonomous Notch signaling.

Stage-specific Arf tumor suppression in Notch1-induced T-cell acute lymphoblastic leukemia

Frequent hallmarks of T-cell acute lymphoblastic leukemia (T-ALL) include aberrant NOTCH signaling and deletion of the CDKN2A locus, which contains 2 closely linked tumor suppressor genes (INK4A and ARF). When bone marrow cells or thymocytes transduced with a vector encoding the constitutively activated intracellular domain of Notch1 (ICN1) are expanded ex vivo under conditions that support T-cell development, cultured progenitors rapidly induce CD4+/CD8+ T-ALLs after infusion into healthy syngeneic mice. Under these conditions, enforced ICN1 expression also drives formation of T-ALLs in unconditioned CD-1 nude mice, bypassing any requirements for thymic maturation. Retention of Arf had relatively modest activity in suppressing the formation of T-ALLs arising from bone marrow-derived ICN1+ progenitors in which the locus is epigenetically silenced, and all resulting Arf (+/+) tumors failed to express the p19(Arf) protein. In striking contrast, retention of Arf in thymocyte-derived ICN1+ donor cells significantly delayed disease onset and suppressed the penetrance of T-ALL. Use of cultured thymocyte-derived donor cells expressing a functionally null Arf-GFP knock-in allele confirmed that ICN1 signaling can induce Arf expression in vivo. Arf activation by ICN1 in T cells thereby provides stage-specific tumor suppression but also a strong selective pressure for deletion of the locus in T-ALL.

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

Mutations 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 gamma-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.

Oncogenic Kras initiates leukemia in hematopoietic stem cells

How oncogenes modulate the self-renewal properties of cancer-initiating cells is incompletely understood. Activating KRAS and NRAS mutations are among the most common oncogenic lesions detected in human cancer, and occur in myeloproliferative disorders (MPDs) and leukemias. We investigated the effects of expressing oncogenic Kras^G12D from its endogenous locus on the proliferation and tumor-initiating properties of murine hematopoietic stem and progenitor cells. MPD could be initiated by Kras^G12D expression in a highly restricted population enriched for hematopoietic stem cells (HSCs), but not in common myeloid progenitors. Kras^G12D HSCs demonstrated a marked in vivo competitive advantage over wild-type cells. Kras^G12D expression also increased the fraction of proliferating HSCs and reduced the overall size of this compartment. Transplanted Kras^G12D HSCs efficiently initiated acute T-lineage leukemia/lymphoma, which was associated with secondary Notch1 mutations in thymocytes. We conclude that MPD-initiating activity is restricted to the HSC compartment in Kras^G12D mice, and that distinct self-renewing populations with cooperating mutations emerge during cancer progression.

Ras proteins act as molecular switches that relay growth signals from outside the cell. This mechanism is often subverted in cancer, and Ras proteins are activated directly by RAS gene mutations in approximately one-third of human malignancies. We have modeled this in mice engineered to have a Ras mutation. These mice develop a disease similar to chronic leukemias in humans called myeloproliferative disorders. It is marked by a fatal accumulation of mature and immature cells in the blood and bone marrow. We investigated whether some or all of these neoplastic cells were immortal. In agreement with the “cancer stem cell” hypothesis, we found that immortal cells were extremely rare in the bone marrow of diseased mice. They were found only in the same cell populations that contain normal bone marrow stem cells. However, these cells had high rates of replication and produced large numbers of daughter cells. Furthermore, many mice went on to develop acute lymphoid leukemia after acquiring additional mutations in maturing lymphoid cells. These studies exemplify the evolution of malignant stem cells during cancer progression. They also highlight the importance of rare, long-lived cells in the genesis and, potentially, therapy of high-risk chronic leukemias caused by abnormal Ras proteins.

TheKras^G12D oncogene causes excessive proliferation of stem cells, promoting their preferential expansion and initiating myeloproliferative disease.Kras^G12D does not alter self-renewal, but secondary mutations in lymphoid progenitors allow progression to acute leukemia.

Heterozygosity for hypoxia inducible factor 1alpha decreases the incidence of thymic lymphomas in a p53 mutant mouse model

Hypoxia inducible factors (HIF) are critical mediators of the cellular response to decreased oxygen tension and are overexpressed in a number of tumors. Although HIF1alpha and HIF2alpha share a high degree of sequence homology, recent work has shown that the two alpha subunits can have contrasting and tissue-specific effects on tumor growth. To directly compare the role of each HIFalpha subunit in spontaneous tumorigenesis, we bred a mouse model of expanded HIF2alpha expression and Hif1alpha(+/-) mice to homozygotes for the R270H mutation in p53. Here, we report that p53(R270H/R270H) mice, which have not been previously described, develop a unique tumor spectrum relative to p53(R270H/-) mice, including a high incidence of thymic lymphomas. Heterozygosity for Hif1alpha significantly reduced the incidence of thymic lymphomas observed in this model. Moreover, reduced Hif1alpha levels correlated with decreased stabilization of activated Notch1 and expression of the Notch target genes, Dtx1 and Nrarp. These observations uncover a novel role for HIF1alpha in Notch pathway activation during T-cell lymphomagenesis.

The molecular programme of tumour reversion: the steps beyond malignant transformation

How cells become malignant has preoccupied scientists for over a century. However, the converse question is also valid: are tumour cells capable of reverting from their malignant state? Askanazy's studies in 1907 indicated that teratoma cells could differentiate into normal somatic tissues and current evidence indicates that some tumour cells have acquired the molecular circuitry that results in the negation of chromosomal instability, translocations, oncogene activation and loss of tumour suppressor genes. Studying these extremely rare events of tumour reversion and deciphering these pathways, which involve SIAH1, presenilin 1, TSAP6 and translationally controlled tumour protein (TCTP), could lead to new avenues in cancer treatment.

Spa-1 (Sipa1) and Rap signaling in leukemia and cancer metastasis

Although Rap GTPases of the Ras family remained enigmatic for years, extensive studies in this decade have revealed diverse functions of Rap in the control of cell proliferation, differentiation, survival, adhesion, and movement. With the use of genetic engineering strategies, we have uncovered essential roles of Rap signaling in normal lymphohematopoietic cell development as well as its crucial involvement in the development of a wide spectrum of leukemia in manners highly dependent on the contexts of cell lineages. Incidentally, recent results also indicate an important role of Spa-1, a Rap GTPase-activating protein, in invasion and metastasis in human cancers. While it is unlikely that Rap can function as a classic oncogene by itself, like Ras, emerging findings unveil crucial involvements of Rap GTPases in the distinct aspects of malignancy, including leukemia genesis and cancer metastasis.

gamma-Secretase in biology and medicine

gamma-Secretase is a membrane-embedded proteolytic complex composed of presenilin and three other subunits. The gamma-secretase complex generates the amyloid beta-peptide of Alzheimer's disease but also plays important roles in normal physiology, especially in signaling from the Notch receptor. How this hydrolytic enzyme works in a hydrophobic environment is largely unanswered, but mutagenesis and chemical probes have offered insight. gamma-Secretase is an important therapeutic target, although mechanism-based toxicity presents a serious obstacle. Agents that lower amyloid beta-peptide production while leaving important normal functions of gamma-secretase intact are promising therapeutic leads. Inhibition of Notch signaling by gamma-secretase inhibitors, which is undesirable for the prevention or treatment of Alzheimer's disease, may be beneficial for the treatment of a variety of cancers.

NOTCH1/FBXW7 mutation identifies a large subgroup with favorable outcome in adult T-cell acute lymphoblastic leukemia (T-ALL): a Group for Research on Adult Acute Lymphoblastic Leukemia (GRAALL) study

Many somatic genetic abnormalities have been identified in T-cell acute lymphoblastic leukemia (T-ALL) but each individual abnormality accounts for a small proportion of cases; therapeutic stratification consequently still relies on classical clinical markers. NOTCH1 and/or FBXW7 mutations both lead to activation of the NOTCH1 pathway and are among the most frequent mutations in T-ALL. We screened 141 adult diagnostic T-ALL samples from patients treated on either the Lymphoblastic Acute Leukemia in Adults (LALA)-94 (n = 87) or the GRAALL-2003 (n = 54) trials. In 88 cases (62%) there were demonstrated NOTCH1 mutations (42% heterodimerization [HD], 10% HD+proline glutamate serine threonine [PEST], 6% PEST, 2% juxtamembrane mutations, 2% transactivation domain [TAD]) and 34 cases (24%) had FBXW7 mutations (21 cases had both NOTCH1 and FBXW7 mutations); 40 cases (28%) were wild type for both. There was no significant correlation between NOTCH1 and/or FBXW7 mutations and clinico-biologic features. Median event-free survival (EFS) and overall survival (OS) were 36 versus 17 months (P = .01) and not reached versus 32 months (P = .004) in patients with NOTCH1 and/or FBXW7 mutations versus other patients, respectively. Multivariate analysis showed that the presence of NOTCH1/FBXW7 mutations was an independent good prognostic factor for EFS and OS (P = .02 and P = .01, respectively). These data demonstrate that NOTCH1 pathway activation by either NOTCH1 or FBXW7 mutation identifies a large group of patients with a favorable outcome that could justify individual therapeutic stratification for T-ALL.

Defective Notch activation in microenvironment leads to myeloproliferative disease

Despite the great importance of nonhematopoietic cells constituting the microenvironment for normal hematopoiesis, the cellular interactions between nonhematopoietic cells themselves are largely unknown. Using the Cre-loxP system in mice to inactivate Mind bomb-1 (Mib1), an essential component for Notch ligand endocytosis, here we show that the development of an MPD is dependent on defective Notch activation in the microenvironment. Our 2 independent Mib1 conditional knockout (CKO) mouse lines each developed a myeloproliferative disease (MPD), with gradual accumulations of immature granulocytes. The mutant mice showed hepatosplenomegaly, anemia, granulocytosis, and leukocyte infiltration in multiple organs and finally died at approximately 20 weeks of age. We were surprised to find that the transplantation of wild-type bone marrow cells into the Mib1-null microenvironment resulted in a de novo MPD. Moreover, by introducing the constitutively active intracellular domain of Notch1 in the Mib1-null background, we show that active Notch1 expression in the Mib1-null microenvironment significantly suppressed the disease progression, suggesting that the MPD development in the Mib1 CKO mice is due to defective Notch activation in the nonhematopoietic cells. These findings demonstrate that normal hematopoiesis absolutely requires Notch activation through the Notch ligand-receptor interaction between microenvironmental cells themselves and shed light on the microenvironment that fosters hematopoietic disorders.

Notch signaling mediates G1/S cell-cycle progression in T cells via cyclin D3 and its dependent kinases

Notch signaling plays a role in normal lymphocyte development and function. Activating Notch1-mutations, leading to aberrant downstream signaling, have been identified in human T-cell acute lymphoblastic leukemia (T-ALL). While this highlights the contribution of Notch signaling to T-ALL pathogenesis, the mechanisms by which Notch regulates proliferation and survival in normal and leukemic T cells are not fully understood. Our findings identify a role for Notch signaling in G(1)-S progression of cell cycle in T cells. Here we show that expression of the G(1) proteins, cyclin D3, CDK4, and CDK6, is Notch-dependent both in vitro and in vivo, and we outline a possible mechanism for the regulated expression of cyclin D3 in activated T cells via CSL (CBF-1, mammals; suppressor of hairless, Drosophila melanogaster; Lag-1, Caenorhabditis elegans), as well as a noncanonical Notch signaling pathway. While cyclin D3 expression contributes to cell-cycle progression in Notch-dependent human T-ALL cell lines, ectopic expression of CDK4 or CDK6 together with cyclin D3 shows partial rescue from gamma-secretase inhibitor (GSI)-induced G(1) arrest in these cell lines. Importantly, cyclin D3 and CDK4 are highly overexpressed in Notch-dependent T-cell lymphomas, justifying the combined use of cell-cycle inhibitors and GSI in treating human T-cell malignancies.

Mechanisms and clinical prospects of Notch inhibitors in the therapy of hematological malignancies

Activation of Notch signaling has been implicated in pathogenesis of various hematologic tumors including leukemias, lymphomas, and multiple myeloma. Pre-clinical studies have suggested that inhibition of Notch could be an attractive new approach to treatment of hematologic malignancies. This review discusses most recent findings in the field and potential role of Notch signaling as a therapeutic target focusing on the effects of gamma-secretase inhibitors.

Notch1 is a frequent mutational target in chemically induced lymphoma in mouse

Activating Notch1 mutations have been reported in human T-lineage acute lymphoblastic leukemia (T-ALL) and lymphomas from genetically modified mice. We report that Notch1 is a prevalent and major mutational target in chemically induced mouse lymphoma. The regions of the gene that are frequently mutated are the heterodimerization domain and the N-terminal ligand-binding region, important for protein stability, and the polypeptide rich in proline, glutamate, serine and threonine (PEST) domains, which is critical for protein degradation. Another gene, CDC4, is also involved in Notch1 degradation and shows frequent mutations. Mutations in the heterodimerization and the ligand-binding regions may cause ligand-independent signaling, whereas mutations preventing protein degradation result in accumulation of intracellular Notch1. We analyzed 103 chemical-induced mouse lymphomas for mutations in the Notch1 gene using single strand conformation analysis (SSCA) and DNA sequencing. Genetic alterations resulting in premature truncation of Notch1 were identified in 28 tumors, whereas 8 revealed alterations in the heterodimerization and 16 harbored deletions in the ligand-binding region. Dideoxycytidine-induced lymphomas displayed the highest frequency of Notch1 mutations (49%), whereas in butadiene- and phenolphthalein-induced tumors showed lower frequencies (26 and 10%, respectively). In total, 26 novel and 3 previously reported mutations were detected. This report shows that Notch1 is a prevalent and major mutational target for 2',3'-dideoxycytidine and butadiene-induced lymphoma.

Rag-dependent and Rag-independent mechanisms of Notch1 rearrangement in thymic lymphomas of Atm(-/-) and scid mice

The pathways of thymic lymphomagenesis are classified as Rag-dependent or -independent according to their dependence on recombination-activating gene (Rag1/2) proteins. The role of the two-lymphoma pathways in oncogene rearrangements and the connection between lymphoma pathways and rearrangement mechanisms, however, remain obscure. We compared the incidence and latency of thymic lymphomas, and associated rearrangements of the representative oncogene Notch1 among Rag2(-/-), ataxia telangiectasia mutated (Atm)(-/-), and severe combined immune deficiency (scid) mice combined with Rag2 deficiency. Contrary to expectations, Rag2(-/-) mice were prone to thymic lymphoma development, suggesting the existence of a Rag2-independent lymphoma pathway in Rag2(-/-) mice. The lymphoma incidence in Rag2(-/-)Atm(-/-) mice was lower than that in Atm(-/-) mice, but higher than that in Rag2(-/-) mice, indicating that Atm(-/-) mice develop lymphomas through both pathways. Scid mice developed lymphomas with an incidence and latency similar to Rag2(-/-)scid mice, suggesting that Rag2-mediated V(D)J recombination-driven events are not necessarily required for lymphomagenesis in scid mice. Notch1 rearrangement mechanisms were classified as Rag2-dependent or Rag2-independent based on the presence of recombination signal-like sequences at rearranged sites. In Rag2(-/-) lymphomas, Notch1 must be rearranged independently of Rag2 function, implying that Rag2(-/-) mice are susceptible to lymphomagenesis due to the presence of other rearrangement mechanisms. The results in Atm(-/-) mice suggest that Notch1 was rearranged through both lymphoma pathways. In scid mice, the frequency of Rag2-mediated rearrangements was relatively low compared with that in wild-type mice, suggesting that the Rag2-independent lymphoma pathway prevails in the development of thymic lymphomas in scid mice. Thus, two rearrangement mechanisms underlie the lymphoma pathways and constitute the mechanistic bases for lymphomagenesis, thereby providing the molecular criteria for distinguishing between Rag2-dependent and Rag2-independent lymphoma pathways.

Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients

X-linked SCID (SCID-X1) is amenable to correction by gene therapy using conventional gammaretroviral vectors. Here, we describe the occurrence of clonal T cell acute lymphoblastic leukemia (T-ALL) promoted by insertional mutagenesis in a completed gene therapy trial of 10 SCID-X1 patients. Integration of the vector in an antisense orientation 35 kb upstream of the protooncogene LIM domain only 2 (LMO2) caused overexpression of LMO2 in the leukemic clone. However, leukemogenesis was likely precipitated by the acquisition of other genetic abnormalities unrelated to vector insertion, including a gain-of-function mutation in NOTCH1, deletion of the tumor suppressor gene locus cyclin-dependent kinase 2A (CDKN2A), and translocation of the TCR-beta region to the STIL-TAL1 locus. These findings highlight a general toxicity of endogenous gammaretroviral enhancer elements and also identify a combinatorial process during leukemic evolution that will be important for risk stratification and for future protocol design.

Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients

X-linked SCID (SCID-X1) is amenable to correction by gene therapy using conventional gammaretroviral vectors. Here, we describe the occurrence of clonal T cell acute lymphoblastic leukemia (T-ALL) promoted by insertional mutagenesis in a completed gene therapy trial of 10 SCID-X1 patients. Integration of the vector in an antisense orientation 35 kb upstream of the protooncogene LIM domain only 2 (LMO2) caused overexpression of LMO2 in the leukemic clone. However, leukemogenesis was likely precipitated by the acquisition of other genetic abnormalities unrelated to vector insertion, including a gain-of-function mutation in NOTCH1, deletion of the tumor suppressor gene locus cyclin-dependent kinase 2A (CDKN2A), and translocation of the TCR-beta region to the STIL-TAL1 locus. These findings highlight a general toxicity of endogenous gammaretroviral enhancer elements and also identify a combinatorial process during leukemic evolution that will be important for risk stratification and for future protocol design.

Leukemia-associated NOTCH1 alleles are weak tumor initiators but accelerate K-ras-initiated leukemia

Gain-of-function NOTCH1 mutations are found in 50%-70% of human T cell acute lymphoblastic leukemia/lymphoma (T-ALL) cases. Gain-of-function NOTCH1 alleles that initiate strong downstream signals induce leukemia in mice, but it is unknown whether the gain-of-function NOTCH1 mutations most commonly found in individuals with T-ALL generate downstream signals of sufficient strength to induce leukemia. We addressed this question by expressing human gain-of-function NOTCH1 alleles of varying strength in mouse hematopoietic precursors. Uncommon gain-of-function NOTCH1 alleles that initiated strong downstream signals drove ectopic T cell development and induced leukemia efficiently. In contrast, although gain-of-function alleles that initiated only weak downstream signals also induced ectopic T cell development, these more common alleles failed to efficiently initiate leukemia development. However, weak gain-of-function NOTCH1 alleles accelerated the onset of leukemia initiated by constitutively active K-ras and gave rise to tumors that were sensitive to Notch signaling pathway inhibition. These data show that induction of leukemia requires doses of Notch1 greater than those needed for T cell development and that most NOTCH1 mutations found in T-ALL cells do not generate signals of sufficient strength to initiate leukemia development. Furthermore, low, nonleukemogenic levels of Notch1 can complement other leukemogenic events, such as activation of K-ras. Even when Notch1 participates secondarily, the resulting tumors show "addiction" to Notch, providing a further rationale for evaluating Notch signaling pathway inhibitors in leukemia.

Inhibition of gamma-secretase affects proliferation of leukemia and hepatoma cell lines through Notch signaling

Notch signaling is a well-conserved pathway playing crucial roles in regulating cell fate decision, proliferation, and apoptosis during the development of multiple cell lineages. Aberration in Notch signaling is associated with tumorigenesis of tissues from various origins. To investigate the role Notch signaling plays in the proliferation of cancer cell lines, the expression profiles of Notch1 in six human cancer cell lines (Jurkat, HepG2, SW620, KATOIII, A375, BT474) were examined. All cell lines differentially expressed Notch1, and only Jurkat and SW620 expressed cleaved Notch1 (Val1744). Among the six cell lines tested, only Jurkat and HepG2 showed a decrease in cell proliferation during 4 days of treatment with a gamma-secretase inhibitor (GSI). This is the first report on the anti-proliferative effects of GSI on a human hepatoma cell line. These two cell lines expressed Notch1-3, Jagged1, Jagged2, Dlk1 and Hes1. GSI treatment led to a decrease in Hes1 expression in both cell lines. Surprisingly, GSI treatment resulted in the accumulation of Notch1 protein upon treatment. During this period, GSI treatment did not induce apoptosis, but caused cell cycle arrest in both cell lines. This was also correlated with decreased c-myc expression. Forced expression of activated intracellular Notch1 completely abrogated GSI sensitivity in both cell lines. These results clearly demonstrate that Notch signaling positively regulates cell proliferation in Jurkat and HepG2 cell lines and that GSI treatment inhibits tumor cell proliferation through the suppression of Notch signaling.

K-RasG12D-induced T-cell lymphoblastic lymphoma/leukemias harbor Notch1 mutations and are sensitive to gamma-secretase inhibitors

To study the impact of oncogenic K-Ras on T-cell leukemia/lymphoma development and progression, we made use of a conditional K-Ras(G12D) murine knockin model, in which oncogenic K-Ras is expressed from its endogenous promoter. Transplantation of whole bone marrow cells that express oncogenic K-Ras into wild-type recipient mice resulted in a highly penetrant, aggressive T-cell leukemia/lymphoma. The lymphoblasts were composed of a CD4/CD8 double-positive population that aberrantly expressed CD44. Thymi of primary donor mice showed reduced cellularity, and immunophenotypic analysis demonstrated a block in differentiation at the double-negative 1 stage. With progression of disease, approximately 50% of mice acquired Notch1 mutations within the PEST domain. Of note, primary lymphoblasts were hypersensitive to gamma-secretase inhibitor treatment, which is known to impair Notch signaling. This inhibition was Notch-specific as assessed by down-regulation of Notch1 target genes and intracellular cleaved Notch. We also observed that the oncogenic K-Ras-induced T-cell disease was responsive to rapamycin and inhibitors of the RAS/MAPK pathway. These data indicate that patients with T-cell leukemia with K-Ras mutations may benefit from therapies that target the NOTCH pathway alone or in combination with inhibition of the PI3K/AKT/MTOR and RAS/MAPK pathways.

The role of NF-kappaB in lymphoid malignancies

Nuclear Factor kappaB (NF-kappaB) transcription factors are central regulators of lymphocyte proliferation, survival and development. Although normally subject to tight control, constitutive activation of NF-kappaB promotes inappropriate lymphocyte survival and proliferation, and is recognised as key pathological feature in various lymphoid malignancies. Inhibition of NF-kappaB may be an attractive therapeutic approach in these diseases. This review focuses on the mechanisms and functional consequences of NF-kappaB activation in lymphoid malignancies and potential therapeutic strategies for inhibition of NF-kappaB.

The Fbxw7/hCdc4 tumor suppressor in human cancer

Fbxw7/hCdc4 is a member of the F-box family of proteins, which function as interchangeable substrate recognition components of the SCF ubiquitin ligases. SCF(Fbxw7/hCdc4) targets several important oncoproteins including c-Myc, c-Jun, cyclin E1, and Notch, for ubiquitin-dependent proteolysis. Recent studies have shown that FBXW7/hCDC4 is mutated in a variety of human tumor types, suggesting that it is a general tumor suppressor in human cancer. Alteration of Fbxw7/hCdc4 function is linked to defects in differentiation, cellular proliferation, and genetic instability. In this review, we summarize what is known about Fbxw7/hCdc4-mediated degradation in the regulation of cellular proliferation and discuss how alteration of its function contributes to human tumorigenesis.

Notch signaling in leukemia

Recent discoveries indicate that gain-of-function mutations in the Notch1 receptor are very common in human T cell acute lymphoblastic leukemia/lymphoma. This review discusses what these mutations have taught us about normal and pathophysiologic Notch1 signaling, and how these insights may lead to new targeted therapies for patients with this aggressive form of cancer.

Cooperation of Gata3, c-Myc and Notch in malignant transformation of double positive thymocytes

Gata transcription factors are critical regulators of proliferation and differentiation implicated in various human cancers, but specific genes activated by Gata proteins remain to be identified. We previously reported that enforced expression of Gata3 during T cell development in CD2-Gata3 transgenic mice induced CD4(+)CD8(+) double-positive (DP) T cell lymphoma. Here, we show that the presence of the DO11.10 T-cell receptor transgene, which directs DP cells towards the CD4 lineage, resulted in enhanced lymphoma development and a dramatic increase in thymocyte cell size in CD2-Gata3 transgenic mice. CD2-Gata3 DP cells expressed high levels of the proto-oncogene c-Myc but the Notch1 signaling pathway, which is known to induce c-Myc, was not activated. Gene expression profiling showed that in CD2-Gata3 lymphoma cells transcription of c-Myc and its target genes was further increased. A substantial fraction of CD2-Gata3 lymphomas had trisomy of chromosome 15, leading to an increased c-Myc gene dose. Interestingly, most lymphomas showed high expression of the Notch targets Deltex1 and Hes1, often due to activating Notch1 PEST domain mutations. Therefore, we conclude that enforced Gata3 expression converts DP thymocytes into a pre-malignant state, characterized by high c-Myc expression, whereby subsequent induction of Notch1 signaling cooperates to establish malignant transformation. The finding that Gata3 regulates c-Myc expression levels, in a direct or indirect fashion, may explain the parallel phenotypes of mice with overexpression or deficiency of either of the two transcription factors.

Notch-1 mutations are secondary events in some patients with T-cell acute lymphoblastic leukemia

PURPOSE

Activating Notch-1 mutations are frequent in T-cell acute lymphoblastic leukemia (T-ALL), occurring in >50% of patients. In murine models of T-ALL, Notch-1 activation can both directly initiate leukemia and cooperate secondarily to other primary events. Whether acquisition of Notch-1 mutations is an early initiating event or a secondary event in the pathogenesis of human T-ALL is unclear.

EXPERIMENTAL DESIGN

We used denaturing high-performance liquid chromatography, sequencing, and fragment analysis to analyze Notch-1 mutational status and mutant level in 62 patients at presentation as well as 16 matched presentation-relapse samples.

RESULTS

We detected Notch-1 mutations in 47 patients (76%). Seven of these were low-level mutations (quantified at < or =10%), despite high blast counts, suggesting that they were acquired as a secondary event in a subclone. Of 16 matched presentation-relapse samples studied, 7 were wild-type at both presentation and relapse. Five of nine mutant-positive patients at presentation relapsed with the same mutation(s) at the same high level. Four patients had evidence of a change in mutant at relapse. One lost a PEST mutation and became wild-type. Two others lost mutations at relapse but acquired different mutations, despite unchanged T-cell receptor rearrangements, suggesting that the latter event predated the acquisition of the Notch-1 mutation. One relapsed with a secondary T-cell leukemia and different Notch mutation.

CONCLUSIONS

These results suggest that Notch-1 mutations can sometimes be acquired as secondary events in leukemogenesis and must be used cautiously as solitary minimal residual disease markers.

Translating insights from the cancer genome into clinical practice

Cancer cells have diverse biological capabilities that are conferred by numerous genetic aberrations and epigenetic modifications. Today's powerful technologies are enabling these changes to the genome to be catalogued in detail. Tomorrow is likely to bring a complete atlas of the reversible and irreversible alterations that occur in individual cancers. The challenge now is to work out which molecular abnormalities contribute to cancer and which are simply 'noise' at the genomic and epigenomic levels. Distinguishing between these will aid in understanding how the aberrations in a cancer cell collaborate to drive pathophysiology. Past successes in converting information from genomic discoveries into clinical tools provide valuable lessons to guide the translation of emerging insights from the genome into clinical end points that can affect the practice of cancer medicine.

Ikaros directly represses the notch target gene Hes1 in a leukemia T cell line: implications for CD4 regulation

Ikaros and Notch1, two regulators of gene transcription, are critically important at many stages of T cell development. Deregulation of Ikaros and Notch activities cooperate to promote T cell leukemogenesis, providing evidence that they function in converging pathways in developing T cells. In this report, a mechanism for Ikaros:Notch cooperativity is described, revealing a non-redundant role for Ikaros in regulating expression of the Notch target gene Hes1 in a leukemia T cell line. We provide evidence that Ikaros directly represses Hes1 in concert with the transcriptional repressor, RBP-Jkappa, allowing for cross-talk between Notch and Ikaros that impacts regulation of CD4 expression. Taken together, these data describe a potential mechanism for Ikaros' function during T cell development and define Ikaros as an obligate repressor of Hes1.