Notch1-dependent lymphomagenesis is assisted by but does not essentially require pre-TCR signaling

Interactions between the DNA Damage Response and the Telomere Complex in Carcinogenesis: A Hypothesis

Contrary to what was once thought, direct cancer originating from normal stem cells seems to be extremely rare. This is consistent with a preneoplastic period of telomere length reduction/damage in committed cells that becomes stabilized in transformation. Multiple observations suggest that telomere damage is an obligatory step preceding its stabilization. During tissue turnover, the telomeres of cells undergoing differentiation can be damaged as a consequence of defective DNA repair caused by endogenous or exogenous agents. This may result in the emergence of new mechanism of telomere maintenance which is the final outcome of DNA damage and the initial signal that triggers malignant transformation. Instead, transformation of stem cells is directly induced by primary derangement of telomere maintenance mechanisms. The newly modified telomere complex may promote survival of cancer stem cells, independently of telomere maintenance. An inherent resistance of stem cells to transformation may be linked to specific, robust mechanisms that help maintain telomere integrity.

IL-15 Prevents the Development of T-ALL from Aberrant Thymocytes with Impaired DNA Repair Functions and Increased NOTCH1 Activation

We previously reported that NOD.Scid mice lacking interleukin-15 (IL-15), or IL-15 receptor alpha-chain, develop T-acute lymphoblastic leukemia (T-ALL). To understand the mechanisms by which IL-15 signaling controls T-ALL development, we studied the thymocyte developmental events in IL-15-deficient Scid mice from NOD and C57BL/6 genetic backgrounds. Both kinds of mice develop T-ALL characterized by circulating TCR-negative cells expressing CD4, CD8 or both. Analyses of thymocytes in NOD.Scid.Il15^-/- mice prior to T-ALL development revealed discernible changes within the CD4^-CD8^- double-negative (DN) thymocyte developmental stages and increased frequencies of CD4⁺CD8⁺ double-positive cells with a high proportion of TCR-negative CD4⁺ and CD8⁺ cells. The DN cells also showed elevated expressions of CXCR4 and CD117, molecules implicated in the expansion of DN thymocytes. T-ALL cell lines and primary leukemic cells from IL-15-deficient NOD.Scid and C57BL/6.Scid mice displayed increased NOTCH1 activation that was inhibited by NOTCH1 inhibitors and blockers of the PI3K/AKT pathway. Primary leukemic cells from NOD.Scid.Il15^-/- mice survived and expanded when cultured with MS5 thymic stromal cells expressing Delta-like ligand 4 and supplemented with IL-7 and FLT3 ligand. These findings suggest that IL-15 signaling in the thymus controls T-ALL development from aberrant thymocytes with an impaired DNA repair capacity and increased NOTCH1 activation.

Notch Partners in the Long Journey of T-ALL Pathogenesis

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological disease that arises from the oncogenic transformation of developing T cells during T-lymphopoiesis. Although T-ALL prognosis has improved markedly in recent years, relapsing and refractory patients with dismal outcomes still represent a major clinical issue. Consequently, understanding the pathological mechanisms that lead to the appearance of this malignancy and developing novel and more effective targeted therapies is an urgent need. Since the discovery in 2004 that a major proportion of T-ALL patients carry activating mutations that turn NOTCH1 into an oncogene, great efforts have been made to decipher the mechanisms underlying constitutive NOTCH1 activation, with the aim of understanding how NOTCH1 dysregulation converts the physiological NOTCH1-dependent T-cell developmental program into a pathological T-cell transformation process. Several molecular players have so far been shown to cooperate with NOTCH1 in this oncogenic process, and different therapeutic strategies have been developed to specifically target NOTCH1-dependent T-ALLs. Here, we comprehensively analyze the molecular bases of the cross-talk between NOTCH1 and cooperating partners critically involved in the generation and/or maintenance and progression of T-ALL and discuss novel opportunities and therapeutic approaches that current knowledge may open for future treatment of T-ALL patients.

Early expression of mature αβ TCR in CD4-CD8- T cell progenitors enables MHC to drive development of T-ALL bearing NOTCH mutations

T cell development and immune responses are directed by major histocompatibility complex:T cell antigen receptor (MHC:TCR) signaling, but aberrant signals can cause T cell tumors to form. We show that in mice and humans, a low-frequency progenitor cell population expresses early αβ TCR while coreceptor double-negative (EADN), and these EADN cells can transform to thymic leukemia. Mouse models showed that EADN cells did not require MHC to develop but when presented with MHC they could respond with high sensitivity. Transformation to leukemia occurred and required MHC, although with extended tumor growth this requirement could be lost. Thus, MHC:TCR signaling can initiate a leukemia phenotype from an understudied developmental state that appears to be represented in the mouse and human disease spectrum.

During normal T cell development in mouse and human, a low-frequency population of immature CD4⁻CD8⁻ double-negative (DN) thymocytes expresses early, mature αβ T cell antigen receptor (TCR). We report that these early αβ TCR+ DN (EADN) cells are DN3b-DN4 stage and require CD3δ but not major histocompatibility complex (MHC) for their generation/detection. When MHC - is present, however, EADN cells can respond to it, displaying a degree of coreceptor-independent MHC reactivity not typical of mature, conventional αβ T cells. We found these data to be connected with observations that EADN cells were susceptible to T cell acute lymphoblastic leukemia (T-ALL) transformation in both humans and mice. Using the OT-1 TCR transgenic system to model EADN-stage αβ TCR expression, we found that EADN leukemogenesis required MHC to induce development of T-ALL bearing NOTCH1 mutations. This leukemia-driving MHC requirement could be lost, however, upon passaging the tumors in vivo, even when matching MHC was continuously present in recipient animals and on the tumor cells themselves. These data demonstrate that MHC:TCR signaling can be required to initiate a cancer phenotype from an understudied developmental state that appears to be represented in the mouse and human disease spectrum.

Notch-Signaling Deregulation Induces Myeloid-Derived Suppressor Cells in T-Cell Acute Lymphoblastic Leukemia

Notch receptors deeply influence T-cell development and differentiation, and their dysregulation represents a frequent causative event in "T-cell acute lymphoblastic leukemia" (T-ALL). "Myeloid-derived suppressor cells" (MDSCs) inhibit host immune responses in the tumor environment, favoring cancer progression, as reported in solid and hematologic tumors, with the notable exception of T-ALL. Here, we prove that Notch-signaling deregulation in immature T cells promotes CD11b⁺Gr-1⁺ MDSCs in the Notch3-transgenic murine model of T-ALL. Indeed, aberrant T cells from these mice can induce MDSCs in vitro, as well as in immunodeficient hosts. Conversely, anti-Gr1-mediated depletion of MDSCs in T-ALL-bearing mice reduces proliferation and expansion of malignant T cells. Interestingly, the coculture with Notch-dependent T-ALL cell lines, sustains the induction of human CD14⁺HLA-DR^low/neg MDSCs from healthy-donor PBMCs that are impaired upon exposure to gamma-secretase inhibitors. Notch-independent T-ALL cells do not induce MDSCs, suggesting that Notch-signaling activation is crucial for this process. Finally, in both murine and human models, IL-6 mediates MDSC induction, which is significantly reversed by treatment with neutralizing antibodies. Overall, our results unveil a novel role of Notch-deregulated T cells in modifying the T-ALL environment and represent a strong premise for the clinical assessment of MDSCs in T-ALL patients.

Monoallelic Heb/Tcf12 Deletion Reduces the Requirement for NOTCH1 Hyperactivation in T-Cell Acute Lymphoblastic Leukemia

Early T-cell development is precisely controlled by E proteins, that indistinguishably include HEB/TCF12 and E2A/TCF3 transcription factors, together with NOTCH1 and pre-T cell receptor (TCR) signalling. Importantly, perturbations of early T-cell regulatory networks are implicated in leukemogenesis. NOTCH1 gain of function mutations invariably lead to T-cell acute lymphoblastic leukemia (T-ALL), whereas inhibition of E proteins accelerates leukemogenesis. Thus, NOTCH1, pre-TCR, E2A and HEB functions are intertwined, but how these pathways contribute individually or synergistically to leukemogenesis remain to be documented. To directly address these questions, we leveraged Cd3e-deficient mice in which pre-TCR signaling and progression through β-selection is abrogated to dissect and decouple the roles of pre-TCR, NOTCH1, E2A and HEB in SCL/TAL1-induced T-ALL, via the use of Notch1 gain of function transgenic (Notch1ICtg) and Tcf12+/- or Tcf3+/- heterozygote mice. As a result, we now provide evidence that both HEB and E2A restrain cell proliferation at the β-selection checkpoint while the clonal expansion of SCL-LMO1-induced pre-leukemic stem cells in T-ALL is uniquely dependent on Tcf12 gene dosage. At the molecular level, HEB protein levels are decreased via proteasomal degradation at the leukemic stage, pointing to a reversible loss of function mechanism. Moreover, in SCL-LMO1-induced T-ALL, loss of one Tcf12 allele is sufficient to bypass pre-TCR signaling which is required for Notch1 gain of function mutations and for progression to T-ALL. In contrast, Tcf12 monoallelic deletion does not accelerate Notch1IC-induced T-ALL, indicating that Tcf12 and Notch1 operate in the same pathway. Finally, we identify a tumor suppressor gene set downstream of HEB, exhibiting significantly lower expression levels in pediatric T-ALL compared to B-ALL and brain cancer samples, the three most frequent pediatric cancers. In summary, our results indicate a tumor suppressor function of HEB/TCF12 in T-ALL to mitigate cell proliferation controlled by NOTCH1 in pre-leukemic stem cells and prevent NOTCH1-driven progression to T-ALL.

Genome-Wide Analysis Identifies Rag1 and Rag2 as Novel Notch1 Transcriptional Targets in Thymocytes

Recombination activating genes 1 (Rag1) and Rag2 are expressed in immature lymphocytes and essential for generating the vast repertoire of antigen receptors. Yet, the mechanisms governing the transcription of Rag1 and Rag2 remain to be fully determined, particularly in thymocytes. Combining cDNA microarray and ChIP-seq analysis, we identify Rag1 and Rag2 as novel Notch1 transcriptional targets in acute T-cell lymphoblastic leukemia (T-ALL) cells. We further demonstrate that Notch1 transcriptional complexes directly bind the Rag1 and Rag2 locus in not only T-ALL but also primary double negative (DN) T-cell progenitors. Specifically, dimeric Notch1 transcriptional complexes activate Rag1 and Rag2 through a novel cis-element bearing a sequence-paired site (SPS). In T-ALL and DN cells, dimerization-defective Notch1 causes compromised Rag1 and Rag2 expression; conversely, dimerization-competent Notch1 achieves optimal upregulation of both. Collectively, these results reveal Notch1 dimerization-mediated transcription as one of the mechanisms for activating Rag1 and Rag2 expression in both primary and transformed thymocytes. Our data suggest a new role of Notch1 dimerization in compelling efficient TCRβ rearrangements in DN progenitors during T-cell development.

NF-κB1 Regulates Immune Environment and Outcome of Notch-Dependent T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive pediatric malignancy that arises from the transformation of immature T-cell progenitors and has no definitive cure. Notch signaling governs many steps of T cell development and its dysregulation represents the most common causative event in the pathogenesis of T-ALL. The activation of canonical NF-κB pathway has been described as a critical downstream mediator of Notch oncogenic functions, through the sustaining of tumor cell survival and growth. The potential role of Notch/NF-κB partnership is also emerging in the generation and function of regulatory T cells (Tregs) in the context of cancer. However, little is known about the effects of combined mutations of Notch and NF-κB in regulating immune-environment and progression of T-ALL. To shed light on the topics above we generated double-mutant mice, harboring conventional knock-out mutation of NF-κB1/p50 on the genetic background of a transgenic model of Notch-dependent T-ALL. The immunophenotyping of double-mutant mice demonstrates that NF-κB1 deletion inhibits the progression of T-ALL and strongly modifies immune-environment of the disease. Double-mutant mice display indeed a dramatic reduction of pre-leukemic CD4⁺CD8⁺ (DP) T cells and regulatory T cells (Tregs) and, concurrently, the rising of an aggressive myeloproliferative trait with a massive expansion of CD11b⁺Gr-1⁺ cells in the periphery, and an accumulation of the granulocyte/monocyte progenitors in the bone-marrow. Interestingly, double-mutant T cells are able to improve the growth of CD11b⁺Gr-1⁺ cells in vitro, and, more importantly, the in vivo depletion of T cells in double-mutant mice significantly reduces the expansion of myeloid compartment. Our results strongly suggest that the myeloproliferative trait observed in double-mutant mice may depend on non-cell-autonomous mechanism/s driven by T cells. Moreover, we demonstrate that the reduction of CD4⁺CD8⁺ (DP) T cells and Tregs in double-mutant mice relies on a significant enhancement of their apoptotic rate. In conclusion, double-mutant mice may represent a useful model to deepen the knowledge of the consequences on T-ALL immune-environment of modulating Notch/NF-κB relationships in tumor cells. More importantly, information derived from these studies may help in the refinement of multitarget therapies for the disease.

The TCR/CD3 complex in leukemogenesis and as a therapeutic target in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) arises from T cell precursors and is characterized by expression of many lineage-specific proteins. While T-cell antigen receptor (TCR) signaling and its strength are central for thymocyte development, mature T cell homeostasis and immune responses, their roles in T-ALL remain undetermined. Indeed, in contrast to mouse models, in which absence of TCR or major histocompatibility complex binding does not impact on leukemogenesis, other mouse models suggest that basal or weak signaling drives leukemia development. However, recent reports indicate that strong TCR signaling can be detrimental to leukemic cells. Indeed, sustained/high level TCR signaling, stimulated by antigen or CD3 antibody, is strongly anti-leukemic in both murine T-ALL expressing endogenous or transgenic TCR and diagnostic T-ALL cases. As discussed, further work should address the efficacy of T-ALL therapeutic targeting with either TCR/CD3 antibodies or TCR-directed chimeric antigen receptor T cells.

Intrathymic Notch3 and CXCR4 combinatorial interplay facilitates T-cell leukemia propagation

Notch hyperactivation dominates T-cell acute lymphoblastic leukemia development, but the mechanisms underlying “pre-leukemic” cell dissemination are still unclear. Here we describe how deregulated Notch3 signaling enhances CXCR4 cell-surface expression and migratory ability of CD4⁺CD8⁺ thymocytes, possibly contributing to “pre-leukemic” cell propagation, early in disease progression. In transgenic mice overexpressing the constitutively active Notch3 intracellular domain, we detect the progressive increase in circulating blood and bone marrow of CD4⁺CD8⁺ cells, characterized by high and combined surface expression of Notch3 and CXCR4. We report for the first time that transplantation of such CD4⁺CD8⁺ cells reveals their competence in infiltrating spleen and bone marrow of immunocompromised recipient mice. We also show that CXCR4 surface expression is central to the migratory ability of CD4⁺CD8⁺ cells and such an expression is regulated by Notch3 through β-arrestin in human leukemia cells. De novo, we propose that hyperactive Notch3 signaling by boosting CXCR4-dependent migration promotes anomalous egression of CD4⁺CD8⁺ cells from the thymus in early leukemia stages. In fact, in vivo CXCR4 antagonism prevents bone marrow colonization by such CD4⁺CD8⁺ cells in young Notch3 transgenic mice. Therefore, our data suggest that combined therapies precociously counteracting intrathymic Notch3/CXCR4 crosstalk may prevent dissemination of “pre-leukemic” CD4⁺CD8⁺ cells, by a “thymus-autonomous” mechanism.

Regulation of Hematopoietic Cell Development and Function Through Phosphoinositides

One of the most paramount receptor-induced signal transduction mechanisms in hematopoietic cells is production of the lipid second messenger phosphatidylinositol(3,4,5)trisphosphate (PIP₃) by class I phosphoinositide 3 kinases (PI3K). Defective PIP₃ signaling impairs almost every aspect of hematopoiesis, including T cell development and function. Limiting PIP₃ signaling is particularly important, because excessive PIP₃ function in lymphocytes can transform them and cause blood cancers. Here, we review the key functions of PIP₃ and related phosphoinositides in hematopoietic cells, with a special focus on those mechanisms dampening PIP₃ production, turnover, or function. Recent studies have shown that beyond “canonical” turnover by the PIP₃ phosphatases and tumor suppressors phosphatase and tensin homolog (PTEN) and SH2 domain-containing inositol-5-phosphatase-1 (SHIP-1/2), PIP₃ function in hematopoietic cells can also be dampened through antagonism with the soluble PIP₃ analogs inositol(1,3,4,5)tetrakisphosphate (IP₄) and inositol-heptakisphosphate (IP₇). Other evidence suggests that IP₄ can promote PIP₃ function in thymocytes. Moreover, IP₄ or the kinases producing it limit store-operated Ca²⁺ entry through Orai channels in B cells, T cells, and neutrophils to control cell survival and function. We discuss current models for how soluble inositol phosphates can have such diverse functions and can govern as distinct processes as hematopoietic stem cell homeostasis, neutrophil macrophage and NK cell function, and development and function of B cells and T cells. Finally, we will review the pathological consequences of dysregulated IP₄ activity in immune cells and highlight contributions of impaired inositol phosphate functions in disorders such as Kawasaki disease, common variable immunodeficiency, or blood cancer.

Notch in Leukemia

Notch is commonly activated in lymphoid malignancies through ligand-independent and ligand-dependent mechanisms. In T-cell acute lymphoblastic leukemia/lymphoma (T-ALL), ligand-independent activation predominates. Negative Regulatory Region (NRR) mutations trigger supraphysiological Notch1 activation by exposing the S2 site to proteolytic cleavage in the absence of ligand. Subsequently, cleavage at the S3 site generates the activated form of Notch, intracellular Notch (ICN). In contrast to T-ALL, in mature lymphoid neoplasms such as chronic lymphocytic leukemia (CLL), the S2 cleavage site is exposed through ligand-receptor interactions. Thus, agents that disrupt ligand-receptor interactions might be useful for treating these malignancies. Notch activation can be enhanced by mutations that delete the C-terminal proline (P), glutamic acid (E), serine (S), and threonine (T) (PEST) domain. These mutations do not activate the Notch pathway per se, but rather impair degradation of ICN. In this chapter, we review the mechanisms of Notch activation and the importance of Notch for the genesis and maintenance of lymphoid malignancies. Unfortunately, targeting the Notch pathway with pan-Notch inhibitors in clinical trials has proven challenging. These clinical trials have encountered dose-limiting on-target toxicities and primary resistance. Strategies to overcome these challenges have emerged from the identification and improved understanding of direct oncogenic Notch target genes. Other strategies have arisen from new insights into the "nuclear context" that selectively directs Notch functions in lymphoid cancers. This nuclear context is created by factors that co-bind ICN at cell-type specific transcriptional regulatory elements. Disrupting the functions of these proteins or inhibiting downstream oncogenic pathways might combat cancer without the intolerable side effects of pan-Notch inhibition.

From the outside, from within: Biological and therapeutic relevance of signal transduction in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological cancer that arises from clonal expansion of transformed T-cell precursors. In this review we summarize the current knowledge on the external stimuli and cell-intrinsic lesions that drive aberrant activation of pivotal, pro-tumoral intracellular signaling pathways in T-cell precursors, driving transformation, leukemia expansion, spread or resistance to therapy. In addition to their pathophysiological relevance, receptors and kinases involved in signal transduction are often attractive candidates for targeted drug development. As such, we discuss also the potential of T-ALL signaling players as targets for therapeutic intervention.

Activation of an endothelial Notch1-Jagged1 circuit induces VCAM1 expression, an effect amplified by interleukin-1β

The Notch1 and Notch4 signaling pathways regulate endothelial cell homeostasis. Inflammatory cytokines induce the expression of endothelial adhesion molecules, including VCAM1, partly by downregulating Notch4 signaling. We investigated the role of endothelial Notch1 in this IL-1β-mediated process. Brief treatment with IL-1β upregulated endothelial VCAM1 and Notch ligand Jagged1. IL-1β decreased Notch1 mRNA levels, but levels of the active Notch1ICD protein remained constant. IL-1β-mediated VCAM1 induction was downregulated in endothelial cells subjected to pretreatment with a pharmacological inhibitor of the γ-secretase, which activates Notch receptors, producing NotchICD. It was also downregulated in cells in which Notch1 and/or Jagged1 were silenced.Conversely, the forced expression of Notch1ICD in naïve endothelial cells upregulated VCAM1 per se and amplified IL-1β-mediated VCAM1 induction. Jagged1 levels increased and Notch4 signaling was downregulated in parallel. Finally, Notch1ICD and Jagged1 expression was upregulated in the endothelium of the liver in a model of chronic liver inflammation.In conclusion, we describe here a cell-autonomous, pro-inflammatory endothelial Notch1-Jagged1 circuit (i) triggering the expression of VCAM1 even in the absence of inflammatory cytokines and (ii) enhancing the effects of IL-1β. Thus, IL-1β regulates Notch1 and Notch4 activity in opposite directions, consistent with a selective targeting of Notch1 in inflamed endothelium.

Non-canonical antagonism of PI3K by the kinase Itpkb delays thymocyte β-selection and renders it Notch-dependent

β-selection is the most pivotal event determining αβ T cell fate. Here, surface-expression of a pre-T cell receptor (pre-TCR) induces thymocyte metabolic activation, proliferation, survival and differentiation. Besides the pre-TCR, β-selection also requires co-stimulatory signals from Notch receptors - key cell fate determinants in eukaryotes. Here, we show that this Notch-dependence is established through antagonistic signaling by the pre-TCR/Notch effector, phosphoinositide 3-kinase (PI3K), and by inositol-trisphosphate 3-kinase B (Itpkb). Canonically, PI3K is counteracted by the lipid-phosphatases Pten and Inpp5d/SHIP-1. In contrast, Itpkb dampens pre-TCR induced PI3K/Akt signaling by producing IP₄, a soluble antagonist of the Akt-activating PI3K-product PIP₃. Itpkb^-/- thymocytes are pre-TCR hyperresponsive, hyperactivate Akt, downstream mTOR and metabolism, undergo an accelerated β-selection and can develop to CD4⁺CD8⁺ cells without Notch. This is reversed by inhibition of Akt, mTOR or glucose metabolism. Thus, non-canonical PI3K-antagonism by Itpkb restricts pre-TCR induced metabolic activation to enforce coincidence-detection of pre-TCR expression and Notch-engagement.

DOI:http://dx.doi.org/10.7554/eLife.10786.001

T cells defend our body against cancer and infectious agents such as viruses. However, they can also cause rheumatoid arthritis and other autoimmune diseases by attacking healthy tissue. T cells recognize target cells via receptor proteins on their surface. To maximize the variety of infections and cancers our immune system can recognize, we generate millions of T cells with different T cell receptors every day.

To ensure T cells work correctly, T cell receptors are tested at various checkpoints. The first checkpoint involves a process called beta (β) selection, during which T cells produce their first T cell receptor – the so-called pre-T cell receptor. This receptor causes T cells to divide and mature, and sets their future identity or “fate”. To complete β-selection, T cells must also receive signals from another surface receptor – one that belongs to the Notch family, which determines cell fate in many different tissues.

The Notch receptor and the pre-T cell receptor both activate an enzyme called PI3K – a key mediator of β-selection. But the pre-T cell receptor also activates another enzyme called Itpkb that is required for T cell development. Westernberg, Conche et al. have now investigated how these different proteins and signaling processes work and interact during β-selection, using mice that lack several immune genes, including the gene that produces Itpkb.

The results of the experiments show that during β-selection, Itpkb limits the ability of PI3K to activate some of its key target proteins. This “dampened” PI3K signaling ensures that both the pre-T cell receptor and the Notch receptor must be activated to trigger T cell maturation. Without Itpkb, β-selection can occur in the absence of Notch signaling.

As Notch signaling is important for determining the fate of many different cell types, Westernberg, Conche et al.’s findings raise the possibility that Itpkb might also regulate cell fate determination in other tissues. Moreover, Itpkb may suppress tumor development, because excessive PI3K signaling drives many cancers.

DOI:http://dx.doi.org/10.7554/eLife.10786.002

Placing ion channels into a signaling network of T cells: from maturing thymocytes to healthy T lymphocytes or leukemic T lymphoblasts

T leukemogenesis is a multistep process, where the genetic errors during T cell maturation cause the healthy progenitor to convert into the leukemic precursor that lost its ability to differentiate but possesses high potential for proliferation, self-renewal, and migration. A new misdirecting "leukemogenic" signaling network appears, composed by three types of participants which are encoded by (1) genes implicated in determined stages of T cell development but deregulated by translocations or mutations, (2) genes which normally do not participate in T cell development but are upregulated, and (3) nondifferentially expressed genes which become highly interconnected with genes expressed differentially. It appears that each of three groups may contain genes coding ion channels. In T cells, ion channels are implicated in regulation of cell cycle progression, differentiation, activation, migration, and cell death. In the present review we are going to reveal a relationship between different genetic defects, which drive the T cell neoplasias, with calcium signaling and ion channels. We suggest that changes in regulation of various ion channels in different types of the T leukemias may provide the intracellular ion microenvironment favorable to maintain self-renewal capacity, arrest differentiation, induce proliferation, and enhance motility.

Breakpoint sites disclose the role of the V(D)J recombination machinery in the formation of T-cell receptor (TCR) and non-TCR associated aberrations in T-cell acute lymphoblastic leukemia

Aberrant recombination between T-cell receptor genes and oncogenes gives rise to chromosomal translocations that are genetic hallmarks in several subsets of human T-cell acute lymphoblastic leukemias. The V(D)J recombination machinery has been shown to play a role in the formation of these T-cell receptor translocations. Other, non-T-cell receptor chromosomal aberrations, such as SIL-TAL1 deletions, have likewise been recognized as V(D)J recombination associated aberrations. Despite the postulated role of V(D)J recombination, the extent of the V(D)J recombination machinery involvement in the formation of T-cell receptor and non-T-cell receptor aberrations in T-cell acute lymphoblastic leukemia is still poorly understood. We performed a comprehensive in silico and ex vivo evaluation of 117 breakpoint sites from 22 different T-cell receptor translocation partners as well as 118 breakpoint sites from non-T-cell receptor chromosomal aberrations. Based on this extensive set of breakpoint data, we provide a comprehensive overview of T-cell receptor and oncogene involvement in T-ALL. Moreover, we assessed the role of the V(D)J recombination machinery in the formation of chromosomal aberrations, and propose an up-dated mechanistic classification on how the V(D)J recombination machinery contributes to the formation of T-cell receptor and non-T-cell receptor aberrations in human T-cell acute lymphoblastic leukemia.

Regulation of immune responses and tolerance: the microRNA perspective

Much has been learned about the molecular and cellular components critical for the control of immune responses and tolerance. It remains a challenge, however, to control the immune response and tolerance at the system level without causing significant toxicity to normal tissues. Recent studies suggest that microRNA (miRNA) genes, an abundant class of non-coding RNA genes that produce characteristic approximately 22 nucleotides small RNAs, play important roles in immune cells. In this article, we discuss emerging knowledge regarding the functions of miRNA genes in the immune system. We delve into the roles of miRNAs in regulating signaling strength and threshold, homeostasis, and the dynamics of the immune response and tolerance during normal and pathogenic immunological conditions. We also present observations based on analyzes of miR-181 family genes that indicate the potential functions of primary and/or precursor miRNAs in target recognition and explore the impact of these findings on target identification. Finally, we illustrate that despite the subtle effects of miRNAs on gene expression, miRNAs have the potential to influence the outcomes of normal and pathogenic immune responses by controlling the quantitative and dynamic aspects of immune responses. Tuning miRNA functions in immune cells, through gain- and loss-of-function approaches in mice, may reveal novel approach to restore immune equilibrium from pathogenic conditions, such as autoimmune disease and leukemia, without significant toxicity.

Modulating the strength and threshold of NOTCH oncogenic signals by mir-181a-1/b-1

Oncogenes, which are essential for tumor initiation, development, and maintenance, are valuable targets for cancer therapy. However, it remains a challenge to effectively inhibit oncogene activity by targeting their downstream pathways without causing significant toxicity to normal tissues. Here we show that deletion of mir-181a-1/b-1 expression inhibits the development of Notch1 oncogene-induced T cell acute lymphoblastic leukemia (T-ALL). mir-181a-1/b-1 controls the strength and threshold of Notch activity in tumorigenesis in part by dampening multiple negative feedback regulators downstream of NOTCH and pre-T cell receptor (TCR) signaling pathways. Importantly, although Notch oncogenes utilize normal thymic progenitor cell genetic programs for tumor transformation, comparative analyses of mir-181a-1/b-1 function in normal thymocyte and tumor development demonstrate that mir-181a-1/b-1 can be specifically targeted to inhibit tumor development with little toxicity to normal development. Finally, we demonstrate that mir-181a-1/b-1, but not mir-181a-2b-2 and mir-181-c/d, controls the development of normal thymic T cells and leukemia cells. Together, these results illustrate that NOTCH oncogene activity in tumor development can be selectively inhibited by targeting the molecular networks controlled by mir-181a-1/b-1.

Oncogenes elicit driving signals required for tumor initiation, development, and maintenance and are valuable targets for cancer therapy. However, oncogenes often have essential functions in normal cellular physiology and produce intracellular proteins that are difficult to inhibit with small molecule drugs without causing significant toxicity to normal tissues. Thus, one of the challenges in cancer therapy is to identify downstream networks that can be targeted to specifically dampen the oncogenic signals in tumor cells without harming normal tissues. In this study we demonstrate that deletion of a microRNA (miRNA) gene, mir-181a-1/b-1, specifically inhibits the activity of the Notch oncogene in tumorigenesis without causing significant defects in normal development. Although earlier studies have elegantly shown the essential role of NOTCH and pre-TCR signals in NOTCH-induced tumorigenesis, neither NOTCH nor pre-TCR signals can be targeted effectively for treatment of T-ALL with available drugs due to their weak therapeutic effects and severe toxicities. Our findings illustrate that dissecting the downstream targets of miRNAs can reveal the molecular networks that can be targeted to control tumor transformation caused by oncogenes. More importantly, our results illustrate that comparative studies on the pathways utilized by normal cells and tumor cells may reveal novel insights into how tumorigenic pathways may be selectively inhibited with limited damage to normal tissues.

Vital roles of mTOR complex 2 in Notch-driven thymocyte differentiation and leukemia

Rictor is essential in Notch-driven T-ALL pathogenesis.

Notch plays critical roles in both cell fate decisions and tumorigenesis. Notch receptor engagement initiates signaling cascades that include a phosphatidylinositol 3-kinase/target of rapamycin (TOR) pathway. Mammalian TOR (mTOR) participates in two distinct biochemical complexes, mTORC1 and mTORC2, and the relationship between mTORC2 and physiological outcomes dependent on Notch signaling is unknown. In this study, we report contributions of mTORC2 to thymic T-cell acute lymphoblastic leukemia (T-ALL) driven by Notch. Conditional deletion of Rictor, an essential component of mTORC2, impaired Notch-driven proliferation and differentiation of pre-T cells. Furthermore, NF-κB activity depended on the integrity of mTORC2 in thymocytes. Active Akt restored NF-κB activation, a normal rate of proliferation, and differentiation of Rictor-deficient pre-T cells. Strikingly, mTORC2 depletion lowered CCR7 expression in thymocytes and leukemic cells, accompanied by decreased tissue invasion and delayed mortality in T-ALL driven by Notch. Collectively, these findings reveal roles for mTORC2 in promoting thymic T cell development and T-ALL and indicate that mTORC2 is crucial for Notch signaling to regulate Akt and NF-κB.

Therapeutic potential of Notch inhibition in T-cell acute lymphoblastic leukemia: rationale, caveats and promises

T-cell acute lymphoblastic leukemia (T-ALL) is a malignancy that presents with poor prognosis. Treatment relies on the application of aggressive therapies that produce deleterious side-effects, justifying the quest for novel, more efficient and selective molecular targeting agents. Mutations leading to abnormal Notch-1 activity are present in more than half of the T-ALL patients, underscoring the potential therapeutic relevance of targeting Notch-1 inhibition and further reinforcing the need to better comprehend the mechanisms by which Notch-1 drives T cell leukemogenesis. Clinical application of γ-secretase inhibitors to block Notch signaling in T-ALL revealed new challenges that involve improvement of the therapeutic benefit and reduction of intestinal toxicity. Here, we review the latest advances in the development and use of Notch antagonists and summarize the current knowledge on Notch function in T-ALL to understand how it may translate into novel therapeutic strategies that increment the efficiency of Notch inhibition.

Notch1 and IL-7 receptor signalling in early T-cell development and leukaemia

Notch receptors are master regulators of many aspects of development and tissue renewal in metazoans. Notch1 activation is essential for T-cell specification of bone marrow-derived multipotent progenitors that seed the thymus, and for proliferation and further progression of early thymocytes along the T-cell lineage. Deregulated activation of Notch1 significantly contributes to the generation of T-cell acute lymphoblastic leukaemia (T-ALL). In addition to Notch1 signals, survival and proliferation signals provided by the IL-7 receptor (IL-7R) are also required during thymopoiesis. Our understanding of the molecular mechanisms controlling stage-specific survival and proliferation signals provided by Notch1 and IL-7R has recently been improved by the discovery that the IL-7R is a transcriptional target of Notch1. Thus, Notch1 controls T-cell development, in part by regulating the stage- and lineage-specific expression of IL-7R. The finding that induction of IL-7R expression downstream of Notch1 also occurs in T-ALL highlights the important contribution that deregulated IL-7R expression and function may have in this pathology. Confirming this notion, oncogenic IL7R gain-of-function mutations have recently been identified in childhood T-ALL. Here we discuss the fundamental role of Notch1 and IL-7R signalling pathways in physiological and pathological T-cell development in mice and men, highlighting their close molecular underpinnings.

Cooperation between somatic Ikaros and Notch1 mutations at the inception of T-ALL

To understand the interactions between Notch1 and Ikaros in the evolution of T cell acute lymphoblastic leukemia (T-ALL), we traced the evolution of T-ALL in mice with an inherited Ikaros mutation, Ikzf1(Plstc) which inactivates DNA binding. DNA-binding Ikaros repressed Notch1 response in transfected cell lines and in CD4(+)8(+) (DP) thymocytes from young pre-leukemic Ikzf1(Plstc) heterozygous mice. In DP thymocytes, a 50-1000 fold escalation in mRNA for Notch1 target genes Hes1 and Dtx1 preceded thymic lymphoma or leukemia and was closely correlated with the first detectable differentiation abnormalities, loss of heterozygosity (LOH) eliminating wild-type Ikzf1, and multiple missense and truncating Notch1 mutations. These findings illuminate the early stages of leukemogenesis by demonstrating progressive exaggeration of Notch1 responsiveness at the DP thymocyte stage brought about by multiple mutations acting in concert upon the Notch1 pathway.

Cell of origin strongly influences genetic selection in a mouse model of T-ALL

Identifying the normal cell from which a tumor originates is crucial to understanding the etiology of that cancer. However, retrospective identification of the cell of origin in cancer is challenging because of the accumulation of genetic and epigenetic changes in tumor cells. The biologic state of the cell of origin likely influences the genetic events that drive transformation. We directly tested this hypothesis by performing a Sleeping Beauty transposon mutagenesis screen in which common insertion sites were identified in tumors that were produced by mutagenesis of cells at varying time points throughout the T lineage. Mutation and gene expression data derived from these tumors were then compared with data obtained from a panel of 84 human T-cell acute lymphoblastic leukemia samples, including copy number alterations and gene expression profiles. This revealed that altering the cell of origin produces tumors that model distinct subtypes of human T-cell acute lymphoblastic leukemia, suggesting that even subtle changes in the cell of origin dramatically affect genetic selection in tumors. These findings have broad implications for the genetic analysis of human cancers as well as the production of mouse models of cancer.

NOTCH and phosphatidylinositide 3-kinase/phosphatase and tensin homolog deleted on chromosome ten/AKT/mammalian target of rapamycin (mTOR) signaling in T-cell development and T-cell acute lymphoblastic leukemia

Activating mutations in NOTCH1 consitute the most prominent genetic abnormality in T-cell acute lymphoblastic leukemia (T-ALL). However, most T-ALL cell lines with NOTCH1 mutations are resistant to treatment with γ-secretase inhibitors (GSIs). The spotlight is now shifting to the phosphatidylinositide 3-kinase (PI3K)/phosphatase and tensin homolog deleted on chromosome ten (PTEN)/AKT/mammalian target of rapamycin (mTOR) pathway as another key potential target. These two signaling routes are deregulated in many types of cancer. In this review we discuss these two pathways with respect to their signaling mechanisms, functions during T-cell development, and their mutual roles in the development of T-ALL.

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.

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 targets and their regulation

The proteolytic cleavages elicited by activation of the Notch receptor release an intracellular fragment, Notch intracellular domain, which enters the nucleus to activate the transcription of targets. Changes in transcription are therefore a major output of this pathway. However, the Notch outputs clearly differ from cell type to cell type. In this review we discuss current understanding of Notch targets, the mechanisms involved in their transcriptional regulation, and what might underlie the activation of different sets of targets in different cell types.

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.

Pre-TCR signaling inactivates Notch1 transcription by antagonizing E2A

Precise control of the timing and magnitude of Notch signaling is essential for the normal development of many tissues, but the feedback loops that regulate Notch are poorly understood. Developing T cells provide an excellent context to address this issue. Notch1 signals initiate T-cell development and increase in intensity during maturation of early T-cell progenitors (ETP) to the DN3 stage. As DN3 cells undergo beta-selection, during which cells expressing functionally rearranged TCRbeta proliferate and differentiate into CD4(+)CD8(+) progeny, Notch1 signaling is abruptly down-regulated. In this report, we investigate the mechanisms that control Notch1 expression during thymopoiesis. We show that Notch1 and E2A directly regulate Notch1 transcription in pre-beta-selected thymocytes. Following successful beta-selection, pre-TCR signaling rapidly inhibits Notch1 transcription via signals that up-regulate Id3, an E2A inhibitor. Consistent with a regulatory role for Id3 in Notch1 down-regulation, post-beta-selected Id3-deficient thymocytes maintain Notch1 transcription, whereas enforced Id3 expression decreases Notch1 expression and abrogates Notch1-dependent T-cell survival. These data provide new insights into Notch1 regulation in T-cell progenitors and reveal a direct link between pre-TCR signaling and Notch1 expression during thymocyte development. Our findings also suggest new strategies for inhibiting Notch1 signaling in pathologic conditions.

Oncogenesis of T-ALL and nonmalignant consequences of overexpressing intracellular NOTCH1

Mutations resulting in overexpression of intracellular Notch1 (ICN1) are frequently observed in human T cell acute lymphoblastic leukemia (T-ALL). We have determined the consequences of ICN1 overexpression from retroviral vectors introduced into bone marrow cells. Early consequences are the generation of polyclonal nontumorigenic CD4(+)8(+) T cell receptor (TCR)-alphabeta(+) cells that do not qualify as tumor precursors despite the observation that they overexpress Notch 1 and c-Myc and degrade the tumor suppressor E2A by posttranslational modification. The first tumorigenic cells are detected among more immature CD4(-)8(+)TCR-alphabeta(-) cells that give rise to monoclonal tumors with a single, unique TCR-beta chain and diverse TCR-alpha chains, pinpointing malignant transformation to a stage after pre-TCR signaling and before completion of TCR-alpha rearrangement. In T-ALL, E2A deficiency is accompanied by further transcriptional up-regulation of c-Myc and concomitant dysregulation of the c-Myc-p53 axis at the transcriptional level. Even though the tumors consist of phenotypically heterogeneous cells, no evidence for tumor stem cells was found. As judged by array-based comparative genomic hybridization (array CGH) and spectral karyotype (SKY) analysis, none of the tumors arise because of genomic instability.

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.

Notch signaling and diseases: an evolutionary journey from a simple beginning to complex outcomes

Notch signaling pathway regulates a wide variety of cellular processes during development and it also plays a crucial role in human diseases. This important link is firmly established in cancer, since a rare T-ALL-associated genetic lesion has been initially reported to result in deletion of Notch1 ectodomain and constitutive activation of its intracellular region. Interestingly, the cellular response to Notch signaling can be extremely variable depending on the cell type and activation context. Notch signaling triggers signals implicated in promoting carcinogenesis and autoimmune diseases, whereas it can also sustain responses that are critical to suppress carcinogenesis and to negatively regulate immune response. However, Notch signaling induces all these effects via an apparently simple signal transduction pathway, diversified into a complex network along evolution from Drosophila to mammals. Indeed, an explanation of this paradox comes from a number of evidences accumulated during the last few years, which dissected the intrinsic canonical and non-canonical components of the Notch pathway as well as several modulatory extrinsic signaling events. The identification of these signals has shed light onto the mechanisms whereby Notch and other pathways collaborate to induce a particular cellular phenotype. In this article, we review the role of Notch signaling in cells as diverse as T lymphocytes and epithelial cells of the epidermis, with the main focus on understanding the mechanisms of Notch versatility.

Higher doses of lenalidomide are associated with unacceptable toxicity including life-threatening tumor flare in patients with chronic lymphocytic leukemia

PURPOSE

Lenalidomide is a novel therapeutic agent with uncertain mechanism of action that is clinically active in myelodysplastic syndrome (MDS) and multiple myeloma (MM). Application of high (MM) and low (MDS) doses of lenalidomide has been reported to have clinical activity in CLL. Herein, we highlight life-threatening tumor flare when higher doses of lenalidomide are administered to patients with CLL and provide a potential mechanism for its occurrence.

PATIENTS AND METHODS

Four patients with relapsed CLL were treated with lenalidomide (25 mg/d for 21 days of a 28-day cycle). Serious adverse events including tumor flare and tumor lysis are summarized. In vitro studies examining drug-induced apoptosis and activation of CLL cells were also performed.

RESULTS

Four consecutive patients were treated with lenalidomide; all had serious adverse events. Tumor flare was observed in three patients and was characterized by dramatic and painful lymph node enlargement resulting in hospitalization of two patients, with one fatal outcome. Another patient developed sepsis and renal failure. In vitro studies demonstrated lenalidomide-induced B-cell activation (upregulation of CD40 and CD86) corresponding to degree of tumor flare, possibly explaining the tumor flare observation.

CONCLUSION

Lenalidomide administered at 25 mg/d in relapsed CLL is associated with unacceptable toxicity; the rapid onset and adverse clinical effects of tumor flare represent a significant limitation of lenalidomide use in CLL at this dose. Drug-associated B-cell activation may contribute to this adverse event. Future studies with lenalidomide in CLL should focus on understanding this toxicity, investigating patients at risk, and investigating alternative safer dosing schedules.

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.

Notch signaling in CD4 and CD8 T cell development

Because Notch often acts in concert with other signaling pathways, it is able to regulate a diverse set of biological processes in a cell-context dependent manner. In lymphocytes, Notch is essential for specifying the T cell fate and for promoting early stages of T cell differentiation. At later stages of development, Notch signaling is proposed to direct CD4 versus CD8 T lineage commitment. This hypothesis has been challenged by recent studies of conditional Presenilin-deficient mice showing that Notch promotes the selection and maturation of CD4 and CD8 T cells by potentiating TCR signal transduction in immature thymocytes. While similar conclusions have not been reported with conditional mutation of other downstream mediators of Notch activation, it appears that functional inhibition may not have been achieved at a comparable stage of development and/or analogous issues have not been addressed. The differences also question whether in thymocytes Notch signals only through the canonical pathway. Further study of conditional mutants, signaling intermediates, and transcriptional regulators are needed to elucidate how Notch facilitates TCR signaling in generating mature T cells.

Development of Notch-dependent T-cell leukemia by deregulated Rap1 signaling

SPA-1 (signal-induced proliferation associated gene-1) functions as a suppressor of myeloid leukemia by negatively regulating Rap1 signaling in hematopoietic progenitor cells (HPCs). Herein, we showed that transplantation of HPCs expressing farnesylated C3G (C3G-F), a Rap1 guanine nucleotide exchange factor, resulted in a marked expansion of thymocytes bearing unique phenotypes (CD4/CD8 double positive [DP] CD3− TCRβ−) in irradiated recipients. SPA-1−/− HPCs expressing C3G-F caused a more extensive expansion of DP thymocytes, resulting in lethal T-cell acute lymphoblastic leukemia (T-ALL) with massive invasion of clonal T-cell blasts into vital organs. The C3G-F+ blastic thymocytes exhibited constitutive Rap1 activation and markedly enhanced expression of Notch1, 3 as well as the target genes, Hes1, pTα, and c-Myc. All the T-ALL cell lines from C3G-F+ SPA-1−/− HPC recipients expressed high levels of Notch1 with characteristic mutations resulting in the C-terminal truncation. This proliferation was inhibited completely in the presence of a γ-secretase inhibitor. Transplantation of Rag2−/− SPA-1−/− HPCs expressing C3G-F also resulted in a marked expansion and transformation of DP thymocytes. The results suggested that deregulated constitutive Rap1 activation caused abnormal expansion of DP thymocytes, bypassing the pre-T-cell receptor and eventually leading to Notch1 mutations and Notch-dependent T-ALL.

The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone-marrow-resident stem cells. However, whereas most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals is well established and rooted in highly conserved pathways involving Notch, morphogenetic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remains under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis, with an emphasis on the nature of thymus-derived signals delivered to T cell progenitors that support the commitment and differentiation of T cells toward the alphabeta and gammadelta T cell lineages.

Constitutive Notch signalling promotes CD4 CD8 thymocyte differentiation in the absence of the pre-TCR complex, by mimicking pre-TCR signals

Notch1 signalling is essential for the commitment of multipotent lymphocyte precursors towards the alphabeta T-cell lineage and plays an important role in regulating beta-selection in CD4(-)CD8(-) double-negative (DN) thymocytes. However, the role played by Notch in promoting the development of CD4(+)CD8(+) double-positive (DP) thymocytes is poorly characterized. Here, we demonstrate that the introduction of a constitutively active Notch1 (ICN1) construct into RAG(-/-) lymphocyte precursors resulted in the generation of DP thymocytes in in vitro T-cell culture systems. Notably, developmental rescue was dependent not only on the presence of an intact Notch1 RAM domain but also on Delta-like signals, as ICN1-induced DP development in RAG(-/-) thymocytes occurred within an intact thymus or in OP9-DL1 co-cultures, but not in OP9-control co-cultures. Interestingly, ICN1 expression in SLP-76(-/-) precursors resulted in only a minimal developmental rescue to the immature CD8(+) single-positive stage, suggesting that Notch is utilizing the same signalling pathway as the pre-TCR complex. In support of this, ICN1 introduction resulted in the activation of the ERK-MAPK-signalling cascade in RAG(-/-) thymocytes. Taken together, these studies demonstrate that constitutive Notch signalling can bypass beta-selection during early T-cell development by inducing pre-TCR-like signals within a T-cell-promoting environment.

An in vivo functional genetic screen for suppressors of the Rag1-/- T-cell defect

Functional genetic screens on mutant backgrounds have been successfully used in lower organisms to investigate biological processes. However, few identical screens have been performed in mice. Recombinase activating gene-1 deficient (Rag1-/-) mice have a severe T-cell developmental block owing to lack of rearrangement of their T-cell receptor (TCR) genes. Using a retroviral cDNA library derived from wild-type embryonic thymocytes we performed a suppressor screen in Rag1-/- hematopoietic cells and recovered TCRbeta. This is the first demonstration that targeted genetic screens are feasible using transduced primary cells in vivo. Consequently, this technique can be used to interrogate multiple blood lineages using diverse hematopoietic mouse mutants.

New treatments for chronic lymphocytic leukemia

An enhanced understanding of the important pathways governing chronic lymphocytic leukemia (CLL) cell survival and the critical role played by the microenvironment in the pathogenesis of the disease has brought new opportunities for drug development in CLL. Several new targets have been identified, and novel agents are under intense investigation in clinical trials. Some of these agents are already demonstrating promising anti-CLL activity on their own, whereas others hold promise in combination with existing therapeutic options. As the use of monoclonal antibodies for chemoimmunotherapy becomes standard clinical practice, the future holds promise for concurrent targeting of the tumor cell as well as its microenvironment.

Notch1 co-opts lymphoid enhancer factor 1 for survival of murine T-cell lymphomas

Oncogenic Notch1 mutations are found in most T-lineage acute lymphoblastic leukemias in humans and T-cell lymphomas in mice. However, the mechanism by which Notch1 promotes transformation or maintains malignant cell survival has not been determined fully. Here, we report that expression of the transcription factor lymphoid enhancer factor 1 (Lef1) is Notch dependent in murine T-cell lymphomas in vitro and in vivo, and that the intracellular domain of Notch1 (ICN1) is present at the Lef1 promoter. Lef1 expression is not Notch dependent in primary T-cell progenitors, but Lef1 mRNA is increased by ectopic expression of ICN1 in these cells. We show that Lef1 is required for survival of T-cell lymphoma lines, and that ectopic expression of Lef1 delays lymphoma cell death in the absence of Notch signaling, indicating that Lef1 is an important Notch target in these cells. Therefore, Notch1 co-opts Lef1 during the process of transformation to maintain survival of T-cell lymphomas.

p16INK4A tumor suppressor gene expression and CD3epsilon deficiency but not pre-TCR deficiency inhibit TAL1-linked T-lineage leukemogenesis

Inactivation of the CDKN2 genes that encode the p16(INK4A) and p14(ARF) proteins occurs in the majority of human T-cell acute lymphoblastic leukemias (T-ALLs). Ectopic expression of TAL1 and LMO1 genes is linked to the development of T-ALL in humans. In TAL1xLMO1 mice, leukemia develops in 100% of mice at 5 months. To identify the molecular events crucial to leukemic transformation, we produced several mouse models. We report here that expression of P16(INK4A) in developing TAL1xLMO1 thymocytes blocks leukemogenesis in the majority of the mice, and the leukemias that eventually develop show P16(INK4A) loss of expression. Events related to the T-cell receptor beta selection process are thought to be important for leukemic transformation. We show here that the absence of the pTalpha chain only slightly delays the appearance of TAL1xLMO1-induced T-ALL, which indicates a minor role of the pTalpha chain. We also show that the CD3epsilon-mediated signal transduction pathway is essential for this transformation process, since the TAL1xLMO1xCD3epsilon-deficient mice do not develop T-ALL for up to 1 year.

Molecular basis for pre-TCR-mediated autonomous signaling

The pre-T-cell receptor (pre-TCR) is a multimeric complex composed of a nascent TCRbeta chain, an invariant pre-TCRalpha (pTalpha) chain and CD3 molecules, and is crucial for early T-cell development. Despite its structural similarity to the mature alphabetaTCR, which requires MHC-antigen for receptor triggering, the pre-TCR is proposed to initiate signals in a ligand-independent manner. However, the molecular mechanism underlying the autonomous signaling is still unclear. Recent studies have revealed that pTalpha possesses unique characteristics that promote autonomous signaling. In this review, we summarize current data relating to the molecular mechanism underlying the initiation of pre-TCR-mediated autonomous signaling.

Pre-TCR expression cooperates with TEL-JAK2 to transform immature thymocytes and induce T-cell leukemia

The TEL-JAK2 gene fusion, which has been identified in human leukemia, encodes a chimeric protein endowed with constitutive tyrosine kinase activity. TEL-JAK2 transgenic expression in the mouse lymphoid lineage results in fatal and rapid T-cell leukemia/lymphoma. In the present report we show that T-cell leukemic cells from EmuSRalpha-TEL-JAK2 transgenic mice present an aberrant CD8(+) differentiation phenotype, as determined by the expression of stage-specific cell surface markers and lineage-specific genes. TEL-JAK2 transforms immature CD4(-)CD8(-) double-negative thymocytes, as demonstrated by the development of T-cell leukemia with full penetrance in a Rag2-deficient genetic background. This disease is similar to the bona fide TEL-JAK2 disease as assessed by phenotypic and gene profiling analyses. Pre-TCR signaling synergizes with TEL-JAK2 to transform immature thymocytes and initiate leukemogenesis as shown by (1) the delayed leukemia onset in Rag2-, CD3epsilon- and pTalpha-deficient mice, (2) the occurrence of recurrent chromosomal alterations in pre-TCR-deficient leukemia, and (3) the correction of delayed leukemia onset in Rag2-deficient TEL-JAK2 mice by an H-Y TCRalphabeta transgene that mimics pre-TCR signaling. Although not affecting leukemia incidence and mouse survival, TCRalphabeta expression was shown to facilitate leukemic cell expansion in secondary lymphoid organs.

The multifaceted role of Notch in cancer

The diverse roles that Notch signals play during the development and maintenance of normal tissues are recapitulated in different forms of cancer. Depending on the tumor type, Notch can variously promote or limit tumor growth through either cell autonomous or cell non-autonomous effects on differentiation, cellular metabolism, cell cycle progression, angiogenesis, and possibly self-renewal and immune function. Of particular interest, recent findings indicate that a high fraction of T-cell acute lymphoblastic leukemias and lymphomas have activating mutations in the Notch 1 receptor, and that Notch signaling might have a role in the maintenance of normal and malignant stem cells.