Leukemia (2006) 20, 1197–1205 & 2006 Nature Publishing Group All rights reserved 0887-6924/06 $30.00 www.nature.com/leu REVIEW Notch and Wnt signaling in T-lymphocyte development and acute lymphoblastic leukemia F Weerkamp, JJM van Dongen and FJT Staal Department of Immunology, Erasmus MC University Medical Center Rotterdam, Rotterdam, The Netherlands Many acute lymphoblastic leukemias can be considered Wnt proteins, these are factors of the Tcf/Lef family, in as malignant counterparts of cells in the various stages thymocytes mainly Tcf-1. of normal lymphoid development in bone marrow and thymus. T-cell development in the thymus is an ordered and tightly Both Notch and Wnt proteins were first identified in controlled process. Two evolutionary conserved signaling Drosophila and named after their effect in the development pathways, which were first discovered in Drosophila, of the Drosophila wing: heterozygous mutation of Notch control the earliest steps of T-cell development. These are resulted in a nicked wing phenotype,1 whereas the ‘wingless’ the Notch and Wnt-signaling routes, which both are deregu- mutation of Wnt dramatically shortened the wing.2 Attention lated in several types of leukemias. In this review we discuss was attracted to these genes in the mammalian system when it both pathways, with respect to their signaling mechanisms, was discovered that both function as proto-oncogenes. Nusse functions during T-cell development and their roles in devel- 3 opment of leukemias, especially T-cell acute lymphoblastic and Varmus discovered the Wnt gene (then termed Int1)asan leukemia. integration site for mouse mammary-tumor viruses. The first Leukemia (2006) 20, 1197–1205. doi:10.1038/sj.leu.2404255; mammalian homologue of the Drosophila Notch gene was published online 11 May 2006 identified as TAN1, the fusion partner of the T-cell receptor beta Keywords: Notch; Wnt; T-cell development (TCRB) gene in t(7;9) causing T-cell acute lymphoblastic leukemia (T-ALL).4 Later it was found that both Notch and Wnt signaling play Introduction critical roles in normal T-cell development. Both cascades are thought to mediate different processes in the thymus: It is becoming increasingly clear that most cancers arise Notch in differentiation and Wnt in proliferation, although from cancer stem cells and follow differentiation pathways we will show in this review that this is not as clear-cut as that closely resemble the developmental and ontogenetic assumed. Despite differences in function and their very different pathways of normal cells and tissues. This is in particular molecular mechanisms, many parallels exist for both pathways. the case for antigen-specific cells of the immune system, the Absence of either causes a block at the earliest stages of lymphocytes. There is no other developmental system that T-cell development, but both are thought to be important for has been described to such detail as the differentiation of later stages as well. Interestingly, the two signaling pathways B lymphocytes in the bone marrow (BM) and of T lymphocytes also play a role in hematopoietic stem cell (HSC) self-renewal. in the thymus. In order to fully comprehend the malignant Moreover, dysregulated signaling of both cascades is involved in processes underlying lymphoid leukemias, the malignant leukemogenesis. counterpart of lymphocytes, thorough knowledge of normal differentiation pathways is a prerequisite. T cells develop in the thymus, from BM-derived multipotent Overview of T-cell development in the thymus progenitors. The ‘thymus-seeding’ cells encounter molecules that initiate and guide their development: adhesion molecules, T-cell development takes place in the thymus, a primary cytokines, chemokines and morphogens largely produced by lymphoid organ that provides a unique microenvironment for the thymic stroma. These molecules initiate signaling path- the development of hematopoietic progenitors into mature ways that lead to the activation of transcription factors that T cells.5 In adult mice and humans, these precursor cells are change the transcriptional program of a cell. Two exogenously derived from the BM, whereas during fetal development the activated signaling pathways that are crucial for thymocyte liver is the principal source. The exact nature of the progenitors development and have attracted much attention recently is unclear, but some level of multipotency is now commonly are the Notch and the Wnt-signaling cascades. Both act as accepted.6,7 Development into a mature T cell comprises the molecular switches: the executing transcription factor that is ordered rearrangement of the different TCR genes and positive normally bound to the DNA as a transcriptional repressor is and negative selection events that finally lead to the expression converted into an activator. For the Notch pathway, set in of mature TCRab and TCRgd molecules recognizing non-self motion by ligands of the Delta and Jagged family, this peptides in context of self-major histocompatibility complex transcription factor is CSL (see section on ‘Notch signaling’ (MHC) or equivalent molecules. These processes take place below). For the Wnt pathway, which is activated by secreted during a series of discrete phenotypic stages that can be recognized by expression of several key membrane molecules, Correspondence: Dr FJT Staal, Department of Immunology, Erasmus most notably CD4 and CD8 (Figure 1a). In both man and mouse, MC University Medical Center Rotterdam, Dr Molewaterplein 50, thymocytes are successively CD4ÀCD8À (double negative 3015 GE Rotterdam, The Netherlands. þ À E-mail: [email protected] (DN)), CD4 CD3 (human immature single positive (ISP)) or þ À þ þ Received 3 March 2006; revised 3 April 2006; accepted 6 April 2006; CD8 CD3 (mouse ISP), CD4 CD8 (double positive (DP)) published online 11 May 2006 and finally CD4 þ CD3 þ or CD8 þ CD3 þ (single positive Notch and Wnt signaling in T-cell development and T-ALL F Weerkamp et al 1198 Figure 1 Schematic representation of three thymic lobules. (a) The different thymocyte subsets and their migration through the anatomic niches. The cells that enter the thymus are CD4ÀCD8À double negative (DN). The thymocytes subsequently develop into immature single positive (ISP), double positive (DP) and single positive (SP) stages. Finally, mature naive T cells leave the thymus into the blood. (b) The primary events during ab T-cell development. The earliest DN cells in the thymus massively proliferate. In the DN3 stage, rearrangements of the TCRB genes start. Production of a successful TCRb chain leads to membrane expression of the pre-TCR in the ISP cells, after which proliferation and initiation of TCRA rearrangements occurs. During the DP stage, positive selection (for a functional TCR) and negative selection (against self-reactivity) takes place. Surviving cells become SP. (c) The stromal compartments of the thymus. Each compartment contains specialized epithelial cells (TECs), which provide Wnt proteins, Notch ligands, and other growth and differentiation factors for developing thymocytes. The earliest thymocytes enter the thymus at the cortico-medullary junction and migrate to the subcapsular zone. As they mature, they pass through the cortex and the medulla. (SP)).8,9 In the mouse, the DN subset can be further subdivided Notch signaling in the thymus into four stages: CD44 þ CD25À (DN1), CD44 þ CD25 þ (DN2), CD44ÀCD25 þ (DN3) and CD44ÀCD25À (DN4).10 In the The Notch signal transduction pathway, depicted in Figure 2, is human thymus, CD34 þ CD1aÀ cells correspond to murine an evolutionary conserved mechanism that regulates cell DN1 and 2 thymocytes and CD34 þ CD1a þ cells are homo- fate determination during developmental processes.1 The Notch logous to DN3 cells 9,11 (Figure 1a). The human DN4 family of transmembrane receptors consists of four members, population is hard to distinguish from ISP cells that express named Notch1 to 4. Signaling is initiated when the large low level CD3. extracellular domain of the Notch receptor binds a membrane- As the numbers of progenitors that enter the thymus are bound ligand on a neighboring cell. The five Notch ligands in limited, an enormous expansion takes place during the DN1 mammals are Delta1, 3 and 4 and Jagged1 and 2. The and DN2 stages of T-cell development12 (Figure 1b). In the glycosylation status of the Notch receptor can be modified by DN3 stage, cells stop proliferating and start rearranging the Fringe proteins, thereby influencing ligand binding.17 TCRB loci, resulting in a complete TCRb chain during Interaction of Notch with a ligand can induce proteolytic the DN4/ISP stage.11 When the thymocytes have developed cleavage of Notch, successively by metalloprotease and into ISP cells, the functionality of the TCRb chain is tested g-secretase activities,18,19 releasing the intracellular part of the by expression on the cell surface together with a pre-TCR protein (intracellular (IC-) Notch). IC-Notch then translocates alpha (pTa) chain, a process called b-selection.13 Signals to the nucleus and binds to the nuclear transcription factor emanating from this pre-TCR complex induce the cells first to CSL (CBF1¼ c-promoter binding factor in humans, suppressor proliferate and subsequently to initiate TCRA gene rearrange- of hairless in Drosphila, Lag-1 in Caenorhabditis elegans; also ments. The thymocytes now become DP cells. Productive called RBP-k in mouse).20 Binding of IC-Notch to CSL induces TCRA rearrangements lead to the expression of a TCRab the dislocation of co-repressors (coR) such as Mint and Nrarp, complex on the cell surface, which is then tested for the and recruitment of co-activators (coA), such as Mastermind recognition of self-MHC molecules (positive selection) and (Maml), consequently stimulating the transcription of Notch absence of reactivity against self-antigens (negative selection).14 target genes20 (Figure 2). It has been estimated that 95% of developing thymocytes die The ligands that induce Notch signaling are present on the by apoptosis.15 The surviving cells become SP thymocytes membrane of stromal cells such as TECs.