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)-$\alpha \beta^+$ 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 $CD4^−8^+TCR-\alpha \beta^-$ 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

Pre-TCRα and TCRα Are Not Interchangeable Partners of TCRβ during T Lymphocyte Development

In contrast with the αβ T cell receptor (TCR), the pre-TCR spontaneously segregates to membrane rafts from where it signals in a cell-autonomous fashion. The disparate behaviors of these two receptors may stem either from differences inherent to the distinct developmental stages during which they are expressed, or from features intrinsic and unique to the receptor components themselves. Here, we express TCRα precisely at the pre-TCR checkpoint, at levels resembling those of endogenous pre-TCRα (pTα), and in the absence of endogenous pTα. Both in isolation and more dramatically when in competition with pTα, TCRα induced defective proliferation, survival, and differentiation of αβ T lymphocyte precursors, as well as impaired commitment to the αβ T lymphocyte lineage. Substitution of TCRα transmembrane and cytoplasmic domains with those of pTα generated a hybrid molecule possessing enhanced competitive abilities. We conclude that features intrinsic to the pre-TCR, which are absent in TCRα, are essential for its unique function

Differential synergy of Notch and T cell receptor signaling determines αβ versus γδ lineage fate

Thymic precursors expressing the pre–T cell receptor (TCR), the γδTCR, or the αβTCR can all enter the CD4+8+ αβ lineage, albeit with different efficacy. Here it is shown that proliferation and differentiation of precursors with the different TCRs into αβ lineage cells require Notch signaling at the DN3 stage of thymic development. At the DN4 stage, Notch signaling still significantly contributes to the generation of αβ T cells. In particular, in αβ lineage commitment, the pre-TCR synergizes more efficiently with Notch signals than the other two TCRs, whereas γδTCR-expressing cells can survive and expand in the absence of Notch signals, even though Notch signaling enhances their proliferation. These observations suggest a new model of αβ versus γδ lineage choice in which lineage fate is determined by the extent of synergy between TCR and Notch signaling and in which the evolutionarily recent advent of the cell-autonomously signaling pre-TCR increased the efficacy of αβ T cell generation

A far downstream enhancer for murine Bcl11b controls its T-cell specific expression

Bcl11b is a T-cell specific gene in hematopoiesis that begins expression during T-lineage commitment and is required for this process. Aberrant expression of BCL11B or proto-oncogene translocation to the vicinity of BCL11B can be a contributing factor in human T-ALL. To identify the mechanism that controls its distinctive T-lineage expression, we corrected the identified Bcl11b transcription start site and mapped a cell-type–specific differentially methylated region bracketing the Bcl11b promoter. We identified a 1.9-kb region 850 kb downstream of Bcl11b, “Major Peak,” distinguished by its dynamic histone marking pattern in development that mirrors the pattern at the Bcl11b promoter. Looping interactions between promoter-proximal elements including the differentially methylated region and downstream elements in the Major Peak are required to recapitulate the T-cell specific expression of Bcl11b in stable reporter assays. Functional dissection of the Major Peak sequence showed distinct subregions, in which TCF-1 sites and a conserved element were required for T-lineage–specific activation and silencing in non-T cells. A bacterial artificial chromosome encompassing the full Bcl11b gene still required the addition of the Major Peak to exhibit T-cell specific expression. Thus, promoter-proximal and Major Peak sequences are cis-regulatory elements that interact over 850 kb to control expression of Bcl11b in hematopoietic cells

Harnessing of the Nucleosome Remodeling Deacetylase complex controls lymphocyte development and prevents leukemogenesis

Cell fate decisions depend on the interplay between chromatin regulators and transcription factors. Here we show that activity of the Mi-2β nucleosome remodeling and deacetylase (NuRD) complex was controlled by the Ikaros family of lymphoid-lineage determining proteins. Ikaros, an integral component of the NuRD complex in lymphocytes, tethered this complex to active lymphoid differentiation genes. Loss in Ikaros DNA binding activity caused a local increase in Mi-2β chromatin remodeling and histone deacetylation and suppression of lymphoid gene expression. The NuRD complex also redistributed to transcriptionally poised non-Ikaros gene targets, involved in proliferation and metabolism, inducing their reactivation. Thus, release of NuRD from Ikaros regulation blocks lymphocyte maturation and mediates progression to a leukemic state by engaging functionally opposing epigenetic and genetic networks

Lef-1: NOTCHed up in T-cell lymphomas

In this issue of Blood, Spaulding and colleagues show that Lef-1, one of the transcription factors mediating Wnt signaling, is a transcriptional target of Notch in T-cell lymphomas. Notch-activating mutations are commonly found in human T-lineage acute lymphoblastic leukemia (T-ALL) cases, while activation of Wnt/β-catenin signaling has recently been shown to induce T-cell leukemia in mice. The proposed regulation of Lef-1 transcription by Notch suggests the intriguing possibility that the Notch and Wnt pathways are closely intertwined in the etiology of T-cell leukemia