Tcf1 is essential for initiation of oncogenic Notch1-driven chromatin topology in T-ALL

NOTCH1 is a well-established lineage specifier for T cells and among the most frequently mutated genes throughout all subclasses of T cell acute lymphoblastic leukemia (T-ALL). How oncogenic NOTCH1 signaling launches a leukemia-prone chromatin landscape during T-ALL initiation is unknown. Here we demonstrate an essential role for the high-mobility-group transcription factor Tcf1 in orchestrating chromatin accessibility and topology, allowing aberrant Notch1 signaling to convey its oncogenic function. Although essential, Tcf1 is not sufficient to initiate leukemia. The formation of a leukemia-prone epigenetic landscape at the distal Notch1-regulated Myc enhancer, which is fundamental to this disease, is Tcf1-dependent and occurs within the earliest progenitor stage even before cells adopt a T lymphocyte or leukemic fate. Moreover, we discovered a unique evolutionarily conserved Tcf1-regulated enhancer element in the distal Myc-enhancer, which is important for the transition of preleukemic cells to full-blown disease.

DN2 Thymocytes Activate a Specific Robust DNA Damage Response to Ionizing Radiation-Induced DNA Double-Strand Breaks

For successful bone marrow transplantation (BMT), a preconditioning regime involving chemo and radiotherapy is used that results in DNA damage to both hematopoietic and stromal elements. Following radiation exposure, it is well recognized that a single wave of host-derived thymocytes reconstitutes the irradiated thymus, with donor-derived thymocytes appearing about 7 days post BMT. Our previous studies have demonstrated that, in the presence of donor hematopoietic cells lacking T lineage potential, these host-derived thymocytes are able to generate a polyclonal cohort of functionally mature peripheral T cells numerically comprising ~25% of the peripheral T cell pool of euthymic mice. Importantly, we demonstrated that radioresistant CD44⁺ CD25⁺ CD117+ DN2 progenitors were responsible for this thymic auto-reconstitution. Until recently, the mechanisms underlying the radioresistance of DN2 progenitors were unknown. Herein, we have used the in vitro “Plastic Thymus” culture system to perform a detailed investigation of the mechanisms responsible for the high radioresistance of DN2 cells compared with radiosensitive hematopoietic stem cells. Our results indicate that several aspects of DN2 biology, such as (i) rapid DNA damage response (DDR) activation in response to ionizing radiation-induced DNA damage, (ii) efficient repair of DNA double-strand breaks, and (iii) induction of a protective G₁/S checkpoint contribute to promoting DN2 cell survival post-irradiation. We have previously shown that hypoxia increases the radioresistance of bone marrow stromal cells in vitro, at least in part by enhancing their DNA double-strand break (DNA DSB) repair capacity. Since the thymus is also a hypoxic environment, we investigated the potential effects of hypoxia on the DDR of DN2 thymocytes. Finally, we demonstrate for the first time that de novo DN2 thymocytes are able to rapidly repair DNA DSBs following thymic irradiation in vivo.

Abstract LB-034: Tcf7: an essential mediator in Notch-induced T-ALL

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer caused by the malignant transformation of immature thymocytes. Notch1 gain-of-function mutations are present in >50% of T-ALL cases and result in the constitutive activation of Notch1 intracellular domain (N1ICD) in mutated cells. The mechanisms by which deregulated Notch1 signalling leads to T-ALL development are currently unresolved and stringently investigated. T cell factor-1 (TCF-1) is a transcription factor encoded by Tcf7. Not only playing crucial roles in Wnt signalling, Tcf7 is (i) a direct Notch1 target gene and (ii) required for driving early T lineage commitment of hematopoietic progenitors. Together, these findings suggest that Tcf7 may play a role in Notch1-driven T-ALL. Herein, we show that the conditional loss of Tcf7 in hematopoietic progenitors prevents T-ALL development in both genetic- and retroviral-based mouse models of N1ICD over-expression. However, T-ALL development was unaffected by conditional loss of the Wnt signalling mediator, β-catenin. Interestingly, conditional Tcf7 deficiency partially restored the B lineage potential of N1ICD over-expressing bone marrow progenitors. Similar to Tcf7-/- mice, conditional Tcf7 deficiency in bone marrow progenitors abrogated T cell development in adult thymi. However, unexpectedly, Tcf7-deficient hematopoietic progenitors adopted a B cell fate in these thymi, a phenotype that was found to be comparable to conditional loss of Notch1. Overall, our results indicate that Tcf7 is an important mediator of Notch1-driven T-ALL. Furthermore, our data suggests that, in addition to promoting T lineage commitment, Tcf7 may also function as a Notch1-dependent transcriptional repressor of B lineage commitment in adult hematopoiesis. We performed RNAseq to identify the molecular players involved in Tcf7-mediated lineage repression and complement this approach with CHIP_Seq.

Citation Format: Ute Koch, Tara Sugrue, Christelle Dubey, Freddy Radtke. Tcf7: an essential mediator in Notch-induced T-ALL. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-034.

Hypoxia enhances the radioresistance of mouse mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) are radioresistant bone marrow progenitors that support hematopoiesis and its reconstitution following total body irradiation. MSCs reside in hypoxic niches within the bone marrow and tumor microenvironments. The DNA damage response (DDR) represents a network of signaling pathways that enable cells to activate biological responses to DNA damaging agents. Hypoxia-mediated alterations in the DDR contribute to the increased radioresistance of hypoxic cancer cells, limiting therapeutic efficacy. The DDR is important in mediating mouse MSC radioresistance. However, the effects of hypoxia on MSC radioresistance are currently unknown. In this report, hypoxia was found to (a) increase MSC proliferation rate and colony size; (b) increase long-term survival post-irradiation (IR), and (c) improve MSC recovery from IR-induced cell cycle arrest. DNA double-strand break (DSB) repair in MSCs was upregulated in hypoxia, accelerating the resolution of highly genotoxic IR-induced DNA DSBs. In addition, HIF-1α was found to contribute to this enhanced DSB repair by regulating (a) the expression of DNA ligase IV and DNA-PKcs and (b) Rad51 foci formation in response to DNA DSBs in hypoxic MSCs. We have demonstrated, for the first time, that hypoxia enhances mouse MSC radioresistance in vitro. These findings have important implications for our understanding of MSC functions in supporting allogeneic bone marrow transplantation and in tumorigenesis.