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

Abstract

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.

Regulation of the Signal-Dependent E Protein HEBAlt Through a YYY Motif Is Required for Progression Through T Cell Development

“…Therefore, decreasing HEBAlt activity may function to allow b-selection once DN3 cells as they move into the outer cortex where concentrations of IL-7 are low. This would be consistent with the observation that HEB protein is degraded in thymocytes undergoing leukemic transformation, resulting in a decrease in Cdkn1a expression and dysregulated proliferation, in both mice and humans (49).…”

“…Progenitors from ALT-Tg mice in OP9-DL4 co-cultures showed a much stronger defect in T cell development than we observed in vivo , akin to what has been previously observed for HEB-deficient and TCF1-deficient mice ( 21 , 53 ). Given our findings, this may be due in part to the movement of progenitors through different niches in the intact thymus, resulting in modulation of IL-7 availability and regulation of signaling environments that contribute to HEB protein stability ( 49 ). In ALT-Tg mice, the mild differences observed in the ISP and CD8 SP subsets were not reflected in the subsets representing progress through positive selection in a background with a polyclonal TCR repertoire.…”

High exposure variance enables candidate biomarker detection in a small EWAS of methylmercury-exposed Peruvian adults

“…1A ) and interleukin-3 signaling ( Supplemental Table S1 ), cells further develop into neutrophils and macrophages 96 . In contrast, RUNX1 97,98 , RUNX3 97,98 , TCF3 99 , and TCF12 99 ( Fig. 1A ) promote T cell lineage commitment.…”