Notch

Notch in T cell development, function & transformation

Notch proteins are a conserved family of receptors that regulate cell fate decisions in organisms ranging from Drosophila to humans. Because hematopoietic development requires multiple cell fate decisions to generate mature cells from a single hematopoietic stem cell, Notch proteins are likely to control some of these developmental switches. Together with the Radtke lab, my lab established that Notch signaling is essential to generate T cells from a multipotent progenitor (Pui et al., Immunity 1999). In subsequent studies, we showed that Notch signaling controls multiple steps during intrathymic T cell differentiation. In particular, we showed that Notch1 signals are essential up to and including DN3 stage but are no longer required by the DP stage (Allman et al., J. Exp. Med. 2001; Maillard et al., J. Exp. Med, 2006). We also identified a number of the key transcriptional targets regulated by Notch in T cell development, which include Hes1, the pTα receptor, CD25, Dtx1, and Myc, and showed how pre-T cell signaling turns off Notch signaling at the β-selection checkpoint. Notch signaling is also utilized in peripheral T cells and have particularly important functions in helper T cells. We identified important functions for Notch in Th2 generation (Tu et al., J. Exp. Med., 2005) and proposed a new hypothesis for Notch function in the periphery whereby Notch simultaneously orchestrates multiple lineage programs, rather than restricting alternate outcomes. We also generated novel reagents including “dominant negative mastermind” mice, which allow for conditional shutdown of Notch signaling.

Failure to precisely regulate Notch signaling during T cell development may lead to T cell leukemia, as evidenced by Notch-dependent T cell leukemias in humans and mice. Oncogenic Notch mutations are now recognized as the most common mutation in T-ALL. Utilizing methodology that we established to rapidly generate high titer retroviral supernatants (Pear et al., PNAS, 1993), we established the first murine model of Notch-induced T-ALL that closely mimics the human disease (Pear et al., J. Exp. Med, 1996). Much of this work was performed in collaboration with the labs of Jon Aster and Steve Blacklow (Ann Rev Path, 2017). Recent work has focused on delineating the impact of Notch signaling on the epigenetic landscape in T-ALL and identifying important transcriptional targets of Notch signaling in T-ALL. Utilizing Notch1 antibodies generated by us, we showed that Notch preferentially binds enhancers (Wang et al., PNAS, 2011, Wang et al., PNAS, 2014). We also found that Notch-induced T-ALL depends on the ability of Notch to form dimeric Notch transcriptional complexes. One particularly important target is Myc and we identified a distal enhancer through which Notch regulates Myc in T cells (Yashiro-Ohtani et al., PNAS 2014). These studies show that Notch regulates the H3K27Ac over a broad “super enhancer” region (>600 kb). Recently, we found that distinct regions within this Notch-dependent super enhancer mediate drug resistance. We also identified a common Notch signature in three tumor types (breast cancer, B cell lymphoma, T-ALL) that contain Notch1 activating mutations. Surprisingly, there were only 5 transcripts in common, of which one was Myc (Stoeck et al. Cancer Discov, 2014). In close collaboration with the Faryabi lab, we are currently trying to understand how Notch influences oncogenesis in these different tumors, particularly Myc, which we showed when constitutively expressed suppresses the selective drive for Notch1 gain-of-function mutations in a T-ALL prone genetic background (Chiang et al., Blood 2016, PMC5095757). In our collaborative work with the Faryabi lab, we found that Notch was capable of enhancer repositioning and that cells can develop resistance to Notch inhibitors by reactivating a B cell-like program that maintains oncogenic Myc expression (Petrovic et al., Mol Cell 2019, Zhou et al., 2022).