Oncogenic Notch signaling in T-cell and B-cell lymphoproliferative disorders

Abstract

Purpose of review: This article highlights recent discoveries about Notch activation and its oncogenic functions in lymphoid malignancies, and discusses the therapeutic potential of Notch inhibition.

Recent findings: NOTCH mutations arise in a broad spectrum of lymphoid malignancies and are increasingly scrutinized as putative therapeutic targets. In T-cell acute lymphoblastic leukemia (T-ALL), NOTCH1 mutations affect the extracellular negative regulatory region and lead to constitutive Notch activation, although mutated receptors remain sensitive to Notch ligands. Other NOTCH1 mutations in T-ALL and NOTCH1/2 mutations in multiple B-cell malignancies truncate the C-terminal proline (P), glutamic acid (E), serine (S), threonine (T)-rich (PEST) domain, leading to decreased Notch degradation after ligand-mediated activation. Thus, targeting Notch ligand-receptor interactions could provide therapeutic benefits. In addition, we discuss recent reports on clinical testing of Notch inhibitors in T-ALL that influenced contemporary thinking on the challenges of targeting Notch in cancer. We review advances in the laboratory to address these challenges in regards to drug targets, the Notch-driven metabolome, and the sophisticated protein-protein interactions at Notch-dependent superenhancers that underlie oncogenic Notch functions.

Summary: Notch signaling is a recurrent oncogenic pathway in multiple T- and B-cell lymphoproliferative disorders. Understanding the complexity and consequences of Notch activation is critical to define optimal therapeutic strategies targeting the Notch pathway.

Figure 1. Structure of Notch1/2 receptors depicting domain organization and key mutation sites observed in lymphoid malignancies

A. Wild-type Notch1/2 receptors depicting extracellular Notch (ECN) and intracellular Notch (ICN) domains. EGF11/12 repeats (red) are important for ligand binding. Sites of proteolytic cleavage are indicated as S1 (furin-like protease), S2 (ADAM10 metalloprotease) and S3 (γ-secretase complex). EGF, epidermal growth factor like domain; LNR, Lin12/Notch repeats; HD, heterodimerization domain; N, N-terminal portion of HD; C, C-terminal portion of HD; NRR, negative regulatory region; RAM, RBPJ-associated molecule domain; NLS, nuclear localization signal; ANK, ankyrin repeats; TAD, transactivation domain; PEST, proline (P), glutamic acid (E), serine (S) and threonine (T)-rich sequence. B. Mutated Notch1 and Notch2 receptors with corresponding disease associations. Lightly colored areas over HD and PEST domains represent key mutation sites. PEST mutations typically truncate the PEST domain. The most frequent disease associations of individual mutations are shown as follows: T-ALL, T cell acute lymphoblastic leukemia; CLL, chronic lymphocytic leukemia; MCL, mantle cell lymphoma; FL, follicular lymphoma; SMZL, splenic marginal zone lymphoma; DLBCL, diffuse large B cell lymphoma.

Figure 2. Mechanisms of Notch pathway activation in lymphoid malignancies

A. Notch1 heterodimerization domain mutations in T-ALL. HD mutations destabilize the receptor and lead to constitutive ligand-independent proteolytic activation (right). HD-mutated Notch1 receptors also remain sensitive to ligand-mediated activation (left). Both ligand-independent and ligand-dependent inputs can contribute to Notch signaling in malignant T cells. B. Notch1/2 PEST domain mutations in T-ALL and B cell lymphoproliferative disorders. PEST mutations truncate the PEST domain, leading to decreased proteasomal degradation and increased half-life of cleaved ICN1/2. Notch signaling through PEST-mutated receptors requires ligand-dependent activation. The most frequent disease associations are shown as follows: T-ALL, T cell acute lymphoblastic leukemia; CLL, chronic lymphocytic leukemia; MCL, mantle cell lymphoma; FL, follicular lymphoma; SMZL, splenic marginal zone lymphoma; DLBCL, diffuse large B cell lymphoma.

Figure 3. Emerging insights into transcriptional regulation of Notch target genes

Cleaved ICN translocates to the nucleus and interacts with the DNA-binding transcription factor RBPJ, a member of the Mastermind-like (MAML) family and other transcriptional coactivators (CoA). In T-ALL and possibly in other contexts, activation of key target genes such as MYC involves long-range interactions between the basal promoter and a distant superenhancer. Additional transcription factors (TF) such as ETS1, RUNX1, HEB, E2A, GABPA, and TAL1 converge with ICN to superenhancer sites. The PIAS-like coactivator ZMIZ1 binds ICN1/RBPJ and facilitates the recruitment of ICN to the MYC superenhancer in cancer cells.