Notch signaling in lymphoid cell development and transformation.

SY17-01 The human NOTCH1 gene was originally identified through its involvement in a rare 7;9 chromosomal translocation that is found in approximately 1% of human T cell acute lymphoblastic leukemia/lymphoma (T-ALL), an aggressive tumor comprised of immature T cells. Although NOTCH1 was subsequently shown to have an essential role in T cell development and to be a potent inducer of T-ALL in animal models, the extent of its importance in T-ALL was uncertain until recently, when gain-of-function NOTCH1 mutations were identified in more than 50% of human acute T cell lymphoblastic leukemias and lymphomas (T-ALL) (1). NOTCHes are large type I transmembrane proteins that function as surface receptors and transcription factors, leading some to tout the NOTCH pathway as the most direct communication pipeline between the extracellular milieu and the nucleus. This unusual form of signal transduction is accomplished by way of a series of regulated proteolytic cleavages. The first, catalyzed by furin, occurs at a site in the extracellular domain of NOTCH1 during receptor maturation. As a result of this cleavage, most NOTCH1 receptors on the cell-surface are heterodimers comprised of extracellular and transmembrane subunits, which are held together non-covalently by contacts made through a juxtamembrane heterodimerization domain that flanks the furin cleavage site. Binding of ligands to the extracellular subunit induces two successive additional proteolytic cleavages. The first occurs in the extracellular portion of the transmembrane subunit and is catalyzed by ADAM-type metalloproteases. This cleavage creates a short-lived transmembrane intermediate that is rapidly recruited to a multiprotein complex called gamma-secretase, which cleaves NOTCH within its transmembrane domain. This final cleavage releases the intracellular domain of NOTCH1 (ICN1) and allows it to translocate to the nucleus, where it forms a short-lived transcriptional activation complex with the DNA-binding factor CSL and co-activator proteins of the Mastermind family. Turnover of this nuclear complex is regulated in part by the C-terminal part of ICN1, which contains a PEST “degron” domain. The gain-of-function mutations in NOTCH1 that are commonly observed in primary lymphoblasts and established T-ALL cell lines take two forms: i) juxtamembrane mutations involving the extracellular heterodimerization domain, which increase proteolysis and ICN1 production; and ii) frameshift and stop codon mutations in the intracellular portion of NOTCH1, which delete the C-terminal “degron” domain and thereby sustain ICN1 action. Not uncommonly, both types of mutations are found in single NOTCH1 alleles in T-ALL cells, an alignment that leads to synergistic increases in NOTCH signaling. Remarkably, these types of NOTCH1 mutations have now been found in concert with virtually every other genetic lesion linked to human or experimental T-ALL, indicating that aberrant NOTCH1 signaling has a central, non-redundant role in many forms of this disease. Of possible therapeutic importance, human and murine T-ALL cell lines with ongoing NOTCH-receptor activation often undergo growth arrest followed by variable degrees of apoptosis upon exposure to NOTCH pathway inhibitors, suggesting that NOTCH1 signaling has a role in tumor maintenance as well as tumor induction. These data raise a number of mechanistic questions. One is whether leukemogenic mutations in the extracellular heterodimerization domain merely weaken contacts that are important for maintaining the integrity of NOTCH1 heterodimers, or act by directly increasing the access of the transmembrane subunit to activating cleavages. It also remains to be determined whether heterodimerization domain mutations cause ligand-independent activation of NOTCH1, or render the receptor hypersensitive to ligand. Other emerging areas include the identity and function of the negative regulatory sequences that are impacted by the C-terminal deletions, the identity of the genes lying immediately downstream of NOTCH1 that induce leukemias and maintain leukemia cell growth, and the interaction of NOTCH1 mutations, alone and in tandem, with other proteins that are commonly dysregulated in T-ALL. Ongoing work that aims to fill these gaps is likely to provide important new insights into physiologic and pathophysiologic NOTCH1 receptor activation, regulation, and function. Independent of these basic questions, the serendipitous availability of gamma-secretase inhibitors has enabled the opening of a clinical trial of a gamma-secretase inhbitor in patients with refractory/relapsed T-ALL. Each of these mechanistic issues, as well as prospects for NOTCH pathway therapeutics in T-ALL and other human cancers, will be discussed.