The Notch cell signaling pathway – named for its contribution to Drosophila wing indentations, which were first described in 1914 by John Dexter, a college biology professor who spent several summers working with legendary geneticist Thomas Hunt Morgan at the Marine Biology Laboratory in Woods Hole, MA – is a highly conserved signal transduction cascade present in all metazoan organisms, critical for normal embryological development and definitive hematopoiesis. Although Notch pathway activity is dysregulated in several cancers and congenital disorders, hematologists know Notch best from T-cell acute lymphocytic leukemia (T-ALL), where constitutively activating mutations of NOTCH1 are detectable in more than 50 percent of cases.1 The more recent identification of recurrent activating mutations in NOTCH1 in 12 percent of patients with chronic lymphocytic leukemia (CLL), particularly among the poor prognosis subset of patients with unmutated immunoglobulin heavy chain genes, highlighted NOTCH1’s status as an oncogene and underscored the relevance of the Notch pathway in human lymphoid neoplasia.2
Recently, experiments designed to explore the role of the Notch pathway in hematopoiesis unexpectedly revealed another face of Notch signaling: a tumor suppressive effect. Apostolos Klinakis and Iannis Aifantis from Athens, Greece, led an international group of investigators who knocked out one of the few non-redundant members of the Notch pathway, nicastrin, in hematopoietic cells in a murine model. Nicastrin is a member of the γ secretase enzyme complex that normally cleaves an intracellular domain from cell-surface Notch receptors upon ligandreceptor binding; the cleaved Notch intracellular domain then migrates to the nucleus, where it modifies the expression of a broad range of genes via interactions with several DNA-bound factors and recruitment of mastermind-like (MAML) proteins. Notably, none of the mice with nicastrin loss lived more than 20 weeks, and all developed monocytosis, splenomegaly, and myeloid proliferation suggestive of human chronic myelomonocytic leukemia (CMML). Furthermore, deletion of either Notch1 or Notch2 receptors (but not Notch3) in murine hematopoetic cells resulted in a similar CMML-like proliferation, whereas forced Notch1 expression rescued the nicastrin knockout phenotype. The hematopoietic effects of Notch disruption were cell-autonomous and mediated by the MAML1 transcriptional co-activator and Hes1 transcriptional repressor.
The relevance of these findings to human CMML was confirmed when the investigators discovered inactivating mutations in Notch pathway members in five of 42 patients with CMML. These patients had heterozygous somatic mutations in one of four Notch pathway genes: NCSTN (encoding nicastrin), APH1 (anterior pharynxdefective 1, a member of the γ secretase complex), MAML1, or NOTCH2. Additional polymorphisms that might be somatic mutations were detected in other patients with CMML; the possibility of epigenetic silencing was not explored. Transcriptional reporter and in vitro differentiation assays confirmed that the MAML1 and NCSTN mutations were either null or dominant negative. Notch pathway mutations were not present in 47 patients with polycythemia vera or primary myelofibrosis. The five CMML patients with Notch cascade signaling mutations also had mutations in other genes previously described in myeloid neoplasia, including JAK2, KRAS, ASXL1, and TET2, suggesting that these mutations may be cooperative and are not mutually exclusive.
In Brief
The discovery of inactivating Notch pathway mutations in a human myeloid neoplasm provides evidence of a novel pathobiologic role for deregulated Notch signaling. Notch-based experimental therapeutics in lymphoid leukemia have thus far focused on reducing pathologically upregulated Notch signaling, primarily by inhibition of γ secretase. The discovery that Notch pathway members might act as tumor suppressors rather than oncogenes suggests that augmenting Notch signaling might also have therapeutic benefit; it also indicates that effects on non-lymphoid leukocyte subsets should be monitored during longer-term Notch pathway inhibition in lymphoid leukemias.
