Notch activation is pervasive in SMZL and uncommon in DLBCL: implications for Notch signaling in B-cell tumors

Notch receptors participate in a signaling pathway in which ligand-induced proteolysis frees the Notch intracellular domain (NICD), allowing it to translocate to the nucleus, form a transcription complex, and induce target gene expression. Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), splenic marginal zone B-cell lymphoma (SMZL), and distinct subsets of diffuse large B-cell lymphoma (DLBCL) are strongly associated with mutations in the 3′ end of NOTCH1 or NOTCH2 that disrupt a proline, glutamic acid, serine, and threonine (PEST) degron domain and stabilize NICD1 and NICD2. By contrast, mutations leading to constitutive Notch activation are rare in primary B-cell neoplasms, suggesting that Notch activation is confined to ligand-rich tumor microenvironments, or that cryptic strong gain-of-function mutations have been missed in prior analyses. To test these ideas, we used immunohistochemical stains to screen a broad range of B-cell tumors for Notch activation. Our analyses reveal that among small B-cell neoplasms, NICD2 is primarily detected in SMZL and is a common feature of both NOTCH2 wild-type and NOTCH2-mutated SMZLs, similar to prior findings with NOTCH1 in CLL/SLL. The greatest NOTCH2 activation was observed in NOTCH2-mutated SMZLs, particularly within splenic marginal zones. By contrast, little evidence of NOTCH2 activation was observed in DLBCL, even in NOTCH2-mutated tumors, suggesting that selective pressure for NOTCH2 activation is mainly confined to low-grade B-cell neoplasms, whereas DLBCLs with NOTCH1 mutations frequently showed evidence of ongoing NOTCH1 activation. These observations have important implications for the pathogenic role of Notch and its therapeutic targeting in B-cell lymphomas.