Abstract 4293: Notch expression expands the pre-malignant pool of T-cell acute lymphoblastic leukemia clones without affecting self-renewal

Abstract

Abstract Over 60% of human T-cell acute lymphoblastic leukemias (T-ALL) harbor NOTCH1 activating mutations, and the cMYC pathway is also commonly activated in this disease. Notch and cMyc have been shown to collaborate in mouse transgenic models of T-ALL, suggesting that these oncogenes function in parallel pathways to enhance tumor growth; however, the exact role of each in T-ALL is unknown. Here, we show that co-expression of Notch and cMyc in zebrafish T cells significantly enhances T-ALL progression compared to cMyc or Notch alone (p<0.001). However, Notch co-expression with cMyc does not enhance proliferation, alter cell cycle kinetics, or modify apoptosis in leukemic cells when compared to T-ALLs expressing cMyc alone. Clonality assays using RT-PCR analyses for T-cell receptor-beta rearrangements indicate that Notch increases the number of T-ALL clones contained within the primary tumor when compared to cMyc expressing leukemais. A large portion of T-ALL clones present in primary Notch and Notch/cMyc leukemias cannot transfer disease into transplanted animals, while all cMyc-alone expressing T-ALLs clones are capable of engraftment and reinitiation of leukemia. Large scale limiting dilution cell transplantation analyses using syngeneic zebrafish demonstrated that primary T-ALLs expressing either Notch or Notch/cMyc have 10-fold less leukemia-initiating frequency when compared to T-ALLs that express only cMyc; however, following serial passaging, both the Notch and Notch/cMyc leukemias exhibit similar leukemia-initiating frequency as cMyc-induced T-ALLs, indicating that Notch signaling does not enhance self-renewal of T-ALL initiating cells. Taken together, our data supports a model where Notch expands a pool of pre-malignant T-ALL clones within the primary tumor, a subset of which acquire additional mutations to confer a fully transformed phenotype. By contrast, cMyc alone is insufficient to increase the overall pool of pre-malignant clones but confers a fully-transformed phenotype to leukemic cells, accounting for the longer latency that likely reflects the acquisition of additional genetic changes in clones. Our data may explain why a subset of relapsed human T-ALLs develop from an underrepresented clone found in the primary leukemia, in that primary human T-ALLs likely have a large pool of pre-malignant clones resulting from NOTCH-pathway activation that are unable to self-renew and thus, cannot give rise to relapsed T-ALL. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4293. doi:10.1158/1538-7445.AM2011-4293