Targeting the Leukemia Initiating Cells in a Mouse T-ALL Model.

Abstract 3474

Poster Board III-411

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy that is largely caused by aberrant activation of the TAL1/SCL, LMO1/2 and NOTCH1 oncogenic pathways. Although most patients respond to cytotoxic therapy, 20-30% relapse and currently we cannot distinguish children likely to relapse from those who will respond to therapy. Evidence is emerging that some malignancies are driven by a rare initiating population that is capable of extensive self-renewal and is resistant to conventional chemotherapy. Although the leukemia-initiating cells (L-ICs) have been well documented in AML and CML, it is unclear whether T-ALL is driven by a rare L-IC and whether relapse reflects an inability to eliminate L-ICs. To determine whether T-ALL is driven by a rare L-IC, we utilized a murine model of T-ALL in which expression of the Tal1 and Lmo2 oncogenes arrests thymocyte development via E47/HEB inhibition and 75% of tumors develop spontaneous gain of function mutations in Notch1. We have shown that treatment with γ-secretase inhibitors (GSIs) to inhibit Notch1 activity induces apoptosis of mouse T-ALL cells in vitro and when administered in vivo extends the survival of leukemic mice. We demonstrate that clonal mouse T-ALL tumors are phenotypically heterogeneous, containing immature CD4- and CD8-negative, double negative (DN) 3 and 4 thymic progenitors and differentiated double positive (DP) and/or single positive (SP) leukemic blasts. Importantly, the DN3 or DN4 progenitors are maintained upon serial transplantation of the tumor into syngeneic recipient mice. Injection of serial dilutions of murine T-ALL cells reveal that the tumors are also functionally heterogeneous; with 1/5000-1/50,000 tumor cells exhibiting leukemia initiating activity. We found the CD44-, CD25+ DN3 progenitors enriched in disease potential, whereas mice injected with DP leukemic blasts failed to develop leukemia. Consistently, our preleukemic studies reveal a 3.5-fold increase in the percentage of Notch1 active, DN3/4 thymic progenitors, raising the possibility that Notch1 drives L-IC growth. Collectively, these studies suggest that activation of the Notch1-c-Myc pathway may confer self-renewal capabilities on committed thymic progenitors. The effects of Notch inhibitors on mouse L-IC survival and activity will be discussed.