PO-068 Determining how distinct microenvironments alter BCL-2 family dependence in T-cell acute lymphoblastic leukaemia

Abstract

IntroductionT-cell acute lymphoblastic leukaemia (T-ALL) is an aggressive hematologic malignancy resulting from the transformation of T-cell progenitors. Early T-cell progenitor ALL (ETP-ALL) subgroup accounts for 5% to 10% of T-ALL cases and associated with a poor prognosis and a very high risk for relapse. T-ALL leukemias develop a dependence on anti-apoptotic BCL-2 family proteins that is linked to the maturation stage of the blasts. Typical T-ALL is mostly dependent on BCL-XL, whereas ETP-ALL is more dependent on BCL-2. In recent years, there has been the development of a series of specific BH3 mimetics (ABT-199, ABT-263, WEHI 539) that can inhibit the function of anti-apoptotic BCL-2 proteins and induce apoptosis. T-ALL patients have blasts disseminated in the blood, bone marrow and spleen. Each niche may provide distinct extrinsic signals that could offer a survival advantage to leukemic cells.Material and methodsWe aim to assess in both ETP-ALL (Loucy cell line) and typical T-ALL (CEM-CCRF cell line) if distinct microenvironments affect the BCL-2 family proteins expression and the sensitivity to BH3 mimetics. To study the anti-apoptotic BCL-2 dependencies we will use the cutting-edge technology mitochondrial BH3 profiling. In brief, BH3 profiling is a functional assay that measures the response of mitochondria to exposure of known concentrations of synthetic BH3 peptides by measuring loss of mitochondrial membrane potential or cytochrome c release.Results and discussionsWe found that ETP-ALL cell line was BCL-2 dependent and sensitive to ABT-199 (BCL-2 specific BH3 mimetic). While the Typical T-ALL cell line was BCL-XL dependent and sensitive to both ABT-263 (BCL-2,BCL-XL and BCL-w) and WEHI 539 (BCL-XL) BH3 mimetics. Interestingly, when the ETP–ALL cell line Loucy was co-cultured with the human splenic fibroblast (HSF) cell line we found a reduction in BCL-2 dependency, as assessed by BH3 profiling, and a reduced sensitivity to ABT-199 (BCL-2 specific BH3 mimetic). Remarkably, we found the inverse in the typical T-ALL cell line co-cultured with the HSF. We found an increase in BCL-2 dependence and an increase in sensitivity to ABT-199.ConclusionCurrently, we are trying to identify the signalling pathways which are activated to cause this switch in BCL-2 family dependence in the different subtypes of T-ALL. In addition, we are assessing the BCL-2 family dependence of a xenograft cell line isolated from the blood and spleen of animals to determine if this switiching of BCL-2 dependence occurs in vivo.