ASPP2 Expression In Acute Leukemia Modulates Response to Therapy

Abstract 3143

Acute myeloid leukemias (AML) remain difficult to treat and therapy outcome is far from satisfactory for most disease subgroups. Inactivation of the p53 tumor suppressor pathway by mutation is a frequent event in many cancers that promotes tumorigenesis and resistance to chemotherapy. Although p53 mutations are rare in AML it is not well studied whether the p53 pathway is influenced by other mechanisms instead. ASPP2 (Apoptosis Stimulating Protein of p53 2) is a highly regulated member of a family of p53-binding proteins that enhance apoptosis at least in part through stimulation of p53-transactivation of selected pro-apoptotic target genes. We previously demonstrated in a mouse model that ASPP2 is a haploinsufficient tumor suppressor (Kampa et al., PNAS 2009), and low ASPP2 expression levels have been associated with aggressive courses of different tumors such as breast cancer and lymphoma. We have now studied how ASPP2 expression correlates with response to therapy using in vitro models as well as patient-derived acute leukemia cells collected pre- and post- chemotherapy. We first analyzed changes in ASPP2 protein expression after treatment with chemotherapy or small molecule tyrosine-kinase inhbitors. We found that ASPP2-induction was cell-type specific in various established leukemia lines including Jurkat and HL60 (ASPP2 levels induced) and MOLM14 cells (ASPP2 levels unchanged). To test if ASPP2 levels modulated growth or response to therapy of leukemia cells, we generated three different ASPP2-siRNA constructs and transiently introduced them by lipofection into various lymphoid and myelogenous leukemia cell lines, including K562, Kasumi1, HL60, MOLM14 and Jurkat. After ASPP2 silencing, we observed, with the exception of MOLM14 cells, an up to 3-fold increase in cell proliferation measured by an XTT-assay compared to an empty vector control. We also treated siRNA-silenced K562, Kasumi1, HL60 and Jurkat cell lines with daunorubicin or small molecules targeting cell line-specific mutations in FLT3, KIT or ABL. Cell lines with attenuated ASPP2 expression displayed a significantly (∼50%) lower rate of apoptosis-induction in AnnexinV-assays after chemotherapy as well as small molecule inhibitor treatment as compared to a negative control. Interestingly, treated and siRNA-silenced leukemic cells frequently demonstrated enlarged morphology consistent with mitotic catastrophe. To study ASPP2 expression in humans, we quantified ASPP2 levels by qRT-PCR and intracellular immunophenotyping in circulating leukemic cells derived from patients at several timepoints before and during induction-chemotherapy (n=63). We found that pre-treatment ASPP2 basal levels were variable in acute leukemias. Additionally, we found that induction-chemotherapy increased ASPP2 expression in leukemic cells in a subgroup of patients. Univariate and multivariate analysis of the correlation of available clinical data and patient outcomes with ASPP2 expression in these patient datasets is ongoing. Taken together, our results demonstrate that dysfunctional regulation of ASPP2 expression may contribute to the biology of leukemogenesis and to primary therapy resistance in a subgroup of patients with acute leukemia. This data provides important and clinically relevant insight into how the p53 pathway can be inactivated in acute leukemia and opens new avenues for investigation. Prospective clinical studies are warranted in order to further define the role of the ASPP2 pathway as a therapeutic target and as biomarker for response to therapy.