Attenuation of Apoptosis-Stimulating Protein of p53 2 (ASPP2 or TP53BP2) Associates with High Risk Disease and Therapy Failure in Acute Leukemia

Inactivation of the p53 pathway is a universal event in human cancers and promotes tumorigenesis and resistance to chemotherapy by impaired induction of apoptosis. Unlike in solid tumors, inactivating p53 mutations are uncommonly found in de novo acute leukemias, and the p53-pathway thus must be inactivated by other mechanisms instead. ASPP2 is a damage-inducible p53-binding protein that enhances apoptosis at least in part through a p53-mediated pathway. Moreover, ASPP2 is a haploinsufficient tumor suppressor (Kampa et al. PNAS 2009) that is suppressed in many human cancers, correlating with poor clinical outcomes. Using a global gene-chip approach evaluating attenuated ASPP2 expression in mouse embryonic fibroblasts, we found evidence that loss-of ASPP2 alters pathways involved in hematologic tumorigenesis. We now show that ASPP2 is attenuated in high-risk leukemias, which may have implications for further therapy decisions.

Quantitative RT-PCR was performed to analyze ASPP2 mRNA levels in leukemia patients and healthy volunteers. Protein expression levels were evaluated in treatment naïve as well as anthracycline-treated patient samples using flow cytometry and immunoblotting. Small-interference (si)RNA experiments were employed to reveal involvement of ASPP2 in therapy response and tumorigenesis in functional viability assays using primary patient samples.

Analysis of ASPP2 mRNA levels of 50 patients diagnosed with acute leukemia revealed wide expression variance ranging from 0.03 to 97–fold (normalized) compared to GAPDH as house keeping gene control. Notably, patients considered as prognostic good risk (n=24) showed a trend to higher levels (mean 8.5, range 0.8–97.1) as compared to the high risk cohort (n=26, mean 6.1, range 0.03–31.5). Even more, within the high risk cohort 31% revealed expression levels below 1 (27% below 0.8) – compared to 13% in the good risk population (0% below 0.8). Subanalysis of the high risk cohort revealed that none of the patients achieving early complete remission after initial induction chemotherapy belonged to those patients presenting with attenuated ASPP2 expression levels. In accordance, intracellular ASPP2 protein expression levels were higher in the good risk patient cohort. Additionally, we found that low ASPP2 protein expression levels in ex vivo cultured patient-derived blasts with high-risk features did not increase after chemotherapy-induced damage, while upregulation of ASPP2 levels – indicating apoptosis induction – was observed in blasts from good-risk patients. Failure to upregulate ASPP2 upon chemotherapy may thus contribute to therapy resistance and indeed, ASPP2 attenuation in in vitro and ex vivo leukemia models using siRNA lipofection resulted in increased cellular proliferation and resistance towards anthracyclines and tyrosin kinase inhibition.

Our results suggest that attenuated ASPP2 expression levels contribute to leukemogenesis of high-risk leukemia and therapy failure. Thus, ASPP2 expression should be further explored as a potential prognostic and/or predictive biomarker in AML.

No relevant conflicts of interest to declare.