ASPP2 Attenuation Is an Early Step in Leukemogenesis Facilitating Acquisition of Mutations

Inactivation of the p53 pathway is an universal event in human cancers. As p53 mutations are rare in de novo acute leukemias, the p53-pathway must be inactivated by other mechanisms instead. The Apoptosis stimulating Protein of p53-2 (ASPP2) is a haploinsufficient tumor suppressor, which we have shown to be attenuated in acute leukemia. We now provide evidence that ASPP2 regulates pathways involved in early hematopoiesis, and attenuated expression perturbs malignant transformation.

Gene-chip microarrays were performed on ASPP2+/+ versus +/- mouse embryonic fibroblasts (MEFs) (-/- knockouts are not viable) and pathway analysis was performed using Affimetrix software. ASPP2 expression was silenced in IL3-dependent Ba/F3 pro-B cells and leukemia cell lines using standard siRNA protocols. Cellular proliferation was assessed using XTT-based assays. To evaluate for malignant transformation, long term cell cultures with stably retrovirally silenced Ba/F3 cells (+/- stress inforcement using gamma irradiation, 4x 5Gy) were employed. IL3 weaning was performed as an indicator of autoactivated cellular proliferation, and analysis of chromosomal defects or gene mutations were performed.

Microarray mRNA analysis revealed that attenuated ASPP2 expression resulted in significant alterations of pathways involved in mediating cellular growth, proliferation and tissue differentiation. Subanalysis proofed involvement of genes playing a major role in embryogenesis, the development of the hematopoietic system and leukemogenesis (homeobox (HOX) family members, NOTCH, BCL, IRF7 and EGR1). Population doubling times were significantly increased in ASPP2+/- MEFs compared to the parental cells. Modeling ASPP2 attenuation via specific siRNA knockdown in leukemia cell lines (K562 and Kasumi1) resulted in significantly increased cellular proliferation. Similarly, ASPP2-interference in pro-B Ba/F3 cells lead to perturbed proliferation and an increase of polyploid cells with a dramatic change in morphology with megaloid cells, indicating mitotic failure. This effect was even more pronounced after stress induction with daunorubicin. Lentiviral shRNA-ASPP2 stable knock-out was performed in Ba/F3 cells to follow cells in long term cultures for malignant transformation. Again, we confirmed signs of mitotic failure as observed in the siRNA approach. Even more, gamma irradiated ASPP2-lacking strains were successfully IL3 weaned, which indicates autoactivated cellular proliferation. A screen for acquired oncogenic mutations was performed and analyses are ongoing.

In summary, we provide evidence that ASPP2 has a pivotal role in early hematopoiesis, and loss of ASPP2 is a driver mechanism to fuel leukemogenesis.