Apoptosis Stimulating Protein of p53 (ASPP2) Heterozygous Mice Are Lymphoma-Prone and Display Attenuated Apoptosis and Cell Cycle Checkpoints

Introduction: p53 is an important tumor suppressor, and loss of p53 pathway function is a common event in human cancer. ASPP2 is a damage-inducible p53 binding protein that enhances apoptosis, at least in part, by selective stimulation of p53 pro-apoptotic target genes. Low ASPP2 expression occurs in many human tumors and as we have previously demonstrated, correlates with poor clinical outcome in patients with B-cell lymphomas. However, the mechanisms by which ASPP2 suppresses tumor formation remain unknown.

Methods: To rigorously study ASPP2 in vivo function, we targeted the ASPP2 allele in a mouse model by homologous recombination by replacing exons 10–17 with a neoR gene. Aging mice were followed for spontaneous tumor formation. Additionally, six week old mice were irradiated at different doses and followed for tumor development. To explore the functional consequences of low ASPP2 expression, ASPP2+/− and ASPP2+/+ thymocytes were subjected ex vivo to 5Gy ionizing irradiation and apoptosis was assessed by Annexin-V/PI staining and flow cytometry. We also irradiated ASPP2+/+ and ASPP2+/−MEFs (mouse embryonic fibroblasts) with 5Gy and performed cell cycle analysis. Furthermore, we conducted global gene expression profiling between the unirradiated and irradiated ASPP2+/+ and ASPP2+/−MEFs using an Affymetrix GeneChip® Mouse Gene 1.0 ST Array. Moreover, phosphoproteomic analysis was performed on unirradiated and irradiated ASPP2+/+ and ASPP2+/−MEFs using 2-dimensional gel electrophoresis, fluorescent phosphoprotein dye Pro-Q Diamond staining, and liquid chromatography tandem mass spectroscopy.

Results: We were unable to generate ASPP2−/− mice due to an early embryonic lethal defect. However, ASPP2+/− mice appear developmentally normal and reproduce. We observed an increased formation of spontaneous tumors in aging ASPP2+/− mice compared to ASPP2 +/+ mice (43% vs. 22%, at 115 weeks, p=0.011). Additionally we were able to show that after ionizing radiation, ASPP2+/− mice develop high-grade lymphomas in a dose-dependent manner at a significantly higher incidence at 50 weeks compared to ASPP2+/+ mice (p = 0.024 and p = 0.045, 6 Gy and 10.5 Gy respectively). Immunophenotyping demonstrated that these were high-grade T-cell lymphomas of thymic origin. Since ASPP2 is damage-inducible and promotes apoptosis, we wished to determine the extent to which reduction in ASPP2 expression attenuated the cellular damage response. We therefore gamma-irradiated ex vivo ASPP2+/+ and ASPP2+/−thymocytes in short-term culture and found a two-fold reduction in apoptosis in ASPP2+/− thymocytes compared to ASPP2+/+ thymocytes. Additionally, after 5 Gy gamma-irradiation, ASPP2+/− MEFs exhibited an attenuated G0/G1 checkpoint compared to ASPP2+/+ MEFs. Preliminary analysis of global gene expression in ASPP2+/+ and ASPP2+/− MEFs shows specific differences in gene expression patterns after damage. Likewise, preliminary analysis of phosphoproteomics between ASPP2+/+ and ASPP2+/− MEFs after damage, demonstrate differences in the phosphophorylation status of 170 proteins.

Conclusions: ASPP2 is a haploinsufficient tumor suppressor, and reduction in ASPP2 expression attenuates both cell cycle checkpoints and apoptosis-induction after damage. These results suggest that reduction of ASPP2 levels modulate the cellular damage response, possibly at transcriptional as well as post-transcriptional levels, and provide a functional explanation for the increased tumor incidence in ASPP2+/− mice---since attenuated damage-response thresholds would lead to an impaired ability to eliminate cells that have accumulated tumorigenic mutations. Our study provides important insights into the p53-ASPP2 axis, and opens new avenues for investigation into its role in tumorigenesis and response to therapy.