ASPP2 Haploinsufficiency Promotes Tumor Formation in a Mouse Model.

ASPP2 interacts with the tumor suppressor protein p53 and promotes damage-induced apoptosis in part through stimulation of p53-mediated apoptosis. We have previously demonstrated that low ASPP2 levels correlate with poor clinical outcome in patients with diffuse large B-cell lymphoma treated with anthracycline-based chemotherapy. Moreover, reduced ASPP2 expression has been demonstrated in other tumor types. These findings led us to hypothesize that ASPP2 may function as a tumor suppressor. To further explore this, we targeted the ASPP2 allele in a mouse by homologous recombination using a knockout vector that replaced exons 10–17 with a neoR gene. Two separate ES clones were used for blastocyst injections to generate several chimeras that were used to generate ASPP2 heterozygous mice. ASPP2+/− mice appear developmentally normal and reproduce. However ASPP2−/− mice could not be generated. Genotype analysis as early as Ed 6.5 did not detect ASPP2−/− embryos---which implies an early embryonic lethal defect in the homozygote. ASPP2+/− (n=135) and ASPP2+/+ (n=63) sibling mice were observed for spontaneous tumor formation. Overall median tumor-free survival was 117 weeks in the ASPP2+/− mice verses 125 weeks in the ASPP2+/+mice (p = 0.035 log-rank test). Overall tumor incidence (at 115 weeks) for ASPP2+/− and ASPP2+/+ mice was 43% and 22%, respectively. The incidence of tumor types, from all tumors detected, was similar between ASPP2+/− and ASPP2+/+ mice: 34% versus 33% (lymphoma), 18% versus 14% (sarcoma), and 47% versus 52% (carcinoma), respectively. Compound p53+/−;ASPP2+/− mice did not exhibit accelerated tumor formation relative to p53+/−;ASPP2+/+ mice. Additionally, a tet-Myc:ASPP2+/− lymphoma mouse model did not exhibit accelerated lymphomagenesis. However, preliminary data suggests that ASPP2+/− mice may have an increased incidence of irradiation-induced leukemia/lymphoma when compared to ASPP2+/+ mice, and confirmatory studies are ongoing. In response to ionizing radiation, doxorubicin, or serum-starvation, preliminary analysis reveals a G0/G1 checkpoint defect in ASPP2+/− MEFs compared to ASPP2+/+ MEFs. Our results provide in vivo evidence that ASPP2 can function as a tumor suppressor. Further studies are underway to determine the mechanism of this observation.