Abstract 2189: ΔN-ASPP2 is a novel isoform of the ASPP2 tumor suppressor and a regulator of ASPP2 function

Abstract

Abstract ASPP2 (Ankyrin-repeats, SH3 domain, Poly-proline domain-2) is a pro-apoptotic member of a family of p53 binding proteins that share homology in the C-terminus. ASPP2 expression is frequently suppressed in human cancers, and mouse models targeting the ASPP2 allele have shown that ASPP2 is a bonafide tumor suppressor. Although ASPP2 is a damage-inducible protein that stimulates apoptosis at least in part through a p53-mediated mechanism, ASPP2 also modulates p53-independent pathways to inhibit the tumorigenic phenotype. However, the complex mechanisms that control ASPP2 regulation and function remain unclear. Using 5’RACE and RT-PCR, we have discovered a new ASPP2 isoform (that we have named ΔN-ASPP2) that is generated from an internal transcription start site in mouse, rat, and human libraries. ΔN-ASPP2 is a predicted 880 amino acid protein that is missing the highly structured N-terminus important for apoptotic function. Western blotting with epitope specific antibodies, and immunoprecipitation followed by LC-MC, confirmed a 95 kD ASPP2 protein. Moreover as predicted, MEFs derived from our ASPP2+/- mouse targeted at exons 10-17, demonstrated decreased levels of ΔN-ASPP2 mRNA and 95 kD ASPP2 immunoreactive protein. Since the N-terminus of ASPP2 is important for pro-apoptotic and tumor suppressor function, we hypothesized that N-terminal truncated ΔN-ASPP2 could inhibit apoptosis and promote tumorigenesis. Indeed, overexpression of ΔN-ASPP2 inhibited UV-induced apoptosis, as well as promoted RasV12-induced 3T3 colony formation in cell culture. Interestingly, we found complex binding dynamics between ΔN-ASPP2, p53, and full-length ASPP2, suggesting that ΔN-ASPP2 could function as a dominant-negative. Importantly, and consistent with a role in promoting tumorigenesis, we found that ΔN-ASPP2 was highly overexpressed in a panel of 20 human breast tumor samples compared to matched adjacent normal tissue. In stark contrast, full-length ASPP2 expression was suppressed in these breast tumors. Together these data suggest that the novel ΔN-ASPP2 isoform may play a role in antagonizing ASPP2 and/or p53 function, and promoting resistance to therapy. These findings open new areas for investigation into a potential new target for therapy in human cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2189. doi:10.1158/1538-7445.AM2011-2189