Abstract 3804: The pathobiology of perturbed SEPT9 expression in ovarian neoplasia

Abstract

Abstract We have made observations of the deregulated expression of certain SEPT9 transcripts and we wish to place these in a pathobiological context. Septins are an evolutionarily conserved family of GTP binding proteins which have been implicated in a wide range of cellular processes. SEPT9 was initially identified through mapping of a region of chromosome 17q which demonstrated allelic imbalance in sporadic ovarian tumours. The large SEPT9 locus is complex since up to 18 splice variants are possible through alternate splicing of six 5’ ends and three 3’ ends. Of note are two transcripts SEPT9_v4 and v4* differing only in their 5’UTRs but encoding the same polypeptide SEPT9 isoform 4 (SEPT9_i4). Ovarian neoplasia is accompanied by down regulation of SEPT9_v4 and up regulation of SEPT9_v4*. We have also shown that translation of the former is by cap-dependant mechanisms while translation of the later is cap-independant and stress responsive (Hum Mol Gen 2007;6:742-52). We therefore sought to understand the mechanism of control of transcription of these two transcripts and how this is perturbed in neoplasia. Using bioinformatic analysis we have identified multiple putative p53 responsive elements upstream of the transcription start site of SEPT9_v4*. Such sites are rare upstream of all other SEPT9 transcripts. In wild type p53 cells but not an isogenic p53 null cell line, the SEPT9_v4* transcript and isoform 4 are induced under conditions in which p53 is induced. No other SEPT9 transcript responded in this way. We used chromatin immunoprecipitation assays to demonstrate the validity of at least some of these putative p53 sites. Using a luciferase assay we show that these same sites bind and respond to wild type but not mutant p53. Previous studies from our lab have shown that over expression of isoform 4 perturbs microtubule dynamics in cells such that depolymerisation of microtubules (in response to cold) and their subsequent repolymerisation is delayed. We therefore sought to determine if the p53 dependant upregulation of SEPT9_v4* might also be associated with altered microtubule dynamics and resistance to microtubule-interacting drugs. In a tissue culture model we observation that induction of p53 and SEPT9_v4* using a DNA damaging agent reduces the cellular response to paclitaxel. Further, the transfection of SEPT9_v4* specific siRNA (with SEPT9_i4 knock down) leads to dramatically enhanced cell kill after taxol treatment compared with scrambled siRNAs. The clinical relevance of this is emphasised by the observation that SEPT9_v4* expression is associated with disease stage in ovarian cancer. Our data provide novel insight into septin function and the role of stress pathways including p53 in its regulation and has clinical significance as a determinant of drug sensitivity. 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 3804. doi:10.1158/1538-7445.AM2011-3804