Abstract 260: Elucidating the roles of the alternatively spliced transcripts of Septin 9

Abstract

Abstract The septins are a family of 14 genes, the products of which are thought to function primarily as scaffolds which recruit other proteins to particular cellular locations. This gene family is highly conserved and encodes proteins involved in a variety of cellular functions including cytokinesis, apoptosis and vesicle trafficking. Septin colocalisation with actin and tubulin has also been reported. Previous work from this laboratory mapped the SEPT9 gene to a region of allelic imbalance on chromosome 17q in sporadic epithelial ovarian tumours. SEPT9 undergoes extensive alternate splicing at both the 5’ (v1, v2 v3, v4, v4* and v5) and 3’ (a, b and c) ends, with SEPT9_v4 and _v4* differing only in their 5’UTRs, therefore encoding the same polypeptide. We have shown perturbed expression of SEPT9 transcripts in neoplasia, specifically up-regulation of SEPT9_v1 and _v4* transcripts. In a range of normal tissues we noted three alternatively spliced 3’ transcripts (a, b & c) were expressed. However, in ovarian tumour development expression of 2 of these (b & c) is decreased. This suggests that expression of the various SEPT9 transcripts is independently regulated. We therefore sought to gain some insight into control of expression of these various splice variants and why specific patterns of deregulation might contribute to neoplasia. With respect to the cell cycle, we observed by immunofluoresence that SEPT9 protein is visible around the cell membrane during anaphase and into telophase. During cytokinesis the SEPT9 protein localises to the cleavage furrow and is part of the ‘ring’ structure of dividing cells. We also used a double thymidine block to synchronise HeLa cells but there was no alteration in levels of transcripts with specific 5’ or 3’ ends in these cells. Next we looked at expression of the SEPT9 5’ and 3’ ends in response to cellular stress, in particular DNA damage following doxorubicin treatment. We observe an up-regulation of the 5’ v4* transcript and 3’ c variant after doxorubicin treatment in a p53 wild type background but not in p53 null cells. We then used a model of ovarian surface epithelial cells immortalised with telomerase and a temperature sensitive SV40 large T antigen which proliferate at 330C but senesce when shifted to the non-permissive temperature (370C). We see that SEPT9_v2 and v4 mRNA levels increase significantly as cells undergo senescence. Finally, we note that the b and c variants share the same short 3’UTR (57 bases) while the a variant has a long 3’UTR (1.938kb). Bioinformatics of both 3’UTRs identified several putative microRNA binding sites present in the 3’UTR of the a variant but only one in that of b and c. We show that the 3’UTR of a drives a reporter gene when cloned upstream of luciferase and in a sequence dependant manner. In summary, our data supports the hypothesis that the alternatively spliced transcripts of SEPT9 play individual roles within the cell and that the 3’UTRs appear to be important in the regulation of SEPT9. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 260.