Investigating the role of Septin 9 in breast cancer: A complex analysis of this novel oncogene and its isoform variants

Abstract

Septins are a family of GTPases known for their involvement in various cellular processes and interactions with the cytoskeleton. Septin 9 ( SEPT9) is of particular interest because of its status in breast tumorigenesis. Our group previously identified SEPT9 as a novel oncogene amplified and overexpressed in breast cancer. Analysis of SEPT9 is complex due to the presence of 18 putative isoform transcripts. Studies show that SEPT9 isoforms are differentially expressed between normal and tumor breast cells, but their relation to stages of breast cancer development and subtype is still unclear. We approached this by analyzing isoform expression at the genomic, proteomic and epigenetic levels. Using mouse models for breast cancer progression, cell lines, and human tumor and matching adjacent normal breast tissue along with normal breast from reduction mammoplasty, we have detected gene amplification and expression changes of SEPT9 isoforms among the different tissue types. We show that pre-malignant cells have a SEPT9 expression profile that distinguishes them from both normal breast and advanced adenocarcinomas. Moreover, we found that the differential expression of at least one isoform (SEPT9_v3 ) is due to epigenetic regulation via the methylation status of an alternative promoter region. Our data suggest that additional alternative promoters within this gene could also be regulated by DNA methylation and thus dramatically affect the expression of SEPT9 isoforms in tumorigenesis. We have generated stable MCF-7 clones, each expressing one of five isoforms of interest. Analysis of these clones has revealed phenotypic differences within morphology, cellular localization and migration, supporting that the SEPT9 isoforms maintain specific roles in cancer.