The Role of SEPT9_V1 in Breast Cancer: Insights into the Development of Pro-Oncogenic Phenotypes and Genomic Instability.

Abstract

Breast cancer is a heterogeneous disease stemming from alterations in multiple genes. SEPT9, a gene important in cell division, encodes multiple SEPT9 splicing isoforms, SEPT9_v1-SEPT9_v7. SEPT9 maps to a region of allelic imbalance in breast cancers and SEPT9 isoforms are differentially expressed in breast cancer. One isoform, SEPT9_v1, is over-expressed in >50% of breast cancers, suggesting an important role in tumorigenesis. To examine the role of SEPT9_v1 in mammary tumorigenesis, we established immortalized human mammary epithelial cells (IHMECs) models and analyzed SEPT9_v1 expression in a human breast cancer tumor microarray. High SEPT9_v1 was observed with >50% of hormone receptor positive tumors. Retroviral expression of SEPT9_v1 in IHEMCs resulted in the development of pro-oncogenic phenotypes such as increased cell proliferation, decreased apoptosis, enhanced motility and invasiveness, epithelial to mesenchymal transition, and increased aneuploidy. Ablation of SEPT9_v1 in breast cancer cells reduced proliferation and decreased motility and invasiveness.

To explore SEPT9_v1 in genomic stability, IHMEC models expressing SEPT9_v1 were analyzed for mitotic spindle checkpoint function, chromosome segregation and cytokinesis. SEPT9_v1 promoted both mitotic spindle defects and cytokinesis failure. SEPT9_v1 interacted with two components of the mitotic spindle, alpha-tubulin and gamma-tubulin, promoting alpha-tubulin filament formation defects and centrosome amplification. Transient expression of SEPT9_v1 drove chromosome instability independent of increased cell proliferation. SEPT9_v1 could increase cellular proliferation rates by interacting with the c-jun-N-terminal kinase (JNK), stabilizing JNK, and increasing the transcriptional activation of target genes important in cell cycle progression. Finally, in an effort to identify novel interacting partners of SEPT9 isoforms, we identified, SEPT14, by yeast two-hybrid analysis. SEPT14 is localized to stress fibers, is not expressed in cancer cell lines and has expression limited to testicular tissues. Together, these findings indicate that SEPT9_v1 has oncogenic potential in mammary cells and may be a biomarker for disease aggressiveness. SEPT9_v1 impacts genomic instability by affecting mitotic spindle function, chromosome segregation and cytokinesis. Its interaction with the JNK signaling pathway is relevant to understanding its role in cell cycle regulation. This study provides novel insights into the mechanisms by which SEPT9_v1 expression may lead to the disruption of cellular processes important in breast cancer development.