SEPT9_i1 and genomic instability: Mechanistic insights and relevance to tumorigenesis

Abstract

Septins are highly conserved cytoskeletal GTP-binding proteins implicated in numerous cellular processes from apoptosis to vesicle trafficking. Septins have been associated with leukemia and solid tumor malignancies, including breast, ovarian, and prostate. We previously reported that high SEPT9_i1 expression in human mammary epithelial cell lines (HMECs) led to malignant cellular phenotypes such as increased cell proliferation, invasiveness, motility, and genomic instability. Our goal here was to better understand how SEPT9_i1 expression might contribute to genomic instability and malignant progression. First, we confirmed that even transient expression of SEPT9_i1 was sufficient to increase aneuploidy in HMECs. We then analyzed SEPT9_i1 by immunoprecipitation and immunofluorescence studies and found that SEPT9_i1 interacts with both α and γ tubulin. SEPT9_i1 expressing cells demonstrated dramatic chromosome segregation defects, centrosome amplification and cytokinesis defects, suggesting two possible molecular mechanisms contributing to the development of genomic instability. This suggests that SEPT9_i1 may promote genomic instability through both cytokinesis and mitotic spindle defects which lead to chromosome missegregation.

Septin 9 amplification and isoform-specific expression in peritumoral and tumor breast tissue

“…The lack of mutations detected in breast tumorigenesis suggests that genomic instability that causes SEPT9 mutations are selected against during tumorigenesis, whereas genomic amplifications are selected for. Our findings show that high-grade tumors associated with poor clinical outcome have high levels of SEPT9 amplification, which may be associated with the increased genomic instability reported in SEPT9_v1-overexpressing cells (Peterson et al, 2011). Based on the aforementioned information, we could reason that SEPT9 amplification, an event that occurs more frequently in high-grade tumors, promotes genomic instability and ultimately results in worse clinical outcome.…”

“…Evidence for SEPT9 isoform-specific function is emerging (Chacko et al, 2005(Chacko et al, , 2012Gonzalez et al, 2009;Peterson et al, 2011;Golan and Mabjeesh, 2013), but despite published reports that have clearly demonstrated the altered expression of SEPT9 isoforms during tumorigenesis (Scott et al, 2005;Gonzalez et al, 2007;McDade et al, 2007;Stanbery et al, 2010;Connolly et al, 2011b), a highly sensitive analysis that quantifies and compares multiple isoform variant mRNA expression levels in tumor-free breast epithelium and matching tumor tissue is lacking. Therefore, in the present study, we propose that SEPT9 amplification occurs more frequently in highgrade, or poorly differentiated, breast tumors that morphologically lack or exhibit altered features of normal breast tissue such as tubule formation, nuclear grade, and mitotic rate.…”

“…Yet, a definitive analysis of SEPT9 DNA copy number relative to breast cancer tumor subtypes is lacking. At the transcriptional level, SEPT9 is overexpressed in breast, ovarian, and head and neck cancer (Burrows et al, 2003;Stanbery et al, 2010;Peterson et al, 2011); SEPT9 DNA methylation changes are currently utilized as a biomarker in colorectal carcinogenesis (Grutzmann et al, 2008); and MLL-SEPT9 fusion transcripts frequently occur in acute myeloid leukemia (Cerveira et al, 2011). Some SEPT9 isoforms are known to be altered in tumorigenesis: SEPT9_v1 is upregulated in breast and head and neck cancer (Stanbery et al, 2010;Connolly et al, 2011b); SEPT9_v4* (currently SEPT9_v5) is upregulated in ovarian tumors (Burrows et al, 2003), and our group has previously reported that SEPT9_v3 is downregulated in about half of the breast cancer patients analyzed (Connolly et al, 2011b).…”

Septin Mutations in Human Cancers

“…Alterations in septin expression affect chromosome alignment and segregation as well as cytokinesis, which may result in the loss or gain of whole chromosomes (Spiliotis et al, 2005; Estey et al, 2010; Thompson et al, 2010; Menon et al, 2014). Indeed, over-expression of SEPT9_i1 in human mammary epithelial cells increases aneuploidy, which correlates with defects in centrosome duplication, chromosome segregation and cytokinesis (Peterson et al, 2011). In cancer cell lines, elevated expression of SEPT9 isoforms (e.g., SEPT9_i1/i4) and down-regulation of SEPT10 have been reported to confer resistance to the anti-cancer microtubule-targeting drug paclitaxel (Amir and Mabjeesh, 2007; Froidevaux-Klipfel et al, 2011; Chacko et al, 2012; Xu et al, 2012).…”

Mammalian septins: dynamic heteromers with roles in cellular morphogenesis and compartmentalization

“…The up‐regulation of SEPT9_v1 , a transcript encoding an isoform with the largest N‐terminal extension has been linked to several tumour‐associated phenotypes. There is now evidence that this isoform can localize to the nucleus where it stabilizes HIF1‐α and leads to the activation of HIF‐regulated target genes, thus linking SEPT9_i1 to angiogenesis 53, 93, 94. Over‐expression of this isoform in immortalized human mammary epithelial cells lead to increased growth kinetics, cell motility and invasion, as well as cytokinesis defects and disruption of tubulin microfilaments 92.…”