Since the Septin family of highly conserved proteins was discovered in budding yeast 40 years ago, homologous proteins have been continuously identified in nearly every eukaryotic lineage except higher plants[18, 19]. The number of Septin isoforms varies from species to species, with two in Caenorhabditis elegans, five in drosophila, and thirteen in humans, respectively, SEPT1-12 and SEPT14. Septin gene is a kind of GTP-binding protein with molecular weight between 30-65kDa, and its GTP-binding domain is very similar to that of Ras protein[12, 21]. The N-terminal sequence of Septin is rich in proline, and the C-terminal is a coiled spiral structure[21, 22]. A single Septin protein monomer can be polymerized into heterologous complexes called septum filaments, which are further assembled to form advanced complex structures such as fiber bundles and participate in the physiological process of cytokinesis[23, 24]. As a non-traditional cytoskeletal component, Septin protein can construct cellular scaffold or diffusion barrier on cell membrane, affecting diverse physiological functions. Septin is involved in the formation of cilia and sperm flagellum structures, neuronal development, and plays an important role in the interaction of pathogenic bacteria invading the body[26–28]. For example, Septin1, which locates to spindle poles in mitosis, was reported to participate in spindle assembly and chromosome congression. Zhang et al. found that Septin4 was a novel PARP1 interaction protein essential in oxidative stress induced vascular endothelial cell injury.
In this study, we first explored the expression of SEPT1-11 in colon cancer, and found that compared with normal tissues, the expression of SEPT2/3/7/8/9 was increased while the expression of SEPT1/6 was decreased. Evaluation of the relationship between Septin and pathological stage of colon cancer patients showed that the expression of SEPT7 declined significantly with the progression of colon cancer. Then, we assessed the prognostic value of Septin in colon cancer. It was found that high expression of SEPT2/9/10/11 while low expression of SEPT5 were associated with better OS.
As for the methylation status of SEPT1-11, the results demonstrated that the methylation level of SEPT2/5/7/9/11 was markedly elevated in colon cancer. We also found that the methylation level of SEPT9/10/11 decreased gradually with the development of colon cancer. And SEPT9’s methylation expression decreased with the increase of patients’ age. These results seem to contradict existing research and deserve further study. Exploring the prognostic value of the DNA methylation of Septin, we found that 2 CpG of SEPT1, 2 CpG of SEPT3, 5 CpG of SEPT4, 3 CpG of SEPT5, 8 CpG of SEPT6, 3 CpG of SEPT7, 5 CpG of SEPT8, 20 CpG of SEPT9, 3 CpG of SEPT10, and 7 CpG of SEPT11 were significantly associated with prognosis of colon cancer patients. In addition, the results presented that patients with colon cancer in the low-risk group of SEPT1/3/4/6/9 or in the high-risk group of SEPT5/7/8 survived longer.
Next, the molecular characteristics of SEPT1-11 in colon cancer were the focus of our investigation. In the exploration of genetic alteration, SEPT1-11 was found to have 0%, 2.9%, 1.9%, 2.9%, 0%, 1.9%, 1%, 1%, 9%, 1%, and 0% alterations in colon cancer samples, respectively. For the correlation analysis of Septin, we found that the expression of SEPT1, SEPT3, SEPT8, SEPT10 and SEPT11 were moderately to strongly correlated, indicating that they may function synergistically in the occurrence and development of colon cancer.
Then, the function of Septin was performed with GO and KEGG pathway enrichment analyses. Not surprisingly, the functions of SEPT1-11 genes were mainly related to cell cycle, cell division, cytokinesis, GTPas activity, and bacterial invasion of epithelial cells. We also analyzed the downstream targets of SEPT1-11 in depth. Potential key transcription factors (such as FosB, PITX2, PAX4a) and miRNA targets of Septin were identified. Furthermore, we assessed the correlation between SEPT1-11 and immune cell infiltration in colon cancer. The results showed that SEPT2/6/7/9/10/11 was significantly correlated with B cells, CD8 + T cells, CD4 + T cells, macrophages, neutrophils and dendritic cells. The Cox proportional hazard model for SEPT1-11 and tumor-infiltrating immune cells showed that SEPT4 and SEPT5 were significantly correlated with the clinical outcome of colon cancer patients. The above results suggested that Septin may participate in the progression of colon cancer by influencing immune status.
More and more evidences show that Septin plays an indispensable role in the growing tumors. For example, the down-regulated miR-744-5P in colon cancer lost its inhibitory effect on SEPT2, which functions as an oncogene and promotes the proliferation of colon cancer cells. Notch target gene SEPT4 is involved in the Notch signaling pathway, and inactivation of this pathway has been confirmed to be associated with the development of cancers. Recent studies revealed that SEPT6 was the upstream gene of UBC in prostate cancer and inhibited tumor growth by regulating UBC. Other evidences showed that SEPT6 was also participated in the pathogenesis of breast cancer and hepatocellular carcinoma, which suggested that targeting SEPT6 may be a new therapeutic strategy for cancers[34, 35]. Overexpression of SEPT7 plays a crux role in inhibiting the growth and migration of glioma cells[36, 37]. And there was reported that SEPT7 and SEPT2 could synergically promote the proliferation, migration and invasion of breast cancer cells. SEPT9, the most well-known member of the Septin family, has been studied for many years, with much emphasis on methylation and cancer biomarkers[39–41]. SEPT9 methylation analysis has great potential and usefulness as a non-invasive marker for early colon cancer, and might be used in combination with other conventional markers to detect disease recurrence[42, 43]. Other functions of SEPT9 are also worth discussing and exploring, which is what we did in our study.
The data in this study came from multiple online bioinformatics databases, it is understandable that there are biases or conflicts in the analysis results. These contradictions instead become the focus of our study. Next, we plan to verify the analysis results in vitro and in vivo, and further explore the specific mechanism of the Septin family in the occurrence and development of colon cancer.
In conclusion, our research provides valuable information on the role of septin family members as biomarkers, clinical therapeutic targets, and immune targets for colon cancer. It is hoped that these results will help in early screening and clinical treatment of colon cancer, and contribute to the development of novel targets or drugs.