Association of SEPT9 Gene Methylation with the Clinicopathologic Features and Fusobacterium nucleatum Infection in Colorectal Cancer Patients

Background/Objectives: Colorectal cancer (CRC) is a significant global health issue. The identification of methylated Septin 9 (mSEPT9) as a biomarker for CRC represents a significant advancement in cancer diagnostics. On the other hand, Fusobacterium nucleatum (FN) is one of the most studied cancer-related microbes in CRC. This study provided cohort evidence on the association of mSEPT9 with clinicopathologic characteristics and FN infection in CRC patients. Methods: Paired formalin-fixed paraffin-embedded (FFPE) tissue DNA (cancerous and adjacent non-cancer tissues) of eighty-three CRC patients was collected. Methylation-specific qPCR targeting the v2 promoter region of mSEPT9 was carried out on bisulfite-converted FFPE DNA. For FN detection, a TaqMan probe-based method targeting the 16S rRNA gene was used. The differences in mSEPT9 levels and FN expression between cancer and non-cancer tissues were evaluated. Association studies between mSEPT9 in the tumor and relative mSEPT9 levels with FN infection and available clinical data were conducted. Results: Higher mSEPT9 levels were found in the cancerous tissue compared to non-cancerous tissue (p < 0.0001). High mSEPT9 levels in the tumor were significantly associated with older patients (p < 0.001) and larger tumor size (p = 0.048) but not with other clinicopathologic variables. In double-positive patients where mSEPT9 was detected in both cancerous and non-cancerous tissue, the expression fold-change in mSEPT9, calculated using the 2−ΔΔCT formula, was significantly higher in patients with tumor size equal to or greater than 5 cm (p = 0.042). High levels of mSEPT9 in tumor were not associated with FN infection. However, high levels of FN infection were associated with mSEPT9 (p < 0.021). Conclusions: High levels of mSEPT9 are found in CRC tumor tissue and are associated with older age and larger tumor size, while high levels of FN infection are associated with mSEPT9 in this single-center cohort study.