Although the screening and treatment strategies for CRC have been greatly improved in recent years, mortality from colorectal cancer remains high14. The existing colonoscopy and fecal occult blood test (FOBT) or FIT have certain limitations as a screening method for colorectal cancer15–17. Therefore, robust diagnostic, prognostic and predictive biomarkers are clearly and urgently needed to detect advanced colon polyps and early-stage CRC, which are most effectively treated with current therapies, and to identify the most effective treatments for specific CRC patients.
DNA methylation was first found in cancer. Aberrant DNA methylation is thought to occur at very early stages of cancer development and specific genes seem to be methylated at different tumor stages9. SFRP1 has been classified as a tumor suppressor gene due to the loss of its expression in many human cancers18. Under physiological conditions, SFRP1 inactivates the canonical and non-canonical WNT/β-catenin pathway by directly binding to WNT proteins or the frizzled receptor19. Promoter methylation down-regulates the expression of SFRP1 in CRC, which relieves the negative regulation of Wnt/β-catenin, consequently altering the proliferation and differentiation of tumor cells20. Xinyan Liu, Jinming Fu and others provide that SFRP1 promoter methylation may be related to the pathogenesis of CRC by cohort study method, and indicate that SFRP1 may become a valuable biomarker for early diagnosis of CRC21. Furthermore, Salehi et al. also supported the theory that aberrant promoter methylation of the SFRP1 gene may be used as a novel epigenetic biomarker in the diagnosis of CRC, especially as a noninvasive screening method for the detection of CRC22. In this experiment, we detected all methylation sites on the SFRP1 promoter in fecal samples of normal patients and CRC patients. Combined with the construction of the prediction model, three methylation sites with good accuracy and specificity were found.
Fecal samples are easier to obtain, non-invasive to patients and have better compliance compared with tissue samples. Most importantly, colorectal tumor cells continue to fall off and enter the cavity for CRC patients23. Logically, the shedding of tumor cells into the feces occurs earlier during the progression of CRC development24. This is why we did not select tissue and blood samples from patients with CRC. In addition, Massarray technology plays an important role in cancer biological diagnosis and individualized treatment, and it has been widely used in the detection of somatic mutation, genotyping, gene methylation and so on25. MassARRAY has high accuracy and sensitivity for detecting DNA methylation, and can detect all sites at one time. This provides high reliability for us to determine the methylation sites of the SFRP1 promoter with differential expression in CRC patients. In our experiment, the methylation level of 12 methylation sites on the SFRP1 promoter in CRC patients was significantly higher than that in normal patients detected by MassARRAY.
Furthermore, a random forest model was constructed to seek out specific methylation sites and the model achieved high accuracy and specificity. The Random Forest regression model is a relatively new computer learning method, which is widely used to evaluate the significance of important predictors26. In many current data sets, it has great advantages over other algorithms and performs well. A large number of studies have used bioinformatics to excavate clinical data27. In our research, we use clinical experimental data combined with bioinformatics analysis to draw conclusions that may be more reliable. To predict early CRC lesions, Random Forest regression model was established based on 12 methylation sites on the SFRP1 promoter, with high accuracy and confirmation by the validation set. Random forest analysis identified the 3 highly discriminating methylation sites among 12 methylation sites. ROC analysis was conducted and showed an AUC of 0.742, implying high specificity and sensitivity.
Locked nucleic acid has higher stability and affinity compared with common fluorescent PCR28. To validate the significance of methylation sites as potential CRC biomarkers, locked nuclear acid was incorporated in the PCR probe. Comparing three characteristic methylation sites in stool samples from 30 CRC patients and 30 normal patients as predictors, SFRP1_CpG_16.17.18 performed best and the AUC of the ROC curve is 0.791 (SFRP1_CpG_12: 0.728, SFRP1_CpG_13: 0.636). Research shows that mSEPT9 is a reliable blood-based marker in CRC detection, particularly advanced CRC10. In addition, Yoon DAE Han et al verified the capability of stool DNA based-SDC2 methylation for early detection of CRC patients with high specificity11. Even though these two biological factors have been applied in clinics, each has certain disadvantages. Through clinical sample detection and bioinformatics analysis, it is concluded that the methylation site of SFRP1_CpG_16.17.18 on the SFRP1 promoter may be used as a potential biological site for early prevention of CRC. The methylation sites were shown to be associated with PFS and DFS of the patients. And Cg15839448 methylation is negatively correlated with immune checkpoints with immunotherapy sensitivity, indicating which might be a potential marker for immunotherapy.
However, some limitations need to be emphasized in the study. First, the samples included in the study and surveillance cohort were small, especially positive patient samples, so the data needs to be validated in a larger multicenter prospective study. Secondly, the effect of SFRP1 promoter site methylation on SFRP1 expression and function has not been deeply studied, so further experiments need to be carried out in the later stage.