Global changes of 5-hydroxymethylcytosine and 5-methylcytosine from normal to tumor tissues are associated with carcinogenesis and prognosis in colorectal cancer

Aberrant DNA methylation has raised widespread attention in tumorigenesis. In this study, we aimed to investigate the changes of global DNA methylation and hydroxymethylation from normal to tumor tissues in colorectal cancer (CRC) and their association with the prognosis. The levels of genomic 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) in cancerous tissues were significantly lower than those in corresponding adjacent normal tissues. The genomic levels of 5mC were significantly positively correlated with 5hmC in normal and cancerous tissues (all P < 0.05). The ratio of 5mC in cancerous tissues to matched normal tissues (C/N-5mC) was also significantly positively correlated with the ratio of 5hmC in cancerous tissues to matched normal tissues (C/N-5hmC) (P=0.01). The 5mC levels and C/N-5mC ratios decreased with age (all P < 0.05). Higher 5mC and 5hmC levels were found in rectal than in colon tissues (all P < 0.05). High levels of 5mC in cancerous tissues and high C/N-5hmC ratios were each associated with lymph node metastasis (all P < 0.05). Survival analysis indicated that the C/N-5mC ratio (P=0.04) is an independent protective factor for overall survival. The data showed that patients with a combination of high C/N-5hmC and low C/N-5mC ratios tended to have a worse prognosis (P < 0.01). Our findings showed that the C/N-5mC ratio may be an independent prognostic factor for CRC outcome. Patients with both a high C/N-5hmC ratio and a low C/N-5mC ratio exhibited the worst survival, suggesting that 5mC and 5hmC can be used as critical markers in tumorigenesis and prognosis estimation.