Abstract
BACKGROUND: Circulating tumor DNA (ctDNA) detected before surgery disappears after complete surgical resection of the cancer. Residual ctDNA indicates minimal residual disease (MRD), which is a cause of recurrence. The presence of long-fragment circulating cell-free DNA (cfDNA) or methylated cfDNA also implies the presence of cancer. In this study, we evaluated the prognostic value of cfDNA methylation and long-fragment cfDNA concentration in gastric cancer patients undergoing curative surgery METHODS: Ninety-nine gastric cancer patients were included. Peripheral blood samples were collected before and 1 month after surgery. In patients administered chemotherapy, samples were collected before starting chemotherapy. qPCR was performed to detect long- and short-fragment LINE-1. A plasma HELP (HpaII tiny fragment Enrichment by Ligation-mediated PCR) assay to determine the concentration of HpaII small fragments was performed using ligation-mediated PCR and HpaII was quantified as the HpaII:MspI ratio to detect methylation levels of cfDNA. RESULTS: Overall survival (OS) of patients with low methylation levels before starting treatment was significantly worse than that of patients with high methylation levels (P = 0.006). In the 90 patients who underwent curative surgery, recurrence-free survival (RFS) and OS of patients with low methylation levels before surgery were worse than those with high methylation levels (P=0.08 and P = 0.11, respectively). RFS and OS of patients with high concentrations of long-fragment LINE-1 after surgery were significantly worse than those with low concentrations of long-fragment LINE-1 (P = 0.009, P = 0.04). CONCLUSIONS: Pre-surgical low methylation levels of LINE-1 are a negative prognostic factor. Post-surgical high concentrations of long-fragment LINE-1 indicate MRD and a high risk of recurrence.