418 Genome-wide Methylation Profiling of Circulating Cell-Free DNA in Advanced Stage Epithelial Ovarian Cancer

Introduction/Background Circulating DNA methylation markers in blood are promising non-invasive biomarkers in cancer. The aims of this study are to identify differentially methylated regions in cell-free DNA (cfDNA) between healthy persons and patients with advanced stage epithelial ovarian cancer (EOC) and differences in cfDNA methylation throughout the course of the disease. Methodology Plasma-derived cfDNA was analyzed by a high-throughput genome wide DNA methylation sequencing technique: MeD-seq. A training set of therapy naïve samples of patients with advanced stage EOC (≥FIGO stage IIIB) was compared with healthy controls to define a cfDNA methylation signature. Analysis of differentially methylated regions was performed using Chi-square test with Bonferroni correction for multiple testing. A cumulative hypermethylation score was constructed and validated in a set of pre- and postoperative samples. Tumor tissue samples were correlated with cfDNA results. Results Forty-nine patients with advanced stage EOC and ten healthy controls were included. Patients with advanced stage EOC showed a clear distinct cfDNA methylation signature from healthy controls (p-value <0.0001). Preoperative hypermethylation scores (135; interquartile range 110–163) were significantly higher than postoperative hypermethylation scores (91; interquartile range 76–101) (p-value <0.001). cfDNA methylation signature at baseline differed from tumor tissue and was closer related to DNA methylation of immune-related cells (T-lymphocytes, neutrophil granulocytes, monocytes and B-lymphocytes) than to the primary tumor. Conclusion MeD-seq provides a promising method for genome wide methylation profiling on cfDNA. Patients with advanced stage EOC could clearly be distinguished from healthy controls and differed pre- and postoperatively. It is a remarkable finding that the cfDNA methylation signature was not correlated with tumor tissue DNA methylation, but with immune-related cells. This led to the hypothesis that advanced stage EOC, which mainly spreads in the peritoneal cavity, leads to a cfDNA methylation profile mainly based on an immune response instead of a reflection of tumor DNA methylation. Disclosures No.