Normal and Cancerous Tissues Release Extrachromosomal Circular DNA (eccDNA) into the Circulation

Cell-free circulating linear DNA is being explored for noninvasive diagnosis and management of tumors and fetuses, the so-called liquid biopsy. Previously, we observed the presence of small extrachromosomal circular DNA (eccDNA), called microDNA, in the nuclei of mammalian tissues and cell lines. Now, we demonstrate that cell-free microDNA derived from uniquely mapping regions of the genome is detectable in plasma and serum from both mice and humans and that they are significantly longer (30 to 60 percent greater than 250 bases) than cell-free circulating linear DNA ( around 150 bases). Tumor-derived human microDNA is detected in the mouse circulation in a mouse xenograft model of human ovarian cancer. Comparing the microDNA from paired tumor and normal lung tissue specimens reveals that the tumors contain longer microDNA. Consistent with human cancers releasing microDNA into the circulation, serum and plasma samples (12 lung and 11 ovarian cancer) collected prior to surgery are enriched for longer cell-free microDNA compared with samples from the same patient obtained several weeks after surgical resection of the tumor. Thus, circular DNA in the circulation is a previously unexplored pool of nucleic acids that could complement miRNAs and linear DNA for diagnosis and for intercellular communication.