Shotgun sequencing of 5-hydroxymethylcysotine circulating cell-free DNA from the blood distinguishes solid tumor tissues of origin

5-Hydroxymethylcytosine (5hmC) is an important mammalian DNA epigenetic modification that has been linked to gene regulation and cancer pathogenesis. Here we explored the diagnostic potential of 5hmC in cell-free DNA (cfDNA), the circulating DNA found in human plasma which represents a noninvasive window into the health status of the body. We showed that the genome-wide 5hmC distribution in cfDNA can be reliably sequenced by chemical labeling-based 5hmC enrichment. We sequenced cell-free 5hmC from 49 patients of seven different cancer types and found distinct features that can be used for monitoring disease status and progression. Specifically, we discovered that lung cancer leads to a stage-dependent hypohydroxymethlation in cfDNA, whereas hepatocellular carcinoma (HCC) and pancreatic cancer lead to disease-specific changes in the cell-free hydroxymethylome. Our results demonstrate that cell-free 5hmC can be used not only to track the stage of cancer but also to identify tissue of origin in some solid tumors.