Profiling disease and tissue-specific epigenetic signatures in cell-free DNA

Programmed cell death, accidental cell degradation and active extrusion constantly lead to the release of DNA fragments into human body fluids from virtually all cell and tissue types. It is widely accepted that these cell-free DNA (cfDNA) molecules retain the cell-type specific genetic and epigenetic features. Particularly, cfDNA in plasma or serum has been utilized for molecular diagnostics. The current clinically implemented liquid biopsy approaches are mostly based on detecting genetic differences in cfDNA molecules from healthy and diseased cells. Their diagnostic potential is limited to pathologies involving genetic alterations, by the low proportion of cfDNA molecules carrying the mutation(s) relative to the total cfDNA pool, and by the detection limit of employed techniques. Recently, research efforts turned to epigenetic features of cfDNA molecules and found that the tissue-of-origin of individual cfDNA molecules can be inferred from epigenetic characteristics. Analysis of, e.g., methylation patterns, nucleosome or transcription factor binding site occupancies, fragment size distribution or fragment end motifs, and histone modifications determined the cell or tissue-of-origin of individual cfDNA molecules. With this tissue-of origin-analysis, it is possible to estimate the contributions of different tissues to the total cfDNA pool in body fluids and find tissues with increased cell death (pathologic condition), expanding the portfolio of liquid biopsies beyond genetics and towards a wide range of pathologies, such as autoimmune disorders, cardiovascular diseases, and inflammation, among many others. In this review, we give an overview on the status of tissue-of-origin approaches and focus on what is needed to exploit the full potential of liquid biopsies towards minimally invasive screening methods with broad clinical applications.