Circulating free DNA

Circulating free DNA (cfDNA) (also known as cell-free DNA) are degraded DNA fragments released to body fluids such as blood plasma, urine, cerebrospinal fluid, etc. Typical sizes of cfDNA fragments reflect chromatosome particles (~165bp), as well as multiples of nucleosomes, which protect DNA from digestion by apoptotic nucleases.[1] The term cfDNA can be used to describe various forms of DNA freely circulating in body fluids, including circulating tumor DNA (ctDNA), cell-free mitochondrial DNA (ccf mtDNA), cell-free fetal DNA (cffDNA) and donor-derived cell-free DNA (dd-cfDNA).[2] Elevated levels of cfDNA are observed in cancer, especially in advanced disease.[3] There is evidence that cfDNA becomes increasingly frequent in circulation with the onset of age.[4] cfDNA has been shown to be a useful biomarker for a multitude of ailments other than cancer and fetal medicine. This includes but is not limited to trauma, sepsis, aseptic inflammation, myocardial infarction, stroke, transplantation, diabetes, and sickle cell disease.[5] cfDNA is mostly a double-stranded extracellular molecule of DNA, consisting of small fragments (50 to 200 bp) [6][7] and larger fragments (21 kb) [8] and has been recognized as an accurate marker for the diagnosis of prostate cancer and breast cancer.[9]

Recent studies have laid the foundation for inferring gene expression from cell-free DNA, with EPIC-seq emerging as a notable advancement.[10] This method has substantially raised the bar for the noninvasive inference of expression levels of individual genes, thereby augmenting the assay’s applicability in disease characterization, histological classification, and monitoring treatment efficacy.[10][11][12]

Other publications confirm the origin of cfDNA from carcinomas and cfDNA occurs in patients with advanced cancer. Cell‐free DNA (cfDNA) is present in the circulating plasma and in other body fluids.[13]

The release of cfDNA into the bloodstream appears by different reasons, including apoptosis, necrosis and NETosis. Its rapidly increased accumulation in blood during tumor development is caused by an excessive DNA release by apoptotic cells and necrotic cells. Active secretion within exosomes has been discussed, but it is still unknown whether this is a relevant or relatively minor source of cfDNA.[14]

cfDNA circulates predominantly as nucleosomes, which are nuclear complexes of histones and DNA.[15] cfDNA can also be observed in shorter size ranges (e.g. 50bp) and associated with regulatory elements. [16] They are frequently nonspecifically elevated in cancer but may be more specific for monitoring cytotoxic cancer therapy, mainly for the early estimation of therapy efficacy.