Cell-Free DNA in Serum and Plasma: Comparison of ELISA and Quantitative PCR

Although circulating DNA has generally been referred to as cell-free DNA, it is likely that a significant proportion is bound to protein molecules, possibly as nucleosomes. This conclusion is supported by theory and by observations. Theoretically, circulating DNA is mostly released from degrading cells after cleavage by endonucleases that cut the chromatin into the basic nucleosomal elements (1)(2). Empirically, DNA fragments in circulation are mainly sized in multiples of the nucleosomal DNA (3)(4). Filtration experiments have shown that circulating RNA seems to be associated with particles, whereas DNA is not (5). This might be attributable to the arrangement of DNA in nucleosomes, which conserves them from proteolytic digestion in blood. Nucleosomal complexes consist of duplicate copies of the histones H2A, H2B, H3, and H4 as core proteins, with ∼146 bp of DNA on the outside (6). In various pathologic conditions, qualitative and quantitative changes in circulating DNA have been shown. Only small amounts of serum or plasma DNA have been observed in healthy individuals, whereas high concentrations have been described in patients with various malignancies and in those with several benign diseases, such as infections, sepsis, trauma, stroke, and autoimmune diseases (3)(7)(8)(9)(10)(11)(12)(13). Because most of these disorders are associated with increased rates …