Recently, there has been renewed interest in understanding the origins of DNA found in the cell-free fraction of blood (i.e. plasma) and other body fluids. Analysis of cfDNA by NGS and other methods is being used for monitoring recurrence of malignancy in cancer patients undergoing treatment, as well as for non-invasive prenatal diagnostics and other clinical applications. Conventional thinking has been that most of the cfDNA in plasma is derived from mononucleosomes, and to a lesser extent, from nucleosome multimers (di- and tri-nucleosomes), produced by cells undergoing normal process of apoptosis (1-3). The predominant size of cfDNA recovered from some plasma samples is ~170 bp, with lesser amounts present as multimers of this size, reflecting the length of DNA complexed with the histone proteins that comprise nucleosomes. In addition to apoptosis, circulating cfDNA is also believed to originate from necrotic tissue, and cfDNA derived from necrotic cells may be elevated in plasma of individuals with malignant disease (2). The fragments of cfDNA derived from necrotic cells may be larger compared to cfDNA originating from nucleosomes. In addition to apoptosis and necrosis, active release of DNA by leukocytes has been shown to contribute to the cfDNA pool. It has been proposed that the bulk of cfDNA may originate during normal metabolism, and be released as lipoprotein complexes (“virtosomes”) after undergoing apoptotic breakdown (4). Circulating virtosomes may be able to penetrate other cells and deliver DNA capable of modifying the biology of recipient cells. Another potential source of DNA found in the circulation is DNA adhered to the outer surface of cells, extracellular vesicles, and platelets. Regarding concentrations of cfDNA, it should be kept in mind that the steady-state level of circulating cfDNA is influenced not only by processes responsible for its release, but also by processes responsible for cfDNA clearance. Analysis of cfDNA extracted from plasma and urine using the NextPrep-Mag™ cfDNA Isolation Kit and the NextPrep-Mag™ Urine cfDNA Isolation Kit shows that the size distribution of cfDNA varies considerably between samples from different donors, and also between samples collected at different times from the same donor.

References

  1. Nagata S et al. Cell Death Differentiation 2003; 10:108-116.
  2. Anker P et al. Cancer Metastasis Rev 1999;18:65-73
  3. Stroun M et al. Clin Chim Acta 2001;313:139-142
  4. Van der Vaart M and Pretorius P. Clinical Chemistry 2007; 53:2215