Introduction to cell-free DNA

cfDNA biology

What are the basics of cell-free DNA biology? In healthy individuals, most cfDNA comes from cells of hematopoietic origin (1), primarily through DNA shedding during apoptosis. cfDNA of apoptotic origin typically circulates in fragments ranging of 120–220 base pairs (bp), with a predominant fragment size of around 166–170 bp. This corresponds to the length of DNA that can wrap around a nucleosome (147 bp), plus an additional stretch of DNA to link two nucleosome cores.  

Apoptosis can also produce longer cfDNA fragments that correspond to di-, tri- or poly-nucleosomes (2). Besides apoptosis, cfDNA can be generated by releasing genomic or mitochondrial DNA from any cell type via various mechanisms, including necrosis, active secretion, pyroptosis, autophagy, mitotic catastrophes and NETosis. 

The half-life of cfDNA in blood ranges from 15 minutes to 2.5 hours (3), and when it originates from tumor cells, we call it circulating tumor DNA (ctDNA). 

Scientists discussing cfDNA

Q: Why is cfDNA mostly a similar size? 

A: It’s because cfDNA is protected when it is wrapped around nucleosomes.  

Q: So each fragment corresponds to the length that is protected? 

A: Yes, plus we see a smaller amount that corresponds to two nucleosomes – that is twice as long.