The origin of circulating free DNA.

To the Editor: Almost every report on circulating DNA identifies apoptosis or necrosis or both as the main source of free circulating DNA in serum and plasma. A hallmark of apoptosis is DNA degradation, in which chromo-somal DNA is 1st cleaved into large fragments (50 –300 kb) and subsequently into multiples of nucleoso-mal units (180 –200 bp) (1). This ladder pattern is also visible after electrophoresis of circulating DNA and is frequently considered to be evidence that apoptosis may be the source of the observed DNA fragments in plasma (2, 3). It has been shown, however, that the characteristic ladder pattern can also be observed for actively released DNA (3). The contents of apoptotic cells are rapidly ingested by professional phagocytes or neighboring cells through mechanisms that are not fully understood (4), and the DNA is consequently completely digested by DNase II in lysosomes (1). Thus the possibility exists that DNA fragments released by apopto-sis are removed before appearing in the circulation. If this engulfment of apoptotic bodies is impaired or cell death is increased enough to produce substantial amounts of circulating DNA, inflammation would definitely be a problem and autoimmunity would occur frequently in cancer and other conditions involving increased circulating DNA (1, 4). Radiotherapy, chemotherapy, and other cancer treatments cause cell death by apoptosis, and less circulating DNA is found in cancer patients after treatment than before treatment , possibly because of the inhibi-tory effect of treatment on the proliferation of cancer cells. Furthermore, in the early stages of cancer, when little cell death seems to occur, circulating DNA may already be present in higher than normal concentrations. As the cancer burden increases , so does the rate of cell death and the amount of proliferating cancer cells, with a concomitant increase in circulating DNA. In addition, many cancer cells are resistant to apoptosis, and to escape the immune system proliferating cells lose the ability to become apoptotic (2, 3), a process that also runs counter to the notion of apoptosis as the main mechanism for generating free DNA. Necrosis, on the other hand, produces large DNA fragments. Even with no cells dying, the DNA concentration in culture medium increases in proportion to the proliferation of cultured cancer cells. Human lymphocytes have also been observed to actively release double-stranded DNA into culture medium to a certain concentration (5), irrespective of incubation time. A similar observation was made in …