While various clinical applications especially in oncology are now in progress such as diagnosis, prognosis, therapy monitoring, or patient follow-up, the determination of structural characteristics of cell-free circulating DNA (cirDNA) are still being researched. Nevertheless, some specific structures have been identified and cirDNA has been shown to be composed of many “kinds.” This structural description goes hand-in-hand with the mechanisms of its origins such as apoptosis, necrosis, active release, phagocytosis, and exocytose. There are multiple structural forms of cirDNA depending upon the mechanism of release: particulate structures (exosomes, microparticles, apoptotic bodies) or macromolecular structures (nucleosomes, virtosomes/proteolipidonucleic acid complexes, DNA traps, links with serum proteins or to the cell-free membrane parts). In addition, cirDNA concerns both nuclear and/or mitochondrial DNA with both species exhibiting different structural characteristics that potentially reveal different forms of biological stability or diagnostic significance. This review focuses on the origins, structures and functional aspects that are paradoxically less well described in the literature while numerous reviews are directed to the clinical application of cirDNA. Differentiation of the various structures and better knowledge of the fate of cirDNA would considerably expand the diagnostic power of cirDNA analysis especially with regard to the patient follow-up enlarging the scope of personalized medicine. A better understanding of the subsequent fate of cirDNA would also help in deciphering its functional aspects such as their capacity for either genometastasis or their pro-inflammatory and immunological effects.

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Origins, structures, and functions of circulating DNA in oncology

Small amounts of free DNA circulate in both healthy and diseased human plasma/serum, and increased concentrations of DNA are present in the plasma of cancer patients. Characteristics of tumour DNA have been found in genetic material extracted from the plasma of cancer patients. These features include decreased strand stability and the presence of specific oncogene, tumour suppressor gene and microsatellite alterations. Point mutations of the ras genes have been detected in the plasma DNA of patients suffering from haematopoetic malignancies, colorectal and pancreatic cancer, sometimes prior to clinical diagnosis. Rearranged immunoglobulin heavy chain DNA has been found in the plasma of patients with non-Hodgkins lymphoma and acute B cell leukaemia. Microsatellite instability, expressed either as a new allele or a loss of one allele (LOH) occurs in the plasma and serum DNA of patients suffering from head and neck, lung and renal cell cancer. The results obtained in many different cancers have opened a new research area indicating that plasma DNA might eventually be a suitable target for the development of non-invasive diagnostic, prognostic and follow-up tests for cancer.

Detection of circulating tumour DNA in the blood (plasma/serum) of cancer patients.

Although it is evident that DNA circulates freely in blood plasma both in disease and in health, the source of this DNA remains enigmatic. It can be presumed that circulating DNA in healthy subjects derives from lymphocytes or other nucleated cells. Yet, it is not known why cancer patients have such large quantities of plasma DNA. There is no doubt that a proportion seems to originate from nucleated blood cells since wild-type DNA has been detected in the plasma of all cancer patients studied as well as in that of healthy controls. However, a substantial proportion of plasma DNA in cancer patients derives from tumor cells. This latter concept is supported by both quantitative and qualitative observations. Plasma DNA levels are not only greater in cancer patients than in normal subjects,1–5 but also correlate inversely with outcome and tend to fall with effective treatment.1,5 In addition, the ability to detect LOH in plasma DNA6–13 suggests that tumor-related DNA is the predominant subtype in at least some cancer patients. What is responsible for the presence of tumor DNA shed in the bloodstream? It could be due to lysis of circulating cancer cells or of micrometastases, to DNA leakage resulting from tumor necrosis or apoptosis, or to a new mechanism of active release.

The origin and mechanism of circulating DNA.

Human blood lymphocytes released DNA in vitro in the absence of any stimulation. Once purified from the complex appearing in the supernatant, this DNA exhibited typical characteristics as shown by its UV absorption curve, its deoxyribose coloration, and its sensitivity to DNase. Elution patterns on hydroxyapatite columns indicated that the excreted DNA is double stranded. The released DNA was smaller than the cellular DNA; its molecular weight ranged from 3.5 × 105 to 3.7 × 106 daltons.

The DNA appearing in the supernatant does not seem to be due to dead or dying cells since: ( a ) the same amount of DNA was found in the medium whether the incubation lasted 2 hr or as long as 16 hr; ( b ) cell death rate had no effect on the amount of extracellular DNA; ( c ) when the lymphocytes were centrifuged and placed in a new medium several times in a row, a similar amount of extracellular DNA was isolated from each of the successive supernatants, whereas if, after centrifugation, the lymphocytes were put back in their original medium, no increase in the amount of extracellular DNA was observed, suggesting an active regulatory mechanism independent of a mechanical effect; ( d ) it took more than 1 hr for extracellular DNA to reach its maximum concentration, a fact that also argues against a mechanical effect; ( e ) the specific activity of the released DNA was different from that of the cellular DNA, depending on the time of labeling; and ( f ) the cells that had excreted DNA kept their functional integrity, as shown by their fully maintained capacity to increase DNA synthesis after stimulation.

The extracellular DNA hybridized specifically with cellular DNA. The hybridization curves indicate that the DNA excreted is highly complex, and they suggest that it is composed of part of the newly synthesized DNA. The higher specific activity of the released DNA, compared with that of the cellular DNA after a long labeling period, can be explained only by a preferential release of the newly synthesized DNA.

https://cancerres.aacrjournals.org/content/canres/35/9/2375.full.pdf

Spontaneous Release of DNA by Human Blood Lymphocytes as Shown in an in Vitro System

Studies on a range of prokaryote and eukaryote cells and tissues have shown that a newly synthesized DNA/RNA-lipoprotein complex is released in a regulated manner. This complex, termed a virtosome, is a novel cytosolic component of eukaryote cells. The released virtosomes can readily enter other cells where they can modify the biology of the recipient cells. Such modifications include immunological changes and transformation from normal to cancer cells. The virtosomes form a normal component of the circulating nucleic acids in plasma and serum currently used for clinical diagnostic purposes. Given the transformative powers of virtosomes released from tumour cells, the presence of such a complex in human plasma could readily offer the basis of an alternative mechanism for the initiation of metastases.

The virtosome-a novel cytosolic informative entity and intercellular messenger.

Purified Agrobacterium tumefaciens deoxyribonucleic acid (DNA) does not produce crown gall tumors in growing plants, conditioned by wounding, as the living bacteria do. Purified bacterial DNA migrates in the plant and replicates, but it is not transcribed in our experimental conditions. On the contrary, when DNA is released naturally from bacteria into plant cells, a bacterial ribonucleic acid (RNA) can be found in these cells. There seems to be a direct relation between the appearance of A. tumefaciens RNA in the plant cells and the induction of the tumor.

Agrobacterium tumefaciens Ribonucleic Acid Synthesis in Tomato Cells and Crown Gall Induction

L'effet du citrate de sodium sur la translation d'ADN d'origine bacterienne chez la tomate a ete etudie biochimiquement et par autoradiographie. Les plantes pretraitees au citrate prennent plus d'ADN etranger que les plantes temoins. En revanche le citrate n'a pas d'effet sur la depolymerisation du DNA pris par la plante. Les relations entre ces phenomenes et les differents modes possibles de penetration du DNA etranger sont examines.

P. Gahan

Papers

Agrobacterium tumefaciens Ribonucleic Acid Synthesis in Tomato Cells and Crown Gall Induction

Effect of sodium citrate on the translocation of bacterial DNA inSolanum lycopersicum esc.