Quantitation and characterization of plasma DNA in normals and patients with systemic lupus erythematosus

Using the in vitro DNA labeling technique of nick translation on purified plasma DNA, we have estimated the plasma DNA concentration in three normal individuals to be 266 +/- 57 ng/ml (mean +/- SD). This was not significantly different in three patients with a chronic inflammatory disease (209 +/- 14 ng/ml) or in five patients with steroid-inactivated systemic lupus erythematosus (SLE) (293 +/- 57 ng/ml). In two untreated, newly diagnosed, active SLE patients, however, the plasma DNA concentration was considerably higher (4,024 and 2,437 ng/ml, respectively). Characterization of these in vitro labeled DNA preparations by neutral sucrose-gradient sedimentation analysis showed a sedimentation coefficient of 6-8S, corresponding to a molecular weight of similar to or approximately 0.2-0.45 x 10(6). No difference was observed between normal subjects or patients. In addition, the relative size uniformity of these DNA molecules might suggest some form of specific protection of the DNA from blood DNAases. Further characterization in terms of buoyant density in cesium chloride did not reveal a difference between normal or SLE plasma and the human (HEp-2 cell) DNA used as marker. Taking into account the limitations of the method, no indication of a possible exogenous origin of the DNA circulating in SLE patients could be found. The physiological or pathophysiological role of this plasma DNA remains to be determined.