Circulating DNA in Systemic Lupus Erythematosus

Immunoprecipitable double-stranded (dsDNA) was previously shown to persist in the circulation ofa clinically recognizable subgroup of patients with systemic lupus erythematosus (SLE). Plasma from 10 such patients was subjected to a DNA isolation procedure that used a combination ofproteolysis, phenol extraction, and hydroxylapatite adsorption and elution in the presence of urea. The isolated dsDNA was radiolabeled by nick translation and then characterized by isopyknic ultracentrifugation in CsCl under both neutral and alkaline conditions, as well as after digestion with S,-endonuclease. These experiments demonstrated essential identity in nucleotide base composition between the plasma-derived DNA and human genomic DNA. The presence of specific human base sequences in the plasma DNA was demonstrated by finding that authentic human genomic DNA accelerated the renaturation of plasma DNA when compared with the effect of nonhuman, control DNA. The proportion of such sequences in plasma DNA was estimated by attempting to renature the plasma DNA in the presence ofhuman DNA under conditions shown to result in complete renaturation of human DNA in model experiments. In this way, a minimum of 47% of plasma DNA base sequences could be shown also to be present in human genomic DNA. However, an average of 1020% ofthe plasma-derived DNA failed to renature under these conditions, a result that was further confirmed by comparing the renaturation ofthe tritium-labeled plasma DNA specimens, in double-label experiments, with inThis work was presented in preliminary form at the 43rd Annual Meeting ofThe American Rheumatism Association, May 30, 1979 and published in abstract form (1979. Arthritis Rheum. 22:664). Received for publication 30 August 1982 and in revisedform 7 November 1983. ternal controls consisting of '4C-labeled authentic human DNA. Attempts to drive the reaction to completion with human DNA led to a similar conclusion. The relative nonrenaturability of this fraction ofplasma DNA did not appear to be attributable to extensive chain breakage, although adequate analysis of this DNA subfraction was limited by reagent availability. It was therefore concluded that, in this group ofSLE patients, persistently circulating DNA consisted largely of base sequences also found in human genomic DNA. The additional presence in plasma of a DNA subfraction that differed in its renaturation behavior from human genomic DNA was recognized, although its significance could not be established with certainty.