Background: Cell-free DNA from dying cells recently has been discovered in human blood plasma. In experiments performed on animals and humans, we examined whether this cell-free DNA can cross the kidney barrier and be used as a diagnostic tool. Methods: Mice received subcutaneous injections of either human Raji cells or purified 32P-labeled DNA. DNA was isolated from urine and analyzed by measurement of radioactivity, agarose gel electrophoresis, and PCR. In humans, the permeability of the kidney barrier to polymeric DNA was assessed by detection in urine of sequences that were different from an organism bulk nuclear DNA. Results: In the experiments on laboratory animals, we found that ∼0.06% of injected DNA was excreted into urine within 3 days in a polymeric form and that human-specific Alu sequences that passed through the kidneys could be amplified by PCR. In humans, male-specific sequences could be detected in the urine of females who had been transfused with male blood as well as in DNA isolated from urine of women pregnant with male fetuses. K-ras mutations were detected in the urine of patients with colon adenocarcinomas and pancreatic carcinomas. Conclusions: The data suggest that the kidney barrier in rodents and humans is permeable to DNA molecules large enough to be analyzed by standard genetic methodologies.

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