Although the biological functions of tumor-originated cell-free DNA have not been previously clarified, such molecular characteristics as mutations, hypermethylation, and microsatellite instability have confirmed its tumor origin. Therefore, to investigate the use of plasma DNA level as a biomarker of lung cancer, we compared plasma DNA concentrations in 102 patients with lung cancer and 105 healthy individuals using quantitative PCR analyses. The median plasma DNA concentrations for the healthy and cancer groups were 10.4 and 22.6 ng/ml, respectively (P < 0.0001), and elevated plasma DNA levels were also detected in patients with either stage I or II disease. Neither smoking status nor the number of packs per year had an effect on the level of circulating cell-free DNA. Increased concentrations of circulating cell-free DNA showed the potential power to discriminate lung cancer (area under the receiver operating characteristic curve = 0.86, 95% CI = 0.81 to 0.91). When subjects were classified into three groups based on their plasma DNA concentrations, subjects in the upper tertile (ie, those with the highest concentration) had a significantly increased risk of lung cancer as compared with those in the lowest tertile (adjusted odds ratio = 50.6, P < 0.001). These results suggest that elevated circulating plasma DNA levels may serve as a potential diagnostic indicator and be an important risk factor for lung cancer
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