Detection of tumor DNA in serum of colorectal cancer patients.

We previously found p16 promoter methylation in DNA in the sera of 13 colorectal cancer patients out of 44 (30%) whose tumor DNA exhibited the methylation, using methylation-specific PCR (MSP). To examine whether the cancer detection rate could be improved by using a different tumor marker, we examined the K-ras status in 90 colorectal cancer patients using a mismatch ligation assay. Among the 31 patients showing K-ras gene mutations in their tumors, the same mutations were observed in serum DNA of 11 patients (35%). Among the 90 patients, 63 showed tumors positive for K-ras mutation or p16 promoter methylation, or both, and 22 had serum DNA positive for one or both. K-ras mutation was found even in serum DNA of patients with Dukes A cancer, suggesting that colorectal cancer might be detected even in patients without symptoms by using this ligation assay.