Redefined complement C3c structures have significant increase in the plasma of ovarian cancer patients

Ovarian cancer (OC) ranks as the third most common gynecological cancer. Like other cancers, there still a need for effective biomarkers. To explore if plasma proteolytic products could serve as biomarkers, we devised a platform incorporating CyDye labeling, macroporous reversed-phase liquid chromatography, reducing/non-reducing SDS-PAGE, and fluorescence imaging. With paired preoperative and postoperative plasma samples, our analyses revealed a circulating complement C3 derivative present only at the disease state. This 145-kDa species under non-reducing condition split into 72-, 39-, and 29-kDa fragments upon reduction, reminiscent of the C3c structure. Mass spectrometric analyses showed multiple C-terminal ends in the C3c α’1 fragment, which were used differently among OC patients. The variety of ends were also observed in serum samples prepared using various complement activators, redefining C3c as a mix of multiple molecular entities. A commercialized ELISA assay targeting only the canonical C3c showed a strong correlation between increased plasma levels and the OC occurrence and pro-gression. Our results suggest that plasma proteolysis in complement deactivation is involved in ovarian tumorigenesis, and the resulting protein changes may help develop next-generation cancer biomarker.