Proteomics-based identification of RS/DJ-1 as a novel circulating tumor antigen in breast cancer.

PURPOSE: We used a proteomics-based approach to identify tumor proteins that elicit a humoral response in breast carcinoma and that may occur as circulating antigens. EXPERIMENTAL DESIGN: The breast cell line SUM-44 was used as a source of tumor cell proteins for two-dimensional PAGE (2-D PAGE) and for Western blot analysis in which individual sera were analyzed for primary antibodies. RESULTS: Sera from 30 newly diagnosed patients with breast cancer were screened for IgG antibodies to tumor cell proteins. Sera from 116 patients with other cancers and from 25 healthy subjects served as controls. Restricted reactivity against a set of three proteins, identified by mass spectrometry as isoforms of a novel oncogenic protein that regulates RNA-protein interaction (designated RS/DJ-1), was observed in four patients with breast cancer, but not in healthy subjects. The identity was further confirmed by Western blotting with specific antibodies. RS/DJ-1 was found to be secreted in the breast cell line SUM-44, which led us to determine whether RS/DJ-1 was found in circulation in breast cancer. Interestingly, unlike in controls, RS/DJ-1 was readily detectable in sera from 37% of newly diagnosed patients with breast cancer. CONCLUSION: The presence of autoantibodies and/or circulating RS/DJ-1 protein in sera from patients with breast cancer may have clinical utility.