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 tha t elicit a humoral response in breast carcinoma and that may occur as circu lating antigens. Experimental Design: The breast cell line SUM-44 was used as a source of tu mor 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 scre ened 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 intera ction (designated RS/DJ-1), was observed in four patients with breast cance r, but not in healthy subjects. The identity was further confirmed by Weste rn blotting with specific antibodies. RS/DJ-1 was found to be secreted in t he breast cell line SUM-44, which led us to determine whether RS/DJ-1 was f ound in circulation in breast cancer. Interestingly, unlike in controls, RS /DJ-1 was readily detectable in sera from 37% of newly diagnosed patients w ith breast cancer. Conclusion: The presence of autoantibodies and/or circulating RS/DJ-1 prote in in sera from patients with breast cancer may have clinical utility.