Calnuc is a calcium (Ca2+) binding protein found in both Golgi and cytoplasm, and it may play a role in G protein- and Ca2+-regulated signal transduction events. This study was designed to investigate the possibility of whether Calnuc protein might be a tumor-associated antigen (TAA) that induces autoantibody response in human cancers, and to evaluate the feasibility of the Calnuc antigen-antibody system as a marker in cancer detection. Purified full-length recombinant Calnuc protein was used as an antigen in enzyme-linked immunoassay and Western blotting for the detection of autoantibodies in cancers. Sera from 447 patients with 9 different types of cancer were analyzed. Although the frequency of autoantibody to Calnuc was found to be 4.7% in total groups of cancer, it was not significantly different to that of normal individuals (1.2%). However, the frequency of autoantibody to Calnuc in colon cancer (11.5%) was significantly higher than that in normal individuals (1.2%). The expression analysis of Calnuc in multiple colon cancer tissues by immunohistochemistry on tissue array further confirmed the high specificity of Calnuc in colon cancer. Of 69 colon cancer tissue specimens examined, 41 tissues (59.4%) overexpressed Calnuc, while normal colon tissues did not show any expression of Calnuc. The subcellular distribution analysis of Calnuc examined by subcellular fractionation and immunofluorescence indicates that Calnuc is a membrane associated protein and mostly distributed in Golgi, which is consistent with previous reports. With adding Calnuc into a TAA array (including p53, c-myc, cyclin B1, cyclin D1), the cumulative frequency of antibody to multiple TAAs in colon cancer was raised to 65.4% which is significantly higher than the cumulative frequency in normal individuals (6.1%). This indicates that a mini-array of multiple TAAs which includes Calnuc might provide a novel non-invasive approach to enhance antibody detection for colon cancer diagnosis.

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