A novel serum assay for breast epithelial antigen using a fusion protein.

Serum levels of breast epithelial antigens (BrE-Ags) are presently used in the follow-up of breast cancer patients. Available assays do not have optimal sensitivity and rely on reagents that could vary in their source and purity. A novel competitive solid-phase radioimmunoassay was developed for BrE-Ags that consists of the NP5 fusion protein, produced in Escherichia coli, that is composed of beta-galactosidase and polypeptide sequence obtained from a breast carcinoma cell line cDNA library, and anti-human milk fat globule monoclonal antibody Mc5. The fusion protein carries an altered epitope sequence (mimotope) that is similar, but not identical, to that found in the native antigen. This new competitive assay configuration has two essential features, a solid-phase affinity step that purifies the fusion protein carrying the mimotope for Mc5 and a competitive step that provides quantitation. Serum values for this assay show high specificity and sensitivity for breast cancer patients when compared to normal subjects and post-surgical breast cancer patients during their disease-free period.