RBP1L1, a retinoblastoma-binding protein-related gene encoding an antigenic epitope abundantly expressed in human carcinomas and normal testis

Background: Antibodies isolated from cancer patients have been used to identify genes encoding tumor-associated antigen epitopes relevant to immune responses in cancer patients. In this report, we used an immunoglobulin G (IgG) purified from serum of a patient with breast cancer to identify its corresponding epitope, gene, and protein-retinoblastoma-binding protein-l-like protein-1 (RBP1L1)-and determined whether it is a potential molecular marker for various cancers. Methods: IgG purified from the serum of a patient with breast cancer was used to screen an MCF-7 breast cancer cell complementary DNA (cDNA) expression library for immunoreactive clones. The cDNAs identified were cloned and sequenced. Immunoreactivity of specific amino acids in the epitope was determined by western blot analysis and enzyme-linked immunosorbent assay. The cellular location of the antigen was determined by immunoperoxidase staining with purified RBP1L1-specific IgG. Gene expression in various human carcinomas and normal tissues was examined by northern blot analysis and the reverse transcription-polymerase chain reaction. Results: Our purified IgG recognized just one epitope on RIBP1L1. The complete 5802-base-pair RBP1L1 cDNA encodes a 1226-amino acid protein containing the antigenic epitope IKPSLGSKK. The derived protein sequence of RBP1L1 shares 74% and 37% amino acid identity, respectively, with a partial sequence of the retinoblastoma-binding protein and the complete sequence of retinoblastoma-binding protein-1. The RBP1L1 epitope was localized to the cytoplasm of MCF-7 cells but was not detected in peripheral blood mononuclear cells. High expression of RBP1L1 messenger RNA was found in human breast, lung, colon, pancreatic, and ovarian cancers and in normal testis, but expression was limited in other normal tissues. Conclusions: RBP1L1 appears to be a molecular marker associated with a broad range of human malignancies.