Mage-b4, a novel melanoma antigen (MAGE) gene specifically expressed during germ cell differentiation

The MAGE genes were initially isolated from different kinds of tumors, and based on their virtually exclusive tumor-specific expression in adult tissues, they have been used as targets for cancer immunotherapy. However, although a large number of MAGE genes have now been identified and extensively studied in tumors of various origin, their functions in normal cells remain unknown. Here we describe the isolation and characterization of a novel murine MAGE homologue, Mage-b4. mRNA expression studies in a wide variety of adult and embryonic tissues revealed that Mage-b4 is specifically expressed in fetal and adult gonads. An antibody specific to Mage-b4 was developed, and using this antibody, we found that the Mage-b4 protein was confined to the cytoplasm of germ cells. Double-labeling experiments using antibodies against the meiosis-specific SCP3 protein and the Mage-b4 protein showed that Mage-b4 is down-regulated as the germ cells enter meiosis in adult testis. In contrast, Mage-b4 was expressed in female germ cells throughout meiosis, and the protein was also found in dormant primary oocytes.