Expression of MAGE-A protein, a family of cancer/testis antigens, was investigated in normal and neoplastic canine tissues. Immunohistochemical analysis of cross-reactions between a mouse anti-human MAGE-A proteins including MAGE-A1, -A2, -A3, -A4, -A6, -A10, and -A12 monoclonal antibody and canine proteins, showed positive immunoreactivity only in testicular spermatogonia and spermatocytes, and ovary oocytes. The immunoreaction was negative in all other tissues tested, including normal tissues of the skin, gingiva, muscle, adipose, connective, salivary gland, lymph node, intestinal mucosa, mammary gland, liver, cartilage, oviduct, endometrium, cerebrum and cerebellum. Use of a scoring system in the investigated tumors showed positive immunoreactivity in 75% (21/28) of melanomas including oral, cutaneous, eyelid, and interdigital melanomas; in 68.7% (22/32) of oral and nasal tumors; in 52.5% (21/40) discrete round cell tumors; and in 40.5% (15/37) of soft tissue sarcomas. Different tumor types also showed large difference in percentage of MAGE-A expression. Although oral squamous cell carcinomas, multicentric lymphomas and extraosseous osteosarcomas showed no expression, overexpression occurred in oral melanomas (81.82%, 18/21), malignant nasal tumors (100%, 3/3) and in transmissible venereal tumors (100%, 10/10). Based on the characteristic expression of MAGE-A in canine germ cells and in various neoplasms, MAGE-A has potential use as an indicator of malignancy but is probably unsuitable for strictly diagnostic purposes (i.e., diagnosis of tumor type)
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