Melan A (A103) is not a marker of mesothelioma

Although a large number of immunohistochemical markers have been proven to be valuable in the differential diagnosis between epithelioid mesotheliomas and metastatic carcinomas involving the serosal membrane, no single antibody has been found that is absolutely sensitive and/or specific in making this distinction. A recent study reported melan A positivity in all 12 of the epithelioid mesotheliomas stained with a melan A antibody (clone A103). To fully determine the practical value of this antibody for assisting in the differential diagnosis of mesotheliomas, we investigated the expression of melan A (A103) in 40 mesotheliomas (27 epithelioid, 6 sarcomatoid, and 7 biphasic), 10 lung adenocarcinomas, and 10 serous carcinomas of the ovary. None of the mesotheliomas, lung adenocarcinomas, or serous carcinomas of the ovary were melan A (A103) positive. Similar staining results were observed in the 20 mesotheliomas immunostained in another institution using the same antibody clone from a different commercial source. On the basis of these results, it is concluded that in contrast to the initial report, melan A (A103) is not expressed in mesotheliomas and therefore, immunostaining with this antibody has no utility in the diagnosis of mesothelioma. The possible cause of the discrepancies between the results obtained in the present investigation and those of the initial study is discussed.