Staining pattern of specific and cross‐reacting Melan‐A antibodies: A comparative study on 15,840 samples from 133 human tumor types

The Melan‐A (melanocyte antigen) protein, also termed ‘melanoma antigen recognized by T cells 1’ (MART‐1) is a protein with unknown function whose expression is specific for the melanocyte lineage. Antibodies against Melan‐A are thus used for identifying melanocytic tumors, but some Melan‐A antibodies show an additional – diagnostically useful – cross‐reactivity against an unspecified protein involved in corticosteroid hormone synthesis. To comprehensively compare the staining patterns of a specific and a cross‐reactive Melan‐A antibody in normal and neoplastic tissues, tissue microarrays containing 15,840 samples from 133 different tumor types and subtypes as well as 608 samples of 76 different normal tissue types were analyzed by immunohistochemistry. For the Melan‐A‐specific antibody ‘Melan‐A specific’ (MSVA‐900M), Melan‐A positivity was seen in 96.0% of 25 benign nevi, 93.0% of 40 primary and 86.7% of 75 metastatic melanomas, 82.4% of 85 renal angiomyolipomas as well as 96.4% of 84 neurofibromas, 2.2% of 46 granular cell tumors, 1.0% of 104 schwannomas, and 1.1% of 87 leiomyosarcomas. The cross‐reactive antibody ‘Melan‐A+' (MSVA‐901M+) stained 98.1% of the tumors stained by ‘Melan‐A specific’. In addition, high positivity rates were seen in sex‐cord‐stroma tumors of the ovary (35.3%–100%) and the testis (86.7%) as well as for adrenocortical neoplasms (76.3%–83.0%). Only nine further tumor groups showed Melan‐A+ staining, including five different categories of urothelial carcinomas. Our data provide a comprehensive overview on the staining patterns of specific and cross‐reactive Melan‐A antibodies. The data demonstrate that both antibodies are highly useful for their specific purpose. It is important for pathologists to distinguish these two Melan‐A antibody subtypes for their daily work.