BAP-1 Expression Status by Immunohistochemistry in Cellular Blue Nevus and Blue Nevus-like Melanoma

The family of blue nevi includes the common blue nevus (BN), cellular blue nevus (CBN), and atypical BN, while melanomas with BN-like morphology can either arise in association with a blue nevus (MABN) or in the de novo setting mimicking cellular blue nevus (MMCBN). Recent molecular and immunohistochemical studies have demonstrated loss of BAP-1 in MABN/MMCBN but not in BN/CBN, suggesting that loss of BAP-1 correlates with a malignant phenotype in these lesions. In this study, we applied anti-BAP-1 antibodies to a series of CBN/BN (n = 11) and MABN/MMCBN (n = 4). Nuclear BAP-1 expression was detected in the majority of CBN/BN (n = 10/11) but was lost in 1 case. Most cases of MABN/MMCBN showed loss of nuclear BAP-1 expression (n = 3/4), with one case of MMCBN showing preserved BAP-1 expression. Demonstration of BAP-1 loss in a single case of CBN and preservation of BAP-1 expression in 1 case of MMCBN may indicate that detection of alterations in BAP-1 protein expression by immunohistochemistry may not be a completely reliable biomarker for the distinction of BN/CBN from MABN/MMCBN. Further investigation of the significance of BAP-1 loss/preservation in BN-like tumors is warranted.