S100 protein expression in PKC-fused blue naevi, cellular blue naevi and PRKAR1A-inactivated pigmented epithelioid melanocytomas.

Recent data have redefined the genetic spectrum of pigmented epithelioid melanocytomas (PEMs). PEM is now defined by a secondary genetic event, a protein kinase cAMP-dependent type I regulatory subunit alpha (PRKAR1A) inactivation, that confers the specific cytomorphology of the entity, but this event can arise within a naevus with a genetic background of common, blue or Spitz type. PKC-fused melanocytic proliferations, although they can exhibit PEM-like morphological features, have now been regrouped within the blue group of tumours. Past studies have shown that the latter group tends to lose S100 expression. Herein, we studied the nuclear expression of S100 by immunohistochemistry in 73 PKC-fused benign blue naevi. Histologically, the most frequent pattern found in PKC-fused blue naevi (51%) was a dermal biphasic architecture associated with a horizontal band of medium-sized bland melanocytes in the upper papillary dermis, with a deeper dermal expansion combining spindled and dendritic melanocytes with occasional small nests of bland melanocytes within a fibrous background. A PEM-like hyperpigmented hyperplasia of the epidermis was seen in 32% of cases. The immunohistochemical study found in 31 of the 37 (84%) dermal biphasic PKC-fused melanocytic tumour cases a significant loss of nuclear expression of S100 (in more than 50% of cells) in the superficial horizontal dermal band area and in 68% of the biphasic dermal component. However, the hyperpigmented PEM-like junctional components were not assessable by immunohistochemistry. An exploratory analysis of S100 expression in 21 blue naevi and in 25 PEM with confirmed PRKAR1A inactivation was also performed. In blue naevi, a loss of nuclear S100 expression in more than 50% of melanocytes was found in over 70% of these lesions both in the dendritic and epithelioid dermal components. By contrast, nuclear expression of S100 was most often preserved in PEM with PRKAR1A inactivation (85% preservation in the epithelioid component). These results suggest that searching for S100 expression loss by immunohistochemistry may be helpful in the diagnosis of PKC-fused blue naevus similarly as in dendritic and cellular blue naevi. This simple test, especially if a band-like structure is present in the upper dermis, can effectively support this diagnosis, as a genetic confirmation of these benign tumours is not warranted.