Histopathologic and molecular characterization of BAP-1-inactivated melanoma

BRCA1-associated-protein-1 inactivated melanocytic tumours: characterisation of the clinicopathological spectrum and immunohistochemical expression pattern of preferentially expressed antigen in melanoma

AIMS

BRCA1-associaed protein-1 (BAP1) inactivated tumours (BIMT) are rare melanocytic tumours that may be mistaken for Spitz tumours or melanoma. They occur sporadically or in association with the BAP1 tumour predisposition syndrome (BAP1-TPDS), which may be complicated by uveal or cutaneous melanoma, mesothelioma, basal cell carcinoma and renal cell carcinoma. The aim of this study was to characterise the clinicopathological features and the immunohistochemical expression pattern of preferentially expressed antigen in melanoma (PRAME) of BIMT in a large patient cohort.

METHODS AND RESULTS

Ethical approval was obtained, haematoxylin and eosin-stained slides were reviewed, PRAME immunohistochemistry was performed and clinical follow-up was obtained from patient records. Sixty-five BIMT from 38 patients (F:M = 4.4:1) were identified. BIMT were typically located on the trunk and head and neck (median size = 0.5 cm). Seven patients with BAP1-TPDS (range = 16-66 years, median = 25) had multiple BIMT (median = 5), while sporadic BIMT were solitary (median patient age = 39 years). One of seven patients with BAP1-TPDS developed additional malignancies (mesothelioma and cutaneous spindle cell melanoma) and died of complications of mesothelioma. All other patients are alive without recurrence of BIMT (median follow-up = 42 months). BIMT presented as intradermal, nodular aggregates of epithelioid melanocytes with low mitotic activity and moderate to severe cytological atypia in 63% of cases. A background conventional naevus was present in 64%. PRAME immunohistochemistry showed negative or weakly patchy positive staining in all BIMT.

CONCLUSIONS

BIMT are more common in a sporadic setting and behave indolently, despite worrying cytological atypia. PRAME immunohistochemistry is a reassuring tool in distinguishing BIMT from melanoma.

Beyond typical histology of BAP1-inactivated melanocytoma

BAP1-inactivated melanocytoma (BIM) is a novel subgroup of melanocytic neoplasm listed in the 5th edition of WHO classification of skin tumor. BIM is characterized by two molecular alterations, including a mitogenic driver mutation (usually BRAF gene) and the loss of function of BAP1, a tumor suppressor gene located on chromosome 3p21, which encodes for BRCA1-associated protein (BAP1). The latter represents a nuclear-localized deubiquitinase involved in several cellular processes including cell cycle regulation, chromatin remodeling, DNA damage response, differentiation, senescence and cell death. BIMs are histologically characterized by a population of large epithelioid melanocytes with well-demarcated cytoplasmic borders and copious eosinophilic cytoplasm, demonstrating loss of BAP1 nuclear expression by immunohistochemistry. Recently, we have published a series of 50 cases, extending the morphological spectrum of the neoplasm and highlighting some new microscopic features. In the current article, we focus on some new histological features, attempting to explain and link them to certain mechanisms of tumor development, including senescence, endoreplication, endocycling, asymmetric cytokinesis, entosis and others. In light of the morphological and molecular findings observed in BIM, we postulated that this entity unmasks a fine mechanism of tumor in which both clonal/stochastic and hierarchical model can be unified.