Primary cutaneous epithelioid mesenchymal neoplasm with ACTB-GLI1 fusion: a case report.

Mesenchymal tumors harboring GLI1 gene abnormalities are a rare but distinctive group of neoplasms whose clinicopathologic features are currently evolving. In particular, examples of this tumor with ACTB-GLI1 gene fusion, tentatively termed ACTB-GLI1 epithelioid mesenchymal neoplasm (EMN), show a distinctive monomorphic round-to-epithelioid morphology, nested to trabecular pattern of growth, and S100+/SOX10-/SMA- immunophenotype. We report the first case of this entity arising exclusively in the skin. A 69-year-old man with no prior history of neoplasia presented with a 1.5-cm raised lesion on the left buttock. Histopathologic examination revealed a diffuse dermal proliferation of small, monomorphic, round-to-ovoid cells with hyperchromatic nuclei, focally enlarged nucleoli, and minimal eosinophilic to clear-staining cytoplasm. These cells were arranged in confluent nests and trabeculae in a background of fibrocollagenous to focally myxoid stroma. Immunohistochemical analysis revealed strong positivity for S100 and CD56, and negativity for SOX-10, SMA, Melan-A, HMB-45, and a variety of other markers. Based on the morphology and immunophenotype, molecular studies were performed, which revealed the presence of an ACTB-GLI1 fusion transcript, confirming the diagnosis. Given the morphologic overlap of this tumor with other cutaneous round cell neoplasms and its potential for malignant behavior, ACTB-GLI1 EMN is an important entity for pathologists to recognize.