Primary Cutaneous SWI/SNF-Deficient Carcinomas: Morphologic, Immunohistochemical, and Molecular Analysis of Seven Cases.

Mutations in genes encoding proteins of the SWI/SNF complex are highly recurrent in select tumor entities. Among primary cutaneous tumors, only 4 SMARCA4- deficient undifferentiated neoplasms and 3 primary cutaneous SMARCB 1 -deficient carcinomas have been documented. In this report, we describe the morphologic, immunohistochemical, genomic, and methylation profile features of 5 additional SMARCA4 -deficient undifferentiated neoplasms and 2 SMARCB1 -deficient carcinomas of the skin. The patients (6M;1F) had a median age of 82 years (range: 60 to 94). Morphologically, all lesions showed poorly differentiated, dermal-based neoplasms, 5 of which were associated with subcutaneous involvement. The tumor cells were organized into nests, strands, and solid sheets. These cells displayed moderate to abundant cytoplasm, large, round vesicular nuclei, and prominent nucleoli. Immunohistochemical analysis revealed expression of cytokeratin AE1/AE3 in all cases, along with loss of SMARCA4 expression in 5 cases. Loss of SMARCB1 expression was identified in 2 cases and was mutually exclusive with SMARCA4 alterations. DNA sequencing revealed a high tumor mutation burden and a prominent UV signature in 4 of the 5 analyzed cases. Methylation analysis, in which tumors were compared with a control group of 68 SNF/SWI-deficient neoplasms and 18 cutaneous squamous cell carcinomas, revealed that primary cutaneous SMARCA4 -deficient and SMARCB1- deficient neoplasms, along with SMARCA4 -deficient carcinomas of other organs, constitute a unique group of neoplasms, distinct from other analyzed tumor entities. These results support the theory that these primary cutaneous SWI/SNF-deficient tumors represent a distinctive group of morphologically undifferentiated cutaneous carcinoma.