Primary cutaneous non neuroendocrine small cell carcinoma POU2F3 subtype: morphologic, immunohistochemical, transcriptomic and methylation analysis of two cases.

Up to 10% of small cell lung carcinomas do not express any neuroendocrine markers and are characterized by an expression of the transcription factor POU2F3. Recently, few cases of poorly differentiated carcinomas harboring POU2F3 expression have been described in the breast, cervix, and bladder. Herein, we report the morphological, immunohistochemical, and molecular characterization of two primary cutaneous POU2F3-expressing small cell carcinomas. The cases arose in the temple and on the occipital region in a 92-year-old woman and a 78-year-old man respectively. Microscopic examination revealed in both cases a large and ulcerated poorly differentiated neoplasm infiltrating the full thickness of the dermis with extension into the subcutaneous tissues in case #1. The tumors harbored solid and trabecular growth patterns and were composed of poorly differentiated highly mitotic cells. Immunohistochemical investigation revealed diffuse pancytokeratin positivity while no expression of cytokeratin 20, chromogranin A, synaptophysin, CD56, or INSM1 was detected. Diffuse nuclear expression of POU2F3 was observed. Transcriptomic analysis revealed a SBS7 mutation signature related to UV-induced DNA damages in one case and evidenced an expression profile similar to the one observed in POU2F3-expressing small cell lung cancers in both. Methylation analysis confirmed the proximity of the two cases with other skin cancers. To conclude, we report herein the first description of primary cutaneous small cell carcinoma POU2F3 subtype.