Abstract

NUT carcinoma, molecularly defined by NUTM1 gene rearrangement with fusion partners including BRD3, BRD4 and NSD3, are most commonly reported in young adults in the sinonasal tract, nasopharynx, and thorax. At these sites, NUT carcinoma is an extremely aggressive malignancy with dismal prognosis. Recently, three cases of primary cutaneous NUT adnexal carcinoma have been reported with BRD3 and NSD3 fusion partners. Although NUT adnexal carcinomas are shown to have metastatic potential, they may behave less aggressively than extracutaneous NUT carcinomas with BRD4::NUTM1 rearrangement. We report a case of a 59-year-old man who underwent a biopsy of a 3cm fungating plantar mass, which showed BRD4::NUTM1 fusion. The tumor is a poorly-differentiated dermal neoplasm arranged in nodules and poorly-formed glands. Immunohistochemically, tumor cells show positivity for keratins, EMA, and SOX10, with patchy smooth muscle actin, which initially led to a diagnosis of myoepithelial carcinoma. However, molecular testing revealed BRD4::NUTM1 rearrangement, which was confirmed by positive NUT immunoreactivity. With no alternative primary identified by imaging, and SOX10 and cytokeratin co-immunoreactivity previously reported in NUT carcinoma, a diagnosis of primary cutaneous NUT carcinoma was favored. To our knowledge, this is the first reported case of primary cutaneous NUT carcinoma with BRD4 fusion, which may have prognostic implications. We hope to contribute to the limited body of knowledge on this entity, with emphasis on recognition as well as studying and defining its prognostic differences from extracutaneous NUT carcinomas.