YAP1::MAML2 Fusion in Adnexal Tumors Combining Poroma and Sebaceous Components: Report of Two Cases

Poroma is a benign sweat gland tumor harboring YAP1::MAML2 and YAP1::NUTM1 fusion transcripts in most of the cases. Recently, YAP1 fusions have also been detected in trichoblastomas and in 1 sebaceoma. To expand the spectrum of YAP1-fused adnexal tumors, we report herein 2 cases of adnexal tumors with YAP1::MAML2 fusion transcript combining a poroma and a sebaceous component. These 10- and 7-mm diameter neoplasms arose in a 55-year-old man and in a 90-year-old woman and were located on the back and the right breast, respectively. Microscopically, both lesions harbored a biphasic appearance resulted from a superficial poroma component associated with relatively well-demarcated areas of sebaceous differentiation located either in the epidermal (case #1) or in the intradermal part of the lesions (case #2). Immunohistochemical investigation of the cases revealed diffuse positivity of the androgen receptor in the sebaceous areas together with loss of YAP1 in both poroid and sebaceous parts. Accordingly, molecular analysis of the cases revealed the presence of an in-frame YAP1::MAML2 fusion transcript. To conclude, our cases confirmed that YAP1 fusions are the oncogenic drivers of a subset of adnexal tumors that may harbor poroid, follicular but also sebaceous differentiation.