YAP1::MAML2, YAP1::NUTM1, and RNF13::PAK2 rearrangements in trichoblastomas and adnexal tumors with panfollicular differentiation: expanding the spectrum of YAP1/PAK-fused skin adnexal tumors

In 2019, YAP1::MAML2 and YAP1::NUTM1 rearrangements were demonstrated in the majority of poromas and in porocarcinomas. Recently, our group demonstrated recurrent fusions of PAK (p21 (RAC1) activated kinase) 1/2/3 genes in a subset of poromas and porocarcinomas frequently harboring hair follicle and sebaceous differentiation. To expand the spectrum of YAP1/PAK-fused tumors, we report six adnexal neoplasms with follicular differentiation with in frame YAP1::MAML2, YAP1::NUTM1, or RNF13::PAK2 fusion transcripts. Four cases of trichoblastoma and two neoplasms with panfollicular differentiation were included. Median age was 66 (range 39–76). Two patients were female. Tumors were located on the head (n = 4), chest (n = 1), or leg (n = 1). Microscopically, tumors were located in the dermis (n = 4) and/or subcutaneous tissue (n = 5), and showed macro (n = 6), micronodular (n = 5), and cystic (n = 4) structures. A delicate fibrillar stroma was present in 5 cases. Infundibulocystic structures, cell balls, and sebocytes were observed in 5, 1, and 3 cases, respectively. Immunohistochemistry revealed BerEP4 expression and Merkel cell hyperplasia in 5 and 2 cases, respectively. YAP1 (C-terminal) loss was observed in 5 cases. Accordingly YAP1::MAML2 (n = 2), YAP1::NUTM1 (n = 1), or RNF13::PAK2 (n = 1) inframe fusion transcripts were detected in the four trichoblastomas. YAP1::MAML2 fusions were also detected in the two tumors with panfollicular differentiation. In conclusion, we report six cases of follicular tumors with YAP1::MAML2, YAP1::NUTM1, or RNF13::PAK2 fusions and therefore suggest that in addition to poroid tumors, YAP1 and PAK2 fusions might be the oncogenic driver in a subset of adnexal tumors with prominent follicular differentiation.