FOS and FOSB Immunohistochemistry and Molecular Testing Reveal Novel Patterns in Proliferative Fasciitis and Myositis, FOSB Rearrangement in Proliferative Funiculitis, and FOSL1 :: RELA Fusion in Pediatric Proliferative Fasciitis.

作者：Sanhong Yu, F. Cordier, Esther Baranov, I. Odintsov, L. Magnusson, J. Van der Meulen, Siebe Loontiens, L. Ferdinande, J. Van Dorpe, F. Haglund de Flon, Fredrik Mertens, C. Fletcher, D. Creytens, David J. Papke · 发表于：American Journal of Surgical Pathology · 年份：2026 · DOI：10.1097/pas.0000000000002540 · 研究领域：Medicine

Proliferative fasciitis (PFA) and proliferative myositis (PM) are related benign neoplasms that show large ganglion-like neoplastic cells and harbor recurrent FOS fusions. Proliferative funiculitis (PFU) is a benign mesenchymal lesion of the spermatic cord, with small ganglion-like cells and unknown etiology. Ischemic fasciitis (IFA) is a possibly reactive lesion that shows cytomorphologic overlap with PFA/PM. Here, we assessed FOS and FOSB immunohistochemistry (IHC) in PFA (adult and pediatric), PM, PFU, and IFA. Diffuse nuclear FOS expression was detected in adult PFA (15/26 cases; 58%) and PM (6/10; 60%), while FOSB was consistently negative (19 adult PFA, 5 PM). FOS FISH demonstrated rearrangement in only 4 of 12 tested adult PFA/PM, all of which were FOS-positive by IHC. A FOS fusion ( FOS::TSHZ2 ) was detected in only 1 of 9 adult PFA/PM (11%) tested by RNA sequencing (RNAseq); 5 tumors without detected FOS fusions by RNAseq were FOS-positive by IHC. The pediatric PFA lacked expression of FOS (all 9 tumors) and FOSB (4 tested tumors) by IHC. Compared with FOS-positive PFA/PM, pediatric PFA and adult FOS-negative PFA/PM were more frequently cellular, and pediatric tumors were less infiltrative. DNA sequencing demonstrated FOSL1 :: RELA fusions in 2 of 5 tested pediatric PFAs, both in infants. Six of 18 PFU (33%) expressed FOS and/or FOSB by IHC. FISH demonstrated FOSB rearrangement in 1 of 6 PFU, while all 6 tested PFU were negative for FOS rearrangement. Nine of 20 IFA ...