Aims: USP6 rearrangement underpins self-limiting fibroblastic/myofibroblastic neoplasms, including nodular fasciitis (NF), myositis ossificans (MO), aneurysmal bone cyst (ABC), and related variants. The aim of this study was to characterise UPS6 and fusion partners in order to delineate the clinicopathological, genetic and bone-forming features in such lesions of soft tissue (ST). Methods and results: Break-apart fluorescence in-situ hybridisation (FISH) validated USP6 rearrangement in 31 of 35 NF [comprising three of three fasciitis ossificans (FO) cases, seven of eight cellular variant of fibroma of tendon sheath (C-FTS), four of six MO, three of three ST-ABC, and two of two fibro-osseous pseudotumours of digits (FOPD)]. As determined with FISH and reverse transcription polymerase chain reaction, MYH9–USP6 was the commonest fusion in four C-FTS and 20 NF, including one intravascular case and two infantile (one retroperitoneal) cases. The presence of MYH9–USP6 confirmed the diagnosis of two NFs> 50 mm with prominent ischaemic necrosis. COL1A1–USP6 was predominant in ossifying lesions, including all FO, MO, ST-ABC and FOPD with identified partner genes, and was also present in non-ossifying head and neck NF (HN-NF) and C-FTS in two cases each. A cervical NF of a 14-month-old girl harboured the novel COL1A2–USP6. Ossifying lesions showed considerable genetic and morphological overlaps. Sharing COL1A1–USP6, FO and FOPD showed similar central or haphazard bone matrix deposition. Besides zonation of outward bone maturation, four COL1A1–USP6-positive MO had incipient to sieve-like pseudocysts reminiscent of ST-ABC. Conclusion: MYH9–USP6 is present in some C-FTS and most NF, including rare variants, but is unrelated to bone formation. All bone-forming USP6-rearranged lesions adopt COL1A1 as the 5′ partner, indicating close genetic kinships. However, COL1A1/COL1A2 also contributes to the pathogenesis of minor subsets of non-ossifying USP6-rearranged HN-NF and C-FTS.
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