To describe two cases of intra-articular nodular fasciitis (NF) which developed within the knee joint and were associated with the expression of the MYH9-USP6 gene fusion.
Despite overlap in histological and immunohistochemical features between pseudosarcomatous myofibroblastic proliferation and nodular fasciitis, these tumours lack the recently recognised USP6 rearrangements that occur in nodular fasciitis, as well as alternative fusions found in ALK-negative inflammatory myofibroblastic tumours.
Previously, it was observed that USP6 is sufficient to drive formation of tumors mimicking ABC and nodular fasciitis, and that it functions through JAK1/STAT3 signaling.
Ectopic expression of USP6 is known to drive formation of tumors, which recapitulate key features of ABC and nodular fasciitis; however, the identity of USP6's relevant substrates has been obscure.