Intraarticular nodular fasciitis-detection of USP6 gene fusions in three cases by targeted RNA sequencing

Intraarticular nodular fasciitis arising in the joint synovium is an uncommon lesion. Most cases have been reported in the knee and rarely in other joints. A USP6 gene fusion has so far been documented in only four cases of intraarticular nodular fasciitis, three were located in the knee and one in the proximal interphalangeal joint. In all three cases located in the knee, MYH9 was detected as a USP6 fusion partner. We analysed three cases of intraarticular nodular fasciitis for the presence of USP6 fusion by targeted RNA sequencing. Two cases were located in the hip (a 25-year-old female and 48-year-old male) and one in the shoulder (a 38-year-old male). We detected a MYH9-USP6 fusion in the two hip cases and a COL1A1-USP6 fusion in the shoulder case. Our findings provide additional evidence that intraarticular nodular fasciitis is a form of nodular fasciitis arising in the joint synovium, harbouring a USP6 fusion. Although a MYH9-USP6 fusion seems to predominate in intraarticular nodular fasciitis, other fusion partners of the USP6 gene may also be involved. Detection of a USP6 fusion by targeted RNA sequencing may assist in confirming the diagnosis in selected cases.