Diagnostic Utility of USP6 RNA Chromogenic In Situ Hybridization in the Differential Diagnosis of Nodular Fasciitis and Aneurysmal Bone Cyst.

Recurrent USP6 gene rearrangements underlie a subset of mesenchymal neoplasms, including nodular fasciitis and aneurysmal bone cyst. These rearrangements place USP6 under the regulatory control of another gene's constitutively active promoter, resulting in USP6 overexpression. Recently, it has been demonstrated in a limited number of cases that detection of USP6 overexpression by RNA chromogenic in situ hybridization (CISH) can serve as a surrogate marker for USP6-rearranged tumors. To further assess the diagnostic utility of USP6 RNA CISH, we evaluated its application in 1210 bone and soft tissue tumors, using a combination of whole tissue sections and tissue microarrays. USP6 overexpression, defined as RNA CISH staining in >1% of lesional cells, was observed in 92% (36/39) of nodular fasciitis cases and in 85% (17/20) of primary aneurysmal bone cysts. All examples of nodular fasciitis (15/15) and primary aneurysmal bone cyst (8/8) with USP6 fusion confirmed by molecular genetic testing were positive for overexpression. Other tumors staining positive for USP6 included cellular fibroma of tendon sheath (3/3, 100%) and myositis ossificans (3/3, 100%). Although USP6 RNA CISH proved to be highly specific for USP6-rearranged lesions when assessed in 1145 negative control cases reflecting 53 other tumor types, positivity was rarely seen in isolated cases of angiosarcoma (1/16, 6%), malignant peripheral nerve sheath tumor (1/80, 1%), and carcinosarcoma (1/7, 14%). Taken together, these findings indicate that USP6 RNA CISH represents a useful diagnostic tool; careful interpretation remains critical to its successful implementation, given that isolated examples of non-USP6-rearranged tumors rarely stain positive.