Recurrent and novel USP6 fusions in cranial fasciitis identified by targeted RNA sequencing.

Cranial fasciitis is a benign myofibroproliferative lesion of the scalp and underlying bones typically occurring in the pediatric population. Histologically, it is characterized by loose fascicles of stellate cells in a fibromyxoid background, findings similar to those described in the closely related variant nodular fasciitis. Previously characterized as a reactive process, the identification of USP6 translocations in over 90% of nodular fasciitis cases prompted their reclassification as a clonal neoplastic process. Unlike nodular fasciitis, the molecular underpinnings of cranial fasciitis are less clear. While a subset of cranial fasciitis has been associated with Wnt/β-catenin pathway dysregulation, recent case reports suggest that this entity may also harbor USP6 fusions, a finding we sought to further investigate. We identified fifteen archival cases of cranial fasciitis, five females and ten males ranging in age from 3 months to 9 years (median 11 months), composed of formalin-fixed paraffin-embedded and fresh frozen tissues (11 and 4 cases respectively). Samples were evaluated on an RNA-based targeted sequencing panel targeting genes recurrently rearranged in neoplasia, including USP6. Five of fifteen cases (33%) were positive for USP6 rearrangements predicted to result in the fusion of the entire USP6 coding region to the promoter of the 5' partner, (three of which were novel):  two SERPINH1-USP6 (novel) and one each of COL3A1-USP6 (novel), SPARC-USP6, and MYH9-USP6. These results demonstrate the recurrent nature of USP6 rearrangements in cranial fasciitis, and highlight the success of targeted RNA sequencing in identifying known and novel fusion partners. The identification of USP6 promoter-swapping rearrangements is helpful in understanding the underlying biology of cranial fasciitis, and reinforces its biologic relationship to nodular fasciitis. Targeted RNA sequencing is a helpful tool in diagnosing this pseudosarcomatous lesion.