Evaluation of EWSR1/FUS rearrangements by FISH and NKX2.2 immunoexpression in simple bone cysts of the jaw.

OBJECTIVE:Recent studies have identified recurrent rearrangements of EWSR1 or FUS with NFATC2 in extragnathic simple bone cysts (SBCs). We investigated the presence of EWSR1 or FUS rearrangements and the immunophenotypic expression of NKX2.2 and CD99 in a series of SBCs of the jaw. STUDY DESIGN:We retrieved 10 cases of SBC of the jaw from the University of Pittsburgh archives. Of the 10 cases, we were able to evaluate 8 by immunohistochemistry for CD99 and NKX2.2 and 7 by fluorescence in situ hybridization (FISH) for EWSR1 and FUS rearrangement using EWSR1 and FUS break-apart probes. RESULTS:All 8 cases evaluated by immunohistochemistry expressed CD99 but were negative for NKX2.2, and all 7 cases assayed using FISH were negative for EWSR1 and FUS rearrangements. CONCLUSIONS:In contrast to the SBC of extragnathic sites, we found no presence of EWSR1 and FUS rearrangements by FISH in the SBC of the jaw, suggesting that this entity may be etiologically/molecularly distinct and reflects a non-neoplastic reactive process. However, as these lesions tend to be paucicellular, FISH may not be the appropriate technique for identifying EWSR1/FUS fusions. Other techniques should be used to evaluate them in future studies.