Detection of EWS-FLI-1 fusion in Ewing's sarcoma peripheral primitive neuroectodermal tumor by fluorescence in situ hybridization using formalin-fixed paraffin-embedded tissue

The balanced translocation t(11;22)(q24;q12) is specific for the Ewings sar coma/peripheral primitive neuroectodermal tumors (ES/PNETs) and results in the EWS/FLI-1 fusion transcript, which can be detected by reverse transcrip tion polymerase chain reaction (RT-PCR). Recent studies also have used fluo rescence in situ hybridization (FISH) to show the translocation; however, m ost of these have been performed on cell lines or touch preparations and sh ort-term cultures of tumors. Moreover, the existing probes generally have s hown only the break in the specific chromosomes rather than the translocati on itself. We describe our findings with a new set of probes that localize to 22q12 (EWS) and 11q24 (FLI-1) and directly show the translocation as jux taposed red-green signals on der(22) in nuclei extracted from formalin-fixe d, paraffin-embedded tissues. After establishing the specificity of the pro bes (on metaphase spreads and interphase nuclei in two translocation-positi ve cell lines and normal peripheral blood lymphocytes), we evaluated 11 ES/ PNETs and 10 other tumors (four alveolar rhabdomyosarcomas, three neuroblas tomas, two lymphomas, one extramedullary myeloid tumor) using a two-color F ISH assay. All 11 ES/PNETs showed fusion signals in 20% to 80% of evaluable nuclei. In two lymphoma cases, random overlapping signals were present in 2% and 4% of nuclei, whereas the remaining eight tumors were negative. The presence of t(11;22) was confirmed by RT-PCR in 10 of 11 ES/PNETs. We concl ude that FISH analysis with this newly designed probe pair is a specific an d sensitive method of detecting t(11;22) on routinely processed tissue and can be useful in the differential diagnosis of ES/PNETs from other small ro und blue cell tumors when only fixed tissue is available. HUM PATHOL 30:324 -330. Copyright (C) 1999 by W.B. Saunders Company.