Detection of EWS/FLI‐1 by Immunostaining. An Adjunctive Tool in Diagnosis of Ewing′s Sarcoma and Primitive Neuroectodermal Tumour on Cytological Samples and Paraffin‐Embedded Archival Material

Purpose. Recently we showed that the 68-kDa fusion protein derived from the EWS/FLI1 hybrid gene can be specifically detected by Western blotting using a polyclonal antibody to the C-terminal of FLI1 on biopsy material from Ewing's sarcoma. The aim of this study was to investigate whether this antibody also could be used for immunocytochemistry and immunohistochemistry in diagnosis of Ewing's sarcoma.Methods. Immunostaining on paraffin-embedded archival material, fine-needle aspirates and tumour touch imprints from Ewing's sarcomas and primitive neuroectodermal tumours (PNET) for detection of the fusion protein was performed. Most cases were also analysed by Western blotting.Tumours of differential diagnostic importance were also included.Results. Eighty per cent (12/15 cases) of the Ewing tumours exhibited a positive immunoreactivity for the FLI1 antibody. The signal was mainly localised in the nuclei of the tumour cells, which seems reasonable since EWS/FLI1 is a transcription factor. The signal was found to be specific since it did not appear when the blocking peptide was added to the antibody solution.Moreover, two other types of small-round cell tumours (i.e. neuroblastoma and alveolar rhabdomyosarcoma) were negative as well as most normal tissues.Discussion. Immunostaining of histological and cytological specimens with the FLI1 antibody can be of diagnostic relevance in Ewing tumours carrying t(11;22).The absence of immunoreactivity in non-Ewing cells is most likely due to a low expression of the wild-type FLI1 protein.