EWS FLI-1 FUSION TRANSCRIPT DETECTION AND MIC2 IMMUNOHISTOCHEMICAL STAINING IN THE DIAGNOSIS OF EWINGS-SARCOMA/

Ewing's sarcoma (ES) and other primitive peripheral neuroectodermal tu mors (pPNETs) can present a significant diagnostic problem, as they ma y morphologically resemble other small round cell tumors (SRCTs) of ch ildhood. However, ES/pPNET is known to carry a characteristic t(11;22) (q24;q12), the detection of which may aid diagnosis. The recent identi fication of the EWS and FLI-1 genes flanking the translocation break p oint has enabled reverse transcriptase-polymerase chain reaction (RT-P CR) to be used to detect the putative chimeric transcription factor mR NA produced by the fusion gene. We have assessed the RT-PCR method of detection by examining 40 cases of ES for the presence of EWS/FLI-1 tr anscripts. Twenty-six (76%) of the 34 cases with intact mRNA yielded f usion transcripts. Four different transcript sizes were detected and t wo tumors contained two transcripts of different size. No transcripts wee detected in a control group of non-ES/pPNET SRCTs. Eight cases wit h intact mRNA were transcript negative. The MIC2 cell surface antigen, which is reported to be present in over 95% of ES/pPNETs, was present in 32 of 33 tumors (97%), including all 24 EWS/FLI-1 transcript-posit ive cases examined. Hence MIC2 is a useful screen for ES, with RT-PRC detection of t(11;22) being the optimal method for confirming the diag nosis.