In contrast to the EWS/FLI1 fusion which is represented by a t(11;22)(q24;q12), EWS/ERG fusions are frequently cytogenetically not detectable. Three Ewing tumors (ET), two with apparently normal chromosomes 21 and 22, and one ET with a t(2;22)(p25;q12), were studied by FISH on interphase nuclei, metaphase chromosomes and on DNA fibers. EWS/ERG transcripts were detected by RT-PCR in all cases. FISH, using cosmids located proximally (F10, G9) and distally (F7) to the EWS breakpoint region, revealed no detectable separation of these probes in two cases. In contrast, co-hybridization of probe PT1526 containing the ERG breakpoint region with G9 revealed the juxtaposition of two signals per interphase nucleus in all three cases indicating the EWS/ERG fusions. Chromosome preparations displayed the juxtaposed signals on the der(22), and hybridization signals of the probes PT1526 and G9 on the non-rearranged chromosomes 21 and 22 in all cases, respectively. The PT1526 signal on the der(21) was seen only in cases 1 and 2. These results were confirmed by triple-target FISH on tumor DNA fibers. In all three cases, the hybridization pattern F10 - G9 - PT1526 indicates a centromere to telomere orientation. This finding suggests that EWS/ERG fusions in ETs may be generated by an inversion of the ERG gene or a part thereof followed by an insertion into the EWS gene on the der(22). Double-target FISH on interphase nuclei using probes flanking the EWS breakpoint region and probe PT1526 enables the detection of virtually all 22q12 rearrangements in ETs, thus providing a reliable diagnostic assay.
Copyright 2001 S. Karger AG, Basel