Complex rearrangement of chromosomes 19, 21, and 22 in Ewing sarcoma involving a novel reciprocal inversion–insertion mechanism of EWS–ERG fusion gene formation: a case analysis and literature review

EWS–ERG Ewing sarcoma (ES) gene fusions often result from complex chromosomal rearrangements. We report an unusually aggressive case of ES with an EWS–ERG fusion gene that appeared to be a result of a simple balanced and reciprocal translocation, t(19;22)(q13.2;q12.2). Subsequent molecular investigation of the primary tumor, the metastasis, and a cell line generated from this ES permitted reconstruction of each genomic step in the evolution of this complex EWS–ERG fusion. We elucidated a new mechanism of reciprocal insertion inversion between chromosome 21 and 22, involving cryptic alterations to both the ERG and EWS genes. Molecular cytogenetic investigation, using systematic analysis with locus-specific probes, identified the cognate genomic breakpoints within chromosome 21 and 22, mandatory for the excision and exchange of both 3′ERG and 3′EWS, resulting in the formation of the EWS–ERG fusion gene present on the der(22). Array comparative genomic hybridization and fluorescence in situ hybridization studies of the ES cell line derived from this tumor identified additional acquired chromosomal and genomic abnormalities, likely associated with establishment and adaptation to in vitro growth. Notably, the cell line had lost one copy of the RB1 gene within the 13q13.1∼q14.2 region, and also had a near-tetraploid karyotype. The significance of these findings and their relationship to other reports of variant and complex ES translocations involving the ERG gene are reviewed.