Molecular characterization of an EWSR1–POU5F1 fusion associated with a t(6;22) in an undifferentiated soft tissue sarcoma

We report a soft tissue sarcoma from the thigh with morphologic features resembling Ewing sarcoma, clear cell sarcoma, and myoepithelial tumor of soft tissue. In addition, the genetic and immunohistochemical findings do not correspond to any established pattern, so the tumor does not clearly fit into any one classification. The karyotype analysis revealed a rare chromosomal rearrangement, t(6;22)(p22;q12), that previously has been reported in bone and epithelial tumors. Molecular studies confirmed the presence of an EWSR1–POU5F1 fusion creating a chimeric gene with the N-terminal transcriptional activation domain of EWSR1 and the C-terminal POU DNA binding domain of POU5F1. This report is novel in that to our knowledge, it is the first complete molecular characterization of an EWSR1–POU5F1 fusion in a soft tissue sarcoma. Evaluation of existing data on the known EWSR1–POU5F1 tumors suggests that the fusion gene functions in a wide variety of cell types and may modify the differentiation state of cells, resulting in susceptibility to tumorigenesis.