Molecular Genetic Analysis of Rhabdomyosarcoma and its Pathologic Differential Diagnosis

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Rhabdomyosarcoma (RMS) is a highly malignant soft-tissue sarcoma that usually occurs in childhood. The diagnosis of RMS depends on the identification of skeletal differentiation, however, with conventional histologic techniques it is sometimes difficult to distinguish RMS from other sarcomas. We have studied the immunohistochemical characteristics and molecular analysis in RMS cases.In the results of immunohistochemical studies most of rhabdomyosarcoma cells were positive for desmin and sarcomeric actin, but myoglobin was detected in a few tumor cells. CD99 (MIC2 gene product) was weakly positive in 70% of RMS cases. These results suggest that desmin and sarcomeric actin are more valuable markers for RMS than myoglobin and CD99.Alveolar type RMS is associated with specific translocations, t(2 ; 13) and t(1 ; 13), resulting in the fusion genes PAX3-FKHR and PAX7-FKHR. We detected the PAX3-FKHR or PAX7-FKHR fusion transcripts in all cases of alveolar RMS cases. However, there were no chimeric gene fusions in cases of malignant lymphoma, Ewing's sarcoma/PNET family, malignant fibrous histocytoma and embryonal RMS. One of seven pleomorphic RMSs which were histologically composed of pleomorphic spindle cells and tumor giant cells without the presence of alveolar pattern, showed the chimeric PAX3-FKHR gene. This evidence suggests a specific similarity in the mechanism of tumorgenesis between tumors classified as alveolar and pleomorphic RMS.