Background: Polymerase chain reaction (PCR) assays that detect fusion genes resulting from consistent chromosomal translocations have been used to detect minimal residual disease, primarily in hematologic malignancies. Molecular assays have been developed recently that detect the PAX3-FKHR or PAX7-FKHR fusion transcript resulting from the t(2; 13) or t(1; 13) translocation consistently observed in alveolar rhabdomyosarcoma. Because of the tumor's propensity to disseminate widely, our aim was to determine whether or not reverse transcriptase-PCR assays could detect submicroscopic disease in bone marrow or peripheral blood specimens.
Methods: We analyzed 19 bone marrow samples from 11 patients with a known gene fusion in their primary tumor. Specimens were collected at diagnosis, remission, and relapse. Fourteen peripheral blood samples were obtained serially from 5 of these patients. A reverse transcriptase-PCR assay was used to detect the presence of the PAX3-FKHR or PAX7-FKHR fusion transcript. These results were compared with the results of microscopic examination of the bone marrow. Medical records of all 11 patients were reviewed.
Results: Adequate amplifiable RNA was obtained in 17 of 19 marrow samples. The PCR assay detected fusion products in all 4 specimens obtained from 3 patients with histologic evidence of bone marrow involvement. In addition, PAX3-FKHR fusion products were detected in bone marrows from 2 patients for whom there was no histologic evidence of disease. The fusion transcripts were not detected in any of the peripheral blood samples.
Conclusions: The detection of submicroscopic disease is possible in alveolar rhabdomyosarcoma patients using PCR. These assays may have a role in staging, monitoring therapy, purging protocols for autologous bone marrow transplantation, and post therapy follow-up. Larger prospective studies are needed to address the clinical relevance of these results.