Status of topoisomerases in pediatric high grade osteosarcomas and their therapeutic and prognostic role.

10030 Background: High grade osteosarcomas are the most common malignant bone tumors at adolescence. In the French group, they were treated until 2005 with OS94 guidelines, using a first line therapy of combined high dose-methotrexate and VP16- ifosfamide courses. In the poor response to these drugs, a second line therapy was administered associating doxorubicin and cisplatine. Both therapeutic strategies were based on topisomerase IIα inhibitor. Therefore, a retrospective and prospective molecular study of topoisomerase genes (TOP2A, TOP2B and TOP1) was conducted on diagnostic tumors consecutively collected from this population to understand their therapeutic and prognostic role.,91 patients were included. Paired normal and tumor biopsy tissues were analyzed by allelotyping, using microsatellites flanking on both sides each locus containing TOP1, TOP2A and TOP2B, coding for topoisomerase genes. These loci are respectively 20q12, 17q21 and 3p24. In addition to these genomic data, quantitative real-time PCR (QPCR) was added to target specifically each gene comparatively to two reference genes (DCK and APP). As frequently co- expressed with TOP2A at 17q21 locus in breast cancer, ERBB2 was also studied by QPCR.,Only allelic imbalance were showed by allelotyping but these loci seem to be frequently rearranged : 20q12 is abnormal in 44.4%, 17q21 in 55.3% and 3p24 in 67.5%. QPCR results were concordant with these DNA results, showing mainly gene deletions (22% for TOP1, 34% for TOP2A and 50.5% for TOP2B) and few amplifications (respectively, 14%, 11.5% and 11.5%). No amplification, nor deletion of ERBB2 was detected with QPCR. In a prognostic and therapeutic point of view, amplifications of TOP2A and TOP1 were significantly associated with the group of good responders to chemotherapy (p=0.001 and p=0.064, respectively). Furthermore, a significant prognostic correlation was found between an abnormal TOP2A gene and the overall survival (p=0.045).,TOP2A seems to appear at diagnosis as a therapeutic and prognostic marker involved frequently in pediatric high grade osteosarcomas. No significant financial relationships to disclose.