Gains of chromosome 17q common in high-risk neuroblastomas do not involve the topoisomerase IIα gene: FISH and quantitative-PCR (Q-PCR) studies of six neuroblastoma cell lines

2724 Neuroblastoma (NB) is the most common extra cranial solid tumor of childhood. Only 20-40% of children diagnosed with stage 4 disease survive 5 years. Tumors from these patients frequently have non-reciprocal translocations that result gain of genetic material from chromosome 17q21-25. The topoisomerase IIα gene is located in this region of 17q (17q21-22). Since topo IIα inhibitors are commonly used for NB treatment, we asked whether the 17q translocations in NB cells resulted in increased copy number of the topo IIα gene and/or increased levels of topo IIα protein. We also examined whether topo IIα gene copy number correlated with sensitivity to the topo IIα inhibitors VP-16 and doxorubicin. We evaluated six NB cell lines (NB 1691, SK-N-AS, SK-N-SH, SJNB-1, IMR-32, and NB 1643) by fluorescence in situ hybridization (FISH) analysis using a 17q “paint” probe or a BAC probe (RP11-45506) to the terminus of 17q (17q25.3). We then compared FISH results with karyotype analyses to identify specific 17q translocations. Then Q-PCR was done to determine whether the topo IIα gene was amplified in NB cell lines containing gains of 17q. FISH analyses with both probes showed that all six NB cell lines harbored non-reciprocal translocations of 17q. The karyotype analyses showed translocations of 17q to chromosomes 1, 3, 11, 19 and 22. However, although all six NB cell lines contained gains of 17q material, only one NB cell line, SK-N-AS, showed an increased topo IIα gene dose of 2. Further, increased gene copy number did not correspond to an increase in protein expression or in increased sensitivity to VP-16 or doxorubicin. This study illustrates that the topo IIα gene does not participate in 17q translocations in 5/6 NB cell lines examined and that an increased topo IIα gene copy number does not predict sensitivity to topo IIα inhibitors. (Supported by CA79763, CA76202, CA21765 and by the American Lebanese Syrian Associated Charities.)