Amplification and deletion of topoisomerase II alpha associate with ErbB-2amplification and affect sensitivity to topoisomerase II inhibitor doxorubicin in breast cancer

Topoisomerase II alpha (topoII alpha) is a key enzyme in DNA replication an d a molecular target for many anti-cancer drugs called topoII inhibitors, T he topoII alpha gene is located at chromosome band 17q12-q21, close to the ErbB-2 oncogene (HER-2/neu), which is the most commonly amplified oncogene in breast cancer, Because of the physical proximity to ErbB-2, copy number aberrations may also occur in the topoII alpha gene. These topoII alpha gen e copy number aberrations may be related to the altered chemosensitivity to topoII inhibitors that breast cancers with ErbB-2 amplification are known to have. We used fluorescence in situ hybridization to study copy number ab errations of both topoII alpha and ErbB-2 in nine breast cancer cell lines and in 97 clinical breast tumors, which were selected for the study accordi ng to their ErbB-2 status by Southern blotting. TopoII alpha-protein expres sion was studied with Western blot and sensitivity to doxorubicin (a topoII inhibitor) with a 96-well clonogenic in vitro assay. Two of the five cell lines with ErbB-2 gene amplification (SK-BR-3 and UACC-812) showed amplific ation of topoII alpha. In MDA-361 cells, ErbB-2 amplification (14 copies/ce ll) was associated with a physical deletion of topoII alpha (four copies of chromosome 17 centromere and two copies of topoII alpha). The topoII alpha amplification in UACC-812 cells was associated with 5.9-fold-increased top oII alpha protein expression and 2.5-fold-increased sensitivity to the topo II inhibitor, doxorubicin, whereas the deletion in MDA-361 leads to decreas ed protein expression (45% of control) and a 2.4-fold-increased chemoresist ance in vitro. Of 57 ErbB-2-amplified primary breast carcinomas, 25 (44%) s howed ErbB-2-topoII alpha coamplification and 24 (42%) showed a physical de letion of the topoII alpha gene. No topoII alpha copy number aberrations we re found in 40 primary tumors without ErbB-2 amplification. TopoII alpha ge ne amplification and deletion are common in ErbB-2-amplified breast cancer and are associated with increased or decreased sensitivity to topoII inhibi tors in vitro, respectively. These findings may explain the altered chemose nsitivity to topoII inhibitors reported in ErbB-2-amplified breast cancers.