Overexpression of human DNA topoisomerase II alpha by fusion to enhanced green fluorescent protein

DNA topoisomerase (topo) II alpha is a major target for many anticancer age nts. However, progress towards understanding how these agents interact with this enzyme in human cells and how resistance to these agents arises is gr eatly impeded by difficulties in expressing this gene. Here, we report on a chieving a high level of expression of a full-length human topo II alpha ge ne in human cells. We started with the topo II alpha cDNA driven by a stron g cytomegalovirus promoter and transiently transfected HeLa cells. Although topo II alpha mRNA was consistently detected in transfected cells, no exog enous topo II alpha protein was detected. By contrast, when the same cDNA w as fused to an enhanced green fluorescent protein (EGFP), we detected a hig h level of expression at both mRNA and protein levels. The exogenous topo I I alpha was localized to cell nuclei as expected, indicating that the fusio n protein is properly folded. Furthermore, overexpression of the EGFP-topo II alpha fusion protein increased the sensitivity of the transfected cells to teniposide, suggesting that it functions as the endogenous counterpart. Thus, in addition to being used as a gene tag, the GFP fusion approach may be generally applicable for expressing genes, such as topo II alpha, that a re difficult to express by conventional methods.