Targeted therapy of DNA tumor virus-associated cancers using virus-activated transcription factors.

DNA tumor virus-mediated tumorigenic processes typically involve functional inactivation of cellular tumor suppressors pRB and p53 by viral oncoproteins, with concomitant activation of oncogenic transcription factors such as E2Fs. This feature could be exploited to design a treatment for corresponding malignancies. Here, we report a gene therapy strategy for DNA tumor virus-associated cancers using a synthetic, E2F-regulated gene expression system named pESM6. This system contains multimerized E2F-responsive elements in combination with the binding sites for ubiquitous transcription factors Sp1 and CTF/NF1. pESM6 could drive a high-level transgene expression comparable to that of the CMV IE promoter and exert constitutive activity in cells expressing DNA tumor viral oncogenes. In contrast, it was effectively repressed by pRB and thus only minimally active in nontransformed cells. Expression of cytosine deaminase from pESM6 resulted in a highly efficient and specific killing of HPV-transformed fibroblasts (C3) after treatment with the prodrug 5-fluorocytosine. Also, an effective tumor mass reduction was observed when the vector was injected directly into C3 tumors implanted in C57BL/6 mice. pESM6 showed a superior performance throughout these experiments compared to the previously known E2F-regulated gene vector. These results clearly demonstrate the potential usability of pESM6 for the gene therapy of DNA tumor virus-associated cancers.