Combinatorial control of suicide gene expression by tissue-specific promoter and microRNA regulation for cancer therapy.

Transcriptional targeting using a tissue-specific cellular promoter is proving to be a powerful means for restricting transgene expression in targeted tissues. In the context of cancer suicide gene therapy, this approach may lead to cytotoxic effects in both cancer and nontarget normal cells. Considering microRNA (miRNA) function in post-transcriptional regulation of gene expression, we have developed a viral vector platform combining cellular promoter-based transcriptional targeting with miRNA regulation for a glioma suicide gene therapy in the mouse brain. The therapy employed, in a single baculoviral vector, a glial fibrillary acidic protein (GFAP) gene promoter and the repeated target sequences of three miRNAs that are enriched in astrocytes but downregulated in glioblastoma cells to control the expression of the herpes simplex virus thymidine kinase (HSVtk) gene. This resulted in significantly improved in vivo selectivity over the use of a control vector without miRNA regulation, enabling effective elimination of human glioma xenografts while producing negligible toxic effects on normal astrocytes. Thus, incorporating miRNA regulation into a transcriptional targeting vector adds an extra layer of security to prevent off-target transgene expression and should be useful for the development of gene delivery vectors with high targeting specificity for cancer therapy.