Recent progress in neural stem cell- (NSC-) based tumor-targeted gene therapy showed that NSC vectors expressing an artificially\nengineered viral fusogenic protein, VSV-G H162R, could cause tumor cell death specifically under acidic tumor microenvironment\nby syncytia formation; however, the killing efficiency still had much room to improve. In the view that coexpression of another\nantitumoral gene with VSV-G can augment the bystander effect, a synthetic regulatory system that triggers transgene expression\nin a cell fusion-inducible manner has been proposed. Here we have developed a double-switch cell fusion-inducible transgene\nexpression system (DoFIT) to drive transgene expression upon VSV-G-mediated NSC-glioma cell fusion. In this binary system,\ntransgene expression is coregulated by a glioma-specific promoter and targeting sequences of a microRNA (miR) that is highly\nexpressed in NSCs but lowly expressed in glioma cells.Thus, transgene expression is ââ?¬Å?switched offââ?¬Â by the miR in NSC vectors, but\nafter cell fusionwith glioma cells, the miR is diluted and loses its suppressive effect.Meanwhile, in the syncytia, transgene expression\nis ââ?¬Å?switched onââ?¬Â by the glioma-specific promoter. Our in vitro and in vivo experimental data show that DoFIT successfully abolishes\nluciferase reporter gene expression in NSC vectors but activates it specifically after VSV-G-mediated NSC-glioma cell fusion.
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