[Development of recombinant adenovirus carrying microRNA-regulated gene expression system].

Target tissue-specific delivery and transcription of foreign genes are desirable for safe and effective gene therapy. Two approaches for this purpose, "Targeted Delivery" and "Targeted Expression", have been mainly reported. Among "Targeted Expression" approaches, microRNA (miRNA)-mediated "post-transcriptional de-targeting" has been recently demonstrated, and much attention has been focused on this approach. MiRNAs are an approximately 22-nt length non-coding RNA, and bind to imperfectly complementary sequences in the 3'-untranslated region (UTR) of target mRNA, leading to suppression of gene expression via post-transcriptional regulation. First, in order to reduce the hepatic transduction by Ad vectors, complementary sequences of liver-specific miRNA, miR-122a, were inserted into the 3'-UTR of the transgene expression cassette. Intratumor injection of this Ad vector resulted in approximately 100-fold lower hepatic expression than that of the conventional Ad vector, without reducing gene expression in the tumor. Second, complementary sequences for miRNAs selectively down-regulated in tumor cells were inserted into the E1 gene expression cassette in oncolytic Ads, which exhibit tumor cell-specific replication and antitumor effects. Recent studies demonstrated that expression of several miRNAs is exclusively reduced in tumor cells. Oncolytic Ads containing the miRNA complementary sequences showed reduced replication in the normal cells, without altering the antitumor effects. MiRNA-regulated gene expression system mediates "post-transcriptional de-targeting", in which translation of transgene is suppressed in a tissue-specific manner; however, tissue-specific transgene expression can be achieved by taking tropism of gene delivery vehicles into consideration and reducing the transgene expression in untargeted organs via miRNA-regulated gene expression system.