Regulation of HSVtk Gene by Endogenous microRNA-122a in Liver Cell Lines as Suicide Gene Therapy

Aim: Here, we use miR-122a that exhibits liver-specific expression for developing a post-transcriptional regulative system mediated by microRNAs.
Gene therapy with adenovirus (Ad) vectors that express herpes simplex virus thymidine kinase (HSVtk) and ganciclovir (GCV) have been suggested as a therapeutic strategy to cancer. However, Ad vectors injected into tumors are dispersed into the systemic circulation and transduce liver cells, resulting in severe hepatotoxicity. To be effective, the delivery and expression of suicide genes to cancer treatment ought to be specific to tumor cells, and avoid death of healthy cells. Researchers have demonstrated that expression of transgene could be suppressed in healthy cells with use of vectors that are reactive to microRNA regulation.
We constructed an Ad vector carrying four tandem copies of target sequences of miR-122a that were incorporated into 3'-UTR of HSVtk gene. The expression level of miR-122a was quantified in HepG2 and Huh7 cell lines.
Quantitative RT- PCR analysis demonstrated that Huh7 cells express large amounts of miR-122a compared to HepG2 cells. The viability of Huh7 cells and HepG2 cells after infection by Ad-tk-122aT vector was 83% and 23.5%, respectively. The viability of Huh7 cells was not reduced in the presence of GCV after infection by Ad-tk-122a vector. In contrast, cytotoxicity of HSV-tk/GCV was similar in Huh7 cells and HepG2 cells by Ad-tk vector, with 35.3% and 27% viability, respectively.
Inclusion of the miR-122a target sequences in the HSVtk expression cassette yielded a feasible strategy for reducing cytotoxicity of suicide gene in a liver cell line with high miR-122a expression.