Cancer‐selective induction of cytotoxicity by tissue‐specific expression of targeted trans ‐splicing ribozyme

For suicide gene therapy to be successfully applied for clinical settings, cancer‐restricted expression of such suicide gene should be required. We previously showed that group I intron from Tetrahymena can induce new RNA that exerts anti‐cancer activity through RNA replacement by trans ‐splicing reaction with high fidelity and specificity onto targeted human telomerase reverse transcriptase (hTERT) RNA in cancer cells, and hence the ribozyme can selectively retard growth of the cells in vivo as well as in vitro. However, the shortage of complete tumor‐selectivity due to telomerase expression of highly proliferating normal cells can limit therapeutic applicability of the hTERT‐targeting approach. In this study, to explore the possibility of improving specificity of cancer therapy, we have attempted to stimulate anticancer gene activity specifically in liver cancer cells by tissue‐specific expression of the hTERT‐targeting trans ‐splicing ribozyme using liver‐specific promoters. Transient transfection experiments demonstrated that the expression of transgene such as luciferase gene was specifically and highly triggered from hTERT‐expressing liver cancer cells transfected with the ribozyme. Moreover, liver‐specific expression of the ribozyme with diphtheria toxin A or herpes simplex virus thymidine kinase gene as 3′ exon could specifically and highly retard the growth of the hTERT‐expressing liver cancer cells. In conclusion, we can greatly improve specificity of cancer cytotoxicity by combination of transcriptional targeting for tissue‐specific transgene expression with RNA replacement for cancer‐specific anticancer gene induction.