Stable inhibition of human thymidylate synthase expression following retroviral introduction of an siRNA gene.

Thymidylate synthase (TS) is an essential enzyme that synthesizes thymidylic acid in the de novo biosynthetic pathway. Inhibiting TS enzyme activity with substrate or cofactor analogs leads to inhibition of DNA replication and cell death. For this reason, TS is an important target enzyme for cancer chemotherapeutic drugs. We describe an alternative approach to reducing cellular TS enzyme activity using short interfering RNA (siRNA) technology to lower TS mRNA levels. Plasmids that direct the synthesis of siRNAs that target nucleotides 898-916 and 965-983 (relative to the A of the translational start codon) of human TS mRNA were highly effective at reducing TS enzyme levels in transient transfection assays. Infection of HeLa cells with retroviruses that contain the effective siRNA genes led to a stable 80-95% reduction of TS enzyme and mRNA. A similar percent reduction in TS expression was observed in a cell line that overproduces TS enzyme 100-fold due to TS gene amplification. Cells that exhibited the greatest reduction in TS enzyme level grew poorly in medium that lacked thymidine. These observations suggest that siRNA approaches may provide an alternative therapeutic strategy to reduce TS enzyme levels.