Stable reduction of thymidine kinase activity in cells expressing high levels of anti-sense RNA.

Anti-sense thymidine kinase (TK) RNA was expressed as part of a chimeric dihydrofolate reductase (DHFR) anti-sense TK transcript. High level expression was obtained by selection of cells resistant to progressively higher levels of methotrexate, provided by overproduction of DHFR. The result was a concomitant increase in intracellular anti-sense TK RNA level owing to its presence on the same transcription unit as DHFR. In several cell lines expressing high levels of anti-sense TK RNA, thymidine kinase activity was reduced by 80%-90%. RNA:RNA duplexes were detected in the nuclear fraction. The results suggest a mechanism for diminution of TK activity; anti-sense RNA hybridizes with sense TK RNA in the nucleus, and duplex containing TK transcripts fail to enter the cytoplasm with normal efficiency.