Telomerase protein rather than its RNA is the target of phosphorothioate-modified oligonucleotides.

Human telomerase is a ribonucleoprotein which uses its internal RNA moiety as a template for telomeric DNA synthesis. This enzyme is up-regulated in most malignant tumors and is therefore considered as a possible cancer target. Here we examined the effects of differently modified oligomers on telomeraseactivity from HL-60 cell extracts (TRAP-ezetrade mark assay). Phosphorothioate-modified oligonucleotides (PS-ODNs) inhibited telomerase activity at subnanomolar concen-trations and proved to be more efficient than peptide nucleic acids. In contrast to all the investigated oligomers, PS-ODNs were found to bind to the protein motif of telomerase called the primer binding site but poorly to its RNA. This is suggested by kinetic investigations demonstrating a competitive interaction of PS-ODNs and TS primer at the primer binding site. The K m value of the TS primer was 10.8 nM, the K i value of a 20mer PS-ODN was 1.6 nM. When the TS primer was PS-modified a striking increase in the telomerase activity was found which correlates with the number of phosphodiesters replaced. The K m value of a completely PS-modified TS primer was 0.56 nM. Based on these results the design of chimeric ODNs is proposed consisting of a 5'-PS-modified part targeting the primer binding site and a 3'-terminus part targeting the telomerase RNA.