Abstract

To maintain the telomeres at the ends of the chromosomes, telomerase in human cells adds a repeating sequence of nucleotides (TTAGGG) to the 3′-end of each chromosome using an RNA component of the enzyme as the template for DNA synthesis. Because of the selective expression of this enzyme in cancer cells, we have evaluated the interaction of human telomerase with several deoxyguanosine nucleotides of clinical importance. 2′,3′-dideoxyguanosine 5′-triphosphate, 6-thio-2′-deoxyguanosine 5′-triphosphate (T-dGTP), carbovir 5′-triphosphate, and d-carbocyclic-2′-deoxyguanosine 5′-triphosphate (d-CdG-TP) inhibited telomerase activity by 50% when these analogs were present at only 2 to 9 times the dGTP concentration. The l-enantiomer of CdG-TP was far less inhibitory, thereby demonstrating the stereoselectivity of telomerase for nucleotide substrates. T-dGTP was incorporated into the DNA by telomerase in the absence of dGTP, but unlike dGTP there was little extension of the DNA chain after its incorporation. These results indicate that the metabolites of three clinically useful agents (6-mercaptopurine, 6-thioguanine, and Abacavir) can inhibit human telomerase activity, and it is possible that the effect of these nucleotides on telomerase activity or telomere function could contribute to the mechanism of action of these agents.