PREFERENTIAL INCORPORATION OF 3'-AZIDO-2',3'-DIDEOXYTHYMIDINE (AZT) IN TELOMERIC SEQUENCES OF CHO CELLS

3'-Azido-2',3'-dideoxythymidine (AZT), the thymidine analogue used aga inst human immunodeficiency virus 1 (HIV-1), exhibits bone marrow and blood toxicity in humans, presumably as the result of genotoxic mechan isms induced by incorporation of AZT into eukaryotic DNA. Preferential incorporation of AZT into telomeric regions of DNA of Chinese hamster ovary (CHO) cells has been previously demonstrated by immunofluoresce nce using anti-AZT antibodies. We quantitatively compared the amount o f [H-3]-AZT bound to telomeric and non-telomeric sequences of CHO cell DNA. DNA from cells exposed to [H-3]-AZT was digested by a mixture of restriction enzymes, frequent cutters in the overall genome, without restriction sites in the telomeric repeat. As a result, the telomeric fraction (TF): isolated by separation columns, comprised longer sequen ces (> 2 kb) than the non-telomeric fraction (NTF). Radioactivity asso ciated with each fraction revealed a three fold increase in [H-3]-AZT incorporated in the TF compared with the NTF. No preferential telomeri c binding was detected for [H-3]-thymidine (Tdr) or [H-3]-5'bromodeoxy uridine (BrdU) in similar experiments or in DNA of AZT-treated mouse p rimary fibroblasts, cells with large telomeric repeats that lack telom erase. When the chromosomal ends of high molecular weight [H-3]-AZT-DN A were digested with Pal 31, the radioactivity was double in the TF co mpared with the NTF. Therefore incorporation of AZT in CHO immortalize d cells but not in primary fibroblasts (that lack telomerase) indirect ly shows that AZT incorporation could be telomerase-mediated.