Effect of phosphorothioate modifications on the ability of GTn oligodeoxynucleotides to specifically recognize single-stranded DNA-binding proteins and to affect human cancer cellular growth

We have previously identified phosphodiester oligonucleotides exclusively m ade of G and T bases, named GTn, that significantly inhibit human cancer ce ll growth and recognize specific nuclear single-stranded DNA binding protei ns. We wished to examine the ability of the modified GTn oligonucleotides w ith different degrees of phosphorothioate modifications to bind specificall y to the same nuclear proteins recognized by the GTn phosphodiester analogu es and their cytotoxic effect on the human T-lymphoblastic CCRF-CEM cell li ne. We showed that the full phosphorothioate GTn oligonucleotide was neithe r able to specifically recognize those nuclear proteins, nor cytotoxic. In contrast, the 3'-phosphorothioate-protected GTn oligonucleotides can mainta in the specific protein-binding activity. The end-modified phosphorothioate oligonucleotides were also able to elicit the dose-dependent cell growth i nhibition effect, but a loss in the cytotoxic ability was observed increasi ng the extent of sulphur modification of the sequences. Our results indicat e that phosphorothioate oligonucleotides directed at specific single-strand ed DNA-binding proteins should contain a number of phosphorothioate end-lin kages which should be related to the length of the sequence, in order to ma intain the same biological activities exerted by their phosphodiester analo gues. (C) Society francaise de biochimie et biologie moleculaire/Editions s cientifiques et medicales Elsevier SAS.