Positively charged oligonucleotides overcome potassium-mediated inhibition of triplex DNA formation

The formation of triplex DNA using unmodified, purine-rich oligonucleotides (ODNs) is inhibited by physiologic levels of potassium. Changing negative phosphodiester bonds in a triplex forming oligonucleotide (TFO) to neutral linkages causes a small increase in triplex formation. When phosphodiester bonds in a TFO are converted to positively-charged linkages the formation of triplex DNA increases dramatically. In the absence of KCl, a 17mer TFO containing 11 positively-charged linkages at a concentration of 0.2 microM converts essentially all of a 30 bp target duplex to a triplex. Less than 15% of the target duplex is shifted by 2 microMolar of the unmodified TFO. In 130 mM KCl, triplex formation is undetectable using the unmodified TFO, while triplex formation is nearly complete with 2 microM positively-charged TFO. With increasing potassium, TFOs containing a higher proportion of modified linkages show enhanced triplex formation compared with those less modified. In contrast with unmodified TFOs, triplex formation with more heavily modified TFOs can occur in the absence of divalent cations. We conclude that replacement of phosphodiester bonds with positively-charged phosphoramidate linkages results in more efficient triplex formation, suggesting that these compounds may prove useful for in vivo applications.