A Bis-Alkylating Triplex Forming Oligonucleotide Inhibits Intracellular Reporter Gene Expression and Prevents Triplex Unwinding Due to Helicase Activity

Triplex forming oligonucleotides (TFOs) have the ability to site specifically modulate gene expression through the formation of triple helix DNA. The HER-2/neu promoter contains a strategically located triplex target sequence, and has been successfully targeted in vitro, with little success in vivo. A TFO was conjugated at both its 5' and 3' ends to an alkylating agent (phenylacetate mustard) in an attempt to stabilize the triple helix intracellularly. In vitro assays demonstrated that the bis-conjugate bound the duplex and alkylated the target guanine residues with high efficiency. The bis-conjugate suppressed promoter activity by 60-70% in cancer cells using a plasmid with a preformed triple helix, and the suppression was minimal when the nitrogen mustard was conjugated at only one end. Helicase assays demonstrated that helicase activity can unwind the TFO at the unalkylated end of the triple helix, which may leave the unwound oligonucleotide susceptible to nuclease degradation or ineffective at inhibiting transcription initiation. Our findings indicate that dual alkylation of the target sequence is required to suppress the intracellular activity of a reporter plasmid with a preformed triple helix, likely due to greater stability of the triple helix within cells and inhibition of helicase activity.