Cationic oligonucleotides can mediate specific inhibition of gene expression in Xenopus oocytes

Base-specific hydrogen bonding between an oligonucleotide and the purines in the major groove of a DNA duplex provide an approach to selective inhibition of gene expression. Oligonucleotide-mediated tripler formation in vivo may be enhanced by a number of different chemical modifications. We have previously described an in vitro analysis of tripler formation using oligonucleotides containing internucleoside phosphate linkages modified with the cation N,N-diethyl-ethylene-diamine (DEED), When compared with unmodified oligonucleotides of identical base composition, DEED-modified oligonucleotides were better able to form DNA triplexes under conditions that approximate the pH, magnesium and potassium levels found in vivo. Here we report the ability of DEED-modified oligonucleotides to inhibit the expression of plasmid DNA injected into Xenopus oocytes. Inhibition is specific to plasmids containing a tripler formation target and sensitive to sequence alteration in the tripler forming target site. Inhibition of gene expression was nearly complete when oligonucleotide and plasmid were mixed together prior to injection. Inhibition was partial when oligonucleotide was injected first and not evident when plasmid was injected and allowed to form chromatin prior to oligonucleotide injection, Thus, access to DNA is a determining factor in effective tripler inhibition of gene expression.