Triplex-forming oligonucleotides as potential tools for modulation of gene expression.

Triplex-forming oligonucleotides (TFOs) bind in the major groove of duplex DNA at polypurine/ polypyrimidine stretches in a sequence-specific manner. The binding specificity of TFOs makes them potential candidates for use in directed genome modification. A number of studies have shown that TFOs can introduce permanent changes in a target sequence by stimulating a cell's inherent repair pathways. TFOs have also been demonstrated to inhibit gene expression providing a possible role for these compounds in cancer therapy. This review summarizes the dual roles of TFOs for use in delivering DNA reactive compounds to a specific site in the genome or for introducing permanent changes in the target sequence through the introduction of an altered helical structure. In addition to compiling the ways in which TFOs have been successfully utilized, this review will explore conflicting reports of TFO bioactivity focusing on the variables which affect the efficacy in vitro of TFO mediated genomic modification which in turn may represent the obstacles encountered using TFOs to modulate gene expression in vivo.