Inhibition of aromatase expression by a psoralen-linked triplex-forming oligonucleotide targeted to a coding sequence

The cytochrome P450 enzyme aromatase (P450arom) is an important target in breast cancer treatment. We have designed a 20-base pyrimidine oligodeoxynucleotide (ODN) which forms a sequence-specific triple helix (triplex) with a purine-rich tract in the P450arom coding sequence. The psoralen-linked ODN (Pso20T) formed photo-induced cross-linked products with target double-stranded DNA. Cross-linked adducts formed in vitro between ODNs and P450arom expression constructs were used to transfect COS and human MCF-7 breast cancer cells. Levels of aromatase transcripts and enzyme activity were significantly lower in cultures transfected with Pso20T-treated cDNA relative to controls. Pso20T had a lesser inhibitory effect on aromatase expression from a mutant P450arom construct, consistent with predicted effects of the mutations on triplex formation. These results are compatible with triplex-mediated interruption of transcription within intact cells.