Sequence-Specific Targeting of GC-Rich Gene Loci by Parallel Triplex-Forming Oligonucleotides Containing a Modified Nucleobase

Targeting GC-rich gene loci is a major challenge owing to their high duplex stability, repetitive sequence composition, and propensity to adopt alternative DNA structures. Triplex-forming oligonucleotides (TFOs) provide a programmable strategy towards the recognition of GC-rich DNA, but their application is restricted by the limited recognition capabilities of natural nucleobases in a cellular setting. Here, we overcome this barrier using parallel-binding TFOs containing the synthetic nucleobase 6-amino-5-nitropyridin-2-one (Z), which enables pH-independent recognition of G-C base pairs. Using two structurally distinct regulatory elements within the MYC promoter, we show that Z-modified TFOs form stable, sequence-selective triplexes that repress promoter activity by 50-80% in both episomal reporter assays and at endogenous gene loci. Notably, the greatest repression was observed at a GC-rich quadruplex-forming element that functions as a structural hub for transcription factor recruitment. To our knowledge, this represents the first demonstration that a simple nucleobase modification alone is sufficient to enable parallel-binding TFOs to repress expression of an endogenous gene, establishing a general strategy for targeting GC-rich regulatory elements through programmable DNA recognition.