Triple Helix-Forming Oligonucleotides
Triple helix-forming oligonucleotides, also known as TFO, act at the level of transcription of mRNA. Triple helix formation occurs when a polypurine or poly-pyrimidine DNA or RNA oligonucleotide binds to a polypurine/polypyrimidine region of genomic DNA. Triple helix-forming ONs can bind specifically in the major groove of such stretches of DNA to the polypurine strand, forming (reverse) Hoogsteen hy-drogen bonds (Fig. 3) (Rogers et al., 2005).
As a result of this binding, triplex-forming ONs can prevent transcription initiation or elongation by binding promoter, gene or regulatory DNA regions (Fig. 4). The concept has been validated in vivo,
Triplex-forming ONs have also been used for site-directed mutagenesis with or without the use of coupled mutagens, as well as homologous-site-specific recombination using triple-forming ONs alone or in combination with a donor fragment to correct genetic disorders. This application will be discussed in the section on gene repair.