Antisense oligonucleotides (ASOs) are used to inhibit gene expression levels both in vitro and in vivo. Recent improvements in design and chemistry of antisense compounds have enabled this technology to become a routinely used tool in basic research, genomics, target validation, and drug discovery. It is becoming increasingly popular to confirm phenotypes seen using RNAi by gene silencing antisense DNA oligos. A nucleic acid sequence, usually 15–25 bases long, is designed in antisense orientation to the mRNA of interest; the sequence is made as a synthetic oligonucleotide and is introduced into the cell or organism. Hybridization of the antisense oligo to the target mRNA results in RNase H cleavage of the message, which prevents protein translation and thereby blocks gene expression. Antisense oligonucleotides containing a native DNA or phosphorothioate-modified DNA segment of at least six bases long will bind the target mRNA and form an RNA/DNA heteroduplex, which is a substrate for endogenous cellular RNases H [1–2]. The decrease in mRNA levels can be measured using real-time PCR.
Phosphorothioates and chimeric oligos
LNA, 2′-O-methyl RNA, and 5-methyl dC
State-of-the-art antisense design employs chimeras with both DNA and modified-RNA bases. The use of modified RNA, such as 2′-O-methyl (2′OMe) RNA or LNAs (Exiqon) in chimeric antisense designs, increases both nuclease stability and affinity (Tm) of the antisense oligo to the target mRNA [3–5]. These modifications, however, do not activate RNase H cleavage. The preferred antisense design incorporates 2′-O-modified RNA or LNA in chimeric antisense oligos that retain an RNase H activating domain of DNA (or phosphorothioate DNA). As LNA bases confer significant nuclease resistance, we recommend phosphorothioate modification of only the DNA gap, leaving the LNA flanks as phosphodiester linkages in chimeric LNA antisense oligos. For synthesis reasons, a 3′-phosphate is preferred when an LNA base is at the 3′ end.
It can also be beneficial to substitute 5-Methyl-dC for dC in the context of CpG motifs. Substitution of 5-Methyl dC for dC will slightly increase the Tm of the antisense oligo. Use of 5-Methyl dC in CpG motifs can also reduce the chance of adverse immune responses in vivo. We recommend HPLC purification and Na+ salt exchange for all antisense oligos before use in cells or live animals to ensure that salts used in purification are removed.