Is irrelevant cleavage the price of antisense efficacy?

Antisense oligonucleotides are useful reagents for the suppression of gene expression. Their mechanism of action in eukaryotic cells appears to depend heavily on the activity of RNase H, a ubiquitous enzyme that cleaves the mRNA strand of an RNA-DNA duplex. However, the stringency requirements of RNase H are very low, and as little as a 5-base complementary region of oligomer to target may be sufficient to elicit RNase H activity. This would result in scission of nontargeted mRNAs, or what is known as "irrelevant cleavage." One strategy to reduce RNase H competency that has been employed is modification of the oligonucleotide backbone, replacing phosphodiester linkages with uncharged methylphosphonates, which are not RNase H competent. Another strategy involves replacement of deoxyribonucleic acid with 2'-O-alkylribonucleic acid. A third strategy, eliminating RNase H dependency entirely, requires activation of RNase P. The relative merits of these strategies will be discussed in the context of selective inhibition of gene function.