Antisense Oligonucleotides and RNA Interference - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 340

The lone exceptions so far validated are the morpholino-oligonucleotides [83] . When
placed near the 5 end of the mRNA, morpholino-oligonucleotides have been shown
to specifically reduce translation.
7.5.3 Inhibition of Splicing
Splicing is an important and specific step in the translation of an mRNA to a pro-
tein and requires spliceosomes. The AS ODNs act by binding to specific splicing
sequence of mRNA, thereby inhibiting its translation and the production of related
protein [84,85] . 2- O -methyl phosphorothioate oligonucleotides have been reported
to inhibit protein expression by inhibiting splicing [86,87] .
7.5.4 Translational Arrest
Many AS ODNs bind to the translational initiation codon, thereby inducing transla-
tional arrest. PNAs and morpholino-oligonucleotides have been reported to act via
translational arrest by binding to the translational initiation codon or 5-UTR [88,89] .
Translational arrest has been reported to be a mechanism of action to inhibit replica-
tion of viruses like HIV and vesicular stomatitis virus [90,91] .
7.5.5 Inhibition of Polyadenylation
Polyadenylation is an intermediate step in the protein synthesis requiring addition
of long tracts of polyadenylate to the pre-mRNA molecules, thereby stabilizing it.
Capping of the 3-terminal of pre-mRNA could inhibit polyadenylation and destabi-
lize it. However, to date no study reports polyadenylation as a mechanism of action
for antisense drugs [92] .
7.5.6 Steric Block
This mechanism involves physical blockage of the RNA, thereby preventing protein
expression by an RNA-DNA duplex formation. This can be achieved by binding to
the 5 end or the translational initiation codon of mRNA [93] . Other RNA-processing
events such as nuclear splicing and polyadenylation are also inhibited by steric block-
ade of mRNA. PNAs and morpholino-oligonucleotides have been reported to inhibit
mRNA translation by steric blockade [93,94] .
7.5.7 Activation of Double-Strand RNase
Some AS ODNs inhibit mRNA translation by activating a double-strand RNase
enzyme called Dicer, thereby cleaving a dsRNA [95] . RNAi is an antisense mecha-
nism of action that utilizes the enzyme Dicer to promote hydrolysis of the target
RNA. siRNA oligonucleotide duplexes have been reported to inhibit protein expres-
sion through RNAi pathway by activation of double-strand RNase [96] . The potency,
maximal effectiveness, duration of action, and sequence specificity of siRNA oligo-
nucleotide duplexes have been found to be comparable to those of RNase H-dependent