Bioresponsive Nanoparticles for the Intracellular Delivery of RNAi Therapeutics - Nanotechnology for the Delivery of Therapeutic Nucleic Acids - page 131

gene silencing supports an ancient defence mechanism against
unwanted cytoplasmic double stranded RNA e.g. of viral origin
[12]. A closely related class of endogenously expressed non-coding
RNA termed microRNA (miRNA) that plays a key role in post-
transcriptional control of cellular processes has been found [13-15].
The exogenous siRNA and endogenous miRNA pathway converge
downstream through cytoplasmic RISC and DICER involvement.
Understanding the RNAi pathway is pivotal to identify therapeutic
substrates to harness RNAi effects and to design a delivery strategy
that enables engagement within the RNAi pathway.
The conventional therapeutic approach is introduction of
synthetic 21-22 nt siRNA into the cell cytoplasm to target RISC and
mediate enzymatic cleavage of mRNA located at perinuclear sites.
The upstream cytoplasmic endonuclease DICER facilitates cleavage
of long duplexes into 21-22 nt siRNA in preparation for RISC
processing [16]. This has resulted in the development of synthetic
DICER-substrates 25-27 nt in length that show potent RNAi effect
attributed to more effective RISC incorporation as part of the natural
processing scheme [17, 18]. The drawback of non-specific induction
of cellular innate immune responses exhibited by longer duplexes
can be overcome by incorporation of chemical modification such as
2

′

MeO within the DICER-substrate siRNA [19, 20].
MicroRNAs are transcribed as hairpin structured primary RNA
transcripts (pri-miRNAs) and processed in the nucleus by the
RNase enzyme Drosha into shorter 50-70 nt precursor miRNAs
(pre-miRNAs). The pre-miRNAs are transported across the nuclear
pore complex into the cytoplasm by the enzyme Exportin-5 (XPO5)
[21] where they are processed into mature miRNAs by DICER [22]
that mediates potent gene silencing by translational repression
or mRNA cleavage. Recent attention has been directed towards
tumour suppressive miRNA and tumour inductive miRNA (termed
oncomiRs) [23, 24]. Nuclear delivery of pri- or pre-miRNA transcripts
or miRNA mimics introduced directly into the cytoplasm can be
used to facilitate miRNA upregulation. Alternatively, cytoplasmic
delivery of antisense oligonucleotides (termed Antagomirs) [25, 26]
can be used to inhibit homologous miRNA. The selected strategy is
dependent on the role of the miRNA in disease. In addition to the
central role in controlling cellular processes, miRNAs are attractive
RNAi therapeutics due to the improved potency of gene silencing
[27] and avoidance of interferon activation as a result of endogenous
miRNA processing [28].