Introduction
Since its discovery, RNA interference (RNAi) has been studied
with great interest for its ability to knock down genes that participate
in a variety of disease processes. Translation of RNAi to
the clinical arena has been hampered by issues involving stability,
targeting and uptake, and immunogenicity.1 Nanomedicine,
a rapidly expanding field that utilizes nanoscale materials for the
development of therapeutic modalities, has been evaluated as a
solution to these impediments as a means to translate RNAi to
the bedside.2 A variety of nanomaterial carriers for RNAi therapy
are currently being investigated as a means of enhancing stabilization
and improving delivery of the enclosed small interfering
RNA (siRNA).1 The synergistic effect of encapsulating siRNA in
nanoparticle vehicles permits 2 powerful scientific discoveries to
be harnessed for more effective therapeutic modalities in order
to treat a variety of dermatologic conditions, including malignancy
and impaired wound healing.
Mechanism of RNA Interference
RNAi is an innate cellular mechanism of post-transcriptional
gene silencing in which small double-stranded RNAs (dsRNA)
target specific complementary sequences of messenger RNA
(mRNA) for degradation.3-5 RNAi has been evolutionarily conserved
in many organisms. By silencing viruses and other genetic
elements that synthesize dsRNA intermediates,4 it functions as a
mechanism of genome protection. For example, about half the
human genome contains transposable elements and repetitive
genes, duplicated and inserted in sequences that represent nonfunctional,
or "junk," DNA. The silencing of these components
by endogenous RNAi pathways is essential to preserving the
stability of the genome. This sequence of events begins when
long pieces of dsRNA are cleaved into segments called small interfering
RNA (siRNA). This is accomplished by ribonuclease III,
an enzyme from the Dicer family, yielding fragments composed
of 21 to 23 nucleotides.6 These RNA fragments are much shorter
than usual mRNAs and ribosomal RNAs. siRNA can be manufactured
synthetically and inserted directly into cells. By doing so,
the potential for long dsRNA pieces to overwhelm endogenous
RNA machinery and disrupt normal protein expression mechanisms
is avoided.7 Once siRNA is introduced into the cytoplasm,
it is integrated into a protein complex called RNA-induced silencing
complex (RISC) (Figure 1).7 A component of the RISC unwinds
the siRNA and cleaves the sense strand. The now-activated RISC
complex contains the remaining antisense strand of siRNA and
selectively targets and degrades the complementary mRNA sequence.
The resultant gene silencing persists for 3 to 7 days in
rapidly diving cells and for several weeks in nondividing cells.8
The cessation of siRNA's therapeutic effects is due to the dilution
of siRNA below a certain threshold as a result of cell division, in
addition to degradation within the cell.8
Role of siRNA in Medicine and Dermatology
It was only recently that RNAi became a prominent field of
research, when in 1999, Fire et al9 discovered an siRNA that silences
gene expression in the nematode Caenorhabditis elegans.
In 2001, Elbashir and colleagues10 published a "proof-of-principle"
experiment that demonstrated that synthetic siRNA could
successfully mediate RNAi by knocking down sequence-specific
genes in mammalian cell lines. Soon after these groundbreaking
experiments, the first use of siRNA for gene silencing was
achieved in a murine model for the therapeutic purpose of targeting
a hepatitis C viral sequence.11 The field of RNAi rapidly
diversified as gene therapy presented innovative therapeutic approaches,
capitalizing on these new mechanisms.12
With the progression of siRNA research, applications in dermatology
have emerged in efforts to treat various cutaneous
diseases. Skin diseases are particularly amenable to RNAibased
therapy, as the therapeutic targets are easily accessible
to topical treatments.12,13 Local therapy can be limited to the
affected skin, minimizing systemic absorption and toxicity of
siRNA as compared with intravenous delivery.13,14 In addition,
cutaneous siRNA therapy benefits from the capability to tangibly
examine and biopsy the treated region as necessary during
treatment. There is a broad spectrum of cutaneous diseases for
which gene therapy is an attractive modality, including heritable
skin disorders, tumors, metabolic disorders, and infectious
diseases.12,15 For example, there is particular interest in applying
RNAi in the treatment of heritable skin diseases, where
knockdown of a disease-causing gene may demonstrate clinical
benefits.13 RNAi has also been researched for treatment of viral
skin diseases; for example, siRNA targeting herpes simplex virus
proteins has the potential to interfere with active infection.16
However, limitations still exist and must be addressed for RNAi
to successfully translate to the bedside.
A major obstacle to utilizing gene silencing via RNAi for therapeutic
purposes lies in the delivery of siRNA to specific targets
