Download MicroRNA: A novel class of master regulators of gene expression

MicroRNA: A novel class of master regulators
of gene expression
Figure 1
MicroRNAs are a recently discovered class of eukaryotic, endogenous,
non-coding RNAs that play a key role in the regulation of gene
expression. Acting at the post-transcriptional level, these fascinating
molecules may fine-tune the expression of as much as 30% of all
mammalian protein-encoding genes.
Nucleus
Transcription
RNA Pol II
Microprocessing
Drosha
DGCR8
pre-miRNA
(A)n
pri-miRNA(s)
Cytoplasm
Transcription and processing of microRNA
MicroRNA genes are transcribed by RNA polymerase II in the nucleus
as large primary transcripts (pri-microRNA) that are processed by a
protein complex containing the RNase III enzyme Drosha. The resulting
approximately 70 nucleotide hairpin-structured precursor microRNA
(pre-microRNA) is subsequently transported to the cytoplasm where
a second RNase III enzyme DICER processes the pre-microRNA to a
transient microRNA duplex of approximately 22 nucleotides. This duplex
is then loaded into the RNA-induced silencing complex miRISC and
the mature, single-stranded microRNA is selectively retained by this
complex. The mature microRNA then binds to complementary mRNA
sites to suppress protein production by preventing translation of the
messenger and/or by catalyzing degradation of the mRNA.
Exportin-5
pre-miRNA
Dicing
Dicer
miRNA duplex
Asymmetric unwinding
Dicer
TRBP
RISC loading
RISC
mature miRNA
mRNA target selection
?
?
(A)n
v
v
Near perfect complementarity
(A)n
Partial complementarity
Argonaute
P-bodies
(A)n
(A)n
mRNA cleavage
Translation repression
?
RNA decay
Figure 1. MicroRNA biogenesis.
Based on Wienholds and Plasterk,
FEBS Letters 2005, 579: 5911-5922.
Mature microRNAs are short, single-stranded RNA molecules
approximately 22 nucleotides in length. Some microRNA are encoded by
multiple loci and occasionally microRNA genes are organized in tandem
co-transcribed clusters.
MicroRNA and gene expression
MicroRNAs silence gene expression by binding to target sites in the
target mRNA, usually within the 3’ UTR. This interaction attenuates
protein production by preventing active translation of the messenger or
by destabilizing the mRNA. Since most target sites on the mRNA have
only partial base complementarity with their corresponding microRNA,
individual microRNAs may target as many as 100 different mRNAs.
Additionally, individual mRNAs may contain multiple binding sites for
different microRNA, leading to a complex network of gene regulation.
MicroRNA and disease
MicroRNAs have been shown to be involved in a wide range of
biological processes such as the cell cycle control, apoptosis and
several developmental and physiological processes including stem cell
differentiation, haematopoiesis, hypoxia, cardiac and skeletal muscle
development, neurogenesis, insulin secretion, cholesterol metabolism,
immune response and viral replication. In addition, highly tissuespecific expression and distinct temporal expression patterns during
embryogenesis suggest that microRNA play a key role in differentiation
and maintenance of tissue identity.
Furthermore, aberrant expression of microRNAs has been implicated in
a number of diseases including a broad range of cancers, heart disease
and Parkinsons disease. MicroRNAs are also intensely studied as
promising candidates for diagnostic and prognostic biomarkers of cancer
and predictors of drug response.
MicroRNA research
MicroRNAs were first reported in mammalian systems in 2001. Since
then, close to 5000 microRNAs have been identified in vertebrates,
invertebrates, and plants. However, the function of most microRNAs is
still poorly characterized.
The challenges of studying microRNA are two-fold. First, the short
nature (~22 nt) of microRNA sequences makes it difficult for traditional
DNA-based analysis tools to achieve the required target sensitivity.
Second, closely related microRNA family members differ by as little as
one nucleotide, emphasizing the need for high specificity and single basepair mismatch discrimination.
Exiqon’s microRNA tools
Exiqon has pioneered the development of microRNA tools for the
research and diagnostics community with leading-edge products and
services, based on proprietary Locked Nucleic Acid (LNA™) technology.
The use of LNA™ technology significantly increases both affinity and
specificity of the probe for its microRNA target, thereby addressing both
of the research challenges mentioned above.
All of Exiqon’s miRCURY LNA™ products are based on LNA™ technology.