MicroRNAs and Biomarker Discovery - Proteomic and Metabolomic Approaches to Biomarker Discovery - page 370

FIGURE 1 Biogenesis and mechanism of action of miRNAs. After transcription of the miRNA gene mostly by RNA
Polymerase II into a primary miRNA (pri-miRNA), the hairpin structure is processed in the nucleus by the microprocessor
complex (Drosha, DGCR8) into a precursor miRNA (pre-miRNA). After export into the cytoplasm by Exportin-5, the pre-
miRNA is processed by the RNaseIII enzyme Dicer into a mature double-stranded miRNA (miRNA) that is about 18 to 25
nt in length. After association with the RNA-induced silencing complex (RISC), consisting of one of the four Argonaute
proteins, the miRNA strand is separated: while the passenger strand is degraded, the leading strand in the RISC binds
predominantly to the 3 0 UTR of mRNAs, leading either to decapping, deadenylation and degradation, cleavage, and
degradation or to the restriction of translation of partially complementary mRNAs.

Upon binding of the RISC to the mRNA,
several scenarios are possible. Either the
mRNA is degraded by removing the cap struc-
ture and shortening of the poly(A) tail, leading
to subsequent degradation of the mRNA or, in
cases of perfect complementarity between
miRNA and mRNA, the miRNA can act like
a small interfering RNA (siRNA), leading to
the cleavage of the mRNA molecule. Alterna-
tively, RISC binding to the mRNA can also
lead to restriction of translation of this mRNA.
In all cases, miRNA regulation leads to a decline
in mRNA levels and subsequently protein abun-
dance, thus opening up the possibility to

Identifying miRNA-target mRNAs and
thereby deciphering the cellular pathways most
likely controlled by the respective miRNAs, has
proven to be dif cult, as base pairing between

miRNAs and mRNAs is only partial and can
occur with different bases of the miRNA mole-
cule. Several computer algorithms have been
devised to aid in the identi

cation of target
genes in silico, 4 but still additional validation
experiments are necessary. Currently, there are
1,600 human and 855 murine miRNAs listed in
miRBase ( http://www.mirbase.org/ ; release 19
from August 2012) and numbers are still rising
more than ten years after the discovery of the

ne-
tune the expression of many genes by one single
miRNA molecule.

first human miRNA. 5 In addition to unique
miRNA genes, miRNA families like the let-7/