Biology Reference
In-Depth Information
miRNAs,
3,19,20
and even in the identification of piRNAs.
13
Recent devel-
opments in sequencing technologies
21,22
allow one to obtain millions of
small RNAs from a single sample, making the task of small RNA identi-
fication much easier. These technologies have been used in the discovery
of U21 RNAs in worm, piRNAs in mammals,
14
and several miRNAs that
appear to have evolved rapidly in flies.
23
In parallel, approaches that start
with computational miRNA gene prediction followed by microarray-
based validation have also been developed and used for miRNA gene
discovery.
24
Both types of methods (cloning and sequencing, and microarrays)
have also been used to determine the expression profile of miRNAs across
tissues. The largest database of miRNA expression profiles obtained
through cloning was developed by Landgraf
et al
.,
25
and can be accessed
at http://www.mirz.unibas.ch/smiRNAdb/. Expression profiling is more
commonly done using microarrays, with many different platforms — using
modified and unmodified DNA, RNA, and locked-nucleic acid (LNA)
probes
26-32
— having been developed.
3. Classes of Small Regulatory RNAs
3.1. miRNAs
miRNAs are noncoding RNAs that regulate gene expression in a
sequence-specific manner (reviewed by Bartel
33
). They are generated from
the genome-encoded precursor hairpins
3
by the sequential action of two
complexes containing RNase III-type nucleases: Drosha in the nucleus,
34
with its partner Pasha
35
; and Dicer in the cytoplasm,
36
with its partner
known as Loquacious in
Drosophila melanogaster
37
and the human
immunodeficiency virus (HIV) transactivating response RNA-binding
protein (TRBP) in humans.
38,39
The founding member of the miRNA family, lin-4, was discovered in
nematode
C. elegans
through a genetic screen for defects in the tempo-
ral control of postembryonic development.
40
Mutations in lin-4 disrupt
the temporal regulation of larval development, causing the first larval
stage-specific cell-division pattern (L1) to reiterate at later developmen-
tal stages.
40,41
The opposite developmental phenotype is observed in
