Biology Reference
In-Depth Information
later, as the sequencing of genomes and their miR components soared to an
unprecedented pace, it has become apparent that lin-4 is also deeply con-
served (
Christodoulou
et al
., 2010
). However, unlike let-7 whose sequence is
almost identical between
C. elegans
and human, the conservation between
lin-4 and its human orthologues miR-125a/b is limited to the 5
0
end of the
miR. Despite these variations, the miR pathway has emerged as one of the
most ancient pathways that are deeply involved in animal evolution
(
Christodoulou
et al
., 2010; Grimson
et al
., 2008
). Notably, miR-100 is likely
to be the most deeply conserved miR as its orthologues are identified from
Nematostella
to human (
Christodoulou
et al
., 2010; Grimson
et al
., 2008
).
Because of the ease for genetic manipulation,
C. elegans
has long served
as a fertile ground for functional characterization of novel genes by the loss-
of-function studies. Recently, the roles of most individual miRs and miR
families have been investigated by examining developmental defects and
viability in a large collection of miR knockout (KO) models in
C. elegans
(
Alvarez-Saavedra and Horvitz, 2010; Miska
et al
., 2007
). Surprisingly, very
few miRs either individually (7 out of 95) or collectively as a family (3 out
of 15 families) were found to be essential for development and viability in
these studies (
Alvarez-Saavedra and Horvitz, 2010; Miska
et al
., 2007
).
However, when challenged with sensitized genetic background, many
miR mutants (25 out of 31 miRs) manifest strong defects (
Brenner
et al
.,
2010
). These interesting findings are also echoed by studies with miR KO
mouse models where miR KO animals often only show discernible defects
under stress conditions (
Liu and Olson, 2010
). Together, these results
support the view that the miR-mediated regulatory network is complex
and suggest that rather than having absolute functions, miRs may instead
operate to more rapidly reinforce or enhance a pathway, a feature that may
only surface under circumstances like stress, where the sense of urgency in
responding is more acutely felt by a tissue.
3. The Biogenesis Pathways of miR
In the past decade, the miR biogenesis pathway has been extensively
investigated. For the clarity of this chapter, we reiterate some of the key
steps that are important to our discussion but refer our readers to several
recent reviews for detailed analysis (
Kim
et al
., 2009; Krol
et al
., 2010
).
Inmammals,most miRs are transcribed byRNApolymerase II but some of
them are generated by RNA polymerase III (
Borchert
et al
., 2006; Cai
et al
.,
2004
). During the transcription of the primary transcript, the flanking
sequences of the miR fold into a hairpin structure, called pre-miR, character-
istic of miRcoding genes (
Fig. 7.1
). For stereotypical miRs that are represented
by most abundantly expressed miRs, the hairpin is first processed by the
