Biology Reference
In-Depth Information
Dicer as the two important enzymes in maturation machinery
[
57
]. All types of epithelial tumors presented dysregulated expres-
sion levels of either Drosha or Dicer or both of them compared to
healthy tissue. Especially for Dicer, many more associations to dis-
ease, particularly cancer, have been reported [
58
]. Interestingly in
this context, the transcription factor Sox4, which is associated with
certain cancers like melanoma, has been shown to drive Dicer
expression [
59
]. However, up to now it remains quite unclear how
such alterations in Dicer expression—usually probed by qRT-PCR
of its mRNA—in detail affect human disease. The concentration of
miRNA dramatically differs between individual miRNAs and it is
unclear if miRNAs expressed at very low concentrations have sig-
nifi cant impact on gene regulation. The large differences in expres-
sion levels of individual miRNA also suggest that miRNAs expressed
at high levels may be more affected by damped dicing activity. In
Sox4 knockdown cells, 34.9 % of miRNAs were down-regulated,
whereas 26.5 % of miRNAs were up-regulated [
59
]. A rescue of
Dicer expression increased less than two-thirds of the previously
down-regulated miRNAs. In fact, it remains to be consistently
shown that dicing is a rate-limiting step for the majority of miRNA
concentrations. However, a good hint is provided by a study in
which siRNA activity, as well as miRNA maturation, was elevated
by enoxacin [
60
]. Enoxacin is a small molecule and belongs to a
family of synthetic antibacterial compounds. By using a chemical
screen, it was identifi ed to promote RNA interference pathway by
enhanced siRNA-mediated mRNA degradation. Furthermore,
cells stably expressing a pri-miRNA transcript (pri-miR-125a)
showed increase in the mature form of miR-125a after addition of
enoxacin. Also, decreases in total levels of pre- and pri-miRNA
were observed. Results indicate that enoxacin targets TRBP, which,
together with Dicer, processes and loads miRNA onto RISC.
Notably, miRNA levels were only elevated when precursor miRNA
is generally abundant in the untreated cells. The fi ndings suggest
that in those cases the maturation pathway and not transcription of
primary miRNA is the rate-limiting step in miRNA biogenesis.
These studies underline the theory that maturation machinery is a
tightly controlled and by that very slow process that can be
enhanced by certain effectors. In addition, enoxacin provides a
good example of how small molecules can be used to gradually
understand the RNA maturation pathway.
In the Dicer-independent steps of miRNA maturation, it is
much more evident how dysregulation can affect only a limited
subpopulation of cellular miRNAs. The Drosha-mediated cleav-
age of pri-miRNA can take place by alternative mechanisms, as
this enzyme is a component in multiple complexes that often
also contain helicases like p68 and p72 which are implicated in
diverse miRNA maturation pathways [
61
]. Furthermore, selectivity for
certain miRNAs or miRNA families is often defi ned by RNA-binding
