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misexpression generates tissue overgrowths ( Nairz et al ., 2006 ), a property
that was used in a reversion screen. EP-mir-278 flies were mutagenized and
screened for loss of the ability to induce large eyes. A single-point mutant in
the seed region of miR-278 was isolated as a revertant from
10,000
animals ( Nairz et al ., 2006 ), a substantial but by no means daunting number
for Drosophila genetics ( Fig. 8.1 A). One could imagine that additional
reversion screening of this and other EP-miRNA loci could produce
insights regarding the necessity of 3 0 regions of miRNAs, for example.
3.2. Directed analysis of UAS-miRNA transgenes
The realization that miRNAs can be ectopically expressed effectively using
the Gal4-UAS system has led to the production of many UAS-miRNA
transgenes and extensive illustrations of the detrimental consequences of
ectopic miRNAs ( Fig. 8.2 A). One notable set of phenotypes from miRNA
gain-of-function emerged from studies of the Drosophila Bithorax-Complex
(BX-C) locus mir-iab-4/mir-iab-8 , which generates miRNAs from bidirec-
tional transcription and processing of the same genomic hairpin locus
( Bender, 2008 ; Stark et al ., 2008 ; Tyler et al ., 2008 ). Among the targets of
these miRNAs are several homeobox genes in the BX-C, which specify the
identities of various abdominal segments ( Lewis, 1978 ). One such gene is
Ultrabithorax ( Ubx ), which normally represses the wing development pro-
gram in the segment that generates the haltere; thus Ubx mutants bear
homeotic transformation of halteres into an extra pair of wings. Misexpres-
sion of mir-iab-4 can directly repress Ubx in developing halteres, causing
them to transform partially into wings ( Ronshaugen et al ., 2005 ); while
many miRNAs can target Hox genes, this was the first demonstration that a
miRNA could induce a homeotic segment transformation in the animal.
More strikingly, mir-iab-8 has even stronger capacity to repress Ubx , and
correspondingly, its ectopic expression generates a fuller haltere-to-wing
transformation ( Stark et al ., 2008 ; Tyler et al ., 2008 ). Notably, ectopic
expression of other miRNAs with conserved seed matches in the Ubx
3 0 UTR does not generate extra wings, indicating that there is a practical
difference between the existence of conserved miRNA-binding sites and
their ability to mediate sufficient downregulation to yield mutant
phenotypes.
Another particularly compelling set of miRNA misexpression pheno-
types comes from those that inhibit apoptosis or that promote tissue growth.
Both of these activities are central to the process of oncogenesis and are
mediated by distinct effectors, since the inhibition of apoptosis by itself does
not promote tissue overgrowth. Reciprocally, promotion of cell prolifera-
tion is usually accompanied by excess cell death.
Misexpression of the bantam miRNA provided early evidence for the
notion of a miRNA oncogene, since this miRNA locus could both induce
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