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Journal of Virology, January 2009, p. 489-492, Vol. 83, No. 1
0022-538X/09/$08.00+0 doi:10.1128/JVI.01166-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
A Functional MicroRNA-155 Ortholog Encoded by the Oncogenic Marek's Disease Virus
,
Yuguang Zhao,1
Yongxiu Yao,1
Hongtao Xu,1
Luke Lambeth,1
Lorraine P. Smith,1
Lydia Kgosana,1
Xiaowei Wang,2 and
Venugopal Nair1*
Viral Oncogenesis Group, Division of Microbiology, Institute for Animal Health, Compton, Berkshire RG20 7NN, United Kingdom,1
Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 631102
Received 4 June 2008/
Accepted 13 October 2008

ABSTRACT
Kaposi's sarcoma-associated herpesvirus-encoded microRNA (miRNA)
MiR-K12-11 was recently shown to be a functional ortholog of
miR-155, a miRNA that plays a major role in lymphoid malignancies
and the modulation of immune responses. Here we show that miR-M4,
encoded by the highly oncogenic Marek's disease virus of chickens,
shares common targets with miR-155 and thus is also a functional
ortholog of miR-155, the first one identified in an alphaherpesvirus.
The observation that two distinct oncogenic herpesviruses associated
with distinct types of lymphomas in different species encode
functional miR-155 orthologs suggested the importance of this
miRNA in regulatory pathways and the biology of lymphomagenesis.

TEXT
MicroRNAs (miRNAs) constitute a large family of small noncoding
RNAs functioning as major regulators of gene expression in several
eukaryotes (
1,
22) as well as in a number of viruses, particularly
in the members of the family
Herpesviridae (
10,
17,
20). Among
the numerous miRNAs expressed in hematopoietic cells, miR-155
was shown to have the most wide-ranging effects on the biology
of lymphocytes (
7,
29,
30). An association of miR-155 with various
types of malignancies has also been demonstrated in several
studies (
8,
9,
15,
21,
26-
28). Although the precise molecular
mechanisms by which miR-155 modulates lymphocyte transformation
are not clear, it is suggested to be a combinatorial repression
of a broad range of genes such as the PU.1, BACH-1, and CEBPβ
genes (
18,
22).
Compared to the metazoan miRNAs, which are often highly conserved between species, virus-encoded miRNAs generally do not share sequence homologies with other virus- or host-encoded miRNAs (6, 17, 34). However, partial sharing of sequences, particularly in the target interaction region, can result in the conservation of miRNA functions between virus- and host-encoded miRNAs. Recent studies have demonstrated that Kaposi's sarcoma herpesvirus (KSHV)-encoded KSHV-miR-K12-11 can modulate the some of the target genes that are repressed by miR-155, thereby acting as a functional ortholog of miR-155 (11, 16, 24). As part of a study to look at the functional conservation of virus- and host-encoded miRNAs, we examined the miRNAs encoded by the oncogenic Marek's disease virus (MDV) (3-5, 34, 35) for any sequence homologies with miRNAs listed in miRBase (http://microrna.sanger.ac.uk/). One of the MDV type 1 (MDV-1)-encoded miRNAs, MDV-miR-M4, shared perfect seed sequence with gga-miR-155 and with KSHV-miR-K12-11, demonstrating its potential as a functional ortholog of miR-155. We examined whether MDV-1-miR-M4 and gga-miR-155 shared a common set of target genes by use of a recently developed miRNA target prediction algorithm, MirTarget2 (32, 33). Several of the predicted targets of MDV-1-miR-M4 (see Table S1 in the supplemental material) were common to those already identified as gga-miR-155 targets (http://mirdb.org/cgi-bin/search.cgi). Among the predicted targets, PU.1 (SPI-1), C/EBPβ, and HIVEP2 (Schnurri-2) have been validated experimentally as targets of miR-155 and the KSHV-miR-K12-11 ortholog (11, 16, 24, 30, 36). Nearly all of the predicted targets showed high sequence homology to the complementary target miRNA response element (MRE), with the sequences showing conservation between chicken and human genes (see Fig. S1 in the supplemental material), demonstrating the potential of MDV-1-miR-M4 to regulate at least some of the gga-miR-155 target genes.
In order to validate the predicted targets experimentally, we generated expression vectors for both miRNAs (Fig. 1). Sequences of all the oligonucleotides used are shown in Table S2 in the supplemental material. In the gga-miR-155 expression vector, the EF1
promoter drives a partial BIC sequence from exon 2, with sequences 50 bp upstream and
300 bp downstream of the miR-155 precursor (Fig. 1E and F). An identical vector driving the expression of MDV-1-miR-M4 from the EF1
promoter was also constructed with sequences
100 bp upstream and
500 bp downstream of the precursor (Fig. 1C). We also generated an expression vector of the whole miRNA cluster (miR-M12, miR-M5, miR-M3, miR-M2, and miR-M4) driven by the cytomegalovirus (CMV) promoter in the pcDNA3.1/myc-His vector (Fig. 1B). For the construction of a miRNA-negative expression vector, we synthesized a 1,445-nucleotide NgoMIV-EcoRV fragment (CodonDevices) corresponding to the position of 134780 to 136225 in the RB-1B strain (accession number EF523390) of the MDV sequence (25) in which all the miRNAs were mutated to prevent the formation of a miRNA hairpin, at the same time retaining the potential R-LORF8 open reading frame in the antisense direction from that region (Fig. 1D). The mutant region was amplified by PCR using MDV-miR cluster For and Rev primers and cloned into the pcDNA3.1/myc-His vector. The expression of miRNAs from these constructs was confirmed by Northern blotting or quantitative real-time PCR analysis (not shown).
We next examined the biological activity of a specific miRNA
by use of an indicator reporter assay by placing perfect complementary
target sequence of the miRNA into the 3' untranscribed region
(3'UTR) of a reporter gene so that it can function as a small
interfering RNA target. We constructed sensor vectors for both
miR-M4 and miR-155 by introducing antisense complementary binding
sites within the 3'UTR of the
Renilla luciferase reporter in
the psiCHECK-2 vector (Fig.
1G). Luciferase reporter assays
were carried out with DF-1 cells (
12) in 96-well plates (10
5 cells/well) transfected with Lipofectamine 2000 (Invitrogen)
according to the manufacturer's protocols. Firefly and
Renilla luciferase activities were measured consecutively with the dual
luciferase reporter assay system (Promega) by use of a Lucy
1 luminometer (Anthos Labtec). In all cases, a constitutively
expressed firefly luciferase activity in the psiCHECK-2 vector
served as a normalization control for transfection efficiency.
Cotransfection of each miRNA expression vector with the respective
miR-M4 and miR-155 sensor vectors (10:1 ratio) resulted in 75
to 85% knockdown of
Renilla luciferase activity for pEF6-miR-M4:pmiR-M4
sensor (M4-4s) or pEF6-miR-155:pmiR-155 sensor (155-155s) combinations
(Fig.
2). The pEF6-miR-M4:pmiR-155 sensor (M4-155s) and pEF6-miR-155:pmiR-M4
sensor (155-4s) combinations showed reciprocal knockdown efficiencies
of approximately 60 to 65%. Notably, the negative controls,
pEF6miR-M4:pmiR-M2 sensor (M4-2s) and mutant miRNA:pmiR-M4 sensor
(Mut-4s), showed no repression effect (Fig.
2). These assays
demonstrated that miR-M4 and miR-155 can inhibit the expression
of their own and each other's sensor vectors.
Having shown that miR-M4 and miR-155 are functional in the chicken
cells through indicator assays, we asked whether these miRNAs
can downregulate any of the predicted targets. For the functional
identification of miRNA target genes, we used reporter constructs
containing three copies of the MRE from the 3'UTR of the predicted
target gene downstream of the
Renilla luciferase in the psiCHECK-2
vector (Fig.
1G). The use of such reporter constructs with multiple
MREs has proven to be a valid functional method for the identification
of several miRNA targets (
13,
14,
19,
23,
24,
31). Reporter
assays using high ratios (20:1) of miRNA expression to reporter
plasmids showed very good silencing of the predicted targets
for both gga-miR-155 and MDV-1-miR-M4, respectively, when expressed
either individually or as part of the MDV-1-miR cluster 1 (Fig.
3). Restoration of the
Renilla luciferase levels in the mutant
reporter constructs with MRE (seed mutant) or miRNA (miR mutant)
mutations confirmed the specificity of the suppression of luciferase
by the two miRNAs. Thus, the reporter assays demonstrated that
targets such as PU.1, CEBPβ, HIVEP2, BCL2L13, and PDCD6
can be negatively regulated by both gga-miR-155 and MDV-1-miR-M4.
In order to obtain more evidence that MDV-miR-M4 and miR-155
are true functional orthologs, we carried out further analysis
on one of the predicted targets, PU.1. First, we repeated the
reporter assay using a 110-bp fragment (see Table S1 in the
supplemental material) of the chicken PU.1 3'UTR transcript
(accession no. NM_205023) that contained the predicted MRE.
A second potential MRE in the 468-nucleotide-long 3'UTR with
only a partial seed match at positions 2 to 7 that failed to
match the MirTarget2 prediction criteria was not included in
the assay. A mutant reporter construct (PU.1-mu2) in which two
nucleotide positions of the MRE were mutated was included for
validating the specificity of the assay. Reporter assays with
DF-1 cells transfected with the miRNA expression constructs
showed that miR-M4 expressed alone or as part of the miR cluster,
as well as miR-155, induced nearly 50% silencing of luciferase
(Fig.
4a), confirming the findings obtained from the PU.1 multi-MRE
reporter construct shown in Fig.
3. The absence of silencing
of the mutant PU.1-mu2 reporter further validated the specificity
of the assay. Next, we measured miR-M4- and miR-155-mediated
silencing by directly measuring the levels of PU.1 expression
in the chicken myelomonocytic cell line HD11 (
2), stably expressing
these miRNAs. Western blot analysis of PU.1 levels using rabbit
anti-SPIB polyclonal antibody (Aviva Systems Biology) showed
that miR-M4 expressed alone or as part of the cluster and miR-155
had a negative effect on PU.1 levels compared to what was seen
for control HD11 cells or mutant constructs (Fig.
4b and c).
At least two oncogenic herpesviruses are known to exploit miR-155
regulatory pathways. Epstein-Barr virus upregulates cellular
miR-155 expression in latently infected B cells (
37), and KSHV
encodes a functional miR-155 ortholog (
11,
24). Here we demonstrate
that MDV also utilizes miR-155 pathways by encoding the functional
ortholog miR-M4. Demonstration of a functional ortholog of miR-155
encoded by two distantly related herpesviruses associated with
two distinct types of tumors in different host species strongly
indicated the significance of the miR-155 pathway in lymphomagenesis.
Interestingly, the expression of endogenous miR-155 was significantly
reduced in cells transformed by both these viruses (reference
24 and our unpublished data). The reasons for the downregulation
of endogenous miR-155 in these transformed cells are not known.
However, a demonstration of the activation of miR-155 expression
through conserved AP-1 elements would suggest a role for autoregulatory
mechanisms (
37). Although the precise roles and molecular pathways
of miR-155 in neoplastic transformation are not fully known,
its repressive function on transcriptional factors such as PU.1
can have wide-ranging effects on the cellular milieu and the
global gene expression profiles seen for lymphocytes. Similarly,
the repression of some of the other target genes, such as the
CEBPβ, HIVEP2, BCL2L13, and PDCD6 genes, are also likely
to contribute to the induction of hematopoietic cell malignancy.

ACKNOWLEDGMENTS
We thank Malik Yousef for the prediction of targets using NBmiRTAR,
Mick Watson and Dennis Prickett for organizing the chicken 3'UTR
dataset, and Mick Gill for assistance in digital imaging and
graphics.
This work was funded by BBSRC, United Kingdom.

FOOTNOTES
* Corresponding author. Mailing address: Viral Oncogenesis Group, Division of Microbiology, Institute for Animal Health, Compton, Berkshire RG20 7NN, United Kingdom. Phone: 441635 577356. Fax: 441635 577263. E-mail:
venu.gopal{at}bbsrc.ac.uk 
Published ahead of print on 22 October 2008. 
Supplemental material for this article may be found at http://jvi.asm.org/. 

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Journal of Virology, January 2009, p. 489-492, Vol. 83, No. 1
0022-538X/09/$08.00+0 doi:10.1128/JVI.01166-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
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