RESEARCH ARTICLE Open Access

Regulatory coordination of clustered microRNAsbased on microRNA-transcription factorregulatory networkJin Wang1, Martin Haubrock2, Kun-Ming Cao1, Xu Hua1, Chen-Yu Zhang1*, Edgar Wingender2* and Jie Li1*

Abstract

Background: MicroRNA (miRNA) is a class of small RNAs of ~22nt which play essential roles in many crucialbiological processes and numerous human diseases at post-transcriptional level of gene expression. It has beenrevealed that miRNA genes tend to be clustered, and the miRNAs organized into one cluster are usuallytranscribed coordinately. This implies a coordinated regulation mode exerted by clustered miRNAs. However, howthe clustered miRNAs coordinate their regulations on large scale gene expression is still unclear.

Results: We constructed the miRNA-transcription factor regulatory network that contains the interactions betweentranscription factors (TFs), miRNAs and non-TF protein-coding genes, and made a genome-wide study on theregulatory coordination of clustered miRNAs. We found that there are two types of miRNA clusters, i.e. homo-clusters that contain miRNAs of the same family and hetero-clusters that contain miRNAs of various families. Ingeneral, the homo-clustered as well as the hetero-clustered miRNAs both exhibit coordinated regulation since themiRNAs belonging to one cluster tend to be involved in the same network module, which performs a relativelyisolated biological function. However, the homo-clustered miRNAs show a direct regulatory coordination that isrealized by one-step regulation (i.e. the direct regulation of the coordinated targets), whereas the hetero-clusteredmiRNAs show an indirect regulatory coordination that is realized by a regulation comprising at least three steps (e.g. the regulation on the coordinated targets by a miRNA through a sequential action of two TFs). The direct andindirect regulation target different categories of genes, the former predominantly regulating genes involved inemergent responses, the latter targeting genes that imply long-term effects.

Conclusion: The genomic clustering of miRNAs is closely related to the coordinated regulation in the generegulatory network. The pattern of regulatory coordination is dependent on the composition of the miRNA cluster.The homo-clustered miRNAs mainly coordinate their regulation rapidly, while the hetero-clustered miRNAs exertcontrol with a delay. The diverse pattern of regulatory coordination suggests distinct roles of the homo-clusteredand the hetero-clustered miRNAs in biological processes.

BackgroundMicroRNA (miRNA) is a class of ~22 nt small non-cod-ing RNAs (ncRNAs) which inhibit gene expression atpost-transcriptional stage by binding to the 3’UTR ofmRNAs. They play essential roles in many crucial biolo-gical processes, including development, differentiation,

apoptosis and cell proliferation [1-4], as well as numer-ous human diseases, such as chronic lymphocytic leuke-mia, fragile X syndrome, and various types of cancers[5-8].The studies on the biogenesis of miRNAs [9,10] show

that miRNA is firstly transcribed as pri-miRNA (i.e. pri-mary miRNA) in the nucleus, then exported to the cyto-plasm after being cleaved into pre-miRNA (precursormiRNA). In cytoplasm, pre-miRNA is processed intomature miRNA and incorporated into the RNA-inducedsilencing complex (RISC) which subsequently binds tothe 3’UTR of mRNAs. The conversion of pre-miRNA to

* Correspondence: cyzhang@nju.edu.cn; e.wingender@med.uni-goettingen.de; jlee@nju.edu.cn1The State Key Laboratory of Pharmaceutical Biotechnology and JiangsuEngineering Research Center for MicroRNA Biology and Biotechnology2Department of Bioinformatics, Medical School, Georg August University ofGöttingen, Goldschmidtstrasse 1, D-37077 Göttingen, GermanyFull list of author information is available at the end of the article

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© 2011 Wang et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction inany medium, provided the original work is properly cited.

mature miRNA is generally one-to-one (i.e. one pre-miRNA once generates one mature miRNA), althoughboth strands of pre-miRNAs could potentially becomemature miRNAs [11]. However, the splicing of one pri-miRNA could result in multiple pre-miRNAs. This isbecause miRNA genes tend to be clustered in the gen-ome [12] and one pri-miRNA could be a cluster of sev-eral miRNAs [13].A set of miRNAs that are closely distributed in gen-

ome is termed as the miRNA cluster. The clusteringpropensity of miRNAs was first been discovered bylarge-scale surveys of small ncRNAs [14,15]. At present,it has been confirmed that miRNA clusters are widelydistributed in animal genomes [16,17]. The conservationof miRNA clusters across species [18] indicates thatmiRNA clusters adapt special regulatory functions inbiological processes. In addition, it has been shown byexpression studies that the clustered miRNAs are oftenco-expressed [19-21], suggesting that they are jointlytranscribed as a polycistron. Thus, the hypothesis comesup that the genomic coordination of clustered miRNAgenes, which further leads to their coordinated tran-scription, will consequently result in a functional coordi-nation. However, it is still unclear how the clusteredmiRNAs function coordinately.Recently, Yuan X-Y et al. studied the functional coor-

dination of clustered miRNAs based on the protein-pro-tein interaction (PPI) network [22]. They found that theclustered miRNAs tend to target the mRNA of proteinsthat are located in the same functional module. Whilethis kind of correlation supports the view of functionalcoordination of clustered miRNAs, little is known aboutthe underlying mechanisms. In addition, the PPI net-work which is composed of direct protein-protein inter-actions cannot provide the successive regulation detailsof miRNAs. For example, the successive regulation on aprotein by a miRNA through one or more TFs is notincluded in the PPI network.Here, we studied the regulatory coordination of clus-

tered miRNAs based on the miRNA-Transcription fac-tor (miRNA-TF) regulatory network that comprises theinteractions between transcription factors (TF), miRNAsand non-TF protein-coding genes. We found that thereare two types of miRNA clusters, i.e. homo-clusters(miRNA clusters composed of miRNAs from a singlemiRNA family) and hetero-clusters (miRNA clusterscomposed of miRNAs from multiple miRNA families).In general, both the homo- and the hetero-clusteredmiRNAs show the behavior of regulatory coordination.However, the ways of regulatory coordination of bothtypes of clustered miRNAs are different. The homo-clustered miRNAs show a direct regulatory coordinationwhich is realized by a single regulation step (i.e. directregulation), and tend to be involved in emergency

processes, whereas the hetero-clustered miRNAs showan indirect regulatory coordination which is accom-plished by 3 or more steps, and tend to participate inmore complex processes.

ResultsHomologous and heterologous miRNA clustersThere are in total 66 miRNA clusters of human (seeMethods), which can be classified into two types. One isthe homologous cluster (homo-cluster) composed ofseveral miRNAs from the same family, and the other isthe heterologous cluster (hetero-cluster) composed ofmiRNAs of various families (see Figure 1a and Addi-tional File 1). We found that there are 25 homo-clustersand 41 hetero-clusters. The detailed family diversity ofthe miRNA clusters is characterized by the familyentropy (Efam, see Methods). It is seen that the distribu-tion of Efam shows a polarized behavior (see Figure 1b).Most of the family entropies are either 0 or 1. Thismeans most of the 66 miRNA clusters are composed ofthe miRNAs from either one family or completely differ-ent families, suggesting that there is a fundamental dif-ference of family composition between the homo-clusters and the hetero-clusters.Usually, the miRNAs of the same family target a simi-

lar set of mRNAs, since they have the same seed regions[23] which predominantly determine the targets ofmiRNA [24]. Thus, it is suggested that the homo-clus-tered and the hetero-clustered miRNAs may have quitedifferent regulatory features.

Clustered miRNAs in network modulesIn the miRNA-TF regulatory network containing theinteractions between TF, miRNA and non-TF protein-coding genes (see Methods), 39 modules that havedense interactions were found using MCODE plus [25]of the network tool Cytoscape [26]. These modules alto-gether contain 47 homo-clustered miRNAs, 132 hetero-clustered miRNAs and 232 isolated miRNAs that are farfrom other miRNAs and do not belong to any clusters.The statistical analysis (see Methods) shows that thehomo-clustered miRNAs and the hetero-clustered miR-NAs are both significantly enriched (P < 0.01) in these39 modules. Moreover, more than 50% of the modules(i.e. 22 out of 39) contain at least one miRNA pair thatcomes from the same miRNA cluster. The average clus-ter entropy (see Methods) of the 39 modules whichdescribes the diversity of the miRNA clusters within amodule is significantly lower (P < 0.01) than that in therandom case in which miRNAs were randomly assignedto groups of the same number as human miRNA clus-ters (see Methods). These findings suggest that the miR-NAs located within one genomic cluster tend to beinvolved preferentially in the same module. Such

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preference means that, in general, the clustered miRNAsexert a coordinated regulation since a module in a bio-logical network usually represents a special or relativelyindependent biological function [27].The preference of homo-clustered miRNAs in the same

module is not surprising. Since the miRNAs in a homo-cluster bind to similar targets, they tend to form a localcommunity of dense interactions associating them withcommon targets (see the orange circles in Figure 2a).Nevertheless, not all the homo-clustered miRNAs areinvolved in modules. The reason is that some homo-

clustered miRNAs have such a small number of targetsthat the local community composed of these miRNAs andtheir targets is not dense enough to be included in a mod-ule. In addition, some homologous members of hetero-clustered miRNAs are found to be involved in the samemodules (see the blue circles in Figure 2a). It seems thatthese homologous members of hetero-clustered miRNAshave the same type of coordinated regulation with that ofthe homo-clustered miRNAs. However, there are about10% hetero-clusters in which the heterologous miRNAmembers appear in the same modules (see the green

Figure 1 Homo-clusters and hetero-clusters of miRNA. a) Illustration of miRNA distributions in genome. The miRNAs of the same family arerepresented as squares of the same color; b) family entropy distribution of miRNA clusters.

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circles in Figure 2b). Since the coordinated regulation bythese heterologous miRNAs obviously does not workthrough similar target sequences, it is assumed that thereare distinct mechanisms underlying the coordinated regu-lation by hetero- and homo-clustered miRNAs.

Distinctive regulatory coordination of homo- and hetero-clustersTo measure the regulatory coordination of clusteredmiRNAs quantitatively, we studied the target overlap ofmiRNA pairs in the same cluster. The target overlap(TO) of miRNA i and miRNA j is defined as:

TOij =

N∑k

SikSjk

min(li, lj) + 1(1)

where li is the number of targets that are regulated bymiRNA i, N is the total number of targets, and sik is theelement of the connecting matrix. sik equals to 1 whenmiRNA i regulates target k directly (or indirectly), other-wise it equals to 0. Clearly, TOij varies in the range of 0to 1. The closer TOij is to 1, the more targets miRNA iand miRNA j share, and the stronger is the coordinatedregulation by miRNA i and miRNA j.We first studied the TO distribution by checking the

direct targets of miRNAs. As expected, the homo-

clustered miRNA pairs have high TOs. More than 75%miRNA pairs in homo-clusters are of the TOs higherthan 0.8 (see Figure 3a). The average TO of homo-clus-ters (0.70) is significantly higher (P < 0.05) than those forhetero-clusters (0.15) and random clusters (0.10). Never-theless, the TOs of the hetero-clustered miRNA pairs areas low as those of the miRNA pairs in random clusters (i.e. the clusters that are randomly generated by keepingthe total number of miRNAs in each cluster, see Meth-ods). About 90% miRNA pairs in hetero-clusters and ran-dom clusters are of the TOs lower than 0.3. Thisindicates that there is no regulatory coordination of het-ero-clustered miRNAs within one step. However, the TOfeatures of hetero-clustered miRNA pairs change whenthe indirect targets of miRNAs are additionally consid-ered. The behavior of TO distribution for hetero-clusterslooks more like that for the homo-clusters than the ran-dom clusters. Specifically, the relative frequency of het-ero-clusters is apparently higher than that for randomclusters when TO > 0.8 (see Figure 3b). The average TOsof homo-clusters and hetero-clusters which are 0.97 and0.96 respectively are both significantly higher than that ofrandom clusters as 0.83 (P < 0.05). This indicates that theregulatory coordination of the hetero-clustered miRNAscomes up by indirect regulations.Furthermore, we analyzed the dependence of the aver-

age TO on the number of regulation steps for homo-

Figure 2 Clustered miRNAs in two examples of modules. a) A module containing homologous miRNAs from a homo-cluster (orange) and ahetero-cluster (blue); b) a module involving heterologous miRNAs from a hetero-cluster (green).

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clusters, hetero-clusters and random clusters. A regula-tion step equal to 1 means that all the considered tar-gets are directly regulated by miRNAs. As shown inFigure 4, the TOs of homo-clusters and hetero-clusters,which are both consistently higher than those of therandom clusters, saturate when the regulation steps ismore than 5. However, there are distinct differencesbetween the curves of homo-clusters and hetero-clus-ters. While the TO curve of hetero-clusters is first closeto that of random clusters for regulation steps less than3, it approaches the curve of homo-clusters and becomesignificantly higher than that of random clusters (P <0.05) as the regulation step is larger than 3. This meansthat the regulatory coordination of hetero-clusteredmiRNAs occurs after 3 steps (see example in Figure 4b),unlike that of homo-clustered miRNAs, where it isalready effective at the first step (i.e. the direct regula-tion). Such distinction between regulatory coordinationmechanisms may indicate distinct roles of the homo-and hetero-clustered miRNAs in biological processes.

Functional analysis of targets of homo- and hetero-clustersThe function of miRNA targets was analyzed using thetool DAVID (http://david.abcc.ncifcrf.gov/, see Meth-ods). We found that the targets of homo-clustered miR-NAs are significantly involved in emergency processesthat need to be preceded rapidly, such as response to

stimuli and the intrinsic apoptotic pathway that involvesmitochondria. The latter represents an emergency pro-cess since the intrinsic apoptotic pathway is usually acti-vated to induce a rapid cell death [28]. In contrast, thehetero-clustered miRNAs are involved in complex biolo-gical processes including the metabolism and the extrin-sic apoptotic pathway that happens primarily in thecytoplasm. These processes are generally of less urgency,but they are more complex than those of homo-clusters.It has been revealed that thousands of genes and reac-tions are involved in metabolic processes [29], and theextrinsic apoptotic pathway is composed of several com-plex caspase processes [28].Moreover, the extrinsic apoptotic process is also more

complex than the internal apoptotic process since itcomprises significantly more reactions [28]. Besides,three functions (i.e. signal transduction, developmentand transport) are shared targets of homo- and hetero-clustered miRNAs.

DiscussionMicroRNA clusters, which are groups of tandemmiRNA genes that are closely located in the genome,are abundantly and widely distributed in animal gen-omes. It has been revealed that about 50% of themiRNA genes in Drosophila [15] and over 30% of themiRNA genes in human, mouse, rat and chicken arelocated in clusters [30]. Co-expression experiments of

Figure 3 Distribution of target overlaps for homo-clustered, hetero-clustered and random-clustered miRNAs. a) Targets under directregulation only; b) targets including indirect regulation.

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clustered miRNA genes show that one miRNA cluster isusually transcribed as a single transcriptional unit [21].This suggests the existence of some kind of regulatorycoordination between the clustered miRNAs. However,it is still unclear how the clustered miRNAs coordinatetheir regulation. Here, we describe a genome-wide studyon the regulatory coordination of clustered miRNAsbased on the miRNA-TF regulatory network.The miRNA clusters are classified into homo-clusters

that contain miRNAs of the same family and hetero-clusters that contain miRNAs of multiple families. Mostof the miRNA clusters are either homo-clusters or het-ero-clusters of miRNAs with completely differentfamilies. Such polarized behavior indicates that thefamily composition of clustered miRNAs may be animportant characteristic that is closely related to theregulatory features of miRNA clusters.In this study, we have used a miRNA-TF regulatory

network that represents the regulation exerted by TFsand miRNAs on gene expression of target genes. Thisregulatory network presents a comprehensive view onthe regulations of miRNAs since it involves the tran-scriptional regulation of miRNAs genes by TFs as wellas the direct regulation of the targeted mRNAs by miR-NAs. Such regulatory network has previously been usedto study the combinatory regulation of miRNAs andTFs on gene expression. For example, different types ofmiRNA-TF co-regulations have been revealed based on

the miRNA-TF regulatory network [31,32]. In addition,it is reported by Kang et al. that there are two-layer reg-ulations on the gene expression, where TFs function asimportant mediators of miRNA-initiated regulatoryeffects [33]. These studies suggest that the miRNA-TFregulatory network is a good substrate for studying thecomplex regulatory features of miRNAs.The result that the clustered miRNAs, whether they

are from homo- or hetero-clusters, preferably exert theireffect in one module suggests a general regulatory coor-dination of clustered miRNAs. Intuitively, the regulatorycoordination of homo-clustered miRNAs is ascribed tothe high sequence similarity of homologous miRNAs.However, not all the sequences of homologous miRNAsare similar enough to bind to the same targets. Morethan half of the homologous miRNA pairs share lessthan 50% of the targets (see Additional File 2). Anexample is miR-329 family, in which any pairs of thethree members (i.e. hsa-mir-543, hsa-mir-329, hsa-mir-495) share less than 20% of the targets. The homo-clus-tered miRNAs are the homologous ones that share largeamount of targets. This suggests that the homo-clus-tered miRNAs are not the arbitrary homologous miR-NAs, but the ones finely designed for the regulatorycoordination. The target overlaps of hetero-clusteredmiRNAs are much smaller than the homo-clusteredmiRNAs, but they similarly appear in the same modules.This indicates that the homo-clustered miRNAs and the

Figure 4 regulatory coordination by regulation steps. a) Variation of target overlap (TO) with the number of regulation steps; b) illustrationof 3-step regulations.

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hetero-clustered miRNAs have distinctive ways of coor-dinated regulation. In addition, there are some casesthat not all the members of miRNAs in homo-/hetero-clusters are found in the same modules. One possiblereason is that the size of modules depends on the para-meter that scales the density of interactions. If the para-meter is strict, the resulting modules, which aregenerally of small size, will include few miRNA clustermembers. Another reason may be that 10 kb is not anaccurate cutoff for the definition of miRNA clusters.The miRNA cluster members that are not found in thesame modules may not be included anymore in thecluster when there is a little deviation of the cutoff.It is clear that the regulatory coordination of homo-

clustered miRNAs is achieved by one regulation step (i.e. at the level of direct targets of miRNAs), since thehomologous miRNAs in homo-clusters have almost thesame targets. However, the regulatory coordination ofhetero-clustered miRNAs is realized by at least 3 steps.Thus, the regulatory coordination of homo-clusteredmiRNAs is direct, while that of hetero-clustered miR-NAs is indirect. These two types of coordinated regula-tion both have their own advantage. Since there is nointermediate regulator between the miRNAs and theirtargets, the direct coordinated regulation has the advan-tage of accuracy and quickness. Whereas, the indirectcoordinated regulation has the advantage of varietysince diverse types of coordinated regulation can be rea-lized by inducing additional cross-regulations betweenintermediated regulators (see Additional File 3).In general, the regulatory coordination of clustered miR-

NAs is to guarantee the validity and efficiency of miRNAregulations in a certain biological process. The direct regu-latory coordination means that the effective regulation is arapid one which is capable to cope with an emergencysituation (see Figure 5). Thus, the homo-clusters could beinvolved in biological processes that match this require-ment, such as response to certain stimuli. However, arapid coordinated regulation pushing the affected systeminto a certain direction is accompanied by the risk to letthe corresponding biological processes run out of controlunless there are additional control processes counter-act-ing this push. Furthermore, the incorporation of moreintermediates may increase the flexibility of regulation.Therefore, the indirect regulatory coordination that resultsin a delayed regulation may be adopted by complex biolo-gical processes such as metabolism.

ConclusionOur study is focused on the internal coordination ofgenomic-clustered miRNAs. The results suggest thatthere are two types of miRNA clusters, i.e. homo-clus-ters that contain miRNAs of the same family and het-ero-clusters that contain miRNAs of multiple families.

These two types of miRNA clusters show distinct beha-viors of regulatory coordination in the gene regulatorynetwork that represents the direct interactions betweenmiRNA, TF and non-TF protein-coding genes. Thehomo-clusters show a direct regulatory coordinationand tend to be involved in biological processes of emer-gency situations, while the hetero-clusters show anindirect regulatory coordination and tend to take part inmore complex biological processes.Our study shows the diversity of miRNA regulations

responding to the complex requirements of biologicalfunctions and contributes to understand the complexfunction and regulatory mechanism of miRNAs at a net-work scale.

MethodsmiRNA clustersAll the human miRNAs along with their genomic infor-mation were retrieved from miRBase 13.0 [34]. TwomiRNAs that are consecutively located within 10 kb ofeach other were considered to belong to one miRNAcluster. This definition about miRNA clusters is basedon the study of miRNA genomic distribution. In pre-vious, it has been revealed that the distances betweenmiRNA pairs located consecutively in genome are fol-lowing a biomodal distribution. The valley between thetwo peaks is located at around 10 kb, suggesting that 10kb may be the reasonable cutoff to define miRNA clus-ters [31]. In total, there are 718 human precursor miR-NAs (pre-miRNAs) of distinct genomic locations.Among these 718 pre-miRNAs, about 36% form 66miRNA clusters. The number of miRNA clusters isstable as the cutoff deviates from 10 kb. It varies at therange of 64-67 as the cutoff increases from 5 kb to 15kb. The miRNAs in the same miRNA cluster composedof intronic/extronic miRNAs are of the same host genes(see Additional File 4), suggesting that the miRNA clus-ters tend to be transcribed into one transcript. For theintergenic miRNA clusters, it is suggested that the inter-genic miRNA cluster may have short transcript less than4 kbp [35]. This means our definition about intergenicmiRNA clusters has the risk to draw an inaccurate con-clusions since some of our intergenic miRNA clusters(about 30%) are much longer than 4 kbp. To detectwhether our definition about intergenic miRNA clusterstake a bias impact on the conclusion, we compared thecoordinated behavior of our intergenic miRNA clusterswith that of a newly-defined clusters which follow theconstraint that the length of every intergenic miRNAcluster should be less than 4 kbp (i.e. the 4 kbp-con-straint intergenic miRNA cluster). The results show thatthe direct and indirect coordinated behaviors of thesetwo types of intergenic clusters are both very similar(see Additional File 5 and 6 for details). This suggests

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our definition on intergenic miRNA clusters is goodenough for our study, and it does not take any biasimpact on the final conclusion.

MiRNA familiesThe information on miRNA families has been derivedfrom Rfam database, in which various kinds of RNAsare classified into families based on their sequence andstructural alignment [36]. A miRNA family generallymeans a collection of miRNAs that are derived from acommon ancestor.

Family entropy of a miRNA clusterThe family entropy, which is defined to characterize thediversity of miRNA families in a miRNA cluster, isdefined as follows:

Efam = − 1ln Nfam

Nfam∑

i

pi ln pi (2)

Given that a cluster comprises miRNAs of Nfam

families, pi is the probability that any miRNA in thecluster belongs to family i. Efam varies in the range of 0-1. Efam = 0 means that all the miRNAs in the clustercome from the same family, and Efam = 1 means that allclustered miRNAs come from different families.

Construction of the regulatory networkTo study the regulatory feature of the clustered miR-NAs, we constructed the miRNA-TF regulatory network,

which contains the regulatory relationships betweenTFs, miRNAs and non-TF protein-coding genes forhuman (see Additional File 7). Generally, there are twokinds of interactions in the network which respectivelystart from TFs and miRNAs. An interaction startingfrom a TF means that the TF regulates the transcriptionof the target, while an interaction starting from amiRNA means that the miRNA represses the translationof the target. We predicted the interactions startingfrom a TF by searching the conserved TF binding sites(TFBSs) within a putative promoter area 1 kb upstreamthe transcriptional start site of the target. Firstly, all thepotential human TFBSs are collected from TRANSFAC(version 2009.4) [37] based on the position weightedmatrix (PWM). Secondly, the conserved TFBSs arederived from the conserved promoter area across the 5species of human, mouse, dog, cow and opossum basedon RefSeq annotation of UCSC hg18 http://genome.ucsc.edu/index.html. Finally, the conserved relationshipsbetween TFBSs and TFs are predicted using the Match-algorithm provided by TRANSFAC. Note that the pre-diction of the TFBSs is done based on phylogeneticfootprinting from five mammalian genomes in order tolimit the false positive of the TFBS prediction. Since ithas been discussed that the miRNAs within the samecluster are often transcribed simultaneously, we took awhole cluster of miRNAs as a single unit, and searchedthe TFBSs in the 10 kb upstream to the start point ofthe first miRNA in the cluster. In addition, we predictedthe interactions starting from miRNAs by the three

Figure 5 Illustration of regulations for homo-clusters and hetero-clusters. Homo-clustered miRNAs are represented as lined squares of asame color, and hetero-clustered miRNAs as lined squares of different colors. Eclipses are TF/non-TF genes. Solid arrows denote directregulation, and dash arrows indirect regulation.

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tools of Targetscan [24], Pictar [38] and Tarbase [39].The union results predicted by these three tools aretaken to give a comprehensive regulatory network sothat many potential instants of regulatory coordinationwill be dug out to help further studies in experiment.Replacing the union results with the results of Tarbasewhich are verified by experiments does not affect ourconclusion.

Statistic analysis on the clustered miRNAs in modulesEnrichment of homo-/hetero- clustersThe enrichment of homo-/hetero-clusters in the 39modules is evaluated based on the random distributionof the average homo-/hetero-clustered miRNA numbersin modules. Firstly, 1000 random cases are generated byrandomly re-participating miRNAs into the 39 moduleswith the total miRNA number in each module kept.Then the random distribution is obtained by calculatingthe average homo-/hetero-clustered miRNA numbers inmodules. Finally, the significance of the enrichment ofhomo-/hetero-clustered miRNAs in modules is evalu-ated based on the random distribution.miRNA cluster entropy of a moduleThe miRNA cluster entropy of a module (Ec) is definedas follows:

Ec = − 1ln Nc

Nc∑

i

pci ln pc

i (3)

Note that pci is the probability of miRNA cluster ioccurring in a module and Nc is the total number ofmiRNA clusters in the module. Similar as Efam, Ec variesat the range of 0-1. Ec = 0 means that all the miRNAsin the module are in a same cluster, and Ec = 1 meansthat the miRNAs are from completely different clusters.

Random clusters of miRNAsThere are totally 458 isolated miRNAs and 66 miRNAclusters in genome. If the isolated miRNAs are consid-ered as the pseudo-cluster, there are 524 (i.e. 458 + 66)clusters of miRNAs. Thus, a set of random clusters aregenerated by re-assigning miRNAs into these 524

clusters with the number of miRNAs in each clustermaintained. All the distributions for the random miRNAclusters are the ones averaged over 1000 random sets.

Functional analysis of miRNA targetsThe public tool DAVID is used to analyze the functions ofthe targets of homo-clustered and hetero-clustered miR-NAs. GO [40] is selected as the annotated database. Theannotated level is set at “GOTERM_BP_5”. “FunctionalAnnotation Clustering” with default classification strin-gency is applied to derive all the related functions associat-ing with their enrichment scores and P-values. Theenriched functions are defined as the ones with P-valueless than 0.01 and enrichment score more than 1.0 [41].All of the enriched functions for the targets of homo-clus-tered and hetero-clustered miRNAs are respectively listedin Additional File 8 and 9. To make the feature ofenriched functions more clear, we manually curated andre-categorized the enriched functions as shown in Table 1(Details in Additional File 8 and 9). Each enrichment scoreand P-value for a functional category are the average onthe values of all the included functions.

Additional material

Additional file 1: miRNA clusters. All of the 66 miRNA clusters arelisted associating with their types, i.e. homo-cluster or hetro-cluster.

Additional file 2: Distribution of target overlaps for homologousmiRNAs. The black is for the homologous miRNAs in homo-clusters, andthe red for all the homologous miRNAs.

Additional file 3: Examples of 3-steps coordinated regulation ofhetero-clustered miRNAs. Six examples embedding the cross-regulationbetween intermediate regulators are illustrated.

Additional file 4: MiRNA clusters composed of intronic/extronicmiRNAs. The information about miRNA types and host genes ofintronic/extronic clustered miRNAs is derived from miRBase 13.0.

Additional file 5: Direct regulatory coordination for two types ofintergenic miRNA clusters. Target overlap (TO) distribution describingthe direct regulatory coordination. a) for our intergenic miRNA clusters;b) for 4 kbp-constraint intergenic miRNA clusters. The black, red and blueplots are respectively for the homo-clustered, hetero-clustered andrandomized miRNA pairs (i.e. the miRNA pairs randomly selected fromdifferent intergenic miRNA clusters) that are located in intergenic miRNAclusters. The TO distributions show a similar behavior in figure a) and b),suggesting that the definition of the intergenic miRNA clusters has noimpact on the property of the direct regulatory coordination.

Table 1 Enriched functions of miRNA targets

Homo-cluster Hetero-cluster

Functional categories Enrichment score P-value Functional categories Enrichment score P-value

Response to stimuli 1.73 0.004 Metabolism 2.60 0.002

Internal apoptotic pathway 1.64 0.002 External apoptotic pathway 2.08 0.003

Signal transduction 1.43 0.008 Signal transduction 2.20 0.005

Development 1.37 0.004 Development 1.50 0.007

Transport 1.21 0.007 Transport 1.25 0.005

The enriched function, enrichment scores and P-values are given. The left three columns are for the targets of homo-clustered miRNAs, and the right threecolumns are for the targets of hetero-clustered miRNAs.

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Additional file 6: Indirect regulatory coordination for two types ofintergenic miRNA clusters. TO distribution describing the indirectregulatory coordination. a) for our intergenic miRNA clusters; b) for 4kbp-constraint intergenic miRNA clusters. The black, red and blue plotsare respectively for the homo-clustered, hetero-clustered andrandomized miRNA pairs that are located in intergenic miRNA clusters.The TO distributions show a similar behavior in figure a) and b),suggesting that the definition of the intergenic miRNA clusters has noimpact on the property of the indirect regulatory coordination.

Additional file 7: Regulatory network. The whole regulatory network isprovided in SIF format.

Additional file 8: Enriched functions of targets of homo-clusteredmiRNAs. Detailed information about the enriched functions of thetargets of homo-clustered miRNAs is given.

Additional file 9: Enriched functions of targets of hetero-clusteredmiRNAs. Detailed information about the enriched functions of thetargets of hetero-clustered miRNAs is given.

AcknowledgementsThis work was supported by the National Natural Science Foundation ofChina (Grant no. 30890044), the National Basic Research Program of China(Grant no. 2007CB814806), and the German-China joint project (Grant no.CHN08/031).

Author details1The State Key Laboratory of Pharmaceutical Biotechnology and JiangsuEngineering Research Center for MicroRNA Biology and Biotechnology.2Department of Bioinformatics, Medical School, Georg August University ofGöttingen, Goldschmidtstrasse 1, D-37077 Göttingen, Germany.

Authors’ contributionsJW did the network analysis on the regulatory coordination of clusteredmiRNAs. MH, KMC and XH participated in the construction of the miRNA-TFregulatory network. CYZ, EW and JL conceptualized and designed the study,coordinated among research collaborators, and helped to draft themanuscript. All authors read and approved the final manuscript.

Received: 12 April 2011 Accepted: 16 December 2011Published: 16 December 2011

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doi:10.1186/1752-0509-5-199Cite this article as: Wang et al.: Regulatory coordination of clusteredmicroRNAs based on microRNA-transcription factor regulatory network.BMC Systems Biology 2011 5:199.

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