Genome-Wide Co-Expression Analysis in Multiple Tissues
Ian C. Grieve1, Nicholas J. Dickens1,2, Michal Pravenec3,4, Vladimir Kren3,4, Norbert Hubner5, Stuart A.
Cook1,6, Timothy J. Aitman1, Enrico Petretto1,7*, Jonathan Mangion1
1MRC Clinical Sciences Centre, Imperial College, Hammersmith Hospital, London, United Kingdom, 2Institute of Cancer Research, Belmont, Sutton, Surrey, United
Kingdom, 3Institute of Biology and Medical Genetics, First Faculty of Medicine and General Teaching Hospital, Charles University, Prague, Czech Republic, 4Institute of
Physiology, Czech Academy of Sciences, Prague, Czech Republic, 5Max-Delbru ˝ck-Center for Molecular Medicine, Berlin-Buch, Berlin, Germany, 6National Heart and Lung
Institute, Imperial College, London, United Kingdom, 7Division of Epidemiology, Public Health and Primary Care, Imperial College, London, United Kingdom
Expression quantitative trait loci (eQTLs) represent genetic control points of gene expression, and can be categorized as cis-
and trans-acting, reflecting local and distant regulation of gene expression respectively. Although there is evidence of co-
regulation within clusters of trans-eQTLs, the extent of co-expression patterns and their relationship with the genotypes at
eQTLs are not fully understood. We have mapped thousands of cis- and trans-eQTLs in four tissues (fat, kidney, adrenal and
left ventricle) in a large panel of rat recombinant inbred (RI) strains. Here we investigate the genome-wide correlation
structure in expression levels of eQTL transcripts and underlying genotypes to elucidate the nature of co-regulation within
cis- and trans-eQTL datasets. Across the four tissues, we consistently found statistically significant correlations of cis-
regulated gene expression to be rare (,0.9% of all pairs tested). Most (.80%) of the observed significant correlations of cis-
regulated gene expression are explained by correlation of the underlying genotypes. In comparison, co-expression of trans-
regulated gene expression is more common, with significant correlation ranging from 2.9%–14.9% of all pairs of trans-eQTL
transcripts. We observed a total of 81 trans-eQTL clusters (hot-spots), defined as consisting of $10 eQTLs linked to a
common region, with very high levels of correlation between trans-regulated transcripts (77.2–90.2%). Moreover, functional
analysis of large trans-eQTL clusters ($30 eQTLs) revealed significant functional enrichment among genes comprising 80%
of the large clusters. The results of this genome-wide co-expression study show the effects of the eQTL genotypes on the
observed patterns of correlation, and suggest that functional relatedness between genes underlying trans-eQTLs is reflected
in the degree of co-expression observed in trans-eQTL clusters. Our results demonstrate the power of an integrative,
systematic approach to the analysis of a large gene expression dataset to uncover underlying structure, and inform future
Citation: Grieve IC, Dickens NJ, Pravenec M, Kren V, Hubner N, et al. (2008) Genome-Wide Co-Expression Analysis in Multiple Tissues. PLoS ONE 3(12): e4033.
Editor: Cathal Seoighe, University of Cape Town, South Africa
Received June 16, 2008; Accepted November 24, 2008; Published December 29, 2008
Copyright: ? 2008 Grieve et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: We acknowledge funding to ICG from the Wellcome Trust (069962/Z/02/Z); to TJA from the Medical Research Council Clinical Sciences Centre, the
Wellcome Trust Cardiovascular Functional Genomics award; to TJA and SAC from the Fondation Leducq; to EP, NJD, SAC and JM from the Medical Research
Council; to SAC from The Department of Health UK and British Heart Foundation; to NH from the German Ministry of Science and Education (BMBF/NGFN); to MP
from the Grant Agency of the Czech Republic; to MP and VK from the Ministry of Education of the Czech Republic; MP is an international research scholar of the
Howard Hughes Medical Institute; and to NH, MP and TJA from the EU-funded EURATools Integrated Project. Sponsors/funders played no direct role in the design
or conduct of this study.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com
The use of linkage analysis in combination with genome-wide
expression profiling by microarray, also known as ‘genetical
genomics’ , enables the genetic control points of gene
expression to be mapped to the genome. These have come to be
referred to as expression quantitative trait loci (eQTLs) . This
study design has in the past five years been applied to the
investigation of regulatory processes in, among others, yeast,
rodents, plants and humans [3,4,5,6,7].
A major strength of the eQTL approach is the inherent ability
to distinguish between local and distant regulation  of gene
expression, by classifying eQTLs as cis and trans. Cis-eQTLs are
those in which the eQTL maps to the physical location of the
transcript . Cis-eQTLs have been shown generally to be highly
heritable and to have a larger genetic effect than trans-eQTLs .
Of particular interest are those cis-eQTLs that co-localise with
mapped physiological quantitative trait loci (pQTLs) [5,10,11,12],
as these can be considered to be strong candidates for the genetic
regulation underlying variation in the physiological trait. To
prioritize cis-eQTLs, Schadt et al applied data resulting from the
correlation of expression profiles with phenotypic measurements to
identify candidate genes underlying physiological QTLs in mice
. Similar procedures have been carried out in other model
organisms [7,11,13,14,15]. Using these approaches in humans,
Deutsch et al  and Goring et al  identified candidate genes
for Down’s syndrome phenotypes and plasma HDL cholesterol
Trans-eQTLs are those in which the expression of a given gene
maps to a location remote from the physical location of the gene
itself. Trans-eQTLs are considered indicative of gene expression
levels that are under polygenic control . Because trans-eQTLs
generally have small genetic effects and often a correspondingly
high False Discovery Rate (FDR), they are relatively difficult to
detect [9,17]. They are however of interest as potential indirect
regulators of gene expression . It has been observed that trans-
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eQTLs form clusters, or ‘hot-spots’ , where mRNA levels of
transcripts across the genome show linkage to the same genetic
locus. Such trans-eQTL clusters have also been detected through
the application of quite straightforward correlation-based methods
[19,20] or functionally-informed correlation analysis . The
possibility of functional enrichment within trans-eQTL clusters has
been explored in other datasets [20,21], with outcomes suggesting
that the presence of clusters is predictive of biological relationships
between the underlying genes.
Co-expressed, functionally enriched trans-eQTL clusters are
suggestive of co-ordinated regulation, perhaps by a ‘master’
transcriptional regulator located within the linkage region [3,22].
Previous attempts to prioritise candidate regulators in silico have
involved the correlation of the expression levels of the genes
comprising the trans-eQTL cluster with those of candidate
regulators co-localising with the cluster locus .
Correlation-based methods have been applied to prioritise
candidate genes or to infer co-expression networks, but in many
cases genotype information is not utilised in the analysis [23,24].
Others have taken into account the effect of correlation due to
linkage disequilibrium [12,25] but not long-range allelic associa-
Here, we extend these analyses, taking into account the
correlation of the genotypes underlying cis- and trans-eQTLs in
multiple tissues. We carried out a full-scale integrative co-
expression analysis of gene expression in a large panel of rat
recombinant inbred (RI) strains, the BXH/HXB panel .
Genotypic and genetic map information was fully integrated into
the analysis. We show that taking into account genetic distance
data is requisite in the prediction of candidate regulators of trans-
eQTL clusters. The outcomes of this study provide new insight
into the relationship between gene expression and underlying
genotype in cis- and trans-eQTLs across multiple tissues.
Correlation of gene expression levels of cis- and trans-
Pairwise correlation analysis was performed in all eQTLs
detected with genome-wide significance p,0.05 in fat, kidney,
adrenal and left ventricle tissues (Table 1). The correlation structure
in genes that form cis-eQTLs were consistently found to differ
markedly from that in genes that form trans-eQTLs in all tissues.
Gene expression levels of cis-eQTLs were found to be significantly
(FDR,0.05) correlated to one another in 0.6 to 0.9% of all pairs
(Table 2). 80.4 to 91.5% of these were found to have correlated
genotypes across the RI strains (i.e., Strain Distribution Patterns
(SDPs)), most of which (.67.9% (Table S1)) could be explained by
linkage disequilibrium (LD) (Figure 1). Of the significantly
correlated pairs of cis-eQTLs that can not be explained by LD,
39.6 to 55.6% have correlated SDPs (Table S1), even though the
eQTLs are often located on different chromosomes. The remainder
of the correlated pairs of cis-eQTL genes cannot be explained by
similarity of the underlying genotypes.
Consistently across all tissues, we found a greater proportion of
pairs of trans-eQTL genes (2.9 to 14.9%) that showed significant
correlation of expression levels (Table 3), compared with cis-
eQTLs. Unlike cis-eQTLs, a substantial overrepresentation of
pairs of trans-eQTLs whose underlying genes are both located on
the same chromosome was not observed (Figure 2). However,
when the SDPs at the significantly correlated trans-eQTLs were
tested for correlation (Figure 3), we detected a high proportion
(47.5 to 63.7%) of significantly (p,0.05) similar SDPs (Table 3).
This was investigated more closely by analysing the distances
between the map locations of these trans-eQTLs. We found that
23.1 to 39.1% of significantly correlated trans-eQTL genes form
eQTLs that are located within 1 cM of one another (Table S2).
This suggests the presence of groups, or clusters, of co-regulated
Co-regulation analysis of trans-eQTL clusters
A total of 81 trans-eQTL clusters, defined as 10 or more co-
localising trans-eQTLs, were identified across all four tissues (Table
S3). Levels of significant correlation within trans-eQTL clusters
(Figure 4) were found to be much greater than in the rest of the
trans-eQTL dataset. Significant correlations ranged from 77.2 to
Table 1. Numbers of cis- and trans- expression QTLs
identified in each of the four rat tissues at genome-wide
TissueNo. of cis-eQTLs*No. of trans-eQTLs*
Kidney 718 1,033
Left Ventricle (LV)1,3622,140
*cis- and trans-eQTLs are defined as follows: A cis-eQTL is one in which the peak
of linkage is within 10 Mb of the transcript. Any other eQTL is defined as a
Table 2. Outcomes of correlation analysis of cis-eQTL genes in each of the four tissues.
Total Number of
Pairs of cis-eQTLs
No. Significantly Correlated
Pairs of cis-eQTL genesa
Correlated Pairs of
No. (%) Correlated Pairsawith
Significantly Correlated Genotypesbat
Peaks of Linkage
Fat 155,403 1,3860.9 1,114 (80.4%)
Kidney 257,4032,284 0.9 1,979 (86.6%)
Adrenal 180,9011,443 0.81,294 (89.7%)
LV926,841 5,3710.64,912 (91.5%)
aSignificantly correlated pairs are defined as those in which the absolute Pearson correlation of the gene expression levels was empirically found to be statistically
significant (FDR,0.05), following control for multiple testing.
bCorrelation of genotypes at a pair of eQTLs across the RI strain panel was assessed by calculating the matching coefficient. Genotypes are described as significantly
correlated if the total number of matched pairs is found to be significant by a two-tailed test of probability (p,0.05).
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90.2% of all pairs (Table 4) whereas in the trans-eQTL dataset as a
whole they ranged from 2.9 to 14.0% of all pairs (Table 3).
We tested for functional enrichment within trans-eQTL clusters
through Gene Ontology (GO) annotations using DAVID, a web-
based functional annotation tool . This analysis was performed
on 15 large trans-eQTL clusters consisting of 30 or more transcripts,
because smaller clusters were considered unlikely to provide
sufficient statistical power for such an analysis (see Methods). Of
these, 12 (80%) were found to have at least one significant biological
process or metabolic function GO term at an uncorrected
significance level of p,0.01 and almost half have at least one
significant term at uncorrected p,0.001 (Table 5). To assess the
significance of these findings, simulation studies were carried out
using DAVID on 200 random sets of 47 unique trans-eQTL genes.
Wefounda significantlyincreasedfunctionalenrichment atallthree
GO p-value thresholds (p,0.001, p,0.01, p,0.05) in the trans-
eQTL datasets relative to the random sets (Kolmogorov-Smirnov
Exact test p-values: p=0.001, p=0.002, p=0.004).
Figure 1. Scatter plots showing pairs of cis-eQTL genes with significantly correlated expression profiles by genetic map location.
Significantly correlated pairs of cis-eQTL genes identified in a) fat, b) kidney, c) adrenal and d) left ventricle are plotted according to genetic map
location (cM), indicating significance or otherwise of correlation of strain distribution patterns (SDPs) at the peak of linkage. Colour coding indicates
significance of correlation of SDPs (p,0.05; see Methods). Pairs consisting of probesets located close to one another, which correspondingly have
nearby peaks of linkage, are disproportionately represented among significantly correlated pairs as indicated by the red diagonal.
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Figure 2. Scatter plots showing pairs of trans-eQTL genes with significantly correlated expression profiles by transcript genetic map
location. Significantly correlated pairs of trans-eQTL genes identified in a) fat, b) kidney, c) adrenal and d) left ventricle are plotted according to
genetic map location of the transcripts. Colour coding is as in Figure 1.
Table 3. Outcomes of correlation analysis of trans-eQTL genes in each of the four tissues.
of Pairs of
Correlated Pairs of
Correlated Pairs of
No. (%) Correlated
at Peaks of Linkage
No. (%) Correlated Pairsa
with Significantly Correlated
Fat425,503 63,47714.9 31,288 (49.3%) 6,375 (10.0%)
Kidney 533,02839,2577.4 18,628 (47.5%) 4,601 (11.7%)
Adrenal434,77812,4822.9 7,945 (63.7%) 1,370 (11.0%)
LV 2,288,730 78,8263.4 44,110 (56.0%)8,450 (10.7%)
a bAs described in Table 2.
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Prediction of candidate ‘master regulators’
In order to prioritize cis-eQTLs as candidate ‘master regulators’
 of the trans-eQTL clusters, expression profiles of all cis-eQTL
genes located within 50 Mb either side of the cluster locus were
tested for co-expression with those of the trans-eQTL cluster genes.
We found levels of correlation to be dependent on the distance of
the cis-eQTL from the cluster peak of linkage. Upon linear
regression of correlation against distance between gene and peak
of linkage, R2in all four tissues was found across clusters to show a
negative relationship (Figure 5) with distance from the peak;
ranging from 0.15 to 0.43.
The analysis was subsequentlycarried out on all transcripts located
in the same region, regardless of whether they form eQTLs or not.
Regression analysis between distance and average correlation with
the trans-eQTL cluster genes showed no relationship (R2: 0–0.01)
(Figure S1), indicating that the relationship observed in cis-eQTLs is
explained by the linkage of those probesets to the genetic region
expression in the cis-eQTLs in the region, we also calculated
transcript heritability as previously described . There was found to
be no relationship between distance of the cis-eQTL from the cluster
linkage region and heritability of the cis-eQTL (Figure S2).
Figure 3. Scatter plots showing pairs of trans-eQTL genes with significantly correlated expression profiles by genetic map location
of peak of linkage. Significantly correlated pairs of trans-eQTLs identified in a) fat, b) kidney, c) adrenal and d) left ventricle are plotted according to
genetic map position of the peak of linkage. Colour coding is as in Figure 1. Over-representation of pairs of trans-eQTLs with nearby peaks of linkage
among the significantly correlated set can be observed (in the red diagonal) in all four tissues.
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These findings suggest that correlation of expression levels of cis-
eQTLs in the vicinity of the peak of linkage of a trans-eQTL cluster
with those of the trans-eQTLs making up said cluster, without taking
into account factors relating to genetic linkage is inappropriate for the
purpose of identifying putative regulators. Analysis of outliers of the
relationship between distance and correlation may hold more
promise to identify candidates than simply looking at highly
correlated eQTLs. For this purpose, Z-scores (based on the outcome
of the regression analysis) were calculated for each tissue. Cis-eQTLs
with significant Z-scores (defined as Z.2) in each tissue are shown in
Table S5. These cis-eQTLs may represent candidates for the
regulation of the trans-eQTL clusters.
Our genome-wide studies of expression levels of genes
underlying cis- and trans-eQTLs provide strong support for the
hypotheses that the categorisation of eQTLs has a genuine
biological basis that can be detected in transcript expression levels,
and that trans-eQTL clusters consist of functionally related and co-
ordinately regulated transcripts.
We observed that patterns of correlation of expression profiles
and of SDPs at the genetic location of the transcript whose
expression was measured are strikingly different between the sets
of cis- and trans-eQTLs in all four tissues (Figures 1–3). Significant
correlation between pairs of cis-eQTLs was found to be rare
(,1%, Table 2). A significant relationship has previously been
shown between the absolute correlation coefficient of pairs of
eQTLs derived from genes located on the same chromosome and
the distance apart of those genes [10,24]. Hence it was
hypothesised that correlation of cis-eQTL genes can be explained
in terms of linkage disequilibrium. We found that most of the
observed correlations between cis-eQTL genes’ expression profiles
for co-localised genes can be explained by similarity of underlying
genotypes. We also found that most significant correlations
between cis-eQTLs located on different chromosomes can be
explained by long-range allelic association. Such patterns of
association have previously been observed in a SNP map of a
mouse inbred strain panel , and hypothesised to have the
potential to produce spurious associations between transcripts’
expression profiles. Our findings suggest that correlation of
expression profiles, in isolation, is not a suitable basis for the
analysis of relationships between cis-eQTLs.
Much higher levels of significant correlation, encompassing 2.9–
14.9% of pairs, were observed between trans-eQTL genes. This
was not found to be explained by correlation of the genotypes at
the regions to which the transcripts map. However, when
genotypes at the trans-eQTL are similar or the same, the gene
pairs were disproportionately correlated (47.5 to 63.7% of
significantly correlated pairs). This observation provides strong
support for the hypothesis, formulated in studies of genetic
regulation of gene expression in S. cerevisiae , that trans-eQTL
clusters are indicative of co-regulation of the transcripts. In our
dataset, the level of significant correlation within trans-eQTL
clusters was found to be far higher than across the trans-eQTL
dataset as a whole, averaging 83.5% across the 81 clusters
Figure 4. Boxplots showing percentage within-cluster correlation of genes underlying trans-eQTLs forming trans-eQTL clusters. For
each of the four tissues, the percentages of significantly correlated pairs of trans-eQTL genes within trans-eQTL clusters are displayed. The boxplots
indicate the median, interquartile range, and range of within-cluster correlation in each tissue. The percentage significant correlation among all trans-
eQTLs is shown for each of the four tissues as horizontal lines, for purpose of comparison with the trans-eQTL clusters. One outlier is shown, a cluster
in adrenal found to have 100% significant pairwise correlation of gene expression levels (see Table S3).
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(Table 4). Additionally, we observed that much of the variability in
the levels of trans-eQTL correlation between tissues can be
explained by differences in the distribution and size of the trans-
eQTL clusters (data not shown).
Functional investigation of the large trans-eQTL clusters showed
significant over-representation of Gene Ontology (GO) terms in
80% of these clusters. While GO analysis may have limitations in
scope and accuracy, owing to the annotation challenges posed by
genes which often have complex and imprecisely defined roles and
functions , the findings presented here are in agreement with
those of Ghazalpour et al , who observed that genes in
functionally related ‘pathway sets’ are typically highly correlated.
KEGG pathway analysis of the trans-eQTL clusters was carried
out through the DAVID interface, and pathways of potential
interest were identified in a minority of the clusters (Table S6). It
has previously been shown in a study of co-regulation in a mouse
F2 intercross  that groups of highly correlated transcripts
linked to the same genomic location can be identified in a
functionally informed genome-wide correlation analysis. Here we
show across multiple tissues that highly correlated expression
profiles are a consistent motif of such trans-eQTL clusters. These
results suggest a significant degree of functional relatedness of the
genes making up the cluster (Table 5).
Expression correlation between trans-eQTL cluster transcripts
and cis-eQTL genes located in the linkage region has been
previously described as a method of identifying candidate
regulators and applied in yeast  and Arabidopsis . Here we
show a strong relationship between the distance of the cis-eQTL
from the cluster peak of linkage and the strength of correlation
(Figure 5), which was not observed when unlinked transcripts
located in the region were similarly tested (Figure S1). This, along
with the observation that there is no association between distance
from the peak of linkage and heritability of the cis-eQTL genes
(Figure S2), suggests that genotype similarity at the linkage region,
rather than the genetic influence on gene expression underlies the
relationship. On this basis, correlation-based methods of prioritis-
ing candidates for genetic regulation of trans-eQTL clusters would
be improved by taking into account the distance between the
cluster linkage region and the map location of the candidate
transcript. We therefore suggest that it may be of interest instead
to identify outliers; those cis-eQTL genes whose average
correlation with the trans-eQTL cluster genes significantly deviates
positively from the general negative regression trend. We were
able to find 54 cis-eQTLs with significant positive Z-scores (Z.2)
(Table S4), and consider that these may be worthy of further
investigation into the possibility that their correlation with the
cluster transcripts has a biological basis.
In this study, we demonstrate the power of computational analysis
of eQTL datasets across multiple tissues to provide new insights into
the genome-wide correlation structure in gene expression data. Our
findings consistently show that correlation of cis-eQTL genes’
expression profiles is primarily indicative of similarity of genetic
inheritance, as measured by the correlation of SDPs at the transcript
location. Whereas, correlation between trans-eQTLs can frequently
be explained in terms of co-regulation by a common linkage region
even though correlated transcripts forming trans-eQTL clusters are
located throughout the genome. The observation of functional
Table 4. Summary of outcomes of pairwise correlation of
trans-eQTL cluster genes.
No. of Trans-eQTL
Cluster Size (no.
Ave. % Significantly
Fat 1130.8 90.2
LV38 25.1 77.2
Overall 8123.7 83.5
aAs described in Table 2.
Table 5. Outcome of GO analysis of trans-eQTL clusters consisting of 30 or more eQTLs using DAVID, a functional annotation
Marker at Cluster
Peak of Linkage
terms at p,0.001
terms at p,0.01
terms at p,0.05
Adrenal D11Rat16 31027
Fat Cacna1s 33014
LV Ckb4334 12
LVD15Ucsf1 4602 12
LV D15Utr2512 13 20
LVD15Rat29 548 15 28
KidneyD15Rat69 579 13 20
FatD17Rat1 1463 2053
LV Crabp116504 14
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enrichment within clusters is suggestive of a relationship between co-
expression and function. Finally, we inform investigations of
candidate regulators of trans-eQTL clusters by indicating that genetic
linkage strongly influences co-expression of trans-eQTL cluster genes
and candidate regulatory genes.
Materials and Methods
The eQTL dataset
The gene expression dataset used in this study derives from the
BXH/HXB panel of 29 RI strains produced from progenitor
strains SHR and BN.Lx previously as described [5,9]. Gene
expression was measured in four tissues using Affymetrix
GeneChips with appropriate biological and technical replication.
For the retroperitoneal fat, kidney and adrenal gland tissues, Rat
Expression Array 230A GeneChips were used, on which 15,923
transcripts are represented. Rat Genome 230 2.0 GeneChips, on
which 31,099 transcripts are represented (including the same
15,923 as on 230A), were used for left ventricle. The summary
value for each expression trait (as described in ) was used in the
generation of the eQTLs upon which this analysis was performed.
This was achieved through the application of a genome-wide
Figure 5. Scatter plots showing correlation of genes underlying cluster trans-eQTLs and cis-eQTLs located in the window region,
plotted against their distance from the peak of linkage. Each point on the scatter plot represents a cis-eQTL within the defined window region
of a trans-eQTL cluster. The average Pearson coefficient of correlation of the underlying transcript expression levels of the cis-eQTL with those of the
cluster trans-eQTLs is shown to be strongly negatively correlated with distance in a) fat, b) kidney, c) adrenal, d) LV, R2for this relationship ranges
from 0.15 to 0.43. The Z-score, calculated from the vertical distance of each cis-eQTL from the regression line, is indicated by the colour of each point,
indicated by the colour legend in the top right of each plot.
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linkage methodology, testing for linkage between each expression
trait and the same 1,011 microsatellite markers in each of the four
study tissues. QTL Reaper (http://www.genenetwork.org/qtlreaper.
html) was used to carry out this analysis. The software calculates
a likelihood ratio statistic (LRS) for each combination of marker
and probeset, and uses permutation to estimate an empirical
genome-wide probability of obtaining such a score, accounting for
multiple testing . The eQTLs used in this analysis are significant
at genome-wide p,0.05.
Cis- and trans-eQTLs
The categorisation of eQTLs as cis or trans was determined,
following the merging of linkages to tightly linked markers, by
eQTL Explorer  (http://web.bioinformatics.ic.ac.uk/eqtlexplorer).
Cis-eQTLs were defined empirically as having a peak of linkage
within 10 Mb of the physical location of the probeset as described
previously . Those eQTLs mapped to locations that are 10 Mb
or more apart, or where the probeset is located on a different
chromosome to the peak of linkage are defined as trans-eQTLs.
Probeset location data is obtained from Affymetrix NetAffx (http://
www.affymetrix.com/analysis/index.affx) and checked using EnsEMBL
(http://www.ensembl.org/) (v42) data. Probesets that map to more
than one place in the genome according to the latter were
removed from the analysis using SCAMPA (http://microarray.csc.
mrc.ac.uk/scampa/), as were those that do not map at all – in which
case the cis/trans definition may be inaccurate. The resultant
filtered set used in the correlation analysis consists of a total of
8,269 eQTLs across the four tissues, 3,240 of which are defined as
cis-eQTLs and the remaining 5,029 as trans-eQTLs.
A ‘trans-eQTL cluster’ was defined as 10 or more co-localising
trans-eQTLs. Trans-eQTL clusters within 1 Mb of one another
were merged following initial detection of co-ordinated linkage of
at least 10 transcripts to the region. For the purpose of prioritising
‘master regulator’ candidates, the ‘window region’ of a trans cluster
was conservatively defined as the region of the genome 50 Mb
either side of the physical map location of the peak of linkage of
the cluster prior to any merging.
Correlation of Expression Data
Correlation of expression levels of genes underlying eQTLs was
carried out using Matlab. A raw Pearson coefficient and associated
nominal p-value were calculated for each pair of eQTLs.
Additionally, an empirical p-value was determined using a
permutation-based method robust against non-normality of the
underlying distribution . In each tissue, all pair combinations
of cis-eQTLs and of trans-eQTLs were correlated. Separately, all
pairs of trans-eQTLs within each of the 81 trans-eQTL clusters
were correlated with each other and with the cis-eQTLs in the
50 Mb ‘window region’. All of these analyses entailed the testing of
substantial numbers of simultaneous hypotheses. Statistical
significance of obtained correlation coefficients, taking this into
account, was assessed using the q-value (http://faculty.washington.
edu/jstorey/qvalue/)  method of estimating the False Discovery
Rate (FDR), and defined as q,0.05.
Correlation Analysis of Marker SDPs
The correlation of the SDPs of the markers at the peaks of
linkage of the eQTLs in each correlated pair was assessed using a
matching coefficient. For each strain, a match in the marker
genotypes was scored 1, and a mismatch scored 0. Strains with
missing genotype data at the marker locus were not used in the
calculation of the matching coefficient, which was assessed by
dividing the total number of matches by the total number of strains
with no missing genotypes. Because both correlation and anti-
correlation of SDPs can explain significant correlation of eQTL
transcript expression levels, the two-tailed significance (p,0.05) of
the resulting coefficient was found by assessing the probability of
the occurrence of all combinations, given the distribution of
Assessment of Functional Coherence of Trans-eQTL
The possibility of functional enrichment in the genes that make
up the observed trans-eQTL clusters was assessed through testing
of their GO annotations using the functional annotation clustering
component of the web-based genetic data analysis tool DAVID
(http://david.abcc.ncifcrf.gov) [28,34]. This analysis was performed on
15 clusters consisting of 30 or more eQTLs, since only these
clusters provide enough statistical power considering that not all
transcripts are annotated. Pathway analysis was performed
through the same interface, using data from KEGG (http://www.
genome.jp/kegg) . 200 control groups of 47 probesets (47 being
the median size of the 15 large trans-eQTL clusters) were randomly
generated from the set of all trans-eQTLs across all four tissues and
each was tested for enrichment of gene ontology terms in the same
way as were the real clusters using DAVID. A Kolmogorov-
Smirnov test was used to test the statistical significance of the
degree of enrichment found in the 15 large trans-eQTL clusters,
compared to the control groups. This analysis was carried out
using SPSS v14.
In the investigation of the correlation of expression levels of
trans-eQTL clusters and colocalising cis-eQTLs, the relationship
between correlation coefficient and distance of eQTL from the
peak of linkage of the trans-eQTL cluster was tested by performing
a regression analysis as follows: For each cluster, the genetic
distance between the peak of linkage and each of the cis-eQTLs in
the window region was found. The distance was plotted against the
averaged absolute correlation coefficient of the expression profile
measured by that probeset and those of the trans-eQTLs making
up the cluster. The data for all of the clusters in each tissue was
then combined to produce the plots shown in Figure 5, and linear
regression was performed on the data. In addition, the relationship
between heritability data (as described in Petretto et al ) and cis-
eQTL distance from the peak of linkage was tested (Figure S2). Z-
scores were calculated from the vertical distance of each cis-eQTL
from the regression line, following testing for normality of the
distribution of vertical distances. For comparative purposes, the
analysis as described was also performed using all of the probesets
in the ‘window region’ (as opposed to just cis-eQTLs) (Figure S1).
cluster trans-eQTLs with all probesets mapped to within 50 Mb of
the linkage region, plotted against the physical distance of the
probeset from the peak of linkage in a) fat, b) kidney, c) adrenal, d)
Found at: doi:10.1371/journal.pone.0004033.s001 (6.79 MB TIF)
Scatter plots showing correlation of genes underlying
(Figure 5) plotted against probeset heritability in a) fat, b) kidney, c)
adrenal or d) LV.
Found at: doi:10.1371/journal.pone.0004033.s002 (3.24 MB TIF)
Z-score of cis-eQTL distance from regression line
PLoS ONE | www.plosone.org9 December 2008 | Volume 3 | Issue 12 | e4033
genes (q,0.05), whose peaks of linkage are located more than
50 cM apart.
Found at: doi:10.1371/journal.pone.0004033.s003 (0.03 MB
Outcomes of correlation analysis of pairs of cis-eQTL
(q,0.05) whose peaks of linkage are located 50 cM or less apart.
Found at: doi:10.1371/journal.pone.0004033.s004 (0.05 MB
Outcomes of correlation analysis of trans-eQTL genes
of genes forming trans-eQTL clusters
Found at: doi:10.1371/journal.pone.0004033.s005 (0.14 MB
Outcomes of correlation analysis of expression profiles
‘window regions’ of trans-eQTL clusters with cluster-forming
Found at: doi:10.1371/journal.pone.0004033.s006 (0.14 MB
Outcomes of correlation of cis-eQTL genes in the
trans-eQTL cluster positively deviating (Z.2) from regression of
cluster-averaged correlation coefficient against distance of cis-
eQTL from linkage region (Figure 5).
cis-eQTL genes located within the window region of a
Found at: doi:10.1371/journal.pone.0004033.s007 (0.08 MB
analysis of large (.30 transcripts) trans-eQTL clusters.
Found at: doi:10.1371/journal.pone.0004033.s008 (0.04 MB
Supplementary information on functional enrichment
We thank the MRC CSC Computing Department for providing
computational facilities including support for large-scale statistical analysis
using MatLab and SPSS software and general technical assistance, and
Centre for Bioinformatics at Imperial College for supplying server space
and general technical assistance. We also thank Anuj Goel for helpful
comments and suggestions, and Rizwan Sarwar and Liqun He for critical
reading of the manuscript.
Conceived and designed the experiments: ICG JM. Performed the
experiments: ICG. Analyzed the data: ICG NJD EP JM. Contributed
reagents/materials/analysis tools: MP VK NH SC TJA. Wrote the paper:
ICG. Assisted in the development of statistical and computational methods:
EP NJD JM. Assisted in revision of the manuscript: EP TJA. Supervised the
writing of the paper: EP. Advised on study design and implementation:
TJA. Supervised the analysis: JM.
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