GWENA - Tutorial

Gwenaëlle Lemoine

Contents

1 Overview

GWENA (Gene Whole co-Expression Network Analysis) is an R package to perform gene co-expression network analysis in a single pipeline. This pipeline includes functional enrichment of modules of co-expressed genes, phenotypcal association, topological analysis and comparisons of networks between conditions.

Using transcriptomics data from either RNA-seq or microarray, the package follows the steps displayed in Figure 1:

1. Input: data is provided as a data.frame or a matrix of expression intensities (pre-normalized).
2. Gene filtering: data is filtered according to the transcriptomic technology used.
3. Network building: a matrix of similarity score is computed between each gene with Spearman correlation, then transformed into an adjacency matrix, and finally into a topological overlap matrix.
4. Modules detection: groups of genes with closest similarity scores are detected as modules.
5. Biological integration: gene set enrichment analysis and phenotypic association (if phenotypes are provided) are performed on modules.
6. Graph visualization and topological analysis: hub genes are identified, as well as visualization of modules.
7. Networks comparison: if multiple conditions are available (time points, treatments, phenotype, etc.), analysis of modules preservation/non-preservation between conditions can be performed.

This document gives a brief tutorial using a subset of a microarray data set to show the content and value of each step in the pipeline.

.

Figure 1. Analysis pipeline of GWENA, from expression data to characterization of the modules and comparison of conditions.

2 Main steps of the pipeline

2.1 Starting with GWENA

Installation can either be from:

1. the official version of the last Bioconductor release (recommended).
2. the last stable version from the Bioc Devel branch.
3. the day-to-day development version from the Github repository.

if (!requireNamespace("BiocManager", quietly=TRUE))
  install.packages("BiocManager")

# 1. From Bioconductor release
BiocManager::install("GWENA")

# 2. From Bioconductor devel
BiocManager::install("GWENA", version = "devel")

# 3. From Github repository
BiocManager::install("Kumquatum/GWENA")
# OR
if (!requireNamespace("devtools", quietly=TRUE))
  install.packages("devtools")
devtools::install_github("Kumquatum/GWENA")

Package loading:

library(GWENA)
library(magrittr) # Not mandatory, we use the pipe `%>%` to ease readability.

threads_to_use <- 2

2.2 Input data

2.2.1 The expression data

GWENA support expression matrix data coming from either RNA-seq or microarray experiments. Expression data have to be stored as text or spreadsheet files and formatted with genes as columns and samples as rows. To read this file with R, use the appropriate function according to the data separator (e.g. read.csv, read.table). Moreover, the expression data have to be normalized and transcripts expression reduced to the gene level (See How can I reduce my transcriptomic data to the gene level ? since GWENA is designed to build gene co-expression networks.

In this vignette, we use the microarray data set GSE85358 from the Kuehne et al. study. This data was gathered from a skin ageing study and has been processed and normalized with the R script provided in Additional data n°10 of the corresponding article.

# Import expression table
data("kuehne_expr")
# If kuehne_expr was in a file :
# kuehne_expr = read.table(<path_to_file>, header=TRUE, row.names=1)

# Number of genes
ncol(kuehne_expr)
#> [1] 15801
# Number of samples
nrow(kuehne_expr)
#> [1] 48

# Overview of expression table
kuehne_expr[1:5,1:5]
#>                  A_19_P00325768 A_19_P00800244 A_19_P00801821 A_19_P00802027
#> 253949420929_1_1       10.27450       5.530172       10.75672       16.78277
#> 253949420929_1_2       10.23440       5.712894       11.05393       16.25480
#> 253949420929_1_3       10.54336       5.889068       10.92150       16.39615
#> 253949420929_1_4       10.32649       5.646343       10.55770       16.37210
#> 253949420929_2_1       10.13626       5.726866       11.23012       16.41413
#>                  A_19_P00802201
#> 253949420929_1_1       8.549254
#> 253949420929_1_2       8.313369
#> 253949420929_1_3       8.469018
#> 253949420929_1_4       7.983723
#> 253949420929_2_1       7.521542

# Checking expression data set is correctly defined
is_data_expr(kuehne_expr)
#> $bool
#> [1] TRUE
#> 
#> $reason
#> NULL

2.2.2 The metadata

To be able to perform the phenotypic association step of the pipeline (optional), we need to specify in another matrix the information associated with each sample (e.g. condition, treatment, phenotype, experiment date…). This information is often provided in a separate file (also text or spreadsheet) and can be read in R with read.csv or read.table functions.

# Import phenotype table (also called traits)
data("kuehne_traits")
# If kuehne_traits was in a file :
# kuehne_traits = read.table(<path_to_file>, header=TRUE, row.names=1)

# Phenotype
unique(kuehne_traits$Condition)
#> [1] "young" "old"

# Overview of traits table
kuehne_traits[1:5,]
#>          Slide Array Exp Condition Age
#> 1 253949420929     1 1_1     young  23
#> 2 253949420929     2 1_2       old  66
#> 3 253949420929     3 1_3     young  21
#> 4 253949420929     4 1_4       old  62
#> 5 253949420929     5 2_1     young  25

2.2.3 Using SummarizedExperiment object

GWENA is also compatible with the use of SummarizedExperiment. The previous dataset can therefore be transformed as one and used in the next steps

se_kuehne <- SummarizedExperiment::SummarizedExperiment(
  assays = list(expr = t(kuehne_expr)),
  colData = S4Vectors::DataFrame(kuehne_traits)
)

S4Vectors::metadata(se_kuehne) <- list(
  experiment_type = "Expression profiling by array",
  transcriptomic_technology = "Microarray",
  GEO_accession_id = "GSE85358",
  overall_design = paste("Gene expression in epidermal skin samples from the",
                         "inner forearms 24 young (20 to 25 years) and 24 old",
                         "(55 to 66 years) human volunteers were analysed", 
                         "using Agilent Whole Human Genome Oligo Microarrays",
                         "8x60K V2."),
  contributors = c("Kuehne A", "Hildebrand J", "Soehle J", "Wenck H", 
                   "Terstegen L", "Gallinat S", "Knott A", "Winnefeld M", 
                   "Zamboni N"),
  title = paste("An integrative metabolomics and transcriptomics study to",
                "identify metabolic alterations in aged skin of humans in", 
                "vivo"),
  URL = "https://www.ncbi.nlm.nih.gov/pubmed/28201987",
  PMIDs = 28201987
)

2.3 Gene filtering

Although the co-expression method implemented within GWENA is designed to manage and filter out low co-expressed genes, it is advisable to first reduce the dataset size. Indeed, loading a full expression matrix without filtering for uninformative data will result in excessive processing time, CPU and memory usage, and data storage. However, the author urges the users to proceed carefully during the filtering as it will impact the gene network building.

Multiple filtration methods have been natively implemented :

• For RNA-seq and microarray:
  • filter_low_var : Filtering on low variation of expression
• For RNA-seq data:
  • filter_RNA_seq(<...>, method = "at least one"): only one sample needs to have a value above the minimal count threshold in the gene
  • filter_RNA_seq(<...>, method = "mean"): the means of all samples for the gene needs to be above min_count
  • filter_RNA_seq(<...>, method = "all"): all samples for the gene need to be above min_count

NB: The authors of WGCNA (used in GWENA for network building) advise against using differentially expressed (DE) genes as a filter since its module detection method is based on unsupervised clustering. Moreover, using DE genes will break the scale-free property (small-world network) on which the adjacency matrix is calculated.

In this example, we will be filtering the low variable genes with filter_low_var function.

kuehne_expr_filtered <- filter_low_var(kuehne_expr, pct = 0.7, type = "median")

# Remaining number of genes
ncol(kuehne_expr_filtered)
#> [1] 11060

2.4 Network building

Gene co-expression networks are an ensemble of genes (nodes) linked to each other (edges) according to the strength of their relation. In GWENA, this strength is estimated by the computation of a (dis)similarity score which can start with a distance (euclidian, minkowski, …) but is usually a correlation. Among these, Pearson’s one is the most popular, however in GWENA we use Spearman correlation by default. It is less sensitive to outliers which are frequent in transcriptomics datasets and does not assume that the data follows the normal distribution.

The co-expression network is built according to the following sub-steps :

1. A correlation (or distance) between each pair of genes is computed.
2. A power law is fitted on the correlation matrix. This step can be performed by itself through the function get_fit.expr if needed.
3. An adjacency score is computed by adjusting previous correlations by the fitted power law.
4. A topological overlap score is computed by accounting for the network’s topology.

These successive adjustments improve the detection of modules for the next step.

# In order to fasten the example execution time, we only take an 
# arbitary sample of the genes. 
kuehne_expr_filtered <- kuehne_expr_filtered[, 1:1000]

net <- build_net(kuehne_expr_filtered, cor_func = "spearman", 
                 n_threads = threads_to_use)

# Power selected :
net$metadata$power
#> [1] 8

# Fit of the power law to data ($R^2$) :
fit_power_table <- net$metadata$fit_power_table
fit_power_table[fit_power_table$Power == net$metadata$power, "SFT.R.sq"]
#> [1] 0.917733

2.5 Modules detection

At this point, the network is a complete graph: all nodes are connected to all other nodes with different strengths. Because gene co-expression networks have a scale free property, groups of genes are strongly linked with one another. In co-expression networks these groups are called modules and assumed to be representative of genes working together to a common set of functions.

Such modules can be detected using unsupervised learning or modeling. GWENA use the hierarchical clustering but other methods can be used (kmeans, Gaussian mixture models, etc.).

modules <- detect_modules(kuehne_expr_filtered, 
                            net$network, 
                            detailled_result = TRUE,
                            merge_threshold = 0.25)

Important: Module 0 contains all genes that did not fit into any modules.

Since this operation tends to create multiple smaller modules with highly similar expression profile (based on the eigengene of each), they are usually merged into one.

# Number of modules before merging :
length(unique(modules$modules_premerge))
#> [1] 6
# Number of modules after merging: 
length(unique(modules$modules))
#> [1] 3

layout_mod_merge <- plot_modules_merge(
  modules_premerge = modules$modules_premerge, 
  modules_merged = modules$modules)

Resulting modules contain more genes whose repartition can be seen by a simple barplot.

ggplot2::ggplot(data.frame(modules$modules %>% stack), 
                ggplot2::aes(x = ind)) + ggplot2::stat_count() +
  ggplot2::ylab("Number of genes") +
  ggplot2::xlab("Module")

Each of the modules presents a distinct profile, which can be plotted in two figures to separate the positive (+ facet) and negative (- facet) correlations profile. As a summary of this profile, the eigengene (red line) is displayed to act as a signature.

# plot_expression_profiles(kuehne_expr_filtered, modules$modules)

2.6 Biological integration

2.6.1 Functional enrichment

A popular way to explore the modules consists of linking them with a known biological function by using currated gene sets. Among the available ones, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), WikiPathways, Reactome, Human Phenotype Ontology (HPO) put modules into a broader systemic perspective.

In oppositions, databases references like TRANSFAC, miRTarBase, Human Protein Atlas (HPA), and CORUM give more details about tissue/cell/condition information.

Using the over-representation analysis (ORA) tool GOSt from g:Profiler, we can retrieve the biological association for each module and plot it as follows.

enrichment <- bio_enrich(modules$modules)
plot_enrichment(enrichment)

2.6.2 Phenotypic association

If phenotypic information is available about the samples provided, an association test can help to determine if a module is specifically linked to a trait. In this case, module 1 seems to be strongly linked to Age.

# With data.frame/matrix
phenotype_association <- associate_phenotype(
  modules$modules_eigengenes, 
  kuehne_traits %>% dplyr::select(Condition, Age, Slide))

# With SummarizedExperiment
phenotype_association <- associate_phenotype(
  modules$modules_eigengenes, 
  SummarizedExperiment::colData(se_kuehne) %>% 
    as.data.frame %>% 
    dplyr::select(Condition, Age, Slide))

plot_modules_phenotype(phenotype_association)

Combination of phenotypic information with the previous functional enrichment can guide further analysis.

2.7 Graph visualization and topological analysis

Information can be retrieved from the network topology itself. For example, hub genes are highly connected genes known to be associated with key biological functions. They can be detected by different methods :

• get_hub_high_co: Highest connectivity, select the top n (n depending on parameter given) highest connected genes. Similar to WGCNA’s selection of hub genes
• get_hub_degree: Superior degree, select genes whose degree is greater than the average connection degree of the network. Definition from network theory.
• get_hub_kleinberg: Kleinberg’s score, select genes whose Kleinberg’s score is superior to the provided threshold.

Manipulation of graph objects can be quite demanding in memory and CPU usage. Caution is advised when choosing to plot networks larger than 100 genes. Since co-expression networks are complete graphs, readability is hard because all genes are connected with each other. In order to clarity visualization, edges with a similarity score below a threshold are removed.

module_example <- modules$modules$`2`
graph <- build_graph_from_sq_mat(net$network[module_example, module_example])

layout_mod_2 <- plot_module(graph, upper_weight_th = 0.999995, 
                            vertex.label.cex = 0, 
                            node_scaling_max = 7, 
                            legend_cex = 1)

As modules also follow a modular topology inside, it may be interesting to detect the sub clusters inside them to find genes working toward the same function through enrichment. The sub cluster can then be plotted on the graph to see their interaction.

net_mod_2 <- net$network[modules$modules$`2`, modules$modules$`2`] 
sub_clusters <- get_sub_clusters(net_mod_2)

layout_mod_2_sub_clust <- plot_module(graph, upper_weight_th = 0.999995,
                                      groups = sub_clusters,
                                      vertex.label.cex = 0, 
                                      node_scaling_max = 7, 
                                      legend_cex = 1)