QC and downstream analysis for differential expression RNA-seq

                      message=FALSE, error=FALSE,

Lorena Pantano Harvard TH Chan School of Public Health, Boston, US


General QC figures from DE analysis

We are going to do a differential expression analysis with edgeR/DESeq2. We have an object that is coming from the edgeR package. It contains a gene count matrix for 85 TSI HapMap individuals, and the gender information. With that, we are going to apply the glmFit function or r Biocpkg("DESeq2") to get genes differentially expressed between males and females.

idx <- c(1:10, 75:85)
dds <- DESeqDataSetFromMatrix(assays(humanGender)[[1]][1:1000, idx],
                              colData(humanGender)[idx,], design=~group)
dds <- DESeq(dds)
res <- results(dds)

We need to extract the experiment design data.frame where the condition is Male or Female.

counts <- counts(dds, normalized = TRUE)
design <- as.data.frame(colData(dds))

Size factor QC

A main assumption in library size factor calculation of edgeR and DESeq2 (and others) is that the majority of genes remain unchanged. Plotting the distribution of gene ratios between each gene and the average gene can show how true this is. Not super useful for many samples because the plot becomes crowed.

degCheckFactors(counts[, 1:6])

Mean-Variance QC plots

p-value distribution gives an idea on how well you model is capturing the input data and as well whether it could be some problem for some set of genes. In general, you expect to have a flat distribution with peaks at 0 and 1. In this case, we add the mean count information to check if any set of genes are enriched in any specific p-value range.

Variation (dispersion) and average expression relationship shouldn't be a factor among the differentially expressed genes. When plotting average mean and standard deviation, significant genes should be randomly distributed.

In this case, it would be good to look at the ones that are totally outside the expected correlation.

You can put this tree plots together using degQC.

degQC(counts, design[["group"]], pvalue = res[["pvalue"]])

Covariates effect on count data

Another important analysis to do if you have covariates is to calculate the correlation between PCs from PCA analysis to different variables you may think are affecting the gene expression. This is a toy example of how the function works with raw data, where clearly library size correlates with some of the PCs.

resCov <- degCovariates(log2(counts(dds)+0.5),

Covariates correlation with metrics

Also, the correlation among covariates and metrics from the analysis can be tested. This is useful when the study has multiple variables, like in clinical trials. The following code will return a correlation table, and plot the correlation heatmap for all the covariates and metrics in a table.

cor <- degCorCov(colData(dds))

QC report

A quick HTML report can be created with createReport to show whether a DE analysis is biased to a particular set of genes. It contains the output of degQC,degVB and degMB.

createReport(colData(dds)[["group"]], counts(dds, normalized = TRUE),
             row.names(res)[1:20], res[["pvalue"]], path = "~/Downloads")

Report from DESeq2 analysis

Here, we show some useful plots for differentially expressed genes.


DEGSet is a class to store the DE results like the one from results function. r Biocpkg("DESeq2") offers multiple way to ask for contrasts/coefficients. With degComps is easy to get multiple results in a single object:

degs <- degComps(dds, combs = "group",
                 contrast = list("group_Male_vs_Female",
                                 c("group", "Female", "Male")))

degs contains 3 elements, one for each contrast/coefficient asked for. It contains the results output in the element raw and the output of lfcShrink in the element shrunken. To obtain the results from one of them, use the method dge:


By default it would output the shrunken table always, as defined by degDefault, that contains the default table to get.

To get the original results table, use the parameter as this:

deg(degs[[1]], "raw", "tibble")

Note that the format of the output can be changed to tibble, or data.frame with a third parameter tidy.

The table will be always sorted by padj.

And easy way to get significant genes is:

significants(degs[[1]], fc = 0, fdr = 0.05)

This function can be used as well for a list of comparisons:

significants(degs, fc = 0, fdr = 0.05)

And it can returns the full table for a list:

significants(degs, fc = 0, fdr = 0.05, full = TRUE)

Since log2FoldChange are shrunken, the method for DEGSet class now can plot these changes as follow:

degMA(degs[[1]], diff = 2, limit = 3)

The blue arrows indicate how foldchange is affected by this new feature.

As well, it can plot the original MA plot:

degMA(degs[[1]], diff = 2, limit = 3, raw = TRUE)

or the correlation between the original log2FoldChange and the new ones:

degMA(degs[[1]], limit = 3, correlation = TRUE)

Volcano plots

Volcano plot using the output of r Biocpkg("DESeq2"). It mainly needs data.frame with two columns (logFC and pVal). Specific genes can be plot using the option plot\_text (subset of the previous data.frame with a 3rd column to be used to plot the gene name).

res[["id"]] <- row.names(res)
show <- as.data.frame(res[1:10, c("log2FoldChange", "padj", "id")])
degVolcano(res[,c("log2FoldChange", "padj")], plot_text = show)

Note that the function is compatible with DEGset. Using degVolcano(degs[[1]]) is valid.

Gene plots

Plot top genes coloring by group. Very useful for experiments with nested groups. xs can be time or WT/KO, and group can be treated/untreated. Another classification can be added, like batch that will plot points with different shapes.

degPlot(dds = dds, res = res, n = 6, xs = "group")

Another option for plotting genes in a wide format:

degPlotWide(dds, rownames(dds)[1:5], group="group")

Markers plots

Markers can be used to show whether different conditions are enriched in different markers. For instance, in this example, Females and Males show different total expression for chromosome X/Y markers

degSignature(humanGender, geneInfo, group = "group")

Full report

If you have a DESeq2 object, you can use degResults to create a full report with markdown code inserted, including figures and table with top de-regulated genes, GO enrichment analysis and heatmaps and PCA plots. If you set \Rcode{path_results}, different files will be saved there.

resreport <- degResults(dds = dds, name = "test", org = NULL,
                        do_go = FALSE, group = "group", xs = "group",
                        path_results = NULL)

Interactive shiny-app

Browsing gene expression can help to validate results or select some gene for downstream analysis. Run the following lines if you want to visualize your expression values by condition:

degObj(counts, design, "degObj.rda")
shiny::runGitHub("lpantano/shiny", subdir="expression")

Detect patterns of expression

In this section, we show how to detect pattern of expression. Mainly useful when data is a time course experiment. degPatterns needs a expression matrix, the design experiment and the column used to group samples.

ma = assay(rlog(dds))[row.names(res)[1:100],]
res <- degPatterns(ma, design, time = "group")

Useful functions

This section shows some useful functions during DEG analysis.

Filter genes by group

degFilter helps to filter genes with a minimum read count by group.

cat("gene in original count matrix: 1000")
filter_count <- degFilter(counts(dds),
                          design, "group",
                          min=1, minreads = 50)
cat("gene in final count matrix", nrow(filter_count))

Generate colors for metadata variables

This functions allows you to create colors for metadata columns to be used as annotation for columns in a heatmap figure.

th <- HeatmapAnnotation(df = colData(dds),
                        col = degColors(colData(dds), TRUE))
Heatmap(log2(counts(dds) + 0.5)[1:10,],
        top_annotation = th)

pheatmap(log2(counts(dds) + 0.5)[1:10,], 
         annotation_col = as.data.frame(colData(dds))[,1:4],
         annotation_colors = degColors(colData(dds)[1:4],
                                       con_values = c("white",

Session info


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DEGreport documentation built on Nov. 8, 2020, 7:23 p.m.