Differential expression analysis
Last updated on 2024-03-05 | Edit this page
Overview
Questions
- What are the steps performed in a typical differential expression analysis?
- How does one interpret the output of DESeq2?
Objectives
- Explain the steps involved in a differential expression analysis.
- Explain how to perform these steps in R, using DESeq2.
Differential expression inference
A major goal of RNA-seq data analysis is the quantification and statistical inference of systematic changes between experimental groups or conditions (e.g., treatment vs. control, timepoints, tissues). This is typically performed by identifying genes with differential expression pattern using between- and within-condition variability and thus requires biological replicates (multiple sample of the same condition). Multiple software packages exist to perform differential expression analysis. Comparative studies have shown some concordance of differentially expressed (DE) genes, but also variability between tools with no tool consistently outperforming all others (see Soneson and Delorenzi, 2013). In the following we will explain and conduct differential expression analysis using the DESeq2 software package. The edgeR package implements similar methods following the same main assumptions about count data. Both packages show a general good and stable performance with comparable results.
The DESeqDataSet
To run DESeq2 we need to represent our count data as
object of the DESeqDataSet class. The
DESeqDataSet is an extension of the
SummarizedExperiment class (see section Importing and annotating quantified data
into R ) that stores a design formula in addition to the
count assay(s) and feature (here gene) and sample metadata. The
design formula expresses the variables which will be used in
modeling. These are typically the variable of interest (group variable)
and other variables you want to account for (e.g., batch effect
variables). A detailed explanation of design formulas and
related design matrices will follow in the section about extra exploration of design matrices.
Objects of the DESeqDataSet class can be build from count
matrices, SummarizedExperiment
objects, transcript
abundance files or htseq
count files.
Load packages
R
suppressPackageStartupMessages({
library(SummarizedExperiment)
library(DESeq2)
library(ggplot2)
library(ExploreModelMatrix)
library(cowplot)
library(ComplexHeatmap)
library(apeglm)
})
Load data
Let’s load in our SummarizedExperiment object again. In
the last episode for quality control exploration, we removed ~35% genes
that had 5 or fewer counts because they had too little information in
them. For DESeq2 statistical analysis, we do not technically have to
remove these genes because by default it will do some independent
filtering, but it can reduce the memory size of the
DESeqDataSet object resulting in faster computation. Plus,
we do not want these genes cluttering up some of the visualizations.
R
se <- readRDS("data/GSE96870_se.rds")
se <- se[rowSums(assay(se, "counts")) > 5, ]
Create DESeqDataSet
The design matrix we will use in this example is
~ sex + time. This will allow us test the difference
between males and females (averaged over time point) and the difference
between day 0, 4 and 8 (averaged over males and females). If we wanted
to test other comparisons (e.g., Female.Day8 vs. Female.Day0 and also
Male.Day8 vs. Male.Day0) we could use a different design matrix to more
easily extract those pairwise comparisons.
R
dds <- DESeq2::DESeqDataSet(se,
design = ~ sex + time)
WARNING
Warning in DESeq2::DESeqDataSet(se, design = ~sex + time): some variables in
design formula are characters, converting to factorsNormalization
DESeq2 and edgeR make the following
assumptions:
- most genes are not differentially expressed
- the probability of a read mapping to a specific gene is the same for all samples within the same group
As shown in the previous section
on exploratory data analysis the total counts of a sample (even from the
same condition) depends on the library size (total number of reads
sequenced). To compare the variability of counts from a specific gene
between and within groups we first need to account for library sizes and
compositional effects. Recall the estimateSizeFactors()
function from the previous section:
R
dds <- estimateSizeFactors(dds)
Statistical modeling
DESeq2 and edgeR model RNA-seq counts as
negative binomial distribution to account for a limited
number of replicates per group, a mean-variance dependency (see exploratory data analysis) and a
skewed count distribution.
Dispersion
The within-group variance of the counts for a gene following a negative binomial distribution with mean \(\mu\) can be modeled as:
\(var = \mu + \theta \mu^2\)
\(\theta\) represents the
gene-specific dispersion, a measure of variability or
spread in the data. As a second step, we need to estimate gene-wise
dispersions to get the expected within-group variance and test for group
differences. Good dispersion estimates are challenging with a few
samples per group only. Thus, information from genes with similar
expression pattern are “borrowed”. Gene-wise dispersion estimates are
shrinked towards center values of the observed distribution of
dispersions. With DESeq2 we can get dispersion estimates
using the estimateDispersions() function. We can visualize
the effect of shrinkage using plotDispEsts():
R
dds <- estimateDispersions(dds)
OUTPUT
gene-wise dispersion estimatesOUTPUT
mean-dispersion relationshipOUTPUT
final dispersion estimatesR
plotDispEsts(dds)
Testing
We can use the nbinomWaldTest()function of
DESeq2 to fit a generalized linear model (GLM) and
compute log2 fold changes (synonymous with “GLM coefficients”,
“beta coefficients” or “effect size”) corresponding to the variables of
the design matrix. The design matrix is directly
related to the design formula and automatically derived from
it. Assume a design formula with one variable (~ treatment)
and two factor levels (treatment and control). The mean expression \(\mu_{j}\) of a specific gene in sample
\(j\) will be modeled as following:
\(log(μ_j) = β_0 + x_j β_T\),
with \(β_T\) corresponding to the log2 fold change of the treatment groups, \(x_j\) = 1, if \(j\) belongs to the treatment group and \(x_j\) = 0, if \(j\) belongs to the control group.
Finally, the estimated log2 fold changes are scaled by their standard error and tested for being significantly different from 0 using the Wald test.
R
dds <- nbinomWaldTest(dds)
Explore results for specific contrasts
The results() function can be used to extract gene-wise
test statistics, such as log2 fold changes and (adjusted) p-values. The
comparison of interest can be defined using contrasts, which are linear
combinations of the model coefficients (equivalent to combinations of
columns within the design matrix) and thus directly related to
the design formula. A detailed explanation of design matrices and how to
use them to specify different contrasts of interest can be found in the
section on the exploration of design
matrices. In the results() function a contrast can be
represented by the variable of interest (reference variable) and the
related level to compare using the contrast argument. By
default the reference variable will be the last
variable of the design formula, the reference level
will be the first factor level and the last level will be used
for comparison. You can also explicitly specify a contrast by the
name argument of the results() function. Names
of all available contrasts can be accessed using
resultsNames().
In the lesson example the last variable of the design formula is
time. The reference level (first in
alphabetical order) is Day0 and the last
level is Day8
R
levels(dds$time)
OUTPUT
[1] "Day0" "Day4" "Day8"No worries, if you had difficulties to identify the default contrast
the output of the results() function explicitly states the
contrast it is referring to (see below)!
To explore the output of the results() function we can
use the summary() function and order results by
significance (p-value). Here we assume that we are interested in changes
over time (“variable of interest”), more specifically genes
with differential expression between Day0 (“reference
level”) and Day8 (“level to compare”). The model we used
included the sex variable (see above). Thus our results
will be “corrected” for sex-related differences.
R
## Day 8 vs Day 0
resTime <- results(dds, contrast = c("time", "Day8", "Day0"))
summary(resTime)
OUTPUT
out of 27430 with nonzero total read count
adjusted p-value < 0.1
LFC > 0 (up) : 4472, 16%
LFC < 0 (down) : 4282, 16%
outliers [1] : 10, 0.036%
low counts [2] : 3723, 14%
(mean count < 1)
[1] see 'cooksCutoff' argument of ?results
[2] see 'independentFiltering' argument of ?resultsR
# View(resTime)
head(resTime[order(resTime$pvalue), ])
OUTPUT
log2 fold change (MLE): time Day8 vs Day0
Wald test p-value: time Day8 vs Day0
DataFrame with 6 rows and 6 columns
baseMean log2FoldChange lfcSE stat pvalue
<numeric> <numeric> <numeric> <numeric> <numeric>
Asl 701.343 1.117332 0.0594128 18.8062 6.71212e-79
Apod 18765.146 1.446981 0.0805056 17.9737 3.13229e-72
Cyp2d22 2550.480 0.910202 0.0556002 16.3705 3.10712e-60
Klk6 546.503 -1.671897 0.1057395 -15.8115 2.59339e-56
Fcrls 184.235 -1.947016 0.1277235 -15.2440 1.80488e-52
A330076C08Rik 107.250 -1.749957 0.1155125 -15.1495 7.63434e-52
padj
<numeric>
Asl 1.59057e-74
Apod 3.71130e-68
Cyp2d22 2.45431e-56
Klk6 1.53639e-52
Fcrls 8.55406e-49
A330076C08Rik 3.01518e-48R
## Male vs Female
resSex <- results(dds, contrast = c("sex", "Male", "Female"))
summary(resSex)
OUTPUT
out of 27430 with nonzero total read count
adjusted p-value < 0.1
LFC > 0 (up) : 51, 0.19%
LFC < 0 (down) : 70, 0.26%
outliers [1] : 10, 0.036%
low counts [2] : 8504, 31%
(mean count < 6)
[1] see 'cooksCutoff' argument of ?results
[2] see 'independentFiltering' argument of ?resultsR
head(resSex[order(resSex$pvalue), ])
OUTPUT
log2 fold change (MLE): sex Male vs Female
Wald test p-value: sex Male vs Female
DataFrame with 6 rows and 6 columns
baseMean log2FoldChange lfcSE stat pvalue
<numeric> <numeric> <numeric> <numeric> <numeric>
Xist 22603.0359 -11.60429 0.336282 -34.5076 6.16852e-261
Ddx3y 2072.9436 11.87241 0.397493 29.8683 5.08722e-196
Eif2s3y 1410.8750 12.62513 0.565194 22.3377 1.58997e-110
Kdm5d 692.1672 12.55386 0.593607 21.1484 2.85293e-99
Uty 667.4375 12.01728 0.593573 20.2457 3.87772e-91
LOC105243748 52.9669 9.08325 0.597575 15.2002 3.52699e-52
padj
<numeric>
Xist 1.16684e-256
Ddx3y 4.81149e-192
Eif2s3y 1.00253e-106
Kdm5d 1.34915e-95
Uty 1.46702e-87
LOC105243748 1.11194e-48Multiple testing correction
Due to the high number of tests (one per gene) our DE results will contain a substantial number of false positives. For example, if we tested 20,000 genes at a threshold of \(\alpha = 0.05\) we would expect 1,000 significant DE genes with no differential expression.
To account for this expected high number of false positives, we can
correct our results for multiple testing. By default
DESeq2 uses the Benjamini-Hochberg
procedure to calculate adjusted p-values (padj) for
DE results.
Independent Filtering and log-fold shrinkage
We can visualize the results in many ways. A good check is to explore
the relationship between log2fold changes, significant DE
genes and the genes mean count. DESeq2
provides a useful function to do so, plotMA().
R
plotMA(resTime)
We can see that genes with a low mean count tend to have larger log fold changes. This is caused by counts from lowly expressed genes tending to be very noisy. We can shrink the log fold changes of these genes with low mean and high dispersion, as they contain little information.
R
resTimeLfc <- lfcShrink(dds, coef = "time_Day8_vs_Day0", res = resTime)
OUTPUT
using 'apeglm' for LFC shrinkage. If used in published research, please cite:
Zhu, A., Ibrahim, J.G., Love, M.I. (2018) Heavy-tailed prior distributions for
sequence count data: removing the noise and preserving large differences.
Bioinformatics. https://doi.org/10.1093/bioinformatics/bty895R
plotMA(resTimeLfc)
Shrinkage of log fold changes is useful for visualization and ranking of
genes, but for result exploration typically the
independentFiltering argument is used to remove lowly
expressed genes.
Challenge
By default independentFiltering is set to
TRUE. What happens without filtering lowly expressed genes?
Use the summary() function to compare the results. Most of
the lowly expressed genes are not significantly differential expressed
(blue in the above MA plots). What could cause the difference in the
results then?
R
resTimeNotFiltered <- results(dds,
contrast = c("time", "Day8", "Day0"),
independentFiltering = FALSE)
summary(resTime)
OUTPUT
out of 27430 with nonzero total read count
adjusted p-value < 0.1
LFC > 0 (up) : 4472, 16%
LFC < 0 (down) : 4282, 16%
outliers [1] : 10, 0.036%
low counts [2] : 3723, 14%
(mean count < 1)
[1] see 'cooksCutoff' argument of ?results
[2] see 'independentFiltering' argument of ?resultsR
summary(resTimeNotFiltered)
OUTPUT
out of 27430 with nonzero total read count
adjusted p-value < 0.1
LFC > 0 (up) : 4324, 16%
LFC < 0 (down) : 4129, 15%
outliers [1] : 10, 0.036%
low counts [2] : 0, 0%
(mean count < 0)
[1] see 'cooksCutoff' argument of ?results
[2] see 'independentFiltering' argument of ?resultsGenes with very low counts are not likely to see significant differences typically due to high dispersion. Filtering of lowly expressed genes thus increased detection power at the same experiment-wide false positive rate.
Visualize selected set of genes
The amount of DE genes can be overwhelming and a ranked list of genes can still be hard to interpret with regards to an experimental question. Visualizing gene expression can help to detect expression pattern or group of genes with related functions. We will perform systematic detection of over represented groups of genes in a later section. Before this visualization can already help us to get a good intuition about what to expect.
We will use transformed data (see exploratory data analysis) and the top differentially expressed genes for visualization. A heatmap can reveal expression pattern across sample groups (columns) and automatically orders genes (rows) according to their similarity.
R
# Transform counts
vsd <- vst(dds, blind = TRUE)
# Get top DE genes
genes <- resTime[order(resTime$pvalue), ] |>
head(10) |>
rownames()
heatmapData <- assay(vsd)[genes, ]
# Scale counts for visualization
heatmapData <- t(scale(t(heatmapData)))
# Add annotation
heatmapColAnnot <- data.frame(colData(vsd)[, c("time", "sex")])
heatmapColAnnot <- HeatmapAnnotation(df = heatmapColAnnot)
# Plot as heatmap
ComplexHeatmap::Heatmap(heatmapData,
top_annotation = heatmapColAnnot,
cluster_rows = TRUE, cluster_columns = FALSE)
Output results
We may want to to output our results out of R to have a stand-alone
file. The format of resTime only has the gene symbols as
rownames, so let us join the gene annotation information, and then write
out as .csv file:
R
head(as.data.frame(resTime))
head(as.data.frame(rowRanges(se)))
temp <- cbind(as.data.frame(rowRanges(se)),
as.data.frame(resTime))
write.csv(temp, file = "output/Day8vsDay0.csv")