RNA sequencing data analysis - Counting, normalization and differential expression


This analysis was performed using R (ver. 3.1.0).

Introduction

RNA-Seq is a valuable experiment for quantifying both the types and the amount of RNA molecules in a sample.

In this article, we will focus on comparing the expression levels of different samples, by counting the number of reads which overlap the exons of genes defined by a known annotation.

Counting reads in genes

We will work with a count matrix, which has genes along the rows and samples along the columns. The elements in the matrix give the number of reads which could be uniquely aligned to a given gene for a given sample.

Load data

We will work with the Hammer et al dataset, as prepared by the ReCount website

ReCount is an online resource consisting of RNA-seq gene count datasets built using the raw data from 18 different studies. The raw sequencing data (.fastq files) were processed with Myrna to obtain tables of counts for each gene.

This is really helpful for us, so we don’t have to download all the FASTQ files and map them ourselves. If you use this resource, you should cite Frazee (2011), and cite the appropriate paper for the experimental data that you download.

Here we read in the Eset hosted by ReCount, and turn it into a SummarizedExperiment. The matrix is an expression set.

#download data
link <- "http://bowtie-bio.sourceforge.net/recount/ExpressionSets/hammer_eset.RData"
if (!file.exists("hammer_eset.RData")) download.file(link, "hammer_eset.RData")
#Load the matrix
load("hammer_eset.RData")
library(Biobase)
library(GenomicRanges)
# the SimpleList() part below is only necessary for Bioc <= 2.13
#se <- SummarizedExperiment(SimpleList(counts=exprs(hammer.eset)))
se <- SummarizedExperiment(exprs(hammer.eset))
colData(se) <- DataFrame(pData(hammer.eset))

Column data contains some informations about sample caracteristics. We have the eight samples (4 controls and four of which had spinal nerve ligation). There’s sequencing after two months after spinal nerve ligation and sequencing after two weeks of spinal nerve ligation.

#column data
colData(se)
## DataFrame with 8 rows and 5 columns
##                   sample.id num.tech.reps protocol         strain     Time
##                                  
## SRX020102         SRX020102             1  control Sprague Dawley 2 months
## SRX020103         SRX020103             2  control Sprague Dawley 2 months
## SRX020104         SRX020104             1   L5 SNL Sprague Dawley 2 months
## SRX020105         SRX020105             2   L5 SNL Sprague Dawley  2months
## SRX020091-3     SRX020091-3             1  control Sprague Dawley  2 weeks
## SRX020088-90   SRX020088-90             2  control Sprague Dawley  2 weeks
## SRX020094-7     SRX020094-7             1   L5 SNL Sprague Dawley  2 weeks
## SRX020098-101 SRX020098-101             2   L5 SNL Sprague Dawley  2 weeks
#We need to fix a typo in the Time column:
colData(se)$Time[4] <- "2 months"
colData(se)$Time <- factor(colData(se)$Time)
colData(se)$Time
## [1] 2 months 2 months 2 months 2 months 2 weeks  2 weeks  2 weeks  2 weeks 
## Levels: 2 months 2 weeks

Normalization

Why RNAseq data should be normalized ?

The counts of the summarized experiment are in the assay slot.

#The count of experiment
head(assay(se))
##                    SRX020102 SRX020103 SRX020104 SRX020105 SRX020091-3 SRX020088-90 SRX020094-7 SRX020098-101
## ENSRNOG00000000001         2         4        18        24           7            4          93            77
## ENSRNOG00000000007         4         1         3         1           5            4           9             4
## ENSRNOG00000000008         0         1         4         2           0            5           2             6
## ENSRNOG00000000009         0         0         0         0           0            0           0             0
## ENSRNOG00000000010        19        10        19        13          50           57          45            58
## ENSRNOG00000000012         7         5         1         0          31           26          12             9
#column sums of the count
colSums(assay(se))
##     SRX020102     SRX020103     SRX020104     SRX020105   SRX020091-3  SRX020088-90   SRX020094-7 SRX020098-101 
##       5282855       4562100       4897807       5123782      17705411      17449646      23649094      23537179

If you look at the column sums of the counts, you can see that each sample had a different amount of reads which could be aligned to the different genes. So if we’re going to do some analysis, we definitely need to take care of the fact that the different samples had different sequencing depth.

It’s not a great idea to use the number of reads which map to genes as a normalizing factor. There have been a number of papers showing that, this number is susceptible to being altered by a single gene. You might have one or a dozen genes which have very high gene expression, and those genes play the largest role in determining this sum. There are a number of more robust estimators. Probably using any of those robust estimators is better than using this sum.

We’re going to use the median ratio method, which is in the DESeq2 package.

Normalization using DESeq2 (size factors)

We will use the DESeq2 package to normalize the sample for sequencing depth. For now, don’t worry about the design argument.

In order to use this normalization method, we have to build a DESeqDataSet, which just a summarized experiment with something called a design (a formula which specifies the design of the experiment). So we’ll explain this in the next unit, but we just need to specify something. So I’m going to specify tilde 1.

# biocLite("DESeq2")
library(DESeq2)
dds <- DESeqDataSet( se, design = ~ 1 )
#names(assays(dds))

The following code estimates size factors to account for differences in sequencing depth. So the value are typically centered around 1. If all the samples have exactly the same sequencing depth, you expect these numbers to be near 1. But instead, we see that the first sample and the 7th sample have about a difference of more than 4.

#Estimate size factors
dds <- estimateSizeFactors( dds )
sizeFactors(dds)
##     SRX020102     SRX020103     SRX020104     SRX020105   SRX020091-3  SRX020088-90   SRX020094-7 SRX020098-101 
##        0.5211        0.4479        0.5122        0.5323        1.6893        1.6874        2.4138        2.3777
colSums(counts(dds))
##     SRX020102     SRX020103     SRX020104     SRX020105   SRX020091-3  SRX020088-90   SRX020094-7 SRX020098-101 
##       5282855       4562100       4897807       5123782      17705411      17449646      23649094      23537179
#Plot column sums according to size factor
plot(sizeFactors(dds), colSums(counts(dds)))
abline(lm(colSums(counts(dds)) ~ sizeFactors(dds) + 0))

plot of chunk size-factors

Now we can divide the columns by the size factor and take the log2 of these normalized counts plus a pseudocount of 1. We transpose in order to run PCA.

#The argument normalized equals true, divides each column by its size factor.
logcounts <- log2( counts(dds, normalized=TRUE) + 1 )
pc <- prcomp( t( logcounts ) )

Exploratory data analysis

You can see that the samples are basically clustering by the first principal component, which is in this case the protocol. More variance explained by protocol than by time. You could also do a hierarchical clustering, and you could see that here again, they’re separated more by protocol than by time. You can see they don’t actually cluster discreetly by time.

#library(devtools)
#install_github("rafalib", "ririzarr")
library(rafalib)
#PC 1 VS PC 2
plot(pc$x[,1], pc$x[,2], 
     col=colData(dds)$protocol, # color is the protocol
     pch=as.numeric(colData(dds)$Time)+15)# point shape by time
#hierarchical clustering
plot(hclust(dist(t(logcounts))), labels=colData(dds)$protocol)
plot(hclust(dist(t(logcounts))), labels=colData(dds)$Time)

plot of chunk exploratory-data-analysisplot of chunk exploratory-data-analysisplot of chunk exploratory-data-analysis

And finally, we can just take a look at the log accounts of the first two samples against each other. What I’m trying to point out here is, as we saw in an earlier unit, you can see there’s a spread of the points. So there’s extra variance in the log space when you get down to low values, which is sometimes called Poisson noise.

#log count of the first two samples
plot(logcounts[,1], logcounts[,2], cex=.1)

plot of chunk logcount

Now we will use a normalization method, which is similar to the variance stablizing normalization method mentioned. It uses the variance model to shrink together the sample values for lowly expressed genes with high variance.

Normalization to stabilize variance (regularized logarithm)

The data is in the assay slot, and needs to be transposed as before to run PCA.

# this takes ~15 seconds
# normalization to stabilize variance (regularized logarithm)
rld <- rlog( dds ) # object of class GenomicRanges 
# if using Bioc 2.13:
# rld <- rlogTransformation( dds )

We see again, the protocol drives the variance. But if you look at the time dimension, now the times cluster together discreetly. So instead of before, we had two months joining with the two weeks, now they join each other first, and then they join as a protocol. We could look at the plot of the first sample against the second sample. We can see what, the values near 0 were shrunk towards each other.

We can look at the same plots now using this transformed data:

pc2 <- prcomp( t( assay(rld) ) )
plot(pc2$x[,1], pc2$x[,2],  col=colData(rld)$protocol,pch=as.numeric(colData(rld)$Time)+15)
plot(hclust(dist(t(assay(rld)))), labels=colData(rld)$protocol)
plot(hclust(dist(t(assay(rld)))), labels=colData(rld)$Time)
plot(assay(rld)[,1], assay(rld)[,2], cex=.1)

plot of chunk variance-stabilizationplot of chunk variance-stabilizationplot of chunk variance-stabilizationplot of chunk variance-stabilization

Differential gene expression

A number of methods for assessing differential gene expression from RNA-Seq counts use the Negative Binomial distribution to make probabilistic statements about the differences seen in an experiment. A few such methods are edgeR, DESeq, DSS and many others. A very incomplete list of other methods is provided in the footnotes.

We will use DESeq2 to perform differential gene expression on the counts. This also uses a Negative Binomial distribution to model the counts. It performs a similar step to limma, in using the variance of all the genes to improve the variance estimate for each individual gene. In addition, it shrinks the high variance fold changes, which will be seen in the resulting MA-plot.

Before we specified a design for this, which was just tilde 1. And that represented just modeling these counts on an intercept value. But for differential expression analysis, we want to model them based on the design of the experiment. The design of the experiment was two different treatments, spinal nerve ligation and control, and sequencing was done at two different times.

First, we setup the design of the experiment, so that differences will be considered across time and protocol variables. The last variable is used for the default results tables and plots, and we make sure the “control” level is the first level, such that log fold changes will be treatment over control, and not control over treatment.

If control was not the base level, because the factoring is done alphabetically, you could use the relevel function in R, and give it the level that you want to be the first. So this would just then set control as the first level, if it wasn’t already so.

colData(dds)$protocol
## [1] control control L5 SNL  L5 SNL  control control L5 SNL  L5 SNL 
## Levels: control L5 SNL
# if control was not already the "base level", we would do:
# set control as the first level, if it wasn't already so.
colData(dds)$protocol <- relevel(colData(dds)$protocol, "control")
levels(colData(dds)$protocol)
## [1] "control" "L5 SNL"
#Change the design
design(dds) <- ~ Time + protocol

We put the protocol as the last variable of the design, because this is the variable that will be used to generate log fold changes. So we want to know, controlling for time, what genes were changed according to the protocol.

The following line runs the model, and then we can extract a results table for all genes:

# this takes ~20 seconds
dds <- DESeq( dds )
res <- results( dds )
head(res)
## log2 fold change (MAP): protocol L5 SNL vs control 
## Wald test p-value: protocol L5 SNL vs control 
## DataFrame with 6 rows and 6 columns
##                     baseMean log2FoldChange     lfcSE      stat    pvalue      padj
##                              
## ENSRNOG00000000001    21.304         2.9267    0.4055    7.2180 5.275e-13 4.219e-12
## ENSRNOG00000000007     3.548        -0.1309    0.6373   -0.2054 8.373e-01 8.886e-01
## ENSRNOG00000000008     2.514         0.7699    0.7314    1.0526 2.925e-01 4.039e-01
## ENSRNOG00000000009     0.000             NA        NA        NA        NA        NA
## ENSRNOG00000000010    28.340        -0.3193    0.2899   -1.1013 2.708e-01 3.798e-01
## ENSRNOG00000000012     8.633        -1.9587    0.4943   -3.9628 7.409e-05 2.411e-04

baseMean is the average mean expression stat = log2FoldChange/ lfcSE

We can also make other results tables, such as control over SNL, or for comparing the time variable.

#switch the numerator and the denominator
#For the protocol variable I want the control over L5 SNL
head(results(dds, contrast=c("protocol","control","L5 SNL")))
## log2 fold change (MAP): protocol control vs L5 SNL 
## Wald test p-value: protocol control vs L5 SNL 
## DataFrame with 6 rows and 6 columns
##                     baseMean log2FoldChange     lfcSE      stat    pvalue      padj
##                              
## ENSRNOG00000000001    21.304        -2.9267    0.4055   -7.2180 5.275e-13 4.219e-12
## ENSRNOG00000000007     3.548         0.1309    0.6373    0.2054 8.373e-01 8.886e-01
## ENSRNOG00000000008     2.514        -0.7699    0.7314   -1.0526 2.925e-01 4.039e-01
## ENSRNOG00000000009     0.000             NA        NA        NA        NA        NA
## ENSRNOG00000000010    28.340         0.3193    0.2899    1.1013 2.708e-01 3.798e-01
## ENSRNOG00000000012     8.633         1.9587    0.4943    3.9628 7.409e-05 2.411e-04
#You can also build result for time variable
head(results(dds, contrast=c("Time","2 months","2 weeks")))
## log2 fold change (MAP): Time 2 months vs 2 weeks 
## Wald test p-value: Time 2 months vs 2 weeks 
## DataFrame with 6 rows and 6 columns
##                     baseMean log2FoldChange     lfcSE      stat    pvalue      padj
##                              
## ENSRNOG00000000001    21.304         0.2950    0.3227    0.9143    0.3605    0.5496
## ENSRNOG00000000007     3.548         0.3728    0.4307    0.8656    0.3867        NA
## ENSRNOG00000000008     2.514         0.3814    0.4162    0.9163    0.3595        NA
## ENSRNOG00000000009     0.000             NA        NA        NA        NA        NA
## ENSRNOG00000000010    28.340         0.1863    0.2753    0.6766    0.4986    0.6726
## ENSRNOG00000000012     8.633        -0.5253    0.4018   -1.3073    0.1911        NA

We can now contruct an MA-plot of the fold change over the average expression level of all samples.

# Bioc 2.13
plotMA(dds, ylim=c(-5,5))
# Bioc 2.14
plotMA(res, ylim=c(-5,5))

plot of chunk plotma

And you can see, highlighted in red, are those genes which had an adjusted p-value less than 0.1.

Suppose we are not interested in small log2 fold changes even if they are significantly differentially expressed. We can also test for log2 fold changes larger than 1 in absolute value.

So there’s a way to specify this to the results function, to say I’m only interested in genes which have a log 2 fold change greater in absolute value than 1. And this is not just filtering based on the fold change, but it’s actually performing a statistical test.

resBigFC <- results(dds, lfcThreshold=1, altHypothesis="greaterAbs")
plotMA(resBigFC, ylim=c(-5,5))
abline(h=c(-1,1),lwd=5)

plot of chunk plotMA2

Let’s examine the top gene, sorting by p-value:

#Top genes : sort by pvalue
resSort <- res[order(res$pvalue),]
head(resSort)
## log2 fold change (MAP): protocol L5 SNL vs control 
## Wald test p-value: protocol L5 SNL vs control 
## DataFrame with 6 rows and 6 columns
##                     baseMean log2FoldChange     lfcSE      stat     pvalue       padj
##                                
## ENSRNOG00000004805    1127.9          4.068   0.10665     38.15  0.000e+00  0.000e+00
## ENSRNOG00000004874    1271.5          2.498   0.08466     29.50 2.532e-191 2.016e-187
## ENSRNOG00000015156    1166.1          3.383   0.12076     28.01 1.090e-172 5.784e-169
## ENSRNOG00000004731     892.4         -2.588   0.09352    -27.67 1.620e-168 6.450e-165
## ENSRNOG00000003745    2495.9          3.196   0.12372     25.83 3.707e-147 1.181e-143
## ENSRNOG00000032884    3219.1         -2.466   0.09607    -25.66 2.889e-145 7.669e-142
#Count for the first gene: the unnormalized count
k <- counts(dds)[rownames(resSort)[1],]
#Make a stripchart by combining time and protocol
cond <- with(colData(se), factor(paste(Time, protocol)))
stripchart(log2(k + 1) ~ cond, method="jitter", vertical=TRUE, las=2)

plot of chunk topgenes

You can see why this was given such a low p-value is because if you look at this change from control to spinal nerve ligation, it’s a very large log2 fold change. And it’s consistent, also, across time. And that leads to this very low p-value.

We can then check the annotation of these highly significant genes:

# biocLite("org.Rn.eg.db")
library(org.Rn.eg.db)
#The keys we can query on Ensembl
keytypes(org.Rn.eg.db)
##  [1] "ENTREZID"     "PFAM"         "IPI"          "PROSITE"      "ACCNUM"       "ALIAS"        "CHR"         
##  [8] "CHRLOC"       "CHRLOCEND"    "ENZYME"       "PATH"         "PMID"         "REFSEQ"       "SYMBOL"      
## [15] "UNIGENE"      "ENSEMBL"      "ENSEMBLPROT"  "ENSEMBLTRANS" "GENENAME"     "UNIPROT"      "GO"          
## [22] "EVIDENCE"     "ONTOLOGY"     "GOALL"        "EVIDENCEALL"  "ONTOLOGYALL"
head(rownames(dds))
## [1] "ENSRNOG00000000001" "ENSRNOG00000000007" "ENSRNOG00000000008" "ENSRNOG00000000009" "ENSRNOG00000000010"
## [6] "ENSRNOG00000000012"
#The first 20 genes according to the lowest p-value
geneinfo <- select(org.Rn.eg.db, keys=rownames(resSort)[1:20],
                   columns=c("ENSEMBL","SYMBOL","GENENAME"), 
                   keytype="ENSEMBL")
geneinfo
##               ENSEMBL     SYMBOL                                                           GENENAME
## 1  ENSRNOG00000004805      Stac2                                     SH3 and cysteine rich domain 2
## 2  ENSRNOG00000004874      Flrt3                   fibronectin leucine rich transmembrane protein 3
## 3  ENSRNOG00000015156        Gal                                         galanin/GMAP prepropeptide
## 4  ENSRNOG00000004731       Ano3                                                        anoctamin 3
## 5  ENSRNOG00000003745       Atf3                                  activating transcription factor 3
## 6  ENSRNOG00000032884     Scn11a              sodium channel, voltage-gated, type XI, alpha subunit
## 7  ENSRNOG00000018808        Vip                                      vasoactive intestinal peptide
## 8  ENSRNOG00000033531   Cacna2d1         calcium channel, voltage-dependent, alpha2/delta subunit 1
## 9  ENSRNOG00000032473     Scn10a               sodium channel, voltage-gated, type X, alpha subunit
## 10 ENSRNOG00000019486      Trpv1 transient receptor potential cation channel, subfamily V, member 1
## 11 ENSRNOG00000009186      Stmn4                                                    stathmin-like 4
## 12 ENSRNOG00000012448     Chrnb3                 cholinergic receptor, nicotinic, beta 3 (neuronal)
## 13 ENSRNOG00000019574     Cuedc2                                            CUE domain containing 2
## 14 ENSRNOG00000031997                                                                      
## 15 ENSRNOG00000002595      Dpp10                                             dipeptidylpeptidase 10
## 16 ENSRNOG00000027331      Ppef1              protein phosphatase, EF-hand calcium binding domain 1
## 17 ENSRNOG00000015055       Scg2                                                   secretogranin II
## 18 ENSRNOG00000000129 RGD1559864                                       similar to mKIAA1045 protein
## 19 ENSRNOG00000003386     Rbfox3                  RNA binding protein, fox-1 homolog (C. elegans) 3
## 20 ENSRNOG00000020136       Tgm1                                                 transglutaminase 1

Footnotes

Introduction

Mortazavi A, Williams BA, McCue K, Schaeffer L, Wold B., “Mapping and quantifying mammalian transcriptomes by RNA-Seq”, Nat Methods. 2008. http://www.nature.com/nmeth/journal/v5/n7/full/nmeth.1226.html

John C. Marioni, Christopher E. Mason, Shrikant M. Mane, Matthew Stephens, and Yoav Gilad, “RNA-seq: An assessment of technical reproducibility and comparison with gene expression arrays” Genome Res. 2008. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2527709/

Hammer et al

Hammer P, Banck MS, Amberg R, Wang C, Petznick G, Luo S, Khrebtukova I, Schroth GP, Beyerlein P, Beutler AS. “mRNA-seq with agnostic splice site discovery for nervous system transcriptomics tested in chronic pain.” Genome Res. 2010 http://www.ncbi.nlm.nih.gov/pubmed?term=20452967

ReCount

Frazee AC, Langmead B, Leek JT. “ReCount: a multi-experiment resource of analysis-ready RNA-seq gene count datasets”. BMC Bioinformatics 12:449 http://www.ncbi.nlm.nih.gov/pubmed/22087737

Negative Binomial methods for differential expression of count data

All the following methods are available on Bioconductor:

  • edgeR

Mark D. Robinson, Davis J. McCarthy, and Gordon K. Smyth, “edgeR: a Bioconductor package for differential expression analysis of digital gene expression data” Bioinformatics 2010. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2796818/

  • DESeq (the latest version is a separate package, DESeq2)

Simon Anders and Wolfgang Huber, “Differential expression analysis for sequence count data”, Genome Biology 2010. http://genomebiology.com/2010/11/10/r106

  • DSS

Hao Wu, Chi Wang, Zhijin Wu, “A new shrinkage estimator for dispersion improves differential expression detection in RNA-seq data” Biostatistics 2013. http://biostatistics.oxfordjournals.org/content/14/2/232

Transformation followed by linear model methods

voom in the limma Bioconductor package

Charity W Law, Yunshun Chen, Wei Shi and Gordon K Smyth, “voom: precision weights unlock linear model analysis tools for RNA-seq read counts”, Genome Biology. 2014. http://genomebiology.com/2014/15/2/R29

Resampling-based methods

SAMseq in the samr package on CRAN

Jun Li and Robert Tibshirani, “Finding consistent patterns: A nonparametric approach for identifying differential expression in RNA-Seq data”, Stat Methods Med Res. 2013. http://smm.sagepub.com/content/22/5/519.short

Incorporating isoform-abundance

  • Cuffdiff (the latest version is Cuffdiff2)

Trapnell C, Hendrickson DG, Sauvageau M, Goff L, Rinn JL, Pachter L., “Differential analysis of gene regulation at transcript resolution with RNA-seq” Nat Biotechnol. 2013. http://www.ncbi.nlm.nih.gov/pubmed/23222703

  • BitSeq

Peter Glaus, Antti Honkela, and Magnus Rattray, “Identifying differentially expressed transcripts from RNA-seq data with biological variation”, Bioinformatics. 2012. http://bioinformatics.oxfordjournals.org/content/28/13/1721

Licence

Licence


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