Contents

1 Overview of DEqMS

DEqMS builds on top of Limma, a widely-used R package for microarray data analysis (Smyth G. et al 2004), and improves it with mass spectrometry specific data properties, accounting for variance dependence on the number of quantified peptides or PSMs for statistical testing of differential protein expression.

2 Load the package

library(DEqMS)
## Loading required package: ggplot2
## Loading required package: limma

3 Differential protein expression analysis

3.1 Example Dataset 1

We analyzed a protemoics dataset (TMT10plex labelled) in which A431 cells (human epidermoid carcinoma cell line) were treated with three different miRNA mimics (Yan Z. et al. Oncogene 2016). The raw MS data was searched with MS-GF+ (Kim et al Nat Communications 2016) and post processed with Percolator (Kall L. et al Nat Method 2007). A tabular text output of PSM intensity table filtered at 1% peptide level FDR is used as input for downstream analysis.

3.2 Loading the data and preprocess

Read a tabular input in which PSMs are in rows and samples are in columns. Intenisty values were log transformed since systematic effects and variance components are usually assumed to be additive on log scale (Oberg AL. et al JPR 2008; Hill EG. et al JPR 2008).

### retrieve example dataset
library(ExperimentHub)
## Loading required package: BiocGenerics
## Loading required package: parallel
## 
## Attaching package: 'BiocGenerics'
## The following objects are masked from 'package:parallel':
## 
##     clusterApply, clusterApplyLB, clusterCall, clusterEvalQ,
##     clusterExport, clusterMap, parApply, parCapply, parLapply,
##     parLapplyLB, parRapply, parSapply, parSapplyLB
## The following object is masked from 'package:limma':
## 
##     plotMA
## The following objects are masked from 'package:stats':
## 
##     IQR, mad, sd, var, xtabs
## The following objects are masked from 'package:base':
## 
##     Filter, Find, Map, Position, Reduce, anyDuplicated, append,
##     as.data.frame, basename, cbind, colMeans, colSums, colnames,
##     dirname, do.call, duplicated, eval, evalq, get, grep, grepl,
##     intersect, is.unsorted, lapply, lengths, mapply, match, mget,
##     order, paste, pmax, pmax.int, pmin, pmin.int, rank, rbind,
##     rowMeans, rowSums, rownames, sapply, setdiff, sort, table,
##     tapply, union, unique, unsplit, which, which.max, which.min
## Loading required package: AnnotationHub
eh = ExperimentHub()
## snapshotDate(): 2018-10-30
query(eh, "DEqMS")
## ExperimentHub with 1 record
## # snapshotDate(): 2018-10-30 
## # names(): EH1663
## # package(): DEqMS
## # $dataprovider: Swedish BioMS infrastructure
## # $species: Homo sapiens
## # $rdataclass: data.frame
## # $rdatadateadded: 2018-09-11
## # $title: microRNA treated A431 cell proteomics data
## # $description: High resolution isoelectric focusing LC-MS/MS data for A4...
## # $taxonomyid: 9606
## # $genome: hg38
## # $sourcetype: TXT
## # $sourceurl: ftp://ftp.pride.ebi.ac.uk/pride/data/archive/2016/06/PXD004...
## # $sourcesize: NA
## # $tags: c("Proteomics", "MassSpectrometry", "Preprocessing",
## #   "DifferentialExpression", "MultipleComparison", "Normalization",
## #   "Bayesian") 
## # retrieve record with 'object[["EH1663"]]'
dat.psm = eh[["EH1663"]]
## see ?DEqMS and browseVignettes('DEqMS') for documentation
## downloading 0 resources
## loading from cache 
##     '/home/biocbuild//.ExperimentHub/1663'
dat.psm.log = dat.psm
dat.psm.log[,3:12] =  log2(dat.psm[,3:12])
head(dat.psm.log)
##                                           Peptide     gene tmt10plex_126
## 1         +229.163EK+229.163EDDEEEEDEDASGGDQDQEER    RAD21      16.75237
## 2      +229.163LGLGIDEDEVAAEEPNAAVPDEIPPLEGDEDASR HSP90AB1      10.83812
## 3          +229.163TEGDEEAEEEQEENLEASGDYK+229.163    XRCC6      14.50514
## 4       +229.163GDAEK+229.163PEEELEEDDDEELDETLSER   TOMM22      15.03117
## 8       +229.163APLATGEDDDDEVPDLVENFDEASK+229.163     BTF3      12.91406
## 9 +229.163LEEEDEDEEDGESGC+57.021TFLVGLIQK+229.163    CAPN2      14.98558
##   tmt10plex_127N tmt10plex_127C tmt10plex_128N tmt10plex_128C tmt10plex_129N
## 1       16.58542       17.26731       16.89528       17.01872       17.57275
## 2       10.13673       11.11384       11.07480       10.94694       10.79556
## 3       14.24282       15.23424       14.89867       14.74940       14.97737
## 4       14.91910       15.41189       15.28130       15.28605       15.41345
## 8       12.95097       13.00558       13.42184       12.63930       13.62308
## 9       14.97605       15.30197       15.27980       15.10410       15.31982
##   tmt10plex_129C tmt10plex_130N tmt10plex_130C tmt10plex_131
## 1       17.17815       16.86259       17.10233      17.75614
## 2       11.12758       11.14692       10.82071      11.21737
## 3       15.15130       15.09598       15.01059      15.46618
## 4       15.25668       15.39181       15.26238      15.79845
## 8       13.12886       12.19316       12.90018      13.52949
## 9       15.25234       15.51071       15.72660      16.06220

Generate sample annotation table.

cond = c("ctrl","miR191","miR372","miR519","ctrl",
"miR372","miR519","ctrl","miR191","miR372")

sampleTable <- data.frame(
row.names = colnames(dat.psm)[3:12],
cond = as.factor(cond))
sampleTable
##                  cond
## tmt10plex_126    ctrl
## tmt10plex_127N miR191
## tmt10plex_127C miR372
## tmt10plex_128N miR519
## tmt10plex_128C   ctrl
## tmt10plex_129N miR372
## tmt10plex_129C miR519
## tmt10plex_130N   ctrl
## tmt10plex_130C miR191
## tmt10plex_131  miR372

3.3 Generate a summary of the data

Since each row is one PSM, to count how many PSMs per protein, we simply use function table in R to count the number of occurance that each protein ID appears.

psm.count.table = as.data.frame(table(dat.psm$gene))
rownames(psm.count.table)=psm.count.table$Var1

plot(sort(log2(psm.count.table$Freq)),pch=".",
    xlab="Proteins ordered by PSM count",ylab="log2(PSM count)")

3.4 Summarization and Normalization

Here, median sweeping is used to summarize PSMs intensities to protein log2 ratios. In this procedure, we substract the spectrum log2 intensity from the median log2 intensities of all samples. The relative abundance estimate for each protein is calculated as the median over all PSMs belonging to this protein. Assume the log2 intensity of PSM i in sample j is \(y_{i,j}\), its relative log2 intensity of PSM i in sample j is \(y'_{i,j}\): \[y'_{i,j} = y_{i,j} - median_{j'\in ctrl}\ y_{i,j'} \] Relative abundance of protein k in sample j \(Y_{k,j}\) is calculated as: \[Y_{k,j} = median_{i\in protein\ k}\ y'_{i,j} \]

Correction for differences in amounts of material loaded in the channels is then done by subtracting the channel median from the relative abundance (log2 ratio), normalizing all channels to have median zero.

dat.gene.nm = medianSweeping(dat.psm.log,group_col = 2)

boxplot(dat.gene.nm,xlab="TMT 10 channels",
        ylab="log2 relative protein abundance")

3.5 Ordinary T test

We first apply t.test to detect significant protein changes between ctrl samples and miR372 treated samples, both have three replicates.

pval.372 = apply(dat.gene.nm, 1, function(x) 
t.test(as.numeric(x[c(1,5,8)]), as.numeric(x[c(3,6,10)]))$p.value)

logFC.372 = rowMeans(dat.gene.nm[,c(3,6,10)])-rowMeans(dat.gene.nm[,c(1,5,8)])

Generate a data.frame of t.test results, add PSM count values and order the table by p-value.

ttest.results = data.frame(gene=rownames(dat.gene.nm),
                    logFC=logFC.372,P.Value = pval.372, 
                    adj.pval = p.adjust(pval.372,method = "BH")) 

ttest.results$PSMcount = psm.count.table[ttest.results$gene,]$Freq
ttest.results = ttest.results[with(ttest.results, order(P.Value)), ]
head(ttest.results)
##          gene      logFC      P.Value   adj.pval PSMcount
## CCNE2   CCNE2  0.5386427 6.522987e-07 0.00599593        1
## CPSF2   CPSF2  0.1077977 7.633799e-06 0.03508494       53
## PPOX     PPOX -0.2464418 2.546510e-05 0.07802507        6
## RELA     RELA -0.5617739 5.078761e-05 0.11670992       31
## IFIT1   IFIT1  0.6375060 7.300925e-05 0.13422020       33
## MAGEA6 MAGEA6  0.4625733 1.093648e-04 0.16178031        6

3.6 Anova analysis

Anova analysis is equivalent to linear model analysis. The difference to Limma analysis is that estimated variance is not moderated using empirical bayesian approach as it is done in Limma. The purpose here is to compare the statistical power to that when Limma is applied.

We first make a design matrix and then use lmFit function in Limma for linear model analysis. The input for lmFit function is a matrix of relative protein abundance (log2 ratios) and a design matrix containing sample annotation. Use head(fit$coefficients) to see effect size(fold change) of genes for different conditions.

library(limma)
gene.matrix = as.matrix(dat.gene.nm)
colnames(gene.matrix) = as.character(sampleTable$cond)
design = model.matrix(~cond,sampleTable)

fit1 <- lmFit(gene.matrix,design)

ord.t = fit1$coefficients[, 3]/fit1$sigma/fit1$stdev.unscaled[, 3]
ord.p = 2*pt(-abs(ord.t), fit1$df.residual)
ord.q = p.adjust(ord.p,method = "BH")
anova.results = data.frame(gene=names(fit1$sigma),
                            logFC=fit1$coefficients[,3],
                            t=ord.t, 
                            P.Value=ord.p, 
                            adj.P.Val = ord.q)

anova.results$PSMcount = psm.count.table[anova.results$gene,]$Freq
anova.results = anova.results[with(anova.results,order(P.Value)),]

head(anova.results)
##            gene      logFC         t      P.Value   adj.P.Val PSMcount
## SH3BP1   SH3BP1  0.3426263  18.84397 1.442660e-06 0.005882983       20
## ANKRD52 ANKRD52 -1.1917809 -18.66205 1.527812e-06 0.005882983       17
## TGFBR2   TGFBR2 -1.3474739 -17.38307 2.323406e-06 0.005882983        7
## TBC1D2   TBC1D2 -0.5272092 -16.85506 2.786690e-06 0.005882983       26
## PHLPP2   PHLPP2 -0.7969800 -16.46379 3.200056e-06 0.005882983        8
## CROT       CROT -1.1997155 -15.92937 3.885625e-06 0.005952777       20

3.7 Limma analysis

Limma is essentially a combination of linear model analysis and empirical bayeisan estimation of variance, the latter is applied to increase the statistical power by borrowing informations across genes to shrink the variance.

fit2 <- eBayes(lmFit(gene.matrix,design))
head(fit2$coefficients)
##        (Intercept)  condmiR191  condmiR372   condmiR519
## A2M    0.235296527 -0.29793777 -0.51680730 -0.368015917
## A2ML1 -0.151670026  0.29957250  0.10781670  0.373261252
## AAAS   0.082921445 -0.11106003 -0.07255767 -0.156424479
## AACS   0.001687942 -0.06661920 -0.04126341  0.004051871
## AAED1 -0.117523764  0.09149186  0.25393152  0.076939663
## AAGAB -0.022929404  0.18415988  0.11458149  0.002356680

Extract limma results using topTable function, coef = 3 allows you to extract the contrast of specific condition (miR372), option n= Inf output all rows. Add PSM count values in the table.

limma.results = topTable(fit2,coef = 3,n= Inf)
limma.results$gene = rownames(limma.results)
limma.results$PSMcount = psm.count.table[limma.results$gene,]$Freq

head(limma.results)
##              logFC      AveExpr         t      P.Value    adj.P.Val        B
## ANKRD52 -1.1917809 -0.093887723 -18.26692 2.608305e-08 0.0001326236 9.113399
## TGFBR2  -1.3474739  0.083901815 -18.05371 2.885630e-08 0.0001326236 9.041434
## CROT    -1.1997155 -0.060459201 -16.41503 6.530463e-08 0.0002000934 8.438659
## PHLPP2  -0.7969800 -0.001069459 -14.37066 2.029693e-07 0.0004664235 7.543032
## PDCD4   -0.7800666  0.007360661 -13.07496 4.512500e-07 0.0008295781 6.874678
## ZKSCAN1 -0.8816149 -0.056612793 -11.95070 9.591643e-07 0.0012877234 6.218545
##            gene PSMcount
## ANKRD52 ANKRD52       17
## TGFBR2   TGFBR2        7
## CROT       CROT       20
## PHLPP2   PHLPP2        8
## PDCD4     PDCD4       40
## ZKSCAN1 ZKSCAN1       10

3.8 Variance dependence on spectra count

We observed that the variance of gene across samples gradually decreases as the number of spectra count increases.

dat.temp = data.frame(var = fit2$sigma^2,
                        PSMcount = psm.count.table[names(fit2$sigma),]$Freq)
dat.temp.filter = dat.temp[dat.temp$PSMcount<21,]

op <- par(mfrow=c(1,2), mar=c(4,4,4,1), oma=c(0.5,0.5,0.5,0))
plot(log2(psm.count.table[rownames(dat.gene.nm),2]),
    dat.gene.nm[,1]-dat.gene.nm[,5],pch=20,cex=0.5,
    xlab="log2(PSM count)",ylab="log2 ratio between two ctrl replicates")

boxplot(log2(var)~PSMcount,dat.temp.filter,xlab="PSMcount",
        ylab="log2(Variance)")

3.9 DEqMS analysis

Limma assumes equal prior variance for all genes, the function spectraCounteBayes in DEqMS package is able to correct the biase of prior variance estimate for proteins quantified by different number of PSMs. It works in a similar way to the intensity-based hierarchical Bayes method (Maureen A. Sartor et al BMC Bioinformatics 2006). Outputs of spectraCounteBayes:
object is augmented form of “fit” object from eBayes in Limma, with the additions being: sca.t - Spectra Count Adjusted posterior t-value
sca.p - Spectra Count Adjusted posterior p-value
sca.dfprior - estimated prior degrees of freedom
sca.priorvar- estimated prior variance sca.postvar - estimated posterior variance loess.model - fitted loess model

fit2$count = psm.count.table[rownames(fit2$coefficients),]$Freq

fit3 = spectraCounteBayes(fit2)
DEqMS.results = outputResult(fit3,coef_col = 3)
head(DEqMS.results)
##              logFC      AveExpr         t      P.Value    adj.P.Val        B
## ANKRD52 -1.1917809 -0.093887723 -18.26692 2.608305e-08 0.0001326236 9.113399
## CROT    -1.1997155 -0.060459201 -16.41503 6.530463e-08 0.0002000934 8.438659
## TGFBR2  -1.3474739  0.083901815 -18.05371 2.885630e-08 0.0001326236 9.041434
## PDCD4   -0.7800666  0.007360661 -13.07496 4.512500e-07 0.0008295781 6.874678
## TRPS1   -0.8122847 -0.050833888 -11.70281 1.142391e-06 0.0013126073 6.063183
## PHLPP2  -0.7969800 -0.001069459 -14.37066 2.029693e-07 0.0004664235 7.543032
##            gene count     sca.t  sca.P.Value sca.adj.pval
## ANKRD52 ANKRD52    17 -18.97949 3.769163e-10 3.147510e-06
## CROT       CROT    20 -17.59956 8.866592e-10 3.147510e-06
## TGFBR2   TGFBR2     7 -17.37181 1.027255e-09 3.147510e-06
## PDCD4     PDCD4    40 -14.25841 9.397591e-09 2.159566e-05
## TRPS1     TRPS1    30 -12.78539 3.133624e-08 4.919434e-05
## PHLPP2   PHLPP2     8 -12.75696 3.211119e-08 4.919434e-05

Check if the fitted variance ~ PMS count model is correct.

plotFitCurve(fit3,type = "boxplot",xlab="PSM count")

4 Comparing Limma and DEqMS

4.1 Compare prior variance and posterior variance in Limma and DEqMS

Visualize the prior variance fitted by Limma and DEqMS

x = fit3$count
y = fit3$sigma^2

limma.prior = fit3$s2.prior
DEqMS.prior = fit3$sca.priorvar

plot(log2(x),log(y),pch=20, cex=0.5,xlab = "log2(PSMcount)",
     ylab="log(Variance)")
abline(h = log(limma.prior),col="green",lwd=3 )
k=order(x)
lines(log2(x[k]),log(DEqMS.prior[k]),col="red",lwd=3 )
legend("topright",legend=c("Limma prior variance","DEqMS prior variance"),
        col=c("green","red"),lwd=3)