In statOmics/msqrob2: Robust statistical inference for quantitative LC-MS proteomics

Background

In this vignette, we assess the performance of the MSqRob Hurdle workflow for differential expression analysis using a publicly available spike-in study (PRIDE identifier: PXD003881 Shen et al. [2018]). E. Coli lysates were spiked at five different concentrations (3%, 4.5%, 6%, 7.5% and 9% wt/wt) in a stable human background (four replicates per treatment). The samples were run on an Orbitrap Fusion mass spectrometer. Raw data files were processed with MaxQuant (version 1.6.1.0, Cox and Mann [2008]) using default search settings unless otherwise noted. Spectra were searched against the UniProtKB/SwissProt human and E. Coli reference proteome databases (07/06/2018), concatenated with the default Maxquant contaminant database. Carbamidomethylation of Cystein was set as a fixed modification, and oxidation of Methionine and acetylation of the protein amino-terminus were allowed as variable modifications. In silico cleavage was set to use trypsin/P, allowing two miscleavages. Match between runs was also enabled using default settings. The resulting peptide-to-spectrum matches (PSMs) were filtered by MaxQuant at 1% FDR.

concentrations <- (2:6) * 1.5
names(concentrations) <- letters[1:5]

Data

We first import the peptides.txt file. This is the file that contains your peptide-level intensities. For a MaxQuant search [6], this peptides.txt file can be found by default in the "path_to_raw_files/combined/txt/" folder from the MaxQuant output, with "path_to_raw_files" the folder where raw files were saved. In this tutorial, we will use a MaxQuant peptides file of the ecoli spike-in study that is stored in the msdata package. We use the QFeatures package to import the data.

With the grepEcols function we find the columns that are containing the expression data of the peptides in the peptides.txt file.

library(tidyverse)
library(limma)
library(QFeatures)
library(msqrob2)
library(gridExtra)

myurl <-
    "https://raw.githubusercontent.com/statOmics/MSqRobSumPaper/master/spikein/data/maxquant/peptides.zip"
download.file(myurl, "peptides.zip", method = "curl", extra = "-L")
unzip("peptides.zip")
peptidesFile <- "peptides.txt"
ecols <- MSnbase::grepEcols(peptidesFile, "Intensity ", split = "\t")
pe <- readQFeatures(
    table = peptidesFile, fnames = 1, ecol = ecols,
    name = "peptideRaw", sep = "\t"
)
pe

We can extract the spikein condition from the raw file name.

cond <- which(strsplit(colnames(pe)[[1]][1], split = "")[[1]] == "a") # find where condition is stored
colData(pe)$condition <- substr(colnames(pe), cond, cond) %>%
    unlist() %>%
    as.factor()

We calculate how many non zero intensities we have per peptide. This will be useful for filtering.

rowData(pe[["peptideRaw"]])$nNonZero <- rowSums(assay(pe[["peptideRaw"]]) > 0)

Peptides with zero intensities are missing peptides and should be represent with a NA value instead of 0.

pe <- zeroIsNA(pe, i = "peptideRaw")

Information on species

In the spikin-study there are peptides from ecoli and human proteins. The ecoli peptides are spiked.

myurl <-
    "https://raw.githubusercontent.com/statOmics/MSqRobSumPaper/master/spikein/data/fasta/ecoli_up000000625_7_06_2018.fasta"
download.file(myurl, "ecoli.fasta", method = "curl", extra = "-L")
myurl <- "https://raw.githubusercontent.com/statOmics/MSqRobSumPaper/master/spikein/data/fasta/human_up000005640_sp_7_06_2018.fasta"
download.file(myurl, "human.fasta", method = "curl", extra = "-L")

id <- list(
    ecoli = "ecoli.fasta",
    human = "human.fasta"
) %>%
    map(~ {
        read_lines(.x) %>%
            {
                .[str_detect(., "^>")]
            } %>%
            str_extract(., "(?<=\\|).*(?=\\|)")
    })

Data exploration

We can inspect the missingness in our data with the plotNA() function provided with MSnbase. r format(mean(is.na(assay(pe[["peptideRaw"]])))*100,digits=2)% of all peptide intensities are missing and for some peptides we do not even measure a signal in any sample. The missingness is similar across samples.

MSnbase::plotNA(assay(pe)) +
    xlab("Peptide index (ordered by data completeness)")

Preprocessing

We normalize the data using vsn normalisation. Note, that the data should not be log-transformed then.

Filtering

Handling overlapping protein groups

In our approach a peptide can map to multiple proteins, as long as there is none of these proteins present in a smaller subgroup.

pe[["peptideRaw"]] <- pe[["peptideRaw"]][rowData(pe[["peptideRaw"]])$Proteins %in% smallestUniqueGroups(rowData(pe[["peptideRaw"]])$Proteins), ]

Remove reverse sequences (decoys) and contaminants

We now remove the contaminants, peptides that map to decoy sequences and proteins, which were only identified by peptides with modifications.

pe[["peptideRaw"]] <- pe[["peptideRaw"]][rowData(pe[["peptideRaw"]])$Reverse != "+", ]
pe[["peptideRaw"]] <- pe[["peptideRaw"]][rowData(pe[["peptideRaw"]])$
    Potential.contaminant != "+", ]

Drop peptides that were only identified in one sample

We want to keep peptide that were at least observed twice.

pe[["peptideRaw"]] <- pe[["peptideRaw"]][rowData(pe[["peptideRaw"]])$nNonZero >= 2, ]
nrow(pe[["peptideRaw"]])

We keep r nrow(pe[["peptideRaw"]]) peptides upon filtering.

Normalize the data using the vsn method

pe <- normalize(pe, i = "peptideRaw", method = "vsn", name = "peptideNorm")

Explore quantile normalized data

Upon normalisation the density curves for all samples coincide. We have not run the code to reduce the size of the vignette. Uncomment the line below to generate the plot.

# limma::plotDensities(assay(pe[["peptideNorm"]]))

We can visualize our data using a multi-dimensional scaling plot, eg. as provided by the limma package. We have not run the code to reduce the size of the vignette. Uncomment the line below to generate the plot.

# limma::plotMDS(assay(pe[["peptideNorm"]]), col = as.numeric(colData(pe)$condition))

The first axis in the plot is showing the leading log fold changes (differences on the log scale) between the samples. We notice that the leading differences (log FC) in the peptide data seems to be driven by the spike-in condition.

Summarization to protein level

Use the standard summarisation in aggregateFeatures (robust summarisation)

pe <- aggregateFeatures(pe, i = "peptideNorm", fcol = "Proteins", na.rm = TRUE, name = "protein")

We can visualize the summarized data using a multi-dimensional scaling plot, eg. as provided by the limma package. We have not run the code to reduce the size of the vignette. Uncomment the line below to generate the plot.

# plotMDS(assay(pe[["protein"]]), col = as.numeric(colData(pe)$condition))

Data Analysis

Estimation

Here, we will illustrate three different workflows that are available in MSqRob.

Default workflow:

1. Robust summarisation

2. Estimation with a robust linear model of the protein expression values

pe <- msqrob(object = pe, i = "protein", formula = ~condition)

Robust summarisation followed by robust ridge regression

1. Robust summarisation

2. Estimation with robust ridge regression.

3. This is done by setting the argument ridge = TRUE.

4. The performance of ridge regression generally improves for more complex designs with multiple conditions.
5. In two-group designs ridge regression does not improve much. The method is computationally complex!

pe <- msqrob(
    object = pe,
    i = "protein",
    formula = ~condition,
    modelColumnName = "ridge",
    ridge = TRUE)

Hurdle workflow

The hurdle model has two components:

• A count component modelling absence/presence of a peptide
• A intensity component modelling the expression level given that expression occurs.

The count component models the number of peptides that were used to summarize to protein level expression values using a quasibinomial model. The model component is stored by default in the rowData under the name "msqrobHurdleCount".

For the intensity component, the hurdle model estimates the model parameters by default using robust regression. This model component is stored in the rowData under name "msqrobHurdleIntensity".

pe <- msqrobHurdle(object = pe, i = "protein", formula = ~condition)

Inference

What are the parameter names of the model?

getCoef(rowData(pe[["protein"]])$msqrobModels[[1]])

Spike-in condition a is the reference class. So the mean log2 expression for samples from condition a is '(Intercept). The mean log2 expression for samples from condition b-e is '(Intercept)+conditionb',...,'(Intercept)+conditione', respectively. Hence, the average log2 fold change (FC) between condition b and condition a is modelled using the parameter 'conditionb'. Thus, we assess the contrast 'conditionb = 0' with our statistical test. The same holds for comparison c-a, d-a, e-a.

comparisonsRef <- paste0(paste0("condition", letters[2:5]), " = 0")
comparisonsRef

The test for average log2 FC between condition c and b will assess the contrast 'conditionc - conditionb = 0', ...

comparisonsOther <- paste0(
    apply(
        combn(paste0("condition", letters[2:5]), 2)[2:1, ],
        2,
        paste,
        collapse = " - "
    ),
    " = 0"
)
comparisonsOther

comparisons <- c(comparisonsRef, comparisonsOther)

We make the contrast matrix using the makeContrast function

L <- makeContrast(comparisons, parameterNames = paste0("condition", letters[2:5]))
L

And we adopt the hypothesis tests for each contrast.

Inference

Default workflow

We use the argument resultsColumnNamePrefix = "default" because we will evaluate the contrasts with multiple models and store all results in the rowData.

pe <- hypothesisTest(object = pe, i = "protein", contrast = L,overwrite=TRUE, resultsColumnNamePrefix = "default")
contrastNamesDefault <- paste0("default",colnames(L))

We do not make plots to reduce the vignette size.

pe <- hypothesisTestHurdle(object = pe, i = "protein", contrast = L)

Compare it with inference on MSqRob native, i.e. the intensity component only

pe <- hypothesisTest(object = pe, i = "protein", contrast = L, modelColumn = "msqrobHurdleIntensity")

Plots

Heatmaps

for (i in colnames(L)) {
    sigNames <- rowData(pe[["protein"]])[[paste0("hurdle_", i)]] %>%
        rownames_to_column("protein") %>%
        filter(fisherAdjPval < 0.01) %>%
        pull(protein)
    heatmap(assay(pe[["protein"]])[sigNames, ], main = i)
}

Sensitivity FDP plots

Because we are analysing a spike-in study we know the ground truth, i.e. we know that only the spike-in proteins (ecoli) are differentially expressed. We can therefore evaluate the performance of the method, i.e. we will assess

• the sensitivity or true positive rate (TPR), the proportion of actual positives that are correctly identified, in the protein list that we return $$TPR=\frac{TP}{\text{#actual positives}},$$ here TP are the true positives in the list. The TPR is thus the fraction of ups proteins that we can recall.

• false discovery proportion (FPD): fraction of false positives in the protein list that we return: $$FPD=\frac{FP}{FP+TP},$$ with FP the false positives. In our case the yeast proteins that are in our list.

Instead of only calculating that for the protein list that is returned for the chosen FDR level, we can do this for all possible FDR cutoffs so that we get an overview of the quality of the ranking of the proteins in the protein list.

We first add the ground truth data to the rowData of the object.

accessions <- rownames(pe[["protein"]]) %>%
    data_frame(protein = .)

accessions <- accessions %>%
    transmute(protein = as.character(protein), proteins = strsplit(protein, ";")) %>%
    unnest() %>%
    mutate(human = proteins %in% id$human, ecoli = proteins %in% id$ecoli) %>%
    group_by(protein) %>%
    summarise(human = any(human), ecoli = any(ecoli)) %>%
    right_join(accessions)
rowData(pe[["protein"]])$accession <- accessions

Check that all accessions are either human or ecoli:

nrow(accessions)
sum(accessions$human)
sum(accessions$ecoli)
sum(accessions$human) + sum(accessions$ecoli)

Function to calculate TPR and FDP

tprFdp <- function(pval, tp, adjPval) {
    ord <- order(pval)
    return(data.frame(
        pval = pval[ord],
        adjPval = adjPval[ord],
        tpr = cumsum(tp[ord]) / sum(tp),
        fdp = cumsum(!tp[ord]) / 1:length(tp)
    ))
}

tprFdpDefault <- list()
tprFdpHurdle <- list()
tprFdpPlots <- list()
for (i in colnames(L)) {
    tprFdpDefault[[i]] <- tprFdp(
        rowData(pe[["protein"]])[[i]]$pval,
        rowData(pe[["protein"]])$accession$ecoli, rowData(pe[["protein"]])[[i]]$adjPval
    )
    tprFdpHurdle[[i]] <- tprFdp(
        rowData(pe[["protein"]])[[paste0("hurdle_", i)]]$fisherPval,
        rowData(pe[["protein"]])$accession$ecoli, rowData(pe[["protein"]])[[paste0("hurdle_", i)]]$fisherAdjPval
    )
    hlp <- rbind(cbind(tprFdpDefault[[i]], method = "default"), cbind(tprFdpHurdle[[i]], method = "hurdle"))
    tprFdpPlots[[i]] <- hlp %>%
        ggplot(aes(x = fdp, y = tpr, color = method)) +
        geom_path() +
        theme_classic(base_size = 14) + # guides(size = FALSE, alpha = FALSE)
        ggtitle(paste0(i, " = 0"))
}
grid.arrange(grobs = tprFdpPlots[1:9], ncol = 3)

The performance of the hurdle method is better then of the default method, especially for the comparisons involving low concentrations.

References

statOmics/msqrob2 documentation built on May 8, 2024, 4:38 p.m.