library(TargetDecoy)
library(ggplot2)

1 Basics

1.1 Installing TargetDecoy

TargetDecoy is an R package available via the Bioconductor repository for packages. R can be installed on any operating system from CRAN after which you can install TargetDecoy by using the following commands in your R session:

if (!requireNamespace("BiocManager", quietly = TRUE)) {
      install.packages("BiocManager")
  }

BiocManager::install("TargetDecoy")

## Check that you have a valid Bioconductor installation
BiocManager::valid()

The latest development version of TargetDecoy can be installed from GitHub by running the following in an R session:

BiocManager::install("statOmics/TargetDecoy")

1.2 Citing TargetDecoy

We hope that TargetDecoy will be useful for your research. Please use the following information to cite the package and the overall approach. Thank you!

## Citation info
citation("TargetDecoy")
#> 
#> To cite package 'TargetDecoy' in publications use:
#> 
#>   Debrie E, Clement L, Malfait M (2022). _TargetDecoy: Diagnostic Plots
#>   to Evaluate the Target Decoy Approach_.
#>   https://www.bioconductor.org/packages/TargetDecoy,
#>   https://statomics.github.io/TargetDecoy/,
#>   https://github.com/statOmics/TargetDecoy/.
#> 
#> A BibTeX entry for LaTeX users is
#> 
#>   @Manual{,
#>     title = {TargetDecoy: Diagnostic Plots to Evaluate the Target Decoy Approach},
#>     author = {Elke Debrie and Lieven Clement and Milan Malfait},
#>     year = {2022},
#>     note = {https://www.bioconductor.org/packages/TargetDecoy, https://statomics.github.io/TargetDecoy/, https://github.com/statOmics/TargetDecoy/},
#>   }

2 Introduction

A first step in the data analysis of Mass Spectrometry (MS) based proteomics data is to identify peptides and proteins. For this, a huge number of experimental mass spectra typically have to be assigned to theoretical peptides derived from a sequence database. This task is carried out by specialised algorithms called search engines, which compare each of the observed spectra to theoretical spectra derived from relevant candidate peptides obtained from a sequence data base and calculate a score for each comparison. The observed spectrum is then assigned to the theoretical peptide with the best score, which is also referred to as a peptide-to-spectrum match (PSM). It is of course crucial for the downstream analysis to evaluate the overall reliability of these matches. Therefore False Discovery Rate (FDR) control is used to return a sufficiently reliable list of PSMs. The FDR calculation, however, requires a good characterisation of the score distribution of PSMs that are matched to an incorrect peptide (bad target hits). In proteomics, the target decoy approach (TDA) is typically used for this purpose. The TDA method matches the observed spectra to a database of real (target) and nonsense (decoy) peptides, with the latter typically generated by reversing protein sequences in the target database. Hence, all PSMs that match to a decoy peptide are known to be bad hits and the distribution of their scores is used to estimate the distribution of the bad scoring target PSMs. A crucial assumption of the TDA is that decoy PSM hits have similar properties as bad target hits so that decoy PSM scores are a good simulation of target PSM scores. Users, however, typically do not evaluate these assumptions. To this end we developed TargetDecoy to generate diagnostic plots to evaluate the quality of the target decoy method, thus allowing users to assess whether the key assumptions underlying the method are met.

3 Concepts

3.1 Basic Statistical Concepts

We first introduce some notation. With \(x\) we denote the PSM and without loss of generality we assume that larger score values indicate a better match to the theoretical spectrum. The scores will follow a mixture distribution:

\[f(x) = \pi_0f_0(x)+(1-\pi_0)f_1(x),\]

with \(f(x)\) the target PSM score distribution, \(f_0(x)\) the mixture component corresponding to incorrect PSMs, \(f_1(x)\) the mixture component corresponding to the correct PSMs and \(\pi_0\) the fraction of incorrect PSMs. Based on the mixture distribution we can calculate the posterior probability that a PSM with score \(x\) is a bad match:

\[ P[\text{Bad hit} \mid \text{score }x]=\frac{\pi_0 f_0 (x)}{f(x)}, \]

which is also referred to as the posterior error probability (PEP) in mass spectrometry based proteomics. With the mixture model, we can also calculate the posterior probability that a random PSM in the set of all PSMs with scores above a score threshold t is a bad hit:

\[ P[\text{Bad hit} \mid \text{score }x>t]=\pi_0 \frac{\int\limits_{x=t}^{+\infty} f_0(x)dx}{\int\limits_{x=t}^{+\infty} f(x)dx}, \]

with \(\int\limits_{x=t}^{+\infty} f_0(x) dx\) the probability to observe a bad PSM hit above the threshold and, \(\int\limits_{x=t}^{+\infty} f(x) dx\) the probability to observe a target PSM hit above the threshold. The probability \(P[\text{Bad hit} \mid \text{score }x>t]\) is also referred to as the False Discovery Rate for peptide identification (FDR) of the set of PSMs with scores above the threshold \(t\). Hence, the FDR has the interpretation of the expected fraction of bad hits in the set of all target hits that are returned in the final PSM list, so bad PSM hits with scores above the threshold.

In order to estimate the FDR, we thus have to estimate the distribution of the bad hits and of the targets. In proteomics this is typically done by the use of the Target Decoy Approach (TDA).

3.2 Target Decoy Approach

A competitive target decoy search involves performing a search on a concatenated target and decoy database (as, for example, obtained from a reversed target database). Typically, one will ensure that there are as many targets as there are decoys. If the decoy hits are a good simulation of the bad target hits, it is equally likely that a bad hit will go to the targets as as it is to go to the decoys. With a competitive target decoy approach, it is therefore assumed that a bad hit matches a bad target equally likely as a decoy.

The distribution of bad target hits (\(f_0(t)\)) and the marginal distribution of all target hits (\(f(t)\)) is empirically estimated using the decoy scores and all target scores respectively. With the TDA, the FDR of the set of returned PSMs with scores above a threshold t, is estimated by dividing the number of decoy hits with a score above t by the number of target PSMs with a score above t.

\[ \widehat{\text{FDR}}(t) = \frac{\#\widehat{\text{ bad hits}} \mid x>t}{\#\text{ targets} \mid x>t} \stackrel{(*)}{=} \frac{\#\text{ decoys} \mid x>t}{\#\text{ targets} \mid x>t} \\ (*) \text{ Assumption TDA}: \text{bad targets} \stackrel{d}{=} \text{decoys} \]

This can be rewritten as:

\[\widehat{\text{FDR}}(t)=\frac{\#decoys}{\#targets} \cdot \frac{\frac{\# decoys \mid x>t}{\#decoys}}{\frac{\#targets \mid x>t}{\#targets}}\]

\[\widehat{\text{FDR}}(t) = {\widehat{\pi}}_0 \frac{\widehat{\int\limits_t^{+\infty} f_0(x) dx}}{\widehat{\int\limits_t^{+\infty} f(x)dx}}\]

Hence, the proportion of bad hits \(\pi_0\) is estimated as the number of decoys divided by the number of targets, since the competitive TDA assumes that it is equally likely that a bad hit matches to a bad target hit or to a decoy. The probability on a (bad) target PSM hit above the threshold is estimated based on the empirical cumulative distribution in the sample, i.e. as the fraction of targets (decoys) that are above the threshold. Hence, a second assumption is that the decoy scores provide a good simulation of the bad target scores.