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Statistical control of peptide and protein error rates in large-scale targeted data-independent acquisition analyses

Abstract

Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the main method for high-throughput identification and quantification of peptides and inferred proteins. Within this field, data-independent acquisition (DIA) combined with peptide-centric scoring, as exemplified by the technique SWATH-MS, has emerged as a scalable method to achieve deep and consistent proteome coverage across large-scale data sets. We demonstrate that statistical concepts developed for discovery proteomics based on spectrum-centric scoring can be adapted to large-scale DIA experiments that have been analyzed with peptide-centric scoring strategies, and we provide guidance on their application. We show that optimal tradeoffs between sensitivity and specificity require careful considerations of the relationship between proteins in the samples and proteins represented in the spectral library. We propose the application of a global analyte constraint to prevent the accumulation of false positives across large-scale data sets. Furthermore, to increase the quality and reproducibility of published proteomic results, well-established confidence criteria should be reported for the detected peptide queries, peptides and inferred proteins.

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Figure 1: Estimation of q-values at the peptide-query level, the peptide level and the protein level.
Figure 2: Schematic illustration of the different context-dependent error-rate estimation strategies.
Figure 3: Analyte accumulation across multiple runs.
Figure 4: Comparison between peptide queries with varying target prevalence.

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Acknowledgements

Please note that M.H., C.L.H., Y.L., M.J.M., B.X.M., A.I.N., P.G.A.P., L.R., H.L.R., S.T. and Y.S.T. were added to the author list in alphabetical order. We thank the authors of the SWATH-MS interlaboratory study and of the human blood plasma data set for providing the data to conduct this study. We also thank the Scientific IT Support (ID SIS) and the high-performance computing (HPC) teams of ETH Zurich for support and maintenance of the computing infrastructure. M.H. was supported by a grant from the Institut Mérieux; A.I.N. was funded by the US National Institutes of Health (NIH; grant R01GM094231); H.L.R. was funded by the Swiss National Science Foundation (SNSF; grant P2EZP3 162268); B.C.C. was supported by a SNSF Ambizione grant (PZ00P3_161435); and R.A. was supported by ERC Proteomics v3.0 (AdG-233226 Proteomics v.3.0) and AdG-670821 Proteomics 4D), the PhosphonetX project of SystemsX.ch and the Swiss National Science Foundation (SNSF) grant 31003A_166435.

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Authors

Contributions

G.R., I.B. and R.A. wrote the paper with feedback from all authors; G.R. and B.C.C. developed the methods; I.B. analyzed the data set; U.S. and G.R. implemented the PyProphet extension; M.H., C.L.H., Y.L., M.J.M., B.X.M., A.I.N., P.G.A.P., L.R., H.L.R., S.T. and Y.S.T. provided critical input on the project; and B.C.C. and R.A. designed and supervised the study.

Corresponding authors

Correspondence to Ben C Collins or Ruedi Aebersold.

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Competing interests

C.L.H. and S.T. are employees of SCIEX, which operates in the field of quantitative proteomics by data-independent acquisition covered by the article. M.J.M. is a paid consultant for Thermo Fisher Scientific, which operates in the field of quantitative proteomics by data-independent acquisition covered by the article. L.R. is employee of Biognosys AG, which operates in the field of quantitative proteomics by data-independent acquisition covered by the article. R.A. holds shares of Biognosys AG.

Integrated supplementary information

Supplementary Figure 1 Estimation of q-values at the peptide-query level, the peptide level and the protein level.

The peptide-query-level (left), peptide-level (middle) and protein-level (right) discriminant score density plots (a) and p-value histograms (b) for one DIA run of the SWATH-MS interlaboratory study that was analyzed with the combined human assay library (CAL) are shown. a) The distributions indicate a large (false target)/(total target) ratio (π0 0.6) on the peptide-query level. The q-value estimation was adapted for peptide and protein level by using the best scoring peak group per peptide or protein across all samples for both targets and decoys. The (false target)/(total target) ratio decreases slightly on peptide level and more on protein level (π0 0.5), compared to the peptide-query level. b) On the peptide-query level and the peptide level, the estimation of π0 is anticonservative, indicated by a lower density of p-values after the p-value threshold of λ=0.4. On the protein level, the estimation of π0 is more accurate with a consistent density of p-values.

Supplementary Figure 2 Influence of protein length on the peptide-query-level and protein-level q-value estimation.

a) Protein length distribution of all proteins in the combined human assay library (CAL), all proteins inferred at 1% peptide-query-level FDR in the global context of all 229 DIA runs of the SWATH-MS interlaboratory comparison study, and all proteins inferred at 1% global protein FDR respectively. b) Histogram of protein length distribution for the differently filtered protein subsets of the CAL. The distributions show that there is no bias for protein length when selecting the best peak group as proxy for protein-level q-value estimation.

Supplementary Figure 3 Decoy accumulation across multiple runs.

The number of cumulatively detected peak group decoys (a), peptide decoys (b) and protein decoys (c) is shown for 229 DIA runs of the SWATH-MS interlaboratory study data set.

Supplementary Figure 4 Analyte accumulation across multiple runs (5% FDR).

The number of cumulatively detected peak groups (a), peptides (b) and proteins (c) is shown for 229 DIA runs of the SWATH-MS interlaboratory study data set.

Supplementary Figure 5 Decoy accumulation across multiple runs (5% FDR).

The number of cumulatively detected peak group decoys (a), peptide decoys (b) and protein decoys (c) is shown for 229 DIA runs of the SWATH-MS interlaboratory study data set.

Supplementary Figure 6 Combined human and M. tuberculosis spectral library analysis.

a) The peptide-level discriminant score density of human targets, human decoys, M. tuberculosis (Mtb) targets, and Mtb decoys is shown for global analysis of the 229 DIA runs of the SWATH-MS interlaboratory study data set applying the combined human and Mtb spectral library. The Mtb targets and decoys show a similar distribution compared to the human decoys and the fraction of false human targets. The number of cumulatively detected peptides is shown for human targets (b), human decoys (c), Mtb targets (d), and Mtb decoys (e) from the combined human and Mtb spectral library with different error rate control strategies. The Mtb decoy to target ratio is 0.82, explaining the absolute higher number of the accumulated Mtb targets.

Supplementary Figure 7 Analyte accumulation across multiple runs in the plasma data set (1% FDR).

The number of cumulatively detected peak groups (a), peptides (b) and proteins (c) is shown for the 246 DIA runs of the plasma data set analyzed with the nonparametric model for q-value estimation.

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Rosenberger, G., Bludau, I., Schmitt, U. et al. Statistical control of peptide and protein error rates in large-scale targeted data-independent acquisition analyses. Nat Methods 14, 921–927 (2017). https://doi.org/10.1038/nmeth.4398

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