Search Engine Result

Combining

Search Engine Result

Combining

Nathan EdwardsDepartment of Biochemistry and Molecular & Cellular BiologyGeorgetown University Medical Center

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Peptide Identification Results

• Search engines provide an answer for every spectrum...• Can we figure out which ones to believe?

• Why is this hard? • Hard to determine “good” scores• Significance estimates are unreliable• Need more ids from weak spectra• Each search engine has its strengths ...

... and weaknesses• Search engines give different answers

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Mascot Search Results

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Translation start-site correction

• Halobacterium sp. NRC-1• Extreme halophilic Archaeon, insoluble

membrane and soluble cytoplasmic proteins• Goo, et al. MCP 2003.

• GdhA1 gene:• Glutamate dehydrogenase A1

• Multiple significant peptide identifications• Observed start is consistent with Glimmer 3.0

prediction(s)

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Halobacterium sp. NRC-1ORF: GdhA1

• K-score E-value vs PepArML @ 10% FDR• Many peptides inconsistent with annotated

translation start site of NP_279651

0 40 80 120 160 200 240 280 320 360 400 440

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Translation start-site correction

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Search engine scores are inconsistent!

Mascot

Tan

dem

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Common Algorithmic Framework – Different Results

• Pre-process experimental spectra• Charge state, cleaning, binning

• Filter peptide candidates• Decide which PSMs to evaluate

• Score peptide-spectrum match• Fragmentation modeling, dot product

• Rank peptides per spectrum• Retain statistics per spectrum

• Estimate E-values• Appy empirical or theoretical model

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Comparison of search engines

• No single score is comprehensive

• Search engines disagree

• Many spectra lack confident peptide assignment

4%

OMSSA10%

2%

5%9%

69%

2%

X!Tandem

Mascot

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Lots of techniques out there

• Treat search engines as black-boxes• Generate PSMs + scores, features

• Apply supervised machine learning to results• Use multiple match metrics

• Combine/refine using multiple search engines• Agreement suggests correctness

• Use empirical significance estimates• “Decoy” databases (FDR)

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Machine Learning

• Use of multiple metrics of PSM quality:• Precursor delta, trypsin digest features, etc

• Requires "training" with examples• Different examples will change the result• Generalization is always the question

• Scores can be hard to "understand"• Difficult to establish statistical significance

• Peptide Prophet's discriminant function• Weighted linear combination of features

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Combine / Merge Results

Threshold peptide-spectrum matches from each of two search engines• PSMs agree → boost specificity• PSMs from one → boost sensitivity• PSMs disagree → ?????

• Sometimes agreement is "lost" due to threshold...• How much should agreement increase our confidence?

• Scores easy to "understand"• Difficult to establish statistical significance

• How to generalize to more engines?

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Consensus and Meta-Search

• Multiple witnesses increase confidence• As long as they are independent• Example: Getting the story straight

• Independent "random" hits unlikely to agree• Agreement is indication of biased sampling• Example: loaded dice

• Meta-search is relatively easy• Merging and re-ranking is hard• Example: Booking a flight to Denver!

• Scores and E-values are not comparable• How to choose the best answer?• Example: Best E-value favors Tandem!

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Searching for Consensus

Search engine quirks can destroy consensus

• Initial methionine loss as tryptic peptide

• Charge state enumeration or guessing

• X!Tandem's refinement mode

• Pyro-Gln, Pyro-Glu modifications

• Difficulty tracking spectrum identifiers

• Precursor mass tolerance (Da vs ppm)

Decoy searches must be identical!

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Configuring for Consensus

Search engine configuration can be difficult:

• Correct spectral format

• Search parameter files and command-line

• Pre-processed sequence databases.

• Tracking spectrum identifiers

• Extracting peptide identifications, especially modifications and protein identifiers

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Peptide Identification Meta-Search

• Simple unified search interface for:• Mascot, X!Tandem, K-

Score, S-Score, OMSSA, MyriMatch, InsPecT

• Automatic decoy searches

• Automatic spectrumfile "chunking"

• Automatic scheduling• Serial, Multi-Processor,

Cluster, Grid

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Peptide Identification Grid-Enabled Meta-Search

NSF TeraGrid1000+ CPUs

UMIACS250+ CPUs

Edwards LabScheduler &80+ CPUs

Securecommunication

Heterogeneouscompute resources

Single, simplesearch request

Scales easily to 250+ simultaneous

searches

X!Tandem,KScore,OMSSA,

MyriMatch,Mascot(1 core).

X!Tandem,KScore,OMSSA.

X!Tandem,KScore,OMSSA.

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PepArML

• Peptide identification arbiter by machine learning

• Unifies these ideas within a model-free, combining machine learning framework

• Unsupervised training procedure

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PepArML Overview

X!Tandem

Mascot

OMSSA

Other

PepArML

Feature extraction

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Dataset Construction

T),( 11 PS

F),( 21 PS

T),( 12 PS

X!Tandem Mascot OMSSA

T),( mn PS

……

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Voting Heuristic Combiner

• Choose PSM with most votes

• Break ties using FDR• Select PSM with min. FDR of tied votes

• How to apply this to a decoy database?

• Lots of possibilities – all imperfect• Now using: 100*#votes – min. decoy hits

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Supervised Learning

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Feature Evaluation

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Application to Real Data

• How well do these models generalize?

• Different instruments• Spectral characteristics change scores

• Search parameters• Different parameters change score values

• Supervised learning requires• (Synthetic) experimental data from every instrument• Search results from available search engines• Training/models for all

parameters x search engine sets x instruments

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Model Generalization

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Unsupervised Learning

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Unsupervised Learning Performance

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Unsupervised Learning Convergence

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Peptide Atlas A8_IP – LTQ

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OMICS 17 Protein Mix – LCQ

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Feature Selection (InfoGain)

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Conclusions

• Combining search results from multiple engines can be very powerful• Boost both sensitivity and specificity• Running multiple search engines is hard

• Statistical significance is hard• Use empirical FDR estimates...but be

careful...lots of subtleties• Consensus is powerful, but fragile

• Search engine quirks can destroy it• "Witnesses" are not independent