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Finding, Aligning and Analyzing Non Coding RNAs Cédric Notredame Comparative Bioinformatics Group Bioinformatics and Genomics Program
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They are Everywhere… And ENCODE said… “nearly the entire genome may be represented in primary transcripts that extensively overlap and include many non-protein-coding regions” Who Are They? – tRNA, rRNA, snoRNAs, – microRNAs, siRNAs – piRNAs – long ncRNAs (Xist, Evf, Air, CTN, PINK…) How Many of them – Open question – 30.000 is a common guess – Harder to detect than proteins.
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Searching “…When Looking for a Needle in a Haystack, the optimistic Wears Gloves…”
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ncRNAs can have different sequences and Similar Structures
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ncRNAs Can Evolve Rapidly CCAGGCAAGACGGGACGAGAGTTGCCTGG CCTCCGTTCAGAGGTGCATAGAACGGAGG **-------*--**---*-**------** GAACGGACCGAACGGACC CTTGCCTGGCTTGCCTGG G G A A CC A C G G A G A C G CTTGCCTCCCTTGCCTCC GAACGGAGGGAACGGAGG G G A A CC A C G G A G A C G
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ncRNAs are Difficult to Align --CCAGGCAAGACGGGACGAGAGTTGCCTGG CCTCCGTTCAGAGGTGCATAGAACGGAGG-- * * *** * * *** * CCAGGCAAGACGGGACGAGAGTTGCCTGG CCTCCGTTCAGAGGTGCATAGAACGGAGG **-------*--**---*-**------** Regular Alignment
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ncRNAs are Difficult to Align Same Structure Low Sequence Identity Small Alphabet, Short Sequences Alignments often Non- Significant
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Obtaining the Structure of a ncRNA is difficult Hard to Align The Sequences Without the Structure Hard to Predict the Structures Without an Alignment
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The Holy Grail of RNA Comparison: Sankoff’ Algorithm
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The Holy Grail of RNA Comparison Sankoff’ Algorithm Simultaneous Folding and Alignment – Time Complexity: O(L 2n ) – Space Complexity: O(L 3n ) In Practice, for Two Sequences: – 50 nucleotides: 1 min.6 M. – 100 nucleotides 16 min.256 M. – 200 nucleotides 4 hours 4 G. – 400 nucleotides3 days3 T. Forget about – Multiple sequence alignments – Database searches
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The next best Thing: Consan Consan = Sankoff + a few constraints Use of Stochastic Context Free Grammars – Tree-shaped HMMs – Made sparse with constraints The constraints are derived from the most confident positions of the alignment Equivalent of Banded DP
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Consan for Databases: Infernal Infernal is a Faster version of Consan For Database Search Sill Very Slow Receiver operating characteristicReceiver operating characteristic (ROC) Comparison of Infernal with BLAST
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Consan for Databases: Infernal BLAST: 360 s. Fast Infernal: 182 000 s. Slow Infernal: 5 320 000 s.
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Searching Databases for New RNAs
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Rfam: In practice Rfam contains RNA families – Families Multiple Sequence Alignment Models – Models are like Pfam Profiles Use Consan or Cmsearch rather than HMMer Much Slower – Too expensive to search the models Models are used to build Rfam People usually BLAST Rfam
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Where do Rfam Families Come From? Infernal Requires a Model Models requires an MSA The MSA requires a Family It all starts with a BlastN Rfam, Gardner et al. NAR 2008
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Can we make BlastN more accurate ? BlastN is not very accurate because: – Poor substitution models for Nucleic Acids – Low information density (4 symbols) BlastN assumes – Equal evolution rates for all nucleotides – Independence form Neighbors
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Love Thy Neighbor Measured Nearest Neighbor Dependencies on Rfam sequences
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High Rate of CpG mutations
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Measuring Di-Nucleotide Evolution Each Nucleotide can be made more informative It can incorporate the “name” of its Neighbor – AA => a – AG => b – AC => c – AT => d – … A 16 Letter alphabet can be used to recode all nucleotide sequences We name these extended Nucleotides
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Blosum-R and eRNA
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Substitutions ?? How much does it cost to turn one nucleotide into another one ? Blosum/Pam style matrix Matrices estimated on Rfam families
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Blosum-R and eRNA
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Using BlastR When Nucleic Acids look like Proteins They can be aligned with Protein Methods – BlastN BlastP – BlastP with eRNA is BlastR
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Validating Blast-R
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Benchmarking BlastR Rfam PP PN EVALUESEVALUES Blast Query
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Benchmarking BlastR Rfam 001 Rfam 002 Rfam … Rfam 001 Rfam 002 Rfam … Blast ROC
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Benchmarking BlastR Good Bad False Positives True Positive Good Bad
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Benchmarking BlastR False Positives True Positive Good Bad Area Under Curve Small AUC Better
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BlastR vs The World
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The 3 Components of Blast R BlastP is better than BlastN BlosumR makes BlastP a little bit better Blast: wuBlast
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The 3 Components of Blast R BlastP is better than BlastN BlosumR makes BlastP a little bit better And Faster
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BlastR and Clustering Given all Rfam in Bulk How good is BlastR at reconstituting all the families Sensitivity 1-Specificty
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BlastR and Clustering Given all Rfam in Bulk How good is BlastR at reconstituting all the families Sensitivity 1-Specificty
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BllastR: In Practice
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E-Value Threshold: 10 -20 BlastN BlastR
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Take Home Searching Nucleotides is Difficult BlastN is not a very good algorithm Simple Adaptations can improve the situation – Changing the algorithm (BlastP) – Changing the Scoring Scheme (BlastP-Nuc) – Changing the alphabet (BlastR)
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