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CAP5510 – Bioinformatics Database Searches for Biological Sequences or Imperfect Alignments. Tamer Kahveci CISE Department University of Florida. Goals. Understand how major heuristic methods for sequence comparison work FASTA BLAST Understand how search results are evaluated.
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CAP5510 – BioinformaticsDatabase Searches for Biological Sequences or Imperfect Alignments Tamer Kahveci CISE Department University of Florida
Goals • Understand how major heuristic methods for sequence comparison work • FASTA • BLAST • Understand how search results are evaluated
What is Database Search ? • Find a particular (usually) short sequence in a database of sequences (or one huge sequence). • Problem is identical to local sequence alignment, but on a much larger scale. • We must also have some idea of the significance of a database hit. • Databases always return some kind of hit, how much attention should be paid to the result? • A similar problem is the global alignment of two large sequences • General idea: good alignments contain high scoring regions.
Imperfect Alignment • What is an imperfect alignment? • Why imperfect alignment? • The result may not be optimal. • Finding optimal alignment is usually to costly in terms of time and memory.
Database Search Methods • Hash table based methods • FASTA family • FASTP, FASTA, TFASTA, FASTAX, FASTAY • BLAST family • BLASTP, BLASTN, TBLAST, BLASTX, BLAT, BLASTZ, MegaBLAST, PsiBLAST, PhiBLAST • Others • FLASH, PatternHunter, SSAHA, SENSEI, WABA, GLASS • Suffix tree based methods • Mummer, AVID, Reputer, MGA, QUASAR
History of sequence searching • 1970: NW • 1980: SW • 1985: FASTA • 1990: BLAST
Hash Table • K-gram = subsequence of length K • Ak entries • A is alphabet size • Linear time construction • Constant lookup time
FASTP Lipman & Pearson, 1985
FASTP • Three phase algorithm • Find short good matches using k-grams • K = 1 or 2 • Find start and end positions for good matches • Use DP to align good matches
FASTP: Phase 1 (1) position 1 2 3 4 5 6 7 8 9 10 11 protein 1 n c s p t a . . . . . protein 2 . . . . . a c s p r k position in offset amino acid protein A protein B pos A - posB ----------------------------------------------------- a 6 6 0 c 2 7 -5 k - 11 n 1 - p 4 9 -5 r - 10 s 3 8 -5 t 5 - ----------------------------------------------------- Note the common offset for the 3 amino acids c,s and p A possible alignment can be quickly found : protein 1 n c s p t a | | | protein 2 a c s p r k
FASTP: Phase 1 (2) • Similar to dot plot • Offsets range from 1-m to n-1 • Each offset is scored as • # matches - # mismatches • Diagonals (offsets) with large score show local similarities • How does it depend on k?
FASTP: Phase 2 • 5 best diagonal runs are found • Rescore these 5 regions using PAM250. • Initial score • Indels are not considered yet
FASTP: Phase 3 • Sort the aligned regions in descending score • Optimize these alignments using Needleman-Wunsch • Report the results
FASTP - Discussion • Results are not optimal. Why ? • How does performance compare to Smith-Waterman? • What is the impact of k? • How does this idea work for DNAs ? • K = 4 or 6 for DNA
FASTA – Improvement Over FASTP Pearson 1995
FASTA (1) • Phase 2: Choose 10 best diagonal runs instead of 5
FASTA (2) • Phase 2.5 • Eliminate diagonals that score less than some given threshold. • Combine matches to find longer matches. It incurs join penalty similar to gap penalty
FASTA Variations • TFASTAX and TFASTAY: query protein against a DNA library in all reading frames • FASTAX, FASTAY: DNA query in all reading frames against protein database
BLAST Altschul, Gish, Miller, Myers, Lipman, 1990
BLAST (or BLASTP) • BLAST – Basic Local Alignment Search Tool • An approximation of Smith-Waterman • Designed for database searches • Short query sequence against long database sequence or a database of many sequences • Sacrifices search sensitivity for speed
MCGPFILGTYC CGP MCG BLAST Algorithm (1) • Eliminate low complexity regions from the query sequence. • Replace them with X (protein) or N (DNA) • Hash table on query sequence. • K = 3 for proteins
BLAST Algorithm (2) • For each k-gram find all k-grams that align with score at least cutoff T using BLOSUM62 • 20k candidates • ~50 on the average per k-gram • ~50n for the entire query • Build hash table PQGMCGPFILGTYC QGM PQG PQG PQG 18 PEG 15 PRG 14 PSG 13 PQA 12 T = 13
BLAST Algorithm (3) • Sequentially scan the database and locate each k-gram in the hash table • Each match is a seed for an ungapped alignment.
BLAST Algorithm (4) • HSP (High Scoring Pair) = A match between a query word and the database • Find a “hit”: Two non-overlapping HSP’s on a diagonal within distance A • Extend the hit until the score falls below a threshold value, X
BLAST Algorithm (5) • Keep only the extended matches that have a score at least S. • Determine the statistical significance of the result
What is Statistical Significance? • Two one-on-one games, two scores. • Which result is more significant? • Expected: maybe a random result. • Unexpected: significant, may have significant meanings. 13 : 15 13 : 15
Statistical Significance • E-value: The expected number of matches with score at least S • E = Kmne-lambda.S • m, n : sequence lengths • S : alignment score • K, lambda: normalization parameters • P-value: The probability of having at least one match with score at least S • 1 – e-E • The smaller these values are, the more significant the result • http://www.ncbi.nlm.nih.gov/Education/BLASTinfo/glossary2.html
K (k-gram) Lower: more sensitive. Slower. T (neighbor cutoff) Lower: Find distant neighbors. Introduces noise X (extension cutoff) Higher: lower chances of getting into a local minima. Slower. BLAST - Analysis
Sample Query • http://www.ncbi.nlm.nih.gov/BLAST/ Dhal_ecoli I D R A M S A A R G V F E R G D W S L S S P A K R K A V L N K L A D L M E A H A E E L A L L E T L D T G K P I R H S L R D D I P G A A R A I R W Y A E A I D K V Y G E V A T T S S H E L A M I V R E P V G V I A A I V P W N F P L L L T C W K L G P A L A A G N S V I L K P S E K S P L S A I R L A G L A K E A G L P D G V L N V V T G F G H E A G Q A L S R H N D I D A I A F T G S T R T G K Q L L K D A G D S N M K R V W L E A G G K S A N I V F A D C P D L Q Q A A S A T A A G I F Y N Q G Q V C I A G T R L L L E E S I A D E F L A L L K Q Q A Q N W Q P G H P L D P A T T M G T L I D C A H A D S V H S F I R E G E S K G Q L L L D G R N A G L A A A I G P T I F V D V D P N A S L S R E E I F G P V L V V T R F T S E E Q A L Q L A N D S Q Y G L G A A V W T R D L S R A H R M S R R L K A G S V F V N N Y N D G D M T V P F G G Y K Q S G N G R D K S L H A L E K F T E L K T I W I
BLASTN • BLAST for nucleic acids • K = 11 • Exact match instead of neighborhood search.
Even More Variations • PsiBLAST (iterative) • BLAT, BLASTZ, MegaBLAST • FLASH, PatternHunter, SSAHA, SENSEI, WABA, GLASS • Main differences are • Seed choice (k, gapped seeds) • Additional data structures
Suffix Tree • Tree structure that contains all suffixes of the input sequence • TGAGTGCGA • GAGTGCGA • AGTGCGA • GTGCGA • TGCGA • GCGA • CGA • GA • A
Suffix Tree Analysis • O(n) space and construction time • 10n to 70n space usage reported • O(m) search time for m-letter sequence • Good for • Small data • Exact matches
Suffix Array • 5 bytes per letter • O(m log n) search time • Better space usage • Slower search
Other Sequence Comparison Tools • Reputer, MGA, AVID • QUASAR (suffix array)