Sequence Alignment Cont’d. Needleman-Wunsch with affine gaps Initialization:V(i, 0) = d + (i –...

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Sequence AlignmentCont’d

Needleman-Wunsch with affine gaps

Initialization: V(i, 0) = d + (i – 1)eV(0, j) = d + (j – 1)e

Iteration:V(i, j) = max{ F(i, j), G(i, j), H(i, j) }

F(i, j) = V(i – 1, j – 1) + s(xi, yj)

V(i – 1, j) – d G(i, j) = max

G(i – 1, j) – e

V(i, j – 1) – d H(i, j) = max

H(i, j – 1) – e

Termination: similar

HINT: With similar idea we can do arbitrary (non-convex) gaps

Linear-space alignment

• Iterate this procedure to the left and right!

M/2M/2

The Four-Russian Algorithm

Heuristic Local Alignerers

1. The basic indexing & extension technique

2. Indexing: techniques to improve sensitivityPairs of Words, Patterns

3. Systems for local alignment

State of biological databases

http://www.genome.gov/10005141

http://www.cbs.dtu.dk/databases/DOGS/

State of biological databases

• Number of genes in these genomes:

Mammals: ~25,000 Insects: ~14,000 Worms: ~17,000 Fungi: ~6,000-10,000

Small organisms: 100s-1,000s

• Each known or predicted gene has one or more associated protein sequences

• >1,000,000 known / predicted protein sequences

Some useful applications of alignments

• Given a newly discovered gene, Does it occur in other species? How fast does it evolve?

• Assume we try Smith-Waterman:

The entire genomic database

Our new gene

1010 - 1012

Some useful applications of alignments

• Given a newly sequenced organism,• Which subregions align with other organisms?

Potential genes Other biological characteristics

• Assume we try Smith-Waterman:

The entire genomic database

Our newly sequenced mammal

1010 - 1012

Indexing-based local alignment

(BLAST- Basic Local Alignment Search Tool)

Main idea:

1. Construct a dictionary of all the words in the query

2. Initiate a local alignment for each word match between query and DB

Running Time: O(MN)

However, orders of magnitude faster than Smith-Waterman

Indexing-based local alignment

Dictionary:

All words of length k (~10)

Alignment initiated between words of alignment score T

(typically T = k)

Alignment:

Ungapped extensions until score

below statistical threshold

Output:

All local alignments with score

> statistical threshold

……

Indexing-based local alignment—Extensions

A C G A A G T A A G G T C C A G T

Example:

The matching word GGTC initiates an alignment

Extension to the left and right with no gaps until alignment falls < T below best so far

Output:

GTAAGGTCC

GTTAGGTCC

Gapped extensions

• Extensions with gaps in a band around anchor

Output:

GTAAGGTCCAGTGTTAGGTC-AGT

Gapped extensions until threshold

• Extensions with gaps until score < T below best score so far

Output:

GTAAGGTCCAGTGTTAGGTC-AGT

Indexing-based local alignment—The index

• Sensitivity/speed tradeofflong words

(k = 15)

short words

(k = 7)

Sensitivity

Kent WJ, Genome Research 2002

Indexing-based local alignment—The index

Methods to improve sensitivity/speed

1. Using pairs of words

2. Using inexact words

3. Patterns—non consecutive positions

……ATAACGGACGACTGATTACACTGATTCTTAC……

……GGCACGGACCAGTGACTACTCTGATTCCCAG……

……ATAACGGACGACTGATTACACTGATTCTTAC……

……GGCGCCGACGAGTGATTACACAGATTGCCAG……

TTTGATTACACAGAT T G TT CAC G

Measured improvement

Kent WJ, Genome Research 2002

Non-consecutive words—Patterns

Patterns increase the likelihood of at least one match within a long conserved region

3 common

5 common

7 common

Consecutive Positions Non-Consecutive Positions

6 common

On a 100-long 70% conserved region: Consecutive Non-consecutive

Expected # hits: 1.07 0.97Prob[at least one hit]: 0.30 0.47

Advantage of Patterns

11 positions

10 positions

Multiple patterns

• K patterns Takes K times longer to scan Patterns can complement one another

• Computational problem: Given: a model (prob distribution) for homology between two regions Find: best set of K patterns that maximizes Prob(at least one match)

TTTGATTACACAGAT T G TT CAC G T G T C CAG TTGATT A G

Buhler et al. RECOMB 2003Sun & Buhler RECOMB 2004

How long does it take to search the query?

Variants of BLAST

• NCBI BLAST: search the universe http://www.ncbi.nlm.nih.gov/BLAST/• MEGABLAST:

Optimized to align very similar sequences• Works best when k = 4i 16• Linear gap penalty

• WU-BLAST: (Wash U BLAST) Very good optimizations Good set of features & command line arguments

• BLAT Faster, less sensitive than BLAST Good for aligning huge numbers of queries

• CHAOS Uses inexact k-mers, sensitive

• PatternHunter Uses patterns instead of k-mers

• BlastZ Uses patterns, good for finding genes

Example

Query: gattacaccccgattacaccccgattaca (29 letters) [2 mins]

Database: All GenBank+EMBL+DDBJ+PDB sequences (but no EST, STS, GSS, or phase 0, 1 or 2 HTGS sequences) 1,726,556 sequences; 8,074,398,388 total letters

>gi|28570323|gb|AC108906.9| Oryza sativa chromosome 3 BAC OSJNBa0087C10 genomic sequence, complete sequence Length = 144487 Score = 34.2 bits (17), Expect = 4.5 Identities = 20/21 (95%) Strand = Plus / Plus

Query: 4 tacaccccgattacaccccga 24 ||||||| |||||||||||||

Sbjct: 125138 tacacccagattacaccccga 125158

Score = 34.2 bits (17),

Expect = 4.5 Identities = 20/21 (95%) Strand = Plus / Plus

>gi|28173089|gb|AC104321.7| Oryza sativa chromosome 3 BAC OSJNBa0052F07 genomic sequence, complete sequence Length = 139823 Score = 34.2 bits (17), Expect = 4.5 Identities = 20/21 (95%) Strand = Plus / Plus

Example

Query: Human atoh enhancer, 179 letters [1.5 min]

Result: 57 blast hits1. gi|7677270|gb|AF218259.1|AF218259 Homo sapiens ATOH1 enhanc... 355 1e-95 2. gi|22779500|gb|AC091158.11| Mus musculus Strain C57BL6/J ch... 264 4e-68 3. gi|7677269|gb|AF218258.1|AF218258 Mus musculus Atoh1 enhanc... 256 9e-66 4. gi|28875397|gb|AF467292.1| Gallus gallus CATH1 (CATH1) gene... 78 5e-12 5. gi|27550980|emb|AL807792.6| Zebrafish DNA sequence from clo... 54 7e-05 6. gi|22002129|gb|AC092389.4| Oryza sativa chromosome 10 BAC O... 44 0.068 7. gi|22094122|ref|NM_013676.1| Mus musculus suppressor of Ty ... 42 0.27 8. gi|13938031|gb|BC007132.1| Mus musculus, Similar to suppres... 42 0.27

gi|7677269|gb|AF218258.1|AF218258 Mus musculus Atoh1 enhancer sequence Length = 1517 Score = 256 bits (129), Expect = 9e-66 Identities = 167/177 (94%),

Gaps = 2/177 (1%) Strand = Plus / Plus Query: 3 tgacaatagagggtctggcagaggctcctggccgcggtgcggagcgtctggagcggagca 62 ||||||||||||| ||||||||||||||||||| |||||||||||||||||||||||||| Sbjct: 1144 tgacaatagaggggctggcagaggctcctggccccggtgcggagcgtctggagcggagca 1203

Query: 63 cgcgctgtcagctggtgagcgcactctcctttcaggcagctccccggggagctgtgcggc 122 |||||||||||||||||||||||||| ||||||||| |||||||||||||||| ||||| Sbjct: 1204 cgcgctgtcagctggtgagcgcactc-gctttcaggccgctccccggggagctgagcggc 1262

Query: 123 cacatttaacaccatcatcacccctccccggcctcctcaacctcggcctcctcctcg 179 ||||||||||||| || ||| |||||||||||||||||||| |||||||||||||||

Sbjct: 1263 cacatttaacaccgtcgtca-ccctccccggcctcctcaacatcggcctcctcctcg 1318

Hidden Markov Models

x1 x2 x3 xK

Outline for our next topic

• Hidden Markov models – the theory

• Probabilistic interpretation of alignments using HMMs

Later in the course:

• Applications of HMMs to biological sequence modeling and discovery of features such as genes

Example: The Dishonest Casino

A casino has two dice:• Fair die

P(1) = P(2) = P(3) = P(5) = P(6) = 1/6• Loaded die

P(1) = P(2) = P(3) = P(5) = 1/10P(6) = 1/2

Casino player switches back-&-forth between fair and loaded die once every 20 turns

Game:1. You bet $12. You roll (always with a fair die)3. Casino player rolls (maybe with fair die,

maybe with loaded die)4. Highest number wins $2

Question # 1 – Evaluation

A sequence of rolls by the casino player

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QUESTION

How likely is this sequence, given our model of how the casino works?

This is the EVALUATION problem in HMMs

Question # 2 – Decoding

1245526462146146136136661664661636616366163616515615115146123562344

QUESTION

What portion of the sequence was generated with the fair die, and what portion with the loaded die?

This is the DECODING question in HMMs

Question # 3 – Learning

1245526462146146136136661664661636616366163616515615115146123562344

QUESTION

How “loaded” is the loaded die? How “fair” is the fair die? How often does the casino player change from fair to loaded, and back?

This is the LEARNING question in HMMs

The dishonest casino model

FAIR LOADED

0.950.95

P(1|F) = 1/6P(2|F) = 1/6P(3|F) = 1/6P(4|F) = 1/6P(5|F) = 1/6P(6|F) = 1/6

P(1|L) = 1/10P(2|L) = 1/10P(3|L) = 1/10P(4|L) = 1/10P(5|L) = 1/10P(6|L) = 1/2

Definition of a hidden Markov model

Definition: A hidden Markov model (HMM)• Alphabet = { b1, b2, …, bM }• Set of states Q = { 1, ..., K }• Transition probabilities between any two states

aij = transition prob from state i to state j

ai1 + … + aiK = 1, for all states i = 1…K

• Start probabilities a0i

a01 + … + a0K = 1

• Emission probabilities within each state

ei(b) = P( xi = b | i = k)

ei(b1) + … + ei(bM) = 1, for all states i = 1…K

A HMM is memory-less

At each time step t,

the only thing that affects future states

is the current state t

P(t+1 = k | “whatever happened so far”) =

P(t+1 = k | 1, 2, …, t, x1, x2, …, xt) =

P(t+1 = k | t)

A parse of a sequence

Given a sequence x = x1……xN,

A parse of x is a sequence of states = 1, ……, N

x1 x2 x3 xK

Likelihood of a parse

Given a sequence x = x1……xN

and a parse = 1, ……, N,

To find how likely is the parse:

(given our HMM)

P(x, ) = P(x1, …, xN, 1, ……, N) =

P(xN, N | N-1) P(xN-1, N-1 | N-2)……P(x2, 2 | 1) P(x1, 1) =

P(xN | N) P(N | N-1) ……P(x2 | 2) P(2 | 1) P(x1 | 1) P(1) =

a01 a12……aN-1N e1(x1)……eN(xN)

x2 x3 xK

Example: the dishonest casino

Let the sequence of rolls be:

x = 1, 2, 1, 5, 6, 2, 1, 6, 2, 4

Then, what is the likelihood of

= Fair, Fair, Fair, Fair, Fair, Fair, Fair, Fair, Fair, Fair?

(say initial probs a0Fair = ½, aoLoaded = ½)

½ (1/6)10 (0.95)9 = .00000000521158647211 = 0.5 10-9

So, the likelihood the die is fair in all this run

is just 0.521 10-9

OK, but what is the likelihood of

= Loaded, Loaded, Loaded, Loaded, Loaded, Loaded, Loaded, Loaded, Loaded, Loaded?

½ P(1 | Loaded) P(Loaded, Loaded) … P(4 | Loaded) =

½ (1/10)8 (1/2)2 (0.95)9 = .00000000078781176215 = 0.79 10-9

Therefore, it somewhat more likely that the die is fair all the way, than that it is loaded all the way

Let the sequence of rolls be:

x = 1, 6, 6, 5, 6, 2, 6, 6, 3, 6

Now, what is the likelihood = F, F, …, F?

½ (1/6)10 (0.95)9 = 0.5 10-9, same as before

What is the likelihood

= L, L, …, L?

½ (1/10)4 (1/2)6 (0.95)9 = .00000049238235134735 = 0.5 10-7

So, it is 100 times more likely the die is loaded

The three main questions on HMMs

1. Evaluation

GIVEN a HMM M, and a sequence x,FIND Prob[ x | M ]

2. Decoding

GIVEN a HMM M, and a sequence x,FIND the sequence of states that maximizes P[ x, | M ]

3. Learning

GIVEN a HMM M, with unspecified transition/emission probs.,and a sequence x,

FIND parameters = (ei(.), aij) that maximize P[ x | ]

Let’s not be confused by notation

P[ x | M ]: The probability that sequence x was generated by the model

The model is: architecture (#states, etc)

+ parameters = aij, ei(.)

So, P[x | M] is the same with P[ x | ], and P[ x ], when the architecture, and the parameters, respectively, are implied

Similarly, P[ x, | M ], P[ x, | ] and P[ x, ] are the same when the architecture, and the parameters, are implied

In the LEARNING problem we always write P[ x | ] to emphasize that we are seeking the * that maximizes P[ x | ]

Sequence Alignment Cont’d. Needleman-Wunsch with affine gaps Initialization:V(i, 0) = d + (i –...

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