Learning gene regulatory networks Statistical methods for haplotype inference Part I

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1 Learning gene regulatory networks Statistical methods for haplotype inference Part I Input: Measurement of mrn levels of all genes from microarray or rna sequencing Samples (e.g. 200 patients with lung cancer) Lecture 3 May 21 th, 2013 ENOME 541, Spring 2013 Su In Lee S & SE, UW suinlee@uw.edu 1 oal: Reconstruct the gene regulatory network underlying genome wide gene expression Method: Probabilistic models to represent the regulatory network e 1 e 6 e p 2 Unknown structure, complete data E B P( E,B) H H?? H L?? L H?? L L?? E, B, <H,L,L> <H,L,H> <L,L,H> <L,H,H>.. <L,H,H> E B Learner E B E B P( E,B) H H.9.1 H L.7.3 L H.8.2 L L Score based learning Define scoring function that measures how well a certain structure fits the observed data. E, B, <Y,N,N> <Y,Y,Y> <N,N,Y> <N,Y,Y>.. <N,Y,Y> Score( 1 ) = 10 Score ( 2 ) = 1.5 Score ( 3 ) = 0.01 E 1 B E 2 3 B B E Network structure is not specified Learner needs to estimate both structure and parameters Data does not contain missing values 3 Search for a structure that maximizes the score. 4 1

2 hallenges oo large search space What is the number of possible structures of n genes? omputationally costly Heuristic approaches may be trapped to local maxima. Biologically motivated constraints can alleviate the problems Module based approach ~ 3 n Only the genes in the candidate regulators list can be parents of other variables 5 2 / 2-3 x x he Module networks concept X3 X5 = Module3 X2 Repressor X4 ctivator X3 Linear PDs X 1 Module 1 Module 2 X4 Module 3 X6 activator expression repressor expression target gene expression induced repressed ctivator X3 false true Repressor X4 false ree PDs true regulation program Module 3 genes 6 Segal et al. Nat enet 03, JMLR 05 Linear module network andidate regulators (X 1,,X N ) Expression levels of genes that have regulatory roles ranscription factors, signal transduction proteins, mrn processing factors, M1 M22 Feature selection via regularization ssume linear aussian PD MLE: solve maximize w (Σw i x i Y) 2 KEM1 MK1 HP1 MSK1 DHH1 SS1 SE59 EM18 S7 ME3 UH1 P1 MF1 E1 Regulatory Program? argets PHO3 PHO5 PHO84 RIM15 PHM6 PHO2 PHO4 SS5 SPL2 I1 V3 x 1 x 2 w 1 w 2 w N parameters Y x N Regulatory network andidate regulators (features) Yeast: 350 genes Mouse: 700 genes P(Y x:w) = N(Σw i x i, ε 2 ) Problem: his objective learns too many regulators Lee et al., PLoS enet

3 L 1 regularization Select a subset of regulators ombinatorial search? Effective feature selection algorithm: L 1 regularization (LSSO) [ibshirani, J. Royal. Statist. Soc B. 1996] minimize w (Σw i x i Y) 2 + w i : convex optimization! Induces sparsity in the solution w (Many w i s set to zero) Linear module network Iterative procedure -3 x P1 + Learn a regulatory program for each module 0.5 x MF1 luster genes into modules = Module M1 MSK1 EM18 KEM1 UH1 P1 DHH1 MK1 E1 M22 S7 MF1 ME3 x 1 x 2 w 1 w 2 w N Y x N andidate regulators (features) Yeast: 350 genes Mouse: 700 genes P(Y x:w) = N(Σw i x i, ε 2 ) HP1 SS1 SE59 Better? PHO3 PHO5 PHO84 RIM15 L 1 regularized optimization PHM6 minimize PHO2 w (Σw i x i -E argets PHO4 )2+ w i SS5 SPL2 I1 V3 9 Lee et al., PLoS enet Summary Haplotype inference (5/21, 5/23) Basic concepts on Bayesian networks Probabilistic models of gene regulatory networks Learning algorithms Parameter learning Structure learning Structure discovery Evaluation Recent probabilistic approaches to reconstructing the regulatory networks 11 Background & motivation Problem statement Statistical methods for haplotype inference lark s algorithm Expectation Maximization (EM) algorithm oday oalescent based methods and HMM Haplotype inference on sequence data Example applications 12 3

4 enetic variation Single nucleotide polymorphism (SNP) Each variant is called an allele; each allele has a frequency chromosome SNP 1 SNP 2 SNP 3 P(SNP 1 = ) = 4/8 P(SNP 1 = ) = 4/8 : How about the relationship between alleles of neighboring SNPs? We need to know about haplotype 13 Haplotype combination of alleles present in a chromosome Each haplotype has a frequency, which is the proportion of chromosomes of that type in the population here are 2 N possible haplotypes But in fact, far fewer are seen in human population chromosome haplotype 123 SNP 1 SNP 2 SNP 3 Haplotype frequencies P(haplotype 123 = ) = 3/8 P(haplotype 123 = ) = 2/8 P(haplotype 123 = ) = 2/8 P(haplotype 123 = ) = 1/8 14 History of two neighboring alleles lleles that exist today arose through ancient mutation events Before mutation History of two neighboring alleles One allele arose first, and then the other Before mutation fter mutation Mutation fter mutation Mutation 15 Haplotype: combination of alleles present in a chromosome 16 4

5 Recombination can create more haplotypes Without recombination No recombination (or 2n recombination events) With recombination Recombination (2n+1 recombination events) 17 Recombinant haplotype 18 Haplotype What determines haplotype frequencies? Recombination rate (r) between neighboring alleles in the population r is different for different regions in genome Linkage disequilibrium (LD) Non random association of alleles at two or more loci, not necessarily on the same chromosome. haplotype 123 SNP 1 SNP 2 SNP 3 Haplotype frequencies P(haplotype 123 = ) = 3/8 P(haplotype 123 = ) = 2/8 P(haplotype 123 = ) = 2/8 P(haplotype 123 = ) = 1/8 How can we measure haplotypes? Haplotypes can be generated through laboratorybased experimental methods X chromosome in males Sperm typing Hybrid cell lines Other molecular techniques omputational approaches Input: enotype data from individuals in a population Output: Haplotypes of each individual in the population chromosome

6 Haplotype inference problem Sequence and SNP array data generally take the form of unphased genotypes Markers 21 Motivation Enormous amounts of genotype data are being generated Inexpensive genome wide SNP microarrays Whole genome and whole exome sequencing tools Determination of haplotype phase is increasingly important haracterizing the relationship between genetic variation and disease susceptibility Imputing low frequency variants 22 Why useful in WS? In a typical short chromosome segment, there are only a few distinct haplotypes arefully selected markers can determine status of others We can test for association of untyped SNPs Example Holm et al. Nat enet (2011) Used the inferred haplotypes for accurate imputation of a putative rare causal variant in other individuals, to obtain a stronger association signal Haplotype 1 Haplotype 2 Haplotype 3 Haplotype 4 Haplotype 5 S 1 S 2 S 3 S 4 S 5 S N Different alleles of each SNP 30% 20% 20% 20% 10% Marker (ag SNP)

7 Example Sick sinus syndrome (SSS) haracterized by slow heart rate, sinus arrest and/or failure to increase heart rate with exercise enome wide association scan of 7.2M SNPs with 792 SSS cases and 37,592 controls he association analysis yielded association with several correlated SNPs in and near MYH6 MYH7 (never before associated with SSS) 25 Outline Background & motivation Problem statement Statistical methods for haplotype inference lark s algorithm Expectation Maximization (EM) algorithm oday oalescent based methods and HMM Haplotype inference on sequence data Example applications 26 ypical genotype data wo alleles for each individual for each marker hromosome origin for each allele is unknown Observation marker 1 {} {} {} marker 2 marker 3 Multiple haplotype pairs can fit observed genotype Possible states / / / / 27 Use information on relatives? Family information can help determine phase at many markers an you propose examples? enotype: {} {} {} Maternal genotype: {} {} {} Paternal genotype: {} {} {} hen the haplotype is / / / 28 7

8 Example inferring haplotypes Still, many ambiguities might not be resolved Problem more serious with larger numbers of markers enotype: {} {} {} Maternal genotype: {} {} {} Paternal genotype: {} {} {} annot determine unique haplotype What if there are no relatives? Rely on linkage disequilibrium (LD) LD: non random association of variants at different sites in the genome ssume that population consists of small number of distinct haplotypes Problem Determine haplotypes without parental genotypes Haplotype reconstruction lso called, phasing, haplotype inference or haplotyping Data enotype on N markers from M individuals oals Frequency estimation of all possible haplotypes Haplotype reconstruction for individuals How many out of all possible haplotypes are plausible in a population? Individual i marker 1 marker 2 marker 3 31 Statistical methods for haplotypes inference Let s focus on the methods that are most widely used or historically important Browning and Browning, Nat Rev enet

9 Outline Background & motivation Problem statement Statistical methods for haplotype inference lark s algorithm Expectation Maximization (EM) algorithm oalescent based methods and HMM Haplotype inference on sequence data Example applications lark s haplotyping algorithm lark (1990) Mol Biol Evol 7: One of the first published haplotyping algorithms omputationally efficient Very fast and widely used in 1990 s More accurate methods are now available lark s haplotyping algorithm Find unambiguous individuals Initialize a list of known haplotypes Unambiguous vs. ambiguous Haplotypes for 2 SNPs (alleles: /a, B/b) What kinds of genotypes will these have? Unambiguous individuals Homozygous at every locus (e.g. {} {} {}) Haplotypes: Heterozygous at just one locus (e.g. {} {} {}) Haplotypes: or

10 lark s haplotyping algorithm Find unambiguous individuals Initialize a list of known haplotypes Parsimonious phasing example Notation (more compact representation) 0/1: homozygous at each locus (00,11) h: heterozygous at each locus (01) Resolve ambiguous individuals If possible, use two haplotypes from the list Otherwise, use one known haplotype and augment list If unphased individuals remain ssign phase randomly to one individual ugment haplotype list and continue from previous step h h 0 1 h h h 1 h Parsimony algorithm Outline Pros Very fast an deal with very long sequences ons No homozygotes or single SNP heterozygotes in the data Some haplotypes may remain unresolved Outcome depends on order in which lists are transversed Naïve, not very accurate (no modeling) Background & motivation Problem statement Statistical methods for haplotype inference lark s algorithm Expectation Maximization (EM) algorithm oalescent based methods and HMM Haplotype inference on sequence data Example applications

11 he EM haplotyping algorithm ssume that we know haplotype frequencies Excoffier and Slatkin Mol Biol Evol (1995); Qin et al. m J Hum enet (2002); Excoffier and Lischer Molecular ecology resources (2010) Why EM for haplotyping? EM is a method for MLE with hidden variables. What are the hidden variables, parameters? Hidden variables: haplotype state of each individual Parameters: haplotype frequencies Individual n Haplotype status (hidden variable) z =0 z =1 Haplotype frequencies (parameters) p b, p ab, p B, p ab 41 Probability of first outcome: 2P b P ab = 0.06 Probability of second outcome: 2P B P ab = 0.18 For example, if P B = 0.3 P ab = 0.3 P b = 0.3 P ab = onditional probabilities are onditional probability of first outcome: 2P b P ab / (2P b P ab + 2P B P ab ) = 0.25 For example, if P B = 0.3 P ab = 0.3 P b = 0.3 P ab = 0.1 ssume that we know the haplotype state of each individual omputing haplotype frequencies is straightforward Individual 1 Individual 2 Individual 3 p B =? P ab =? p b =? p ab =? onditional probability of second outcome: 2P B P ab / (2P b P ab + 2P B P ab ) = Individual

12 EM as hicken vs Egg If we know haplotype frequencies p s (parameters), we can estimate the haplotype status of individuals z s (hidden variables) If we know the haplotype state of each individual z s (hidden variables), we can estimate the haplotype frequencies p s (parameters) EM as hicken vs Egg If we know haplotype frequencies p s (parameters), we can estimate the haplotype states of individuals z s (hidden variables) If we know the haplotype state of individuals z s (hidden variables), we can estimate the haplotype frequencies p s (parameters) Individual n Haplotype status (hidden variable) z =0 z =1 Haplotype frequencies (parameters) p b, p ab, p B, p ab 45 BU we know neither; iterate Expectation step: Estimate z s, given haplotype frequencies p s Maximization step: Estimate p s, given the haplotype states of individuals z s Overall, a clever hill climbing strategy 46 Phasing By EM EM: Method for maximum likelihood parameter inference with hidden variables Parameters (haplotype frequencies) p s) Maximize uess Likelihood Inferring haplotype state of each individual E M Estimating haplotype frequencies Hidden variables (haplotype states of individuals z s) Find expected values 47 EM algorithm for haplotyping 1. uesstimate haplotype frequencies 2. Use current frequency estimates to replace ambiguous genotypes with fractional counts of phased genotypes 3. Estimate frequency of each haplotype by counting 4. Repeat steps 2 and 3 until frequencies are stable 48 12

13 Phasing by EM Phasing by EM Data: 1 0 h h 1 h h 1 h h Data: 1 0 h h 1 h h 1 h h Frequencies / / / /12 3/ /12 2/12 1 / /12 50 Phasing by EM Phasing by EM Maximization Data: 1 0 h h 1 h h 1 h h Haplotypes Frequencies / / / /12 3/ /12 2/12 1 / /12 Expectation 51 Data: 1 0 h h 1 h h 1 h h Haplotypes Frequencies / / / / / /12.1 2/ / /12.1 Expectation 52 13

14 Phasing by EM Data: 1 0 h h 1 h h 1 h h Haplotypes Frequencies / / / /6 53 omputational cost (for SNPs) onsider sets of m unphased genotypes Markers 1..m If markers are bi allelic 2 m possible haplotypes 2 m 1 (2 m + 1) possible haplotype pairs 3 m distinct observed genotypes 2 n 1 reconstructions for n heterozygous loci For example, if m=10 = 1024 = 524,800 = 59,049 = EM algorithm Outline Pros More accurate than lark s method Fully or partially phased individuals contribute most of the information ons Estimate depends on starting point: need to run multiple times on different starting points Implementation may become computationally expensive: cost grows rapidly with number of markers For each individual, the number of possible haplotypes is 2 m, where m is the number of makers ypically run for short sequences with < 25 SNPs Background & motivation Problem statement Statistical methods for haplotype inference lark s algorithm Expectation Maximization (EM) algorithm oalescent based methods and HMM Haplotype inference on sequence data Example applications No modeling on haplotypes