handling and analyzing data from a genome-wide association study

Handling and analyzing data from a genome-wide association study

Laura Scott

Biostatistics DepartmentCenter for Statistical Genetics

University of Michigan

Outline

• Storing large amounts of genotype data

• Quality control

• Generating initial association analysis

• Viewing results

• Imputation of missing SNP genotypes

• Storing results and planning specialized analysis

Genotype data is huge

• 500,000 SNPs * 2000 cases + controls = 1,000,000,000 genotypes!

• Need compact ways to store data– If store each genotype as 00, 01, or 11 will

have file that looks like Person 100011001010001001001100….

000000100001000010001000….




have file that looks like SNP 100011001010001001001100….

000000100001000010001000….




have file that looks like 100011001010001001001100….

000000100001000010001000….

– Total file space for 300K SNPs: 4 Gigabytes– Largest chromosome file: .4 Gigabytes

Need to do extensive planning for genotype data before it arrives

• Chromosome datasets are too large for SAS and other commonly used analytic packages

• Need programs to select and write out genotype data in multiple formats

• Tests of procedures with large-scale trial datasets

Gather other data needed for analysis

• SNP information– Chromosome– Position

• SNP annotation– Gene– Function

• Translation of called allele to a standard allele– Example forward strand of given genome build

How good is the data?

• Identify and remove bad samples and SNPs

• Compute summary statistics– Percent successfully genotyped samples– Average genotyping success rate– Duplicate sample error rate– Non-Mendelian inheritance error rates (errors

not consistent with normal transmission of chromosomes in family members)

Identify bad samples and remove

• Poor quality samples– Sample genotype success rate < 95 to 97.5%– Greater proportion of heterozygous genotypes

than expected

• Related individuals (if independent samples) – Based on pair-wise comparisons of similarity of

genotypes

• Sample switches– Wrong sex

• Regions of homozygosity in cell line

Identify poor quality SNPs and remove

• Expected proportions of genotypes are not consistent with observed allele frequency (Hardy Weinberg Equilibrium (HWE))

• HWE p-value < 10-4 to 10-6

• Look for deviation from expected distribution of p-values under the null

• Genotyping success rate < 95%• Duplicate sample or Non-Mendelian error rate

is elevated• Differential missingness in cases and controls

Programs are available for large scale quality control analysis

• Plink– Duplicate error rates, sample relatedness, HWE,.. – Develop by Shaun Purcell– http://pngu.mgh.harvard.edu/~purcell/plink/

• GAINQC Software used for the quality control analysis of the GAIN project – Duplicate error rates, sample relatedness, HWE,..– Developed by Shyam Gopalakrishnan and Goncalo

Abecasis– Available from [email protected]

Initial analysis is straightforward once have everything in place

• Case/control association– Use test that is not affected by deviations

from HWE• Cochran-Armitage test for trend• Equivalent to score test in logistic regression

• TDT or other family-based test

• Quantitative trait association

Programs are available for large scale case-control or family-based analysis• Plink

– Case/control, tdt, quantitative traits– Develop by Shaun Purcell– http://pngu.mgh.harvard.edu/~purcell/plink/

• Merlin– Quantitative traits in independent samples or families,

ability to impute genotypes for untyped individuals based on genotyped family members

– Developed by Goncalo Abecasis– http://www.sph.umich.edu/csg/abecasis/Merlin/

Are the results believable?

• Are stronger associations correlated with poorer quality control measures?

• Is there a strong deviation from expected distribution of p-values?

• Is there confounding from differences in the genetic origins of case and control samples (population stratification)?– Genomic control– Eigenstrat analysis

Seeing from many different angles is believing (sometimes)

• Plink graphical output

• User added custom tracks in the UCSC browser– http://genome.ucsc.edu/– http://genome.ucsc.edu/goldenPath/help/

hgTracksHelp.html#CustomTracks

• Homemade graphes

FUSION T2D association

Many different ways to display similar data

Zeggini et al. (2007) Science 316: 13361341; Diabetes Genetic Initiative (2007) Science 316:1331-1336; Scott et al., (2007) Science 316:1341-1345

Getting more for your genotyping dollars: Imputation of SNP genotypes

• Impute/predict genotypes for:– Missing data within genotyped markers– Untyped markers

• Uses haplotype structure of existing sample such as HapMap samples to infer data for samples with sparser marker set

Observed genotypes

Observed Genotypes

. . . . A . . . . . . . A . . . . A . . .

. . . . G . . . . . . . C . . . . A . . .

Reference Haplotypes

C G A G A T C T C C T T C T T C T G T G CC G A G A T C T C C C G A C C T C A T G GC C A A G C T C T T T T C T T C T G T G CC G A A G C T C T T T T C T T C T G T G CC G A G A C T C T C C G A C C T T A T G CT G G G A T C T C C C G A C C T C A T G GC G A G A T C T C C C G A C C T T G T G CC G A G A C T C T T T T C T T T T G T A CC G A G A C T C T C C G A C C T C G T G CC G A A G C T C T T T T C T T C T G T G C

Study Sample

HapMap

Gonçalo Abecasis

Identify match among reference

Observed Genotypes

. . . . A . . . . . . . A . . . . A . . .

. . . . G . . . . . . . C . . . . A . . .



Gonçalo Abecasis

Phase chromosomes, impute missing genotypes

Observed Genotypes

c g a g A t c t c c c g A c c t c A t g gc g a a G c t c t t t t C t t t c A t g g



Gonçalo Abecasis

Imputing genotype data allows much more thorough analysis

• Allows testing of untyped variation

• Allows easy combination of data across genotyping platforms

• Provides complete data for analysis with multiple SNPs

Imputed data takes care to generate, analyze and understand

• Requires large scale computing resources• Need to assess quality of imputation

– Compare imputed gentoypes to actual genotypes

• Error rates are higher than for genotyped SNPs• Works less well for rarer alleles• Best to take account of uncertainty imputed

SNPs in analysis– Need ways to take into account fractional genotype

counts

Imputation programs are available

• IMPUTE– Developed by Jonathan Marchini– Nature Genetics, Advance online publication– http://www.stats.ox.ac.uk/~marchini/#software

• Mach 1.0, Markov Chain Haplotyping– Developed by Goncalo Abecasis– http://www.sph.umich.edu/csg/abecasis/MACH/

Need to store results and prepare for large scale specialized analysis

• System to store, view, merge results– SQL database– Plink

• Testing speed of specialized analyses in different statistical packages

• Potential development of software to run large scale specialized analysis

Summary: Ideally what needs to happen before getting the data

• Ability to store, select and write out genotype data in multiple formats for quality control and association analysis

• Identification of primary quality control and analysis programs

• Systems to store, view, merge results• Adequate computing resources to do

intensive computing • Testing of standard and specialized

processes with large-scale trial datasets

FUSION study

U Michigan

Michael BoehnkeKaren ConneelyCharles Ding William DurenTerry GliedtLarry HuAnne JacksonXiao-Yi LiAndrew SkolHeather StringhamPeggy WhiteCristen WillerFang XiangRui Xiao

National Public Health Institute Helsinki

Jaakko TuomilehtoTimo Valle

USCRichard BergmanThomas BuchananRichard Watanabe

Randall PruimCalvin College

Kimberly DohenyElizabeth Pugh

CIDR

Francis CollinsLori BonnycastlePeter ChinesMichael ErdosNarisu NarisuL. Prokunina-OlssonNancy RiebowAndrew SprauAmy SwiftMaurine Tong

NHGRI / NIH

Karen MohlkeKyle GaultonJason Luo Li Qin

UNC-Chapel Hill

U Michigan

Gonçalo AbecasisYun LiJun DingPaul Scheet

handling and analyzing data from a genome-wide association study

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