QTL Studies- Past, Present and Future. David Evans

Transcription

1 QTL Studies Past, Present and Future David Evans

2 Genetic studies of complex diseases have not met anticipated success Glazier et al, Science (2002) 298:

3 Korstanje & Pagan (2002) Nat Genet Korstanje & Pagan (2002) Nat Genet

4 Reasons for Failure? BUT Not much success in mapping complex diseases / traits! Individual environment Linkage Marker Linkage Gene 1 disequilibrium Linkage Association Complex Phenotype Mode of inheritance Gene 2 Gene 3 Common environment Polygenic background Weiss & Terwilliger (2000) Nat Genet

5 LD Patterns and Allelic Association Type 1 diabetes and Insulin VNTR Alzheimers and ApoE4 Bennett & Todd, Ann Rev Genet, 1996 Pattern of LD unpredictable Extent of common genetic variation unknown Roses, Nature 2000

6 Genomewide Association? Risch & Merikangas, Science 1996

7 Multiple Rare Variant Hypothesis? GWA assumes that common variants underlie common diseases

8 Enabling Genomewide Association Studies HAPlotype MAP High throughput genotyping Large cohorts ALSPAC

9 Wellcome Trust Case Control Consortium

10 Wellcome Trust CaseControl Consortium GenomeWide Association Across Major Human Diseases DESIGN Collaboration amongst 26 UK disease investigators 2000 cases each from 9 diseases 1000 cases from 4 diseases GENOTYPING Affymetrix 500k SNPs Illumina Human NS_12 SNP chip CASES 1. Type 1 Diabetes 2. Type 2 Diabetes 3. Crohn s Disease 4. Coronary Heart Disease 5. Hypertension 6. Bipolar Disorder 7. Rheumatoid Arthritis 8. Malaria 9. Tuberculosis 10. Ankylosing Spondylitis 11. Grave s Disease 12.Breast Cancer 13.Multiple Sclerosis CONTROLS 1. UK Controls A (1, BC) 2. UK Controls B (1,500 NBS) 3. Gambian controls (2000)

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12 Ankylosing Spondylitis Autoimmune arthritis resulting in fusion of vertebrae Prevalence of 0.4% in Caucasians. More common in men. Often associated with psoriasis, IBD and uveitis Ed Sullivan, Mike Atherton

13 Ankylosing Spondylitis GWAS WTCCC (2007) Nat Genet IL23R ARTS1

14 Successes MSMB JAZF1 CTBP2 HNF1B 10q21 PTPN2 FAM92B NKX23 IFIH1 CTLA4 KCNJ11 ERBB3 TCF2 WFS1 TCF7L2 Common genes = common etiology? Some in gene deserts Large relative risks does not = success 11q13 NCF4 CD25 SLC30A8 Drug targets 2q35 LSP1 3des. NUDT11 PHOX2B IRGM PTPN22 KIAAA 035D PPARG HHEX MAPKI3 SLC22A3 BSN IL2RA IGF2BP2 TNRC9 KLK3 ATG16L1 18p11 CDKAL1 2q36 8q24 LMTK2 8q24 5p13 12q24 FTO 6q251 IL21 IL23R FGFR2 8q24 SMAD7 NOD2 IL23R INS 9p21 FTO GCKR 9p21 IL2 PTPN22 ARTS GALP LOXL1 ORMDL Breast ca Prostate ca Colorectal ca Crohn s Dis IBD T1D T2D Obesity Triglycerides CAD Coeliac Dis Rheumatoid A Ankylosing S Alzheimer Dis Glaucoma Asthma

15 What About Quantitative Traits? 1 Gene 3 Genotypes 3 Phenotypes 2 Genes 9 Genotypes 5 Phenotypes 3 Genes 27 Genotypes 7 Phenotypes 4 Genes 81 Genotypes 9 Phenotypes Quantitative genetics theory suggests that quantitative traits are the result of many variants of small effect Unselected samples Central Limit Theorem Normal Distribution The corollary is that very large sample sizes will be needed to detect these variants in UNSELECTED samples

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17 FTSO WTCCC T2D Scan FTO FTO produces a moderate signal in WTCCC T2D scan But, no signal in an American T2D scan? American cases and controls matched on BMI

18 Replication is critical!!! FTO

19 Height The Archetypal Polygenic Dizygotic Twins Trait Monozygotic Twins Twin, family and adoption studies suggest that, within a population, 90% of variation in height is due to genetic variation Twins separated at birth Borjeson, Acta Paed, 1976

20 GWA of Height A 1914 Cases (WTCCC T2D) B 4892 Cases (DGI) C 6788 Cases (WTCCC HT) D 8668 Cases (WTCCC CAD) E Cases (EPIC) F Cases (WTCCC UKBS) Weedon et al. (in press) Nat Genet Large numbers are needed to detect QTLs!!! Significant results Other loci? Collaboration is the name of the game!!!

21 A: 1,900 B: 5,000 C: 7,200 D: 9,100 E: 12,600 F: 14,000 Some real hits sit in the bottom of the distribution Weedon et al. (in press) Nat Genet Some hits initially look interesting but then go away

22 Hedgehog signaling, cell cycle, and extracellular matrix genes overrepresented Candidate gene Monogenic Knockout mouse Details* ZBTB38 Transcription factor. CDK6 Yes Involved in the control of the cell cycle. HMGA2 Yes Yes Chromatin architectural factors GDF5 Yes Yes Involved in bone formation LCORL May act as transcription activator LOC Not known EFEMP1 Yes Extracellular matrix C6orf106 Not known PTCH1 Yes Yes Hedgehog signalling SPAG17 Not known SOCS2 Yes Regulates cytokine signal transduction HHIP Hedgehog signaling ZNF678 Transcription factor DLEU7 Not known SCMH1 Yes Polycomb protein ADAMTSL3 Extracellular matrix IHH Yes Yes Hedgehog signaling ANAPC13 Cell cycle ACAN Yes Yes Extracellular matrix DYM Yes Not known Weedon et al. (in press) Nat Genet

23 The combined impact of the 20 SNPS with a P < 5 x 10 7 The 20 SNPs explain only ~3% of the variation of height Lots more genes to find but extremely large numbers needed Weedon et al. (in press) Nat Genet

24 Height Linkage Regions Perola et al, Plos Genetics, 2007; data available at Weedon et al.; unpublished data

25 Perola et al, Plos Genetics, 2007; data available at Weedon et al.; unpublished data

26 What s Going On? Loci identified by GWAs don t have linkage peaks over them Linkage analysis lacks power? Areas identified by linkage don t have significant assocation hits over them Type I error? Power? BUT what if linkage analysis and association analysis identify different types of loci?

27 What next? Genomewide Sequencing Functional Studies Animal models Other ethnic groups Epigenetics Fine mapping Initial Genome Wide Scans More genes Transcriptomics Mendelian Randomisation Genomic Profiling CNVs

28 Distribution of MAFs in HapMap Genomewide panels and HapMap biased towards common variants Common variants don t tag rare variants well

29 Complex Disease Tree??????????????? High hanging fruit??? ARTS1 TCF7L2 FTO IL23R PPARG Low hanging fruit WFS1 TCF2 HLAB27 SEMA5A Rotten Fruit

30 Methods of gene hunting rare, monogenic (linkage) Effect Size? common, complex (association) Frequency

31 Genomewide Sequencing Sequence individuals genomes Will identify rare variants But will we have enough power? Evans et al. (2008) EJHG

32 Genomic Profiling The idea of using genetic information to inform diagnosis Predictive testing in the case of monogenic diseases has been used for years (1300+ tests available) (e.g. Phenylketonuria) Not possible in complex diseases as effects of an individual variant is so small BUT if we consider several predisposing genetic and environmental factors, can we predict disease?

33 Genomic Profiling (from Janssens et al AJHG) => Give up and go home? (from Yang et al AJHG)

34 Ankylosing Spondylitis POSTERIOR PROBABILITY OF DISEASE D+/B27+ D+/B27 D+/B27+,ARTS1+,IL23+ D+/B27/ARTS1/IL23R PRIOR DISEASE PROBABILITY (Brown & Evans, in prep) Prevalence of B27+, ARTS1+,IL23R+ is 2.4% Prevalence of B27, ARTS1, IL23R is 19%

35 Using Genetics to Inform Classical Epidemiology

36 Observational Studies Fanciful claims often made from observational studies In a casecontrol study, a group of diseased individuals are recruited (Cases); A group of individuals without disease are gathered (Controls); Both groups are then measured retrospectively on an exposure of interest; A test of association is performed Example: Obesity (Exposure) and Coronary Heart Disease (Outcome) CHD Control Obesity Yes No Odds of obesity in cases: 200/100 = Odds of obesity in controls: 50/250 = 0.2 Odds Ratio: 2/0.2 = 10

37 Confounding Classic limitations to observational science Reverse Causation Bias

38 Randomized Control Trials Individuals free from exposure and disease Randomization Treatment Group Control Group Measure Outcome Measure Outcome Randomization controls for confounding Reverse causation impossible Gold standard for assessing causality

39 Mendelian Randomization RCTs not always ethical or possible Fortunately nature has provided us with a natural randomized control trial! Mendel s law of independent assortment states that inheritance of a trait is independent (randomized) with respect to other traits Therefore individuals are randomly assigned to three groups based on their genotype (AA, Aa, aa) independent of outcome Assessing the relationship between genotype, environmental risk factor and disease informs us on causality

40 Mendelian Randomization Genetic Variant (FTO) Confounding Variables (smoking, diet etc.) β FTOObesity Modifiable Environmental Exposure (Obesity) β ObesityCHD Disease (Coronary Heart Disease) If obesity causes CHD then the relationship between FTO and CHD should be estimated by the product of β FTOObesity and β ObesityCHD

41 Mendelian Randomization Genetic Variant (FTO) Confounding Variables (smoking, diet etc.) β FTOObesity Modifiable Environmental Exposure (Obesity) β ObesityCHD Disease (Coronary Heart Disease) If CHD causes obesity then β FTOCHD should be zero.

42 Mendelian Randomization Genetic Variant (FTO) Confounding Variables (smoking, diet etc.) β FTOObesity Modifiable Environmental Exposure (Obesity) Disease (Coronary Heart Disease) If the relationship between Obesity and CHD is purely correlational (i.e. due to confounding) then β FTOCHD should be 0

43 Mendelian Randomization Genetic Variant (FTO) Confounding Variables (smoking, diet etc.) Modifiable Environmental Exposure (Obesity) Disease (Coronary Heart Disease) Genotype is associated with the environmental exposure of interest Genotype is NOT associated with confounders Genotype is only related to its outcome via its association with the modifiable environmental exposure

44 Mendelian Randomization Mendelian Randomization is a way of using a genetic variant(s) to make causal inferences about (modifiable) environmental risk factors for disease and health related outcomes Environmental exposures (e.g. Obesity) can be modified! Genetic factors cannot (at least for the moment ) Still a relatively new approach that has problems (i.e. finding genetic proxies for environmental exposures multiple instruments?) but a LOT of scope for development

45 Could SEM be used to enhance MR? Genetic Variant (FTO) Confounding Variables (smoking, diet etc.) Modifiable Environmental Exposure (Obesity) Disease (Coronary Heart Disease) Genetic Variant (MC4R) Genetic Variant (?)