INTRODUCTION TO GENETIC EPIDEMIOLOGY (Antwerp Series) Prof. Dr. Dr. K. Van Steen

Transcription

1 INTRODUCTION TO GENETIC EPIDEMIOLOGY (Antwerp Series) Prof. Dr. Dr. K. Van Steen

2 POPULATION BASED GENOME-WIDE ASSOCIATION STUDIES 1 Setting the pace 1.a A hype about GWA studies 1.b Genetic terminology revisited 1.c Genetic association studies 2 Study Designs 2.a Marker level 2.b Subject level 2.c Gender level

3 3 Preliminary analyses 3.a Quality Control 3.b Population genetics 3.c Linkage disequilibrium, haplotypes and SNP tagging 3.d Confounding: population stratification 4 Tests of association 4.a Single SNP 4.b Repeated single SNP tests: Multiple testing correction 4.c Replication 5 Interpretation and follow-up

4 1 Setting the pace 1.a A hype about GWA studies May he live in interesting times. Like it or not we live in interesting times. Robert Kennedy, June 7, 1966

5 How much (sequence) data are available? The complete genome sequence of humans and of many other species provides a new starting point for understanding our basic genetic makeup and how variations in our genetic instructions result in disease.

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13 The pace of the molecular dissection of human disease can be measured by looking at the catalog of human genes and genetic disorders identified so far in OMIM, which is updated daily ( (V. A. McKusick, Mendelian Inheritance in Man (Johns Hopkins Univ. Press, Baltimore, ed. 12, 1998))

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15 What is OMIM? Online Mendelian Inheritance in Man (OMIM ) is a continuously updated catalog of human genes and genetic disorders and traits, with particular focus on the molecular relationship between genetic variation and phenotypic expression. It is thus considered to be a phenotypic companion to the Human Genome Project. OMIM is a continuation of Dr. Victor A. McKusick's Mendelian Inheritance in Man, which was published through 12 editions, the last in OMIM is currently biocurated at the McKusick-Nathans Institute of Genetic Medicine, The Johns Hopkins University School of Medicine. Frequently asked questions:

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20 1.b Genetic terminology revisited What is genetic epidemiology? (Ziegler and Van Steen, Brazil 2010)

21 Where is the genetic information located? (Ziegler and Van Steen, Brazil 2010)

22 Where is the genetic information located? (Ziegler and Van Steen, Brazil 2010)

23 Where is the genetic information located? (Ziegler and Van Steen, Brazil 2010)

24 What is recombination? (Ziegler and Van Steen, Brazil 2010)

25 How much do individuals differ with respect to genetic information? (Ziegler and Van Steen, Brazil 2010)

26 How much do individuals differ with respect to genetic information? Genotype: The two alleles inherited at a specific locus. If the alleles are the same, the genotype is homozygous, if different, heterozygous. In genetic association studies, genotypes can be used for analysis as well as alleles or haplotypes. Haplotype: Linear arrangements of alleles on the same chromosome that have been inherited as a unit. A person has two haplotypes for any such series of loci, one inherited maternally and the other paternally. A haplotype may be characterized by a single allele unless a discrete chromosomal segment flanked by two alleles is meant.

27 Are haplotypes always better in association studies for disease? Analyses based on phased haplotype data rather than unphased genotypes may be quite powerful Test 1 vs. 2 for M1: Test 1 vs. 2 for M2: Test haplotype H1 vs. all others: D + d vs. d D + d vs. d D vs. d If the Disease Susceptibility Locus (DSL) is located at a marker, haplotype testing can be less powerful

28 How can individual differences be detected? (Ziegler and Van Steen, Brazil 2010)

29 What are microsatellite markers? (Ziegler and Van Steen, Brazil 2010)

30 What are single nucleotide polymorphisms? (Ziegler and Van Steen, Brazil 2010)

31 Why are SNPs preferred over STRs? (Ziegler and Van Steen, Brazil 2010)

32 Which genotyping methods are currently being used? (Ziegler and Van Steen, Brazil 2010)

33 Which genotyping methods are currently being used? (Ziegler and Van Steen, Brazil 2010)

34 1.c Genetic association studies What is a genome-wide association study? It refers to a method / methodology for interrogating all 10 million variable points across the human genome. Since variation is inherited in groups, or blocks, not all 10 million points have to be tested. Blocks are shorter though (so need for testing more points) the less closely people are related.

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37 What is a genome-wide association study? Hence, a genome-wide association study is an approach that involves rapidly scanning markers across the complete sets of DNA, or genomes, of many people to find genetic variations associated with a particular disease. Once new genetic associations are identified, researchers can use the information to develop better strategies to detect, treat and prevent the disease. ( The impact on medical care from genome-wide association studies could potentially be substantial. Such research is laying the groundwork for the era of personalized medicine, in which the current one size-fits-all approach to medical care will give way to more customized strategies.

38 What do we need to carry out a genome-wide association study? The tools include - computerized databases that contain the reference human genome sequence, - a map of human genetic variation and - a set of new technologies that can quickly and accurately analyze whole-genome samples for genetic variations that contribute to the onset of a disease. (

39 What do we need to carry out a genome-wide association study?

40 What do we need to carry out a genome-wide association study? To distinguish between true and chance effects, there are several routes to be taken: - Set tight standards for statistical significance - Only consider patterns of polymorphisms that could plausibly have been generated by causal genetic variants (use understanding of and insights into human genetic history or evolutionary processes such as recombination or mutation) - Adequately deal with distorting factors, including missing data and genotyping errors (quality control measures)

41 What is the flow of a genome-wide association study? The genome-wide association study is typically (but not solely!!!) based on a case control design in which single-nucleotide polymorphisms (SNPs) across the human genome are genotyped... (Panel A: small fragment)

42 What is the flow of a genome-wide association study? Panel B, the strength of association between each SNP and disease is calculated on the basis of the prevalence of each SNP in cases and controls. In this example, SNPs 1 and 2 on chromosome 9 are associated with disease, with P values of and 10 8, respectively (Manolio 2010)

43 What is the flow of a genome-wide association study? The plot in Panel C shows the P values for all genotyped SNPs that have survived a quality-control screen, with each chromosome shown in a different color. The results implicate a locus on chromosome 9, marked by SNPs 1 and 2, which are adjacent to each other (graph at right), and other neighboring SNPs. (Manolio 2010)

44 What is the flow of a genome-wide association study? (Ziegler 2009)

45 2 Study Designs What are the components of a study design for GWA studies? The design of a genetic association study may refer to - study scale: Genome-wide Genomic - marker design: Which markers are most informative? Microsatellites? SNPs? CNVs? Which platform is the most promising? - subject design

46 Does scale matter? candidate gene approach Can t see the forest for the trees vs genome-wide screening approach Can t see the trees for the forest

47 Does scale matter?

48 Which genetic markers to select? The Common Disease/Common Variant hypothesis (CDCV) Continuous distribution of genetic variants, shaped by mutation and selection (Ziegler and Van Steen, 2010)

49 Dichotomous Traits Quantitative Traits Observations: The higher the MAF (minor allele frequency), the higher the detection rate? The higher the MAF, the lower the penetrance?

50 Types of genetic diseases Monogenic diseases are those in which defects in a single gene produce disease. Often these disease are severe and appear early in life, e.g., cystic fibrosis. For the population as a whole, they are relatively rare. In a sense, these are pure genetic diseases: They do not require any environmental factors to elicit them. Although nutrition is not involved in the causation of monogenic diseases, these diseases can have implications for nutrition. They reveal the effects of particular proteins or enzymes that also are influenced by nutritional factors (

51 Oligogenic diseases are conditions produced by the combination of two, three, or four defective genes. Often a defect in one gene is not enough to elicit a full-blown disease; but when it occurs in the presence of other moderate defects, a disease becomes clinically manifest. It is the expectation of human geneticists that many chronic diseases can be explained by the combination of defects in a few (major) genes. A third category of genetic disorder is polygenic disease. According to the polygenic hypothesis, many mild defects in genes conspire to produce some chronic diseases. To date the full genetic basis of polygenic diseases has not been worked out; multiple interacting defects are highly complex!!! (

52 Complex diseases refer to conditions caused by many contributing factors. Such a disease is also called a multifactorial disease. - Some disorders, such as sickle cell anemia and cystic fibrosis, are caused by mutations in a single gene. - Common medical problems such as heart disease, diabetes, and obesity likely associated with the effects of multiple genes in combination with lifestyle and environmental factors, all of them possibly interacting. Challenge for many years to come

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54 Which genetic markers to select? Linkage exists over a very broad region, entire chromosome can be done using data on only DNA markers Broad linkage regions imply studies must be followed up with more DNA markers in the region (Figure: courtesy of Ed Silverman) Must have family data with more than one affected subject E.g., microsatellites

55 Which genetic markers to select? Association exists over a narrow region; markers must be close to disease gene - The basic concept is linkage disequilibrium (LD) see later in this chapter Initially used for candidate genes or in linked regions Can use population-based (unrelated cases) or familybased design E.g., SNPs

56 Which DNA SNPs to select? Costs may play a role, but a balance is needed between costs and chip performance as well as coverage (e.g., exonic regions only?) Some of the fundamental principles of array technology (see future class)

57 Which DNA SNPs to select? (adapted from Manolio 2010)

58 How can technology bias be avoided? (Ziegler and Van Steen, Brazil 2010)

59 How can technology bias be avoided? (Ziegler and Van Steen, Brazil 2010)

60 How can technology bias be avoided? (Ziegler and Van Steen, Brazil 2010)

61 Next generation sequencing will overtake array technology? The competing hypothesis to the CDCV hypothesis is the Common Disease/Rare Variant (CDRV) hypothesis. It argues that multiple rare DNA sequence variations, each with relatively high penetrance, are the major contributors to genetic susceptibility to common diseases. Although some common variants that underlie complex diseases have been identified, and given the recent huge financial and scientific investment in GWA studies, there is no longer a great deal of evidence in support of the CDCV hypothesis and much of it is equivocal... Hence, nowadays, both CDCV and CDRV hypotheses have their place in current research efforts.

62 Next generation sequencing will overtake array technology?

63 Next generation sequencing will overtake array technology?

64 Crucial question: How to best capture disease predisposition? (Gut 2012)

65 Which study subjects to select? (Cordell and Clayton 2005)

66 Which study subjects to select? (Ziegler and Van Steen, 2010)

67 Which study subjects to select? (Ziegler and Van Steen, 2010)

68 Which study subjects to select? (Ziegler and Van Steen, 2010)

69 Which study subjects to select? (Ziegler and Van Steen, 2010)

70 Which study subjects to select?

71 Which study subjects to select? Rare versus common diseases (Lange and Laird 2006)

72 3 Preliminary analyses Is there a standard file format for GWA studies?

73 Is there a standard file format for GWA studies?

74 3.a Quality control Why is quality control important? BEFORE (false positives!!!!): (Ziegler and Van Steen 2010)

75 Why is quality control important? AFTER: (Ziegler and Van Steen 2010)

76 What is the standard quality control? Quality control on different levels: o Subject or sample level o SNP level o X-chromosomal SNP level

77 What are standard filters on the sample level? (Ziegler and Van Steen 2010)

78 What are standard filters on the SNP level? (Ziegler and Van Steen 2010)

79 Reading genotype calling cluster plots

80 What are standard filters on the gender level? (Ziegler and Van Steen 2010)

81 When genotypes are missing: Is there a power advantage in imputing?

82 Is there a power advantage in imputing?

83 Is there a power advantage in imputing?

84 Is there a power advantage in imputing? (Spencer et al 2009)

85 What are the Travemünde criteria? (Ziegler 2009)

86 What are the Travemünde criteria? (Ziegler 2009)

87 3.b Population genetics What does population genetics mean? The concept of frequency lies at the heart of population genetics:

88 What does population genetics mean? A gene by itself is a constant entity (but perhaps harbors mutation) Alternative forms of a gene (allele) can exist at certain frequencies in a population These frequencies can change (via genetic drift, selection, mutation) Resulting in adaptation and evolution There are two major facets: - Describing the pattern of genetic diversity - Investigating the processes that generate this diversity

89 What is it useful for? Breeding - Plant and animal breeding - Pesticide and herbicide resistance - Effects of release of GM organisms People - Forensic analysis (DNA fingerprinting) - Identification of genes for complex human traits - Genetic counseling - Study of human evolution and human origin Ecology and evolution - Inferring evolutionary processes - Preservation of endangered species

90 Theoretical population genetics General theoretical models predict evolution of gene frequencies and other things Highly dependent upon assumptions May or may not be realistic Mathematically satisfying h r ti iri ri t

91 Empirical population genetics Apply statistical models to real data to infer underlying processes Again, adequate sampling is necessary to achieve statistical power Empirical population genetics is often emphasized due to the enormous volumes of genetic data that is out there. h r ti iri ri t

92 Experimental population genetics Test hypotheses in population genetics using controlled experiments Design such that alternative outcomes possible, some of which can reject hypothesis Need controls, replicates and adequate sample size Usually restricted to model organisms - Drosophila, Neurospora, some crop plants h r ti iri ri t

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94 How does evolution take place? Charles Darwin ( ) In the 19th century, a man called Charles Darwin, a biologist from England, set off on the ship HMS Beagle to investigate species in exotic places (e.g., Galapagos islands). After spending time on the islands, he soon developed a theory that would contradict the creation of man and imply that all species derived from common ancestors through a process called natural selection. Natural selection is considered to be the biggest factor resulting in the diversity of species and their genomes.

95 Darwin s principles One of the prime motives for all species is to reproduce and survive, passing on the genetic information of the species from generation to generation. When species do this they tend to produce more offspring than the environment can support. The lack of resources to nourish these individuals places pressure on the size of the species population, and the lack of resources means increased competition and as a consequence, some organisms will not survive. The organisms that die as a consequence of this competition were not totally random. Darwin found that those organisms more suited to their environment were more likely to survive. This resulted in the well-known phrase survival of the fittest, where the organisms most suited to their environment had more chance of survival if the species falls upon hard times. (see also (

96 Darwin s tree of life Those organisms who are better suited to their environment exhibit desirable characteristics, which is a consequence of their genome being more suitable to begin with. The Tree of Life image that appeared in Darwin s On the Origin of Species by Natural Selection, It was the book's only illustration

97 A group at the European Molecular Biology Laboratory (EMBL) in Heidelberg has developed a computational method that resolves many of the remaining open questions about evolution and has produced what is likely the most accurate tree of life ever:

98 What are the basic characteristics of evolution? Some terminology: A population refers to a group of interbreeding individuals of the same species sharing a common geographical area A species is a group of populations that have the potential to interbreed in nature and produce viable offspring Gene pool is the sum total of all the alleles within a population Evolution refers to the changes in gene (allele) frequencies in a population over time; Evolution takes place at the population, not species level. So populations, not species evolve

99 There are 4 processes of evolution Natural variation Mutation: refers to changes in nucleotide sequences of DNA. Mutations provide new alleles, and therefore are the ultimate source of variation Recombination: refers to reshuffling of the genetic material during meiosis Other variation types Natural selection Reproductive isolation

100 What is natural selection? Natural selection refers to differential reproduction. Organisms with more advantageous gene combinations secure more resources, allowing them to leave more progeny. It is a negative force, nature selects against, not for. Ultimately natural selection leads to adaptation - the accumulation of structural, physiological or behavioral traits that increase an organism's fitness.

101 There are 3 types of natural selection Stabilizing Directional Disruptive Stabilizing Selection maintains an already well adapted condition by eliminating any marked deviations from it. As long as the environment remains unchanged the fittest organisms will also remain unchanged. - Human birth weight averages about seven pounds. Very light or very heavy babies have lower chances of survival. - Stabilizing selection accounts for "living fossils" - organisms that have remained seemingly unchanged for millions of years.

102 Directional Selection favors one extreme form over others. Eventually it produces a change in the population. Directional selection occurs when an organism must adapt to changing conditions. - Industrial melanism: The peppered moths (Biston betularia) fly by night and rest during the day on lichen covered tree trunks where they are preyed upon by birds. Prior to the industrial revolution most of the moths were light colored and well camouflaged. A few dark (melanistic) were occasionally noted. During the industrial revolution soot began to blacken the trees and also cause the death of the lichens. The light colored moths were no longer camouflaged so their numbers decreased quite rapidly. With the blackening of the trees the numbers of dark moths rapidly increased. The frequency of the dark allele increased from less than 1% to over 98% in just 50 generations. Since the 1950's, attempts to reduce industrial pollution in Britain have resulted in an increase in numbers of light form. - Antibiotic resistance in bacteria several resistant strains exist due to overuse

103 Disruptive Selection occurs when two or more character states are favored. - African butterflies (Pseudacraea eurytus) range from orange to blue. Both the orange and blue forms mimic (look like) other foul tasting species (models) so they are rarely eaten. Natural selection eliminates the intermediate forms because they don't look like the models.

104 What is reproductive isolation? Reproductive isolation is the inability of a species to breed successfully with related species due to geographical, behavioral, physiological, or genetic barriers or differences Reproductive isolating mechanisms may restrict the gene flow: - Ecological isolation: The geographic ranges of two species overlap, but their ecological needs or breeding requirements differ enough to cause reproductive isolation - Temporal isolation: when two species whose ranges overlap have different periods of sexual activity or breeding seasons - Behavioral isolation: Species with complex courtship rituals (breeding calls, mating dances, etc.) usually exhibit a stereotyped "give-and-take" between male and female before actual mating takes place. - Mechanical isolation: Morphological differences prevent mating (examples: see

105 What is Hardy-Weinberg Equilibrium (HWE)? The concept HWE arises from the following problem: How do allele frequencies relate to genotype frequencies in a population? If we have genotype frequencies, we can easily compute allele frequencies: -

106 Example on cystic fibrosis

107 Example on cystic fibrosis (continued)

108 From allele frequencies to genotype frequencies Discrete generations No migration or selection No mutation No population stratification Equal initial genotype frequencies in the two sexes Infinite population size Random mating

109 From allele frequencies to genotype frequencies Early in the 20th century biologists believed that natural selection would eventually result in the dominant alleles driving out or eliminating the recessives. Therefore, over a period of time genetic variation would eventually be eliminated in a population At some point the geneticist Punnett was asked to explain the prevalence of blue eyes in humans despite the fact that it is recessive to brown. He couldn't do it so he asked a mathematician colleague named Hardy to explain it. A physician named Weinberg came up with a similar explanation, describing the genetics of non-evolving populations The Hardy-Weinberg law was born: The frequencies of alleles in a population will remain constant unless acted upon by outside agents or forces. A non-evolving population is said to be in Hardy-Weinberg equilibrium

110 What does E stand for in HWE? Equilibrium

111 What does E stand for in HWE?

112 What does E stand for in HWE?

113 What does E stand for in HWE?

114 Computations under HWE are simple Punnett s square

115 Can HWE be visualized? A useful way to look at frequencies in HWE is the following

116 What are signs of deviations from HWE? Decreased or increased HET

117 What are signs of deviations from HWE? Inbreeding

118 Inbreeding coefficient F for an individual X The probability of 2 alleles at a randomly chosen locus being IBD can be computed via the probabilities Hence, F X =4 1/16=1/4

119 Inbreeding coefficient F for an individual X The shortcut loop method determines for a loop (path through common ancestor) a contribution of (½) n to F, where n is the number of individuals in the loop, X excluded Hence, for more than one loop, determine each time (½) n, and sum the contributions to F In the example, the loops are: - ADXC: (½) 3 - BDXC: (½) 3 leading to F X = (½) 3 + (½) 3 = ¼

120 Change in genotype frequencies in response to inbreeding Inbreeding increases expression of recessive alleles: - Genotype frequencies for non-inbred: p 2, 2pq, q 2 - Genotype frequencies for inbred: p 2 +Fpq, 2pq-2Fpq, q 2 +Fpq For instance, if q=0.02, then

121 Change in genotype frequencies in response to inbreeding (continued) For instance, if p=q=0.5 Observe that the allele frequencies f(a)=q and f(a)=p do not change: - f(a) = (q 2 + pqf) + ½ (2pq-2pqF) = q 2 +pq=q(q+p)=q - f(a) = (p 2 + pqf) + ½ (2pq-2pqF) = p 2 +pq=p(p+q)=p

122 Are there other genetic similarity coefficients? F IT is the inbreeding coefficient of an individual (I) relative to the total (T) population; F IS is the inbreeding coefficient of an individual (I) relative to the subpopulation (S) and F ST is the effect of subpopulations (S) compared to the total population (T)

123 Population subdivision

124 Wright s F statistics

125 What are generalized Hardy-Weinberg deviations?

126 Generalized Hardy-Weinberg deviations - example

127 Generalized Hardy-Weinberg deviations - example

128 Generalized Hardy-Weinberg deviations example

129 How can F ST for subpopulations be estimated?

130 How can F ST for subpopulations be estimated?

131 How can F ST for subpopulations be estimated?

132 How can F ST for subpopulations be estimated?

133 How can F ST for subpopulations be estimated?

134 How can HWE be tested?

135 Chi-square goodness-of-fit test

136 Chi-square goodness-of-fit test

137 HWE exact test

138 HWE exact test

139 HWE exact test

140 How do both HWE test compare? Type I error

141 How do both HWE test compare? Type I error

142 How do both HWE test compare?

143 How to interpret HWE test results? A useful tool for interpreting the results of HWE and other tests on many SNPs is the log quantile quantile (QQ) p-value plot: - the negative logarithm of the i-th smallest p-value is plotted against log (i / (L + 1)), where L is the number of SNPs. The 0.3 (or 30%) quantile is the point at which 30% percent of the data fall below and 70% fall above that value. A 45-degree reference line is also plotted as visualization tool: - If the two sets come from a population with the same distribution, the points should fall approximately along this reference line. - The greater the departure from this reference line, the greater the evidence for the conclusion that the two data sets have come from populations with different distributions.

144 How to interpret HWE test results? (Balding 2006)

145 Genetic Association Studies 3.c Linkage disequilibrium, haplotypes and SNP tagging Distances among cities Boston New York Providence Philadelphia Baltimore Providence 59 New York Philadelphia Baltimore Washington

146 Distances among cities

147 Distances among SNPs If a causal polymorphism is not genotyped, we can still hope to detect its effects through Linkage Disequilibrium (LD) with polymorphisms that are typed (key principle behind doing genetic association analysis ). LD is a measure of co-segregation of alleles in a population: Two alleles at different loci that occur together on the same chromosome (or gamete) more often than would be predicted by random chance. In general, LD is taken to be a measure of allelic association. Among the measures that have been proposed for two-locus haplotype data, the two most important are D (Lewontin s D prime) and r 2 (the square correlation coefficient between the two loci under study).

148 Distances among SNPs (Christensen and Murray 2007)

149 Distances among SNPs (Samani et al 2007)) These plots can be generated using the free software Haploview, but also in R! We talk about LD blocks 1 and 2

150 What is linkage disequilibrium (LD)? - more formal definition

151 What does D represent as a measure of LD?

152 What does D represent as a measure of LD? Can similarly show that D Ab = -D AB and D ab = D AB LD is a property of two loci, not their alleles The magnitude of D does not depend on the choice of alleles, thus the magnitude of the coefficient is important, not the sign The range of values the linkage disequilibrium coefficient can take on, varies with allele frequencies

153 What does D represent as a measure of LD?

154 How can D be estimated?

155 How can D be estimated?

156 How to perform a test for LD with D?

157 What does D (Lewontin s D prime ) represent as a measure of LD?

158 How far does linkage disequilibrium extend? (Hecker et al 2003)

159 What does r 2 represent as a measure of LD?

160 How to perform a test for LD with r 2? r 2

161 How to choose between measures of LD? Sample size must be increased by a factor of 1/r 2 to detect an unmeasured variant, compared with the sample size for testing the variant itself. Hence, r 2 has a very nice and useful interpretation! (Jorgenson and Witte 2006) If two loci both have very rare alleles but the loci are not in high LD, it is possible for D to be 1 and r 2 to be small. D is problematic to interpret with rarer alleles.

162 Why does LD occur? Genetic drift: random events due to small/finite population size. E.g., consider a population of 1 million almond trees with a frequency of an allele r at 10%. If a severe ice storm wiped out half, leaving 500,000, it is very likely that the r allele would still be present in the population. However, suppose the initial population size of almond trees were 10 (with the same frequency of r at 10%). It is likely that the same ice storm could wipe the r allele out of the small population. Scenario 1: Intense natural selection or a disaster can cause a population bottleneck, a severe reduction in population size which reduces the diversity of a population. The survivors have very little genetic variability and little chance to adapt if the environment changes. E.g., by the 1890's the population of northern elephant seals was reduced to only 20

163 individuals by hunters. Even though the population has increased to over 30,000 there is no genetic variation in the 24 alleles sampled. A single allele has been fixed by genetic drift and the bottleneck effect. In contrast southern elephant seals have wide genetic variation since their numbers have never reduced by such hunting. Scenario 2: Bottleneck effect, combined with inbreeding, is an especially serious problem for many endangered species because great reductions in their numbers have reduced their genetic variability. This makes them especially vulnerable to changes in their environments and/or diseases. The Cheetah is a prime example.

164 Scenario 3: Sometimes a population bottleneck or migration event can cause a founder effect. A founder effect occurs when a few individuals unrepresentative of the gene pool start a new population. E.g., a recessive allele in homozygous condition causes Dwarfism. In Switzerland the condition occurs in 1 out of 1,000 individuals. Amongst the 12,000 Amish now living in Pennsylvania the condition occurs in 1 out of 14 individuals. All the Amish are descendants of 30 people who migrated from Switzerland in The 30 founder individuals carried a higher than normal percentage of genes for dwarfism. Mutation - if a new mutation occurs in a population then alleles at loci linked with the mutant locus will maintain linkage disequilibrium for many generations. LD lasts longer when linkage is greater (that is, the recombination fraction is much smaller than ½ - very close to 0).

165 Why does LD occur? (continued)

166 How to interpret LD data? The patterns of LD observed in natural populations are the result of a complex interplay between genetic factors and the population's demographic history (Pritchard, 2001). LD is usually a function of distance between the two loci. This is mainly because recombination acts to break down LD in successive generations (Hill, 1966). When a mutation first occurs it is in complete LD with the nearest marker (D' = 1.0). Given enough time and as a function of the distance between the mutation and the marker, LD tends to decay and in complete equilibrium reached D' = 0 value. Thus, it decreases at every generation of random mating unless some process is opposing to the approach to linkage 'equilibrium'.

167 How to interpret LD data? Therefore, the key concept in a (population-based) genetic association study is linkage disequilibrium.

168 How to interpret LD data? It gives the rational for performing genetic association studies Phenotype: The visible or measurable (expressed) characteristic of an organism Trait: Coded phenotype

169 How can one tag SNP serve as proxy for many? (adapted from Manolio 2010)

170 How can one tag SNP serve as proxy for many? (adapted from Manolio 2010)

171 Where is the true causal variant? (Duerr et al 2006)

172 3.d Confounding What is spurious association? Spurious association refers to false positive association results due to not having accounted for population substructure as a confounding factor in the analysis

173 What is spurious association? Typically, there are two characteristics present: - A difference in proportion of individual from two (or more) subpopulation in case and controls - Subpopulations have different allele frequencies at the locus.

174 What are typical methods to deal with population stratification? Methods to deal with spurious associations generated by population structure generally require a number (at least >100) of widely spaced null SNPs that have been genotyped in cases and controls in addition to the candidate SNPs. These methods large group into: o Genomic control methods o Structured association methdos o Principal component-based methods

175 What is genomic control? In Genomic Control (GC), a 1-df association test statistic is computed at each of the null SNPs, and a parameter λ is calculated as the empirical median divided by its expectation under the chi-squared 1-df distribution. Then the association test is applied at the candidate SNPs, and if λ > 1 the test statistics are divided by λ.

176 What is genomic control? The motivation for GC is that, as we expect few if any of the null SNPs to be associated with the phenotype, a value of λ > 1 is likely to be due to the effect of population stratification, and dividing by λ cancels this effect for the candidate SNPs. GC performs well under many scenarios, but can be conservative in extreme settings (and anti-conservative if insufficient null SNPs are used). There is an analogous procedure for a general (2 df) test; The method can also be applied to other testing approaches.

177 What is a structured association method? Structured association (SA) approaches are based on the idea of attributing the genomes of study individuals to hypothetical subpopulations, and testing for association that is conditional on this subpopulation allocation. Several clustering algorithms exist to estimate the number of subpopulations. These approaches (such as Bayesian clustering approaches) are computationally demanding, and because the notion of subpopulation is a theoretical construct that only imperfectly reflects reality, the question of the correct number of subpopulations can never be fully resolved.

178 What is principal components analysis? When many null markers are available, principal components analysis provides a fast and effective way to diagnose population structure. Principal components are linear combinations of the original variables (here SNPs) that optimized in such a way that as much of the variation in the data is retained.

179 In European data, the first 2 principal components nicely reflect the N-S and E-W axes! Y-axis: PC2 (6% of variance); X-axis: PC1 (26% of variance)

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181 Does the same hold on a global (world) level? (Paschau 2007)

182 4 Tests of association What is the causal model underlying genetic association? (Ziegler and Van Steen 2010)

183 4.a Single SNP What are common association tests (dichotomous traits)? (Ziegler and Van Steen 2010)

184 What are common association tests (dichotomous traits)? (Ziegler and Van Steen 2010)

185 What are common association tests (dichotomous traits)? Penetrances for simple Mendelian inheritance patterns Trait T: coded phenotype Penetrance: P(T Genotype) Complete penetrance: P(T DD) = 1 (simplified definition)

186 What are common association tests (dichotomous traits)?

187 What are common association tests (dichotomous traits)? The Cochran-Armitage trend test measures a linear trend in proportions weighted by general measure of exposure dosage: variable x in regression model =#alleles Max test: computes maximum over standardized tests for different genetic models, providing a global test

188 Which test should be used in applications?

189 How are genetic effects measured? RR being

190 Which odds ratios (measures of effect) can we expect? (A and B) Histograms of susceptibility allele frequency and MAF, respectively, at confirmed susceptibility loci.. (Iles 2008)

191 Which odds ratios (measures of effect) can we expect? (C) Histogram of estimated ORs (estimate of genetic effect size) at confirmed susceptibility loci. (Iles 2008)

192 4.b Repeated single SNP tests The regression framework Regression analysis is used for explaining or modeling the relationship between a single variable Y, called the response, output or dependent variable, and one or more predictor, input, independent or explanatory variables, X 1,, X m. When m=1 it is called simple regression but when m > 1 it is called multiple regression. When there is more than one Y, then it is called multivariate regression The basic syntax for doing regression in R is lm(y~model) to fit linear models and glm() to fit generalized linear models (e.g. logistic regression models in the dichotomous trait setting before). Next slide: syntax!

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202 Can screening for 1000nds of SNPs be performed automatically in R? GenAbel is designed for the efficient storage and handling of GWAS data with fast analysis tools for quality control, association with binary and quantitative traits, as well as tools for visualizing results. pbatr provides a GUI to the powerful PBAT software which performs family and population based family and population based studies. The software has been implemented to take advantage of parallel processing, which vastly reduces the computational time required for GWAS. SNPassoc provides another package for carrying out GWAS analysis. It offers descriptive statistics of the data (including patterns of missing data!) and tests for Hardy-Weinberg equilibrium. Single-point analyses with binary or quantitative traits are implemented via generalized linear models, and multiple SNPs can be analyzed for haplotypic associations or epistasis.

203 Is there one tool that fits it all? NO (

204 Other analytic methods Recursive Partitioning (CART; Breiman 1984, Foulkes 2005) Random Forests (Pavolov 1997) Combinatorial Partitioning (Nelson 2001) Multifactor-Dimensionality Reduction (Ritchie 2001) Permutation-Based Procedures (Trimming/Weighting; Hoh 2000) Multivariate Adaptive Regression Splines (Friedman 1991) Boosting (Schapire 1990) Support Vector Machines (Vapnik 2000) Neural Networks (Friedman & Tukey 1974, Friedman & Stuetzle 1981) Bayesian Pathway Modeling (Conti 2003, Cortessis & Thomas 2004) Clique-Finding (Mushlin 2006)

205 What is a multiple testing correction? Simultaneously test m null hypotheses, one for each SNP j H 0j : no association between SNP j and the trait Every statistical test comes with an inherent false positive, or type I error rate which is equal to the threshold set for statistical significance, generally However, this is just the error rate for one test. When more than one test is run, the overall type I error rate is much greater than 5%.

206 What is a multiple testing correction? Suppose 100 statistical tests are run when (1) there are no real effects and (2) these tests are independent, then the probability that no false positives occur in 100 tests is = So the probability that at least one false positive occurs is =0.994 or 99.4% There is not a single measure to quantify false positives (Hochberg et al 1987). Several multiple testing corrections have been developed and curtailed to a genome-wide association context, when deemed necessary: Bonferroni (highly conservative) [divide each single SNPbased p-value by the nr of tests before comparing to the nominal sign level 0.05] vs permutation-based (highly computational demanding) [keep the LD structure, but swap the trait labels among the subjects]

207 4.c Replication May J. Hirschhorn and D. Altshuler J Clin Endo Metab Am J Hum Genet July Am J Hum Genet July PLoS Biol Sept Nat Genet July 2006

208 What does replication mean? Replicating the genotype-phenotype association is the gold standard for proving an association is genuine Most loci underlying complex diseases will not be of large effect. It is unlikely that a single study will unequivocally establish an association without the need for replication SNPs most likely to replicate: - Showing modest to strong statistical significance - Having common minor allele frequency - Exhibiting modest to strong genetic effect size Note: Multi-stage design analysis results should not be seen as evidence for replication...

209 Guidelines for replication studies Replication studies should be of sufficient size to demonstrate the effect Replication studies should conducted in independent datasets Replication should involve the same phenotype Replication should be conducted in a similar population The same SNP should be tested The replicated signal should be in the same direction Joint analysis should lead to a lower p-value than the original report Well-designed negative studies are valuable check the NHGRI Catalog of GWA studies

210 What does validation mean? (Igl et al. 2009)

211 5 Interpretation: functional genomics Are there criteria for assessing the functional significance of a variant? (Rebbeck et al 2004) Criterion Nucleotide Sequence Evolutionary Conservation Population Genetics Experimental Exposures Strong Support V ri t disru ts fu ti k w tif Str g s rv ti r ss s i s, u tig f i y Str g d vi ti s fr t d fr qu i s C sist t vid i hu t rg t tissu V ri t ff ts r v t t b is i t rg t tissu Moderate Support S iss s h g, disru ts ut tiv fu ti tif s rv ti r ss s i s r u tig f i y Some deviations from expected frequencies Neutral Information - Evidence Against N -fu ti h g N t k w N s rv ti N t k w S vid N d t v i b Variant affects metabolism N d t v i b N d vi ti s fr t d fr qu i s N fu ti ff t Variant does not affect metabolism Epidemiology C sist t r d r r du ib rts R r rts with ut i ti N d t v i b N ss i ti

212 ( 2016)

213 (ENCODE: finding all functional elements in the human genome)

214 The more we find, the more we see, the more we come to learn. The more that we explore, the more we shall return. Sir Tim Rice, Aida, 2000

215 A selection of consulted references: Balding D A tutorial on statistical methods for population association studies. Nature Reviews Genetics, 7, Kruglyak L The road to genomewide association studies. Nature Reviews Genetics 9: 314- Li Three lectures on case-control genetic association analysis. Briefings in bioinformatics 9: Peltonen L and McKusick VA Dissecting human disease in the postgenomic era. Science 291, Rebbeck et al Assessing the function of genetic variants in candidate gene association studies 5: 589- Robinson Common Disease, Multiple Rare (and Distant) Variants. PLoS Biology 8(1): e URL: courses.washington.edu/b516 Wang et al Genome-wide association studies: theoretical and practical concerns. Nature Reviews Genetics 6: 109- Ziegler A and Van Steen K 2010: IBS short course on Genome-Wide Association Studies