Imaging Genetics - Reading PDF

Imaging Genetics - Reading 25 November 2023 18:44 Source Notes A neurogenetics approach to understanding individual differences in brain, behavior, and risk for psychopathology Introduction Individual differences in personality, mood, cognition, and environmental experience play a significant role i...

Imaging Genetics - Reading 25 November 2023 18:44 Source Notes A neurogenetics approach to understanding individual differences in brain, behavior, and risk for psychopathology Introduction Individual differences in personality, mood, cognition, and environmental experience play a significant role in shaping complex human behavior and influencing susceptibility to psychopathology. Integrating neuroscience, psychology, and psychiatry has revealed links between brain structure, connectivity, resting activi ty, task-elicited activation, and peripheral indices of circuit function to individual differences in behavior and psychopathology. Direct manipulation of these circuits can induce behavioral and clinical changes, further supporting the biological basis of complex behavior and psychopathology. A deeper understanding of psychopathology can lead to improved treatment and prevention. (Bogdan et al., 2013) Identifying Sources of Individual Variability in Neural Signalling Pathways ○ A critical step in understanding complex behavior is to identify sources of individual variability in neural signalling pathw ays, such as neurotransmitter systems. ○ Differences in protein availability and function shape variability in emergent neural pathways, highlighting the importance o f understanding the connection between brain chemistry and circuitry. The Role of Molecular Genetics in Neurogenetics ○ Molecular genetics provides tools to tap into variability in brain chemistry through the identification of common DNA sequenc e variations, called polymorphisms. ○ These polymorphisms allow for modeling individual differences in brain chemistry and neural signalling pathways, representing the first step in a cascade leading from genetic differences to neural differences to behavioral differences. Neurogenetics: Linking Genetic Polymorphisms to Behavior and Psychopathology ○ Neurogenetics integrates genetics, neuroscience, psychology, and psychiatry to link genetic polymorphisms to variability in p rotein expression, brain structure, function, connectivity, and behavior. ○ A neurogenetics approach provides a plausible, observable, and testable mechanism through which genes influence behavior. ○ By focusing on dimensional and relatively objective intermediate phenotypes, neurogenetics research avoids the limitations of broad nosological psychiatric definitions. The Role of Environmental Experience in Neurogenetics ○ Epigenetics and gene-by-environment interaction (GxE) research have emphasized the importance of including measures of environmental experience along side genetic polymorphisms. ○ An interdisciplinary neurogenetics approach has led to a better understanding of how genetic differences and environmental ex periences interact to shape human behavior and the underlying molecular mechanisms. Challenges in Neurogenetics Research ○ Neurogenetics research faces several challenges, including small effects of individual variables, lack of detailed mechanisms , and the need to translate findings to the clinic. Utilizing an Integrative Neurogenetics Approach ○ An integrative neurogenetics approach can provide valuable insights into complex human behavior and psychopathology. ○ Techniques and emergent developments can be used to confront the challenges faced by neurogenetics research. The challenge of small effects Neurogenetics research has successfully linked several polymorphisms to differences in brain function, behavior, and psychopathology. One example is the serotonin-transporter-linked polymorphic region (5-HTTLPR) genotype, which has been shown to influence threat-related amygdala reactivity. The catechol-O-methyltransferase gene (COMT) Val158Met polymorphism also affects enzyme function and synaptic catecholamine concentrations, leading to variability in emotion, cognition, and related brain function. Despite such robust findings, individual common polymorphisms typically have only a small effect on brain function and behavi or, presenting a major challenge to the field. This weak penetrance is difficult to detect and can lead to non-replications, especially in small samples. Non-replications may arise from original false-positive associations, a lack of standards in neurogenetics research, or differences in experimental paradigms and analysis s trategies. Neurogenetics research has consistently replicated core genotype-phenotype associations, despite the challenge of small effects. Large-scale studies, multisite data pooling protocols, and data-sharing networks are being developed to better capture small genetic effects. Incorporating environmental and epistatic relationships into neurogenetics research can improve the detection of molecular genetic effects. Constructing biologically informed multilocus profiles can more holistically represent genetically driven variability within a specific neural system. Considering the environment Gene-by-Environment Interaction (GxE) ▪ GxE occurs when the relationship between an environmental experience and a phenotype is contingent on individual differences in genetic make-up. ▪ GxE also occurs when the association between genetic make-up and a phenotype is dependent upon environmental experience. ▪ GxE research emphasizes an interaction between genetic variation and experience. ▪ GxE research holds promise to confront the problem of small effects in neurogenetics research. ▪ GxE research provides face validity as it represents a more plausible model of disease. Example of GxE Study by Caspi et al. (2003) ▪ Caspi et al. demonstrated that the depressogenic effects of stress are contingent upon 5-HTTLPR genotype. ▪ Short allele carriers had a strong and positive relationship between life stress and depression, whereas long allele homozygotes had little or no relationship between stress and depression. ▪ This finding has been well replicated and is supported by meta-analytic data. Incorporating Measures of Environmental Experience into Neurogenetics Research ▪ Including measures of environmental experience in neurogenetics research can improve power to detect effects and clarify mechanisms. ▪ One study showed that genetic variation affecting HPA axis function moderates the association between childhood emotional neglect and threat-related amygdala reactivity. ▪ This study highlights several advantages of incorporating measures of environmental experience into neurogenetics research. Challenges of GxE Research ▪ Traditional GxE psychiatric research suffers from publication bias, low power, and a high false-discovery rate. ▪ Replication of neurogenetics GxE research is critical. ▪ Assessing environmental exposure is fraught with difficulties. ▪ Neurogenetics should carefully select measures of environmental exposure and, when possible, use controlled manipulations of exposure. Considerations for Future GxE Neurogenetics Research ▪ Recent theoretical developments within GxE research suggest that polymorphisms may be more accurately conceptualized as markers of ‘plasticity’ to the environment. ▪ Neurogenetics GxE research should include measures of adversity and enrichment. The importance of epistasis Epistasis in Neurogenetics Research ▪ Epistasis refers to the interaction between two or more polymorphisms such that the observed phenotype differs from what would be expected by either polymorphism alone. ▪ Capturing epistatic (i.e., gene-by-gene, G x G) interactions has the potential to clarify relationships between genetic variation and brain function. Epistasis and Monoamine Transporter Variants ▪ Monoamine transporter variants that result in reduced reuptake can enhance synaptic neurotransmitter concentrations. ▪ A genetic variant of a presynaptic inhibitory auto-receptor that confers a loss of binding can reduce negative feedback. ▪ The combined effects of these variants can result in greater postsynaptic signalling than either variant would confer alone. Epistasis and COMT Val158Met Polymorphism ▪ Several epistatic relationships have been documented with the COMT Val158Met polymorphism in relation to prefrontal function and working memory performance. ▪ In one study, an interaction between the rs951436 polymorphism of the gene encoding regulator of G-protein signalling 4 (RGS4) and the COMT polymorphism was observed. ▪ This study highlights how genetic variation within two genes can influence dopamine function and working memory performance. Challenges and Future Directions ▪ Replication of epistatic relationships is important due to statistical instability resulting from small groupings of genotype combinations. ▪ Large-scale neurogenetics studies are needed to adequately power studies of epistatic interactions. ▪ Additive genetic effects may dominate variability in emergent biological and behavioral phenotypes, even if epistatic interactions occur at the level of individual genes. Biologically informed multilocus profiles ○ The majority of neurogenetics research has examined single polymorphic loci to predict differences in brain, behavior, and ps ychopathology. ○ A single functional polymorphism affects a single protein's function and/or expression within a complex neural system. ○ Biologically informed multilocus profiles represent the cumulative effect of multiple polymorphic loci on a specific signalli ng mechanism. ○ This approach was recently demonstrated in a study showing that five functional dopaminergic polymorphisms predicted 11% of t he variance in reward-related ventral striatal reactivity. ○ The use of profile scores can account for significant proportions of variability, presumably by better characterizing genetic ally driven variability in overall signalling. ○ This approach holds tremendous potential for neurogenetics research and could be applied across neural systems. ○ This approach is limited by our functional understanding of polymorphisms, as the vast majority have unknown functional conse quences or have not been well replicated. Complementing Biologically Informed Multilocus Profiles ▪ Data-driven profiles derived from genome-wide association studies (GWASs) can complement biologically informed profiles. ▪ Hypothesized gene-group analyses (i.e., clustering polymorphisms of unknown biological function within genes of similar function) can also complement biologically informed profiles. ▪ These approaches may inform who is at risk for certain phenotypic expressions, but they will not reveal mechanisms underlying these individual differences without follow-up research. Benefits of Multilocus Approaches ▪ These approaches emphasize the polygenic nature of complex traits. ▪ They highlight the benefit of more completely modeling genetic variation to capture the small effects conferred by single polymorphisms. ▪ Novel statistical approaches such as regression trees, recursive partitioning, and machine learning can identify genetic interactions empirically (without hypotheses). Furthering our understanding of molecular mechanisms Utilizing Neurogenetics to Identify Complex Biological Mechanisms ○ Functional genetic polymorphisms represent individual differences in brain chemistry and associated neural signalling pathway s. ○ Neurogenetics can inform our understanding of the detailed and complex biological mechanisms that give rise to the diversity of human behavior and related risk for psychopathology. Enhancing Neurogenetics Approaches for Mechanistic Insights ○ A more comprehensive neurogenetics approach should incorporate assessments and manipulations of brain chemistry. ○ Assessments: ligand positron emission tomography (PET) ○ Manipulations: pharmacological challenge ○ Collaboration with non-human animal research for more detailed and controlled assessments and manipulations Non-Human Animal Research for Molecular Mechanism Investigation ○ Non-human animal research can target specific molecular mechanisms that would otherwise be inaccessible in humans. ○ Genetic manipulation of specific genes or signalling pathways ○ Direct manipulation of neural circuitry ○ Measurement of brain chemistry and behavior under controlled conditions Epigenetics and Gene-Environment Interactions ○ Epigenetics studies the mechanisms that control gene expression without altering the underlying DNA sequence. ○ Epigenetics offers insights into how experience shapes biology. ○ Neurogenetics research can incorporate epigenetic measures to understand gene -environment interactions. Genome-Wide Association Studies (GWASs) ○ GWASs identify genetic variants associated with complex traits. ○ GWASs can identify novel or understudied proteins critical to neural and behavioral phenotypes of interest. Integration with non-human animal models Non-human animal models provide opportunities to directly manipulate and measure brain chemistry and gene function. ○ Non-human animal models allow for controlled genetic and environmental manipulation. ○ Non-human animal models allow for the identification of specific molecular mechanisms for behaviors related to human psychopathol ogy. Orthologous Genetic Variants ○ Orthologous genetic variants are similar but not identical to human polymorphisms. Orthologous genetic variants allow for more direct comparison of non -human animals and humans. PSYC0036 Genes and Behaviour Page 1 ○ Orthologous genetic variants allow for more direct comparison of non -human animals and humans. ○ The rhesus macaque monkey ortholog of the 5 -HTTLPR has been used to study the effects of maternal separation and chronic selective serotonin reuptake inhibitor administr ation on behavior and brain chemistry. Transgenic Mouse Models ○ Transgenic mouse models have provided insights into the genetic and molecular components of complex behavior and related psyc hopathology. ○ The serotonin transporter (5-HTT) knockout mouse model has informed our understanding of how the short allele in humans may contribute to depression. ○ Conditional transgenic models allow experimenters to control the exact timing of gene knockout. Limitations of Non-Human Animal Research ○ Non-human animal research is constrained by the phenotypes available for study. ○ Non-human animal models cannot always model complex human behavior and psychopathology. ○ Non-human animal models cannot be used to model all aspects of disorders. ○ The direct translation of non-human animal research to human studies is not fully understood. Focus on Neural Phenotypes ○ A focus on neural phenotypes provides a phenotype that is highly conserved across humans and non -human animal models. ○ Neural phenotypes can be measured with comparable techniques. Epigenetics ○ Epigenetics refers to changes in gene expression that are caused by factors other than the underlying DNA sequence. ○ These changes can be influenced by experiences, especially those occurring early in life. ○ Epigenetic changes can have long-lasting effects on brain structure, function, and behavior. Maternal Care and Epigenetics ○ Maternal care can influence the epigenetic regulation of the HPA axis, which is responsible for regulating stress responses. ○ Rat pups that receive elevated maternal licking and grooming (LG -ABN) have increased stress resilience. ○ This is due to epigenetic changes that increase the expression of glucocorticoid receptors (GRs) in the hippocampus. ○ These epigenetic changes persist throughout the rat's lifespan and promote increased maternal care in subsequent generations. Gene-Environment Interactions ○ Epigenetic factors can be influenced by genotype. ○ The COMT Val158 allele has a CpG island that is absent in the Met allele. ○ Methylation of this CpG island is associated with reduced stress and improved working memory performance. ○ These findings suggest that environment -related methylation can regulate gene expression and behavior. Implications for Psychopathology ○ Epigenetic regulation can explain how experiences can have long -lasting effects on biology and behavior. ○ This suggests that the impact of genetic variation on psychopathology will be experience and context dependent. Peripheral Blood Measures of Methylation ○ peripheral blood measures of methylation may not accurately reflect methylation patterns in the brain. ○ More research is needed to determine how well these measures represent methylation in the human brain. GWAS: identification of novel proteins and pathways ○ GWASs have the potential to identify novel genes that could play important mechanistic roles within distinct neural systems. ○ GWASs overcome some of the impedances to traditional psychiatric GWASs (e.g., low penetrance, diagnostic heterogeneity, self -report bias). ○ Half of neurogenetics GWASs have identified single -nucleotide polymorphisms (SNPs) reaching stringent genome -wide significance. ○ Obtaining adequate sample sizes to detect the small effects of common polymorphisms is challenging. ○ Data-driven profiles from GWAS data could be developed and applied to independent data sets for replication. ○ The inclusion of environmental measures of stress experience may aid the detection of polymorphic variants that are only asso ciated with individual differences under certain circumstances. ○ Given the increasing economy of genome-wide genotyping, investigators may wish to collect DNA to pool with other investigators for GWAS analyses and/or to use for c andidate gene investigations. Mechanisms ○ Recent developments in our understanding of gene transcription and translation will undoubtedly affect all research in geneti cs. ○ Recent work has documented widespread sequence differences between RNA and DNA. ○ Emerging expression quantitative trait loci (eQTL) research suggests that gene transcripts can be influenced by multiple and even distal genomic regions. ○ A number of studies have documented that epigenetic fingerprints are themselves heritable. Clinical Relevance Clinical Relevance of Neurogenetics Research ○ Neurogenetics research has the potential to improve mental health. ○ Neurogenetics research can predict individual differences in brain, behavior, risk for psychopathology, and treatment respons e. ○ Neurogenetics research can inform the development of novel strategies for treatment and prevention. Pathway Identification ○ Neurogenetics research can identify pathways mediating relationships between genes, behavior, and clinical symptoms. ○ This information can be used to identify novel therapeutic targets. ○ These targets can be tailored to specific individuals in the context of personalized medicine. Research Domain Criteria (RDoC) Project ○ The National Institute of Mental Health has launched the RDoC project to integrate findings from neurogenetics research into future diagnostic systems and treatment options. Predicting behaviour Using Neurogenetics Linking Genetic Differences to Brain Function and Behavior ▪ Neurogenetics research has shown that genetic differences are associated with differences in brain function and behavior. ▪ For instance, the short allele of the 5-HTTLPR is linked to increased amygdala reactivity, which has been associated with behavioral responsiveness to stress and threat. Indirect Pathways: Gene-Brain-Behavior ▪ Mediation analyses can be used to establish meaningful links between genes, brain, and behavior. ▪ These analyses model indirect pathways between genetic variation and behavior via the brain. Example: 5-HT1A Gene Polymorphism and Trait Anxiety ▪ A study by the author's research group examined a polymorphism in the promoter region of the 5-HT1A gene. ▪ The G allele of this polymorphism is associated with increased gene expression, 5-HT1A auto-receptor density, and decreased serotonin signaling. ▪ The study found that this polymorphism indirectly accounted for over 9% of the variance in trait anxiety through its effects on threat-related amygdala reactivity. Indirect Pathway Insights ▪ Neurogenetics research can detect indirect associations between genes and behavior through the brain, even when no direct gene-behavior link is evident. ▪ The indirect pathway, gene-brain-behavior, can advance our understanding of both etiologic and pathophysiologic mechanisms in psychiatry. Treatment and prevention Identifying Novel Therapeutic Targets ▪ Neurogenetics research can identify novel therapeutic targets by deconstructing the molecular mechanisms underlying gene-brain-behavior pathways. ▪ For instance, combining evidence linking HTR1A rs6295 with anxiety through amygdala reactivity with prior work on the polymorphism's effects on negative feedback inhibition suggests that targeting 5-HT1A autoreceptors may enhance the effectiveness of selective serotonin reuptake inhibitor treatment. ▪ Neurogenetics research on TREK1 has not only identified a novel therapeutic target (antagonism of TREK1 autoreceptors) but also a marker for individualized treatment (C allele homozygotes). Informing Treatment and Prevention Strategies ▪ Neurogenetics research can inform treatment and prevention strategies by identifying genetic variants associated with treatment response. ▪ For example, variation in the human TREK1 gene (KCNK2) has been linked to antidepressant treatment response. Examples: HTR1A rs6295 and Anxiety □ HTR1A rs6295 is associated with increased gene expression, 5-HT1A autoreceptor density, and decreased serotonin signalling. □ This polymorphism indirectly accounts for over 9% of the variance in trait anxiety through its effects on threat -related amygdala reactivity. □ Targeting 5-HT1A autoreceptors may produce greater clinical effect, especially in combination with selective serotonin reuptake inhibitor treatment. TREK1 and Depression □ TREK1 is a background potassium channel. □ Deletion of TREK1 in mice results in a depression-resistant phenotype. □ Variation in the human TREK1 gene (KCNK2) has been linked to depression, blunted striatal response to reward, and antidepress ant treatment response. □ TREK1 antagonists may be a promising novel antidepressant treatment. Summary Challenges in Neurogenetics Research ○ Small effect sizes of individual variables ○ Lack of detailed mechanisms ○ Need for clinical translation of research findings Addressing Challenges ○ Incorporate environment and epistatic interactions into neurogenetics models ○ Construct multilocus genetic profiles ○ Utilize non-human animal and epigenetics research ○ Apply GWASs to identify novel proteins involved in neural function Clinical Relevance of Neurogenetics Research ○ Informing ongoing efforts to improve treatment ○ Revolutionizing our understanding of individual differences in genes, brain, behavior, and psychopathology ○ Manipulating these systems for the benefit of individuals and society Genome-wide association studies of brain imaging phenotypes in UK Biobank (Elliott et al., 2018) Introduction Brain structure and function ○ Can be measured non-invasively using magnetic resonance imaging (MRI) ○ Varies between individuals ○ Effects of neurological and psychiatric disorders can be seen in MRI data MRI modalities Structural MRI ○ Measures brain anatomy ○ Includes tissue and structure volumes Other MRI modalities ○ Map different biological markers ○ Include venous vasculature, microbleeds, and aspects of white matter microstructure Brain function ○ Typically measured using task-based functional MRI (fMRI) ○ Uses imaging sensitive to local changes in blood oxygenation and flow caused by brain activity in grey matter Brain connectivity ○ Can be divided into functional connectivity and structural connectivity Functional connectivity PSYC0036 Genes and Behaviour Page 2 Functional connectivity ▪ Measures spontaneous temporal synchronizations between brain regions using fMRI with subjects scanned at rest Structural connectivity ▪ Measures the physical connections between brain regions based on how water molecules diffuse within white matter tracts ▪ Measured using diffusion MRI (dMRI) UK Biobank ○ A rich, long-term prospective epidemiological study of 500,000 volunteers ○ Participants were 40–69 years old at recruitment ○ One aim is to acquire as rich data as possible before disease onset ○ Identification of disease risk factors and early markers will increase over time with emerging clinical outcomes Brain and body imaging extension ○ Will scan 100,000 participants by 2020 ○ Brain imaging includes three structural modalities, resting and task -based fMRI, and diffusion MRI Automated image processing pipeline ○ Removes artefacts and renders images comparable across modalities and participants ○ Generates thousands of image-derived phenotypes (IDPs) Genetic data ○ Collected using a purpose-designed genotyping array ○ Consists of about 96 million genetic variants in almost 500,000 participants Joint analysis of genetic and brain imaging datasets ○ Presents a unique opportunity for uncovering the genetic bases of brain structure and function ○ In this study, we carried out genome-wide association studies (GWASs) for 3,144 IDPs Previous large-scale GWAS imaging studies ○ Have focused on narrower ranges of phenotypes ○ We expect that the homogeneous image acquisition and genetic data assay in UK Biobank will boost the power of our study Ethics ○ The UK Biobank has approval from the North West Multi -centre Research Ethics Committee (MREC) to obtain and disseminate data and samples from the participants ○ Written informed consent was obtained from all participants Results ○ All results are available on the Oxford Brain Imaging Genetics (BIG) web browser ○ The browser also includes GWAS results from more than 2,500 other traits and diseases Heritability and genetic correlations of IDPs SNP heritability of IDPs ○ 1,578 of 3,144 IDPs show significant SNP heritability ○ Volumetric measures are the most heritable structural MRI IDPs, while cortical thicknesses are the least ○ Tractography-based diffusion MRI IDPs show lower heritability than tract -skeleton-based IDPs ○ Resting-state fMRI functional connectivity edges show the lowest levels of SNP heritability, with only 235 of 1,771 IDPs being signif icant ○ Four of the six resting fMRI features identified by independent component analysis (ICA) are much more highly heritable ○ The task-related fMRI IDPs do not show significant evidence of SNP heritability ○ Subcortical volumes show lower levels of SNP heritability than previously estimated in twin studies Comparison of GWAS results with ENIGMA consortium ○ There was a strong correlation between the studies, suggesting that there were no major differences in how these phenotypes w ere measured ○ In all cases a perfect genetic correlation of 1 lies within the 95% confidence intervals Genetic correlations between IDPs ○ There is a range of both strong and weak, positive and negative genetic correlations between the IDPs Significant associations and IDPs and SNPs SNP associations ○ Identified 1,262 significant associations between SNPs and the 3,144 IDPs ○ The associations spanned all classes of IDPs, except task -related fMRI ○ 844 and 455 associations replicated at the 5% significance level using two smaller replication datasets ○ Estimated that there are approximately 427 distinct associated genetic regions (clusters) ○ Found 368 significant associations between genetic regions and distinct IDPs at a threshold of −log10(P) > 11 ○ Found no appreciable change in these GWAS results when they included a set of potential body confound measures in addition to the main set of imaging confound measures Replication ○ Replicated the majority of the loci identified by the ENIGMA consortium in two separate GWASs of seven brain subcortical volu me IDPs in up to 13,171 subjects ○ Replicated an association between volume of white matter hyperintensities (‘lesions’) and SNPs in TRIM47 SNP function ○ Most of the SNPs in the 38 loci are either in genes or in high linkage disequilibrium with SNPs that are themselves in the ge nes of interest ○ Many are significant expression quantitative trait loci (eQTLs) in the GTEx database ○ Found 17 genetic loci that can be linked to genes that broadly contribute to brain development, patterning and plasticity Examples of SNP associations ○ Identified many associations between T2* in the caudate nucleus, putamen and pallidum and SNPs in genes or near genes that ar e known to affect iron transport and storage ○ Identified four SNPs that either encode or are eQTLs of genes involved in transport of nutrients and minerals ○ Found effects of rs4428180 (TF) on T2* not just in the putamen and pallidum, but also in additional, smaller regions of subco rtical structures not included as IDPs ○ Found grey matter volume effects across the entire brain associated with rs13107325 (SLC39A8) in the anterior cingulate corte x ○ Found that the vast majority of forebrain white matter -related dMRI IDPs were associated with SNPs related to genes that encode proteins involved in the extracellular matrix and ep idermal growth factor signalling Additional examples of meaningful correspondences between brain IDPs and significantly associated genes ○ Found a significant association between the volume of the fourth ventricle and a SNP in, and eQTL of, ALDH1A2 (rs2642636) ○ Found two SNPs associated with dMRI IDPs of the crossing pontine tract in genes that regulate axon guidance and fasciculation during development Multi-phenotype association tests Multi-trait tests for IDPs ○ An alternative strategy for analysing large numbers of IDPs is to use multi -trait tests that fit joint models of associations to groups of IDPs. ○ These approaches can use estimates of genetic correlation to boost power. ○ In addition, by analysing P traits in one GWAS, these tests can avoid the need to correct for multiple genome -wide scans. Results of multi-trait tests ○ We used a multi-trait test to analyse 23 groups of IDPs with up to 243 IDPs per group. ○ These IDP groups were chosen to cover the majority of the IDP classes with significant IDP correlations in each grouping. ○ Overall, across these 23 groups, we found 278 SNPs at about 160 loci associated with –log10(P) >7.5. ○ Of these 278 SNPs, 170 survived a correction for 23 scans with –log10(P) >8.86 and 138 of these 170 SNPs had a P value