Colocalization of GWAS and eQTL Signals Detects Target Genes

Farhad Hormozdiari recently developed a method for combining genome-wide association studies (GWASs) and quantitative trait loci (eQTL) studies in a statistical framework that quantifies the probability of each variant to be causal while allowing an arbitrary number of causal variants. Together with collaborators at the University of Oxford and Broad Institute of MIT and Harvard, we present a paper in The American Journal of Human Genetics. Here, we describe eQTL and GWAS CAusal Variants Identification in Associated Regions (eCAVIAR). We apply our approach to datasets from several GWASs and eQTL studies in order to assess its accuracy and potential contributions to colocalization and fine-mapping.

Integrating GWASs and eQTL studies is a promising way to explore the mechanism of non-coding variants on diseases. Integration of GWAS and eQTL data is challenging due to the uncertainty induced by linkage disequilibrium (LD), the non-random association of alleles at different loci, and presence of loci that harbor multiple causal variants (allelic heterogeneity). Current methods assume that each locus contains a single causal variant and expect loci to be independent and associated randomly.

eCAVIAR is a novel probabilistic model for integrating GWAS and eQTL data that extends the CAVIAR (Hormozdiari et al. 2014) framework to explicitly estimate the posterior probability of the same variant being causal in both GWAS and eQTL studies, while accounting for allelic heterogeneity and LD. Our approach can quantify the strength between a causal variant and its associated signals in both studies, and it can be used to colocalize variants that pass the genome-wide significance threshold in GWAS. For any given peak variant identified in GWAS, eCAVIAR considers a collection of variants around that peak variant as one single locus.

We apply eCAVIAR to the Meta-Analyses of Glucose and Insulin-related traits Consortium (MAGIC) dataset and GTEx dataset to detect the target gene and most relevant tissue for each GWAS risk locus. When applied to the MAGIC dataset’s 2 phenotypes, eCAVIAR identifies genetic variants that are causal in both eQTL and GWAS. Further, eCAVIAR detects a large number of loci where the GWAS causal variants are clearly distinct from the causal variants in the eQTL data. Interestingly, eCAVIAR also identifies genes that colocalize in one tissue yet can be excluded in others. For the majority of loci in which we identify a single variant causal for both GWAS and eQTL, eCAVIAR implicates more than one causal variant across the 45 tissues.

We observe that eCAVIAR outperforms existing methods even when there are different values of non-colocalization. Using simulated datasets, we compared accuracy, precision, and recall rate of eCAVIAR to RTC (Nica et al. 2010) and COLOC (Giambartolomei et al. 2014), two current methods for eQTL and GWAS colocalization. Our results show that eCAVIAR has high confidence for selecting loci to be colocalized between the GWAS and eQTL data and is conservative in selecting a locus to be colocalized.

We hope that future applications of eCAVIAR will advance identification of specific GWAS loci that share a causal variant with eQTL studies in a tissue, thus providing insight into presently unclear disease mechanisms.

Figure2

Overview of eCAVIAR.

 

eCAVIAR was created by Farhad Hormozdiari, Ayellet V. Segre, Martijn van de Bunt, Xiao Li, Jong Wha J Joo, Michael Bilow, Jae Hoon Sul, Bogdan Pasaniuc and Eleazar Eskin. The article is available at: http://www.cell.com/ajhg/abstract/S0002-9297(16)30439-6.

Visit the following page to download CAVIAR and eCAVIAR: http://genetics.cs.ucla.edu/caviar/

The full citation to our paper is:

Hormozdiari, Farhad; van de Bunt, Martijn; Segrè, Ayellet V; Li, Xiao; Joo, Jong Wha J; Bilow, Michael; Sul, Jae Hoon; Sankararaman, Sriram; Pasaniuc, Bogdan; Eskin, Eleazar

Colocalization of GWAS and eQTL Signals Detects Target Genes. Journal Article

In: Am J Hum Genet, 2016, ISSN: 1537-6605.

Abstract | Links | BibTeX

Our paper builds upon a method introduced in a previous publication:

Hormozdiari, Farhad; Kostem, Emrah ; Kang, Eun Yong ; Pasaniuc, Bogdan ; Eskin, Eleazar

Identifying causal variants at Loci with multiple signals of association. Journal Article

In: Genetics, 198 (2), pp. 497-508, 2014, ISSN: 1943-2631.

Abstract | Links | BibTeX

Chromosome conformation elucidates regulatory relationships in developing human brain

Farhad Hormozdiari, a recent ZarLab alumni, contributed to a paper published this week in Nature. Our paper reports new findings on genetic factors related to human cognition and neurodevelopmental disorders, the result of a collaboration with UCLA’s David Geffen School of Medicine and the School of Biotechnology and Biomolecular Sciences at University of New South Wales. Farhad implemented the software package CAVIAR which was utilized to identify the causal variants and interpretation of data.

Neurodevelopmental disorders such as autism and schizophrenia are thought to originate during embryonic development of the cerebral cortex. The project focused on the 3D interactions of genome-wide chromatin contacts, the areas of a cell’s nucleus that package chromosomes into DNA and influence cell replication. Chromatin contacts regulate gene expression in specific tissues, and mapping their interactions within chromosomes provides important biological insights into the malfunctioning gene regulatory mechanisms that drive these disorders.

The project generated high-resolution 3D maps of chromatin contacts active during development of the cortex region of the human brain. These maps enabled a large-scale annotation of previously uncharacterized regulatory mechanisms tied to the evolution of human cognition and disease. Using this data, the paper identified hundreds of genes involved with human cognitive function. Next, the paper integrated chromatin contacts with noncoding variants previously identified in schizophrenia genome-wide association studies (GWAS) and performed several analyses to explore the relationships of interactions between chromatin and biological function.  One of the uses of CAVIAR in the paper was to verify that the causal variants involved in schizophrenia GWAS are in fact compatible with the 3D maps of chromatin contacts.

The paper also found several highly interacting chromatin regions that correlate with levels of gene expression and are associated with promoters, positive transcriptional regulators, and enhances—areas of the genome that shape cell replication and neurological development. The paper identified specific sets of genes enriched in known intellectual disability risk genes, including mutations known to cause autosomal recessive primary microcephaly. The GWAS results identified approximately 500 genome-wide significant schizophrenia-associated loci, about 30% of which interact with schizophrenia SNPs exclusively in developing brain tissue. Genome editing in human neural progenitors suggests that one of these distal schizophrenia GWAS loci regulates FOXG1 expression, supporting its potential role as a schizophrenia risk gene.

This work provides a framework for understanding the effect of non-coding regulatory elements on human brain development and the evolution of cognition, and highlights novel mechanisms underlying neuropsychiatric disorders. Read the paper for a detailed account of our data, methods, and results: http://www.nature.com/nature/journal/vaop/ncurrent/full/nature19847.html

The CAVIAR program was developed by Farhad Hormozdiari and is freely available for download on the following webpage: http://genetics.cs.ucla.edu/caviar/

The full citation to our paper is:

Won, Hyejung; de la Torre-Ubieta, Luis; Stein, Jason L; Parikshak, Neelroop N; Huang, Jerry; Opland, Carli K; Gandal, Michael J; Sutton, Gavin J; Hormozdiari, Farhad; Lu, Daning; Lee, Changhoon; Eskin, Eleazar; Voineagu, Irina; Ernst, Jason; Geschwind, Daniel H

Chromosome conformation elucidates regulatory relationships in developing human brain. Journal Article

In: Nature, 538 (7626), pp. 523-527, 2016, ISSN: 1476-4687.

Abstract | Links | BibTeX

 

figure

Annotation of schizophrenia-associated loci identified by a GWAS of chromatin contact data.

Identification of causal genes for complex traits (CAVIAR-gene)

Although genome-wide association studies (GWAS) have identified thousands of variants associated with common diseases and complex traits, only a handful of these variants are validated to be causal. We consider ‘causal variants’ as variants which are responsible for the association signal at a locus. As opposed to association studies that benefit from linkage disequilibrium (LD), the main challenge in identifying causal variants at associated loci lies in distinguishing among the many closely correlated variants due to LD. This is particularly important for model organisms such as inbred mice, where LD extends much further than in human populations, resulting in large stretches of the genome with significantly associated variants. Furthermore, these model organisms are highly structured and require correction for population structure to remove potential spurious associations.

In our recently published work, we propose CAVIAR-Gene (CAusal Variants Identification in Associated Regions), a novel method that is able to operate across large LD regions of the genome while also correcting for population structure. A key feature of our approach is that it provides as output a minimally sized set of genes that captures the genes which harbor causal variants with probability q. Through extensive simulations, we demonstrate that our method not only speeds up computation, but also have an average of 10% higher recall rate compared with the existing approaches. We validate our method using a real mouse high-density lipoprotein data (HDL) and show that CAVIAR-Gene is able to identify Apoa2 (a gene known to harbor causal variants for HDL), while reducing the number of genes that need to be tested for functionality by a factor of 2.

In the context of association studies, the genetic variants which are responsible for the association signal at a locus are referred to in the genetics literature as the ‘causal variants.’ Causal variants have biological effect on the phenotype.

CAVIAR-Gene provides better ranking of the causal genes for Outbred, F2, and HMDP datasets. Panels a and b illustrate the results for Outbred genotypes for case where we have one causal and two causal genes, respectively. Panels c and d illustrate the results for F2 genotypes for case where we have one causal and two causal genes, respectively. Panels e and f illustrate the results for Outbred genotypes for case where we have one causal and two causal genes, respectively.

CAVIAR-Gene provides better ranking of the causal genes for Outbred, F2, and HMDP datasets. Panels a and b illustrate the results for Outbred genotypes for case where we have one causal and two causal genes, respectively. Panels c and d illustrate the results for F2 genotypes for case where we have one causal and two causal genes, respectively. Panels e and f illustrate the results for Outbred genotypes for case where we have one causal and two causal genes, respectively.

Generally, variants can be categorized into three main groups. The first group is the causal variants which have a biological effect on the phenotype and are responsible for the association signal. The second group is the variants which are statistically associated with the phenotype due to LD with a causal variant. Even though association tests for these variants may be statistically significant, under our definition, they are not causal variants. The third group is the variants which are not statistically associated with the phenotype and are not causal.

CAVIAR-Gene is a statistical method for fine mapping that addresses two main limitations of existing methods. First, as opposed to existing approaches that focus on individual variants, we propose to search only over the space of gene combinations that explain the statistical association signal, and thus drastically reduce runtime. Second, CAVIAR-Gene extends existing framework for fine mapping to account for population structure. The output of our approach is a minimal set of genes that will contain the true casual gene at a pre-specified significance level.  The output of our approach is a minimal set of genes that will contain the true casual gene at a pre-specified significance level. This gene set together with its individual gene probability of causality provides a natural way of prioritizing genes for functional testing (e.g. knockout strategies) in model organisms. Through extensive simulations, we demonstrate that CAVIAR-Gene is superior to existing methodologies, requiring the smallest set of genes to follow-up in order to capture the true causal gene(s).

Building off our previous work with CAVIAR,  CAVIAR-Gene takes as input the marginal statistics for each variant at a locus, an LD matrix consisting of pairwise Pearson correlations computed between the genotypes of a pair of genetic variants, a partitioning of the set of variants in a locus into genes, and the kinship matrix which indicates the genetic similarity between each pair of individuals. Marginal statistics are computed using methods that correct for population structure.  We consider a variant to be causal when the variant is responsible for the association signal at a locus and aim to discriminate these variants from ones that are correlated due to LD.

In model organisms, the large stretches of LD regions result in a large number of variants associated in each region, thus making CAVIAR computationally

infeasible. Instead of producing a rho causal set of SNPs, CAVIAR-gene detects a ‘q causal gene set’ which is a set of genes in the locus that will contain the actual causal genes with probability of at least q.

For further details of our new method, CAVIAR-gene, view our full paper here:

Hormozdiari, Farhad; Kichaev, Gleb; Yang, Wen-Yun Y; Pasaniuc, Bogdan; Eskin, Eleazar

Identification of causal genes for complex traits. Journal Article

In: Bioinformatics, 31 (12), pp. i206-i213, 2015, ISSN: 1367-4811.

Abstract | Links | BibTeX