Genome-wide association studies point scientists toward chromosome loci that influence disease risk, but for the most part they yield ZIP codes rather than specific genetic addresses, many of them in non-coding regions. “GWAS have undoubtedly provided insight … but they do not necessarily tell us the important disease-linked genes,” said Mina Ryten of University College London. In the August 31 Nature Neuroscience online, Ryten and the U.K. Brain Expression Consortium describe a new resource to help do just that. They have created a database of expression-quantitative trait loci (eQTLs)—single-nucleotide polymorphisms (SNPs), either within or outside protein-coding genes, that somehow influence the expression of one or more genes. Covering 10 different regions of the brain, the Brain eQTL Almanac includes some that are important in neurodegeneration, such as the hippocampus and substantia nigra.
“Once people start rolling up their sleeves and digging around in the data to look at their favorite genes, we will see the value of this [database],” commented Alison Goate of Washington University in St. Louis, who was not involved in the study. Researchers may start with an SNP and look for the genes it affects, or start with a gene and use the data to build a network of eQTLs that regulate it and related genes, Ryten explained.
Ryten and co-senior author Michael Weale of King’s College London led the work as part of a large collaboration with the U.K. Brain Expression Consortium and North American Brain Expression Consortium to analyze postmortem tissues from 134 neurologically healthy brain donors. They selected the 10 brain regions based on the collaborators’ interest in neurological disease. In addition to the hippocampus and substantia nigra, they looked at the cerebellar cortex, frontal cortex, inferior olivary nucleus, occipital cortex, putamen, temporal cortex, thalamus, and intralobular white matter. Ryten would have liked to test even more regions, but funding was limited. However, the authors continue to collect tissues to add to the database. First author Daniah Trabzuni of University College London isolated both DNA and RNA from those samples to determine SNP profiles and gene-expression levels.
To identify eQTLs, co-first author Adaikalavan Ramasamy, now at Oxford University in the United Kingdom, led a large and complex genome-wide association study (GWAS). Traditionally a GWAS looks for SNPs that are associated with a binary phenotype—disease or no disease. In this case, Ramasamy considered a series of continuous phenotypes, that is, the RNA levels of all genes in each tissue type. In addition, he correlated SNPs to splicing patterns. From this analysis, co-first author Sebastian Guelfi of UCL assembled the Brain eQTL Almanac database. One shortfall is that while eQTLs may occur far from their target gene—even on different chromosomes—the study lacked power to detect these distant modifiers, Ryten said. The authors limited their analysis to so-called cis-eQTLs, which are located within one megabase of the gene they influence.
The researchers found about 30,000 cis-eQTLs that regulated 8,573 genes in some fashion. Because they were interested in neurological disease, they compared their eQTLs to a list of 385 previously published SNPs from the U.S. National Human Genome Research Institute catalog. These 385 associate with brain-related traits such as Parkinson’s. About 17 percent of these SNPs appeared in the eQTL database, as well. About 23 percent of SNPs linked specifically to adult-onset neurological disorders also appeared in the eQTLs almanac. In this paper, the study authors examined just a few of those SNPs in detail, offering a glimpse into what their database can really do.
Sometimes the eQTL showed up within the gene it regulated. For example, a risk SNP for Alzheimer’s disease that lies within the complement receptor 1 (CR1) gene on chromosome 1 turned out to be an eQTL for the CR1 gene (see Jun 2010 news story). No big surprise.
More exciting to Ryten were eQTLs that pointed at the existence of novel disease genes at some distance from the SNP itself. There were several such cases: 61 out of 149 cis-eQTL signals for brain diseases were completely distinct from their target gene. For example, a chromosome 19 risk SNP for amyotrophic lateral sclerosis lies in an intron of the gene UNC13A, which regulates neurotransmitter release and seemed a shoo-in for an ALS risk gene (see Sep 2009 news story). However, the Brain eQTL Almanac indicates that the ALS SNP regulates not UNC13A, but a potassium channel called the potassium intermediate/small conductance calcium-activated channel, subfamily N, member 1 (KCNN1). KCNN1 controls neuron excitability.
Curiously, the eQTL correlated with KCNN1 expression only in the frontal cortex. The authors do not know how a frontal cortex eQTL could influence ALS risk. Perhaps the SNP is also an eQTL for KCNN1 in motor neurons of the spinal cord, suggested Ryten, who noted that spinal cord tissue was excluded from the analysis because it is hard to obtain. Based on their functions, either UNC13A or KCNN1 could be the real ALS risk factor, the authors wrote, but they argued that the eQTL data make a stronger case for KCNN1.
The authors also examined a Parkinson’s disease-related SNP (Pankratz et al., 2012). Because it sits in a non-coding region of chromosome 6 that is chock-full of genes, researchers have struggled to determine which gene it affects. In the eQTL database, that SNP points to the gene for major histocompatibility complex, class II, DQ alpha 2 (HLA-DQA2). Several studies have previously linked immune cells and inflammation to PD (see Apr 2009 news story; Feb 2009 news story; Jul 2008 news story).
In addition to investigating GWAS hits, scientists can use the almanac to understand the basic biology of gene regulation in the brain. For example, each brain tissue contained a cluster of eQTLs that were most influential (see image above). That means every brain region has its own particular way of regulating genes. This finding may help to explain why an inherited mutation, though it occurs in every cell in the body, can cause disease in a specific part of the brain. For example, Parkinson’s genes may be subject to specific regulation in the substantia nigra, making cells in that region more vulnerable. If substantiated, this approach might help scientists understand the frequently discussed problem of selective vulnerability in neurodegenerative disease.
With these examples, the study authors have “just scratched the surface,” said Goate. Philip De Jager of Harvard University, who did not participate in the work, agreed. “This study significantly extends the range of brain regions that have been profiled and examined to identify eQTLS,” he wrote in an email to Alzforum. “It will serve as an excellent resource for future investigations, particularly those involving neuropsychiatric diseases that have regionally specific effects” (see full comment on the Alzforum site).
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